119
112
Earthquakes Tsunami Volcanoes Landslides
219 285
Subsidence Climate Floods Beaches Hurricanes Tornadoes Wildfires Asteroids Case in Point Locations Numbers on map refer to page numbers in book.
483
56 56
174
317
465
349
341 88
146
216
345
351 340
241
85
381
319 220
88
462
316
244
380
322
54 483 463
420 321 425 447 315
342
411
418 423
57
Natural Hazards and Disasters Second Edition
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Natural Hazards and Disasters Second Edition
Donald Hyndman University of Montana
David Hyndman Michigan State University
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To Shirley and Teresa for their endless encouragement and patience
About the Authors DONALD HYNDMAN is an emeritus professor in the department of geology at the University of Montana, where he has taught courses in natural hazards, regional geology, igneous and metamorphic petrology, volcanology, and advanced igneous petrology. He continues to lecture on natural hazards. Donald is co-originator and co-author of six books in the Roadside Geology series and one on the geology of the Pacific Northwest, and is also the author of a textbook on igneous and metamorphic petrology. His B.S. in geological engineering is from the University of British Columbia, and his Ph.D. in geology is from the University of California, Berkeley. He has received the Distinguished Teaching Award and the Distinguished Scholar Award, both given by the University of Montana. He is a fellow of the Geological Society of America. DAVID HYNDMAN is an associate professor in the department of geological sciences at Michigan State University, where he teaches courses in natural hazards, environmental geology, physical geology, and advanced hydrogeology. His B.S. in hydrology and water resources is from the University of Arizona, and his M.S. in applied earth sciences and Ph.D. in geological and environmental sciences are from Stanford University. David is an associate editor for the journal Ground Water, is a former associate editor for water resources research, was selected as a Lilly Teaching Fellow, and has received the Ronald Wilson Teaching Award. He was the 2002 Darcy Distinguished Lecturer for the National Groundwater Association and is a fellow of the Geological Society of America.
vi
Brief Contents
1 Natural Hazards and Disasters 1 2 Plate Tectonics and Physical Hazards 12 3 Earthquakes and Their Causes 32 4 Earthquake Prediction and Mitigation 61 5 Tsunami 97 6 Volcanoes: Tectonic Environments and Eruptions 124 7 Volcanoes: Hazards and Mitigation 150 8 Landslides and Other Downslope Movements 188 9 Sinkholes, Land Subsidence, and Swelling Soils 225 10 Climate Change and Weather Related to Hazards 249 11 Streams and Flood Processes 290 12 Floods and Human Interactions 325 13 Waves, Beaches, and Coastal Erosion 357 14 Hurricanes and Nor’easters 386 15 Thunderstorms and Tornadoes 430 16 Wildfires 451 17 Impact of Asteroids and Comets 471 18 The Future: Where Do We Go From Here? 489 APPENDIX
1
Geological Time Scale 500
APPENDIX
2
Mineral and Rock Characteristics Related to Hazards 501
APPENDIX
3
Conversion Factors 512
Glossary 514 Index 527 vii
Contents
Preface xvi
1 Natural Hazards and Disasters Living in Harm’s Way
1
1
Catastrophes in Nature 2 Human Impact of Natural Disasters Predicting Catastrophe
Earthquake Waves 44 Types of Earthquake Waves Seismographs 45 Locating Earthquakes 46
3
3
Relationships Among Events
6
Key Points 10 Key Terms 11 Questions for Review
41
44
Earthquake Size and Characteristics Earthquake Intensity 47 Earthquake Magnitude 48
Mitigating Hazards 7 Land Use Planning 7 Insurance 8 The Role of the Government 8 The Role of Public Education 9 Living with Nature
Continental Spreading Zones Intraplate Earthquakes 42
47
Ground Motion and Failure During Earthquakes 50 Ground Acceleration and Shaking Time Secondary Ground Effects 52
9
50
CASE IN POINT
A Major Earthquake on a Blind Thrust Fault— Northridge Earthquake, California, 1994 54
11
CASE IN POINT
2 Plate Tectonics and Physical Hazards The Big Picture
CASE IN POINT
12
Development of a Theory Earth Structure
12
Paleoseismology Provides a Record of a Giant Event—Pacific Northwest Earthquake, 1700 56
13
17
CASE IN POINT
Hazards and Plate Boundaries Divergent Boundaries 21 Convergent Boundaries 23 Collision of Continents 25 Transform Boundaries 26 Hotspot Volcanoes 27 Key Points 30 Key Terms 30 Questions for Review
Damage Mitigated by Depth of Focus— Nisqually Earthquake, Washington, 2001 56
21
CASE IN POINT
Amplified Shaking over Loose Sediment— Mexico City Earthquake, 1985 57 Key Points 59 Key Terms 59 Questions for Review
60
31
3 Earthquakes and Their Causes Earthquake Devastates South Asia
32
Faults and Earthquakes 33 Causes of Earthquakes 33 Tectonic Environments of Faults Transform Faults 37 Subduction Zones 39
viii
A Major Subduction-Zone Earthquake— Chile, 1960 55
36
32
4 Earthquake Prediction and Mitigation Predicted Earthquake Arrives on Schedule Predicting Earthquakes 62 Earthquake Precursors 63 Early Warning Systems 64 Prediction Consequences 64
61 61
Earthquake Probability 65 Forecasting Where Faults Will Move Populations at Risk 70 The San Francisco Bay Area The Los Angeles Area 74
Tsunami Hazard Mitigation 105 Tsunami Warnings 106 Surviving a Tsunami 108
65
Future Giant Tsunami 108 Pacific Northwest Tsunami: Historical Record of Giant Tsunami 108 Kilauea, Hawaii: Potentially Catastrophic Volcano Flank Collapse 110 Canary Islands: Potential Catastrophe in Coastal Cities Across the Atlantic 111
72
Minimizing Earthquake Damage 74 Structural Damage and Retrofitting 75 Earthquake Preparedness 81 Land Use Planning and Building Codes 82 CASE IN POINT
CASE IN POINT
Earthquake Fills a Seismic Gap—Loma Prieta Earthquake, California, 1989 85
Immense Local Tsunami from a Landslide— Lituya Bay, Alaska, 1958 112
CASE IN POINT
CASE IN POINT
One in a Series of Migrating Earthquakes— Izmit Earthquake, Turkey, 1999 87
An Ocean-Wide Tsunami from a Giant Earthquake—Chile Tsunami, 1960 114
CASE IN POINT
CASE IN POINT
A Case of Equal-Interval Earthquakes— Parkfield Earthquakes, California 88
Lack of Warning and Education Costs Lives— Sumatra Tsunami, 2004 116
CASE IN POINT
CASE IN POINT
Devastating Fire Caused by an Earthquake— San Francisco, California, 1906 88
Subduction-Zone Earthquake Generates a Major Tsunami—Anchorage, Alaska, 1964 119
CASE IN POINT
Critical View
Damage Depends on Building Design— Kobe Earthquake, Japan, 1995 90
Key Points 122 Key Terms 123 Questions for Review
CASE IN POINT
Collapse of Poorly Constructed Buildings— Kashmir Earthquake, Pakistan, 2005 91 Building Code Not Enforced—Bhuj Earthquake, India, 2001 93
96
97 97
Tsunami Generation 98 Earthquake-Generated Tsunami 98 Tsunami Generated by Volcanic Eruptions 99 Tsunami from Fast-Moving Landslides or Rockfalls 100 Tsunami from Volcano Flank Collapse 100 Tsunami from Asteroid Impact 101 Tsunami Movement
124
Introduction to Volcanoes: Generation of Magmas 125 Magma Properties and Volcanic Behavior
5 Tsunami Swept Away
Cascade Range Volcanoes Are Active
94
Key Points 95 Key Terms 95 Questions for Review
123
6 Volcanoes: Tectonic Environments and Eruptions 124
CASE IN POINT
Critical View
121
102
Tsunami on Shore 103 Coastal Effects 103 Run-Up 104 Period 104
Tectonic Environments of Volcanoes Spreading Zones 129 Subduction Zones 130 Hotspots 130
125
129
Volcanic Eruptions and Products 131 Nonexplosive Eruptions: Lava Flows 131 Explosive Eruptions: Pyroclastic Materials 131 Styles of Explosive Eruptions 134 Types of Volcanoes 135 Shield Volcanoes 135 Cinder Cones 139 Stratovolcanoes 140 Lava Domes 141 Giant Continental Calderas
141
CONTENTS
ix
8 Landslides and Other Downslope Movements
CASE IN POINT
Deadly Lahar—Mount Pinatubo, Philippines, 1991 142
Falling Mountains
CASE IN POINT
CASE IN POINT
Future Eruptions of a Giant Caldera Volcano— Yellowstone Volcano, Wyoming 146
Slope Material 193 Internal Surfaces 193 Clays and Clay Behavior
148
7 Volcanoes: Hazards and Mitigation Mount St. Helens Erupts
150 150
Volcanic Hazards 152 Lava Flows 152 Pyroclastic Flows and Surges Ash and Pumice Falls 155 Volcanic Mudflows 157 Poisonous Gases 160
164
Mitigation of Damage 165 Controlling Lava Flows 165 Warning of Mudflows 165 Populations at Risk 165 Vesuvius and Its Neighbors 165 The Cascades of Western North America A Look Ahead 174
168
CASE IN POINT
CASE IN POINT
Pyroclastic Flows Can Be Deadly—Mount Pelée, Martinique, West Indies 179 CASE IN POINT
The Catastrophic Nature of Pyroclastic Flows— Mount Vesuvius, Italy 181 CASE IN POINT
Even a Small Eruption Can Trigger a Major Debris Avalanche—Nevado del Ruiz, Colombia, 1985 183
x
CONTENTS
Hazards Related to Landslides 208 Earthquakes 208 Failure of Landslide Dams 209
Slippery Smectite Deposits Create Conditions for Landslide—Forest City Bridge, South Dakota 216 CASE IN POINT
A Coherent Translational Slide Triggered by Filling a Reservoir—The Vaiont Landslide, Italy 217 CASE IN POINT
A Rockfall Triggered by Blasting—Frank Slide, Alberta 219 CASE IN POINT
Cliffs Above Houses Can Pose a Severe Rockfall Hazard—Rockville Rockfall, Southwestern Utah 220 A High-Velocity Rock Avalanche Buoyed Up by Air—Elm, Switzerland 221
187
212
CASE IN POINT
CASE IN POINT
185
Key Points 186 Key Terms 186 Questions for Review
Causes of Landslides 194 Oversteepening 194 Overloading 195 Adding Water 195 Overlapping Causes 195
Mitigation of Damages from Landslides Record of Past Landslides 212 Landslide Hazard Maps 212 Engineering Solutions 214
Volcanic Precursors—Mount St. Helens Eruption, Washington, 1980 174
Critical View
193
Types of Downslope Movement 196 Rockfalls 196 Debris Avalanches 198 Rotational Slides and Slumps 201 Translational Slides 203 Lateral-Spreading Slides 204 Soil Creep 204 Snow Avalanches 205
153
Predicting Eruptions 162 Examining Ancient Eruptions 162 Eruption Warnings: Volcanic Precursors
188
Slope Processes 189 Slope and Load 189 Frictional Resistance 190 Cohesion and Water 191
Long Periods Between Collapse–Caldera Eruptions—Santorini, Greece 145
Key Points 148 Key Terms 148 Questions for Review
188
Critical View
Global Air Circulation Weather Fronts 254 Jet Stream 255
222
Key Points 223 Key Terms 224 Questions for Review
224
9 Sinkholes, Land Subsidence, and Swelling Soils Shrinking Ground
225
225
Types of Ground Movement
226
Sinkholes 226 Processes Related to Sinkholes 226 Types of Sinkholes 227 Areas That Experience Sinkholes 229 Land Subsidence 231 Mining Groundwater and Petroleum 231 Drainage of Organic Soils 232 Drying of Clays 234 Permafrost Thaw and Ground Settling 236 Swelling Soils
238
CASE IN POINT
Excessive Mining Causes Roof Collapse—Genessee Valley, New York State 241 CASE IN POINT
Subsidence Due to Groundwater Extraction— Venice, Italy 242 Subsidence Due to Groundwater Extraction— Mexico City, Mexico 244 CASE IN POINT
Differential Expansion over Layers of Smectite Clay—Denver, Colorado 244
260
Hazards Related to Weather and Climate Drought 262 Growing Deserts 264 Heat Waves 266 Snow and Ice 267 Atmospheric Cooling 268
262
Global Warming and the Greenhouse Effect 270 The Greenhouse Effect 270 Rising Levels of Greenhouse Gases 271 Consequences of Climate Change Warming Oceans 274 Precipitation Changes 274 Arctic Thaw 275 Sea-Level Rise 276 Global Ocean Circulation 277
273
281
CASE IN POINT
Climate Cooling from a Major Volcanic Eruption— Mount Tambora, Indonesia 283
246
Key Points 247 Key Terms 247 Questions for Review
Climatic Cycles 256 Days to Seasons 256 El Niño 257 North Atlantic Oscillation 259 Atlantic Multidecadal Oscillation Long-Term Climatic Cycles 260
Mitigation of Climate Change 278 The Kyoto Protocol 279 Alternative Energies 280 Sequestration of Greenhouse Gases Carbon Trading 282 The Political Side of the Emissions Problem 282
CASE IN POINT
Critical View
253
CASE IN POINT
Rising Sea Level Heightens Risk to Populations Living on a Sea-Level Delta—Bangladesh and Calcutta, India 283
248
10 Climate Change and Weather Related to Hazards Rapid Melting in the Arctic
249
249
Basic Elements of Climate and Weather 250 Hydrologic Cycle 250 Adiabatic Cooling and Condensation 251 Atmospheric Pressure and Weather 252 Coriolis Effect 252
CASE IN POINT
CO2 Sequestration Underground—The Weyburn Pilot Project 285 Critical View
286
Key Points 287 Key Terms 288 Questions for Review
288
CONTENTS
xi
11 Streams and Flood Processes Too Close to a River
290
290
Stream Flow and Sediment Transport 291 Stream Flow 291 Sediment Load and Grain Size 292 Sediment Transport and Flooding 294
Prolonged Summer Storms on Thick Soils— Blue Ridge Mountains Debris Flows 322 Key Points 323 Key Terms 324 Questions for Review
Flood Intensity 302 Stream Order 302 Flood Crests Move Downstream Flash Floods 303
The Great Flood of 1993
301
302
Flood Frequency and Recurrence Intervals 304 100-Year Floods and Floodplains 304 Recurrence Intervals and Discharge 305 Paleoflood Analysis 305 Problems with Recurrence Intervals 308 Mudflows, Debris Flows, and Other Flood-Related Hazards 309 Mudflows and Lahars 309 Debris Flows 310 Glacial Outburst Floods: Jökulhlaups 312 Other Hazards Related to Flooding 314 CASE IN POINT
Heavy Rainfall on Near-Surface Bedrock Triggers Flooding—Guadalupe River Upstream of New Braunfels, Texas, 2002 315
xii
324
12 Floods and Human Interactions
299
Flooding Processes 300 Bankfull Channel Width, Depth, and Capacity 300 Precipitation Intensity and Surface Runoff Floods on Water-Saturated or Frozen Ground 301
Desert Debris Flows and Housing on Alluvial Fans—Tucson, Arizona, Debris Flows, 2006 321 CASE IN POINT
Channel Patterns 295 Meandering Streams 295 Braided Steams 296 Bedrock Streams 298 Climate Controls on Stream Flow
CASE IN POINT
325
325
Development Effects on Floodplains 326 Urbanization 326 Fires, Logging, and Overgrazing 327 Mining 328 Bridges 329 Levees 329 Levee Failure 330 Unintended Consequences of Levees Wing Dams 332
331
Dams and Stream Equilibrium 332 Floods Caused by Failure of Human-Made Dams 332 Reducing Flood Damage 334 Land Use on Floodplains 334 Flood Insurance 335 Environmental Protection 337 Reducing Damage from Debris Flows Early Warning Systems 338 Trapping Debris Flows 339
337
CASE IN POINT
Addition of Sediment Triggers Flooding—Hydraulic Placer Mining, California Gold Rush, 1860s 340 CASE IN POINT
Streambed Mining Causes Erosion and Damage— Healdsburg, California 341
CASE IN POINT
CASE IN POINT
Major Flooding from a Minor Hurricane—Hurricane Agnes, June 1972 316
The Potential for Catastrophic Avulsion— New Orleans 342
CASE IN POINT
CASE IN POINT
Spring Thaw from the South on a North-Flowing River—The Red River, North Dakota 317
A Long History of Avulsion—Yellow River of China 344
CASE IN POINT
CASE IN POINT
A Flash Flood from an Afternoon Thunderstorm— Big Thompson Canyon, Northwest of Denver 319
Repeated Flooding in Spite of Levees—Mississippi River Basin Flood, 1993 345
CONTENTS
14 Hurricanes and Nor’easters
CASE IN POINT
Dams Can Fail—Failure of Teton Dam, Idaho 349
Costliest Natural Disaster in U.S. History
CASE IN POINT
Catastrophic Floods of a Long-Established City— Arno River Flood, Florence, Italy, 1966 350 CASE IN POINT
Proposed Development on a Floodplain— Sacramento–San Joaquin Valley, California 351 CASE IN POINT
Alluvial Fans Are Dangerous Places to Live— Venezuela Flash Flood and Debris Flow, 1999 353 Critical View
354
Key Points 355 Key Terms 355 Questions for Review
13 Waves, Beaches, and Coastal Erosion
357
357
Living on Dangerous Coasts
359
Waves and Sediment Transport 359 Wave Refraction and Longshore Drift Waves on Irregular Coastlines 362 Rip Currents 362 Beaches and Sand Supply 363 Beach Slope: An Equilibrium Profile Loss of Sand from the Beach 364 Sand Supply 366
361
Hurricanes, Typhoons, and Cyclones 387 Formation of Hurricanes and Cyclones 387 Hurricane-Strength Winds 388 Areas at Risk 388 Storm Damages 391 Storm Surges 392 Waves and Wave Damage 395 Winds and Wind Damage 397 Rainfall and Flooding 400 Deaths 400 Social and Economic Impacts 401 Climate Change and Hurricane Damage
402
Managing Future Damages 405 Natural Protections 405 Building Codes 406 Flood Insurance 407 Homeowners Insurance 407 Extratropical Cyclones and Nor’easters
408
CASE IN POINT
City Drowns in Spite of Levees—Hurricane Katrina 411 364
Erosion of Gently Sloping Coasts and Barrier Islands 366 Development on Barrier Islands 367 Dunes 369 Sea-Cliff Erosion
386
Hurricane Prediction and Planning 403 Uncertainty in Hurricane Prediction 403 Planning for Hurricanes 403 Evacuation 404
356
Coastal Cliff Collapse
386
371
Human Intervention and Mitigation of Coastal Change 374 Engineered Beach Protection Structures 374 Beach Replenishment 376 Zoning for Appropriate Coastal Land Uses 379
CASE IN POINT
Trapped on a Barrier Island—Galveston Hurricane, 1900 418 CASE IN POINT
Back-to-Back Hurricanes Amplify Flooding— Hurricanes Dennis and Floyd, 1999 420 CASE IN POINT
Floods, Landslides, and a Huge Death Toll in Poor Countries—Hurricane Mitch, Nicaragua and Honduras 421 CASE IN POINT
Unpredictable Behavior of Hurricanes—Florida Hurricanes of 2004 423
CASE IN POINT
CASE IN POINT
Extreme Beach Hardening—New Jersey Coast 380
Choosing to Ignore Evacuation During a Major Hurricane—Hurricane Hugo 425
CASE IN POINT
Repeated Beach Nourishment—Long Island, New York 381 Critical View
383
Key Points 384 Key Terms 385 Questions for Review
Critical View
427
Key Points 428 Key Terms 429 Questions for Review
429
385 CONTENTS
xiii
15 Thunderstorms and Tornadoes Twister Demolished Kansas Town Thunderstorms 431 Lightning 432 Downbursts 434 Hail 434 Safety During Thunderstorms
430
430
The Ultimate Catastrophe?
435
CASE IN POINT
448
450
16 Wildfires
451
A Deadly Wildfire
451
Consequences of Impacts with Earth 479 Immediate Effects of Impact 479 Impacts as Triggers for Other Hazards 479 Mass Extinctions 480 Evaluating the Risk of Impact 480 Your Personal Chance of Being Hit by a Meteorite 481 Chances of a Significant Impact on Earth
CASE IN POINT
CASE IN POINT
455
CASE IN POINT
Debris Flows Follow a Tragic Fire—Storm King Fire, Colorado, 1994 462 CASE IN POINT
Firestorms Threaten a Major City—Southern California Firestorms, 2003 and 2007 463
A Nickel Mine at an Impact Site—The Sudbury Complex, Ontario 483 CASE IN POINT
An Impact Sprays Droplets of Melt—Ries Crater in Germany 484 CASE IN POINT
A Close Grazing Encounter—Tunguska, Siberia 485 Critical View
486
Key Points 487 Key Terms 487 Questions for Review
488
18 The Future: Where Do We Go From Here?
CASE IN POINT
We Are the Problem
A Major Wildfire after Years of Fire Suppression— Bitterroot Valley Fires, Montana, 2000 465
Hazard Assessment and Mitigation
Critical View
Key Points 469 Key Terms 469 Questions for Review CONTENTS
470
489
489
Societal Attitudes 491 After a Disaster 492 Education 493
468
481
A Round Hole in the Desert—Meteor Crater, Arizona 483
Wildfire Management and Mitigation 456 Government Policy 457 Fighting Wildfires 457 Risk Assessments and Warnings 458 Protecting Homes from Fire 458 Public Cost of Fires 461
xiv
476
What Could We Do about an Incoming Asteroid? 482
Fire Process and Behavior 452 Fuel 452 Ignition and Spreading 453 Topography 454 Weather Conditions 454 Secondary Effects of Wildfires Erosion Following Fire 455 Mitigation of Erosion 456
471
Evidence of Past Impacts 475 Impact Energy 475 Impact Craters 476 Shatter Cones and Impact Melt Fallout of Meteoric Dust 478 Multiple Impacts 479
Tornado Safety—Jarrell Tornado, Texas, 1997 447 Key Points 449 Key Terms 450 Questions for Review
471
Projectiles from Space 472 Asteroids 472 Comets 473 Meteors and Meteorites 473 Identification of Meteorites 474
Tornadoes 435 Tornado Development 438 Tornado Damages 441 Fujita Tornado Scale 443 Safety During Tornadoes 446
Critical View
17 Impact of Asteroids and Comets
491
Different Ground Rules for the Poor Worse Problems to Come? Critical View
Appendix 2
493
Mineral and Rock Characteristics Related to Hazards
495
497
Key Points 498 Key Terms 498 Questions for Review
Appendix 3 499
Appendix 1 Geological Time Scale
501
500
Conversion Factors
512
Glossary
514
Index
527
CONTENTS
xv
Preface
Living with Nature The further you are from the last disaster, the closer you are to the next.
Why We Wrote This Book In teaching large introductory environmental and physical geology courses for many years—and, more recently, natural hazards courses—it has become clear to us that topics involving natural hazards are among the most interesting for the students. Thus, we realize that employing this thematic focus can stimulate students to learn basic scientific concepts, to understand how science relates to their everyday lives, and to see how such knowledge can be used to help mitigate both physical and financial harm. For all of these reasons, natural hazards and disasters courses should achieve higher enrollments, have more interested students, and be more interesting and engaging than those taught in a traditional environmental or physical geology framework. A common trend is to emphasize the hazards portions of physical and environmental geology texts while spending less time on subjects that do not engage the students. Students who have previously had little interest in science can be awakened with a new curiosity about Earth and the processes that dramatically alter it. Science majors experience a heightened interest, with expanded and clarified understanding of natural processes. In response to years of student feedback and discussions with colleagues, we have reshaped our courses to focus on natural hazards. Students who take a natural hazards course greatly improve their knowledge of the dynamic Earth processes that will affect them throughout their lives. They should be able to make educated choices about where to build houses, business offices, or engineering projects. Perhaps some of those who take this course will become government officials or policy makers who can change some of the current culture that contributes to major losses from natural disasters. Undergraduate college students, including nonscience majors, should find the writing clear and stimulating. Our emphasis is to provide them a basis for understanding important hazard-related processes and concepts. This book encourages students to grasp the fundamentals while still appreciating that most issues have complexities that are
xvi
beyond the current state of scientific knowledge and involve societal aspects beyond the realm of science. Students not majoring in the geosciences may find motivation to continue studies in related areas and to share these experiences with others. Natural hazards and disasters can be fascinating and even exciting for those who study them. Just don’t be on the receiving end!
Natural Hazards and Society Natural hazards, and the disasters that accompany many of them, are an ongoing societal problem. We continue to put ourselves in harm’s way, through ignorance or a naïve belief that a looming hazard may affect others but not us. We choose to live in locations that are inherently unsafe. A series of major disasters in recent years claimed hundreds of thousands of lives and billions of dollars in damages. A giant subduction-zone earthquake off the coast of Sumatra in late December, 2004, triggered a major tsunami that swept ashore minutes later to kill about 168,000 coastal residents. An hour or two later and with no warning, the same waves killed 5,400 more in Thailand, 31,000 in Sri Lanka and 10,700 in India. Although such events are not frequent, thousands could have been spared if they immediately ran upslope because they recognized that such major earthquakes can cause tsunami. In October, 2005, a major earthquake in the continent–continent collision zone of the Himalayas killed about 87,000 people as the shaking collapsed their weakly constructed homes built from stone and brick, with heavy concrete floors and roofs. Others died when rockslides swept down from steep mountainsides. In late August, 2005, Hurricane Katrina moved north from the Gulf of Mexico to drown New Orleans and obliterate coastal areas of Louisiana, Mississippi, and Alabama. In spite of ample warning in an affluent country with abundant resources, tens of thousands did not evacuate in time. Most were poor and lacked transportation; others stayed because they thought the levees would protect them or because they had endured false alarms before. Days later, with thousands still stranded, help arrived only slowly because of lack of planning and poor leadership. Inherent to New Orleans’ problem was its location in a depression below sea level, “protected” by inadequate levees.
Unfortunately, politics also enters the equation. Disaster assistance continues to be provided without a large costsharing component from states and local organizations. Thus, local governments continue to lobby Congress for funds to pay for losses but lack incentive to do much about the causes. The Federal Emergency Management Agency is charged with rendering assistance following disasters; it continues to provide funds for victims of earthquakes, floods, hurricanes, and other hazards. It remains reactive to disasters, as it should be, but is only beginning to be proactive in eliminating the causes of future disasters. Congress continues to fund multimillion-dollar Army Corps of Engineers projects to build levees along rivers and replenish sand eroded from beaches. The Small Business Administration’s disaster loan program continues to subsidize credit to finance rebuilding in hazardous locations. The federal tax code also subsidizes building in both safe and hazardous sites. Real estate developers benefit from tax deductions, and ownership costs such as mortgage interest and taxes can be deducted from income taxes. A part of uninsured “casualty losses” can still be deducted from the disaster victim’s income taxes. Such policies encourage future damages from natural hazards. Following tens of billions of dollars of losses from 2004 hurricanes Charley, Ivan, Frances, and Jeanne that pounded Florida and nearby states, and 2005 hurricanes Katrina, Rita, and Wilma that decimated Louisiana and other Gulf Coast areas, smaller insurance companies pulled out of southeast coastal states. The few remaining very large companies dramatically increased premiums charged to their policy holders and added many restrictions. People in especially risky areas were denied coverage entirely. Individual states stepped in to insure those people but with high premiums. Complaints from policyholders led to somewhat lower premiums but that shifted heavy costs to other state taxpayers. The expectation that we can control nature through technological change stands in contrast to the fact that natural processes will ultimately prevail. We can choose to live with nature or we can try to fight it. Unfortunately, people who choose to live in hazardous locations tend to blame either “nature on the rampage” or others for permitting them to live there. People do not often make such poor choices willfully, but rather through their lack of awareness of natural processes. Even when they are aware of an extraordinary event that has affected someone else, they somehow believe “it won’t happen to me.” These themes are revisited throughout the book, ensuring that our discussion of principles and processes is frequently related to societal behavior and attitudes. People often decide on their residence or business location based on a desire to live and work in scenic environments without understanding the hazards around
them. Once they realize the risks, they often compound the hazards by attempting to modify the environment. Students who read this book should be able to avoid such decisions. Toward the end of the course, our students sometimes ask, “So where is a safe place to live?”We often reply that you can choose hazards that you are willing to deal with and live in a specific site or building that you know will minimize impact of that hazard.
Our Approach This text begins with an overview of the dynamic environment in which we live and the variability of natural processes, emphasizing the fact that most daily events are small and generally inconsequential. Larger events are less frequent, though most people understand that they can happen. Fortunately, giant events are infrequent; regrettably, most people are not even aware that such events can happen. Our focus here is on Earth and atmospheric hazards that appear rapidly, often without significant warning. The main natural hazards covered in the book are earthquakes and volcanic eruptions; extremes of weather, including hurricanes; and floods, landslides, tsunami, wildfires, and asteroid impacts. For each, we examine the nature of the hazard, the factors that influence it, the dangers associated with the hazard, and the methods of forecasting or predicting such events. Throughout the book, we emphasize interrelationships between hazards, such as the fact that building dams on rivers often leads to greater coastal erosion. Similarly, wildfires generally make slopes more susceptible to floods, landslides, and mudflows. We attempt to provide balanced coverage of natural hazards across North America. The book includes chapters on dangers generated internally, including earthquakes, tsunami, and volcanic eruptions. Society has little control over the occurrence of such events but can mitigate their impacts through a deeper understanding that can afford more enlightened choices. The landslides section addresses hazards influenced by both in-ground factors and weather, a topic that forms the basis for many of the following chapters. A chapter on sinkholes, subsidence, and swelling soils addresses other destructive in-ground hazards that we can to some extent mitigate, and that are often subtle yet highly destructive. The following hazard topics depend on an understanding of the dynamic variations in climate and weather, so we begin with a chapter to provide that background and an overview of global warming. Chapters on streams and floods begin with the characteristics and behavior of streams and how human interaction affects both a stream and the people around it. Chapters follow on wave and beach processes, hurricanes and Nor’easters, thunderstorms and tornadoes, and wildfires. The final chapters discuss asteroid
P R E FA C E
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impacts and future concerns related to natural hazards. Appendixes present the geological time scale and a brief discussion of the nature of rocks and minerals, primarily as background for some of the physical hazards. The book is up to date and clearly organized, with most of its content derived from current scientific literature and from our own personal experience. It is packed with relevant content on natural hazards, the processes that control them, and the means of avoiding catastrophes. Numerous excellent and informative color photographs, many of them our own, illustrate concepts in a manner that is not possible without color. The diagrams are clear, straightforward, and instructive. Extensive illustrations and Case in Point examples bring reality to the discussion of principles and processes. These cases tie the process-based discussions to individual cases and integrate relationships between them. They emphasize the natural processes and human factors that affect disaster outcomes. Illustrative cases are interwoven with topics as they are presented. End-of-Chapter material also includes a list of Key Points, Key Terms, and Questions for Review.
p Chapter 8, Landslides and Other Downslope
p
p
Changes in This Edition The primary organization of natural hazards into chapters and their sequence remains largely as it was in the first edition. However, within most chapters we have heavily reorganized the text to begin each chapter or pair of chapters with emphasis on the environments and natural processes that lead to a particular type of hazard, followed by the human impacts, consequences, and means of mitigation of the hazard. Because our approach to hazards is very processoriented, we have done considerable rewriting to simplify and clarify important concepts. With such a fast-changing and evolving subject as natural hazards, we have extensively revised and added to the content, with emphasis not only on recent events but on those that best illustrate important issues. New hazard maps help the reader quickly determine the locations of important events, including those of Cases in Point. Some significant additions to individual chapters include the following:
p Chapter 1, Natural Hazards and Disasters, adds p p p
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coverage of increases in world populations and associated increases in natural hazard impacts. Chapters 3 and 4, Earthquakes, include coverage of the deadly Kashmir earthquake. Chapter 5, Tsunami, adds new material on the disastrous Sumatra tsunami. Chapters 6 and 7, Volcanoes, include new coverage of poisonous gases released by volcanoes.
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p
p p p
p
Movements, now includes a section on snow avalanches, danger signs, and how to avoid them. Rockfalls threatening homes, as well as recent landslides in southern California, South Dakota, and New York, are important additions. Chapter 9, Sinkholes, Land Subsidence, and Swelling Soils, contains a new section on permafrost thawing and ground subsidence, problems that increasingly threaten higher-latitude areas because of global warming. Chapter 10, Climate Change and Weather Related to Hazards, is almost entirely new and includes much greater depth, reflecting rapid advances in the understanding of that field and the publication of the 2007 report from the International Panel on Climate Change. Major additions include greenhouse-gas emissions, arctic melting, the climate effect of permafrost melting, methane hydrate under the continental slopes, thermohaline circulation in the Atlantic, intercontinental dust storms, drought and growing deserts, urban heat islands, lake-effect snows and ice storms, carbon-dioxide sequestration, and the Atlantic Multidecadal Oscillation. Glacial outburst floods that take on more prominence in the light of global warming concerns and flash floods and debris flows in desert areas of southern Arizona are new. Chapters 11 and 12, Streams and Floods, now include extensive material on debris flows and mudflows, moved and amplified from the Landslides chapter. In Chapter 13, Waves, Beaches, and Coastal Erosion, a new section explains rip currents and how to escape one. Chapter 14, Hurricanes and Nor’easters, includes an extensive and much updated Case in Point on Hurricane Katrina. Chapter 15, Thunderstorms and Tornadoes, includes the new Enhanced Fujita Scale, peak tornado season in different areas, and a review of the devastating 2007 Greenburg, Kansas, tornado. Chapter 16, Wildfires, is extensively changed with analysis of the importance of dry grass and needles on the ground in igniting wildland homes, as well as discussion of the distance at which a wall of flame can ignite dry wood structures and can cause severe burns on skin.
The art program has also been significantly enhanced. Recognition and analysis of natural hazards is intensely visual. To help visualize hazards and their consequences, we added or replaced more than 200 photos, many of them
our own. We also added or revised many of the illustrations to better demonstrate concepts. Several pedagogical features have been added or changed to keep students engaged in the material. To immediately engage the students, most chapters now begin with a newspaper-like account of an important recent event pertaining to that hazard. Important Cases in Point for each chapter are cited in appropriate places but have been moved to the chapter end to improve continuity of the subject matter discussions. New descriptive titles draw attention to the connection between the concepts in the chapter and the specific case, while new maps help students locate the site of the event easily. A new Critical View exercise at the end of most chapters prompts student discussion and analysis of natural hazards. For each of the photos we suggest that the student indicate the hazard or hazards and any damaging events that have happened, explain why the event should have been foreseen, what could have been done to prevent it, and, where appropriate, evaluate what can be done to stabilize the area or mitigate the hazard. By the Numbers boxes (called Sidebars in the first edition) include more quantitative explanations of concepts, including formulas, for those who prefer a more quantitative approach.
Acknowledgments We are grateful to many people for assistance in gathering material for this book, far too many to list individually here. However, we especially appreciate the help we received from the following colleagues.
p For editing and suggested additions: Dr. Dave Alt (Uni-
p
versity of Montana, emeritus); Ted Anderson; Shirley Hyndman; Teresa Hyndman; Dr. Duncan Sibley, Dr. Kaz Fujita, and Dr. Tom Vogel (Michigan State University); Dr. Kevin Vranes, University of Colorado, Center for Science and Technology Policy Research; Peter Adams, executive editor for Earth Sciences at Brooks/Cole. We especially wish to thank Rebecca Heider, developmental editor at Brooks/Cole, who not only expertly managed and organized the logistical aspects of the second edition but suggested innumerable and important changes in the manuscript. Much of the improvement you see is due to her skillful editing. For information and photos on specific sites: Dr. Brian Atwater, USGS; Dr. Rebecca Bendick, University of Montana; Karl Christians, Montana Dept. Natural Resources and Conservation; Susan Cannon, USGS; Jack Cohen, research physical scientist, Fire Sciences Laboratory, U.S. Forest Service; Dr. Joel Harper, University of Mon-
tana; Bretwood Hickman, University of Washington; Peter Bryn, Hydro.com (StoreggaSlide); Dr. Dan Fornari, Woods Hole Oceanographic Institution, MA; Dr. Kaz Fujita, Michigan State University; Dr. Benjamin P. Horton, University of Pennsylvania, Philadelphia; Dr. Roy Hyndman, Pacific Geoscience Institute, Saanichton, British Columbia; Bernt-Gunnar Johansson photo, Sweden; Sarah Johnson, Digital Globe; Walter Justus, Bureau of Reclamation, Boise, ID; Ulrich Kamp, Geography, University of Montana; Dr. M. Asif Khan, Director, National Center of Excellence in Geology, University of Peshawar, Pakistan; Karen Knudsen, executive director, Clark Fork Coalition; Dr. David Loope, University of Nebraska; Martin McDermott, P.G., McKinney Drilling Co.; Dr. Ian Macdonald, Texas A&M University, Corpus Christi; Andrew MacInnes, coastal zone administrator, Plaquemines Parish, LA; Dr. Jamie MacMahan, Naval Postgraduate School, Monterey, CA; Andrew Moore, Kent State University; Jenny Newton, Fire Sciences Laboratory, U.S. Forest Service; Dr. Mark Orzech, Naval Postgraduate School, Monterey, CA; Jennifer Parker, Geography, University of Montana; Dr. Steve Running, Numerical Terradynamic Simulation Group, University of Montana; Todd Shipman, Arizona Geological Survey; Dr. Duncan Sibley, Center for Integrative Studies, Michigan State University; Robert B. Smith, University of Utah; Rick Stratton, fire modeling analyst, Fire Sciences Laboratory, U.S. Forest Service; Donald Ward, head, Roads and Bridges, Travis Col, TX; Karen Ward, Terracon Consultants, Austin, TX; John M. Thompson; Dr. Robert Webb, USGS, Tucson, AZ; Vallerie Webb, Geoeye.com, Thornton, CO; Ann Youberg, Arizona Geological Survey.
p For providing access to the excavations of the Minoan culture at Akrotiri, Santorini: Dr.Vlachopoulos, head archaeologist, Greece.
p For assisting our exploration of the restricted excavations at Pompeii, Italy: the site’s chief archaeologist.
p For logistical help: Roberto Caudullo, Catania, Italy; Brian Collins, University of Montana; and Keith Dodson, Brooks/Cole’s earth sciences textbook editor at the time the first edition was published. In addition, we are indebted to chapter reviewers who helped focus our attention on issues and specifics that led to many improvements in the Second Edition: Eric M. Baer at Highline Community College; David M. Best, Northern Arizona University; M. Stanley Dart, University of Nebraska at Kearney; Richard W. Hurst, California State University, Los Angeles; Mary Leech, San Francisco State University; Tim Sickbert, Illinois State University; Christiane Stidham, Stony Brook University; Kent M. Syverson, University of Wisconsin– Eau Claire.
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We thank the following reviewers that contributed to the development of the First Edition: Ihsan S. Al-Aasm, University of Windsor; Jennifer Coombs, Northeastern University; Jim Hibbard, North Carolina State University; Mary Leech, Stanford University; David Evans, California State University, Sacramento; Wang-Ping Chen, University of Illinois at Urbana-Champaign; Stephen Nelson,Tulane University; Alan
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Lester, University of Colorado at Boulder; Luther Strayer, California State University, East Bay; Katherine Clancy, University of Maryland; and James Harrell, University of Toledo.
Donald Hyndman and David Hyndman December 2007
Natural Hazards and Disasters
Jocelyn Augustino photo, FEMA.
Chapter
1
i
Flooding during Hurricane Katrina covered many homes to their rooftops with contaminated water coated with an oil slick. Some homes floated off their foundations to lodge against other homes.
Those who cannot remember the past are condemned to repeat it. —GEORGE SANTAYANA (SPANISH PHILOSOPHER), 1905
Living in Harm’s Way
L
Disaster
arge numbers of people around the world live and work in notoriously dangerous places—near volcanoes, in floodplains, or on deep fault lines. Some are ignorant of potential disasters, but others even rebuild homes destroyed in previous disasters. Why would people choose to put their lives and property at risk? Sometimes the reasons are cultural or economic. The areas around volcanoes make good farmland because volcanic ash degrades into richly productive soil. Large floodplains attract people because they provide good agricultural soil, inexpensive land, and natural transportation corridors. For understandable reasons, such people are living in the wrong places. More people also crowd into dangerous areas for frivolous reasons. They build homes at the bases or tops of large cliffs for a scenic view, not realizing that large portions of the cliffs can give way in landslides or rockfalls (p Figure 1-1). They long to live along edges of sea cliffs where they can enjoy the ocean view, or they want to live along the beach to experience the ocean more intimately. Others build beside rivers that are picturesque or seem soothing. Far too many people build houses in the woods because they enjoy the seclusion and scenery of this natural setting.
1
Some experts who are concerned with natural catastrophes say that such people have chosen to live in an “idiot zone.” People who deliberately choose to live in hazardous areas might as well choose to park their cars on a rarely used railroad track. Trains don’t come frequently, but the next one might come any minute. Catastrophic natural hazards are much harder to avoid than passing freight trains; we may not recognize the signs of imminent catastrophes because these events are infrequent. So many centuries may pass between eruptions of a large volcano that most people forget it is active. Many people live so long on a valley floor without seeing a big flood that they forget it is a floodplain. The great disaster of a century ago is long forgotten, so people move into the path of disaster without a thought for the tragic sequel that will occur on some unknowable future date. The hazardous event may not arrive today or tomorrow, but it is just a matter of time.
Catastrophes in Nature
Bill Lund photo, Utah Geological Survey.
Everyday geologic processes like erosion have produced large effects over the enormous course of Earth’s history, carving out valleys or changing the shape of coastlines. While some processes operate slowly and gradually, infrequent catastrophic events have sudden and major impacts. For instance, streams that run clear throughout most of the year will be muddy during the few days or weeks of high water when they carry most of their annual load of sediment. That sediment reflects a short and intense erosion period.
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Major floods occurring once every ten or twenty years do far more damage and move more material than all of the intervening floods put together. Soil moves slowly downslope by creep, but occasionally a huge part of a slope may slide. Mountains rise, sometimes slowly, but more commonly by sudden movements within the Earth. During an earthquake, a mountain can abruptly rise several meters above the adjacent valley. Some natural events involve disruption of a temporary “equilibrium” between opposing influences. Unstable slopes, for example, may hang precariously for thousands of years, held there by friction along a slip surface until some small perturbation such as water soaking in from a large rainstorm sets them loose. Similarly, the opposite sides of a fault may stick until the continuing stress along them finally tears them loose, triggering an earthquake. A bulge may form on a volcano as molten magma slowly rises into it; then the bulge collapses as the volcano erupts. The behavior of these natural systems is somewhat analogous to a piece of fabric or plastic wrap that remains intact as it stretches until it suddenly tears. People who watch Earth processes proceed at their normal and unexciting pace rarely pause to imagine what might happen if that slow pace were suddenly punctuated by a major event. The fisherman enjoying an afternoon pursuing trout in a quiet stream can hardly imagine how a 100-year flood might transform the scene. Someone gazing at a serene, snow-covered mountain can hardly imagine
p
FIGURE 1-1. This four-year-old house near Zion National Park in southern Utah was built near the base of a steep rocky slope capped by a sandstone cliff. Early one morning in October 2001, the owner awoke with a start as a giant boulder 4.5 meters (almost 15 feet) across crashed into his living room and bedroom, narrowly missing his head.
destructive mudflows racing down its flanks (p Figure 1-2). Large or sometimes gigantic events are a part of nature. Such abrupt events produce large effects that can be disastrous if they affect people.
Human Impact of Natural Disasters
Donald Hyndman photo.
When a natural process poses a threat to human life or property, we call it a natural hazard. Many geologic processes are potentially hazardous. For example, streams flood, as part of their natural process, and become a hazard to those living nearby; if not every year, then every two or three years. A hazard leads to a natural disaster when an event causes significant damage to life or property. A moderate flood that spills over the floodplain every few years does not often create a disaster, but when a major flood strikes, it may lead to a disaster, with many people killed or displaced. When a natural event kills or injures large numbers of people or causes major property damage, it is called a catastrophe. The potential impact of a natural disaster is related not only to the size of the event, but also to where the event takes place. A natural event in a thinly populated area can hardly pose a major hazard. For example, the magnitude 6.9 Borah Peak earthquake that struck central Idaho in 1983 was severe but posed little hazard because it happened in a region with few people or buildings. However, the magnitude 7.6 Kashmir earthquake occurred in heavily populated valleys of the southern Himalayas and killed more than 80,000 people (p Figure 1-3). The eruption of Mount St. Helens in 1980 caused few fatalities and remarkably little property damage simply because few people lived in the area surrounding the mountain. On the other hand, a similar eruption of Vesuvius, on the outskirts of Naples, Italy, could kill hundreds of thousands of people and cause property damage beyond reckoning. The average annual cost of natural hazards has increased dramatically in the last several decades (p Figure 1-4a). This is especially true because population increases in urban and coastal settings result in more people occupying land that is subject to major natural events (p Figure 1-4b). In effect, people place themselves in the path of unusual, sometimes catastrophic events. Economic centers of society are increasingly concentrated in larger urban centers. As a result, those urban centers tend to expand into areas that were previously considered undesirable, including those with greater exposure to natural hazards. The type of damage sustained as a result of a natural disaster also depends on the economic development of the area where it occurs. In developed countries, there are typically greater economic losses, while in developing countries, there are increasing numbers of deaths from natural disasters.
p
FIGURE 1-2. Orting, Washington, with spectacular views of Mount Rainier, is built on a giant, ancient mudflow from the volcano. If mudflows happened in the past, they almost certainly will happen again.
Image not available due to copyright restrictions
Predicting Catastrophe A catastrophic natural event is unstoppable, so the best way to avoid it would be to predict its occurrence and get out of its way. Unfortunately, dejection comes easily to those who would predict the occurrence of a natural disaster on a particular date. So far, there have been few well-documented cases of accurate prediction, and even those may have involved luck. Use of the same techniques
N AT U R A L H A Z A R D S A N D D I S A S T E R S
3
200 Kobe, Japan Earthquake
180
Hurricane Katrina
140
Sumatra Tsunami
120 100 80 60 Modified from Munich Re.
Billions of U.S. dollars
160
40 20 0 1950
1955
1960
1965
1970
1975
Overall losses (2006 values)
1980
1985
1990
1995
Trend overall losses
2000
2005
Trend insured losses
a 10 2048 2028
8 2013
7 1999
6
1987
5 1974
4 1959
3 1922
2 1
Modified from World Population Data Sheet.
World population (billions)
9
1804
0 1800
1850
1900
1950
2000
2050
b
p
FIGURE 1-4. a. The cost of natural hazards is increasing worldwide. b. World population has been rising rapidly, especially in the last four decades, though the rate of increase is slowing.
in similar circumstances has involved false alarms or failed to correctly predict a disaster when it came. Many people have sought to find predictable cycles in natural events. Natural events that occur at predictable intervals are called cyclic events. However, even most recurrent events are generally not really cyclic. Too many variables control the behavior of natural events. Even with cyclic events, overlapping cycles would make the resultant extremes noncyclic. That would affect the predictability of the event. So far as anyone can tell, most events, large and
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small, occur at seemingly random and essentially unpredictable intervals. The calendar does not predict them. Nevertheless, the people who make it their business to understand natural disasters have learned enough about them to provide some guidance to people who are at risk. They cannot predict exactly when an event will occur. However, based on past experience, they can often forecast that a hazardous event will occur in a certain area within the next several decades or hundreds of years with an approximate percentage probability. They can forecast that
Stream discharge (cubic meters/sec.) (linear scale)
record. Nonetheless, the knowledge that scientists have of the pattern of occurrences here helps them assess the risk and prepare for the eventual earthquake. To estimate the recurrence interval of a particular kind of natural event, we typically plot a graph of each event size versus the time interval between sequential individual events. Such plots often make curved lines that cannot be reliably extrapolated to larger events that might lurk in the future (p Figure 1-5). Plotting the same data on a logarithmic scale often leads to a straight-line graph that can be extrapolated to values larger than those in the historical record. Whether the extrapolation produces a reasonable result is another question. The probability of the occurrence of an event is related to the magnitude of the event. We see huge numbers of small events, many fewer large events, and only a rare giant event (By the Numbers 1-1: “Relationship between Frequency and
Stream discharge (cubic meters/sec.) (log scale)
there will be a large earthquake in the San Francisco Bay region over the next several decades, or that Mount Shasta will likely erupt sometime in the next few hundred years. In many cases, their advice can greatly reduce the danger to lives and property. Ask a stockbroker where the market is going, and you will probably hear that it will continue to go wherever it has been going during recent weeks. Ask a scientist to predict an event, and he or she will probably look to see what has happened in the geologically recent past and predict more of the same. Most predictions of any kind are based on linear projections of past experience. Of course, past experience is not always a good indicator of what will happen in the future, which explains why so many people lose money in the stock market. Statistical predictions are simply a refinement of past recorded experiences. They are typically expressed as recurrence intervals that relate to the probability that a natural event of a particular size will happen within a certain period of time. For example, the past history of a fault may indicate that it is likely to produce an earthquake of a certain size once every hundred years on average. A recurrence interval is not, however, a fixed schedule for events. Recurrence intervals can tell us that a 50-year flood is likely to happen sometime in the next several decades but not that such floods occur at intervals of 50 years. Most people do not realize the inherent danger of an unusual occurrence, or they believe that they will not be affected in their lifetimes because such events occur infrequently. That inference often incorrectly assumes that the probability of another severe event is lower for a considerable length of time after a major event. In fact, even if a 50-year flood occurred last year, that does not indicate that there will not be another one this year or next or for the next ten years. To understand why this is the case, take a minute to review probabilities. Flip a coin, and the chance that it will come up heads is 50 percent. Flip it again, and the chance is again 50 percent. If it comes up heads five times in a row, the next flip still has a 50 percent chance of coming up heads. So it goes with earthquakes, floods, and many other kinds of apparently random natural events. The chance that anyone’s favorite fishing stream will stage a 50-year flood this year and every year is 1 in 50, regardless of what it may have done during the last 50 years. As an example of both the usefulness and the limitations of recurrence intervals, consider the case of Tokyo. This enormous city is subject to devastating earthquakes that for more than 500 years came at intervals of close to 70 years. The last major earthquake ravaged Tokyo in 1923, so everyone involved awaited 1993 with considerable consternation. The risk steadily increased as the population grew during those years and the strain across the fault zone grew. More than 15 years later, no large earthquake has occurred. Obviously, the recurrence interval does not predict events at equal intervals, in spite of the 500-year Japanese historical
1,000 800 600 400 200 0 1,000 100 10 1 0.1
1
High
10 100 Recurrence interval (years) (Log scale) Frequency
1,000
Low
p
FIGURE 1-5. On a graph of magnitude (e.g., stream discharge) versus the frequency of such magnitude, a logarithmic plot is often equivalent to a straight-line graph that can then be extrapolated to larger values. Also note that small-magnitude events tend to be frequent, whereas large magnitude events tend to be infrequent.
1-1 By the Numbers Relationship between Frequency and Magnitude M α 1/f Magnitude (M) of an event is inversely proportional to frequency (f) of the type of event.
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Relationships Among Events
p
FIGURE 1-6. The branching of streams is fractal. The general branching of patterns looks similar regardless of scale—from a less-detailed map on the left to the most-detailed map on the right.
Magnitude”).The infrequent occurrence of rare giant events means it is hard to study them, but it is often rewarding to study the small events because they may well be smallerscale models of their infrequent larger counterparts that may occur in future. Many geologic features look the same regardless of their size, a quality that makes them fractal. A broadly generalized map of the United States might show the Mississippi River with no tributaries smaller than the Ohio and Missouri rivers. A more detailed map would show many smaller tributaries. An even more detailed map would show still more. The number of tributaries depends on the scale of the map, but the general branching pattern looks similar across a wide range of scales (p Figure 1-6). Patterns apparent on a small scale quite commonly resemble patterns that exist on much larger scales that we cannot so easily perceive. This means that small events may provide insight into huge events that occurred in the distant past but are larger than any seen in historical time; we may find evidence of these big events if we search. The scale of some natural catastrophes that have affected the Earth, and will do so again, is almost too large to fathom. Examples include catastrophic failure of the flanks of oceanic volcanoes or the impact of large asteroids. For these, reality is more awesome than fiction.Yet each is so well documented in the geologic record that we need to be aware of the potential for such future extreme events. It is impossible in our current state of knowledge to predict most natural events, even if we understand in a general way what controls them.The problem of avoiding natural disasters is like the problem drivers face in avoiding collisions with trains. They can do nothing to prevent trains, so they must look and listen. We have no way of knowing how firm the natural restraints on a landslide, fault, or volcano may be. We also do not generally know what changes are occurring at depth. But we can be confident that the landslide or fault will eventually move or that the volcano will erupt. And we can reasonably understand what those events will involve when they finally happen.
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Although randomness is a factor in forecasting disasters, not all natural events occur quite as randomly as floods or tosses of a coin. Some events are directly related to others— formed as a direct consequence of another event (p Figure 1-7). For example, the slow movement of the huge outer layers of the Earth colliding or sliding past one another clearly explains the driving forces behind volcanic eruptions and earthquakes. Heavy or prolonged rainfall can cause a flood or a landslide. But are some events unrelated? If an earthquake happens at the time of a volcanic eruption, did the eruption cause the earthquake or did the earthquake cause the eruption—or neither? Or did the earthquake not cause the eruption but merely trigger the final eruption? Could any of the arrows in Figure 1-7 be reversed? Given all of the interlocking possibilities, the variability, and the uncertainties, we could call Figure 1-7 a “chaos net” for natural hazards. Past events can also create a contingency that influences future events. It is certainly true, for example, that sudden movement on a fault causes an earthquake. But the same movement also changes the stress on other parts of the fault and probably on other faults in the region, so the next earthquake will likely differ considerably from the last. Similar complex relationships arise with many other types of destructive natural events. Sometimes major natural events are preceded by a series of smaller precursor events, which may warn of the impending disaster. Geologists studying the stirrings of Mount St. Helens,Washington, in 1980 before its catastrophic eruption monitored swarms of earthquakes and decided that most recorded the movements of rising magma as it squeezed upward, expanding the volcano. Precursor events alert scientists to be on the lookout for a larger event, but
Plate tectonics/Mountain building
Earthquakes
Weather/Climate Hurricanes
Tsunamis
Volcanic eruptions
Flood
Landslides
p
FIGURE 1-7. This flowchart indicates interactions among natural hazards. The bolder arrows indicate stronger influences. Can you think of others?
Ocean height
Land Use Planning One way to reduce losses from natural disasters is to find out where disasters are likely to occur and restrict development there, using land use planning. Ideally, we should prevent development along major active faults by reserving that land for parks and natural areas. We should also limit housing and industrial development on floodplains to minimize damage from floods, and along the coast to minimize hurricane and coastal erosion losses. Limiting building near active volcanoes and the river valleys that drain them can minimize the hazards associated with eruptions. It is hard, however, to impose land use restrictions in many areas because such imposition tends to come too late. Many hazardous areas are already heavily populated, perhaps even saturated with people. Many people want to live as close as they can to a coast or a river and resent being told that they cannot; they oppose any attempt at land use restrictions because they feel it infringes on their property rights. Almost any attempt to regulate land use in the public interest is likely to ignite intense political and legal opposition. Developers, companies, and even governments often aggravate hazards by allowing or even encouraging people to move into hazardous areas. Many developers and private individuals view restrictive zoning as an infringement on their rights to do as they wish with their land. Developers, real estate agents, and some companies are reluctant to admit the hazards that may affect a property for fear of lessening its value and scaring off potential clients (p Figure 1-9). Many local governments consider news of hazards bad for growth and business. They shun restrictive zoning or minimize the possible dangers for fear of inhibiting improvements in their tax base. As in many other venues, different groups have different objectives. Some are most concerned with economics, others with safety, still others with the environment.
Storm surge + tide Storm surge Tide Days
p
FIGURE 1-8. If events overlap, their effects can amplify one another. In this example, a storm surge (green line) can be especially high if it coincides with high tide (blue line). The combination is the orange line.
events that appear to be precursor events are not always followed by a major event. The relationships among events are not always clear. For example, what about the bigger earthquake that occurred at the instant Mount St. Helens exploded during the morning of May 18? The expanding bulge over the rising magma collapsed in a huge landslide. Neither the landslide nor the earthquake caused the formation of molten magma. Once formed, neither caused the magma to rise; but did they trigger the final eruption? If so, which one triggered the other—the earthquake, the landslide, or the eruption? Were the events directly related—that is, did one cause the others? What about the tragic mudflows that immediately followed? It was a beautiful, clear morning before the eruption. Was the falling ash hot enough to melt snow on the volcano, or did the eruption cause it to rain? One or more of these possibilities could be true in different cases. Events can also overlap to amplify an effect. Most natural disasters happen when a number of unrelated variables overlap in such a way that they reinforce each other to amplify an effect. If the high water of a hurricane storm surge happens to arrive at the coast during the daily high tide, the two reinforce each other to produce a much higher storm surge (p Figure 1-8). If this occurs on a section of coast that happens to have a large population, then the situation can become a major disaster. Such a coincidence caused the catastrophic hurricane that killed 8,000 people in Galveston, Texas, in 1900. Bad luck prevailed.
Because natural disasters are not easily predicted, it falls to governments and individuals to assess their risk and prepare for and mitigate the effects of disasters. Mitigation refers to efforts to prepare for a disaster and reduce its damage. Mitigation can include engineering projects like levees, as well as government policy and public education. In each chapter of this book, we examine mitigation strategies related to specific disasters.
David Hyndman photo.
Mitigating Hazards
p
FIGURE 1-9. Some developers seem unconcerned with the hazards that may affect the property they sell. High spring runoff floods this proposed development site in Missoula, Montana.
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Insurance Some mitigation strategies help with recovery after a disaster occurs. Insurance is one way to mitigate the financial impact of disasters after the fact. People buy property insurance to shield themselves from a major loss that they cannot afford. Insurance companies use a formula for risk to establish premium rates for policies. Risk is essentially a hazard considered in the light of its recurrence interval and expected costs (By the Numbers 1-2: “Assessing Risk”). The greater the hazard and the shorter its recurrence interval, the greater the risk. In most cases, the company can estimate the cost of a hazard event to a useful degree of accuracy, but its recurrence interval is hardly better than an inspired guess. The history of experience with a given natural hazard in any area of North America is typically less than 200 years. Large events come around, on average, only every few decades or every few hundred years, or even more rarely. Estimating risk for these events becomes a perilous exercise likely to lose the company large amounts of money. In some cases, most notably floods, the hazard and its recurrence interval are both firmly enough established to support a rational estimate of risk. But the amount of risk and the potential cost to the company can be so large that a catastrophic event would put the company out of business. Such a case explains why private insurance companies are not eager to offer disaster policies. The uncertainties of estimating risk make it impossible for private insurance companies to offer affordable policies to protect against many kinds of natural disasters. As a result, insurance is generally available for events that present relatively little risk, mainly those with more or less dependably long recurrence intervals. The difficulty of obtaining policies from private insurers for certain types of natural hazards has inspired a variety of governmental programs. Earthquake insurance is available in areas such as Texas, where the likelihood of an earthquake is low. In California, where the risks and expected costs are much higher, insurance companies are required to provide earthquake coverage. As a result, the companies now make insurance available through the California Earthquake Authority, a consortium of companies. Similarly, most hurricane-prone southeastern states have mandated insurance pools that
1-2 By the Numbers Assessing Risk Insurance costs are actuarial: They are based on past experience. For insurance, a “hazard” is a condition that increases the severity or frequency of a loss.
Risk α [probability of occurrence] × [cost of the probable loss from the event]
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provide property insurance where individual private companies are unwilling to provide such coverage. Insurance for some natural hazards is simply not available. Landslides, most mudflows, and ground settling or swelling are too risky for companies, and each potential hazard area would have to be individually studied by a scientist or engineer who specialized in such a hazard. The large number of variables makes the risk too difficult to quantify; it is too expensive to estimate the different risks for the relatively small areas involved. A critical question arises for people who lose their houses in landslides and are still paying on the mortgage. They may not only lose what they have already paid into the mortgage or home loan, but may be obligated to continue to pay off the remainder of the loan even though the house no longer exists. However, California, for example, has a law that generally prevents what are called “deficiency judgments” against such mortgage holders. This permits homeowners to walk away from their destroyed homes, and the bank cannot go after them for the remainder of the loan. However, the situation is not always clear because federal law may overrule state law. A federal agency such as the Veterans Administration, which guarantees some mortgages, may pay the bank the balance of the loan and then go after the borrower for the remainder.
The Role of Government The United States and Canadian governments are involved in many aspects of natural hazards. They conduct and sponsor research into the nature and behavior of many kinds of natural disasters. They attempt to find ways to predict hazardous events and mitigate the damage and loss of life they cause. Governmental programs are split among several agencies. The U.S. Geological Survey (USGS) and Geological Survey of Canada (GSC) are heavily involved in earthquake and volcano research, as well as in studying stream behavior and monitoring stream flow. The National Weather Service monitors rainfall and severe weather and uses this and the USGS data to try to predict storms and floods. The Federal Emergency Management Agency (FEMA) was created in 1979, primarily to bring order to the chaos of relief efforts that seemed invariably to emerge after natural disasters. After the hugely destructive Midwestern floods of 1993, it has increasingly emphasized hazard reduction. Rather than pay victims to rebuild in their original unsafe locations, such as floodplains, the agency now focuses on relocating them. Passage of the Disaster Mitigation Act in 2000 signals greater emphasis on identifying and assessing risks before natural disasters strike and taking steps to minimize potential losses. The act funds programs for hazard mitigation and disaster relief through FEMA, the U.S. Forest Service, and the Bureau of Land Management. To determine the level of risk and to estimate potential losses from earthquakes, federal agencies such as FEMA use
a computer system called HAZUS (Hazard United States). It integrates a group of interdependent modules that include potential hazards, inventory of the hazards, direct damages, induced damages, direct economic and social losses, and indirect losses. Unfortunately, some government policy can be counterproductive, especially when politics enter the equation. In some cases, disaster assistance continues to be provided without a large cost-sharing component from states and local organizations. Thus, local governments continue to lobby Congress for funds to pay for losses but lack incentive to do much about the causes. FEMA is charged with rendering assistance following disasters; it continues to provide funds for victims of earthquakes, floods, hurricanes, and other hazards. It remains reactive to disasters, as it should be, but is only beginning to be proactive in eliminating the causes of future disasters. Congress continues to fund multimillion-dollar Army Corps of Engineers projects to build levees along rivers and replenish sand on beaches. The Small Business Administration disaster loan program continues to subsidize credit to finance rebuilding in hazardous locations. The federal tax code also subsidizes building in both safe and hazardous sites. Real estate developers benefit from tax deductions and ownership costs such as mortgage interest, and property taxes can be deducted from income. A part of uninsured “casualty losses” can still be deducted from the disaster victim’s income taxes. Such policies encourage future damages from natural hazards.
The Role of Public Education Much is now known about natural hazards and the negative impacts they have on people and their property. It would seem obvious that any logical person would avoid such potential impacts or at least modify their behavior or their property to minimize such impacts. However, most people are not knowledgeable about potential hazards, and human nature is not always rational. Until someone has a personal experience or knows someone who has had such an experience, most people subconsciously believe “It won’t happen here” or “It won’t happen to me.” Even knowledgeable scientists who are aware of the hazards, the odds of their occurrence, and the costs of an event do not always act appropriately. Compounding the problem is the lack of tools to reliably predict the specific location and timing of many natural hazards. Unfortunately, a person who has not been adversely affected in a major way is much less likely to take specific steps to reduce the effects of a potential hazard. Migration of the population toward the Gulf and Atlantic coasts accelerated in the last half of the twentieth century and still continues. Most of those new residents, including developers and builders, are not very familiar with the power of coastal storms. Even where a hazard is apparent, people are slow to respond. Is it likely to happen? Will I have a major loss?
Can I do anything to reduce the loss? How much time will it take and how much will it cost? Who else has experienced such a hazard? Several federal agencies have programs to foster public awareness and education. The Emergency Management Institute—in cooperation with FEMA, the National Oceanic and Atmospheric Administration (NOAA), USGS, and other agencies—provides courses and workshops to educate the public and governmental officials. Some state emergency management agencies, in partnership with FEMA and other federal agencies, provide workshops, reports, and informational materials on specific natural hazards. Given the hesitation of many local governments to publicize natural hazards in their jurisdictions, people need to educate themselves. Being aware of the types of hazards in certain regions allows people to find evidence for their past occurrence. It also prepares them to seek relevant literature and ask the appropriate questions of knowledgeable authorities. Some people are receptive to making changes in the face of potential hazards. Some are not. The distinction depends partly on knowledge, experience, and whether they feel vulnerable. A person whose house was badly damaged in the 1989 Loma Prieta, California, earthquake is likely to either move to a less earthquake-prone area or live in a house that is well braced for earthquake resistance. A similar person losing his home to a landslide is more likely to live away from a steep slope. The best window of opportunity for effective reduction of a hazard is immediately following a disaster of the same type. Studies show that the window of opportunity is short—generally, not more than two or three months. Successful public education programs such as some of those on earthquake hazards in parts of California and presented by the USGS have shown that information must come from various credible sources and be presented in nontechnical terms that spell out specific steps that people can take. Broadcast messages can be helpful, but they should be accompanied by written material that people can refer to. Discussion among groups of people that would be affected can help them understand the hazard and act on this information. If people think the risk is plausible, they tend to seek additional reliable information to validate what they have heard. And the range of additional sources must be those that different groups of people trust. Some people will believe scientists; others will believe only structural engineers. Some will seek out information online. Successful education programs must include specialists and should adapt the material to the different interests of specific groups, such as homeowners, renters, and corporations. Overall, natural hazard education depends on tailoring the message clearly to different audiences using nontechnical language. It must not only convey the nature of potential events, but also show that certain relatively simple and inexpensive actions can substantially reduce potential losses.
N AT U R A L H A Z A R D S A N D D I S A S T E R S
9
Living with Nature Catastrophic events are natural and expected, but the most common human reaction to a current or potential catastrophe is to try to stop ongoing damage by controlling nature. In our modern world, it is sometimes hard to believe that scientists and engineers cannot protect us from natural disasters by predicting disasters or building barriers. But there are limits to scientific understanding and engineering capabilities. In fact, although scientists and engineers understand much about the natural world, they understand less than many people suppose. Unfortunately we cannot change the behavior of the natural system, because we cannot change natural laws. Most commonly, our attempts tend only to temporarily hinder the natural process while diverting the damaging energy of the natural system to other locations. In other cases, our attempts cause energy to build up and cause more severe damage later. If, through lack of forethought, you find yourself in a hazardous location, what can you do about it? You might build a river levee to protect your land. Or you might build a rock wall into the oceanside surf to stop sand from leaving your beach and undercutting the hill on which your house is built. If you do any of these things, however, you merely transfer the problem elsewhere, to someone else, or to a later point in time. For example, if you build a levee to prevent a river from spreading over a floodplain and damaging
your property, the flood level past the levee will be higher than it would have been without the levee. Constricting river flow with the levee will also back up the floodwater, potentially causing flooding of your upstream neighbor’s property. Deeper water also flows faster past your levee, so it may cause more erosion of your downstream neighbor’s riverbanks. As in the stock market, individual stocks go up and down. If you make money because you bought a stock when its price was low and sold it when its price was high, then you effectively bought it from someone else who lost money. In the stock market, over the short term, the best we can do, from a selfish point of view, is to shift disasters to our neighbors. The same is true in tampering with nature. We need to understand the consequences. Individually and as a society, we need to learn to live with nature, not try to control it. Mitigation efforts typically seek to avoid or eliminate the hazard through engineering. Such efforts require financing, either from the government or from individuals or groups that are likely to be affected. Less commonly but more appropriately, mitigation requires changes in human behavior. Change is commonly much less expensive and more permanent than the necessary engineering work. In recent years, governmental agencies have begun to learn this lesson, generally through their own mistakes. In a few places along the Missouri and Sacramento Rivers, for example, some levees are being moved back away from the river to permit the river to spread out on its floodplain during future floods.
Chapter Review
Key Points Catastrophes in Nature p Many natural processes that we see are slow and gradual, but occasional sudden or dramatic events can be hazardous to humans.
p Hazards are natural processes that pose a threat to people or their property.
p A large event becomes a disaster or catastrophe only when it affects people or their property. Large natural events have always occurred but do not
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become disasters until people place themselves in harm’s way.
p Developed countries lose large amounts of money in a major disaster; poor countries lose larger numbers of lives.
Predicting Catastrophe p Events are often neither cyclic nor completely random.
p Although the precise date and time for a disaster cannot be predicted, understanding the natural processes behind disasters allows scientists to forecast the probability of a disaster striking a particular area.
p There are numerous small events, fewer larger events, and only rarely a giant event. We are familiar with the common small events but tend not to expect the giant events that can create major catastrophes, because they come along so infrequently. Figure 1-5.
p Statistical predictions or recurrence intervals are average expectations based on past experience.
p Many natural features and processes are fractal— that is, they have similarities across a broad range of sizes. Large events tend to have characteristics that are similar to smaller events. Figure 1-6.
Relationships Among Events p Different types of natural hazards often interact with, or influence, one another. Figure 1-7.
p Overlapping influences of multiple factors can lead to the extraordinarily large events that often become disasters. Figure 1-8.
Mitigating Hazards p Mitigation involves efforts to avoid disasters rather than merely dealing with the resulting damages.
p Greater risk is proportional to the probability of occurrence and the cost from such an occurrence. By the Numbers 1-2.
p Most people believe that a disaster will not happen to them. They need to be educated about natural processes and how to learn to live with and avoid the hazards around them.
Living with Nature p Erecting a barrier to some hazard will typically transfer the hazard to another location or to a later point in time.
p Humans need to learn to live with some natural events rather than trying to control them.
Key Terms catastrophe, p. 3 cyclic events, p. 4 forecast, p. 4
fractal, p. 6 insurance, p. 8 land use planning, p. 7
mitigation, p. 7 natural disaster, p. 3 natural hazard, p. 3
precursor events, p. 6 recurrence intervals, p. 5 risk, p. 8
Questions for Review 1. Why do people live in geologically dangerous areas? 2. Is the geologic landscape controlled by gradual and unrelenting processes or intermittent large events with little action in between? Provide an example to illustrate. 3. Some natural disasters happen when the equilibrium of a system is disrupted. What are some examples? 4. What are the main reasons for the ever-increasing costs of catastrophic events? 5. Contrast the general nature of catastrophic losses in developed countries versus poor countries.
6. Why are most natural events not perfectly cyclic, even though some processes that influence them are cyclic? 7. Give an example of a fractal system. 8. If people should not live in especially dangerous areas, what beneficial use is there for those areas? 9. When an insurance company decides on the cost of an insurance policy for a natural hazard, what are the two main deciding factors? 10. When people or governmental agencies try to restrict or control the activities of nature, what is the general result?
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Plate Tectonics and Physical Hazards
Chapter
Jennifer Tidwell photo.
2 i The Himalaya Mountains are the highest in the world because of the immense plate tectonic forces that push India northward into Eurasia.
The Big Picture
W
hy are mountain ranges commonly near coastlines? Why are some of these mountains volcanoes that erupt molten rocks? What causes giant tsunami waves, and why do most originate near coastlines? Why are most of our most devastating earthquakes near certain coastlines? Giant areas of the upper part of Earth move around, grind sideways and collide, or sink into the hot interior of the planet, where they cause melting of rocks and formation of volcanoes. Those collisions squeeze up and maintain high mountain ranges, even though landslides and rivers try to erode them away. Those same collisions lead to giant tsunami waves. To understand where and when these hazards occur, we need to understand the forces that drive them. Without the movements of Earth’s plates, there would be no high ranges of mountains, as we know them, to cause rockfalls or other landslides, or for rivers to flow down. Less directly, those same mountain ranges have a big effect on weather and climate.
12
Hazards
Development of a Theory
N. America
Africa S. America
Murphy and Nance, 1999.
Overlap Gap
a
Fossil reptile Fossil mammal-like reptile Fossil marine reptile Fossil fern
p
FIGURE 2-1. a. Before continental drift a few hundred million years ago, the continents were clustered together as a giant “supercontinent” that has been called Pangaea. The Atlantic Ocean had not yet opened. The pale blue fringes on the continents are continental shelves, which are part of the continents. The areas of overlap and gap (in red and darker blue) are small. b. Some distinctive fossils that seem to lie in belts across the Atlantic Ocean.
Ancient rocks Mountain belts b
P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
13
Modified from Monroe and Wicander.
When you look at a map of the world, you may notice that the continents of South America and Africa would fit nicely together like puzzle pieces. In fact, as early as 1596, Abraham Ortelius, a Dutch map maker, noted the similarity of the shapes of those coasts and suggested that Africa and South America were once connected and had since moved apart. In 1912, Alfred Wegener detailed the available evidence and proposed that the continents were originally part of one giant supercontinent that he called Pangaea. North and South America had drifted apart from Europe and Africa, widening the Atlantic Ocean in the process. He suggested that the continents drifted through the oceanic crust, forming mountains along their leading edges. This hypothesis, called continental drift, remained at the center of the debate about large-scale Earth movements into the 1960s. Wegener noted that the match is especially good if we use the real edge of the continents, including the shallowly submerged continental shelves (p Figure 2-1a). To test this initial hypothesis, Wegener went further; he searched for connections between other aspects of geology across the Atlantic: mountain ranges, rock formations and their ages, and fossil life forms. Continued work showed that ancient rocks, their fossils, and their mountain ranges also matched across the Atlantic Ocean (p Figure 2-1b). This analysis is similar to what you would use when you put a jigsaw puzzle together; the pieces fit and the patterns match across the reconnected pieces. With confirmation of such former connections, he hypothesized that continents that were once together had moved apart by drifting through oceanic crust. Other lines of evidence support the continental drift hypothesis. Exposed surfaces of ancient rocks in the southern parts of Australia, South America, India, and Africa show grooves carved by immense areas of continental glaciers. The grooves show that glaciers with embedded rocks at their bases may have moved from Antarctica into India, eastern South America, and Australia (p Figure 2-2). The rocks were once buried under glacial ice, yet many of these areas now have warm to tropical climates. In addition, the remains of fossils that formed in warm climates are found in areas such as Antarctica and the present-day Arctic: coal with fossil impressions of tropical leaves, the distinctive fossil fern Glossopteris, and coral reefs. Despite this evidence, many scientists rejected Wegener’s whole hypothesis because they could show that his proposed mechanism was not
physically possible. English geophysicist Harold Jeffreys argued that the ocean floor rocks were far too strong to permit the continents to plow through them. Others who were willing to consider other possibilities eventually came up with a mechanism that fit all of the available data.
The first step in understanding how the continents were separating was to learn more about the topography of the ocean floor, what it looked like, and how old it was. Oceanographers from Woods Hole Oceanographic Institute in Massachusetts, who were measuring depths from all over the Atlantic Ocean in the late 1940s and 1950s, found an immense mountain range down the center of the ocean and extending for its full length (p Figure 2-3), a mid-oceanic ridge. Later, scientists recognized that most earthquakes in the Atlantic Ocean were concentrated in that central ridge. Although the anti–continental drift group dominated the scientific literature for years, in 1960 Harry Hess of Princeton University conjectured that the ocean floors acted as giant conveyor belts carrying the continents. New oceanic crust welled up at the mid-oceanic ridges, spread away, and finally sank into the deep oceanic trenches along the edges of some continents, a process later called seafloor spreading. Hess calculated the spreading rate to be approximately 2.5 centimeters (1 inch) per year across the Mid-Atlantic Ridge. If correct, the whole Atlantic Ocean floor would have been created in only 180 million years or so. Confirmation of seafloor spreading finally came in the mid-1960s through work on the magnetic properties of rocks of the ocean floor. We are all aware that the Earth has a magnetic field because a magnetized compass needle points toward the north magnetic pole. Slow convection currents in the Earth’s molten nickel-iron outer core are believed to generate that magnetic field (p Figure 2-4). This field reverses its north-south orientation every 10,000 to several million years (every 600,000 years on average) because of changes in those currents. The ocean floor consists of basalt, a dark lava that erupted at the mid-oceanic ridge and solidified from molten magma.
AFRICA SOUTH AMERICA
South Pole INDIA
ANTARCTICA
AUSTRALIA
a
Donald Hyndman photo.
North America
b
p
FIGURE 2-2. a. Continental masses of the southern hemisphere appear to have been parts of a supercontinent 300 million years ago in which a continental ice sheet centered on Antarctica spread outward to cover adjacent parts of South America, Africa, India, and Australia. After separation, the continents migrated to their current positions. b. The inset photo shows glacial grooves like those found in the glaciated areas of those continents.
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CHAPTER 2
NOAA/NGDC.
Atlantic Ocean
Africa
South America
p
FIGURE 2-3. In this seafloor topographical map for the Atlantic Ocean, shallow depths at the oceanic ridges are shown in orange to yellow, deeper water off the ridge crests are in green, and deep ocean is blue. Shallow continental shelves are shown in red.
North geographic rotational pole
N
North magnetic pole
BRITISH COLUMBIA
Va n
co
uv
er
Isl
an
Jua
nd
Earth's core
d
eF
uca
WASHINGTON
it
Age of oceanic crust (millions of years) Present
Columbia River
Juan de Fuca Ridge
OREGON
2 4 S 6
p
nc
oF
tur e
8 10
Mendocino Fracture Gorda Ridge
CALIFORNIA
Iron atoms crystallizing in the magma orient themselves like tiny compass needles, pointing toward the north magnetic pole. As a result, the rock is slightly magnetized with an orientation like the compass needle. When the magnetic field reverses, that reversed magnetism is frozen into rocks when they solidify. British oceanographers Frederick Vine and Drummond Matthews, studying the magnetic properties of ocean-floor rocks in the early 1960s, showed a striped pattern parallel to the mid-oceanic ridge (p Figure 2-5). Some of the stripes were strongly magnetic; adjacent stripes were weakly magnetic. They realized that the magnetism was stronger where the rocks solidified while Earth’s magnetism was oriented parallel to the present-day north magnetic pole. Where the rock magnetism was pointing toward the south magnetic pole, the recorded magnetism was weak—it was partly canceled by the present-day magnetic field. Earth’s magnetic field imposed a pattern of magnetic stripes as the basalt solidified at the ridge, because the magnetic field reversed from time to time. As the ridge spread apart, ocean floor formed under alternating periods of north- versus southoriented magnetism to create the matching striped pattern on opposite sides of the ridge. These magnetic anomalies provide relative ages of the ocean floor; their mapped widths match across the ridge, and the rocks were assumed to get progressively older as they moved away from mid-oceanic ridges. Determination of the true ages of ocean-floor rocks eventually came from drilling in the deep-sea floor by research ships of the Joint Oceanographic Institute for Deep Earth Sampling (JOIDES),
Bla
rac
W. J. Kious and R. I. Tilling, USGS.
FIGURE 2-4. Earth’s magnetic field is shaped as if there were a huge bar magnet in the Earth’s core. Instead of a magnet, Earth’s rotation is thought to cause currents in the liquid outer core. Those currents create a magnetic field in a similar way in which power plants generate electricity when steam or falling water rotates an electrical conductor in a magnetic field.
Stra
Cape Mendocino
p
FIGURE 2-5. The magnetic polarity, or orientation, across the Juan de Fuca Ridge in the Pacific Ocean shows a symmetrical pattern, as shown in this regional survey (a similar nature of stripes exists along all spreading centers). Basalt lava erupting today records the current northward-oriented magnetism right at the ridge; basalt lavas that erupted less than 1 million years ago recorded the reversed, southward-oriented magnetic field at that time. The south-pointing magnetism in those rocks is largely canceled out by the present-day north-pointing magnetic field, so the ocean floor shows alternating strong (north-pointing) and weak (south-pointing) magnetism in the rocks.
funded by the National Science Foundation (p Figure 2-6). The ages of basalts and sediments dredged and drilled from the ocean floor showed that those near the Mid-Atlantic Ridge were young (1 million years old) and had only a thin coating of sediment. Both results contradicted the prevailing notion that the ocean floor was extremely old. In contrast, rocks from deep parts of the ocean floor far from the ridge were consistently much older (up to 180 million years) (p Figure 2-7). All of this evidence supports the modern theory of plate tectonics, which describes the big picture of movements of Earth’s plates. We now know that the world’s landmasses once formed one giant supercontinent, called Pangaea, 225 million years ago. As the seafloor spread, Pangaea began to break up, and the plates slowly moved the continents into their current positions (p Figure 2-8). P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
15
0 50
ATLANTIC OCEAN FLOOR
100 150 180 Age of ocean floor (millions of years)
NOAA/NGDC.
National Science Foundation.
PACIFIC OCEAN FLOOR
p p
FIGURE 2-6. The JOIDES deep-seafloor drilling ship still does deep-sea drilling of the ocean floor that contributes to understanding ocean-floor processes.
FIGURE 2-7. Ocean-floor ages are determined by their magnetic patterns. Red colors at the oceanic spreading ridges grade to yellow at 48 million years ago, to green 68 million years ago, and to dark blue some 155 million years ago.
EURASIA
AFRICA
Kious and Tilling, USGS.
Equator
Equator
Equator S. Amer.
AUSTRALIA ANTARCTICA
Present day
Cretaceous 65 million years ago
Jurassic 135 million years ago
FIGURE 2-8. The supercontinent Pangaea broke up into individual continents starting approximately 225 million years ago.
Midoceanic ridge Trench Ocean Subduction zone
Modified from Monroe and Wicander, 2001.
Oceanic lithosphere
Cold
Continental lithosphere
Upwelling
Asthenosphere
Outer core
Hot
Mantle
Inner core
p
FIGURE 2-9. A generalized cross section through the Earth shows its main concentric layers. The more rigid lithosphere moves slowly over the less rigid asthenosphere, which is thought to circulate slowly by convection. The lithosphere pulls apart at ridges and sinks at trenches.
CHAPTER 2
INDIA
SOUTH AMERICA
Africa
Permian 225 million years ago
16
ASIA
Equator
India Australia Antarctica
p
EUROPE
NORTH AMERICA
N. Amer.
Those plates continue to pull apart at the Mid-Atlantic Ridge to make the ocean floor wider. In the Pacific Ocean, the plates pull apart at the East Pacific Rise; their oldest edges sink in the deep ocean trenches near the western Pacific continental margins (p Figure 2-9). As it turns out, Wegener’s hypothesis that the continents moved apart was confirmed by the data, although his assumption that the continents plowed through the ocean was not. The development of this theory is a good example of how the scientific method works. The scientific method is based on logical analysis of data to solve problems. Scientists make observations and develop tentative explanations—that is, hypotheses—for their observations. A hypothesis should always be testable, because science evolves through continual testing with new observations and experimental analysis. Alternate hypotheses should be developed to test other potential explanations for observed behavior. If observations are inconsistent with a hypothesis, it can either be rejected or revised. If a hypothesis continues to be supported by all available data over a long period of time, it becomes a theory. After a century of testing, Wegener’s initial hypothesis of continental drift was modified to be the foundation for the modern theory of plate tectonics. Plate tectonics is supported by a large mass of data collected over the last century. Modern data continue to support the concept that plates move and substantiate the mechanism of new oceanic plate generation at the mid-oceanic ridges and destruction of the plates at oceanic trenches. This theory is a fundamental foundation for the geosciences and is important for understanding why and where we have a variety of major geologic hazards, such as earthquakes and volcanic eruptions.
