Physical Geology Earth Revealed: Fourth Edition

Charles Plummer

David McGeary, Diane Carlson, and Charles Plummer at an outcrop ofa Sierra Nevadan intrusivebody.

Physical Geology

Professor Charles "Carlos" Plummer grew up in the shadows of volcanoes in Mexico City. There, he developed a love for mountains and mountaineering that eventually led him into geology. He received his B.A. degree from Dartmouth College. After graduation, he served in the U.S. Army as an artillery officer. He resumed his geological education at the Un iversity of Washington where he received his M.S. and PhD degrees. His geologic work has been in mountainous and polar regions, notably Antarctica (where a glacier is named in his honor) . He taught at Olympic Community Col lege in Washington before joining th e faculty at California State University, Sacramento. At CSUS, he teaches introductory geology courses as well as optical mineralogy, metamorphic petrology, and field courses. (plummercc'ecsus.edu)

Fourth Edition

David McGeary Emeritus ofCalifornia State University at Sacramento

David McGeary

Charles C. Plummer California State University at Sacramento

Professor David McGeary was born and raised in central Pennsylvania, learning to love the Nittany Lions as well as shoofly pie for breakfast. He earned his B.S. degree at Williams College, M.S. at University of Illinois, and PhD at Scripps Institution of Oceanography. He helped pay for grad school by teaching scuba diving. David has been married to his wife for more than 30 years, has two grown sons, and a handmade house he built with his bro ther on 10 country acres. He taught geology at California State University for more than 23 years, specializing in physical geology, sedimentary petrology, oceanography, and field methods . He retired in 1995, turning his portion of the text over to the excellent hands of fellow professor Diane Carlson. He is now a theater actor, appearing in 5 productions in 1997. His favorite role was as Colonel Jessep in A Few GoodMen.

Diane H. Carlson California State University at Sacramento

Diane Carlson Professor Diane Carlson grew up on the glaciated Precambrian shield of northern Wisconsin and received an A.A. degree at Nicolet College in Rhinelander and her B.S. in geology at the University of Wisconsin at Eau Claire. She continued her studies at the University of Minnesota-Duluth where she studied the structural complexities of high-grade metamorphic rocks along the margin of the Idaho batholith for her master's thesis. The lure of the West and an opportunity to work with the U.S. Geological Survey to map the Colville batholith in northeastern Washington, led her to Washington State University for her PhD. Dr. Carlson accepted a position at California State Uni versity, Sacram ento after her PhD and teaches physical geology, str uctural geology, environmental and applied geology, and field geology. Professor Carlson is a recent recipient of the Outstanding Teacher Award from the CSUS School of Arts and Sciences. She is also actively engaged in researching the str uctural and tecto nic evolution of part of the Foothill Fault System in the northern Sierra Nevada of California. ([email protected])

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PHYSICAL GEOLOGY: EARTH REVEALED , FOURTH EDIT ION Published by McGraw-Hill , an imprint of The McGraw-Hill Companies, Inc., 122 1 Avenu e of the Americas, New York, N Y 10020. Copyright © 200 1,1998, 1994, 1992 by The McGraw-Hill Companies, Inc. All rights reserved. No part of this publ ication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior writt en consent of The McGraw-Hill Companies, Inc., including, bu t not limited to, in any network or other electronic storage or transmission, or broadcast for distance learni ng. Some ancillaries, includin g electronic and print components, may not be available to customers outside the United States.

*

Meet theAuthors ii Preface x The LearningSystem xu

CHAPTER 1 Introduction to Physical Geology

This book is printed on recycled, acid-free paper containing 10% postconsumer waste.

1 2 3 4 5 67 8 9 0 QPD/QPD 0 9 8 7 6 5 43 2 1 0 ISBN 0-07- 36618 3-X ISBN 0-07-118082-6 (ISE) Vice president and editor -in-chief: Kevin T Kane Publisher: JP Lenney Sponsoring editor: RobertSmith Developmental editor: LisaLeibold Editorial assistant: Jenni Lang Senior marketing manager: LisaL. Gottschalk Project manager: VickiKrug Associate med ia producer: Judi David Production supervisor: Enboge Chong Coordinator of freelance design: David W Hash Cover designer: Maureen McCutcheon Cover image: ©FPG International/Alan Kearney Senior photo research coordinator: CarrieK Burger Photo research: LouAnn K Wilson Supplement coordinator: BrendaA. Ernzen Compositor: Shepherd, Inc. Typeface: 11/12 A Garamond Printer: Quebecor Printing Book Group/Dubuque, IA

CHAPTER 2

Library of Congress Cataloging-in-Publication Data

The Earth's Interior 26

McGeary, David. Physical geology: Earth revealed. - 4th ed. I David McGeary, Charles C. Plummer, Diane H. Carlson. p. cm. "This book contains the same text and illustrations as the upda ted version of the eighth edit ion of Physical geology by Plummer, McGeary and Carlson. The chapter order has been chang ed so that. featured as the companion text to 'Earth revealed intro ductory geology,' a PBS television course and video resource produced in collaboration with the Annenburg/CPB Projeet"-Pref Includes index . ISBN 0-07-366183-X (acid-free paper ) 1. Physical geology. 1. Plummer, Charl es c., 1937II. Carlson, Diane H. III . Earth revealed (Television program) . IV Titl e. QE28.2.M34 551-dc21

2001 00-0339 37 CIP

INTERNATIONAL EDITION ISBN 0-07-118082-6 Copyright © 2001. Exclusive rights by The McGraw-Hill Companies, Inc., for manufacture and export. This book cannot be re-exported from the country to which it is sold by McG raw-H ill. The International Edition is not available in North America. www.mhhe .com

2

Who Needs Geology? 4 Avoiding Geologic Hazards 6 Supplying Things We Need 7 Protecting the Environment 8 Understanding Our Surroundings 11 An Overview of Physical Geology-Important Concepts 11 Internal Processes: How the Earth's Internal Heat Engine Works 14 T he Earth's Inter ior 15 The Theory of Plate Tectonics 16 Surficial Processes: The Earth's External Heat Engine 18 Geologic Time 21 Summary 22 Terms to Remember* 23 Testing Your Knowledge* 23 Expanding Your Knowledge* 24 Exploring Resources" 24 Interacting with Journey T hrough Geology CD -ROM* 25

Evidence from Seismic Waves 28 The Earth's Int ernal Structure 29 The Earth's Crust 30 The Mantle 30 The Core 32 Isostasy 35 Gravity Measurements 37 The Earth's Magnetic Field 39 Magnetic Reversals 40 Magnetic Anomalies 41 Heat Within the Earth 44 Geothermal Gradient 44 Heat Flow 45 Summary 45

*T hese end-of-chap ter sectio ns appear in every chap ter.

CHAPTER 3 The Sea Floor 48 Ori gin of the Ocean 50 Methods of Studying the Sea Floor 51 Featu res of the Sea Floor 52 Continental Shelves and Continental Slopes 52 Submarine Canyons 54 Turbidity Currents 55 Passive Continental Margins 55 The Continental Rise 55 Abyssal Plains 56 Active Continental Margins 56 Oceanic Trenches 57 T he Mid-O ceanic Ridge 57 Geologic Activity on the Ridge 58 Biologic Activity on the Ridge 58 Fracture Zones 61 Seamounts, Guyots, and Aseismic Ridges 61 Reefs 62 Sediments of the Sea Floor 64 O ceanic Crust and Ophiolites 64 The Age of the Sea Floor 66 The Sea Floor and Plate Tectonics 67 Sum mary 67

CHAPTER 4 Plate Tectonics 70 The Early Case for Continental Drift 72 Skepticism about Continental Drift 74 Paleomagnetism and the Revival of Continental Drift 75 Recent Evidence for Continental Drift 76 History of Continental Positions 77

v

Sea-Floor Spreading 77 Hess's Dr iving Force 78 Explanations 78 Plates and Plate Motion 79 How Do We Know T hat Plates Move? 80 Marine Magnetic Anomalies 80 Ano ther Test: Fracture Zones and Transform Faults 83 Measuring Plate Motion D irectly 84 Divergent Plate Boundaries 84 Transform Boun daries 87 Conve rgent Plate Boundaries 88 O cean-O cean Convergence 89 O cean-Continent Convergence 90 Continent -Continent Convergence 92 Backarc Spreading 93 The Motion of Plate Boundaries 94 Plate Size 94 The Attractiveness of Plate Tectonics 94 What Causes Plate Motions? 95 Mantle Plumes and Hot Spots 98 The Relationship Between Plate Tectonics and Ore Deposits 100 A Final Note 104 Summary 104

CHAPTERS Mountain Belts and the Continental Crust 108 Characteristics of Major Mountain Belts III Size and Alignment III Ages of Mountain Belts and Continents III Thickness and Characteristics of Rock Layers 113 Patterns of Folding and Faulting 114 Metamorphism and Plutonism 116 Normal Faulting 116 Thickness and Density of Rocks 116 Features of Active Mountain Ranges 117 The Evolution of a Mountain Belt 117 The Accumulation Stage 117 The Orogenic Stage 118 T he Uplift and Block-faulting Stage 121 The Growth of Continents 126 Suspect and Exotic Terranes 127 Summary 130

CHAPTER 6 Geologic Structures 132 Tectonic Forces at Work 134 Stress and Strain in the Earth's Crust 134 Behavior of Rocks to Stress and Strain 135 Present Deformation of the Crust 136 Structures as a Record of the Geologic Past 137 Geologic Maps and Field Methods 137 Contents

Folds 139 Geometry of Folds 139 Interpreting Folds 142 Fractures in Rock 143 Joints 144 Faults 146 Summary 155

CHAPTER 7 Earthquakes 158 Causes of Earthquakes 160 Seismic Waves 162 Body Waves 162 Surface Waves 162 Locating and Measuring Earthquakes 162 Determi ning the Location of an Earthquake 165 Measuring the Size of an Earthquake 166 Location and Size of Earthquakes in the United States 169 Effects of Earthquakes 171 Tsunamis 175 World Distribution of Earthquakes 177 First-Motion Studies of Earthquakes 180 Earthquakes and Plate Tectonics 180 Earthquakes at Plate Boundaries 181 Subduction Angle 184 Earthquake Prediction 184 Earthquake Control 185 Summary 187 t

CHAPTER 8 Time and Geology 190 The Key to the Past 192 Relative Time 192 Principles Used to Determine Relative Age 192 Correlation 197 The Standard Geologic Time Scale 200 Unconfo rmities 201 Disconformities 201 Angular Unconformities 203 Nonconformities 204 Numerical Age 204 Isotopic Dating 204 Uses of Isotopic Dating 208 Combining Relative and N umerical Ages 208 Age of the Earth 209 Comprehending Geologic Time 209 Summary 213

CHAPTER 9 Atoms, Elements, and Minerals 216 Atoms and Elements 218 Chemical Activity 220

Ions 220 Chemical Composition of the Earth's Crust 221 Crystallinity 222 The Silicon-Oxygen Tetrahedron 223 Minerals 227 Crystalline Solids 227 Natural and Inorganic Substances 227 Definite Chemical Composition 227 The Important Minerals 228 The Physical Properties of Minerals 230 Color 230 Streak 230 Luster 230 Hardness 230 External Crystal Form 231 Cleavage 233 Fracture 235 Specific Gravity 235 Other Properties 236 Simple Chemical Tests 236 The Rock Cycle 237 Summary 239

CHAPTER 10 Volcanism and Extrusive Rocks 242 Effects on Humans 244 The Growth of Hawaii 244 Geothermal Energy 244 Effect on Climate 244 Volcanic Catastrophes 247 Eruptive Violence and Physical Cha racteristics of Lava 248 Extrusive Rocks and Gases 249 Scientific Investigation of Volcanism 249 Gases 250 Extrusive Rocks 250 Types of Volcanoes 253 Shield Volcanoes 254 Cinder Cones 254 Composite Volcanoes 256 Volcanic Domes 259 Lava Floods 261 Subma rine Eruptions 262 Pillow Basalts 263 Summary 265

CHAPTER 11 Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks 268 Igneous Rocks 271 Identification ofIgneous Rocks 271 Varieties of Granite 274 Chemistry of Igneous Rocks 274 Intrusi ve Bodies 275

Shallow Intrusive Structures 276 Intrusives That Crystallize at Depth 277 Abundance and Distribution of Plutonic Rocks 279 How Magma Forms 279 Heat for Melting Rock 280 Factors That Control Melting Tempera tures 280 How Magmas of Different Compositions Evolve 282 Differentiation and Bowen's Reaction Theory 282 Partial Melting 284 Assimilation 285 Mixing of Magmas 285 Explaining Igneous Activity by Plate Tectonics 286 Igneous Processes at Divergent Boundaries 286 Int raplate Igneous Activity 286 Igneous Processes at Convergent Boundaries 287 Summary 289

CHAPTER 12 Weathering and Soil 292 Weathering, Erosion, and Transportation 294 How Weatheri ng Alters Rocks 294 Effects of Weathering 294 Mechanical Weathering 294 Frost Action 294 Abrasion 296 Pressure Release 297 Chemical Weathering 298 Role of Oxygen 298 Role of Acid 299 Solution Weathering 300 Chemical Weathering of Feldspar 301 Chemical Weathering of Other Minerals 302 Weathering and Climate 303 Weathering Products 303 Soil 303 Soil Horizons 304 Residual and Transported Soils 304 Soils, Parent Rock, Time, and Slope 305 Organic Activity 306 Soils and Climate 306 Buried Soils 307 Summary 308

CHAPTER 13 Mass Wasting 310 Classification of Mass Wasting 312 Rate of Movement 312 Type of Material 312 Type of Movement 314 Controlling Factors in Mass Wasting 315 Gravity 316 Water 316 Common Types of Mass Wasting 317 Creep 317 Debris Flow 318 Contents

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Rockfalls and Rockslides 324 Preventing Landslides 327 Preventing Mass Wasting of Debris 327 Preventing Rockfalls and Rockslides on Highways 328 Summary 330

CHAPTER 14 Sediments and Sedimentary Rocks 332 Sediment 334 Transportation 334 Dep osition 335 Preservation 336 Lith ification 337 Types of Sedimentary Rocks 338 Clastic Rocks 338 Breccia and Conglomerate 338 Sandstone 339 The Fine-Gr ained Rocks 340 Chemical Sedimentary Rocks 342 Carbonate Rocks 342 Chert 345 Evaporites 346 Organic Sedimentary Rocks 347 Coal 347 The Origin of Oil and Gas 347 Sedimentary Structures 347 Formations 351 Interpretation of Sedimentary Rocks 352 Source Area 352 Environment of Dep osition 354 Plate Tectonics and Sedimentary Rocks 357 Summary 358

CHAPTER 16

CHAPTER 18

Streams and Floo ds

Deserts and Wind Action 446

384

The Hydrologic Cycle 386 Channel Flow and Sheet Flow 386 Drainage Basins 388 Drainage Patterns 388 Factors Affecting Stream Erosion and D eposition 389 Velocity 389 Gradient 390 Channel Shape and Roughness 390 Discha rge 391 Stream Erosion 392 Stream Transportation of Sediment 393 Stream Deposi tion 394 Bars 394 Braided Streams 395 Meandering Streams and Point Bars 399 Flood Plains 401 Deltas 403 Alluvial Fans 405 Flooding 406 Urban Flooding 406 Flash Floods 409 Cont rolling Floods 410 T he Great Flood of 1993 411 Stream Valley Development 412 Downcutting and Base Level 412 The Concept of a Graded Stream 414 Lateral Erosion 415 Headward Erosion and Stream Piracy 415 Stream Terraces 416 t Incised Meanders 417 Superposed Streams 419 Summary 419

CHAPTER 15 Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks 362 Factors Controlling the Characteristics of Metamorphic Rocks 364 Composition of the Parent Rock 364 Temperature 365 Pressure 365 Fluids 367 Time 368 Classification of Metamorphic Rocks 368 Types of Metamorphism 368 Contact Metamorphism 368 Regional Metamorphism 370 Plate Tectonics and Metamorphism 374 Hydrothermal Processes 375 Hydrothermal Activity at Divergent Plate Boundaries 375 Metasomatism 377 Hydrothermal Rocks and Minerals 378 Sources of Water 379 Summary 381

iii

Contents

CHAPTER 17 Ground Water 422 Porosity and Permeability 424 The Water Table 424 The Movement of Ground Water 426 Aquifers 427 Wells 428 Springs and Streams 430 Pollution of Ground Water 430 Balancing Withdrawal and Recharge 436 Effects of Ground-Water Action 438 Caves, Sinkho les, and Karst Topography 438 Other Effects 440 Ho t Water Unde rground 441 Geothermal Energy 442 Summ ary 443

Distribution of Deserts 448 Some Characteristics of Deserts 448 Desert Features in the Southwestern Unit ed States 451 Wind Action 454 Wind Erosion and Transportation 456 Wind Deposition 459 Summary 464

CHAPTER 19 Glaciers and Glaciation 468 T he T heory of Glacial Ages 470 Glaciers-Where They Are, How T hey Form and Move 470 Distribution of Glaciers 470 Types of Glaciers 471 Formation and Growth of Glaciers 471 Movement of Valley Glaciers 474 Movement of Ice Sheets 479 Glacial Erosion 480 Erosional Landscapes Associated with Alpine Glaciation 481 Erosional Landscapes Associated with Continental Glaciation ' 484 Glacial Deposition 485 Moraines 486 Outwash 488 Glacial Lakes and Varves 489 Effects of Past Glaciation 489 The Glacial Ages 489 Direct Effects of Past Glaciation in North America 491 Indirect Effects of Past Glaciation 496 Evidence for Older Glaciation 496 Summary 497

CHAPTER 20 Waves, Beaches, and Coasts 500 Water Waves 502 Surf 503 Nearshore Circu lation 504 Wave Refraction 504 Longshore Currents 504 Rip Currents 505 Beaches 505 Longshore Drift of Sediment 506 Human Interference with Sand Drift 508 Sources of Sand on Beaches 508 Coasts and Coastal Features 508 Erosional Coasts 509 Depo sitional Coasts 512 Drowned Coasts 513 Uplifted Coasts 516

Coasts Shaped by Organisms 516 Summary 517 .

CHAPTER 21 Geologic Resources 520 Types of Resources 522 Resources and Reserves 523 Energy Use 523 O il and Natu ral Gas 523 The O ccurrence of Oil and Gas 524 Recovering the Oil 526 H ow Much Oi l Do We Have Left? 527 Heavy Crude and O il Sands 528 Oi l Shale 528 Coal 529 Varieties of Coal 529 Occ urrenc e of Coal 530 Environment al Effects 532 Reserves and Resources 532 Uraniu m 532 Alternative Sources of Energy 533 Metals and Ores 533 Origin of Metallic Ore Deposits 533 Ores Associated with Igneous Rocks 534 Ores Formed by Surface Processes 535 Metal Ores and Plate Tectonics 536 Mining 536 Environmental Effects 537 Some Important Metals 538 Iron 538 Copper 538 Aluminum 538 Lead 539 Zinc 539 Silver 539 Gold 539 Other Metals 540 Nonmetallic Resources 540 Construction Materials 540 Fertilizers and Evaporites 541 Other Nonmetallics 542 Some Future Trends 542 Summary 543

Appendixes A-G 546 Glossary 556 Index 570

Contents

ix

hysical Geology: Earth Revealed is a straightforward, easy-

P

to-read introduction to geology both for nonscience majors and for students contemplating majoring in geology. This book contains the same text and illustrations as the updated version of the eighth edition of Physical Geology by Plumm er, McGeary, and C arlson. The chapter order has been changed so that internal processes (plate tectonics, earthqu akes, etc.) are covered in the first part of the book and external process (rivers, glaciers, etc.) are described toward the end of the book. This ordering is favored by many geology instructors. 'Physical Geology: Earth Revealed is featured as the com panion text to Earth Revealed Introductory Geology, a PBS television cour se and video resource produced in collaboration with the Annenberg/CPB Project. Earth Revealed is a series of twenty-six half-hour video programs organized around the chapte rs of this text. The television programs document eviden ce of geologic pri nciples at geograph ically diverse sites, .often using a case study approach. Videocassett es can be pur chased individually or as a thirteen- tape set. A Study Guide and Faculty Guide are also available to supp lement the programs. For information regarding the use of Earth Revealed Introductory Geologyas a television course, or to purchase videocassett es for institutio nal or classroom use, contact The Annenbergl CP B Mu ltimedia Co llection at I-S00-LEARNER. The book contains more information than can normall y be covered duri ng a college term . This provides flexibility for the instruc tor who wishes to emphasize some topics while covering other topi cs superficially. It is also useful to the student who wants to pursue topi cs beyond what is covered in the classroom. T his edition greatly expands and improves upon the use of electro nic resources. Also int egrated into this edition is David McCo n nell's The Good Earth, an Internet Resource for Int rodu cto ry Geology. This digital method of teaching will give students a more "h ands -on" approach to learning geology. The Good Earth is organized into chapters, with animations used to explain certain proc esses. Chapter summaries, quizzes, exercises, and web links to related websites are also included. With the purchase of a new textb ook, the student will gain access to this resource, whi ch can be found at http:// www.mhhe .com /earthsci/geology/m cconnell/ . Look for the Post-it notes on chapter opener pages to find out how The GoodEarth can help you understand geology. Journey Through Geology (the two CD-ROMs that accompany this book) is an exciting supp lem ent. This was pro duced in partnership with The Smithsonian Institution. "Int eracting with Journey Through Geology" at the end of each chapter has que stions to help the stu dent get the maximum ben efit from use of th e CDs. The Internet section at the end of each chapter should make it easy and meaningful for the student to enrich his or her knowledge through the World Wide Web. We have listed websites that we personally checked for usefulness. T he universal resource locators (URLs) are printed in blue and easy to read. However, typing in a lengthy URL will not be necessary as we have the sites listed as links on the book's website. T he user need only click on the link. Appropriate new websites that are discovered after publication will be added to this book's website.

x

Obsolete or defunct websites will be so noted. To help students effectively and efficiently use the Inte rnet from the website, we include step-by -step procedures and pose questions. The primary purpose of the questions is to guid e students through th inking about the topic at hand. We expect that many students will explore top ics beyond where we have let them. Some of the changes we made for the fourth edition follow. Recent major disasters, such as the devastating earthquake in Turkey and the tsunami in New Guinea are described. We have taken a number of descriptions and examples of geologic resources from the final chapter of the book and integrated them into appropriate chapters elsewhere in the book. The rock cycle (in chapter 9) has been expanded to include a plate tectonic example . The discussion of the origin of magmas at convergent boundaries (chapter 11) places more emphasis on the current view by researchers that mafic magmas are generated in the asthenosphere above the subducted oceanic crust. All diagrams showing magma generation at convergent boundaries were redone . In the chapter on geologic time we have int roduced the term "actualisrn" and discussed why it might be preferable to "uniformitarianism." Lateral continuity and inclusions have been added to th e princ iples used for determining relative time relationships. In the met amorphic chapter we have related foliation to the modern concept of gravitational collapse and spreading. We moved unconformities from the structural geology chapter to the geological time chapter. We now use "numerical age" rather than "absolute age" in the geologic time chapter. A section on changing concepts of the age of the earth has been added. The relationsh ip between isotop ic dating and the geologic time scale has been expanded. T he 1996 (and 1999) rockfall at Yosemite is used as an example of mass wasting. The stream chapter underwent a major revision and now includes an expanded discussion of flooding with examples from the 1997 floods in the upper Midwest and California. In the glaciation chapte r we clarify and expand on the conversion of snow to glacier ice. In the structure chapter, the section on stress and strain and the behavior of rocks was rewritten and new examples and figures are included to clarify these difficult concepts. Love and Rayleigh waves are now discussed and illustrated in the earthquake chapter. In the chapter on mountains and the cont inental crust we added a section on the disparity of the height of the Rocky Mountains and the thickness of the crust and describe recent work th at indicates that the Basin and Range Mountains were three kilometers higher than at present . An appendix listing commonly used prefixes, suffixes, and root words was added . The geologic map of North Ame rica was moved from the append ix to the inside front cover. We added new boxes on water and ice- m olecules and crystals, flight hazards associated with volcanoes, the eruptions on Montserrat compared to the disastrous eruption that destroyed St. Pierre on Martinique in 1902, the Bingham Canyon copp er m ine, highlights of biological evoluti on th rough tim e, and the meteorite from M ars with possible signs of fomer life. A box on water bene ath glaciers describes the recentl y discovered lake beneath the East An tarctic Ice Sheet, surging glaciers, and subglacial volcanism and floodi ng in Iceland . T he stream chapter includes boxes on the plan ned flood in th e Gra nd Canyo n

and how the recurrence interval of large floods is calculated. The structure chapter includes a box on how to find oil and the salt dome box has been expan ded. The inte rior of the earth chapter now includes a box on the spinnin g inner core. In th e mountains chapter we add ed a box on a systems approach to understanding mountains and expanded a former box, retitling it "Dance of th e Continents (with SWEAT )." T he fourth edition has a new look with many of the diagrams redone or replaced. New photos have also been add ed and include the volcanic erup tio n at Soufriere, Montserrat, the 1997 Yosemite rockfall, glacially carved features in the Teton range, and giant stream ripples. Photos of many rocks and min erals and geologic structures have been replaced.

