PHYSICAL
GEOLOGY EXPLORING THE EARTH
James S. Monroe
ReedWicander
The Rock Cycle
(Figure 1-15)
Ridge axis '
The...
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PHYSICAL
GEOLOGY EXPLORING THE EARTH
James S. Monroe
ReedWicander
The Rock Cycle
(Figure 1-15)
Ridge axis '
The
Transform
Subduction zone
Zones
of extension within continents
Earth's Plates (Figure 1-13)
gp"
Upwelling
Asthenospnere Upwelling Lithosphere
"^ Three
Principle
Types of Plate Boundaries (Figure
1
-
14)
Uncertain plate boundary
PHYSICAL
GEOLOGY EXPLORING THE EARTH James
S.
Monroe
Reed Wicander Central Michigan University
WEST PUBLISHING COMPANY St.
Paul
New York
Los Angeles
San Francisco
PRODUCTION CREDITS Copyediting and indexing
Patricia
COPYRIGHT ©
Lewis
Interior and cover design
Artwork
Diane Beasley Darwen and Vally Hennings, Carlyn
Iverson, Precision Graphics, Rolin Graphics,
Communications, Ltd. ImageSmythe, Inc. Cover image Frederic Edwin Church, detail of Cotopaxi (1862). Oil on canvas, 48 in. x 7 ft. in. Copyright © The Detroit Institute of Arts, Founders Society Purchase with funds from Mr. and Mrs. Richard A. Manoogian, Robert H. Tannahill Foundation Fund, Gibbs-Williams Fund, Dexter M. Ferry, Jr. Fund, Merrill Fund, and Beatrice W. Rogers Fund.
endeavor.
MN 55164-0526
Printed in the United States of America
J. Farr,
99 98 97 96 95 94 93 92
8
7 6 5 4 3 2
1
Edwin Church was one of America's premier landscape painters of the mid-nineteenth century. His paintings were magnificent in scope and sought to integrate realism with the majesty of nature. Cotopaxi, which shows the Ecuadoran volcano erupting, is an excellent example of Church's work. This painting was chosen for the cover because of its realism and to show how geology plays an integral part in the human Frederic
Paul,
COMPANY
All rights reserved
Carlisle
David
St.
and
Victor Royer. Individual credits follow index.
Composition Page layout
1992 By WEST PUBLISHING 610 Opperman Drive P.O. Box 64526
LIBRARY OF CONGRESS CATALOGING-INPUBLICATION DATA Monroe,
J. S.
(James'S.)
Physical geology
:
exploring the Earth
/
James
S.
Monroe,
Reed Wicander. cm. p.
ISBN 0-314-00559 1.
Physical geology.
QE28.2.M655
550-dc20
-5
I.
Wicander, Reed, 1946-
.
II.
Title.
1992 91-29160
CIP (go)
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BRIEF CONTENTS
Preface
xvii
Developing Critical Thinking and Study 1
Understanding the Earth: Physical Geology 2
2
A
An
Skills
xxiii
Introduction to
17 Groundwater
80
6 Weathering, Erosion, and
158
Metamorphism and Metamorphic Rocks
9 Geologic Time
450 484
214
546
Shorelines and Shoreline Processes
190
Answers to Multiple-Choice and Review Questions 599 Glossary
250
11 The Interior of the Earth
514
572
134
Soil
7 Sediment and Sedimentary Rocks
10 Earthquakes
376
19 The Work of Wind and Deserts
20
340
and the
Building,
414
18 Glaciers and Glaciation
5 Igneous Rocks and Intrusive Igneous Activity 110
8
Unifying Theory
14 Deformation, Mountain
16 Running Water
54
4 Volcanism
A
Plate Tectonics:
Evolution of Continents
26
and Planets
314
13
15 Mass Wasting
History of the Universe, Solar System,
3 Minerals
12 The Sea Floor
Index
286
ys
$?*
Credits
601
617 633
Fill-in-the-Blank
1
ryyy%3
CONTENTS Preface
Chapter Summary 22 Important Terms 23 Review Questions 23 Additional Readings 24
xvii
Developing Critical Thinking and Study xxiii
Skills
CHAPTER
1
CHAPTER
UNDERSTANDING THE EARTH: An
Introduction to Physical Geology
Prologue 3 Introduction 5 What Is Geology?
5
Geology and the Human Experience 7 How Geology Affects Our Everyday Lives
Perspective 1—1: Public
Need
The Earth
as a
to
How Much
Know?
Dynamic
Science
8
Does the
10 Planet
1
Perspective 1-2: The Gaia Hypothesis
12
Geology and the Formulation of Theories 12 The Formulation of Plate Tectonic Theory 14
Guest
Essay: Science:
Plate Tectonic
Theory
Our Need
to
Know
16
The Rock Cycle 16 Geologic Time and Uniformitarianism
15
2
A HISTORY OF THE UNIVERSE, SOLAR SYSTEM, AND PLANETS Prologue 27 Introduction 28 The Origin of the Universe 29 The Origin and Early Development of the Solar System 30 General Characteristics of the Solar System 30 Current Theory of the Origin and Early History of 31 the Solar System Meteorites 33 The Planets 35
The Terrestrial Planets Mercury 35
35
Perspective 2—1: The Tunguska Event 21
Venus
36
38 Contents
vii
Perspective 2—2: The Evolution of Climate on the Terrestrial Planets 40
Mars 43 The Jovian Planets Jupiter 44 Saturn 44
Ferromagnesian Silicates 66 Nonferromagnesian Silicates 67 Carbonate Minerals 67
Perspective 3-1: Quartz— A
44
Useful Mineral
Other Mineral Groups
Uranus 45 Neptune and Pluto 45 The Origin and Differentiation of the Early Earth The Origin of the Earth-Moon System 49 Chapter Summary 51 Important Terms 52 Review Questions 52 Additional Readings 53
68
Physical Properties of Minerals
46
Common
68
Color and Luster 69 Crystal Form 70 Cleavage and Fracture Hardness 72 Specific Gravity 72
69
71
Perspective 3-2: Diamonds and
Pencil Leads
73
Other Properties 74 Important Rock-Forming Minerals 74 Mineral Resources and Reserves 76 Chapter Summary 78 Important Terms 78 Review Questions 78 Additional Readings 79
CHAPTER MINERALS Prologue 55 Introduction 56 Matter and Its Composition Elements and Atoms 56
56
Bonding and Compounds 58 Ionic Bonding 58 Covalent Bonding 59 Metallic and van der Waals Bonds Minerals 60
VOLCANISM Prologue
60
Naturally Occurring, Inorganic Substances
Guest
Essay: Mineralogy:
Pursuits
A
61
The Nature of
Crystals 62 Chemical Composition 62 Physical Properties 64 Mineral Diversity 64 Mineral Groups 64 Silicate Minerals 65
Contents
81
84 and Lava 85 Composition 85 Temperature 86 Viscosity 86 Volcanism 87 Volcanic Gases 87 Lava Flows and Pyroclastic Materials Introduction
Magma
60
Career with Diverse
Perspective 4—1: Volcanism System
87
in the Solar
88
Perspective 4-2: Volcanic Gases and Climate
90
Volcanoes 92 Shield Volcanoes
Perspective 5-1: Ultramafic Lava Flows 93
Andesite-Diorite
Perspective 4—3: Monitoring Volcanoes and Forecasting Eruptions 94 Cinder Cones 97 Composite Volcanoes Lava Domes 98 Fissure Eruptions 99
Guest
Other Igneous Rocks
122
Intrusive Igneous Bodies: Plutons
Dikes and
98
Laccoliths
Essay: Monitoring Volcanic Activity
Pyroclastic Sheet Deposits
Rhyolite-Granite
100
101
102 102 Plate Tectonics and Volcanism Volcanism at Spreading Ridges 103 Volcanism at Subduction Zones 105 106 Intraplate Volcanism Chapter Summary 107 Important Terms 107 Review Questions 108 Additional Readings 109 Distribution of Volcanoes
Sills
120
121 121 123
123
125
Volcanic Pipes and Necks 125 Batholiths and Stocks 125
Mechanics of Batholith Emplacement 126 Pegmatites 128 Plate Tectonics and Igneous Activity 129
Perspective 5-2: Complex Pegmatites
130
Chapter Summary 132 Important Terms 132 Review Questions 133 Additional Readings 133
CHAPTER
CHAPTER
5
WEATHERING, EROSION,
AND IGNEOUS ROCKS AND INTRUSIVE IGNEOUS ACTIVITY Prologue 111 Introduction 112 Igneous Rocks 113 Textures 113
Composition 115 Bowen's Reaction
Series
116 Assimilation 117 Magma Mixing 118 Classification 118 Ultramafic Rocks 119 Basalt-Gabbro 119 Crystal Settling
115
SOIL
Prologue 135 Introduction 136 Mechanical Weathering 137 Frost Action 138 Pressure Release 139 Thermal Expansion and Contraction
139
Perspective 6 — 1: Bursting Rocks and 140 Sheet Joints Activities of Organisms 141 Chemical Weathering 141 Solution 141 Oxidation 142 Hydrolysis 143
Perspective 6-2: Acid Rain
144
Contents
Chemical Sedimentary Rocks Limestone-Dolostone 168
Factors Controlling the Rate of Chemical
Weathering Particle Size
144 145
Climate 146 Parent Material Soil
The
Perspective 7—1: The Mediterranean Desert
146
Chert 171 Coal 172 Sedimentary Facies
148
Factors Controlling Soil Formation
Climate
149
149
Parent Material
Organic Activity
151 151
-"-Guest Essay: Environmental Geology: Sustaining
152
the Earth
and Slope 153 Time 153 153 Soil Erosion Weathering and Mineral Resources Chapter Summary 155 Important Terms 156 Review Questions 156 157 Additional Readings Relief
154
CHAPTER
Perspective 7-2: Persian Gulf Petroleum
CHAPTER METAMORPHISM AND METAMORPHIC ROCKS 162
Guest
Gas
Essay: Exploring for Oil and Natural
164 Sedimentary Rocks 165 166 Detrital Sedimentary Rocks Conglomerate and Sedimentary Breccia Sandstone 166
167
184
7
Prologue 159 Introduction 160 Sediment Transport and Deposition 160 Lithification: Sediment to Sedimentary Rock
Contents
173 Marine Transgressions and Regressions 174 Environmental Analysis 175 Sedimentary Structures 175 Fossils 177 Environment of Deposition 179 Sediments, Sedimentary Rocks, and Natural Resources 180 Petroleum and Natural Gas 181 Uranium 183 Banded Iron Formation 183 Chapter Summary 187 Important Terms 188 Review Questions 188 189 Additional Readings
SEDIMENT AND SEDIMENTARY ROCKS
Mudrocks
170
170
Evaporites
147 Soil Profile
168
Prologue 191 Introduction 193 The Agents of Metamorphism Heat 193 Pressure
194
Fluid Activity
166
193
Perspective
195
8 — 1:
Asbestos
196
Types of Metamorphism 197 Contact Metamorphism 197
1
Dynamic Metamorphism 200 Regional Metamorphism 200 Classification of Metamorphic Rocks 201 Foliated Metamorphic Rocks 201 Nonfoliated Metamorphic Rocks 205 Metamorphic Zones and Facies 206 Metamorphism and Plate Tectonics 208 Metamorphism and Natural Resources 208 Perspective 8—2: Graphite
210
Chapter Summary 211 Important Terms 211 Review Questions 211 Additional Readings 212
Radiocarbon Dating Methods
Perspective 9-2: Radon: The
239 Silent Killer
"•-Guest Essay: Paleontology: Tracing Life through
Time
244
Chapter Summary 247 Important Terms 248 Review Questions 248 Additional Readings 249
CHAPTER
CHAPTER
Prologue
251
Introduction
Prologue 215 Introduction 216 Early Concepts of Geologic Time and the Age of the Earth 216 James Hutton and the Recognition of Geologic
218
Methods 219 Fundamental Principles of Relative Dating 219 Unconformities 222 Applying the Principles of Relative Dating to the
Relative Dating
Reconstruction of the Geologic History of
223 227 Absolute Dating Methods an Area
Correlation
23 Atoms, Elements, and Isotopes
Perspective 9-1: Subsurface Correlation and the Search for Oil and Natural Gas 232
234
Long-Lived Radioactive Isotope Pairs
253
Rebound Theory 254 Seismology 255 The Frequency and Distribution of Earthquakes Elastic
Guest
Essay: Geology Meets Public Policy
Seismic Waves
258
260
261
Body Waves 261 Surface Waves 263 Locating an Earthquake 263 Measuring Earthquake Intensity and Magnitude Intensity 264 Magnitude 266 The Destructive Effects of Earthquakes 269 Ground Shaking 269
264
Perspective 10-1: Designing Earthquake-Resistant Structures 270
231
Radioactive Decay and Half-Lives Sources of Uncertainty 235
10
EARTHQUAKES
9
GEOLOGIC TIME
Time
240
Tree-Ring and Fission Track Dating Methods 242 The Development of the Geologic Time Scale 243
239
273 Tsunami 274 Fire
Ground Failure 275 Earthquake Prediction 276 Earthquake Precursors 276 Contents
xi
Dilatancy
Model
278
Earthquake Prediction Programs 279 Earthquake Control 280 -^Perspective 10-2: A Predicted Earthquake That Didn't Occur
Chapter Summary 312 Important Terms 312 Review Questions 312 Additional Readings 313
281
Chapter Summary 283 Important Terms 284 Review Questions 284 Additional Readings 285
THE SEA FLOOR Prologue
THE INTERIOR OF THE EARTH Prologue 287 Introduction 288
The Discovery of the Earth's Core 290 Density and Composition of the Core -•-Guest Essay: Geology:
Rewarding Career
An Unexpected But
293
297 Internal Heat
295
Earth's Crust Earth's
297
the Mantle
298
^Perspective 11-2: Seismic Tomography
302
303 Earth's Magnetic Field 306 Inclination and Declination of the Magnetic Field 307 Magnetic Anomalies 309 Magnetic Reversals 310
The The
Principle of Isostasy
Contents
320
322
323
329
Seamounts, Guyots, and Aseismic Ridges 329 -^Perspective 12-2: Maurice Ewing and His Investigation of the Atlantic
300
Measuring Gravity