Earth Structure At the center of the Earth is its core, surrounded by the thick mantle and covered by the much thinner crust. Note that these distinctions are based on rock composition. In addition, we distinguish between lithosphere and asthenosphere based on rock rigidity or strength.The stiff, rigid outer rind of the Earth is called the lithosphere, and the inner, hotter, more easily deformed part is called the asthenosphere. The lithosphere makes up the tectonic plates and is 60 to 200 kilometers (47 to 124 miles) thick (p Figure 2-10). Continental lithosphere includes silica-rich crust 30 to 50 kilometers thick, underlain by mantle (see Appendix 2 for detailed rock compositions). Oceanic lithosphere is generally only about 60 kilometers thick. The top 7 kilometers are a low-silica crust. Continental crust is largely composed of high-silica-content minerals, which give it the lowest density (2.7 g/cm3) of the major regions on Earth. Oceanic crust has a higher density (3 g/cm3) because it contains more iron- and magnesium-rich minerals. As shown in the right-hand diagram of Figure 2-10, the low-density continental crust is thicker and stands higher than the denser oceanic crust. The concept of isostacy, or buoyancy, explains this elevation difference. Although Earth’s mantle is not liquid, its high temperature (above 450°C or 810°F) permits it to flow slowly as if it were a viscous liquid. A floating solid object will displace a liquid of the same mass. As a result, the proportion of a material immersed in the liquid can be calculated from the density of the floating solid divided by the density of the liquid. For example, when water freezes, it expands to become a lower density (ice density is 0.9 g/cm3 relative to liquid water at 1.0 g/cm3). Thus, 90 percent of an ice cube or iceberg will be underwater; similarly, approximately 84 percent of a Oceanic crust (rigid) (3 g/cm3) Ocean water (1 g/cm3)
km 370
=3
Depth (km)
i
1m
,98
Inner core
Continental crust (rigid) (2.7 g/cm3)
7–65
6,
Lithosphere (rigid)
60–200
Modified from Garrison, 2002.
e Outer cor
2,9 0 1,8 0 km = 13 mi
Upper Asthenosphere mantle (deformable, (3.2 g/cm3) capable of flow)
Mantle Crust
360–650
Lower mantle (4.5 g/cm3)
Lithosphere (strong “plate” layer): Crust and upper part of mantle; relatively cool and brittle Asthenosphere (weak layer): Small amount of melt; weak, plastic behavior
p
FIGURE 2-10. A slice into the Earth shows a solid inner core and a liquid outer core, both composed of nickel-iron. Peridotite of the Earth’s mantle makes up most of the volume of the Earth. The Earth’s crust, on which we live, is as thin as a line at this scale.
P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
17
Iceberg: 10% above water
Major mountain range
Mountain root (crust)
90% of volume submerged
Water
Earth’s mantle
a
b
p
FIGURE 2-11. a. An iceberg sinks 90 percent of its mass into water because the ice (90 percent of the density of water) displaces an equivalent mass of water (1.0 g/cm3). b. Similarly, the load of a thick mass of a continental mountain range sinks the continental crust (2.7 g/cm3) into the underlying denser mantle (3.2 g/cm3) to provide a “mountain root.”
mountain range of continental rocks (2.7 g/cm3) will submerge into the mantle (3.2 g/cm3) as a deep mountain root (p Figure 2-11). If the weight of an extremely large glacier is added to a continent, the crust and upper mantle will slowly sink deeper into the mantle. A map of Earth’s topography clearly shows the continents standing high relative to the ocean basins because of isostacy. The thin lithosphere of the ocean basins stands low; the continents with their thick lithosphere sink deep into the asthenosphere and float high. The thickest parts of the continental lithosphere sink deepest and stand highest
as major mountain ranges such as the Rockies, Alps, and Himalayas (p Figure 2-12). Because we do not have direct observations of crustal thickness, scientists measure the gravitational attraction of the Earth (greater over denser rocks) and analyze seismic waves as they propagate away from the locations of earthquakes to provide indirect evidence of the density, velocity, and thickness of subsurface materials. The boundary between Earth’s crust and mantle has been identified as a difference in density that we call the Mohoroviçic Discontinuity, or Moho (p Figure 2-13).
Atlantic Ocean Pacific Ocean Indian Ocean Sea-floor ridge
NOAA.
Trench
p
FIGURE 2-12. This shaded relief map shows the continents standing high. Mountain ranges in red tones concentrate at converging margins. Light blue ridges in oceans are generally spreading centers, and trenches are visible at Pacific continental margins where oceanic crust is subducting below continental crust.
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CHAPTER 2
Oceanic ridge
Oceanic crust (basalt over gabbro) ≈ 7 km thick Density ≈ 3.0 g/cm3
Continental crust (granite, gneiss, schist, sedimentary, and M o h volcanic rocks) ≈ 30-50 km thick Oceanic lithosphere o Density ≈ 2.7 g/cm3 (strong and rigid) ≈ 60 km thick Mantle (peridotite) LVZ Density ≈ 3.2 g/cm3 (conta Upwelling ins sm a ll mantle p of bas alt pa ercentage rtial m elt) Continental lithosphere (strong and rigid) Asthenosphere (peridotite) ≈ 200 km thick (weak, easily deformed) density ≈ 3.2 g/cm3
Moho
Deeper in the mantle, the next major change in material properties occurs at the boundary between the strong, rigid lithosphere and the weak, deformable asthenosphere. This boundary was first identified as a near-horizontal zone of lower velocity of seismic or earthquake waves that move at several kilometers per second. The so-called low-velocity zone (see Figure 2-13) is concentrated at the top of the asthenosphere and may contain a small amount of molten basalt over a zone several hundred kilometers thick. The cold,
p
FIGURE 2-13. Earth’s lithosphere and asthenosphere are distinguished by their strength: strong or weak. Earth’s crust and mantle are distinguished by composition: basalt or peridotite. These rock types are described in Appendix 2. Moho Mohoroviçic Discontinuity boundary between crust and mantle. LVZ low-velocity zone, where seismic waves are slower due to partially melted rocks.
rigid lithosphere rides on that asthenosphere made weak by its higher temperatures and perhaps also by small melt contents. The lithosphere moves over the weak, deforming asthenosphere at a few centimeters per year. The lithosphere is not continuous like the rind on a melon. It is broken into about a dozen or so large plates and another dozen or so much smaller plates (p Figure 2-14). Even though they are uneven in size and irregular in shape, the plates fit neatly together in a mosaic that covers the
Eurasian Plate 5.4 7.9
6.9 Pacific Plate
10.5
Monroe and Wicander, modified from NOAA.
7.1
3.7
3.0
2.0 2.5
7.0 Nazca Plate
Indian-Australian Plate
7.2
Caribbean Plate
5.5
17.2
4.0
7.3
2.3
11.7 Cocos Plate
2.0
6.2
Eurasian Plate
1.8 North American Plate
18.3
10.1
South American Plate
3.8 African Plate
11.1
7.4 4.1
10.3
3.7
3.3
7.7
1.3
1.7
5.7 Antarctic Plate
Ridge axis
Subduction zone
Hot spot
Direction of movement
p
FIGURE 2-14. Most large lithospheric plates consist of both continental and oceanic areas. Although the Pacific Plate is largely oceanic, it does include a slice of western California and the eastern part of New Zealand. General direction and velocities of plate movement (compared with hotspots that are inferred to be anchored in the deep mantle), in centimeters per year, are shown with arrows. P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
19
Plate
Plate Plate
Asthenosphere
Asthenosphere
Asthenosphere
Convergent boundary Transform Divergent Subduction (subduction zone) plate boundary plate boundary zone
Continental divergent boundary
Tre n
Trench
ch
Continental crust
Lithosphere Oceanic crust Asthenosphere Hot spot
Su
bd
uc
p
FIGURE 2-15. This threedimensional cutaway view shows the different types of lithospheric plate boundaries: convergent, divergent, and transform.
tin
gp
lat
USGS.
e
entire surface of the Earth. The plates do not correspond to continent versus ocean areas; most plates consist of a combination of the two. Even the Pacific Plate, which is mostly ocean, includes a narrow slice of western California and part of New Zealand. Earth’s plates move up to 11 centimeters (4.2 inches) per year, as recently confirmed by satellite Global Positioning System measurements. Many move in roughly an east-west
direction, but not everywhere. Some separate, others collide, and still others slide under or over or past one another (p Figure 2-15). In some cases, their encounters are head on; in others, the collisions are more oblique. Plates move away from each other at spreading or divergent boundaries, most commonly at mid-oceanic spreading zones (p Figure 2-16). Plates move toward each other at collision or convergent boundaries. In cases where one or
p
Aluminum Company of America, in Garrison, Oceanography.
FIGURE 2-16. The spreading Mid-Atlantic Ridge, fracture zones, and transform faults are dramatically exhibited in this exaggerated topography of the ocean floor.
20
CHAPTER 2
both of the plates are oceanic lithosphere, the denser plate will slide down, or be subducted, into the asthenosphere, forming a subduction zone. When two continental plates collide, neither side is dense enough to be subducted deep into the mantle, so the two sides typically crumple into a thick mass of low-density continental material. This type of convergent boundary is where the largest mountain ranges
BRITISH COLUMBIA
ca
Ri dg e
Vancouver
JUAN DE FUCA PLATE
WASHINGTON
Portland
Cas subdu cadia ction zone
Ju
an
de
Fu
Seattle
Mendocino fracture
OREGON
NORTH AMERICAN PLATE CALIFORNIA
on Earth, such as the Himalayas, are built. In the remaining category of plate interactions, two plates slide past each other at a transform boundary such as the San Andreas Fault. In some places, different types of plate boundaries join at triple junctions. For example, the Cascadia subduction zone off the Washington-Oregon coast joins both the San Andreas transform fault and the Mendocino transform fault at the Mendocino triple junction just off the northern California coast. The north end of the same subduction zone joins both the Juan de Fuca spreading ridge and the Queen Charlotte transform fault at a triple junction just off the north end of Vancouver Island (p Figure 2-17).
Hazards and Plate Boundaries Most of Earth’s earthquake and volcanic activity occurs along or near moving plate boundaries (p Figures 2-18 and 2-19). Most of the convergent boundaries between oceanic and continental plates form subduction zones along the Pacific coasts of North and South America, Asia, Indonesia, and New Zealand. Collisions between continents are best expressed in the high mountain belts extending across southern Europe and Asia (see Figure 2-12). Most rapidly spreading divergent boundaries follow oceanic ridges. In some cases, slowly spreading boundaries such as the East African Rift zone pull continents apart. Each type of plate boundary has a distinct pattern of natural events associated with it.
NEVADA eas ndr n A Sa
San Francisco
lt fau
PACIFIC PLATE
Monroe and Wicander, 2002.
Los Angeles
Oceanic ridge
p
Subduction zone
Transform fault
FIGURE 2-17. Three types of plate boundaries are found at the western edge of North America: The San Andreas Fault runs through the western edge of California; the Cascadia subduction zone parallels the coast off Oregon, Washington, and southern British Columbia; and the Juan de Fuca spreading ridge lies farther offshore. Spreading at the Juan de Fuca Ridge carries ocean floor of the Juan de Fuca Plate down the Cascadia subduction zone.
Divergent Boundaries Divergent boundaries where plates pull apart, by sinking of heavy lithosphere at the oceanic trenches, make a system of more or less connected oceanic ridges that wind through the ocean basins like the seams on a baseball. Iceland is the only place where that ridge system rises above sea level; elsewhere, it is submerged to an average depth of a few thousand meters. A broad valley doglegs from south to north across Iceland. The hills east of it are on the Eurasian Plate that extends all the way east to the Pacific Ocean; the hills to the west are on the North American Plate. Repeated surveys over several decades have shown that the valley is growing wider at a rate of several centimeters per year. The movement is the result of the North American and Eurasian Plates pulling away from each other, making the Atlantic Ocean grow wider at this same rate. Iceland’s long recorded history shows that a broad fissure opens in the floor of the central valley of Iceland every 200 to 300 years. It erupts a large basalt lava flow that covers as much as several thousand square kilometers. The last fissure opened about the time of the American Revolution, and another such event could happen anytime. Fortunately,
P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
21
Pacific Ocean
Monroe and Wicander, modified from NOAA.
Indian Ocean Atlantic Ocean
Deep-focus earthquake
Intermediate-focus earthquake
Shallow-focus earthquake
p FIGURE 2-18. Most earthquakes are concentrated along boundaries between major tectonic plates, especially subduction zones and transform faults, with fewer along spreading ridges.
Aleutian Islands Eurasian plate Juan de Fuca
Eurasian plate
North American plate Cascade Range Caribbean Arabian plate
Hawaiian volcanoes
Indian plate
Monroe and Wicander, modified from NOAA.
Pacific plate
Cocos
Nazca plate
Australian plate
South American plate
African plate
Antarctic plate
Divergent plate boundary
p
Transform plate boundary
Convergent boundary
Volcano
FIGURE 2-19. Most volcanic activity also occurs along plate tectonic boundaries. Eruptions tend to be concentrated along the continental side of subduction zones and along divergent boundaries such as rifts and mid-oceanic ridges.
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CHAPTER 2
Convergent Boundaries SUBDUCTION ZONES If the Earth generates new oceanic crust at boundaries where plates pull away from each other, then it must destroy old oceanic crust somewhere else. It swallows old oceanic crust in subduction zones (see Figure 2-15). If not, our planet would be growing steadily
Donald Hyndman photo.
a
Rio Grande Rift Modified from NOAA.
the sparse population of the region limits the potential for a great natural disaster. It now seems clear that similar events happen fairly regularly all along the oceanic ridge system. These spreading centers are the source of the basalt lava flows that cover the entire ocean floor, roughly two-thirds of Earth’s surface, to an average depth of several kilometers. The molten basalt, or magma, rises to the surface, where it comes in contact with water. It then rapidly cools to form pillowshaped blobs of lava, with an outer solid rind initially encasing molten magma. As the plate moves away from the spreading center, it cools, shrinks, and thus increases in density. This explains why the hot spreading centers stand high on the subsea topography. New ocean floor continuously moves away from the oceanic ridges as the oceans grow wider by several centimeters every year (Figure 2-19). The only place where frequent earthquakes and volcanic eruptions along oceanic ridges pose a danger to people or property is in Iceland, where the oceanic ridge rises above sea level. Spreading centers in the continents pull apart at much slower rates and do not generally form plate boundaries. The Rio Grande Rift of New Mexico and the Basin and Range of Nevada and Utah are active North American examples (p Figure 2-20). The East African Rift zone extends north-south through much of that continent (p Figure 2-21). A few earthquakes, sometimes large, and volcanic eruptions accompany the up-and-down,“normal fault” movements. Volcanic activity is varied, ranging from large rhyolite calderas in the Long Valley Caldera of the Basin and Range of southeastern California and the Valles Caldera of the Rio Grande Rift of New Mexico to small basaltic eruptions at the edges of the spreading center. The Red Sea Rift, at the northeastern edge of Africa, is a location where the rift forms a plate boundary. Continental rifts such as the Rio Grande Rift of New Mexico spread so slowly that they cannot split the continental plate to form new ocean floor. Most of the magmas that erupt in continental rift zones are either ordinary rhyolite or basalt with little or no intermediate andesite (see Appendix 2). But some of the magmas are peculiar, with high sodium or potassium contents. Some of the rhyolite ash deposits in the Rio Grande Rift and in the Basin and Range provide evidence of extremely large and violent eruptions of giant rhyolite volcano activity. But those events appear to be infrequent, and much of the region is sparsely populated, so they do not pose much of a volcanic hazard.
b
p
FIGURE 2-20. a. This Basin and Range terrain is found southwest of Salt Lake City, Utah. b. This broad area of spreading in the western United States is marked by prominent basins and mountain ranges. Centered in Nevada and western Utah, it gradually decreases in spreading rate to the north across the Snake River Plain, near its north end. Its western edge includes the eastern edge of the Sierra Nevada Range, California, and its main eastern edge is at the Wasatch Front in Utah. An eastern branch includes the Rio Grande Rift of central New Mexico.
P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
23
Modified from NASA Space Shuttle photo.
b
Rift Rift valley Oceanic crust Monroe and Wicander, 2006.
Stretched continental crust
a
p
FIGURE 2-21. a. The East African Rift Valley spreads the continent apart at rates 100 times slower than typical oceanic rift zones. This rift forms one arm of a triple junction, from which the Red Sea and the Gulf of Aden form somewhat more rapidly spreading rifts. b. The northern end of the Red Sea spreading zone connects with the Dead Sea transform fault.
larger at the same rate as new oceanic crust forms. That is clearly not the case. The idea of two plates colliding is truly horrifying at first thought—the irresistible force finally meets the immovable object. But the Earth solves its dilemma as one plate slides beneath the other and dives into the hot interior (p Figure 2-22). Grinding rock against rock is not easy, since the slippage zone sticks and occasionally slips, with an accompanying earthquake. The plate that sinks is the denser of the two, the one with oceanic crust on its outer surface. It absorbs heat as it sinks into the much hotter rock beneath, until it finally heats up at a depth of several hundred kilometers. Where an oceanic plate sinks in a subduction zone, a line of picturesque volcanoes rises inland from the trench (p Figures 2-22a and 2-23). The process begins at the oceanic spreading ridge, where fractures open in the ocean floor. Seawater penetrates the dense peridotite of the upper mantle, where the two react to make a greenish rock
24
CHAPTER 2
called serpentinite that contains a lot of water. That altered ocean floor eventually sinks through an oceanic trench and descends into the upper mantle, where the serpentinite heats up, breaks down, releases its water, and reverts back to peridotite. The water rises into the overlying mantle, which it partially melts to make basalt magma that rises. If the basalt passes through continental crust, it can heat and melt some of those rocks to make rhyolite magma. The basalt and rhyolite may erupt separately or mix in any proportion to form andesite and related rocks, the common volcanic rocks in stratovolcanoes. The High Cascades volcanoes in the Pacific Northwest are a good example; they lie inland from an oceanic trench, the surface expression of the active subduction zone. Because sudden slippage of the continental plate over the ocean floor is generally under water, the sudden movement of a lot of water can cause a huge tsunami wave. The wave both washes onto the nearby shore and races out across the ocean to endanger other shorelines.
Oceanic crust
Trench Trench
Volcanic arc
Island arc
Continental crust
Oceanic crust
Modified from Kious and Tilling, USGS.
Continental crust
Asthenosphere Asthenosphere
a
Lithosphere
Lithosphere
b
p
FIGURE 2-22. a. A continental volcanic arc forms on the continent where the oceanic lithosphere descends beneath the continental margin. b. An oceanic island arc forms on oceanic lithosphere where the lithosphere descends beneath more oceanic lithosphere. In both cases, earthquakes are generated in the subduction zone where the overriding lithosphere sticks against the descending lithosphere and then suddenly slips.
The sinking slab of lithosphere also generates many earthquakes, both shallow and deep. Earth’s largest earthquakes are generated along subduction zones; some of these cause major natural catastrophes. Somewhat smaller earthquakes occur in the overlying continental plate between the oceanic trench and the line of volcanoes.
Collision of Continents Where two continental plates collide, called a collision zone, the results can be catastrophic. Neither plate sinks, and so high mountains such as the Himalayas are pushed up in fits and starts, accompanied by large earthquakes (p Figure 2-24). Earthquakes regularly kill thousands of
Modified from NOAA.
John Pallister photo, USGS.
In some cases, an oceanic plate descends beneath another section of oceanic plate attached to a continent (Figure 2-22b). The same melting process described above generates a line of basalt volcanoes because there is no overlying continental crust to melt and form rhyolite. Volcanoes above a subducting slab present major hazards to people who live on or near them and to their property. It is hard to deter people from settling near these hazards because volcanoes are very scenic, and the volcanic rocks break down into rich soils that support and attract large populations.They are prominent all around the Pacific basin and in Italy and Greece, where the African Plate collides with Europe.
a
p
b
FIGURE 2-23. a. Mount St. Helens (in foreground) and Mount Rainier (behind) are two of the picturesque active volcanoes that lie inland from the Cascadia subduction zone. b. The Cascade volcanic chain forms a prominent line of peaks parallel to the oceanic trench and 100 to 200 kilometers inland.
P L AT E T E C T O N I C S A N D P H Y S I C A L H A Z A R D S
25
High plateau
Continental crust
Continental crust
Lithosphere Lithosphere Asthenosphere a
Geoff Edwards photo.
Modified from Kious and Tilling, USGS.
Mountain range
b
p
FIGURE 2-24. a. Collision of two continental plates generally occurs after subduction of oceanic crust. The older, colder, denser plate may continue to sink, or the two may merely crumple and thicken. Collision promotes thickening of the combined lithospheres and growth of high mountain ranges. b. The Himalayas, which are the highest mountains on any continent, were created by collision between the Indian and Eurasian Plates.
people from the continuing collision between India and Asia during the ongoing rise of the Himalayas, and between the Arabian Plate and Asia during the formation of the Caucasus (p Figures 2-25 and 2-26). These earthquakes are distributed across a wide area because of the thick, stiff crust in these mountain ranges.
Transform Boundaries In some places, plates simply slide past each other without pulling apart or colliding. Those are called transform plate boundaries or transform faults. Some of them offset the mid-oceanic ridges. Because the ridges are spreading zones, the plates move away from them. As shown in p Figure 2-27a, the section of the fault between the offset ends of the spreading ridge has significant relative movement. Lateral movement between the ridge ends (area of earthquakes shown by red stars) occurs in the opposite direction compared with beyond the ridges, where there is no relative movement across the same fault. Note also that the offset between the two ridge segments does not
p
FIGURE 2-25. Collision of the Indian Plate with the Eurasian Plate has thickened the continental lithosphere and continues to push up the Himalayas, Caucasus, and Zagros mountains.
A S I A
Zagros Mountains Himalayas
NASA.
Saudi Arabia
26
CHAPTER 2
India
indicate the direction of relative movement on the transform fault. Oceanic transform faults generate significant earthquakes without causing casualties because no one lives on the ocean floor. On continents it is a different story. The San Andreas Fault system in California is a well-known continental example. The North Anatolian Fault in Turkey is another that is even more deadly. The San Andreas Fault is the dominant member of a swarm of more or less parallel faults that move horizontally. Together, they have moved a large slice of western California, part of the Pacific Plate, north more than 500 kilometers so far. Transform plate boundaries typically generate large numbers of earthquakes, a few of which are catastrophic. A sudden movement along the San Andreas Fault caused the devastating San Francisco earthquake of 1906, with its large toll of casualties and property damage. The San Andreas system of faults passes through the metropolitan areas south of San Francisco (p Figure 2-27b) and just east of Los Angeles. Both areas are home to millions of people, who live at risk of major earthquakes that have the
China
Subduction before collision Ancient oceanic crust INDIAN PLATE
EURASIAN PLATE
Early collision Tip of Indian plate
After
EURASIAN PLATE
Reference point
Rising Himalayas
Reference point
Ancient Rising oceanic crust Tibetan Plateau
EURASIAN PLATE
INDIAN PLATE
p
FIGURE 2-26. Collision between two continental plates deforms the edge of both plates, causing frequent earthquakes (red stars) in the broad zone of collision.
Hotspot Volcanoes Some volcanoes erupt in places remote from any plate boundary; most of these are hotspot volcanoes (p Figure 2-28). These are the surface expressions of hot columns of partially molten rock anchored in the deep mantle. Their origin is unclear, but many scientists infer that they arise from the core/mantle boundary. Because hotspots are anchored deep in the Earth, they burn a track in the overlying lithospheric plates that move over the hotspot. Typical hotspot volcanoes therefore erupt at the active end of a long chain of extinct volcanoes that become progressively older
Plate boundary Oceanic ridge
Adjacent sections Sections here here move in same move in opposite direction directions
Fracture zone (inactive)
Garrison, 2002.
Transform fault (active part of fracture zone)
Adjacent sections here move in same direction
Fracture zone (inactive)
Lithosphere Asthenosphere
a
p
David Hyndman photo.
Modified from Kious and Tilling, USGS.
INDIAN PLATE Very old rock, 2 to 2 1/2 billion years old
Ancient oceanic crust
potential to cause enormous numbers of casualties and substantial property damage with little or no warning. Even moderate earthquakes in 1971 and 1994 near Los Angeles, in 1989 near San Francisco, and in 2003 near Paso Robles killed some people. The threat of such sudden havoc in a still larger event inspires much public concern as well as major scientific efforts to find ways to predict large earthquakes. For reasons that remain mostly unclear, some transform plate boundaries are also associated with volcanic activity. Several large volcanic fields have erupted along the San Andreas system of faults during the last 15 million or so years. One of those, in the Clear Lake area north of San Francisco, erupted recently enough to suggest that it may still be capable of further eruptions.
b
FIGURE 2-27. a. In this perspective view of an oceanic spreading center, earthquakes (stars) occur along spreading ridges and on transform faults offsetting the ridge. b. The heavily populated area, here viewed south from above San Francisco, California, straddles the San Andreas Fault, under San Andreas Lake and Crystal Springs Reservoir here.
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27
Iceland
Yellowstone
Azores
Hawaii Canary Is. Galapagos Plotted from various sources on NASA base map.
Tahiti Reunion Easter Is.
p
FIGURE 2-28. The main hotspot volcanoes of the world are shown as white dots. A few of the more prominent locations are named. Note that most, like Hawaii, are in ocean basins, at the ends of hotspot tracks.
with increasing distance from the active volcano. Mauna Loa and Kilauea, for example, erupt at the eastern end of the Hawaiian Islands, a chain of extinct volcanoes that become older westward toward Midway Island. Beyond Midway, the Hawaiian-Emperor chain doglegs to a more northerly course. It continues as a long series of defunct volcanoes that are now submerged. They form seamounts to the western end of the Aleutian Islands west of Alaska. The rising column or plume of hot rock appears to remain fixed in its place as one of Earth’s plates moves over it. So far as anyone knows, this may continue almost indefinitely as the hotspot track of dead volcanoes lengthens. Eventually the volcanoes and the plate carrying them slide into a subduction zone and disappear. Hotspot volcanoes leave a clear record of the direction and rate of movement of the lithospheric plates. Remnants of ancient hotspot volcanoes show the direction of movement in the same way that a saw blade traces the direction of movement of the board being sawn. The ages of those old volcanoes provide the rate of movement of the lithospheric plate (p Figure 2-29). The assumption, of course, is that the mantle containing the hotspot is not itself moving. Comparison of different hotspots suggests that this is generally valid. Plumes of abnormally hot but solid rock rising within Earth’s mantle begin to melt as the rock pressure on them drops. Wherever peridotite of the asthenosphere partially
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melts, it releases basalt magma that fuels a volcano on the surface. If the hotspot is under the ocean floor, the basalt magma erupts as basalt lava. If the hot basalt magma rises under continental rocks, it partially melts those rocks to form rhyolite magma; that magma often produces violent eruptions of ash. The melting temperature of basalt is more than 300°C above that of rhyolite, so a small amount of molten basalt can melt a large volume of rhyolite. The molten rhyolite rises in large volumes, which may erupt explosively through giant rhyolite calderas such as in Yellowstone National Park in Wyoming and Montana, Long Valley Caldera in eastern California, and Taupo Caldera in New Zealand. The Snake River Plain of southern Idaho is probably the best example of a continental hotspot track. Along this track is a series of extinct resurgent calderas, depressions where the erupting giant volcano collapsed. Those volcanoes began to erupt some 14 million years ago. They track generally east and northeast in southern Idaho, becoming progressively younger northeastward as the continent moves southwestward over the hotspot (p Figure 2-30). They are a continental hotspot track that leads from its western end near the border between Idaho and Oregon to the Yellowstone resurgent caldera at its active northeastern end in northwestern Wyoming within Yellowstone National Park.
Seamount Kaua’i Ni’ihau
Moti
on o
O’ahu
f Pa
cific
plate
Maui
Moloka’i Lana’i
Kaho’olawe
Hawai’i
Modified from NOAA.
Lo’ihi Volcano
PACIFIC OCEAN a
Volcanoes are progressively older
NW
Ni’ihau Kaua’i (5.6–4.9 Ma)
O’ahu (3.4 Ma)
Moloka’i (1.8 Ma)
Maui (1.3 Ma)
SE
Hawai’i (0.7–0 Ma)
Seamount
Lithosphere
PAC IFIC P LATE Motion of Pacific plate drags the plume head
USGS.
Asthenosphere
Mauna Loa Kilauea Lo’ihi
b
p
FIGURE 2-29. a. The relief map of the Hawaiian-Emperor chain of volcanoes clearly shows the movement of the crust over the hotspot that is currently below the big island of Hawaii, where there are active volcanoes. Two to three million years ago, the part of the Pacific Plate below Oahu was over the same hotspot. The approximate rate and direction of plate motion can be calculated using the common belief that the hotspot is nearly fixed in space through time. The distance between two locations of known ages divided by the time (age difference) indicates a rate of movement of about 9 cm per year. b. The lithospheric plate, moving across a stationary hotspot in the Earth’s mantle (to the left in this diagram), leaves a track of old volcanoes. The active volcanoes are over the hotspot.
pl
ric
e
th
Am
2.2–0.6 Ma 6.5–4.3 Ma
M
Boise
Sn
13.8 Ma
River Twin Falls 12–10.5 Ma
40
ly
ol
ve
in Pla
i ss
re
e
10.5–8.6 Ma
s er
ar
og pr
nt
c ni
ce
ca
l Vo
IDAHO UTAH
NEVADA
0
r
de
10–7 Ma
ake
FIGURE 2-30. This shaded relief map of the Snake River Plain shows the outlines of ancient resurgent calderas leading northeast to the present-day Yellowstone caldera. Caldera ages are shown in millions of years before present.
WYOMING
n
io
ot
of
r No
p
Yellowstone caldera
e
at
an
OREGON IDAHO
Modified from USGS, Pierce & Morgan, 1992, Beranek and others, 2006.
NA NTA HO IDA
MO
Yellowstone National Park
80 km Great Salt Lake
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29
Chapter Review
Key Points Development of a Theory p Continental drift was proposed by matching shapes of the continental margins on both sides of the Atlantic Ocean, as well as the rock types, deformation styles, fossil life forms, and glacial patterns. Figures 2-1 and 2-2.
p Continental drift evolved into the modern theory of plate tectonics based on new scientific data, including the existence of a large ridge running the length of many deep oceans, matching alternating magnetic stripes in rock on opposite sides of the oceanic spreading ridges, and age dates from oceanic rocks that confirmed a progressive sequence from very young rocks near the rifts and older oceanic rocks toward the continents. Figures 2-3 to 2-7.
p The scientific method involves developing tentative hypotheses that are tested by new observations and experiments, which can lead to confirmation or rejection.
Earth Structure p The concept of isostacy explains why the lowerdensity continental rocks stand higher than the denser ocean-floor rocks and sink deeper into the underlying mantle. This behavior is analogous to ice (lower density) floating higher in water (higher density). Figure 2-11.
p A dozen or so nearly rigid lithospheric plates make up the outer 60 to 200 kilometers of the Earth. They slowly slide past, collide with, or spread apart from each other. Figures 2-14 and 2-15.
Hazards and Plate Boundaries p Much of the tectonic action, in the form of earthquakes and volcanic eruptions, occurs near the boundaries between the lithospheric plates. Figures 2-18 and 2-19.
p Where plates diverge from each other, new lithosphere forms. If the plates are continental material, a continental rift zone forms. As this process continues, a new ocean basin can form, and the spreading continues from a mid-oceanic ridge, where basaltic magma pushes to the surface. Figures 2-20 to 2-21.
p Subduction zones, where ocean floors slide beneath continents or beneath other slabs of oceanic crust, are areas of major earthquakes and volcanic eruptions. These eruptions form mountains on the overriding plate. Figures 2-22 and 2-23.
p Continent–continent collision zones, where two continental plates collide, are regions with major earthquakes and the tallest mountain ranges on Earth. Figures 2-24 to 2-26.
p Transform faults are where two lithospheric plates slide laterally past one another. Where these faults cross continents, such as along the San Andreas Fault through California, they cause major earthquakes. Figure 2-27.
p Hotspots form chains of volcanoes within individual plates rather than near plate boundaries. They grow as a trailing track of progressively older extinct volcanoes because lithosphere is moving over hotspots fixed in the Earth’s underlying asthenosphere. Figures 2-28 to 2-30.
Key Terms asthenosphere, p. 17 collision zone, p. 25 continental drift, p. 13 convergent boundaries, p. 20 divergent boundaries, p. 20
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hotspot volcanoes, p. 27 hypothesis, p. 17 isostacy, p. 17 lithosphere, p. 17 low-velocity zone, p. 19
magnetic field, p. 14 mid-oceanic ridge, p. 14 Mohoroviçic Discontinuity (Moho), p. 18 Pangaea, p. 13
plates, p. 19 plate tectonics, p. 15 plumes, p. 28 resurgent calderas, p. 28 rift zones, p. 23
scientific method, p. 17
spreading centers, p. 23
theory, p. 17
trenches, p. 14
seafloor spreading, p. 14
subduction zone, p. 21
transform boundary, p. 21
triple junctions, p. 21
Questions for Review 1. Before people understood plate tectonics, what evidence led some scientists to believe in continental drift? 2. If the coastlines across the Atlantic Ocean are spreading apart, why isn’t the Atlantic Ocean deepest in its center? 3. What evidence confirmed seafloor spreading? 4. Distinguish among Earth’s crust, lithosphere, asthenosphere, and mantle.
7. Why does oceanic lithosphere almost always sink beneath continental lithosphere at such convergent zones? 8. Along which type(s) of lithospheric plate boundary are large earthquakes common? 9. Along which type(s) of lithospheric plate boundary are large volcanoes common? Provide an example. 10. What direction is the Pacific Plate currently moving, based on Figure 2-29a? How fast is this plate moving?
5. What does oceanic lithosphere consist of and how thick is it? 6. What are the main types of lithospheric plate boundaries in terms of relative motions? Provide a real example of each (by name or location).
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Earthquakes and Their Causes Chapter
Jennifer Parker photo.
3 i Heavy concrete floors and roofs collapsed on people in the 2005 Kashmir earthquake.
Earthquake Devastates South Asia
S
uddenly at 8:51 a.m., on October 8, 2005, in Pakistani Kashmir, the ground shifted, then continued shaking violently, knocking some people off their feet. Many homes, stores, and schools collapsed, crushing the occupants. Huge boulders and landslides crashed down from the steep mountainsides onto more houses. Landslides and rockfalls closed many highways and mountain roads for days and in some places for months, cutting off access to injured and buried people. Other roads were open for only one or two hours per day because of heavy rains and the danger of continuing slides. Tens of thousands migrated to shelters at lower elevations, but many refused to leave their homes, in part because they feared that others would occupy them and take their few belongings. In late November, temperatures dropped below freezing, and snow fell; hundreds of thousands of people who remained in villages in remote mountain valleys were without tents, warm clothes, blankets, and sufficient heat; about 2,000 died from the cold. People burned pieces of furniture to keep warm. Food saved for the winter months was buried in their collapsed houses. Corrugated iron sheets were needed to keep snow from collapsing light tents.
32
Earthquakes
Helicopters brought in food, medicine, and other relief supplies but were hindered by weather and steep terrain with no flat areas to land. Many boxes of supplies dropped on slopes merely slid downslope to be lost in rivers, sometimes a thousand meters below. Trucks carried in supplies where roads were reopened, and mule trains were used where only makeshift trails across the landslides were available. Hospitals treated many people for the flu, pneumonia, hypothermia, measles, and tetanus after lack of treatment for open wounds and broken bones. Some $580 million for earthquake relief was pledged by dozens of countries, including the United States and Canada, but only $15.8 million was immediately available, much of it in the form of goods and services provided directly by a country, rather than money that could cover numerous needs. Unfortunately, much of the pledged aid was typically never delivered, and millions of dollars of relief funds sometimes just disappeared—diverted to unrelated uses or into officials’ pockets. In all, about 87,000 people died and tens of thousands more were injured. About 3.5 million people lost their homes. It was by far the most deadly earthquake on record in India, Pakistan, and surrounding areas (see Case in Point: “Collapse of Poorly Constructed Buildings—Kashmir Earthquake, Pakistan, 2005,” p. 91).
Faults and Earthquakes To understand why earthquakes happen, remember that the plates of Earth’s crust move, new crust forms, and old crust sinks into subduction zones. It is these movements that give rise to earthquakes, which form along faults, or ruptures, in the Earth’s crust. Faults are simply fractures in the crust along which rocks on one side of the break move past those on the other. Faults are measured according to the amount of displacement along the fractures. For example, over several million years the rocks west of the San Andreas Fault of California have moved at least 450 kilometers north of where they started. Thousands of other faults have moved much less than 1 kilometer in the same amount of time. Some faults produce earthquakes when they move; others produce almost none. Some faults have not moved for such a long time that we consider them inactive; others are clearly still active and potentially capable of causing earthquakes. Active faults are rare in regions such as the American Midwest and central Canada, where the continental crust has been stable for hundreds of millions of years. Such stable regions contain many faults that geologists have yet to recognize. Some of those first announce their presence when they cause an earthquake; others are marked by the line of a fresh break near the base of a mountainside (p Figure 3-1). Earthquakes are common in the mountainous western parts of North America, where the rocks are deformed into complex patterns of faults and folds. Faults can be classified according to the way the rocks on either side of the fault move in relation to each other
(p Figure 3-2). Normal faults move on a steeply inclined surface. Rocks above the fault surface slip down and over the rocks beneath the fault. Normal faults move when Earth’s crust pulls apart, during crustal extension. Reverse faults move rocks on the upper side of the fault up and over those below. Thrust faults are similar to reverse faults, but the fault surface is more gently inclined. Reverse and thrust faults move when Earth’s crust is pushed together, during crustal compression. Strike-slip faults move horizontally as rocks on one side of the fault slip laterally past those on the other side. If rocks on the far side of the fault move to the right, as in Figure 3-2a, it is a right-lateral fault. If they moved in the opposite direction, it would be a leftlateral fault.
Causes of Earthquakes At the time of the great San Francisco earthquake of 1906 (see Case in Point: “Devastating Fire Caused by an Earthquake—San Francisco, California, 1906,” p. 88), the cause of earthquakes was a complete mystery. The governor of California at the time appointed a commission to find the cause of earthquakes. The director of this commission, Andrew C. Lawson, was a distinguished geologist and one of the most colorful personalities in the history of California. Lawson and his students at the University of California (UC)– Berkeley had already recognized the San Andreas Fault and mapped large parts of it, but until the 1906 event they had no idea that it could cause earthquakes. During their investigation, members of the commission found numerous EARTHQUAKES AND THEIR CAUSES
33
National Park Service photo.
USGS.
a
b
p
FIGURE 3-1. a. This fault scarp near West Yellowstone, Montana, formed during the 1959 Hebgen Lake, Montana, earthquake. Such lines indicate an active fault. b. Fault scarp (steep slope in shadow) at the eastern base of the Grand Teton Range, Wyoming. Green arrows point to both fault scarps.
Modified from Pipkin & Trent, 2001.
places where roads, fences, and other structures had broken during the 1906 earthquake just where they crossed the San Andreas Fault. In every case, the side west of the fault had moved north as much as 7 meters. That led to the theory of how fault movement causes earthquakes. The earthquake commission hypothesized that as the Earth’s crust moved, the opposite sides of the fault had stuck for many years, with the rocks on either side of the fault bending, or deforming, instead of slipping. As the rocks on opposite sides of the fault bent, they accumulated energy. When the stuck segment of the fault finally slipped, the bent
Reverse/thrust fault
rocks straightened with a sudden snap, releasing energy in the form of an earthquake (p Figure 3-3). Imagine pulling a bow taut, bending it out of its normal shape, and then releasing it. It would snap back to its original shape with a sudden release of energy capable of sending an arrow flying. This explanation for earthquakes, called the elastic rebound theory, has since been confirmed by rigorous testing. We now know enough about the behavior of rocks in response to stress to explain why faults either stick or slip. We think of rocks as brittle solids, but rocks are elastic, like a spring, and can bend when a force is applied. We use
Strike-slip fault
Normal fault
a
p Donald Hyndman photo.
FIGURE 3-2. a. Block diagrams indicating movement on the basic types of faults. b. A normal fault near Challis, Idaho, that moved in the 1983 earthquake.
b
34
CHAPTER 3
Fault Modified from Pipkin & Trent, 2001.
Fence
(a) Original position
(b) Deformation
(c) Rupture and release of energy
(d) Rocks rebound to original undeformed shape
p
FIGURE 3-3. Elastic rebound theory: Rocks near a fault are slowly bent elastically until the fault breaks during an earthquake; the rocks on each side then slip past each other, relieving the stress. Distortion of the Earth’s crust can extend over tens of kilometers outward from the main fault.
the term stress to refer to the forces imposed on a rock, and strain to refer to the change in shape of the rock in response to the imposed stress. The larger the stress applied, the greater the strain. Rocks bend, or deform, in broadly consistent ways in response to stress. Typical rocks will deform elastically under low stress, which means that they revert to their former shape when the force is relieved. At higher stress, these rocks will deform plastically, which means they permanently change shape or flow when forces are applied. Deformation experiments show that most rocks near Earth’s surface, where they are cold and not under much pressure from overlying rocks, deform elastically when affected by small forces. Under other conditions, such as deep in the Earth where they are hot and under high pressure imposed by the overlying load of rocks, it is much more likely that these rocks will deform plastically. Rocks can bend, but they also break if stretched too far. In response to smaller stresses, rocks may merely bend, while
in response to large stresses, they fracture or break. As stress levels increase, rocks ultimately succumb to brittle failure, causing fault slippage or an earthquake (p Figure 3-4). In the laboratory, scientists compress a rock from its ends to simulate differential stress, or stress where forces are greater in one direction. Such a differential stress generally breaks the rock in a diagonal orientation (p Figure 3-5). Along a fault, differential plate motions apply stresses continuously. Because those plate motions do not stop, elastic deformation progresses to plastic deformation within meters to kilometers of the fault, and the fault finally ruptures in an earthquake. Under these conditions, a fault may slip more than once, with smaller slips, called foreshocks, preceding the main earthquake and additional slips after the event, called aftershocks. The size of an earthquake is related to the amount of movement on a fault. The displacement or offset is the distance of movement across the fault, and the surface rupture length is the total length of the break (p Figure 3-6).
σ1
Stress
Elastic limit
σ1
Brittle failure: fault slips (earthquake) Plastic deformation (not reversible)
Compression of a brittle rock causes it to break along a diagonal fault
Elastic deformation (reversible)
σ1
Strain
p
FIGURE 3-4. With increasing stress, a rock deforms elastically, then plastically, before ultimately failing or breaking in an earthquake. A completely brittle rock fails at the elastic limit.
σ1
p
FIGURE 3-5. Deformation of a cylinder of rock by compression from the top and bottom breaks the rock on diagonal shear planes. Shear is generally on one plane only, as shown by the red line. 1 is the maximum principal stress.
EARTHQUAKES AND THEIR CAUSES
35
Offset or displacement
Offset line
Not displaced
Fault Surface rupture length
G. K. Gilbert photo.
a
San An
dreas F ault
b
p
FIGURE 3-6. a. This diagram shows displacement and surface rupture length on a fault. Beyond the ends of the rupture, the fault does not break or offset. b. This fence near Point Reyes, north of San Francisco, was offset approximately 2.6 meters in the 1906 San Francisco earthquake on the San Andreas Fault.
36
CHAPTER 3
Dec. 2004 a
p
Donald Hyndman photo.
2004 2006 Sue Hirschfield photo.
The largest earthquake expected for a particular fault generally depends on the total fault length or the longest segment of the fault that typically ruptures. A 1,000-kilometer-long (620-mile) subduction zone such as that off the coast of Oregon and Washington or a 1,200-kilometer-long transform fault such as the San Andreas Fault of California, if broken along its full length in one motion, would generate a giant earthquake. Although the full length of such faults does occasionally break in a single earthquake, shorter segments commonly break at different times. Normal faults such as the Wasatch Front of Utah generally break in shorter segments, so somewhat smaller earthquakes are to be expected in such areas (compare Figure 4-13). This relationship puts a theoretical limit on the size of an earthquake at a given fault. A short fault only a few kilometers long can have many small earthquakes but cannot have an especially large earthquake because the whole fault is not long enough; it does not break a large area of rock. The 1,000-kilometer-long San Andreas Fault, however, could conceivably break its whole length in one shot. Certainly, the San Andreas has had some large earthquakes, but those that have been observed have broken only as much as half
Dec. 2006 b
FIGURE 3-7. This curb in Hayward, California, has been offset by creep along the Hayward Fault. The right photo shows further offset in two years after December 2004.
the length of the fault at one time. The potential is still there for a much larger earthquake to break the full length of the fault. For those living in the region of the San Andreas, that would be truly catastrophic. Sometimes a fault moves continuously, rather than suddenly snapping. A segment of the central part of the San Andreas Fault south of Hollister slips fairly continuously without causing significant earthquakes. In this zone, strain in the fault is released by creep and thus does not accumulate to cause large earthquakes. Why that segment of the fault slips without causing major earthquakes, whereas other segments stick until the rocks break during an earthquake, is not entirely clear. The Hayward Fault also creeps but can produce large earthquakes (p Figure 3-7). Presumably, the rocks at depth are especially weak, such as might be the case with shale or serpentine; or perhaps water penetrates the fault zone to great depth, making it weak. The continuously creeping section of the fault almost halfway between San Francisco and Los Angeles seems to be a zone of weak rocks that would be unable to build up a significant stress. Perhaps that means that no more than half of the length of the fault is likely to break in one sudden movement. Slip of only half the length of the fault could still generate a catastrophic earthquake.
Tectonic Environments of Faults Because earthquakes are triggered by the motion of the Earth’s crust, it follows that earthquakes are associated with plate boundaries.The sense of motion during a future earthquake is dictated by the relative motion across the plate boundary—strike-slip faults move along transform bound-
p Table 3-1
Tectonic Environments for the Largest World Earthquakes* Since 1700
May 22, 1960 Mar. 28, 1964 Dec. 26, 2004 Nov. 4, 1952 Jan. 26, 1700
9.5 9.2 9.15 9.0 9.0
Locked segment Creeping segment
lt Fau
Chile Anchorage, Alaska Northern Sumatra Kamchatka Cascadia
MAGNITUDE
reas And
DATE
San
EARTHQUAKE
ey F. s Vall Owen
*All are subduction-zone earthquakes.
d ar yw Ha
San Francisco
F.
Sa
Robert E. Wallace, USGS, 1990.
n
ck F. Garlo as Fa ult Sa nJ ac in
a
San Andreas Fault
Robert E. Wallace, USGS, 1990.
The most important example of a transform fault in the United States is the San Andreas Fault, a zone that slices through a 1,200-kilometer length of western California, from just south of the Mexican border to Cape Mendocino in northern California (p Figure 3-8). The trace of the San Andreas Fault appears from the air and on topographic maps as lines of narrow valleys, some of which hold long lakes and marshes (see Figure 2-27b). The San Andreas Fault is a continental transform fault in which the main sliding boundary marks the relative motion between the Pacific Plate, which moves northwest, and the North American Plate, which moves slightly south of west. As shown in p Figure 3-9, the total motion of the westernmost slice of California moves more than the slices closer to the continental interior. The greatest difference between those arrow lengths, which represent movement rates across a fault, suggests the greatest likelihood of new fault slippage or an earthquake. The west side of the northwest-trending fault moves northwestward at an average rate of 3.5 centimeters per year or 3.5 meters every 100 years, relative to the east side. The rupture length for a magnitude 7 earthquake for a 3.5meter offset would be 50 kilometers; release of all the strain accumulated in 100 years would require a series of such earthquakes along the length of the fault. However, earthquakes frequently occur in clusters separated by periods of relative seismic inactivity. The San Andreas Fault stretches from the San Francisco Bay area southward toward Los Angeles. The northward
Los Angeles
dre
F.
Transform Faults
An
to
aries, thrust faults are typically associated with subduction zones and continent–continent collision boundaries, and normal faults move in spreading zones. p Table 3-1 gives the tectonic environment for the largest earthquakes since 1700. This section discusses examples of faults in these major tectonic environments, as well as fault systems isolated from plate boundaries. Chapter 4 will explore the human impact of earthquake activity in some of these earthquake zones.
b
p
FIGURE 3-8. a. The San Andreas Fault and other major faults nearby show as a series of straight valleys slicing through the Coast Ranges in this shadedrelief map of California. b. In this view from the air, streams jog abruptly (yellow arrows) where they cross the San Andreas Fault in the Carrizo Plain north of Los Angeles. The 1857 Fort Tejon earthquake caused 9.5 meters of this movement.
drag of the Pacific Plate against the continent is slowly crushing the Los Angeles basin—and the sedimentary rocks that fill it—northward at roughly 7 millimeters per year. The sedimentary formations buckle into folds and break along thrust faults, both of which shorten the basin as they move (p Figures 3-10 and 3-11). The thrust faults are called blind thrusts because they do not break the surface. A tight fold
EARTHQUAKES AND THEIR CAUSES
37
at the surface marks the shallow end of the fault. Blind thrusts are quite dangerous because many of them remain unknown until they cause an earthquake (Case in Point: “A Major Earthquake on a Blind Thrust Fault—Northridge Earthquake, California, 1994,” p. 54). A total of seventeen earthquakes greater than magnitude 4.8 shook the Los Angeles region between 1920 and 1994. Especially dangerous faults include the Sierra Madre– Cucamonga Fault system that follows 100 kilometers of the northern edge of the San Fernando and San Gabriel Valleys. The blind thrust along the westernmost 19 kilometers of the fault moved to cause the 1971 San Fernando Valley earthquake. The fault systems beneath downtown Los Angeles include thrust faults that dip down to the northeast. The Santa Monica Mountains fault zone near downtown Los Angeles extends west along the Malibu coast for 90 kilometers. It includes blind thrust faults that do not break the surface and strike-slip faults that do. The Oak Ridge Fault system north of the Malibu coast generated the 1994 Northridge earthquake. The Palos Verdes thrust fault, along the coast south of downtown Los Angeles, slips approximately 3 millimeters per year; this progressive slip releases accumulating strain that might someday cause an earthquake on an adjacent fault segment. The San Andreas Fault appears to have accumulated a total displacement of 235 kilometers in the approximately 16 million years since it began to move. The fault has been stuck along its big bend, south of Parkfield, since the Fort Tejon earthquake of 1857. More recent earthquakes near the southern San Andreas Fault are associated with blind thrusts, which means that some of the crustal movement is being taken up in folding and thrust faults near the main fault instead of in slippage along the main fault itself (Figure 3-11).
NORTH AMERICAN PLATE
PACIFIC PLATE Faults Major faults 0
2 in./year Motion
0
5 cm/year
USGS.
0 0
30 miles 30 kilometers
p
FIGURE 3-9. The San Andreas Fault system is a wide zone that includes nearly the entire San Francisco Bay area. The black arrows are proportional to the rates of ground movement relative to the stable continental interior based on Global Positioning System measurements. Energy builds up when there is a differential movement, as can be seen across each of the major faults shown in orange.
p
Compression
Compression folds and thrust faults
Sa
Santa
Monic
San Fernando Vly
An
dr
b ea s
San Gabriel . Mts.
a Mts
Los Angeles
a
38
n
CHAPTER 3
Diagonal shear direction
Mojave Desert
Fa ul t
FIGURE 3-10. a. Major faults in the Los Angeles area. b. A diagram showing the stresses that cause movement on blind thrust faults in the area. Note that the San Andreas Fault is parallel to the shear direction and that the blind thrusts are oriented perpendicular to the compression direction.
Santa Susana Mountains Santa Monica Mountains
San Gabriel Mountains
Santa Susana Mountains– Veteran Fault system (ruptured in 1971)
Northridge
?
Hollywood–Santa Monica Fault
Compressive stress
4
Santa M onic thrust fa a ult
8
?
Sedimentary rocks
Focus
Pipkin and Trent, 2001, modified from USGS.
12 16 Depth (km)
20
and Ancient igneous ks metamorphic roc
p
FIGURE 3-11. This cutaway block diagram shows the blind thrust fault movement that caused the 1994 Northridge earthquake. Yellow arrows indicate the relative direction of movement.
Subduction Zones Subduction zones are another tectonic environment in which earthquakes occur, including the largest earthquake on record, which struck the coast of Chile in 1960 (Case in Point: “A Major Subduction-Zone Earthquake—Chile, 1960,” p. 55). An 1868, magnitude 9 event in Peru killed several thousand. In 2001, a magnitude 8.4 earthquake on the same subduction zone may have increased stress on nearby parts of the boundary. It was followed on August 15, 2007, by a magnitude 8.0 event that struck the coast of Peru and killed more than 510 people, many from collapse of their adobe-brick homes. The most important example of subduction-zone faults in the United States is in the Pacific Northwest. We know from several lines of evidence that an active subduction zone lies off the coast for the 1,200 kilometers between Cape Mendocino in northern California and southern British Columbia (p Figure 3-12). The magnetic stripes parallel to the Juan de Fuca Ridge show that the plate on the east is moving to the southeast; the Yellowstone hotspot track shows that the North American Plate is moving southwest. The collision zone between ocean floor and continent is the Cascadia subduction zone. In addition, the line of active Cascade volcanoes about 100 kilometers inland indicates an active subduction zone at depth. We also know that subduction zones often generate giant earthquakes and that such sudden shifts of the ocean floor can generate huge ocean waves, tsunamis. A comparable zone in Sumatra in 2004 generated a giant earthquake and tsunami that killed about 230,000 people
(Chapter 5). On March 6, 2007, a magnitude 6.3 earthquake struck southeast of the 2004 event, killing 70 people. Then on September 12, 2007, a magnitude 8.4 earthquake farther southeast on the same zone, near Panang, Sumatra, caused considerable damage and a 3-meter tsunami. These prompted concern that the post-2004 events could be precursors to a still larger earthquake. Much farther southeast along the same plate boundary in May 2006, a magnitude 6.3 earthquake in Java killed more than 6,000 people and left about 650,000 homeless. Such slabs of oceanic lithosphere sinking through an oceanic trench at subduction zone boundaries typically generate earthquakes from as deep as several hundred kilometers, so the apparent absence of those deep earthquakes in the Pacific Northwest has worried geologists for years. Several lines of evidence now show that major earthquakes do indeed happen but at such long intervals that none have struck within the 200 or so years of recorded Northwest history. Radiocarbon dating of the peat and buried trees covered by beach-derived sand in the Pacific Northwest bays helped establish a record of major earthquakes in that region that could be a precedent for future events (Case in Point: “Paleoseismology Provides a Record of a Giant Event—Pacific Northwest Earthquake, 1700,” p. 56). The oceanic plate sinking through the trench off the Northwest coast is now stuck against the overriding continental plate. The continental plate is bulging up, as shown by precise surveys (p Figure 3-13). The locked zone is 50 to 100 kilometers off the coasts of Oregon, Washington, and southern British Colum-
EARTHQUAKES AND THEIR CAUSES
39
130°
126°
122°W
NORTH AMERICA PLATE
Queen Charlotte Fault
52°N
Exp lo Rid rer ge
British Columbia
EXPLORER PLATE
Vancouver Victoria
Nootka Fault
48° Seattle
Jua Fuc n de aR idg e
Washington
Cascadia subduction zone
Cascade volcanoes
Portland JUAN DE FUCA PLATE
Blanco Fault zone
44° Oregon
Mendocino Fault
a
200 kilometers
40°
s
0
California
rea nd n A ault F
p
GORDA PLATE
Sa
NOAA/NGDC.
Gorda Ridge
PACIFIC PLATE
b
FIGURE 3-12. The Cascadia oceanic trench to the north and the San Andreas transform fault to the south dominate the Pacific continental margin of the United States. a. Seafloor topography. b. Map of plate boundaries. The December 2004 magnitude 9.15 Sumatra earthquake rupture zone is comparable to the size of the potential Cascadia subduction zone slip (shaded area). Uplift
48°
0
0.3 Coast
1 2
Between events
British Columbia
48°
10 20
3 Shortening CONTINENT
46°
2 Washington Washington
h Trenc
Locked
~100 kilometers
a
42°
Vertical (mm/yr) 0
Oregon
42°
Trench
P. Fluck, R. D. Hyndman, and K. Wang.
Extension
Zone of maximum uplift of bulging edge of continent
Washington Washington
Zone of maximum eastward movement of western edge of continent
Oregon Oregon
EARTHQUAKE
15
Trench
44°
Subsidence
Rupture
30
4
44°
Roy Hyndman, Pacific Geoscience Centre.
25
46°
3
OCEAN PLATE
British 2 Columbia
5
Horizontal (mm/yr)
California
100 kilometers
0
40°
California
100 kilometers
40° 126°W
124°
126°W
124°
b
p
FIGURE 3-13. a. Denser oceanic plate sinks in a subduction zone. As strain accumulates, a bulge rises above the sinking plate while an area landward sinks. Those displacements reverse when the fault slips to cause an earthquake. b. The subduction zone is locked between the oceanic trench at the landward edge of the Juan de Fuca Plate and halfway to the coast.
40
CHAPTER 3
bia. Just inland, the margin is now rising at a rate between 1 and 4 millimeters per year and shortening horizontally by as much as 3 centimeters per year. In contrast, most of the earthquakes in the Puget Sound area of northwestern Washington State do not involve slip on the collision boundary at the oceanic trench offshore. Instead they accompany movement at shallow depth on faults that trend west or northwest and straddle Puget Sound. A similar set of active faults exists east of the Cascades in Washington and Oregon (p Figure 3-14a). Every three or four years, the Puget Sound area feels the jolt of a moderate to large earthquake with Richter magnitude 5 to 7. The Seattle Fault is the best known, and perhaps the most dangerous, fault in the region. It trends east through the southern end of downtown Seattle, almost through the interchange between Interstate 5 and Interstate 90 (see Figure 3-14b). Seventy kilometers of the fault are mapped; the part that reaches the surface dips steeply down to the south. Studies show that the rocks south of the fault rose 15.6 meters in a large earthquake about 900–930 A.D. That movement generated large tsunami waves in the water in Puget Sound and caused landslides into Lake Washington at the eastern edge of downtown Seattle (also discussed in Chapter 5). In 2001, movement on this fault during the Nisqually earthquake caused more than $2 billion* in property damage (Case in Point: “Damage Mitigated by Depth of Focus—Nisqually Earthquake, Washington, 2001,” p. 56).
Continental Spreading Zones In western North America, the best-known area of continental extension and associated normal faults is the Basin and Range of Nevada, Utah, and adjacent areas (p Figure 3-15; see also Chapter 2, Figure 2-20). This broad region is laced with numerous north-trending faults that separate raised mountain ranges from dropped valleys. The Wasatch Front, the eastern face of the Wasatch Range of central Utah, is a high fault scarp that faces west across the Salt Lake basin and defines the eastern margin of the Basin and Range (p Figure 3-16). It is the eastern counterpart to the Sierra Nevada front of California. It overlooks the deserts of Utah in the same way that the Sierra Nevada front overlooks those of Nevada. Many small earthquakes shake the Wasatch Front, but none of any consequence have been felt since Brigham Young’s party founded Salt Lake City in 1847. One way to interpret the modest size of many deposits of stream sand and gravel at the mouths of canyons at the base of the Wasatch Front is to suggest that the fault movement has dropped the valley relative to the Wasatch Range during the geologically recent past, probably within tens of thousands of years. That would roughly correspond to the time in which the Sierra Nevada last rose. In fact, both faults remain active as their ranges rise.The active fault zone extends from central Utah, north to southeastern Idaho. The central section near Weber, Salt Lake City, Provo, and Nephi is the most active but even the end segments are capable of causing magnitude 6.9 earthquakes.
Donald Hyndman photo.
Logan & Walsh (1995), Bourgeois & Johnson (2001), Haugerud & others (2003), Kelsey & others (2004).
*For the sake of comparison, 2002 dollars are used in discussion of earthquake damages.
a
p
b
FIGURE 3-14. a. This map of northwestern Washington shows the Seattle Fault and related major recent active fault zones in the Seattle area. b. The Seattle Fault runs east-west through the interchange of I-90 (foreground) and I-5 (middle right) at the southern edge of Seattle.
EARTHQUAKES AND THEIR CAUSES
41
c– pi a m ly ow O all W
FARALLON PLATE
he ot Br rs
San
P A C IF IC P
Alt & Hyndman, 1995.
Wasatch Front
Basin and Range
ek re eC ac e rn an Fu rL t lke ul Wa DA Fa EVA RA N reas SIER And
N
L T E
USGS.
A
a
p
b
Bill Case photo, Utah Geological Survey.
FIGURE 3-15. a. The north-south faults of the Basin and Range of Nevada, western Utah, and adjacent areas occupy a spreading zone accompanying the northwestward drag of the Pacific Ocean floor. That drag also causes shear to form the San Andreas Fault. The western margin of the Basin and Range is marked by the precipitous eastern edge of the Sierra Nevada; the eastern margin is the equally precipitous Wasatch Front at Salt Lake City. b. Most of the earthquake activity of the Basin and Range is concentrated along the east face of the Sierra Nevada and the Wasatch Front.
p
FIGURE 3-16. A prominent fault scarp crosses the East Bench, out from the base of the Wasatch Front, in a completely built-up part of the Salt Lake City metropolitan area. Another marks the Wasatch Fault at the base of the range in the background. Arrows mark the faults.
Intraplate Earthquakes Earthquakes occasionally strike without warning in places that lack any recent record of earthquakes and are remote from any plate boundary. These intraplate, or withincontinent, earthquakes can be devastating, especially because most local people are unaware of their threat. Some of these isolated earthquakes are enormous, easily capable
42
CHAPTER 3
of causing a major natural catastrophe. Although many geologists have offered tentative explanations for these earthquakes, their causes remain generally obscure. The intraplate earthquakes that struck southeastern Missouri in 1811 and 1812 were among the most severe to strike North America during its period of recorded history. The three great earthquakes that struck near New Madrid, Missouri, in December 1811, January 1812, and February 1812 were felt throughout the eastern United States, toppling chimneys in Ohio, Alabama, and Louisiana and causing church bells to ring in Boston. Although there has not been another large earthquake in the region since then, the area is seismically active enough that people as far away as St. Louis and Memphis, the nearest big cities, occasionally hear the ground rumble as their dishes and windows rattle (p Figure 3-17). A repetition of an earthquake in this magnitude range would cause enormous loss of life and major property damage in Memphis, St. Louis, Louisville, Little Rock, and many smaller cities that have older masonry buildings. Few of the buildings in such cities are designed or built to resist significant earthquakes. Another isolated earthquake, which struck Charleston, South Carolina, in 1886, caused many casualties and heavy property damage. The Charleston event, near the eastern coast of the United States, was along what has been called a “trailing continental margin.” This is not a current plate margin but the margin between the North American continent and the Atlantic Ocean basin; it was originally a plate
Area of Detail
MO
KY TN
AR
90°
89°
Missouri
New Madrid
J. K. Hillers photo, USGS.
91° 37°
Kentucky
p
FIGURE 3-18. The structural brick walls of this house at 157 Tradd Street in Charleston collapsed during the 1886 earthquake.
Reelfoot Scarp Caruthersville
36°
Arkansas
Richmond
Marked Tree
ECF
S
Ri
37°N
pi
ip
s sis
0
North Carolina t n o Raleigh m d e i P
25 kilometers
is
M
Memphis
35°
p
FIGURE 3-17. Recent microearthquake epicenters in the New Madrid region appear to outline three fault zones responsible for the earthquakes of 1811 and 1812. Two northeast-trending lateral-slip faults were offset by a short fault that pushed the southwestern side up over the northeastern side.
North Carolina
l
33°N
South Georgia Basin
a
i
n
P
FA
ECF S
South Carolina Florence Basin
C
margin when the Atlantic Ocean floor began to spread more than 150 million years ago. An earthquake struck 20 kilometers northwest of Charleston on August 31, 1886, sending hundreds of people into the streets and toppling several chimneys in Lancaster, Ohio, 800 kilometers away. It shook plaster from the walls on the fourth floor of a building in Chicago, 1,200 kilometers away; 14,000 chimneys fell, and many buildings were destroyed on both solid and soft ground. One hundred people were killed, most of them in areas where the soil liquefied (p Figure 3-18). The Charleston earthquake and many others may be the result of movements along segments of the East Coast fault system, a swarm of aligned segments that trend generally northeast near the modern East Coast (p Figure 3-19). They
35°N
Albemarle Embayment
FS
Blytheville Arch
NFA
Virgina
r
ve Ri
EC
Re
Tennessee
ft
ot
o elf
Modified from Marple & Talwani, 2000.
D. Russ, USGS, modified by Pipkin and Trent, 2001.
Blytheville
o
a
s
t
a
l
N
C
Charleston 79°W
0
50
100 kilometers 77°W
p
FIGURE 3-19. This map of the East Coast fault system between South Carolina and Virginia shows how the fault zone lies close to the buried boundary between the continental crust of the Piedmont and the Atlantic oceanic crust. The Piedmont is rising.
EARTHQUAKES AND THEIR CAUSES
43
Explanation
Shaking felt Area of damage WASH
VT NH MONT
N. DAK
OREG
MINN
S. DAK
IDAHO
WIS
WYO NEBR
NEV CA
UTAH
ARIZ
COLO
N. MEX
1994: Magnitude 6.7
OKLA
MICH PA
IOWA IL
KANS
NY
MO
IND
OHIO
KY
Epicenter
NC
TENN
SC
ARK MISS ALA
TEXAS
W. VA VA
MAINE MA RI CT NJ DEL MD
Fault
GA
Focus
LA
USGS.
FLA
0
500 miles
0
800 kilometers
1895: Magnitude 6.8
p
FIGURE 3-21. The epicenter of an earthquake is the point on the Earth’s surface directly above the focus where the earthquake originated.
p
FIGURE 3-20. A comparison of similar-magnitude earthquakes shows that the damage would be much greater for an earthquake in the Midwest than in the mountainous West.
were first recognized between South Carolina and Virginia but may extend much farther. The fault zone is near the buried boundary between continental crust of the Piedmont and Atlantic oceanic crust, with the coastal plain dropping. Renewed movement on faults may be associated with the early stages of opening of the Atlantic Ocean more than 150 million years ago. Earthquake hazards may be significant in at least some parts of the eastern United States, where past earthquakes have left little memory or lasting concern. Although large earthquakes are more frequent in the West, the few large earthquakes that have occurred in eastern North America have been much more damaging than those in the West because the Earth’s crust in eastern North America transmits earthquake waves more efficiently, with less loss of energy, than the continental crust of the west, which is hotter and more broken along faults. That explains why the area of significant damage for an earthquake of a given size is greater in the East than in the West (p Figure 3-20). Good land use planning and building codes for new structures cost little anywhere and may someday save enormous loss of life and property damage in a city that does not now suspect it is living dangerously.
Earthquake Waves When a fault slips, the released energy travels outward in seismic waves from the place where the fault first slipped, called the focus, or hypocenter, of the earthquake. The epicenter is the point on the map directly above the focus (p Figure 3-21). The behavior of earthquake waves explains both how we experience earthquakes and the types of damage they cause.
44
CHAPTER 3
Types of Earthquake Waves Observant people have noticed for centuries that many earthquakes arrive as a distinct series of shakings that feel different.The first event is the arrival of the P waves, the primary or compressional waves, which come as a sudden jolt. People indoors might wonder for a moment whether a truck just hit the house. P waves consist of a train of compressions and expansions (p Figure 3-22a). P waves travel roughly 5 to 6 kilometers per second in the less dense continental crust and 8 kilometers per second in the dense, less compressible rocks of the upper mantle. People sometimes hear the low rumbling of the P waves of an earthquake. Sound waves are also compressional and closely comparable to P waves, but they travel through the air at only 0.34 kilometer per second. After the P waves comes a brief interval of quiet while the cat heads under the bed and plaster dust sifts down from the cracks in the ceiling. Then come the S waves (secondary or shear waves), moving with a wiggling motion like that of a rhythmically wriggle shaking rope (p Figure 322b) that makes it hard to stand. Chimneys may snap off and fall through the floors to the basement. Streets and sidewalks twist and turn. Buildings jarred by the earlier P waves distort and may collapse. S waves are slower than P waves, traveling at speeds of 3.5 kilometers per second in the crust and 4.5 kilometers per second in the upper mantle. These shear waves do not travel through liquids. Their wiggling motions make them more destructive than P waves. The P and S waves are called body waves because they travel through the body of the Earth. After the body waves, the surface waves arrive—a long series of rolling motions (p Figure 3-22c). Surface waves travel along Earth’s surface and fade downward. Surface waves include Love and Rayleigh waves, which move in perpendicular planes. Love waves move from side to side, and Rayleigh waves move up and down in a motion that somewhat resembles swells on the ocean.
P-wave propagation
Wavelength (1 cycle) P waves
Dilation
Compressio
n
Dilation
Amplitude Waves travel past a point
Compressio
a
n Unstresse d condition
p
FIGURE 3-23. The definitions of wavelength and amplitude for earthquake waves.
S waves
Pipkin and Trent, 3rd ed., 2001.
b
Vertical plane
Surface waves Surface-wav e propagation
c
p
FIGURE 3-22. a. Compressional P-wave propagation (see front face of diagram): Waves of compression alternating with extension move through the rock. b. Shear (S) wave propagation: S waves travel in a wiggling motion perpendicular to the direction of wave travel (only the horizontal direction is shown here). c. In the rolling motion of surface waves, individual particles at the Earth’s surface move in a circular motion, opposite the direction of travel; see front face of diagram, both in a vertical plane and a horizontal plane.
Surface waves generally involve the greatest ground motion, so they cause a large proportion of all earthquake damage. Surface waves find buildings of all kinds loosened and weakened by the previous body waves, ready for the final blow. Inertia tends to keep people and loose furniture in place as ground motion yanks the building back and forth beneath them. Shattering windows spray glass shrapnel as plaster falls from the ceiling. If the building is weak or the ground loose, it may collapse. Although there are more complex internal refractions of waves as they pass between different Earth layers, those complications do not much affect the damage that earthquakes inflict because the direct waves are significantly stronger. The differences people feel during this series of earthquake waves can be explained by the different characteristics of those waves. To describe the vibrations of earthquake waves, we use a variety of terms (p Figure 3-23). The time for one complete cycle between successive wave peaks to pass is the period; the distance between wave crests is the wavelength; and the amount of positive or negative wave motion is the amplitude. The number of peaks per second is the frequency in cycles per second or Hertz (Hz).
When you bend a stick until it breaks, you hear the snap and feel the vibration in your hands. When the Earth breaks along a fault, it vibrates back and forth with the frequency of a low rumble, although the frequencies of earthquake waves are generally too low to be heard with the human ear. P and S waves generally cause vibrations in the frequency range between 1 and 30 cycles per second (1–30 Hz). Surface waves generally cause vibrations at much lower frequencies, which dissipate less rapidly than those associated with body waves. That is why they commonly damage tall buildings at distances as great as 100 kilometers from the epicenter.
Seismographs A seismograph records the shaking of earthquake waves on a record called a seismogram. When recording seismographs finally came into use during the early part of the twentieth century, it became possible to see those different shaking motions as a series of distinctive oscillations that arrive in a predictable sequence (p Figure 3-24). Imagine the seismograph as an extremely sensitive mechanical ear clamped firmly to the ground, constantly listening for noises from the depths. It is essentially the geologist’s stethoscope. We normally stand firmly planted on the solid Earth to watch things move, but how do we stand still and watch the Earth move? Seismographs consist of a heavy weight suspended from a rigid column that is firmly an-
100 50 0 –50
Wald and others, USGS.
agation
Amplitude (mm)
S-wave prop
Surface
P
S
–100 –150 8
10
12 14 16 Time (seconds × 100)
18
20
22
p
FIGURE 3-24. A seismogram for the 1906 San Francisco earthquake shows the main seismic waves—P, S, and surface waves.
EARTHQUAKES AND THEIR CAUSES
45
Vertical
Cable North Suspended mass Marker Rotating drum
Yamaguchi, USGS.
Base anchored into bedrock and moves with it
Hinge Base anchored into bedrock and moves with it
b
p
Suspended mass Rotating drum
Marker
Bedrock
Bedrock
a
Spring
Support
c
FIGURE 3-25. a. Although many seismograph stations now record earthquake waves digitally, a recording drum seismograph like this one is especially useful for visualizing the nature of earthquake waves: their amplitude, wavelength, and frequency of vibration. Different seismographs are used to measure, for example, b. horizontal versus c. vertical motion, and small versus large earthquakes.
seismographs are designed to measure those various directions of earthquake vibrations—north to south, east to west, and vertical motions. Knowing the directions of ground motion makes it possible to infer the direction of fault movement from the seismograph records.
chored to the ground. The whole system moves with the earthquake motion except the suspended weight, which stays relatively still because of its inertia. In seismographs designed to measure horizontal motion, the weight is suspended from a wire, whereas in those designed for vertical motions, it is suspended from a weak spring (p Figure 3-25). The first seismograph used in the United States was at UC Berkeley in 1887. It used a pen attached to the suspended weight to make a record on a sheet of paper that was attached to the moving ground. Most modern seismographs work on the same basic principle but detect and record ground motion electronically. Seismograms can help scientists understand more about how a fault slipped, as well as where it slipped and how much. Faults with different orientations and directions of movement generate various patterns of motion. Specialized
Locating Earthquakes The time interval between the arrivals of the P and S waves recorded by a seismograph can also help scientists locate the epicenter of the earthquake. Imagine the P and S waves as two cars that start at the same place and at the same time, one going 100 kilometers per hour, the other at 90 kilometers per hour. The faster car gets farther and farther ahead with time. An observer who knows the speeds of the two cars could determine how far they are from their starting point simply by timing the interval between their passage. In exactly the same way, since we know the velocity of the waves, the time interval between the arrivals of the P and S waves reveals the approximate distance between the seismograph and the place where the earthquake struck (p Figure 3-26). This calculation is explained in greater detail in By the Numbers 3-1:“Earthquake-Wave Velocities.”
0 Travel-time curves P Wave S Wave
Tepich, Mexico (TEIG)
1000 1.5 minutes = 900 kilometers P Wave
S Wave Isla Socorro, Mexico (SOCO)
2000
Station SSPA
3 minutes = 1800 kilometers 3300 Kilometers
P Wave
1800 Kilometers
S Wave Standing Stone, Pennsylvania (SSPA)
Station TEIG Station SOCO
900 Kilometers
Earthquake Location
5 minutes = 3300 kilometers 4000 3 a
6
9
IRIS/USGS.
IRIS/USGS.
3000
b
p
FIGURE 3-26. a. The difference in arrival time between the P and S waves reveals the distance from a seismograph to an earthquake. This plot shows records from seismographs at different distances from a single earthquake. b. Circles of distance to the earthquake drawn from at least three seismograph stations locate the earthquake on a map, in this case in the Mexico trench.
46
CHAPTER 3
Monroe and Wicander, 2001.
Horizontal Support
3-1 By the Numbers Earthquake-Wave Velocities Surface wave arrival times increase linearly with distance from the earthquake because the waves travel with nearly constant velocity in shallow rocks. In contrast, P waves travel at 5–6 km/sec in the continental crust but about 8 km/sec in more dense rocks of the mantle. S waves travel at about 3.5 km/sec in the crust but about 4.5 km/sec in the mantle. Since P and S waves travel faster deeper in the Earth, waves at greater depths can reach a seismograph faster along those curving paths (see Figure 3-26a).
The arrival times of P and S waves at a single seismograph indicates how far from the seismograph the earthquake originated, but it does not indicate in which direction the earthquake occurred. This means the earthquake could have happened anywhere on the perimeter of a circle drawn with the seismograph at its center and the distance to the earthquake as its radius. In order to better pinpoint the location of the earthquake, this same type of data is needed from at least three different seismograph stations. The three circles will intersect at only one location, and that is where the earthquake struck (Figure 3-26b). In fact, because the earthquake waves travel at slightly different velocities through different rocks on their way to the seismograph, their apparent distances are slightly different, and the circles intersect in a small triangle of error. In practice, seismograph stations communicate the basic data to a central clearing house that locates the earthquake, evaluates its magnitude, and issues a bulletin to report when and where it happened. The bulletin is often the first news of the event. That is why we so often find the news media reporting an earthquake before any information arrives from the scene of the earthquake itself.
Earthquake Size and Characteristics A question that comes to mind when people feel an earthquake or see the wild scribbling of a seismograph recording its ground motion is “How big is it?” This question can be answered two ways—by describing its perceived effects, its intensity, or by measuring the amount of energy released, its magnitude.
Italian scientist Giuseppe Mercalli formalized the system of reporting in 1902 with his development of the Mercalli Intensity Scale. It is based on how strongly people feel the shaking and the severity of the damage it causes. The original Mercalli Scale was later modified to adapt it to construction practices in the United States. The Modified Mercalli Intensity Scale is still in use. The U.S. Geological Survey sends postcard or email questionnaires to people it considers qualified observers who live in the area of an earthquake and then assembles the returns into an earthquake intensity map, on which the higher Roman numerals record greater intensities (p Table 3-2). Mercalli Intensity Scale maps reflect both the subjective observations of people who felt the earthquake and an objective description of the level of damage. They typically show the strongest intensities in areas near the epicenter and areas where ground conditions favor the strongest shaking. In the case of the Loma Prieta earthquake, shown in p Figure 3-27, the map indicates that zones of greater intensity are, as expected, elongated parallel to the San Andreas Fault. The greater intensities shown along San Francisco Bay can be explained by the fact that its loose, wet muds amplify the shaking. Such maps are especially useful in land use planning because they predict the pattern of shaking in future earthquakes along the same fault. The maps shown in Figure 3-27 are examples of recently developed computer-generated maps of ground motion called ShakeMaps, which show the distribution of maximum acceleration and maximum ground velocity for many potential earthquakes; they can be used to infer the likely level of damage. Such real-time maps can help send emergency-response teams quickly to areas that have likely suffered the greatest damage.
p Table 3-2 MERCALLI INTENSITY (APPROX.) AT EPICENTER I–II III IV–V VI–VII
Earthquake Intensity
VIII–IX
After the great Lisbon earthquake of 1755, the archbishop of Portugal sent a letter to every parish priest in the country asking each to report the type and severity of damage in his parish. Then the archbishop had the replies assembled into a map that clearly displayed the pattern of damage in the country. Jesuit priests have been prominent in the study of earthquakes ever since.
X–XI XII XII
Mercalli Intensity Scale
EFFECT ON PEOPLE AND BUILDINGS Not felt by most people. Felt indoors by some people. Felt by most people; dishes rattle, some break. Felt by all; many windows and some masonry cracks or falls. People frightened; most chimneys fall; major damage to poorly built structures. People panic; most masonry structures and bridges destroyed. Nearly total damage to masonry structures; major damage to bridges, dams; rails bent. Nearly total destruction; people see ground surface move in waves; objects thrown into air.
EARTHQUAKES AND THEIR CAUSES
47
35˚
PALMDALE
34.5˚ WRIGHTWOOD VENTURA
NORTHRIDGE
PASADENA
LOS ANGELES
SANTA CRUZ.IS
34˚
MALIBU
LONG BEACH IRVINE
33.5˚
TriNet, 2003.
km 0 10 20 30
-119˚ USGS.
INSTRUMENTAL INTENSITY POTENTIAL DAMAGE
-118˚
I
II-III
IV
V
VI
VII
VIII
IX
X+
none
none
none
Very light
Light
Moderate
Moderate/Heavy
Heavy
Very Heavy
b a
p
FIGURE 3-27. a. This ShakeMap of earthquake intensities of the Loma Prieta earthquake near Santa Cruz, California, in 1989 shows a Mercalli intensity VIII at the epicenter northeast of Santa Cruz. b. This TriNet ShakeMap shows the distribution of shaking during the 1994 Northridge earthquake. These maps were created after the earthquake because the system was not available at the time.
Earthquake Magnitude Suppose you were standing on the shore of a lake on a perfectly still evening admiring the flawless reflection of a mountain on the opposite shore.Then a ripple arrives, momentarily marring the reflection. Did a minnow jump nearby, or did a deranged elephant take a flying leap into the distant opposite shore of the lake? Nothing in the ripple as you would see it could answer that question.You also need to know how far it traveled and spread before you saw it, because the size of the wave decreases with distance. Useful as it is, the Mercalli Intensity Scale does not help answer some of those basic questions. That is the problem that Charles Richter of the California Institute of Technology addressed when he first devised a new earthquake magnitude scale in 1935. Richter developed an empirical scale, called the Richter Magnitude Scale, based on the maximum amplitude of earthquake waves measured on a seismograph of a specific type, the Wood-Anderson seismograph. Although wave amplitude decreases with distance, Richter designed the magnitude scale as though the seismograph were 100 kilometers from the epicenter. Seismograms vary greatly in amplitude for earthquakes of different sizes. To make that variation more manageable,
48
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Richter chose to use a logarithmic scale to compare earthquakes of different sizes. At a given distance from the earthquake, an amplitude 10 times as great on a seismograph records a magnitude difference of 1.0—an earthquake of magnitude 6 sends the seismograph needle swinging 10 times as high as one of magnitude 5 (p Figure 3-28). The Richter Magnitude Scale is simple in principle and easy to use, but actual practice leads to all sorts of complications, which have inspired many modifications. Seismo-
10 Magnitude 6
0
p
Magnitude 5
FIGURE 3-28. An earthquake of magnitude 6 registers with 10 times the amplitude as an earthquake of magnitude 5 from the same location and on the same seismograph. That difference is an increase in 1 on the Richter scale. The horizontal axis on these seismograms is time, and the vertical axis is the ground motion recorded.
10 20 30
P
S
Amplitude = 23 mm
0 10 20 P − S = 24 seconds 500 400 300 200 100 60 40
20 Monroe & Wicander, 2001.
graphs, like buildings and people, sense shaking at different frequencies. Tall buildings, for example, sway back and forth more slowly than short ones—they have longer periods of oscillation. P waves, S waves, and surface waves have different amplitudes and different periods. With this variability in earthquake waves, Richter chose to use as the standard for his local magnitude, ML, waves with periods, or back-andforth sway times, of 0.1 to 3.0 seconds. The Richter magnitude is now known as ML. Seismologists, the scientists who study earthquakes, use different magnitude scales, specifically based on the amplitudes of surface waves, P waves, or S waves, or the moment magnitude, MW (see below). Larger earthquakes have longer periods, so different seismographs are used to measure short- and long-period wave motion. Distant earthquakes travel through Earth’s interior at higher velocities and frequencies. To work with distant earthquakes, Beno Gutenberg and Charles Richter developed two more-specific magnitude scales in 1954. MS, the surface-wave magnitude, is calculated in a similar manner to that described for ML. The number quoted in the news media is generally the surface-wave magnitude, as it is in this book, unless specified otherwise. Surface waves with a period of 20 seconds or so generally provide the largest amplitudes on seismograms. Special seismographs record earthquake waves with such long periods. MB, the bodywave magnitude, is measured from the amplitudes of P waves. To estimate the magnitude of an earthquake, we need the amplitude (from the S wave or surface wave). Since the amplitude of shaking decreases with distance, we also need the distance to the epicenter (from the P minus S time). These calculations can be made using a graphical method, the earthquake nomograph, on which a straight line is plotted between the P – S time and the S-wave amplitude (p Figure 3-29). This line intersects the central line at the approximate magnitude of the earthquake. For earthquakes with ML above 6.5, the strongest earthquake oscillations, which have a lower frequency, may lie below the frequency range of the seismograph. This may cause saturation of earthquake records, which occurs when the ground below the seismograph is still going in one direction while the seismograph pendulum, which swings at a higher frequency, has begun to swing back the other way. Then the seismograph does not record the maximum amplitude. So the Richter magnitude becomes progressively less accurate above ML 6.5, and a different scale, such as that of MW, becomes more appropriate. An earthquake of magnitude 6 indicates ground motion or seismograph swing 10 times as large as that for an earthquake of magnitude 5, but the amount of energy released in the earthquakes differs by a factor of 32 (By the Numbers 3-2: “Energy of Different Earthquakes”). Below ML 6 or 6.5, the various measures of magnitude differ little; but above that, the differences increase with magnitude. For larger earth-
50 6
50
30
5
20 10
20 10 6
4
5
3
2 1
4
2
0.5 0.2
1 2
5
0 Distance (km)
100
40
0.1 0 Magnitude
Amplitude (mm)
P−S (seconds)
p
FIGURE 3-29. A nomograph chart uses the distance from the earthquake (P S time, in seconds) and the S-wave amplitude (in mm) to estimate the earthquake magnitude.
quakes, the energy released is a better indicator of earthquake magnitude than ground motion. MW is essentially a measure of the total energy expended during the earthquake. It is determined from long-period waves taken from broadband seismic records that are controlled by three major factors that affect the energy expended in breaking the rocks. Calculation of MW depends
3-2 By the Numbers Energy of Different Earthquakes To compare energy between different earthquakes, a Richter magnitude difference of: 0.2 is 2 times the energy 0.4 is 4 times the energy 0.6 is 8 times the energy 1.0 is 32 times the energy 2.0 is 1,000 times the energy (32 32 1,024) 4.0 is 1,000,000 times the energy (324 1,148,576)
EARTHQUAKES AND THEIR CAUSES
49
on the seismic moment, which is determined from the shear strength of the displaced rocks multiplied by both the surface area of earthquake rupture and the average slip distance on the fault. The largest of these variables and the one most easily measured is the offset or slip distance. Small offsets of a fault release small amounts of energy and generate small earthquakes. If the length of fault and the area of crustal rocks broken is large, then it will cause a large earthquake. Because the relationships are consistent, it is possible to estimate the magnitude of an ancient earthquake from the total surface rupture length (p Figure 3-30). A fault offset of 1 meter would generate an earthquake of approximately MW 6.5, whereas a fault offset of 13 meters would generate an earthquake of approximately MW 9 for typical rupture thicknesses. If you find a fault with a measurable offset that occurred in a single earthquake, then you can infer the approximate magnitude of the earthquake it caused. In 1954, Gutenberg and Richter worked out the relationship between frequency of occurrence of a certain size of earthquake and its magnitude. Recall from Chapter 1 that there are many small events, fewer large ones, and only rarely a giant event (see Figure 1-5). Quantitatively, that translates as a “power law.” Plotted on a graph of earthquake frequency versus magnitude, the power law can be plotted as a log scale: 101 or 10 to the power of 1 is 10; 102 or 10 to the second power is 10 10 100; 103 10 10 10 1,000, and so on. The Gutenberg-Richter frequency–magnitude relationship tells us that if we plot all known earthquakes of a certain size against their frequency of occurrence (on a logarithmic axis), we get a more or less straight line that
we can extrapolate to events larger than those on record (p Figure 3-31a.). Small earthquakes are far more numerous than large earthquakes, and giant earthquakes are extremely rare, which is presumably why we have not had many in the historical record (p Figure 3-31b). Most of the total energy release for a fault occurs in the few largest earthquakes. Each whole-number increase in magnitude corresponds to an increase in energy release of approximately 32 times. Thus, 32 magnitude 6 earthquakes would be necessary to equal the total energy release of 1 magnitude 7 earthquake. And more than 1,000 earthquakes of magnitude 6 would release energy equal to a single earthquake of magnitude 8 (32 32 1,024).