Supplements to Accompany Physical Geology: Earth Revealed, Fourth Edition: • • • •

Journey Through Geology two CD-ROM set Instructor's Manual computerized testing software 224 transparenciesand 350 slides Visual Resource Library CD-ROM • Student Study Guide • Physical Geology and Journey Through Geology websites

John M. Alderson Marymount College N. L. Archbold western Illinois University Victor R. Baker The UniversityofTexas Joan Baldwin EI Camino College Alexander R. Ball Los AngelesValley College Paul G. Bauer Cuesta College Kenneth A. Beem Montgomery College Robert E. Behling western Virginia University Terrill R. Berkland Central Missouri State University David J. Berner NormandaleCommunity College Peter E. Borella Riverside City College Ted Bornhorst Mi chigan Technological University David P. Bucke University of Vermont Reid L. Buell Caltrans Gary Carlson Midland Lutheran College Roseann Carlson Tidewater Community College Greg S. Conrad Sam Houston State University Peter Copeland UniversityofHouston Kevin Cornw ell California State Universityat Sacramento Larry Davis St. John's University Paul A. Dike Glassboro State College Steven F. Dodin Community Collegeof Allegheny County Robert ] , Elias University of Manitoba Stanley C. Fagerlin Southwest Missouri State University Peter Fisher California State University Ronald C. Flemal Northern Illinois University Richard A. Flory California State University at Chico Robert D . Forester Collin County Community College M. G. Frey University ofNew Orleans John S. Galehouse San Francisco State University Heath er L. Gallacher ClevelandState University Lloyd Glawe Northeast Louisiana University Andrew J. Hajash Texas A&M

Additional classroom tools include: TheAmerican GeologicalInstitute's Videodisc • JLM Visuals Physical Geology Photo CD • Interactive Plate Tectonics CD-ROM, Annual Editions: Geology98/99 • Student Atlas ofEnvironmental Issues • McGraw-Hill Learning Architecture For addi tional information on Physical Geology: Earth Revealed, or Journey Through Geology CD-ROM, please visit our Websites at http://www.mhhe.com/earthsci/geology/plummer or www.mhhe.com/jtg. We have tried to write a book that will be useful and exciting to students (and instructors). We would be grateful for any com m ents by users, especially regarding mistakes within th e text or sources of good geological photographs . We would like to tha nk Susan Slaymake r for writing the original boxed material on planetary geology, and Judi Kushick for writing the qu estions to accompany The Smithsonian In stitution'sJourney Through Geology CD-ROM. We are also very grateful to the following reviewers of this text for their careful evaluat ion and useful suggestions for improvement.

Frank M. Hanna California State UniversityNorthridge Stephen B. Harper East Carolina University Stephen L. Harris California State University at Sacramento Barry Haskell Los A ngeles PierceCollege Miles O. Hayes University of South Carolina Richard A. Heimlich Kent State University Timothy Horner California State University at Sacramento Mary S. Hubbard Kansas State University Roy L. Ingram University ofNorth Carolina Clark M. Johnson Universityof WisconsinM adison Norris W Jones UniversityofWisconsinOshkosh Manfred Kehlenbeck Lakehead University James G. Kirchner Illinois State University Lawrence W Knigh t William Rainey Harper College Ronald H. Konig University ofArkansas Albert M. Kudo The UniversityofNew Mexico Howard Level Venttl1tl College David N. Lumsden M emphis State University Harmon D. Maher, Jr. Universityof Nebraska at Omaha Donald Marchand, J r. Old Dominion Uniuersity Kathleen Marsaglia University ofTexas at EI Paso James McLelland Colgate University William S. McLoda M ountain View College Margaret E. McMillan Universityof A rkansas at Little Rock C. Daniel Miller U.S. Geological Survey William D. Orndorff Concord College Bruce C. Panuska M ississippi State University Jacqueline Patterson CaliforniaState University, Fullerton David R. Privette Central Piedmont Community College Frederick J . Rich Georgia Southern University

Gary D. Rosenberg Indiana UniversityPurdue University at Indianapolis Robert A. Schiffman Bakersfield College Vernon P. Scott Oklahoma State University Barbara Sherriff University of Winnipeg Charles R. Singer Youngstown State University Kenneth G. Smith Dallas Baptist University Wtlliam A. Smith Grand Valley State University Richard Smosna Universityofwest Virginia Steven Stearns College ofCharleston Don W Steeples UniversityofKansas Dion C. Stewart Adams State College M. Ali Tabidian California State University, N orthridge Norman W Ten Brink Grand Valley State College J. Robert Thompson Glendale Community College Daniel R. Tucker UniversityofSouthwestern Louisiana Sherwood D. Tuttle UniversityofIowa Kenneth J. Van Dellen Macomb Community College W R. Van Schmu s UniversityofKansas Stephen Wareham California State University at Fullerton Steph en H. Watts Sir Sanford Fleming College Will iam J. Wayne UniversityofNebraska Thomas H . Wolosz SUNY College of Plattsburgh A special thank you to the '97 GSA Focus Group: Scott Babcock western Washington University Drew Coleman Boston University Ralph Davis UniversityofArkansas Megan Jones UniversityofMinnesota and Inver Hills Community College Michael Katuna College of Charleston Peter Kresan UniversityofArizona Jame s Miller SouthwestMissouri State University

Preface

xi

The Good Earth 'Website has been integrated into the text. This web-based course provides interactive activities, animations, and current information that you can't receive anywhere else.

--.-----_-=:

T

he proven Learning System that has been successful for over 25 years now has the most advanced technology resources.

This updated Internet edition of Physical Geology has been thoroughly enhanced to bring you the most current information on how physical geology is working in your world today. O ne of the most exciting additions to this package is the incorporation of The

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Good Earth Website. T his visionary method of teaching gives students a more hands-on approach to learning geology by allowing them to view animations that explain processes, access quizzes and exercises, and explore web links. XlI

Visit this site at http://www.mhhe.com/plummer High-tech, interactive learning from The Smithsonian Institution. This two CD-ROM set is packaged free with every new text.

On every chapter opener page students will find a "post-it note" that directs them to The Good Earth Website. This interactive presentation of material will not only help students understand some of the more difficult concepts being presented, but it will intrigue them with geological phenomena.

xiii

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A hallmark of Physical Geology is the inclusion of timely topics. Recent geologic events like the devastating earthquake in Turkey and the tsunami in Papua, New Guinea are incorporated.

Additional support helps you make the grade. Use these helpful end-of-chapter learning aids to prepare for tests and quizzes.

Summary-overviews of chapter content. Terms to Remember-important terms to review and remember.

Testing fOur Knowledge- realistic sample tests you can use to prepare for exams and improve your grades.

Ifxpanding fOur Knowledge---questions that help you develop critical thinking skills.

Exploring Resources-Supplemental references in a number of different media.

\D Textbook reference To support these timely top ics, you'll find the text is filled with updated resources. These "ExploringResources" are found at the end of every chapter and provide you and your students with opportunities to expand you r kn owledge. Every chapter also includes "Interacting withJourney Through Geology CD-ROM" questions that help tie the concepts of the chapter to modules of Th e Smithsonian Institution's Journey Through Geology two CD-ROM set.

~ CD-ROM a::::D Videotape

L~~ World Wide Web addresses

Interacting withJourney Through Geology CD-ROM---questions ~that

help tie the concepts of the chapter to modules of The Smithsonian Institution's Journey Through Geology two CD-ROM set.

Learn more about this text. Visit the Physical GeologyWebsite: www.

xiv

1

Introduction to Phx-sica! Geology

G

eology uses the scientific method to · explain natural aspects of the earth-for example, how mountains form or why oil resources are concentrated in some rocks and not in others. This chapter briefly

explains now and why the earth's surface, and its interior, are constantly changing. It relates this constant change to the major geological topics of interaction of the atmosphere, water and rock, the modern theo ry of plate tectonics, and geologic time. These concepts form a framework for the rest of the book. Understanding the "big picture" presented here will aid you in comprehending the chapters that follow.

Natural rock scu lpture in Paria Plateau , Arizona. Sandstone formed from ancient sand dunes. Running water has eroded the rock into the present distinctive shapes. Photo © Kerric k Jame s

3

R

Strategy for Using T~s Textbook • As authors, we try to be thorough in our coverage of

• •

•

• •

•

topics so the textbook can serve you as a resource. However, your instructor may choose to concentrate only on certain topics for your course. Find out which topics and chapters you should focus upon in your studying and concentrate your energies there. Your instructor may present additional material that is not in the textbook. Take good notes in class. Do not get overwhelmed by terms. (Every discipline has its own language.) Don't just memorize each term and its definition. If you associate a term with a concept or mental picture, remembering the term comes naturally when you understand the concept. (You remember names of people you know because you associate personality and physical characteristics with a name.) You may find it helpful to learn the meanings of frequently used prefixes and suffixes for geologic terms. These can be found in Appendix G. Boldfaced terms are ones you are likely to need to understand because they are important to the entire course. Italicized terms are not as important, but may be necessary to understand the material in a particular chapter. Pay particular attention to illustrations. Geology is a visually oriented science and the photos and artwork are at least as important as the text. You should be able to sketch important concepts from memory. Find out to what extent your instructor expects you to learn the material in the boxes. They offer an interesting perspective on geology and how it is used, but much of the material might well be considered optional for an introductory course and not vital to your understanding of major topics. Many of the "In Depth" boxes are meant to be challenging-do not be discouraged if you

need your instructor's help in understanding them. • Read through the appropriate chapter before going to class. Reread it after class, concentrating on the topics covered in the lecture or discussion. Especially concentrate on concepts that you do not fully understand. Return to previously covered chapters to refresh your memory on necessary background material. • Use the end of chapter material for review. The Summary is just that, a summary. Don't expect to get through an exam by only reading the summary but not the rest of the chapter. Use the Terms to Remember to see if you can visually or verbally associate the appropriate concept with each term. Answer the Testing Your Knowledge questions in writing. Be honest with yourself If you are fuzzy on an answer, return to that portion of the chapter and reread it. Remember that these are just a sampling of the kind of questions that might be on an exam. • Geology, like most science, builds on previously acquired knowledge. You must retain what you learn from chapter to chapter. If you forget or did not learn significant concepts covered early in your course, you will find it frustrating later in the course. (To verify this, turn to chapter 20 and you will probably find it intimidating; but if you build on your knowledge as you progress through your course, the chapter material will fall nicely into place.) • The Smithsonian Institution's Journey Through Geology CD-ROM accompanying this textbook is an exciting innovation. Try it out. The animation and film clips in it should provide a real boost to visualizing difficult concepts. • Be curious. Geologists are motivated by a sense of discovery. We hope you will be too.

Who Needs Geology? Imagine yourself as a student at California State University, Northridge (CSUN ) in greater Los Angeles . At 4:30 A.M. on January 17, 1994, you are jolted awake when your apartment begins to sway violently. Dishes, bookcases, and ceiling plaster crash to the floor. The noi se and shaking are terrifying as you struggle to stand up . In less than a minute the shaking stops and silence momentarily returns. You realize you survived an earthquake, but you are still scared and disoriented. This seismic event took place because of a sudden shift of bedrock eighteen kilometers beneath Northridge. Shock waves spread in all directions with damaging effects in many parts of Los Angeles. Northridge and vicinity suffered the heaviest damage. You feel fortunate, you are safe and your apartment building was not destroyed. Others nearby were not so fortunate. Two University students were among the sixteen killed in

4

Chapter 1

one of the forty apartment buildings that collapsed (figure 1.1). (Emergency crews drove right past one apartment building because it appeared undamaged; but it had collapsed straight downward, completely crushing the first floor but leaving the upper floors standing.) After the earthquake you and your neighbors do not have electricity (power was lost, temporarily, as far away as Edmonton, Canada). Water is in short supply because of broken water mains. Leaving the area is not a good option. Eleven major highway structures have been destroyed, including a segment of the Santa Monica Freeway, the nation's busiest highway. Despite the deprivations and the jitters caused by frequent aftershocks, you are comforted by how people pitch in to help each other. A tremendous sense of community grows from the shared adversity. http://www.mhhe.com/earthsci/geologylplummer

B

Figure 1.1 Damage from the Northridge earthquake. (A) The Northridge Meadows apartments where 16 people or roughly one-third of all those killed in the quake died when the first floor was crushed by the weight of the top two floors. (8) The parking garage at California State University, Northridge that was destroyed by the earthquake. Photo A © Michael Edwards/Los Angeles Times Syndicate; Photo B by Frank M. Hanna

Weeks later, the spring semester begins at CSUN. You are among the 27,000 students that must cope with the fact that all the buildings on campus were damaged and some were destroyed. But at least the most severely mangled building was a large, prefabricated parking structure (figure l.IB). Repairing your campus will take time and an estimated $350 million. Meanwhile, some of your classes are held outdoors, others in hastily erected temporary buildings. As the weeks pass, you share the pride of the university community and are determined that the quality of your education shall not be undermined by the earth-

quake and its damage. But your earthquake jitters continue even' though the aftershocks now are infrequent and barely perceptible. Sixty people were killed in the Northridge earthquake. The monetary cost was estimated at between $20 and $40 billion-financially the most costly natural disaster in North America's history. The loss in lives could have been far worse. For instance, 8,000 people died in the 1985 Mexico City earthquake and 5,000 were killed in 1995 at Kobe, Japan. The Northridge death toll was low due mainly to good planning. Preparing for major earthquakes has been an ongoing process in California for decades. The quake was expected because of what geologists have learned about how the earth works. Skyscrapers in the Los Angeles area survived because they were built to meet high standards for seismicresistance-the fruits of many decades of engineering studies and design. The apartment buildings that collapsed in Northridge probably would have survived if, during their construction, building codes had been enforced. It is likely that thousands of lives were saved because of adherence to building codes. Pillars for freeway bridges and underpasses were known to be vulnerable. The Santa Monica and other freeways were scheduled to have their pillars reinforced later in 1994, but the quake came first . Luck also played a role in keeping the casualties down. Los Angeles' normally densely packed freeways were almost empty, because of the early hour and it being a holiday, Martin Luther King 's birthday. (The San Francisco Bay area was equally lucky when, four years earlier, the Loma Prieta earthquake took place as the area's two baseball teams were about to begin a World Series game; the normally heavy rush hour traffic was the lightest in anyone's memory when a freeway and part of a bridge collapsed.) The Northridge earthquake was a reminder that our solid earth does not stand still. According to the theory of plate tectonics, the earth's rigid outer shell is broken into a series of plates. Adjoining plates may slide past, move away from, or collide with one another. Plates generally move from 1 to 18 centimeters a year. But the motion may not be smooth and continuous. Plates may be "locked" against one another for many years and move suddenly. Sudden motion along a fault caused the Northridge earthquake (figure 1.2). The mountain ranges that rise above Los Angeles are another product of relentless plate motion. It's as if a giant vise is slowly closing, forcing bedrock upward into mountains. The Santa Susana Mountains that border Northridge grew higher by 38 centimeters during the earthquake. The awesome energy released by an earthquake is a product of forces within the earth that move firm rock. Earthquakes are only one consequence of the ongoing changing of the earth. Ocean basins open and close. Mountain ranges rise and are worn down to plains through slow, but very effective, processes. Studying how the earth works can be as exciting as watching a great theatrical performance. Understanding th e changes that take place in and on the earth, and th e reasons for Introduction to Physical Geology

5

n contrast to the disaster in Colombia, geologists were much more successful at saving lives when Mount Pinatubo erupted in the Philippines in 1991. When minor steam eruptions began in April, 1991, Pinatubo was a vegetation-covered mountain that had last erupted 400 years earlier. As the eruptions intensified, Filipino geologists thought a major eruption might be developing. Geologic fieldwork completed in earlier years indicated that prehistoric eruptions of the volcano tended to be violent. Under a previous arrangement for cooperation, American geologists joined their Philippine colleagues and deployed portable seismographs to detect and locate small earthquakes within the volcano and tiltmerers for measuring the bulging of the volcano. Data were analyzed by state-of-the-art computer programs. Fortunately, it took two months for the volcano to reach its climactic eruption, allowing time for the scientiststo work with local officials and develop emergency evacuation plans. Geologists had to educate the officials about the principal hazardsmudflows and pyroclastic flows. Pyroclastic flows (as explained in the chapter on volcanoes) are hot, turbulent mixtures of exploding gases and volcanic ash that flow rapidly down the flanks of a volcano. Pyroclastic flows can reach speeds of over 100 kilometers per hour and are extremely destructive.

I Figure 1.2 Compression (indicated by large arrows) in the Los Angeles area causes movement along a fault and the January 17, 1994 earthquake.

those changes, is the challenging objective of geology, the scientific study of the earth. Physical geology is the division of geology concerned with earth materials, changes in the surface and interior of the earth, and the dynamic forces that cause those changes. Put another way, physical geology is about how the earth works. Earthquakes and other aspects of geology are interesting, but how does geology benefit you, as an inhabitant of this planet? Some of the ways are discussed next.

In June, explosions, ash eruptions and minor pyroclastic flows indicated that magma (molten rock) was not far underground and a major eruption was imminent. Some 80,000 people were evacuated from the vicinity of the volcano. The U.S. military evacuated and later abandoned Clark Air Force Base, which was buried by ash. The climactic eruption occurred on June 15 when huge explosions blasted the top off the volcano and resulted in large pyroclastic flows. Volcanic debris was propelled high into the atmosphere. A typhoon 50 km away brought heavy rains, which mixed with the ash and resulted in numerous, large mudflows. The estimated volume of magma that erupted from the climactic eruption is 5 km", making it the world's largest eruption since 1917. Its effects extended beyond the Philippines. Fine volcanic dust and gas blasted into the high atmosphere were carried around the world and would take years to settle out. For a while, we got more colorful sunsets worldwide. Because of the -filtering effect for solar radiation, worldwide average temperature was estimated to drop by 0.5°C for two years, more than countering the long-term warming trend of the earth's climate. The death toll from the eruption was 374. Of these, 83 were killed in mudflows. Most of the rest died because roofs collapsed from the weight of ash. In addition, 358 people died from illness' related to the eruptions. More than 108,000 homes were partly or totally destroyed. The death toll probably would have been in the tens of thousands had the prediction and warning system not been so successful. Related Web Resources: Volcano World http://volcano.und.nodak.edu/ The site contains a wealth of information on volcanoes, including Pinatubo.

Avoiding Geologic Hazards Geology can have a direct application in ensuring people's safety and well-being. For example, if you were building a house in an earthquake-prone area, you would want to know how to minimize danger to yourself and your home. You would want to build the house on a type of ground not likely to be shaken apart by an earthquake. You would want the house designed and built to absorb the kind ofvibrations given off by earthquakes. People instinctively regard volcanoes as dangerous. They are, but the hazards are not immediately apparent to the nongeologist. (One is not likely to get killed by a lava flow or by a boulder ejected from a volcano.) The 1985 eruption of Nevado del Ruiz in Colombia is a tragic example. No one died from the eruption itself, but 23,000 people were killed from the indirect effects. The hot rocks blasted out of the volcano caused part of the ice and snow capping the peak to melt. The

6

Chapter 1

gists had previously predicted such a mudflow could occur and published maps showing the location and extent of expected mudflows. The actual mudflow that wiped out the town matched that shown on the geologists' map almost exactly. Unfortunately, government officials had ignored the map and the geologists' report; otherwise the tragedy could have been averted. A case where geologists were successful at saving lives is described in Box 1.1. Geologic hazards, other than earthquakes and volcanoes, that geologists investigate include floods, wave erosion at coastlines, collapsing ground surfaces, and landslides. (In the United States and Canada, far more property and lives have been lost due to landslides and floods than to earthquakes and volcanoes.)

Figure 1.3 Armero, Colombia, after the 1985 mudflow. The buildings are the small portion of the town that survived the mudflow. Photo by U.S. Geological Survey

water mixed with loose rock on the flank of the mountain and flowed down stream channels as a mudflow. At the base of the volcano, the mudflow overwhelmed the town of Armero, killing most of its inhabitants (figure 1.3). Colombian geolo-

Supplying Things We Need We depend upon the earth for energy resources and the raw materials we need for survival, comfort, and pleasure. The earth, at work for billions of years, has distributed material into localized concentrations that humans can mine or extract. By learning how the earth works and how different kinds of substances are distributed and why, we can intelligently search for metals, sources of energy, and gems. Even maintaining a supply of sand and gravel for construction purposes depends upon geology. The economic systems of Western civilization currently depend on abundant and cheap energy sources. Nearly all our vehicles and machinery are powered by petroleum, coal, or nuclear power and depend on energy sources concentrated unevenly in

Introduction to Physical Geology

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7

hen the tanker Exxon Valdez ran aground in 1989, over 240,000 barrels of crude oil were spilled into the waters of Alaska's Prince William Sound. It was the worst ever oil spill in U.S. waters. The spill, with its devastating effects on wildlife and the fishing industry, dramatically highlighted the conflicts between maintaining the energy demands of the American economy and conservation of the environment. The Exxon Valdez was only one of many tankers that carry oil from the southern end of the Alaskan pipeline to refineries on the west coast of the United States. Oil spills, such as this one, were predicted by the 1972 environmental impact statement for the Alaska pipeline prepared by the U.S. Geological Survey. In the late 1970s the United States was importing almost half its petroleum, at a loss of billions of dollars per year to the national economy. (By 1997, the United States was importing more than half of the petroleum it uses.) This drain on the country's economy and the increasing cost of energy can be major causes of inflation, lower industrial productivity, unemployment, and the erosion of standards of living. In the 1960s, geologists discovered oil beneath the shores of the Arctic Ocean on Alaska's North Slope. It is now the United States' largest oil field. Thanks to the Alaska pipeline, completed in 1977, Alaska has supplied as much as 200/0 of the United States' domestic oiL Despite its important role in the American economy, some people consider the Alaska pipeline and the use of tankers as unacceptable threats to the area's ecology. The

Box 1.1 Figure 1 The major eruption of Mount Pinatubo on June 15, 1991 as seen from Clark Air Force Base, Philippines. Photo by Robert Lapo inte . U.S. Air Force

the earth. The United States economy in particular is geared to petroleum as a cheap source of energy. In a few decadesAmericans have used up most of the country's known petroleum reserves, which took nature hundreds of millions of years to store in the earth. Americans are now heavily dependent on imported oil. (The GulfWar of 1991 was at least partially fought becauseof the industrialized nations' petroleum requirements.) To find more of this diminishing resource will require more money and increasingly sophisticated knowledge of geology. Although many people are not aware of it, we face similar problems with diminishing resources of other materials, notably metals such as iron, aluminum , copper, and tin, each of which has been concentrated in a particular environment by the action of the earth'sgeologicforces.

Protecting the Environment Our demands for more energy and metals have, in the past, led us to extract them with lit tle regard for effects on th e 8

Chapter 1

balance of nature within the earth, and therefore on us, earth's residents. Mining of coal, if done carelessly, for example, can release acids into water supplies. Understanding geology can help us lessen or prevent damage to the environment-just as it can be used to find the resources in the first place. The environment is further threatened because these are nonrenewable resources. Petroleum and metal deposits do not grow back after being harvested. As demands for these commodities increase, so does the pressure to disregard the ecological damage caused by the extraction of the remaining deposits. Problems involving petroleum illustrate this. Oil companies employ geologists to discover new oil fields, while the public and government depend on other geologists to assess the potential environmental impact of petroleum's removal from the ground, the transportation of petroleum (see Box 1.2), and disposal of any toxic wastes from petroleum products.

Box 1.2 Figure 1 Road bulldozed during the winter in Arctic Alaska became a useless quagmire after thawing. Photo by U.S. Geological Survey

1989 oil spill demonstrated the hazards of the marine portion of the oil transportation system. Oil spilled from a ruptured pipe could have a devastating effect on the fragile Arctic plant and animal life. Geologists with the U.S. Geological Survey conducted the official environmental impact investigation of the proposed pipeline route. After an exhaustive study, they recommended against its construction, partially because of the hazards to oil tankers and partially because of the geologic hazards of the pipeline route. Their report was overruled. The Congress and the President of the United States exempted the pipeline from laws that require a favorable environmental impact statement before a major project can begin. The 1.5 to 2 million barrels of oil a day that flow from the Arctic oil fields mean that over $10 billion a year that would have been lost through the purchase of foreign oil instead remains in the American economy. The 1,250-kilometer-Iong pipeline, through which 88,000 barrels of oil an hour flow, crosses regions of icesaturated, frozen ground and major earthquake-prone

Introduction to Physical Geology

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9

Box 1.2 Figure 2 The Alaska pipeline. Photo © Steve McCutcheon/Alaska Pictorial Service

mountain ranges that geologists regard as serious hazards to the structure. Building anything on frozen ground creates problems. For example, the road in box figure 1 was built during the exploration of Alaska's North Slope oil fields. When the road was being built, the protective vegetation was scraped off During the summer thaw the road became a quagmire. As thawing continued, the flooded tracks shown in the pho to grew into ponds so that the road will never be passable.