Submarine Fans 322 Types of Continental Margins The Deep-Ocean Basin 325 Abyssal Plains 325 Oceanic Trenches 326 Oceanic Ridges 326 Fractures in the Sea Floor
-^Perspective 11-1: Kimberlite Pipes -Windows to
Heat Flow
Rise
Turbidity Currents, Submarine Canyons, and
291
Structure and Composition of the Mantle
The The
-
The Continental Slope and
294
The Mantle
316 '
Oceanographic Research 317 Continental Margins 318 The Continental Shelf 319 ^Perspective 12-1: Lost Continents
289
Seismic Waves
315
Introduction
301
Deep-Sea Sedimentation
Ocean
330
330
332 Composition of the Oceanic Crust Resources from the Sea 334 Chapter Summary 337 Important Terms 338 Review Questions 338 Additional Readings 339
Reefs
334
and the Distribution of
Plate Tectonics
Natural Resources 371 Chapter Summary 373 Important Terms 373 Review Questions 374 Additional Readings 375
CHAPTER
13
PLATE TECTONICS:
A Unifying Prologue
Theory
341
CHAPTER
342
Introduction
Alfred Wegener and the Continental Drift
Hypothesis
The Evidence
DEFORMATION, MOUNTAIN AND THE EVOLUTION OF CONTINENTS
344
BUILDING,
345
for Continental Drift
Continental Fit
345
Rock Sequences and Mountain Ranges 346 Glacial Evidence 347 Fossil Evidence 349 Paleomagnetism and Polar Wandering 349 Similarity of
Sea-Floor Spreading 351 "^ Perspective 13 — 1: Paleogeographic Maps
Prologue 377 Introduction 378
Deformation 379 Strike and Dip 379 Folds
352 """
384
Domes and Joints Faults
Basins
385
386 389
^"Perspective 14—1: Folding, Joints, and
Convergent Boundaries 361 Oceanic-Oceanic Boundaries 362 Oceanic-Continental Boundaries 363 Continental-Continental Boundaries 364
"^ Guest Essay: Geoscience Careers— The Diversity Unparalleled 365 Plate
368
Plate Tectonics
381
Guest Essay: Studying the Earth: Reflections of an Enthusiast
^Perspective 13-2: Tectonics of the Terrestrial Planets 358
The Driving Mechanism of
380
Monoclines, Anticlines, and Synclines Plunging Folds 383
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading 355 Plate Tectonic Theory 357 Plate Boundaries 357 Divergent Boundaries 357
Transform Boundaries 366 Movement and Motion 366 Hot Spots and Absolute Motion
14
343
Early Ideas about Continental Drift
369
Arches
390
Dip-Slip Faults Strike-Slip Faults
is
391 393
Oblique-Slip Faults 394 Mountains 395 Types of Mountains 396 Mountain Building: Orogenesis 397 Plate Boundaries and Orogenesis 397
Orogenesis at Oceanic-Oceanic Plate Boundaries 397
Contents
xiii
Orogenesis at Oceanic-Continental Plate Boundaries 399 Orogenesis at Continental-Continental Plate Boundaries 399 ^"Perspective 14—2:
The Origin of Rocky Mountains 400
the
The Origin and Evolution of Continents Shields, Cratons, and the Evolution of Continents 405
Flows
433
Complex Movements
437
Recognizing and Minimizing the Effects of
Mass Movements ""'Perspective
439
15-2: The Vaiont
Dam
Disaster
440
Chapter Summary 448 Important Terms 448 Review Questions 449 Additional Readings 449
405
^Perspective 14—3: Plate Tectonic History of the Appalachians 406 Microplate Tectonics and Mountain Building Chapter Summary 410 Important Terms 411 Review Questions 411 Additional Readings 412
408
CHAPTER
16
RUNNING WATER
CHAPTER
Prologue 451 Introduction 452 The Hydrologic Cycle
15
MASS WASTING
452 Running Water 454 Sheet Flow versus Channel Flow Stream Gradient 456 Velocity and Discharge 457
455
"^ Guest Essay: Managing Our Water Resources Prologue 415 Introduction 417
Mass Wasting 418 419 Weathering and Climate 420 Water Content 420 Vegetation 420 Overloading 421 Geology and Slope Stability 421 Triggering Mechanisms 421 "^ Perspective 15—1: The Tragedy at Aberfan, Wales 422
Factors Influencing
Slope Gradient
Types of Mass Wasting Falls
Slides
424
425 426
"•'Guest Essay: Cleansing the Earth— Waste
Management xiv
Contents
427
Stream Erosion 459 Transport of Sediment Load 460 Stream Deposition 461 Braided Streams and Their Deposits 462 Meandering Streams and Their Deposits 463 Floodplain Deposits
464
"^ Perspective 16—1: Predicting and Controlling Floods 465 Deltas
466
Alluvial Fans
469
Drainage Basins and Drainage Patterns Base Level 472 The Graded Stream 474 Development of Stream Valleys 475 Superposed Streams 476 Stream Terraces 477 Incised
Meanders
478
470
458
"^ Perspective 16—2: Natural Bridges
479
Chapter Summary 480 Important Terms 480 Review Questions 481 Additional Readings 482
CHAPTER GLACIERS
CHAPTER
17
Prologue 485 Introduction 486
524 U-Shaped Glacial Troughs 524 Hanging Valleys 526 Cirques, Aretes, and Horns 526 Erosional Landforms of Continental Glaciers 528 Glacial Deposits 528 Landforms Composed of Till 528 End Moraines 528 Lateral and Medial Moraines 530 Drumlins 530 Landforms Composed of Stratified Drift 531 Outwash Plains and Valley Trains 531 Karnes and Eskers 531 532 Glacial Lake Deposits Pleistocene Glaciation 533
Groundwater and the Hydrologic Cycle 486 Porosity and Permeability 487 The Water Table 488 Groundwater Movement 489 Springs, Water Wells, and Artesian Systems 489 Springs 490 Water Wells 491 "^ Perspective 17—1: Mammoth Cave National
492
493 Groundwater Erosion and Deposition 495 Sinkholes and Karst Topography 495 Caves and Cave Deposits 496 Modifications of the Groundwater System and Their Effects 498 Lowering of the Water Table 500 Saltwater Incursion 500 Subsidence 502 Groundwater Contamination 504 "^ Perspective 17—2: Radioactive Waste Disposal Artesian Systems
Hot
Springs and Geysers
506 Geothermal Energy 509 Chapter Summary 511 Important Terms 512 Review Questions 512 Additional Readings 513
AND GLACIATION
Prologue 515 Introduction 516 Glaciers and the Hydrologic Cycle 516 The Origin of Glacial Ice 517 Types of Glaciers 518 The Glacial Budget 519 Rates of Glacial Movement 520 Glacial Erosion and Transport 522 Erosional Landforms of Valley Glaciers
GROUNDWATER
Park, Kentucky
18
^Perspective 18 — 1: Glacial Lake Missoula and the Channeled Scablands 534
536 and Proglacial Lakes
Pleistocene Climates Pluvial
506
"^ Perspective 18—2: Great Lakes 538
A
537
Brief History of the
539 540 Causes of Glaciation 540 The Milankovitch Theory 541 Short-Term Climatic Events 541 Chapter Summary 542 Changes
in
Sea Level
Glaciers and Isostasy
Contents
xv
Important Terms 543 Review Questions 543 Additional Readings 544
CHAPTER
20
SHORELINES AND SHORELINE PROCESSES
CHAPTER
19
Prologue 573 Introduction 574
THE WORK OF WIND
Wave Dynamics 575 Wave Generation 576
AND DESERTS Prologue 547 Introduction 549 Sediment Transport by
^Guest
Wind
549
on Mars
Wind
Wave
Wind
Activity
552
^Perspective 19—2: Death Valley National
562
Weathering and
Soils 564 Mass Wasting, Streams, and Groundwater Wind 566 Desert Landforms 566 Chapter Summary 569 Important Terms 570 Review Questions 570
Additional Readings xvi
Contents
Refraction and Longshore Currents Rip Currents 580 Shoreline Deposition 581 Beaches 582 Seasonal Changes in Beaches 583
and Bay mouth Bars 584 585 The Nearshore Sediment Budget Shoreline Erosion 587
580
Spits
552 The Formation and Migration of Dunes 553 Dune Types 554 Loess 556 Air Pressure Belts and Global Wind Patterns 558 The Distribution of Deserts 559 Characteristics of Deserts 561 Temperature, Precipitation, and Vegetation 561 Deposits
Monument
577
^Perspective 20—1: Waves and Coastal Flooding 579
Bed Load 549 Suspended Load 550 Wind Erosion 550 Abrasion 550 Deflation 551 ^Perspective 19 — 1: Evidence of
576
Essay: Geophysics and the Search for Oil
Shallow- Water Waves and Breakers Nearshore Currents 578
571
565
Barrier Islands
587
^ Perspective 20—2: Rising Sea Level and Coastal
Management
588
Wave-cut Platforms and Associated Landforms Types of Coasts 592 Submergent and Emergent Coasts 592 Tides 594 Chapter Summary 596 Important Terms 597 Review Questions 597 Additional Readings 598
591
Answers to Multiple-Choice and Fill-in-the-Blank Review Questions 599 Glossary 601 Index 617 Credits 633
T^^^^^^mj^^r» ^^m. ^^^^^^^^^^K^^m.^^ ^^^^^^^^ ^
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PREFACE
The Earth
dynamic planet that has changed contin4.6 billion years of existence. The size, shape, and geographic distribution of the continents and ocean basins have changed through time, as have the atmosphere and biota. Over the past 20 years, bold new theories and discoveries concerning the Earth's origin and how it works have sparked a renewed interest in geology. We have become increasingly aware of how fragile our planet is and, more importantly, how inter-
students can see, through relevant and interesting exam-
dependent all of its various systems are. We have learned that we cannot continually pollute our environment and that our natural resources are limited and, in most cases, nonrenewable. Furthermore, we are coming to realize how central geology is to our everyday lives. For these and other reasons, geology is one of the most important college or university courses a student can take. Physical Geology: Exploring the Earth was designed for a one-semester introductory course in geology that serves both majors and nonmajors in geology and the Earth sciences. It was written with the student in mind. One of the problems with any introductory science course is that the students are overwhelmed by the amount of material that must be learned. Furthermore, most of the material does not seem to be linked by any unifying theme and does not always appear to be rele-
logic
is
a
uously during
vant to their
One
its
lives.
of the goals of this book
ples,
how
geology impacts our
lives.
^ TEXT ORGANIZATION is the unifying theme of geology book. This theory has revolutionized geology because it provides a global perspective of the Earth and allows geologists to treat many seemingly unrelated geo-
Plate tectonic theory
and
this
phenomena
as part of a total planetary system.
Because plate tectonic theory
duced
in
Chapter
1,
and
is
is
so important,
it is
intro-
discussed in most subsequent
chapters in terms of the subject matter of that chapter.
We have organized Physical Geology: Exploring the Earth into several informal categories. Chapter 1 is an introduction to geology,
its
relevance to the
human
perience, plate tectonic theory, the rock cycle, logic time
ex-
and geo-
and uniformitarianism. Chapter 2 discusses and planets,
the origin of the universe, the solar system
and the Earth's place in the evolution of this larger system. Chapters 3-8 examine the Earth's materials (minerals and igneous, sedimentary, and metamorphic rocks) and the geologic processes associated with them including the role of plate tectonics in their origin and distribution. Chapter 9 discusses geologic time, introduces several dating methods, and explains how geologists
10—14
is to provide students with a basic understanding of geology and its processes
correlate rocks. Chapters
and, more importantly, with an understanding of how geology relates to the human experience; that is, how geology affects not only individuals, but society in gen-
deformation and mountain building, and plate tectonics. Chapters 15-20 cover the Earth's surface processes.
eral.
With
this goal in
mind,
we
introduce the major
themes of the book in the first chapter to provide students with an overview of the subject and enable them to see how the various systems of the Earth are interrelated. We also discuss the economic and environmental aspects of geology throughout the book rather than treating these topics in separate chapters. In this
way
deal with the related
topics of the Earth's interior, the sea floor, earthquakes,
We have found, as have many of the reviewers of this book, that presenting the material in this order works well for most students. We know, however, that many instructors prefer an entirely different order of topics depending on the emphasis in their course. We have therefore written this
book
so that instructors can present
the chapters in any order that suits the needs of their course.