Ground Motion and Failure During Earthquakes How much and how long the ground shakes during an earthquake is related to how much and where the fault moved. p Table 3-3 summarizes the relationship between earthquake magnitude and ground motion. Local conditions can also amplify shaking and increase damage.
Ground Acceleration and Shaking Time Sometimes it helps to think of ground motion during an earthquake as a matter of acceleration, that is, the strength of the shaking. Acceleration is normally designated as some proportion of the acceleration of gravity ( g); 1 g is the acceleration felt by a freely falling body, such as what you feel when you step off a diving board. Most earthquake
Strike slip fault Normal fault
8
Reverse fault
Moment magnitude (Mw) (linear scale)
Moment magnitude (Mw) (linear scale)
Theoretical maximum magnitude for Earth
10
7
6
Chile, 1960
9
Circumference of Earth = 40,000 km
8 e.g., 7.53 7 160 km 6
4 10−2 (1 cm)
10−1 (10 cm)
1 (meter)
10 (meters)
Displacement (log scale)
a
p
Wells & Coppersmith, USGS.
Wells & Coppersmith, USGS.
5 5
4 1 b
10
100
1,000
10,000
100,000
Surface rupture length along fault (km) (log scale)
FIGURE 3-30. a. This graph shows the relationship between the maximum fault offset (see Figure 3-20) during earthquakes on all types of faults to the magnitude of the earthquake. b. The relationship between the surface rupture length (see Figure 3-20) on all types of faults is graphed here to the magnitude of the earthquake.
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CHAPTER 3
Regional magnitude–frequency relation 1
0.1
10
0.01
100
1,000
0.001
0.0001 6.0
6.5
7.0 7.5 Magnitude
Approximately 1 earthquake every how many years
Annual frequency (earthquakes/year)
Susan Hough, USGS.
1
10,000 8.5
8.0
a Magnitude
Energy Release (equivalent kilograms of explosive)
Ear thquakes
10 9 8 7 6
Energy Equivalents 56,000,000,000,000
Chile (1960) Great earthquake: near total destruction, massive loss of life Major earthquake: severe economic impact, large loss of life Strong earthquake: damage ($ billions), loss of life Moderate earthquake: property damage
5 Light earthquake: some property damage
4
Alaska (1964) 0°
Ground level
p
FIGURE 10-27. An ice storm can form when snow in the cold upper atmosphere falls through a warmer layer, melts, then refreezes as it continues down into a cold ground layer.
knocking out power for hundreds of thousands of people. At least 16 people died and 43 were hospitalized by carbon monoxide poisoning. Another ice storm in mid-January 2007 left ice coatings on everything in Texas to Maine. At least 39 people died, including those in Texas, Oklahoma, Missouri, Iowa, New York, and Maine.
Atmospheric Cooling Volcanic eruptions produce large amounts of sulfur dioxide (SO2), along with other aerosols that cool the earth by reflecting sunlight back into outer space. The SO2 dissolves in water droplets in clouds to form reflective droplets that can remain in the atmosphere for a few years until rain washes them out. The eruption of Mount Tambora in Indonesia in 1815 caused New England’s “Year without a Summer” in 1816. The eruption of Mount Pinatubo in the Philippines in 1991 lowered the northern hemisphere’s temperatures by less than 1°F for a few years.
AP Images/Ron Heflin
NOAA
a
p
b
FIGURE 10-28. a. A classic ice storm in the northeast coats power lines. b. An ice storm in Geneva on January 28, 2005, coats everything with a thick layer of ice.
However, it also reduces the heat to the lower atmosphere and cools the surface of the Indian Ocean, which reduces evaporation. This in turn increases the intensity of regional droughts because less moisture is available to fall as rain. As mentioned above, a more intermittent but locally important contributor appears to be volcanic eruptions that produce large amounts of ash (Case in Point: “Climate Cooling from a Major Volcanic Eruption—Mount Tambora, Indonesia,” p. 283). Environmental effects have followed many large eruptions of rhyolite ash since 1815. The violent eruption of Krakatau near the island of Java in Indonesia in 1883 injected large volumes of ash into the upper atmosphere, where it spread westward around the Earth. It cooled the climate for several years but did not cause widespread crop failures and famines such as those that devastated populations following the eruption of Tambora in 1815.
1 Overlap of reconstructed temperatures Anthropogenic changes (thick lines = greenhouse gases from power plants, industry, cars, etc.)
0 10 20 30 40 50 60 70 80 90 % 0.5
Natural changes (thin lines = solar input, volcanic input)
–0.0
IPCC, 2007.
Temperature anomaly (°C wrt 1500-1899)
The atmosphere can be cooled by anything that puts particulates such as soot and dust in the air. Forest fires, both natural and human-caused, and dust blowing off deserts and drought-affected croplands are major sources of particulates (p Figure 10-29). Industrial smokestacks contribute, especially in underdeveloped countries. Pollution drifting east from coal-fired plants in China has even shifted normal rainfall patterns, causing more rain in the south and worse droughts in the north. Burning wood, coal, and peat provide significant amounts of particulates in poor countries. A 3-kilometer-thick brown cloud of smoke, soot, and dust roughly the size of the United States was recently discovered over part of the Indian Ocean. Part of that pollution is due to the huge population of people in India who use dried cow dung as a cheap fuel source for cooking. Dark soot in the cloud absorbs heat and warms the upper atmosphere.
–0.5 1000
1200
1400
Year
1600
1800
2000
p
FIGURE 10-29. Temperatures have risen sharply since 1900, compared with records from the previous 1,700 years. The broad gray area covers the range of uncertainty, with higher degree of confidence indicated by darker shades. The confidence is lower for earlier measurements that were based on tree ring and glacial ice core records. Instrumental records since 1902. Different colors of lines refer to different models.
C L I M AT E C H A N G E A N D W E AT H E R R E L AT E D T O H A Z A R D S
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Large eruptions of rhyolite ash also cause spectacular sunsets, which are especially notable for their vivid streaks of green, a color not ordinarily observed in sunsets. These typically continue for several years after such eruptions.Weather is confined mainly to elevations below 15 kilometers, so ash lingers high in the stratosphere, where rain does not wash it out. Any huge eruption of rhyolite ash is likely to develop into a global climatic catastrophe. No warning or evacuation scheme can mitigate this type of danger. People everywhere should dread a major eruption of any large resurgent caldera such as Yellowstone Volcano or Long Valley Caldera.
The Greenhouse Effect The vast majority of scientists attribute most of the recent rise in atmospheric temperature to increases in atmospheric greenhouse gases, including carbon dioxide (CO2), methane (CH4), chlorofluorocarbons (CFCs), ozone (O3), and nitrous oxide (NO2). All of these greenhouse gases trap heat in the atmosphere in about the same way that a greenhouse permits the sun to shine through glass, but prevents much of the heat from escaping, in a phenomenon called the greenhouse effect (p Figure 10-32). Surface temperatures rise on the Earth as greenhouse gases enter the atmosphere and insulate our planet. Sunlight is short-wave radiation that passes easily through the atmosphere (or the glass of a greenhouse), heating up the Earth’s surface. The heat and other long-wave radiation produced at the surface heat molecules of water, CO2 and other greenhouse gases in the atmosphere (p Figure 10-33). The individual total greenhouse effect of these gases depends on their effectiveness in blocking the outgoing long-wave radiation (heat) and their abundance in the atmosphere. CO2 has the greatest total effect, followed by CH4, but the others are also significant (p Figure 10-34). The most dramatic effect on temperatures occurs in the polar regions because the polar air is drier than tropical air and is thus more sensitive to the concentrations of such greenhouse gases. It is important to remember that without some natural greenhouse effect, Earth would be uninhabitable, as the average temperature would fall below freezing. Water vapor is the most important greenhouse gas, but there is little we can do about its levels in the atmosphere. Most atmospheric water vapor has evaporated from the oceans, which cover about two-thirds of Earth’s surface. The greenhouse effect explains why temperature tends to drop more quickly on a clear night than on a cloudy night because the heat radiates to outer space unhindered by clouds. The water vapor of clouds absorbs huge amounts of heat, preventing its loss
IPCC, 2007. Temperature anomaly (°C wrt 1961-1990)
Earth’s average surface temperature has been rising since the Industrial Revolution began in the late 1700s. The temperature increased by about 1°C (1.8°F) over the past century, with the most dramatic increase since 1970 (p Figure 10-30). Earth’s surface temperatures have been rising at an alarming rate, especially since about 1920, and especially at high latitudes in the Arctic. In the past 90 years, they have gone up about 8°C (more than 14°F). The Intergovernmental Panel on Climate Change (IPCC) in 2007 estimated that the Earth’s average surface temperature will likely rise by 1.6°–3.4°C over the next century and possibly more than 5°C (9°F) in some places. This change compares with a rise of 4°C since the peak of the last ice age 20,000 years ago. In North America, by 2080 temperatures in the Arctic are expected to rise as much as 7°C (12°F) in winter and 3°C in summer (p Figure 10-31). Temperatures in the Great Lakes region and eastern Canada are expected to rise by 4°C in winter and 6°C in summer. Temperatures in most of the central United States, including the already hot desert southwest, are expected to rise by more than 3°C in winter and about 4°C in summer.
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FIGURE 10-30. Changes in Earth’s surface temperature since 1850 and projected future increase in Earth’s surface temperature. The increase through the twentieth century is well known from measurements. Projected increases for the twenty-first century depend on various scenarios for amounts of fossil-fuel burning and various feedback mechanisms. Even with no additional greenhouse gas content in the atmosphere (orange line), temperatures will increase very slowly because of heat slowly released from the oceans. Scenario A1B (green line) envisions a world population that peaks near 2050, rapid world economic growth, decreased economic differences among world regions, and a balanced use of fossil and nonfossil energy sources. B1 assumes an unlikely low rate of economic growth, social and environmental sustainability, and dominance of clean and resource-efficient technologies.
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from the atmosphere. Actually, during the day, different types of clouds have quite different effects. Thin, high-altitude clouds, above the flight paths of commercial aircraft, permit the sun to shine through unhindered to heat the ground. But they prevent heat loss back to outer space. Thick, low clouds, however, have white tops that reflect the sun back away from the Earth; they prevent the sun from reaching the ground, so those cloudy days stay cooler.
FIGURE 10-31. Projected changes in world temperature for the period from 2080 to 2099 relative to the average from the 1980–1999 period, for the most likely scenario, A1B (compare Figure 10-30), which envisions rapid world economic growth, decreased economic differences among world regions, and a balanced use of fossil and nonfossil energy sources. Note that the largest increase in winter temperature is in northern latitudes.
Rising Levels of Greenhouse Gases In recent years the levels of greenhouse gases in the atmosphere have been rising, trapping more heat and raising the Earth’s temperature. Some greenhouse gases are natural. Carbon dioxide and CH4 are emitted by erupting volcanoes, animals, and decaying vegetation. NO2 is generated by oxidation of nitrogen in the atmosphere during lightning storms. However, neither volcanic expulsion of carbon dioxide nor increase in solar radiation shows sufficiently
Incoming short-wave radiation passes through glass Sun
Some solar radiation is reflected by the Earth and the atmosphere.
At m About half the solar radiation is absorbed by the Earth’s surface and warms it.
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USGCRP.gov
Donald Hyndman photo.
The Greenhouse Effect
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FIGURE 10-32. A greenhouse heats up inside because the shortwavelength energy of incoming sunlight passes through glass. The longer-wave reflected radiation—heat—cannot pass through glass and is trapped inside.
Some of the infrared radiation passes through the atmosphere but most is absorbed and re-emitted in all directions by greenhouse gas molecules and clouds. This warms the Earth’s surface and the lower atmosphere.
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FIGURE 10-33. Carbon dioxide and other greenhouse gases in Earth’s atmosphere act in about the same way as the glass covering a greenhouse. C L I M AT E C H A N G E A N D W E AT H E R R E L AT E D T O H A Z A R D S
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FIGURE 10-34. a. Global greenhouse gas concentrations over the past 650,000 years show a distinct series of cycles about every 100,000 years (one of the Milankovitch cycles) that correlates with the orbital eccentricity of the Earth around the sun. b. Carbon emissions have dramatically increased in the last 150 years. Since 1700, carbon dioxide levels measured in the atmosphere on the top of Mauna Kea, Hawaii, have risen dramatically. Values for the previous 1,700 years were relatively stable, derived from air samples trapped in the Antarctic ice sheet.
values are far higher than the maximum of 300 ppm during the past 650,000 years. Concentrations of CH4 and NO2 have also risen far above their pre-industrial values. Few remaining scientists argue that the temperature increase of the last century is mostly natural; all agree that CO2 concentration has increased some 30 percent since before the Industrial Revolution, less than 300 years ago. Most of the CO2 increase is human-caused, from burning of fossil fuels (coal, oil, and natural gas used to produce heat and electricity), from land-use changes, and as a by-product of cement production (p Figure 10-35).The main ingredient in cement, calcium oxide (lime), is produced from calcium carbonate (limestone) in a reaction that generates carbon
large increases in recent decades to explain rising levels of greenhouse gases and the rise in temperatures. There is also some evidence of cyclic variation of CO2 levels. Carbon dioxide amounts in the atmosphere have varied in the distant past. Measurements of tiny amounts of air trapped in Greenland and Antarctic ice provide a record of CO2 through the last 400,000 years (see Figure 10-34a). They show significant cyclic variation from less than 200 to almost 300 parts per million (ppm), leading some people to argue that the current increase in CO2 is merely natural variation, not human-caused. However, the pre-industrial value of about 280 ppm skyrocketed to about 380 ppm by 2005, with current increases of almost 2 ppm per year. These
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FIGURE 10-35. a. A small coal-fired factory belches black smoke in Spain—soot and CO2. b. Total CO2 emissions by use. Note that electricity production involves burning coal, oil, and natural gas.
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Donald Hyndman photo.
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dioxide (CaCO3 → CaO CO2). Millions of motor vehicles that travel roads across the world generate large amounts of CO2 and other greenhouse gases. Thousands of jet aircraft traversing our skies burn fuel that generates large amounts of CO2 and water vapor. They provide only about 3 percent of the CO2, but they contribute 6 to 12 percent of the total greenhouse effect due to the increase in water vapor. Additional smaller amounts come from forest and range fires, some of which are natural. For example, the 2007 Southern California wildfires generated about 8.7 million tons of CO2, which is small compared with other sources. The increase in methane and nitrous oxide is primarily from farm animals and decay of agricultural materials. Decay in landfills also generates methane but some of that gas is now being captured, cleaned, and used to generate electricity. The relative temperature effect of greenhouse gases and ozone is partially countered by the effect of aerosols, but the total overall anthropogenic (human-caused) effect is very large (p Figure 10-36). By far the largest proportion of world energy involves burning fossil fuels, which generates large amounts of CO2. Fossil fuels, from natural gas (methane) to petroleum oil and coal, are all hydrocarbons, meaning that they are made of various combinations of carbon (C), hydrogen (H), and oxygen (O). Of those fuels, CH4 generates less CO2 when Radiative forcing of climate between 1750 and 2005 Radiative Forcing Terms CO2
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FIGURE 10-36. The relative temperature effect of greenhouse gases, ozone, and aerosols.
burned because it has a smaller proportion of carbon to hydrogen (1.0 to 4 parts H) compared with about 1.8 for liquid petroleum, about 5.6 for soft coal, and 10.7 for hard coal. Soft coal, although far more abundant than other fossil fuels, contains much more moisture, so it generates less heat per unit of weight. For the same energy, burning CH4 releases 30 percent to 40 percent less CO2 than burning coal or oil. Coal-fired power plants are the largest contributors of CO2 into the atmosphere. Coal burning also typically generates more of other pollutants such as sulfur and mercury. Unfortunately, coal, the most-polluting source, produces most of the energy in China, India, and many other developing countries. Coal produced half of the electricity used in the United States in 2005, followed by 19 percent for nuclear power and for natural gas, 6.5 percent for hydropower, and 3 percent for oil. Biofuels, which are mostly ethanol from corn, are used as a gasoline supplement. Their price dramatically increased because demand increased the price of corn used in its production. Some greenhouse gases are absorbed by natural buffers, but unfortunately, these buffers are also becoming more limited in their capacity to absorb CO2. Carbon dioxide is used by plants, primarily during the growing season from spring to fall. Thus atmospheric CO2 levels are lower in the northern hemisphere in summer. Large-scale deforestation reduces the ability of plant matter to absorb CO2. However, this contribution may be less than previously thought; new studies show that plants take up significant CO2 only when provided with sufficient soil nitrogen to aid in that growth. Some CO2 dissolves into ocean water, which means that the oceans also serve as a natural buffer to soak up the greenhouse gases that we emit. However, warm water dissolves less CO2 than cold water, so as water temperatures rise, less CO2 is absorbed by the ocean. A 2007 study published by the National Academy of Sciences showed a much faster rate of increase in CO2 emissions than expected—a 35-percent increase since 1990, not only because of coal burning but because of decreased absorption of CO2 by the oceans.
Consequences of Climate Change The warming of Earth’s surface brings changes to weather patterns and other aspects of climate. The most significant change is greater variability of weather, with more extremes of temperature, winds, and precipitation. The 2007 IPCC report paints a bleak picture of what may lie ahead in some areas. Abrupt changes in regional climate, in either cooling or drying, would have disastrous consequences for today’s world populations. If the change were to occur in a decade or two, as has happened in the past, the consequences would be catastrophic. Extreme weather events such as torrential rainfalls are expected to become
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more frequent because atmospheric heat, the engine that drives storms, gets stronger. Intense precipitation events would couple with longer and more severe drought periods. In large areas, the drop in temperature and rainfall would limit agricultural production, and water supplies would be severely curtailed. Food availability would be disrupted, resulting in spreading malnutrition; public health would suffer from spreading insect-borne diseases such as malaria, cholera, and dengue fever to higher latitudes and higher elevations. Economic disruption, population migration, political upheaval, and conflicts over resources seem likely.
Warming Oceans The increase in atmospheric temperature has been moderated by the fact that most of our planet is covered with oceans that soak up more than 80 percent of the effect of global warming. The problem, however, rests in the capacity of water to soak up heat. It takes a large amount of heat to warm a pot of water on the stove. By contrast, it takes a much smaller amount of heat to warm the same volume of air by the same amount. The oceans are slowly soaking up heat from the warming atmosphere. The average change in ocean temperature over the past 40 years was 0.3°C in the top 300 meters and 0.06°C in the top 3.5 kilometers. This small warming of the seas is more ominous than it appears because ocean heat contributes significantly to the energy that drives storms. Hurricanes, for example, form and strengthen where SSTs are above 25°C (77°F). Warmer seas may cause these storms to be stronger and more frequent in the future. In addition, water expands as it warms. As a result, sea level will rise with global warming even if no glaciers and ice sheets melted (discussed further in Chapter 13). Warming of the atmosphere also causes more evaporation from the oceans, increasing the water vapor content in the atmosphere and thereby causing still more global warming—an unfortunate feedback effect. The feedback associated with water vapor is thought to roughly double the warming effect from carbon dioxide increases alone. Warming of the oceans is a trend that cannot easily be reversed. Warm oceans could be cooled only if the atmosphere above them were significantly cooler. Unfortunately, the oceans are such a huge heat sink, covering about twothirds of the Earth’s surface. We can’t easily cool the atmoWinter
sphere enough to begin cooling the oceans. Even with reduction in manmade CO2 emissions, atmospheric amounts would not level off for about 200 years, and temperatures would continue to increase for another 200 years. Even with no further manmade emissions, it would take hundreds to thousands of years to cool our environment to levels of a century ago.
Precipitation Changes An increase in Earth’s surface temperature also has an impact on precipitation. In general, areas closer to the poles and near the equator will get wetter, and the warmer midlatitude regions will get drier (p Figure 10-37). Thus many areas that are now dry will get drier, and many wet areas will get wetter. By 2080, much of Canada is expected to be wetter in winter, whereas Mexico is expected to be 15–50 percent drier. In summer, the precipitation across most of the United States may not change much, but the Pacific Northwest is expected to be 20–30 percent drier. Most of Earth’s near-surface heat is concentrated in the oceans, and warmer oceans lead to more evaporation and thus greater rainfall. More energy in the atmosphere often leads to more storms. Atmospheric pressure would show higher highs and lower lows. That would cause winds to flow faster between them, with associated stronger and more frequent storms. Natural disasters such as hurricanes and tornadoes would likely be more frequent and stronger. Warming sea temperatures may be responsible for the recent increase in frequency and strength of especially stormy weather in the southwestern United States. Floods would be more frequent and more intense, even without other influences. Mudflows and coastal erosion would become more severe on the West Coast. Lower rainfall and drier vegetation would lead to more thunderstorms and more fires. Although populations in some northern climates might welcome modest increases in temperature, detrimental effects would include increased incidence of both droughts and floods. It might seem strange that there could be more drought with more precipitation, but drought depends on the balance between precipitation and evaporation; higher temperatures cause more evaporation, which may have a larger effect than the increase in precipitation. More precipitation
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FIGURE 10-37. The relative expected change in precipitation in the century between 2000 and 2100 (Scenario A1B), a. for winter (December to February) and b. for summer (June to August). Stippled areas are where more than 90 percent of models agree in the direction of change.
in the ice have to be abandoned because they are settling and deforming. Permafrost that once remained frozen through the summer at a depth of 0.5 meter finally began thawing in the early 1990s. Roads and railroads deform and become unusable (see Figure 9-20b), spruce trees tilt at odd angles in forests, and sinkholes form in ice thawed beneath the surface (see Figure 10-39b). Permafrost around Fairbanks, in central Alaska, averages about 50 meters thick, increasing to about 200 meters in northern Alaska, and more than 600 meters near the Arctic coast. Studies suggest that more than one-half of the top 3 or more meters may thaw by 2050, and the rest by 2100. Wind-blown dust and organic materials contained in permafrost in Siberia and Alaska contain about three-quarters as much carbon as is tied up in living vegetation on Earth. When permafrost thaws, its organic material is warm enough to decay, and some of its carbon can return to the atmosphere as CH4 and CO2. Much of this carbon, sealed in permafrost for more than 10,000 years, may be released into the atmosphere over the next century. This would cause a huge increase in global warming, especially since CH4 has about 20 times as much warming effect as CO2 for an equivalent amount of gas. About one million square kilometers of western Siberia permafrost has begun to thaw, along with huge areas of northern Canada and central Alaska. Permafrost melting is feeding more water into northflowing rivers and into the Arctic Ocean, accelerating warming and melting there. Northern rivers are thawing several weeks earlier than in past decades. Glaciers are progressively melting, not only in Alaska and northern British Columbia, but in the North Cascades of Washington and Glacier Park in Montana. The loss of mountain glaciers leads to faster runoff and depletion of water supplies in summer, the peak growing season for many areas. Even in the southern hemisphere, in the southern Andes of Argentina and in Antarctica, huge glaciers are dramatically shrinking (p Figure 10-40).
would fall as rain rather than snow in western North America; this would mean that less moisture would be stored until late spring, when its stream flow from groundwater would be available for the dry summers. Warmer winters in the Midwest would reduce the seasonal frozen surface area of the Great Lakes, which would lead to more winter evaporation from the open water surface and thus lower the lake levels. Snowpacks are expected to be smaller, and runoff would generally be earlier; thus summer stream flow would be lower on average. Areas that already have water shortages would likely be in desperate shape during similar periods in the future. Water access in places such as the Middle East would spark even more conflict, if not wars. Some countries, such as Turkey, have already diverted irrigation water with dams, depriving arid Iraq and Syria, which are plagued by still lower river levels.
Arctic Thaw
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USGCRP images.
USGCRP images.
The surface of the Arctic Ocean has been frozen for as long as anyone can remember, preventing ships from using the Arctic as a summer pathway between the Pacific and Atlantic oceans. Since the late 1970s, however, Arctic sea ice has dwindled by about 8 percent per decade, for a total loss of more than 20 percent of its area (p Figure 10-38). Arctic ice, about 3 meters thick in the 1960s, has thinned to about 1.5 meters today. Unfortunately, there is also a feedback mechanism that accelerates melting of Arctic Ocean ice. White ice reflects 90 percent of the sun’s energy back into space. The fraction of energy reflected away from the Earth’s surface is called its albedo. Once some ice has melted, however, the dark surface of the ocean absorbs 90 percent of the sun’s energy, heating the water and in turn melting more ice. Frozen ground (permafrost) that covers much of northern Canada, central Alaska, and northern Asia is thawing (p Figure 10-39). Buildings that were once solidly anchored
2003 b
FIGURE 10-38. Ice cover in the Arctic Ocean in 1979 and 2003. Note the reduced and thinned ice at left and open water around northern islands of Canada.
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Sharon Smith, Geological Survey of Canada.
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V. Romanovsky photo.
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FIGURE 10-39. a. Permafrost distribution, northern hemisphere. b. Permafrost ice under a parking lot at the University of Alaska, Fairbanks, is melting, leaving cavities in the ice and potholes in the ground.
Image not available due to copyright restrictions
Sea-Level Rise Global warming leads to sea-level rise from two primary factors—about half from water added from melting of ice on land and half from heating and expansion of sea water. Melting of Arctic Ocean ice does not raise sea level because floating ice merely displaces the same volume of water as the submerged part of the ice. This is why melting ice cubes in a completely full glass of water will not make the water overflow. Melting of temperate-climate glaciers will continue. If all the Greenland ice were to melt, the added water would raise sea level by about 7 meters; the Greenland ice cap is now melting and calving off rapidly, three times faster in
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2006 than in 1996. Melting of all of the Antarctic ice would raise it about 66 meters. Fortunately, wholesale melting of the Greenland and Antarctic ice sheets, which would cause global sea level to rise a phenomenal 80 meters, is not likely in the foreseeable future. As water warms, it expands, so the increase in ocean temperature will fill the oceans beyond that expected based solely on the volume of melted ice from the continents. Expansion occurs because heating causes the water molecules to vibrate faster and therefore take up more space. The actual rise in sea level from both thermal expansion and glacier melting was about 19 centimeters between 1870 and 2000 (0.15 centimeter per year), but the
will take hundreds to thousands of years for temperatures to stabilize, as discussed earlier. Unfortunately, sea-level rise would take several centuries to stabilize, due to the long duration of heat uptake. It is clear that the world needs to act quickly and decisively to prevent further warming, given the long lag time once the oceans warm.
rate increased to about 0.3 centimeter per year between 1993 and 2005. Rise of sea level will ultimately cause the flooding of coastal areas, including many of the world’s largest cities; a rise of 3.5 to 12 meters, for example, would endanger New York City, Tokyo, and Mumbai (Bombay), India. The current rate of sea-level rise is roughly 30 centimeters per 100 years, and sea level is expected to rise by between 30 and 50 centimeters over the next century (p Figure 10-41). The consequences of major sea-level rise would be serious for low-lying areas. With 60 centimeters of rise, beaches in the southeastern United States would move about 60 meters landward, and almost 30,000 square kilometers would submerge. The problem is even more acute in poor countries, which lack the resources to build and maintain protective barriers. Bangladesh, which occupies the immense near-sealevel delta of the Ganges and nearby rivers, is already subjected to severe flooding during storms. Bangladesh would lose more than 17 percent of its land if sea level were to rise 1 meter.That would have a disastrous effect on its agriculture and the millions of people who depend on it. Deaths could run into the tens to hundreds of thousands during strong cyclones (Case in Point: “Rising Sea Level Heightens Risk to Populations Living on a Sea-Level Delta—Bangladesh and Calcutta, India,” p. 283). Even a moderate sea-level rise of 18 to 35 centimeters would accelerate erosion of coastal areas near Boston and Atlantic City. People live at sea level for diverse reasons, sometimes because cities grew there for historic reasons. Some in wealthy, industrialized countries choose to live close to beaches for views and recreation. Others, in poor countries, live in places such as deltas near sea level because fertile soils there facilitate growing crops to feed large numbers of people. Even if we are successful in the worldwide goal of stabilizing greenhouse gas concentrations in the atmosphere, it
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Global Ocean Circulation Changes in ocean currents resulting from changes in salinity could rapidly change climate in some areas. Changes in local ocean salinity from factors such as melting glaciers and ice sheets could alter the path and strength of the Gulf Stream, the ocean current that keeps Europe abnormally warm for its northerly location. Any reduction in the flow of this warm current to the north could thus cool Europe and some other northern hemisphere climates. Some scientists argued that the current warming trend has the potential to rapidly push parts of northern Europe and northern Asia into a little ice age for a few hundred years. This would have devastating effects on humans as well as natural ecosystems and the economies of most industrialized nations; clearly we should be proactive about the effects of human activities on climate change. The large-scale circulation in the Atlantic Ocean (socalled thermohaline circulation) involves a current of warm, shallow water moving northward away from the equator. Closer to the Arctic, where it cools and sinks, the current pulls more warm surface water behind it (p Figure 10-42). The sinking water moves south at depth. The warm, salty Gulf Stream, which moves northeast along the east coast of North America and across to Europe, is part of this circulation. Westerly winds carry heat from the warm ocean to keep Europe warm in winter, in spite of its northern latitude. If that warm-water circulation were to weaken or stop, as believed by some researchers, northern Europe could cool
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FIGURE 10-42. The Global Circulation System, with red arrows as surface warm currents and blue arrows as deep cold currents.
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significantly, as documented in past climate records. Parts of northern Europe could see frozen rivers and harbors, dramatic increases in energy needs, and loss of agricultural productivity. With global warming, ocean waters gradually warm. However, Arctic sea ice, Greenland glaciers, and frozen ground in Arctic lands melt faster, pouring fresh water into the North Atlantic. In 2005, 221 cubic kilometers of Greenland’s ice melted, compared with 91.7 cubic kilometers in 1995. As glacial ice melts, water pours down crevasses and holes in the ice, some refreezes; some flows below the glaciers, causing immense subglacial floods and faster flow of glaciers that would calve off more icebergs into the ocean (p Figure 10-43). Kangerdlugssuaq Glacier in Greenland, which contains four times the water in the Great Lakes, flowed downslope at 3.5 kilometers per year in 1996; it had accelerated to more than 16 kilometers per year by 2005. In the ocean, that lower-density fresh water cannot sink, so circulation of the ocean current will slow or stop. Less warm water will reach eastern North America and northern Europe, causing both to turn colder in winter, possibly very quickly. Although the flow of this current has slowed some-
what and shallow subpolar seas around the North Atlantic have been cooling, the rate varies with time and is not presently a cause of concern. Studies in the last few years suggest that the effects of global warming will counterbalance the ocean-cooling effect so that cooling in Europe is not expected to be a major problem. Past climate studies show, however, that the warm oceanic conveyor belt has happened before. It shut down 12,700 years ago in the cold period known as the Younger Dryas and then restarted 1,800 years later. Each of these changes occurred within only about a decade. Another, less severe abrupt cooling 8,200 years ago lasted only about 100 years. Lesser variation in temperature continued. A medieval warm period 1,000 to 700 years ago was followed by the “Little Ice Age” between the years 1300 and 1850. Winters were severe, glaciers advanced, crops failed, famine and disease were widespread, and large numbers of people migrated from northern Europe. Simultaneous drought in the southwestern United States has been linked to the collapse of the Mayan civilization in about 900 A.D. and the Anasazi just before 1300. Because it affects ocean circulation worldwide, cooling has also been linked to disruption of the monsoons of Southeast Asia that are critical to supporting the huge populations of the region. Monsoons are warm, with moist winds drawn off an ocean by warm air rising against a coastal mountain range. The moisture-laden air rises, cools, and dumps its moisture as heavy rains. Chilling of the continent prevents the mountain air from rising, and moist air is not pulled off the ocean.
Joel Harper photo.
Mitigation of Climate Change
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FIGURE 10-43. A moulin or melt hole in a glacier carries large amounts of water to the glacier base, where it increases buoyancy and lubrication of the glacier.
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Atmospheric CO2 is expected to double in the next century, and there is no clear way to stop this trend.Weather patterns will definitely change, and some economists and politicians argue that we need to “learn to live with” those changes. If average temperatures rise as anticipated, predictions indicate that snowpack in the Rockies could decrease dramatically. This would create severe problems. Northern Europe could see increases in rainfall, while southern Europe would likely become drier. Warming of some northern climates would permit the northward migration of insects bearing diseases such as malaria and West Nile virus because fewer insects are killed off in warmer winters. Longer warm seasons also permit northward migration of pine-bark beetles that kill forests. Infestations that had been infrequent and small already appear to be happening on a larger scale. Policy changes are necessary to reduce vulnerability to some of the effects of global warming, including those that reduce energy generation by burning of fossil fuels. This could be accomplished by increased conservation, improved efficiency of power production and use, regulation of emissions, carbon taxes and carbon trading, subsidies and tax credits, and capture and sequestration of greenhouse gases, along with use of alternative fuels such
Carbon dioxide emissions (billions of metric tons per year)
Data from U.S. DOE, Carbon Dioxide Information Analysis Center.
With the increase in atmospheric CO2 correlated to global warming, 111 countries, including most industrialized nations but neither the United States nor Russia, signed the Kyoto Protocol in 1997 to reduce emissions of six greenhouse gases beginning in 2008. Russia signed the protocol in 2004, bringing the percentage of industrialized-nation signatories to 55 and thus meeting the treaty’s threshold for activation. The United States declined to agree to such targets despite being the largest contributor to the emissions at that time (p Figure 10-45), arguing that China, India, and other large contributors should also have to agree and that the costs would have a detrimental effect on U.S. economic growth. Instead, the United States said it would reduce the rate of increase of greenhouse gas emissions to less than the rate of increase in U.S. economic growth. Several states, however, have decided to do their part in spite of the stand of the federal government. In 2006, California mandated a cap on global-warming greenhouse gas emissions and intends to cut them to 1990 levels by 2020. In April 2007,
a
p
p
FIGURE 10-44. Simple but effective solar collectors that heat water for domestic use are almost ubiquitous in some countries such as Turkey.
the U.S. Supreme Court ruled that carbon dioxide from cars and trucks is considered a pollutant. The Environmental Protection Agency (EPA) is planning to provide regulations for new motor vehicles by the end of 2008. The EPA recently required fuel importers and refiners to reduce the sulfur content of diesel fuel by 97 percent, thereby permitting building of clean diesel cars and light trucks that can be 30-percent more fuel efficient. The debate around the Kyoto Protocol shows the high stakes involved in changes to global energy policies. Nations are polarized over who is to blame and what to do about global warming. The countries that contributed most historically to CO2, such as the United States, profited from unrestrained industrial development to become the wealthiest nations in the world. The countries that are today
7 6 United States China Russia Japan India Germany Canada United Kingdom
5 4 3 2 1 0 1980
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Carbon dioxide emissions per person (metric tons per year)
The Kyoto Protocol
Donald Hyndman photo.
as wind, solar collectors, and possibly nuclear power. Conservation is an inexpensive way to save large amounts of electric power and therefore reduce carbon output used in its production. An estimated 4 million tons of carbon is used per year in the United States to power electronic devices in standby mode—devices such as TVs, phones, computers, alarm clocks, and microwave ovens. The Department of Energy estimates that the “standby” portion of each person’s electric bill will reach 20 percent by 2010! Some solar collectors generate electricity from silicon panels, and excess electricity can charge batteries for various domestic uses. These can be quite effective but are expensive to manufacture. New “thin-film” solar materials, which can be used to coat windows and roofs, use much less silicon and promise to reduce costs significantly. Other much less expensive solar heating panels are merely waterfilled tubes that are heated by the sun to provide hot water for washing or domestic heating (p Figure 10-44).
25 20 15 10 5 0 1980
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FIGURE 10-45. a. Total greenhouse gases for countries that have highest total emissions. b. Per person emissions in the same countries.
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going through periods of rapid industrial development, such as China and India, feel that they too should be allowed to develop without environmental restraints. They argue that the industrialized nations must first reduce their emissions because they can more afford the associated costs. Huge populations in Southeast Asia are so poor that they cannot afford to purchase more efficient fuels. How do you tell a family that has little money for food to stop cooking with the only fuel that it can afford? Without the cooperation of large emitters like China and India, the prospect for significant reduction of emissions is bleak. China still generates far less greenhouse gases than the United States on a per capita basis, but their production is rapidly increasing because of their immense populations and rapid development. China overtook the United States as the largest emitter of greenhouse gases in 2006 due to minimal pollution controls, and a dramatic increase in number of heavily polluting coal-fired power plants, cement production, and very rapid economic growth. The average American produces 16 times as much CO2 as the average person in India, but this too is changing. China’s and India’s only abundant energy source is coal, the least efficient fuel, which generates the most greenhouse gases; the prospects are alarming. Even with one of the most optimistic scenarios for reversing the increase in CO2 with strong limits on emissions, the total CO2 in the atmosphere is not expected to level off until after 2060, at a level of about 475 ppm compared with 386 ppm in 2007. Without immediate and drastic countermeasures, China’s rate of emissions increase threatens to greatly exacerbate the greenhouse gas problem in the near future. On September 8, 2007, the leaders of 21 Asia-Pacific countries agreed to a 25-percent reduction in their “energy intensity,” the energy to produce a dollar of economic product, by 2030. Thus countries are permitted to increase their emissions as long as they become more efficient and their emissions per dollar decrease. Unfortunately, although the rate of increase in emissions would slow under this agreement, the total greenhouse gas emission would continue to increase. The agreement does little to solve the problem.
Alternative Energies In 2002, energy consumption of fossil fuels worldwide was 410 quadrillion British Thermal Units, which released 2.6 billion tons of CO2 into the atmosphere. A large focus of the current effort to mitigate climate change is on the exploration of alternative energy sources that have lower emissions. For example, nuclear power does not generate CO2. Other alternatives that do not generate CO2 include hydroelectric power from falling water, wind turbines, solar energy, geothermal power, and in some places tidal energy. With so much demand, the cost of energy is what drives most usage. Least expensive are coal and natural gas, costing about 5¢ per kilowatt-hour, wind at about 6¢, and nuclear at about 6.5¢. Coal is most abundant but dirty—if hidden costs of health effects from air pollution were included,
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coal would cost about 6.3¢. Solar panels cost about 22¢ per kilowatt-hour, presently well out of the running. Arguments against wind are that the giant turbines are noisy and unsightly and can kill birds. Coal burning not only pollutes but strip-mining of coal that removes waste rock from above the coal typically lays waste to huge areas of landscape. The newest coal gasification plants, now in the experimental stage, are more efficient, would reduce pollution, use less water, and more easily collect CO2 for underground disposal. Japan and countries in western Europe that have little fossil fuel generate significant amounts of nuclear energy— about 16 percent of worldwide energy supply. France generates about 78 percent of its power from nuclear fission—the breakup of uranium atoms—and it recycles much of the nuclear waste to make additional fuel. Nuclear power presently produces about 20 percent of electrical power in the United States, from 103 plants, all built before 1980. Problems with nuclear power include limited deposits of uranium fuel, projected to last only about 50 more years, and disposal of nuclear waste; most of that has low levels of radioactivity. Nuclear power earned a bad name from the power-plant disasters at Three Mile Island, Pennsylvania, in 1979 and Chernobyl in the Soviet Union in 1986; but Chernobyl was a poorly designed, constructed, and monitored plant. The problems of safety and disposal of nuclear waste now appear be less of a hazard to mankind than further increases in greenhouse gases, and the power industry is under pressure to reduce reliance on coal-fired plants. As a result, new nuclear fission power plants are again being planned in the United States. The U.S. Congress is providing incentives in the form of risk insurance and production tax credits for the first few new reactors. As a result, several companies are applying to construct new nuclear plants. Nuclear fusion—the combining of parts of atoms at temperatures of a hundred million degrees Celsius, as occurs in our sun—is feasible but presently very inefficient and likely at least 50 years away from commercial production. Tidal power systems are pollution free but expensive to build.Various schemes involve either tidal currents running underwater turbines in narrow fjords with large tidal ranges or capture of the potential energy difference between the heights of high versus low tides. Most existing installations are small, but one operating on the Rance River in France since 1967 generates about 68 megawatts of average power. Tidal power areas are under consideration in a few promising areas, from the Bay of Fundy between Maine and New Brunswick, to Cook Inlet near Anchorage, Alaska. Hydrogen fuel, touted as clean and pollution free, generates electric power by burning hydrogen with oxygen. Unfortunately, it must be generated by separating hydrogen and oxygen using large amounts of traditional energy sources.The advantage is that hydrogen can be generated in centralized plants where CO2 produced by burning can be captured and hopefully sequestered underground. Support and government subsidies to add ethanol in gasoline to increase fuel supplies and make it burn cleaner
may be somewhat misguided. Recent studies show that although ethanol burns cleaner than gasoline, the number of miles per gallon of fuel is lower. If you include the fertilizer and machine-fuel energy used to produce the crops, it takes at least 25 percent more energy to get ethanol or biodiesel from corn, soybeans, and other commonly used plants than is available in those fuels. Ethanol must be transported by truck or rail, rather than pipeline, thereby using more fuel. In addition, it would take an estimated 43 percent of all crop land in the United States to replace only 10 percent of gasoline and diesel fuel supplies. Although not yet commercially viable, a cellulose source (e.g., grass or logging residues) produces about four times as much ethanol with the same energy input. However cellulose growth requires a lot of land. Hopefully, new technologies will develop to make this a net gain rather than a net loss. Ethanol has other downsides. Its primary source is corn, which is important for animal feed and as a food export to poor countries. In a single year, the world price of corn quadrupled and that of wheat doubled, leaving poor countries without enough food for their people. In addition, the huge amounts of water used in producing ethanol are depleting worldwide aquifers and polluting water supplies. Methane, a natural gas, is an efficient fuel used for heating homes and driving power plants. A potential major source of greenhouse gas is methane hydrate, frozen methane-ice “compound” (p Figure 10-46), trapped in layers deeper than 1 kilometer in continental permafrost commonly at depths of 0.5 to 3 kilometers under the seafloor of many of the world’s continental slopes. Most, though not all, are in the sediment wedge of the subduction zones, where soft sediments are scraped up against the edge of the continent.
Energy companies are interested in the possibility of tapping the vast reserves of methane in the hydrates. Current estimates are that methane hydrate holds about twice as much energy as all other fossil fuels combined—gas, oil, oil shale, and coal. Methane in hydrate concentrates 160 times as much methane as in the gas form. If only 1 percent of the methane hydrate were recovered, that would double the current methane reserves of the United States. Methane under U.S. waters is about 1,100 times current U.S. reserves and 53 times the world’s current land reserves. Unfortunately, heating the hydrate at depth to release the methane uses much of the heat value that could be obtained. Technology to tap those offshore reserves economically is not presently available. CONSEQUENCES OF UNINTENDED RELEASE OF METHANE Stability of methane hydrate depends on pressure (depth of burial) and temperature (thawing of the hydrate at temperatures above 0° to 15° or 20°C). Unintended release of methane from hydrate, however, is a potential hazard, since methane is a greenhouse gas twenty times more potent than CO2. Present estimates are that about 3 percent of global methane emissions now naturally come from the seafloor. Possible triggers for release of large amounts of methane include major earthquakes, hurricanes, seafloor slumping, and drilling activities that initiate landslides on the continental slope. Small landslides triggered by an earthquake can destabilize larger areas of gas hydrate that in turn weaken the slope and cause much larger areas of landsliding. A few giant submarine landslides, probably triggered by earthquakes that destabilized the methane hydrate layers in the past, have caused major tsunami that inundated coastal regions (see Chapter 5).
Image not available due to copyright restrictions
p
Ian MacDonald photo.
FIGURE 10-46. a. Methane hydrate (pale orange) layer on the ocean floor.
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Sequestration of Greenhouse Gases Another method of reducing atmospheric greenhouse gas concentrations is to remove CO2 from the air and pump it into underground storage reservoirs (Case in Point: “CO2 Sequestration Underground—The Weyburn Pilot Project,” p. 285). This is currently done to repressurize oil fields and increase the amount of recoverable oil and gas. If all of the world’s oil fields used such CO2 reinjection, roughly half of the new CO2 we produce could be confined to the subsurface. In fact, pumping CO2 into underground sites such as anticlines (arches in sedimentary rocks) that were once well-enough sealed to hold reservoirs of natural gas may be one of the more permanent places to lock it away. Another option may be to pump liquefied CO2 into deep ocean-floor sediments, where it would be trapped because of its higher density. One concern is whether pumping of large amounts of fluid under pressure could trigger earthquakes in some areas, as was the case with the pumping of waste fluids underground near Denver in the early 1960s (see Figure 4-3). Another concern is the acidification of groundwater because CO2 water produces carbonic acid, a weak acid. An additional strategy would be to react CO2 with magnesium-rich silicate minerals such as olivine and serpentine to produce magnesium carbonates. That would permanently remove CO2 from the air; however, it is not yet clear how such a process could be carried out commercially on a large scale. Alternatively, CO2 could be disposed of in the deep ocean, since it dissolves in water. Disposal could use fixed pipelines from plants onshore to inject CO2 into the ocean at relatively shallow depths of, say, 1 or 2 kilometers so that the gas would bubble up and dissolve in the water. If it were piped to deeper levels of, say, 4 kilometers, the higher pressure on the CO2 would compress it into liquid that would be heavy enough to sink to still deeper levels in the ocean. Then again, if the CO2 were piped directly to the ocean bottom from offshore drilling platforms, the liquid CO2 could pool as a “lake” on the ocean bottom. There it would only slowly dissolve back into the ocean.
Carbon Trading Many aspects of mitigation involve political decisions and political solutions. Some industrialized countries, such as the United States, have been reluctant to cut back on carbon emissions for fear of hindering economic growth. Some countries, such as Sweden, use a carbon tax (a tax
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on burning fossil fuels); others use voluntary schemes. Still other countries, such as most members of the European Union, try to limit emissions by encouraging “trading of carbon emissions” by companies that burn large amounts of fossil fuels. In that scenario, companies are limited to a certain level of carbon emissions based on past highlevel periods of emissions; that is, their emissions levels are “grandfathered.” A company that emits a smaller amount of greenhouse gases than it did in the past may sell its permitted emissions credits to one that generates more than it did in the past. The idea is to encourage companies to produce fewer emissions, even though the company may be growing. Unfortunately, the baseline for individual companies does not distinguish whether the company uses old, heavily polluting, “dirty” power generation or new, state-of-the-art low-pollution power.
The Political Side of the Emissions Problem Most large companies are focused on making money and much less concerned with greenhouse gases and global warming. This recently began to change as the public became more aware of the problem and expressed more and more concern. A few petroleum companies now are beginning to cut greenhouse gas emissions but most are not. Most do not support mandatory emissions caps, but seeing state emissions caps coming from California and the Northeast, they want a say in the design of a national policy. For example, by 2007, General Electric (GE), the giant multinational company that manufactures everything from light bulbs to jet engines to coal-fired power plants to wind turbines had seen enough to polish its environmental image. It recognizes that more-stringent constraints on carbon emissions are coming and can’t afford to fall behind. It has also found that increasing its own efficiency and manufacturing more efficient and lower-emission equipment has saved money. The federal government, in conjunction with GE and Philips Electronics, is working on legislation to phase out the common, inefficient tungsten-filament light bulbs over the next seven years; they would be replaced mostly by compact fluorescent bulbs. In 2007, Canada decided to ban the sale of non-fluorescent light bulbs by 2012. The October 2007 award of the Nobel Peace Prize jointly to Al Gore and the IPCC added more fuel to the push for cutting climate-warming emissions.
Case in Point Climate Cooling from a Major Volcanic Eruption Mount Tambora, Indonesia u
In April 1815, a gigantic eruption reduced the peak of Tambora volcano on the densely populated Indonesian island of Sumbawa from an elevation of 4,300 meters to 2,900 meters. The eruption (with a Volcanic Explosivity Index of 7) produced 40 cubic kilometers of ash and pumice, an even larger volume than was produced in the eruption that reduced Mazama to Crater Lake almost 8,000 years earlier (see Chapter 7). The caldera is roughly 7 kilometers wide and more than 600 meters deep. The eruption killed some 10,000 people directly by pyroclastic flows and another 80,000 in the famine and epidemics that followed. Brightly colored orange and red sunsets were seen worldwide in the summer and fall of 1815. A persistent “dry fog” with reddish sunsets continued into the
summer of 1816 because of sulfate aerosols in the stratosphere. The environmental aftermath developed into a global catastrophe of famine and misery. The rhyolite ash that Tambora injected into the upper atmosphere blocked enough sunshine throughout the northern hemisphere to reduce the average temperature about 0.5°C (~0.9°F) within a few weeks. That seems a small drop, but it was enough to inflict agricultural havoc. Freezing temperatures in New England throughout the middle of 1816, “the year without a summer,” caused widespread famine. On June 6, 1816, snow fell in parts of New York and Maine. Summer frosts ruined crops as far south as Virginia, including, by some accounts, Thomas Jefferson’s corn. Meanwhile, abnormally cold and rainy weather caused wide-
spread crop failure and famine in Europe. It was a hard year. Some historians argue that the eruption of Tambora helped inspire a large migration from New England to the region west of the Ohio River, as well as considerable movement from Europe to North America.
Case in Point Rising Sea Level Heightens Risk to Populations Living on a Sea-Level Delta Bangladesh and Calcutta, India u The vast delta regions of Bangladesh and the northern coastal region of India around Calcutta are low lying and especially fertile—and one of the most densely populated regions on Earth, with some 130 million people in an area not much larger than New York State. People are exceptionally poor; their livelihoods are based directly on agriculture and therefore are intimately tied to weather and its associated hazards. The delta lands on which people live and farm are virtually at sea level (p Figure 10-47). Most people live on farms rather than in the cities, and the few rail lines connect only the larger cities. Roads are narrow and crowded with
heavy trucks, buses, and rickshaws. Bridges span only a few of the smaller river channels. People get around on bicycles, small boats, and ferries. Traffic jams in the cities are frequent on normal days. Imagine the chaos of trying to evacuate hundreds of thousands or millions of people with a couple of days’ warning given such limited transportation. Even when there is adequate satellite warning to evacuate populations in the region, most people do not leave because they believe that others will steal their belongings, that since they have lived through one major cyclone, the average time between cyclones suggests to them
that the next major one “will not come for many years,” or that if they die, it is “God’s will.” The problem is enormous:
p In October 1737, 300,000 died in a surge that swept up the Hooghly River in Calcutta.
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(continued) p In 1876, 100,000 died in another surge
Villages were flooded when a flood protection embankment failed, and rail service was disrupted when the floods swept away a small bridge.
near the mouth of the Meghna River, and thousands more in 1960 and 1965.
p In November 1970, cyclone winds of 200 kilometers per hour accompanied a 12-meter surge that swept across the low-lying delta of the Ganges and Brahmaputra Rivers in Bangladesh. Four hundred thousand people died, many of them in the span of only 20 minutes. Whole villages disappeared, along with all of their people and animals.
p In late November 2007, Tropical Cyclone Sidr, the worst to hit Bangladesh in many years, came directly north from the Bay of Bengal to strike the lowlying coast. More than three million people evacuated. A combination of 250 km/hr winds, a 5-meter storm surge, huge waves, and flooding due to torrential rain swept at least 3,500 people to their deaths. Coastal fishing villages were especially hard hit.
p Again, on April 30, 1990, a cyclone with 233-kilometer-per-hour winds and a 6-meter surge swept into Bangladesh, drowning 143,000 people. The 1990 Bangladesh population of 111 million has been projected to double in 30 years, placing millions more on the delta.
p On June 6, 2001, nearly 100,000 people were stranded up to their chests in water following heavy monsoon rains.
Catastrophic floods in Bangladesh are caused by several factors. The heavy monsoon rains from April through October are carried by warm, moist Indian Ocean winds from the southwest and magnified by the orographic effect of the air mass rising against the Himalayas. The rains are torrential and widespread in the drainage areas of the Ganges and Brahmaputra riv-
p
AFP/Getty Images.
Bruce Bander/Photo Researchers, Inc.
FIGURE 10-47. a. Clusters of houses on higher ground are completely surrounded by floodwaters. October flooding in Bangladesh. b. An Indian boy reaches his house near Guwahati, India, in the Brahmaputra flood of July 8, 2003.
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ers, which come together in the broad delta region of Bangladesh. Rivers swell annually to twenty times their normal width. Those same drainage areas have been subjected to widespread deforestation and plowing of the land surface, which causes massive erosion and heavy siltation of river channels. The decrease in channel capacity causes the rivers to overflow more frequently. In the delta region of Bangladesh, gradual compaction and subsidence of delta sediments coupled with gradual rises in sea level compound the problem. Relative sea-level rise compared with the delta surface raises the base level of the rivers, reduces their gradient, and raises the water level everywhere on the delta. Cyclonedriven storm surges can put virtually the whole delta—75 percent of the country— underwater in a few hours (compare Figure 14-12). Given that people live on the sea-level delta of two major rivers and in the path of frequent monsoon floods and frequent tropical cyclones, their options are few. The government plans to dredge and channelize more than 1,000 kilometers of riverbeds to improve navigation and build homes for flood victims on the new levees. Unfortunately, the impetus appears to be to improve food production rather than ensure safety and will do little to prevent the problem. Ironically, it is those same floods that bring new fertile soils to the land surface. Dredging will increase the chances of flooding downstream; and the new levees, built from those fine-grained materials dredged from the delta’s river channels, will be easily eroded during floods. Levee breaches will lead to avulsion of channels (formation of new river pathways), widespread flooding, and destruction of fields. One useful suggestion is to construct high-ground refuges and provide flood warning systems.
Case in Point CO2 Sequestration Underground The Weyburn Pilot Project u
One of the possible solutions to the abundance of greenhouse gases is permanent removal from the atmosphere and storage underground. Oil companies have been pumping high-pressure CO2 into the ground for years to help push out residual oil from rock pore spaces during declining production. However, much of that CO2 eventually leaks back out to the surface. The world’s largest-scale experiment to test the viability of permanent underground storage is being undertaken in southern Saskatchewan. The Weyburn pilot project of the International Energy Agency pumps CO2 under pressure into a partly depleted oil reservoir 1,450 meters below, both to permit recovery of some of the remaining oil and to permanently remove CO2 from the atmosphere. Initial injection in September 2000 was of 2.69 million cubic meters per day, increasing to 3.39 million cubic meters in 2004. At the end of 2006, it was disposing of about 1 million tons per year. The CO2 is pumped by a 320-kilometer pipeline from a plant in nearby North Dakota. Apparent faults cut the essentially horizontal sedimentary layers, but their
potential as paths for leaking fluids is unknown. Local deformation caused by injection of the high-pressure fluids complicates the situation by widening fractures or opening new ones. Another experimental sequestration project is in the Colorado Plateau of southeastern Utah and adjacent Colorado, New Mexico, and Arizona, an area that surrounds many large coal-fired power plants that generate enormous amounts of CO2. In fact, such power plants are the largest contributors of atmospheric CO2. The area consists of horizontal sedimentary rocks with structural domes that naturally trap CO2. Some of the natural CO2 has reacted with rock minerals to form solid carbonate minerals, thereby trapping the CO2 in the solid form. Preliminary data and modeling are encouraging, suggesting that after 1,000 years, 70 percent of the CO2 should remain trapped in the rock. A third experiment pumped CO2 into sandstones sealed against a salt dome near the Gulf Coast of Texas. The CO2 remained well sealed but reacted with mineral grains to mobilize iron, manganese,
and toxic elements, displacing them in the saltwater that originally filled the spaces between the grains. Such toxic materials could contaminate groundwater in the region. Given the amounts of CO2 being produced, it is clear that new cleaner, higherefficiency, coal-fired plants now being built will need to capture the CO2 directly at the point of generation, then transport it to sites for permanent burial underground. Unfortunately, given the amounts of CO2 from both present and planned coal-fired plants, it seems doubtful that a large proportion of it will ever be safely and permanently stored in subsurface rock formations.
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1. For each of the following photos, indicate the hazard or hazards and any damaging events that have happened.
Critical View
2. Why should the event have been foreseen, and what could be done to prevent it?
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G.
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Donald Hyndman photo.
Donald Hyndman photo.
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Donald Hyndman photo.
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NOAA.
D. Peterson, USGS.
Donald Hyndman photo.
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Mike Shumate, NOAA.
NOAA.
Donald Hyndman photo.
3. Where plausible, evaluate what can be done to stabilize the area.
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I.
J.
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Chapter Review
Key Points Basic Elements of Climate and Weather p Water continuously evaporates from oceans and other water bodies, falls as rain or snow, is transpired by plants, and flows through streams and groundwater back to the oceans. Figure 10-1.
p Rising air expands and cools adiabatically, that is, without loss of total heat. The rate of temperature decrease with elevation—that is, the adiabatic lapse rate—in dry air is twice the rate in humid air. Figures 10-2 and 10-3.
p Cool air can hold less moisture; thus, as moist air rises over a mountain range and cools, it often condenses to form clouds. This is the orographic effect of mountain ranges. Figure 10-4.
p Warm air rises, cools, and condenses to form an atmospheric low-pressure zone that circulates counterclockwise in the northern hemisphere. Cool air sinks, warms, and dries out to form a highpressure zone that circulates clockwise in the northern hemisphere. Air moves from high to low pressure, producing winds. Figure 10-5.
p West-to-east rotation of the Earth causes air and water masses on its surface to lag behind a bit. Because the rotational velocity at the equator is greater than at the poles, the lag is greater near the equator. This causes air and water masses to rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. Figure 10-6.
p The prevailing winds are the westerlies north of about 30 degrees north (wind blows to the northeast) and the trade winds farther to the south (blowing to the southwest). Cells of warm air rise near the equator and descend at 30 degrees north and south. Figure 10-7.
p The subtropical jet stream meanders eastward across North America, across the interface between warm equatorial air and colder air to the north and between high- and low-pressure cells. Figure 10-11.
p Warm fronts, where a warm air mass moves up over a cold air mass, and cold fronts, where a cold air mass pushes under a warm air mass, both cause thunderstorms. Such fronts often intersect at a low-pressure cell. Figures 10-8 to 10-11.
Climatic Cycles p Earth’s climate has cycles from days and seasons to those that come thousands of years apart. Figure 10-12.
p Equatorial oceanic circulation normally moves from east to west, but every few years the warm bulge in the Pacific Ocean drifts back to the east in a pattern called El Niño, bringing winter rain to the west coast of equatorial South and North America, including southern California. Figures 10-13 to 10-15.
p The North Atlantic Oscillation is a comparable shift in winter atmospheric pressure cells that affects weather in the North Atlantic region. It also shifts every few years, but the times do not correspond to those of El Niño. Figures 10-16 and 10-17.
Hazards Related to Weather and Climate p Because flooding depends on water in the atmosphere and tropical air contains ten times as much water as cold polar air, tropical air masses bring the wettest storms. Warm, moist, tropical air moves to much higher latitudes during the summer.
p Vegetation is abundant in wet climates, so rain falls on leaves and soaks slowly into the ground to feed groundwater and year-round streams. Lack of vegetation in dry climates permits rain to fall directly on the ground, where most of it runs off the surface. Especially heavy rainfall can cause floods, as can prolonged rainfall that saturates surface soil to prevent further rapid infiltration.
p The atmosphere can be cooled by particulates from sources such as industrial smokestacks, forest fires, and volcanic ash eruptions.
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Global Warming and the Greenhouse Effect p Greenhouse gases such as carbon dioxide and methane trap heat in the Earth’s atmosphere much as the glass in a greenhouse permits the sun to shine in but prevents most heat from escaping. Figure 10-33.
p Atmospheric carbon dioxide and temperature are increasing, especially since 1970. Figures 10-34 and 10-35.
Mitigation of Climate Change p Reductions in the release of greenhouse gases into the atmosphere could be accomplished by increased conservation, improved efficiency of power production and use, regulation of emissions, carbon taxes and carbon trading, subsidies and tax credits, capture and sequestration of greenhouse gases, and use of alternative fuels such as wind, solar collectors, and possibly nuclear power.
Consequences of Climate Change p Consequences of global warming include more frequent and stronger storms, smaller snowpacks and earlier runoff, drier vegetation and more fires, and warming and expansion of the oceans that leads to rise of sea level. Figures 10-37 to 10-42.
Key Terms adiabatic cooling, p. 251 adiabatic lapse rate, p. 251 albedo, p. 275
drought, p. 262 El Niño, p. 257 frostbite, p. 267
hypothermia, p. 267 ice ages, p. 260 jet stream, p. 255
orographic effect, p. 251 relative humidity, p. 250 right-hand rule, p. 253
Atlantic multidecadal oscillation (AMO), p. 260 Chinook winds, p. 254
global warming, p. 270 greenhouse effect, p. 270 greenhouse gases, p. 271 groundwater, p. 251 heat-island effect, p. 266 high-pressure system, p. 253 hydrologic cycle, p. 250
Kyoto Protocol, p. 279 La Niña, p. 258 lake-effect snow, p. 267 low-pressure system, p. 253 methane hydrate, p. 281 North Atlantic oscillation (NAO), p. 259
Santa Ana winds, p. 254 seasons, p. 256 trade winds, p. 253 warm front, p. 254 weather, p. 250 weather fronts, p. 254 westerly winds, p. 253
climate, p. 250 cold front, p. 254 Coriolis effect, p. 252 desertification, p. 265
Questions for Review 1. If a humid air mass has 100-percent relative humidity and is 20°C at sea level, what would the temperature of this same air package be if it were pushed over a 2,000-meter-high mountain range before returning again to sea level? Explain your answer and show your calculations. 2. Explain the orographic effect on weather. 3. An area of low atmospheric pressure is characterized by what kind of weather? 4. Why do the oceans circulate clockwise in the northern hemisphere?
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5. Explain the right-hand rule as it applies to rotation of winds around a high- or low-pressure center. 6. What is the main distinction between a cold front and a warm front? 7. What causes the Earth’s seasons, such as winter and summer? 8. What main changes occur in an El Niño weather pattern? 9. Why do streams flow year-round in a wet climate? Explain clearly.
10. How much has the Earth’s atmosphere increased in temperature in the last 1,000 years? When did most of the increase begin? 11. About how much has carbon dioxide increased in the atmosphere? When did the increase begin? 12. What are the main sources of important greenhouse gases? 13. Other than an increase in temperature, what would be the most prominent changes in weather with global warming?
14. Approximately how much is sea level expected to rise in the next 100 years? What country is expected to see the largest loss of life as a result of rise in sea level? Why? 15. What is the main contributor to the rise in worldwide sea level? Be specific as to what makes the level rise—not just “global warming.”
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11
Karl Christians photo, Montana Department of Natural Resources and Conservation.
Chapter
i
Too Close to a River
A
resident of Plains, Montana, had just finished building a new house in a picturesque location on the outside of a big meander bend, next to the Clark Fork River. It had a great view of the river and the surrounding mountains. Although the site was on the floodplain, he thought it would be amply safe because it was about 10 meters back from the bank. In late May of 1997, rising water in the spring runoff rapidly eroded the steep bank on the bend of the river next to the house. The fast-moving water caused progressive caving of the bank until it undercut the edge of the house. The owner hired a company to move his house, but he was too late. Before they could move the house, erosion had progressed so far that the moving company was not willing to risk loss of its equipment on the unstable riverbanks. Finally, local authorities decided to burn the house rather than find it floating down the river in pieces. The situation was especially embarrassing because the owner was also the local disaster relief coordinator.
290
No, this house was not built overhanging the cut bank of the river. When it was built, it was 10 meters back from the bank of the Clark Fork River near Plains, Montana. Authorities finally burned the house before it fell into the river.
Processes
Stream Flow and Sediment Transport
Stream Flow Streams and rivers collect water and carry it across the land surface to the ocean. Streams in humid regions collect most of their water from groundwater seepage—water percolating down through porous soils to reach the stream. Flow generally increases in the downstream direction as additional flow from tributary streams and groundwater enters the channel. Streams accumulate surface water from their watershed (or drainage basin), the entire upstream area from which surface water will flow toward the channel (p Figure 11-1). The discharge of a stream, or total volume of water flowing per unit of time, is the average water velocity multiplied by the cross-sectional area of the stream (By the Numbers 11-1: “Total Flow of Stream”). Because there is no easy way to measure the average velocity, point velocities are measured at equal intervals and depths across the stream channel, and each such velocity is multiplied by a cross-sectional area surrounding that measurement (p Figure 11-2a). New instruments called acoustic doppler current
11-1 By the Numbers Total Flow of Stream The total flow of water in a stream depends on the average velocity of the water times the cross-sectional area through which it flows:
Q VA where: Q discharge or total flow (e.g., m3/sec) V average velocity (e.g., m/sec) A cross-sectional area (e.g., m2) width depth (m)
David Hyndman photo.
What the homeowner in the preceding account didn’t understand is that a river is not a fixed structure like a highway but is subject to natural processes, including changing its course or flooding. The first step to understanding flooding is to understand the natural processes by which rivers and streams transport water and sediment. Rivers are complex networks of interconnected channels with many small tributaries flowing to a few large streams, which in turn flow to one major river. They flow in valleys that they have eroded over thousands or millions of years. Rivers respond to changes in regional climate and local weather through the amount and variability of flow and to the size and amount of sediment particles supplied to their channels.
p
FIGURE 11-1. A drainage basin near Boise, Idaho. The “drainage basin” of a river includes all of the slopes that drain water downslope to feed the river.
profilers have been developed to more accurately approximate stream flow by measuring water velocity at hundreds of locations based on the shift in sound frequencies due to moving particles (see Figure 11-2b). SEDIMENT TRANSPORT AND STREAM EQUILIBRIUM Along with moving water, rivers and streams carry sediment downstream, eroding material in one place and depositing it in another. Streams change to maintain a dynamic equilibrium in which the inflow and outflow of sediment is in balance. A stream that is able to maintain this equilibrium is called a graded stream. The cross section of a stream adjusts to accommodate its flow, as well as the sediment volume and the grain sizes supplied to the channel. The geometry of a channel cross section is controlled by flow velocities and the associated ability of a stream to carry sediment. Most streams are wide and shallow, with nearly flat bottoms, but the cross section of a stream adjusts based on the erodibility of the bottom and banks and the nature of the transported sediment. Streams flowing through easily eroded sand and gravel at low flow generally have steep banks and broad, nearly flat bottoms. Streams flowing through bedrock or fine silt and clay tend to be narrow and deep because these materials are less easily eroded. A stream also adjusts its gradient in response to water velocity, sediment grain size, and total sediment load in
STREAMS AND FLOOD PROCESSES
291
90 80
Area (A)
70 60
100
50 150
40
Velocity (cm/s)
lo Ve
Depth (cm)
y
cit
50
) (V
a
p
Donald Hyndman photo.
order to be able to transport its supplied sediment over time. The gradient of a stream, or its channel slope, is the steepness with which it descends from its highest elevation to its lowest, typically expressed in meters per kilometer. Most streams begin high in their drainage basins, surrounded by steeper slopes and often by harder, less easily eroded rocks. Coarser grain sizes, such as those supplied to a stream from the steeper slopes of mountainous regions, require a steeper gradient or faster water to move the grains (p Figure 11-3). There the stream moves the rocks and sediment on down the valley. The greater discharges and smaller grain sizes that exist downstream lead to gentler slopes in those channels. Thus, the gradient generally decreases downstream as sediment is worn down to smaller sizes and the larger flow there is capable of transporting the particles on a gentler slope. Ultimately, the stream will reach a lake or the ocean, a base level below which the stream cannot erode. Where a tributary stream descends from a steeper gradient in mountains onto a broad valley bottom, it leaves the
p
FIGURE 11-3. Coarse gravel brought in by the small tributary on the right creates steepening and rapids in the Middle Fork of the Smith River north of Crescent City, California.
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30 200 20 10
250 10 b
20
30 40 Distance (m)
50
60
USGS.
FIGURE 11-2. a. The cross-sectional area of a simple stream channel can be approximated by dividing it into a rectangular grid. With individual velocity measurements for each such box, the total flow would be the sum of V1 A1 V2 A2 . . . , where A1 and V2 are the areas of individual boxes. b. Stream flow through the San Joaquin River in California was measured using an acoustic doppler current profiler. The stream flow is then calculated by summing the velocity of each cell by the cross-sectional area of that cell.
70
narrow valley that it eroded to reach a local base level of a larger valley. The rapid decrease in its gradient causes it to drop much of the sediment it was carrying. Thus where the slope decreases, the stream changes from an erosional mode, where it is picking up sediment, to a depositional mode, where it is depositing sediment. The excess sediment may spread out in a broad fan-shaped deposit called an alluvial fan. The term alluvial implies the transport of loose sediment fragments, and fan refers to the shape of the deposit in map view. Similarly, where a river reaches the base level of a lake or ocean, the abrupt drop in stream velocity at nearly still water causes it to drop most of its sediment in the form of a delta. The delta is like an alluvial fan except that the delta sediments are deposited underwater.
Sediment Load and Grain Size Eroding riverbanks and landslides can supply a stream with particles of any size from mud to giant boulders. The velocity and volume of the flow limit both the size and the amount of sediment that can be carried by the stream. Empirical curves can be used to estimate the maximum particle size that can be picked up or transported for a given water velocity (p Figure 11-4). Note that the velocity required to mobilize particles is appreciably greater than that to transport the same particles. The relationship of larger grain sizes requiring higher velocities for movement does not hold where fine silt and clay particles make up the streambed. In that case, the fine particles lie entirely within the zone of smoothly flowing water at the bottom and do not protrude into the current far enough to be moved. Clay-size particles also have electrostatic surface charges that help to hold them together. In general, coarser particles in a stream channel provide greater roughness or friction against the flowing water (By the Numbers 11-2: “Velocity in Channel”). Thus, coarser particles also slow the water velocity along the base of a stream. For this reason, mountain streams with coarse pebbles or
1000
Erosion
Gravel 10
Sand
Transportation
Don Hyndman photo.
Flow velocity (cm/sec)
100
Silt 1
Deposition of particles
Clay
a
0.1 1 ––– 256
1 ––– 16
2.0
100
Particle size (diameter in mm)
p
boulders in the streambed often appear to be flowing fast but actually flow more slowly than most large, smoothflowing rivers such as the Missouri and Mississippi. Note that the water velocity depends on increases with water depth and slope (p Figure 11-5). Although a stream is capable of carrying particles of a certain size, there can still be a limit to the volume of sediment a stream can carry, or its load. Large volumes of sand dumped into a stream from an easily eroded source or sediment provided by a melting glacier will overwhelm
Donald Hyndman photo.
FIGURE 11-4. This diagram shows the approximate velocity required to pick up (erode) and transport sediment particles of various sizes. Note that both axes are log scales, so the differences are much greater than it seems on the graph.
b
Stream
11-2 By the Numbers
slope
Finer gra in
s
Velocity in Channel c
The velocity multiplied by the channel roughness is proportional to the average water depth of the channel multiplied by the square root of its slope:
V n 1.49R2/3 s1/2
p
FIGURE 11-5. a. The Smith River south of Great Falls, Montana, has a lower gradient and carries finer particles. b. The turbulent, high-energy Lochsa River in the mountains of northeastern Idaho. c. Grain sizes decrease downstream as slope decreases in the stream channel.
where: R hydraulic radius is proportional to average water depth n Manning roughness coefficient: 0.03 for straight, small streams or grassy floodplains with no pebbles 0.05 for sinuous small streams with bouldery bottoms or floodplains with scattered brush 0.10 to 0.15 for brushy flood zones or floodplains with trees s slope of the channel
its carrying capacity. The excess sediment will be deposited in the channel. Floodwaters do, of course, carry more sediment. As water depth and velocity increase during a flood, shear or drag at the bottom of the channel increases. That extra shear picks up more sediment. Leopold and Maddock demonstrated that the suspended load or carrying capacity of a stream depends upon the discharge (By the Numbers 11-3 “Carrying Capacity of a Stream”).
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293
11-3 By the Numbers Carrying Capacity of a Stream Carrying capacity is proportional to discharge:
L Qn where: L suspended load transport rate (e.g., cm3/sec) Q discharge (e.g., cm3/sec) n an exponent, generally between 2.2 and 2.5
Sediment Transport and Flooding Flooding is a natural part of the process by which rivers move sediment and maintain equilibrium.When peak flood velocity and depth are high enough to develop significant turbulence, the erosive power of a stream becomes very large, temporarily increasing both the size and volume of sediment it can carry. At low water, virtually all of the material brought into the stream stays put, backing up the water behind it. When water rises in the stream, such as during flood, it has a higher velocity and thus can carry more and larger sediment particles. During floods, coarser particles are gradually broken down to smaller-sized particles in the channel and are then flushed downstream. Similarly, if coarser material is added to the channel, such as from a steeper tributary or a landslide, it accumulates in the stream channel until a large flood with sufficient velocity occurs or the gradient of the channel increases sufficiently to move that size material (p Figure 11-6). Factors that produce unusually high stream flows cause dramatic changes in flood turbulence or energy and therefore erosion. Channel scour, the depth of sediment eroded
during floods, affects the shape of the stream channel and distribution of sediment. The grain size a stream can carry is proportional to its velocity; thus, rising water first picks up the finest grains, then coarser and coarser particles. Sediment is carried in suspension as long as grains sink more slowly than the upward velocity of turbulent eddies. Fine sediment is first picked up in eddies. At higher flows, pebbles or boulders may tumble along the bottom and even be heard as they collide with one another. This causes more erosion from the stream banks and bottom and deepens the channel. For this reason, bridge pilings in a river channel must be set deep enough that major floods will not undermine the pilings and cause a bridge to collapse (see Figures 12-6 and 12-7). As water velocity increases, the water drags much more strongly against the bottom. Increase in frictional drag on the stream bottom provides more force on particles on the streambed and thus more erosion. That friction also slows down the water (By the Numbers 11-4 “Drag on Stream Bottom”). As the flood flow wanes, the coarser sand and gravel in suspension progressively drop out, thereby raising the streambed (p Figure 11-7). Thus, as water level rises, the
11-4 By the Numbers Drag on Stream Bottom Drag or total friction on the stream bottom is proportional to velocity squared:
0 v2 where: 0 friction V velocity
Channel fills with water, then begins eroding bottom (185 m3/s)
Low water (18 m3/s)
Deepest erosion during highest flood stage (1670 m3/s)
Channel fills back in as flood recedes (512 m3/s)
October 14
October 26 September 15
Donald Hyndman photo.
September 9
p p
FIGURE 11-6. Giant granite boulders dumped by a landslide into the Feather River in the Sierra Nevada range of California can be moved only in an extreme flood.
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FIGURE 11-7. Channel depth of scour in response to a flood on the San Juan River near Bluff, Utah, in September 1941. With early water rise (September 15), sediment from upstream deposited to raise the channel bottom. As water rose further to the maximum level (October 14), the channel eroded to its deepest point. As the flood level waned (October 26), sediment again deposited to raise the channel bottom. m3/s meters per second.
Robert Weidman photo.
because the water has forward momentum that drives it into the bank, where the higher velocity can mobilize more sediment. The sediment is then deposited as a point bar downstream in the slow water along the inside corner of the bend. The flow commonly alternates between deep pools on the outside bends where sediment has been eroded and shallow riffles between there and the next outside bend where sediment has been deposited (p Figure 11-9). Meanders sometimes come closer together until floodwater breaks through the narrow neck between. When a stream cuts across a meander bend, either naturally or by human interference, the new stretch of channel follows a shorter path for the same drop in elevation. This creates
p
stream begins eroding and the water gets muddy (p Figure 11-8); as the level falls, the sediment in transit begins to deposit. For this reason, a cross section of a flood deposit shows the largest grains at the bottom, grading upward to finer sediments. Stream channel changes are insignificant with normal flows or even small floods, regardless of their frequency. Significant changes occur only when flow reaches a threshold level that mobilizes large volumes of material from the streambed and channel sides.
Donald Hyndman photo.
FIGURE 11-8. Clark Fork River near Missoula, Montana, spilled muddy water over its floodplain in the June 1964 flood.
a Riffle
Channel Patterns
Meandering Streams Any irregularity that diverts water toward one bank moves more water to help erode that bank. As the water swings back into the main channel, it sweeps toward the opposite bank, much like a skier making slalom turns. This deep and highest-velocity part of the stream is the thalweg. The flow preferentially erodes the outside of meander bends
Pool
Deposition C
Erosion
Deposition Pool
Riffle B Erosion
Erosion Riffle Modified from Mount, 1995.
The way in which streams pick up and deposit sediment also determines the pattern of the channel and the way the channel moves over time, which in turn determines the type of flooding characteristic of each type of stream. Meandering streams, which sweep from side to side in wide turns called meanders, are most common. Multichannel braided streams are much less common, and naturally straight streams are rare. Meandering and braided river types form end members of rivers that exhibit a complete range of behavior. Meandering streams are more typical of wet climates with their finer-grained sediments, and braided streams are common in dry climates with abundant coarse sediment. Even within one river, some reaches may meander and others may braid, depending on the erodibility of the banks and the amount of supplied sediment.
Pool
D
A Deposition
b
p
FIGURE 11-9. a. The Carson River in Nevada illustrates the eroding cut bank on the outside of meanders and the depositional gravelly point bar on the inside of a meander. Flow is toward the right. b. Cross sections of a typical meandering stream channel from a riffle at A, downstream through pools at B and C, then finally through a riffle at D. Note that the river erodes on the outside of the meander bends where it has a deep channel, and it deposits on the inside of bends where it has a shallow channel. STREAMS AND FLOOD PROCESSES
295
FIGURE 11-10. a. Meanders in this river near Houston, Texas, eroded the outsides of bends and migrated until one meander bend spilled over to one farther downstream, leaving an abandoned oxbow lake in the center of the photo. b. Prominent point bars deposit on the inside of meander bends along Beaver Creek, north of Fairbanks, Alaska.
Kevin Wyatt photo.
David Hyndman photo.
p
a
b
a short, steeper cutoff and leaves behind the abandoned meander as an oxbow lake (p Figure 11-10). The steepening of the channel slope causes increased velocity and thus greater erosion. The size and shape of river meanders follow some general relationships (see p Figure 11-11 for descriptions of terms):
p Meander wavelength, which approximately equals p p
12 channel width or 1.6 meander belt width or 4.5 meander radius of curvature Meander belt width, which approximates 2.9 meander radius of curvature Length of a meander arc, which is related to the bankfull channel width and depth
These relative proportions of meander wavelength, radius of curvature, and meander belt width hold regardless of the stream size, whether a small stream only 2 meters across or the lower Mississippi River 1,000 meters across. Thus, artificial attempts to change a channel by narrowing it or straightening it will be met by the river’s attempts to return to a more natural equilibrium channel cross section and meander path.
Meandering streams flood in a typical way as a result of their patterns of erosion and deposition. Meanders erode outward and slowly migrate downstream. That process of erosion and deposition over a period of centuries gradually moves the river back and forth to erode a broad valley bottom. At high water, the flooding river spills out of its channel and over that broad area—its floodplain. As a stream spills over its floodplain, it moves from a deep, high-velocity channel to a shallow, broad, low-velocity floodplain. As water velocity slows at the edge of the deeper channel, sediment deposits to form a natural levee (p Figure 11-12). These features form a nearly continuous low ridge along the edge of the channel that may keep small floods within the channel. The floodplain, with its relatively slow moving water, is part of the overall river path; it carries a significant flow during floods. Think back to the vignette that opened this chapter, about the homeowner who built his house 10 meters from the river but was surprised to find the river one day right at his doorstep. He lacked two key pieces of information when he built his home at the outside of a meander bend: He hadn’t considered that meanders erode outward and move over time and that the rising water at spring runoff would speed up the erosion process.
Braided Streams Meander wavelength
Meander arc length
Meander belt width Radius of curvature
p
FIGURE 11-11. This diagram shows the relationship between meander characteristics.
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Braided streams do not meander but form broad, multichannel paths. These streams are overloaded with sediment, which they deposit in the stream channel, locally clogging it so that water must shift to one or both sides of the deposit. With further deposition, water shifts to form new channels. That behavior is promoted by dry climates in which little vegetation grows to protect slopes from erosion. Such braided streams are characterized by eroding banks, a steep gradient, and abundant stream bedload, which is the sediment carried along the stream bottom. With waning flow, bedload is deposited in the channel and flow diverges
Pipkin & Trent, 2001.
Floodplain
0
1
2 kilometers
Channel Levee
p
FIGURE 11-12. a. The main channel of a river has the coarsest gravels at the bottom, grading to finer grains above; the natural levees are still finer grains settled out in shallower water, and floodplain forms from very fine-grained muds that settled out from almost still water during floods. b. Natural levees with houses along a channel and floodplain in the Mississippi River delta.
Marli Miller photo.
a
a
p
FIGURE 11-13. a. This braided river channel of the Wairou River is southwest of Blenheim, New Zealand. b. The strikingly braided Tanana River near Fairbanks, Alaska.
David Hyndman photo.
Donald Hyndman photo.
b
around it, splitting into separate channels. Depending on flow, some channels are abandoned, while others temporarily become dominant (p Figure 11-13). Braided channels are characteristic of meltwater streams flowing from sediment-laden glaciers and of arid Basin and Range valleys in which heavy, intermittent rains from the mountains carry abundant sediment across valley alluvial fans. When a stream channel moves from an area of high slope to one of lower slope, sediment will generally deposit in an alluvial fan because the stream can no longer carry its full load (p Figure 11-14). Active alluvial fans are always marked by braided streams. Alluvial fans are also somewhat arched, with higher elevations along their midlines and with lower elevations toward the side edges because they are built by deposition of sediment flushed out of a mountain canyon at the apex of the fan (p Figure 11-15). Like other braided environments, most alluvial fans tend to be in dry climates and lack significant vegetation. People tend to build on alluvial fans because
Donald Hyndman photo.
b
p
FIGURE 11-14. Water runs off this steep pile of sand in heavy rains despite its high permeability. Overland flow erodes gullies and carries sediment down onto depositional fans. Note that water drains into the gullies on the eroding area but spreads out over wider areas on the depositional zone.
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297
p
FIGURE 11-15. Flood hazard zones vary on alluvial fans. All of the flow from a canyon concentrates at the apex of the fan and then spreads out, decreasing in depth and intensity downslope. At different times, though, the flow tends to concentrate on different parts of the fan as it finds the easiest path down the slope. Shown here: Copper Canyon, Death Valley.
(Small area with concentrated hazard)
High hazard
(Large area with spread-out hazard)
Moderate hazard
USGS photo.
Low hazard
A bedrock stream, develops when streams erode down to resistant bedrock. Sections of bedrock streams tend to abruptly steepen. These abrupt changes in gradient from gentle to steep are called knickpoints. Upstream, the gentle gradient has low energy for the river and may deposit sediment. Downstream, the steeper channel provides high energy, and sediment erodes. High-gradient bedrock channels generally have deep, narrow cross sections that carry turbulent and highly erosive flows during floods (p Figure 11-16a). Their high energy and turbulence allow them to transport all of the loose material in the channel. This can include large boulders, which impact and abrade the channel sides. Bedrock is resistant to erosion, so only major floods can scour or pluck fragments from the rock.
p
FIGURE 11-16. a.The turbulent, high-energy Colorado River in Grand Canyon, Arizona. b. Potholes in streambed of McDonald Creek, Glacier National Park.
a
Donald Hyndman photo.
Bedrock Streams
David Hyndman photo.
they are inviting areas, with gentle slopes at the base of steep, often rocky, mountainsides. They typically do not realize that the fan is an active area of stream deposition because the streams rarely flow. In fact, alluvial fans are particularly dangerous flooding areas. Torrential floods can wash out of the canyon above with little or no warning. They destroy, bury, or flush houses and cars downslope. Deaths are common, especially during rainstorms at night, when people do not hear the telltale rumble of an approaching flood.
b
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Floods in bedrock channels have excess energy that cannot be dissipated by changing channel roughness or by sediment transport. Extreme turbulence is typical, and large-scale vortexes or whirlpools can appear. These can be effective in swirling rocks on the bottom to drill potholes in the bedrock (p Figure 11-16b). Recreational rafting or boating at high water in such channels, although exciting, is especially hazardous because of the extreme turbulence and vortexes. Flotation devices can be insufficient to raise a person against the effect of a whirlpool.