Understanding Our Surroundings

Building the pipeline over such terrain also presented enormous engineering problems. If the pipeline were placed on the ground, the hot oil flowing through it could melt the frozen ground. On a slope, mud could easily slide and rupture the pipeline. Careful (and costly) engineering minimized these hazards. Much of the pipeline is elevated above the ground (box figure 2). Radiators conduct heat out of the structure. In some places refrigeration equipment in the ground protects against melting. Records indicate that a strong earthquake can be expected every few years in the earthquake belts crossed by the pipeline. An earthquake could rupture a pipelineespecially a conventional pipe as in the original design. However, when the Alaska pipeline was built, in several places sections were specially jointed to allow the pipe to shift as much as 6 meters without rupturing. The original estimated cost of the pipeline was $900 million, but the final cost was $7.7 billion, making it the costliest privately financed construction project in history. The redesigning and construction that minimized the potential for an environmental disaster were among the reasons for the increased cost. There have been some minor spills from the pipeline. For instance, in January 1981, 5,000 barrels of oil were lost when a valve ruptured. Thus far, the pipeline company's claim that there is virtually no chance of a major spill from a pipeline rupture seems vindicated. However, the risks of marine transportation remain. In hindsight, a pipeline through Canada to the American Midwest might have been better in the long run (as advocated by geologists who prepared the environmental impact statement). Although longer, it would traverse less hazardous terrain, would avoid the long marine leg, and would bring the oil to refineries in the central part of the United States. A joint venture with Canada would have benefited both countries, particularly since Canada has discovered major petroleum resources in its Arctic territory.

Understanding geology leads to a greater appreciation of your surroundings. If, for instance, you were traveling through the Canadian Rockies: you might see the scene in figure 1.4 and wonder how the landscape came to be. You might wonder: (1) why there are layers in the rock exposed in the cliffs; (2) why the peaks are so jagged; (3) why there is a glacier in a valley carved into the mountain; (4) why this is part of a mountain belt that extends northward and south ward for tho usands of kilometers; (5) why there are mountain ranges here and not in the central part of the continent. After completing a course in physical geology, you should be able to answer these questions as well as understand how other kinds of landscapes formed. People who have completed a physical geology course find that their understanding of geological processes makes travel much more interesting. Figure 1.5 is a map showing the landforms of the contiguous United States and southernmost Canada. Major features discussedor depicted in this book are named with black lettering. (In addition, on the inside, front cover is a generalized geologicmap of North America that will be a useful reference as you progress through this book.) In your future air or surface travel, you may want to take along your book with these maps to enhance your appreciation of the scenery.

An Overview of Physical Geology-Important ' Concepts

Related Web Resources http ://www.alyeska-pipe.com/

Figure 1.4 Mount Robson, the highest peak in the Canadian Rocky Mountains. Photo © Superstoc k

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Chapter 1

www.mhhe.com/earthsci/geology/plummer

The remainder of this chapter is an overview of physical geology that should provide a framework for most of the material in this book. Although the concepts probably are totally new to you, it is important that you comprehend what follows. You may want to reread portions of this chapter while studying later chapters when you need to expand or reinforce your comprehension of this basic material. The earth can be visualized as a giant machine driven by two engines, one internal and the other externaL Both are heat engines, devices that convert heat energy into mechanical

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energy. Two simple heat engines are shown in figure 1.6 . An automobile is powe red by a heat engine. When gasoline is ignited in th e cylinders, the resulting hot gases expand, driving pistons to th e far end of cylinders. In this way, the heat energy of the expa nding gas has been converted to the mechanical energy of the moving pistons, then tra nsferred to the wheels , where the energy is put to work m oving the car. The internal heat engine ofthe earth is powered by heat moving from the hot interior of the earth toward the cooler exterior. Moving plates and earthquakes are produ cts of this heat engine. The earth's external heat engine is driven by solar power and gravity. Heat from the sun provides the ene rgy for circulating the atmosphere and oceans. Water, especially from the 14

Chap ter 1

Movement of wax due to density differences caused by heating and cooling.

oceans, is evaporated due to solar heating. When moist air cools, rain or snow falls. Over long periods of time, moisture at the earth's surface helps rock disintegrate. Water washing down hillsides and flowing in streams loosens and carries away the rock particles. In this way mountains originally raised by the earth's internal forces are worn away by processes driven by the external heat engine. We will look at how the earth's heat engines work and show how some of the major topics of physical geology are related to the surficial (on the earth's surface) and internal processes powered by die heat engines.

Figure 1.8 Cross section through the earth. Expanded .section shows the relationship between the two types of crust , the lithosphere and the asthenosphere, and the mantle. The crust ranges from five to seventy-five kilome ters thick.

Internal Processes: How the Earth's Internal Heat Engine Works The earth's internal heat engine works because hot, buoyant material deep within the earth moves slowly upward toward the cool surface and cold, denser material moves downward. Visualize a vat of hot wax, heated from below (figure 1.7). As the wax immediately above the fire gets hotter, it expands, becomes less dense (that is, a given volume of the material will weigh less) and will rise. Wax at the top of the vat loses heat to the air, cools, contracts, becomes denser, and will sink. A similar process takes place in the earth's interior. Rock that is deep within the earth and is very hot rises slowly toward the surface while rock that has cooled near the surface is denser and sinks downward. Instinctively, we don't want to believe that rock can flow like hot wax. However, experiments have shown that, under the right conditions, rocks are capable of being molded (like wax or putty). The hotter a roc k and the greater the pressure on the rock, the mo re likely a rock is to defo rm . Pressure and temperature, even a few kilom eters beneath the eart h's surface, can be high enough so th at rock can flow very slowly. http://www.mhhe.com/earthsci/geologylplummer

Photo by NASA

The Earth's Interior Rock flowage is believed to take place in part of the interior of the earth in the zone known as the mantle, the largest, by volume, of the earth's three majo r concentric zones (see figure 1.8). The mantle is solid (except in a few spots) and probably composed of rock not very different from some kinds of rock found at the earth's surface. The other two zones are the crus t and the core. The crust of the earth is analogous to the skin on an apple. The thickness of the crust is insignificant compared to the whole earth. We have direct access to only th e crust, and not much of the crust at that. We are like microbes crawling on an apple, without the ability to penetrate its skin. Because it is our home and we depend on it for resources, we are concerned more with the crust than with the inacce ssible mantle and core . The crust varies in th ickn ess. Two major types of crust are ocean ic crust and continental crust. The crust under the oceans is much thin-

nero It is made of rock that is som ewh at denser than the rock that underlies the continents. The lower parts of the crust and the entire mantle are inaccessible to direct observation. No mine or oil well has penetrated through the crust, so ou r con cept of the earth's interior is based on indirect evidence (the topic of chapter 2). The crust and the uppermost part of the mantle are relatively rigid. Collectively th ey m ake up th e lithosphere. (To help you remember terms, the meanings of commonly used prefixes and suffixes are given in Appendix G. For example, lith means "rock" in Greek. You will find lith to be part of many geologic terms.) The upper mantle underlying th e lithosphere, called the asthenosphere, is soft and therefore flows. It provides a "lubricating" layer over which the lithosphere move s (asthenos me an s "weak" in G reek). W here hot mantle material wells upwa rd , it will upl ift the litho sphere. W here the lithosphere is coldest and densest, it will sink down th rough the asthen osphere and into th e deep er mantle Introduction to Physical Geology

15

forces strong enough to outdo gravitational forces . (Mount Everest, the world's highest peak, is m ade of rock that formed beneath an ancient sea.) Mountain ranges are built over extended periods, as portions of the earth's crust are squeezed, stretched, and raised. Most tectonic forces are mechanical forces. Some of the energy from these forces is put to work defo rm ing rock, bending and breaking it, and raising mountain ranges. The mechanical energy may be stored (an earthquake is a sudden release of stored mechanical energy) or converted to heat energy (rock may melt, resulting in volcanic eruptions). The working of the machinery of the earth is elegantly demonstrated by plate tectonics.

Core

Figure 1.9 Hot mant le travels upward, Cold crust and mantle sink.

The Theory of Plate Tectonics From time to time a theory emerges within a science that revolutionizes that field. The plate tectonic theory, currently accepted by virtually all geologists, is a unifying theory that accounts for many seemingly unrelated geological phenomena. The theory is as important to geology as the theory of relativity is to physics, the atomic theory to chemistry, or evolution is to biology.

(figure 1.9). The effect of this internal heat engine on the crust is of great significance to geology. The forces generated inside th e eart h, called tectonic forces, cause deformation of rock as well as vertical and horizontal mo vement of portions of the earth's crust. Mountain ran ges are evidence of tectonic

A

Figure 1.11 (A) The sea floors of the world. (B) A diverging boundary at a mid-oceanic ridge. Hot asthenosphere wells upward beneath the ridge crest. Magma forms and squirts into fissures. Solid material that does not melt remains as mantle in lower part of lithosphere. As lithosphere moves away from spreading axis, it cools, becomes denser, and sinks to a lower level.

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(A) Plates of the world . Large arrows indicate direction of plate motion. (B) Plate motio n away from a divergent boundary toward a conve rge nt boundary. After W. Hamilton , U.S. Geolog ica l Sur vey .

16

Chapter 1

Asthenosphere

B

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Plate tectonics was seriously proposed as a hypothesis in the early 1960s, though the idea was based on earlier work-notably, the hypothesis of continental drift. In the chapters on igneous, sedimentary, and metamorphic rocks, as in the chapter on earthquakes, we will expand upon what you learn about the theory here to explain the origin of some rocks and why volcanoes and earthquakes occur. Chapter 4 is devoted to plate tectonics and will show that what you learned in many previous chapters is interrelated and explained by plate tectonic theory. Plate tectonics regards the lithosphere as broken into plates that are in motion (figure 1.10). The plates, which are much like segments of the cracked shell on a boiled egg, move relative to one another, sliding on the underlying asthenosphere. Much of what we observe in the rock record can be explained by what takes place along plate boundaries, where

two plates are pulling away from each other, sliding past each other, or moving toward each other. According to plate tectonics, divergent boundaries exist where plates are moving apart. Most divergent boundaries coincide with the crests of submarine mountain ranges, called mid-oceanic ridges (figures 1.10B and 1.11). A mid-oceanic ridge is higher than deep ocean floor (figure 1.11A) because the rocks, being hotter there, are less dense . Tensional cracks develop along the ridge crest (figure 1.11B). These cracks tap localized magma (mo lten rock) chambers in the underlying asthenosphere, and the magma squeezes into fissures (cracks through the lithosphere). Some magma erupts along the ridge crest, and the rest solidifies in the fissure. Continued pulling apart of the ridge crest develops new cracks, and the process of filling and cracking continues indefinitely. Thus, Introduction to Physical Geology

17

Sea level

4. O ne predicts what would occur in given situations if a hypothesis were correct. S. Predic tions are tested. Incorrect hypotheses are discarded. 6. A hypothesis that passes the testing becomes a theory, which is regarded as having an excellent chance of being true. In science, howeve r, nothing is considered pro ven absolutely. All theories remain open to scrutiny, furthe r testing , and refinement.

Folded sedimentary rocks

Oceanic crust (lithosphere) •

Mantle (lithosphere)

• ~

Q,

Magma moving upward

~

Mantle (asthenosphere)

oI

Mantle (lithosphere)

Magma created here Mantle (asthenosphere)

Kilometers

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Figure 1. 12 A convergent boundary.

new oceanic crust is continuously created at a diverging boundary. Not all of the ma ntle ma ter ial melts; a solid residue remains under the newly created crust. N ew crust and underlying solid m antle make up the lithosphere that moves away from the ridge crest, traveling like the top of a conveyor belt . The rate of motion is generally 1 to 18 centimeters per yearslow in human terms but quite fast by geologic standards. As the lithosphere moves away from the divergent boundary, the material slowly cools. As it cools it contracts and becomes denser. The contraction of the lithosphere and its slow sinking (because of the increased density) cause the floors beneath oceans to deepen away from ridge crests . The top of a plate may be composed exclusively of oceanic crust or include a continent or part of a continent. For example, ifyou live on the North American plate, you are riding westward relative to Europe because the plate's divergent boundary is along th e mid-oceanic ridge in the North Atlantic Ocean (figures 1.10 and 1.11). The western half of the No rth Atlantic sea floor and North America are moving together in a westerly direction away from the mid-Atlantic ridge plate boundary. A second type of boundary, a transform bound ary, occurs whe re two plates slide past each other. The San Andreas fault in California is an example of this type of boundary, and the earthquakes along the fault are a result of plate motion. The third type of boundary, one resulting in a wide range of geologic activ ities, is a convergent boundary, where plates move toward each other (figure 1.12). If one plate is capped by oceanic crust and the other by continental crust, th e less dense, more bu oyant continental plate will override the denser, oceanic plate. The oceanic plate sinks along what is known as a subduction zone, a zone where an oceanic pla te descends into the mantle ben eath an overridi ng plate. In the

18

Chap ter 1

region where the top of the subducting plate slides beneath the asthenosphere, melting takes place and magma is created. Magma is less dense than the overlying solid rock. Therefore, the magma created along the subduction zone works its way upward and either erupts on the earth's surface to solidify as extrusive igneous rock or solidifies within the crust to become intrusive igneous rock Hot rock, under high pressure, near the subduction zone that does not melt may change in the solid state to a new rock-metamorphic rock. In addition to con taining igneous and metamorphic rocks , major mountain belts show the effects of squeezing caused by plate convergence (for instance, the "folded sedimentary rocks" shown on figure 1.12). In the process, rock that may have been below sea level might be squee zed upward to become part of a mountain range. Box 1.3 describes how plate tectonic theory was developed through the scientific method. If you do not have a thorough comprehension of how the scientific method works, be sure to study the box .

Surficial Processes: The Earth's External Heat Engine When tectonic forces shove a portion of the earth's crust above sea level, rocks are exposed to the atmosphere. The earth's external heat engine, driven by solar power and gravity, then comes into play. Our weather patterns are largely a product of this heat engine. For instance, hot air rises near the equator and sinks in cooler zones to the north and south. Solar heating of air creates wind; ocean waves are, in turn , p roduced by wind. When moist air cools, it rains or snows. Rainfall on hillsides flows down slopes and into streams. Streams flow to lakes http://www.mhhe.com/earthsci/geology/plummer

lthough the hypothesis was proposed only a few decades ago, plate tectonics has been so widely accepted and disseminated that most people have at least a rough idea ofwhat it is about. Most nonscientists can understand the television and newspaper reports (and occasional comic strip, such as that in box figure 1) that include plate tectonics in reports on earthquakes and volcanoes. Our description of plate tectonics implies little doubt about the existence of the process. T he theory of plate tectonics has been accepted by nearly all geologists (this does not mean it is "true"). Plate tectonic theory, like all knowledge gained by science, has evolved through the processes of the scientific method. We will illustrate the scientific method by showing how plate tectoni cs has evolved from a vague idea into a plausible theory. The basis for the scientific method is the belief that th e universe is orderly and that by objectively analyzing phenomena, we can discover its workings. The technique is best illustrated as a series of steps , although a scientist does not ordinarily go through a formal checklist.

A

1. A question is raised or a problem is presented. 2. Available information pertinent to the question or problem is analyzed. Facts, which scientists call data, are gathered . 3. After the data have been analyzed, tentative explanations or solutions, called hypotheses, are proposed.

Like any h uma n endeavor, the scientific method is not infallible. Objectivity is needed throu ghout. Someone can easily become attached to the hypot hesis he or she has created and so tend subconsciously to find only supporting evidence. As in a court of law, every effort is made to have observers objectively examine the logic of both procedures and concl usions. Courts sometimes make wrong decisions; science, likewise, is not immune to erro r. How the concept of plate tectonics evolved into a theory is outlin ed below. Ste p 1: A question asked or problem raised. Actually, a number of questions were being asked about seem ingly unrelated geological phenomena. What caused the submarine ridge that extends through most of the oceans of the world? Why are rocks in mountain belts intensely deformed? What sets off earthquakes? What causes rock to melt underground and 'erup t as volcanoes? Step 2: Gatheri n g of data. Early in the century, the amo unt of data was limited. But through the decades, the information gathered increased eno rmo usly. N ew data, most notably information gained from exploration of the sea floor in .the mid1900s, forced scientists to discard old hypotheses and come up with new ideas. Step 3: Hypotheses proposed. Most of the questions being asked were treated as separate problems wanting separate hypotheses. Some appeared

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Introduction to Physical Geology

19

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areas of geology and will keep the attention of future generations of scientists.

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Mid-oceanic ridge

Important Note Words used by scientists do not always have the same meaning when used by the general public. A case in point is the word "theory." To most people, a "theory" is what scientists regard as an "hypothesis." You may remember news reports about an airliner that exploded offshore from N ew York in 1996. A typical statement on television was: "One theo ry is that a bomb in the plane exploded; a secon d theory is that

the plane was shot down by a missile fired from a ship at sea; a third theory is that a spark ignited in a fuel tank and the plane exploded." Clearly, each "theory" is an hypothesis in the scientific sense of th e word. This has led to considerable confusion for nonscientists about science . You have probably heard the expression, "It's just a theory." Statements, such as, "Evolution is just a theory," are used to imply that scientific support is weak. The reality is that theories such as evolution and plate tect onics have been so overwhelmingly verified that th ey come as close as po ssible to wha t scientists accept as being proved true.

Bo x 1.3 Figure 2 Ages of rocks from holes drilled into the oceanic crust. (Vertical scale of diagram is exaggerated.)

interrelated. One hypothesis, continental drift, did address several questions. It was advocated by Alfred Wegener, a German scientist, in a boo k published in th e early 1900s. Wegener postulated tha t the continent s were all once part of a single supercont inent called Pangaea. The hypothesis explained why the coastlines of Africa and South America look like separated parts of a jigsaw puzzle. Some 200 million years ago this supercontinent broke up, and the various continents slowly drifted into th eir present positions. The hypothesis suggested that the rock within mountain belts becomes deformed as the leading edge of a continental crust moves against and over the stationary oceanic crust. Earthquakes were presumably caused by continuing movement of the continents. It was not until th e 1960s, after new data on the nature of the sea floor became available, th at the idea of continental dr ift was incorporated into the concept of plate tectonics. What was added in the plate tectonic hypothesis was the idea that oceanic crust, as well as continental cru st, was shifting. Step 4: Prediction. An obvious prediction, if plate tectonics is correct, is that if Europe and North America are moving away from each other, the distance measured between the two continents is greater from one year to the next. But, we can't stretch a tape measure across oceans and, until recently, we have not had the technology to accur ately measure distances betwe en continents. So, in th e 1960s, other testable predictions had to be m ade. O ne prediction was that the rocks of the oceanic crust will be progressively olde r the farther th ey are from the crest of a mi d-oceanic ridge. Step 5: Predictions are tested. Experiments were conducted in which hole s were drilled in the deep sea floor from a specially designed ship . Rocks and

20

Chapter1

or seas. Glaciers grow where there is abundant snowfall at colder, high elevations and flow downhill due to gravity. Where moving water, ice, or wind loosens and removes material, erosion is taking place. Streams flowing toward oceans remove some of the land over which they run. Crashing waves carve back a coastline. Glaciers grind and carry away underlying rock as they move. In each case, rock originally raised by the earth's internal processes is worn down by surficial processes (figure 1.13). Rocks Iormed-at high temperature and under high pressure deep within the earth and pushed upward by tectonic forces are unstable in their new environment. Air and water tend to cause the once deep-seated rocks to break down and form new materials. The new materials, stable under conditions at the earth's surface, are said to be in equilibrium, that is, adjusted to the physical and chemical conditions of their environment so that they do not change or alter with time. For example, much of an igneous rock (such as granite) that formed at a high temperature tends to break down chemically to clay. Clay is in equilibrium, that is to say it is stable, at the earth's surface. The product of the breakdown of rock is sediment, loose material. Sediment may be transported by an agent of erosion, such as running water in a stream. Sediment is deposited when the transporting agent loses its carrying power. For example, when a river slows down as it meets the sea, the sand being transported by the stream is deposited as a layer of sediment. In time a layer of sediment deposited on the sea floor becomes buried under another layer. This process may continue burying our original layer progressively deeper. The pressure from overlying layers compresses the sediment, helping to consolidate the loose material. With the cementation of the loose particles, the sediment becomes lithified (cemented or otherwise consolidated) into a sedimentary rock.

sediment were collected from these holes , and th e ages of these materials were determined. As the hypothesis predicted, the youngest sea floor (generally less than a million years old) is near th e mid-oceanic ridges, wh ereas the oldest sea floor (up to about 200 million years old) is farthest from th e ridges (box figure 2) . This test was only on e of a series. Various other tests, described in some detail later in this book, tended to confirm the hypothesis of plate tectonics. Som e tests did not work out exactly as predicted. Because of this, and more detailed study of data, the original concept was, and continues to be, modified. The basic premise, however, is generally regarded as valid. Step 6: The hypothesis becomes a theory. Most geologists in the world considered the results of this and other tests as positive, indicating that the concept is probably true. It can now be called the plate tectonic theory. During the last few years, plate tectonic theory has been further confirmed by the results of very accur ate satellite surveys that determine where points on separate continents are relative to one another. The results indicate that the continents are indeed moving relative to one another. Europe and North America are moving farther apart. Although it is unlikely that plate tectonic theory will be replaced by something we haven't thought of yet, aspects that fall under plate tectonics' umbrella continue to be analyzed and revised as new data become available. For instance , the question of how magma is generated deep down in a subduction zone has been addressed by several hypo th eses (and is discus sed in chapter 10). Does melting take place in the descending plate or in the overlying ma ntle? Is water carried down by the descending plate an aid to m elti ng? Simil ar questions are raised in many other

Geologic Time We have mentioned the great amount of time required for geological processes. As humans, we think in units of time related to personal experience-seconds, hours, years, a human lifetime. It stretches our imagination to contemplate

http://www.mhhe.com/earthsci/geologylplummer

Sea

Igneous rock

Older rock

A Sediment transported to sea

.. . ..... .

Tectonic uplift

: .'

Layers of sediment collect on the sea floor and will Iithify to sedimentary rock

B

Figure 1.13 Uplift, erosion, and deposition. (A) Magma has solidified undergroun d to become igneous rock. (8) Land is uplifted . Upper portion is eroded. Sediment is transported to the sea to become sedimentary rock.

ancient history that involves 1,000 or 2,000 years. Ge ology involves vastly greater amounts of time, often referred to as

deep time.

.

To be sure, some geological processes occur quickly, such as a great landslide or a volcanic eruption. These events occur when stored energy (like the energy stored in a car battery) is suddenly released. Most geological processes, howIntroduction to Physical Geology

2

-,

21

Table 1.1

~

Some Important Ages in the Development of Life on Earth

move toward convergent boundaries. Convergence results in subduction of one plate. Plates slide past one another at transform boundaries. Erosion takes place at the earth's surface where rocks are exposed to air and water.

Rocks that formed under high pressure and temperature inside the earth are out of equilibrium at the earth's surface and tend to alter to substances that are stable at the earth's surface. Sediment is transported to a lower elevation where it is deposited (com-

monly on a sea Hoor in layers). When sediment is cemented it becomes sedimentary rock. Although the earth is changing constantly, the rates of change are generally extremely slow by human standards.

asthenosphere 15

geology 6

plate tectonics 17

continental drift 20

hypothesis 19

scientific method 19

convergent boundary 18

igneous rock 18

sediment 21

core 15

lithosphere 15

sedimentary rock 21

crust 15

magma 17

subduction zone 18

data 19

mantle 15

tectonic forces 16

divergent boundary 17

metamorphic rock 18

theory 19

equilibrium 21

mid-oceanic ridge 17

transform boundary 18

erosion 21

physical geology 6

Use the questions below to prepare for exams based on this chapter.

ever, are slow but relentless, reflecting the pace at which the heat engines work. It is unlikely that a hill will visibly change in shape or height during your lifetime (unless through human activity). However, in a geologic time frame, the hill probably is eroding away quite rapidly. "Rapidly" to a geologist may mean that within a few million years the hill will be reduced nearly to a plain. Similarly, in the geologically "recent" past of several million years ago, a sea may have 'existed where the hill is now. Some processes are regarded by geologists as "fast" if they are begun and completed within a million years. The rate of plate motion is relatively fast. If new magma erupts and solidifies along a mid-oceanic ridge, we can easily calculate how long it will take that igneous rock to move 1,000 kilometers away from the spreading center. At the rate of 1 centimeter per year, it will take 100 million years for the presently forming part of the crust to travel the 1,000 kilometers.

Geology is the scientific study of the earth. Geological investigations indicate that the earth is changing because of internal and surficial processes. Internal processes are driven mostly by temperature differences within the

1. Plate tectonics is a result of the earth's internal heat engine, powered by (choose all that apply) (a) the sun (b) gravity (c) heat Rowing from the earth's interior outward

Although we will discuss geologic time in detail in chapter 8, table 1.1 shows some reference points to keep in mind. The earth is estimated to be about 4.5 billion years old (4,500,000,000 years). 1Fossils in rocks indicate that complex forms of animal life have existed in abundance on the earth for about the past 545 million years. Reptiles became abundant about 230 million years ago. Dinosaurs evolved from reptiles and became extinct about 65 million years ago. Humans have been here only about the last 3 million years. The eras and periods shown in table 1.1 comprise a kind of calendar for geologists into which geologic events are placed (as explained in the chapter on geologic time). Not only are the immense spans of geologic time difficult to comprehend, but very slow processes are impossible to duplicate. A geologist who wants to study a certain process cannot repeat in a few hours a chemical reaction that takes a million years to occur in nature. As Mark Twain wrote in Life on theMississippi, "Nothing hurries geology."

earth's mantle. Surficial processes are driven by solar energy and gravity. Internal forces cause the crust of the earth to move. Plate tectonic theory visualizes the lithosphere (the crust and uppermost mantle) as broken into

2. What tectonic plate are you presently on? Where is the nearest plate boundary and what kind of boundary is it?

3. What is the most likely geologic hazard in your part of the country? 4. What are the three major types of rocks? 5. What are the relationships among the mantle, the crust, the asthenosphere, and the lithosphere? 6. What would the surface of the earth be like if there were no tectonic activity?