Text Organization
xvii
^ CHAPTER ORGANIZATION All chapters have the
Prologues
same organizational format. Each
chapter opens with a photograph relating to the chapter material, a detailed outline,
and a prologue, which
is
designed to stimulate interest in the chapter material by discussing
The
some aspect of
text
is
introductory prologues focus on the
human
aspects of geology such as the eruption of Krakatau
(Chapter
1),
the
Loma
Prieta earthquake (Chapter 10),
or the story of Floyd Collins (Chapter 17).
the chapter in detail.
written in a clear informal style,
comprehend
easy for students to
Many of the
making
it
Numer-
the material.
Economic and Environmental Geology
ous diagrams and photographs complement the text, providing a visual representation of the concepts and
The
information presented. Each chapter contains at least two Perspectives that present a brief discussion of an
in separate chapters at the
interesting aspect of geology or geological research.
nomic and environmental geology with the chapter material helps students see the importance and relevance of
The end-of-chapter
materials begin with a concise
topics of environmental
and economic geology are
discussed throughout the text rather than being treated
many
end of the book as
is
done
in
other physical geology books. Integrating eco-
many
review of important concepts and ideas in the Chapter
geology to their
Summary. The Important Terms, which are printed in boldface type in the chapter text, are listed at the end of each chapter for easy review, and a full glossary of important terms appears at the end of the text. The Review
with a section on resources, further emphasizing the im-
book; they include multiple-choice questions with answers as well as short answer and essay questions. Each chapter Questions are another important feature of
concludes with a
list
which are written
lives. In
addition,
portance of geology in today's world.
Perspectives
this
of Additional Readings,
many
of
at a level appropriate for beginning
students interested in pursuing a particular topic.
The chapter
perspectives often focus
asbestos and graphite (Chapter 8), radioactive waste dis-
posal (Chapter 17), and wind activity on 19).
The
it
The
many
fascinating
perspectives can be assigned as
part of the chapter reading, used as the basis for lecture
number of special
or discussion topics, or even used as the starting point features that set
apart from other physical geology textbooks.
them
Mars (Chapter
topics for the Perspectives were chosen to pro-
aspects of geology.
» SPECIAL FEATURES
on aspects of en-
vironmental, economic, or planetary geology such as
vide students with-tan overview of the
This book contains a
chapters close
Among
and study skills section, the chapter prologues, guest essays by people who chose
for student papers.
are a critical thinking
geology or geologically related the integration of
fields for their careers,
economic and environmental geologic
throughout the book, and a set of multiple-choice questions with answers for each chapter.
Guest Essays
A number of guest essays
are interspersed throughout the
book. These essays focus on three themes— how and
issues
the individuals
became
career, their current areas of research,
and the possible
ciopolitical ramifications of their specific field.
Study
why
interested in geology as a potential so-
The essayists
Randolph H. Bromery (University of MassachuAmherst and former president of the Geological Society of America), Susan M. Landon (a consulting geologist), Michael L. McKinney (a paleontologist at the University of Tennessee), Malcolm Ross (United States Geological Survey), and Steve Stow (head of nuclear waste include
Skills
setts at
Immediately following the Preface is a section devoted to developing critical thinking and study skills. This section contains hints to help students improve their study habits, prepare for
exams, and generally get the most tips can be
out of every course they take. While these helpful in any course,
relevant to geology.
many
Whether you
are just beginning col-
about to graduate, take a few minutes to read over this section as these suggestions can help you in your studies and later in life. lege or
xviii
Preface
disposal at
Oak
Ridge National Laboratories).
of them are particularly
Planetary Geology Planetary geology at the
is
discussed in Chapter 2 rather than
end of the book as
it is
in
many
other physical
geology textbooks. This early coverage of comparative planetary geology allows meaningful examples to be in-
try
troduced later in the book. Furthermore,
priate topical films.
student to understand
it
enables the
how the origin and early evolution
organized by region, all images from the textbook, animated sequences, quiz frames, and clips from appro-
Two
slide sets will
be provided. The
first set will
and
include 150 of the most important and attractive figures
The book has been planned,
however, so that Chapter 2 can be covered at any time
and photographs of rocks and minerals, as well as photographs from the book, and the second set will contain
in the course or omitted altogether
at least
of the Earth
fit
into the larger context of the origin
history of the solar system.
wishes.
The planetary examples
if
the instructor
later in the
book are not
dependent on the student having read Chapter
300 slides illustrating important geologic feaThe majority of these photographs will be from
North America, but examples from around the world and the solar system will also be provided.
2.
Transparency masters of the important charts, graphs, and figures will be available as well as a set of full-color
Review Questions Most
tures.
physical geology books have a set of review ques-
transparency acetates to provide clear and effective
illus-
An important
end of each chapter. This book, however, includes not only the usual essay and thought-provoking
trations of important
questions, but also a set of multiple-choice questions,
same
something not found in other physical geology textbooks. The answers to the multiple-choice questions are at the end of the book so that students can check their answers and increase their confidence before taking an
example, volcano and earthquake distributions and plate
examination.
disclosures. This will ensure that
tions at the
artwork from the
feature of the transparencies size,
is
that the
text.
maps will
all
be the
so they can be used as overlays to show, for
boundaries.
A
Newsletter will be provided to adopters each year book with recent and relevant research
to update the
most current information
your students have the
available.
Lastly, in addition to publishing a separate student
Unique
Illustrations
study guide,
we
have incorporated
much
of the material
usually found in such guides into the
depicting geologic processes or events are block dia-
book itself. This saves students time and money and also makes the book a more valuable learning tool. For those students who want fur-
grams rather than cross sections so that students can
ther study aid, a study guide
The
figures include
many
pieces of original artwork de-
signed especially for this book.
Many
of the illustrations
more
easily visualize the salient features of these pro-
cesses
and
human
on the
events. In an effort to focus attention
aspects of geology,
paintings, drawings,
and
we have
also included
many
also available.
^ ACKNOWLEDGMENTS As the authors, we
historical photographs.
is
are, of course, responsible for the
organization, style, and accuracy of the text, and any mistakes, omissions, or errors are our responsibility.
» INSTRUCTOR ANCILLARY
finished product
MATERIALS To
assist
you
in
teaching this course and supplying your
students with the best in teaching aids, West Publishing
Company
has prepared a complete supplemental pack-
age available to
all
Instructor's
Manual
will include
teaching ideas, lecture outlines (including notes on ures
and photographs available
videodisc for use in lecture has been developed to
accompany
the text.
work during which we received numerous comments and advice from many geologists who reviewed parts of the text.
We
wish to express our sincere appreciation to whose contributions were in-
the following reviewers
The videodisc
includes,
Gary C. Allen
fig-
as slides), teaching tips,
Consider This lecture questions, Enrichment Topics, global examples, slides, transparency masters and acetates as well as a computerized test bank.
A
The
the culmination of several years of
valuable:
adopters.
The Comprehensive
is
among
other things, a wealth of images from around the coun-
University of
New
Orleans
R. Scott Babcock
Western Washington University
Kennard Bork Denison University
Thomas W. Broadhead University of Tennessee at Knoxville
Acknowledgments
xix
Anna
James F. Petersen Southwest Texas State University
Buising
Hayward
California State University at F. Howard Campbell HI James Madison University
Katherine H. Price
Larry E. Davis
Washington State University
William D. Romey St. Lawrence University
Noel Eberz
Gary Rosenberg
California State University at San Jose
Indiana University, Purdue University at Indianapolis
Allan A. Ekdale
David B. Slavsky Loyola University of Chicago
DePauw
University of Utah
Stewart
S.
Edward
Farrar
University
F.
Stoddard
Eastern Kentucky University
North Carolina
Richard H. Fluegeman,
Charles
Jr.
J.
State University
Thornton
Pennsylvania State University
Ball State University
William
P.
Samuel
Fritz
B.
Upchurch
Georgia State University
University of South Florida
Kazuya Fujita Michigan State University
John R. Wagner Clemson University
Norman Gray
We
University of Connecticut
Jack Green
also wish to
thank Professor Emeritus Richard
V.
Dietrich of Central Michigan University for reading var-
California State University at
Long Beach
David R. Hickey Lansing Community College
ious drafts of the book, providing us with several pho-
tographs, and discussing various aspects of the text with
on numerous occasions.
us
In addition,
we
are grateful
University of Texas at Austin
Geology Department of Central Michigan University for reading various drafts and providing us with photographs. They are David J. Matty, Jane M. Matty, Wayne E. Moore, and Stephen D. Stahl. We also thank Mrs. Martha Brian of the Geology Department, whose word processing skills and general efficiency were invaluable during the preparation of the manuscript, and Bruce M. C. Pape of the Geography Department for providing photographs. David Hickey de-
Richard H. Lefevre
serves special thanks for his assistance with the devel-
Grand
opment of many of
R.
to the other membtJrs of the
W. Hodder
University of Western Ontario
Cornells Klein University of
New
Mexico
W
Lawrence Knight William Rainey Harper College Martin
I. P.
B.
Lagoe
Valley State University
Martini
University of Guelph, Ontario
Michael McKinney University of Tennessee
at Knoxville
California State University at Fresno
Carleton Moore Arizona State University P.
Morris
Harold Pelton
Preface
are also grateful for the generosity of the various
Community
College
many
countries
who
pro-
vided photographs.
must go to Jerry Westby, college ediWest Publishing Company, who made many valuable suggestions and patiently guided us Special thanks
torial
University of Texas at San Antonio
Seattle Central
We
agencies and individuals from
Robert Merrill
Alan
the excellent ancillaries for the text,
and for proofing all of the illustrations in the text. Additionally, we wish to acknowledge the fine efforts of Kathleen Chiras in coordinating the Guest Essay feature.
manager
for
through the entire project. His continued encouragement provided constant inspiration and helped us pro-
duce the best possible book. We are equally indebted to our production manager, Barbara Fuller, whose atten-
tion to detail
and consistency
is
greatly appreciated as
are her unflagging efforts and diligence in securing
many
sponsible for
We would
of the photographs and paintings used in the book. Bar-
them.
bara was especially helpful in responding to our
tion manager,
last-
minute concerns as she guided the book through final We would also like to thank Patricia Lewis
production.
for her excellent copyediting
and indexing
skills.
We
appreciate her help in improving our manuscript. Be-
cause geology
is
such a visual science,
thanks to Carlyn Iverson
and
to the artists
who
we extend
of the rest of the art program. They
we enjoyed working with
also like to
Ann
acknowledge our promo-
Hillstrom, for her help in the devel-
opment of
the promotional poster that is available with book, and Maureen Rosener, marketing manager, who developed the excellent videodisc that accompanies this book. this
Our
special
rendered the reflective art at Precision Graphics who were re-
much
did an excellent job, and
families
were patient and encouraging when most
of our spare time and energy were devoted to this book.
We
thank them for their support and understanding.
Acknowledgments
xxi
DEVELOPING CRITICAL THINKING AND STUDY SKILLS * INTRODUCTION
beneficial, waiting until the last
demanding and important time, a time when your values will be challenged, and you will try out new ideas and philosophies. You will make personal and career decisions that will affect your entire life. With this new freedom you will enjoy, one of the most important things you must learn is how to balance your time among work, study, and recreation. If you develop good time management and study skills early in your college career, you will find that your college years will be successful and rewarding. This section offers some suggestions to help you maximize your study time and develop critical thinking and College
study
is
a
skills
that will benefit you, not only in college, but
throughout your course
and
is
life.
While mastering the content of a
obviously important, learning
to think critically
portant. Like
is,
most things
in
many ways,
how far
to study
more im-
in life, learning to think crit-
and study efficiently will initially require addiand effort, but once mastered, these skills save you time in the long run.
ically
tional time will
You may already be gestions
and may
familiar with
find that others
to you. Nevertheless,
if
many
do not
specific goals
basis,
It is easy to fall into the habit of eating nothing but junk food and never exercising. To be mentally alert, you must be physically fit. Try to develop a program of fit.
regular exercise. ergy, feel better,
to read this
avoiding pro-
While procrastination provides temporary you have avoided doing something you did not want to do, in the long run procrastination leads to stress. While a small amount of stress can be crastination.
satisfaction because
You
will find that
and study more
you have more en-
efficiently.
^ GENERAL STUDY SKILLS Most courses, and geology vious material, so
it is
in particular, build
upon
pre-
extremely important to keep up
with the coursework and
set aside regular time for study each of your courses. Try to follow these hints, and you will find you do better in school and have more time
in
for yourself:
tively. is
greatly reduce the temptation to procras-
better to
of the sug-
and apply the appropriate suggestions to your we are confident that you will become a better and more efficient student, find your classes more rewarding, have more time for yourself, and get better grades. We have found that the better students are usually also the busiest. Because these students are busy with work or extracurricular activities, they have had to learn to study efficiently and manage their time effecof the keys to success in college
is
which is usually what happens when you procrastinate. Another key to success in college is staying physically
•*»
situation,
One
clear,
and working toward them on a regular
work efficiently for short periods of time than to put in long, unproductive hours on a task,
section
own
you can
tinate. It
directly apply
you take the time
minute usually leads to
mistakes and a subpar performance. By setting
»
»
Develop the habit of studying on a daily basis. Set aside a specific time each day to study. Some people are day people, and others are night people. Determine when you are most alert and use that time for study. Have an area dedicated for study. It should include a well-lighted space with a desk and the study materials you need, such as a dictionary, thesaurus, paper, pens and pencils, and a computer if you have one. Study for short periods and take frequent breaks, usually after an hour of study. Get up and move around and do something completely different. This will help you stay alert, and you'll return to your studies with renewed vigor.