Climate Controls on Stream Flow
Modified from USGS.
Because atmospheric moisture is the main source of water for floods, large floods generally depend upon large areas of humid air. Water stored in snow and ice is a secondary reservoir. Evaporation from oceans, and secondarily from continents, constantly resupplies the atmosphere. Because warm air can hold more water vapor, the atmosphere at low latitudes contains much more moisture. More than 60 percent of water that has evaporated into the atmosphere is between the equator and 30 degrees north or south. Even with global air circulation, tropical air contains roughly ten times more moisture than polar air. In tropical regions, temperature and pressure gradients are weak, as are regional winds. Rising air, often resulting from localized heating, causes condensation, formation of towering convection clouds, and frequent thunderstorms. Rainfall rates are high but do not last long. In winter, the area of tropical moisture reaches only as far north as Mexico City and Miami. In summer, it pushes north into southern California and Arizona. East of the Rockies, the tropical moisture loops north through the High Plains into north-central and southeastern Canada, as well as all of the eastern United States. Tropical cyclones, including hurricanes, move westward in the belt of trade winds. In the Atlantic Ocean, they drift westward and then northward and eastward along the eastern fringe of the United States, where they interact with midlatitude frontal systems. Reaching the Atlantic coastal plain, Hurricanes Camille in 1969 and Agnes in 1972, for example,
caused significant flooding, with 25 to 50 centimeters of rain in many areas. Especially heavy rainfall will cause floods almost anywhere. In most cases, such heavy rainfall accompanies thunderstorms, which last for a few minutes or an hour or two. A line of thunderstorms may prolong the deluge for several hours, whereas tropical cyclones may stretch out the rains for several days. Areas such as Southeast Asia, which have seasonal monsoons, may experience extreme rains for several weeks or even months. The maximum amounts of rain recorded for various periods of time are truly amazing (see Table 10-1). Most of these result from an extraordinary weather system such as a hurricane, coupled with the orographic effect of a system rising against a mountain range. The maximum recorded amounts are at Cherrapunji in northeastern India, where monsoon-driven winds, laden with moisture from the Bay of Bengal, rise against the eastern Himalayas. Imagine more than a meter of rainwater trying to flow off the ground surface all at once! Floods generally occur after prolonged soaking of the ground by either rainfall or snowmelt, when the nearsurface soil is water saturated. Only during uncommon torrential rainstorms will water run directly off the surface to streams. Except in arid regions, groundwater flows out to streams, keeping many of them flowing year-round (p Figure 11-17a). Stream flow in more-humid regions is high during wet weather periods and low during dry weather periods. Groundwater in such areas does not respond rapidly to changes in rainfall, and inflow to streams from groundwater is slow and relatively constant. Thus, wet climates tend to provide streams that flow year-round and flood in prolonged heavy rains. In northern latitudes, they commonly flood during the spring snowmelt period. With low annual rainfall, little or no vegetation can grow to soften the impact of raindrops and slow their infiltration. Rain falls directly on the ground, packing it tightly to permit less infiltration and cause more surface runoff. The rain also kicks up sediment from the surface, permitting it to be carried off by streams. During dry seasons, less water gets into the ground, even during the less frequent rainstorms, so less of it feeds the groundwater (Figure 11-17b). Dry climates with no year-round streams can see flash floods after any major or prolonged rainfall.
Gaining stream (wet climate)
Losing stream (dry climate)
Flow
Flow
Water table
Water table
p
FIGURE 11-17. Groundwater flow directions depend on climate.
Unsaturated zone
Aquifer a
b STREAMS AND FLOOD PROCESSES
299
Streams in desert basins generally flow only during and shortly after a rainstorm but then dry up until the next rainstorm. Because more sediment is supplied to the dwindling amount of water, sediment deposits in the gullies.This progressively chokes the flow, causing some of the water to spill over and follow another path. This gully in turn fills with sediment, and so on. The result is a braided alluvial fan that continuously deposits sediment that builds with time. Flood behavior partly depends on surface water interaction with groundwater. Streams in all but arid regions are fed mostly by groundwater. In areas of moderate to high annual rainfall, groundwater levels stand higher than most streams and thus continuously feed them. During low-water periods, the stream surface is generally at the groundwater surface, which is the exposed water table (see Figure 11-17). In such areas of gaining streams, the rate at which water flows from groundwater into a stream depends upon both the slope of the water table and the ease of flow through the water-saturated sediments or rocks. Storm discharge includes this groundwater flow as well as any overland flow. New groundwater does not need to flow all the way from its infiltration source during a storm to a river but merely has to raise the water table and displace “old water” into the stream. In semi-arid to arid regions, however, losing streams lose water into the ground and often dry up between storms. When they do flow, they drain water into the ground and therefore raise the water table.
The bankfull channel width appears to have a simple relationship to bankfull stream discharge over a large range of channel sizes, so you can infer the approximate discharge of a stream just below its flood level by measuring the average bankfull channel width. Channel widths of 6, 12, and 30 meters generally carry, respectively, bankfull discharges of approximately 0.5–1, 14–28, and 70–250 cubic meters per second. This relationship between channel form and discharge seems appropriate for some streams, especially those in humid areas that produce mostly fine-grained sediment. Larger floods fill the channel and spill out over its floodplain (p Figure 11-18).This relationship does not seem to hold for semiarid environments with gaining streams. In these environments, normal flows and even modest floods in the following few years typically fail to restore the “damage” to the channel. It may take many years of smaller floods to restore the channel to something resembling its pre–major flood form. An increase in discharge during a flood involves an increase in water velocity, water depth, and sometimes width of the stream. Streams in humid regions adjust their channels to carry the typical annual flows that fill them. Channels in semiarid regions adjust their channels to less frequent large floods because smaller flows do not significantly affect channel shapes.
Low flow: 95% of time
Flooding Processes Rivers and streams collect water when rain soaks into the ground and percolates down through the soil to groundwater and then to the streams. The amount of precipitation on the land surface varies from year to year. Some of that water evaporates from the ground and vegetation surfaces, some is taken up by vegetation, and some soaks into the soil to eventually reach rivers from groundwater. During torrential rainfall, some may flow across the ground as surface runoff directly into streams. Floods are most commonly initiated by especially heavy or prolonged rain but can also form by rapid snowmelt, rain on frozen ground, or ice-dam formation when river ice breaks up.
Mean annual flow: 30% of time
Bankfull flow: 2 times in 3 years on average
Moderate flood: every 10 years on average
Bankfull Channel Width, Depth, and Capacity The bankfull level of a stream is the level at which the water spills over the banks. Every 1.5 to 3 years, streams generally reach bankfull. Large rivers can have large flows without being above flood level. Small streams can flood with fairly small flows. It is not simply the size of the flow that makes a flood, but how unusual such a flow is for that particular channel.
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Floodplain
Channel erosion during bankfull stage
Channel erosion during moderate flood
p
FIGURE 11-18. Generalized cross sections of a stream channel at various flows.
rated with water, large floods occurred (Case in Point: “Major Flooding from a Minor Hurricane—Hurricane Agnes, June 1972,” p. 316). The largest floods in cold-climate regions are generated during rainstorms, just as they are in warmer climates. Regions north of the influence of warm, moist, tropical airflow are affected by snowpack on the ground for more than a month in most years. Floods in these areas can result from melting snow or glaciers, a rain event, or a combination. At high elevations, much of the winter precipitation falls as snow. This stores the water at the surface until snowmelt in the spring. If the snowpack melts gradually, much of the water can soak into the ground. If it melts rapidly, either with prolonged high temperatures or especially with heavy warm rain, large volumes of water will flow off the surface directly into streams. When the snowpack warms to 0˚C, a large proportion of meltwater remains in the pore spaces and the snowpack is “ripe.” Water draining through the pack concentrates in channels at the base of the snow. The largest floods develop when heavy warm rain accelerates melting of a ripe snowpack. Water has a large heat capacity, so dispersal of warm rainwater effectively melts snow. Under these conditions, or if the ground is already saturated with snowmelt, water from a rainstorm may entirely flow off the surface. If the ground is frozen, as is sometimes the case with minimal snowpack, heavy rains quickly run off the surface because the water cannot soak into the ground. During winter and early spring, ice coating rivers can block stream channels; as the ice begins to melt, it breaks up and moves. A sudden warm spell that causes ice to break up can dam the channel at constrictions such as at bridges (p Figure 11-19). Large rivers flowing northward have the additional problem that the ice upstream thaws before that downstream. As upstream meltwater flows north, it encounters ice jams and restricts downstream flow, causing floods (Case in Point: “Spring Thaw from the South on a NorthFlowing River—The Red River, North Dakota,” p. 317).
Because rainfall depends on moisture in the atmosphere, heavy rainfall tends to develop where moist tropical or subtropical air over an ocean moves onto land or rises against a coastal mountain range. Or the moist air mass may collide with a cold front, where it rises and condenses. In North America, the Gulf Coast and southern Atlantic coast have those characteristics. On the west coast from northern California through southern British Columbia, the westerly winds bring moist Pacific air eastward to collide with coastal mountain ranges, where these air masses rise and shed their rain. Coastal parts of southern California and adjacent Mexico, in the belt of trade winds, which blow from dry land areas out over the ocean, are generally very dry. During El Niño, however, the trade winds weaken, and moist Pacific air comes ashore. California then sees repeated heavy rainstorms that cause floods and landslides. The intensity of precipitation plays a significant role in the rate of runoff to streams and, in turn, floods. Light precipitation can generally be absorbed into the soil without surface runoff. Heavy precipitation can overwhelm the nearsurface permeability of soils, leading to rapid runoff over the surface, a process called overland flow. Even where the permeability of the soil is relatively high, heavy rainfall, especially over a long period or in multiple storms, can saturate near-surface sediments, thus forcing the water to rapidly flow over the surface to streams. The part of the water that soaks into the soil will raise the local groundwater level; that adds pressure to the groundwater and forces more water back out to the surface downslope into the stream. Rapid flood peaks during large storms are most common in areas with fine-grained soils or desert soils, especially tight clay hardpan or soils with a shallow, nearly impervious calcium carbonate–rich layer. The same is true of areas with near-surface bedrock or shallow groundwater that has little capacity to absorb rainfall (Case in Point: “Heavy Rainfall on Near-Surface Bedrock Triggers Flooding—Guadalupe River Upstream of New Braunfels, Texas, 2002,” p. 315). At the other extreme, decomposed granite soils dominated by coarse sand have high permeability and high infiltration capacity, which can cause stream levels to rise rapidly. Flat areas that contain many depressions can temporarily store enough water to delay runoff.
Floods on Water-Saturated or Frozen Ground Floods are common when heavy rain falls on ground that is either saturated from earlier storms or frozen. In both of these cases, most of the water that reaches the ground surface runs off directly to the stream. For example, when Hurricane Agnes dumped 5 to 7 centimeters of rain in Pennsylvania in June 1972 on ground that was already satu-
New Hampshire Bureau of Emergency Management photo.
Precipitation Intensity and Surface Runoff
p
FIGURE 11-19. An ice jam built up at a river constriction threatens a bridge at Gorham, New Hampshire. The power shovel tries to get the river flowing again.
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Flood Intensity
Stream Order The number of tributaries of a stream (its stream order) has a significant effect on the rate of rise of floodwaters during and following a storm. Small streams that lack tributaries are designated first-order streams (p Figure 11-21). Firstorder streams join to form second-order streams; secondorder streams join to form third-order streams; and so on.
Stream discharge (m3/sec)
500
400
Flood in paved-over urban area
300 Typical flood 200
100
2 1 1 3 1 3 2 34
1
3 5
Modified from NASA basemep.
The destructive effect of a flood depends primarily on its intensity. The intensity of a flood can be measured by the discharge of floodwater and the rate of rise of water. Flood intensity varies over time according to the rate of runoff, the shape of the channel, distance downstream, and the number of tributaries it has. For example, floods in small, narrowly confined drainage basins are typically much more violent than those along major rivers such as the Mississippi. We use a hydrograph to represent the intensity of flooding as a plot of the volume of water flowing in a stream over a period of time. A typical flood hydrograph rises steeply to the flood crest, where the flood reaches its peak discharge, and then falls more gently (p Figure 11-20). Anything that causes more rapid transfer of water to a stream will heighten and steepen the hydrograph. In areas where the rainfall saturates the soil and is forced to run over the surface to rapidly feed streams, the hydrograph peaks much more quickly and rises to a greater maximum discharge. Such rapid large runoff can develop in urban areas with large areas of pavement, houses, and storm sewer systems, or in natural areas that have been deforested. Deforestation can increase the volume of storm runoff by roughly 10 percent.
Elevation (m) 250
500
750
1000
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FIGURE 11-21. Stream order: First-order streams have no tributaries but join to form a second-order stream, and so on.
Low-order streams tend to respond rapidly to storms with steep hydrographs because water has to travel only a short distance to the stream. Such streams provide less flood warning time for downstream residents. They have smaller drainage basins and carry coarser and larger amounts of sediment for a given area. A storm in a headwaters area may cause flooding in several first-order streams. As the flood crest of each moves downstream, the length of time it takes to reach the secondorder stream varies, so each first-order flood crest arrives at somewhat different times. The flood peak for the secondorder stream will therefore begin later and be spread over a longer period. The same goes for several second-order streams coming together in a third-order stream; its flood peak will again begin later and be spread over a still longer period. Thus, high-order streams with numerous tributaries have longer lag times between a storm and a downstream flood; their hydrographs are less peaked and cover longer time periods. Flood warning time for downstream residents is longer.
Flood Crests Move Downstream 0
0
1
2
3
4
5
6
7
Time (days)
p
FIGURE 11-20. This hydrograph is a plot of stream discharge versus time for a similar eighteen-hour rainfall event for the same area before and after urbanization. Note that the area under the two curves is similar, that is, approximately the same total volume of water for both floods. Actually, because less water infiltrates, the flood volume after urbanization will be a little larger.
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Even with local intense rainstorms on a small drainage basin, there will be a lag between a storm and the resulting flood peak. A torrential downpour may last for only ten minutes, but it takes time for the water to saturate the surface layers of soil and to percolate down to the water table. More time is required for overland flow to collect in small gullies and for water in those gullies to flow down to a stream. In turn, it takes time for the water in small streams to combine
crest is still lower (location C). In cases where the precipitation occurs over the whole drainage basin, flows will increase downstream. For flood hazards, we are normally more concerned with the maximum height of the flood crest than when the first water arrives from a storm. The flood crest moves downstream more slowly than the leading water in the flood wave. Note in Figure 11-22b that the first rise in floodwater from the storm appears downstream later at locations A, B, and C. Note that the flood crest or peak discharge also takes longer to move downstream. In general, the flood crest or flood wave moves downstream at roughly half the speed of the average water velocity.
and cause flooding in a larger stream. The length of the lag time depends on many factors, including slope steepness, basin area and shape, spacing of the drainage channels, vegetation cover, soil permeability, and land use. Flood intensity depends on similar characteristics. If a storm occurs only in an upstream portion of a watershed, the peak height of the hydrograph will be lower and the flood duration will be longer farther downstream from the storm area (p Figure 11-22). At the downstream edge of the main rainfall area, water levels rise as water flows in from slopes, tributaries, and upstream. The level often continues to rise until about the time the rainfall event stops (location A in Figure 11-22b). At that time, water levels on the upstream slopes begin to fall, and flood level at that point in the stream peaks and begins to fall. Farther downstream, some of the earlier rainfall has already begun to raise the water level. Water continues to arrive from upstream, but the size of the stream channel is larger downstream because it is adjusted to carry the flow from all upstream tributaries. Thus, the flood flow fills a wider cross section to shallower depth (location B). Even farther downstream, the channel is still larger and the flood
Flash Floods
Rates of precipitation and discharge
Any of these conditions increase the likelihood of a flash flood, which comes on suddenly with little warning. Any type of flood can be dangerous, but flash floods are especially so because they often appear unexpectedly, and water levels rise rapidly (Case in Point: “A Flash Flood from an Afternoon Thunderstorm—Big Thompson Canyon, Northwest
Precipitation
Discharge at A
Discharge at B Discharge at C
Time (hour) Lag time
Christopher Magirl photo.
Modified from Luna Leopold.
Area of storm
a
p
A B C
b
FIGURE 11-22. a. Localized afternoon rainfall over Tucson, Arizona. b. Storm rainfall entering the stream precedes the flood crest that it causes. The flood hydrograph nearest the rainfall area is highest and narrowest. Farther downstream at location B, the flood hydrograph crests at a lower level but lasts longer. Still farther downstream at C, the flood crests at an even lower level and lasts longer.
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Relative Flash Flood Hazard >0.8
Modified from Beard, 1975.
N
of Denver,” p. 319). A map of flash-flood hazard tendency for the United States shows that high flash-flood danger areas are primarily in the semiarid Southwest—southern California to western Arizona and West Texas (p Figure 11-23). Moderate flash-flood dangers exist in areas such as the eastern Rocky Mountains, the Dakotas, western Nebraska, eastern Colorado, New Mexico, and central Texas. This is not to say that other areas are not prone to flooding; rather, the floods there tend to be less extreme compared with the normal stream flows. Deaths are frequent in flash floods because of little warning and because of their violence. Even under a clear blue sky, floodwaters may rush down a channel from a distant storm. On many occasions, people have been caught in a narrow, dry gorge because they were not aware of a storm far upstream. At night, people in their homes have been swept away.
Flood Frequency and Recurrence Intervals Flood frequency is commonly recorded as a recurrence interval, the average time between floods of a given size. Larger flood discharges on a given stream have longer recurrence intervals between floods.
100-Year Floods and Floodplains A 100-year flood is used by the U.S. Federal Emergency Management Agency (FEMA) to establish regulations for building near streams. A 100-year flood has a 1-percent chance of happening in any single year, although it also has a 1-percent chance of happening in the year following a similar-magnitude event. A 100-year floodplain is the area
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0.7-0.8 0.6-0.7 0.5-0.6 0.4-0.5 0.3-0.4 0.2-0.3 3.5
Jocelyn Augustino photo, FEMA.
Modified from USGS and U.S. Army Corps of Engineers.
is
b
Pumping station
Breach direction
Main water plant
a
p
Donald Hyndman photo.
Peter Nicholson photo, NSF.
FIGURE 14-32. a. This map of New Orleans for September 2, 2005, shows shallow flood depths on the natural levee areas at the north bank of the Mississippi River, increasing to 3 or 3.5 meters over much of the city. The flood area abruptly ends on the west at the 17th Street Canal; levees breached to the east. b. Almost all of the city lies below the average annual high-water level of the river and below the level of Lake Pontchartrain, so these giant pumps are used to drain the city.
a
b
p
Donald Hyndman photo.
Donald Hyndman photo.
FIGURE 14-33. a. This sheetpile floodwall catastrophically failed during the storm surge from Hurricane Katrina. b. This canal wall in the Gentilly area of New Orleans collapsed outward from the canal. The Corps of Engineers erected the corrugated steel wall as a temporary barrier to further flooding.
a
p
b
FIGURE 14-34. a. Modest houses in the Lower 9th Ward were floated off their cinder-block posts and deposited on cars, other houses, or streets. b. This brick home on a thick concrete slab in the Chalmette area at the east edge of New Orleans floated in the surge and was carried five blocks to be deposited in the middle of a residential street.
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Les Harder photo.
Donald Hyndman photo.
(continued)
a
b
p
FIGURE 14-35. a. Home in the Chalmette area at the east edge of New Orleans was trashed by Katrina. b. Three people died in this 9th Ward home during the hurricane. Many waited for rescue on rooftops.
and Alabama to the Florida panhandle. Because of power failures, destruction of base stations, and breaks in lines, telephones and cell phones wouldn’t work. The lack of communication greatly hampered rescue efforts. Broadcasts from reporters in nearby cities and communication over the Internet became important. Ten major hospitals were forced onto backup power. The water came in so fast that within minutes it was over people’s knees, forcing them up to second floors and attics or onto roofs. A few of those who retreated into attics thought to grab an axe or a saw so they could break through the roof if the water continued to rise. They waited there, sometimes for days, but had no means to contact authorities or rescuers. Searchers rescued at least 1,500 people from roof-
tops and heard others beating on roofs from inside attics. Rescuers doing careful house-to-house searches found a few bodies, but fortunately not many (p Figure 14–35). Engineers dammed the breached 17th Street Canal with steel sheet pilings 15 meters long to stop the flow of more water from Lake Pontchartrain through the breach into the city (p Figure 14-36). It took two weeks to fill the breaches, using helicopters to drop giant sandbags. One plan was to deliberately breach some lower levees to let water drain back out and to use smaller replacement pumps that were available. Contamination, Disease, and Mold The floodwaters were littered with pieces of houses, old tires, garbage cans, all
p
manner of trash, sewage, coatings of oil and gasoline from ruptured tanks, and even bodies. A few people were fortunate to grab hold of larger pieces of trash or drifting boats to stay afloat. Many of those rescued had problems related to the polluted water, especially gastrointestinal illnesses, dehydration, and skin infections. Aircraft sprayed pesticides to kill mosquitoes because of the danger of malaria, West Nile virus, and St. Louis encephalitis. Even after September 16, some neighborhoods were still flooded (p Figure 14-37). Mold grew in most buildings in contact with the warm, contaminated water (see Figure 14-37b). Despite being structurally sound, many buildings were so affected that they have to be bulldozed. As the water receded, dark gray muck coated everything. In French Quarter buildings on
U.S. Army Corps of Engineers photo.
FIGURE 14-36. The 17th Street Canal crosses much of New Orleans south from Lake Pontchartrain in the foreground. The breach and flooded homes are visible on the far side of the canal to the right of the bridge. The Corps of Engineers are driving pilings at the bridge to block water flowing down the canal from the lake while they also drop giant sandbags into the breach.
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Debbie Randolph photo.
Marvin Nauman photo, FEMA.
(continued)
a
p
b
FIGURE 14-37. a. Some neighborhoods were still flooded weeks after the storm. Many already damaged homes floated off their foundations to collide with other homes. Fetid water was everywhere. b. Mold growing on the ceiling and walls of a home in the Lakeview District of New Orleans, submerged by the flood for about two weeks.
the natural levee of the Mississippi, mold was growing a month later in rooms that were not flooded but were still without airconditioning. Wallboard, insulation, rugs, bedding, and almost anything else that had gotten wet had to be discarded. Even the bare studs of walls needed to be sanded, disinfected with bleach, and then dried with fans. Any wood frame structure standing in water for more than two or three weeks had to be demolished because mold was impossible to remove from deep in the wood. The effects of mold on people with allergies, asthma, or weak immune systems can be serious. Relief Came Slowly, Many Victims Died After Katrina, people waited for four days before anyone brought food, water, medical supplies, or vehicles for evacuation. Clearly emergency response to the catastrophe was a dismal failure. Day after day, federal Homeland Security and FEMA officials promised National Guard troops, supplies, and buses for evacuation—help that rarely materialized. Distribution of aid for disasters is logistically complex, but giant building materials and grocery companies have become very efficient at distribution on a global scale. Since the Bam, Iran, earthquake disaster of 2003, aid organizations such as the International Red Cross have used those techniques to distribute aid quickly
to victims of major disasters. FEMA presumably could have used such procedures. As of August 2006, confirmed deaths totaled 1,723, including 1,464 in Louisiana and 238 in Mississippi. Thirty-nine percent of the people who died were more than 75 years old. As water rapidly rose, people were swept away by surge flow and drowned because they couldn’t swim, or they drowned in their houses after retreating to a higher floor or an attic and becoming trapped. Five people reportedly died from illness caused by bacteria related to cholera. Some patients in hospitals died when electricity necessary to power hospital equipment such as respirators and dialysis machines failed and backup generators ran out of fuel; doctors and nurses squeezed hand-held ventilators for patients who couldn’t breathe on their own. There was no running water or ventilation; seriously ill patients died in the 41°C (106°F) heat. Thirty-four nursing-home patients died in the flood; the owners said they never received the mandatory evacuation order and that relocating would have killed some of the frail patients. Some hospitals were so damaged by flooding and mold that they will never reopen. Truly, anything that could go wrong did go wrong. These should be prominent lessons for any future potential disaster.
Impacts Farther South and East: The Hurricane Winds, Surge, and Waves News media focused on the storm damage in New Orleans because its flooding and destruction was so dramatic and catastrophic. However, elsewhere downriver and along the coast to the east, the storm’s effects were no less disastrous. Southeast of New Orleans, near the main dredged Mississippi shipping channel, the high winds, huge surge, and waves floated houses like matchboxes, dropping them on roads or on other houses. Some buildings were moved with their concrete foundation slabs still attached. The surge lifted shrimp boats of all sizes and dumped them onto nearby levees and roads (p Figure 14-38). Nests of poisonous water moccasins and other snakes swept in from the bayous added to the dangers. Rotting animal carcasses were scattered near the roads. Northeast of New Orleans, the high surge and waves from Katrina lifted segments of a 10-kilometer-long Interstate 10 causeway and dropped them into Lake Pontchartrain. The U.S. 90 causeway, across St. Louis Bay (p Figure 14-39) and the east end of the Back Bay of Biloxi, Mississippi, collapsed as a similar series of tilted road panels. Katrina’s eye tracked almost due north, making landfall on August 29 at 10 a.m. at the border of Louisiana and Mississippi, causing collapse of an apartment complex
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(continued)
p
U.S. Army Corps of Engineers photo.
FIGURE 14-38. A pair of 100-foot-long oil service vessels from the Mississippi River shipping channel ended up on Highway 23.
p
John Fleck photo, FEMA.
FIGURE 14-39. The U.S. 90 bridge across St. Louis Bay, near the western end of the Mississippi coast, collapsed in the massive surge and giant waves as Katrina arrived. The surge and waves must have lifted the bridge deck segments and then dropped them either onto their supports or into the bay.
and killing dozens of people. Some people survived the fast-rising surge by climbing into treetops. Given its counterclockwise rotation, the strongest onshore winds, as high as 224 kilometers per hour at the deadly eastern edge of the eyewall, hammered the Mississippi coast near Bay St. Louis, where the surge reached its highest level of about 9 meters, the height of a three-story building. On the mainland the surge, extreme winds, and high waves crushed houses, toppled trees, and severed power lines; transformers exploded, and sailboats broke
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loose and were thrown across the coastal highway. Cars were scattered like toys. A few homes, specially built to withstand major storms, survived even where all of the neighboring houses were obliterated. Buildings all along the beachfront from the border of Louisiana, through Mississippi from Bay St. Louis, Gulfport, and Biloxi to Pascagoula, were leveled, leaving only the tall posts and concrete pads to show where they once stood. Many beachfront houses were washed out to sea or undermined and toppled; others over large areas were reduced to kindling,
even as far as 2 kilometers from the beach (p Figure 14-40). In Gulfport, the surge rose 3 meters in a half hour; fierce winds tore the roofs off eight schools that were being used as shelters, and a hospital was heavily damaged, as were two huge casinos. In Biloxi, seven giant casinos, floating just offshore, were wrecked, including the state’s largest; it was carried more than a kilometer inland. The effect of the storm surge on houses in Gulfport is clear where the lumber that once made up the houses was swept up and stacked against the remaining heavily damaged houses. Buildings lifted from their foundations and slammed into nearby buildings are clear evidence of storm surge damage, as are huge piles of building debris banked up against one side of buildings with none on the downcurrent side. In Alabama, a huge oil-drilling platform moored at a shipyard floated away and slammed into a suspension bridge across the Mobile River. Downtown Mobile saw severe flooding, not only from the surge but from heavy rainfall as the storm moved north. Following damage from each previous storm in coastal Mississippi and Alabama, developers took advantage of the destruction to build larger and more expensive structures right to the edge of the beach. Although many of the high-rise hotels survived the storm, some of the beaches that attracted them and on which they were built disappeared, as did most homes behind them. Predictions, Preparation, and Response In 2004 FEMA prepared a simulation of a major flood in New Orleans. The results, although unfinished, were unnervingly accurate: The simulation left much of the city under 3.5 meters of water, and transportation would be a major problem. Batteries in emergency radios used by the mayor’s staff, police, and firefighters would quickly drain and could not be recharged because the power was out. These were unlike radios used by teams fighting wildfires, which can be powered by ordinary disposable batteries. A big contributor to the poor response to this disaster was lack of coordination
U.S. Navy photo.
(continued)
p
FIGURE 14-40. a. A nearly new subdivision in a coastal area of Gulfport, Mississippi, was leveled, leaving its remains piled up against the battered houses farther inland. b. Two-story apartment buildings were lifted from their foundations to crash into adjacent buildings.
U.S. Navy photo.
a
b
among government groups with different responsibilities. Clear-cut lines of authority and communication were not in place. In some cases, the head of an agency said to proceed with a plan, but lower-level employees wanted signed papers to protect themselves from later criticism. Some governmental organizations were afraid of being sued if they stepped beyond their authority or made a mistake. Different federal agencies, such as FEMA, charged with separate duties, communicated poorly with one another. Some did nothing because it was “not part of their jurisdiction.” Interagency squabbles on the federal level and quarrels among federal, state, and local governments appeared to involve protecting each organization’s turf. Much of FEMA’s problems originated when the federal government reduced the funding for FEMA and relegated it to a role of responding to disasters instead of preparing for or preventing them. It couldn’t even do that; it tragically neglected to stage adequate water, food, medical supplies, and transportation nearby in preparation for post-storm response. Individual medical and emergency organizations were tragically underprepared. FEMA, charged with handling response to disasters, proved tragically unprepared and inept. Five hours after landfall, FEMA’s director decided to send 1,000 federal employees to deal with the storm’s effects, but supplies were very slow to arrive. With thousands of people sheltered by the Red
Cross in the Convention Center, FEMA said it had no “factual knowledge” of its use as a shelter until September 1. Arranging temporary housing for an estimated 300,000 displaced people in the wake of Katrina was an immediate and enormous task. By September 4, 220,000 refugees were sheltered in Houston, San Antonio, Dallas, and other cities across the country. Outside New Orleans, FEMA provided army-style wood-frame tents and some travel trailers and mobile homes but insisted that before the tens of thousands of trailers could be moved, the sites that would receive them had to have water, sewer, and electricity hooked up, services not available in many areas for months. In most of the city, even a month after the hurricane, power lines still dangled, tree branches and other debris still clogged the streets, and no stores or gas stations had reopened. The Future of New Orleans? An important question is whether New Orleans should be rebuilt in essentially its previous form. Two years later, little had been accomplished in most of the city. Should people be permitted to rebuild in a huge sinking depression several meters below sea level and below the Mississippi River, or should aid for reconstruction come with the requirement that any new homes be situated above sea level and outside the floodplain? The latter was one of FEMA’s main requirements for people and companies seeking funds for rebuild-
ing structures along rivers; this rule was put into effect after the disastrous 1993 upper Mississippi River flood. Certainly the higher-elevation areas of New Orleans, those on the natural levees of the Mississippi River, should be restored. These areas provide the shipping and industrial facilities that serve not only the Mississippi River basin but much of the rest of the country. Some port facilities could be moved upriver about 150 kilometers to Baton Rouge, which is also a dredged deepwater port. The prospect of a permanent relocation of much of New Orleans’ population to Baton Rouge brings to mind the migration of people and businesses from Galveston, Texas, to the then small town of Houston after the disastrous 1900 Galveston hurricane. Most of those people never returned to Galveston. New Orleans’ natural-levee areas, including the lightly damaged, famous French Quarter, also make up the cultural and historical part of the city frequented by tourists, who provide a large portion of the city’s income. Even in areas that should be rebuilt, where do you start? Months after the storm, large devastated areas had no gas stations; no open grocery, hardware, or building-supply stores; no schools; and no funds from property or sales taxes with which to pay the city or parish employees needed to repair roads and utilities. For people with jobs in rebuilding or at restarted refineries, where will they live, get groceries and gas, or find schools for their children? Without people
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(continued) in the area, there are no jobs; without jobs, people cannot return. Katrina was the costliest natural disaster to strike North America to date. Insured costs reached $60 billion as of August 2006; federal government appropriations reached $71 billion within three months following the storm. Total costs reached $125 billion as of 2006, and some estimates suggest they may top $200 billion, including payments to businesses and individuals, but the federal government seems to be backing off on initial promises. After several months, FEMA had not allocated most of the federally appropriated funds. The Corps of Engineers plans to repair 60 kilometers of the 480-kilometer levee system to withstand a Category 3 storm. Improved sections will be 5.2 meters high
rather than the previous 3.8 meters high. Rebuilding the system to withstand a Category 5 storm would cost more than $32 billion—that is $66,000 for each of the 485,000 original residents, $264,000 per family of four, much more than that for the many fewer who are expected to return! Hurricanes have affected New Orleans before. Betsy, a Category 3 hurricane, submerged almost half of New Orleans in 1965; some places were under as much as 6 meters of water. The storm left 60,000 people homeless. Congress then authorized a gigantic construction project to raise the levees and link them to those of the Mississippi River to prevent such flooding ever again. As the city and its levees continue to sink, it will be just a matter of time before the next catastrophic flood.
Some of those levees along Lake Pontchartrain were built years ago by local governments or private groups and were not well engineered. Following a disastrous Mississippi River flood in 1927, the levees were built higher and strengthened. Additional levees were constructed in the 1940s and 1950s, and shipping canal walls were added in the 1960s. Unfortunately, the city on the floodplain continues to slowly sink, as groundwater is withdrawn for municipal and industrial uses, and buildings continue to compress the underlying peat. A total of 148 giant pumps remove water from the spongy sediments. Now much of the city lies 3 meters below Lake Pontchartrain’s normal level and 4 meters below the Mississippi River—not a good place for hundreds of thousands of people to live.
Case in Point Trapped on a Barrier Island Galveston Hurricane, 1900 u
In 1900, the science of meteorology was in its infancy. Weather forecasters in Galveston, Texas, were crudely tracking the hurricane as it moved northwest over Cuba to south of Louisiana. Because there were no planes or satellites, much of the tracking was provided by radio reports from ships at sea. They expected that it would swing northeast to move up the eastern United States, as most hurricanes did. However, a stationary high-pressure cell over Florida forced it to swing west instead—toward Galveston, which was a prosperous center of shipping and resort activity. Huge 4.5-meter-high sand dunes along the edge of the city had been removed to provide better beach access. The highest part of the island was only 2.5 meters above sea level.
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The local weather bureau had been receiving storm warnings from headquarters in Washington, D.C., for a couple of days, but the first sign of a change was a heavy swell from the southeast, beginning in the afternoon of September 7. By 4 a.m. on the morning of September 8, an especially high tide flooded the low parts of town, and skies began to darken, winds strengthened, heavy rain fell, and the barometer dropped. Water rose to cover bridges to the mainland, and the rough weather precluded using boats. By the time people realized that it was a hurricane, they were trapped on the island. Houses crumbled in the waves; roofing slates, whole roofs, and pieces of houses flew through the air, and debris filled the streets. Injured people waded and climbed
through debris to reach sturdy buildings on the highest ground. Those in sturdy buildings retreated to the upper floors as the water rose. An estimated 1,000 people survived in the elegant Tremont Hotel. Early on the morning of September 7, the steamship Pensacola left Galveston harbor, heading east for Pensacola,
(continued) Florida. Captain Simmons was not aware that storm warnings were issued for Galveston Island 30 minutes after departure. Shortly after leaving the harbor, he ran into high seas and winds but continued east. That evening he dropped anchor, but the chain broke in the huge waves, so the ship was adrift for all of the next day. When Simmons finally regained control of the ship, he headed back, reaching Galveston on September 9, only to find battered ships and thousands of dead bodies. The hurricane, a Category 4 storm upon landfall with winds up to 193 kilometers per hour during a high tide, created a 6-meter storm surge (p Figure 14-41). It covered the offshore barrier island on which the city is built with 3 to 6 meters of water. Waves and winds destroyed boats, bridges, more than half of the wooden buildings, and even some brick ones. The only thing that saved some of the remaining business district six blocks back from the beach was a wall of debris from the shattered buildings closer to the beach, the remains of 3,600 houses. Thousands evacuated before the storm, but 8,000 to 12,000 died, mostly in the storm surge. Survivors marooned with the loss of their only bridge to the mainland lacked water,
food, medical supplies, and electricity. The third of the city closest to the Gulf completely disappeared. Innumerable bodies were found in the jumbled debris; 70 a day turned up for a month. Survivors doused the bodies with oil and burned them on the spot because they could not be buried in the water-saturated ground. Looting was rampant until martial law was imposed five days later. To provide protection from future hurricanes, Galveston and the U.S. Army Corps of Engineers raised the island surface by some 3 meters and built a giant seawall along the beachfront of the city (see Figure 13-15, p. 367). The seawall has grown in length over the years and is now 16 kilometers long. As would be expected, the beach in front of the wall largely disappeared. Galveston regularly dumps truckloads of sand over the seawall to provide a narrow beach. West of the west end of the seawall, rapid development in the 1970s permitted homes to be built on the beach at an average elevation of only 1.5 meters. Hurricane Alicia, a Category 3 storm that arrived in 1983, destroyed or severely damaged 99 of 207 homes on the beach. Although Texas law claims all land seaward of the vegetation
line, lawsuits and pressure from real estate groups led the state and county to relent and provide building permits to repair and reoccupy damaged homes. Permits have also been provided for additional new homes on the beach near the west end of Galveston Island, so that there are now 100 or so more beachfront homes than before Hurricane Alicia. New developments on the west end of Galveston Island are now as vulnerable as Galveston was before the 1900 hurricane. A storm equivalent to that of 1900 would likely bring a storm surge 4.6 meters high at the seawall and 7.6 meters at the north (inland) edge of Galveston Island. Seawater would be 1.5 to 3 meters deep in buildings in the center of town. West of the seawall, all 300 beachfront homes would be destroyed, their wreckage becoming battering rams that would destroy homes landward of them. It would take almost 36 hours to evacuate those at risk. The population of coastal counties in the area was 1.6 million in 1961; it is now almost 5 million, a large proportion being new residents who have no experience with hurricanes. A similar storm today would be expected to cause over $100 billion in damages.
p
State of Texas photo.
FIGURE 14-41. The 1900 Galveston hurricane piled up debris from buildings closer to the coast into a huge ridge that partly protected those behind.
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Case in Point Back-to-Back Hurricanes Amplify Flooding Hurricanes Dennis and Floyd, 1999 u Hurricane Dennis, a Category 3 storm, moved up the East Coast offshore beginning August 30, wandered back and forth erratically for a few days, then made landfall as a much weaker tropical storm on September 4. Unlike many hurricanes that move quickly across the shoreline, minimizing the time available for wave damage, Dennis remained 125 kilometers off the North Carolina coast for days, generating big waves that progressively eroded the beaches through a dozen high tides. Sand overwashed the North Carolina Outer Banks at numerous points, and erosion was equivalent to that of a Category 4 hurricane. A large frontal dune built in the 1930s for erosion control along 150 kilometers of beachfront had been progressively eroded by storms in the past decade. Hundreds of buildings now rest on the seaward-sloping beachfront, where they are directly exposed to storm attack, but the houses would have to be removed to replace the old dune. Hurricane Dennis removed sand from beneath some buildings and stranded others below high tide level. Ten days later, from September 14 to 18, Hurricane Floyd hit the coast from
South Carolina to New Jersey. However, it was only a strong Category 2 hurricane in September, when it reached the mainland near Cape Fear, North Carolina. Buildings on Oak Island, largely destroyed in a hurricane in 1954, had been rebuilt in their original locations well back from the beach. After 45 years of landward beach migration, some 20 to 45 meters in total, half of them were again destroyed by Floyd (p Figure 14-42). Its big waves came on top of a 1.5- to 2.5-meter storm surge. Topsail Island, which is low and thin, was covered with a sheet of overwash sand. The artificial frontal dune along much of the island, constructed from sand bulldozed from the beach, largely disappeared. The dune had been replaced at least five times in the previous ten years. It will again be replaced—primarily at federal taxpayer expense. At least 77 people died, most of those in flooding. Military helicopters and emergency personnel in boats rescued thousands of people from flooded houses, rooftops, and even trees. Damages were in the billions of dollars. Some 2.6 million people evacuated from Florida to the Car-
p
Dave Gatley photo, FEMA.
FIGURE 14-42. Sand eroded from under the shallowly anchored posts under this house, causing it to topple during Hurricane Floyd on September 17, 1999, at Long Beach, Oak Island, North Carolina.
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olinas, the largest such evacuation in U.S. history. At the peak of the evacuation, almost every east-to-west highway was jammed with traffic, some almost at a standstill. On Interstate 26 west of Charleston, South Carolina, both directions were converted to only westbound traffic to handle the vehicles. Some people took two and a half hours to cover fifteen miles. Most gas stations and restaurants were closed. Floyd covered a larger area and lasted longer than many larger hurricanes, so its heavy rains lasted much longer. The storm dumped more than 50 centimeters of rain on coastal North Carolina, an area where Dennis had saturated the ground only two weeks earlier. Floyd’s torrential rains had nowhere to go but to run off the surface. Flood levels from eastern North Carolina to New Jersey rose above the 100-year flood stage. Twenty-four-hour rainfall totals reached 34 centimeters in Wilmington, North Carolina, and 35.5 centimeters at Myrtle Beach, South Carolina (p Figure 14-43). Total rainfall along part of the coast reached 53.3 centimeters. As if that were not enough, with rivers still high, heavy runoff carried sediment, organic waste, and pesticides from farms; hazardous chemicals from industrial sites; and raw sewage into coastal lagoons and bays and onto beaches. According to FEMA, this was the worst flood disaster ever recorded in the southeastern states. For many people, the trauma and mess of the storms were just
(continued)
p
the beginning. Many thousands of dollars of appliances, computers, stereo equipment, clothing, and other belongings were ruined beyond repair. Far more than half of the people flooded out did not have flood insurance. Common reactions were, “No one told me I was in a flood area” and “I didn’t know my insurance didn’t cover floods.”
Dave Gatley photo.
FIGURE 14-43. This family returns to its almost completely submerged home in a community near the Tar River just north of Greenville, North Carolina.
Case in Point Floods, Landslides, and a Huge Death Toll in Poor Countries Hurricane Mitch, Nicaragua and Honduras u Mitch formed as a tropical storm in the Caribbean Sea on October 21, 1998, then rapidly strengthened to Category 5 from October 26 through 28. Maximum sustained surface winds reached 290 kilometers per hour. Weakening, Mitch hovered near the north coast of Honduras before moving southwest and inland as a tropical storm on October 29 (p Figure 14-44).
Waves north of Honduras probably reached heights of more than 13 meters. For the next two days, the storm continued westward over Honduras, Nicaragua, and then Guatemala, producing torrential rains and floods. Some mountainous areas received 30 to 60 centimeters of rain per day, with storm-total rainfalls as high as 1.9 meters. After several days, a weak-
p
NOAA satellite image.
FIGURE 14-44. Hurricane Mitch, October 26, 1998, neared Honduras with winds at 180 miles per hour. Its cloudless eye is well defined. The storm track is shown as a green line that starts on the lower right and progresses to the upper right over this two-week period.
ened Mitch turned northeastward across Yucatán and over the Gulf of Mexico, where it strengthened to tropical storm status before pounding Florida’s Key West on November 4–5. Larger towns throughout Central America were concentrated in valleys near rivers, where businesses and homes were flooded, buried in mudflows, or washed away (p Figure 14-45). Mitch was the deadliest hurricane in the western hemisphere since 1780; it killed more than 11,000 people in Central America, most in floods and mudflows. Whole villages disappeared in floodwaters and mudflows, and 18,000 people were never found. More than 3 million others lost their homes or were otherwise severely affected.
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(continued) their furniture onto the roof, leave the house, and set up temporary shelters along a nearby raised highway until the water recedes in a couple of weeks. This time, Mitch took away nearly everything and left behind several feet of mud (see Figure 14-45). There was one surprising outcome from Hurricane Mitch. Near the Pacific coast of Honduras, roads, bridges, adobe huts, sugar cane fields, and dairy farms that were damaged or destroyed by the hurricane’s winds and floods in the formerly destitute area have been rebuilt and upgraded. Less than ten years later, the attention brought by the destruction brought entrepreneurs that planted melon fields and ultramodern seafood farms and built new concrete bungalows, motels, and textile factories. The profits, however, go to the foreign capitalists, lured by government and national labor union agreements that keep wages below the legal minimum for several years. The poor still struggle to survive. Was the devastation of Hurricane Mitch a freak event that is not likely to repeat itself? Unfortunately, the combination of rapid population growth, widespread poverty, and lack of access to usable land
makes Central America increasingly vulnerable to natural disasters. In 1995, 75 percent of Guatemala’s and 50 percent of Nicaragua’s populations were living in poverty, defined as living on less than $1 per day at 1985 prices. Those levels were worse after Hurricane Mitch. In 1974, 63 percent of Honduran farmers had access to only 6 percent of the farmable land. Large corporate farms took over most of the fertile valley floors and gentle slopes for growing cotton, bananas, and irrigated crops and for raising livestock. Peasants were forced onto steep slopes where they cleared forests for agriculture, building materials, and firewood; this caused increased soil erosion and the addition of sediment to rivers. Those who cannot survive by growing crops on those slopes move to cities in search of jobs. Lacking access to safe building sites there, they build shelters on steep, landslide-prone slopes or flood-prone riverbanks. Improved hurricane forecasts are of limited use for these poverty-stricken people. Even if warned of approaching storms, most are reluctant to leave what little they own. In addition, they lack the resources to leave and have no way to survive when they get there.
USGS photo.
USGS photo.
During and after the storm, there were critical shortages of food, medicine, and water. Dengue fever, malaria, cholera, and respiratory illnesses were widespread in the warm climate. Roads were impassable, and there were so few helicopters for distributing relief supplies that some areas did not receive help for more than a week. Survivors surrounded by mudflows had to wait days for the mud to dry enough for them to walk to rescuers. A large percentage of all crops were destroyed, including most of the banana and melon crops, as were shrimp farms in Honduras. Coffee and other crops in Nicaragua, Guatemala, and El Salvador suffered severe damage. Big agricultural companies that employed many of the people lost huge areas of farmland, so they had to lay off many employees. Damages came to more than $5.5 billion (in 2002 dollars) in Honduras and Nicaragua, two of the poorest countries in the world. It will take decades for these nations to get back to where they were before Hurricane Mitch. Many people live and work on the floodplains of large rivers and are used to flooding during hurricane season almost every year. When the river rises, they pull
a
b
p
FIGURE 14-45. a. Many homes in Honduras were buried by mudslides. b. Washed-out bridges crippled transportation and the distribution of food and fuel. This truck collapsed a bridge across the Manacal River—its underpinnings were weakened by incessant rains.
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Case in Point Unpredictable Behavior of Hurricanes Florida Hurricanes of 2004 u
The 2004 hurricane season showed that even with our greatly improved prediction abilities, hurricanes still behave unpredictably. This was the first time that four hurricanes came onshore in a single state since 1886, when Texas was the unfortunate victim. Charley arrived on August 13 as a Category 4 storm, Frances arrived on September 5 as a Category 2, Ivan (p Figure 14-46) made landfall on September 15 as a Category 4, and then finally Jeanne dealt the final blow as a Category 3, following virtually the same track through Florida as Frances. In less than a month and a half, these storms wreaked havoc across much of Florida. In some places, later storms hammered areas destroyed from the earlier storms as they were in the middle of repairs. This was the second costliest hurricane season on record; Charley inflicted $6.3 billion in damages, Frances $9.7 billion, Ivan $15.4 billion, Jeanne $7.5 billion. Those hurricanes were, respectively, the 4th, 11th, 5th, and 13th costliest. The first storm, Charley, began on August 9 as a tropical storm just north of South America, gradually strengthening as it curved from west to north. It ap-
proached the Gulf Coast of Florida as a Category 2 storm and was not expected to do severe damage. Much to the surprise of forecasters, before landfall on August 13, just north of Fort Myers, Florida, it rapidly strengthened to a Category 4 hurricane with surface wind speeds of 232 kilometers per hour. In Florida, 22 people died, 12,000 buildings were destroyed, and 2 million customers lost electric power (p Figure 14-47). It weakened to a Category 1 as it crossed Florida into the Atlantic, then made landfall again north of Charleston, South Carolina. It weakened over land to a tropical storm and then moved northeast up the coast. Hurricane Frances reached Category 4 strength before hitting the Bahamas, then paused for a day, long enough for 2.5 to 3 million people in Florida to evacuate. It weakened to a Category 2 before crossing Florida on September 5. It weakened further as it turned north through Georgia. In North Carolina and Virginia, it dumped about 37 centimeters of rain and caused heavy flooding. It spun off 137 tornadoes, and six people died in the United States. Hurricane Ivan, locally dubbed Ivan the Terrible, grew in the warm waters of the
Atlantic Ocean and strengthened to a Category 4 as it grazed the north coast of South America, then strengthened to a Category 5 with a core pressure of 910 millibars and winds of 260 kilometers per hour. It heavily damaged Grenada, Jamaica, and western Cuba before striking the Gulf Coast of Alabama at 2 a.m. on September 16, as a Category 3 storm (p Figure 14-48). In response to warnings that the hurricane could hit New Orleans, more than half of the city’s residents evacuated. In Mobile, Alabama, about one-third of the residents evacuated, but many people were delayed up to 12 hours because of
Canada
Charley Aug. 9 to 15
United States
N
Ivan weakens
Cuba
Jeanne Sept. 13 to 29
North Atlantic Ocean Frances Aug. 25 to Sept. 10
Africa
NOAA data.
NCDC-NOAA.
Central America
a
p
South America
Ivan Sept. 2 to Sept. 18
b
FIGURE 14-46. a. As Hurricane Ivan roared into the Gulf Coast on September 15, 2004, its eye crossed the west end of the Florida panhandle, while its strongest onshore winds, waves, and storm surge pounded areas just east of the eye. State boundaries are superimposed on this natural color image. b. Four major hurricanes decimated Florida in August and September 2004, striking many areas more than once.
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FEMA photo.
(continued)
FEMA photo.
b
a
p
USGS photo.
USGS photo.
FIGURE 14-47. Hurricane Charley in Florida. a. Pre-engineered long-span metal building designed for use as a storm shelter. Civic Center in Arcadia. b. This manufactured home provided no protection from a falling tree. Pine Island.
a
b
p
J. Augustino photo, FEMA.
Florida State Department of Transportation photo.
FIGURE 14-48. Many beachfront homes in Orange Beach, Alabama, vanished in Hurricane Ivan in September 2004, along with the beach that had attracted their owners. Even some behind the first row of buildings were destroyed; note the ground-level house to the right and behind the larger building on the right.
a
b
p
FIGURE 14-49. a. Hurricane Ivan wrecked beachfront homes in Pensacola, Florida. b. It completely lifted off the eastbound lanes of I-10 near Pensacola. Here cranes on barges begin rebuilding.
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(continued) traffic congestion on highways. There it caused major damage before turning northeast, dumping heavy rains, and weakening. Looping east into the Atlantic, it turned south and then west across the southern part of Florida, into the Gulf of Mexico, and again headed inland as a weak tropical storm near the LouisianaTexas border. Tornadoes spun off from the
leading edge of the hurricane, causing localized severe damage in northwesternmost Florida—more than 100 tornadoes in all of the eastern United States. Thirty-nine people died in Grenada, 17 in Jamaica, 14 in Florida, and 8 in North Carolina. Damages in the Caribbean area reached $3 billion. Much of the eastbound lanes of the Interstate 10 bridge across a wide bay
near Pensacola, Florida, were destroyed (p Figure 14-49). Rains brought major flooding to Georgia, the western Carolinas, and Pennsylvania. Jeanne hammered the same area of Florida only three weeks after Frances. At least 3,132 people died, mostly from Jeanne in Haiti.
Case in Point Choosing to Ignore Evacuation During a Major Hurricane Hurricane Hugo u
On September 21, 1989, residents in and around Charleston, South Carolina, braced for the onslaught of Hurricane Hugo. The National Weather Service forecast a 5.2-meter-high surge that would inundate most barrier islands in South Carolina, since most were less than 3 meters above sea level. The storm, coincident with high tide, was a Category 4 with minimum pressure of 934 millibars and a forward speed of 40 to 48 kilometers per hour. In March 1988, Gwenyth and William Reid completed their home 30 kilometers northeast of Charleston, overlooking the intracoastal waterway, which was a long boat channel excavated from lagoons behind the barrier bar. They took special care to build with storm-resistant design and materials. The year-old house was built with its floor 6 meters above sea level, well anchored onto dozens of concrete pilings pounded 3 to 4 meters into the ground. In anticipation of the storm, they attached plywood over the windows and doors. Most neighbors evacuated, but the Reids elected to stay with one neighbor in another well-built house with 300 meters of woods between the house and the water. By 7 p.m. the night of the storm, the wind whipped the trees over, and by 8
p.m. the area lost power. By 11:30 p.m. the water rose to 20 to 30 centimeters above the ground, and the house trembled and shook in the fierce wind. By 11:45, the water was 1 meter deep, and the new car parked under the house between the pilings floated free; they opened its doors to keep it from rising and bashing through the floor above it. By 12:30 a.m., water rose above the floor inside the house; lumber and branches floated by; and all of the trees were broken off above water level. The Reids and their neighbor were terrified that they would not survive, but fortunately the water began to drop at 1 a.m., and the wind eased a little. The only other neighbors to remain through the storm, in their house about a kilometer farther west, were forced upstairs to their den, and then with water rising to chest height, they climbed onto cabinets. In the morning, when they tried to go downstairs, they realized that the den in which they’d found shelter was resting on the ground 10 meters from where it belonged. The rest of the house was gone. Of the 42 homes in the immediate area, all less than six years old, 21 were destroyed and 17 were severely damaged. Those who stayed said they would never do so again. They empha-
sized that you must leave, take a chain saw so you can clear trees to return quickly, keep records in pencil because ink generally runs, and spread family pictures and vital papers around to other parts of the country. They would build still higher above base flood elevation and build even more sturdily. Hugo came ashore on a northwesterly track, just north of Charleston near midnight on September 21. A huge surge of 6.1 meters caused extensive flooding; waves severely eroded sand from under most buildings and flattened most oceanfront dunes (p Figure 14-50). Even on the landward side of the lagoon behind the barrier island, homes were severely damaged, and pleasure boats in marinas were stacked up on shore like piles of fish. The storm caused 105 deaths, and property damage was $13.4 billion (in
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S.J. Williams photo, USGS.
U.S. Army Corps of Engineers photo.
(continued)
a
b
p
FIGURE 14-50. Hurricane Hugo caused rampant destruction of beachfront homes on Sullivan’s Island, South Carolina. It destroyed homes behind the two in the foreground, which were also damaged. It even wrecked nearly new homes on 3-meter-high pilings. b. After Hurricane Hugo, pleasure boats moored in the lagoon behind the barrier island lay stacked like fish in a basket.
2006 dollars). Wind did the greatest damage. Electric power lines and poles were undermined on the barrier islands. Only 23 percent of customers in the Charleston area had power eight days later; some had none for two to three weeks. The main bridge to the mainland from Sullivan’s Island and the Isle of Palms failed. Many of the newcomers were not aware of the long history of hurricanes and their
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effects along the coast. Much of the rural population in South Carolina lives in mobile homes. South Carolina lacked restrictions over the use of coastal sites and the quality of construction. That was left to local communities, some of which imposed controls, while others provided few or none. Most adopted a building code only when they wanted to participate in the NFIP.
Well-designed buildings generally survived Hurricane Hugo with little damage, but others lost roofing and wall siding or were completely destroyed. The high level of damages was caused by the lack of standards held by many groups, including governments, developers, builders, lenders, insurers, and homeowners.
1. For each of the following photos, indicate the hazard or hazards and any damaging events that have happened.
Critical View
2. Why should the event have been foreseen, and what could be done to prevent damages?
NOAA/NWS.
USGS.
A. Booher, FEMA.
3. Where plausible, evaluate what can be done to stabilize the area.
Donald Hyndman photo.
E.
H.
J.
I.
NWS.
FEMA.
Donald Hyndman photo.
G.
F.
Donald Hyndman photo.
C. Hunter/FEMA. Donald Hyndman photo.
D.
C.
Donald Hyndman photo.
B.
Donald Hyndman photo.
A.
K.
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Chapter Review
Key Points Hurricanes, Typhoons, and Cyclones p Hurricanes, typhoons, and cyclones are all major storms that circulate counterclockwise with winds from 120 to more than 260 kilometers per hour.
p Hurricanes rotate counterclockwise, as in any lowpressure area, but track clockwise, as with ocean currents.
p The central core, or eye, of the hurricane is 20 to 50 kilometers across, out of a total diameter of 160 to 800 kilometers. Clear, calm air in the lowpressure eye is surrounded by the highest winds and stormy skies. Figures 14-1 and 14-2.
p Hurricanes that affect the southeastern United States form as atmospheric lows over warm subtropical water. They grow off the west coast of Africa, then move westward with the trade winds. Most hurricanes occur between August and October because it takes until late summer to warm the ocean sufficiently. They strengthen over warmer water and weaken over cool water or land.
p The strongest hurricanes, Category 5 on the SaffirSimpson Hurricane Scale, have the lowest atmospheric pressure (less than 920 millibars) and the highest wind speeds (more than 249 kilometers per hour). Table 14-1.
p Hurricanes impacts are greatest from North Carolina to Florida to Texas. Annual damages run from hundreds of millions to billions of dollars.
Storm Damages p Hurricanes and other major storms cause severe coastal damage. Beaches and dunes are eroded, and buildings are severely damaged by wind, waves, and flooding. In addition to buildings and bridges, damages include the deaths of farm animals, agricultural damage, contaminated drinking water, and landslides.
p Damages from the hurricane depend on its path compared with shore orientation, presence of bays, forward speed of the hurricane, and the height of dunes and coastal vegetation. Figure 14-8.
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p Storm surges, as much as 7.3 meters high and 160 kilometers wide, result from a combination of low atmospheric pressure that permits the rise of sea level and prolonged winds pushing the sea into a broad mound. By the Numbers 14-1 and 14-2.
p Surge hazards include significant rise of sea level and waves on top of the higher sea level. The surge and associated winds are concentrated in the northeast forward quadrant of the hurricane because of wind directions. Figure 14-12.
p High waves have much more energy and erode the beach and dunes lower and to a flatter profile, especially if the waves are not slowed by shallow water offshore. Figure 14-14.
p Wind damages include blown-in windows, doors, and walls; lifted-off roofs; blown-down trees and power lines; and flying debris. Figures 14-17 and 14-18.
p Rain and flooding from hurricanes can be greater with large-diameter, slower-moving storms.
p Thousands of people in poor mountainous countries such as in Central America die in floods and landslides triggered by hurricanes because they live in poorly constructed houses on floodplains and unstable steep slopes.
p Thousands of people living on low-lying deltas of major rivers die in floods and storm surges.
Hurricane Prediction and Planning p Hurricane predictions and warnings include the time of arrival, location, and magnitude of the event. The National Hurricane Warning Center tries to give 12 hours of warning, but evacuations can take as many as 30 hours.
p Many people think they can evacuate quickly and leave too late. Storm surges and downed trees and power lines often close roads.
Managing Future Damages p The National Flood Insurance Program requires that buildings in low-elevation coastal areas be landward of mean high tide and raised above heights that could be impacted by 100-year floods, including those imposed by storm surges.
p The nature and quality of building construction
Extratropical Cyclones and Nor’easters p Nor’easters and other extratropical cyclones are similar to hurricanes except that they form in winter, lack a distinct eye, are not circular, and spread out over a large area. Like hurricanes, they are characterized by high winds, waves, and storm surges.
have a major effect on damages. Floors, walls, and roofs need to be well anchored to one another, and buildings should be well attached to deeply anchored stilts.
Key Terms cyclones, p. 387
hurricane, p. 387
Nor’easter, p. 408
storm surges, p. 392
extratropical cyclones, p. 408 eye, p. 387
hurricane warning, p. 403 hurricane watch, p. 403
Saffir-Simpson Hurricane Scale, p. 388
typhoons, p. 387
fetch, p. 409
Questions for Review 1. What causes a tropical cyclone or hurricane? Where does a hurricane get all of its energy? 2. Where do hurricanes that strike North America originate? Why there? Why do they track toward North America? 3. Why are coastal populations so vulnerable to excessive damage (other than the fact that they live on the coast)? 4. Where in a hurricane is the atmospheric pressure lowest, and approximately how low might that be? 5. When is hurricane season (which months)? Why then? 6. What two main factors cause increased height of a storm surge? 7. What effects does the wind have on buildings during a hurricane? 8. What effects do the higher waves of hurricanes have on the coast? 9. If the forward speed of a hurricane is greater, what negative effect does that have? What positive effect does it have?
10. Which part of a hurricane does the greatest damage? (In other words, if the eye of a west-moving hurricane were to go right over Charleston, South Carolina, where would the greatest damage be?) 11. What is the difference in damage if a hurricane closely follows another hurricane—for example, by a week? Why? 12. Why is there more coastal damage if the sand dunes are lower? 13. What shape of roof is most susceptible to being lifted off by a hurricane? Why? 14. Why is it so important to cover windows and doors with plywood or shutters? 15. Why do developers, builders, local governments, and many members of the public oppose higher standards for stronger houses? 16. Examine Figure 14-3 and explain why the Atlantic coast of northern Florida and Georgia has fewer hurricane strikes than coastal areas farther north and south.
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Thunderstorms and Tornadoes
Chapter
Greg Rudl photo, U.S. Air Force.
15 i
Greensburg, Kansas, emergency vehicles lie buried in rubble from a severe tornado on May 4, 2007.
Twister Demolishes Kansas Town
T
he small town of Greensburg in southwestern Kansas was effectively wiped off the map by a powerful tornado just before 10 p.m. on May 4, 2007. It was an EF-5, the strongest tornado category, the first since the Oklahoma outbreak of May 3, 1999. The National Weather Service managed to provide 30 minutes of warning that a major tornado was headed for Greensburg; then with a more precise track, they activated tornado sirens and an emergency message 10–15 minutes before it struck, for people to take shelter immediately. In spite of this, nine were killed in Greensburg. The 2.7-kilometer-wide tornado decimated the town in 15–20 minutes (p Figure 15-1), following a northeastward path on the ground for 35 kilometers. Nearly an hour later the twister touched down again 46 kilometers to the northeast, causing fatal injuries to a sheriff in his patrol car. Winds were estimated at between 320 and 335 kilometers per hour. The storm developed when a major low-pressure system moving east from northern Nevada and Utah
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Greg Henshall, FEMA.
Greg Henshall, FEMA.
collided with a stream of moist air moving north from the Gulf of Mexico into Oklahoma and Kansas. The resulting cold front spun off days of severe storms. The May 4 tornado demolished every building on the main street in Greensburg, a town of 1,600. It stripped the branches from trees and flattened most homes, churches, and other buildings. Even solid buildings were destroyed, including the high school, which was built in 1939 using two layers of bricks and mortar on the outside, with concrete blocks and mortar inside (p Figure 15-2). Searchers dug through the debris to find people trapped in their basements. One survivor was pulled from under rubble two days later. A tank of anhydrous ammonia, used in fertilizer, ruptured during cleanup, requiring further evacuation of part of the town. Many residents suggested that the town itself may not survive. Rebuilding first required repairing utility poles, lines, and meters, along with water and sewer lines. First they had to be dug out from under the piles of debris. Damage estimates reached $153 million.
p
FIGURE 15-1. A deadly tornado swept through and destroyed nearly all of Greensburg, Kansas.
Thunderstorms Thunderstorms, as measured by the density of lightning strikes, are most common in latitudes near the equator, such as central Africa and the rain forests of Brazil (p Figure 15-3). For its latitude, the United States has an unusually large number of lightning strikes and severe thunderstorms. These storms are most common from Florida and the southeastern United States through the Midwest because of the abundant moisture in the atmosphere that flows north from the Gulf of Mexico. Thunderstorms form as unstable, warm and moist air rapidly rises into colder air and condenses. As water vapor condenses, it releases heat. Because warm air is less dense
p
FIGURE 15-2. The heavily built Greensburg high school was mostly destroyed.
than cold air, this added heat causes the rising air to continue to rise in an updraft. This eventually causes an area of falling rain in an outflow area of the storm when water droplets get large enough through collisions. If updrafts push air high enough into the atmosphere, the water droplets freeze in the tops of cumulonimbus clouds; these are the tall clouds that rise to high altitudes and spread to form wide, flat, anvil-shaped tops (p Figure 15-4). This is where lightning and thunder commonly form. Cold air pushing under warm, moist air along a cold front is a common triggering mechanism for these storm systems, as the warm humid air is forced to rapidly rise over the advancing cold air. Isolated areas of rising humid air from localized afternoon heating, or warm, moist air rising against a mountain front or pushing over cold air at the surface
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–150
–120
–90
–60
–30
0
30
60
90
120
Strokes per km2 per year
60
50 40 30 20 10 8 6 4 2 1 .8 .6 .4 .2 .1
30
0
Modified from NOAA.
–30
–60
FIGURE 15-3. This worldwide map shows the average density of annual lightning flashes per square kilometer.
Eric Helgeson, NWS photo.
p
p
FIGURE 15-4. A classic cumulonimbus anvil buildup over Utah, August 26, 2004.
can have similar effects. Individual thunderstorms average 24 kilometers across, but coherent lines of thunderstorm systems can travel for more than 1,000 kilometers. Lines of thunderstorms commonly appear in a northeast-trending belt from Texas to the Ohio River valley. Cold fronts from the northern plains states interact with warm moist air from the Gulf of Mexico along that line, so the front and its line of storms move slowly east. Thunderstorms produce several different hazards. In 1940, lightning strikes in the United States killed about 400 people per year. The number dropped continuously to an average of 44 people per year from 1997 to 2006, likely in part due to increased awareness of the hazard and forecasts. Twice as many deaths were in Florida as any other state. Among weather-related events, only floods cause more deaths.
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Strong winds can down trees, power lines, and buildings. Severe thunderstorms cause numerous wildfires, and sometimes large damaging hail and tornadoes. Major insurance companies reported in 2007 that lightning-associated claims in the previous six years rose 77 percent. Costs escalated because of the growing number of electronic devices in people’s homes.The voltage surge in a home’s wiring from a lightning strike can destroy personal computers, HD TV sets, VCRs, game consoles, other devices, and heating systems. U.S. lightning strikes cause about $144 million in direct property damage and 6,100 house fires every year. Lightning-protection systems for a whole home or a surge suppressor installed at the main circuit-breaker panel can reduce the excess voltage. Individual surge suppressors that you plug into an electrical outlet should have a Suppressed Voltage Rating of 330 or less.
Lightning Lightning results from a strong separation of charge that builds up between the top and bottom of cumulonimbus clouds. Atmospheric scientists commonly believe that this charge separation increases as water droplets and ice particles are carried in updrafts toward the top of cumulonimbus clouds and collide with the bottoms of downwardmoving ice particles or hail. The smaller, upward-moving particles tend to acquire a positive charge, while the larger, downward-moving particles acquire a negative charge. Thus, the top of the cloud tends to carry a strong positive charge, while the lower part of the cloud carries a strong negative charge (p Figure 15-5). This is a much larger but similar effect to static electricity that you build up by dragging your feet on carpet during dry weather, a charge that is discharged as a spark when you get near a conductive object.
+ + + + ++ + + + + + – – – – – – – –
+ + + + + + ++ + +++ + + + + + + + + + + + +
Modified from NOAA, National Severe Storm Lab photo.
– – –– – – – – – – – – – – – – – –– – – – – – – – – –– –– – – – –– –– – –
++ + + + + + +++++ + + ++ ++ ++ + +
+
to-ground lightning is generated when charged ions in a thundercloud discharge to the best conducting location on the ground. Negatively charged step leaders angle their way toward the ground as the charge separation becomes large enough to pull electrons from atoms. When this occurs, a conductive path is created that in turn creates a chain reaction of downward-moving electrons. These leaders fork as they find different paths toward the ground; as they move closer, positive leaders reach upward toward them from elevated objects on the ground (see lower right side of Figure 15-6a). If you ever feel your hairs pulled upward by what feels like a static charge during a thunderstorm, you are at high risk of being struck by lightning. When one of the pairs of leaders connects, a massive negative charge follows the conductive path of the leader stroke from the cloud to the ground. This is followed by a bright return stroke moving back upward to the cloud along the one established connection between the cloud and ground. The enormous power of the lightning stroke instantly heats the air in the surrounding channel to extreme temperatures approximating 28,000°C (50,000°F). The accompanying expansion of the air at supersonic speed causes the boom that we hear as thunder. The lightning channel itself appears to be only about 2 or 3 centimeters in diameter, based on holes produced in fiberglass screens and long narrow tubes fused in loose sand. In fewer cases, lightning will strike from the ground to the base of the cloud; this can be recognized as an upwardly forking lightning stroke (p Figure 15-6b) rather than the more common downward forks observed in cloud-to-ground strokes. Lightning also strikes from cloud to cloud to equalize its charges, although there is little hazard associated with such cloud-to-cloud strokes. Of the more than 100,000 thunderstorms in the United States each year, the National Weather Service classifies about 10,000 as severe. Those severe storms spawn up to 1,000 tornadoes each year. The weather service classifies a
+ + ++ + + + +
p
FIGURE 15-5. In a thunderstorm, the lighter, positive-charged rain droplets and ice particles rise to the top of a cloud while the heavier, negative-charged particles sink to the cloud’s base. The ground has a positive charge. In a lightning strike, the negative charge in the cloud base jumps to join the positive charge on the ground.
David Hyndman photo.
C. Clarke photo, NOAA.
The strong negative charges near the bottom of the clouds attract positive charges toward the ground surface under the charged clouds, especially to tall objects such as buildings, trees, and radio towers.Thus, there is an enormous electrical separation or potential between different parts of the cloud and between the cloud and ground. This can amount to millions of volts; eventually, the electrical resistance in the air cannot keep these opposite charges apart, and the positive and negative regions join with an electrical lightning stroke (Figures 15-5 and p 15-6). Because negative and positive charges attract one another, a negative electrical charge may jump to the positivecharged cloud top or to the positive-charged ground. Air is a poor conductor of electricity, but if the opposite charges are strong enough, they will eventually connect. Cloud-
a
p
b
FIGURE 15-6. a. The return stroke on the left side of this photo is much brighter than both the small leader coming up from the ground and the cloud-to-cloud stroke on the right. b. This ground-to-cloud lightning stroke was observed near East Lansing, Michigan, in spring 2004.
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Downbursts Several airplane accidents in the 1970s spurred research into the winds surrounding thunderstorms. This research demonstrated that small areas of rapidly descending air, called downbursts, can develop in strong thunderstorms. Downburst winds as fast as 200 kilometers per hour and microburst winds (downbursts less than 4 kilometers radius) of up to 240 kilometers per hour are caused by a descending mass of cold air, sometimes accompanied by rain. These severe downdraft winds pose major threats to aircraft takeoffs and landings because they cause wind shear, which results in planes plummeting toward the ground as they lose the lift from their wings. Once Dr. Tetsuya (Ted) Fujita proved this phenomenon and circulated the information to pilots and weather professionals, the likelihood of airplane crashes because of downbursts was greatly reduced. When these descending air masses hit the ground, they cause damage that people sometimes mistake as having been caused by a tornado. On close examination, downburst damage will show evidence of straight-line winds: Trees and other objects will lie in straight lines that point away from the area where the downburst hit the ground (p Figure 15-7). This differs from the rotational damage that is observed after tornadoes, where debris lies at many angles due to the inward flowing winds.
Dr. Tetsuya T. Fujita photo; courtesy of Dr. Kazuya Fujita.
storm as severe if its winds reach 93 kilometers per hour, it spawns a tornado, or it drops hail larger than 1.9 centimeters in diameter. Flash flooding from thunderstorms causes more than 140 fatalities per year (see Chapter 11).
p
FIGURE 15-7. Downburst winds in Bloomer, Wisconsin, blew these trees down on July 30, 1977.
Hail Hail causes $2.9 billion in annual damages to cars, roofs, crops, and livestock (p Figure 15-8). Hailstones appear when warm and humid air in a thunderstorm rises rapidly into the upper atmosphere and freezes. Tiny ice crystals waft up and down in the strong updrafts, collecting more and more ice until they are heavy enough to overcome updrafts and fall to the ground. The largest hailstones can be larger than a baseball and are produced in the most violent storms. Hailstorms are most frequent in late spring and early summer, especially April to July, when the jet stream
p
Fred Phillips photo.
Fred Phillips photo.
FIGURE 15-8. a. A violent storm over Socorro, New Mexico, on October 5, 2004, unleashed hailstones, many larger than golf balls and some 7 centimeters in diameter. b. Most cars caught out in the open suffered severe denting and broken windows. In some cases, hailstones went right through car roofs and fenders.
a
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b
Safety During Thunderstorms When someone is fatally struck by lightning, the immediate cause of death is heart attack, with deep burns at lightning entry and exit points. Seventy percent of survivors have residual effects, including damage to nerves, the brain, vision, and hearing. The maximum number of deaths from lightning strikes occurs at around 4 p.m., with a significant increase on Sunday—presumably because more people, such as golfers, are outside. Most lightning-strike victims can survive with medical help such as application of CPR. However, rescuers should not put themselves in danger of another strike. A lightning-strike victim does not carry an electrical charge and does not endanger you. To reduce your risk of being struck, gauge your distance from the lightning. Lightning is visible before you hear the clap of thunder. The speed of light is almost instantaneous, whereas sound takes roughly three seconds to travel a kilometer. Thus, the time between seeing the lightning and hearing the thunder is the time it takes for the sound to get to you. Every three seconds means a kilometer between you and the lightning; if the time difference is 12 seconds, then the lightning is about 4 kilometers away. The National Weather Service recommends that you take cover if you hear thunder within 30 seconds of the lightning and stay in a safe place until you do not see lightning flash for at least 30 minutes—the “30-30 rule.” Danger from lightning strikes can be minimized by observing the following precautions during a thunderstorm:
p Take cover in an enclosed building; its metal plumbing p
p p p
and wiring will conduct the electrical charge around you to the ground. A picnic shelter is not safe. Do not touch anything that is plugged in including video games. Do not use a phone with a cord; cordless phones and cell phones are okay. One of us was struck by lightning through a corded phone—not something you want to experience. Do not take a shower or bath or wash dishes. Stay away from high places or open fields or open water. Water conducts electricity. Stay away from tall trees, power poles, or other tall objects. If there are tall trees nearby, staying under low bushes away from the trees is a better plan.
NOAA photo.
migrates northward across the Great Plains. The extreme temperature drop from the ground surface up into the jet stream promotes the strong updraft winds. Hailstorms are most common in the plains of northern Colorado and southeastern Wyoming but rare in coastal areas. Hail suppression using supercooled water containing silver iodide nuclei has been used successfully to reduce crop damage; however, this practice was discontinued in the United States in the early 1970s because of environmental concerns.
p
FIGURE 15-9. Reality can be gruesome. These cows were probably spooked by thunder and ran over against the barbed wire fence, where they were electrocuted by a later lightning strike. Note that they were at the base of a hill but out in the open.
p If you are trapped in the open, your skin tingles, or
p
p
your hair stands on end, you are in immediate danger of being struck. Crouch on the balls of your feet, away from other people. Keep your heels touching to minimize the chance that a lightning strike will kill you as it goes up one foot, through your body, and back to the other. Do not lie down because that increases your contact with the ground. Remember that the ground conducts electricity.You can be burned many meters away from the site of a strike. Stay away from metal objects, such as fences, the outside of cars and trucks, golf clubs, golf carts, umbrellas, and farm machinery (p Figure 15-9). Avoid tall objects such as trees or high elevation areas such as hills or mountains. Stay inside a car with the windows rolled up and do not touch any metal. Pull over and stop; do not touch the steering wheel, gearshift, or radio. The safety of a car is in the metal shield around you, not in any insulation from the tires.
Tornadoes Tornadoes, narrow funnels of intense wind, typically have rapid counterclockwise rotation, though 1 percent or so rotate clockwise. The mesocyclone, a rotating cylindrical wind within a big anvil-shaped thundercloud, sinks below the flat base of the cloud to form a wall cloud. Rotation within the mesocyclone accelerates and tightens to form the tornado, which descends to create havoc on the ground. As shown in p Figure 15-10, the wall cloud and its tornado often descend from the base of the storm cloud after the heaviest rain and hail have passed. Tornadoes are nature’s most
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Overshooting top Anvil Wind
Mammatus
Rainfree base Inflow Inflow Hail Tornado Wall cloud Southwest
Heavy rain
Light rain Northeast
a
© Eric Nuygen photo.
Modified from Ahrens, 2002.
Mesocyclone Rear flank downdraft
b
p
FIGURE 15-10. a. In this lateral view of a classic supercell, the system is moving to the right. b. A perfectly formed “wall cloud” descends from the main cloud base. Heavy rain is visible to the right.
violent storms and the most significant natural hazard in much of the Midwestern United States. They often form in the right-forward quadrant of hurricanes, in areas where the wind shear is most significant. Even weak hurricanes spawn tornadoes, sometimes dozens of them. The United States has an unusually high number of large and damaging tornadoes relative to the rest of the world, over 1,000 per year on average. Canada is second, with only about 100 per year; northern Europe has a moderate number. Some Canadian tornadoes are strong, such as F4 tornado that touched down west of Winnipeg, Manitoba in 2007. The storms that lead to tornadoes are created through the collision of warm, humid air moving north from the Gulf of Mexico with cold air moving south from Canada. Because there is no major east-west mountain range to keep these air masses apart, they collide across the southeastern and Midwestern United States. These collisions of contrasting air masses cause intense thunderstorms that sometimes turn into deadly tornadoes.
The average number of tornadoes is highest in Texas, followed by Oklahoma, Kansas, Florida, and Nebraska. Tornado Alley, covering parts of Texas, Oklahoma, Arkansas, Missouri, and Kansas, marks the belt where cold air from the north collides frequently in the spring with warm, humid air from the Gulf of Mexico to form intense thunderstorms and tornadoes.Tornadoes are rare in the western and northeastern states (p Figure 15-11). An individual tornado outbreak—that is, a series of tornadoes spawned by a group of storms—has killed as many as several hundred people and covered as many as thirteen states (p Table 15-1). One of the most severe tornado outbreaks in recent years was that of May 3, 1999, in central Oklahoma (p Figure 15-12). Eight storms producing 58 tornadoes moved northeastward along a 110-kilometer-wide swath through Oklahoma City. Eighteen more tornadoes continued up through Kansas. Individual tornadoes changed in strength as they churned northeast. Fifty-nine people were killed, and damage reached $800 million.
40 35 30 Tornadoes
25
15 10 NOAA-NSSL.
Weak (F0-F1)
Barbato, Texas Tech. University.
20
5 0 a
Strong (F2-F3) Violent (F4-F5) Weak tracks Strong tracks Violent tracks
b
p
FIGURE 15-11. a. The areas of greatest tornado risk include much of the eastern half of the United States for F2 and greater, 1921–1995. The scale indicates the number of significant tornado days per year. b. In this map of the paths for all recorded tornadoes in the United States from 1950 to 1995, the paths in yellow and blue are for smaller tornadoes (F0 to F2), while the paths in red are for larger tornadoes (F3 to F5).
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p Table 15-1
Deadliest U.S. Tornado Outbreaks on Record* NUMBER OF TORNADOES (NUMBER OF STATES AFFECTED)
NAME OR LOCATION
DATE
Tristate: MO, IL, IN Tupelo-Gainesville (MS, GA) Northern Alabama Superoutbreak, E. US and Ontario LA, MS, AL, GA St. Louis, MO Palm Sunday Flint, MI; MA, OH, NE AR, TN Easter Sunday PA, OH, NY, Ontario Carolinas AL, AR, KY, MS, TN OK-KS (F5) Southeastern U.S. Jarrell, TX (F5) KS, MO, AL, GA
March 18, 1925 April 5–6, 1936 March 21–22, 1932 April 3–4, 1974 April 24–25, 1908 May 27, 1896 April 11–12, 1965 June 8-9, 1953 March 21–22, 1952 March 23, 1913 May 31, 1985 March 28, 1984 Feb. 5–6, 2008 May 3–4, 1999 March 27, 1994 May 27, 1997 Feb. 28- March 1 2007
7 (6) 17 (5) 33 (7) 148 (13) 18 (5) 18 (3) 51 (6) 10 (4) 28 (4) 8 (3) 41 (3) 22 (2) 62 (5) 76 (2) 2 (2) 1 (1) 57 (4)
DEATHS 695 419 334 330 310 306 256 247 204 181 75 57 59 49 42 30 20
ESTIMATED DAMAGE IN MILLIONS, 2007 $ 196 242 49 1,860 53 380 695 554 57 105 1,142 507 Unknown 1,600 271 203 520
*From FEMA, NOAA, and other sources.
A tornado rarely stays on the ground for more than 30 minutes, leaving a path less than 1 kilometer wide and up to 30 kilometers long. Typical speeds across the ground range from 50 to 80 kilometers per hour, but internal winds can be as high as 515 kilometers per hour—the most intense winds on Earth. Tornado season varies, depending on location. The number of tornadoes in Mississippi reaches a maximum in April,
The largest known tornado outbreak to date started just after noon on April 3, 1974. A total of 148 tornadoes scored tracks from Mississippi all the way north to Ontario and New York, with an overall storm path length of 4,180 kilometers. This superoutbreak lasted more than seventeen hours, killed 315 people, and injured 5,484 others. The map of the storm tracks (p Figure 15-13) shows that several of these tornadoes ended in downbursts.
Tornado Tracks colored by Fujita intensity scale
35
Wind speed Scale estimate (mph) F0 & F1 Under 112 F2 & F3 113-206 F4 & F5 207-318
44
40
Modified from National Weather Service.
240
A. Booher photo, FEMA.
40
b
35
0 0
20
40 kilometers 25 miles
p
FIGURE 15-12. a. This map of the May 3, 1999, tornadoes shows their paths and intensities around Oklahoma City. b. An Oklahoma tornado on May 4, 1999, threw these cars into a crumpled heap.
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9
0 0
100 200
200
300 miles 400
500 kilometers
Tornado Downburst
6 5 4 3 2 1
p
FIGURE 15-13. This map of the 148 tornado paths from the superoutbreak of April 1974 was compiled by Dr. Ted Fujita and his team of graduate students at the University of Chicago, along with others from the National Severe Storms Lab and other institutions.
with a secondary peak in November (p Figure 15-14). An unusual tornado outbreak on February 5–6, 2008, killed at least 55 people in southeastern states (Table 15-1). Farther north, the maximum is in May, and in Minnesota it is in June. These are the periods that people should be particularly vigilant for tornadoes. At these northern latitudes, tornadoes are virtually absent from November to February. Even late-season tornadoes can be deadly, especially for those in mobile homes. Just after 6:30 a.m. on November 16, 2006, and without warning, a thunderstorm spawned a tornado that killed eight in mobile homes west of Wilmington, North Carolina. At about 10:30 p.m. on October 19, 2007, a tornado just east of Lansing, Michigan, blew a mobile home into a nearby pond, drowning its two occupants. On December 29, 2006, a line of severe storms east of Waco, Texas, spawned tornadoes that killed one man, and on February 1, 2007, tornados killed 19 northwest of Orlando, Florida. Most, though not all, tornadoes track toward the northeast. Storm chasers, individuals who are trained to gather storm data at close hand, know to approach a tornado from the south to southwest directions so they will not be in its path. They also know that it is safer to chase them on the
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.45 .40 .35 .30 .25 .20 .15 .10 .05
.00 0 1 1-Jan 29-Jan 26-Feb 25-Mar 22-Apr 20-May 17-Jun 15-Jul 12-Aug 9-Sep 7-Oct 4-Nov 2-Dec 30-Dec Date
b
p
FIGURE 15-14. a. Tornado season varies by region. The dotted line marks the northern extent of most tornadoes. b. Tornado occurrence rises quickly in springtime to a peak in May and then gradually falls off through early winter. Data from 1980–1999. The heavy black line is the average for the United States.
flat plains rather than along the Gulf Coast, where the lower cloud base can hide the funnel from their view.
Tornado Development Tornadoes derive their energy from the latent heat released when water vapor in the atmosphere condenses to form raindrops. Since latent heat is the amount of heat required to change a material from solid to liquid or from liquid to gas, as in boiling water, the same amount of heat is released in the opposite change from gas to liquid. In a storm, when high humidity or a large amount of water vapor (gas) in the atmosphere condenses to liquid, the amount of heat released is the same as that required to boil and vaporize the same amount of liquid. Since the heat released is localized in the area of condensation, it creates instability in the atmosphere, in some cases fueling tornadoes. Note that in p Figure 15-15, the storm is moving to the right; the rainstorm passes over an area on the ground before arrival of the tornado.