12. Which is a geologic hazard? (a) earthquake (b) volcano (c) mudflows (d) Roods (e) wave erosion at coastlines (f) landslides (g) all of the above 13. The largest zone of the earth's interior by volume is the (a) crust (b) mantle (c) outer core (d) inner core 14. Oceanic and continental crust differ in (a) composition (b) density (c) thickness (d) all of the above 15. The forces generated inside the earth that cause deformation of rock as well as vertical and horizontal movement of portions of the earth's crust are called (a) erosional forces (b) gravitational forces (c) tectonic forces (d) all of the above

7. Explain why cavemen never saw a dinosaur.

16. Plate tectonics is a (a) conjecture (b) opinion (c) hypothesis (d) theory

8. What is meant by equilibrium? What happens when rocks are forced out of equilibrium?

17. Which is a type of a plate boundary? (a) divergent (b) transform (c) convergent (d) all of the above

9. A typical rate of plate motion is (a) 3-4 meters per year (b) 1 kilometer per year (c) 1-10 centimeters per year (d) 1,000 kilometers per year

18. The lithosphere is (a) the same as the crust (b) the layer beneath the crust (c) the crust and uppermost mantle (d) only part of the mantle

10. Earthquakes may be caused by (a) movement of plates (b) motion along faults (c) shifting of bedrock (d) all of the above

plates that move relative to each other over the asthenosphere. The plates are moving away from divergent boundaries usually located at the crests of mid-oceanic ridges where new crust is being created. Plates

dynamic forces that cause those changes is (a) physical geology (b) historical geology (c) geophysics (d) paleontology

11. The division of geology concerned with earth materials, changes in the surface and interior of the earth, and the

19. Erosion is a result of the earth's external heat engine, powered by (choose all that apply) (a) the sun (b) gravity (c) heat Rowing from the earth's interior outward

Introduction to Physical Geology

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Chapter1

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1. Why are some parts of the lower mantle hotter than other parts? 2. According to plate tectonic theory, where are crustal rocks created? Why doesn't the earth keep getting larger if rock is continually created?

3. What percentage of geologic time is accounted for by the last century? 4. What wou ld the earth be like without solar heating? 5. What are some of the technical difficulties you would expect to

encount er if you tried to drill a hole to the center of the earth?

Expand your knowledge of the concepts presented in this chapter by usingthe CD-ROM to answer thefollowingquestions. 1. The earth has changed greatly over time through the processesof plate tectonics. Watch the animations on the Changing Earth module to find examples of each of these plate processes:

• continents splitting apart at divergent boundaries • oceans shrinking as the ocean floor is consumed at subduction zones • continents colliding at convergent boundaries

"Transform Faults," then click on "Earthquake Footage." What were the effects of the Lorna Prieta earthquake? How was this earthquake related to plate processes?

2. Go to the Transform Faults module. After the introduction, click on

Exploring Reso"Y$ees ~ ...•.......... Allegre, c. 1988. The behavior ofthe ~ Earth. Boston: Harvard University

a::::aEarth Revealed. Southern California

Press.

Consortium. A series of videotapes produced for a televised physical geology course.

Drake, E. T., and W M. Jordan, eds. 1985.

/".

Geologists and ideas: A history ofNorth Americangeology. Boulder, Colo.: Geological

Plummer/McGeary. 1996. WCB/McGraw-Hill.

Society of America.

http://www.mhhe.com/earthsci/geology/ plummer This is the ded icated website for this book. You can go to it for new and updated information . The universal resource locators (URL) listed in this book are also given as links on the website, making it easy to go to those websites without typing in the URL. Links to additional websites can also be found. We have added questions to some of the links to allow you to get the most of your exploration of the web. Using the web is an enjoyable way of enhancing your knowledge of geology.

McPhee, J. 1981. Basinand range. New York: Farrar, Straus & Giroux. This is one of several outstanding books about geology written for the layman by John McPhee. Moores, E. M., ed. 1990. Shapingthe earth: Tectonics ofcontinents and oceans. New York: W H. Freeman and Company. Nuhfer, E. B., R. J. Proctor, and P. H. Moser. 1993. The citizens guide togeologic hazards. Arvada, Colorado: American Institute of Professional Geologists. Officer, C. B. and]. Page. 1993. Tales ofthe

earth: Paroxysms and perturbations ofthe blue planet. New York: Oxford University Press. Pirsig, R. M. 1974. Z en and the art of motorcycle maintenance. New York: Bant am Books (paperback). This book con tains an exception ally good exposition of the scientific method as well as considerable insight int o the philosoph y of science. Rhodes , F. H . T., and R. O . Stone, eds. 1981. Language ofthe earth. New York: Pergamon Press.

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Chapter 1

Interactive Plate Tectonics by

http://pubs. usgs.gov/publications/text/ dynamic.html This Dynamic Earth by the U.S. Geological Survey is an online, illustrated publication explaining plate tectonics. You may want to go to the section "Understanding plate motion." This will help reinforce what you read about plate tectonics in this chapter. However, it goes into plate tectonics in greater depth, covering material that is in chapter 4 of this textbook.

http://www.uh.edu/ - jbutler/anon/ anontrips.html Virtual Field Trips. The site provides access to geologic sites throughout the world. Many are field trips taken by geology classes. Check the alphabetical listing and see if there are any sites near you. Or watch a video clip in one of the Quick Time field trips. One of the well-done trips in the alphabetical listing is the Oneonta to the Hudson River field trip in Central New York. http://www.usgs.gov The Us. Geologic Surveys home page. Use this as a gateway to a wide range of geologic information. http://earthrise.sdsc.edu/

E(lrthRISE provides access to photographs taken from the space shuttle. You may retrieve photos by requesting a geologic feature (e.g., volcano) or of a specific geographic portion of the earth. http://www.nrcan.gc.ca/gscl The Geological SurveyofCanada home page.

Introduction to Physical Geology

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p

25

The Earth's Interior

T

he only rocks that geologists can study directly in place are those of the crust; and the earth's crust is but a thin skin of rock, making up less than 1% of the earth's total volume. Mantle rocks brought to

the earth's surface in basalt flows, in diamond-bearing kimberlite pipes, and also the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust, give geologists a glimpse of what the underlying mantle might look like. Meteorites also give clues about the possible composition of the core of the earth. However, to learn more about the deep interior of the earth, geologists must study it indirectly, largely by using the tools of geophysics-that is, seismic waves and the measurem~nt of gravity, heat flow, and earth magnetism. The evidence from geophysics suggests that the earth is divided into three major layers-the crust on the earth's surface, the rocky mantle beneath the crust, and the metallic core at the center of the earth. The study of plate tectonics has shown that the crust and uppermost mantle can be conveniently divided into the brittle lithosphere and the plastic asthenosphere. You will learn in this chapter how gravity measurements can indicate where certain regions of the crust and upper mantle are being held up or held down out of their natural position of equilibrium. We will also discuss the earth's magnetic field and its history of reversals. We will show how magnetic anomalies can indicate hidden ore and geologic structures. We close with a discussion of the distribution and loss of the earth's heat.

Because diamonds form in the mantle and are brought to the surface in kimberlite pipes , they give geolog ists a glimpse of the earth's interior. Neg. no. K17579 Diamond in Kimb erlite, Courtes y Department of Library Services , Ame rican M.useum of Natura l History

27'

Geologists who have considerable contact with the public (such as in government offices and universities) occasionally encoun ter people who are convinced that geologists are misled about the interior of the earth . These people hold strong convictions about what is deep inside the earth. Many believe, for example, that the earth is hollow and is a storage place for water or even flying saucers. What do geologists know about the earth's interior? How do the y obtain information about the parts of the earth ben eath the surface? Is there even a slight chance that geologists are wrong, that the earth really is hollow? Geologists, in fact, are not able to sample rocks very far below th e earth's surface. Some deep mines penetrate 3 kilometers (2 miles) into the earth, and a deep oil well may go as far as 8 kilometers ben eath the surface; the deepest scientific well has reached 12 kilometers in Russia. Rock samples can be brought up from a mine or a well for geologists to study. A direct look at rocks from deeper levels can be gotten where mantle rocks have been brought up to the surface by basalt flows (see box 2.2), by the int rusion and erosion of diamond-bearing kimberlite pip es (see chapter 12), or where the lower part of the oceanic lithosphere (see chapter 3) has been tectonically attached to the continental crust at a convergent plate boundary. However, the earth has a radius of about 6,370 kilome ters, so it is obvious that geologists can only scratch the surface when they try to study directly the rocks beneath their feet. D eep parts of th e earth are studied indirectly, however, largely through th e branch of geology called geophysics, which is the application of physi cal laws and principles to a study of the earth. G eophysics includes the study of seismic waves and the earth's magnetic field, gravity, and heat. All these things tell us som ething about th e nature of th e deeper parts of the eart h . Together th ey create a convincing picture of what makes up th e earth's interior.

Seismograph station

Earthquake

Minimum distance from epicenter __ for refracted wave to arrive before direct wave

28

Chapter 2

"I

Surface

Layer B

A

Reflecting boundary

Figure 2.1 Seismic reflection. Seismic waves reflect from a rock boundary deep within the earth and return to a seismograph station on the surface.

Layer B

Fig ure 2.3 Path of seismic wave

Layer in which seismic waves travel slowly (low-velocity layer)

Seismic refraction can be used to detect bounda ries between rock layers. See text for explanation.

Layer in which seismic waves travel rapidly (high-velocity layer)

though the refracted wave travels farther, it can arrive at a station first because most of its path is in the high-velocity layer B. The distance between this point of transform ation and the epicen ter of the earthquake is a function of the depth to the rock boundary between layers A and B. A series of portable seismographs can be set up in a line away from an explosion (a seismic shot) to find this distan ce, and the depth to the boundary can the n be calculated. T he velocities of seismic waves within the layers can also be foun d . Figure 2.2 shows how waves bend as they travel downward into high er velocity layers. But why do Waves return to th e surface, as shown in figu re 2.3?The answer is that advancing waves give off energy in all direc tions. M uch of th is energy contin ues to travel horizontally within layer B (figure 2.3). This ene rgy passes beneath station 2 and out of th e figure toward the righ t. A small part of th e energy "leaks" upward in to layer A, an d it is this pathway that is shown in the figure . There are many other pathways for this wave's energy that are not shown here. A sharp rock boundary is not necessary for the refraction of seismic waves. Even in a thick layer of uniform rock, the increasing pressure with dept h tends to increase the velocity of th e waves. T he waves follow curved paths thro ugh such a layer, as shown in figure 2.4 . To understand the reason for the curving path, visualize the thick rock layer as a stack of very thin layers, each with a slightly higher velocity than the one above. The curved path results from ma ny small changes in direction as the wave passes through the many layers.

A

High-velocity layer

Low-velocity layer

B

Figure 2.2 Seismic refraction occurs when seismic waves bend as they cross rock boundaries. (A) Low-velocity layer above high-velocity layer. (B) High-velocity layer above low-velocity layer.

Evidence from Seismic Waves Seismic waves from a large earthquake may pass through the entire earth. A nuclear bomb explosion also generates seismic waves. Geo logists obtain new inform ation about the earth's interior after every large earthquake and bomb test. One important way of learning about the earth's interior is the study of seismic reflection, the return of some of the energy of seismic waves to the earth's surface after the waves bou nce off a rock boundary. If two rock layers of differing densities are separated by a fairly sharp boundary, seismic waves reflect off that boundary just as light reflects off a mirror (figure 2.1) . These reflected waves are recorded on a seismogram, which shows the amount of time the waves took to travel down to the boundary, reflect off it, and return to the surface. From th e amo unt of time necessary for the round trip , geologists calculate th e depth of the boundary. Another method used to locate rock boundaries is the study of seismic refraction, the bending 'of seismic waves as they pass from one material to another, which is sim ilar to the way th at light waves ben d when they pass through th e lenses of eyeglasses. As a seismic wave strikes a rock boundary, much of

Epicenter

the energy of the wave passes across the boundary. As the wave crosses from one rock layer to another, it changes direction (figure 2.2). This change of direction, or refraction, occurs only if the velocity of seismic waves is different in each layer (which is generally true if the rock layers differ in density or strength). The boundaries between such rock layers are usually distinct enough to be located by seismic refraction techniques, as shown in figure 2.3. Seismograph station 1 is receiving seismic waves that pass directly through the upper layer A. Stations farther from the epicenter, such as station 2, receive seismic waves from two pathways: (1) a direct path straight through layer A and (2) a refracted path through layer A to a higher-velocity layer B and back to layer A. Station 2 therefore receives the same wave twice. Seismograph stations close to station 1 receive only the direct wave or possibly two waves, the direct (upper) wave arriving before the refracted (lower) wave. Stations near station 2 receive both the direct and the refracted waves. At some point between station 1 and station 2 there is a transformation from receiving the direct wave first to receiving the refracted wave first. Even http://www.mhhe.com/earthsci/geologylplummer

B

Figure 2.4 Curved paths of seis mic waves caused by uniform rock with increasing seismic velocity with depth. (A) Path betwe en earthquake and recordin g station. (B) Waves spreading out in all directions from earthquake focus.

The Earth's Internal Structure It was the study of seismic refraction and seismic reflection that enabled scientists to plot the three main zones of the earth's interior (figure 2.5). The crust is the outer layer of rock, which forms a thin

Crust (7 - 50 km thick)

Figure 2.5 The earth' s interior. Seismic waves show the three main divisions of the earth: the crust, the mant le, and the core. Photo by NASA

The Earth's Interior

29

30-50 km thick 7 km thick

Table 2.1

I

Characteristics of Oceanic Crust and Continental Crust Oceanic Crust

Continental Crust

Average thickness

7km

30 to 50 km (thickest under mountains)

Seismic P-wave velocity

7 km/second

6 km/second (higher in lower crust)

Figure 2.6

Density

3.0 gm/cm 3

2.7 gm/cm 3

Thin crust with a P-wave velocity of 7 kilometers per second underlies the ocea n. Thic k crust with a lower veloci ty makes up continents. Mantl e velocities are about 8 kilometers per second.

Probable composition

Basalt underlain by gabbro

Granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover)

Mantle

8 km/sec

skin on the earth's surface. Below the crus t lies th e mantle, a thick shell of rock that separates the crust above from the core below. The core is the central zone of the earth. It is probably me tallic and the source of the earth's magnetic field.

The Earth's Crust Studies of seismic waves have shown (1) that the earth's crust is thinner beneath the oceans than beneath the continents (figure 2.6) and (2) that seismic waves travel faster in oceanic crust than in continental crust. Because of this velocity difference, it is assumed that the two types of crust are made up of different kinds of rock. Seismi c P waves travel through oceanic crust at about 7 kilometers per second, which is also the speed at which they travel th rough basalt and gabbro (the coarse-grained equivalent of basalt). Samples of rocks taken from the sea floor by oceanographic ships verify that the upper part of the oceanic crust is basalt and suggest that the lower part is gabbro. The oceanic crust averages 7 kilometers in thickness, varying from 5 to 8 kilometers (ta le 2.1). Seismic P waves travel more slowly through continental crust-about 6 kilometers per second, the same speed at which they travel through granite and gneiss. C on tinental crust is often called "granitic," but the term sho uld be put in quotatio n marks because most of the rocks expo sed on land are not granite. The continental crust is highly variable and complex, consisting of a crystalline basement com posed of granite, other pluto nic rocks, gneisses and schists, all capped by a layer of sedimentary rocks, like icing on a cake. Since a single rock term cannot accurately describe crust that varies so greatly in composition, some geologists use the term sial (rocks high in silicon and aluminum) for continental crust an d sima (rocks high in silicon and magnesium) for oceanic crust. Continental crust is much thicker than oceanic crust, averaging 30 to 50 kilometers in thickness, thou gh it varies from 10 to 70 kilometers. Seismic waves show th at th e crust is th ick-

30

Chapter2

T

he structure and composition of most of the continental crust is unknown. Surface mapping and seismic reflection and refraction suggesr that the continents are ' largely igneous and metamorphic rock, such as granite and gneiss, overlain by a veneer of sedimentary rocks. This sedimentary cover is generally thin, like icing on a cake, but it may thicken to 10 kilometers or more in giant sedimentary basins where the underlying "basement rock" has subsided (see map in front inside cover). Although oil companies have drilled as deep as 8 kilometers on land, they drill in the sedimentary basins. The igneous and metamorphic basement, which averages 4 0 kilometers thick and makes up most of the continental crust, has rarely 'been sampled deeper than 2 or 3 kilometers (although uplift and erosion have exposed some rocks widely thought to have been formed much deeper in the crust).

est under geologically young mountain ranges, such as the Andes and Himalayas, bulging downward as a mountain root into the mantle (figure 2.6). The continental crust is also less dense than oceanic crust, a fact that is important in plate tectonics (table 2.1). The boundary that separates the crust from the mantle beneath it is called the Mohorovicic discontinuity (Moho for short). Note from figure 2.6 that the mantle lies closer to the earth's surface bene ath the ocean than it does beneath continents. The idea behind an ambitious program called Project Mohole (begun during the early 1960s) was to use specially equipped, ships to drill through the oceanic crust and obtain samples from the mantle. Although the project was abandoned because of high costs, ocean-floor drilling has become routine since then, but not to the great depth necessary to sample the mantle. Perhaps in the future the original concept of drilling to the mantle through oceanic crust will be revived. (Ocean drilling is discussed in more detail in the following two chapters.)

The earth's crust and uppermost mantle together form th e lithosphere, the outer shell of the earth that is relatively str ong and brittle. The lithosphere makes up the plates of plate-tecto nic theory. The lithosphere averages about 70 kilome ters thick beneath oceans and may be 125 to 250 kilometers thick beneath continents. Its lower boundary is marked by a curious mantle layer in which seismic waves slow down (figure 2.7). Generally, seismic waves increase in velocity with depth as inc reasing pressure alters the properties of the rock. Beginning at a depth of 70 to 125 kilometers, however, seismic waves travel more slowly than they do in shallower layers, and so this zone has been called the low-velocity zone (figure 2.7). This zone, extending to a depth of perhaps 200 kilometers, is also called, in plate-tectonic theory, the asthenosphere. The rocks in this zone may be closer to their melting point than the rocks above or below the zone. (The rocks are probably not hotter than the rocks below-melting points are controlled by pressure as well as temperature.) Some geologists think that these r?c~s may actually be partially melted, forming a crystal-andliqui d slush; a very small percentage of liquid in the asthen osphere could help explain some of its physical properties. . If th~ rocks of the asthenosphere are close to the ir melting point, thi s zone may be important for two reasons: (1) it may represent a zone where magma is likely to be generated; and

The Mantle Because of the way seismic waves pass through the mantle, geologists believe that it, like the crust, is made of solid rock. Localized magma chambers of melted rock may occur as isolated pockets of liquid in both the crust and the upper mantle, but most of the mantle seems to be solid. Because P waves travel at about 8 kilometers per second in the upper mantle, it appears that the mantle is a different type of rock from either oceanic crust or continental crust. The best hypothesis that geologists can make about the composition of the upper mantle is that it consists of ultramafic rock such as peridotite. Ultramafic rock is dense igneous rock made up chiefly of ferromagnesian minerals such as olivine and pyroxene (see box 2.2). Some ultramafic rocks contain garnet, and all of them lack feldspar. http://www.mhhe.com/earthsci/geologylplummer

Since 1970 Russia has been drilling a superdeep hole on the Kola Peninsula near Murm ansk north of the Arctic Circle. The hole penetrates ancient Precambrian basement rocks, and is currently 12 kilometers deep. The Kola hole is the deepest of 11 holes currently being drilled in Russia to depths of 10 to 15 kilometers. D eep drilling is as technically complex as space exploration. H igh pr essures and 300°C temperatures require special equip men t and techniques. The Russians use a turbodrill that rotates under the pressure ~f circulating drilling mud. Unlike no rm al dri lling operanons, the lightweight aluminum drill pipe does not turn. The drilling at Kola shows th at seismi c models for th is area are wrong. The Russians expected 4.7 kilometers of metamorphosed sedimentary and volcani c rock, then a granitic layer to a depth of 7 kilo m eters, and a "basaltic" layer below that. The granite, however, appeared at 6.8 kilometers and extends to more than 12 kilometers; the "basalt" has not yet been found. T hese results (and other deep holes) show that seismic surveys of continental crus t are being systematically misinterpreted. The Russians unexpectedly fou nd open fractures and circulating fluids throughout the borehole. The fluids incl ude hydrogen, helium , and m ethan e (natural gas), as well as mineralized wate rs forming ore bodies. Copper-nickel ore was found deeper than the ory predicted, and gold mineralization was present from 9.5 to 11 kilometers down. These results will change geologists' models of ore formation and fluid circulation underground .

(2) the rocks here may have relatively little strength and therefore are likely to flow. If mantle rocks in the asthenosphere are weake r than the y are in th e overlying lithosphere, then the asthenosphere can deform easily by plastic flow. Plates of brittle lithospher e pr ob ably move easily over the asthenosphere, which may act as a lubricating layer below. There is widespread agreement on the existence and depth of the asthenosphere under oceanic crust, but considerable disagreement about asthenosphere under continental crust. Figure 2. 7 sho ws asthenosphere at a depth of 125 kilometers below the continents. Some geologists think that the lithosphere is much thicker beneath continents than shown in the figure , an d that the asthenosphere begi ns at a depth of 250 kilometers (or even more). A few geologists say that there is no asthenosphere beneath continents at all. T he reasons for this disagreemen t are the results of the rap idly developing field of seism ic tomography, which is described in a box in chapter 4 . T he figu res throughout this book match the model in figu re 2.7. Data from seismic reflection and refraction indicate several concent ric layers in the mantle (figure 2.7), with prominent boundaries at 400 and 670 kilometers (670 kilometers is also the depth of the deepest earthquakes). It is doubtful that the layering is du e to th e presence of several different kinds of rock. Most geologists think tha t the chemical composition of the mantle The Earth's Interior

p

31

Quake

103'

xenolith is an inclusion of foreign rock within an igneous rock. Plutons sometimes contain xenoliths of nearby country rock, but xenoliths within some basalts may have come from deep in the earth's mantle. Numerous basalts contain xenoliths of the relatively rare rocks peridotite and duni te. Peridotite is an igneous rock com posed mos tly of olivine and pyroxe ne; it lacks feldspar, which is common in most igneous rocks. Dunite is a related rock that is almost entirely olivine (box figure 1). Because basalts appear to be generated in the mantle's asthenosphere, and because peridotite and dunite are unco mmon at the earth's surface, most geologists suspect that these xenoliths may be solid pieces of th e earth's mantle brought to

A

Box 2.2 Figure 1 Dunite xenoliths in basalt. The xenoliths are thought by many geologists to be pieces of mantle rock torn off and brought to the surface by ascending basaltic magma. Scale is 7 cm long.

p W. "'e~e aVes received \\

the surface by the erupting basalt. The density, seismic velocity, and chemical composition of peridotite and dunite make them reasonable candidates for mantle rock. Peridotites (usually serpentinized by metamorphism) also occur at the base of ophiolite sequences on land (see next chapter), which are widely interpreted to be slivers of oceanic crust and mantle rock attached to continents at convergent plate boundaries.

Velocity of P waves (km/sec) 6

o

8

•

10

Ocean

Continent

________ ~ _ _ _ _

r,~~ : :~~'~ ~~~ '::~:~~~~~'~ ::~ :~~~~~~~~f~U~~,!-7,:~,,~ : -~rnnmmillIDnm~ 70km

V:ithosphere

Mantle 125 km

100

Asthenosphere

Figure 2.7

{I

300

t3

400

Upper mantle

t

-

Chapter 2

400 km

------ -- - -- ------ -- -------- - Boundaries that may represent pressure COllapse of minerals

500

600 670 km

700

L-

rock is about th e same throughout the mantle . Because pressure increases with depth into the earth, the bou ndaries between mantle layers possibly represent depths at which pressure collapses the internal structure of certain minerals into denser minerals (figure 2.7). For example, at a pressure equ ivalent to a depth of about 670 kilometers, the mineral olivine should collapse into the denser structure of the mineral perovskite. If the boundaries between mantle layers represent pressure-caused transformations of minerals , the entire mantle may have the same chemical composition throughout, although not the same

32

200 km

I-----?'- - - -- - - -----I

200

The concentric shell structure of the upper mantle . The velocity of seismic P waves generally incre ases with depth except in the low-velocity zone. The lithosphere contains the cr ust and the uppermost mantle. The plastic asthenosphere slows down seismic waves. Velocity increases at 400 and 670 kilomete rs may be caused by mineral collapse.

103'

=

/ ------ -- ------------------ --

mineral compos ition. However, some geologists think that the 670-kilom eter boundary represents a chemical change as well as a physical change and separates th e upper mantle from the chemically different lower mantle below.