General Study
Skills
xxiii
Try to review each subject every day or at least the day of the class. Develop the habit of reviewing lecture material from a class the same
example, pt (plate tectonics), iggy (igneous), meta (metamorphic), sed (sedimentary), rx
day.
years),
"v Become familiar with the vocabulary of the course. Look up any unfamiliar words in the glossary of your textbook or in a dictionary.
(rock or rocks), ss (sandstone),
and
my
(million
gts (geologic time scale).
Rewrite your notes soon after the lecture. Rewriting your notes helps reinforce what you heard and gives you an opportunity to
Learning the language of the discipline will help
determine whether you understand the material.
you learn the
^ GETTING THE MOST FROM
By learning the vocabulary of the discipline before the lecture, you can cut down on the amount you have to write— you won't have to write down a definition if you already know
YOUR NOTES
the word.
material.
you are to get the most out of a course and do well on exams, you must learn to take good notes. This does not mean you should try to take down every word your If
good note taker is knowing what is important and what you can safely leave out. Early in the semester, try to determine whether the
professor says. Part of being a
lecture will follow the textbook or be
predominantly
much
covered in the
new
material.
If
when
the material
is
new. In any case, the
is
make you
following suggestions should
taker and enable you to derive the
a better note
maximum amount of
information from a lecture: -^-
would appear on a
(They were usually
to class regularly,
what
if
the screen, If
somewhat
familiar with the
everything. Later a few key
words or phrases
your memory as to what was said. Before each lecture, briefly review your notes from the previous lecture. Doing this will refresh your memory and provide a context for will jog
material.
own style of note taking. Do not down every word. These are notes
It is
sit
near the front of
easier to hear
and there are fewer
the professor allows
it,
distractions.
tape record the
but don't use the recording as a
lecture,
is
down
and
and see on the board or projected onto
possible.
written
is
chapter the lecture will cover before class. This
substitute for notes. Listen carefully to the
and write down the important points; in any gaps when you replay the
lecture
then
fill
tape.
and they are available, These are usually taken by a graduate student who is familiar with the
If
your school allows
buy
it,
class lecture notes.
Develop your
material; typically they are quite
try to write
comprehensive. Again use these notes to supplement your own. Ask questions. If you don't understand
you're taking, not a transcript. Learn to abbreviate and develop your
own set of common words
abbreviations and symbols for
example, w/o (without), w (equals), (above or increases),
and phrases: (with),
=
for
A
(below or decreases),
a
Pay particular attention to the professor's examples. These usually elucidate and clarify an important point and are easier to remember
way you
new
test.
Check any unclear points in your notes with classmate or look them up in your textbook.
Go
as the textbook or supplements
being said rather than trying to write
xxiv
when I stated something twice during a lecture, they knew it was important and probably
the class
the
down and highlight it told me (RW) that
it
some way. Students have
same material
concepts and can listen critically to what
-*«•
he or
than an abstract concept.
the reading assignment, read or scan the
-w-
in
If
important or repeats a
point, be sure to write
Regardless of whether the lecture discusses the
will be
is
right!)
of the material
textbook, your notes do not have to be as extensive or detailed as
Learn the mannerisms of the professor. she says something
V
(greater
something, ask the professor. are reluctant to
do
lecture hall, but
if
Many
students
this, especially in a large
you don't understand
a
point, other people are probably confused as
you can't ask questions during
than), &c (and), u (you).
well. If
Geology lends itself to many abbreviations that can increase your note-taking capability: for
lecture, talk to the professor after the lecture or
Developing Critical Thinking and Study
Skills
during office hours.
a
^
GETTING THE MOST OUT OF
Whenever you encounter new facts, ideas, or concepts, be sure you understand and can
WHAT YOU READ
define all of the terms used in the discussion.
"you get out of something what you put into it" is very true when it comes to reading textbooks. By carefully reading your text and following these suggestions, you can greatly increase your under-
Determine
how
derived.
the facts were derived from
standing of the subject:
repeated?
The old adage
that
fusion
is
an excellent example.
Two
scientists
claim to have produced cold fusion reactions using simple experimental laboratory
chapter before you start to read in depth.
apparatus, yet other scientists have as yet been unable to achieve the same reaction by repeating the experiments. •-
logical or
bold face or
on previous
material,
it is
Look
What
critically
is
particularly important in learning
it to what you already know. Although you can't know everything, you can learn to question effectively and arrive at conclusions consistent with the facts. Thus, these suggestions for critical thinking can help you in all your courses:
material and relating
how
dam
how
across a river that
will be the
consequences to the beaches
One of the most important lessons you can learn from your geology course is how interrelated the various systems of the Earth river?
When you alter one numerous other features are.
Thinking
determine
that will be deprived of sediment from the
if you were taking a test. Only when you see your answer in writing will you know if you really understood the material.
and white, and it is important to be able to examine an issue from all sides and come to a logical conclusion. One of the most important things you will learn in college is to think critically and not accept everything you read and hear at face value.
at the big picture to
flows to the sea affect the stream's profile?
imperative that you
are black
the underlying
were known were not accepted until of overwhelming evidence.
will constructing a
out your answers as
life
all,
various elements are related. For example,
over the end-of-chapter questions. Write
things in
flawed?
ideas. After
the 1970s in spite
Because geology builds
^ DEVELOPING CRITICAL THINKING SKILLS
somehow
early in this century, yet
understand the terminology.
Go
is it
principles of plate tectonic theory
of the key terms, especially those italic type.
the source?
Be open to new
make
you don't highlight everything. Make notes in the margins. If you don't understand a term or concept, look it up in the glossary. »• Read the chapter summary carefully. Be sure you in
is
Consider whether the conclusions follow from the facts. If the facts do not appear to support the conclusions, ask questions and try to determine why they don't. Is the argument
sure
all
not accept any statement at face value. is the source of the information? How
reliable
unconformities.
understand
Do
What
As you read your textbook, highlight or underline key concepts or sentences, but
new
Can they be The current controversy over cold
executed and free of bias?
is
•^ Pay particular attention to the tables, charts, and figures. They contain a wealth of information in abbreviated form and illustrate important concepts and ideas. Geology, in particular, is a visual science, and the figures and photographs will help you visualize what is being discussed in the text and provide actual examples of features such as faults or
Few
was
about and how it flows from topic to topic. If you have time, skim through the material
^
the facts or information
experiments, were the experiments well
"» Look over the chapter outline to see what the
^
If
feature,
IMPROVING YOUR is
affect
MEMORY
Why do you remember some things reason
you
as well.
and not others? The
that the brain stores information in different
ways and forms, making it easy to remember some things and difficult to remember others. Because college requires that you learn a vast amount of information, any suggestions that can help you retain more material will help you in your studies: "» Pay attention to what you read or hear. Focus on the task at hand, and avoid daydreaming. Repetition of any sort will help you remember
Improving Your
Memory
xxv
Review the previous
material.
lecture before
"•"
important.
questions as you read.
Try to
Use mnemonic devices to help you learn unfamiliar material. For example, the order of the Paleozoic periods (Cambrian, Ordovician,
facts to
Devonian, Mississippian,
Pennsylvanian, and Permian) of the geologic time scale can be remembered by the phrase,
Campbell's Onion Soup Does Make Peter Pale, or the order of the Cenozoic epochs (Paleocene, Eocene, Oligocene, Miocene, Pliocene, and Pleistocene) can be remembered by the phrase,
example, pyroclastic comes from pyro meaning fire and clastic meaning broken pieces. Hence a pyroclastic rock is one formed by volcanism
and composed of pieces of other rocks.
remember
much
body of
easier than learning
discrete facts.
Looking
^ The most important advice
particularly helpful in geology because so
t -*
things are interrelated. For example, plate tectonics explains
how mountain
volcanism, and earthquakes are
building,
all
related
(Chapter 13). The rock cycle relates the three major groups of rocks to each other and to subsurface and surface processes (Chapter to tie concepts
1).
•^ Use deductive reasoning
Remember
together.
what you learned as
that geology builds
your foundation and see
material relates to
the
new
If
it.
you can draw
parts,
its
material.
type of
how
a picture and you probably understand the Geology lends itself very well to this
•w Draw a picture. label
on
previously. Use that material
device because so much is example, instead of memorizing a of glacial terms, draw a picture of a
memory
is
to study regularly
cram everything into one massive study session. Get plenty of rest the night before an exam, and stay physically fit to avoid becoming susceptible to minor illnesses that sap your strength and lessen your ability to concentrate on the subject at hand. Set up a schedule so that you cover small parts of the material on a regular basis. Learning some concrete examples will help you understand and remember the material. Review the chapter summaries. Construct an outline to make sure you understand how everything fits together. Drawing diagrams will help you remember key points. Make up flash cards to help you remember terms and concepts.
•*r
many
part of a course.
rather than try to
related material
is
tests are the critical
well
examination:
unconnected and
for relationships
and use the
in the details.
on an exam, you must be prepared. These suggestions will help you focus on preparing for the
To do
Outline the material you are studying. This will help you see how the various components are
is
fill
^ PREPARING FOR EXAMS
their definitions.
interrelated. Learning a
Form
a study group, but
make
sure your group
on the task at hand, not on socializing. Quiz each other and compare notes to be sure you have covered all the material. We have found that students dramatically improved their focuses
grades after forming or joining a study group. -v Write out answers to all of the end-of-chapter questions. Review the key terms. Go over all of the key points the professor emphasized in class. If
you have any questions,
visit
the professor or
review sessions are offered, be sure to attend. If you are having problems with the material, ask for help as teaching assistant.
If
soon as you have difficulty. Don't wait end of the semester. If
what
long
are asked. Find out whether the
list
and label its parts and the type of topography it forms.
Developing Critical Thinking and Study
Skills
all
until the
old exams are available, look at them to see is emphasized and what type of questions
visual. For
glacier
can't
on the
visualize the big picture,
For most students,
We
have provided the roots of many important terms throughout this text to help you
You
so focus
important points of the lecture or the chapter.
Put Eggs On My Plate Please. Using rhymes can also be helpful. »' Look up the roots of important terms. If you understand where a word comes from, its meaning will be easier to remember. For
xxvi
is
remember everything,
class,
Silurian,
^
Focus on what
or look over the last chapter before beginning the next. Ask yourself
going to
objective or
all
exam
will be
essay or a combination.
you have trouble with
a particular type of
If
question (such as multiple choice or essay), practice answering questions of that
study group or a classmate
may
Furthermore, the multiple-choice questions
type— your
contain
question as your opening sentence to the answer. Get right to the point. Jot down a quick outline for longer essay questions to
now
time to take the exam. The most important thing to remember is not to panic. This, of course, is easier said than done. Almost everyone suffers from test anxiety to
exam
some
degree. Usually,
begins, but in
some
cases,
it
passes as soon as the
it is
If
you are one of those people, get help as soon as possible. Most colleges and universities have a program to help students overcome test anxiety or at least keep it in check. Don't be afraid to seek help if you suffer test anxiety. Your success in college depends to a large extent on how well you perform on exams, so by not seeking help, you are only hurting yourself. In addition, the fol-
"w
may
First of all, relax. briefly to see its
Then look over
sure you cover everything. you don't understand a question, ask the examiner. Don't assume anything. After all, it your grade that will suffer if you misinterpret If
If
you have time, review your exam to make you covered all the important points and
sure
»
answered all the questions. you have followed our suggestions, by the time you finish the exam, you should feel confident that you did well and will have cause If
for celebration.
the
exam
format and determine which If it
helps,
^ CONCLUDING COMMENTS
quickly jot
We
afraid
benefit to
down any information you are you might forget or particularly want to remember for a question. *• Answer the questions that you know the best first. Make sure, however, that you don't spend too much time on any one question or on one that is worth only a few points. exam
is a combination of multiple choice answer the multiple-choice questions first. If you are not sure of an answer, go on to the next one. Sometimes the answer to one question can be found in another question.
If
the
and
essay,
is
the question.
be helpful:
questions are worth the most points.
-*"
make
"»-
so debilitating that
the individuals do not perform as well as they should.
lowing suggestions
may
of the facts needed to answer
some of the essay questions. Read the question carefully and answer only what it asks. Save time by not repeating the
be able to help.
^ TAKING EXAMS It is
many
hope that the suggestions we have offered will be of you not only in this course, but throughout your college career. While it is difficult to break old habits and change a familiar routine, we are confident that following these suggestions will make you a better student. Furthermore,
you work more
many
efficiently,
of the suggestions will help
not only in college, but also
throughout your career. Learning is a lifelong process that does not end when you graduate. The critical thinking skills that you learn now will be invaluable throughout your life, both in your career and as an informed citizen.
Concluding Comments
xxvii
PHYSICAL
GEOLOGY EXPLORING THE EARTH
CHAPTER
1
UNDERSTANDING THE EARTH: to
An Introduction Physical Geology ^OUTLINE PROLOGUE INTRODUCTION WHAT IS GEOLOGY? GEOLOGY AND THE HUMAN EXPERIENCE
HOW GEOLOGY AFFECTS OUR EVERYDAY LIVES w Perspective 1-1: How Much
~
'
the Public
THE EARTH
T
Need AS A
Perspective 1-2:
to
Science
Does
Know?
DYNAMIC PLANET The Gaia Hypothesis
GEOLOGY AND THE FORMULATION OFTHEORIES The Formulation of
Plate Tectonic
Theory
IT Guest Essay: Science: Our Need PLATE TECTONIC THEORY
to
Know
THE ROCK CYCLE GEOLOGIC TIME AND UNIFORMITARIANISM CHAPTER SUMMARY
Volcanic peaks of the island of Moorea, part of the French Polynesian Islands chain. These islands formed as a result of volcanic eruptions caused by plate movement.