Modified from Harold Brooks, NOAA.
7
.50 Alabama Arkansas Oklahoma Kansas South Dakota Illinois Ohio Regional average
p(Sig. tor. day)
8
Relative risk of a tornado
Dr. Tetsuya T. Fujita photo; courtesy of Dr. Kazuya Fujita.
a
Tornadoes generally form when there is a shear in wind directions, such as surface winds approaching from the southeast with winds from the west higher in the atmosphere. Such a shear can create a roll of horizontal currents in a thunderstorm as warm and humid air rises over advancing cold air (p Figure 15-16). These currents, rolling on a horizontal axis, are dragged into a vertical rotation axis by an updraft in the thunderstorm to form a rotation cell up to 10 kilometers wide. This cell sags below the cloud base to form a distinctive slowly rotating wall cloud, an ominous sight that is the most obvious danger sign for the imminent formation of a tornado (p Figure 15-17). Strong tornadoes commonly form within and then descend from a slowly rotating wall cloud. A smaller and more rapidly rotating funnel cloud may form within the slowly rotating wall cloud or, less commonly, adjacent to it. If a funnel cloud descends to touch the ground, it becomes a tornado.
NOAA, National Severe Storm Lab photo.
slowly rotating wall cloud
p
FIGURE 15-15. This tornado dropped from a slowly rotating prominent wall cloud south of Dimmitt, Texas, on June 2, 1995; the storm is moving from left to right.
Strong westerly flow aloft
Modified from Ahrens, 2002.
Rotation counterclockwise
W
N
S
E
W
N
S
E
Southeasterly surface winds Updraft
a
b
p
NOAA photo.
NOAA, National Severe Storm Lab photo.
FIGURE 15-16. a. Wind shear, with surface winds from the southeast, and winds from the west aloft. b. This slowly rotating vortex can be pulled up into a thunderstorm, which can result in a tornado.
a
p
b
FIGURE 15-17. a. A slowly rotating wall cloud descends from the base of the main cloud bank, an ominous sign for production of a tornado near Norman, Oklahoma, on June 19, 1980. b. Mammatus clouds are a sign of the unstable weather that could lead to severe thunderstorms and potentially tornadoes. These formed over Tulsa, Oklahoma, on June 2, 1973.
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Mammatus clouds can be another potential danger sign, where groups of rounded pouches sag down from the cloud base (p Figure 15-17b). Tornadoes generally form toward the trailing end of a severe thunderstorm; this can catch people off-guard. Someone in the path of a tornado may first experience wind blowing out in front of the storm cell along with rain, then possibly hail, before the stormy weather appears to subside. But then the tornado strikes. In some cases, people feel that the worst of the storm is over once the strong rain and hail have passed and the sky begins to brighten, unless they have been warned of the tornado by radio, television, or tornado sirens that have been installed in many urban areas that have significant tornado risk. Some tornadoes are invisible until they strike the ground and pick up debris. If you do not happen to have a tornado siren in your area, you may be able to hear an approaching tornado as a hissing sound that turns into a strong roar that many people have characterized as the sound of a loud oncoming freight train. Conditions are favorable for tornado development when two fronts collide in a strong low-pressure center (p Figure 15-18). This can often be recognized as a hook echo, or hook-shaped band of heavy rain, on weather radar. This is a sign that often causes weather experts to put storm spotters on alert to watch for tornadoes.
Typically forming toward the rear of a thunderstorm, tornadoes are generally white or clear when descending and become dark as water vapor inside condenses in updrafts, which pull in ground debris. Growth to form a strong tornado can happen rather quickly, within a minute or so (p Figure 15-19), and last for ten minutes to more than an hour. Comparison of the winds of tornadoes with those of hurricanes (compare Table 15-2 with Table 14-1, p. 389) shows that the maximum wind velocities in tornadoes are twice those of hurricanes. Wind forces are proportional to the wind speed squared, so the forces exerted by the strongest tornado wind forces are four times those of the strongest hurricane winds. In many cases, much of the localized wind damage in hurricanes is caused by embedded tornadoes. As a tornado matures, it becomes wider and more intense. In its waning stages, the tornado then narrows, sometimes becoming ropelike, before finally breaking up and dissipating (p Figures 15-20 and 15-21). At that waning stage, tightening of the funnel causes it to spin faster, so the tornado can still be extremely destructive. Prediction and identification of tornadoes by the National Weather Service’s Severe Storms Forecast Center in Kansas City, Missouri, uses Doppler radar, wind profilers, and automated surface observing systems. A tornado watch is issued when thunderstorms appear capable of producing tornadoes and telltale signs show up on the radar. At this point, storm spotters often watch for severe storms. A tornado warning is issued when Doppler radar shows strong indication of vorticity or rotation, or if a tornado is sighted. Warnings are broadcast on radio and television, and tornado sirens are activated if they exist in the potential path of tornadoes.
Gusty cold front (boundary between cold and warm air)
Light to moderate rain
Rain
Modified from Ackerman & Knox, 2003.
Heavy rain and hail
Typical storm movement
0
5
a
Warm air from environment
Storm updraft region Possible tornado
10 kilometers
Gust front
NOAA-NWS, Norman, OK.
Cold air from storm downdraft
b
p
FIGURE 15-18. a. A common situation for tornado development is the collision zone between two fronts, commonly seen in a “hook echo” of a rainstorm. A pair of curved arrows indicates horizontal rotation of wind in the lower atmosphere. b. Radar image of a classic supercell with an embedded tornado during the 1999 Oklahoma City tornado outbreak.
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27.5
29.6
30.1
27.7
33.1
27.8
34.7
Photos compiled by Dr. Tetsuya T. Fujita; courtesy of Dr. Kazuya Fujita.
28.2
38.0
28.7
0
300
600
p
900 m
0
1000
2000
Tornado Damages In his research on tornadoes, Dr. Ted Fujita of the University of Chicago examined damage patterns. He noticed that there were commonly swaths of severe damage adjacent to areas with only minor damage (p Figure 15-22). He also examined damage patterns in urban areas and cornfields, where swaths of debris would be left in curved paths. This led him to hypothesize that smaller vortices rotate around a tornado, causing intense damage in their paths but allowing some structures to remain virtually unharmed by the luck of missing one of the vortices. Such vortices were later
3000 ft
FIGURE 15-19. This series of photos was taken of the Fargo, North Dakota, tornado on June 20, 1957. The times, in minutes, show that the funnel cloud descended in less than 30 seconds; the tornado then rapidly strengthened for the next minute. Just before the photo at 29.6 minutes, the funnel sheared off before strengthening again into a much wider funnel. This whole sequence took only ten minutes.
photographed on many occasions, supporting this hypothesis. The most intense winds are within these embedded vortices, so the pattern of damage can vary greatly over short distances. Damage to a home generally begins with progressive loss of roofing material, followed by glass breakage from flying debris. Then the roof may lift off and the garage doors may fail. Exterior walls collapse, then interior walls, beginning with upper floors. Small interior rooms, halls, and closets fail last. The ease of destruction depends on how well roofs are attached to walls, and walls to floors. Metal “hurricane straps” connecting horizontal and vertical members make a
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Mini funnel Dust cloud
Modified from T. Fujita.
Harold Richter photos, NOAA-NSSL.
50–100 mph
a
Dust cloud
Deposit Removal and gathering b
p
NOAA-NSSL photo.
NOAA-NSSL photo.
FIGURE 15-20. a. A big tornado south of Dimmitt, Texas, on June 2, 1995, sprays debris out from its contact with the ground. This tornado tore up 300 feet of the highway where it crossed. b. A cross section of the suction vortex of a tornado that Fujita inferred from the behavior of nearby stubble. Debris is drawn into the vortex, lifted, and sprayed out.
a
b
p
FIGURE 15-21. This thin, ropelike tornado was photographed at Cordell, Oklahoma, on May 22, 1981, just before it broke up and dissipated. Note also that the foot of a tornado on the ground can sweep well away from its position at the cloud base.
Modified from Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
Suction vortex Suction vortex
el
f funn
of Path
torna
do ce
Right
Tornado center
nter
edge
a
nel of fun Suction band of debris deposit
Mark Wolfe, FEMA.
dge o
Left e
b
p
FIGURE 15-22. a. Ted Fujita hypothesized that many tornadoes were composed of multiple vortices that rotated around the center of the tornado. b. The Lake County, Florida, tornado of February 3, 2007, shows how selective the damage of tornadoes can be. The hook of debris distribution, in this case clockwise, is clear in the photo.
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© Eric Nguyen/CORBIS.
Although many people believe that the low pressure in a tornado vacuums up cows, cars, and people and causes buildings to explode into the low-pressure funnel, this appears to be an exaggeration. Most experts believe that the extreme winds and flying debris cause almost all of the destruction. Photographs of debris spraying outward from the ground near the base of tornadoes suggest this (p Figure 15-23). However, even large and heavy objects can be carried quite a distance. The Bossier City, Louisiana, tornado ripped six 700-pound I-beams from an elementary school and carried them from 60 to 370 meters away. Another I-beam was carried to the south, where it stuck into the ground in someone’s back yard (p Figure 15-24). In another documented case, several empty school buses were carried up over a fence by a tornado before being slammed back to the ground.
p
FIGURE 15-23. Debris sprays outward from a tight tornado rope in Mulvane, Kansas, June 12, 2004.
Fujita Tornado Scale
big difference. As with many other circumstances, damage and destruction are controlled by the weakest link. Most susceptible to damage and destruction are farm outbuildings, followed by mobile homes, apartment and condo buildings, and then family homes. Winds of 225 kilometers per hour (km/hr) will cause severe damage to a typical home or apartment building and endanger people; 260 km/hr will effectively destroy such structures. Winds of 145–160 km/hr will severely damage a typical mobile home and endanger the occupants; 169 km/hr may destroy it, likely killing any occupants. People may survive in the southwestern corner of a basement with protection from flying debris. High-rise office buildings and hotels framed with structural steel are less susceptible to structural damage, but their exterior walls may be blown out.
G C
F0 and F1 F2 F3 and F4 Debris Damage directions Direction of grass
Angle H 11
E
Meadowview Elem. School
B F
TTo orn rna
add
oo
Ce
en tee rr
C C
a
ard
ard bo bong Divi
Diving
A C
D
Dr. Tetsuya T. Fujita; courtesy of Dr. Kazuya Fujita.
Dr. Tetsuya T. Fujita; courtesy of Dr. Kazuya Fujita.
ddoo
a rn TToo
I beam missile estimated trajectory
p
Dr. Fujita devised a scale, now called the Fujita scale, to classify the severity of tornadoes based on their internal wind speeds and the damage produced. He separated estimated tornado wind speeds into a six-point nonlinear scale from F0 to F5 (p Table 15-2). F0 causes minimal damage and F5 blows away strong frame homes. In addition, Dr. Fujita compiled an F-scale damage chart and photographs corresponding to these wind speeds (p Table 15-3). Reference photographs of damage are distributed to National Weather Service offices to aid in evaluating storm intensities (p Figure 15-25). Wind speeds and damage to be expected in buildings of differing strengths are shown in p Table 15-4. Note that walls are likely to collapse in an F3 tornado in even a strongly built frame house; and in an F4, the house is likely to be blown down. Brick buildings perform better. In an F5 tornado, even concrete walls are likely to collapse.
b
FIGURE 15-24. a. Six 700-pound I-beams were pulled from an elementary school in Bossier City, Louisiana, and carried by a tornado along these paths. Other objects, such as a diving board and a car, were also carried significant distances. b. The beam labeled “D” ended up stuck in the ground.
THUNDERSTORMS AND TORNADOES
443
p Table 15-2
Fujita Wind Scale
WIND STRENGTH
F0
F1
F2
F3
F4
F5
Miles per hour Kilometers per hour
40–73 64–117
74–113 118–182
114–158 183–254
159–207 255–333
208–261 334–420
262–319 421–513
p Table 15-3 FUJITA SCALE VALUE
Fujita Scale Deaths and Damages WIND SPEED, KM/HR [MI/HR]
NUMBER OF TORNADOES (1985–93)
% PER YEAR
% OF DEATHS
DAMAGE
F0
64–118 [40–73]
478
51
0.7
Light: Some damage to tree branches, chimneys, signs
F1
119–181 [74–112]
318
34
7.5
Moderate: Roof surfaces peeled, mobile homes overturned, moving autos pushed off roads
F2
182–253 [113–157]
101
10.8
18.4
Considerable: Roofs torn off, mobile homes demolished, large trees snapped or uprooted. Light objects become missiles.
F3
254–332 [158–206]
28
3
20.5
Severe: Roofs and some walls torn off wellconstructed houses, trains overturned, most forest trees uprooted, heavy cars lifted and thrown
F4
333–419 [207–260]
7
0.8
36.7
Devastating: Well-constructed houses leveled, cars thrown, large missiles generated
F5
420–513 [261–318]
1
0.1
16.2
F6
514
Incredible: Strong frame houses lifted and carried considerable distance to disintegrate. Auto-size missiles fly more than 100 yards; trees debarked. Winds are not expected to reach these speeds.
p Table 15-4
0
Expected Damage for Different Types of Buildings Dependent on Tornado Strength* EXPECTED DAMAGE BY F-SCALE TORNADO
TYPE OF BUILDING
F0
F1
F2
Weak outbuilding
Walls collapse
Blown down
Blown away
Strong outbuilding
Roof gone
Walls collapse
Blown down
Blown away
Weak frame house
Minor damage
Roof gone
Walls collapse
Blown down
Blown away
Strong frame house
Little damage
Minor damage
Roof gone
Walls collapse
Blown down
Blown away
Brick structure Concrete structure
OK OK
Little damage OK
Minor damage Little damage
Roof gone Minor damage
Walls collapse Roof gone
Blown down Walls collapse
*Simplified from Fujita, 1992.
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F3
F4
F5
EF 1-2 Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
EF 0-1 Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
EF-0
a
b
EF 4-5 Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
Dr. Tetsuya T. Fujita, courtesy of Dr. Kazuya Fujita.
EF 3
c
d
p
FIGURE 15-25. Dr. Ted Fujita developed the “F scale” for tornadoes by examining damage and evaluating the wind speeds that caused it. He used this set of photos as his standard for comparison. Conversion of his damage to the EF scale is approximated in the red numbers. EF-5 would level virtually everything except for the strongest, reinforced buildings.
e
shows that particular damages occur at much lower wind speeds. EF5 tornadoes are infrequent, but on May 4, 2007, one ripped through Greensburg in south-central Kansas and flattened 95 percent of the town. Although there was a 20-minute warning, winds reached 328 kilometers per hour and 10 people were killed.
Early in 2007 the scale was updated as the Enhanced Fujita (EF) scale, based on detailed wind measurements and long-term records of damage (p Table 15-5). The new scale uses three-second wind-gust estimates at the site of damage and is considered more reliable than the old scale. The main difference between the scales is that the new EF scale
p Table 15-5
Enhanced Fujita (EF) Scale of Tornado Intensity
ENHANCED FUJITA SCALE (DERIVED EF SCALE)
EF NUMBER
3-SECOND GUST KM/HR (MPH)
FUJITA SCALE
F NUMBER
FASTEST 1⁄4 MILE (MPH)
3-SECOND GUST (MPH)
0
104–136 (65–85)
0
40–72
45–78
1
137–175 (86–109)
1
73–112
79–117
2
176–219 (110–137)
2
113–157
118–161
3
221–267 (138–167)
3
158–207
162–209
4
269–318 (168–189) 320–374 (200–234)
4
208–260
210–261
5
261–318
262–317
5
THUNDERSTORMS AND TORNADOES
445
Safety During Tornadoes People are advised to seek shelter from tornadoes underground or in specially constructed shelters whenever possible (Case in Point: “Tornado Safety—Jarrell Tornado, Texas, 1997,” p. 447). If no such space is available, people should at least go to some interior space with strong walls and ceiling and away from windows. The main danger is from flying debris. People have been saved by going to an interior closet, or even lying in a bathtub while holding a mattress or sofa cushions over them. Unfortunately, in some cases a strong tornado will completely demolish houses and everything in them (p Figure 15-26). Mobile homes are lightly built and are easily ripped apart—certainly not a place to be in a tornado. Car or house windows and even car doors provide little protection from high-velocity flying debris such as two-by-fours from disintegrating houses. Those in unsafe places are advised to evacuate to a strong building or storm shelter if they can get there before the storm arrives. If you cannot get to a safe building,
FEMA recommends that you lie in a ditch and cover your head; that will provide some protection from flying debris. It is as yet unclear whether vehicles provide more protection than mobile homes or lying in a ditch. Although cars are designed to protect their occupants in case of a crash, they can be rolled or thrown or penetrated by flying debris. If you are in open country and can tell what direction a tornado is moving, you may be able to drive to safety at right angles from the tornado’s path. Recall that the path of a tornado is most commonly from southwest to northeast, so being north to east of a storm is commonly the greatest danger zone. Remember also that the primary hazard associated with tornadoes is flying debris, and much to people’s surprise, overpasses do not seem to reduce the winds associated with a tornado. Do not get out of your car under an overpass and think that you are safe. In fact, an overpass can act like a wind tunnel that focuses the winds. Once a few people park under an overpass, this can cause the additional problem of a traffic jam, where helpless people may be stuck in the storm’s path. A radio or television tuned to NOAA’s weather radio network provides severe weather warnings. Typically, these warnings can provide up to ten minutes of lead time before the arrival of a tornado. General guidelines include the following:
p Move to a tornado shelter, basement, or interior room p
Andrea Booher, FEMA.
p
p
p
FIGURE 15-26. A basement, or at least an interior room without windows, would be a better choice for protection than this kitchen, which was destroyed by a tornado in Oklahoma.
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without windows. In some airports, such as Denver International, the tornado shelters are the restrooms. Flying debris is extremely dangerous, so if your location is at all vulnerable, protect your head with a bicycle/motorcycle helmet. In spite of television videos, a highway overpass is not a good location. Do not get out of your car and think you are safe. An overpass acts as a wind tunnel that can amplify the danger. Although cars can overturn, and flying debris can penetrate their windows and doors, they still provide some protection—especially below the window line.
Case in Point Tornado Safety
Jarrell Tornado, Texas, 1997 u On May 27, 1997, around 1 p.m., a tornado watch was issued for the area of Cedar Park and Jarrell, 65 kilometers north of Austin, Texas. Many people heard the announcement on the radio or on television, but most went on with their daily work. Storms are common in the hill country. This case seemed familiar: A cold front from the north had collided with warm, water-saturated air from the Gulf Coast to generate a line of thunderstorms. A tornado warning was issued at 3:25 p.m. Just before 4 p.m., a tight funnel cloud swirled down from the dark clouds 8 kilometers west of Jarrell, a community of roughly 450 people. This tornado moved south-southeast along Interstate 35 at 32 kilometers per hour rather than taking a more typical easterly track. When trained spotters saw a tornado on the ground ten to twelve minutes before the funnel struck, they sounded the alarm, and everyone who could took shelter. Some sought protection in interior rooms or closets; few homes had basements because limestone bedrock was usually close to the surface. People in this area were advised to take shelter in closets and bathtubs with mattresses for cover, but in this case it did not matter.
Within minutes, the F5 tornado wiped 50 homes in Jarrell completely off their foundation slabs (p Figure 15-27). Hail the size of golf balls and torrential rain pounded the area. Wind speeds were 400 to 435 kilometers per hour for the 20–25 minutes the twister was on the ground. At least 30 people died. One woman had hidden under a blanket in her bathtub. Her house blew apart around her, and both she and the tub were thrown more than 100 meters. She survived with only a gash in her leg. Some people watched the tornado approach and decided to outrun it by car. They survived; but in other tornadoes, people have died trying to do this when they would have survived at home. Eyewitnesses reported that the Jarrell tornado lifted one car at least 100 meters before dropping it as a crumpled, unrecognizable mass of metal. This was the second tornado to strike Jarrell; the first was only eight years earlier, on May 17, 1989. One of several tornadoes during the same event moved south through the town of Cedar Park, demolishing a large Albertson’s supermarket, where 20 employees and shoppers huddled in the store’s cooler. One of us hap-
pened to be a few kilometers south of Cedar Park playing golf that hot and humid Texas morning. Thunderstorms began to build on the horizon, and the sky took on a greenish gray cast. Early in the afternoon, golf course attendants quickly drove around the course warning players that two tornadoes had been spotted in the area. Because thunderstorms and tornadoes are fairly common there, many people become complacent; several people thought about finishing their golf rounds. Reaching the car in a drenching downpour, we realized that there was no safe place to go. Our cell phones were useless because all circuits were busy. Fortunately, the tornadoes were north of us, so we drove south into Austin to wait out the storm.
p
AP Images/Ron Heflin.
FIGURE 15-27. The F5 Jarrell tornado stripped many homes in the Double Creek Estates subdivision down to their concrete slabs.
THUNDERSTORMS AND TORNADOES
447
1. For each of the following photos, indicate the hazard or hazards and any damaging events that have happened.
Critical View
2. Why should the event have been foreseen, and what could be done to prevent it?
B.
E.
H.
J.
448
K. CHAPTER 15
I.
A. Lanarre, USACE.
FEMA.
L. Skoogfors, FEMA.
G.
F.
M. Wolfe, FEMA.
NWS, Memphis.
Donald Hyndman photo.
D.
C.
NOAA.
D. Fuller, NOAA.
C. Clark, NOAA/NWS.
A.
M. Austad, USGS.
David Hyndman photo.
D. Bradford, NOAA/NWS.
3. Where plausible, evaluate what can be done to stabilize the area.
L.
Chapter Review
Key Points Thunderstorms p Thunderstorms are most common at equatorial latitudes, but the United States has more than its share for its latitude. Storms form most commonly at a cold front when unstable warm, moist air rises rapidly into cold air and condenses to form rain and hail. Cold fronts from the northern plains states often interact with warm, moist air from the Gulf of Mexico to form a northeast-trending line of storms. Figure 15-3.
p Collisions between droplets of water carried in updrafts with downward-moving ice particles generate positive charges that rise in the clouds and negative charges that sink. Because negative and positive charges attract, a large charge separation can cause an electrical discharge— lightning—between parts of the cloud or between the cloud and the ground. If you feel your hairs being pulled up by static charges in a thunderstorm, you are at high risk of being struck by lightning. Figure 15-5.
p Thunder is the sound of air expanded at supersonic speeds by the high temperatures accompanying a lightning bolt. Because light travels to you almost instantly and the sound of thunder travels 1 kilometer in roughly three seconds, if the time between seeing the lightning and hearing the thunder is three seconds, then the lightning is only 1 kilometer away.
p You can minimize danger by being in a closed building or car, not touching water or anything metal, and staying away from high places, tall trees, and open areas. If trapped in the open, minimize contact with the ground by crouching on the balls of your feet.
p Larger hailstones form in the strongest thunder-
Tornadoes p Tornadoes are small funnels of intense wind that may descend near the trailing end of a thunderstorm; their winds move as fast as 515 kilometers per hour. They form most commonly during collision of warm, humid air from the Gulf of Mexico with cold air to the north. They are the greatest natural hazard in much of the Midwestern United States. The greatest concentration of tornadoes is in Oklahoma, with fewer to the east and north. Figure 15-11.
p The Fujita tornado scale ranges from F0 to F5, where F2 tornadoes take roofs off some wellconstructed houses and F4 tornadoes level them. Tables 15-2 to 15-5.
p Tornadoes form when warm, humid air shears over cold air in a strong thunderstorm. The horizontal rolling wind flexes upward to form a rotating cell up to 10 kilometers wide. A wall cloud sagging below the main cloud base is an obvious danger sign for formation of a tornado. Figures 15-10, 15-15, and 15-17.
p On radar, a hook echo enclosing the intersection of two fronts is a distinctive sign of tornado development. Figure 15-18.
p The safest place to be during a tornado is in an underground shelter or an interior room of a basement. Even being in a strongly built closet or lying in a bathtub can help. If caught in the open while driving a car, you may be able to drive perpendicular to the storm’s path. If you cannot get away from a tornado, your car may provide some protection, or lying in a ditch and covering your head will help protect you from debris flying overhead.
storm updrafts and cause an average of $2.9 billion in damage each year.
THUNDERSTORMS AND TORNADOES
449
Key Terms charge separation, p. 432
hook echo, p. 440
superoutbreak, p. 437
tornado outbreak, p. 436
cumulonimbus clouds, p. 431 downbursts, p. 434
latent heat, p. 438 lightning, p. 432
thunder, p. 433 thunderstorms, p. 431
tornado warning, p. 440 tornado watch, p. 440
Fujita scale, p. 443 hailstones, p. 434
mammatus clouds, p. 440 step leaders, p. 433
tornadoes, p. 435 Tornado Alley, p. 436
wall cloud, p. 435 wind shear, p. 434
Questions for Review 1. When is the main tornado season? 2. How are electrical charges distributed in storm clouds and why? What are the charges on the ground below?
7. In what direction do most midcontinent tornadoes travel along the ground? 8. How fast do tornadoes move along the ground? 9. What is a wall cloud, and what is its significance?
3. What process permits hailstones to grow to a large size?
10. Why does lying in a ditch provide some safety from a tornado?
4. Why do you see lightning before you hear thunder? 5. List the most dangerous places to be in a lightning storm. 6. What should you do to avoid being struck by lightning if caught out in the open with no place to take cover?
11. How do weather forecasters watching weather radar identify an area that is likely to form tornadoes? 12. What is the greatest danger (what causes the most deaths) from a tornado?
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CHAPTER 15
Wildfires
Chapter
Sonny Archuleta photo, USFS.
16 i
The Storm King Mountain fire races upslope toward fleeing firefighters.
A Deadly Wildfire
A
Wildfires
s the Storm King fire spread out below and west of them, firefighters recognized that they were in danger. One group of eight smokejumpers began hiking up toward a burned-out patch that had been identified as a “safety zone.” They abandoned chain saws and fuel used to fight the fire and hiked up a ridge ahead of the fire as fast as they were able. There they deployed their fire shelters; air tankers dropped fire retardant around them and they survived. Another group of firefighters backtracked up the fire line toward the main ridge, but not all of them would reach safety. The fire moved up the canyon at about 1 meter per second, up open slopes at 2 meters per second. A few managed to reach the ridge and followed a steep drainage to the southeast and eventually down to the highway. Some surviving firefighters received first- and second-degree burns on their backs, necks, and elbows.
451
Eleven firefighters, racing upslope ahead of fast-moving flames, were overtaken and died 60 to 85 meters below the main ridge crest that would have slowed the fire’s advance. Another firefighter died 35 meters below it. Two others reached the main ridge and then were forced northwest towards a rocky face. Unfortunately, that led to a deep dead-end gully where they died when hot air and smoke engulfed them (Case in Point: “Debris Flows Follow a Tragic Fire—Storm King Fire, Colorado, 1994,” p. 462).
Fire Process and Behavior Wildland fires are a natural part of forest evolution. They benefit ecosystems by thinning forests, reducing understory fuel, and permitting the growth of different species and age groups of trees. It is only when fires encroach on human environments that they become a hazard. Factors that affect wildfire behavior include fuel, weather, and topography. In some cases, the fuel and conditions foster a firestorm that is virtually impossible to suppress without a change in the conditions.
Fuel
p
OX YG EN
AT HE
Modified from USFS.
A fire requires three components—fuel, oxygen, and heat— and cannot progress without all three (p Figure 16-1). The type of fuel available, its distribution, and its moisture content determine how quickly the fire ignites and spreads, as well as how much energy it releases. Fuel loading refers to the amount of burnable material. Trees and dry vegetation are the primary sources of fuel for a wildfire. They burn at high temperature by reaction with oxygen in the air. The main combustible part of wood is cellulose, a compound of carbon, hydrogen, and oxygen.When
Fire Triangle FUEL
FIGURE 16-1. The “fire triangle”: A fire requires fuel, oxygen, and heat. Without any one of these, a fire cannot burn.
452
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it burns, cellulose breaks down to carbon dioxide, water, and heat. Shrubs and trees also contain natural oils or saps that add to the combustibles. Vegetation with large relative surface areas accelerates ignition and burning because heat that promotes ignition begins at the outer surface of the fuel, then works its way inward. The large relative surface area of dry grasses, tree needles, and to a lesser extent shrubs leads to their greater ease of burning relative to trees. Fires in dry grass spread rapidly. With a small total mass of burnable matter on the ground—that is, grass, dry leaves, shrubs, and litter—a ground fire may move through fast enough to consume the ground cover and understory brush but not ignite the trees. Heavy fuels such as tree trunks have a small surface area compared with their total mass; they ignite with difficulty and burn slowly. Some tree species such as ponderosa pine have evolved with time to survive fires that burn ground vegetation. Low brush and branches are known as ladder fuels because they ignite first and then allow the fire to climb into higher treetops (p Figure 16-2). The ability of fire on the ground to easily reach tree crowns has a major effect on the rate of spread and intensity of a wildfire. Once flames reach into the trees and then to treetops, the smaller branches and fine needles easily ignite to form a crown fire ten or more meters high. A large crown fire will burn out at a single location, such as a cluster of trees, in less than a minute. Burning sparks and embers can blow off to ignite new fires downwind (p Figure 16-3).With only radiant heating, the thermal energy transferred by thermal radiation, without movement of air or contact with flames, 50 seconds of intense crownfire burn is sufficient to ignite dry fuels at a distance of 20 to 25 meters. The intensity of a wildfire depends heavily on different types of vegetation. Lightweight flammable materials, such as dry grass and leaves, ignite easily but burn up quickly without generating very much heat. At the other extreme, heavy materials such as tree trunks are more difficult to ignite but burn much longer and generate much more heat. These include heavy coniferous (softwood) forests. By contrast, deciduous (hardwood) forests burn at somewhat lower intensities.
National Park Service photo.
Hyndman photo.
a
FIGURE 16-2. a. Dense ladder fuels in Glacier National Park, Montana. b. Ladder fuels permit wildfire to climb into the trees; Yellowstone Park fire, 1988.
AP Photo/David J. Phillip.
p
b
p
FIGURE 16-3. Firebrands blow past firefighters in a 2005 fire.
Ignition and Spreading Fires can be naturally started by lightning strikes, intentionally set for beneficial purposes, or set accidentally or maliciously. Nationally, lightning-started fires account for only 13 percent of forest fires, according to U.S. Forest Service
figures. Prescribed debris burns that ran out of control accounted for 24 percent; amazingly, arson accounted for 26 percent. Various unintentional causes make up the remainder. A major heat wave in southern Europe in the summer of 2007 fostered catastrophic wildfires. Some fires in Greece and Italy were ignited by arsonists in protected forests to create new areas for construction. In 2000, Italy passed a law that bans construction for ten years after a wildfire, but the rules are not always enforced. Investigators searching for clues to a fire’s origin look for evidence left behind, including remnants of a campfire, matches, cigarettes, and accelerants such as gasoline. They use wind direction when the fire began and indications of the direction in which the fire moved (p Figure 16-4). Fire ignites and progresses primarily by three mechanisms: radiation, convection, and firebrands or burning embers. Radiant energy decreases with the square of the distance from the heat source, that is, at twice the distance the energy on a surface drops by four times (p Figure 16-5). Convective transfer of heat is more direct and efficient; it involves direct movement of heated air or flames to ignite fuel. Firebrands carried by the wind ignite spot fires, which burn ahead of the main part of the fire but can quickly spread. WILDFIRES
453
National Park Service photo.
Heat
p
Lawrence-Berkeley Lab photos.
FIGURE 16-6. Heat rising from a fire moves upslope, and flames attack new fuels above them. Yellowstone Park fire, 1988.
p
720
100
600 (10 min)
80
480 360
60 Radiant heat
40 20 0
0
10
20 30 40 Distance from flame (meters)
Ignition time (secs)
120 Radiant heat flux (kW/sq m)
Jack Cohen, fire research, U.S. Forest Service.
FIGURE 16-4. The more severely burned white side of this tree indicates the direction from which the fire came; in this case, the fire moved upslope.
240 (4 min) 120 (2 min) 90 secs 0 50 Radiant heat flux Ignition time
p
FIGURE 16-5. Radiant heat (red line) decreases rapidly away from a flame that is 20 meters high and 50 meters wide. The time for flammable material such as wood to ignite (blue line) increases farther from the flame—for a wall of flame 20 meters high.
Topography Local topography, especially canyons, can funnel air, accelerate the fire, and cause more rapid spreading. In a canyon, as sometimes initiated by an untended campfire, can race
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up the valley because of this chimney-like funneling effect. Flames rise because their heat expands air, making it less dense. Thus, fires generally move rapidly upslope but only slowly downslope (p Figure 16-6). On a hillside, especially a steep one, flames and sparks rise with their heat, promoting the upslope movement of fires into new fuel. Although flaming branches and trees can fall downslope, advance in that direction tends to be slow. To be upslope from a fire is to be in serious danger, as were the firefighters whose story introduced this chapter. At a ridge top, fire often slows dramatically because the rising flames run out of fuel. A fire reaching a ridge crest can create an updraft of air from the far side to fan the flames, but the same updraft can reduce the chance that the fire will move down the other side.
Weather Conditions Weather conditions can both ignite a fire and determine the rapidity of its spread. Lightning wildfires are ignited by thunderstorms triggered at weather fronts, especially after a period of dry weather (p Figure 16-7). Lightning strikes in an area of heavy timber, at lower elevations, or during high winds can ignite a fire that progresses rapidly. Lightning strikes on rocky mountain peaks often have limited effect because fire progresses slowly downslope, and fuels there are often limited. Fires start more easily and spread rapidly during dry weather because of low fuel moisture. High humidity makes it more difficult for fires to start and to continue burning, whereas high temperatures and winds progressively reduce moisture over time and increase the fire hazards. A few years of less than normal moisture dehydrates the soil and lowers the water table, providing less moisture for the growth of trees and undergrowth and for all of the vegetation. It may take several wet seasons to replenish the water shortfall below the surface. Thus, surface vegetation during a drought may green up in the rainy season but dehydrate quickly as dry weather returns.
Erosion Following Fire
Mark S. Moak photo, USFS.
Major fires often result in severe slope erosion in the few years following the fire. In normal, unburned areas, vegetation has a pronounced effect in reducing the runoff after rainfall and snowmelt. Tree needles, leaves, twigs, decayed organic material, and other litter on the ground soak up water and permit its slow infiltration rather than turning it into surface runoff. Evapotranspiration by trees and other vegetation also decreases the amount of water soaking deep into the soil. Intense fires burn all of the vegetation, including litter on the forest floor (p Figure 16-8a). The organic material burns into a hydrocarbon residue that soaks into the ground. This material fills most of the tiny spaces between fine soil grains in the top few centimeters of soil and sticks them together (p Figure 16-8b). In some cases, water can no longer infiltrate the surface soils, thus most of it runs off. Such impervious soils are called hydrophobic soils. The hydrophobic nature of soils is often attributed to intense burning, but the relationship to fires is not entirely clear. What is clear is that water more readily runs off the surface of the charred soil. Big rain droplets beat directly on the unprotected ground. When rainfall is heavy and vegetation and ground litter have been removed by fire, the water cannot infiltrate fast enough to keep up with the rainfall. Raindrops splash fine grains off the surface, and the runoff carries them downslope as sheetwash or overland flow. Unprotected soils are easily gullied (p Figure 16-9). The local channels lead to formation of rills and gullies. All of this surface runoff quickly collects downslope in larger gullies and small valleys. Water collecting in a valley with a large enough watershed quickly turns into a flash flood or debris flow. These destructive and
p
FIGURE 16-7. Lightning ignites fires in the Selway-Bitterroot Wilderness area of the northern Rockies on August 9, 2005.
Wind conditions dramatically affect the spread and direction of fires. Winds accelerate fires both by directing the flames at new fuels and by bringing in new oxygen in the air to aid in burning. Once fires reach high into trees and crown, firebrands—burning embers carried downwind from flaming treetops—can ignite fuel as much as a kilometer away. High winds or extreme amounts of dry fuel can initiate firestorms that generate their own winds as the convective updraft of heat from a fire draws in new air from its sides, helping to fan the flames.
Secondary Effects of Wildfires A major fire can lead to other hazards, such as floods and landslides, which are sometimes more disastrous than the fire itself.
Donald Hyndman photo.
Thomas Spittler photo, California Department of Forestry.
Surface of soil sealed by resins formed during fire
a
p
b
FIGURE 16-8. a. This steep slope on the O’Brien Creek drainage at the western edge of Missoula, Montana, had been ravaged by fire one month earlier leaving it vulnerable to erosion. b. In hydrophobic soil such as this produced by the Banning fire in Southern California in 1993, the dry layer lies beneath the dark gray hydrophobic layer, which is composed of soil and ash. WILDFIRES
455
M. Rieger photo, FEMA.
Donald Hyndman photo.
b
p
FIGURE 16-9. a. These gullies were eroded by a short-lived rainstorm one year after the 2000 fires in the southern Bitterroot Valley, Montana. b. This mudslide followed the Hayman, Colorado, fire on July 5, 2002.
a
dangerous floods rip out years of accumulated vegetation, soil, and loose rock, carrying everything down valley in a water-rich torrent (see Case in Point: “Debris Flows Follow Fire—Storm King Fire, Colorado, 1994,” p. 462). Rapid accumulation of water from overland flow following a fire creates a severely shortened and heightened stream hydrograph. This can lead to flash floods that wash out bridges, roads, and buildings. After a region has burned, it is unsafe to drive or walk in small valley bottoms during intense rainstorms because swift runoff funnels most of the precipitation into adjacent gullies and then quickly into sequentially higher-order streams. Even where the storms are several kilometers away in the headwaters of a burned drainage, a flash flood downstream can appear under a clear blue sky. Homes that may have survived a fire in usually dry valley bottoms are vulnerable to debris flows from all of the new sediment washed downslope after a fire.
Wildfire Management and Mitigation Wildfires are necessary to the evolution of forest ecosystems, but out of control they pose hazards to humans. Wildfires occur in most regions of the United States, where they affect wildlands and often encroach on suburban or builtup areas (p Figure 16-10). From 1985 to 1994, an average of 73,000 fires per year burned on federal lands, consuming more than 3,000,000 acres and costing $411 million for fire suppression. From 2000 to 2007, annual firefighting costs
Wildfires in the United States and Puerto Rico
Erosion following a fire cannot be prevented, but it can be minimized. Federal and state agencies and individuals can plant vegetation, grass, shrubs, and trees that prevent the direct impact of raindrops on bare soil. Over large areas, slopes are often seeded soon after a fire. Straw can be packed into tubes or laid out in bales to provide a barrier to overland flow, thus reducing slope erosion. Felling dead trees across the slope can have a similar effect. Drains can direct water flow laterally to valleys to minimize surface gullying. On small steep patches of particularly vulnerable soil, sheets of plastic can be spread to prevent water from reaching the soil.
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U.S. Forest Service and others.
Mitigation of Erosion
p
FIGURE 16-10. A map of large wildfire locations from 1980 to 2003.
suppression have led to a buildup of wildland fuels. Very large wildfires in recent years include the 2002 Biscuit fire in Oregon, the 2002 Rodeo-Chediski fire in Arizona, the 2006 Darby fire in western Montana, and the 2007 Milford Flat fire in Utah, each covering more than 400 square kilometers. Federal agencies now recognize that fire is a natural part of wildland evolution, a process necessary for the health of rangelands and forests. They recently shifted to mitigation and management of fires, including prescribed burns, and now permit many wildfires to burn in wilderness areas as long as they don’t endanger buildings or important resources.
exceeded $1.3 billion in four of the seven years. Additional millions were lost in timber values, and indirect costs resulting from landslides and floods are unknown. Hazards to the public increase as more people move to forest, range, and other wildlands. Over ten days in October 2003, catastrophic fires destroyed 3,452 homes in Southern California (Case in Point: “Firestorms Threaten a Major City— Southern California Firestorms, 2003 and 2007,” p. 463). From October 25 to November 3, 1993, Santa Ana winds fanned 21 major fires in Southern California, burning 76,500 acres and 1,171 buildings. Three people died. As with all hazards, a clear understanding of the natural processes underlying fire behavior allows governments and individuals to take steps to protect lives and property.
Fighting Wildfires When wildfires approach human developments, several approaches are used for fire suppression. Highly trained smokejumpers parachute into remote areas of lightningcaused spot fires to exterminate them before they spread out of control. Bulldozers cut firebreaks to remove vegetation down to bare mineral soil, slowing the spread of fire on the ground. Aircraft drop giant loads of water or fire retardant along the lateral and advancing edges of fires to help direct and contain them (p Figure 16-11). Retardants include ammonium sulfate, ammonium phosphate, borate, or swelling clay. Helicopters scoop huge buckets of water from nearby streams or lakes and dump them on the edge of fires. Firefighters on the ground cut fire lines to prevent creeping ground fires from spreading laterally. For the largest fires that threaten towns or critical facilities, firefighters sometimes deliberately set burnouts (p Figure 16-12). In a burnout, a large area in the path of a big fire is burned under controlled conditions and suitable weather, generally beginning at a road or river; the burnout fire burns back toward the main fire, eliminating fuel and stopping the progress of the main fire. Before lighting a burnout, the forest or range between the burnout and the area to be
Government Policy
David Hyndman photo.
Kelvin May photo, Canadian Forest Service.
The U.S. Forest Service was formed in response to a catastrophic wildfire that burned thousands of square kilometers of northern Idaho and western Montana in the summer of 1910. For most of the twentieth century, their policy was to aggressively fight wildfires. Until a few years ago, the Forest Service and its Smokey Bear mascot maintained that fire was bad and all fires should be extinguished as quickly as possible. Because the United States spends upward of $1 billion per year on fighting fires, you might expect some significant long-term benefit. However, many experts argue that we get no long-term benefit. In fact, preventing fires leads to buildup of wildland fuels, which ultimately leads to worse fires. Forest tree densities and fuel loads have risen to critical levels, creating ideal conditions for fires. Many open, parklike forests became choked with closely spaced smaller trees and dense underbrush, leading to disastrous wildfires, as in the case of the Bitterroot Valley fires in 2000 (Case in Point: “A Major Wildfire After Years of Fire Suppression— Bitterroot Valley Fires, Montana, 2000,” p. 465). Decades of fire
a
p
b
FIGURE 16-11. a. A helicopter fills up at a local pond fire in Sonoma Valley, California, before dropping its load on a fire nearby. b. “Flying boat” used to scoop water from a lake or reservoir before dropping it on a fire.
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Typical weather patterns affecting wind direction or moisture for a region also influence fire risk. Fires tend to advance in the direction of the prevailing winds, so that fires in the northern United States and southern Canada tend to burn toward the northeast with the westerly winds (see Case in Point: “A Major Wildfire After Years of Fire Suppression—Bitterroot Valley Fires, Montana, 2000,” p. 465). In the trade winds belt of the southern United States and northern Mexico, fires tend to burn toward the southwest.Thus, homes in the prevailing downwind direction from fire-prone or high-fuel areas are more at risk than those upwind from such areas. The desert areas in the Southwest, from northwestern Texas to southern California, and northward to southern Wyoming are at higher risk because of their low moisture levels. Areas with more moisture are at less risk. Along the west coast, prevailing westerly winds carry moisture off the Pacific Ocean, and in the southeastern states storms bring abundant moisture north off the Gulf of Mexico. Agencies also monitor shorter-term weather conditions to alert the public of rising risk levels. The favorability of weather conditions for wildfire is called its fire weather potential (see Figure 16-13b). Fire weather potential is posted on the fire danger-level signs along roads by the U.S. Forest Service or Bureau of Land Management. For a local area, it depends primarily on the number of hightemperature days, relative humidity of the air, moisture in available fuels, and wind speed. For large regions, it depends on similar factors over periods from weeks to months. Drought maps provide a record of longer-term moisture deficits. A high-resolution infrared sensor is used to measure greenness of vegetation and inter-fuel moisture content. This greenness is compared with the typical value for each map area. Such estimates are used to infer fire danger and to inform the public through news media and roadside signs.
Fire movement
Main fire
p
Burnout
FIGURE 16-12. Use of a “burnout” to hinder the progress of a wildfire.
protected is thoroughly drenched using planes and helicopters. The technicalities of a burnout are complex and dangerous, but they can work remarkably well. In July 2003, firefighters set a large burnout that saved the town of West Glacier at Glacier National Park from a huge fire that was bearing down on the area.
Risk Assessments and Warnings Several government agencies study patterns in wildfire occurrence and spreading to assess the risk for regions of the United States at different times. Closely monitoring risk levels allows officials to reduce hazards and prepare emergency plans. The vegetation, and in some regions topography, can increase their risk of fire (p Figure 16-13). These include grasslands east of the Rocky Mountains and agricultural lands in the Midwest. Heavy coniferous (softwood) forests in the Northwest, the Rocky Mountain states, northern New England, and eastern Canada ignite more slowly but burn at high intensities. By contrast, deciduous (hardwood) forests of the southeastern states burn at somewhat lower intensities.
Low Moderate High Non Veg
Agriculture Water
a
Extreme Fire Weather Potential
USFS, Rocky Mountain Fire Lab.
USFS, Rocky Mountain Fire Lab.
Potential Fire Exposure
Low Moderate High Extreme
b
p
FIGURE 16-13. a. Potential fire exposure depends upon different vegetation types, how quickly fire spreads, and with what intensity. b. Extreme fire weather potential in the continental United States averaged over 16 years.
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Protecting Homes from Fire People who live in the woods do so to get away from urban congestion and for the beauty of nature. They like to be surrounded by trees and shrubs, but in wildfires, trees and shrubs are fuel. Not only do they put themselves and their property in danger, but they also endanger firefighters and compel fire management officials to expend limited resources protecting individual properties rather than managing the overall fire. The risk to flammable structures, especially people’s houses, depends on the fire weather potential, the regional type of vegetation—how fast fire will spread and how hot it will burn—and the housing density. At one extreme, fire in a wilderness area or unpopulated desert endangers no houses. At the other extreme, a firestorm in a metropolitan area can become a catastrophe because it affects numerous homes and people. Although individual homes scattered throughout the forest might constitute a low-risk area overall, the individual homes could be in severe danger, especially if they are built from flammable materials and surrounded by ground fuels. Even those who live in the urban interface near the forest are in danger, particularly in dry, hot weather, where a fire is driven by high winds. Firebrands carried from burning treetops of crown fires can easily ignite flammable rooftops and decks. Woodshingle roofs, which are favored by many people for their natural appearance, often ignite from falling firebrands, even where a wildfire is not close and there is no nearby vegetation. Once ignited, the fire spreads rapidly across the roof and into the house (p Figure 16-14). Wood walls will not ignite by this process because the burning embers do not remain in contact with the wood long enough to
ignite it. Composition shingles are better than wood shakes because although they may burn if heated enough, the fire will not spread to other areas. Those who live in the forest can minimize the danger by building with flame-resistant materials. With a nonflammable roof and deck and no fuel within 30 meters, a building has a good chance of surviving, even with falling firebrands. Burning embers falling on a metal roof will not ignite it, although if the roof gets sufficiently hot the wood framework supporting it will ignite. Metal roofs, though not as natural, are flame resistant. Trees or ground fuels around a home can also be a danger, either from the spread of fire along the ground or from falling firebrands. Low-intensity fires often burn forest homes, even though surrounding trees and fences may survive, because low-intensity fires burn ground fuel, such as needles, leaves, and twigs, without reaching higher into the trees (p Figure 16-15). In the aftermath of many wildfires, investigators frequently find homes burned to the ground but the trees still mostly green (p Figure 16-16). The best protection for homes in forests is fuel reduction before the fires start. Homeowners should remove underbrush, lowhanging ladder fuels, and especially the dry ground fuels (p Figure 16-17). Even once ground fuels are cleared, radiant heat from nearby burning trees can ignite a home. Since radiant heat decreases rapidly away from a fire, clearing fuels out to about 30 meters from a home will generally protect it from ignition by that mechanism (p Figure 16-18). In fact, even if people are close enough to a wall of flame to be burned severely, dry wood structures may not ignite (p Figure 16-19). Exposure to heat that will produce a second-degree burn on skin in 5 seconds will take more than 27 minutes to ignite wood!
p
© John Gibbons/San Diego Union-Tribune/ZUMA Press.
FIGURE 16-14. Roof fires ignited by firebrands. The homes might otherwise have been saved from low-intensity ground fires nearby.
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Jack Cohen photo, USFS.
Dick Spiess photo, Poudre Fire Authority.
a
b
p
Jack Cohen photo, USFS.
Andrea Booher photo, FEMA.
FIGURE 16-15. a. Missionary Ridge fire, Durango, Colorado, June 2002. Surface fire spread through ground needles to ignite house but did not burn part of wood fence, plastic garbage cans, ponderosa pines, and some low vegetation. b. Summerhaven, Arizona, June 2003. Surface fire spread to house but did not burn low-hanging branches. Note needles on ground.
a
b
p
Donald Hyndman photo.
National Park Service photo.
FIGURE 16-16. a. Low-intensity ground fires moved along ground in needles and twigs to burn these houses near Los Alamos, New Mexico, in May 2000 but did not burn much of the low vegetation, most trees, or a wood fence. b. In the Cerro Grande fire at Los Alamos, New Mexico, in May 2000, houses on one side of a street in a forested community burned to the ground, whereas those on the other side remained unscathed. The ground fire was stopped by the street, indicating that it spread through ground fuels.
a
b
p
FIGURE 16-17. a. Ladder fuels have been removed from this slope to prevent fire from easily reaching the tree branches, but dry grass can provide fire a path to reach the house. b. This forest east of Jackson, California, has been made more resistant to fire by removal of ground vegetation and ladder fuels.
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Jack Cohen, U.S. Forest Service, Missoula, MT.
Jack Cohen, U.S. Forest Service, Missoula, MT.
a
b
p
Rick Trembath photo, USFS.
Rick Trembath photo, USFS.
FIGURE 16-18. a. A wall of heat from a wildfire can ignite flammable material in a home if it gets close enough. Thinning trees and brush around a home minimizes the heat and flame height, and therefore the danger to the home. b. Some ignition sites from firebrands include ground fuels, woodpiles, and flammable roofs.
a
b
p
In many cases, government regulations on building materials can mitigate the damages from future fires. On October 20, 1991, a firestorm in the hills upslope from Oakland, California, destroyed or damaged 3,354 single-family homes and 456 apartments (p Figure 16-20). It killed 25 people, and damage amounted to $2.2 billion. Eighty percent of the homes were rebuilt in the same risky hillside locations with their views of San Francisco Bay and the Golden Gate Bridge. However, new restrictions dictate that roofs be fire resistant, decks and sheds be built with heavier materials that will not burn as quickly, and landscaping use fire-resistant plants.
Public Cost of Fires As with all hazards, the best way to mitigate damages from fire is to keep people from building in high-risk areas. Unfortunately, with the fiercely independent attitudes of many
Lawrence-Berkeley Lab photo.
FIGURE 16-19. A wall of flame at the edge of a forest clearing provides enough radiant heat to severely burn skin but does not burn either wood posts or siding on a home. Wedge Canyon fire, North Fork, Flathead River, Montana, 2003, a. during fire and b. after fire.
p
FIGURE 16-20. Little remained of expensive houses on the steep slopes of the 1991 Oakland-Berkeley hills fire but their chimneys and foundations.
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rural residents, zoning restrictions are considered unacceptable infringements on their freedoms. They insist on their right to do as they wish with their property. Some even build in so-called indefensible locations such as narrow canyons that are too dangerous for firefighters. Who pays for the costs of fighting fires and trying to save the homes of those who choose to live in fire-danger zones? Insurance companies generally set premiums at levels based on risk and replacement costs. Fire-insurance rates, though rising, pay for only a small part of the cost because federal and state governments pay most of the costs of fighting the fires and cleanup and generally pay part of the cost to help people rebuild.When governmental agencies spend millions of dollars fighting fires, and most of those efforts are expended in protecting homes, much of the real cost is on the shoulders of the general public rather than on the people who choose to live in wooded areas. Although fire insurance for homes in the wildland-urban interface is not as high as might be expected given the greater hazard, some insurance companies are now requiring wildland homeowners to clear brush, cut down trees, or install a fireproof roof. They are also dramatically increasing insurance premiums. The country’s second largest insurer
announced in May 2007 that it would no longer provide new homeowner policies in California because of risks from wildfires and earthquakes. One needs to ask whether public funds should be used to help people rebuild in places where wildfires and the associated floods and erosion are ever-present. Our conscience tells us that we should help those in need, but such help merely encourages them and others to live in such vulnerable areas. Perhaps, as FEMA has finally learned, we should help people who are willing to relocate to more suitable and less vulnerable places. An alternative to zoning is to make clear that if people do build in a vulnerable location, they must not expect public help in times of crisis. Such people should not expect help in fighting fire, stabilizing streams or hillsides before or after floods, or rebuilding after catastrophes. If people still insist on living in fire-prone areas, they could create special fire-prevention districts that would tax their members—for example, a few thousand dollars per year—to create a pool of funds to pay the costs of future fire protection for their homes.
Case in Point Debris Flows Follow a Tragic Fire Storm King Fire, Colorado, 1994 u On the afternoon of July 2, 1994, lightning ignited a fire on a ridge flanking Storm King Mountain 7 miles west of Glenwood Springs, Colorado. Dry lightning storms had ignited 40 fires in the general area two days earlier; firefighting teams were assigned to the fires threatening lives, residences, other buildings, and power lines. Firefighters were assigned to the Storm King fire two days later, when it appeared to be spreading. It spread slowly downslope in leaves, twigs, and grass under open pinyon-juniper and oak forest. Bureau of Land Management and U.S. Forest Service firefighters, including smokejumpers, supported by helicopter water drops, had been cutting fire lines flanking the fire and around spot fires. On July 6, as a cold front moved in from the west, the humidity dropped, and winds from the Colorado River Gorge
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below were partly redirected up a side canyon to the north (p Figure 16-21). The combined winds of 65 to 80 kilometers per hour created a shear vortex that lifted and spread burning embers well beyond the fire lines. Individual trees began to torch as the ground fire spread into trees. Strong, erratic winds and heavy smoke hampered effectiveness of the helicopter water drops. Fourteen firefighters working on firelines along one flank of the fire and at the ridge above were endangered as the winds increased in speed and varied direction. They died when they were overrun as they ran upslope ahead of the fast-moving fire. In the weeks that followed the fire, rains washed large amounts of ash and mineral soil downslope into drainage channels. Up to a meter of loose, silty sand and ash accumulated along the sides of most drain-
ages, adding to coarser fragments of colluvium collected by slow downslope movements over the years. This set the stage for erosional events that followed. On the night of September 1, 1994, torrential rains flushed most of this loose material downslope into larger canyons to mobilize debris flows. With no vegetation to slow the flow over the ground surface, heavy rain led to rapid runoff. Rills and
(continued) r th No
r Sto
ing Mountain mK
Free air winds
p
FIGURE 16-21. Wind patterns controlling the Storm King Mountain fire. Regional westerly winds (light blue) and strong winds in the Colorado River Gorge (dark blue) were partly redirected up a side canyon and by rising fire currents in this canyon. Turbulent fire eddies carried burning embers spread spot fires behind the firefighters.
Free air winds
on Up-cany
J. Kautz, USFS.
wi nd s
Colorado River Gorge winds
gullies formed on the bare hillsides, indicating rapid overland flow. These efficiently carried surface runoff to larger channels. The flow came down across Interstate Highway 70 as mud, boulders, and debris to engulf 30 vehicles and push some into the adjacent Colorado River. Fortunately, no one died in these flows. Some debris fans blocked almost half of the river. Every major drainage, in which the headwaters burned, supplied debris flows that reached canyon mouths. Calculated velocities for many flows reached 4.6 to 8.5 meters per second, and discharges were estimated to be from 73 to 113 cubic meters per second.
Case in Point Firestorms Threaten a Major City
Southern California Firestorms, 2003 and 2007 u The first of eleven major fires of the season began in Southern California on October 21, 2003. Hot, dry Santa Ana winds that blew westward out of the high deserts to the east gusted to 100 kilometers per hour. The air compressed and heated up as it descended towards the Pacific, rapidly dehydrating soils and vegetation. Fires ignited by sparks from power lines downed by the wind, by careless handling of campfires, barbeques, and cigarettes, and even by arson. Ten days later, the rapidly spreading fires had burned more than 300,000 hectares, or 3,000 square kilometers, about the area of Rhode Island (p Figure 16-22). They leveled 3,600 homes and killed 24 people. Nine hundred of the homes were in the San Bernardino Mountains east of Los Angeles in a fire likely started by arson on October 25.
The largest, the Cedar fire in San Diego County, burned 2,207 homes and caused 15 deaths (p Figure 16-23). A lost hunter trying to signal rescuers started that fire. Many of those who died in 2003 had ignored evacuation orders, waiting until the last minute to leave. By then, the fastmoving flames overtook their lone evacuation road, cutting off all escape. The worst fires were in San Diego County, where the high cost of available land encouraged developers and individuals to build in areas vulnerable to brush fires. Some neighborhoods were closely spaced wood houses surrounded by pine trees, with some trees wedged against flammable roofs and wood decks. California state law now requires strict standards for building with fireproof materials and clearing brush around homes. However, hundreds of
thousands of people ignore the rules, and enforcement has been limited. Four years of drought in the San Bernardino Mountains northeast of Los Angeles left the trees vulnerable to bark beetle infestation and killed large numbers of them. Although there are tight controls on fire-resistant building materials and brush removal, the cost of tree removal can be as high as $850 per tree, so few people
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NASA Terra Satellite image.
San Diego
a
p
FIGURE 16-22. a. The Southern California fires on October 26, 2003, can be seen in this satellite photo. The largest area of fire at the right edge of the image is on the eastern outskirts of San Diego. The largest area of fires in the northcentral part of the image is on the northern outskirts of Los Angeles. The Santa Ana winds blow the fires and smoke toward the southwest. b. The Southern California fires on October 22, 2007, can be seen in this satellite photo of the same area as in a. c. Fires bear down on San Diego in 2003.
b
© John Gibbons/San Diego Union-Tribune/ZUMA Press.
Los Angeles
NASA Terra Satellite image.
(continued)
c
p
Carol Jandrall, California Department of Forestry.
FIGURE 16-23. Many homes were closely surrounded by trees and brush, which made it impossible for firefighters to save them from the Cedar fire east of San Diego.
removed the dead trees. As one resident put it, “Those dead pine trees are just gasoline on a stick.” Fifteen thousand firefighters fought the fires, along with helicopters equipped with giant water buckets, and airtankers dropped massive volumes of fire retardant. Insured losses from these fires
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amounted to more than $1.25 billion. A change in weather on November 1 calmed the Santa Ana winds and brought moisture from the Pacific. Two or three centimeters of white, powdery snow fell in the Big Bear Lake resort area near San Bernardino, slowing the fires and permitting firefighters
to build fire lines. The only drawback was that the rain and snow caused mudslides that closed highways. Two months later, on December 25, torrential rains fell, with 4.39 inches on the San Bernardino Mountains and 8.57 inches on the San Gabriel Mountains. On the south side of Cajon Pass between them, the route of Interstate Highway 15, the unprotected soils of the burned area unleashed heavy mudslides that raced down canyons and through a trailer camp and recreation center. Fifteen people died in mud as much as 4 to 5 meters deep. Costs of firefighting and cleanup are estimated to reach another $2 billion. Fire insurance is still available; however, insurance companies build the heightened risk into their premiums. In some areas, insurers are becoming less willing to write coverage for fire. In others, homeowners are told to replace roofs and clear brush if they want to be covered. In Hook Canyon on the southeast side of Lake Arrowhead, 350 homes burned.
(continued) Some homeowners’ associations in scenic areas banned property owners from cutting trees, as in the same area in the San Bernardino Mountains. They lifted the ban less than a year before the fire, but by then it was too late to do much even for those inclined to remove trees. Population increases in outlying areas compound the problem. In recent years, as the population in the Sierra Nevada nearly doubled, so did the number of fires and acres burned, including large fires in 2007. Property damage in the same period went up 5,000 percent. In this region’s Mediterranean climate, more than 90 percent of the rainfall comes during the winter and early spring. Chaparral or scrub brush that covers many Southern California hillsides burns in wildfires every 35 to 65 years, primarily during late summer or early fall, when hot, dry Santa Ana winds blow off the deserts to the east. The hydrophobic soil layer produced by fires (see Figure 16-8b), along with the lack of vegetation, lead to greater overland flow and high stream flows. Studies suggest that erosion increases by a factor of 10 following chaparral fires. The frequency of fires and the damage produced are likely to increase dramatically as more people move to the woods. Then exactly four years later, on October 21, 2007, the hot, dry, Santa Ana
winds picked up to again ignite catastrophic fires in nearly the same areas as in 2003 (Figure 16-22b). A wet winter in 2004-05 that permitted abundant growth of trees and brush was followed by a dry 2006 and an extremely dry 2007 with only about one-fifth of normal rainfall that left the brush-covered hills tinder-dry. Some started when high winds downed power lines which sparked fires; a few others appear to have started by arson. One fire near Los Angeles was started by a child playing with matches. Once ignited, high shifting winds reaching gusts of more than 110 kilometers per hour, and essentially no humidity, fanned the intense fastmoving fires. Abundant dry brush, dense groves of eucalyptus trees, and waterstarved and insect-killed trees provided abundant fuel. Under intense conditions even air-drops of water and fire retardant are ineffective because they evaporate before reaching the ground. When flames are longer than about three meters, a fire is considered unstoppable and firefighters must retreat until conditions ease. Four days later when the winds died down, 2,000 square kilometers had burned, more than 800,000 people had been evacuated, and more than 2,300 homes and businesses were destroyed. Damages totaled more than $1 billion in San Diego County. Only seven people
died in the fires—a fact partly credited to aggressive evacuation and to a “reverse 9-1-1” calling system that warned people of the dangerous approach of a fire. The worst fire in the northeastern outskirts of San Diego was dubbed the Witch Fire. Farther north in the Santa Monica Mountains west of Los Angeles, the Canyon Fire burned into the center of Malibu. Erratic winds carried firebrands downwind for more than half a kilometer to ignite new fires. This, along with heavy smoke, made conditions extremely difficult for both firefighters on the ground and for firefighting aircraft. By early November and even with weakening winds, firefighters were still unable to contain two stubborn fires. Compounding the problem in areas such as southern California are two factors: Thousands of people live back in the wooded hills and rugged canyons on narrow, winding roads and federal policies call for firefighters to protect homes, even when they are built in high-risk, poorly accessible areas. Firefighters are under tremendous pressure to protect wildland homes in areas where elsewhere safety would dictate that they leave a fire to burn. Yet, in spite of major wildfires in 1970, 1993, and 2003, even more people rebuilt in the burned areas and with still larger homes. Something needs to change!
Case in Point A Major Wildfire after Years of Fire Suppression Bitterroot Valley Fires, Montana, 2000 u The fire season began in June with a scattering of lightning-caused fires in the mountains flanking the Bitterroot Valley just east of the Montana-Idaho border. By the end of July, five major fires were burning in the mountains around the southern Bitterroot Valley (p Figure 16-24). Then on July 31, a dry lightning storm started 78 new fires. Within a week, some merged into giant forest fire complexes. Many homes in the forest or on its fringes
burned. Eventually, 70 homes and 94 vehicles were lost, and more than 1,500 people had to evacuate their homes. The U.S. Forest Service closed all of the Bitterroot National Forest to public use, and soon thereafter the state closed all public lands until the fire danger subsided. The forest consists of areas of thickbarked ponderosa pine trees that live as open stands of large trees that are resistant to fire. Other areas are thicker stands
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British Columbia
Washington
Alberta
Idaho Montana
John McColgan photo, U.S. Forest Service.
(continued)
b
Missoula
p NASA Satellite Image.
FIGURE 16-24. a. Forest fires spread throughout the northern Rockies on July 15, 2003, as shown by the red areas on the image. Fires in Glacier National Park were just south of the Alberta-Montana border. Fires in the mountains farther south surrounded Missoula, Montana. Smoke plumes, clearly visible as bluish gray areas, are quite different from the white clouds across the region. b. Fires in western Montana on August 6, 2000, chased these two elk into the relative safety of the Bitterroot River’s upper drainage.
a
of Douglas fir and grand fir trees that do not survive most fires. Forest management practices that harvested the large ponderosas and vigorous fire suppression more than doubled the percentage of firevulnerable trees. Fires then burn more intensely and over larger areas than before modern forest management practices. The weather contributed to the extreme conditions, with the strong El Niño of 1998 causing dry weather in the Pacific Northwest and the Southwest. Lower than normal precipitation continued through the summer and early fall of 1999, and the snowpack for much of the mountain west was less than 70 percent of normal. Lower elevations lost their snow cover in early February, causing vegetation to green up early, and spring showers contributed to growth of abundant fuel for fires. Then the normally active storm track across the Pacific Northwest in June weakened and a series of weak low-pressure systems pushed into western Montana. These pro-
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vided little moisture but spawned numerous lightning storms. By late July and August, drought conditions were severe to extreme. Vegetation moisture dropped to critical levels in the forests. Leafy plants were changing colors as if it were fall, and pine trees began dropping needles. The proximity of the Northern Region Forest Service Headquarters and Smoke Jumpers Center in Missoula helped suppression efforts. Initial attack crews managed to control 86 percent of all of the fires within five days. Extremely dry conditions permitted fires to spread rapidly with breezes as little as 5 kilometers per hour. Several large fires in central Idaho and western Montana remained uncontrolled into September and burned a total of more than 1,335 square kilometers. The fires generated their own strong winds, and smoke columns reached as high as 9 kilometers. Burning twigs drifted 2 to 3 kilometers to ignite new fires. Firefighting crews were recruited from across the
United States, and 600 soldiers and crews came from Australia and New Zealand. A total of 3,000 firefighters and twice that many support people worked on the fires at the height of the disaster. With insufficient resources, the highest priority was to protect firefighters and the public, then protect communities, then homes and other structures, and finally critical natural resources. Land use and settlement also changed in recent years, with consequences for both forests and people. Between 1990 and 1998, the population of most of the Bitterroot Valley increased by 40 percent. People are drawn to the natural beauty of the area and the rural environment. Much of the growth is in unincorporated areas within or next to fire-vulnerable forests. Many of the newcomers are not familiar with the risks of forest fires or floods on seemingly peaceful little streams. They build their homes in heavily treed areas right at the edge of little streams with
(continued) almost no floodplain, or on tiny alluvial fans right at the mouths of gullies feeding them. In spite of the unsuitability of these building sites, the owners expect firefighters to concentrate on saving their homes at the expense of the rest of the forest. Some people refused to leave despite mandatory evacuation orders. Several stayed to protect their property; they viewed the evacuation orders as “typical government overreaction.” But when they saw homes burning in other areas, most agreed to leave. Fire’s reputation in leading to floods and mudslides in the following few years held true in the Bitterroot Valley. A series of intense thunderstorms in mid-July 2001 did not lead to many new fires because sufficient rain in June kept the forests
green. But those same storms unleashed short-lived but torrential rains onto unprotected soils. Rain from the torrential downpours, instead of striking tree leaves and needles and collecting in the forest litter before slowly soaking into the ground, struck the ground directly. The fine grains and resinous burned nature of the soil made it almost impervious to water. Water poured off the surface as sheetflow, collecting into gullies and tributary channels, where it roared downslope as flash floods and debris flows. The effect was most extreme where fires burned much of a large drainage basin; water ran off the surface over a large area and collected rapidly in the main channels. In places, a 20- to 30-minute rainfall was intense enough to severely erode gullies in hillsides, even
where there was almost no upslope drainage basin. One woman in a house in the 50meter-wide floodplain of Laird Creek recounted a frightening scene in a little tributary that emptied onto a small alluvial fan almost directly opposite her house. A wall of water 5 meters high came down the small channel with giant turbulent waves coming in rapid pulses. It carried sand, gravel, and boulders up to a half-meter across, leaving sand grains stuck to tree bark as much as 4 meters above the resulting deposit. The whole event lasted fifteen minutes. Surprisingly, the house survived, though the carefully tended yard around it was trashed both by this side channel and by high water in the main stream.
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a. For each of the following photos, indicate the hazard or hazards and any damaging events that have happened.
Critical View
b. Why should the event have been foreseen, and what could be done to prevent it?
USFS.
D.
C.
Jackie Denk, USFS.
B.
Brennan Baldwin, USFS.
A.
USFS.
FEMA.
K. Wattenmaker, USFS.
c. Where plausible, evaluate what can be done to stabilize the area.
E.
F.
NPS.
A. Booher, FEMA.
Image not available due to copyright restrictions
H.
J.
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Sue Cannon, USFS.
J. Peaco, NPS.
Natasha Kotlier, FEMA.
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Chapter Review
Key Points Fire Process and Behavior p Fires can be naturally started by lightning strikes, intentionally set for beneficial purposes, or set accidentally or maliciously. Lightning strikes cause only 13 percent of all large wildfires in the United States, while arson accounts for double that.
p Fire requires fuel, oxygen, and heat. Lacking any one of these, fire cannot burn. The higher surface area of dry grass and needles makes them burn faster and more easily.
Secondary Effects of Wildfires p Hydrocarbons formed in a fire seal soils, making them hydrophobic, and force water to run off the surface rather than soak in; this can lead to flash floods and mudflows.
p White ash formed during some fires forms a calcium-sodium hydrate that swells up when wetted. That seals pore spaces in the soil, leading to runoff and flash floods.
p Black-ash particles lie parallel to the ground, and promote water runoff.
p Reduction in evapotranspiration from vegetation after a fire can increase the amount of water in the ground and thereby promote landsliding.
Wildfire Management and Mitigation p Techniques for fire suppression include cutting firebreaks down to bare ground, having helicopters dump large buckets of water, and having air tankers dump huge loads of fire retardant. In outof-control fires that threaten critical facilities, firefighters sometimes set burnouts to burn back toward the advancing fire and thus deprive it of fuel.
p People who live in the woods are surrounded by fuel for fire. Many build their homes of wood, which provides more fuel for any wildfire. The worst firestorms are pushed by high winds that both bring in oxygen and blow the flames into more fuel.
p Prolonged dry weather reduces fuel moisture and increases fire danger. Thus, satellite imaging for greenness compared with normal conditions provides an indication of regional fire danger. Figure 16-13.
p Some concerns involve costs to the public of fighting fires. Firefighters are directed to first protect human lives, then people’s homes, and after that the forest. Thus, much or most of the cost of protecting a few who choose to live in dangerous places is borne by the vast majority of the public who do not.
Key Terms burnout, p. 457 convective updraft, p. 455 crown fire, p. 452 fire weather potential, p. 458
firebrands, p. 453 firestorms, p. 455 fuel loading, p. 452 fuel moisture, p. 454
hydrocarbon residue, p. 455 hydrophobic soils, p. 455 indefensible locations, p. 462
ladder fuels, p. 452 prescribed burns, p. 457 spot fires, p. 453
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Questions for Review 1. Wildfires are beneficial to forests in what two ways? 2. What are the two main causes of forest fires? 3. What two conditions lead to more fire-prone forests? 4. Why do winds accelerate fires? Give two specific reasons. 5. Why do fires advance faster upslope than downslope? 6. What natural conditions and processes of a fire lead to new fires well beyond the burning area of a large fire? 7. Why is hill-slope erosion more prevalent after severe wildfires? Be specific.
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8. What main techniques are used to minimize postfire erosion? Name two specific and quite different techniques. 9. What are the main differences in a stream hydrograph after a large fire? 10. In addition to pouring water or retardant on fires, what techniques are used to fight fires? Name two specific techniques. 11. On which side of a forest is a home at greater risk to fire, and why? 12. What can people living in the woods or forest do to minimize fire danger to their houses?
Impact of Asteroids and Comets
Chapter
NASA diagram.
17 i
Simulated impact of a giant asteroid on the Earth.
Asteroids
The Ultimate Catastrophe?
A
n asteroid 10 to 15 kilometers in diameter struck the Yucatán peninsula of eastern Mexico 65 million years ago, opening a crater about 80 to 110 kilometers in diameter (p Figure 17-1). Its vertical walls collapsed immediately inward to form a shallower and broader crater basin 195 kilometers in diameter, with a pronounced central uplift. The Chicxulub crater appears to be the impact site of the asteroid that killed the dinosaurs and the majority of other species on Earth at that time. The energy released from such an impact would have been equivalent to that of 100 trillion tons of TNT, or a million 1980 eruptions of Mount St. Helens. An impact large enough to have this effect should theoretically create an initial crater 200 kilometers in diameter and a much larger final diameter, but Chicxulub is the largest crater of the right age that has been found so far. The crater was slowly buried later by the quiet accumulation of limestones on the continental shelf, so it is not exposed at the surface (see image above). However, this burial also preserved some features that tend to be eroded away with time. It has been studied through drilling and geophysical methods.
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The record of the impact is preserved in minute particles and chemical traces in sedimentary rocks deposited around that time. The geological time before the impact is called the Cretaceous Period; that after is the Tertiary Period. The K-T (Cretaceous-Tertiary) boundary is the interface between these periods. Aside from the final crater shape, evidence for the impact includes worldwide distribution of shocked quartz grains, the extremely high pressure silica mineral coesite, and glass spherules near the K-T boundary in Mexico and Haiti. Huge tsunami waves formed in the Gulf of Mexico. At the Brazos River, Texas, these waves left debris 50 to 100 meters above sea level. Massive submarine slope failures were common around the Gulf of Mexico and along the east coast of North America at that time. The Manson impact structure in central Iowa is also 65 million years old, but at 35 kilometers in diameter it is much too small to be the main impact site for the event. Because concurrent impacts are known for other events, breakup of the asteroid may have caused multiple impacts. This impact structure is buried under ice-age glacial deposits but has been studied by drilling and geophysical methods. Basement granite and gneiss under the center of the crater have been raised at least 4 kilometers above their original position. As with most other well-documented asteroid impact sites, shocked mineral grains are present in the target rocks.
Texas Florida
V.L. Sharpton, Lunar & Planetary Institute.
Gulf of Mexico Chicxulub impact site Modified from NOAA.
Yucatan Peninsula
a
Mexico b
p
FIGURE 17-1. a. The Chicxulub Crater, thought to be the impact site that caused the extinction of the dinosaurs, was imaged using geophysical methods because the crater is buried below the sea floor and partly filled with sediment. b. The Chicxulub impact site, 65 million years ago, is at the northern edge of the Yucatán Peninsula of eastern Mexico. The low-lying areas around the current Gulf of Mexico were a shallow continental shelf 65 million years ago.
Projectiles from Space Space objects that cross Earth’s path include asteroids, comets, and meteors. As a group, asteroids and comets are known as bolides.
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Asteroids Planets of our solar system lie in a mathematical progression of distance from the sun except that one planet is missing where the asteroid belt is found. Asteroids are chunks of space rock orbiting the sun just like Earth, and
Direction of comet travel Tail
Magnetic barrier Nucleus Thompson & Turk diagram.
the asteroids appear to be remnants of material that had not coalesced into planets at about the time the other planets formed around our sun. Most asteroids stay in the asteroid belt between the orbits of Mars and Jupiter, where they pose no danger to Earth. Collisions between asteroids and the gravitational influence of the sun and planets pull some asteroids out of their normal orbits and send them hurtling toward the inner solar system, potentially crossing Earth’s path. The dangerous few are difficult to spot because their trajectories toward Earth leave them nearly stationary in the night sky. They are recognized on sequential telescope images as changing position over several weeks, and their approximate paths are then calculated. The majority of these asteroids are less than 3 kilometers in diameter, and most are 100 meters to 1 kilometer in diameter (p Figure 17-2).
UN
S TO
Magnetic field lines
p
FIGURE 17-3. A comet consists of a solid nucleus of a rock and ice mixture surrounded by a “coma” of dust and gas. The tail is a mixture of water, other volatiles, and dust that the solar wind sprays away from the direction of the sun.
NOAO photo.
Comets Comets are similar to asteroids but consist of ice and some rock, essentially “dirty snowballs” (p Figure 17-3). They do not come from the asteroid belt but range far beyond our solar system, where they make up the Oort cloud. The Oort cloud forms a vast spherical region around the sun that extends to more than 100,000 times the distance between Earth and the sun. It contains billions of comets. An inner doughnut-shaped zone of trillions of comets, the Kuiper comet belt, lies in the plane of the solar system and extends to 20,000 times the distance between Earth and the sun. Comets spray off water, other volatiles, and dust to form their glowing tails when they come within the influence of the solar wind from the sun. They gradually become dehydrated, eventually losing virtually all of their water, leaving only their rocky material. At that point, they are not easily distinguished from asteroids. In fact, there may be a continuous gradation between rocky comets and icy asteroids. The tail of a comet does not indicate its direc-
Coma
p
FIGURE 17-4. “Linear comet.”
tion of travel; rather, the tail points away from the sun (see Figures 17-3 and p 17-4). Most comets have heads less than 15 kilometers in diameter, but they travel at speeds up to 60 or 70 kilometers per second. At those velocities, impact with Earth would be a catastrophe. Some comets traverse our solar system as frequently as once every 10 years. These are of greatest concern because they have the highest chance of coming close to Earth, as in the case of Comet Hale-Bopp early in 1997, or even colliding with it.
Bill Arnett photo, NASA.
Meteors and Meteorites
p
FIGURE 17-2. This photo compilation shows Asteroid 460 from four different sides.
Meteors are objects that produce a light streak in the sky as they pass through Earth’s atmosphere, whereas meteorites are the same pieces of rock once they collide with Earth. Most begin in the asteroid belt between Mars and Jupiter. Meteor showers appear in the night sky when small pieces of rock from space enter the atmosphere at high speeds. When this occurs, friction with the air molecules heats the surrounding air to white-hot incandescence. Earth’s atmosphere shields us from the impact of most meteorites, because small ones burn up in the upper atmosphere. The air around a large meteorite heats to become incandescent, but
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D. Ball photo, Arizona State University.
Dave Hyndman photo, at Smithsonian.
a
b
p
FIGURE 17-5. a. The Henbury Iron Meteorite, found in northern Australia, has characteristic small depressions on its surface due to partial melting upon passage through Earth’s atmosphere approximately 10,000 years ago. b. A chondrite meteorite from Romania.
the meteorite interior; in contrast, the weathering rind on an Earth rock is generally not sharply bounded. Weathering of the meteorite may later result in rusting that turns the coating reddish brown. Metallic meteorites probably crystallized slowly in the deep interior of a large, solid body in our solar system. Collision between such bodies, and their breakups, leads to some collisions with Earth. Iron meteorites are extraordinarily heavy, with densities of 7.7 to 8 grams per cubic centimeter. This compares with most of Earth’s surface rocks, which have densities of 2.6 to 3 grams per cubic centimeter, and with water that has a density of 1 gram per cubic centimeter. Stony-iron meteorites make up less than 1 percent of all meteorites (p Figure 17-6). They consist of nearly equal amounts of magnesium and iron-rich silicate minerals such as olivine and pyroxene in a nickel-iron matrix. They prob-
the cores of such meteorites typically do not get especially hot on entry into the atmosphere. Many fall on buildings or dry grass without starting a fire. A meteorite that fell in Colby, Wisconsin, on July 14, 1917, was cold enough to condense moisture from the air and become coated with frost. As with most hazards, there are innumerable small, fewer large, and fortunately only a rare giant one. When a large rock enters Earth’s atmosphere, it forms a fireball that glows for a period of time before it either disintegrates or survives to strike the Earth. Because meteorites travel at speeds much greater than the speed of sound, we hear only those that are relatively close. If we hear no sound, the meteorite is probably more than 100 kilometers away. Relatively few falls are witnessed so their meteorite fragments can be collected; fewer than 1,000 have ever been witnessed in the United States. Only 20 or 30 witnessed falls lead to meteorite finds worldwide each year. Sometimes, large meteorites break up in the Earth’s atmosphere and fall as a strewn field, spread out around the main impact site. The Allende meteorite that fell in Chihuahua, Mexico, in 1968 scattered fragments over an oval-shaped area 50 kilometers long and up to 10 kilometers wide.
Meteorites come in several types, all of which are somewhat similar to rocks thought to make up the deeper interior of the Earth. Iron meteorites make up 6 percent of all meteorites (p Figure 17-5). Because metallic meteorites consist mostly of a nickel-iron alloy, they are attracted to a magnet. Even stony meteorites often contain some iron so they are often magnetic. Most meteorites have a “fusion coating,” a very thin layer of dark glass, formed when friction against Earth’s atmosphere heats it above its melting temperature. The coating is sharply bounded and quite different than
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Meteorite Magazine/Grant Christie.
Identification of Meteorites
p
FIGURE 17-6. This 2004 meteorite that crashed through the roof of a home in Auckland, New Zealand, is approximately 13 centimeters long.
Evidence of Past Impacts Asteroid impacts have been recognized worldwide.The largest proportion presumably fell into the oceans that cover two-thirds of Earth’s surface. Most of those have remained undetected because details of the ocean floor are not well known. Because older parts of the ocean floors have subducted into oceanic trenches, early impacts into the oceans are no longer available as evidence. On continents, impact sites are broadly distributed but have been found concentrated in areas of greater population or in areas well exposed because of lack of vegetation (p Figure 17-7). As with all natural hazards, recognizing and analyzing evidence of past impacts helps scientists determine how often impacts happen and what consequences they have.
Lunar & Planetary Institute.
ably come from a zone between the deeper iron-rich parts of a large asteroid and the outer stony parts. Chondrites are stony meteorites that make up 93 percent of all meteorites. They consist primarily of olivine and pyroxene, magnesium-iron–rich minerals, along with a little feldspar and glass, similar to the overall composition of Earth’s mantle. They have densities of approximately 3.3 grams per cubic centimeter. Millimeter-scale silicate spheres called chondrules enclose nickel-iron inclusions within or surrounding the chondrule. Achondrites are stony meteorites that are similar to basalt, a common rock on Earth. They consist of variable amounts of olivine, pyroxene, and plagioclase feldspar. Iron meteorites are distinctive. They differ from other rocks nearby and are generally black unless oxidation over many years has turned their surfaces brown. Iron meteorites are extremely hard, virtually impossible to break with a hammer. With time out in the weather, they rust to iron oxides. Pieces of manufactured iron are more abundant and may look similar but can be distinguished by polishing a surface of the rock. Iron meteorites show intersecting sets of parallel lines marking the internal structure of the nickeliron minerals. When these lines are accentuated by acid etching, they show the distinctive patterns that are diagnostic of iron meteorites. Even stony meteorites are distinctively heavy. They are made of peridotite that is 15 to 20 percent more dense than most common rocks. Most contain enough metallic iron to be still heavier. Stony meteorites may be broken, exposing the fresh interior of the meteorite. If the broken surface is unaltered, you may see small inclusions of metallic silvergray-colored nickel-iron that are strongly suggestive of a meteorite. Some meteorites are rounded by their passage through the atmosphere. Iron meteorites are commonly more angular and sometimes twisted-looking. Some show rounded thumb-sized depressions. Others have an orientation related to their direction of travel, with a smooth leading end and a pitted rear end.
Recognized impact structures on Earth Diameter (km) 0–4 4–20 20–50 50–100 100–240
p
FIGURE 17-7. Impacts of various sizes have peppered Earth for billions of years. The uneven distribution of these structures is related mainly to how easily they are identified because of population density and land cover.
Impact Energy Because most asteroids travel at velocities of 15 to 25 kilometers per second, they have incredible energies. Recall that the energy of a moving object is equal to its mass times the square of its velocity (By the Numbers 17-1: “Energy, Mass, and Velocity”). Thus, the energy doubles for every doubling of the mass of the asteroid but quadruples for every doubling of the velocity. Because comets are mostly ice, with a density of 0.9 grams per cubic centimeter, their overall densities, including their rock component, tend to be similar to that of water, 1.0 gram per cubic centimeter. Doubling the size of a comet doubles the energy. However, comets tend to travel at much higher velocities than asteroids—for example, 60 to 70 kilometers per second. Because the energy quadruples for every doubling of the velocity, comets can have extremely high energy in spite of their lower densities. On impact, the kinetic energy of the incoming object is converted to heat and vaporization of the asteroid and the target materials. This melts more rock, excavates a crater, and blasts out rock and droplets of molten glass. The result is a huge fireball that heats and melts rock and burns everything combustible.
17-1 By the Numbers Energy, Mass, and Velocity E mC2 where: E energy m mass C velocity
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impactor requires that it explode violently on impact. The effect is more like a missile being fired into a surface of sand. It blasts a nearly round hole regardless of the impact angle (see, for example, Figure 17-8). If the impactor is large enough, the explosion violently compresses material in the bottom of the crater, accelerating it to speeds of a few kilometers per second and ejecting material outward at hypervelocity. The center of the crater rebounds rapidly to form a central cone; that cone and the outer rim almost immediately collapse inward to form a wider but shallower final crater (Figure 17-9b).