The Core Seismic-wave data provide the primary evidence for the existence of the core of the earth. (See chapter 7 for a discussion of seismic P and S waves.) Seismic waves do not reach certain http://www.mhhe.com/earthsci/geologylplummer

Figure 2.8

Figure 2.9

The P-wave shadow zone, caused by refraction of P waves within the earth's core.

The S-wave shadow zone. Because no S waves pass throug h the core, the core is apparently a liquid (or acts like a liquid).

areas on the opposite side of the earth from a large earthquake. Figure 2.8 shows how seismic P waves spread out from a quake until, at 103° of arc (11,500 km) from the epicenter, they suddenly disappear from seismograms. At more than 142° (15,500 km) from the epicenter, P waves reappear on seismograms. The region between 103° and 142°, which lacks P waves, is called the P-wave shadow zone. The P-wave shadow zone can be explained by the refraction of P waves when they encounter the core boundary deep within the earth's interior. Because the paths ofP waves can be accurately calculated, the size and shape of the core can be determined also. In figure 2.8, notice that the earth's core deflects the P waves and, in effect, "casts a shadow" where their energy does not reach the surface. In other words, P waves are missing within the shadow zone because they have been bent (refracted) by the core. The chapter on earthquakes explains that while P waves can tra: el through solids and fluids, S waves can travel only through solids, As figure 2.9 shows, an S-wave shadow zone also exists and is larger than the P-wave shadow zone. Direct S waves are not recorded in the entire region more than 103° away from the epicenter. The S-wave shadow zone seems to indicate that S waves do not travel through the core at all. If this is true, it implies that the core of the earth is a liquid, or at least acts like a liquid. The way in which P waves are refracted within the earth's core (as shown by careful analysis of seismograms) suggests that the core has two parts, a liquid outer core and a solid inner core (figure 2.5).

Comp osition ofthe Core When evidence from astronomy and seismic-wave studies is com bined with what we know about the properties of materials, it appears that the earth's core is made of metal-not silica~e rock-and that this metal is probably iron (along with a minor amount of oxygen, silicon, sulfur, or nickel). How did geologists arrive at this conclusion?

Density (qrn/crn") - -......~

468

2

10

12

14

1000

E

::.

2000

J:

c..

~

3000

!

Mantle

-Outer-core-- ----- - - - - - - - -

4000

5000

Outer core

-~n~com - -- - ---- ---------- --- -

6000

Figure 2.10 Density variations with depth into the earth. Note the great increase in density at the core-mantle boundary.

The overall density of the earth is 5.5 gm/cm 3 , according to astronomers who calculate th e speed of the earth's revolution about the sun and the speed of its rotation on its own axis. The crustal rocks are relatively low den sity, from 2.7 gm /cm 3 for granite to 3.0 gm/cm 3 for basalt. The ultramafic rock thought to make up the mantle probably has a density of 3.3 gm/cm 3 in the upper mantle, although rock pressure should raise this value to about 5.5 gm/cm 3 at the base of the mantle (figure 2.10). The Earths Interior

33

mall solid particles of rock, metal , and/or ice orbiting th e Sun are called meteoroids. When these particles enter the Earth's atmosphere, th ey are heated to incandesence by friction; these glowing particles are called meteors (or "shooting stars" or "falling stars"). Mos t meteors are small and burn up while still in the atmosphere, but about 150 per year are large eno ugh to strike the Earth's surface . T hose that do are called meteorites (box figure 1). The largest fragment of a meteo rite found (in South Africa) weighs 50 tons; much larger meteorites have hit the Earth in the past. T hree basic types of meteorite are iron , stony-iron, and stony meteorites. Stony meteorites are by far the most commo n, but they look like Earth rocks, so they are hard to find. Iron meteorites are rare, but look so unique that they are commonly found; most m useum meteorites are of th e iron type. Iron meteorites are mostly iron alloyed (m ixed) with a small percentage of nickel. Small amoun ts of other metals or minerals may be pr esent. Sto ny- iron meteorites are made of iron-nickel alloy and silicate minerals in abo ut equal parts. Stony meteorites are ma de of silicate m ine rals such as plagioclase, olivine, an d pyroxe ne; th ey may contain a small amo un t of iron-nickel alloy. About 90 0/0 of stony meteorites contai n round silicate grains called chondru les and are called chondrites. T he ot her 100/0 are achondrites, wh ich lack chondrules. Chondrules consist mostl y of olivine and pyroxene, and range from distinct spheres to large bodies with fuzzy outlines. The composition of chondrite me teorites resembles th e ultramafic rock peridotite, but peridotite lacks the chondritic texture and iron-nickel content of the meteorites.

If the crust and the mantle, which have app roximately 85% of the earth's volume, are at or below the average densi ty of the earth, then th e core must be very heavy to bring th e average up to 5.5 grn/cm' . Calculations show th at th e core has to have a density of about 10 grn/crn" at the core-mantle boundary, increasing to 12 or 13 gm/cm3 at the cen ter of the earth (figure 2 .10). This great density would be enough to give the earth an average densi ty of 5.5 gm/crn' . Under the great pressures existing in the core, iron would have a density slightly greater than that required in th e core. Iron

34

Chapter 2

M aterial similar to these meteorites may ha ve helped create th e earth, perhaps settling to the center of the earth because of m etal's high density. The composition of these meteorites, then, may tell us what is in the earth's core . Nickel is de nser th an iron, however, so a mixture of just iron and n ickel would have a density greater than that req uired in the core. (T h e other 90 0/0 of meteorites are mostly ultram afic rock and perhaps represent material that formed the eart h's mantle .) Seismic and density data, together with assum ptions based on meteorite com posit ion , po in t to a core that is largely iron , with at least the ou ter part bein g liqu id. The existence of th e earth's magne tic field, whic h is discussed lat er in thi s chapter, also suggests a me tallic core. Of cour se, no geologist has seen the core, nor is any one likel y to in the foreseeable future . But since so many lines of indirect evidence poin t to a liquid me tal outer core, m ost scientists accep t this th eory as the best conclusion th at can be made abo u t the core's composition. Geologists could be wrong abo ut the eart h's interior, but the current model of a solid rock mantle an d a liquid metallic outer core and a solid inner core is widely accepted beca use it is consistent with all available knowledge. A hollow earth is not.

Astrogeology Box 2.1 Figure 1 A large meteor ite from Meteor Crater, Arizona . Pocketknife for scale. Photo by Frank M. Hanna

O ne kind of chondrite is composed mostly of serpentine or pyroxene and con tains up to 50/0 organic materials, including carb on , hydrocarbon compounds, and amino acids. T hese m eteorites are called carbonaceous chondrites. All available evidence indicates th at the organic compounds were in fact prod uced by inorganic processes. Carbonaceous cho ndrites are of parti cular interest to scien tists because th ey are believed to have th e same composition as the original material from which the solar system was formed . Achondrites are generally sim ilar to ter restrial rock s in composition and t~xture. In com position they are mo st similar to basalts. Some have textures like ordina ry igneo us rocks, an d others are breccias with fragments of different com positions and textures. The origin of meteorites is controve rsial. Many meteorites have a coarse-grained texture, prob ably formed by slow cooling within a larger bo dy, such as a planet. The similarity in iron-nickel composition among iron m eteorites also suggests that the y are fragments from a single, large body. The larger body may have differen tiated into a heavy, iron-rich core and a lighter, rocky m antle before it fragmented into meteoroids. Isotopic dating shows that most m eteorites have the same age, 4 .6 billion years old.

The Core-Mantle Boundary T he boundary be~een the core and mantle is marked by great changes in density (figure 2.10) and temperature, as ,we see later in the chap ter. Here there is a transition zone up to 2 00 kilometers thick and an undulating border between mantle and core. Both the mantle and the core are undergoing convecti on , a circulat ion pattern in which low densi ty material rises and high density material sinks. Heavy portions of the mantle (including subducting plates) might sink to its base, but are un able to penetrate the denser core. Light portions of the core may rise to its top , but not into the mantle above. Continentsized blobs of liquid and liquid-crystal slush may accum ulate here, perhaps int erfering with (or even helping cause) heat loss from the core and causing changes in the earth's magnetic field. This boundary is an exciting frontier for geologic study, bu t data, of course, are sparse and hard to obtain.

Isostasy Is ost asy is a bal~nce or equilibrium of adj acent blocks of brittle crus t "floa ting" on the upper mantle. Since crustal rocks weigh less than mantle rocks, the crust can be thought of as floating on the denser mantle much as wood floats on wa ter (figure 2.1l). . Blocks of wood floating on water rise or sink until they displace an amount of water equal to their own weight. The weigh t of th e displaced water buoys up the wood blocks , allowing them to float. The higher a wood block app ears above

mixed with a small amount of a lighter element, such as oxygen, sulfur or silicon, would have the required density. Therefore, many geologists thi nk that such a mixture makes up the core. But a st udy of density by itself is hardly convincing evidence that the core is mostly iron, for many other heavy substances could be there instead. The choice of iron as the major component of the core comes from looking at meteorites (see Astrogeology Box 2.l) . M eteorites are tho ught by som e scientists to be remnants of th e basic material th at created our own solar system. An estimated 10 0/0 of me teori tes are composed of iron mi xed wi th small amounts of nickel.

Water A Continental crust Ocea nic crust

Mount ain

I I

Man tle . ~~~~~' - - -

B

Dept h of equal pressur e

Figure 2.11 Isostatic bala nce. (A) Wood blocks float in water with most of the ir bulk submerged. (B) Crustal blocks "float" on mantle in approximately the same way. The thicker the block , the deeper it extends into the mantle.

the water surface, the deeper the block extends under water. Thus a tall block has a deep "root." In a greatly simplified way, crustal rocks can be thought of as tending to rise or sink-gradually until they are balanced by th e weight of displaced mantle rocks. This concept of vertical movement to reach equilibrium is called isostatic adjustment. Just as with th e blocks of wood, once crustal blocks have come into isostatic balance , a tall block (a mountain range) extends deep into the mantle (a m oun!ain root, as shown in figure 2 .1l). Figure 2.11 shows both the blocks ofwood and the bloc ks of crustal rock in isostati c balanc e. The weigh t of th e wood is equal to the weight of the displaced water. Similarly, the weigh t of the crus tal blocks is equal to the weight of the displaced mantle. As a result, the rocks (and overlying sea water) in figure 2.11 can be thought of as sepa rated into vert ical columns, each with the same pressure at its base. At some dep th of equal p ressure each column is in balance with the other columns, for each column has the same weight. A column of thick continental crust (a mountain and its root) has the same weight as a column containing thin continental crust an d some of the upper mantle. A column containing sea water, thin oceanic crust , and a thick section of heavy mantle weighs the same as the other two columns. Figure 2.11 shows the crust as isolated blocks free to move pa st each other along vert ical faults , but this is not really a good picture of crustal structure. It is more accurate to think of the cr ust as bending in broad uplifts an d

The Earth's Interior

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35

Mass Wasting

W

hen material on a hillside has weathered (the process described in chapter 12), it is likely to move downslope because of the pull of gravity. Soil or rock moving in bulk at the earth's surface is called

mass wasting. Mass wasting

IS

one of several surficial processes. Other

processes of erosion, transportation, and deposition-involving streams, glaciers, wind, and ocean waves-are discussed in following chapters. Landsliding is the best known type of mass wasting. Landslides destroy towns and kill people. While these disasters involve relatively rapid movement of debris and rock, mass wasting can also be very slow. Creep is a type of mass wasting too slow to be called a landslide. In this chapter we describe how different types of mass wasting shape the land and alter the environment and what factors control the rapidity or slowness of the process. Understanding mass wasting and its possible hazards is particularly important in hilly or mountainous regions .

Earthflow at Stone Canyon, Los Angeles , California, 1980. ©Tom McHugh/Photo Researchers, Inc.

311

Table 13.1

Some Types of Mass Wasting 1 Slowest

Type of Movement

)

Less than 1 em/year

Creep (Debris)

Slide

1 to 5 km/hr

1 mm/day to 1 km/hr

(

Velocities generally greater than 4 km/ho ur )

Mudflow (Watersaturated debris)

Earthflow

(

) Fastest

Deb ris Flow

(

Flow

Increasing Velocities

Debris avalanche (Debris) Rock avalanche (Bedrock)

Debris Slide Rockslide (Bedrock)

Fall

)

)

Rockfall (Bedrock) Debris fall (Debris) (

" Landsli des"

)

1. The type of material at the start of movement is shown in parentheses. Rates given are typical veloci ties for each type of movement.

You may recall from pr evious cha p ters th at mountain s are p roduct s of tecto n ic forces . Most m oun tains are associated with p resent or past co nvergi ng plate bo undaries. If tectonism were not at work, the su rfaces of the continents wo uld lo ng ago have been reduced to fea tu reless plains due to weatherin g and erosion . W e co nsi der th e m ateri al on mountain slopes or h illsides to be o ut of eq uilibr ium w it h respect to grav ity. Because of th e for ce of gravity, th e various age nts of erosio n (moving water, ice, and wind) work to mak e slo pes gen tle r and th er efor e in cre asingly m ore sta ble. The process of erosio n discussed in th is chap te r is mass wasting. Mass wasti ng (also called mass movem ent) is movement in which bedrock, rock deb ris, or soil moves downs lop e in bulk, or as a mass, because of th e pull of gravity. Mass wasting includ es movement so slow that it is almo st imperceptible (called creep ) as well as landslides, a gen eral term for the slow to very rapid descent of rock or soil. M ass wastin g affects humans in ma ny ways. Its effect s ranze fro m the devastation of a killer landslid e (see box 13. 1) to tIle n uisa nce of having a fence slowly pulled apa rt by soil creep. Th e cost in lives and p rop ert y from landslid es is surpri sin gly high . Accord ing to the U. S. G eolo gical Sur vey, more people in th e United Stares d ied fro m landslid es du rin g th e last three mo nths of 1985 than were killed during the last twenty years by all othe r geologic hazards , such as earthqu akes and volca nic eru ptions. O ver time, landslid es have cost Americans triple th e co m bine d costs of earthquakes, h urr icanes, floods, and torn ad oes. On average, th e ann ua l cost of landslid es in the Uni ted States has been 1. 5 billion dollars and 25 lost lives. In many cases of mass wasti ng, a lit-

312

Chapter 13

tie knowledge of geology, along with ap pro p riate pr eventive action, cou ld h ave avert ed destruction .

Classification of Mass Wasting A number of systems are used by geologists, engineers, and others for classif)rin,g mass wastin g, but non e has been un iversally accep ted . Some are very com plex and useful only to the specialist. The classificat ion system used here and summarized in table 13.1 is based on (1) rate of movement, (2) type of material, and (3) nature of the movement.

Rate of Movement A landslide like the on e in Peru (box 13.1) clearly involves rapid movement. Ju st as clearly, movement of soil at a rate of less than a centimeter a year is slow mo vement. Between these extremes is a wide range of velocities.

Type of Material Mass wasting processes are usually distin guished on th e basis of whe ther the descending mass started as bedrock (as in a rockslide) or as debris. The term debris, as applied to mass wasting pro cesses, m eans any un consolidated material at the earth's surface, such as soil and rock fragme nts (weathe red or un weathered) of any size. The amount of water (or ice an d snow) in a descending mass stro ngly influ en ces the rate and typ e of mo vement.

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s a result of a tragic combi nation of geological conditions, on e of the mo st devastat ing landslides in history destroyed the town of Yungay in Peru in 1970 . Yungay was one of the most picturesque towns in the Santa River Valley, whic h runs along the base of the high est peaks of the Peruvian Andes. H eavily glaciated Neva do H uascaran , 6,663 meters (21,860 feet) above sea level, rises steeply above the populated narrow plains along the Santa River. In May 1970 a sharp earthq uake occurred. The earthquake was centered offshore from Peru about 100 kilom eters from Yungay. Although the tremors in this part of the Andes were no stronger than tho se that have done on ly light damage to cities in the United States, many poorly con structed homes collapsed. Because of the steepness of the slopes, thousands of small rockfalls and rockslides were triggered. The greatest tragedy began when a slab of glacier ice about 800 meters wide, perched near the top of Huascaran , was dislodged by the shaking. (A few years earlier American climbers returning from the peak had warned that the ice looked highly unstable. The Peruvian press briefly noted the danger LO the towns below, but the warning was soon forgotten .) The mass of ice rapidly avalanched down the extremely steep slopes, breaking offlarge masses of rock debris, scooping out small lakes and loose rock that lay in its path. Eyewitnesses described the mass as a rapidly moving wall the size of a ten-story buil ding. The sound was deafening. More than 50 mi llion cubic meters of muddy debris traveled 3 .7 kilometers (I2,000 feet) verticall y and 14. 5 kilometers (9 miles) horizontally in less than four minutes, at taining speeds between 200 and 435 kilometers per hour (125 to 27 0 miles per hour). The main mass of mate rial traveled down a steep valley until it came to rest blocking the Santa River and bur ying about 1,800 people in the small village of Ranrahirca (box figure 1). A relatively small part of the mass of mud and debri s that was moving especially rapidly shot up the valley sidewall at a curve and overtopped a ridge. The mass was mornentarilv airborne before it fell on the town of Yungay, completely burying it under several meters of mud and loose rock. Only the top of the church and top s of palm trees were visible, marking wh ere the town center was

Box 13.1 Figure 1 Air photo showing the 1970 deb ris avalanche in Peru, which buried Yungay. The main mass of debris dest royed the small village of Ranrahirca. Photo by Servicio Aerofotografico de Peru. co urtesy of U.S. Geological Survey

buried (box figure 2). Ironically, the cemetery was not buried because it occupied the high ground. The few survivors were people who managed to run to the cemetery. The estimated death toll at Yungay was 17,000. This was considerably more than the town's normal population, becau se it was Sunday, a market day, and many familie s had come in from the country. For several days after tl';e slide the debris was toO muddy for people to walk on , but within three years grass had grown over the site. Except for the church steeple and the tops of palm trees that still protrude above the ground, and the crosses

Mass Wasting

313

Flow

Origina l position of mass

Slide

Tree was here

Original position

B Original position of mass

1--'>'-

Moving mass -

+---

-

-

Moving mass -

-l'--\--

---->,c ~

Fall

Translational slide

-'r--

-

-

-/-- Origin al position of cliff

-1---1--

A

Rotationa l slide

- Falling rock

C

Box 13.1 Figure 2 (A) Yungay is comp letely buried , except for the cemetery and a few houses on the small hill in the lower right of the photograph. (8) Behind the palm trees is the top of a church buried unde r five meters of debris at Yungay's central plaza . (C) Thr ee years later.

Figure 13.1 Flow, slide , and fall.

Photos A and B by George Plafker, U.S. Geolog ica l Survey; photo C by C. C. Plumm er

erected by funilies of tho se buried in the land slide, the former site ofYungay appea rs to be a scenic meadow overlooki ng the Santa River. The U.S. Ceological Survey and Peruvian geologists found evidence th at Yungay itself had been buil t on top of debris left by an even bigger slide in th e recent geologic past. More slides will almost surely occur here in the futu re. Forewarned, the Peruvian govern ment will not allow th e build ing of a new town in the danger area.

Furth er Reading Ericksen, G. E., G. Plafker, and J. Fernan dez Con cha. 1970 Preliminary report on the geologic events associated with the May 31, 1970, Peru earthquake. U.S. Geological Survey Circu lar 639

Controlling Factors in Mass Wasting

Type of Movement In general, th e type of movem ent in mass wasting can be classified as mai nly flow, slide , or fall (figure 13. 1). A flow im plies th at the descending ma ss is m oving down slop e as a viscou s fluid . Slide means the descending m ass rem ains relatively

314

Chapter 13

imately parallel to the slop e of the surface. A rotational slide (also called a slump) involves mov em ent alon g a curv ed surface, the upper part mo ving downward whi le the lower part moves outward.

coherent, mo ving along one or m ore well-defined sur faces. A fall occurs when m ater ial free-falls or bo unces down a cliff. Two kinds of slip are shown in figure 13.1. In a translational slide, the descending mass m oves along a plane approx -

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Table 13.2 sum marizes the factors th at influence th e likelihood and the rate of mov ement of m ass wasting. The table mak es apparent some of the reasons why the landslide (a deb ris av~anche) in Peru (box 13.1) occurred and why it mo ved so rapidly. (l ) The slopes were exceptionally steep, and (2) th e relief (the vert ical distance betw een valley floor and m ountain

summ it) was great, allowing th e mass to pick up speed and mo m entum. (3) Water and ice no t on ly added weigh t to the mass of debris but also acted as lubrican ts. (4) Abund ant loose rock and de bris were available in the course of the slide. (5) Where the slide began, there were no plants with roots to anchor loose material on the slope. Finally, (6) th e area is earthquake prone. Alt hough the slide would have occurred event ually even without an earthquake, it was triggered by an earthquake. Other factors infl uence suscep tibility to m ass wasting as well as its rat e of movement. T he orientation of planes of weakness in bedrock (bedd ing p lan es, foliation planes, etc.) is important if the m ovem ent in volves bedrock rath er th an debris. Fractures or bedding planes ori ented so that slabs of rock can slide easily along the se surfaces greatly increase the likelihood of m ass wastin g.

Mass 1XIasting

315

Table 13.2

Vertical slope

Summary of Controls of Mass Wasting

Fil ms of water

___-

Driving Force: Gravity

Water

Contributing Factors

Most Stable Situation

Most Unstable Situation

Slope angle Local re lief Thickness of deb ris over bedrock Orientation of planes of weakness in bedrock Climatic factors: Ice Water in soil or debris Prec ipitation

Gentle slope s or horizontal surface Low Slight thickness (us ually) Planes at right angles to hillside slopes

Ste ep or vertical High Great thicknes s Planes parallel to hill side slopes

Vegetation

Sand grains

Gravity A

Temperature stays above freezing Film of water around fine particles Frequent but light rainfall or snow Heavily vegetated

Freezing a nd thawing for much of the year Saturation of debr is with water Long periods of drought with rare episodes of heavy precipitation Sparsely vegetated

B

Figure 13.3 The effect of water in sand . (A ) Unsaturated sand held together by surface tension of wate r. (B) Saturated sand grains forced apart by water; mixture flows easi ly.

A

Normal force

Triggering Mechanisms: (1) earthquakes; (2) weight added to upper part of a slope; (3) undercutting of bottom of slope. Shear force C limatic controls inh ibit some types of mass wast ing and aid others (table 13.2). Clima te infl uences how m uch and what kinds of vegetation grow in an area an d what type of weathering occurs. Infrequent but heavy rainfall aids ma ss wast ing because it q uickly satu rates debri s that lacks the protective vegetati on foun d in wett er clim ates. By contrast, rain that dri zzles interm ittently much of th e year results in vegetation th at tends to inhibit m ass wasting. In cold clima tes, freezing an d thawing cont rib ute to downslope move me nt.

Gravity Grav ity is th e driving force for mass wasti ng. Figure 13.2A and B show gravity acting on a block on a slope. T he length of th e vertica l arrow is prop orti on al to th e force- the heavier th e material, th e lon ger the arrow. T he effect of gravity is resolvable into two compone nt forces, in dica ted by th e black arrows . One, the normalforce, is perpendicular to the slope and its value indicates the block's ability to stay in place because of frictio nal considera tions . The other, called th e shear force, is parallel to th e slope, and indicates the block's ability to move. T he length of th e arro ws is proportional to the stre ng th of each force. The steeper th e slope (and the heavier the block), th e greater the shear force and th e greater th e ten dency of the block to slide. Friction coun teracts the shear force. If friction is greater than th e shear force, the block will not move. If the force of friction is red uced (for instance, with wate r) so th at it is less th an th e shear force, th e block will slide. Simi lar forces act on deb ris on a hillside (figure 13.2 C). T he resistance to movement or deformation of th at debris is its shear stren gth. Shear streng th is controlled by facto rs such as the cohesiveness of th e material, fricti on between particles, and the ancho ring effect of plant roots. Shear stre ngth is also related to the norm al force. The larger th e no rm al force, the greate r the shear strength. If the shear strength is greater than the shea r force, the debris will not move or be deformed. O n the other hand, if shear str ength is less than shear force, the debr is will flow or slide.

316

Chapter 13

Partial ly weathered bedrock bends downslope

Building a heavy struc ture high o n a slo pe dem ands special pre cautions. To pr event m ovem ent of both the slope and the building, pilings may have to be sunk th rou gh th e deb ris, perhaps even into bed rock. D evelop ers may have to settle for fewer buildings th an plann ed if the weight of too man y struc tures will make the slop e un safe.

Water

B

Water is a criti cal facto r in mass wasting. When debris is saturated w ith wa ter (as from heavy rain or melting snow), it becomes heavier an d is more likely to flow downslop e. T h e added gravitat ional shear fo rce from th e incre ased weight, however, is p rob ably less imp ortant than the red uctio n in shear strength. This is du e to increase d p orepressure in which water forces grains of debris apart . Paradoxically, a small amo unt of water in soil can actu ally prevent downslope movement. When water does not completely fill th e pore spaces between the grains of soil, it forms a thin film aro und each grain (as shown in figur e 13.3). Loose grains adhere to one ano ther because of th e surface tension created by the film of water and shear strength increases. Surface tension of water between sand grains is what allows you to build a san d castle. T he sides of the castle can be steep or even vert ical because surface tension holds th e moist sand grains in place. D ry sand cannot be shaped into a sand castle because th e sand grains slide back into a pile that generally slopes at an angle of about 30 ° to 35 ° from the hori zontal. On the other hand , an experienced sand castle bu ilder also knows th at it is imp ossible to bui ld anyt hing with sand that is too wet . In this case the water completely occupies th e pore space between sand grains, forcing th em apart and allowin g them to slide easily past one ano the r. When th e tide comes in, or someo ne pours a pail of water on your sand castle, all you have is a puddle of wet sand. Simi larly, as the amount of water in debri s increases, rate of move me nt tends to in crease. D amp deb ris m ay not

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___________________________l.