PROLOGUE On
August 26, 1883, Krakatau, a
small, uninhabited volcanic island in
the
Sunda
between Java and Sumatra, exploded than one day, 18 cubic kilometers of rock were erupted in an ash cloud 80 Straits
(Fig. 1-1). In less
(km 3
)
The explosion was heard as far and Rodriguez Island, 4,653 km to the west in the Indian Ocean. Where the 450 meter (m) high peak of Danan once stood, the water was now 275 m deep, and only one-third of the km island remained above sea level (Fig. 1-2). The explosions and the collapse of the chamber that held kilometers (km) high.
away
as Australia
5x9
the magma (molten rock) beneath the volcano produced giant sea waves, some as high as 40 m. On nearby islands, at least 36,000 people were killed and 165 coastal villages destroyed by the sea waves that hurled ashore coral blocks weighing more than 540
metric tons.
So much ash was blown into the stratosphere that Sunda Straits were completely dark from 10 a.m., August 27, until dawn the next day. Ash was reported the
falling on ships as far away as 6,076 km. The sun appeared to be blue and green as volcanic dust, ash, and aerosols circled the equator in 13 days. As these airborne products spread to higher latitudes, vivid red
sunsets were
common around
three years (Fig. 1-3).
the world for the next
The volcanic dust
in the
stratosphere not only created spectacular sunsets,
it
"^ FIGURE
1-1 Krakatau's climactic explosion in August 1883 was preceded by several smaller eruptions. This photograph was taken on May 27, 1883, one week after Krakatau's initial eruption. It shows ash and steam erupting from a vent at Perbawatan on the south side of the island.
incoming solar radiation back into space; the average global temperature dropped as also reflected
much
as 1/2°C during the following year
and did not
eruption, a few shoots of grass appeared, and three
Why have we chosen the eruption of Krakatau as an introduction to physical geology? The eruption was dramatic and interesting in its own right, but it also illustrates several of the aspects of geology that we will be examining, including the way the Earth's interior, surface, and atmosphere are all interrelated. Sumatra, Java, Krakatau, and the Lesser Sunda
years later 26 species of plants had colonized the
Islands are part of a 3,000
island, thus providing a suitable habitat for animals.
islands that
return to normal until 1888.
Of
animal life was destroyed on Krakatau. The remaining portion of the original island was blanketed by tens of meters of volcanic ash and pumice; two months later, the ash and pumice were still so hot that walking was difficult! A year after the course,
all
The
first creatures to reach Krakatau probably flew or were lofted in by the wind; later, others either swam or were rafted to the island on driftwood or other
flotsam.
Upon
multiplied,
arrival, the various
and today most of the
are widely distributed.
animals rapidly species
on Krakatau
location
is
make up
km
long chain of volcanic
the nation of Indonesia. Their
a result of a collision between
two
pieces
of the Earth's outer layer, generally called the crust.
The theory plates that
that the Earth's crust
move
is
over a plastic zone
divided into rigid is
known
as plate
tectonics (see Chapter 13). This unifying theory
explains and
ties
together such apparently unrelated Prologue
Lampong Bay Krakatau'^
'"•'
FIGURE
Indonesia,
Sumatra,
is
(b)
1-2
(a)
Krakatau, part of the island nation of
located in the Sunda Straits between Java and Krakatau before and after the 1883 eruption.
Krakatau Island-After
After the eruption, only one-third of the island remained
above sea
(b)
level.
"^" FIGURE 1-3 Airborne volcanic ash and dust particles from the eruption of Krakatau soon encircled the globe, producing exceptionally long, beautiful sunsets. This sunset was sketched by William Ascroft in London, England, at 4:40 p.m. on November 26, 1883, three months after Krakatau erupted.
geologic
phenomena
as volcanic eruptions,
earthquakes, and the origin of mountain ranges. In tropical areas such as Indonesia, physical
chemical processes rapidly break lava flows, converting for agriculture (see
them
Chapter
down
ash
and and
falls
into rich, productive soils 6).
These
soils
can
support large populations, and, in spite of the dangers of living in a region of active volcanism, a strong correlation exists between volcanic activity
and
population density. Indonesia has experienced 972 eruptions during historic time, 83 of which have
caused
fatalities.
Yet these same eruptions are also
ultimately responsible for the high food production that can support large
numbers of people.
Volcanic eruptions also affect weather patterns; recall that the eruption of Krakatau caused a global cooling of 1/2°C. More recently, the 1982 eruption of El
Chichon
in
Mexico
resulted in lower global
temperatures and abnormal weather patterns (see
Chapter 4
Chapter
1
An
Introduction to Physical Geology
4).
As you read
book, keep in mind that the you are studying are parts of dynamic
interrelated systems, not isolated pieces of
and surface. These eruptions not only have an immediate effect on the surrounding area, but also contribute to climatic changes that affect the
information. Volcanic eruptions such as Krakatau are
entire planet.
this
different topics
the result of
complex interactions involving the
^ INTRODUCTION One major
benefit of the space age
is
the ability to look
back from space and view our planet in its entirety. Every astronaut has remarked in one way or another on how the Earth stands out as an inviting oasis in the otherwise black void of space
The Earth system
in that
is it
(Fig. 1-4).
unique among the planets of our solar supports life and has oceans of water, a
hospitable atmosphere, and a variety of climates. ideally suited for life as
we know
bination of factors, including
sphere, oceans, and, to
by
life
some
it
crust, oceans, in
processes.
In
and
at-
the Earth's atmocrust have been
turn,
these physical
changes have affected the evolution of life. The Earth is not a simple, unchanging planet. Rather,
complex dynamic body
which innumerable many components. The continual evolution of the Earth and its life makes geology an exciting and ever-changing science in which new discoveries are continually being made. it
is
a
interactions are occurring
among
structural geology, the study of the deformation of the
Earth's crust; geophysics, the application of physical laws and principles to the study of the Earth, particularly its interior; paleontology, the study of fossils; and paleogeography, the study of the Earth's past geographical features.
its
extent,
mineralogy, the study of minerals; petrology, the study of rocks; stratigraphy, the study of the sequence of geologic events as recorded in successive layers of rock;
It is
because of a com-
distance from the Sun
its
and the evolution of its interior, mosphere. Over time, changes influenced
Earth's interior
in
its
Nearly every aspect of geology has some economic or environmental relevance, so it is not surprising that
many
geologists are involved in exploration for mineral
and energy resources. Geologists use
their specialized
"^ FIGURE 1-4 The Earth as seen from Apollo 17. Almost the entire coastline of Africa is visible in this view, which extends from the Mediterranean Sea area to the Antarctic south polar ice cap. The Asian mainland is on the horizon toward the northeast, where the Arabian Peninsula can be seen, and Madagascar is visible off the eastern coast of Africa. In addition, numerous storm systems can be seen over the Atlantic and Indian oceans.
^ WHAT IS GEOLOGY? what is geology and what is it that geologists do? Geology, from the Greek geo and logos, is defined as
Just
"the study of the Earth."
It is
generally divided into
two
broad areas — historical geology and physical geology. Historical geology examines the origin and evolution of the Earth,
its
and
continents, oceans, atmosphere,
However, before one can interpret the Earth's
life.
an understanding of physical geology is needed. This involves the study of Earth materials, such as minerals and past,
rocks, as well as the processes operating within the
Earth and upon
The
its
surface.
discipline of geology
many shows many of
vided into
is
so broad that
it is
subdi-
different fields or specialties. Figure 1-5
the diverse fields of geology
and their chem-
relationship to the sciences of astronomy, physics, istry,
and biology. Some of the
specialties of
geology are
What
is
Geology?
,
Geomorp ho|fogy
**
(landscape " an aP6,t>rn fc>r»—
-T.
»"o!
^ ^"A#
0?V
FIGURE
knowledge
1-5
Some
of geology's
many
subdivisions and their relationship to the other sciences.
to locate the natural resources
industrialized society
is
on which our
based. Such mineral resources as
ways in the search and energy resources (Fig. 1-6). Although locating mineral and energy resources is ex-
geology
in increasingly sophisticated
for mineral
and gravel are nonrenewand once known deposits of them are depleted, new deposits or suitable substitutes must be found. As the world demand for these nonrenewable resources in-
problems.
creases, geologists are applying the basic principles of
water for the ever-burgeoning needs of communities and
coal, petroleum, metals, sand, able,
Chapter
1
An
Introduction to Physical Geology
tremely important, geologists are also being asked to use their expertise to help solve
Some
many
of our environmental
geologists are involved in finding ground-
industries or in monitoring surface ter pollution ical
and suggesting ways
engineering
is
and underground wa-
to clean
it
up. Geolog-
being used to find safe locations for
dams, waste disposal
sites,
and power
plants, as well as to
help design earthquake-resistant buildings.
long-range predictions about earthquakes and volcanic In addition, they are
to help
working with
civil
may
result.
defense planners
draw up contingency plans should such natural
disasters occur.
As
emwide variety of pursuits. As the world's population increases and greater demands are made on the Earth's limited resources, the need for geologists and ployed
this
brief survey illustrates, geologists are
in a
their expertise will
become even
lives
discussion of these topics).
Geologists are also involved in making short- and
eruptions and the potential destruction that
which we depend on geology in our everyday and also at the numerous references to geology in the arts, music, and literature (see the articles by R. V. Dietrich listed at the end of this chapter for an extensive tent to
Rocks and landscapes are realistically represented in sketches and paintings. Examples by famous artists include Leonardo da Vinci's Virgin of the Rocks and Virgin and Child with Saint Anne, Giovanni Bellini's Saint Francis in Ecstasy and Saint Jerome, and Asher Brown Durand's Kindred Spirits (Fig. 1-7). In the field of music, Ferde Grofe's Grand Canyon Suite was, no doubt, inspired by the grandeur and timelessness of Arizona's Grand Canyon and its vast rock exposures. The rocks on the Island of Staffa in the Inner
many
Hebrides
greater.
provided
the
inspiration
for
Felix
Men-
delssohn's famous Hebrides Overture (Fig. 1-8). In literature, references to geology
^ GEOLOGY AND THE HUMAN EXPERIENCE Most people
are aware of the importance of geology in
the search for energy resources
and
abound in The Ger-
man Legends of the Brothers Grimm. Jules Verne's jour-
in the prediction
and
minimization of damage caused by various natural disasters. Many people, however, are surprised at the ex-
ney to the Center of the Earth describes an expedition into the Earth's interior (see Chapter 10 Prologue). On one level, the poem "Ozymandias" by Percy B. Shelley deals with the fact that nothing lasts forever
and even under the ravages of time and weathering. References to geology can even be solid rock eventually disintegrates
^ FIGURE
1-6
(a)
Geologists
measuring the amount of erosion on a glacier in Alaska, (b) Geologists
increasingly use computers in their
search for petroleum and other natural resources.
Geology and the
Human
Experience
found in comics, two of the best known being B.C. by Johnny Hart and The Far Side by Gary Larson (Fig. 1-9). Geology has also played an important role in history. Wars have been fought for the control of such natural resources as oil, gas, gold, silver, diamonds, and other valuable minerals. Empires throughout history have risen and fallen on the distribution and exploitation of natural resources. The configuration of the Earth's surface, or its topography, which is shaped by geologic agents, plays a critical role in military tactics. Natural barriers such as
mountain ranges and
rivers
have
fre-
quently served as political boundaries.
^ HOW GEOLOGY AFFECTS OUR EVERYDAY LIVES Destructive
volcanic
eruptions,
devastating
earth-
quakes, disastrous landslides, large sea waves, floods,
and droughts are headline-making events that affect people (Fig. 1-10). Although we are unable to prevent most of these natural disasters, the more we know about them, the better we are able to predict, and
many
possibly control, the severity of their impact.
FIGURE
Kindred
1-7
Spirits
by Asher Brown Durand
(1849) realistically depicts the layered rocks occurring along gorges in the Catskill Mountains of New York State. Asher Brown Durand was one of numerous artists of the nineteenth-century Hudson River School, who were known for their realistic landscapes.
"^ FIGURE
1-8
Mendelssohn was on the Island of Staffa
Felix
inspired by the rocks
in
when he wrote the famous known as Fingal's Cave)
the Inner Hebrides,
Hebrides (also
Overture. Mendelssohn wrote the opening bars of this overture while visiting Staffa.
8
Chapter
1
An
Introduction to Physical Geology
The
envi-
ronmental movement has forced everyone to take a closer look at our planet and the delicate balance between its various systems. Most readers of this book will not go on to become professional geologists. However, everyone should have a basic understanding of the geological processes that ultimately affect all of us. Such an understanding of geology is important so that one can avoid, for example,
building in an area prone to landslides or flooding. Just
ask anyone
who
purchased a
home
in the
Portuguese
jtted
hits
Caucasus region, 40 de
Bend area of southern California during the 1950s (Fig. 15-31) or who built along a lakeshore and later saw the lake level rise and the beach and sometimes even their house disappear.
As
society
becomes increasingly complex and technowe, as citizens, need an understand-
) Marble, a nonfoliated
metamorphic rock, is formed by metamorphism of the sedimentary rock limestone. (Photos courtesy of Sue Monroe.)
preexisting rocks under the influence of elp varpH tem-
peratures or pressure, or as a consequence p f composi-
brought about by fluid activity (F ig. These changes generally occur beneath the Earth's surface For example, marble, a rock preferred by many sculptors and builders, is a metamorphic roc k produced when the agents of meramnrprikm arp applipH to the sedimentary rock limestone or dolostone tional changes 1-18).