Impact Craters All impacts produce craters that provide evidence about the size and date of past impacts. Relatively small impacts, such as the one that created the 50,000-year-old Meteor Crater in Arizona or recent small craters on the moon (p Figure 17-8), remain as open craters rather than collapsing (Case in Point: “A Round Hole in the Desert—Meteor Crater, Arizona,” p. 483). The largest identified open crater is the 100-kilometer-diameter Popigai Crater of Siberia. The older Manicouagan Crater of eastern Canada is preserved as a striking ring of lakes in the basement rocks of the Canadian Shield (p Figure 17-9). By contrast, complex craters form when the walls of a broad, deep crater collapse inward to create a wider but shallower crater. One might imagine that an asteroid striking the Earth would create a big hole in the ground that has a shape dependent on the incoming angle of the object. However, the incredible velocity, and therefore the energy, of the
Shatter Cones and Impact Melt The same energy that blasts out a crater also has an effect on rocks in the area. As a result of this energy output, rocks on the receiving end of an impact show distinctive features, especially shatter cones. These cone-shaped features, with rough striations radiating downward and outward from the
Transient cavity
David Roddy photo, USGS.
End excavation Ejecta
a Fractured rock
NASA Mars Global Surveyor image.
Bevan French diagram, Lunar & Planetary Institute.
Modification
b
Uplifted rim
Melt-rich material Breccia lens
Ejecta blanket
Fractured rock
Final crater c
p
FIGURE 17-8. a. This panoramic view of Meteor Crater in Arizona shows its relatively small size for an impact crater, 1.2 kilometers across. b. This simple, small, bowl-shaped crater on Mars is a smoothly rounded depression with a raised rim. c. For a moderate-size impact crater: First, a transient crater is excavated, compressed, and fractured, and the base of the cavity melts with the rim raised. Then the ejecta blanket spreads around the cavity, and the rim slumps back into the cavity. Finally, fallback material partly fills the cavity, along with some melt-rich material.
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Excavation Cavity
Transient Cavity
Excavation and compression under cavity
Uplift and excavation
Collapse Central basin
NASA Earth Observatory.
Modified from Morgan diagram.
Megablock zone
a
p
Central peak
Crater rim Melt/allochthonous breccia
Stratigraphic uplift
Final crater b
FIGURE 17-9. a. The giant Manicouagan Crater of the eastern Canadian Shield shows as a dark ring of lakes in this NASA Earth Observatory image. The ring of lakes is 65 kilometers in diameter. b. For a large impact crater: First, a transient crater is excavated and compressed, and the base of the cavity melts. Second, the base of the transient cavity rebounds as excavation continues. Then, the raised rim of the transient crater and central uplift both collapse to form a larger and shallower crater basin partly filled with inward-facing scarps, large blocks, smaller fragments, and melt rocks. The final crater is much broader and shallower than the initial transient crater. “Autochthonous” refers to materials more or less in their original position.
shock effect, range from roughly 10 centimeters to more than a meter long and are considered diagnostic of bolide impact (p Figure 17-10). They form most readily in finegrained massive rocks. Apexes of shatter cones point upward toward the shock source. Those cones directly under the impactor should be vertical; those off to the sides flair down and outward from the source in “horsetail” fashion. Thus, the distribution of orientations of shatter cones provides evidence for the location of the center of the impact site. Individual cones are often initiated at a point of imperfection, sometimes a tiny pebble, as in Figure 17-10b. The Sudbury structure north of Toronto, Ontario, has some of the best-known shatter cones (Case in Point: “A Nickel Mine at an Impact Site—The Sudbury Complex, Ontario,” p. 483). They
are distributed over an area 50 kilometers by 70 kilometers around the intrusion. Sometimes melting of the asteroid produces the glass; sometimes the glass forms by melting the target material (p Figure 17-11). The melt spherules are often altered to green clay. In silicate target rocks, the impact melt may also form sheets, dikes, and ejecta fragments, and be disseminated in breccias. The melt develops under extremely high impact pressures. Impact-melt compositions are a mixture of the compositions of the target rocks that were shocked above their melting temperatures. Because the impacting meteorite is often melted as well, the impact melt may contain small but extraordinary amounts of nickel, iridium, platinum, and other metals that are abundant in iron meteorites.
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FIGURE 17-10. a. Robert Hargraves, who discovered the Beaverhead impact site in Medicine Lodge Valley southwest of Dillon, Montana, points to a shatter cone cluster in an outcrop. Note that the apex of each shatter cone points upward. b. Note the tiny pebble at the apex of the cone (arrow) in this close view of a shatter cone at the Beaverhead impact site.
Donald Hyndman photo.
Donald Hyndman photo.
p
a
Kyte photo.
b
p
FIGURE 17-11. Minute impact spherules are droplets of molten glass sprayed out from the impact site.
Fallout of Meteoric Dust Although the impact site itself is the best source for clues about past impacts, other evidence of impacts can be found far from the impact site. Fragments and dust sprayed out from a large impact site can drift around the Earth. The enormous impact at the end of the Cretaceous period blew out enough material to deposit a thin, dark layer called the Cretaceous-Tertiary boundary (K-T boundary) clay (p Figure 17-12). The dust and elemental carbon soot layer from fires ignited by the impact fireball 65 million years ago is generally only a centimeter or so thick and chocolate brown to almost black in color. The large amount of soot is related to worldwide fires that burned much of the vegetation on the planet. The clay in this boundary layer contains grains of both shocked quartz and other minerals, and tiny,
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now hollow, microtektite-like spherules, the original glass droplets sprayed out as molten droplets during impact. Those droplets of glass, generally a millimeter or two in diameter, are typically the most obvious signatures of an impact feature that are found in sediments (see Figure 17-11 and Case in Point: “An Impact Sprays Droplets of Melt—Ries Crater in Germany,” p. 484). The abundance of quartz in the boundary clay strongly suggests that the dinosaur-killing impact was in quartz-rich continental rocks, probably in an area rich in granite, gneiss, or sandstone. Also found in the boundary clay of the 65-million-yearold event are anomalous amounts of iridium and other platinum-group elements, a phenomenon called the iridium anomaly. The anomalous amounts are tiny, some 0.5 to 10 parts per billion, but those elements are essentially absent in most rocks except for meteorites and dark, dense rocks such as peridotite and other rocks derived directly from the Earth’s mantle. Some iridium is vaporized during the eruption of large basaltic volcanoes such as those in Hawaii, but the amounts are too small to account for the amounts found at the K-T boundary clay. The boundary clay from the initial find contains 30 times as much as would be expected from the normal fallout of meteoric dust that rains down on Earth daily. Elsewhere, as in Denmark, the clay contains up to 340 times as much. Vaporization of the impacting bolide is thought to spread the iridium worldwide, where it has now been found at roughly 100 sites. That boundary in sedimentary rocks marks the demise of the dinosaurs and 60 to 75 percent of all other species.
Donald Hyndman photo.
Multiple Impacts An asteroid would be likely to break up in the atmosphere, so we should expect multiple impacts in a sequence. Most impacts arrive obliquely to Earth’s surface, a significant proportion at 5 to 15 degrees from horizontal. Such bolides ricochet and often break up in the atmosphere into five to ten fragments that still have about 50 percent of the original velocity. A dramatic example of this was seen with multiple impacts on Jupiter. In 1992, the Shoemaker-Levy 9 comet broke up into 21 fragments during a close approach to Jupiter as it was pulled apart by the huge planet’s intense gravitational field. Then in July 1994, the fragments, all less than 1 kilometer in diameter, impacted one after another in an arc across that planet over a period of six days. Comets typically travel at higher velocities than asteroids; this one was traveling at roughly 60 kilometers per second. Apparently simultaneous impacts at different sites on Earth suggest that these may be multiple fragments of larger asteroids or comets. In other cases, multiple impacts may be fragments that broke up after ricocheting off the atmosphere.
Consequences of Impacts with Earth Impact of a modest-sized asteroid 1.5 to 2 kilometers in diameter is thought to be enough to kill perhaps one-quarter of the people on Earth and threaten civilization as we know it.The consequences of such an impact would be disastrous for life. All other hazards and disasters pale in comparison.
Immediate Effects of Impact The fireball or ejecta from the impact would ignite fires within hundreds of kilometers of the impact site. A heavy plume of smoke would linger for years in the atmosphere. Sulfate aerosols and water would be added to the atmo-
p
FIGURE 17-12. The Cretaceous-Tertiary (K-T) boundary layer is exposed at Bug Creek in northeastern Montana. The inconspicuous lowest dark layer (arrow) marks this important boundary.
sphere as well. A large portion of the ozone layer, which protects us from the sun’s ultraviolet rays, probably would be destroyed. If the asteroid broke into fragments, numerous smaller masses still traveling at hypervelocities would provide the heat and energy to cause widespread reaction between nitrogen and oxygen in the atmosphere. That would form nitrates that would combine with water and form nitric acid. The resulting acidic rain would damage buildings, as well as crops and natural vegetation. At about the same time, dust blown into the stratosphere would block sunlight to the equivalent of an especially cloudy day almost worldwide. Large particles would settle out quickly, but dust particles smaller than 1/1,000 millimeter would remain in the stratosphere for months. Any temporary increase in temperature from widespread fires would be quickly replaced by cooling because less solar radiation would reach the surface. The dust would be distributed worldwide because much of it would be blown out of the atmosphere before settling back into it. All agriculture would probably be wiped out for a year, and summertime freezes would threaten most agriculture after that. Many specialists view global-scale wars over food as inevitable. Such desperate conflicts would likely kill a large percentage of the world’s population. Others argue that humans are more resourceful than that. If there were, say, a decade of warning before the event, sufficient food supplies could be grown and stored to outlast the period of darkness. However, that is a pretty big “if”!
Impacts as Triggers for Other Hazards Earthquakes would be generated within hundreds of kilometers of the impact site.Where an asteroid impacts into an ocean, tsunami would form by water sloshing into and back out of the crater. Some of these tsunami could be extremely high. The waves would inundate coastal areas for tens of kilometers inland, areas inhabited by a large proportion of the world’s population. As an example, the wave produced by the Chicxulub impact is calculated to have moved as a
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Mass Extinctions The impact of an asteroid 10 to 15 kilometers in diameter 65 million years ago was associated with the demise of the dinosaurs and the death of between 40 and 70 percent of all species (see chapter introduction). If a bolide of that size were to impact Earth today, it would annihilate virtually everyone on the planet and a large proportion of other species. Almost immediate flash incineration near ground zero would accompany strong shock waves. The 10-kilometerdiameter impactor of 65 million years ago most likely produced sufficient nitric acid rain to be the primary cause of the mass extinction. For those species lucky enough to be far away from the impact, mass extinction would occur after the impact from various indirect causes. Acid rain would kill vegetation and sea life all around the planet. Dust, soot from fires, and nitrogen dioxide would blot out the sun, so animals that were not incinerated would freeze and starve to death. Plants would also die because of the drop in temperature and sunlight. With the impact of a 10-kilometer asteroid, land temperatures would, depending on assumptions, likely drop worldwide to freezing levels within a week to two months. Because of the large heat capacity of water, sea-surface temperatures would drop only slightly. Widespread fires would
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ignite from lightning strikes after much of the vegetation had died from either freezing or from toxic atmospheric effects. The vaporization of a 10-kilometer-diameter chondritic asteroid on impact with Earth would probably not only generate strong acid rain but also spread nickel concentrations of between 130 and 1,300 parts per million, even when diluted with 10 to 100 times the amount of target Earth material. A nickel-iron impactor would generate even more. This is many times the toxic level for chlorophyll production in plants. Seeds and roots would likely not recover for a long time.
Evaluating the Risk of Impact Projectiles from space are not significant natural hazards on most people’s horizon; however, no other physical hazard has such a dire potential. Although the odds of a huge asteroid colliding with Earth are tiny, the consequences of such an impact would be truly catastrophic. A large impact could wipe out civilization on Earth. As with other hazards, small impacts are quite common, while giant events are rare (p Figure 17-13). On average, a 6-meter-diameter bolide collides with Earth every year; one 200 meters in diameter collides every 10,000 years on average. Only 1,500 or so asteroids larger than 1 kilometer in diameter are known to be in Earth-crossing orbits, those that pass through Earth’s orbit around our sun. The largest is 41 kilometers in diameter. Most of these cross the Earth’s orbit only at long intervals, so the chances of a collision with Earth are fortunately extremely small. The orbits of some
Tons of TNT explosive equivalent 10 km
Diameter (log scale)
wave 200 meters high. The runup likely averaged more than 150 meters in height, with a maximum of 300 meters. An impact might also cause volcanic activity. The large, essentially circular features that you see on a dark night that cover most of the surface of the moon are lunar maria or “lunar seas.” Most are filled with basalt. These are recognized as the products of major impacts, mostly during the early evolution of the moon and Earth. Many are enormous, much larger than any known impact sites on Earth. Why have no such giant impact sites been found on the Earth, especially given that the much greater gravitational attraction of the Earth should draw in many more asteroids than the moon? Flood basalts on Earth have a curious habit of forming at about the time of major extinctions of life, so a relationship between them seems possible.The immense Deccan basalts of western India, for example, erupted at about the time of the demise of the dinosaurs and a large proportion of other species 65.7 million years ago. If there is a cause-and-effect relationship, what is it? Most of the other handful of major flood basalt fields on Earth erupted at the times of most other major extinctions of animal species. One imaginative but controversial suggestion is that the impacts of giant asteroids not only caused major extinction events but also triggered mantle melting and eruption of the flood basalts by sudden relief of pressure in the Earth’s mantle. One problem with this proposal is that a layer containing shocked quartz grains has been found recently—not at the base of the 65-million-year-old Deccan flood basalts in India, but between two major periods of basalt eruption.
10 million
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FIGURE 17-13. This log graph plots the approximate chance of impact of an asteroid of a given size hitting Earth.
asteroids vary from time to time because of the gravitational pull of various planets. Innumerable smaller asteroids also cross Earth’s orbit. We live in a cosmic shooting gallery with no way to predict when one will hit the bull’s-eye. We can only estimate the odds.
Your Personal Chance of Being Hit by a Meteorite Meteorites fall from the sky daily, but has anyone ever been struck? The only well-documented case was on November 30, 1954, in Sylacauga, Alabama, when a 3.8-kilogram meteorite crashed through the roof of a house, bounced off a radio, and hit Mrs. Hulitt Hodge, who was sleeping on a sofa. She was badly bruised but otherwise okay. More recently, on Saturday, June 12, 2004, at 9:30 a.m., a 1.3-kilogram stony meteorite crashed through the roof of a home in Auckland, New Zealand. The rock was 7 by 13 centimeters, gray with a black rind, and had rounded edges (see Figure 17-6). It was hot when it landed on the Archer family’s sofa in the living room, but no one was hurt. On June 8, 1997, a 24-kilogram meteorite crashed into a garden 90 kilometers northeast of Moscow, Russia, where it excavated a 1-meter-deep crater. On the night of April 8, 1971, a 0.3-kilogram meteorite came through the roof into the living room of a house in Wethersfield, Connecticut. No one was injured. By incredible coincidence, in 1982 a 2.7kilogram meteorite struck a house just 3 kilometers away. In another unusual case, on October 9, 1992, a bright fireball seen by thousands of people from Kentucky to New Jersey came down in Peekskill, New York, where it mangled the trunk of Michelle Knapp’s car. Hearing the crash, she went outside to find the 11.8-kilogram meteorite in a shallow pit under the car. It was still warm and smelled of sulfur. In 1860, a meteorite falling on a field near New Concord, Ohio, killed a colt. Another killed a dog in Nakhia, Egypt, in 1911. There are no records of a person having been killed—at least so far! Clearly, you need not stay awake at night worrying about the possibility of being hit by a meteorite. There have also been cases of an asteroid or comet making grazing contact with Earth but no actual impact (Case in Point: “A Close Grazing Encounter—Tunguska, Siberia,” p. 485).
Chances of a Significant Impact on Earth Earth is constantly sweeping up stray asteroids, so their number should be decreasing with time. However, collisions in the asteroid belt create new asteroids that leave that belt; some of those fall into Earth-crossing orbits. Also dangerous are comets that have orbits outside our solar system but become visible when they pass close to the sun or Earth. About 10 percent of impacts on the Earth and the moon are from comets.The periodicity of major impacts, based on the ages of impact craters and on theoretical aspects of interacting orbits and other movements within
our galaxy, suggests an average interval of 33 million years. Study of fossils suggests a 26 to 31 million year periodicity of genus extinction events, but some scientists argue against that assertion. Although the number of Earth impacts with time was thought to have slowed some 3.5 billion years ago, a recent study of the ages of glass-melt spherules in lunar soils indicates that after 500 million years ago, the impact rate increased again to previously high levels. The spectacular Hale-Bopp comet, with a diameter of approximately 40 kilometers, was seen by most people on Earth between January and May 1997. It came within Earth’s orbit around the sun, but on the other side of the solar system. Its closest approach to the Earth on March 22 was still 320 million kilometers away (the sun is about 350 million kilometers from Earth). If it had collided with the Earth, the energy expended would have been tens to hundreds of times larger than that of the dinosaur-killing asteroid of 65 million years ago. It is a long-period comet that spends most of its orbit far beyond our solar system before blowing through it again after thousands of years. Whether one of these objects annihilates us is a matter of where Earth is in this shooting gallery when one of these objects passes through. On December 6, 1997, astronomers of the University of Arizona Spacewatch program spotted a huge chunk of space rock, an asteroid 1.5 kilometers in diameter, apparently on a near-collision course with Earth. Asteroids spotted in telescopes are tracked with time until astronomers have enough information to determine how close their trajectories will take them to Earth. In March 1998, the astronomer who spotted this particular rock, Asteroid 1997 XF11, was able to plot that its path would come dangerously close to Earth; the nearest approach to Earth was to be on October 26, 2028. However, in his excitement to announce the event to the press, he neglected to check earlier measurements on the same object. The corrected results indicate that the asteroid will come no closer than 2.5 times the distance to the moon. If an asteroid the size of Asteroid 1997 XF11 ever did strike Earth, the energy expended would be that of 2 million Hiroshima-size atomic bombs. According to Jack G. Hills of the Los Alamos National Laboratory, if it struck the Atlantic Ocean, it would create a tsunami more than 100 meters high that would obliterate most of the coastal cities around that ocean. If it hit on land, the crater formed would be 30 kilometers across and darken the sky for weeks or months with dust and vapor. For comparison, the infamous asteroid that exterminated the dinosaurs and as much as 75 percent of all other species on Earth 65 million years ago was 10 to 15 kilometers in diameter. The chance of that smaller-sized asteroid striking Earth in any one year is estimated at one in several hundred thousand (see Figure 17-13). This probably seems like such a minute chance as to be irrelevant. People do believe, however, in their chances of being dealt a royal flush in poker (1 chance in 649,739) or of winning the multimillion-dollar
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lottery jackpot (1 chance in 10 to 100 million). The chance that Earth will be struck by a civilization-ending asteroid next year is greater than either of those. As with other natural hazards, the low odds of such an event does not necessarily mean that it will be a long time before it happens. It could happen at any time.
What Could We Do about an Incoming Asteroid? What if astronomers were to discover a very large asteroid or comet, at least as large as the one that did in the dinosaurs 65 million years ago and they determined that it was on a collision course with Earth (see chapter-opener photo)? What would we see without a telescope, and when would we first see it? By the time it reached inside the moon’s orbit, it would be three hours from impact. It would first appear like a bright star, becoming noticeably brighter every few minutes. An hour from impact, it would be as bright as Venus. Fifteen minutes from impact, it would appear as an irregular mass, rapidly growing in size.Three seconds from impact, it would enter Earth’s atmosphere with a blinding flash of light, traveling at perhaps 30 kilometers per second. Then instant annihilation! After that, the impact would be as it was recorded in the extinction event 65 million years ago. Clearly we could not survive the scenario outlined above. So what, if anything, can we do about it—that is, other than bury our heads in the sand and wait for the inevitable? Suggestions include blasting the asteroid into pieces with a nuclear bomb or attaching a rocket engine to it to deflect it away from the Earth. Unfortunately, blasting it into smaller pieces might just pepper a large part of Earth with thousands of smaller pieces—not a comforting scenario. Conventional explosives on one side of the asteroid might deflect it rather than shatter it. Another possibility is to deflect the asteroid by changing the amount of heat radiated from one side; for example, we could coat one side with white paint. The effect is weak, but over a long time, it could change its orbit enough to narrowly miss the Earth. NASA catalogs near-Earth objects that are larger than 1 kilometer in diameter. The Jet Propulsion Laboratory in Pasadena, California, can detect objects down to 10 to 20 meters or so in diameter. Smaller ones are not important;
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most flame out before they reach Earth’s surface. However, the impact of a large one would be catastrophic. Sometimes scientists tracking asteroids do not get much warning before one comes close. On December 26, 2001, they spotted an object 0.3 kilometer across. Twelve days later, it came within 800,000 kilometers of Earth, roughly twice the distance to the moon, a frighteningly small distance. If it had collided with the Earth, it would likely have destroyed an area the size of Texas or all of the northeastern states and southern Ontario. Another object 60 meters across came within 460,000 kilometers of Earth on March 12, 2002. Astronomers did not detect it until four days after it passed because it came from the direction of the sun and thus could not be seen. Another 100-meter-diameter object was also first spotted in June 2002, three days after it missed us by only 120,000 kilometers. The impacts of asteroids and comets with Earth only rarely affect individuals and have not killed groups of people in historic time, at least none that have been recorded. A long time without an event, and in this case an especially long time compared with human life spans, leads to the widespread belief that it will never happen, at least not to us. However, we now have enough information on objects in Earth-crossing orbits that we have a pretty good idea of the odds of a significant impact with Earth. It is, of course, quite clear that it will eventually happen—we just do not know when. Are we prepared for that inevitable event? The short answer is, clearly not. There is no point in staying awake at night worrying about being hit by a stray space rock. Certainly, the odds of a person being hit in a human lifetime are extremely small. In addition, worrying about the possibility would do no good. We can neither predict the impact of a small but deadly rock nor see it coming before being struck. That conclusion holds for even larger impactors that could kill thousands of people. For still larger bolides, the difficulty of spotting incoming objects heading directly for Earth creates a major predicament. For a larger doomsday object that is somehow spotted as it grows larger on Earth approach, we have neither decided on a formal plan of action nationally or internationally nor set up the mechanism for implementation of that action. We just do not want to think about the possibility. If you were in a position of power or influence, what would you do?
Case in Point A Round Hole in the Desert Meteor Crater, Arizona u
Meteor Crater is the classic open-crater impact site, 65 kilometers east of Flagstaff, that is so well known to the general public. It is small as impact craters go, only 1.2 kilometers across and 180 meters deep, but it is nicely circular and has distinctly raised rims (see Figure 17-8a). Being only 50,000 years old, it is also well preserved. The projectile was an iron meteorite with a diameter of some 60 meters. It came in at 15 kilometers per second to explode with the energy of 20 million tons of TNT, equivalent to that of the largest nuclear devices. The target rock, Coconino sandstone, shows good evidence of shock features, including shocked quartz and lechatelierite, a fused silica glass. A shepherd found a piece of iron in 1886, and a prospector found many more in 1891. One piece found its way to a mineral dealer in Philadelphia, who recognized it as an iron meteorite. The dealer visited
the site and found numerous fragments of iron meteorites. Unfortunately, no craters were then known to be formed by meteorite impacts, and even the great G. K. Gilbert, chief geologist of the U.S. Geoogical Survey, misinterpreted the crater as of volcanic origin or a limestone sinkhole around which the meteorite fragments had fallen by coincidence. Daniel Barringer, a mining engineer interested in the iron as an ore, filed claim to the site in 1903 and began intensive exploration of it with many drill holes. He found meteorites under rim debris and under boulders thrown out from deep in the crater. Barringer presented scientific papers in which he concluded that the crater had formed by meteorite impact, but few scientists were convinced. A few years later, two astronomers separately visiting the crater concluded that it had been caused by a meteorite, but that given its size and veloc-
ity, the meteorite would have vaporized or disintegrated completely. The scientific community remained unconvinced until Eugene Shoemaker, then a graduate student, studied the crater and its materials in detail, finding shock-melted glass containing meteoritic droplets and the extremely high pressure minerals coesite and stishovite. Stishovite also requires temperatures greater than about 750°C.
Case in Point A Nickel Mine at an Impact Site The Sudbury Complex, Ontario u The Precambrian-age Sudbury intrusion is 140 kilometers in diameter. Although it is not an open crater, its widespread shatter cones lead to essentially universal acceptance that it was caused by asteroid impact. Sudbury is also the largest nickel deposit in the world. It has been suggested that the nickel originated in the impacting meteorite, but there is considerable disagreement on that point.
If the target rocks contain quartz, the extreme shock pressures produced by impact deform the quartz to produce socalled shocked quartz grains that show multiple sets of shock lamellae imposed by more than 60,000 atmospheres of pressure (p Figure 17-14). These are pressures found at depths of 200 kilometers in the Earth, or deep in the planet’s mantle. In 1984, Dr. Bruce Bohor and
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(continued)
Bevan French photo, Lunar & Planetary Institute.
others of the U.S. Geological Survey discovered shocked quartz in the 65-millionyear-old Cretaceous-Tertiary boundary clay. Some workers suggested that shocked quartz had formed by particularly violent volcanic explosions, but volcanic grains show only a single set of deformation lamellae that can be formed at much lower deformation pressures.
p
FIGURE 17-14. This impact-shocked grain of quartz is less than 1 millimeter across. Multiple sets of thinly spaced deformation planes are imposed by a high-intensity impact.
Case in Point An Impact Sprays Droplets of Melt Ries Crater in Germany u
The 15.1-million-year-old, 24-kilometerdiameter Ries Crater formed in limestone, shale, and sandstone over crystalline basement rocks. The impact ejected a blanket of sedimentary rock fragments that were cold when they landed to the east. These fragments contain droplets of frozen melt derived from the explosive melting of underlying crystalline basement rocks. The ejected glass droplets, or tektites, spread to the east of the impact site for 260 to 400 kilometers. Passage through the atmosphere aerodynamically
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shaped the glass, but it landed cold. Shock features are widespread. The interpreted sequence of events begins with high-speed shock waves and vapor blown out at shallow angles to Earth’s surface, quickly followed by highspeed ejection of target material. The tektites formed as melted target material, probably ejected above the atmosphere before falling back to Earth. The crater is circular, but the eastward fallout pattern of the tektites suggests that the impactor arrived from the west.
Case in Point A Close Grazing Encounter Tunguska, Siberia u
An asteroid estimated to be 50 meters in diameter blew down and charred some 1,000 square kilometers of forest on June 30, 1908, but failed to create a crater. It must have been a grazing encounter, approximately 8 kilometers above the ground and traveling some 15 kilometers per second before it exploded and disintegrated. Colliding with even a thin atmosphere at that altitude, its high velocity abruptly encountered severe resistance and caused instant disintegration. Its energy was equivalent to 1,000 Hiroshima-sized atomic bombs. Many people saw the huge fireball as it traveled north over remote villages in central Siberia. At the time
of “impact,” 670 kilometers northeast of Krasnoyarsk, people saw a bright flash and heard loud bangs. The ground shook, windows shattered, and people felt blasts of hot air. One man 58 kilometers southeast of the explosion felt searing heat and was knocked more than 6 meters off his porch by the blast. Some of the reindeer and dogs of native people near the site were killed by the explosion. The first expedition into the remote area, in 1921, showed that the trees were blown radially outward from the site of the explosion. Several intensive expeditions that included trenching and drilling failed to find any meteorite fragments, but mi-
croscope examination of the soil in 1957 showed tiny spheres of iron oxide meteoritic dust. It is now thought that the object was a stony meteorite.
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For each of the following photos indicate:
Critical View
a. What is illustrated in the photo? b. What are the historic or future hazards posed by the objects in the photo?
NASA. David Hyndman photo.
G.
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David Hyndman photo.
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Donald Hyndman photo.
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Cascadia Meteorite Lab, Portland State University.
B.
NASA.
A.
NASA.
NASA.
Donald Hyndman photo.
c. What could we do about these hazards if we knew of the risk in advance?
I.
Chapter Review
Key Points Projectiles from Space
Consequences of Impacts with Earth
p Asteroids are pieces of space rock orbiting the sun
p Although the chances of a giant asteroid striking
like the Earth does. Comets are ice with some rock that occasionally loop through our solar system. Meteors are light-producing objects streaking through the sky. Meteorites are the pieces of rock that actually collide with Earth.
Earth are quite small, such an impact could wipe out civilization on Earth.
p Meteorites include those consisting of iron, chondrites composed of the dark minerals olivine and pyroxene, and achondrites that are similar to the basalt found on Earth. Proportions of the three types are similar to such rocks in the interior of the Earth.
Evidence of Past Impacts p The high velocity of a large incoming asteroid, at tens of kilometers per second, requires that it blast a deep, round crater that compresses and melts the rock below, spraying it outward in all directions.
p Distinctive features at and around the site of a major impact include shatter cones of rock radiating downward and outward, droplets of molten glass, quartz grains showing shock features, and, for giant impacts, a layer of carbon-rich clay containing the metal iridium.
p The asteroid that annihilated the dinosaurs and most other life-forms 65 million years ago was likely 10 to 15 kilometers in diameter and struck in the Yucatán peninsula of eastern Mexico.
p Occasionally, asteroids come relatively close to Earth. An asteroid with a 1.5-kilometer diameter that was spotted in 1997 is expected to come as close as 2.5 times the distance to the moon in October 2028. If calculations are incorrect and it were to strike the Atlantic Ocean, a tsunami more than 100 meters high would obliterate most of the Atlantic coast cities.
p Hazards from a large impact include a firestorm, soot that would block the sun and cause prolonged freezing and death of plants, strong acid rain, and nickel poisoning of plants.
Evaluating the Risk of Impact p Numerous small meteorites, far fewer large ones, and only rarely a giant one strike Earth.
p On average a 200-meter-diameter asteroid would impact Earth once in 10,000 years. Impact of a 10-kilometer-diameter object that could wipe out most or all of civilization is expected to occur once every 100 million years on average. Figure 17-13.
What Could We Do About an Incoming Asteroid? p Spotting an asteroid heading directly for Earth can be almost impossible because it may hardly move in the night sky; coming from the direction of the sun, it would not be seen at all.
Key Terms achondrites, p. 475 asteroid belt, p. 473 asteroids, p. 472 bolides, p. 472
chondrites, p. 475 comets, p. 473 iridium anomaly, p. 478 iron meteorites, p. 474
K-T boundary, p. 478 meteorites, p. 473 meteors, p. 473 near-Earth objects, p. 482
shatter cones, p. 476 stony-iron meteorites, p. 474 strewn field, p. 474
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Questions for Review 1. Where do asteroids originate in space? 2. What is the path of comets around the sun? 3. How is the tail of a comet oriented? (Which way does it point?) 4. Name two general kinds of meteorites and describe their composition. 5. How fast do asteroids travel in space? 6. Why do asteroids create a more or less semicircular hole in the ground, regardless of whether they come in perpendicular to the Earth’s surface or with a glancing blow? 7. What is the sequence of events for the impact of a large asteroid immediately after it blows out of the crater? 8. What evidence is there that some large comets or asteroids break up on close encounter with a planet?
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9. List five quite different direct physical or environmental effects of impacts of a large asteroid in addition to the excavation of a large crater. 10. Roughly what proportion of Earth’s human population would likely be killed by impact of an asteroid 1.5 to 2 kilometers in diameter? 11. Sixty-five million years ago, a large asteroid struck the Earth. Where did it apparently happen? 12. What is the relationship between the size of meteorites and the number of a given size? 13. Why would astronomers have difficulty in recognizing a large incoming asteroid headed for direct impact on Earth?
The Future: Where Do We Go From Here? Chapter
Dave Gatley photo, FEMA.
18 i
A steep, unstable slope is not a good place for a house. This became clear to a resident in Laguna Canyon in southern California in March 1998 when El Niño storms released torrents of mud that engulfed many homes.
Those who cannot remember the past are condemned to repeat it. —GEORGE SANTAYANA, 1905
We Are the Problem
I
Future
n the introductory chapter of this book, we emphasized that a hazard exists where a natural event is likely to harm people or property. Similarly, we noted that a disaster is a hazardous event that affects humans or property. A major natural event—an earthquake, a volcanic eruption, a landslide, a flood, a hurricane, or a tornado—in a remote area is merely a part of ongoing natural processes. Problems arise when people place themselves in environments where they can be impacted by such major events. It makes no sense to blame nature for natural events that have been going on for hundreds of millions of years. The problem is not the natural event, but humans. The solution then would seem simple. Keep people from locations that are hazardous. Unfortunately, that is easier said than done. Nomadic cultures learned to go with the flow; they learned to live
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with nature rather than fight it. Impacted by major events, they merely moved to safer ground, leaving a floodplain when the water began to rise, moving away from a volcano when it began to rumble. Part of the problem now is that there are billions of us humans. As civilization evolved, we moved to where we could live comfortably and where we had the resources to meet our needs for food, water, energy, shelter, and transportation. We settled along coastlines and rivers where water was available, crops would grow, the climate was hospitable, and we could trade goods with other groups of people. We settled near volcanoes where soil was fertile for crops. As time went on, we concentrated in groups for defensive reasons; towns grew larger but essentially in the same locations that were most suitable for our basic needs. As our cities grew, our infrastructures became entrenched. Our shelters, our transportation systems, and our communications networks became permanent—houses, stores, factories, roads, bridges, dams, water supply systems, and power lines. When natural events impacted us, we fought back. We built levees to hold back rising rivers, not realizing that we were living in the natural highwater paths of many rivers. We should not have built in these places to begin with, but back then we did not know any better. We should have built our towns on higher terraces above the level of floods. We could still use floodplains to grow crops but abandon them temporarily whenever the water rose. Unfortunately, that is all in hindsight. Our towns grew up where they did, and we have to live with the consequences. We continue to make poor choices. As the people of industrialized countries became more affluent and as free time and efficient transportation became widely available, we began making choices based on leisure, recreation, and aesthetics. We built homes close to streams (p Figure 18-1), on the edges of sea cliffs, on offshore barrier islands, in the “shelter” and view of spectacular cliffs and majestic volcanoes. When the larger natural events come along, we complain that nature is out of control. But nature is not the problem; we are the problem. We should live in safer places. The same feelings cloud our judgment when we decide to build or buy a house in a location that suits our fancy. That spectacular view of the ocean, that place on a beautiful sandy beach, that tranquil site at the edge of a scenic stream—these are all desirable places to live. Those attractive locations are also subject to natural hazards that endanger our houses and perhaps also our lives. People understand that there are risks in all aspects of life and choose to accept some of them. There is little we can do about a stray meteorite unexpectedly nailing us, and we know that the odds of it happening are remote. Walking in the forest, we know that trees sometimes fall; we know the odds of one hitting us are remote, so we do not think about it. Every time we get in a car and head out on the road, we understand that some other driver may cause a collision with us by being drunk, by falling asleep, by running a red light, or by merely not paying attention. Yet we accept the risk because, although accidents happen, we feel that one is not likely to happen to us. Even people who are knowledgeable of particular natural hazards sometimes succumb to the lure of a great place. Yes, catastrophes have happened to others, “but it won’t happen to me.” Our wanting “something special” clouds our judgment. There are those who are well aware of a particular hazard but are willing to live with the consequences, both physical and financial. In an area prone to earthquakes, they incur the expense of building earthquake-resistant homes and purchasing expensive earthquake insurance. In a landslide-prone area, they install expensive drainage systems and may even pay for rock bolting the bedrock under their houses. Because landslide insurance is not generally available, they are willing to risk losing the complete value of their houses. For most of us, our home is the largest investment we will ever make; we borrow heavily, paying for it over 15 to 30 years. Unfortunately, if the house is destroyed in a landslide, we may be liable for paying off the mortgage, even if the house no longer exists.
Karl Christians, Montana Department of Natural Resources & Conservation.
p
FIGURE 18-1. This house south of Livingston, Montana, was clearly built too close to the Yellowstone River.
Hazard Assessment and Mitigation The underlying problem, revisited elsewhere in this book, is that we believe we can control nature. We built levees and put in riprap to hold back an encroaching hazard. It seemed logical that we could do more of the same as the problem became worse. We did not recognize that our actions actually made the problem still worse. Governmental agencies formed to facilitate our actions came to believe that their short-term “cures” were the solution to nature’s rampages. We the public and many governmental officials view nature as something to be controlled or held at bay. Our overall solution to natural events is to react to them. When asked if New Orleans could survive another hurricane, a senior manager of the U.S. Army Corps of Engineers replied that it certainly could, even a Category 5 because “higher levees would solve the problem.” Unfortunately, for many decades they have raised the Mississippi River levees after each major flood and still some levees fail and the river floods. Many people believe that if a river levee fails or houses and roads are washed away on a barrier island, we just did not build the levee high enough or emplace sufficient protection before the storm. They do not realize that the more we hold back the effect of a large natural event, the worse the effect will eventually be. The natural outcome is inevitable; it is just a matter of time. We need to be proactive rather than reactive. If we understand the natural processes, we can learn to live with them. In the case of New Orleans, emergency managers have long recognized that a major event could lead to flooding of this open bowl and a catastrophe for the city. Hurricane Katrina vividly demonstrated the consequences of a major hurricane impact. Separately, a huge flood on the lower Mississippi River could overtop its big levees, which were built for 100-year flood levels and don’t often fail. As we noted in
Chapters 11 and 12 on streams and floods, 100-year flood events can come anytime. Compounding matters, with each successive flood the channel of the Mississippi River rises because of deposition of sediments from upstream. Major floods also get progressively larger over time because of continued urbanization and building of levees upstream in the drainage basin. A levee breach upstream of New Orleans could flood low ground beyond the levees on the north, which would raise the level of Lake Pontchartrain at the northern edge of the city. If the population did not evacuate because they thought the levees would protect them, hundreds of thousands of people could be endangered after their escape roads become submerged. Although warnings about New Orleans have come with some regularity from many knowledgeable individuals and organizations, no large-scale improvements have been implemented. Governmental groups and the public that they represent are seldom inclined to be proactive. Reacting after a disaster seems more urgent. “Soft” solutions for hazardous areas include zoning to prevent building in certain areas and strict building codes to minimize damage to buildings and their occupants. Over the long term, these are much less expensive than the “hard” solutions, such as installing levees on rivers, concrete barriers and riprap along coasts, catchment basins below debris-flow channels, and drains on a landslideprone slope.
Societal Attitudes Even where lives are lost, collective memories of a disaster go back only a couple of years. People quickly forget that it could happen again. Homes badly damaged by some natural disaster are typically ordered abandoned or removed. Even where such homes are condemned because they are too dangerous to inhabit, individuals and real estate groups sue for permits to rebuild and reoccupy. Often they are successful, and the problem escalates. Homeowners commonly blame, and often sue, others for damage to property that they purchase.Who should be held responsible—the seller, the purchaser, the developer or real estate agent, the government? A good rule of thumb is the old adage caveat emptor, “buyer beware.” In many aspects of society, however, the seller is held responsible if he or she is aware of some aspect of a property that is damaged or endangered but the problem is not obvious. The issue can be a leaky roof, a cracked foundation, high radon levels in the home, previous flood damage, or a house resting on a landslide. If the property is damaged but the buyer is not aware of it, the buyer is not generally held responsible. The same responsibility rests on developers or real estate agents who represent that a property is undamaged or safe from a particular hazard. If developers purchase a property and then find out that it rests on a landslide, they should be required to advise potential buyers of that fact.
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Nuhfer photo, NOAA.
Nuhfer photo, NOAA.
a
b
p
FIGURE 18-2. a. In 1988, the newly remodeled Atlantic House restaurant and gift shop just south of Charleston, South Carolina, sat on the beach on numerous sturdy pilings. b. After Hurricane Hugo in September 1989, the site was almost unrecognizable. Compare the bend in the road and riprap in the upper right and the parking lot. As for the restaurant and its walkway, only the pilings remain.
After a Disaster In many cases the damage is heavily influenced by the homeowner’s own behavior (p Figure 18-3). If a home begins breaking up on a landslide, broken water-supply pipes may leak water into the slope. Did the pipes leak before
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movement began, thereby making the slope more prone to failure? Or did the slope fail, causing the pipes to break? Did a homeowner’s roof and gutter system, heavy watering, and septic drain field feed water into the ground to make a slope less stable? Was the slope destabilized by the cutting and filling used to provide a house site on a slope? Did the building of levees along a river cause flood levels to rise higher than they would have without the levees? Was a debris-flow dam not built high enough or a debris basin not built large enough to protect homes downslope? Or should the homes simply not have been built in such a hazardous site in the first place? Should landowners be permitted to do whatever they wish with their property? Property rights advocates often
Donald Hyndman photo.
Homeowners who face losing major investments such as their homes often try to pin the blame on someone else, especially someone with “deep pockets.” If the developer has disappeared or declared bankruptcy, then the next in line is often a government entity. Did the government provide a permit to the developer to build on a piece of property? Should a county or city be blamed for permitting building on a site that was prone to landsliding or flooding? Many areas have few restrictions on building in hazardous sites. In other areas, local ordinances require that a professional geologist or geotechnical engineer certify that a site is safe from sliding or flooding. Unfortunately, as in any profession, there are those who are less competent or less ethical than others. There are instances where a developer has fired a local expert who provided an assessment that would not permit building on a piece of property for proposed development and then hired another “expert” who would provide a more positive view. In many areas, zoning restrictions concerning natural hazards are limited. In many counties in South Carolina, for example, there are few restrictions on building on active barrier islands (p Figure 18-2) or on the quality of construction to minimize wind damage during a hurricane.Although there have been dramatic advances in hurricane forecasting, increases in coastal populations leave roads jammed with traffic and in some cases too little time for evacuation. In Seattle and many other hilly West Coast cities, real estate agents are not required to disclose the fact that landslides have been occurring in a particular area for decades.
p
FIGURE 18-3. This home was built near Flathead Lake, Montana, by cutting out a portion of the slope for the foundation and filling the downslope area with the cut material. In most places, this is not a safe practice! It is less of a problem in the very dry environment shown here.
say so. If a governmental entity permits building on land within its jurisdiction, should the taxpayers in the district shoulder the responsibility if there is a disaster? Should a landowner be prevented from developing a piece of property that might be subjected to a disaster? If so, has the government effectively taken the landowner’s anticipated value without compensation, a taking characterized in the courts as reverse condemnation? The availability of federal funds in the last several decades to repair and rebuild after a natural disaster has led many states, local governments, and individuals to believe that they are entitled to such help. However, since 1988, federal policy has gradually shifted to emphasize mitigation. Federal funds are still available for rebuilding, but in a safer way or safer location. Almost as important has been the increasing role of insurance companies. After Hurricane Andrew in 1992 and the Mississippi River floods of 1993, many companies have either refused to renew policies or have dramatically increased insurance premiums to cover anticipated losses. After the catastrophic 2004 and 2005 hurricane seasons, insurance rates increased dramatically in areas of the southeastern United States that are likely to be affected by large hurricanes in the future. These changes—coupled with land use regulations, strictly enforced building codes, fines, and additional financial incentives—are all important in reducing losses. Who should bear the cost of hazard mitigation—the developer or landowner who wishes to build on a parcel of land or all taxpayers in the form of local, state, or federal taxes? If one person is permitted to “protect” his or her property from a flood or hurricane, is he or she transferring that hazard to someone else downstream or down the coast? Such questions are not merely hypothetical; they arise all the time. Answers are not simple. They are the subject of lawsuits, countersuits, and court cases at all levels.
Education Given the grim assessment above, it would be easy to despair that not much can be done—that the public will always end up paying for someone else’s greed or poor judgment. Given the anti-taxation feelings of a significant proportion of the public, most people should be receptive to reducing costs in futile efforts to control nature. Billions of tax dollars are expended to protect property in areas that are inherently high risk, efforts that are doomed to ultimate failure. Once people realize that many major projects to protect the property of selective groups entail large costs to great numbers of people but benefit only a few, then the support for such projects should diminish. If so, the solution should be to educate the public about natural hazards and the processes that control them. Many of these problems are political. Politicians are elected to serve their constituents. Their decisions may be made to better the lives of those they serve or for purposes of reelection based on their performance. People at all lev-
els of government have direct control over expenditures for major publicly funded projects, so they need to be educated as to the effects and consequences of such projects. However, because officials serve at the wishes of their constituents, it is the general public that actually holds the cards. Often the issue revolves around bringing money and jobs to a community or state, regardless of the long-term advantages or disadvantages. Members of a community are often happy to have a multimillion-dollar flood-protection or shore-protection project even if the state’s share of the costs is spread to others throughout the state or throughout the country. The dollars brought in provide significant short-term financial benefit to a broad spectrum of the community. The community may also be happy to get such a project even if its members do not clearly understand the long-term effects. Sometimes the lasting effects, such as loss of a beach, are detrimental to the characteristics that drew them to the area in the first place. Clearly, we need to educate people at all levels, from the general public to developers and realtors, business people, banks and insurance companies, and those who hold political office. Education can be a struggle because people do not want to hear that they should not or cannot live in a location that strikes their fancy. The opportune time to effectively convey this message is within a year or so after a major natural catastrophe. Then the cause and effect are clear in people’s minds; we merely need to help clarify cause and effect for them and specify what individuals can do to prevent future similar occurrences. Unfortunately, even then people’s memories of the trauma and costs fade quickly. They begin to believe that it was a rare occurrence that is not likely to affect them again. If we can make people responsible for their own actions, then losses to both property and life can be minimized. The personal responsibility must be passed not only to individuals but also to all involved groups and organizations. We all stand to gain if people can be dissuaded from living in hazardous areas. For people to accept the consequences, we must help them understand the processes of natural hazards and consequences of dealing with them. Instead of addressing natural hazards on a short-term basis, both for gains and losses, we need to consider and deal with the long-term issues. We need to design for the long-term future.
Different Ground Rules for the Poor In countries where poverty is widespread, the forces that drive many people’s behavior are different than those in prosperous countries. In much of Central America and parts of southeastern Asia, for example, millions of people lack the resources to choose to live in certain places for reasons of aesthetics. Food and shelter dictate where they
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Jennifer Parker photo.
forced to live in less suitable areas. Large proportions of those populations do not minimize family sizes for religious and cultural reasons. For such poor societies, the reduction of vulnerability to natural hazards does not depend much on strengthening the zoning restrictions against living in dangerous areas or improving warning systems, though education can help. More so, it depends on cultural, economic, and political factors. Because more affluent individuals and corporations control many of those factors, the poor are left to fend for themselves, even after a major disaster such as the 2005 Kashmir earthquake (p Figure 18-4). The environmental spin-offs, however, affect everyone. The catastrophic December 2004 earthquake and tsunami in Sumatra and nearby areas bring into focus some of the problems faced by poor people living on low-lying coasts in much of the tropics. Large numbers of people with big families survive on fishing, staffing resorts for more affluent vacationers, and providing a broad range of spin-offs from these businesses. Earnings are very low and people live virtually on beaches at the edge of normal high tide, in homes poorly built from mud bricks or light-weight timbers and plaster. Roofs are commonly sheets of corrugated iron. Even without a major earthquake or tsunami, any large storm will rip off roofs and parts of walls to destroy what few belongings people have. The magnitude 9.15 earthquake crumbled many of the coastal homes in Sumatra, killing a lot of people. The giant tsunami that followed a few minutes later crushed most of the remaining homes within hundreds of meters of the beach and swept up the debris and survivors in a churning mass that only a fortunate few survived. Hundreds of kilometers away in Thailand, Sri Lanka, India, and nearby countries, thousands of people living in similar circumstances died because they were unaware of the incoming tsunami (p Figure 18-5).
p
FIGURE 18-4. With winter approaching, survivors of the Kashmir earthquake lived in tent camps, some on the remains of collapsed houses, after everything they owned had been destroyed in the earthquake.
Andrew Moore photo.
David Hyndman photo.
live. In Guatemala and Nicaragua, giant corporate farms now control most of the fertile valley bottoms, leaving the peasants little choice but to work for them in the fields at minimal wages and to provide their own shelter in the steep landslide-prone hillsides. Others who cannot find such work migrate to urban areas but are still relegated to living in steep surrounding hills. Both groups of people clear forests to grow food, provide building materials, and gather firewood for cooking. Their choice of steep hillsides as a place to live is a hazardous one, but they have little choice in order to survive. Compounding the problem is that fertility rates and population growth in desperately poor countries are among the highest in the world, so more people are
a
b
p
FIGURE 18-5. a. Virtually all of the low-lying coastal area around Banda Aceh, Sumatra, was wiped clean in the 2004 tsunami; houses and many roads disappeared, especially near coastal estuaries. b. On the east coast of Sri Lanka, more than 1,500 kilometers from the epicenter, small coastal villages were destroyed. Only a few homes, set well back from the beach and surrounded by trees, survived.
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Donald Hyndman photo.
What could have prevented the catastrophe or at least minimized its effects? Most of the victims lived in the coastal areas because of proximity to their means of livelihood or because the rugged mountainous terrain provided few other suitable building sites. The small number of buildings left standing were generally two-story reinforced concrete structures, built by the few who could afford such luxury. Clearly most of the people had little choice of a better location. In hindsight, building a tsunami warning system would have helped, provided that warnings could be circulated automatically and very quickly to the populace. Since such events generally come at lengthy intervals, the public needs to be aware and reminded regularly of the possibility and what to do when the warning comes. Education on the nature of earthquakes, their relationship to tsunami, and what to do in such an event could have saved tens of thousands. The example of Hurricane Katrina, in New Orleans in 2005, provides additional understanding of the effects of a catastrophic event on poor people (p Figure 18-6). Despite clear prediction of a dangerous storm approaching, many residents had no means to leave the city and nowhere to go even if they could have left. Ten days after the storm, an estimated 10,000 people still refused to leave their homes in flooded and seriously contaminated areas. The predominantly poor people from the eastern part of the city, who were unable to evacuate before arrival of the storm, made up most of the flood survivors who crowded into the Superdome and the Convention Center during the storm. Before Katrina, 23 percent of New Orleans residents were below the poverty line. Many lived from day to day, had no savings or working car, and lived in rented homes or apartments. Less than half of New Orleans residents had flood insurance. Looting was a problem; some people took primarily food and water just to survive. After days isolated from any help,
p
FIGURE 18-6. Why did these people in the Chalmette area of New Orleans not evacuate as Katrina approached? Perhaps the car was not working, or they did not have enough money to evacuate.
many must have been desperate, with nowhere else to turn. Many people apparently shared what they took with others who were also in need. A major problem for many families and especially the poor was separation of family members during evacuation and the storm. For example, a mother or father evacuated with the children but left behind an elderly parent who couldn’t be moved. In some cases, parents were separated from their children during an evacuation, or one parent left to help a relative, friend, or neighbor and was unable to return. Since their home was no longer a point of reference, communication became difficult or impossible. In the case of Hurricane Katrina, evacuees’ medical problems were compounded by the storm. Patients could not find their doctors and doctors could not find their patients. Patients on prescription medications or undergoing specialized treatments often could not remember the names of their medicines or the details of their treatments. Years of medical records were lost.
Worse Problems to Come? The prospect of global warming adds an additional dimension, one of the greatest challenges facing the human race. Scientists agree that global temperatures are rising. As world population grew and large numbers of people became more affluent and used larger amounts of resources, greenhouse-gas emissions increased dramatically. Our generation of greenhouse gases seems likely to cause population collapse in some parts of the world, initially and especially harshly in poor areas most affected by natural hazards. People will be affected by severe disruption of living conditions. Millions will die by increased incidence of storms and coastal flooding, heat stroke, dehydration, famine, and disease, along with wars over water, food, heating fuel, and other resources. Population increase has a cruel feedback mechanism that will cause a reduction in that population to a level that is sustainable. The changes will neither be smooth nor pleasant. We can minimize the adverse aspects by being proactive in reduction of greenhouse gases. It will be difficult, but the consequences of procrastination are magnified for our children and grandchildren. Global warming is expected to lead to more rapid erosion of coastlines, along with more extremes in weather that cause more landslides, floods, hurricanes, and wildfires. Some small islands in the Indian Ocean and far from the earthquake epicenter were completely overwashed by the 2004 tsunami waves. As sea level continues to rise, such low-lying islands will gradually succumb to the sea, even without such a catastrophic event. The number of hurricanes has not increased significantly, but since 1990 the annual number of the most intense storms—Categories 4 and 5—nearly doubled to eighteen worldwide in 2005. Hurricane development and intensity depends on energy provided by higher sea-surface
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250 200 150 100 NOAA data.
Average Annual Deaths in the US
temperatures. An increase of about 0.6°C (1°F) over the last dozen years may not seem like much, but it makes a difference by fueling the storm with warm, humid air. More water vapor drawn into the atmosphere in these intense storms leads to heavier rainfall accompanying the storms—and more flooding. Whether part of the increase in hurricane strength is in response to global warming is still debated, but most of the increase is attributed to a cycle of cooling and warming over a span of 60–70 years called the Atlantic multidecadal oscillation. Since 1995, the Atlantic sea surface has warmed, and hurricane activity has increased significantly. The water temperatures in the Caribbean Sea and the Gulf of Mexico were extraordinarily warm in 2005. Hurricanes are already the deadliest natural hazard affecting the United States (p Figure 18-7). Unfortunately, we don’t know how much worse this problem will become. Finally, we emphasize that developed or prosperous countries lose economic value, but in relation to their total economic viability, poorer countries sustain much greater disaster losses. In addition, underdeveloped or poor countries lose many more lives. After a major disaster, they spend a large proportion of their resources in disaster relief, recovery, and reconstruction without spending those resources on improving their economy or the lives of their people.
50 0 Hurricane
Flood
Tornado
Lightning Winter Storm
Heat
p
FIGURE 18-7. Ten-year average annual deaths in the United States from climate-related hazards.
With each subsequent disaster, the scenario is repeated and those societies remain mired in poverty. Both eventualities are undesirable, but neither seems likely to change in the near future. Both depend on societal attitudes and people’s behavior. We need to begin there. Rather than fighting nature, we need to do more to live with it and accommodate its variable behavior. That would not change major events but would reduce the number that become a hazard or disaster that affects us.
1. For each of the following photos, indicate the hazard or hazards and any damaging events that have happened.
Critical View
2. Why should the event have been foreseen, and what could have been done to prevent or minimize it?
Image not available due to copyright restrictions
Donald Hyndman photo.
Donald Hyndman photo.
.
3. Where plausible, evaluate what can be done to stabilize the area.
C.
E.
S. Brantley, USGS.
H.
Donald Hyndman photo.
Donald Hyndman photo.
G.
J.
F.
I.
R. L. Christiansen, USGS.
NOAA/NWS.
David Hyndman photo.
D.
A. Shlelborad, USGS.
Donald Hyndman photo.
Donald Hyndman photo.
A.
K.
L. T H E F U T UN R AT E: U WRHAELRH E ADZOA RWDES GAON D F RDOI S MA H ST EE RR ES ?
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Key Points Hazard Assessment and Mitigation p A disaster is a hazardous event that affects humans or property. We are the problem. The problem is not the natural event, but humans.
p “Soft” solutions for hazardous areas include zoning to prevent building in certain areas and strict building codes, which minimize damage and are much less expensive in the long run. “Hard” alternatives, including levees on rivers and riprap along coasts, are expensive and create other problems.
Societal Attitudes p The public believes that we can control nature’s rampages. However, most of our solutions are short term and create other problems.
p Federal policy has gradually shifted to emphasize mitigation.
p In many aspects of society, the seller is held responsible if he or she is aware of some aspect of a property that is dangerous or damaged. Homeowners commonly blame and often sue others for damages to property but should remember the old adage of “buyer beware.”
p In many areas, zoning restrictions concerning natural hazards are limited.
p Who should bear the cost of hazard mitigation— the developer or landowner who wishes to build on a parcel of land or all taxpayers in the form of local, state, or federal government taxes?
p We need to educate the general public, developers and realtors, business people, banks and insurance companies, and politicians at all levels.
p If we can make people responsible for their own actions, then losses of both property and life can be minimized.
Different Ground Rules for the Poor p In poor societies, the reduction of vulnerability to natural hazards does not depend on zoning restrictions or improving warning systems, but more on cultural, economic, and political factors.
p Affluent individuals and corporations commonly control many of those factors, so the poor are left to fend for themselves.
p Developed or prosperous countries lose economic value; underdeveloped or poor countries lose lives.
Worse Problems to Come? p Global warming potentially increases the severity of weather-related hazards.
p In some cases, the damage is heavily influenced by the homeowner’s own behavior.
Key Terms buyer beware, p. 491 “hard” solutions, p. 491 mitigation, p. 493
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natural hazards, p. 490 nature’s rampages, p. 491 property rights, p. 492
reverse condemnation, p. 493
“soft” solutions, p. 491 zoning restrictions, p. 492
Questions for Review 1. If someone’s house on a floodplain is damaged or destroyed in a flood, who is to blame? Why? 2. Why do people live in dangerous places like offshore barrier bars? 3. What is the main distinction between “soft” solutions to natural hazards and “hard” alternatives? Provide examples. 4. What is meant by hazard mitigation? 5. If I sell a house that is later damaged by landsliding, who is responsible; that is, what are the main considerations?
6. When the federal government provides funds to protect people’s homes from floods or wildfires, individual homeowners benefit at the expense of taxpayers. What are the two main types of alternatives to elimination of taxpayer expense for natural hazard losses? 7. What is meant by the expression “Those who ignore the past are doomed to repeat it”? Provide an example related to natural hazards. 8. When we note that people need to take responsibility for their own actions in living in a hazardous environment, what is different about the behavior of poor people living in underdeveloped countries?
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Appendix 1
Geological Time Scale
ERA
PERIOD
EPOCH
BEGAN (MILLIONS OF YEARS AGO)
Cenozoic
Quaternary
Holocene Pleistocene
0.01 (10,000 years ago) 1.8
Tertiary
Pliocene Miocene Oligocene Eocene Paleocene
5 24 34 55 65
Mesozoic
Cretaceous Jurassic Triassic
145 213 248
Paleozoic
Permian Pennsylvanian Mississippian Devonian Silurian Ordovician Cambrian
286 325 360 410 440 505 544
PRECAMBRIAN
Proterozoic Archean
500
2,500 4,500
Appendix 2
Mineral and Rock Characteristics Related to Hazards*
The following section outlines the characteristics of many common rocks, especially as they relate to geological hazards. Earthquakes and landslides are among the processes that break down rocks to smaller particles, which are then transported downstream in flowing water. The strength of rocks, therefore, affects when a fault will break and how strong the associated earthquake will be, when a hillside will slide and in what way, and how easily big boulders are broken down to be transported in streams. Whether rocks are at or near Earth’s surface or deep beneath the surface also matters. Rocks at least a few kilometers below the surface are warm or even hot. They are under high pressure because of the load of rocks and sediments above them. Like ice in a glacier, they may be brittle at the surface but deform plastically deep below under such loads. Hard rocks at the Earth’s surface may break during an earthquake or collapse in a landslide. Those same rocks, warmer and persistently stressed at depth, may slowly flow. Some hard taffy seems rigid or brittle if you bite it hard; if you bite more gently and slowly, it bends and flows. Like warm tar, rocks may flow when stressed by tectonic movements. Many rocks deform more easily if they are soaked with water. We discuss these differences in more detail in the separate chapters in which they are most relevant. Geologists classify rocks into three basic groups: igneous, sedimentary, and metamorphic. We focus on their descriptive characteristics here.
Igneous Rocks Igneous rocks are those that solidify from molten magmas. Those that crystallize slowly deep underground are called plutonic rocks; they generally show distinct mineral grains that are easily visible to the unaided eye. Their grains interlock with one another because some grains crystallize first at high temperature within the magma; others crystallize later at somewhat lower temperatures, filling in the spaces between earlier-formed grains so the grains are in direct contact with no spaces in between. Most igneous rocks are massive—that is, they show little or no orientation of their grains. In a few cases, where the magma was moving as it crystallized, the grains may show parallel-oriented grains.
If the magma crystallized within a kilometer or so of the Earth’s surface, gases dissolved in the magma separate as steam bubbles that are trapped between the straight sides of the grains. Such rocks contain cavities bounded by the straight sides of the mineral grains that grew in the magma. Volcanic rocks are formed when a magma reaches the Earth’s surface. If the magma has low viscosity and contains little gas, it may pour out as a lava flow. If it is highly viscous and contains significant dissolved gas, it is likely to blast out as loose particles of volcanic ash. If the ash settles while still hot, its particles may fuse together to form a solid rock. Alternatively, loose ash that sits around for hundreds or thousands of years may become cemented into solid rock because water dissolves the surfaces of particles and then precipitates along their contacts, cementing them together. Blasting ash out of the vent typically chills the shreds of magma so quickly that minerals cannot crystallize into grains; the particles are glass. Volcanic rocks and processes are described in some detail in Chapters 6 and 7 on volcanic rocks.
Sedimentary Rocks Sedimentary rocks form at the Earth’s surface by deposition or precipitation from water or sometimes wind. Weathering processes break down rocks into particles that are transported downslope into streams, lakes, and ultimately the ocean. Loose particles are described as sediments. Chemicals dissolved from the surfaces of sediment grains that are in contact with water for hundreds to millions of years again precipitate to form cements that bind the grains together to form solid sedimentary rocks. Individual rocks of sand-sized grains, for example, become sandstone, and mud-sized grains become mudstone or shale. Where water in a lake or ocean contains large amounts of dissolved constituents, those chemicals may concentrate further by evaporation in a warm environment and precipitate to form chemical sedimentary rocks such as limestone. Marine animals such as clams and coral may take in chemicals dissolved in the water to precipitate them in their shells. Tiny corals often form colonies that precipitate enough calcium carbonate to build whole coral reefs that fringe coastlines in warm-water environments.
*All photos in this appendix appear courtesy of the authors.
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Metamorphic Rocks Metamorphic rocks begin as sedimentary or igneous rocks but have been buried well beneath the Earth’s surface, where they are subjected to higher temperatures and pressures. Heated mineral grains react with one another and with water along their boundaries to form new metamorphic minerals. If sediment grains were not interlocking, the new grains grow to interlock with one another. If the rock undergoing metamorphism is sufficiently deep where it becomes soft and plastic, it often flows in response to tectonic stresses and its mineral grains become oriented. This is one of the distinctive characteristics of many metamorphic rocks. Because a few igneous rocks such as some granites, show such orientation, there can be uncertainty and debate as to whether such a rock is actually igneous or metamorphic. Where sedimentary or igneous rocks are locally heated by contact with an intruding magma, the resulting rock becomes a contact metamorphic rock. Because most contact metamorphic rocks are not buried deeply enough to flow and take on mineral orientation, they are most commonly massive.
Weak Rocks and Strong Rocks Most people think of shale as weak and granite as strong, and in a general way that is quite true. Shale, which is a sedimentary rock that forms when clay-rich muds compact and dry out, splits easily into thin sheets that may crumble in your hand. Shale in a hillside may crumble and slide when the load of the slope above pushes on a shale layer. If the platy sheets of the shale are oriented parallel to the slope, then the shale may split easily and slide on those sheets. Pieces of shale eroded by a stream crumble to tiny fragments as other rocks tumble against them. The tiny scraps are easily picked up by the swirling currents and carried far downstream. In a fault zone, a large mass of shale is so easily broken that it cannot build up a large enough stress to cause a large earthquake. Granite, on the other hand, consists of interlocking grains of hard minerals, quartz, and feldspars. Granite in a hillside is so strong that it rarely fails unless it is exposed in a vertical cliff.Then it may collapse in a rockfall if the cliff is oversteepened or if cracks in the granite fill with water that freezes and expands. Boulders of granite that fall into a stream may tumble along the bottom in high water, but as the water level falls the stream is not energetic enough to move them. They often collect in place to form a steeper gradient and rapids. A large mass of granite in a fault zone does not easily break. Stress on the rocks on opposite sides of the fault needs to build up to a high level before the fault slips. When it does it may produce a large earthquake. Many igneous rocks such as granite, and many metamorphic rocks such as quartzite and gneiss, which are formed by heating at high temperatures far below the Earth’s sur-
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face, can be considered strong. Many sedimentary rocks such as shale or sands in which the grains are not cemented together are weak. As used here, strong rocks are hard to break. They do not collapse as easily in landslides or rockfalls; they do not erode as easily in streams. Rock strength also depends on whether the rock contains weak layers or many fractures.
Common Rock-Forming Minerals To understand the makeup and behavior of rocks, we need to keep clear the distinction that elements such as oxygen, silicon, iron, and aluminum combine chemically to form minerals such as quartz, feldspars, and micas. Groups of minerals go together to form rocks (p Figure A2-1). Some of the more common rocks are discussed below. Rocks are agglomerations of grains of individual minerals. Mineral grains are individual crystals of chemical compounds (Figure A2-1). Each type of mineral has a crystalline structure controlled by the size and electrical charge on the atoms of its elements. Although there are literally thousands of different minerals, only a handful are common rockforming minerals, and those are formed from only a few elements. Some grew to form igneous rocks by crystallization from high temperature melts or magmas. Shale, sandstone, and most other sedimentary rocks were formed by the disintegration of other rocks, followed by erosion and deposition of the sediment. In most cases, the grains are later cemented together to form a solid rock. Some sedimentary rocks grow by precipitation from cold, watery solutions. After deep burial under other rocks, some sedimentary rocks are subjected to the high pressure of other rocks on top and heat up to form metamorphic rocks. Elements in some minerals in the hot rocks diffuse into nearby minerals and react with them to form new metamorphic minerals. Most minerals are recognizable by their visible properties. The abundant elements (and their standard chemical abbreviations) in most rocks are: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). Rocks dominated by light-colored minerals are themselves light-colored or felsic. Rocks dominated by darkcolored minerals are themselves dark-colored or mafic. See p Figure A2-2. Distinctive properties of the common minerals are listed in p Table A2-1. Figure A2-2 shows that igneous rocks range from pale to dark. The rock names are arbitrary divisions that permit us to talk about them efficiently without full descriptions. We use various terms to describe the characteristics of rocks and minerals. Colors are described, as you might expect, in terms of gray, yellow, brown, red, and green, but most colors are imposed by small amounts of iron oxides that are hydrated to varying degrees. Those shades of color are less important than how light or dark the color is. The darkness
Elements: (for example)
1 silicon atom (Si) 2 oxygen atoms (O)
1 1 3 8
potassium atom (K) aluminum atom (Al) silicon atoms (Si) oxygen atoms (O)
1 2 2 8
calcium atom (Ca) aluminum atoms (Al) silicon atoms (Si) oxygen atoms (O)
Minerals: Potassium feldspar (K-f)
Quartz (Q)
Plagioclase feldspar (Pl)
p
FIGURE A2-1. Elements combine chemically to form minerals. Combinations of minerals make rocks.
Type of Magma Texture
Felsic
Aphanitic: Phaneritic:
Rhyolite Granite
Intermediate
Mafic
Andesite Diorite
Basalt Gabbro
p Peridotite
FIGURE A2-2. Percentages of minerals in common igneous rocks. Aphanitic rocks have grains that are too small to see without a magnifier; phaneritic rocks are distinctly grainy looking.
100 Clacium-rich plagioclase
Percentage by volume
Modified from Monroe and Wickander, 2001.
Quartz 80
60
Potassium feldspars
Plagioclase feldspars
40 Sodium-rich plagioclase 20
Pyroxene Olivine
Biotite
Hornblende
0 Darkness and specific gravity increase
Silica increases APPENDIX 2
503
p Table A2.1 MINERAL
The Common Rock-Forming Minerals and Their Main Properties MAIN ELEMENTS*
Quartz
Potassium feldspar
K, Al
Plagioclase feldspar
Na, Ca, Al
Biotite (dark mica)
K, Al, Mg, Fe
Muscovite (white mica)
K, Al
Clay
Al and often Mg, Ca, Na, K
Hornblende (an amphibole)
Ca, Mg, Fe, Al
Pyroxene
Ca, Mg, Fe, Al
Olivine
Mg, Fe
PROPERTIES Generally, medium to pale gray as grains in rocks; commonly white in veins that fill fractures in rocks. Typically glassy looking. Especially hard (cannot scratch with a knife); no flat cleavage surfaces. Abundant in granite, rhyolite, sandstone, schist, gneiss. Pale colors: white, pinkish, beige. Hard, with flat cleavage surfaces in two directions at 90 degrees. Abundant in granite and rhyolite. Pale colors: white, pale gray, sometimes a bit greenish. Hard with flat cleavage surfaces in two directions at 90 degrees. Abundant in granite, gabbro, and most other igneous rocks. Dark brown or black flakes; almost golden if weathered. Soft (can easily scratch with a knife); has one shiny, smooth cleavage direction. Biotite can amount from 5% to 20% of granite and more than 50% in metamorphic rocks such as mica schist. Yellowish white flakes, soft. Has one shiny, smooth cleavage direction. Generally less abundant than biotite but can make up more than 10% of a granite and more than 50% of a schist. Microscopic flakes similar to micas but much softer and weaker. Make up highly variable amounts of soils. Some clay minerals take on water when wet and make a soil susceptible to landsliding (see descriptions of important clay minerals below). A major component of shale. See Figures A2-3 to A2-5. Dark green or black. Short rod-shaped grains. Two shiny cleavage surfaces at 60degree directions to one another. Can amount to 10% to 20% in granite and similar rocks; commonly ~50% in a metamorphosed basalt rock called amphibolite. Dark green, dark brown, or black. Short stubby grains. Two less prominent cleavage surfaces at 90 degrees. Occurs primarily in dark-colored rocks; most abundant in gabbro. Green, glassy looking, hard. Generally occurs in dark-colored rocks, especially in basalt and gabbro.
*In addition to silicon and oxygen.
of the color often reflects the amount of iron in the rock; that also often relates to other aspects of rock composition as well. The cleavage of a mineral refers to the flat breakage surfaces that are controlled by the mineral’s internal arrangement of atoms. The most obvious example is the prominent cleavage of mica flakes. Other minerals such as feldspar have two different orientations of cleavages. Rocks can also have cleavage because of the parallel orientation of flat or elongate mineral grains. Prominent examples include slate or schist in which small to larger grains of micas control the platy breakage of the rock. The hardness of a rock is technically the ease with which you can scratch it, such as with the tip of a pocket knife. Hard rocks are those that contain large amounts of hard minerals such as quartz and feldspars that cannot easily be scratched with a knife. Those that contain large amounts of soft minerals such as micas or calcium carbonate minerals can be easily scratched and are described as soft. The ease of breakage of a rock or mineral is not hardness but brittleness. A poorly cemented sandstone, for example, might be hard because it contains lots of grains of quartz; but it also might be brittle or crumbly because the grains are not well cemented together. Most limestones are soft but not brittle or crumbly.
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APPENDIX 2
The characteristics and strength of a rock depends in large part on the minerals that it contains, and the nature of a mineral depends on the chemical elements and their arrangement within the mineral. Four oxygen atoms surround each silicon atom in a tetrahedral arrangement; six oxygen atoms surround each aluminum atom to make an octahedral array (p Figure A2-3). Those sheets stacked like the pages of a book and more or less weakly bonded together are the basic components of Tetrahedron Oxygen
Si
p
Octahedron Oxygen
Al
FIGURE A2-3. Atomic arrangement of the main building blocks in clay minerals. Oxygen atoms surround either silicon or aluminum atoms.
Si
Sheet of tetrahedra One layer
Tetrahedral sheet Octahedral sheet
One layer
Mg or Al
Sheet of octahedra
Weak bonds Next layer
Oxygen
Al
Si
OH
a
b
p
FIGURE A2-4. a. Kaolinite-type structure in some clays. b. The Kaolinite-type structure in some clays consists of sheets of tetrahedra and octahedra. Compare the tetrahedra (inverted triangles here) and octahedra. Oxygen atoms are at each corner.
the clay minerals. Between these negatively charged combination sheets, some clays have positively charged atoms or ions such as sodium or calcium that hold the sheets together. Kaolinite is the simplest clay. It consists of alternating sheets of silicon tetrahedra and aluminum octahedra (p Figure A2-4). Although they have no overall charge, the layers are weakly positive on one side and weakly negative on the other. These positive and negative charges weakly bind the structure together, resulting in extremely soft crystals. Kaolinite does not absorb water between its layers and does not expand and contract as it wets and dries. It generally forms through weathering in especially wet and warm environments. Kaolinite commonly forms deep soils, some considerably more than 100 meters deep. Because it is so weak, it is frequently involved in landslides. Smectite is a group of complex clay minerals that has silicon, aluminum, and magnesium sheets bound together to make combination layers. Loosely bound atoms of water and sodium or calcium bind the sheets together (p Figure A2-5). When the clay gets wet, water seeps between the layers. Then the clay swells to become plastic and struc-
Tetrahedral sheet Octahedral sheet Tetrahedral sheet Expandable
p
One layer
H2O
H2O
H2O
H2O
Variable water and exchangeable interlayer ions: sodium or calcium
FIGURE A2-5. In the smectite structure, the interlayer cations (positivecharged atoms) are sodium or calcium that can be replaced by water. This causes the interlayer space to expand and the clay to swell.
turally extremely weak. Smectite generally forms through weathering in a dry environment, especially in volcanic ash. Many of the sedimentary formations that lie beneath the High Plains just east of the Rocky Mountains are rich in smectite. Soils developed on them become extremely slippery when wet; they are highly unstable and likely to slide. If this slimy mineral did not exist, landslides would be much less abundant. Because it swells when it gets wet, smectite also wreaks havoc with houses that are built on soils containing this mineral (see Chapter 9’s section on swelling soils).
Common Rocks Granite Granite, a common igneous rock (p Figures A2-1 and A2-6), is found primarily in mountainous areas such as the Rockies and Appalachians and in deeply eroded “basement” rocks such as those of eastern Canada and of the Adirondacks of New York. It is light-colored, grainy, and strong. It breaks to form sharp edges but typically weathers to rounded corners on outcrops. It crystallized from molten magma well below the Earth’s surface to form strongly interlocking grains. Its minerals include abundant plagioclase and sometimes potassium feldspar, moderately abundant quartz, and small amounts of micas or hornblende. Diorite is a darker, less common rock that is otherwise similar to granite. When granite magma reaches the surface to erupt in a volcano, it solidifies as fine-grained rhyolite.
Gabbro Gabbro (p Figure A2-7) is a coarse-grained igneous rock that crystallizes underground in large masses from the same magma that erupts at the surface as basalt lavas. It is medium to dark gray in color and strong. Early crystallizing grains of dark pyroxene often settle in the magma to form black layers as in the photo. Gabbro is not common. APPENDIX 2
505
a
a
b
p
FIGURE A2-7. a. Gabbro is a darker-colored, grainy, massive, strong, and hard igneous rock with few fractures. b. In some cases, early crystallized dark grains of pyroxenes settle out to form dark layers.
Rhyolite
b
Rhyolite (p Figure A2-8) is a common, light-colored, finegrained volcanic rock that has the same composition as granite but which solidified at the surface. It can be white, pink, yellow, or pale green, the color provided by superficial amounts of various iron oxides. Most rhyolite is ash and rather easily eroded, though vertical cliffs are not uncommon. Vertical fractures are well spaced and permit large slabs to break off as dangerous rockfalls. The sample at the bottom shows fragments of pumice.
Andesite and Dacite c
p
FIGURE A2-6. Granite is a light-colored, grainy, massive, strong, and hard igneous rock with few fractures.
506
APPENDIX 2
Andesite (p Figure A2-9) is a volcanic rock with color and characteristics that are intermediate between those of rhyolite and basalt. Its color ranges from gray to pinkish gray, green, or brown. Some andesites contain larger grains
a
c
p
FIGURE A2-8. Rhyolite is a light-colored, fine-grained, massive, generally soft igneous rock with few fractures. a. If crystallized at shallow depths below the surface, rhyolite may be hard. b. Pieces of pumice (slightly darker) may be surrounded by fine-grained ash. c. Pieces of pumice are frothy with gas holes formed in the rising magma as pressure on it decreased.
b
a
p
b
FIGURE A2-9. Andesite is a medium-colored, fine-grained, massive igneous rock with few fractures. Its composition and characteristics are intermediate between those of basalt and rhyolite.
APPENDIX 2
507
p
FIGURE A2-10. a. Basalt is a dark-colored, fine-grained, massive igneous rock with few fractures. Its composition is equivalent to that of gabbro. b. Basalt with gas holes and green crystals of olivine.
of white plagioclase (as in the photo) or greenish black hornblende or pyroxene. Andesite forms strong, hard lava flows but weak ash. Dacite is a paler volcanic rock, related to andesite, and intermediate in composition and appearance between andesite and rhyolite.
Basalt Basalt, the most common volcanic rock, is low in silica and typically forms lava flows. It is usually black but can be slightly brownish or greenish. Most basalts are fine-grained and massive (p Figure A2-10a); some are full of steam holes (p see Figure A2-10b. Some basalts contain larger dark grains of black pyroxene or green olivine. Basalt forms strong, widespread lava flows but less abundant and weak cinder piles.
p
FIGURE A2-11. Some shales or mudrocks are marked by mudcracks that formed as wet mud dried and shrank.
Shale and Mudstone Shale and mudstone are dried mud or mudrocks; some even show ripple marks or mudcracks as in p Figure A2-11. These rocks range widely in color from medium to dark gray, red, green, yellow, and brown. Shale is almost as soft and flaky as the clay-rich mud that forms it. Mudstone is more massive and much stronger; it contains lots of tiny grains of quartz and feldspar. Shale may form thick layers on gentle slopes or thin layers between stronger sedimentary rocks such as sandstone or limestone. Shale and its lightly metamorphosed equivalent, slate, form thin plates that break easily and make weak slopes and weak layers between rocks that easily fail in landslides.
Sandstone As its name implies, sandstone (p Figure A2-12) is made of sand grains—generally quartz but sometimes with feldspar.
508
APPENDIX 2
p
FIGURE A2-12. Sandstone is made of grains of sand cemented together to make a solid rock.
Schist Schist (p Figure A2-15) is a strongly metamorphosed shale or slate. The grains range from 1 to several millimeters across. Mica-rich varieties are weak and prone to sliding on steep slopes. Schist splits into flat sheets; those and individual mica grains lie flat and facilitate sliding in translational landslides. The sample in Figure A2-15 contains dark red garnets.
Gneiss
p
FIGURE A2-13. Light-colored limestone with a rough surface.
Its color ranges from white to yellow, brown, red, or green. It may be rather weak if in thin layers with weak rocks in between or lightly cemented. It may be strong if well-cemented or if metamorphosed to interlocking grains, in which case it is called quartzite.
Limestone
Gneiss (p Figure A2-16) is a generally hard and strong highly metamorphosed rock. It consists of white grains of feldspar, glassy quartz, generally some micas, and sometimes hornblende or garnet. It is characterized by distinct thin layers dominated by dark grains separated by layers of light-colored grains. Gneiss forms bold cliffs that break into slabs or blocks like granite.
Serpentinite Serpentinite (p Figure A2-17) is a low-grade metamorphic rock that forms by hydration of rocks from the Earth’s mantle, dark rocks originally consisting of olivine and some
Limestone (p Figure A2-13) is calcium carbonate, soft to scratch but strong. It dissolves slowly in rainwater to form ragged rock surfaces and underground caverns but does not erode easily. Its color is commonly pale gray, often weathering to a soft-looking but a rough and sandy-feeling surface. Dissolution of limestone under soil is the most common cause of sinkholes.
Slate Slate (p Figure A2-14) is lightly metamorphosed, strongly compressed shale. It is somewhat harder but still splits fairly easily into flat sheets. Those sheets lie flat on steep slopes and facilitate sliding. Its color is most commonly dark gray or green.
b
p
FIGURE A2-14. a. Slate, a lightly metamorphosed but strongly compressed mud rock, breaks into thin plates or flakes that make weak slopes that are subject to landsliding. b. An outcrop view from the Sierra Nevada northwest of Sacramento, California.
a APPENDIX 2
509
p
FIGURE A2-15. Shiny micas in this schist are slightly wavy because of deformation after metamorphism. The little lumps are dark red garnets.
p
FIGURE A2-16. Gneiss is similar to schist but contains pale layers that are dominated by light-colored feldspars alternating with dark mica-rich layers. Some were heated to high enough temperatures to melt and thus produce the pale granite layers.
a
c
p
FIGURE A2-17. a. Dark, thoroughly sheared serpentinite typically contains somewhat bigger but still weak lumps shown in this 5-meter-wide outcrop. b, c. Small samples that show the typical smooth, curving surfaces formed by shear.
b
510
APPENDIX 2
pyroxene. It generally rises toward the Earth’s surface along fault zones, where it can be found in a wide range of shades from medium green to black. Serpentinite is weak, soft, slippery, and creates high risk of landslides. Its weakness and presence in fault zones leaves it strongly deformed. Often only larger blocks of serpentinite remain in a thoroughly sheared matrix, as in the outcrop photo. Serpentinite is found in mountainous areas of western and eastern North America and to a minor extent in areas of basement metamorphic rocks.
Rocks, Landscapes, and Hazards In summary, hard, strong rocks such as granite, gabbro, gneiss, much basalt, and some andesite and sandstone do not erode easily. In cool climates, they form steep slopes, rocky cliffs, and narrow canyons. In dry climates, weathering and formation of clays on the surfaces of feldspar grains in granites causes gradual disintegration of the rocks to form sandy soils that erode easily when it rains. Cliffs of these rocks are prone to rockfalls. In warm, moist climates, they
may weather to soft clays. Fractures in these rocks are typically spaced at decimeters to meters; those fractures help drain water, keeping slopes relatively dry. Rhyolite can be quite hard and resistant to erosion if the rock solidified at high temperature to fuse the grains together. More often it settles out of the air as cold ash that is not well stuck together; if it is not later cemented, it erodes easily. Limestone is soft in the sense that it is easy to scratch with a knife. However, where it forms thick layers, limestone tends to be strong and forms prominent cliffs. It dissolves slowly in rainwater near the water table to form underground caverns. Where cavities above these grow sufficiently large near the surface, the ground may collapse to form sinkholes, sometimes taking roads or houses down with them. Especially soft and crumbly rocks such as shale, slate, and mica schist break up and erode easily to form gentle slopes. They are prone to landsliding because water does not drain out easily and the flat sheets slide easily past one another. Where layers of shale separate layers of stronger rocks, their flat planes form prominent zones of weakness. Where those weak zones slope nearly parallel to a hillside, large masses of rock can be prone to sliding. Serpentinite is simply a slope-failure disaster waiting to happen.
APPENDIX 2
511
Appendix 3
Conversion Factors
Length 1 millimeter (mm) 1 centimeter (cm) 1 meter (m) 1 kilometer (km) 1 inch (in) 1 foot (ft) 1 mile (mi)
0.03937 0.3937 3.281 0.6214 2.540 30.48 1609
inches inches feet miles (5/8 mile) cm cm m 1.609 km
0.1550 10.76 0.3861 2.471 6.452 929.0 2.590
square inches (in2 ) square feet (ft2 ) square miles acres cm 2 cm 2 km 2
0.06102 35.31 61.02 0.02832 0.7646 3.785
cubic inches (in3 ) cubic feet (ft3 ) ⴝ 1.308 cubic yards (yd3) in3 ⴝ 1.057 qt. ⴝ 0.2642 gallons m3 m3 liter 0.1337 ft 3
0.03527 2.205 28.35 453.6
oz ⴝ 0.002205 pounds (lb) lb gm gm 0.4536 kg
Area 1 square centimeter (cm2 ) 1 square meter (m2 ) 1 square kilometer (km2 ) 1 hectare (ⴝ10,000 m2 ) 1 in2 1 ft2 1 mi2
Volume 1 cubic centimeter (cm3 ) 1 cubic meter (m3 ) 1 liter 1 ft 3 1 yd 3 1 gallon U.S.
Mass 1 gram (gm) 1 kg 1 oz 1 pound ( 16 oz.)