Shear force

~

}.y

. \'t ~'-l l ( It~ '

~Shear

strength Figure 13.4 Indicators of creep. After C. F. S. Sharpe.

c Fig ure 13.2 Relationship of shear force and normal force to gravity. (A) For a block on a gent ly inclined surface. (8) For a block on a steep surface. (C) Forces acting at a point in debris. Shear strength is represented by a yellow arrow. If that arrow is longer tha n the one represented by shear force, debr is at that point will not slide or be deformed. move at all, whe reas m oderately wet deb ris mo ves slow ly downslop e. Slow types of ma ss wasting, such as creep, are gener ally cha racte rized by a relatively low ratio of water to de bris. Mudflows always have h igh rati os of wa ter to debris. A m udflow that continues to gain water eventually becom es a m uddy stream .

Common Types of Mass Wasting The common types of ma ss wasting are shown in table 13.1. H ere we wilJ describ e each type in detail.

Creep Creep is very slow, downslop e movem ent of soi l or un consolidated de bris. She ar for ces, over tim e, are only sligh tly greater th an shear str engths. The rate of m ovem ent is usually less th an a centi me te r per year an d ca n be d etected o nly by o bservatio ns taken ove r m onths o r years. W he n co nditions are right, creep can tak e pl ace alo ng nea rly hor izon tal slo p es. Som e indicators of creep are illustrated in fig u res 13 .4 and 13 .5 . M ass Wasting

317

--

Grass -cove red surface

Rock extends ---------==~=downward below freez ing level

A

Stationary debris Flowing debris

A

A

Fi,gure 13.7 Earthflow. (A) Eart hflow involving only flow. (B) Earthflow and rotational sliding.

Earthflow B

~"

Surfa ce level when frozen

~

':..

B

Figure 13.5 (A) Tilted gravesto nes in a churchyard at Lyme Regis, England (someone probably stra ightened the one upright gravesto ne). Grassy slope is inclined gently to the left. (B) Soil and partially weathered, nearly vertical sedimentary strata have crept downslope. Photo A by C. C. Plumme r; pho to B by Frank M. Hanna

c Figure 13.6

Two facto rs that contrib ute sign ifican tly to cree p are water in the soil and da ily cycles of freezing an d thawing. As we have said, wate r-saturat ed grou nd facil ita tes m ovement of soil down hill. W hat keeps downslope m ovem en t fro m becoming more rapid in mo st areas is the presen ce of abu nd ant grass or other plants tha t anc ho r th e soil. (U nd erstanda bly, overg razing can severely dam age slopi ng pastures.) Alt h ough creep does take place in year-ro u nd warm clim ates , the proces s is m o re act ive wh ere the so il freezes and thaws during part of th e yea r. Du ring the wi nte r in region s suc h as the north eastern U ni ted States, th e temperature may rise abo ve and fall below freezing on ce a d ay. Wh en there is mo istur e in t he so il, each freeze- th aw cycle

Downslope movement of soil, illust rated by following two sand grains (each less than a millimeter in size) durin g a freeze-th aw cycle .

m oves soi l pa rt icles a m in ute amount dow nh ill, as shown in figure 13.6.

Debris Flow The genera l term debris Row is used for mass wasting in wh ich motion is takin g place thro ughout the moving mass (Bow). The common varieties earthll ow, mu dllow, and debris avalanche are described in this section.

In an earthRow, debris moves dow nslop e as a viscous Buid; th e process can be slow or rapid. Eart hflows usually occur on hillsides th at have a th ick cover of deb ris, often after heavy rains have satura ted the soil. Typically, the Bowing mass remains covered by a blanket of vegetation , with a scarp (steep cut ) developing where th e mo ving debris has pull ed away from the stationary upper slope. A land slide may be enti rely an earthllow, as in figure 13.7A, with debris particles moving past one another roughly parallel to the slope. Commonly, however, rotational slid ing takes place above the earthfiow as in figures 13.7B and 13.8. This exam ple is a debris slide (upper part ) and an earthjlow (lower part). In such cases, debr is rema ins in a relatively cohe rent block or blocks that rotate downward and outwa rd, forcing the debris below to Bow. A hummocky lobe usually forms at the toe or fron t of the eanhBow where debr is has accumulated. An earthflow can be active over a perio d of hours, days, or months; in some earthflows int ermitt ent slow mo vement cont inues for years. H uma ns can trigger earth Bows by adding roo m uch water to soil from septic tan k systems or by overwateri ng lawns. In one case, in Los Angeles, a man depart ing on a long trip forgot to turn off the sprinkler system for his hillside lawn. The soil became satu rated, and both house and lawn were carried downward on an earthllow whose lobe spread out over th e highway below. Figure 13.8 Earthflow that destroyed several houses in March, 1995 at La Conchita, Califo rnia. Photo by Robe rt L. Sch uster, U.S. Geo logi ca l Survey

3 18

Chapter 1.3

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Mass Wasting

3 19

Figure 13.9

hefOllowing satiricalnewspaper column waswritten by humoristArt Buchwald in 1978, a year, like the 'i l Nino"year of1998, in which southern California had many landslides because ofunusually wet weather.

A Winter

Landslide with houses and roads moved on slide blocks. Point Fermin, Los Angeles, California . Photo by Frank M. Hanna

Zone that thaws during summer

Water-saturated debris

Eart hflows, like other kinds of landslides, can be triggered by undercutt ing at the base of a slope. T he undercuttin g can be caused by waves break ing along shorel ines or streams eroding and steepening the base of a slope. Alon g coastlines, mass wasting commonly destro ys buildings (figure 13.9). Entire ho using developm ent s and expensive ho mes built for a view of the ocean are lost. A home buyer who knows nothing of geology may not realize th at th e sea cliff is there because of the relentless erosion of waves along the shoreline. N or is th e person likely to be aware tha t a steepened slope creates th e potential for land slides. Bulldozers can undercut th e base of a slope mor e rapidly th an wave erosion, and such oversteepe ning of slopes by hu man activity has caused many landslides. Unless careful engineerin g m easures are taken at the tim e a cut is mad e, roadcut s or platform s carved into hillsides for houses may bring about disaster (figure 13.18).

B Summer

Figure 13.10 Solifluction due to thawing of ice-saturated debris.

Solifluction and Permafrost One variety of earr hflow is usually associated with colde r clima tes. Soli fluction is th e flow of wate r-satura ted debr is over imp erm eable material. Becau se th e imperm eable material beneath th e debr is pre vents wa ter from drain ing freely, the deb ris between the vegetation cover an d the impe rmeable ma terial becomes satu rated (figure 13.10). Even a gentle slope is suscep tible to m ovement un der these con dition s (figure 13.11 ). The impermeable material beneath the satura ted soil can be eithe r imp enetrable bedrock or, as is more commo n, perm afrost, ground that remains frozen for many years. Most

solifluction takes place in areas of perm anentl y frozen ground, such as in Alaska and northern Ca nada . Perm afrost occurs at depths ranging from a few cent imet ers to a few meters beneath the surface. T he ice in permafrost is a cement ing agent for the debris. Permafrost is as solid as concrete. Above th e permafrost is a zone that , if the debri s is satu rated, is frozen during the winter and indi stingui shable from

Figure 13.11 A railroad built on permafrost terrain in Alaska. Photo by Lynn A. Yehle, U.S. Geologic al Survey

320

Chapter 13

Los Angeles-I came to Los Angeles last week for rest and recreatio n, only to discover that it had become a rain forest. 1 didn't realize how bad it was unt il I went to dinner at a friend's house . I had the right address, but when I arrived there was nothing there . I went to a neighboring house where I found a man bailin g our his swimm ing pool. I beg your pardon, I said. Co uld you tell me where th e Cables live? "They used to live above us on th e hill. T hen , about two years ago, their house slid down in th e mud, and they lived next do or to us. I think it was last Monday, during th e stor m, th at th eir hou se slid again, and now they live two streets below us, dow n th ere. We were sony to see them go-they were really nice neighb ors." I th anked him and slid straight down the hill to the new location of the Cables' hou se. Ca ble was clearing out the m ud from his car. H e apologized for not giving me th e new address and explained, "Frankly, I didn't know until this morn ing whether the hous e wou ld stay here or continue sliding down a few more blocks." Cable, I said, yOLl and your wife are inte lligent peop le, why do you build your house on the top of a canyon, when you know that during a rainstorm it has a good chance of sliding away? "We did it for the view. It really was fanta stic on a clear night up there . We could sit in our Jacuzzi and see all of Los Angeles, except of course when there were brush fires. "Even when our house slid down two years ago, we still had a great sight of the airport. Now I'm not too sure what

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kind of view we'll have because of the hous e in front of us, which slid down with ours at the same time. " Bur why don't you move to safe ground so th at vou don't have to worr y about rainstorms? . "\Y/e'v.e, thought about it. But ? nce you live high in a canyon , It s hard to move to the plains. Besides, this house is built solid and has about thre e more good mudslides in it. " Still, it must be kind of hair y to sit in your home dming a deluge and wonder where you'll wind up next. Don't vou ever have the desire to just settle down in one place? ' "It's hard for peop le who don't live in California to und erstand how we people out here th ink. Sure we have floods, and fire and drought, bur that's the price you have to pay for living the good life. When Esther and I saw this hous e, we knew it was a dr eam come true . It was located right on the tippy top of the hill, way up there. We would wake up in th e morning and listen to the birds, and eat breakfast out on the patio and look down on all the smog. "T hen, after th e first mudslide, we foun d ourselves living next to peopl e. It was an entirely different experience. But by that time we were ready for a change. N ow we've slid again and we're in a whole new neighborhood . You can't do that if you live on solid ground. Once you move int o a house below Sun set Boulevard, vou're stuck th ere for the rest of your life. ' "W hen you live on th e side of a hill in Los Angeles, you at least know it's not going to last forever." Then , in spite of what's happened, you don 't plan to move out? ''Are you crazy? You couldn't replace a ho use like this in L.A. for $500,000. " What happens if it keeps raining and you slide down the hill again? "It's no problem. Esther and I figure if we slide down too far, we'll just pick up and go back to the top of the hill, and start all over again; that is, if th e hill is still there after the earthquake." Reprinted by permission of the author.

Further Reading John McPhee's The control of nature (see Exp loring Resources ) contains a factual. and highly readable, account of 1978 lands lides in southern California.

th e underlying permafrost. When th is zone thaws d uring th e summer, the water, along with water from rain and run off, canno t percolate downward th rough th e perm afrost, and so the slopes become susceptible to solifluction . As solifluction movement is not rapid eno ugh to break up the overlying blanke t of vegetation int o blocks, the watersaturated debris flows downslope , pulling vegetation along MaJS Wasting

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n Earth mos t un derground wa ter is in the liq uid form, with the only abundant underground ice occurring as permafrost in the polar region s. In these areas underground ice is responsible for producing several distinctive landforms; most no table are surfaces with polygona l cracks called patterned ground (box figure l A ). On Mars, because of its cooler temperatures, ice is the normal state of underground water and is apparently quite abundant. Man y features of the Martian landscape have been attributed to this underground ice, including polygonally patterned ground (box figure IE) , rampart craters, and chaotic terrain . Ramp art craters are M artian meteorite craters that are surrounded by mat erial that appears to have flowed from the point of impact. It has been suggested that the underground ice in the surface layers of Mars melt ed as a result of the impact and , along with rock fragments, sloshed outward from the impac t area.

with it and form ing a wrinkled surface. Gradu ally th e debris collects at the base of th e slope, wh ere th e vegetated surface bulges int o a hummocky lobe. Solifluction is no t th e onl y hazard associated with perma frost. Great expanses of flat terrain in arctic and su barctic clim ates beco m e swampy during the sum m er because of permafrost, makin g overland tr avel very difficult. Building and maintaining roads is an eng ineering headach e. In th e preliminary stages of plan ning th e Alaska pipe lin e, a road was buJldo zed across perm afrost terrain durin g the wi nter, remo vin g th e vegetation from th e rock-hard gro und. It was an excellent tru ck rou te d ur ing the winter, but when summer cam e, th e road becam e a quag mire several hundred kilometers lon g (see box 1.2 , figur e 1). The strip can n ever be used by vehicles as pla nned, nor will th e vegetation return for many decad es. Buildi ng struc tures on perm afro st terr ain presents serious problems. For instance , heat from a bu ild-

Astrogeology Box 13.1 Figure 2

A

Chaotic terrain with associa ted chann els on Mars. Photo is a compos ite of several images . White rectangles are where images were not taken.

Astrogeology Box 13.1 Figure 1

Photo by NASA

(A) Patterned ground in Alaska. Coarse rock forms the edges of polygons; fine-grained materia l in cente r supports vegetation . (8) Pattern ed ground on Mars. The polygons in photo 8 are over 100 times larger than those in photo A. Note the impact craters .

8

Photo A by C. C. Plummer ; Photo B NASA

Patches of jumbled and broken angular slabs and blocks called chaotic terrain occur in some places on Mars (box figure 2). Som e channels originate in these areas, and it is believed that this terrain may be caused by melting of

ing can melt underlyin g permafrost; th e building th en sinks in to th e mud.

Mudflow A mudflow is a flowing mixtu re of debris and water, usually moving down a channel (figu re 13.12). It can be visualized as a stream with the con sisten cy of a thi ck milks hake . Mo st of the solid particles in the slurry are clay and silt (hence the muddy appearance), but coarser sediment comm only is part of the mixture. Usu ally after a heavy rain fall a slur ry of debris and water forms and begin s moving down a slop e. Most mudflows qui ckly become channeled into valleys. They the n move down valley like a stream except that, because of the heavy load of debris, they are more viscou s. Mud moves more slowly th an a stream but, because of its high viscosity, can tran sport boulders, auto mobiles, and even locomotives.

I

permafrost and consequent collapse of the grou nd. Subsidence du e to withdrawal of magma has also been suggested as the cause of chaotic terrain. Some astrogeologists think that th ere is a good chance that Mars is the only other plane t besides Earth that has life. According to their hypothesis, primiti ve organisms evolved

during an earlier tim e when Mars had liquid water. The microorganisms might be dormant in the permafrost and perhaps could be revived by melting the ice. This hypothesis received a boost in 1996 when scientists studying a met eorit e from Mars reported that it revealed possible signs of former life (see box 19.4). H ardly the alien creatures pictured in science fiction movies or books (e.g., figure 9.16), but organ ic life nevertheless.

Houses in the path of a mudflow will be filled with mud, if no t bro ken apart and carried away. Mudflows are most likely to occur in places where debris is not protected by a vegetativ e cover. For this reason , mudflows are mo re likely to occur in arid regions than in wet climates. A hillside in a desert environment, where it may not have rained for many years, may be covered with a blanket of loose material. W ith sparse desert vegetation offering little protection, a sudden thunderstorm with d renching rain can rapidly saturate the loose debris and create a mudflow in minutes. Mudflows frequently occur on young volcanoes that are littered with ash. Water from heavy rains mixes with pyroclastic debris as at Mount Pinatubo in 1991 . Or the water can come from glaciers th at are melted by lava or hot pyroclastic Figure 13.12 A dried mudflow in the Peruvian Andes . Photo by C. C. Plummer

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wedging

Figure 13.13 Man examining a 75-meter-long bridge on Washington state highway 504 , across the North Fork of the Toutle River. The bridge was washed out by mudflow during the May 18, 1980 eruption of Mount St. Helens. The steel structure was carried about 0.5 kilometers downstream and partially buried by the mudflow .

Figure 13.16 Small dust clouds linger high above Yosemite Valley where rock slabs broke loose and fell to the valley floor which, upon impact, created the debris-laden blast of air climbing up the other side of the valley. The photo was taken by a rock climber on a nearby cliff.

Photo by Robe rt L. Schuster, U.S. Geo logical Survey

Photo by Ed Youma ns

debris, as occurred at Mount St. Helens in 1980 (figure 13.13) and at Colombia's Nevado del Ruiz in 1985, which cost 23,000 lives. Mudflows also occur after forest fires destroy slope vegetation that normally anchors soil in place. Burnedover slopes are extremely vulnerable to mudflow if heavy rains fall before the vegetation is restored. The year 1978 was particularly bad for debris flows in southern California. One mudflow roared through a Los Angeles subur b carrying almost as many cars as large boulders. A sturdily built house withstood the onslaught but began filling with muddy debris. Two of its occupants were pinned to the wall of a bedroom and could do nothing as the room filled slowly with mud. The mud stopped rising just as it was reaching their heads . Hours later they were rescued. (John McPhee's The Control ofNature, listed at the end of this chapter, is a highly readable account of this and other debris flows in southern California.)

Figure 13.15 Talus. '\ Photo by C. C. Plummer

Figure 13.14 Two examples of rockfall.

Rockfalls and Rockslides Rockfall

Debris Avalanche The fastest variety of debris flow is a debris avalanche, a very rapidly moving, turbulent mass of debris , air, and water. The best modern example is the one that buried Yungay (see box 13.1 , pgs. 313-314). Some geologists have suggested that in very rapidly moving rock avalanches, air trapped under the rock mass creates an air cushion that reduces friction . This could explain why some landslides reach speeds of several hundred kilometers per hour. Bur other geologists have contended that th e rock mass is too turbulent to permit such an air cushion to form .

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When a block of bedrock breaks off and falls freely or bounces down a cliff, it is a rockfall (figure 13.14) . Cliffs may form naturally by the undercutting action of a river, wave action, or glacial erosion . Highway or other construction projects may also oversteepen slopes. Bedrock commonly has cracks (joints) or other planes of weakness such as foliation (in metamorphic rocks) or sedimentary bedding planes . Blocks of rock will break off along these planes. In colder climates rock is effectively broken apart by frost wedging (as explained in chapter 12). Commonly, an apron of fallen rock fragments, called talus, accumulates at the base of a cliff (figure 13.15).

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A spectacular rockfall took place in Yosemite National Park in the summer of 1996, killing one man and injuring several other people. The rockfall originated from near Glacier Point (the place where the photo for figure 19.1 was taken). Two huge slabs (weighing approximately 80,000 tons) of an overhanging arch broke loose just seconds apart. (The arch was a product of exfoliation, and broke loose along a sheet joint-see chapter 12.) The slabs slid a short distance over steep rock from which they were launched outward, as if from a ski jump, away from the vertical cliffs. The slabs fell free for around 500 meters (1,700 feet) and hit the valley floor 30 meters out from the base of th e cliff (you would not have been hit if you were standing at the base of the cliff) . They shattered upon impact and created a dust cloud (figure 13.16) that obscured visibility for hours. A powerful air blast was created as air between the rapidly falling rock and the ground was compressed. The debris-laden wind felled a swath of trees between the newly deposited talus and a nature center building. In 1999, another rockfall in the same area killed one rock climber and injured three others .

Rockslide A rockslide is, as the term suggests, the rapid sliding of a mass of bedrock along an inclined surface of weakness, such as a bedd ing plane (figure 13.17), a major fracture in the rock, or a foliation plane (box 13.3). Once sliding begins, a rock slab usually breaks up into rubble. Like rockfalls, rockslides can be

caused by undercutting at the base of the slope from erosion or construction. Some rockslides travel only a few meters before halting at the base of a slope. In country with high relief, however, a rockslide may travel hundreds or thousands of meters before reaching a valley floor. If movement becomes vety rapid, the rockslide may break up and become a rock avalanche. A rock avalanche is a very rapidly moving, turbulent mass of broken-up bedrock. Movement in a rock avalanche is flowage on a grand scale. The only difference between a rock avalanche and a debris avalanche is that a rock avalanche begins its journey as bedrock. Ultimately, a rocksIide or rock avalanche comes to rest as the terrain becomes less steep. Sometimes the mass of rock fills the bottom of a valley and creates a natural dam. If the rock mass suddenly enters a lake or bay, it can create a huge wave that destroys lives and property far beyond the area of the originallandslide. An example is a disastrous landslide that took place in northern Italy in 1963. A huge layer of limestone broke loose parallel to its bedding plan es. The translational slide involved around 250 million cubic meters that slid into the Vaiont Reservoir creating a giant wave. The 175-meter (almost two football fields) high wave overtopped the Vaiont Dam (it was the world 's hizhest dam , rising 265 meters above the valley floor). Three thousand people were killed in the villages th at it flooded in the valleys below. The dam was not destroyed , a tribute to excellent engineering, but the men in charge of the building project were convicted of criminal negligence for ignoring the landslide hazards. Mass Wasting

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Wat er softens shale below saturated sandstone

Layer of shale

Slide debris dams river

n 1928 the St. Francis Dam near Los Angeles, California, broke , only a year after it had been completed (box figure 1). The concrete dam was about 60 meters (200 feet) high, and the wall of water that roared down the valley killed about 400 people in two counties. The eastern edge of the dam had been built against a metamorphic rock with foliation planes parallel to the sides of the valley. Landslide scars in th e valley should have been ample warning to the buil ders th at the metamorphic rock moved even under on ly the force of gravity. A competent engineer worri es as much about the stability of the rock against which a dam is bu ilt as about the strength of the dam itself Water pressure at the base of the dam exerted a force of 5.7 tons per square foot against the dam. With pressure such as th is, the dam and part of the bordering foliated rock could easily slide. Movement would be parallel to the weak foliation planes, just as if the dam had been anchored against a gian t deck of cards. Ironically, investigators never found out for sure whether this was what caused the failure of the dam. Many other blunders had been made in construction, and any one of them could have caused the dam to break. The base of the dam was on a fault with ground-up rock; and, incredibly, the other side of th e dam was built against rock th at disintegrates in water. This is but one of man y instances in which ignorance of geology cost lives and mon ey. Had professional zeolozi cal advice been sought, the dam probably wou ld not 0 0 ' have been built in that spot.

I B

A

Figure 13.17 (A ) and (8) Diagram of the Gras Ventre, Wyoming, slide . (e) Photo of Gras Ventre slide. A and B after W. C. Alde n. U.S. Geologica l Survey.

Photo C by D. A. Rahm, c ourtesy of Rahm Memor ial Colle ction, Western Washington University

As in slower mass movements, water can play an important role in causing a rockslide . In 192 5, exceptionally heavy rains in the Gra s Ventre Mo unta ins of Wyoming caused water to seep into a layer of sandstone, wetting the underlying layer of shale, greatly redu cing its shear strength (figure 13.1 7). T he layers of sedimentary rock were inclined roughly parallel to the hillside. W irh th e wet shale acting as a lub ricant, the overlyin g sedimentary rock and its soil cover slid into th e valley, blockin g the river. T he slide itself merely created a lake, but the natural da m broke two years later and the resulting flood destroyed the small town of Kelly several kilometers downs tream . Several res-

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idenrs who were standing on a bridge watching th e floodwaters come down th e valley were killed.

Debris Slides and Debris Falls As th e names suggest, debris slides and debris falls behave similarly to rockslides and rockfalls, except th at th ey involve debris that moves as a coherent mass (at least initiall y). A debris fall is a free-falling mass of debris. A debris slide is a coheren t mass of debris movi ng along a well-defined surface (or surfaces). If the movement is along a http://www.mhhe.com/earthsci/geology/plummer

curved surface the landslide is a rotati onal debris slide. D ebris slides were menti oned earlier with earthflows , with which th ey are commo nly associated (figure 13.7). Debris may slide, however, witho ut an earthflow taking place.

Preventing Landslides Preventing Mass Wasting of Debris Usually m ass movem ents of debris can be prevented . Proper enginee ring is essential whe n the natu ral environment of a

A

B

Box 13.3 Figure 1 (A) The 51. Francis Dam before failure (looking upstream). (8) The 51. Francis Dam after failure (looking downstream) . Note pieces of the dam carried dow nstream and the high-water mar k in the stand ing segment of the dam. Photos by Californ ia Depar tment of Water Resourc es

Related Web Resources hltp://seis.natsci.csulb.edu/VIRTUAL_FIELD/ Francesquito_Dam/fra nmain ,htm 31. Francis Dam virtual field trip

hillside is alte red by construc tion. As shown in figure 13.18, construc tio n gene rally mak es a slope more suscept ible to mass wasting of debris in severa l ways: (1) th e base of th e slope is unde rcut , removing th e natural support for th e up per part of the slope; (2) vegetation is remo ved d uring cons truction ; (3) buildings con structed on the upper pa rt of a slope add weight to the potential slide; and (4) extra water may be allowed to seep into th e debr is. . Som e preventive measu res can be taken during const ructio n. A reta ining wall is usually built where a cut has .b een made in th e slope, but th is alone is seldo m as effective a Mass Wasting

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Water trapped in soil causes movement, pushing down retain ing wal l. Before construction

A Sprinkler adds water to soil

B

Figure 13.20

Building adds weight to slope Water drains through pipe, allowing wall to keep slope from moving.