.
.
^ FIGURE
1-19
As Figure 1-15 and
rock groups are between plates determine, to a certain extent, which one of the three kinds of rock will form (Fig. 1-19). For example, weathering produces sediment that is transported by various means from the continents to the oceans, where it is deposited. This sediment, along with the oceanic crust, is part of a moving plate. When plates converge, heat and pressure interrelated,
illustrates, the three
interactions
Plate tectonics
and the rock cycle. The cross section shows how the three major rock groups, igneous, metamorphic, and sedimentary, are recycled through both the continental and
Sediment
oceanic regions.
Metamorphism Asthenosphere
Upper
Magma and igneous
mantle
activity
Melting
20
Chapter
1
An
Introduction to Physical Geology
generated along the plate boundary
may
lead to igneous
and metamorphism within the descending oceanic plate. Some of the sediment and sedimentary rock is subducted and melts, while other sediments and sedimentary rocks along the boundary of the nonsubducted plate are metamorphosed by the heat and pressure genactivity
Earth formed 4.6 billion years ago corresponds to 12:00 midnight, January 1. On this calendar, we see that the oldest fossils, simple, microscopic bacteria, which first appeared about 3.6 billion years ago, are in mid-March; di-
nosaurs, which existed between 242 million and 66 million years ago, are
erated along the converging plate boundary. Later, the
26; and
mountain range or chain of volcanic islands formed along the convergent plate boundary will once again be worn down by weathering and erosion, and the new sediments will be transported to the ocean to begin yet
last
another rock cycle.
a geologist, recent geologic events
are those that occurred within the last million years or so.
One popular analogy
geologists use to convey the imis
to
compare the
1-1
it
strikes midnight!
scale resulted
nineteenth-century geologists
from the work of
who pieced
covery of radioactivity in 1895, and the development of various radiometric dating techniques, geologists have since been able to assign absolute age dates in years to
the subdivisions of the geologic time scale (Fig. 1-20).
jQne of the cornerstones of geology
is
the principle of
based on the premise tha t present-day processes have operated throughout geouniformitarianism. logic time.
It
Therefore,
pret the rock record,
day processes and
is
in
order to understand and inter-
we must
first
understand present-
their results.
Uniformitarianism
is
a
powerful principle that allows
us to use present-day processes as the basis for inter-
preting the past and for predicting potential future
history of the
evenis^_We should keep in mind that uniformitarianism
when
does not exclude such sudden or catastrophic events as
Earth to a calendar year (Table 1-1). The time
— TABLE
tick of the clock before
the Earth's biota through time. However, with the disis
fundamental to an understanding of geology. Indeed, time is one of the main aspects that sets geology apart from the other sciences. Most people have difficulty comprehending geologic time because they tend to think in terms of the human perspective— seconds, hours, days, and years. Ancient history is what occurred hundreds or even thousands of years ago. When geologists talk of ancient geologic history, however, they are referring to events that happened hundreds of millions or even bil-
mensity of geologic time
history occurs during the
togeth er information from numerous rock exposures and constructed a sequential chronology based on changes in
appreciation of the immensity of geologic time
To
human
few seconds of December 31. Furthermore, all of the scientific and technological discoveries that have brought us to our present level of knowledge take place in the final
many
UNIFORMITARIANISM
lions of years ago.
between December 12 and December
of recorded
The geologic time
^ GEOLOGIC TIME AND An
all
the
We know
constant through time. Era
Epoch
Period
was more
years ago than
Recent Quaternary
0.01
2 5
Miocene
that volcanic activity
North America 5 to 10 million today, and that glaciation has been
intense in it is
more prevalent during the last 3 million years than in the previous 300 million years. What uniformitarianism means is that even though the rates
and
have var-
intensities of geological processes
and chemical laws of nature have remained the same and cannot be violated. Although the Earth is in a dynamic state of change and ied during the past, the physical
24 Oligocene
37
Eocene
has been ever since 58
Paleocene
have shaped
it
it
are the
was formed, the processes that same ones in operation today.
66 Cretaceous
144 Jurassic
208
^ CHAPTER SUMMARY
Triassic
245 1.
286
Carboniferous
Pennsylvanian
Geology is the study of the Earth. two broad areas: physical geology
It is is
divided into
the study of the
composition of Earth materials as well as the processes that operate within the Earth and
upon its and
Missis-
surface; historical geology examines the origin
sippian
evolution of the Earth,
atmosphere, and Devonian
2.
its
continents, oceans,
life.
Geology is part of the human experience. We can examples of it in the arts, music, and literature.
find
Silurian
A
438
basic understanding of geology
for dealing with the
Ordovician
505
and
Cambrian
3.
570
is
also important
many environmental problems
issues facing society.
Geologists engage in a variety of occupations, the
main one being exploration for mineral and energy resources. They are also becoming increasingly involved in environmental issues and making shortand long-range predictions of the potential dangers from such natural disasters as volcanic eruptions and earthquakes. 4.
right of the
1-20
The geologic time
columns are ages
scale.
Numbers
is
differentiated into layers.
The
outermost layer, or crust, is divided into co ntinent al an d oceanic p ortions. Below the crust is the upp er mantle. T he crust and upper mantle comprise the ^lithospherej which is broken into a series of plates.
3800
"^ FIGURE
The Earth
to the
in millons of years before the
present.
The
lithosphere
moves over the asthenosphere,
a
zone that behaves plastically. Below the as thenosphere is the solid lower mantle The Earth's core, which is beneath the lower mantle, is divide d into an outer liquid portion and an inner soli d .
volcanic eruptions, earthquakes, landslides, or flooding that frequently occur.
modern world, and,
These are processes that shape our in fact, some geologists view the
history of the Earth as a series of such short-term or
punctuated events. Such a view is certainly in keeping with the modern principle of uniformitarianism. Furthermore, uniformitarianism does not require that the rates and intensities of geological processes be
22
Chapter
1
An
Introduction to Physical Geology
portion. 5.
--
approach and analyzing facts abou t a pa rticular phenomenon, formulat ing h ypotheses to explain the phenomenon, testipgjh e_hypothgse,s, and
Theftcientific method/is an orderly, lo gical
that involves gathering
finally
proposing a theory. A( theory!? an
explanation for some natural
phenomenon
that has
a large
body of supporting evidence and can be
2.
tested.
many geological features and events Plates can move away from each other, toward each other, or slide past each other. The nteraction between plat es for
.
i
3.
responsible for volcanic eruptions, earthquake sT
is
and the forma tion of mountain ranges and ocean basins. 7.
and metamorphic rocks are major groups of rocks. Jgneous rocks r esult from the crystallization of magma. ^pHimpntary rocks are formed by the consolidation of rock fragments, precipitation of mineral matter from solution, or compaction of plant or animal remains Metamorphic rocks are produced when preexisting frocks are changed in response t o ele vated Igneous,
s edimentar y,
the three
.
temperature, pressure or fl'iiH heneafh the F arth'c cnrfarp
activity,
,
gpnpnll y
8.
The rock
9.
between the internal and external processes of the Earth and among the three major rock groups. Time sets geology apart from the other sciences, is
the
Which
of the following is not a subdivision of geology? a paleontology; b. JC transform; d.
(b)
the United States;
these.
(b)
c.
is in:
b.
e.
transform;
c.
a.
Italy; b.
none of
e.
the hypothesis of
boundary?
^ REVIEW QUESTIONS a
testable;
11. Mid-oceanic ridges are
mantle
Krakatau
it is
The man who proposed continental drift was:
sedimentary rock subduction zone theory transform plate
boundary
lithosphere
10.
method
sea-floor spreading
geologic time scale
is
guess; d.
principle of
core
of the following statements about a scientific
theory a.
1.
b
Earth's core
a.
basic to the
interpretation of Earth histor y. This principle hold s
concentric layers
divided?
The
is
how many
Into
calendar geologists use to date past events. principle of uniformitarianism
stratigraphy.
e.
4.
cycle illustrates the interrelationships
except astronomy. The geologic time scale 10.
of Krakatau: thousands of people; b. created giant sea waves; c. produced spectacular sunsets around the world: d caused a global cooling of about 1/2°C; e. _a_ all of these. killed
a.
Plate tectonic theory provides a unifying explanation
6.
The eruption
and
plate
subduction;
e.
answers
(d). is
composed of
the:
core and lower mantle;
and asthenosphere;
c.
b. lower mantle asthenosphere and upper
Review Questions
23
\
upper mantle and crust; mantle; d. continental and oceanic crust.
24. Briefly describe the Gaia hypothesis. 25. Briefly describe the plate tectonic theory, and explain
e.
14.
Which a.
^V volcanic;
b.
sedimentary;
d.
15.
not a major rock group? igneous; c. metamorphic;
of the following
is
none of
e.
Which rock group forms from magma? "& *>C igneous; b. sedimentary; c.
27.
these.
the cooling of a
all
29.
of these;
e.
none
be
What
is the principle of uniformitarianism? Does allow for catastrophic events? Explain.
it
30. Briefly discuss the importance of having a
of these. 16.
28.
why it is a unifying theory of geology. What are the three types of plate boundaries? What are the three major groups of rocks? Describe the rock cycle, and explain how it may related to plate tectonics.
metamorphic; d.
26.
The premise
that present-day processes have
operated throughout geologic time
is
known
scientifically literate
populace.
as the
principle of: a.
plate tectonics; b.
c.
continental drift; d.
e.*
17.
^X
sea-floor spreading;
Gaia;
uniformitarianism.
The rock
cycle implies that:
metamorphic rocks are derived from magma; rock type can be derived from any other rock type; c. igneous rocks only form beneath
~^ any
the Earth's surface; d.
sedimentary rocks only
form from the weathering of igneous rocks; e
18.
19.
all
Why
of these.
21. 22. 23.
theory.
24
Chapter
1
An
Calif.:
Introduction to Physical Geology
C,
Jr.
1980. The abyss of time. San Francisco,
Freeman, Cooper &c Co.
Dietrich, R. V. 1989.
Rock music. Earth Science 42,
no. 2:
24-25.
&
1990. Rocks depicted in painting and sculpture. Rocks Minerals 65, no. 3: 224-36. 1991. Rocks
Dietrich, R. V.,
and
in literature.
B.
J.
Rocks
Skinner. 1990.
& Minerals Qems,
66.
granites,
and
New
York: Cambridge University Press. Ernst, W. G. 1990. The dynamic planet. Irvington, N.Y.: gravels.
important for people to have a basic understanding of geology? /-_ ^ivJor. Describe some of the ways in which geology affects c Sea '
MONGOLIA
Caspian
Sea
IRAQ,
CHINA
IRAN
"^FIGURE
36
1
Chapter 2
The Tunguska explosion occurred
A
km
in central Siberia in the
History of the Universe, Solar System, and Planets
Soviet Union.
is
-»- FIGURE 2 Evidence of the Tunguska event is still apparent in this photograph taken 20 years later. The destruction was caused by some type of explosion in central Siberia in 1908.
in an extremely remote 1921 that an expedition was launched to investigate. Unfortunately, illness and exhaustion prevented this expedition from reaching the explosion site. Finally, in 1927, 19 years after the explosion, an expedition led by Leonid Kulik successfully reached the Tunguska basin. A vast peat bog called the Southern Swamp was identified as the site above which the explosion occurred; subsequent
because the event occurred
area,
and
it
was not
investigations
and
occurred about 8
until
studies indicate that the explosion
km
above the surface, and estimated to have been about 12.5 megatons (equivalent to 12.5 million tons of
1,000
km 2
it is
TNT). More than
of forest were leveled by the explosion,
from a meteorite impact. In for investigation
fact,
part of the incentive
may have been economic;
the Soviets
was present and could be mined for its iron content. However, when investigators finally reached the site, no evidence of meteor crater was ever identified. During the 1930s, two Americans proposed that the devastation in the Tunguska River basin was caused by a small, icy comet that exploded in the believed that a meteor
atmosphere. According to
perhaps 50
m
this hypothesis, a
in diameter, entered the
began heating up; as
this
a
comet,
atmosphere and
heating occurred, frozen
gases were instantaneously converted to the gaseous state, releasing a
tremendous amount of energy and
and, according to earlier accounts, tens of thousands
causing a large explosion. The comet hypothesis was
of animals perished
subsequently endorsed by E. L. Krinov of the Soviet
(Fig. 2). Fortunately, there were no human casualties. Even before the explosion site was reached, scientists had hypothesized that the explosion resulted
Academy
of Sciences, and currently
is
the
most widely
accepted explanation for the Tunguska event.
The
Planets
37
"""
FIGURE
2-10
(a)
Mercury has
surface that has changed very
little
a heavily cratered
since
its
early history.