Velocity 1 meter per second (m/sec.) 3.281 1 mile per hour 1.609
Temperature °C °F
512
5/9 (°F–32) 9/5 °C 32
feet per second 3.6 km/hr kilometer per hour (km/hr)
Prefixes for SI units Giga Mega Kilo Hecto Centi Milli Micro Nano Pico
109 106 103 102 10 2 10 3 10 6 10 9 10 1 2
1,000,000,000 1,000,000 1000 100 0.01 0.001 0.000,001 0.000,000,001 0.000,000,000,001
times times times times times times times times times
APPENDIX 2
513
Glossary
100-year flood A flood magnitude that comes along once every 100 years on average.
asteroid belt The distribution of asteroids in a disk-shaped ring orbiting the sun.
aa Blocky basalt with a ragged, clinkery surface. acceleration The rate of increase in velocity. During an earthquake, the ground accelerates from being stationary to a maximum velocity before slowing and reversing its movement.
asthenosphere Part of Earth’s mantle below the lithosphere that behaves in a plastic manner.The rigid and brittle lithosphere moves over it.
achondrite A stony meteorite that is similar to basalt in composition. active fault A fault that is likely to move again, especially those that have moved in the last 10,000 years. adiabatic cooling Change in volume and temperature without change in total heat content. adiabatic lapse rate (dry and wet) The rate of change of temperature as an air mass changes elevation. aftershocks Smaller earthquakes after a major earthquake that occur on or near the same fault. They may occur for weeks or months after the main shock. albedo The fraction of energy reflected away from the Earth’s surface. alluvial Related to sand and gravel deposited by running water. alluvial fan A fan-shaped deposit of sand and gravel at the mouth of a mountain canyon, where the stream gradient flattens at a main valley floor. amplitude The size of the back-and-forth motion on a seismograph. andesite An intermediate-colored, fine-grained volcanic rock. It has a silica content of approximately 60 percent, has intermediate viscosity, and forms slow-moving lavas, fragments, and volcanic ash. angle of repose The maximum stable slope for loose material; for dry sand it is 30 to 35 degrees. aquifer A permeable formation saturated with enough water to supply a spring or well; a water-bearing area of rock that will provide usable amounts of water. ash Loose particles of volcanic dust or small fragments. ash fall Volcanic ash falling through the air or collecting loosely on the ground. ash flow A mixture of hot volcanic ash and steam that pours at high velocity down the flank of a volcano. Also called nuée ardenté or pyroclastic flow. asteroid A chunk of rock orbiting the sun in the same way that Earth orbits the sun.
514
Atlantic Multidecadal Oscillation (AMO) The oscillation of the sea-surface temperature of the North Atlantic Ocean by about 0.8° C over several decades, typically about 70 years. atom The smallest particle of matter that takes part in an ordinary chemical reaction. average The usual or ordinary amount of something. Specifically, the sum of a group of values divided by the number of values used. avulsion The permanent change in a stream channel, generally during a flood when a stream breaches its levee to send most of its flow outside of its channel. bankfull capacity The maximum capacity of a stream before it overflows its banks. barrier bar A natural ridge of sand built by waves a few meters above sea level that is parallel to the shoreline and just offshore. The term is commonly used for a sand bar across the mouth of a bay or inlet. barrier island A near-shore, coast-parallel island of sand built up by waves and commonly capped by wind-blown sand dunes. Also called a barrier bar. basalt A black or dark-colored, fine-grained volcanic rock. It has a silica content of approximately 50 percent and low viscosity, and it flows downslope rapidly. base isolation A mechanism to isolate a structure from earthquake shaking in the ground; often flexible pads between a building and the ground. base level The level below which a stream cannot erode, typically at a lake or ocean. base surge The high energy blast of steam and ash that blows laterally during the initial stages of a volcanic eruption. beach The deposit of loose sand along the shore and deposited by waves. It includes sand seaward of either cliffs or dunes and extends offshore to a depth of perhaps 10 meters. beach hardening The placement of beach “protection” structures: riprap, groins, and related features. beach replenishment Addition of sand to a beach to replace that lost to the waves. Also called beach nourishment.
bedding Layers in a sedimentary rock. bedload Heavier sediment in a stream that is moved along the stream bed rather than in suspension. bedrock stream A stream that has eroded down to bedrock. bentonite A soft “swelling” clay that forms by alteration of volcanic ash, swells when wet, and becomes extremely slippery. It is prone to landsliding and can deform houses built on it. berm The uppermost accumulation of beach sand and other sediment left by waves. blind thrust A thrust fault that does not reach Earth’s surface. It is not evident at the surface. block A mass of cold, solid rock ejected from a volcano and larger than 25 centimeters across. body wave A seismic wave that travels through Earth’s interior. P and S waves are body waves. Surface waves travel only near Earth’s surface and are thus not body waves. bolide A general term that includes both asteroids and comets. bomb A 6- to 25-centimeter diameter mass of cold, solid rock that is ejected from a volcano. brackish Water intermediate in salinity between sea water and fresh water. Often true of the water in lagoons connected to the ocean. braided stream A stream characterized by interlacing channels that separate and come together at different places. The stream has more sediment than it can carry, so it frequently deposits some of it. breach Failure of part of a river’s levee, leading to some flow outside the main channel. breakwater An artificial offshore barrier to waves constructed to create calm water for a beach or for anchoring boats. burnout A burnout fire is deliberately set to burn back toward the main fire, eliminating fuel and stopping the progress of the main fire. buyer beware The old saying that warned a buyer to be wary that what he or she was purchasing might not be as good as it appears. The buyer pays the consequences. caldera A large depression, generally more than 1 kilometer across, in the summit of a volcano; formed by collapse into the underlying magma chamber. catastrophe Disaster, calamity. cavern A large, natural underground cave or tunnel, most commonly in limestone. Also used for a soil cavern developed over a limestone cavern. cellulose The woody parts of trees and plants. Celsius (°C) The temperature scale based on 0 for the freezing point of water and 100 for the boiling point of water at Earth’s surface and under normal conditions. Degrees Celsius are 5/9 the size of degrees Fahrenheit, the scale used by the public in the United States.
chance The likelihood of an event. For example, if the chance of an event is 1 percent, then the event will occur one time in 100 tries. channel scour The depth of sediment eroded during floods. channelization Straightening and confining stream flow within artificial barriers along the sides of the stream. chaos theory The theory that seemingly random events can unexpectedly lead to major, seemingly unrelated events. Chance effects can multiply and ultimately lead to major events. charge separation The separation distance or potential between positive and negative charges in a thundercloud. The greater the separation, the more violent the electrical discharge when positive and negative charges connect. Chinook winds Winds that warm by adiabatic compression as they descend from high elevations of a mountain range to low elevations on the plains to the east. chondrite A stony meteorite consisting primarily of the minerals olivine and pyroxene; the most common kind of meteorite. cinder cone A small steep-sided volcano consisting of basaltic cinders. cinders Small fragments of basalt full of gas holes that were blown out of a cinder cone. clay Particles of sediment smaller than 0.004 millimeter. See also clay minerals. clay minerals Exceptionally fine-grained (often claysized), soft, hydrous aluminum-rich silicate minerals with layered molecular structures. climate The weather of an area averaged over a long period of time. coastal bulge The region above a subduction zone in which the overlying continental plate flexes upward before slip on the subduction zone causes a major earthquake. cohesion The attraction between small soil particles that is provided by the surface tension of water between the particles. cold front The line of boundary between a large mass of cold air advancing under an adjacent large mass of warm air. collision zone The zone of convergence between two lithospheric plates. combustible Burnable. comet A mass of ice and some rock material traveling at high velocity in the gravitational field of the sun but traveling outside our solar system before passing through it on occasion. compaction The reduction in volume of clay, soil, or other fine-grained sediments in response to an overlying load. composite volcano A stratovolcano consisting of layers of ash, lava, and assorted volcanic rubble. compound An inorganic substance formed by chemical combination of two or more other substances.
G L O S S A RY
515
constructive interference Where independent highs overlap highs and lows overlap lows; the highs amplify highs and the lows amplify lows. continental caldera Rhyolite volcanoes characterized by their high viscosity and high volatile content but gently sloping flanks.
cyclic events Events that come at evenly spaced times, such as every so many hours or years. cyclone A large low pressure weather system that circulates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. It is equivalent to a hurricane or a typhoon.
continental crust The upper 30 to 60 kilometers of the continental lithosphere that has an average density of 2.7 grams per cubic centimeter and an average composition similar to that of granite.
dacite A light-colored volcanic rock intermediate in composition and appearance between andesite and rhyolite.
continental drift The gradual movement of continents as oceans spread and separate.
debris avalanche A fast-moving avalanche of loose debris that flows out a considerable distance from its source. debris flow A slurry of rocks, sand, and water flowing down a valley. Water generally makes up less than half the flow’s volume. deepwater waves A wave in water deep enough that its water movement does not touch bottom—that is, a wave in water that is deeper than half the wave’s length.
continental margin The boundary between continental lithosphere and oceanic lithosphere. continental shelf The shallowly submerged edges of the continent to a depth of approximately 100 meters below sea level. continent–continent collision Collision between two continental plates. convective updraft The rapid rise of a heated, expanded air mass driven by its lower density. The removal of air from below draws outside air into the rising “chimney.” convergent boundary A tectonic plate boundary along which two plates come together by either subduction or continent–continent collision. core Innermost part of Earth consisting of nickel and iron. The inner part is solid, the outer part liquid. Coriolis effect Rotation of Earth from west to east under a fluid such as the atmosphere or oceans permits that fluid to lag behind Earth’s rotation. Fluid flows shift to the right in the northern hemisphere and to the left in the southern hemisphere as a result of this effect. Thus a southward-moving fluid appears to curve off to the west in the northern hemisphere. cover collapse Collapse of the roof material over an underground cavity, often a soil cavern over a limestone cavern. cover subsidence Gradual depression of the roof material over an underground cavity. crater A depression created as ash blasts out of a volcano. creep (1) Surface layers of soil on a slope move downslope more rapidly than subsurface layers. (2) Slow, more-or-less continuous movement on a fault. Creeping faults either lack earthquakes or have only small ones. cross-bedding Internal layers inclined to the overall layers in a sediment or volcanic ash deposit. crown fire A fire that burns in the treetops. cumulonimbus cloud A thundercloud with a flat anvilshaped top; a sign of a nearby thunderstorm. current The continuous movement of a mass of fluid in a definite direction. cutoff When adjacent meanders in a stream come close together, the stream may cut through the narrow neck between them to bypass and abandon the intervening meander loop. The cut through the narrow neck is a cutoff.
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daylighted bed A layer inclined less steeply than the slope so that it is exposed at the surface down lower on the slope.
deformation The change in shape of an object. delta The accumulation of sediment where a river reaches the base level of a lake or ocean. density The mass (e.g., grams) of a material per area (e.g., cubic centimeter). Water is 1 g/cm3; granite is 2.7 g/cm3. deposition The accumulation of sediment previously carried by a stream. desert A particularly dry climate in which evaporation exceeds precipitation. An area with less than 25 centimeters of precipitation annually. A desert does not need to be hot. desertification The growth of new desert environments in areas where drought prevents regrowth of grass and shrubs stripped away by increased population, intensive cultivation in marginal areas, and overgrazing. destructive interference Where independent highs overlap lows and lows overlap highs; the lows tend to cancel out the effects of the highs. developed countries Industrial countries that are wealthy and have a large middle class. dip The angle of inclination of a plane as measured down from the horizontal. discharge Measured volume of water flowing past a cross section of a river in a given amount of time.Usually expressed in cubic meters per second or cubic feet per second. displacement The distance between an initial position and the position after movement on a fault. dissolution Minerals or rocks dissolving in water. divergent boundary A spreading plate boundary such as a mid-oceanic ridge. dome A bulge on a volcano that is pushed up by molten magma below. downburst A localized, violent, downward-directed wind below a strong thunderstorm. drainage basin The area upslope from a point in a valley that drains water to that point.
dredging Scooping up loose sediment such as gravel from a stream to separate any gold it may contain.
eruptive vent The point of eruption of material from a volcano.
drought A prolonged dry climatic event in a particular region that dramatically lowers the available water below that normally used by humans, animals, and vegetation.
estuary The submerged mouth of a river valley where fresh water mixes with seawater.
dry climate Sometimes described as an area with evaporation greater than precipitation, sometimes as an area with too little precipitation to support significant plant life. dune A ridge or mound of sand deposited by the wind, most commonly at the head of a beach. dynamic equilibrium The condition of a system in which the inflow and outflow of materials is balanced. earthquake The ground shaking that accompanies sudden movement on a fault, movement of magma underground, or a fast-moving landslide.
evaporation The change of a substance from a liquid to a vapor. evapotranspiration Water returned to the atmosphere by a combination of evaporation of water from the soil and leaves, and transpiration of moisture from the leaves. expansive soil Clay-rich soil that expands when it absorbs water. Most expansive soil contains the swelling clay smectite. explosion crater A depression formed by the forceful blasting out of gases and fragments from a volcano.
effluent stream. See gaining stream.
exsolve The process by which gas or other material separates from a solid rock.
elastic A type of deformation in which a rock deforms without breaking. If the deforming stress is relaxed, the rock returns to its original shape. Contrast with plastic.
extratropical cyclone A cyclone formed outside the tropics and commonly associated with weather fronts. Nor’easters are an example.
elastic rebound theory The theory applied to most earthquakes in which movement on two sides of a fault leads to bending of the rocks until they slip to release the bending strain during an earthquake.
eye The small-diameter core of a hurricane; characterized by lack of clouds and little or no wind.
element One of ninety-two naturally occurring materials such as iron, calcium, and oxygen that cannot be separated chemically. Elements combine to make minerals. El Niño Elevated sea-surface temperatures that lead to dramatic changes in weather in some areas every few years— for example, rains in coastal Peru and western Mexico and southwestern California. See also La Niña. emissions Gases given off by a volcano. energy The capacity for doing work. The kinetic energy of a large wave can move huge boulders. The heat energy of a rising mantle plume can melt the overlying Earth crust. ENSO El Niño-southern oscillation; the alternation between El Niño and its opposite extreme La Niña. epicenter The point on the Earth’s surface directly above the focus; the initial rupture point on a fault. equilibrium profile (1) The longitudinal profile or slope of an equilibrium stream (see graded stream); (2) the seaward slope of a beach that is adjusted so that the amount of sediment that waves bring onto the beach is adjusted to the amount of sediment that the waves carry back offshore. erosion The wearing down and transport of loose material at the Earth’s surface, including removal of material by streams, waves, and landslides. eruption The processes by which a volcano expels ash, steam and other gases, and magma.
far-field tsunami A tsunami generated by an earthquake far from the point of impact of the wave. fault An Earth fracture along which rocks on one side move relative to those on the other side. fault creep The slow, more-or-less continuous movement on a fault, in contrast to the sudden movement of a section of fault during an earthquake. felsic Light-colored igneous rocks. fetch The distance over which wind blows over a body of water. The longer the fetch, the larger the waves that will be developed. firebrands Burning embers carried by the wind, potentially to ignite new fires. firestorm A large, extremely hot fire generating a large updraft that pulls in vast amounts of air from the sides to fan the flames. fire weather potential The favorability of weather conditions for wildfire. fissure A fracture at the surface that has opened by widening. See also rift. flash flood A short-lived flood that appears suddenly, generally in a dry climate, in response to an upstream storm. flood A stream flow high enough to overtop the natural or artificial banks of a stream. flood basalt A broad expanse of basalt lava that cooled to fill in low-lying areas of the landscape.
eruption column The more-or-less cylindrical mass of ash and gases forcefully blown up from a volcanic vent during an eruption.
flood crest The point where the flood reaches its peak discharge.
eruptive rift A crack in the ground from which lava erupts.
flood frequency The average time between floods of a given height or discharge.
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flood fringe The fringe or backwater areas of a flood that store nearly still water during a flood and gradually release it downstream after the flood. floodplain Relatively flat lowland that borders a river, usually dry but subject to flooding every few years. floodway Area of floodplain regulated by FEMA for federal flood insurance. fluidization The process of changing a soil saturated with water to a fluid mass that flows downslope. focus The initial rupture point on a fault. The epicenter is the point on Earth’s surface directly above the focus. Also called hypocenter.
global warming Warming of Earth’s surface temperatures, especially over the past 100 years. Much of the recent warming has been attributed to increases in greenhouse gases. graded stream Stream in equilibrium with its environment; its slope is adjusted to accommodate the amount of water and sediment amounts and grain size provided to it. gradient Slope along the channel of a streambed, typically expressed in meters per kilometer or feet per mile. granite Light-colored, grainy igneous rock consisting of the minerals quartz, the two feldspars, orthoclase and plagioclase, and sometimes a dark mineral, either mica or amphibole.
forecast The statement that a future event will occur in a certain area in a given span of time (often decades) with a particular probability. foreshocks Smaller earthquakes that precede some large earthquakes on or near the same fault. They are inferred to mark the initial relief of stresses before final failure.
greenhouse effect Increased atmospheric temperatures caused when atmospheric gases such as carbon dioxide and methane trap heat in Earth’s atmosphere.
fractal Features that look basically the same regardless of size; for example, the coast of Norway looks serrated on the scale of a map of the world or on a map of only part of that coast. fracture Any crack in a rock. frequency The number of events in a given time, such as the number of back-and-forth motions of an earthquake per second. frictional resistance The resistance to downslope movement of a flow or landslide. front (weather) The boundary between one air mass and another of different temperature and moisture content. frostbite The freezing of body tissue as a result of exposure to extreme cold temperature and winds. fuel load The amount of burnable material, such as trees and dry vegetation, available for a fire. fuel moisture Amount of moisture in a fuel. The lower the moisture content, the more readily the fuel will burn and at a higher temperature. Fujita scale The scale of tornado wind speed and damage devised by Dr. Tetsuya Fujita. fumarole Vent that emits steam or other gases. funnel cloud Narrow, rapidly spinning funnel-shaped “cloud” that descends from a violent storm cloud. See also tornado. gaining stream A stream, typically in a climate with abundant rainfall, that lies below the water table and gains water from groundwater. geophysics Study of Earth’s physical properties. geotextile fabric Strong permeable cloth that permits water to pass but not soil and rock particles. geyser Intermittent eruption of hot and boiling water from a vent in an area of volcanic rocks. glacial-outburst flood Flooding of glacial meltwater when ice tunnels or dams collapse. Also called Jökulhlaups.
groin Wall of boulders, concrete, or wood built out into the surf from the water’s edge to trap sand that moves in longshore drift. The intent is to hinder loss of sand from the beach.
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greenhouse gases Atmospheric gases such as carbon dioxide and methane that retain heat much like a greenhouse; solar energy can get in, but the heat cannot easily escape.
groundwater Water in the saturated zone below the ground surface. Gulf Stream Warm oceanic current that moves north from the Gulf of Mexico, past the East Coast of North America, and across the North Atlantic to maintain mild temperatures in northern Europe. gunite. See shotcrete. hailstone Single pellet of hail formed when droplets of water freeze before falling to the ground. “hard” solutions Solutions to a problem that involve building protective structures such as riprap and levees or continuous pumping of water from a landslide-prone slope. harmonic tremor Rhythmic shaking of the ground that often accompanies magma movement. Hawaiian-type lava Fluid basalt lava of the type that erupts from Hawaiian volcanoes. hazard An environment that could lead to a disaster if it affects people. headland An erosionally resistant point of land jutting out into the sea (or a large lake) and often standing high. heat Energy produced by the collisions between vibrating atoms or molecules. The faster the vibration, the greater the heat and the higher the temperature. heat capacity The capacity to hold heat. Metals such as iron heat and cool easily; they have low heat capacity. Water has a high heat capacity. heat-island effect Increased temperatures in urban areas due to buildings and paved areas absorbing more solar heat, while exhaust from cars and factories traps heat. Hertz The number of cycles of an earthquake per second, a measuring unit of frequency of shaking.
high pressure system An area characterized by descending cooler dry air and clear skies. Winds rotate clockwise around a high pressure system (in the northern hemisphere). See also right-hand rule. hook echo A curved, hook-shaped pattern on weather service radar that is indicative of a supercell thunderstorm.The hook shape often indicates development of a tornado. hotspot volcano An isolated volcano, typically not on a lithospheric plate boundary, but lying above a plume or hot column of rock in Earth’s mantle. hurricane A large tropical cyclone in the North Atlantic or east Pacific ocean, with winds greater than 118 kilometers per hour. Similar storms elsewhere are called typhoons or cyclones. hurricane warning A hurricane watch is upgraded to a warning when dangerous conditions of a hurricane are likely to strike a particular area within twenty-four hours. hurricane watch An indication that a hurricane may affect a region within thirty-six hours. hydraulic mining A mining method that used highpressure water hoses to wash gold-bearing gravels down into sluice boxes where gold could be separated from the gravel. hydrograph A graph that shows changes in discharge or river stage with time. hydrologic cycle The gradual circulation of water from ocean evaporation to the continents, where it falls to the ground as rain or snow and soaks into the ground to feed vegetation, groundwater, and streams. From there, some water evaporates and some returns to the oceans. hydrophobic soil Soil sealed by hydrocarbon resins.These soils will not permit water to soak in. hydrothermal Pertaining to hot water, usually of volcanic origin. hypothermia When a person’s core body temperature drops below 35°C/95°F. Symptoms include uncontrollable shivering, drowsiness, disorientation, slurred speech, and exhaustion. hypothesis A proposal to explain a set of data or information, which may be confirmed or disproved by further study. ice age A period of low temperature lasting thousands of years and marked by widespread ice sheets covering much of the northern hemisphere. Two to four ice ages are recorded in North America, and as many as twenty in deep sea sediments of the last 2 million years. igneous rocks Rocks that crystallize from molten magma, either within the Earth as plutonic rocks (e.g., granite and gabbro) or at Earth’s surface as volcanic rocks (e.g., rhyolite, andesite, and basalt). indefensible location A home or building location that is especially vulnerable to wildfire. Particularly important are dry vegetation and other fuels near the building and a location high on a hill, one that fire can easily reach upslope. influent stream. See losing stream.
insurance A service by which people pay a premium, usually annually, to protect themselves from a major financial loss they cannot afford. The insurance company that collects the premiums pays for covered losses. intensity scale The severity of an earthquake in terms of the damage that it inflicts on structures and people. It is normally written as a Roman numeral on a scale of I to XII. ion A charged atom. iridium anomaly A thin clay-bearing layer of sediment 65 million years old that has an abnormally high content of the platinum-like metal iridium. iron meteorite A meteorite consisting of a nickel and iron alloy. isostacy Lower-density crust floats in Earth’s higher density mantle. Also called isostatic equilibrium. jet stream The high speed air current traveling from west to east across North America at an approximate altitude of 10 to 12 kilometers. jetties Walls of boulders, concrete, or wood built perpendicular to the coast at the edges of a harbor, estuary, or river mouth. The intent is to prevent sediment from blocking a shipping channel. joint One of a group of fractures with similar orientation. karst The ragged top of limestone exposed at the surface, resulting from dissolution by acidic rainfall and groundwater. The ground is often marked by sinkholes and caverns. K-T boundary The boundary between the Cretaceous and Tertiary geological time periods, from approximately 65 million years ago. The boundary is marked by a thin layer of sooty clay that contains the iridium anomaly. Kyoto Protocol The 1997 agreement between many countries to reduce their emissions of greenhouse gases into the atmosphere. ladder fuels Fuels of different heights that permit fire to climb progressively from burning ground material to brush, small trees, low branches, and finally to the tops of tall trees. lagoon A narrow, shallow body of seawater or brackish water that parallels the coast just inland from a barrier island. lahar A volcanic mudflow. lake effect snow Increased snowfall in areas downwind from large lakes. land subsidence Settling of the ground in response to extraction of water or oil in subsurface soil and sediments, drying of peat, or formation of sinkholes. land use planning Restriction of development according to practical and ethical considerations, including the risk of natural hazards. landslide Downslope movement of soil or rock. La Niña The opposite of El Niño. lapilli A particle of volcanic ash between 2 millimeters and 6 millimeters across. latent heat The heat added to a gram of a solid to cause melting or to a liquid to cause evaporation. Also the heat produced by the opposite changes of state.
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latitude Imaginary lines on Earth’s surface that parallel the equator. The equator is 0 degrees latitude; the poles are 90 degrees north and south latitude. lava Magma that flows out onto the ground surface. lava dome A large bulge or protrusion, either extremely viscous or solid on the outside, and pushed up by magma. left-lateral One sense of movement on a strike-slip fault; if you stand on one side of the fault, the opposite side has moved to the left. levee An artificial embankment of loose material placed at the edge of a channel to prevent floodwater from spreading over a floodplain. lightning The visible electrical discharge that marks the joining of positive and negative electric charges between clouds or between a cloud and the ground. liquefaction A process in which water-saturated sands jostled by an earthquake rearrange themselves into a closer packing arrangement. The expelled water spouts to form a sand boil. lithosphere Rigid outer rind of Earth approximately 60 to 100 or so kilometers thick; it forms the lithospheric plates. lithospheric plates A dozen or so segments of the lithosphere that cover Earth’s outer part. load (1) Related to landslides, the weight of material on a slope. (2) Related to flood processes, the volume of sediment a stream can carry. longitude Imaginary lines on Earth’s surface oriented north–south and perpendicular to the equator. longshore current A current parallel to the shore caused by refraction of waves coming in to the beach at an angle. longshore drift The gradual migration of sand or gravel along the shoreline resulting from waves repeatedly carrying the sediment grains obliquely up onto shore and then straight back to the water’s edge. losing stream A stream, typically in a dry climate, that lies above the water table and loses water to an aquifer. low pressure system An area of low atmospheric pressure that is characterized by rising warmer and humid air and cloudy skies. Winds rotate counterclockwise around a low pressure cell (in the northern hemisphere). See also right-hand rule. low-velocity zone Zone of low seismic velocity that marks the boundary between the lithosphere and asthenosphere. Presumed to contain a small percentage of partial melt. maar A shallow crater with low rims, often on nearly flat topography, and formed by gas-rich volcanic eruptions. mafic A dark-colored igneous rock. magma Molten rock. When it flows out on the ground surface, it is called lava. magma chamber Large masses of molten magma that rise through Earth’s crust, often erupting at the surface to build a volcano. magnetic field The area around a magnet in which magnetism is felt.
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magnetic stripes Strips of ocean floor, parallel to an oceanic ridge, that alternate between weak and strong magnetism parallel to Earth’s current magnetic field. magnitude The relative size of an earthquake, recorded as the amplitude of shaking on a seismograph. The range of amplitudes is so large that the number (open-ended but always less than 10) is recorded as a logarithm of the amplitude. Different magnitudes are based on amplitudes of different seismic waves: ML Richter or local magnitude; Ms surface wave magnitude; Mb body wave (P or S wave) magnitude; Mw moment magnitude (most important for extremely large earthquakes). mammatus clouds Downward rounded pouches protruding from the base of the overhanging anvil of a thundercloud. mangrove Large leafy bushes or trees that form dense thickets in shallow brackish waters of coastal lagoons and estuaries. mantle The thick layer of material below the thin crust and above Earth’s core. Mostly peridotite in composition in its upper part. Its density approximates 3.2 or 3.3 g/cm3 in the upper part and 4.5 g/cm3 in the lower part. mass A measure of the quantity of matter. Not the same as weight. meandering stream Streams that sweep from side to side in wide turns called meanders. mean sea level The average height of the sea, averaged over a long time, specifically nineteen years. Inferred to be midway between average high and low tides. melting temperature The temperature at which a rock melts. Granite commonly melts between 700° and 900° Celsius. Basalt commonly melts near 1200 to 1400° Celsius. Mercalli Intensity Scale. See intensity scale. metamorphic rocks Original rocks of any kind that have been changed by heat or pressure; sometimes changes occur in chemical composition. meteor A piece of space rock that heats to a white hot incandescence when it streaks through Earth’s atmosphere. meteorite An asteroid that passes through the atmosphere to reach Earth. methane A gaseous hydrocarbon emitted during volcanic eruptions. It can be burned to generate heat; if released into the atmosphere, it is a greenhouse gas. methane hydrate Frozen methane-ice “compound”, trapped in layers deeper than 1 km in continental permafrost and at shallow depths under the sea floor of many of the world’s continental slopes. A potential source of greenhouse gas. microearthquakes Minute underground tremors that may reveal a previously unknown fault system and may someday lead to earthquake predictions. microtektite Translucent droplets of glass that have come through Earth’s atmosphere.
mid-oceanic ridge A high-standing rift or spreading zone in an ocean—for example, the mid-Atlantic Ridge or the East Pacific Rise. migrating earthquakes Earthquakes that occur in a sequential manner along a fault over time. millibar A measure 1/1000 of 1 bar, which is the normal atmospheric pressure at sea level. mineral A naturally occurring inorganic compound with its atoms arranged in a regular crystalline structure. Compare with rock. mining groundwater Removal of water in the ground without replacement. mitigation Changes in an environment to minimize loss from a disaster. Modified Mercalli Intensity Scale Scale measuring the severity of an earthquake in terms of the damage that it inflicts on structures and people. It is normally written as a Roman numeral on a scale of I to XII. Mohoroviçic discontinuity (moho) Boundary between Earth’s crust and mantle. It is detected from the contrast between the slower seismic velocities of the crust (generally 5 to 7 km/sec.) and the upper mantle (approximately 8 km/sec.). molecule The smallest combination of two or more atoms held together by balanced atomic charges. moment magnitude (Mw) The magnitude of an earthquake based on its seismic moment; depends on the rock strength, area of rock broken, and amount of offset across the fault. monsoon A seasonal wind that blows from the southeast for half of the year bringing warm, moist air and heavy rains from the Indian Ocean onto the south Asian continent. mudflow A flow of mud, rocks, and water dominated by clay or mud-sized particles. multipurpose dam A dam justified by perceived multipurpose benefits—for example, flood control, hydroelectric power, water supply, and recreation. National Flood Insurance Program The flood insurance program guaranteed and partly funded by the U.S. government. natural disaster A natural event that causes significant damage to life or property. natural hazards Hazards in nature that endanger our property and physical well-being. natural levee Natural embankment of sediment at the edge of a stream, where sediment is deposited as floodwaters slow and spread over an adjacent floodplain. nature’s rampages The perception of many people that a natural catastrophe involves nature going on an abnormal rampage. Actually, nature is not doing anything that it has not done for millions of years; we humans have merely put ourselves in harm’s way. near-Earth object A space object such as an asteroid or comet that narrowly misses the Earth.
Nor’easter A strong extratropical winter storm that moves up the east coast of North America with high winds and high waves. These can be as damaging as hurricanes. normal fault A fault (generally, steeply inclined) that has the upper block of rock moving down compared with the lower block. normal stress The component of stress perpendicular to (normal to) one of Earth’s planar surfaces. North Atlantic Oscillation (NAO) The winter atmospheric pressure pattern over the North Atlantic Ocean that brings major storms every few years. obsidian Volcanic glass, usually of rhyolite composition. It is generally a water-poor magma that solidified before it could nucleate and crystallize. oceanic crust The upper part of Earth’s lithosphere under the oceans. It consists of basalt and gabbro and is typically 7 kilometers thick. offset The distance of movement across a fault during an earthquake. orographic effect The effect created when moisturebearing winds rise against a mountain range:They condense and form clouds and rain. overburden Soil or other material above the bedrock. Often used in reference to waste material that must be removed to get at desired material below. overland flow Surface runoff of water in excess of that which is able to infiltrate the ground. oxbow lake A small lake left when a meander of a stream is cut off and left after a flood. pahoehoe Basalt lava with a ropy or smooth top. paleo– A prefix referring to events in the past. paleoflood analysis Information on previous floods gathered from erosional and depositional features left by such a flood. paleomagnetism The study of past characteristics (orientation and strength) of Earth’s magnetism as preserved in rocks formed at various times in the past. paleoseismology The study of former earthquakes from examination of offset rock layers below the ground surface. paleovolcanology The study of former volcanic events from examination of offset rock layers below the ground surface. pali The Hawaiian term for giant cliffs; these are now known to be the headscarps of giant submarine landslides. Pangaea Supercontinent that began to break up to form today’s continents 225 million years ago. Peléan eruption A large ash-rich eruption that typically produces ash flows. perforated pipes Pipes full of holes that are pushed into a water-saturated landslide.Water seeps through the holes into the pipes and then down through the gently sloping pipes to the surface. period The time between seismic waves or the time between the peaks recorded on a seismograph.
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permafrost The condition where water in the ground remains frozen.
P wave The compressional seismic wave that shakes back and forth along the direction of wave travel.
permeability The ease with which water can move through soils or rocks.
pyroclastic flow A mixture of hot volcanic ash, coarser particles, and steam that pours at high velocity down the flank of a volcano. Also called nuée ardente or ash flow.
phreatic eruption A volcanic eruption dominated by steam. Phreatomagmatic eruption Similar to a phreatic eruption but with a higher proportion of magma. pillow basalt Basalt that flows into water or wet mud chills on its outside surfaces to form elongate fingers of basalt a meter or so across. piping Water seeping through a dam or levee can begin to carry sand and cause erosion. plastic A type of deformation in which a rock deforms gradually without breaking. If the deforming stress is relaxed, then the rock does not return to its original shape. Contrast with elastic. plate tectonics The theory that lithospheric plates that move relative to one another collide in some places, pull apart in others, and slide past one another in still others. These movements cause earthquakes and volcanic eruptions as well as build mountain ranges. The theory is supported by a wide range of data. Plinian eruption Extremely large eruptions that involve continuous blasts of ash. Examples include Vesuvius in 79 A.D. and Mount St. Helens in 1980. plume A rising, upward-flaring zone of hot rock deep in Earth’s mantle. plutonic rocks A distinctly grainy igneous rock that solidified below Earth’s surface. point bar Deposits of sand and gravel on the inside (concave side) of a meander bend of a stream.
pyroclastic material Fragmental material blown out of a volcano—for example, ash, cinders, and bombs. quick clay Water-saturated mud deposited in salty water tends to consist of randomly oriented flakes of clay with large open spaces between the flakes. If the salt is flushed out, the flakes are unstable and may easily collapse and flow almost like water. quicksand Loose water-saturated sand; if a mass is placed on quicksand, it will sink in the sand and water is pushed to the surface. radiometric The measurement of the age of a rock by the analysis of radioactive constituents in the rock and their products, along with their known rates of decay. rain shadow Drier climate on the downwind side of a mountain range. recurrence interval The average number of years between an event of a certain size in a location—for example, floods, storm surges, and earthquakes. Also known as a return period. refraction. See wave refraction. relative humidity The percentage of moisture in air relative to the maximum amount it can hold (at saturation) under its given temperature and pressure. resisting force The force (such as friction or a load pushing in the opposite direction) that resists downslope movement.
poor sorting A sediment with a mixture of different grain sizes. Compare with sorted.
resurgent caldera A huge collapse depression at the Earth’s surface that sank into a near-surface magma chamber during eruption of the magma.
popcorn clay Clay that swells when wet to take on the surface characteristics of a layer of popcorn.
resurgent dome A bulge in a caldera where magma rose up, which may or may not develop into a new eruption.
pore pressure The pressure of water in pore spaces between soil or sediment grains tends to push the grains apart and reduce the contact force between the grains. That can facilitate landsliding.
retrofitting Modifying existing buildings to minimize the damage during strong earthquake motion.
porosity The percentage of pore space in a sediment or rock. precipitation Water that falls as rain, hail, or snow.
reverse condemnation If the government restricts a property owner from using his or her property for some potential purpose that has effectively taken the value of the property for that purpose.
prediction A statement that a future event will occur at a certain time at a certain place. Compare with forecast.
reverse fault Faults (generally, steeply inclined) that have the upper block of rock moving up compared with the lower block.
prescribed burn Intentional setting of forest fires under controlled conditions to consume fuels and mitigate spread of future fires.
rhyolite A light-colored, fine-grained volcanic rock. It has a silica content of approximately 70 percent and high viscosity, and it generally erupts as volcanic ash and fragments.
pressure The force per unit area; a continuously applied force spread over a body.
Richter Magnitude Scale The scale of earthquake magnitude invented by Charles Richter. See also magnitude.
property rights The right to do what one wishes with one’s personal property.
riffles The part of a stream with shallow rapids—generally, the straighter section between meander bends.
pumice Frothy volcanic rock dominated by gas bubbles enclosed in glass; typically pale in color and floats on water.
rift A spreading zone on the flank of a volcano from which lavas erupt. See also rift zone.
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rift zone An elongate spreading zone in Earth’s lithosphere. right-hand rule The rule used to remember the direction of wind rotation in a low or high pressure cell in the northern hemisphere. With your right thumb pointing in the direction of overall air movement, your fingers point in the direction of wind rotation. right lateral One sense of movement on a strike-slip fault; if you stand on one side of the fault, the opposite side moves to the right during an earthquake.
scoria A lava full of gas bubbles; fewer gas bubbles and heavier pumice. Most commonly basalt composition. seawall Walls of boulders, concrete, or wood built along a beach front and intended to protect shore structures from wave erosion. sedimentary rock A rock deposited from particles in water, ice, or air—or precipitated from solution—and then cemented into a solid mass. Examples are sandstone, shale, and limestone.
ring of fire The zone of volcanoes and arc volcanoes above subduction zones that surround the Pacific Ocean.
seiche The back-and-forth sloshing of a lake or other closed body of water. It can be caused by an earthquake, landslide, or changes in atmospheric pressure.
rip current A short-lived surface current that flows directly off a beach and through the breaker zone. It carries water back to the sea that had piled up onto the beach.
seismic gap A section of an active fault that has not had a recent earthquake. Earthquakes elsewhere on the fault suggest that the gap may have an earthquake in future.
riprap Coarse rock piled at the shoreline in an attempt to prevent wave erosion of the shore or destruction of a nearshore structure or a streambank.
seismic sea wave. See tsunami.
risk The chance of an event multiplied by the cost of loss from such an event. river stage Water height in a river, measured relative to some arbitrary fixed point. rock A mass of interlocking or cemented mineral grains. Compare with mineral. rockbolts Long bolts drilled into and expanded in an unstable rock mass to help keep it from landsliding. rockfall A rock mass that falls from a steep slope. rotational landslide A landslide in which the mass rotates as it slides on a basal slip surface. Also called a slump. runoff The portion of precipitation that flows off the ground surface. run-up The height to which water at the leading edge of a wave rushes up onto shore.Also used for the height to which a tsunami wave rushes up onshore. Saffir-Simpson Hurricane Scale The commonly used five-category scale of hurricane intensity based on wind speeds and seawater damage. sand Particles of rock that range in size from 1/16 to 2 millimeters in diameter. sand boil A pile of sand brought to the surface in water expelled from the ground by liquefaction at shallow depth or during flood when the water pressure under a channel forces groundwater to the surface outside a levee. sand dune An accumulation of wind-blown sand, most commonly along the upper beach above high-tide level. sand sheets Layers of nearly homogeneous, unlayered sand laid down by a tsunami wave that sweeps sand inland from a beach. Santa Ana winds Southern California trade winds that flow southwest, bringing dry air from the continental interior. scientific method The method used by scientists to solve problems. They analyze facts and observations, formulate hypotheses, and test the validity of the hypotheses with experiments and additional observations.
seismic wave A wave sent outward through Earth in response to sudden movement on a fault. See also P wave, S wave, surface wave. seismogram The record of seismic waves from an earthquake or other ground motion as recorded on a seismograph. seismograph The instrument used to detect and record seismic waves. ShakeMap Computer-generated maps of ground motion which show the distribution of maximum acceleration and maximum ground velocity during earthquakes. shallow-water wave A wave in water shallow enough that its water movement touches bottom at a depth shallower than 1/2 of its wavelength. shatter cone Cone-shaped features, with rough striations radiating downward and outward from the shock effect, considered diagnostic of bolide impact. shield volcano An extremely large basalt-lava volcano, such as those in Hawaii, with gently sloping sides. shore profile The slope of the beach. shotcrete A fluid cement-type material that is sprayed on a slope to prevent water penetration. Also called gunite. silica tetrahedra An arrangement of four oxygen atoms in a four-cornered pyramid around a single silicon atom. sinkhole A ground depression caused by collapse into an underground cavern. slip plane, slip surface The sliding surface at the base of a landslide. Also called a slide plane. slope angle The angle of a slope as measured down from the horizontal. slump. See rotational landslide. smectite Clay in which flakes have an open structure between their layers, which when filled with water cause the clay to dramatically expand. snow avalanche Downslope movement of snow. “soft” solutions Solutions to a problem that involve avoiding the problem through restrictive zoning and building codes that minimize damage. G L O S S A RY
523
soil For engineering purposes, all of the loose material above bedrock.
Strombolian eruption Frequent mild eruptions of basalt or andesite scoria, typically forming a cinder cone.
soil creep Slow downslope movement of near-surface soil or rock; caused by numerous cycles of heating and cooling, freezing and thawing, burrowing animals, and trampling feet.
subduction The process in which one lithospheric plate (usually oceanic) descends beneath another.
soluble Able to be dissolved, typically in water. solution The process of dissolving rocks of minerals in water. sorted A sediment is well sorted if its grains are all about the same size. Compare with poor sorting. southern oscillation. See ENSO. spot fire Fires ignited by firebrands which burn ahead of the main fire. spreading zone or rift zone Boundary along which lithospheric plates spread apart or diverge. stalactite A cylindrical deposit of calcium carbonate that grows down from the roof of a cavern by condensation of carbonate-rich water. stalagmite A cone-shaped deposit of calcium carbonate that grows up from the floor of a cavern by evaporation of carbonate-rich water. step leader Electrical charges that advance downward from a thundercloud but do not manage to reach the ground. stony-iron meteorite Essentially a chondrite that contains some nickel-iron. storm surge The rapid sea-level rise caused by both low atmospheric pressure of a major storm and the strong winds that accompany the storm and push water forward. strain Change in size or shape of a body in response to an imposed stress.
subduction zone Convergent boundary along which lithospheric plates come together and one descends beneath the other; often ocean floor descending beneath continent. submarine canyon A deep canyon in the continental shelf and slope offshore and extending about perpendicular to the shore. submarine landslide Subsea level collapse of the flank of an oceanic volcano such as in Hawaii or the Canary Islands. subsidence Settling of the ground in response to extraction of water or oil in subsurface soil and sediments, drying of peat, or formation of sinkholes. Subsidence of the ocean floor occurs by cooling of hot lithosphere. sulfur dioxide A toxic volcanic gas consisting of two oxygen atoms attached to one sulfur atom. supercell A particularly strong rotating thunderstorm that can spin off dangerous tornadoes. superoutbreak A large group of tornadoes produced along a major storm front. surface rupture length The length of a fault broken during an earthquake. surface tension The effect by which grains of sand are held together by the thin films of water between them. surface wave The seismic wave that travels along and near Earth’s surface. These waves include Rayleigh waves (which move in a vertical, elliptical motion) and Love waves (which move with horizontal perpendicular to the direction of wave travel). surge. See base surge, storm surge.
stratovolcano A large, steep-sided volcano consisting of layers of ash, fragmental debris, and lava. Also called a composite volcano.
suspension Sediment grains carried within the water column and buoyed up by turbulent eddies—in contrast to sediment grains transported along a stream bed.
straw wattle A net in the shape of a large hose, filled with straw, and used to hinder surface runoff and erosion on a slope denuded by fire or landslide.
S wave The seismic shear wave that shakes back and forth perpendicular to the direction of wave travel. S waves do not pass through liquids.
streambed mining Excavation of sand and gravel from a stream bed.
swelling soil A soil that expands when wet; generally, a soil that contains the swelling clay smectite.
stream order A way of numbering streams in a hierarchy that designates a small unbranched stream in a head-waters as first order, the stream it flows into as second order, and so on.
talus Coarse, angular rock fragments that fall from a cliff to form a cone-shaped pile banked up against the slope.
stress The forces on a body. These can be compressional, extensional, or shear. strewn field Area in which fragments of large meteorites are spread out around the main impact site.
tectonic. See plate tectonics. tektite Droplets of molten rock—glass—that may have formed by superheated splash from a hypervelocity impact on either the moon or Earth.
strike The compass direction of a horizontal line on a plane such as a fault or a rock layer.
texture The arrangement of minerals in a rock. Flat grains of mica, for example, may be arranged parallel to one another to form a rock that breaks into sheets. Equidimensional grains may form a massive rock.
strike-slip fault A fault (generally vertical) which has relative lateral movement of the two sides.
thalweg The line connecting the deepest parts of the channel along the length of the stream bed.
524
G L O S S A RY
theory A scientific explanation for a broad range of facts that have been confirmed through extensive tests and observations. thrust fault Fault (generally, gently inclined) that has the upper block of rock moving up compared with the lower block. thunderstorm A storm accompanying clouds that generate lightning, thunder, rain, and sometimes hail. tidal current The flow of water through a narrow passage, such as between the ocean and a lagoon, through segments of a barrier island, in response to sea level change between high and low tides. tidal wave A colloquial but incorrect name for tsunami. tide The change in sea level, generally once or twice a day, in response to the sun and moon’s gravitational pull. tiltmeter A device like a carpenter’s level that records any change in slope on the flank of a volcano. tornado A near-vertical narrow funnel (generally only a kilometer or so in diameter) of violently spinning wind associated with a strong thunderstorm. Tornado Alley The region of the central United States, between Texas and Kansas, that is noted for frequent tornadoes. tornado outbreak A series of tornadoes spawned by a group of storms. tornado warning A tornado warning is issued when Doppler radar shows strong indication of vorticity or rotation, or if a tornado is sighted. tornado watch A tornado watch is issued when thunderstorms appear capable of producing tornadoes and telltale signs show up on the radar. At this point, storm spotters often watch for severe storms. trade winds Regional winds that blow from northeast to southwest between latitudes 30 degrees north (or south) and the equator and centered near 15 degrees north (or south). transform fault Boundary along which lithospheric plates slide laterally past one another. translational slide A landslide that moves approximately parallel to the slope of the ground. transpiration The passing of water vapor through the pores of vegetation to the atmosphere. trench An elongate depression in the ocean floor at a subduction zone between two tectonic plates and most commonly at the edge of an active continental margin. Most are at the margins of the Pacific Ocean. trimline The line along a mountainside along which tall trees in the forest upslope are bounded by distinctly shorter trees downslope. triple junction A junction between three lithospheric plates. tropical cyclone A large rotating low pressure cell that originates over warm tropical ocean water. Depending on location, they are called hurricanes, typhoons, or cyclones.
tropics The warm climate area between the Tropic of Cancer (33 degrees north of the equator) and the Tropic of Capricorn (33 degrees south). tsunami An abnormally long wavelength wave most commonly produced by sudden displacement of water in response to sudden fault movement on the seafloor. Can also form when a landslide, volcanic eruption, or asteroid impact displaces water. tsunami warning When a significant tsunami is identified, officials order evacuation of endangered low-lying coastal areas. tsunami watch The alert is issued when a magnitude 7 or larger earthquake is detected somewhere around the Pacific Ocean or some other ocean that may see dangerous tsunamis. tuff A rock formed by consolidation of volcanic ash. typhoon A large low-pressure weather system that circulates counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. It is equivalent to a hurricane or a cyclone. underdeveloped countries Poor countries with a large proportion of the population living in poverty and small proportions of both middle class and wealthy. Uniform Building Code A national code for construction standards to provide safety for people in high hazard zones for earthquakes, hurricanes, and other natural hazards. uniformitarianism The theory that Earth’s features today have been generated by processes that we see going on today and have been going on for millions of years—the “present is the key to the past.” urbanization Change of an area with addition of buildings and pavement such as in a city. VEI. See Volcanic Explosivity Index. viscosity The resistance to flow of a fluid because of internal friction. vog An acidic volcanic smog produced when volcanic gases react with moisture and oxygen in the air, and sun, to produce aerosols. volatiles Dissolved gases in a volcano. volcanic ash Any small volcanic particles; formally, particles less than 2 millimeters across. Volcanic Explosivity Index (VEI) A scale of volcanic eruption violence based on volume, height, and duration of an eruption. volcanic weather Weather generated during a volcanic eruption. The high temperature above an erupting volcano draws in outside air that rises and cools. Moisture in the air condenses to form rain and stormy weather. volcano A mountain formed by the products of volcanic eruptions. Vulcanian eruption A style of eruption that is more violent than Strombolian and less violent than Peléan. wall cloud The rotating area of cloud that sags below the main thundercloud base and from which a tornado may develop.
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525
warm front The boundary between a large mass of cold air advancing under an adjacent large mass of warm air. water pressure The pressure of water spaces between grains in the ground. watershed That part of a landscape that drains water down to a given point on a stream. water table Nearly the level of the top of the saturated zone, the level to which water would rise in a shallow well. water vapor The invisible gaseous form of water. wave base The greatest depth of wave motion that stirs sediment on the bottom—generally, 10 meters or so. wave energy The capacity of a wave to do work—that is, to erode the shore. Because wave energy is proportional to the square of wave height, high waves do almost all coastal erosion. wave height The vertical distance between a wave crest and an adjacent trough. wavelength The distance from crest to crest of a wave— for example, in an earthquake wave or a water wave. wave refraction The bending of wave crests where one end of the wave drags bottom and slows down in shallower water.
526
G L O S S A RY
weather The conditions—temperature, humidity, air motion, and pressure—of the atmosphere at a particular place and time. weather fronts. See cold front, warm front. weathering The gradual destruction of rock materials through physical disintegration and chemical decomposition by exposure to natural agents such as moisture in the atmosphere. welded ash Ash from a hot ash flow that is hot enough when deposited that the particles fuse together to form a solid rock. If it is hot enough and under sufficient load of overlying ash, it may completely fuse to form obsidian. westerly winds Regional winds that blow from southwest to northeast and are centered 45 degrees north (or south) of the equator. wind The movement of air from an area of higher to lower atmospheric pressure. wind shear The short-distance change in wind direction or velocity. wing dam Segments of walls built to protrude into the current of a river to increase the depth of water in the channel to facilitate shipping. zoning restrictions Laws that prohibit building in certain areas.
Index
A 100-year floodplain, 304 in coastal areas, 407 100-year floods, 304, 346 aa lava, 131, 132 accelerations of earthquakes, 71 achondrites, 475 acid rain and solution of rocks, 227 acidic rain from asteroid impact, 479, 480 acoustic-flow monitors, 165 active beach barrier islands, 367 extent of, 364 offshore extent of, 379 active faults, 41, 84 active fault, monitoring, 63 active faults east of Cascades, 40 “active layer” of permafrost, 238 active subduction zones, 125 active volcanoes, 152, 165, 168–169 Cascades, 124–125 actuarial cost of insurance, 8, 408 actuarial risk rates, 336 adding material to top of slope, 195 adding water to a slope, 195, 212 adiabatic cooling, condensation, 251–252 lapse rate, 251 warming, 253, 254 Adriatic Sea, 242 aerosols, 273 from asteroid impact, 479 Africa carbon dioxide hazard, 160 desert spreads, 264, 265 tidal bulge, 242 aftershock and fault break, 35, 66, 88, 91 air pressure and weather, 252 air rises against mountain and rain, 314 air tankers to fight wildfires, 464
aircraft crashes and downbursts, 434 fly into volcanic ash clouds, 157 use in wildfires, 457 air-fall ash, 132, 156, 163 air-pressure gradients, 387 Alabama hurricanes, 391, 394, 423–424 storm surge damage, 416 Alaska, 140, 204, 209, 238, 239 earthquake, 1964, 37, 51–55 earthquake, coastal bulge drop, 119–120 frozen ground, 275 glacial floods, 314 highway bridge piers, 329 landslides, 204 permafrost, 276 rivers, 296, 297 rockfall tsunami, Lituya Bay, 100 subsidence, 237 tsunami 1964, 119–120 tsunami run-up heights, 104 volcano, 157 albedo of ice and water, 275 Alberta landslides, 219 subglacial floods, 314 wildfires, 466 winds, 254 Aleutians, 140, 153 Aleutian trench, 1964 earthquake, 119–120 alluvial fan, 292, 298 alluvial fans and debris flows, 310, 338, 353 building sites, 467 deposit, 297 alternative energy, 278, 280–282 Amazon rain forest, 258 amplified damage in earthquakes, 86 amplitudes, seismic waves, 49 Anasazi civilization, collapse, 278
Anchorage, Alaska, 1964 slide, 204 earthquake, 53, 209 ancient eruptions, 162–164 Andes Mountains debris flows, 310 glaciers melting, 275 La Niña weather, 258 andesite, 127, 129, 130, 140, 158 formation of, 23 lava, 152 lava flows, St. Helens, 169 magma, 126–127 Mount Lassen, 173 volcano, 171 angle of repose, 189, 190, 213 animal behavior before earthquake, 61 annihilation of civilization by asteroid, 482 Antarctic ice sheet, 272, 275 anthropogenic greenhouse gas, 269, 273 anvil-top storm clouds, 431 Appalachians, Hurricane Agnes, 316 aquifer contamination and subsidence, 244 depletion and sinkholes, 226, 229 porosity and subsidence, 232 subsidence, 231, 244 Arabian Plate, collision with Asia, 92 Arctic air and lake effect snow, 267 melting, 236–239, 249–250 sea ice, 238, 249–250, 275, 278 surface temperature rise, 270 thaw, 275–276, See also permafrost Argentina debris flows, 313 glaciers melting, 276 arid regions, groundwater, 299
Bold page numbers indicate section headings. Italic page entries indicate Case in Point features.
INDEX
527
Arizona atmospheric moisture, 299 carbon dioxide sequestration, 285 debris flows, 321–322 drought, 264 flash flood, 304 ground subsidence, 231–232 impact craters, 476, 483 rockfall hazard, 198 shrinking ground, subsidence, 225 volcanoes, 138 wildfires, 457, 460 Arkansas droughts, 264 tornadoes, 436, 438 Armero, Columbia, 183–184 Arno River, Italy, Florence flood, 1966, 350–351 arson and wildfires, 453, 463, 465 artificial dunes, 380–381 artificial fill and earthquakes, 86, 89 artificial replenishment and subsidence, 232 ash clouds and aircraft, 157 collapse of roof, 168 deposits on Naples, 168 eruption, Mount Lassen, 174 ash fall, 131, 152, 155–157 hazard areas, 152 threat, 156–157 recognition of deposits, , 163 tuff, 163 Vesuvius, 167 volcanic record, 162 ash flows, 142, 153 eruption, 154 recognition of deposits, 163 Ash Wednesday Storm, 1962, 409 Asian Plate, Bhuj earthquake, 93 asteroid, 472–473 belt, 472–473, 481 breakup, 472 explode on impact, 476 incoming, what to do, 482 tracking of, 482 velocities, 475 asteroid impacts, 475 65 million years ago, 471–472 chance of a significant, 481–482 Chicxulub, 471–472 consequences, 479–480 craters, 476–477 crater collapse, 476 crater periodicity, 481
528
INDEX
distribution, 475 energy of asteroids, 475 immediate effects, 479 larger than 1 km, 480 mass extinction, 480 melt, 476–477 site, evidence for, 477, 480, 484 spherules, 477 trigger for other hazards, 479 tsunami associated with, 100– 102 asthenosphere, 17, 18, 19, 20 Atchafalaya River avulsion, 342–343 Athens, Greece, earthquake damage, 84 Atlantic City, and sea-level rise, 277, 380–381 Atlantic hurricanes, 390, 401 Atlantic Multidecadal Oscillation, 260, 402, 496 Atlantic Ocean El Niño, 259 circulation, 260, 277 crust, 44 opening, 44 storms, 259–260, 381 Atlantis, legend, Santorini, 146 atmospheric CO2, doubling, 278 cooling, 268–270 moisture and rainfall, 299, 301 pressure and weather, 252 water vapor and warming, 496 Austin Texas, floods, 306, 334 Australia droughts, 263 El Niño weather, 258 hurricane surge height, 396 avalanche causes, 208 chute, 207 danger, tests for, 205 tracks, 207 triggers, 206–207 avulsion, 330 Bangladesh, 284 New Orleans, 342–343 Yellow River, China 344–345 B backwash from waves, 363 Bahamas hurricanes, 2004, 423 Bam, Iran, earthquake, 2003, 75 Banda Aceh, Sumatra, tsunami, 117, 494
Bangladesh cyclones, 283–284 sea-level rise, 277 tsunami, 101 bankfull level of a stream, 300 stream discharge, 300 width, depth, capacity, 300 barometric pressure in hurricane, 388, 389 barrier islands, 366 breaches, 381 bridge access in hurricanes, 404, 418–419 development of, 367–369 early settlement, 359 hazards, 367, 369 ice ages, 366 inlets, 368, 380 migration of, 367 New York, 381 restrictions on development, 492 sea level, influence on, 367 Barringer, Daniel, and Meteor Crater, 483 basalt, 140, 158 Iceland, 21 lava flows, 136–137, 152 lunar maria, 480 melting temperature, 28 oceanic crust, 19 basalt magma, 126–128, 131, 138–139, 145 basalt and meteorites, 475 from Earth’s mantle, 28, 30 heating rhyolite, 128 mingling with rhyolite, 23 of oceanic crust, 19 base isolation for earthquakes, 81, 83 base level of stream, 292 base surge, 134 Basin and Range, 41 debris flows, 310 spreading, 21, 24 valleys, 297 Bay of Bengal and Bangladesh, 284 bays and sandbars, 366 bays and tsunami run-up, 110 beach access by the public, 379 buildings, moving landward, 379 evolution of, 380 gaining sediment, 365 grass and dunes, 371 migration and sea-level rise, 277
processes, early settlements, 359 protection, 375 replenishment, 371, 376–379, 381 sand and cliff erosion, 371 sand supply, 363–366 slope, 364 steepens shoreward, 365 supplied by rivers and cliff erosion, 372 beach cliffs, California, 373–374 coastal erosion of, 371, 375 retreat, 374 beach erosion during hurricanes, 368, 400 during Nor’easters, 409–410 Long Island, 381 seawall affects, 419 sea-level rise influence on, 277 sediment supply, 329 beach hardening, 374–375 alternatives, 379 extreme, 380–381 beach nourishment, 371; See also beach replenishment cost of, 379 repeated, 381–382 beachfront dunes and waves, 369 dunes, absence and hurricanes, 406 homes, 374 riprap, 374 bedload, 296 bedrock streams, 298–299 Beijing, China, dust storms, 265–266 belief in control of nature, 491 benefit from wildfires, long-term, 457 bentonite beds, 245 Bermuda High and drought, 264 Bhuj earthquake, India, 2001, 75, 84, 93 Big One (earthquake), 86 Big Thompson River flood, 1976, 319–320 big waves, energetic, 360 biofuels, 273, 281 Bitterroot Valley, Montana, wildfires, 465–467 Black Hills flood, South Dakota, 333 blind thrust faults, California, 37, 38, 54 Bhuj earthquake, 93 Blue Ridge Mtns. debris flows, 322 body waves of earthquakes, 44
bolides, 472 impact of, 477, 480 vaporization of, boundary clay, 478 bombs, volcanic, 131, 133 boulders as beach protection, 368 boundary clay, impact, 478 braided streams, 296–298 alluvial fans, 300 placer mining, relation to, 341 sediment load of, 327 Bramaputra River, Bangladesh, 284 breach of barrier bar, 368, 382 breach of levees, 330, 346 intentional, 331 Hurricane Katrina, 413 breakaway scarp of avalanche, 206–207 breakwaters, 375, 377 bridges, barrier islands and hurricanes, 404, 422, 425, 426 collapse during Katrina surge, 415–416 floods and, 304, 306, 329 pilings and erosion, 294 pier erosion, 329 Britain, 156 British Columbia, 301 climate, 253 landslide, 198–200 outburst floods, 314 subduction fault, 39 tsunami, 108–110 brittle failure, 34–35 builders, construction after hurricanes, 406 builders inexperience on coasts, 390 building as a pendulum, 80 building codes, 82–84, 87 coastal sites, 369, 394, 405, 407, 426 earthquakes, 79, 93 enforcement after disasters, 493 flood insurance, 337 hurricanes, 406–407 mitigating hazards, 491 Mexico City, 58 buildings coastal on pilings, 370 collapse in earthquake, 81, 91 construction and hurricanes, 395 hazardous locations for, 335, 492 mud or brick, 93 sway in earthquakes, 80 bulge rising on volcano, 164, 171, 174–175
bulging continental plate margin, 39 buoy system for tsunami, 107–108 burning embers and wildfire, 459, 462 burning fossil fuels, 270, 272, 273 burnout and wildfires, 457–458 “buyer beware,” 491 C Calaveras Fault, earthquake risk, 72–73 Calcutta, India, and sea-level rise, 277 caldera, 135, 141, 145 collapse, 142, 146 eruptions, 168 Mount Mazama, 172 subsidence and volcano flank collapse, 111 volcano, 145, 168 California, 139, 140, 172, 192; See also specific places beach cliffs, 373–374 beach replenishment, Santa Monica, 376 Berkeley, earthquakes, 72 blind thrust faults, 37, 38, 54 caldera, hazard, 160–161 Cape Mendocino, 37, 39, 56, 89 Central Valley floods, 340–341 cliff collapse, 357–358, 372 coastal erosion, 366 debris flow traps, 340 drought, 264, 463 earthquakes, 38, 62, 63–65, 66, 70, 76, 79, 81, 88, 89 El Niño mudflows, 489 El Niño and storm waves, 259, 372 emissions cap, 279, 282 faults, see specific fault names flash flood, 304 Gold Rush and flooding, 328 heat wave, 267 homes on beaches, 368, 370, 373 La Conchita landslide, 188–190, 201 Laguna Beach, landslides, 202 Laguna Canyon, mudflow, 489 landslides, 188, 195, 214, 255 levees fail, 351 Loma Prieta earthquake, 66, 70, 72, 72, 76, 78, 85–86 longshore drift, 361 moist Pacific air, 301 moisture, 299
INDEX
529
California—cont’d major 1997 flood, 351–352 Oceanside, California, 361 “pineapple express,” 255 raising levees, 341 rocky coasts, 371 San Andreas Fault, 27, 28, 30, 33, 37, 39 sediment loss, 365 streambed mining, 328, 340–341 subsidence, Long Beach, 231–234 tsunami, 108 volcano, see individual volcano names weather systems, 258 wildfires, 273, 455–457, 459, 460–461, 463–465 winds, 254 Campi Flegrei, 166, 168 Canada, See also British Columbia droughts, 263 eastern, ice storms, 268 eastern, temperature rise, 270 northern, frozen ground, 275 Shield impact craters, 476–477 tornadoes, 436 canal breaches, Hurricane Katrina, 414 Canary Islands collapse and tsunami, 100–101, 108, 111–112 shield volcano, 138 cancellation of insurance, 408 Cancun, Mexico, 363 carbon dioxide, 160–161 in atmosphere, 270–275, 278, 279 capture at power plant, 285 carbonic acid, 227 considered a pollutant, 279 disposal, 282 in magmas, 127–128, 131 from Mount St. Helens, 175 sequestration, 285 from volcanoes, 138, 146 from wildfires, 452 carbon emissions, 282 carbon taxes and trading, 278, 282 carbonic acid, from carbon dioxide, 227 Caribbean countries and hurricanes, 400, 401, 425 Caribbean Sea and warming, 496 Carrizo Plain, San Andreas Fault, 37, 63 carrying capacity of stream, 293 cars float, 326
530
INDEX
Cascades active volcanoes, 164, 170–172, 174 earthquake, 1700, 37 mudflows, 196 oceanic trench, 40, 55 subduction zone, 20, 21, 26, 39, 40 Volcano Observatory, 150 volcanoes, 124–125 126–127, 135, 140, 168–173 casualty losses, 9, see deaths from, for specific hazards catastrophic earthquakes, 75 events, 2–3, 10 fires, 465 floods, 306 floods in Italy, 350–351 slope failures, 218 tornadoes, 437 tsunami, 118 categories of hurricanes, 388–391 causes of landslides, 194–196 caverns and sinkholes, 226–228, 231 cellulose and wildfire fuel, 452 cement production and carbon dioxide, 272 center of rotation of slump, 201, 202 Central American hurricanes, 400, 421–422 Central Valley, California, 231 Challis, Idaho earthquake, 1983, 34 chance of asteroid impact, 480 being hit by meteorite, 481–482 an event, 4 flood, 306 significant Earth impact, 481 changes in world temperature, 271 channel cross-section, 296 erosion after river gravel mining, 350 erosion and bridges, 328 erosion and stream energy, 328 form and stream discharge, 300 gradient of stream, 294 migration on floodplains, 330 patterns, 295–299 scour, 294 shape, 294 slope, 292 channelization of streams, 308, 326 Mississippi River, 346 chaos net for natural hazards, 6 characteristics of volcanic ash, 163 charge separation in clouds, 433
charges, electrical in clouds, 432–433 charges, opposites attract, 433 Chicago, Illinois and earthquakes, 43 heat wave, 266 Chicxulub asteroid impact site, 471–472, 479 Chile 1960 tsunami sand record, 115 earthquake tsunami, 55, 103, 105 earthquake, 1960, 37, 39, 51, 55 landslide tsunami, 2007, 100 subduction-zone tsunami, 1960, 114–116 tsunami run-up heights, 104 tsunami travel times, 106 China coal-fired pollution, 273 desertification, 265–266 droughts, 263 flood-control dams, 344 floods, 2007, 334, 306 greenhouse gases, 278–279, 280 karst limestones, 227 pollution, 269 Yellow River floods, 344 Chinook winds, 254 chlorine gas, poisonous, 160 chlorofluorocarbons, 270 choice of living sites, 490 cholera and climate change, 274 chondrites, 475, 480 chondrules, 475 cinder cones, 130, 134–138, 139–140 cinders, 134 circular motion of waves, 360 circulation in Atlantic Ocean, 277 civilization after major asteroid impact, 480 clathrate, see methane hydrate clay behavior in earthquakes, 192, 193–194 drying and subsidence, 234–235 subsidence, 240 swelling, 226, 240 clearing around homes for wildfires, 463–464 cliffs, coasts and beaches, bound by, 364, 378 collapse of, 357–358, 363, 364 erosion and wave height, 372 erosion of, 357, 359, 371–373 houses on top of, 372, 374 marine terraces, 371
provide sediment to beaches, 363, 371 use of riprap below, 358 climate controls on stream flow, 299–300 cooling from eruptions, 282–283 cooling, Mount Tambora, 283 cycles, 256–261 effect on hazards, 256 influence on rivers, 291 climate change, 249–250, 268–282 consequences, 273–277 influence of volcanic eruptions, 141 link to hurricanes, 402–403 long-term, 261 clouds, atmosphere cooling, 271 formation of, 250, 251 lightning from, 433 storms, 255 coal, 280 burning and carbon dioxide, 273, 280 gasification, 280 coal-fired power plants, 273, 282 carbon dioxide, 285 emissions, 280 coarse grains in stream, 292 coastal bulge and tsunami, 98, 110, 119– 120 cliffs and landslides, 371 coastal construction control line, 369, 407 coastal homes, Oxnard, 370 effects of tsunami, 103–104 elevation change, subduction earthquake, 120 erosion, 365, 366 erosion and weather, 195 flood insurance, 369 flooding and sea-level rise, 276 land uses, 379–380 populations and hurricanes, 391 processes, 366 sand dunes, 367, 370 vegetation and hurricanes, 405–406 Coastal Barrier Resources Act, 1982, 407 coasts gaining sediment, 365 cohesion and water, 191–192 coincidence of events, 7 cold front, 254–255, 301, 431–432 collapse caldera, 145–146 collapse into magma chamber, 135
collapse of buildings in earthquakes, 91, 92 clay soils, 194 coastal cliffs, 357–358 eruption cloud, 154 ground, 226 roofs with volcanic ash, 156, 167, 168 caldera during eruptions, 144–146 sea cliffs, 373 collection basins for debris flows, 340 collision of air masses, 436 continental plates, 26, 27 continents, 20, 24–27 collision zones and earthquakes, 71 colluvium, 312 after wildfires, 462 Colorado, See also Denver Big Thompson flood, 319–320 carbon dioxide sequestration, 285 debris flows, houses on, 339 droughts, 264 earth flow, 203, 210 flash floods, 319–320 landslides,198, 203 rockfall hazards, 198 volcanic ash, 146 wildfires, 456, 460 winds, 254 colors of volcanic rocks, 158 comets, 473 composition of, 475 Hale-Bopp, 473 orbits of, 481 velocity of, 475, 479 communication failures, Katrina, 414, 417 composite volcanoes, 140 compression, of rock, 35 compressional waves of earthquakes, 44, 45 condensation, 254, 299, 438 conditions for tornadoes, 440 consequences of levees, 331 conservation of energy, 278 constraining area of floodplain, 331 construction standards, see building codes contamination during Katrina, 414 continental crust, 17, 18, 19, 130 collision, 37, 92 extension, Basin and Range, 41 spreading zones, 41–42 continental drift, 13–17
continental glaciers and climate, 261 hotspot volcanoes, 136 lithosphere, 19 margin deformation, 57 plate, bulging, 39 rift volcanoes, 136 rocks, 18 shelf and beaches, 364 shelf during ice ages, 366 shelves, 14 slope sediment loss, 365 transform fault, 37 control of nature, 10, 366, 382, 491 controlling lava flows, 165 convection clouds, 299 convective “chimneys,” 387 thrust zone from eruption, 154 transfer of heat, 453 updraft and wildfires, 455 convergent boundaries, 20, 21–27 cooling climate from sulfur dioxide, 160 coordination between groups in storms, 410 lacking after Katrina, 416 coral reefs hurricanes influence on, 397 protect islands, 397 waves, 360 Coriolis effect, 252–253 hurricanes affected by, 390 Coriolis forces, 387 cornice of snow, 207 Corps of Engineers beach projects, 376–377 beach replenishment, 381–382 coastal areas, 368 dams, 332 floods, 346 Hurricane Katrina, 413, 414, 418 levee breach, 347 seawall construction, 419 costs of beach hardening, long-term, 379 beach replenishment, 379 disasters, paid by, 493 energy, 280 evacuations, 404 fighting wildfires, 462, 464 flood-control dams, 332 floods, 334 hazard mitigation, 493 hurricanes, 392 hurricanes, reduction of, 407
INDEX
531
costs of—cont’d landslides, 212 natural hazards, 4 tornadoes, 437 wildfires, 461–462 costliest hurricanes, 391, 423 natural disaster, Katrina, 386–387, 418 cover collapse sinkholes, 228–230 cover subsidence sinkholes, 228 coves collect eroded sand, 371 cracking foundations, smectite, 240, 244–245 crater 131, 135, 138, 139 from asteroid impact, 471–472 Crater Lake, Oregon, 135, 145, 169 crater rebound after impact, 476 Craters of the Moon, 138, 139 craters, Mount St. Helens, 174–177 creep of faults, 88 creep of slope, 189, 204–205, 218 creep on San Andreas Fault, 36 Crescent City, CA, effect of 1964 tsunami, 120 Cretaceous-Tertiary (K-T) boundary, 472 clay along, 484 crevasse failure of levee, 330 cross-sections of a stream, 300 crown fires, 452 cultural factors and disasters, 494 cumulonimbus clouds and lightning, 431–432 cut and fill affects landslides, 194 cut and fill for home site, 492 cutoff low, 255 cycles, long term, 261 in natural events, 4 overlapping, 4 cyclic carbon dioxide variation, 272 cyclones, 253; See also hurricanes in Bangladesh, 283–284 cylindrical surface of rotational slump, 201 cypress forests and hurricanes, 405–406 D dacite, 127, 132, 140, 141 ash, 174–175 dome, St. Helens, 169 Mount Lassen, 173 damage cost reduction in hurricanes, 407
532
INDEX
costs of wildfires, 460 during earthquake, 62 influenced by owner’s behavior, 492 damage from 1997 California flood, 352 2004 hurricanes, 423 debris flows, reducing, 337–340 downbursts, 434 hail, 434 hurricanes, 388, 389–390, 391–403, 404, 419 Hurricanes Dennis and Floyd, 420 Hurricane Hugo, 1989, 425–426 Hurricane Katrina, 412–418 hurricanes, poor countries, 422 Kansas tornado, 431 subsidence, 244 tornadoes, 437, 441–443, 444–445 wave-carried debris, 395 wind in hurricanes, 397 wildfires, 463–465 dams affect floods, 308 beaches, influence on, 357, 366 failure of, 332–333, 348–349 reduce sediment to beaches, 357, 366 stream equilibrium, influence of, 332–333 danger from lightning strikes, 435 danger in tornadoes, 446 dangerous coasts, living on, 359–362 dangerous flooding areas, 298 dangerous places to live, 1 Dauphin Island, Alabama, and Katrina, 394, 395 Davenport, Iowa, 1993 flood, 337 day to season cycles, 256–257 daylighted beds, 193 deadliest hurricane, western hemisphere, 421 deaths from 2004 hurricanes, 423 climate-related hazards, 496 dam failure, 349 earthquakes, 75 flash floods, 304, 434 floods, 298, 334 Galveston Hurricane, 419 heat waves, 266 hurricanes, 387, 391, 392, 393, 400–401, 410, 420, 425 hurricanes, poor countries, 401, 421 Hurricane Agnes, 317 Hurricane Hugo, 1989, 425 Hurricane Katrina, 415
landslides, 212 lightning, 432 sea-level rise, 277 tornadoes, 430, 436–437, 444 tsunami, 111, 118 wildfires, 452, 460, 463, 464, 462, 463, 465 debris avalanche, 131, 158–159, 183–184, 198–201, 219 from Canary Islands, 111 earthquake trigger, 209 from island flank collapse, 110 movement rates, 197 Mount St. Helens, 175–178 from volcano, 183 debris flows, 158, 189, 213, 309, 310–312 after wildfires, 455, 467 alluvial fans, relation to, 310 Arizona, 321–322 collection basins, 340 consequences, 310 damage reduction, 337–340 disaster,Venezuela, 353 early warnings, 338–339 flood insurance, 338 former evidence, 312, 313 initiation, 209, 312 minimizing impact of, 338 movement, 311 movement rates, 197 natural levees, 311, 313 pore water, 312 slopes, 312 source material, 311 surges, 311 trapping, 339–340 velocities estimated, 322 debris slides, 203 deeper water under bridges, 329 deforestation, relation to carbon dioxide, 273 eroding coasts, 359 floods in Bangladesh, 284 floods, 308, 327 runoff, 302 Delaware and Nor’easters, 409 deliberate breach of river levee, 346–347 delta, 292 delta sediments and water extraction, 242 Denali earthquake, Alaska, 2003, 209 Denali Fault, Alaska, and tsunami, 112 density of materials, 17 density of meteorites, 474