Fill Vegetation removed

(A) Cross section of a hill show ing a relatively safe road cut on the left and a hazardou s road cut on the right. (B) The sa me hazardous road cut afte r removal of rock that might slide.

builders mig ht avoid a hazard by choo sing the least dangerous route for the road. If a road cut mus t be made thro ugh bedrock that appears prone to sliding, all of th e rock that might slide could be removed (sometimes at great expense), as shown in figure 13.20B. In some insta nces, slopes prone to rock sliding have been "stitched" in place by th e technique show n in figur e 13.21.

Steep ening of slope for road cut

Figure 13.21 Figure 13.18

B

A hillside becomes vulnerable to mass wasting due to constr uction activities.

deterren t to down -slop e movem ent as peop le hop e. If, in addit ion , drai n pipes are put th rou gh th e retaining wall and into the hillside, wa ter can percolate th rou gh an d d rain away rather than collecting in the debris behi nd the wall (figure 13.19). Without dr ains , excess water results in decreased shear strength and the whole soggy mass can easily burst through the wall. Another practical preventive measure is to avoid oversteepening the slope. T he hillside can be cut back in a series of terraces rat her than in a sing le steep cut. T h is not only redu ces the slope angle bu t also red uces t he shear force by rem oving mu ch of th e overlying material. It also prevent s loose material (such as bould ers dislod ged from the top of the cut) from rolling to the base. Road cuts constructed in th is way are usually reseeded with rapid ly growing grass or plants whose roots help anch or th e slope. A vegetation cover also minimizes erosion from running water.

Figure 13.19 Use of drains to help prevent mass wasting.

"Stitching" a slope to keep bedrock from sliding along planes of weakness . (Al Holes are drilled through unstable layers into stable rock . (B) Expanded view of one hole. A cable is fed into the hole and cemen t is pumpe d into the bottom of the hole and allowed to harden. (C ) A steel plate is placed over the cable and a nut tightene d. (0) Tightening all the nuts pulls unstable layers together and anchors them in stable bedrock. (E) Stabilized road cut along Richardson Highway near Valdez, Alaska. Photo by Paul G. Bauer

Preventing Rockfalls and Rockslides on Highways Rockslides and rockfa lls are a majo r prob lem on highways bui lt through mountainous countr y. Steep slopes and clifts are created wh en road cuts are blasted and bulldo zed into mo unta in sides. If the bedrock has planes of weaknes s (such as joints, bedding plan es, or foliation plan es), th e orientatio n of th ese plan es relative to th e road cut determin es whether there is a rockslide hazard (as in figur e 13.20). If the planes of weakness are incli ned into the hill , th ere is no chance of a rockslide . O n th e other han d, whe re th e planes of weakness are appro ximately parallel to the slope of the hillside, a rockslide may occur. Various techniques are used to prevent rockslides. By doing a detailed geologic study of an area before a road is built, E

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Mass wasting is the movem ent of a mass of debris (soil and loose rock fragments ) or bedrock toward the base of a slope. Movement can take place as a flow, slide , or fall. Gravity is the d riving force. The com po nen t of gravitation al fo rce tha t prop els mass wasting is the shearforce, which occurs parallel to the slopes. The resistance to th at for ce is the shear strength of rock or debri s. If shear force exceeds shear strength, mass wasting takes place. Water is an im port ant factor in mass wasting.

A number of other factor s determine whether movem ent will occur and, if it do es, the rate of movement. The slowest type of mov em ent, creep, occur s mostly on relati vely gen tle slopes, usuall y aided by water in th e soil. In colder climates, repe ated freezing and th awin g of water within th e soil contributes to creep. Landsliding is a general term for m ore rapid mass wastin g of rock, debris , or both. Debris flows include earthflows, mudflows, and debris avalan ches. Earthfl ows vary greatly in velocity alt hough th ey are not as rapid as

debris avalanches, wh ich are tu rbul ent masses of debris, water, and air. Solifluction, a special variety of earrhflow, usuall y takes place in arctic or subarctic climates wher e gro und is perm an ently frozen (permafrost) . A mudflolU is a slur ry of debris, and wat er. Mos t mudflows flow in channels much as streams do. Rockfall is th e fall of broken rock down a vertical or near-vertical slope. A rockslide is a slab of rock sliding down a less-th an-vertical surfac e. Debris fi lls and debris slides involve unconsolidated material rathe r than bedrock.

1. W hy do peopl e fear earthq uakes, hurricanes, and tornadoes more than they fear landslides? 2. If you were building a hou se on a cliff, what would you look for to ensure that

,< ,\\ Brabb, E. E., and B. L. H arrod. ~ 1989 . Landslides: Extent and economic significance. Brookfield , Vermont : A. A. Balkema. Denn en , W H. , and B. R. M oore. 1986. Geology and engineering. Dubuque, Iowa: \'(!m. C. Brown Publi shers.

creep 317

flow 314

rockslide 325

debris 312

landslide 312

rotational slide (slump) 3 15

debris avalanche 324

mass wasting 312

shear force 3 16

debris fall 326

mudflow 322

shear stren gth 3 16

debris flow 3 18

permafrost 320

slide 314

debris slide 326

relief 315

solifluction 320

eart hflow 3 19

rock avalan che 325

talus 324

fall 314

rockfall 324

translational slide 3 14

Use th e que stions below to prep are for exams based on this chapter. ] . Describe the effect o n shear strength of the following: (a) thickness of debri s; (b) orientation of planes of weaknes s; (c) water in debri s; and (d) vegetation. 2.

C om pare the shear force to the for ce of gravity (drawing diagrams sim ilar to figur e 13.2) for the following situ ations: (a) a vert ical clift; (b) a flat horizontal plan e; and (c) a 45° slope.

McPhee, J. 1989. The control of nature. New York: No onday Press. Nuhfer, E. B., R. J. Proctor, and P. H . Mo ser.

1993. The citizens guide to geologic hazards. Arvada, Co lorado: Am erican Institute of Professio nal Ge ologists. Ritter, D . E, R. C. Kochel , and J, R. Mill er. 1995. Processgeomorphology. 3d ed. Dubu qu e, Iowa: Wm. C. Brown Publi shers.

Ex pand )/oltr /mowledge of the concepts p,.esentcd in this chapter by ltSing the CD-ROM to ansu.er thefollowing questions.

10. The driving force behind all mass wasring processes is (a) gravity (b) slope angle (c) type of bedro ck m aterial (d) presen ce of water (e) vegetation

1. Convergent lvlargins module. Go to "Alaska" click all "Earthq uake

11. The resistan ce to movement or deformation of debri s is irs (a) mass (b) shear strength (c) shear force (d) density

4.

What role docs water play in each of th e typ es of mass wasting?

5.

Why is solifluction more common in colde r climates than in temperate clim ates?

14. An apron of fallen rock fragments that accum ulates at the base

List and explain the key Iactors that cont rol mass wasting.

15. How does construc tion destabilize a slope? (a) adds weight

What is the slowest type of mass wasting proce ss? (a) debris flow (b) rockslide (c) creep (d) rockfall (e) avalan che

8.

Any un consolidated material at the earth's surfa ce of any size is called (a) debris (b) sediment (c) soil (d) talu s

9.

A descending mass moving downslope as a viscou s fluid is referred to as a (a) fall (b) landslide (c) flow (d) slide

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Landslides: investigation and mitigation. Transport Research Board Special Report 247 . Washingto n, D. C. : Nati onal Academy of Sciences.

a::::aln thepath ofa killer volcano. Films for the Human ities & Scien ces, Princeton, N .J, C ont ains footage of volcani c mudflow associated with the eruption of M ount Pinatu bo, Philippines. ~E> http://Iandslides.usgs.gov/

~ Geologic hazards, landslides, Us. Geological Survey. You can get to several useful sites from here . Reports on recent land slides can be accessed by clicking on the

Foot age." Earthquakes often trigger landslides. Look for evidence of m ass wastage in this footage of the ] 964 Alaskan earthquake. What typ e of mass wastage happened here?

on es listed. Click on "National Landslide Info rm ation Center" for ph otos of landslides, including some described in thi s chap ter. Watch animation of a landslide. You can access sources of information on landslides and other geologic features for any sta te, usually from a state's geologic sur vey. http: //srs.gsc.nrcan .gc.ca/ page I Igeohl slide.htm

Landslides and snow avalanches in Canada. Geo logical Survey of Ca nada's site has genera lized descrip tio ns and som e photos of significant Canadian lan dslides.

2. Volcanoes module. Click on "Watch a Volcano Grow," then "Lava Dome. " Wh at role docs mass wasting play during formation of a lava dome?

called (a) solifluction (b) flow (c) slide (d) fall

H ow does a rot ation al slide differ from a translational slide?

7.

Turner, A. K., and R. L. Schuster, eds. 1996.

mass wasting as well as by other erosional agents? 4. Ca n any of the indicato rs of creep be explained by processes other th an mass wastin g?

12. Flow of wat er-saturated debris over imp ermeable material is

3.

6.

your house would not be destroyed through mass wasting? 3. Why isn't th e land surface of the earth flat after millions of years of erosion by

13. A flowing mi xture of debris and wat er, usually moving down a channel is called a (a) mudflow (b) slide (c) fall (d) debris flow of a cliff is called (a) debris (b) sediment (c) soil (d) talus to

th e top of th e slope (b) decrea ses water cont ent of the slope (c) add s weigh t to t he bottom of th e slope (d) increases the sh ear strength of the slope 16. How can land slides be prevented during con struction? (choose all that appl y) (a) retaining walls (b) cut steeper slopes (c) install water dr ain age systems (d) add vegetation

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Sediments and Sedimentary Rocks

T

he rock cycle (chapter 9) is a th eoretical model of the constant recycling of rocks as they form, are destro yed, and th en reform. We began our discussion of the rock cycle with igneous rock (chapters

10 and 11), and we now discuss sedimentary rocks. Metamorphic rocks, th e third major rock type , are the subject of the next chapter. Yousaw in chapter 12 how weathering produces sediment. In this chapter we explain more about sediment origin, as well as the erosion, transportation, sorting, deposition, and eventual lithification ofsediments to form sedimentary rock. Because they have such diverse origins, sedimentary rocks are difficult to classify. We divide them into clastic, chemical, and organic sediment ary rocks, but this classification is not entirely satisfactory. Furthermore, despite their great variety, only three sedimentary rocks are very commo n- shale, sandstone, and limestone. Sedimentary rocks contain numerous clues to their origin and the environment of deposition in which they were deposited . Geologists find ou t this information from th e shape and sequence of rock layers and from th e sediment grains and the sedim entary stru ctures such as fossils, cross-beds, ripple marks, and mud cracks th at are contained in the rock. Sedimentary rocks are impo rtant because they are widespread and because many of them, such as coal and limestone, are economically important. About three-fourths of the surface of cont inents is blanketed with a thin skin of sedimentary rocks. C oncentrated in sedimentary rocks are important resources such as crude oil, natural gas, ground water, salt, gypsum, uranium, and iron ore. Wea llleli ng and Erosion

Layers of sedimentary rocks exposed at Capito l Reef National Park, Utah. Photo © David Muenc h Photography

333

Sediment

Table 14.1

Most sedime ntary rocks form from loo se grains of sedim en t. Sediment includ es su ch particles as sand on beaches, mud on a lake bottom, boulders frozen into glaciers, pebbles in streams, and dust particles settli n g out of th e air. An accumulation of clam shell s on the sea bottom offshore is sediment, as are coral fragments broken from a reef by large storm waves. Sediment is the co llective name for loose, solid particles that originate from : 1. Weathering and ero sion of preexisting rocks . 2. Chemical precip itation fro m solut io n, including secretion by organ isms in water. These particles usually collect in layers on th e earth's sur face. An important part of th e definition is th at the part icles are loo se. Sediments are said to be unconsolidated, which means that th e grains are sepa rate, or un attach ed to one another. Sediment particles are classified an d defined according to the size of individual fragm ents. Table 14.1 shows the precise definitions of particles by size. Gravel includes all rounded particles co arser than 2 mm in diameter, th e thickness of a U.S. nickel. (Angular fragments of th is size are called rubble.) Pebbles range from 2 to 64 mm (about 2.5 inc hes, the size of a tennis ball) . Cobbles range from 64 to 256 mm (lO in ches, about th e size of a basketball), and boulders are coarser th an 256 mm . Sand grains are from 1/16 mm (about the th ickn ess of a h uman hair) to 2 mm in diameter. Grains of this size are visible and feel gritty between th e fingers. Silt grains are from 1/256 to 1/16 mm. They are too smal l to see without a magnifYing device, such as a geologist's hand lens. Silt does not feel gritty between the fingers, but it doe s feel gritty between the teeth (geologists often bite sedime nts to test their grain size). Clay is th e finest sediment, at less than 1/256 mm, too fine to feel gritty to finger s or teeth. Mud is a term loosely used for wet silt and clay. Note that we have two different uses of the word clay-a clay-sized particle (table 14.1) and a clay mineral. A clay-sized p article can be com posed of any mineral at all provided its diameter is less than 1/2 56 mm. A clay mineral, on the other hand, is one of a sm all group of silicate minerals w ith a sheet-silicate struc tu re. Cla y minerals usually fall into th e clay-size range. Quite often th e composition of sedim ent in th e clay-size range turns out to be mostly clay mineral s, but this is not always th e case. Becau se of its resistan ce to chemical weathering, quartz may sho w up in this fine-s ize grade. (M ost silt is quartz.) Intense me ch anical weath ering can bre ak down a wide variety of min erals to clay size, and th ese extremely fine particles m ay retain thei r minera l identity for a long tim e if chem ical weathering is slow. The great weight of glaciers is particularly effecti ve at grind ing mi n erals down to the clay-size ran ge, producin g "rock Hour, " w h ich gives a mi lky appearance to glacial meltwater streams (see ch apter 19) . W eathering, erosion, and transportation are so m e of the processes that affect th e ch aracter of sediment. Both weathered and chemically unweathered rock and sedi m en t can be eroded, and weathering does not stop afte r eros ion h as taken place .

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Chapter 14

Diameter (mm)

256 64 2 1/16 1/

256

Sediment Particles and Clastic Sedimentary Rocks

Sedimentary Rock

Sediment Boulder Cobble

Gravel

Pebble Sand

Breccia (angular particles) or Conglomerate (rounded particle s) Sandstone

Silt "Mud"

Clay

Plain

Because rounding du ring transportation is so rap id , it is a mu ch more important process than sphe ro idal weathering (see chap ter 12), which also tends to round sha rp edges. Sorting is th e process by w hich sediment gra ins are selecte~ an d se~arated according to grain size (or gra in shape or sp ecI~c graVIty) by the agents of transportation , espe cially by runnll1g water. Becau se of their high viscosity and man ner of Row, glaciers are poor sorting agents. Gl aciers deposit all sedi~ent sizes In the same place, so glacial sed ime nt usually consists of a m ixture of clay, silt, sand, and gravel. Such glacia l sedim ent is cons ide red poorly sorted. Sediment is considered well-sorted when the grains are nearly all th e same size. A river, for exam ple, is a good sorting agent, separating san d from gravel, and silt and clay from san d . Sorting tak es place bec ause of the gre ater weight of larger particles. Boulders weigh more \th~n pebbles a nd are more diffi cult for the river to transport, so a n ver must Bow more rapidly to move bo ulders than to m ove pebbles. Simil arly, pebbles are harder to mo ve tha n san d , an d sand is harder to move than silt and clay. Figure 14 .2 shows th e sort ing of sed iment by a river as it flows ou t of steep mountai ns onto a gentle p lain, where the water lo:es ener~y and slO\,:"s dow n. As the river loses energy, the heaviest particles of sediment are deposi ted. T he bo uld ers come to rest first (figure 14.3). As the river continues to slow down , co bb les and then pe bbles are deposited. Sand comes to r~st a~ the ri~er loses still more energy (figure 14.4). Finally, the nver IS carrYll1g only the finest sedim ent-silt and clay (figu re 1.4.5). The river has so rted th e or igin al sediment m ix by grain SIze.

Sandstone and shale are quite common ; the others are relatively rare.

Figure 14.1 These boulder s have been rounded by abrasion as wave action rolled them against one another on this beach.

San d beinz transported by a river also can be actively weathering, as can mud on a lake bottom. The ch aracter of sediment can also be altered by rounding and sorting d urin g transportation, and by eventual deposition.

.

Rounding is the gr inding away of sharp edges an d corn ers ~t rock fragmen ts during transportation . Rounding occurs 1~1 sand an d gravel as rivers, glaciers, or wav es cause particles to hit an d scrape against one another (figure 14 .1) or against a rock surface, such as a rocky stream bed. Boulders in a stream mav show su bstan tial rounding in less than one mile of travel.

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Silt and clay

Sorting of sediment by a river. The coarse sediment is deposited first, and the finest sediment is carried the farthest.

Siltston e (mostly silt) Shale or mudstone (mostly clay)

Transportation

Sand

Fig ure 14.2

Deposition ~hen transported m aterial settles or comes to rest, depositl0~ o: curs. Sedi ment is dep osited when running water, glaCIal Ice, waves, or wind lose s energy and can no longer transp ort it s load. Deposition also refers to the accumulation of ch em ical or

organ~c sedim ent, such as clam she lls on the sea Boor, or p lant mater~al on the Boor of a swamp. Su ch sediments may form as organIsms d ie and th eir remains accum ula te, perhaps with no transp ortation at all. Deposition of salt crystal s can take place as seawater evaporates. A change in the temperature, pres sure, Or ~hemistry of a solution may also cause precipitation-hot \Pl'lngs m ay deposit calcite or silica as the warm wa ter cool s.

Figure 14.3 Coarse gravel (boulder size) is depos ited first along a river's course as the river sorts out the various sediment sizes. River gravel is usually deposited in or near steep mountains.

Sediments and SedimentaryRocks

335

Overburden Cement Pore space

A After depositi on

B

Compaction

C

Cementation

Figure 14.6 Lithification of sand grains to become sandstone. (A ) Loose sand grains are depos ited with open pore space between the grains. (B) The weight of overburden compacts the sa nd into a tighter arrangement, reducing pore space. (C) Precipitation of cement in the pores by ground water binds the sand into the rock s andstone, which has a clastic texture.

most likely to be preserved if they are depo sited in a su bsiding (sinking) basin and if th ey are buried by later sediments.

Lithification

The environment of deposition is the location in which deposition occurs. A few examp les of environments of deposition are the deep sea Boor, a desert valley, a river channel, a coral reef, a lake bottom, a beach, and a sand dune. Each enviro nment is marked by characteristic ph ysical, chem ical, and biological conditions. You might expect mud on the sea Boor to differ from mud on a lake bottom. Sand on a beach may differ from sand in a river channel. Some differences are du e to varying sediment sources and transpo rting agents, but some are the result of conditions in the environments of depo sition themselves. One of the most important jobs of geologists studying sedimentary rocks is to try to determine the ancient environment of deposit ion of the sediment tha t formed the rock. Factors th at can help in determining this are a detailed knowledge of mode~n environments, the shape and vertical sequences of rock layers III the field, the features (including fossils) found within the rock, the min eral composition of the rock, and th e size, shape, and surface texture of the individual sediment grains. Later in the chapter we give a few exampl es of inte rpreting sedimentary rocks.

Preservation Figure 14.5 The river on the right is carrying onlys ilt and clay as it enters the clear river on the left. This fine sed iment may come to rest at the mouth of a river where it enters a lake or the sea. Photo byC. W. Montgomery

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Chapter 14

No t all sedime nts are preserved as sediment ary rock. G ravel in a river ma y be depo sit ed whe n a river is low, but th en may be reeroded an d retransported by th e next Bood on th e river. M any sedi ments on land, part icularly th ose well abov e sea level, are easily reerod ed an d carried away, so they are not com monly preserved. Sediments on th e sea Boor are easier to preserve. In general, continental an d marine sediments are http://www.mhhe.com/etlrthsci/ge%gy/pf/lmiller

Lith ification is the general term for a group of processes that convert loose sediment into sed imentary rock. Most sedimentary rocks are lithified by a combination of comp action, which packs loose sediment grains tightly togeth er, and cementation, in wh ich the precipitation of cement around sediment grains binds th em int o a firm, coherent rock. Crystallization of minerals from solution , without passing thro ugh th e loose-sedimen t stage, is another way th at rocks may be lithified. As sediment grains settle slowly in a quiet environment such as a lake bottom, they form an arrangeme nt with a great deal of open space between the grains (figure 14.6A). The op en spaces between grains are called pores, and in a quiet environment, a deposit of sand may have 40% to 50% of its volum e as open pore space. (If the grains were traveling rapidly and imp actin g one ano ther just before depo sition , th e percent age of pore space will be less.) As more and more sediment grains are deposited on top of the original grains, the increasing weight of this overburden packs th e original grains togeth er, reducing the amount of pore space. This sh ift to a tighter packing, with a resulting decrease in pore space, is called compaction (figure 14.6B). As pore space decreases, some of the inters titial water that usually fills sedime nt pores is driven out of the sediment. As underground water m oves th rou gh the remaining pore space, solid material called cement can precipitate in th e pore space and bind the loose sediment grains together to form a solid rock. The cement attac hes very tightly to the grains , holding them in a rigid fram ework. As ceme nt partially or completely fills the pores, th e to tal amo unt of pore space is further redu ced (figure 14.6 C), and the loose sand form s a hard, coherent sandsto ne by cementation. Sedimentary rock cement is often com posed of the mineral calcite or of other carbona te minerals. Dissolved calcium and bicarbonate ions are common in surface and underground waters, as you saw in chapter 12. If the chemic al conditions are

Fi gure 14.7 Crystalline texture .The rock is held togethe r by interlocking crystals , which grew as they precipitated from so lution. Such a rock has no cement or pore space.

right, th ese ions may recombin e to form solid calcite , as shown in th e followin g equation.

Ca' " + 2HC0 3dissolved Ion s

-.

CaC0

3

+

H 20

+

CO 2

calcite

Silica is ano ther common cement. Iron oxides and clay minerals can also act as cement but are less common than calcite and silica. The dissolved ions that precipitate as cement originate from the chemical weathering of min erals such as feldspar and calcite. This weath ering may be local, within the sediments being cemented, or very distant , with the ions being transported tens or even hun dreds of miles by water before precipitating as solid cement . A sed im entary rock th at consists of sed iment grains bound by cement into a rigid framework is said to have a clastic texture. Usually such a rock still has some pore space; cemen t rarely fills th e pores completely (figure 14.6 C). Some sedimentary rocks form by crystallizatio n, the development and growth of crystals by precipi tatio n from solution at or near the earth's sur face (the term is also used for ign eous rocks that crystallize as mag ma cools). T h ese rocks have a crystalline texture, an arrangement of int erlockin g crystals tha t develops as crystals grow and interfere with each other (figure 14.7). Crys talline rocks lack cement. T hey are held toge ther by the interlocking of crystals. Such rocks have no pore space because the crystals have grown until they .fill all available space . Some sedimentary rocks with a crystalline Sediments and Sedimentary Rocks

337

Figure 14.9 Figure 14.8 Breccia is characterized by coarse, angu lar fragmen ts. The cement in this rock is colored by hematite . The wide black and white bars on the sca le are one centimeter long , the sma ll divisions are one millimeter. Note that most grains exceed 2 mm (table 14.1).

texture are the resu lt of recrystallization, th e growth of new crystals that form from and then destro y the or iginal clastic grain s of a rock th at has been buried .

Types of Sedimentary Rocks Sedimentary rock is rock that h as formed fro m (1) lithification of sediment, (2) precipitation from solution, or (3) co nsolida tion of th e remains of plants or anim als. These different types of sedimentary rocks are called, respectively, clastic, chemical, and organic rocks. Mo st sedimentary rocks are clastic sedim entary rocks, formed from cemented sediment grains that are fragmen ts of preexisting rocks. The rock fragm ents can be either identifiable pieces of rock, such as pebbles of granite or shale, or individual mineral grain s, such as sand-sized quartz and feldspa r crystals loosened from rocks by weathering and erosion . Clay m inerals form ed by chemical weathering are also considered fragm ents of preexistin g rocks. In mo st cases th e sediment has been eroded and transported befor e being deposited . D uring transportation the grains may have been rounded and sort ed . Tabl e 14 .1 shows the clastic rocks, such as conglome rate, sandston e, and shale, and shows how the se rocks vary in grain size. Chemical sedimentary rocks are rocks deposited by precipitation of m in erals from solut io n. An example of inorgani c pr ecipitation is th e formation of rock salt as seawater evaporat es. C hemical precipitation can also be induced by or ganisms. T he sedimentary rock limestone, for instance, can form by th e precipitation of calcit e within a coral reef by cor als and algae. Such a rock may be classified as a biochemical lim estone. Chemical rocks mayor m ay not have on ce been sedi m ent. Rock salt may form from sedi me nt ; individual salt crystals forming in evaporating water act as sediment until th ey grow large enough to interlock into a solid rock. Minerals that crystal-

338

Chapter 14

An outcrop of congl omerate. Note the rounding of cobbles, which vary in composition. Long scale bar 10 cm , shor t bars 1 cm.

lize from solution on the sides of a rock cavity, or as a stalacti te in a cave, however, were never a sediment. Neither was lim estone precipitated directly from seawater as a solid rock by corals. Organic sedimentary rocks are rocks that accumulate fro m the rem ains of organisms. Coal is an o rganic rock that form s from th e compressio n of plant remains, such as m oss, leaves, twigs, roots, an d tree trunk s. A lim estone for med fro m th e accum ulation of clam shells on the sea floor m ight also be called an organic rock. Appendi x B describes and help s you identify the common sedi ment ary rocks. The geologic sym bols for these rocks (such as dots for sandstone, and a "b rick-wall" sym bol for lim estone) are shown in Appendix F and will be used in the rem ainder of the book.