Seven scarps (indicated by arrows) can clearly be seen this image. It is thought that these scarps formed when Mercury cooled and contracted early in its history. (c) Internal structure of Mercury, showing its large solid (b)
core relative to
(b)
its
in
overall size.
measurements and observations made during the flybys
bly escaped into space very quickly. Nevertheless, very
of Mariner 10 in 1974 and 1975 (Table 2-2).
high
small quantities of hydrogen and helium, thought to
has a large
have originated from the solar winds that stream by Mercury, were detected by Mariner 10.
overall density of 5.4
g/cm
3
indicates that
metallic core measuring 3,600
accounts for
80%
in
it
diameter; the core
of Mercury's mass (Fig. 2- 10). Fur-
thermore, Mercury has a
1%
km
Its
weak magnetic
field
(about
as strong as the Earth's), indicating that the core
is
Images sent back by Mariner 10 show a heavily cratered surface with the largest impact basins filled with
what appear to be lava flows similar to the lava plains on the Moon. However, the lava plains are not deformed, indicating that there has been little or no tectonic activity. Another feature of Mercury's surface is a large number of long
cliffs,
called scarps (Fig. 2- 10b).
gested that these scarps formed
and contracted. Because Mercury tion
is
38
it
Chapter 2
is
may A
all the planets, Venus is the most similar in size and mass to the Earth (Table 2-2, Figure 2-11). It differs, however, in most other respects. Venus is searingly hot with a surface temperature of 475°C and an oppressively thick atmosphere composed of 96% carbon dioxide and 3.5% nitrogen with traces of sulfur dioxide and
It is
sug-
when Mercury cooled
sulfuric
and hydrochloric
acid.
From information ob-
tained by the various space probes that have passed by,
orbited Venus, and descended to
its
surface,
we know
composed of droplets of planet. Furthermore, winds up
that three distinct cloud layers
so small,
its
gravitational attrac-
atmospheric gases; any athave held when it formed proba-
insufficient to retain
mosphere that
Venus
Of
probably partially molten.
sulfuric acid envelop the
to
360 km/ hour occur
the planet's surface
History of the Universe, Solar System, and Planets
is
at the top of the clouds,
calm.
whereas
'*' FIGURE 2-11 (a) Venus has a searingly hot surface and is surrounded by an oppressively thick atmosphere composed largely of carbon dioxide, (b) This relief map of Venus shows the three major highland areas: Ishtar Terra at the top, Beta Regio at left center,
and Aphrodite Terra
at right center, (c)
The
internal structure of Venus.
The
Planets
39
Perspective 2-2
THE EVOLUTION OF CLIMATE ON THE TERRESTRIAL PLANETS The
origins
and early evolution of the
history,
terrestrial
hold a somewhat different view
planets has acquired a dramatically different climate.
For example,
Why?
water vapor
All four planets were initially alike, with atmospheres high in carbon dioxide and water vapor derived by outgassing, a process whereby light gases from the interior rise to the surface during volcanic
eruptions. Mercury, because of
proximity to the Sun, lost evaporation early
its
its
small size and
in its history.
Venus, Earth, and
all
their early histories to
climate capable of supporting
The reason
is
related to the recycling of
carbon
(carbon-silicate geochemical cycle) as well as their
Carbon dioxide
recycling
is
an
important regulator of climates because carbon dioxide, other gases, and water vapor allow sunlight to pass
"through" them but trap the heat the planet's surface.
Heat
is
reflected
back from
thus retained, and the
temperature of the atmosphere and surface increases in
what is known as the greenhouse effect. Carbon dioxide combines with water in the atmosphere to form carbonic acid. When this slightly acidic rain falls, it decomposes rocks, releasing calcium and bicarbonate ions into streams and rivers and, ultimately, the oceans. In the oceans, marine organisms use some of these ions to construct calcium carbonate.
When
shells of
the organisms die, their
shells
become part of the
some
of which are eventually subducted at convergent
plate boundaries.
total
1—2).
is present in the atmosphere and there is The amount of carbon dioxide leaving the atmosphere thus decreases and less decomposition of rocks occurs. However, there is no overall long term change in the amount of carbon dioxidefeturned to it is
continually replenished
by plate subduction and volcanism. This leads to a temporary increase in carbon dioxide in the atmosphere, greater greenhouse warming, and, thus, higher surface temperatures.
would happen
if
the surface
temperature should increase. Oceanic evaporation
dioxide between the atmosphere and the crust distance from the Sun.
(see Perspective
the Earth's surface cools, less
less rain.
Just the opposite
life.
that these three planets evolved such
different climates
when
the atmosphere because
atmosphere by
were temperate enough during have had fluid water on their surfaces, yet only Earth still has surface water and a Mars, however,
carbonate sediments,
During subduction these carbonate
would then increase, leading to greater rainfall and more rapid decomposition of rock; as a result, carbon dioxide would be removed from the atmosphere. Greenhouse warming would then decrease and surface temperatures would fall. Venus today is almost completely waterless. However, many scientists think that during its early history, when the Sun was dimmer, Venus perhaps had vast oceans. During this time, water vapor as well as carbon dioxide was being released into the atmosphere by volcanism. The water vapor condensed and formed oceans, while carbon dioxide cycled (by plate tectonics) just as it does on Earth. As the Sun's energy output increased, however, these oceans
Once
eventually evaporated. there
was no water
the oceans disappeared,
to return carbon to the crust,
and
carbon dioxide began accumulating in the atmosphere, creating a greenhouse effect and raising temperatures. Mars, like Venus and Earth, probably once had a moderate climate and surface water, as indicated by
network of
on
sediments are heated under pressure and release
the crisscrossing
carbon dioxide gas that reenters the atmosphere primarily through volcanic eruptions (Fig. 1).
it had formed and hence cooled rapidly. Eventually, the interior of Mars became so cold that it no longer released carbon dioxide. As a
The
terrain.
Chapter 2
A
Because Mars
less internal
recyling of carbon dioxide has allowed the
Earth to maintain a moderate climate throughout
40
although proponents of the Gaia hypothesis
planets appear to have been similar, yet each of these
its
History of the Universe, Solar System, and Planets
heat
is
when
valleys
its
oldest
smaller than the Earth,
it
Weathering of continental
rocks
s
Calcium and
Carbon dioxide released back into atmosphere
.bicarbonate ions
by volcanism
carried to
ocean Trench
Marine organisms construct calcium
carbonate shells
Carbonate sediment I
Upper mantle Continental crust
t~^~
Carbon dioxide in
magma
1 The carbon-silicate geochemical cycle illustrates how carbon dioxide is Carbon dioxide is removed from the atmosphere by combining with water and forming slightly acidic rain that falls on the Earth's surface and decomposes rocks. This decomposition releases calcium and bicarbonate ions that ultimately reach the oceans. Marine organisms use these ions to construct shells of calcium carbonate. When they die, the shells become part of the carbonate sediments that are eventually subducted. As the sediments are subjected to heat and pressure, they release carbon dioxide gas back into
FIGURE
recycled.
the atmosphere primarily through volcanic eruptions.
result, the
amount
creased to
its
of atmospheric carbon dioxide de-
current low
level.
The greenhouse
effect
was thus weakened, and the Martian atmosphere became thin and cooled to its present low temperature. If Mars had been the size of Earth or Venus, it very likely would have had enough internal heat to
continue recycling carbon dioxide, thus offsetting the
low sunlight levels caused by its distance from the Sun. In other words, Mars would still have enough carbon dioxide in its atmosphere so that it effects of
could maintain a "temperate climate."
The
Planets
41
Radar images from orbiting spacecraft as well as from the Venusian surface indicate three general types of terrain (Fig. 2-1 lb). Rolling plains, characterized by numerous craters and circular basins, cover about 65% of the planet; lowlands cover another 27%; and highlands, similar to continents, occupy the remaining 8%.
42
Chapter 2
A
Even though no active volcanism has been observed on Venus, the presence of volcanoes, numerous lava flows, folded mountain ranges, and a network of fractures indicate internal and surface activity during the past (see Perspective 12-2). There is, however, no evidence for active plate tectonics such as on Earth.
History of the Universe, Solar System, and Planets
"^ FIGURE 2-12 (a) (left) Dawn rises over Mars as the Viking 2 orbiter passes by. One of the largest volcanoes on Mars, Ascreaus Mons, can be seen near the top of this photograph, while near the bottom is the Argyre basin, formed from the impact of a large meteorite early in the history of Mars. The largest canyon known in the solar system, Valles Marineris, can be seen on the right side of Mars. To gain some perspective on the size of Valles Marineris, consider that it would nearly stretch across the United States and its width and depth would dwarf the Grand Canyon
(see insert), (b)
known volcano
Olympus Mons,
in the solar system,
the largest
can be seen rising above
white clouds of frozen carbon dioxide, (c) To illustrate the size of the Martian volcanoes, a map of the western United States is shown superimposed over Olympus Mons and three companion volcanoes, (d) The internal structure of Mars.
Mars Mars, the red planet, has a diameter of 6,787 km and a mass one-tenth that of the Earth (Table 2-2; Fig. 2-12). It is
differentiated, as are all the terrestrial planets, into
and a silicate mantle and crust. The thin Martian atmosphere consists of 95% carbon dioxide, 2.7% nitrogen, 1.7% argon, and traces of other gases. Rotating once every 24.6 hours, a Martian day is only slightly longer than an Earth day. Mars also has distinct seasons during which its polar ice caps of frozen carbon dioxide expand and recede. Perhaps the most striking aspect of Mars is its surface, many features of which have not yet been satisfactorily explained. Like the surfaces of Mercury and the a metallic core
The
Planets
43
Moon,
the southern hemisphere
is
heavily cratered, at-
bombardment. Hellas, a crater with a diameter of 2,000 km, is the largest known impact structure in the solar system and is found in the Martian southern hemisphere. The northern hemisphere is much different, having large smooth plains, fewer craters, and evidence of extensive volcanism. The largest known volcano in the solar testing to a period of meteorite
system,
Olympus Mons
(Fig.
2-12b), has a basal diameter
27 km above the surrounding plains, and is topped by a huge circular crater 80 km in diameter. The northern hemisphere is also marked by huge canyons that are essentially parallel to the Martian equator. of 600
One
km,
rises
of these canyons, Valles Marineris,
km long, 250 km wide,
and 7
km
is
at least
deep and
is
4,000
the largest
were present on Earth, it would stretch from San Francisco to New York (Fig. 2-12a)! It is not yet known how these vast canyons
yet discovered in the solar system.
If it
formed, although geologists postulate that they may have started as large rift zones that were subsequently modified by running water and wind erosion. Such hypotheses are based on comparison to
rift
structures
found on Earth and topographic features formed by geologic agents of erosion such as water and wind (see Chapters 16 and 19). Tremendous wind storms have strongly influenced the surface of Mars and led to dramatic dune formations (see Perspective 19-1, Fig. 3). Even more stunning than the dunes, however, are the braided channels that appear to be the result of running water (Fig. 16-1). It is currently too cold for surface water to exist, yet the channels strongly indicate that there was running water on Mars during the past.
The fresh-looking strongly suggest that
its
Mars was a and may still
many volcanoes
tectonically active
There is, howno evidence that plate movement, such as occurs
planet during the past ever,
surfaces of
be.
on Earth, has ever occurred.
Jupiter Jupiter
the largest of the Jovian planets (Table 2-2;
is
With its moons, rings, and radiation belts, it most complex and varied planet in the solar sys-
Fig. 2-13). is
the
tem. Jupiter's density
but because (Table 2-2). 2.5 times
it
It is
from the time of
its
formation.
When Jupiter
formed,
heated up because of gravitational contraction
and
the planets) insulates
its
is
still
it
all
cooling. Jupiter's massive size
and hence
interior,
did
(as
it
has cooled very slowly.
Jupiter has a relatively small central core of solid
rocky material formed by differentiation. Above this core is a thick zone of liquid metallic hydrogen followed by a thicker layer of liquid hydrogen; above that is a thin layer of clouds (Fig. 2-13b). Surrounding Jupiter
are a strong magnetic field
and an intense radiation
belt.
Jupiter has a dense atmosphere of hydrogen, helium,
methane, and ammonia, which some believe are the same gases that composed the Earth's first atmosphere.
atmosphere is divided into a series of bands as well as a variety of spots (the Great Red Spot) and other features, all interacting in incredibly complex motions. Revolving around Jupiter are 16 moons varying greatly in tectonic and geologic activity (see Perspective 4-1). Also surrounding Jupiter is a thin, faint ring, a
Jupiter's cloudy
different colored
feature shared by
all
the Jovian planets.
Saturn Saturn
is
slightly smaller
than Jupiter, about one-third as
massive, and about one-half as dense, but has a similar
and atmosphere (Table 2-2; Fig. 2-14). more energy (2.2 times as gets from the Sun. Saturn's most conspic-
internal structure
Saturn, like Jupiter, gives off it
is its
ring system, consisting of thousands
of rippling, spiraling bands of countless particles.
Planets
planets are completely unlike any of the ter-
restrial planets in size
it has 318 times the mass an unusual planet in that it emits almost
more energy than it receives from the Sun. One is that most of the excess energy is left over
uous feature
The Jovian
only one-fourth that of Earth,
explanation
much) than
The Jovian
is
so large,
is
or chemical composition (Table
The composition of Saturn is similar to Jupiter's, but more hydrogen and less helium. Sat-
consists of slightly
urn's core
is
not as dense as Jupiter's, and as
in the case
and followed completely different evolutionary histories. While they all apparently contain a small core in
of Jupiter, a layer of liquid metallic hydrogen overlies
relation to their overall size, the bulk of a Jovian planet
helium, and,
composed of volatile elements and compounds that condense at low temperatures such as hydrogen, helium, methane, and ammonia.
cause liquid metallic hydrogen can exist only at very
2-2)
is
44
Chapter 2
A
the core, followed by a zone of liquid hydrogen and lastly,
a layer of clouds (Fig. 2-14b). Be-
high pressures, and since Saturn
is
smaller than Jupiter,
such high pressures are found at greater depths
History of the Universe, Solar System, and Planets
in Sat-
"^ FIGURE 2-14 Saturn and three of its moons, (a) This image of Saturn was taken by Voyager 2 from several million kilometers away and shows the ring system of the planet as well as its banded atmosphere. Saturn has an atmosphere similar to that of Jupiter, but has a thicker cloud cover and contains little ammonia, (b) The internal structure of Saturn,
(c)
Mimas (392 km in diameter) exhibits Some areas of Enceladus (500 km
a large impact crater, (d) in
diameter) have fewer craters, suggesting recent volcanic Hyperion (350 x 200 km) has an irregular
activity, (e) Little
shape and several impact craters; Saturn.