Denver, Colorado alluvial fans and houses, 338–339 earthquakes, 64, 282 expanding clay, 244–245 rockfall hazards, 198 stream mining, 328 depositional fan, 297 depth of water for sand movement, 364 Des Moines, Iowa, 1993 flood, 325 desertification, 265 deserts of Peru, 258 destruction after floods, 302 after hurricanes, 422 detection of debris flows, 339 detection of near-earth asteroids, 482 developed countries and earthquakes, 84 developed countries and hazards, 3 developers, building standards and, 407 coastal building sites, 426 after Katrina, 416 rebuilding after hurricanes, 406 responsibility of, 491 developing countries and disasters, 3 development on barrier islands, 367–369 on floodplains, 326–329 on floodplains, 335 in high-risk areas, 335 of tornadoes, 435–441 versus aesthetics, 333 dew point, 250 diagonal bracing, 76, 78, 79 diameter of hurricanes and damage, 390 diesel cars and light trucks, 279 differentiated magma, 172 dikes affect floods, 308 dinosaurs asteroid impact, 471–472, 481 demise, 478, 480 direction of tornado movement, 438 disaster aftermath, 492–493 disasters, controlling, 10 potential, 1 prediction of, 4 preparedness, 165, 387 reaction to, 491 relief after a flood, 335 “disaster-resistant” communities, 405 discharge discharge of a stream, 291, 300, 302
estimate for flood, 307 peak annual, 305 disease after hurricanes, 422 Hurricane Katrina, 414 major eruption, 144 disease-bearing insect migration, 278 displacement of fault, 35, 36 disposal of carbon dioxide, 282 dissolution, see dissolving rocks dissolution sinkholes, 227–228 dissolving rocks, 226 distance from lightning, 435 distribution of asteroid impacts, 475 thunderstorms, 431 divergent boundaries, 20–21 Dolan-Davis Nor’easter scale, 409 dome collapse and volcano types, 136 eruption, 175 Mt. St. Helens, 170, 176 volcano, 141 Douglas fir trees and wildfires, 466 downbursts, 434 downpour and soil saturation, 302 drag on stream bottom, 294 drain fields and landslides, 195 drainage basin, 291, 302 drainage of organic soils, 232–234 dredging inlets, 368, 378 dredging navigation channels, 379 drilling platforms and hurricanes, 401–402 driving force of landslide, 189 driving mass of landslide, 189, 201 driving through floodwaters, 405 drought, 260, 261–264, 269, 274 annual losses, 262 Atlantic oscillations, relation to, 264 climate change, influence on, 274 consequences of, 262 during Little Ice Age, 278 evaporation and, 262 extremes, 264 Great Plains, 263 maps, 458 Midwest, 260 southern California, 463 wildfires, 466 drowning of city with levees, 411 dry climates, 295, 296, 297 streams in, 299 dry weather and wildfires, 466 drying of clays, 234–236
dunes, 364 along beaches, 369–371 beach sand, 364 coastal development, 369 coastal erosion, 369 damage to, 394 gone from beaches, 381 height along coasts, 367 modification of coastal, 369 nourishment of, 371 paths across, 394 sand fences on, 370–371 storm surge relation to, 394 vegetation affects, 370–371 “Dust Bowl,” 263–265 dust from asteroid impact, 479, 480, 481 from volcanic eruptions, 145 dust storms, 265–266, 269 E early development on floodplains, 326 early warning of debris flows, 338–339 of earthquakes, 62, 63, 64 earth fissures, 231 Earth process rates, 2 Earth structure, 17–20 Earth’s axis precession, 260 axis tilt, 256–257, 260 core, 14, 17 crust of eastern North America, 44 layers, structure, 14, 16, 17–19 mantle, 17, 18 mantle and meteorites, 475 mantle and water, 23 orbit and seasons, 257 outer layers, 19 plates, tectonic, 15 surface temperature change, 270 tectonic plates, 20 Earth-crossing orbits of asteroids, 480 earthquakes, 32–96 acceleration, 47, 52 81 adjacent building heights, 58 amplified shaking, 52 artificial fill, 52 broke water mains, 86, 88 building construction and codes, 54, 58, 87, 93 causes of, 33–36 clusters of, 37, 74 damage, 44, 47, 52–54, 89 damage, minimize, 74, 81, 82
INDEX
533
earthquakes—cont’d deaths, minimize, 82 depth of focus versus damage, 52 detection of magma, 146 difference in energy released, 49 displaces huge water volume, 98 distance to, 47 early warning of, 62, 63, 64 effect on people and buildings, 47 energy, 47 evacuation, 52 first-floor garages, 54 freeway collapse, 54, 56–57 frequency, 45, 50, 52, 57, 68–70 from asteroid impact, 479 generated tsunami from, 98–99, 108 giant, 50 ground motion, 50–53 ground velocity, maximum, 47 Hawaii, 138, 164 hazard map, 71 insurance, 81 intensity maps, 47–48 intervals, 64, 88 Kashmir, 2005, 4, 32–33, 91–92 landslides, 196 largest, 37 liquefaction, 52–53 locating, 46–47 loss of life, 84 marine terraces raised, 371 methane hydrate, 281 migration of, 67 nature of ground, 88 Nisqually, Washington, 2001, 41, 52, 56–57 nomograph, 49 offset on fault, 52 parking garages, 54 plate collision, 27 pore water pressure, 52 preceding volcanic eruption, 179, 181 precursors, 63–64, 86 prediction, 61, 62–64 preparedness, 81–82 probability, 65–70 retrofit, Menlo Park, 78 relief, Kashmir, 33 risk, 70 rupture length, 52 Santorini volcano, 146 secondary ground effects, 52 shaking and ground type, 52
534
INDEX
sinking lithosphere, 24 size of, 47–50 start fires, 81, 88 strength, versus distance, 52 subduction zones, giant, 39 subsidence, 234 swarms and volcanoes, 174 time of shaking, 50–52 trigger lahars, 310 trigger landslides, 52, 197, 199, 204, 207, 208–209 volcanoes and, 167 water in ground, 52–53 weak first stories, 58 weaken some dams, 333 earthquake magnitude, 47, 48–50 body-wave (MB), 49 depth of focus, 52 estimate ancient events, 50 local (ML), 49 moment (MW), 49 surface (MS), 49 versus fault offset, 50 versus frequency, 51 versus rupture length, 50 earthquake wave(s), 44–47 amplitude, 45, 48 damaging, 44 frequency, 57, 80, 81 magma and shear waves, 146 period, 45 resonance, 57, 80 types of, 44–46 velocity, 47, 52, 80 vibrations, 45 wavelength, 45 East African Rift zone, 20, 21, 25 East Coast fault system, 43, 44 east coast of North America, tsunami risk, 112 East Pacific Rise, 17 East Rift Zone, 137 Easter Sunday tornadoes, 1913, 437 economic impacts of hurricanes, 401–402 losses from hazards, 3 education about hazards, 9, 493 Egypt meteorite kills dog, 481 ejecta blanket of impact, 476 ejected material from impact, 476 El Niño, 257–259 cliff erosion during, 357–358, 372 floods, 301, 308 landslides, 195, 301 mudflows, California, 489
recurrence interval of, 259 trade winds during, 301 weather, North America, 258 wildfires, 466 elastic bending along fault, 63 deformation, 35 energy accumulation before earthquake, 92 limit, 35 rebound theory, 34, 35 rocks, 34 electric power and hurricanes, 402 Hurricane Katrina, 414 electrical separation between particles, 433 Electron mudflow from Mt. Rainier, 159 elevation rise before earthquake, 61 Elm debris avalanche, Switzerland, 198, 201, 220 embers from fires, 452 emergency response to Katrina, 415 emissions caps, 282 emissions from volcanoes, 142 encroaching on floodway, 337 energy, comets, 475 cost and usage, 280 hurricane, 403 release in earthquakes, 50 stream and mining, 328 waves, 395 energy line from eruption, 154–155 “energy intensity” and emissions, 280 engineering for beach protection, 374–376 landsliding, 214–216 Enhanced Fujita scale(EF), 445 environmental protection 337 epicenter of earthquake, 44, 46, 86 distance to, 49 equilibrium among landslide forces, 189 between natural influences, 2 profile of a beach, 364, 379 erosion accelerated by people, 373 beach slope, 365 following wildfires, 455–456 increase after wildfires, 465 mitigation following a wildfire, 456 in poor countries, 422 erosion of beach cliffs, 375
gently sloping coasts, 366–369 headlands, 362 land, beach enlargement, 359 levees on rivers, 330 riverbanks, 292 sand from beaches, 364–366 sea cliffs, 371–373 stream channel, 294 erupting volcanoes and eruption(s) atmosphere affects of, 271 behavior, 130 cloud, 131 column, 131, 181 driving forces, 127 frequency, 134 at hotspots, 130 intervals, 141 prediction, 144 at rifts, 135 at spreading zones, 129–130 styles by volcano type, 134s, 136 at subduction zones, 130 surge, 138 at vents, 135 warnings, 164–165 escape routes in hurricanes, 405 estuary and sand dredging, 378 ethanol in gasoline, 273, 280–281 Eurasian Plate and Atlantic Ocean, 21 European beach grass, 371 European settlement on coasts, 359 evacuation failure to leave, 283, 467 Galveston, TX, 1900, 419 hurricanes, 283, 391, 400, 404–405, 419–420 mandatory, 415, 467 medical problems, 495 separation of family, 495 time, 111, 492 volcanoes, 165, 167–168, 181 wildfires, 463, 465 evaporation from oceans, 250, 299 evaporites, dissolving, 226 evapotranspiration and, 225, 251 droughts, 262 landslides, 214 swelling soils, 240 vegetation, 328 wildfires, 455 evidence of asteroid impacts, 475–479 floods, 307 evolution of barrier islands, 368
exceedance probability, 308 expanding dome, 134 expanding soils and clays, 197, 240–241, 244–245 experience with hurricanes, 419 explosive eruptions, 127–129, 131– 133 styles, 134–135 extratropical cyclones, see Nor’easters, 408–410 extreme events, 5 fire weather potential, 458 rainfalls, 256 weather events, 273–274 winds in tornadoes, 443 eye of hurricane, 387–388, 405 F failure of dams, 332–333 built by landslide material, 209– 212 Idaho, 1976, 349 failure surface of landslide, 202, 203 Fairweather Fault, Alaska, and tsunami, 112 fallout of meteoric dust, 478–479 false predictions, 65 earthquake, 64 family separations during evacuation, 495 famine after volcanic eruptions, 146, 283 famine with climate cooling, 283 far-field tsunami, warning systems, 106 Fargo, North Dakota, flood, 317–318 faults, 33–36 active, 33, 34 differential movement across, 38 displacement and fault magnitude, 65, 99 left-lateral, 33 movements and offsets, 65 normal, 33, 34 reverse, 33, 34 right-lateral, 33 scarps and earthquakes, 34, 65 strike-slip, 33, 34 stuck segment, 34 thrust, 33, 34 federal cost of shore protection, 394 feldspars, weathering to clay, 193 FEMA, 9, 304 appropriated hurricane funds, 418 flood insurance on coasts, 407
Hurricane Katrina, 411, 415, 416–417 hurricanes Dennis & Floyd, 420 tornado recommendation, 446 wildfires, 462 fetch and wave height, 359, 409 fighting nature, alternatives to, 496 fighting wildfires, 457–458 fire(s), See also wildfire advance direction, regional, 458 after asteroid impact, 480 exposure, potential, 458 floods after, 327–328 ignition and wildfires, 453, 463 insurance rates, 462 move upslope, 454 movement direction, clues to, 454 processes and behavior, 452–455 require fuel, oxygen, heat, 452 retardants and wildfires, 464 seal the ground, floods, 327 suppression leads to more fires, 465–467 triangle, 452 triggered by earthquakes, 74, 75, 85 weather potential, 458–459 fireball from asteroid impact, 479, 485 firebrands carry fire downwind, 453, 455, 459 firefighter deaths, 451–452, 462 firefighting costs, average annual, 456, 457 fire-prone areas, 462 fireproof materials and wildfires, 463 firestorms, 455, 463 fire-vulnerable forests, 466 first-order stream, 302 fissures in shrinking ground, 225–226, 231 flame-resistant materials, 459 flames rise, cause of, 454 flammable materials and wildfires, 452, 460 flammable structures, wildfire risk, 459 flank collapse of volcano(es), 196 Mt. St. Helens, 151 oceanic volcano, tsunami, 101, 108 flank eruptions from rift, 138 flash flood(s), 303–304, 319–320 hazard, 304 increasing numbers of, 326 magnitude map, 304 Nor’easters and, 410
INDEX
535
flash flood—cont’d rainfall, 299 thunderstorms, 434 urbanization, 326 Venezuela disaster, 353 wildfires, relation to, 455–456, 467 floated homes, Hurricane Katrina, 415 floods after fires, 327–328 flood(s), See also specific places Bangladesh, 283–284 behavior, 300 bridges, 329 characteristics, 309 climate change, influence of, 274 crests and storm, 302–303, 346 damage, reduction of, 334–337 damages from hurricane, 317 dams, 332 deposits, cross-section, 294 duration, 303 El Niño, relation to, 301 erosion during, 294 evidence of, 307 float cars, 327 frequency, 304–309 fringe, 336 from landslide failure, 209 frozen ground, 301 hazard maps, 336 hazards, 298, 303 high energy, 298 homeowners insurance, 408 hydrograph, 303 intensity, 302–304 large, 299, 301 larger with time, 491 logging, 327–328 minor hurricane, 316–317 rainfall and, 300 saturated ground, 301 sediment transport, 309 storm surges, 387, 401 turbulence, 294 warning, 284, 302 flood basalts, 160, 480 flood insurance, 335–337, 495 along coasts, 369, 421 construction and, 337 development incentives and, 407 hurricanes, 402, 407 rate maps (FIRMs), 336, 407 flood level, 303 determination, 307 levees, 331, 341 near bridges, 329
536
INDEX
flood-control dams, 332, 344–345, 352 flooded roads, driving through, 326 flooded towns, repeated, 337 flooding, 291 behind levees, 330 coastal areas and sea-level rise, 277 coastal building codes, 394 global warming, 495 Hurricane Katrina, 412 intensity and hydrograph, 302 processes, 300–301 tropical storm, 400 floodplain, 290, 297, 300 area and flood height, 346 better uses, 490 development, 326–329, 351–352 homes moved off, 325 homes and hurricane warning, 405 land use, 334–335 living on, 490 maps, 305 Mississippi R. 1993 flood, 347 mudflows, 160 part of stream path, 334 poor countries, 401, 422 floodwalls, New Orleans, 413 floodwater, 302 contamination, Katrina, 414 floodway, 336 Florence, Italy, flood, 1966, 350–351 Florida beach replenishment, 377–378 coastal building, 369 “coastal construction control line,” 407 estuary dredging, 378 Everglades subsidence, 234 groin removal, 376 Hurricane Charley, 424 hurricane deaths, 401 hurricane evacuations, 405 Hurricane Ivan, 424 hurricanes, 259, 39, 423–425 insurance companies leave, 407 insurance rates increase, 408 Keys and hurricanes, 403 lightning deaths, 432 population grows along coast, 359 rebuilding after hurricanes, 406 sinkholes, 226, 229, 230, 231 subsidence, 232 tornadoes, 442, 438 weather oscillations, 260
flow failure, 211 fluid injection, related to, earthquakes, 64 rock deformation, 285 subsidence, 232 fluid movement of debris avalanche, 219 fluidization, 201 fluorine gas, poisonous, 160–161 flying debris in hurricane, 399 in tornadoes, 443, 446 focus of earthquake, 44 fog formation, 250 forecast events, 4 earthquakes, 64, 65 hurricanes, 401, 422 volcanic behavior, 164 where faults will move, 65–68 foreshocks, 88 before earthquake, 35, 63 Forest City, South Dakota, landslide, 216 forest fires, see wildfires Forest Service, and wildfires, 457 forgetting hazards, 491 Fort Tejon, San Andreas, earthquake, 38, 74, 88 fossil-fuel burning, 270, 272, 273 fractals, 4, 6 frame houses shaken off foundation, 79 France heat wave, 267 nuclear power, 280 Frank Slide, Alberta, 219 frequency of, 4 California wildfires, 465 large asteroid, tsunami, 101 major floods, 306 natural catastrophes, 5 oscillation, seismographs and buildings, 49 friction, 193 landslide slip surface, 189, 201 meteorite with atmosphere, 474 resisting sliding, 190–191 stream bottom, 294 frontal systems, 299 frostbite, 267 frozen ground, 275 thawing, 236–239 F-scale damages, 444 fuel for wildfires, 452–453, 465, 466 moisture, 454 wildfire intensity, 459
Fuji, 140 Fujita, Theodore, and tornadoes, 434, 441 Fujita scale and tornado numbers, 443–445, 445 fumarole gases on volcano, 131, 165 funnel cloud, 441 future flood magnitudes, 337 giant tsunami, 108–112 New Orleans, 417 G gaining streams, 299, 300 Galveston, Texas, Hurricane, 1900, 374, 393, 403, 417, 418–419 hurricane deaths, 401 seawall, 374 storm waves, 374 Ganges delta, India, and sea-level rise, 277 Ganges River, floods, 283–284 gaps opened in coastal dunes, 371 garage doors in hurricane, 399 gas(es) dissolved in magma, 125, 127–129, 160 emissions from volcano, 138, 165 Mount St. Helens, 175 separation with pressure decrease, 125 thrust zone from eruption, 154– 155 gently sloping coasts, 364 erosion of, 367–369 Georgia hurricanes, 391 geothermal gradient and permafrost, 236 power, 280 power and earthquakes, 64 giant caldera volcano, 164 continental calderas, 141–142 debris avalanche deposits, 101 earthquakes, tsunami record, 109 landslides and volcano segments, 101 rock avalanche, 173 tsunami of January, 1700, 109 tsunami waves, Hawaii, 101 tsunami, sediment record, 109 Glacial Lake Agassiz, 1997 flood, 318 Bonneville, 53 Missoula, 314
glacial-outburst flood, 160, 312–313 British Columbia, 314 hazards, 314 Glacier Bay, Alaska, tsunami hazard, 114 Glacier National Park, Montana, 207 flood, 298 glaciers melting, 275 wildfires, 458, 465 glass droplets from impact, 478 Global air circulation and winds, 253–254 global ocean circulation, 277 global warming, 316 disasters, 495 effects, 278 glaciers melting, 275 greenhouse effect, 270–282 sea level rise, 276–277 water vapor, 496 Glossopteris, 13 glowing avalanche, 153 Gobi Desert, Mongolia, 266, 344 government, beach replenishment, 379 coastal building, 426 coordination and hurricanes, 410 mitigation of hazards, 493 policy and wildfires, 457 role of, 8–9 GPS measurements along faults, 88 on volcanoes, 164 graded stream, 291 gradient change downstream, 292, 298 grading of layers, 309 Grafton, Illinois, 1993 flood, 325 grain size in debris flows, 312 influence on beach slope, 361, 365 river velocity, 292 stream capacity, 294 waves, 364–366 Grand Canyon, Arizona, 298, 307 Grand Forks, North Dakota flood, 318, 319 Grand Teton Range, Wyoming, 34 Granite, continental crust, 19 magma formation, 126 soils and floods, 301 gravel mining and flooding, 328 pits and stream erosion, 328 on winter beaches, 364
Great Britain, hazard from Canary Is. collapse, 111 Great depression and drought, 263, 264 Great Flood of 1993, 325–326 Great Lakes and climate change, 275 Great Lakes, lake-effect snow, 267–268 Great Plains and drought, 263, 264, 265 Greece, wildfires, 453 greenhouse effect, 270–271, 273 greenhouse gas changes, 269 country emissions, 280 disasters, 495 emission from hydrate, 281 emission reduction, 278 melting, 278 rising levels, 270, 271–273 stabilization, 277 Greenland ice, 272 Greensburg, Kansas, tornado, 2007, 430–431, 445 “greenslide,” 207 groins, 375–376, 382 along beaches, 380 effectiveness of, 376 removal, 376 ground acceleration in earthquakes, 50–52 ground fires, 452, 460 ground fuels and wildfires, 459, 460 ground material amplified shaking, 57 ground motion in earthquakes, 50–53 amplified, 85 ground rules different for the poor, 493 ground rules for the poor, different, 493–495 ground speeds of tornadoes, 437 ground subsidence, 231–238 ground-to-cloud lightning, 433 groundwater and debris flows, 322 flow directions, 299 landslides, 195 level before earthquake, 61, 63 levels and drought, 264 magmas, 134, 138 nature of, 251 recharge, 233 sequestration, 285 shrinking ground, 225 sinkholes, 229
INDEX
537
groundwater and—cont’d streams, 291, 299, 301 subsidence from extraction, 231–232 volcanoes, 158 withdrawal and shrinkage, 225 grout curtain sealing dam, 349 growing deserts, 264–266 Guadalupe River flood, Texas, 315– 316 Guatemala City subsidence, 244 Guatemala, control of fertile land, 494 Guatemala, Hurricane Mitch, 421–422 guidelines for federal flood insurance, 336 Gulf Coast barrier islands, 366 beach erosion, 365 coastal buildings, 370 hurricanes, 391 moisture, 301 salt dome sequestration, 285 swelling soils, 238 Gulf of Mexico Hurricane Katrina, 386–387, 412 hurricanes, 390, 425 moisture and wildfires, 458 moisture from, 264, 333, 346 petroleum and hurricanes, 401 thunderstorms, 432 tornadoes, 430, 436 warming, 496 weather, 316 Gulf Stream, 277 Europe temperature, 277 Temperature relative to hurricanes, 390 Gulfport, Mississippi, Katrina damage, 417 gully formation after wildfires, 455 “gumbo” soils, 240 gunite, 214 Gutenberg, Beno, earthquakes, 49, 50 Gutenberg-Richter frequency-magnitude, 50, 51, 72 gypsum, dissolving, 226, 231 H Haicheng, China, 61–62, 64 hail, 434–435 distribution, 435 formation, 434 largest, 434 seasons of, 434 suppression, 435 thunderstorms, 432
538
INDEX
Hale-Bopp comet, 1997, 481 Halloween Nor’easter, 1991, 409–410 “hard” solutions to hazards, 491 harmonic tremors, magma movement, 138, 142, 147, 164, 174 Hawaii, 130, 132, 135, 137, 139, 152 earthquakes, 164 effect of 1960 Chile tsunami, 114–116 eruptions, 134 flank collapse and tsunami, 108, 110–111 hazard from Kilauea flank collapse, 111 major slumps, debris avalanches, 101 over hotspot, 28, 29 volcanic gases, 162 volcanoes, 164 flank collapse tsunami, 100, 101, 110 Hawaiian hotspot track, 28 rift zone, 128 Hawaiian-Emperor chain, 28, 29 Hayward Fault earthquakes, 36–37, 70–73 hazard assessment, 491 dams, 332 definition, 3 forecasts, volcanoes, 170 loss due to, 8 mitigation, 491 hazardous building sites, restrictions, 422, 492 hurricanes, 410 landslides, 208–212 locations for wildfires, 462 weather, climate, 261–270 HAZUS, 9 headscarp landslide, 201, 202 volcano flank collapse, 110 headlands, rocky, 362 health problems from volcanic ash, 156 heat capacity of water, 301 heat-island effect, 266 heat transfer by convection, 453 heat waves, 266–267 heavy rain, flooding, 400 Hurricane Mitch, 421–422 height of eruption column, 134 former flood, 308
helicopters and wildfires, 457 Herculaneum and Vesuvius, 181–182 Hertz (Hz), vibration frequency, 45 High Cascades, 23 high energy stream flow, 298 high-gradient bedrock channels, 298 high-pressure system, 254, 257 weather under, 252–253 high-rise buildings in hurricanes, 404 high tide and surges, 395 high water and channel spillover, 296 high water marks, 306 higher floods and urbanization, 326 highest dam that ever failed, 349 Hilo Bay, Hawaii and tsunami, 55, 103, 105, 110, 115–116 Himalayas, 299 collision zone, 92 formation of, 24, 25, 26 Kashmir earthquake, 92 orographic effect, 284 plate tectonics, 12 hip roofs in hurricanes, 399 Hokkaido, Japan, earthquake, 64 holding back the sea, 359 hole in glacier (moulin), 278 Hollister, San Andreas Fault creep, 36 home(s) on alluvial fans, 321–322 on barrier bars or islands, 370, 369 on beaches, 368, 373, 394 building quality and disasters, 494 on cliff tops, 372 construction and hurricanes, 395 floated, Hurricane Katrina, 413, 415 hazardous locations for, 492 loans and flood insurance, 337 locations can be hazardous, 490 moved off floodplain, 325 on old debris flows, 321–322 protection from wildfires, 459–461 wildfires near, 463–465 homeowner(s) behavior and damages, 492 insurance coverage, 407–408 Honduras and hurricanes, 401, 402, 421–422 hook echo, 440 Hope Landslide, British Columbia, 198–200 hot ash melting snow or ice, 158 hotspots, 129 plate movement, 27, 28, 29 track on continent, 28 volcanoes, 27–29, 130, 136
“house of cards” clay structure, 193–194 houses, see homes housing temporary after Katrina, 417 Houston area, Texas, housing after Katrina, 417 river, 296 subsidence, 232–233 tropical storm, 400 Huang Ho: see Yellow River human actions along coasts, 359 activities affect wave action, 359 impact of disasters, 3 influence on greenhouse gases, 272, 273 intervention on coasts, 374–380 humans are the problem, 489 humid air, 299 humid region floods, 299, 300 humid, tropical air and rain, 314 humidity and storms, 438 humidity and tornadoes, 438 humidity and wildfires, 454, 462, 465 hummocky landslide surfaces, 212–213 Hurricane Agnes, 1972, 299, 301, 316–317, 390, 391, 400 Alicia, 1983, Gulf Coast, 398, 419 Andrew, 1992, 389–391, 399, 403, 411, 493 Bertha, 1996, 368, 389, 398 Betsy, 1965, New Orleans, 391, 411, 418 Bonnie, 1998, 368 Camille, 1969, 299, 322, 389, 391, 396, 408, 411 Charley, Florida, 2004, 391, 399, 423–424 Dennis, 1999, 400, 420–421 Floyd, 1999, 371, 404, 420 Fran, 1996, 368, 389, 395, 398 Frances, Florida, 2004, 391, 423, 425 Georges, 1998, 394 Gilbert, 1998, 389–390 Hugo, 1989, 388, 389, 391, 404, 406, 425–426, 492 Isabel, 2003, 389–393, 369, 395 Ivan, Florida, 2004, 391, 395, 407, 423–424 Jeanne, 2004, 391, 425 Mitch, 1998, 389, 401, 421–422 Rita, 2005, 391, 407 Wilma, 2005, 363, 391, 402–403, 406
Hurricane Katrina, 2005, 386–387, 391, 394 Alabama, 395 barrier islands consequences, 491 deaths, 317, 401 hurricane cost, 402, 418 insurance, 407, 418 Mississippi, 408, 411, 412, 415–417 New Orleans, 396, 406, 411–418, 495 path, 411 storm surge, 392 hurricane(s), 256, 299, 387–391 back-to-back, 420–421 barrier islands, 367, 369 building codes, 406–407 category 5, 387 damage amplification, 402 damages, 317, 388, 397, 405–408 diameter, affects of, 393 during El Niño, 259 energy and ocean heat, 403 evacuation, 404–405 flood insurance, 407 flooding, 391, 400, 420–421 forecasts, poor countries, 422 formation, 387–388 forward speed, 393 heavy rain, 314 homeowners insurance, 407–408, 418, 493 impact scale, 397 movement direction, 387 natural protections, 405–406 numbers of, 390 path and prediction, 403 planning, 403–404 poor countries, hazards for, 422 prediction, 403–405 preparations, 425 rebuilding after, 405 right-front quadrant, 436 rotation direction, 388 sea-surface temperature, influence of, 402 season, 365, 391 slow moving, 400 storm surge, 365 straps, 441 strength, 388, 390, 402 tracking direction, 253, 388 viewed as abnormal, 359 warm seawater, 387 warning, 403, 404, 405 watch, 403, 404 waves, 394–397
wind damage, 399 wind velocities, 388 winds, 397–400 “hurricane hunter” aircraft, 403 hydraulic placer mining and floods, 340–341 hydrocarbons, 273 hydrocarbon residue after wildfire, 455 hydroelectric power, 280 hydrogen sulfide gas, poisonous, 160–161 hydrograph, 302 Arizona debris flows, 321 Mississippi River, 1993, 345 hydrologic cycle, 250–251 hydrophobic soils and wildfires, 455, 465 hydropower, 273 hyperconcentrated flow, 309 hypocenter of focus of earthquake, 44 hypothermia, 267 hypothesis versus theory, 17 I ice age(s), 260–261 coastal processes during, 366 floods, 308 glacial outbursts, 314 ice-jam floods, 300, 301, 317–319 Red River, Manitoba, 318 Red River, North Dakota, 318 ice storms, 268–269 icebergs from melting glaciers, 278 Iceland basalt eruption, 156, 160, 165 glacial outburst floods, 314 on a spreading ridge, 21 Idaho Basin and Range, 41 dam failure, 349 earthquake, 34 flood basalt, 129 landslides, 213 Snake River Plain, 28, 138, stream, 293 talus slopes, 197 volcanic ash, 146 wildfires, 457, 466 identification of meteorites, 474–475 ignimbrite, see pyroclastic flow ignition and spread of wildfires, 453–454, 463 Illinois 1993 Mississippi flood, 346 and earthquakes, 43
INDEX
539
Illinois—cont’d flood on Illinois River, 1993, 325 heat wave, Chicago, 266 intentional levee breach, 347–348 impact of asteroids, see asteroid impact inclusions of basalt in rhyolite, 128 incoming asteroid, what to do, 482 indefensible locations and wildfires, 462 India, Bhuj earthuake, 2001, 93 coal-fired pollution, 273 droughts, 263 earthquake, Bhuj, 2001, 84 extreme rainfall, 256 greenhouse gas emissions, 278–279, 280 plate tectonics, 12 rainfall maximums, 299 Sumatra tsunami, 117–118, 494 Indian Ocean soot, 269 Indian Plate, Bhuj earthquake from collision, 93 collision with Eurasia, 26, 27 Kashmir earthquake, 92 Indonesia low-pressure cell, 257 mudflows, 160 “normal” weather, 258 tsunami, 1992, 103, 117; See also Sumatra volcanoes, 146 Industrial Revolution and temperature, 270 inexperience of builders on coasts, 390 inflation of volcano summit, 138 injection of water underground, 64 inland progress of surge, 394 inlets across barrier islands, 380 open and close, 368 and shifting sand, 368 and tidal currents, 369 insect-killed trees and wildfires, 465 insects migrating northward, 278 insurance along coasts, 369 cancelled in some areas, 408 claims, 402, 432 companies leave Florida, 407 costs, 8, 418 flood, 335–337 hazards, 493 landslide, 212, 338, 490
540
INDEX
may exclude wind damages, 408 natural hazard, 7–8 premiums for wildfires, 462 rates, 337, 408, 493 wave damage, 407 wildfire, 462–464 intense rainstorm, 255, 302, 322 intensity of wildfires, 452, 459 effect on ground, 455 intentional breach of river levee, 347–348 interactions among natural hazards, 6–7 Intergovernmental Panel on Climate Change, 270–274, 277, 282 internal surfaces in sliding slope, 193 intervals between asteroid impacts, 481 earthquakes, 64 eruptions, 145 intraplate earthquakes, 42–43 investigation of wildfires, 453 Iowa asteroid impact, 472 Mississippi River flood, 345 IPCC, see Intergovernmental Panel on Climate Change Iraq, water and climate change, 266, 275 iridium anomaly after impact, 477, 478 iron meteorites, 474–475, 483 island flank collapse, 110 islands subtropical, hurricanes, 397 wind damage, 397 isostacy or buoyancy, 17, 18 Italian volcanoes, 134 Italy, 140, 153 beach, 365 debris flow, 1998, 338 flood in Florence, 350–351 heat wave, 267 landslides, 203, 216, 217–218 subsidence, 231, 232, 234–235 volcano evacuation, 168 volcano historic record, 162 volcanoes, 166 wildfires, 453 Izmit, Turkey, earthquake, 1999, 53, 75, 78, 87 J Japan, 140 effect of 1960 Chile tsunami, 115 floods, 306 mudflow, 158
nuclear energy, 280 volcano historic record, 162 Jarrell, Texas, 1997 tornado, 437, 446, 447 jet stream, influence on, 255–256 hailstorms, 434 hurricanes, 259 Mississippi River flood, 346 moist air, 346 Nor’easters, 409 “trough,” 255 weather, 264 jetties, 375 longshore drift, 376 sand migration, 378 JOIDES deep-sea drilling ships, 15, 16 jökulhlaups, 312–314 Juan de Fuca Plate, 21, 40 spreading ridge, 15, 20, 21, 39 K Kansas droughts, 264 Mississippi River flood, 345, 347 tornadoes, 430–431, 443, 445 volcanic ash, 146 kaolinite clay, 193 karst, 228, 229, 230 Kashmir earthquake, 2005, 4, 32, 75 landslides, 32–33 Pakistan, 82, 91–92 survival after, 494 Kentucky flood, 327 sinkholes, 228–229 Kilauea Volcano Hawaii, 28, 132–138, 152, 164 earthquakes, 138 failure surfaces, 101 gases, 162 slumping, 111 kinetic energy during rockfall, 199 knickpoints, 298 Kobe earthquake, Japan, 1995, 51, 72, 76, 77, 78, 90 Krakatau eruption, 1883, 51, 134, 135, 152, 155, 269 tsunami from, 99–100 K-T boundary, impact, 478–479 Kwanto, Japan, earthquake, 75 Kyoto Protocol, 278–280 L La Conchita landslide, California, 188–190, 201
La Niña, 258 ladder fuels for wildfires, 452, 453, 459, 460 lag time between storm and flood, 302 lagoons and barrier island migration, 367 Laguna Beach, California, landslides, 202 Laguna Canyon, California mudflow, 489 lahars, 131–132, 133, 142–144, 152, 158 versus mudflow, 309 Lake Erie, lake-effect snow, 268 Michigan, lake-effect snow, 268 Nyos, West Africa, 160–161 Ontario, lake-effect snow, 267–268 Superior, 255 Tahoe, California, tsunami, 100 Washington landslides, 41 lake-effect snow, 267–268 land subsidence, 226, 231–238 land use planning, 7, 82–84 coastal dwellings, 379–380, 405 earthquakes, 44 on floodplains, 334–335 regulations after disaster, 493 restrictions, 7 tsunami, 105 landslide(s), 188–223 along reservoir, 334 California, 255 clay, factor in, 235 cliff collapse and, 373 dams, 209 deaths, 212 earthquake trigger, 86, 91, 92 El Niño and, 301 generated tsunami, 100 hazard maps, 213–214 Hurricane Mitch caused, 1998, 421–422 insurance for, 212, 338, 490 into reservoirs, 333 Mount St. Helens, 151, 175–176 movement rates, 197 North America map of, 212 permafrost influence on, 237 prone slopes, poor areas, 422 provide stream sediment, 328 recognition, 212 scar, old, 189 streams, influence of 292, 328 tsunami triggers, 99
tropical storms and, 400 water in slope, 328, 373 weather, 195 land-use change and wildfires, 466 lapilli, 134 large waves flatten the beach, 371 largest evacuation in history, 420 largest flood on Mississippi River, 346 Las Vegas, Nevada, subsidence, 232 latent heat release and tornadoes, 438 lateral-blast eruptions, 134 ash, from volcano, 154 Mount St. Helens, 175–176 lateral-slip faults, mid-continent, 42 lateral-spreading slides, 204–205 lava, 125, 128, 129, 130, 132 in ancient deposits, 162 dome, 131, 136, 141, 142, 173 lava flows, 131–132, 140, 152–153 from cinder cone, 138, 139 Vesuvius, 167 Leaning Tower of Pisa, 234–236 Leda Clay subsidence, Ontario and Quebec, 194, 235 Lemieux flow, Ontario, 194 levee(s), 329–332 built to hold back rivers, 352, 490 constrict floodplain, 331 fail, California rivers, 351 floods, 9, 10, 341 make people feel safe, 330 materials, 329 moved back from channel, 352 natural, 329–330 New Orleans, 402, 412, 417 overtopping, 346 “protect”, 352, 329 repairs after Hurricane Katrina, 418 repeated flooding, 345–348 sediment supply to coast, alter, 406 levee breaches, 330–332, 346–347 Bangladesh, 284 Mississippi River, 346–347 Hurricane Katrina, 413 light bulbs, 282–283 lightning, 432–434 bolt, energy of, 51 caused wildfires, 453, 454–455 fatalities, 435 insurance claims, 432 risk of being struck, 435 step leaders, 433 storms and nitrous oxide, 271 strikes, 431, 432 stroke temperature, 433 strokes join charges, 433
thunder distance, 435 victims, 435 limestone caverns, dissolving, 227 liquefaction, 204 Bhuj earthquake, 93 clays, 209 during earthquakes, 52–53, 66, 74, 87 soils, 194, 209–211 Lisbon, Portugal, earthquake, 1755, 47, 75, 99 lithosphere, 16, 17, 18, 19, 25 movement rate, 28 moves over hotspot, 29, 30 versus asthenosphere, 20 little ice age, 277, 278 Little Rock, AR, earthquakes, 42 Lituya Bay, Alaska, rockfall tsunami, 100, 112–114 living in hazardous areas, 493 in the woods and wildfires, 459 on dangerous coasts, 359–362 with nature, 10, 489–490, 496 with the sea, 359 load of landslide material, 189 location of homes and disasters, 495 loess, erosion of, China, 344 logging and floods, 327–328 Loma Prieta earthquake, 47–48, 66, 70, 72–73, 76, 78, 79, 85–86 Long Island, New York, cliff erosion, 371 hurricanes, 391 replenishment, 381–382 Long Valley Caldera, California, 28, 134, 160, 270 longshore drift, 329, 359, 361–362, 366, 367 groins, affect, 375–376 long-term cycles, 260–261 long-wave radiation and greenhouse, 270–271 Los Angeles area alluvial fan housing, 338 earthquakes, 70, 74 faults, 37–38 landslide dam, 209 landslides, 202 San Andreas Fault, 36 subsidence, 231–232, 234 wildfires, 463–464, 465 Los Angeles River, channelized, 327 losing stream, 299, 300 loss of life in poor countries, 353 loss of sand from the beach, 364–366
INDEX
541
losses from natural hazards, 4 Louisiana bridge pier failure, 329 hurricanes, 391, 401 Hurricane Katrina, 386–387, 415 tornadoes, 443 tropical storm flooding, 400 Louisville earthquakes, 42 Love waves of earthquakes, 44 low-intensity fires, 459 low-pressure, 255 hurricanes, 400 nor’easters, 268 Pacific Ocean, 255 persistent cell, 346 thunderstorms, 346 tornadoes, 430 weather, 252–253 wildfires, 466 low-velocity zone, 19 lunar maria impact sites, 480 LVZ, low-velocity zone, 19 M maar, 134 Madison rockslide, Montana, 208, 209 Madrid, Spain, hidden wall structure, 84 magma chamber, 125, 129, 135, 145, 146 emptying, 141, 142, 172 Mt. St. Helens, 176 seismographs, 173 magma, 125–127, 142, 144 crystallization, 126 generation, 125–129 groundwater, influence of, 131 mingling, 130 Mount Mazama eruption, 171 properties, 125–129 rise, 125, 164, 165, 168 solidifies, 132 under Yellowstone, 147 viscosity, 125 volume, 145 magnetic field and plate tectonics, 14, 15 magnetic pole, Earth’s, 14, 15 magnetic stripes on ocean floor, 15, 39 magnitude(s) future flood, 337 versus frequency, 4, 5, 6 malaria and climate change, 274 malaria, insect migration, 278
542
INDEX
Malibu, California faults, 38 homes on beach, 373 wildfires, 465 mammatus clouds, 440 Mammoth Mountain, Caldera, hazard, 160–161 management of wildfires, 456–462 mangroves and hurricanes, 394, 405–406 Manicouagan Crater, eastern Canada, 476–477 man-induced subsidence, 243, 244 Manitoba flood, 1997, 317 tornado, 436 Manson impact structure, Iowa, 472 mantle peridotite, 126, 130 marginal levees, 309 Marina district, San Francisco, 86, 89 marine clays, subsidence, 234, 235 marine terraces and earthquakes, 371 Maryland Hurricane Agnes, 316 masonry structures in earthquakes, 91 mass extinctions, 480 Massachusetts, coastal erosion, 365 coastal insurance, 408 hurricanes, 391 nor’easter, 268 Mauna Loa, 132, 135, 136–137 flank collapse, 110 lavas, 165 volcano, Hawaii, 28 maximum fault offset and tsunami, 99 flood height, 306 height of landslide flood, 209 solar radiation, 256 tsunami wave from earthquake, 99 Mayan civilization, collapse, 278 meandering streams, 295–296 arc length, 296 belt width, 296 bend, 290 characteristics, 296 path, 296 wavelength, 296 meandering jet stream, 256 mechanism for debris avalanches, 201 mechanisms of tsunami waves, 102 medical supplies after Katrina, 415
Mediterranean Sea, 242 mega-landsliding from volcanic islands, 100 megathrust earthquake and landslides, 196 mega-thrust earthquake and tsunami, 109 melt droplets, Ries Crater, 484 melting, causes of, 126 continental crust, 126 permafrost, 236–239, 275 rocks, 28 in subduction zone, 126 temperature of rock, 125,126 meltwater streams, 297 memory of disasters fades, 493 Memphis earthquakes, 42 Mendocino transform fault, 20 Mercalli Intensity Scale, 47–48, 89 mesocyclone, 435 metallic meteorites, 474 Meteor Crater, Arizona, 476, 483 meteors and meteorites, 473–474 chance of being hit, 481–482 dust fallout, 478–479 impacts known in 1881, 483 methane in the atmosphere, 270–273, 275 global warming effect, 275 hydrate, 280–281 from volcanoes, 174 “method of slices,” 201 Mexico cinder cone, 139 climate, 253, 258 droughts, 263 earthquakes and seismic gaps, 66 floods, 2007, 335 hurricane damages, 406 meteorites, 474 rip currents, 363 seawall waves, 375 trade winds, 301 Trench, seismic gaps, 66 volcano ash, 156 volcano flank collapse, 173 Mexico City, earthquake, 1985, 52, 57–58, 80 subsidence, 232, 244 tropical moisture, 299 Miami, Florida, beach replenishment, 378 hurricanes, 399, 402 tropical moisture, 299
Michigan flood map, 336 lake-effect snow, 268 tornado, 438 microearthquake, 42, 63 Mid-Atlantic Ridge, 15, 17, 23 Middle East and climate change, 266, 275 mid-oceanic ridge, 14, 15, 16, 17, 22 Midwestern US drought, 260, 263, 264 earthquakes, 44 ice storms, 268 storms, 436 migration of barrier islands, 367 disease-bearing insects, 278 earthquakes, 67 Milankovitch cycles, 261, 272 mingling magmas, 130 mining adding sediment to streams, 328 groundwater and petroleum, 231–232 influence on floods, 328–329 salt and cavern collapse, 241 sand from beaches, 379 sand from streams, 350, 366 Minneapolis, 1993 flood, 346 Minnesota flood, 1997, 317 Mississippi River flood, 345 Mississippi delta subsidence, 234 homes with flood insurance, 408 hurricanes, 391, 408 Hurricane Katrina, 386–387, 412, 415 insurance along coast, 408 Mississippi River 1973 flood, 343 1993 flood, 302, 304, 305, 306, 325–326, 335, 347, 417 Atchafalaya problem, 342–343 avulsion, 330, 342–343 basin floods, 345–348 channelized, 346, 347 delta and Katrina, 411 delta erosion, 365 floodplain, 347 floods, mitigation, 493 Gulf Outlet, 412 levees, 332, 406, 413 levees, raising, 491 meanders, 296
recurrence interval for floods, 332 shipping channel, 415 Missouri, See also St. Louis earthquakes, 42 extreme rainfall, 256 Jefferson City, 1993 flood, 347 limestone cavern, 227 Mississippi River flood, 345 New Madrid, earthquakes, 1811– 1812, 42, 51, 63 Missouri River 1993 flood, 347 landslides, 216 levees, 332 mitigation of climate change, 278–282 coastal change, 374–380 landslide damages, 212–216 erosion after a wildfire, 456 ground subsidence, 232 hazards, 7–9, 10, 493 volcanic damage, 165 wildfires, 456–462 mobile homes hurricanes, 400, 405 tornadoes, 443, 438, 446 Mobile, Alabama, and Katrina, 416 modification of rivers, 326 Mohoroviçic discontinuity (Moho) in Earth, 18, 19 moist air collides with cold front, 301 moisture in atmosphere, lake effect, 267 moisture in wildfire fuels, 454 mold in New Orleans, Katrina, 414–415 molecules of minerals, 126 Molokai flank collapse, 110 moment magnitude (MW), 72 money, jobs and natural hazards, 493 Mongolia, 266 monitoring deformation, 63 monsoons, 299 Asia, disruption of, 278 Bangladesh, 284 Montana, cut and fill for home, 492 debris flows, 313 drought, 264 flood, 8, 295 impact site, 477 landslide, 202 limestone cavern, 228 river destroys house, 290, 491
stream, 293 structural brick walls, 76 swelling soils, 240 talus slope, 198 weather, 252 wildfires, 455–456, 457, 460, 465– 467 winds, 254 Montserrat Island collapse, tsunami, 99 most-catastrophic earthquakes, 75 Moulin in glacier, 278 Mount Adams, 169 Baker, 169 Etna, Italy, 138, 153, 166 Etna, Italy, flank collapse, 101 Lassen, California, 124, 139, 140, 169, 173–174 Mazama (Crater Lake), 171–172, 178 Pelée, Martinique, 1902, 153, 155, 179–181 Ruapehu, New Zealand, 165 Shasta, CA, 169, 172–173, 196 Mt. Hood, Oregon, 169–171 earthquakes, 170 hazard zones, 171 lahars, 171 Mount Pinatubo, 134, 142–144, 158, 164 atmospheric cooling, 268 lahars (mudflows), 195, 310 Philippines, 1991, 142–144, 152, 156 pyroclastic flow, 163 Mount Rainier, 124, 169 debris flow sensors, 339 eruptions, 169 flank collapse, 159 glacial outburst, 314 lahars (mudflows), 3, 158–160, 196, 314 mudflow sensors, 165 outburst floods, 314 possible flank collapse, 196 Mount St. Helens, 140, 165, 168–169, 170 bulge, 150–151 debris flow sensors, 339 eruption, 133–134, 141, 146, 150– 151, 153–154, 157–158, 174–178 eruption, energy of, 51 magma, 150 volcanic gases, 162
INDEX
543
Mount Tambora, Indonesia, 135, 146 eruption, 1815, 268, 269 climate cooling, 283 Mount Vesuvius, Italy, 164, 165–168, 181–182 deaths, 152,153, 155 eruptions, 166 evacuation, 165 mountain range, 18 root, 18 streams, 292 muddy water indicates dam or levee erosion, 349 mudflows, 142, 143, 158–159, 169, 309–312, See also lahars after wildfires, 456 avoiding, 160 climate change and, 274 hazards, 165 Hurricane Mitch, 1998, 421–422 in poor countries, 353 Mount Hood, 170 Mount Lassen, 174 Mount Pelée, 179–181 Mount St. Helens, 165, 177–178 movement rates, 197 Nevado del Ruiz, 183 Orting, Washington, 3, 159, 165 poor countries, 401 versus debris flows, 309–314 versus lahar, 309 Vesuvius, 182 warnings, 165 multiple flood crests, 346 multiple impacts of asteroid, 472, 479 multipurpose reservoirs fill up, 351, 352 multi-use dams, dangers, 333 Mumbai (Bombay) and sea-level rise, 277 N nails in wood during hurricanes, 400 Nantucket Island coastal insurance, 408 Naples, Italy, volcanoes, 166–168, 181–182 National Flood Insurance program, 335, 407, 408, 426 National Hurricane Center, 387, 403 National Weather Service, 412, 430, 433, 435, 443 natural disaster, 3 costliest, 386–387 deaths from, 3
544
INDEX
natural events, 6 are not the problem, 489 holding them back, 491 natural gas power, 273 natural hazards, cost of, 3 definition, 3 living locations, 490 natural levee, 296, 297 debris flows, 311 Mississippi River, 412, 415 natural protection from hurricanes, 405–406 nature control of, 491 on a rampage,” 359, 491 near-earth objects, NASA catalog, 482 Nebraska droughts, 264 Sand Hills, 265 negative-charged particles fall, 433 Nevada Basin and Range, 21, 41 debris flows, 310, 338 river, 295 subsidence, Las Vegas, 232 Nevado del Ruiz debris avalanche, Columbia, 134, 152, 158, 183–184 Nevados Huascarán debris avalanche, 200 New England, crop failure, 156 ice storms, 268–269 insurance along coasts, 408 Nor’easter storm surge, 409 North Atlantic oscillation, 260 rocky coasts, 371 New Hampshire, avalanche, 206 ice jam, 301 rockfall, 196 New Jersey, Ash Wednesday Storm, 409 Atlantic City, and sea-level rise, 277, 380–381 beach hardening, 374, 375–376, 381 Cape May, 370 coastlines, 380–381 Halloween Nor’easter, 410 homes on beaches, 371 homes on barrier bars, 370, 371 New Madrid, Missouri, earthquakes, 1811–1812, 42, 51, 63 New Mexico CO2 sequestration, 285 hailstorm, 434
Rio Grande Rift, 24 wildfires, 460 New Orleans, Louisiana avulsion potential, 342–343 below sea level, 417 catastrophic flood, 343 Corps of Engineers view, 491 floodplain history, 418 future? 417 homes flood insured, 408 hurricanes, 402, 411–418 Hurricane Katrina, 386–387, 396, 406, 495 Hurricane Katrina deaths, 401 levees, 342 Mississippi River, 417 prediction of disaster, 491 subsidence, 234 water depths, Katrina, 413 New York, See also Long Island beach replenishment, 371, 377, 381–382 breach of barrier bar, 368, 382 cliff erosion, 371 coastal erosion, 367 Hurricane Agnes, 316 hurricanes, 391 karst, 231 landslides, 203–204 meteorite hits car, 481 nor’easter, 268 New York City insurance coverage, 408 sea-level rise, 277 New Zealand landslide dammed lake, 209 meteorite hits house, 474, 481 mudflows, 165 rhyolite caldera, 28, 133 river, 297 sheep and erosion, 328 Newberry volcano, Oregon, 135, 139, 169 Newfoundland earthquake tsunami, 99 Nicaragua, control of fertile land, 494 deaths in hurricanes, 401 Hurricane Mitch, 421–422 nickel mine at asteroid impact site, 483 vapor from asteroid impact, and plants, 480 Niigata, Japan, earthquake, 1964, 53 nitrous oxide in atmosphere, 270, 271, 272, 273
NOAA, 9 weather radio, 446 Nor’easters, 268, 365, 408–410 Long Island, New York, 381 Virginia, 1994, 375 normal fault, 33, 34 Basin and Range, 41 in spreading zones, 37 North American Plate, 21 movement, 39 San Andreas, 37 North Anatolian Fault, Turkey, 24, 67, 87 North Atlantic Oscillation (NAO), 259–260 North Carolina beach replenishment, 377–378 Hatteras Island, 369, 394 homes on coasts, 370–371 hurricanes, 393–397, 420–423 Outer Banks, 369, 397, 420 Topsail Beach, 368, 398, 420 tornadoes, 438 North Dakota carbon dioxide, 285 flood, 317–319 tornadoes, 441 northeast quadrant of hurricane, 393 northeast trade winds, 254 Northridge earthquake, 1994, 38, 39, 51, 54–55, 74, 76, 77, 78, 79, 209 ShakeMap, 48 Northwest Territories, Canada, 237 Norway, 194 nuclear power, 273, 279, 280 nuée ardente, 153; See also pyroclastic flow number of hurricanes, distribution, 391 lightning flashes, 431 tornadoes, 431 tornadoes per state, 436 O Oahu, Hawaii, and hotspot track, 29 Oak Ridge Fault, Los Angeles area, 38 Oakland, California, 1991 wildfire, 460, 461 earthquakes, 72; See also Loma Prieta earthquake obsidian, 154 ocean currents, 253, 277 ocean-floor ages, 16 formation of, 15 movement, 21
ocean heat and storms, 274 ocean-effect snow, 268 oceanic circulation, 257 crust, 17, 19, 21, 23 hotspot volcanoes, 135, 136 lithosphere, 19, 25, 39 plate, 23, 40, 130 ridge, 14, 19, 20, 21 rift zone eruptions, 130 trench, and destruction of plate, 17, 20, 23, 25, 39 volcano rifts and tsunami, 101 offset of fault (See also displacement of fault), 35, 36, 65, 89 and earthquake magnitude, 50 offshore drilling platforms and hurricanes, 401–402 island and hurricanes, 405 waves affect the beach, 379 Ogden, Utah, Wasatch Front, 53 Ohio meteorite kills colt, 481 Ohio River valley swelling soils, 238 thunderstorms, 432 weather, 256 oil and water extraction and subsidence, 234 oil-drilling platform and hurricanes, 416 Oklahoma City tornadoes, 436, 437 Oklahoma drought, 264 Oklahoma tornadoes, 431, 439–440, 442, 446 outbreak, 1999, 430, 437 Old River control structure, 343 Ontario, Canada earthflow, 194 subsidence, 235–235 open impact craters, 476, 483 Oregon bay mouth bar, 369 continental hotspot track, 28 climate, 253 coast tsunami hazard, 108, 110 coastal dunes, 371 El Niño, coastal influence, 372 floods, 2006, 335 landslides, 195, 196 lavas, 152 Newport, 195, 203, 364 Portland, debris flows near, 313 rocky coasts, 371 subduction earthquakes, 36, 39 tsunami, record of giant, 109
tsunami warning time, 110 volcanoes, 135, 139, 140, 171 wildfires, 457 organic soils, drainage, 232–234 origin of a fire, clues to, 453 orographic effect, 251–252, 299 Bangladesh, 284 Himalayas, 284 Orting, Washington, mudflows, 3, 159, 165 Osceola mudflows, 159–160 oscillation of buildings in earthquakes, 80 overfilled reservoirs, 350 overgrazing and floods, 327–328 overland flow, 301 and floods, 327 after wildfire, 455–456 overlapping causes of landslides, 195–196 cycles, 257 events, 7 overloaded streams, 296 slopes, 195 oversteepened slopes and landslides, 194–195 overtopping levees, 330, 346 overwash of barrier island, 382, 368, 369, 420 of dunes in Nor’easters, 410 during hurricanes, 391 of sand dunes, 394 oxbow lake, 296 oxygen isotopes and temperatures, 261 oyster shells on beaches, 367 ozone in atmosphere, 270 layer and asteroid impact, 479 P P waves of earthquakes, 44, 46, 47 Pacific Northwest earthquake, 1700, 39, 56 subduction and tsunami, 110 tsunami, 1700, 56 Pacific Ocean, 257 sea temperature, 259 Pacific Plate, lithospheric, 19, 20 moves over hotspot, 29 San Andreas Fault and, 37 Pacific tsunami warning centers, 108, 118 Pacifica, California, cliff collapse, 357 pahoehoe, 131, 132
INDEX
545
paleoflood analysis, 305–306 paleoseismology, 65 earthquake record, 39 giant northwest earthquake, 56 studies, chance of tsunami, 116 paleovolcanology, 162 pali (giant Hawaii cliffs) and tsunami, 101, 110 Palm Springs debris-flow hazard, 338 Palm Sunday tornadoes, 1965, 437 Palmdale, California, 63 Palmer drought index, 262 Palos Verdes thrust fault, Los Angeles, 38 Pangaea, supercontinent of, 13, 15, 16 Papua New Guinea subsea slide, tsunami, 100 parapets and earthquakes, 76, 79, Paricutin Volcano, 139 Parkfield earthquakes, California, 38, 63–64, 88 particulates and atmospheric cooling, 269 Pasadena, California, debris flow traps, 340 past impacts, 475 path lengths of tornadoes, 437 paths of hurricanes, 2004, 423 path of hurricane and prediction, 393, 403, 405 peak discharge, 303 peat blocks in beaches, 368 peat soils and subsidence, 232–234 Peléan eruptions, 134 pendulum oscillation, 80 Pennsylvania, floods, 301 Harrisburg, and Agnes, 316 hurricanes, 316, 391 karst, 228 landslides, 203 nuclear power, 280 sinkholes, 230 Susquehanna River flood, 304, 316–317 people as agents of erosion, 373 expect bailing out after disaster, 351 forget disaster effects, 491, 493 move to the coasts, 359 place themselves in danger, 489 “protected” by dam, 332 settle on floodplains, 334 “Perfect Storm,” Halloween Nor’easter, 409
546
INDEX
perforated pipes to drain landslide, 214–216 peridotite and seawater, 19, 23, 28 periodicity of major asteroid impacts, 481 periods of oscillation, seismographs and buildings, 49 periods, seismic waves, 49 permafrost, Alaska, 276 distribution map, 276 ground settling, 236–238 thaw, 236–239, 275 under sea floor, 281 permanent structures and safety, 490 permeability floodplain, 330 infiltration and, 301 runoff and, 301 soil and landslides, 213–216 personal responsibility for actions, 493 Peru debris avalanche, 200 during El Niño, 257, 259 “normal” weather, 258 petroleum refineries during Katrina, 401 Philadelphia heat wave, 267 Philippines; See also Mount Pinatubo lahars, 195, 310 landslides, 204 Phoenix, Arizona, alluvial fans, 338 shrinkage ground, 225 phreatic eruptions, 134 phreatomagmatic eruption, 134 Piedmont, Atlantic coast faults, 44 Pierre Shale expanding clay, 244–245 landslides, 216 pilings for coastal homes, 370, 399 pillow lavas, 129 “pineapple express” and California, 255 pine-bark beetle migration, 278 piping in levees, 330, 346 planning for hurricanes, 403–404 plastic deformation, 35 plate boundaries earthquakes, 65, 36 faults, 37 hazards, 20–29 plate margin, trailing, 43 plate tectonics, 12–31, 21, 22, 26 development of, 17
Pleistocene and coastal processes, 366 Pleistocene epoch, 260 Plinian eruptions, 134, 138, 142–145, 153–154 , 175–176, 182 Pliny the Younger, 181–182 plumes and lithospheric hotspots, 28 point bars, 295, 296 Point Reyes, 1906 earthquake, 89 poisonous gases from volcano, 159–160, 160–162 polar air, moisture, 299 policy of government and wildfires, 457 political factors and disasters, 493, 494 political side of emissions problem, 282 Pompeii and Vesuvius, 166, 167, 181–182 ponderosa pine trees and wildfires, 465 pools of stream, 295 poor countries disaster losses, 496 hurricanes, 400–401, 421–422 living along coasts, 401 poor people, different ground rules, 493–495 poorly built homes and disasters, 494 popcorn clay, 240 Popigai Crater, Siberia, 476 population(s) at risk, 70–74, 165–173 growth and disasters, 308, 494 increase on coasts, 404 on coast and hurricanes, 391 pore pressure and earthquakes, 64 landslides, 195, 203, 204 Portland, Oregon, area debris flows, 311, 313 Mount Hood, 170–171 mudflow, 178 tsunami risk, 110 Portugal, harbor protection, 374 hazard from Canary Island collapse, 111 Lisbon earthquake, 47 positive charges in clouds, 432–433 on the ground, 433 post-flood flood markers, 306 posts, homes raised along coasts, 394 potential energy of rock on slope, 199
potential hazards, 8, 9 potholes in stream, 298, 299 poverty and natural disasters, 422, 494–495 power law for natural hazards, 50 power lines and wildfires, 465 Prairie du Rocher levee breach, 347–348 precipitation changes with warming, 274–275 Hurricane Agnes, 316 intensity, 273–274 patterns and climate change, 274–275 runoff and, 301 timing, 262 precursors to disasters, 6 of earthquakes, 88 predict disasters, 3–6, 4, 6 to volcanic eruptions, 174 predicting earthquakes, 62–64 eruptions, 162, 164, 170 hurricane path, 405 Hurricane Katrina, 416 hurricanes, 403 maximum flood height, 346 volcanic behavior, 164 preexisting slip surfaces, 212 pre-industrial greenhouse gases, 272 premiums for wind insurance, 408 preparation for hurricanes, 404 Katrina, 416 prescribed burns, 453, 457 pressure of atmosphere in hurricane, 388 pressure release on magma, 176 preventing natural changes, 359 primary waves of earthquakes, 44 Prince William Sound tsunami, 1964, 119–120 probability of events, 5, 8 problems to worsen, 495–496 projectiles from space, 472–475 property damage earthquake, 90 lightning, 432 poor countries, 353 wildfires, 464–465 property rights advocates, 492–493 property values and coastal damage, 390 proposed development on floodplain, 351–352
prosperous countries, disaster losses, 496 protecting coastal sand dunes, 406 homes from wildfires, 459–461 Provo, Utah, and Wasatch Front, 41 Pu’u O’o, 137–138 public access to beaches, 379 public cost of wildfires, 461 public education, role of in hazards, 9 public health and climate change, 274 public tax dollars used to bail out victims, 351 Puerto Rico, earthquake-tsunami risk, 99 Puget Sound, 159 earthquakes, 40 landslides, 195 tsunami waves, 41 pumice, 132, 145, 171–172 falls, 155–157 flows, 155, 168 Mount Vesuvius, 181–182 Plinian eruption, 146 pumping carbon dioxide underground, 282 fluids and earthquakes, 64, 282 groundwater, 242–244 New Orleans, 412, 413 subsidence and, 234, 242–243 pyroclasltic flow(s), 132–134, 141–142, 145, 153–155, 174, 175 deadly, 152, 179, 181–182, 153 eruptions, Italy, 168 Mount Hood, 170 Mount Mazama, 172 Mount Pelée, 179–181 Mount Pinatubo, 143 Mount Rainier, 169 Mount St. Helens, 151, 165, 175, 177 Mount Vesuvius, 166 Nevado del Ruiz, 183 over water, 155 surges, 153 tuff from , 163 Vesuvius, 181–182 volcanic record of, 162 pyroclastic materials, 131–133 Q Quebec, marine clays, 194 subsidence, 235 Queen Charlotte transform fault, 20
quick clays, collapse of, 193–194, 204, 234, 241 quick sand, 204 R Rabaul Caldera, 156 radial ridges, 138 radiant heat and fire ignition distance, 454, 459–460 Radiocarbon dating and earthquakes, 39 rainfall on bedrock, 315–316 rainfall after wildfires, 467 atmospheric moisture and, 301 debris flows and, 322 extreme, 299 formation, 250–251 in hurricanes, 400, 402 in Nor’easters, 410 melts snowpack, 351 on saturated soil causes flood, 350 rates, 299 shadow, 251–252 torrential after wildfire, 462, 464 torrential, 273–274 Raleigh waves of earthquakes, 44 random events and forecasts, 6 Rapid Creek flood, South Dakota, 332, 333 rates of downslope movements, 196–197 reacting to disasters, 491 reactivation of landslide, 212 real estate agents and responsibility, 491 rebuilding in vulnerable locations, 406 receding shorelines, 366 recognizing hazardous places, 2 record of past landslides, 212–213 recurrence intervals, 5, 8, 304–309 eruptions, 164 floods, 305, 309, 320 island flank collapse, 110 Mississippi River floods, 332, 346 problems, 307 Red River, North Dakota flood, 1997, 304, 317–319 Red Sea spreading rift, 25 Redoubt volcano, Alaska, 157 reducing flood damage, 334–337 refraction of waves, 361–363 regulation of emissions, 278 reinforcing steel in earthquakes, 76, 80
INDEX
547
reinjecting fluid into ground, 232 relationships among events, 6–7 relative humidity, 250, 251 wildfire risk, 458 relief after Hurricane Katrina, 415 repeated flooding, 337, 345–348 replenishment of beaches, 376–379, 380–381 reservoir filling and landslides, 195–196, 216 resistant rocks along coasts, 371 resisting force on landslide, 189, 193 resisting load in landslide resisting mass of a landslide, 195, 201 responsibility developers, 491 for damages, 491 for people’s actions, 493 restrictions on building sites, 426, 492 resurgent caldera, 28, 136, 146, 147, 168 Yellowstone, 29 resurgent dome of volcano, 141, 164 Santorini, 146 retardants for wildfires, 457 retrofitting structures for earthquakes, 75–81 Reunion Island, Indian Ocean, collapse, 101 reverse condemnation, 493 reverse fault, 33, 34 rhyolite, 127, 129, 132, 140, 141, 158 ash, 145, 147, 171–172 formation in crust, 23 magma, 28, 126–128 volcano, 146 Richter magnitudes of earthquakes, 48, 52 Richter, Charles 48, 50, 62 Ries Crater, Germany, impact site, 484 riffles of stream, 295 rift zones continental, 21, 136 on volcanoes, 137–139 “right-hand rule,” 253 rights of property owners, 492–493 to build on floodplain, 352 ring fracture in caldera, 142 Rio Grande Rift, spreading, 21, 24 rip currents, 362–363, 365 “ripe” snowpack, 205, 301 riprap can accelerate cliff erosion, 358 erosion at base of cliffs and, 358, 372, 373
548
INDEX
New Jersey coast, 381 protects cliff temporarily, 358 waves and, 373 rising air mass, 255 risk assessment for wildfires, 458 asteroid impact, 480–482 denial of, 490 estimating, 8 insurance, 297 lightning strike, 435 map for earthquakes, 70 river change with climate, 291 meanders, 296 migration on floodplains, 330 provide sediment to beaches, 363 response to added sediment, 340 riverbank eroding, 292 Riverside, California, earthquake risk, 74 roads partly dam floodplains, 329 rock avalanche, Switzerland, 198, 220 rock types along coasts, and erosion, 371 rockbolts and cliffs, 214 rockfall(s), 196–199 cause tsunami in Alaska, 112–113 hazard, 220 triggered by blasting, 219 triggered by earthquakes, 91 Utah, 220 Rockville, Utah, rockfall, 220 rocky headlands, 371 Rocky Mountain winds, 254 Rodgers Creek Fault, earthquake risk, 72–73 “rogue” waves, 359 roof collapse of ground cavity, 229 under volcanic ash, 156, 167, 181 with ash, 168 roof material in hurricane, 399 for wildfires, 459 ropy lava, 131 rotation of tornadoes, 435 rotational landslides and slumps, 189, 190, 201–203 roughness of slip surface, 190 runoff adds water to slope, 218 erosion due to, 300, 455 precipitation intensity and, 301 rate and floods, 302 to streams, 301
runout length, 198, 201 run-up heights of tsunami waves, 102, 104 Lituya Bay, Alaska, 112–113 rupture length of fault, 36, 37, 65 earthquake magnitude and, 50, 65, 66 Russian River, California, stream mining, 341–342 S S waves of earthquakes, 44, 46 velocity of, 44, 47 Sacramento River delta subsidence, 235 floodplain development, 351–352 levees fail, 352 safety during floods, behind levees, 330 hurricanes, 405 thunderstorms, 435 tornadoes, 443, 446 Saffir-Simpson scale, 388, 397 Sahara desert, 260, 264–266 Sahel region, Africa, rainfall, 260, 264 Salt Lake City alluvial fan housing, 338 Basin and Range, 41 debris flows, 310 Wasatch Front, 70 base isolation pads, 81 salt, dissolving, 226, 227, 231 salt mining and cavern collapse, 241 San Andreas Fault, 20–21, 24, 27, 36–38, 66–67, 70, 72, 85, 88 earthquakes, 36, 47 monitoring, 63 movement on, 33, 34 Northridge, 78 risk, 73–74 tectonics, 41 transform boundary, 20, 40 San Andreas Lake, 27 San Bernardino area debris flows, 310 earthquake risk, 74 wildfires, 463, 464–465 San Diego area, California, collapsing cliffs, 372 wildfires, 459, 463–465 San Fernando Valley earthquake, 1971, 38, 53–54 San Francisco Bay area, 27, 70, 72–74 earthquakes, 47, 85 faults, 38
fill liquefied, 211 subsidence, 231 San Francisco 1906 earthquake, 33, 34, 36, 45, 52, 72, 74, 88–90 earthquake zoning, 73 Loma Prieta earthquake, 85 San Gabriel Mountains, wildfires, 464 San Jacinto Fault, earthquake risk, 37, 74 San Joaquin River, California, 292, 304 San Joaquin Valley subsidence, 232–234 San Jose, California, earthquakes, 72 subsidence, 231–232 San Salvador earthquake, 84 sand dunes, 364–367 absorb wave energy, 405–406 beach replenishment, 364, 371 coastal, 369–371 eroded by storms, 420 Galveston Island, 419 height along coasts, 367 Nebraska, 265 stabilized, 379, 381 Sand Hills, Nebraska, 265 sand bars, New York, 381 boils, 66, 74, 209, 211, 330, 331 erosion on beaches, 364–366 fences and dunes, 370 loss from beaches, 364–366 mining along coasts, 366 movement along shore, 361–362, 374 placement for beach replenishment, 378 pumping on beaches, 378 replenishment, 9 sheets in bays and giant tsunami, 110 slurry for beach replenishment, 377 sources for beach replenishment, 377 on summer beaches, 364 supply to coasts and beaches, 363–366 sandbag beach protection, 381 sand blow, see sand boil sandy bays, 362 Santa Ana winds, 254, 464 and wildfires, 457, 463, 465 Santa Catalina debris flows, 321 Santa Catalina Fault and tsunami, 99 Santa Clara area, subsidence, 233
Santa Cruz, 1906 earthquake, CA, 89 Loma Prieta earthquake, 48, 86 Santa Monica Mountains fault, 38, 39 wildfires, 465 Santa Monica, California, beach replenishment, 376 Santa Rosa, California, earthquakes, 70 Santiago, Chile, earthquake, 55 Santorini, Greece, 134, 135, 144–146, 154, 164 eruptions, 145–146 Saskatchewan carbon dioxide sequestration, 285 River avulsion, 343 saturated ground, 192 floods, 316, 345 heavy rains, 350, 353 thunderstorms, 321 scale of natural catastrophes, 5 scalloped shoreline, 362, 363 scientific method, 17 Scotland tsunami from Norway landslide, 100 scour of channel, 294 scour holes near bridge piers, 329 sea level rise, 276–277 after ice ages, 366 barrier islands, 367 Bangladesh, 283–284 Calcutta, India, 283–284 flooding due to, 243 sea water carbon dioxide, 273 sea-cliff erosion, see cliff erosion seafloor spreading, 14 seamounts, remnant of hotspot track, 28 seasons, Earth’s axis tilt and, 257 Nor’easters, 408 tornadoes, 437–438 sea-surface temperature (SST), 258, 259, 260 El Niño, 259 hurricanes, 390, 402 Indian Ocean, 265 oscillation, 260 Seattle-Tacoma area earthquake hazard, 41 earthquakes, 56–57 landslides, 195, 492 mudflows, 159 tsunami risk, 110 volcanoes, Mount Rainier, 169
Seattle Fault earthquake and tsunami, 41, 110 seawalls, 380–381 and barrier islands, 242 beach protection using, 375 Galveston, Texas, 374 sand erosion, 375–376 second homes along coasts, 359 secondary effects of wildfires, 455–456 secondary waves of earthquakes, 44 second-order stream, 302 sediment to beaches from cliffs, 363 to beaches from rivers, 363, 366 concentration in flood, 309 delivered to coasts, 359 load added to river, 327 percent in flood, 309 reduced by dams, 366 supply and beaches, 329 stream mining, 366 trapped behind dams, 332 sediment transport, 294–295 floods and, 309 waves, 359–361 sedimentary structures, 309 seepage under a dam, 332 under levees, 330 segment lengths of faults, 70 seismic gap future earthquake, 66, 74 Kashmir earthquake, 92 Loma Prieta earthquake, 85, 86 seismic moment, determined from, 50 seismic waves, energy, 44 seismogram record, 45 1906 earthquake, 89 seismograph drum, 46 for earthquakes, 45–46 Loma Prieta earthquake, 85, 86 pendulum, 49 Richter Magnitude, 48 station, 46, 47 tsunami warning and, 106 Wood-Anderson type, 48 seismologists and risk maps, 70 semiarid region flood channels, 300 sensitive marine clay, 194 septic drain fields and landslides, 195 sequestration of greenhouse gases, 278, 281–282, 285 hazards, 282
INDEX
549
serpentinite and oceanic peridotite, 23 settling ground, 231 severe weather, 254 warnings, 446 severed barrier islands, 394 Shaanxi, China, earthquake, 75 Shake Maps and acceleration, 47, 48 shaking time in earthquakes, 50–52 shallow slides, environment of, 213 shallow water waves rise, 360 shatter cones, 476–477, 478 Sudbury, Ontario, 483 shear strength of rocks, 50 shear walls and earthquakes, 78 shear waves of earthquakes, 44 sheetwash after wildfires, 455, 467 shelter from tornadoes, 446 shield volcanoes, 135–139 shingle roofs and wildfires, 459 shock features and impact, 484 Meteor Crater, 483 shock waves from asteroid impact, 480 shocked mineral grains and asteroid, 472 shocked quartz, 478 boundary clay, 484 grains and impact, 480 Meteor Crater, 483 Sudbury, Ontario, 483 shock-melted glass, 483 Shoemaker, Eugene, Meteor Crater, 483 Shoemaker-Levy comet, 479 shore nourishment, 379; See also replenishment shore protection projects, 374, 382 federal costs, 394 shorelines eroding, see coastal erosion short-term forecasts, 64 predictions, 65 short-wave radiation and greenhouse, 270, 271 shotcrete, 214, 373; See also gunite shrinking ground, 225–226 Siberia permafrost, 275 asteroid, 485 Sicily, 134; See also Mount Etna Sierra Madre-Cucamonga fault, 38 Sierra Nevada Basin and Range, 41 stream sediment load, 340 wildfires, 465
550
INDEX
silica content of magmas, 127 sinkhole(s), 226–231 areas, 229–230 construction sites, 231 cost of collapses, 228 distribution map, 230 drilling wells, 231 environment of, 227 from salt mining, 241 load on ground, 231 potential for, 229, 230 processes, 226–227 types, 227–229 sirocco wind, 242 size of types of volcanoes, 136 Skykomish River flood, 2006, 334 slab avalanche, 206, 207 slack water deposits, 307, 308 slide plane, 217–218 slip distance, and earthquake magnitude, 50 rates on a fault, 72 surface of landslide, 189, 193, 194, 201, 203, 216–218 slope failure, 189–191 load, 189–190 material, 193–194 processes, 189–192 angle and landslides, 189–190, 194 slopes of beaches, 364, 364 debris flows, 312 stream and sediment added, 342 water table, 300 volcano flanks, 129, 136 slump movement rates, 197 scarp, 209 of slope, 195 of river levees, 346 Small Business Administration, 9 small waves, steeper beach, 372 smectite clay, 193, 240, 244–245 damages, 244–245 landslides, 195, 216 smoke and wildfires, 462, 466 smokejumpers, 451, 457 Storm King Fire, 462 Snake River Plain, Idaho, 24, 28, 29, 147 snow and ice, 267–268 avalanches, 205–208 Nor’easters, 410 “ripe,” snowpack, 205, 301 social impacts of hurricanes, 401– 403
societal attitudes, 491–493 soft sediments and earthquakes, 90 “soft” solutions to hazards, 491 soil cavity collapse and sinkholes, 228 collapse, 241 creep, 189, 197, 204, 205 expansion and shrinkage, 205 flows, 209 permeability and floods, 327 as used in engineering, 193 solar energy, 257 and sea ice, 250 collectors, 279 geothermal power, 280 solar heating panels, 279 solar radiation, 260 after asteroid impact, 479 changes, 271 solid, liquid, and gas, 125 solifluction, 205 soluble rocks, 226 solution cavities and sinkholes, 226 soot from fires after asteroid impact, 480 source of a fire, clues to, 453 of magma, 130 South America climate, 253 El Niño, 257 South Carolina Ash Wednesday Storm, 409 Charleston earthquake, 1886, 42, 51 coastal erosion, 365 East Coast Fault System, 44 hurricanes, 388, 391, 404, 406, 420, 423, 425–426 hurricane deaths, 401 Hurricane Hugo, 492 insurance along coasts, 408 island evacuation, 404 zoning restrictions, 492 South Dakota Black Hills flood, 333 drought, 263 extreme rainfall, 256 landslides, 195, 216 Southeast Asia greenhouse gases, 280 poor people in, 401 rainfall, 299 southeast coast tsunami risk, 99 U.S. tornadoes, 1994, 437 Southern California wildfires, 457, 463–465
space for rivers to flow, 352 Spain, heat wave, 267 soot and carbon dioxide, 272 sparks and embers from fires, 452 spot fires, ignition of, 453 spotting incoming asteroids, 482 spread of wildfires, 453–454 spreading center, oceanic, 16, 18, 21, 22, 23, 27 faults, 37 volcanoes, 129 spring runoff, 296 Sri Lanka, floods, 305 Sumatra tsunami damage, 104, 117–118, 494 SST, see sea-surface temperature St. Francis landslide dam failure, CA, 209 St. Helens, a Cascade volcano, 26 St. Lawrence lowland subsidence, 235 St. Louis, Missouri, floodplain development, 335 1993 Mississippi River flood, 305, 345, 346, 347 Mississippi River, 332 earthquakes, 42 stabilizing landslides, 216 sand dunes, 379 stalactites, 227–228 stalagmites, 227–228 state-run insurance programs, 408 stationary weather front, 346 steam bubbles, 132 eruption from volcano, 181–182 explosions, 133, 134 from lahar, 158 on volcanoes, 174–175 steep coastlines, 360, 362 step leaders in lightning, 433 Stockton, California, 1997 flood, 352 stony-iron meteorites, 474–475 stony meteorites, 475 Tunguska, Siberia, 485 Storegga slide, Norway, tsunami, 100 storm(s) along weather fronts, 255, 256 chasers, tornadoes, 438 coastal dunes, 369 damages from hurricanes, 391– 403, 410 flood crest, 303 global warming, 495 homeowners insurance, 408
Mississippi River flood, 345 North Atlantic, 260 offshore, 360 resistant design, hurricanes, 425 runoff, 302 tide, 392; see storm surge warnings, Galveston Hurricane, 419 wave erosion, 367, 396 waves, Galveston, Texas, 374 storm surge, 255, 365, 392–395; See also surge Ash Wednesday Storm, 409 Bangladesh, 284 Galveston, 419 height in hurricanes, 388, 389–390 home insurance, 408 Hurricane Katrina, 412 Nor’easters and, 410 Storm King Fire, Colorado, 1994, 451–452, 462–463 debris flows following, 462–463 “storm power index” of Nor’easters, 409 strain shown by rocks, 35 stratovolcano, 126, 136, 140, 141, 155, 157–158, 164, 168 stream adjusts its gradient, 291 channel changes, 294 dams on, 332–333 equilibrium, 291 erosion and bridges, 328 flow, 291–294, 308 gradient, 292 grain size carried, 291 hydrograph after wildfire, 456 in deserts, 300 modification, 326 order, 302 peak after storm, 303 roughness, 292 sediment load, 291, 328 slope, 292, 342 turbulence, 294 velocity and load, 294 streambed mining and erosion, 340–341 floods, 328–329 strength of Nor’easters, 408 strength of a slope, 192 stress, differential, 35 stress on rocks, 35 strike-slip fault, 33 earthquakes, 36 offset, 65 strip-mining of coal, 280 Strombolian eruptions, 134
structural damage in earthquakes, 75–81 stumps in beaches, 368 subduction, 16, 20, 21–24 fault and tsunami, 98 generated tsunami, 108 locked, 40 methane hydrate, 281 oceanic plate, 18, 40 tsunami environment, 117 volcanoes, 127, 136, 140, 168–169 subduction zone earthquakes, 33, 36, 37, 39–41, 71 displaces water, 98 January, 1700, 109 mudflow, 159 tsunami, 99 subglacial floods of glaciers, 278, 314 submarine canyon, sediment loss, 365–366 submarine landslide tsunami, 100 subsidence of ground, 211, 226, 231–238, 239, 240 subtropical storms, 255, 256 suction vortex, 442 Sudbury Complex, Ontario, impact, 477, 483–484 sulfate aerosols in stratosphere, 283 sulfur dioxide aerosols, cooling, 268 from volcanoes, 142, 144, 145, 156, 160 sulfuric acid in atmosphere, 160 Sumatra earthquake, 2004, 37, 39, 56, 75 rupture zone, 40 subduction zone and tsunami, 116–119 Sumatra tsunami, 2004, 97, 104, 116–119 aftermath, 494 damage, 118–119 travel times, 117 warning too late, 118–119 warnings, 106 summer waves and beaches, 364 summit elevation change on volcano, 164 sun’s energy, 256 reflection of, 275 supercell storm, 435, 440 supercontinent of Pangaea, 13, 14, 15, 16 Superoutbreak, 1974 tornadoes, 437, 438 supervolcanoes, 147 surface rupture length, 35, 36 INDEX
551
surface tension, 191–192 surface waves of earthquakes, 44 surges, 392–395; See also storm surge amplified with high tide, 396 atmospheric pressure, 392 coastal dunes, 394 deaths in Bangladesh, 284 debris flows, 312 deposit, recognition, 163 during hurricanes, 387, 400 height and wind, 392–393, 402 heights and Nor’easters, 409 high tide, 395 Hurricane Katrina, 411, 415, 416 inland reach of, 393 inlets, 392–393 lakes, 392 pyroclastic, 153 surviving a tsunami wave, 103, 108 suspended load, 293–294; See also sediment swelling soils, 193, 226, 238–240, 244–245 Switzerland debris avalanche, 220 earthquakes, 64 T Tacoma, Washington, 169 Tahiti effect of 1960 Chile tsunami, 115 high-pressure cell, 257, 258, 259 tail of a comet, 473 talus slopes, 197, 199 Tambora volcano, Indonesia, 134, 145, 156 Tangshan, China, earthquake, 1976, 61–62, 75 Taupo Caldera, New Zealand, 28, 133 tax on burning fossil fuels, 282 taxpayers and cost of disasters, 493 tectonic environments of faults, 36–43 of volcanoes, 129–130, 136 tectonic plates, 17, 19 tektites from asteroid impact, 484 temperatures, Caribbean Sea, 496 changes, world, 271 Gulf of Mexico, 496 ocean effect, 274 hurricanes, 390 Tenerife island flank collapse hazard, 111 Teton Dam failure, Idaho, 348–349 tetrahedra, 126
552
INDEX
Texas, See also Galveston, Houston asteroid impact evidence, 472 droughts, 263, 264 flood and moisture, 304, 315 flood Austin, 306, 334 flood, Guadalupe River 315–316 ground subsidence, 231–232 hurricane, 391, 398–401 ice storms, 268 karst, 231 law and coastal rebuilding, 419 limestone cavern, 227 salt dome CO2 sequestration, 285 thunderstorms, 432 tornado, 442, 438, 439, 446, 447 tropical storm, 400 wildfires, 458 Thailand and Sumatra tsunami, 2004, 97, 98, 102, 104, 108, 117–118, 494 thalweg, 295 theory versus hypothesis, 17 thermohaline circulation, 260, 277 thinning trees and brush and wildfires, 460 Thistle landslide, Utah, 203, 209, 210 Three Gorges Dam, China, and floods, 334 Three Sisters volcanoes, Oregon, 169, 171 thrust fault, 33 blind, 37, 38, 54, 93 thunder, cause of, 433 thunderstorms, 253, 299, 431–435 charged droplets, 433 diameters, 432 evolution, 431 heavy rain, 314 hurricanes, 387 lines of, 299 precautions, 435 safety, 435 wildfires, 467 tidal power, 280 sensors and tsunami, 106 tiltmeters in Kilauea, 138, 164 time between tsunami waves, 105 time for ignition at flame distance, 454 timing of precipitation and drought, 262 Tokyo earthquake, 1923, 68 sea-level rise, 277 topography, effect of in wildfires, 454 Tornado Alley, 436
Tornado(es), 253, 256, 435–446 average energy of, 51 best place to be in basement, 443 climate change, 274 damages, 441–443 deadliest, 436 development, 438–441 downbursts and, 437 ground speeds, 437 hurricanes and, 259, 425, 410, 412, 436 movement direction, 438 numbers in United States, 436 outbreak, 436 paths, 436–438 risk map, 436 safety, 446 season, 437, 438 shelter, 446 Superoutbreak, 1974, 437 thunderstorms and, 431, 432, 433–434, 440 watch and warning, 440 wind velocities, 437 Toronto, Ontario, and impact site,477 tracking asteroids, 482 tracks of tornadoes, 437–438 trade winds, 253, 257–258, 299 Atlantic, 260 hurricanes and, 388, 403 trailing continental margin earthquakes, 42 Trans-Alaska Pipeline, subsidence, 238 transform boundaries, 22, 24, 27, 37–38, 70 between plates, 20, 24 continental, 90 earthquakes, 36 transient crater of impact, 476 translational slide, 219, 203, 213 into reservoir, 216–218 Transverse Ranges, earthquakes, 74 trapping debris flows, 339–340 sand with beach grass, 371 sediment behind dams, 332 tree ring damage from flood, 307 trench at edge of oceanic plate, 16, 18 trench dug across a fault, 65 trenches and plate movements, 14, 16, 18 tributaries, 291 flood rise, 302 stream order, 302 triggers of landslides, 195, 207; See also landslides
trimlines, tsunami, 113 TriNet ShakeMap, earthquake warning, 64 Loma Prieta earthquake, 48 triple junction between plates, 20, 25 tropical air, 299 cyclones, 255, 299 low-pressure system, 315 Tropical Storm Allison, Houston, 400 “trough” of jet stream, 255 tsunami, 97–121 from 1960 Chile earthquake, 115 asteroid impact, 481, 479 causes, 108 dangers, 108 drag on bottom, 102 earthquakes driven, 55 frequency of major, 99 generation, 98–102 hazard mitigation, 105–108 largest earthquake on record, 114–116 landslide driven, 112–114 magnitude, first estimation, 119 methane hydrate, 281 on shore, 103–105 in Pacific Northwest, 108–110 run-up, 104 sediments and coastal record, 109 speeds, 117 subduction-zone earthquakes, 39, 56 survival, 108 travel time, 106 velocities, 102 watch issued, 108 tsunami warning, 117, 106–108 buoys map, 107 system, Indian Ocean, 119 time, Pacific coast, 110 tsunami wave(s) from asteroid impact, 472 and coral reefs, 360 hazards, 103 intervals, 105 movement, 102–103 number in sequence, 105, 117 periods, 104–105 Puget Sound, 41 total time of event, 105 travel times, Indian Ocean, 117 Tucson, Arizona, debris flows, 310–312, 338, 339 flood, 304 storms, 303, 321 tuff, 162
Tully Valley landslide, New York, 203–204 tungsten-filament light bulbs, 282 Tunguska, Siberia, asteroid, 485 turbulent stream, 293–294 Turkey, air-fall ash, 163 earthquakes, 87 solar collectors, 279 water and climate change, 275 types of downslope movement, 196–208 of ground movement, 226 typhoons, see hurricanes U unbraced windows, 76, 77, 78 undercut slopes and landslides, 194–195 undertows: see rip currents undeveloped areas and development, 407 Uniform Building Code seismic zone map, 84 unintended release of methane, 281 universal building codes, coastal areas, 407 unpredictable hurricanes, 423 updraft winds and hailstorms, 435 upwelling cold water, Peru, 257 urban planning and floods, 327 urbanization, 302, 308, 326 Utah, base isolation for earthquakes, 81, 83 Basin and Range, 21, 24, 41 carbon dioxide sequestration, 285 flood depths, 294 landslides, 213 rockfall, 2, 198, 220 volcanic ash, 146 wildfires, 457 V Vaiont landslide, Italy, 203, 216, 217–218 Vancouver Island tsunami warning time, 110 Van Norman Dam, collapse, 53 vaporization of chondritic asteroid, 480 vegetation coastal landslides, 371 on dunes, 370–371 removal and transpiration, 328 VEI, 133, 134
velocity asteroids, 475 comets, 473 downslope movements, 197 erode sediment, 293 pyroclastic flows, 153 stream and sediment, 293 tsunami waves tsunami waves, 102, 103 Venezuela flash flood, 1999, 353 Venice, Italy lagoon, 242 subsidence, 231–232, 242–243 tides, 243 vertical movement under water, tsunami, 99 Vesuvius, 134, 140 vibration frequency of ground, 86 violent eruptions, 130, 136 Virginia, active faults, 44 Beach, coastal erosion, 368 breakwaters, 377 coastal erosion, 365 debris flows, 322 hurricane, 316, 423 Nor’easter, 1994, 375 viscosity of magma, 126–127, 140, 141, 175 volatiles in magmas, 127–128 by volcano type, 136, 141 volcanic arc, above descending plate, 25 ash, 132, 133, 143, 157 Ash Advisory Center, 157 behavior, 125–129 “bombs”, 139, 140 cinders, 139 crater, Mt. St. Helens, 151 emissions, 142 sunsets after eruptions, 270 volcanic eruption(s), atmospheric cooling after, 268, 269 climate affects, 282 deaths, 152 gases, 140, 158, 160–162 generate tsunami, 99–100 hazards, 150–185 lahars, 310 mudflows, 157–160 precursors, 164–165, 165, 174–178 products, 131–135 rocks, steam-altered, 174 smog (vog), 160–161 surges, 153
INDEX
553
volcanic eruption(s)—cont’d tree cast, 152 warnings, 167 weather, 156, 310 Volcanic Explosivity Index (VEI), 133, 134 volcanoes, 124–186 characteristics, 130, 136 collapse, Mt. St. Helens, 151 eruption warning, 165 line of over sinking plate, 23 products, 132 slopes of flanks, 129 surface temperatures, 164 tsunami from, 100–101, 108 types, 129, 135-142 volcanologists, 142, 175 volume of ejecta, 134 magma, 129 stream sediment, 293 vortices in tornadoes, 441–442 Vulcanian eruptions, 134 vulnerability to natural disasters, 422, 494 W walkways over dunes, 406 wall cloud and tornado, 435, 439 warm front, 254–255 warm rains on heavy snowpack, 351 warming oceans, 274 warning for debris flows, 338–339 Galveston Hurricane, 419 Hawaiians of volcano collapse, 111 hurricane, what to do, 405 mudflows, 165 systems, hurricane, 401 time for eruption, 168 time, Pacific coast tsunami, 110 Wasatch Front, Utah, Basin and Range, 24, 41 debris flows, 310 earthquakes, 36 liquefaction, 53 segments, 70 Washington Aldercrest slide, Washington, 203–204 Bellevue, Washington, floods, 308 climate, 254 coastal dunes, 371 earthquakes and tsunami, 108 El Nino, coastal influence, 372 flood, 2006, 335
554
INDEX
glaciers, 275 landslides, 195, 196, 203–204 mudflows, 3 outburst floods, 314 Puget Sound area faults, 41 rockfall, 198 rocky coasts, 371 subduction fault, 36, 39 tsunami warning time, 110 volcanoes, 151, 158, 159, 169 water added to ground, landslides, 373 contamination, hurricanes, 404 depth and wave velocity, 102, 360 depths, New Orleans in Katrina, 413 extraction and subsidence, 242– 243 food shortage after hurricanes, 422 lines and hurricanes, 402 movement in a wave, 360 penetration and landslides, 328 pore pressure and landslides, 195 in pore spaces, 192 pressure around reservoir, 211 pressure in soil, 192, 214, 218 removal from slope, 190 saturated ground, 205, 299 saturated mud, 192–193 seeps under levees, 352 supplies, 274–275, 404 water table, slope, 300 solution of rocks, 227 subsidence, 234 water vapor, atmospheric, 250–251, 496 global warming, 496 as greenhouse gas, 270, 273 in magmas, 127–128 warm air holds more, 299 water velocity, estimate, 307 watershed, 291 wave(s) approaching shore, 360 damage, 395–397, 405–406, 409 deep water, 360 drag on bottom, 360–361, 394, 397 energy, 360–361, 405–406 erosion, 364–366, 395, 425 height and wind velocity, 359, 397 hurricanes, 394–397, 402 offshore affect the beach, 379 on irregular coastlines, 362–363 period, 359
refraction, 361–363 run-up, 1964 Alaska tsunami, 119–120 sediment transport from, 359–361 storm centers, 360 tsunami, 102 undercut cliffs, 363, 364, 366 undermine posts of houses, 394, 425 velocity, 360 water movement, 360 wavelength, 359 of tsunami, 102, 104 we are the problem, 490 weak floors and earthquakes, 76, 78, 90 weather, 250–268 elevation in atmosphere, 270 fronts, 254–255, 346 maps, 255 related hazards, 256 satellites and storms, 403, 404 wildfires and, 454–455 weathering of minerals, 193 Weber, Utah, and Wasatch Front, 41 Wegener, Alfred, and continental drift, 13, 17 welded ash, 154 West Nile virus insect migration, 278 westerly winds, 206, 253–254 Atlantic, 259 Nor’easters, 409 wet climates, 295 wet-climate streams, 299 Weyburn sequestration project, 285 whirlpools, 299 White River, Washington, mudflow, 160 wide beach from rivers or cliff erosion, 372 wildfires, area burned in wildfires, 465, 466 average annual number, 456 average firefighting costs, 456 behavior, 452 carbon dioxide from, 273 costs, 461–462 effect of topography, 454 government policy, 457 indirect costs, 457 long-term benefit, 457 management and mitigation, 456–462 regional risk, 458 requirements for insurance, 462 season in Southern California, 465 storm winds, 432
wildland evolution and wildfires, 457 Wilma, Hurricane, 363 wind-blown dust and permafrost, 275 wind(s) driven waves, 359 effects on house in hurricane, 399–400 forces on a home, 397–399, 440 generated by wildfires, 466 of hurricanes, 387–390, 397–400, 402, 410–411, 415 insurance, homeowners, 408 low-lying islands, 397 Nor’easters, 408–409 origin of, 252–253 power lines, and wildfires, 465 power, 279 push storm surge, 365 sand movement on coasts, 366 shear and downbursts, 434 surge height, 392–393 thunderstorms, 432 tornadoes, 437–440 turbines, 282 velocity and wave height, 397 “wall” of hurricane, 387 windows fail in hurricane, 399, 404 wing dams, 332
Winnipeg, Manitoba, flood, 317 winter beaches, 364 Winter Park, Florida, sinkholes, 226 winter storms, California, 358 winter-summer cycles, 256 Wisconsin downburst, 434 meteorite, 474 withdrawal of sea in initial tsunami, 104 witnessed meteorite falls, 474 wood frame houses in earthquakes, 79 world population with time, 4 worst flood disaster in U.S., 325 Wyoming, See also Yellowstone drought, 264 Grand Teton Range, 34 Gros Ventre slide, Wyoming, 210 Yellowstone volcano, 146 Y Yangtze River floods, 334 “year without a summer,” 156, 268, 283 year-round streams, 299 Yellow River avulsion, China, 1855, 343, 344–345 channel siltation, 344–345
flooding, China, 344–345 levees raised, 344 Yellowstone caldera, Wyoming, 29, 147, 154 earthquakes, 1959, 208 hotspot track, 39 landslide, 213 Park wildfires, 453 Yellowstone volcano, Wyoming, 28, 130, 134, 141, 146–147, 164, 168 climate impacts, 270 earthquakes, 147 Yosemite National Park, rockfall, 197 Younger Dryas, 278 Yucatan peninsula asteroid impact, 471–472 hurricanes, 402 Yungay, Peru, debris avalanche, 200 Z zones for slippage, 193 zoning dangerous areas, 494 earthquakes, 84 property, 7 restrictions, South Carolina, 492 to mitigate hazards, 491 San Francisco, for earthquakes, 73
INDEX
555
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484
Earthquakes Tsunami
221 217 242
Volcanoes Landslides Subsidence
350
Climate Floods Beaches
181
87
Hurricanes Tornadoes Wildfires Asteroids Case in Point Locations Numbers on map refer to page numbers in book.
145
485
344 90 91
93
283
142
116
283