Clastic Rocks Breccia and Conglomerate Sedimentary breccia is a coarse-grained sedime nta ry rock form ed by the cementation of coarse, angu lar fragments of rubble (figure 14.8). Because grains are rounded so rapidly during transport, it is unlikely that th e angu lar fragments within breccia have moved very far from their source. Sedimen tary breccia might form , for example, from fragments that have accum ulated at the base of a steep slope of rock th at is being mechanically weathe red. Landslide deposits also might lithi fy int o sedimentary breccia. This type of rock is not partic ularly common. Conglomerate is a coarse-grained sedim ent ary rock formed by the cemen tation of roun ded gravel. It can be distingui shed from breccia by the definite round ness of its parti cles (figure 14.9). Because conglom erates are coarse-grained , the particles may not have traveled far; some transport, however, was necessary to round the particles. Angular fragment.'i that fall from a cliff and then are carried a few miles by a river or pounded by waves crashing in the surf along a beach are qui ckly rounded. Gravel that is transported down steep submarine canyons, or carried by glacial ice as till, however, can be transported tens or even hun dreds of miles before deposition . http://www.mbbe.coIll/earthsci/,geology/plum mer

c Figure 14.1 0 Types of sandstone. (A) Quartz sandstone ; more than 90% of the grains are quart z. (8) Arkose ; the grains are mostly feldspar and quartz. (e ) Graywacke; the grains are surrounded by dark, fin e-grained matrix. (Small sca le divisions are 1 millimete r; most of the sand grains are about 1 mm in diameter.)

Sandstone Sandstone is a medium-grained sedimentary rock formed by the cementation of sand grains (figure 14.10). Any depo sit of sand can lithity to sandstone. Rivers deposit sand in their channels , and wind piles up sand int o dunes. Waves deposit sand on beaches and

in shallow water. Deep-sea currents spread sand over the sea floor. As you might imagine, sandstones show a great deal ofvariation in mineral com position, degree of sorting, and degree of rounding. Quartz sandstone is a sandstone in which more than 90% of the grains are quartz (figure 14. lOA). Because quartz is not subject Sedim ents and Sedim entary Rocks

339

Sand

Clay

Silt

Cliff of feldspar-rich rock such as granite Layer of coarse, angular, feldspar-rich sand

B

Figure 14.12 A poorly sorted sedimen t of sand grains surrounded by a matrix of silt and clay grains. Lithification of suc h a sedim ent would produce a "dirty sand stone."

Figure 14.11 Feldspar-rich sand (arkose ) may accumulate from the rapid erosion of feldspar-containing rock such as granite. Steep terrain accelerates erosion rates so that feldspar may be eroded before it is completely chemically weathered into clay minerals .

Source area of sedimentary. volcanic , and metamorphi c rocks

A

Figure 14.14 (A) An outcrop of shale in the Grand Canyon. Note how this fine-grained rock tends to break into small, flat pieces. (B) Shale pieces; note the very fine grain (scale in centimeters), very thin layers (laminations) on the edge of the large piece, and tendency to break into small, flat pieces (fissility).

B Weight of new sediment

A Wet mud

to chemical weatherin g, it tends to concentra te residually in sand as less resistant minerals such as feldspar are weathered away,probably in a low-lying hu mid region that allows chemical weath ering to continue for a lon g time. The qu artz grains in a quartz sandstone are usually well-sorted and well-roun ded because they have been transported for great distances. Most qua rtz sandston e was deposited as beach sand or dun e sand. A sandstone with more than 25% of the grains consisting of feldspar is called arkose (figure 14.1 OB). Because feldspar grains arc preserved in the rock, the original sed ime nt obviously did not undergo severe chemical weath erin g, or the feldspar would have been destroyed. Cliffs of granite in a desert could be a sour ce for such a sedim ent, for the rapid erosion associated with rugged terrain wou ld allow feldspar to be erod ed before it is chemically weathered (a dry climate slows chemical weathering). Most arkoses contain coarse, angular grains, so transportation distan ces were probably short . An arkose may have been deposited at or near the base of a cliff, as shown in figure 14.11 . Sandstones may contain a substant ial amount of matrix, fine-grained silt and clay foun d in the space between larger sand grains (figure 14.12). M atrix usually consists of fine-grained quartz and clay minerals. A matrix-rich sandsto ne is poorly sorted and often dark in color. It is sometimes called a "dirty sandstone ." Graywacke (prono unced "gray-whacky") is a type of sandsto ne in wh ich more than 15% of the rock's volume consists of fine-grained matrix (figure 14.10 C). Graywackes are 0 ften tough and dense and are generally dark gray or green. The sand grains may be so coated with matrix that they are hard to see, but they typically consist of quartz, feldspar, and sand-sized fragmen ts of fine-grain ed sedimentary, volcanic, and met amorphic rocks. Mos t graywackes probably form ed from sedime nts transported by turbidity currents, den se masses of sedime nt-laden 340

Chapter 14

Turbidity current (sediment- water suspension)

~ "' ~ ~ " ~ '

/ Sediment

Layers of sediment from previous turbidity currents

Figure 14.13 A turbidity current flow downs lope along the sea floor. Dense sedimen t-laden water is heavier than the clear water that it flows beneath.

water that flow downslope along the sea floor. The sedimentwater mixture is heavier than clear water, so it is pulled downslope by gravity until it comes to rest on the flat sea floor at the base of the slope (figure 14.1 3). Turbidity currents may be generated by underwater land slides (perh aps tr iggered by earthquakes) or by violent surface sto rms such as hurricanes, wh ich stir up bottom sediment. Sediment-laden rivers discharging directly into th e sea may also cause turbidity current s.

The Fine-Grained Rocks Rocks con sisting of fine-graine d silt and clay ate called shale, siltstone, claystone, and mudstone. Shale is a fin e-grain ed sedimentary rock not abl e for its splitti ng cap ability (called fissility). Splitting takes place along th e su rfaces of very th in layers (called lami nations) within the sha le (figure 14 .14). Most sh ales contain both silt http://w/llw.•/lhhe.com/earthsci/geology/pIWI/I/lel.

Water loss

- -- - -

- Compac ted sediment

~

Water loss

Shale (after cementation)

Figure 14.15 Lithification of sha le from the compaction and cementation of wet mud. (A) Randomly oriente d silt and clay particles in wet mud. (B) Particles reorien t, water is lost, and pore space decreases during compaction caused by the weight of new sedime nt depos ited on top of the wet mud. (e) Splitting surfaces in ceme nted shale form parallel to the orie nted mineral grains.

and clay (averaging 2/ 3 clay-sized clay minerals, 1/ 3 silt-sized q uartz) and are so fine-grained that th e surface of th e rock feels very sm oo th . The silt and clay deposits that lit hify as sha le accum ulate on lake bottom s, at th e en ds of rivers in deltas, beside river s in flood , an d on qui et parts of th e de ep ocean floor. .Fine-grained rocks such as shale typically undergo pronounced compaction as they lithify. Figure 14. 15 shows the role of compaction in the lithification of shale from wet mud. Before comp action, as much as 80 % of the volume of the wet mud may have been pore space, th e pores bein g filled with water. Th e Hakelike clay minerals were randomly arr anged within the mud. Pressure from overlying material packs th e sedime nt grains together and reduces th e overall volume by squeezing wat er out of the pores. The clay minerals are reorien ted perpendi cular to th e pressure, beco ming parallel to on e

another like a deck of cards. The fissility of shale is due to splittin g between the se parallel clay flakes. C om paction by itself do es not generall y lirhify sediment into sed ime ntary rock. Ir does help consolidate clayey sediments by pressing th e microscop ic clay min erals so closely togethe r that att ractional forces at the ato mic level tend to bind th em togeth er. Even in shal e, howe ver, the primary method oflithification is cem entation. A rock cons isting mostly of silt grains is called siltstone. Somewhat coarser-grained than m ost shales, siltstones lack th e fissility and lam inations of shale. Claystone is a rock composed predominately of clay-sized particles, but lackin g th e fissiliry of shale. Mudstone cont ains both silt and clay, having the same grain size and smoo th feel of shale bu t lackin g shale's lamination s and fissility. Mudstone is massive and blocky, while shale is visibly layered and fissile. Sediments and Sedimentary Rocks

341

Limestone Limestone is a sedi mentary rock composed mostly of calcite (CaCO) . Limestones are either precipitated by th e act ions of orga nisms or are precipitated directly as th e result of inorganic processes. T he two major types of limestone can be classified as either biochemicaL or inorganic Limestone. BiochemicaLLimestones are precipitated th rough th e actions of organisms. Mos t biochemi cal limestones are forme d on continental shelves in warm , shallow water. Biochemi cal limesto ne may be pr ecipitated directly as a solid rock in th e core of a reef by corals and encrustin g algae (figure 14.1 6). Such a rock would have a crystalline texture and wo uld contain the fossil remains of organisms still in growth position. Figure 14.16 The great majority of limestones Corals precipitate calcium carbonate to form limestone in a reef. Water depth about 25 feet are bioc hemical limesto nes for med of (8 meters), San Salvador Island, Bahamas . wave-broken fragme nts of algae, corals, and shells. The fragments may be of any size (gravel, sand, silt, and clay) and are often sorted and roun ded as they are transported by waves and cur rent s across th e sea Boor (figure 14.1 7). T he action of these waves and cur rents and subseC hem ical sedimentary rocks have been precipitated from an qu ent cementation of these fragments into rock give these aqueous enviro n ment. Chem ical sed imentary rocks are eit her limestone s a clastic textu re. These bioclastic (or skeletal) Limeprecipit ated di rectly by inorganic pro cesses or by the actions of stones take a great .variery of appearances. They may be relaorganisms. Chemical rocks include carbonates, chert, and evaptively coarse-g raine d with recognizable fossils (figure 14.1 8) or orates. un iformly fine-grai ned and de nse from the accum ulat ion of microscopi c fragm ents of coralline algae (figures 14.1 8 and 14.19). A variety of limestone called coquina form s from the Carbonate Rocks ceme ntation of shells that accum ulate d on the shallow sea Carbonate rocks contain C0 3 as part of th eir chemical compo Boor near sho re (figure 14.20). It has a clastic texture and is sitio n. T he two main types of carbo nates are lim estone and usually coarse-grained, with easily recognizable shells and shell dolomite.

Chemical Sedimentary Rocks

Fi gure 14.18 Biocl~stic l i~eston ~ s . The two on the left are coars e-grained and contain visible fossils of corals and shells. The limestone on the right consists of fine-grained carbonate mud formed by coralline algae.

Dune Lagoon

== == == == == == ==

~

-----:--. ;:::;".; .. , -;«

.; ::.:.:. :.... .

Reef core (crystalline)

Figure 14.20 Figure 14 . 17

Figure 14.19

A living coral-algal reef sheds bioclastic sediment into the fore-reef and back-reef environments. The fore reef consists of coarse, angular fragments of reef. Coralline algae are the major contributors of carbonate sand and mud in the back reef. Beaches and dunes are often bioclastic sand. The sediments in each environment can lithify to form highly varied limestones.

Coralline algae on the sea floor in 10 feet (3 meters) of water on the Bahama Banks. The "shaving brush" alga is Penicillus, wh ich produces great quantities of fine-grained carbonate mud.

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Chapter 14

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Coquina , a variety of bioclastic limestone , is formed by the cementation of coarse shells.

Sediments and Sedimentary Rocks

343

Table 14.2

Chemical Sedimentary Rocks Inorganic Sedimentary Rocks

Rock

Composition

Texture

Origin

Limestone

CaC0 3

Crystalline

Cementation of oolites (ooids) precipitated chemically from warm shallow seawater (oolit ic limestone). May be precipit ated directly from seawater. Also forms in caves as travertine and in springs, lakes, or percolating ground water as tufa.

Crystalline

Alterati on of limestone by Mg-r ich solutions (usually)

Dolomite

Evaporatio n of seawater or a saline lake.

Evaporites Rock salt Rock gypsum

Figure 14.21

NaC I CaS0 4 2HP 0

Chalk is a fine-grained variety of biocl astic limestone formed of the remai ns of microscopic marine organisms that live near the sea surface.

fragments in it. Chalk is a light-colored , porous, very finegrained variety of bioclastic limestone that forms from the seafloor accumul ation of microscopic marine organisms that drift near the sea surface (figure 14.21 ). Inorganic limestones are precipitated directly as the result of inorganic processes. Oolitic limestone is a distinctive variety of inorgan ic limestone form ed by the cement ation of sand-sized oolites (or ooids) , small spheres of calcite inor ganically precipitated in warm, shallow seawater (figure 14.22). Stro ng tida l curr ents roll the oolites back and forth daily, allowin g them to maintain a sph erical shape as they grow. Wave action m ay also contribute to their growth. Oolitic limestone has a clastic texture. Tuft. and travertine are inorganic limestones th at form from fresh water. Tufa is precipit ated from soluti on in th e water of a cont inent al spring, lake, or from percolatin g ground water. Traverti ne forms in caves when droplets of carbonate-rich water lose COz to the cave atm osphere. Tufa and travertine have a crystalline texture. Lim estones are particularly susceptible to recrystallization, the pro cess by which new cryst als, often of the same com posi ti on as the original grains, deve lop in a rock . C alcite grains recrystallize easily, partic ularly in th e presence of wate r and under th e weight of overlying sed iment . The new crysta ls that form are often large and can be easily seen in a

344

Chapter 14

Crystalline Crystalline

Biochemical Sedimentary Rocks Rock

Composition

Texture

Origin

Limestone

CaC0 3 (calcite)

Clastic or cryst alline

Cementation of fragments of shells, corals, and coralline algae (bioclastic limestone such as coquina and chalk). Also precipitated directly by organism s in reefs.

Chert

Si02 (silica)

Crystall ine (usually)

Cementation of microscopic marine organisms; rock usually recrystallized.

D olom ite B

The term dolomite (table 14.2) is used to refer to both a sedimentary rock and the mineral that composes it, CaMg(CO )z. (Some geologists use dolostone for the rock.) Dolomite o~en forms from limestone as the calcium in calcite is partially replaced by magnesium, usually as water solutions move through the limeston e.

Figure 14.22 (A ) Aerial photo of underwater dunes of oolites (ooids) chem ically precipitated from seawat er on the shallow Baham a Banks, south of Bimini. Tidal currents move the dunes. (B) An oolitic limestone formed by the cementation of oolites (small sphe res). Small divisions on scale are one millimeter wide .

Mg" + magnesIUm in solution rock as light reflects off their broad, flat faces. Because recrystallization often destroys th e original clastic texture and fossils of a rock, replacing the m wi th a new crystalline texture, the geologic history of suc h a rock can be very difficult to determine.

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Regionally extensive layers of dolomite are thought to form in one of rwo ways: 1. As magnesium-rich brines created by solar evaporati on of seawater trickle thro ugh existing layers of limestone. 2. As chemi cal reactions take place at the boundary berween fresh under ground water and seawater; this bound ary could migra te through layers of limestone as sea level rises

or falls. T his replacement process tends to cause recrystallization of th e preexistin g limesto ne, so evidence of the rock's origin is often obscured.

Chert A hard, compact, fine-grained sediment ary rock formed almost ent irely of silica, chert occurs in rwo principal forms- as irregular, lumpy nod ules within other rocks and as layered deposits like other sedime ntary rocks (figure 14.23). T he nodules, often found in limestone, probably formed from inorganic precipitation as underground water replaced part of the original rock with silica. The layered deposits typically form from the accumulation of hard, shell-like parts of microscopic marine organisms on th e sea floor. Microscopic fossils composed of silica are abundant in some cherts. But because chert is susceptible to recrystallization , the original fossils are easily destroyed, and the origin of many cherts rema ins doub tful.

Sediments and Sedimentary Rocks

345

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any sedimentary rocks have uses that make them valuable. Limestone is widely used as building stone and is also th e main rock type quarried for crushed rock for road con struction . Pulverized limestone is the main ingredient of cement and is also used as a soil conditioner in the humid regions of the United States. Coal is a major fuel, used widely for generating electrical power and tor hearing. Plaster and plasterboard for home con strucrion are manufactured from gypsum, which is also used to condition soils in some areas. Huge quantities of rocksalt are con sumed by industry, primarily for th e manufacture of

A Figure 14.23

hydrochloric acid. Mo re familiar uses of rock salt are for table salt and for mel ting ice on roads . Some chalk is used in the manufacture of blackboard chalk, although most classroom chalk is now made from pulverized limestone. The filtering agent for beer brewing and for swimming pools is likely to be made of diatomite, an accumulation of the siliceous remains of microscope diatoms. Clay from shale and other dep osits supplies the basic material for ceramics of all sorts, from hand-thrown pottery and fine porcelain to sewer pipe. Sulfur is used for matches, fungicides, and sulfuric acid; and phosphates and nitrates for fertilizers are extracted from natural occurrences of special sedimentary rocks (although other sources also are used). Potassium for soap manufacture comes largely from evaporites, as does boron for heat-resi stant cookware and fiberglass, and sodium for baking soda, washing soda, and soap . Quartz sandstone is used in glass manufacturing. Many metallic ores, such as the mo st common iron ores, have a sedimentary origin. The pore space of sedimentary rocks acts as a reservoir for ground water (chapter 17), crude oil, and natural gas. In chapter 2 1 we take a closer look at these resources and other useful earth materials.

B

(A ) Chert nodules in Redwall Limestone , Grand Canyon. (8) Bedded chert from the Coast Ranges , California. Camera lens cap (5.5 em) for scale.

Figure 14.24

Figure 14.25

Salt (and mud) deposited on the floor of a dr ied-up desert lake, Bonneville salt flats , Utah .

A bed of coal near Trinidad , Colorado. Hammer at bottom for scale .

Organic Sedimentary Rocks Coal Coal is a sedimentary rock that form s from th e compaction of plant material th at has not completely decayed (figure 14.25). Rapid plant growth and dep osition in water with a low oxygen conte n t are needed, so shallow swamps or bogs in a temperate or trop ical climate are likely environme nts of deposition. The plant fossils in coal beds include leaves, stems, tree trunks, and stumps with roots often extending in to the underlyin g shales, so app arentl y most coal form ed right at the place where the plants grew. Coal usually develops from peat, a brown, lightweight, un con solidated or semiconsolidated deposit of moss and other plant rem ains tha t accumulates in wet bogs. Peat is transformed int o coal largely by compactio n after it has been buried by sed ime nts. Partial decay of the abundant plant mate rial uses up any oxygen in the swamp water, so th e decay stops and th e remaining organic matter is preserved. Burial by sedime nt compresses the plant material, grad ually driving out any water or other volatile compounds. The coal changes from brown to black as the amount of carbo n in it increases. Several varieties of coal are recogn ized on th e basis of the type of original plant mat erial and th e degree of compaction (chapter 2 1).

The Origin of Oil and Gas Evaporites Rocks formed from crystals th at precipitate during evaporatio n of water are called evap orites. T hey form from th e evaporation of seawater or a saline lake (figure 14.24), such as Great Salt Lake in Utah. Rock gypsum, formed from th e mineral gypsum

346

Chap ter 14

(CaS0 4 e 2 H zO ), is a common evaporite. Rock salt, com posed of the mineral halite (NaC l), may also form if evaporatio n contin ues. Other less com mon evapo rites include th e berates, potassium salts, and magnesium salts. All evapo rites have a crystalline texture.

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O il and natural gas seem to originate from organic matter in marine sediment. Mi croscopi c organisms, such as diatoms and other single-celled algae, settle to the sea floor and accumulate in marine mud. The most likely environments for this are restricted basins wit h poor water circulatio n, parti cularly on con tinent al shelves. T he organi c matter may parti ally decompose, using up the dissolved oxygen in the sedimen t. As soon

as the oxygen is gon e, decay sto ps and the remaining organic ma tter is preserved. Cont inued sedime nt ation bu ries the orga nic ma tte r and subjects it to higher temperatures and pressures, whi ch convert the organic matter to oil and gas. As muddy sediments compact, the gas and small droplets of oil may be squeezed out of the mud and may m ove int o more porous and perm eable sandy layers nearby. O ver long period s of tim e large accumulations of gas and oil can collect in the sandy layers. Both oil and gas are less dense th an water, so th ey generally tend to rise up ward through water-saturated rock and sedi ment .

Sedimentary Structures Sedimentary structures are features found within sedimentary rock. Th ey usually form during or shortly after dep osition of the sedime nt , but before lith ification . Str uctures found in sed imentary rocks are important because th ey provide clues that help geologists determine the means by which sediment was transported and also its eventual resting place, or environment of deposition . Sedim entary structures may also reveal the original stratigra phic top , or upward di rection of deposition , whi ch helps geologists unravel th e geometry of rocks th at have been folded and faulted in tectonically active regions. O ne of th e most prominen t structures, seen in m ost large bodi es of sedime nta ry rock, is bed d ing, a series of visible layers within rock (figur e 14.26). Mo st bedding is ho rizontal because th e sediments from whi ch the sedimentary rocks formed were origin ally deposited as horizontal layers. The principle of original ho rizontality states that most water-laid sediment is dep osited in ho rizontal or near-horizontal layers tha t are essentially parallel to th e earth's surface. In ma ny cases thi s is also true for sediments deposited by ice or wind. If each new layer of sed iment buries previous layers, a stack of horizontal layers

Sediments and Sedimentary Rocks

347

Figure 14.26

Bedding in sandstone and shale, Utah. The horizontal layers formed as one type of sediment buried another in the geologic past. The layers get younger upwards .

Figure 14.27

will develop with th e oldest layer on the bottom and the layers becom ing younger upward. Sedime ntary rock s formed from such sed imen ts preserve th e horizontal layering in th e form of beds (figure 14.26). A b edding plane is a nearly flat sur face of deposition separating tWO layers of rock. A change in th e grain size or composition of the particles bein g deposited, or a pause durin g deposition, can create bedding planes. A specialized type of bedding that is not horizontal is cross-be ddi n g, a series of thin, inclined layers within a larger bed of rock. T he cross-beds form a distinct angle to th e hor izont al (or near-hori zont al) bedding planes of th e larger rock un it (figure 14.27). C ross-bedding is found most often in sandstone . It develops as sand is deposited on steep, local slopes (figure 14.28). The distinctive patterns of cross-bedd ing can form in many places- in sand dunes deposited by wind; in sand ridges deposited by ocean currents on th e sea floor ; in sediment bars and dunes deposited by rivers in the ir channels; and in delt as that form at th e mouths of rivers. A graded bed is a layer with a vertical change in particle size, usually from coarse grain s at th e bottom of th e bed to progressively finer grains toward th e top (figure 14 .29 ). A single

bed may have grayel at its base and grade upward th rough sand and silt to fine clay at the top . A graded bed may build up as sedime nt is deposited by a gradually slowing current. This seems particularly likely to happen during deposition from a tur bidi ty current on the deep sea floor (figure 14.1 3). Figure 14.30 shows th e development of a graded bed by turbiditycur rent deposition . Mud cracks are a po lygon al pattern of cracks formed in very fine-gr ained sed ime nt as it drie s (figure 14.31). Because dr ying requires air, mud cracks form only in sediment exposed above water. Mud cracks may form in lake-bottom sediment as the lake dri es up ; in flood -deposited sediment as a river level drops ; or in marine sed iment exposed to the air, perhaps tempor arily by a falling tide. Cracked mud can lirhify to form shale, pr eserving the cracks. The filling of mud cracks by sand can form casts of the cracks in an overlying sandsto ne. Ripple marks are small ridges form ed on the surface oi a sediment layer by moving wind or water. The ridges form perpendicular to th e motion. Rippl e marks, fou nd in gravel, sa~ld. or silt, can be caused by either waves or currents of water or wll1.d eW L (figu re 14.32A,B). Wave-caused rippl e marks are symm

C ross- bedd e~ s an~stone in Zion .National Par~, Utah. Note how the thin layers have formed at an angle to the more extensive beddin planes (also tilted) 111 the rock. This cross-bed dinq was formed in sand dunes deposited by the wind . g

Figure 14 .2 8 The d~ve lopm~nt ~f ~ros.s-bedding in wind-blown sand (A and B ) and current-deposited sand (C and 0 ). (A) Sand. IS deposited 111 II1c.llned !ayers on the downwind side of a dune . (B) Second dune covers first. Crossbedding may chang~ Orientation if wind direction shifts. (C) A water current fills in a depre ssion on river botto m or sea floor With sediment. (0) Continued sedimen tation may cover first set of cross-b eds with another set. Wind

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Chapter 14

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unning water, aided by mass wasting, is the most important geologic agent in eroding, transporting, and depositing sediment. Almost every landscape on earth, shows the results of stream erosion or deposition.

Aithough othe r agents-ground water, glaciers, wind, and waves-can be locally important in sculpturing the land, stream action and mass wasting are the dominant proces ses of landscape development. The first part of this chapter deals with the various ways that streams erode, transport, and deposit sediment. The second part describes landforms produced by stream action, such as valleys, flood plains, deltas , and alluvial fans, and shows how each of these is related

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Grand Forks, North Dakota during the 1997 flooding of the Red River. Photo by Brooks Kraft/Syg ma

385

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