46
Chapter 2
A
History of the Universe, Solar System, and Planets
it
tumbles as
it
orbits
"^ FIGURE 2-15 (a) Images of Uranus taken by Voyager 2 under ordinary' light show a featureless planet, (b) When color is enhanced by computer processing techniques, Uranus is seen to have zonal flow patterns in its atmosphere. (c) The internal structure of Uranus.
With
a diameter of only 2,300
est planet
and,
strictly
Jovian planets (Table 2-2). but recent studies indicate a mixture of
km, Pluto
speaking, Little
it
is
it
is
is
the small-
not one of the
known about
Pluto,
has a rocky core overlain by
methane gas and
ice (Fig. 2-17). It also
has
a thin, two-layer atmosphere with a clear upper layer
overlying a
more opaque lower
Pluto differs from
all
highly eccentric orbit that
plane of the that
is
differ
to those of Jupiter.
The
internal structure of
Neptune
is
Uranus (Table 2-2); it has a rocky core approximately 17,000 km in diameter surrounded by a semifrozen slush of water and liquid methane (Fig. 2-16). Its atmosphere is composed of hydrogen and helium with some methane. Encircling Neptune are three similar to that of
faint rings
and eight moons.
ecliptic. It
nearly half
its
markedly from
layer.
the other planets in that is
tilted
has one
size
it
has a
with respect to the
known moon, Charon,
with a surface that appears to
Pluto's.
^ THE ORIGIN AND DIFFERENTIATION OF THE EARLY EARTH As matter was accreting in the various turbulent eddies that swirled around the early Sun, enough material eventually gathered together in one eddy to form the planet Earth. Recall from Chapter 1 that the Earth is a
The Origin and
Differentiation of the Early Earth
47
24,500
km
The differentiation into a layered planet is probably most significant event in the history of the Earth. Not only did it lead to the formation of a crust and eventually to continents (see Chapter 14), but it was the
probably responsible for the outgassing of light volatile elements from the interior that eventually led to the formation of the oceans and atmosphere.
» THE ORIGIN OF THE EARTH-MOON SYSTEM We
probably
know more about our Moon
than any
other celestial object except the Earth (Fig. 2-19). Nevertheless,
even though the
Moon
centuries through telescopes rectly,
many
has been studied for
and has been sampled
di-
questions remain unanswered.
The Moon
is
one-fourth the diameter of the Earth, has
low density (3.3 g/cm 3 relative to the terrestrial planets, and exhibits an unusual chemistry in that it is bone-dry, having been largely depleted of most volatile elements (Table 2-2). The Moon orbits the Earth and rotates on its own axis at the same rate, so we always see the same side. Furthermore, the Earth-Moon system is unique among the terrestrial planets. Neither Mercury nor Venus has a moon, and the two small moons of Mars— Phobos and Deimos — a
)
FIGURE 2-19 The side of the Moon as seen from Earth. The light-colored areas are the lunar highlands which were heavily cratered by meteorite impacts. The dark-colored areas are maria, which formed when lava flowed out onto the surface.
"'•'
are probably captured asteroids.
The major
surface of the
Moon
can be divided into two
parts: the low-lying dark-colored plains, called
maria, and the light-colored highlands
highlands are the oldest parts of the
""'
FIGURE
2-18
(a)
The
early Earth
(Fig. 2-19).
Moon
The
and are
heavily cratered, providing striking evidence of the massive meteorite
bombardment
was probably of uniform composition and
density throughout, (b) Heating of the early Earth reached the melting point of iron
and
which, being denser than silicate minerals, settled to the Earth's center. At the same time, the lighter silicates flowed upward to form the mantle and the crust. (c) In this way, a differentiated Earth formed, consisting of a dense iron-nickel core, an iron-rich silicate mantle, and a silicate crust with continents and ocean basins. nickel,
that occurred in the solar
system more than four billion years ago.
Study of the several hundred kilograms of rocks returned by the Apollo missions indicates that three kinds of materials dominate the lunar surface: igneous rocks,
and dust. Basalt, a common dark-colored igneous rock on Earth, is one of the several different types of igneous rocks on the Moon and makes up the greater breccias,
The presence of igneous rocks that are essentially the same as those on Earth shows that magmas similar to those on Earth were generated on the part of the maria.
Moon
long ago.
The lunar "soil") that
is
surface
is
covered with a regolith (or thick. This gray
m
estimated to be 3 to 4
composed of compacted aggregates of rock fragments called breccia, glass spherules, and covering, which
is
small particles of dust,
is
thought to be the result of
interior structure of the
Moon
from that of the Earth, indicating a ary history (Fig. 2-20).
The highland
diately following the
are
12%
is
quite different
different evolution-
crust
is
thick (65 to
Moon's volbillion years ago, immeMoon's accretion. The highlands
100 km) and comprises about ume. It was formed about 4.4
thin covering (1 to 2
of the
composed principally of the igneous rock anwhich is made up of light-colored feldspar
km
thick) of basaltic lava
17%
fills
of the lunar surface,
mostly on the side facing the Earth. These maria lavas came from partial melting of a thick underlying mantle of silicate composition. Moonquakes occur at a depth of
about 1,000 km, but below that depth seismic shear waves apparently are not transmitted. Because shear waves do not travel through liquid, their lack of transmission implies that the innermost mantle may be partially molten. There is increasing evidence that the Moon has a small (600 km to 1,000 km diameter) metallic core comprising 2 to 5% of its volume.
The
origin
and
earliest history of the
unclear, but the basic stages in
ment
are well understood.
ago
years
debris formed by meteorite impacts.
The
A
the maria; lava covers about
and shortly
It
its
Moon
are
still
subsequent develop-
formed some 4.6
thereafter
was
billion
partially
or
wholly melted, yielding a silicate melt that cooled and crystallized to form the mineral anorthite. Because of the
low density of the anorthite
crystals
and the lack
of water in the silicate melt, the thick anorthosite
highland crust formed. The remaining
melt
silicate
cooled and crystallized to produce the zoned mantle, while the heavier metallic elements formed the small
orthosite,
metallic core.
minerals that are responsible for their white appearance.
The formation of the lunar mantle was completed by about 4.4 to 4.3 billion years ago. The maria basalts, derived from partial melting of the upper mantle, were extruded during great lava floods between 3.8 and 3.2
^" FIGURE
2-20
The
internal structure of the
Moon
is
from that of the Earth. The upper mantle is the source for the maria lavas. Moonquakes occur at a depth of 1,000 km. Because seismic shear waves are not transmitted below this depth, it is believed that the innermost mantle is liquid. Below this layer is a small metallic core. different
Mare
basalt
billion years ago.
Numerous models have been proposed for the origin Moon, including capture from an independent
of the
formation with the Earth as part of an integrated two-planet system, breaking off from the Earth during
orbit,
and formation resulting from a collision between the Earth and a large planetesimal. These various models are not mutually exclusive, and elements of some occur in others. At this time, scientists cannot agree on a single model, as each has some inherent problems. However, the model that seems to account best for the Moon's particular composition and structure inaccretion,
volves an impact by a large planetesimal with a
Earth
young
(Fig. 2-21).
In this model, a giant planetesimal, the size of
Mars
or larger, crashed into the Earth about 4.6 to 4.4 billion years ago, causing the ejection of a large quantity of hot
Moon. The material that was was mostly in the liquid and vapor phase and came primarily from the mantle of the colliding planetesimal. As it cooled, the various lunar layers crystalmaterial that formed the ejected
lized
50
Chapter 2
A
History of the Universe, Solar System, and Planets
out
in the
order
we have
discussed.
"'' FIGURE 2-21 According to one hypothesis for the origin of the Moon, a large planetesimal the size of Mars crashed into the Earth 4.6 to 4.4 billion years ago, causing the ejection of a mass of hot material that formed the Moon. This computer simulation shows the formation of the Moon as a result of an Earth-planetesimal collision.
CHAPTER SUMMARY
2.
The
universe began with a Big Bang approximately 13 to 20 billion years ago. Astronomers have deduced this age from the fact that celestial objects are
moving away from each other
to be
an ever-expanding universe.
in
what appears
3.
The
universe has a background radiation of 2.7° above absolute zero, representing the cooling remnant of the Big Bang. About 4.6 billion years ago, the solar system formed from a rotating cloud of interstellar matter. As this cloud condensed, it eventually collapsed under the influence of gravity and flattened into a
Chapter Summary
51
The age
counterclockwise rotating disk. Within this rotating disk, the Sun, planets, and moons formed from the turbulent eddies of nebular gases and solids. 4. Meteorites provide vital information about the age and composition of the solar system. The three 5.
major groups are stones, irons, and stony-irons. Temperature as a function of distance from the Sun played a major role in the type of planets that evolved. The terrestrial planets are composed of rock and metallic elements that condense at high
forces ?
electromagnet^; c. strong photon. e. The composition of the universe has been changing since the Big Bang. Yet 98% of it by weight still
hydrogen and carbon; b. helium and hydrogen and helium; d. carbon c. hydrogen and nitrogen. and nitrogen; e.
seem to have had a similar which volcanism and cratering from meteorite impacts were common.
Which
Venus; e Mars. The age of the solar system
and crust, and all had an early atmosphere of carbon dioxide and water vapor. The Jovian planets differ from the terrestrial planets in size and chemical composition and followed
Earth;
c.
is
generally accepted by
scientists as:
4.6 billion years;
a.
10 billion years;
b.
20 billion years; 50 billion years. The major problem that plagued most early theories 15.5 billion years; d.
c.
completely different evolutionary histories. All of the Jovian planets have a small core compared to their overall size, but they are mainly composed of
e.
of the origin of the solar system involved the:
at
distribution of elements throughout the solar
a.
low temperatures, such as hydrogen, helium, methane, and ammonia. The Earth formed from one of the swirling eddies of nebular material 4.6 billion years ago and, by at least 3.8 billion years ago, was differentiated into its present-day structure. It accreted as a solid body and then underwent differentiation during a period of
rotation of the planets around their slow rotation of the Sun; revolution of the planets around the Sun;
system; axes; d.
The
b.
c.
source of meteorites and asteroids.
e.
surface of the
Moon
light-colored highlands
is
divided into
and low-lying, dark-colored
plains called:
internal heating.
Moon
probably formed as a result of a Mars-sized planetesimal crashing into Earth 4.6 to 4.4 billion years ago and ejecting a large quantity of hot material. As it cooled, the various lunar layers crystallized, forming a zoned body.
^ IMPORTANT
not a terrestrial planet?
is
Jupiter;
b.
d.
core, mantle,
The
of the following
Mercury;
a.
7. All the terrestrial planets are differentiated into a
10.
nuclear;
a.
terrestrial planets
and compounds that condense
weak
consists of the elements:
early history during
9.
gravity; b.
a.
nuclear; d.
carbon;
volatile elements
4.6 billion years;
million years; b.
8 to
temperatures.
The
generally accepted by
is
15 billion years; d. 13 to 20 billion greater than 50 billion years. years; e. Which of the following is not one of the four basic
The Jovian planets plus Pluto are composed mostly of hydrogen, helium, ammonia, and methane, all of which condense at lower
8.
570
a. c.
temperatures.
6.
of the universe
scientists as:
a
anorthosites; b
d.
nebulas;
regolith; c
cratons;
maria.
e.
The most widely accepted theory regarding origin of the
Moon
the
involves:
an capture from an independent orbit; b. breaking independent origin from the Earth; c. off from the Earth during the Earth's accretion; formation resulting from a collision between d. none of the Earth and a large planetesimal; e. a.
TERMS
these.
Big Bang greenhouse effect
refractory element
irons
stones
Jovian planets meteorites
stony-irons
outgassing
volatile
10.
solar nebula theory
Images radioed back by Voyagers
1
and 2 revealed
that:
terrestrial planets
1.
52
11
REVIEW QUESTIONS The most abundant meteorites a.
stones; b.
d.
acondrites;
Chapter 2
A
irons; e.
c.
peridotites.
Neptune
is
c. Uranus has and Neptune;
a placid planet;
d.
Pluto has an atmosphere similar to that of
Mars;
e.
The
all
of these.
planets can be separated into terrestrial and
Jovian primarily on the basis of which property? density; atmosphere; c. a. size; b.
are:
stony-irons;
all
b.
a large spot like those of Jupiter
element
planetesimal
^
of the Jovian planets have rings;
a.
d.
12.
It is
color;
was caused by
History of the Universe, Solar System, and Planets
none of
e.
these.
currently believed that the a(n):
Tunguska explosion
meteor;
a.
13.
Which of
asteroid;
b.
the following events did
terrestrial planets
14.
e.
all
21.
comet.
of the
experience early in their history?
a.
accretion; b.
c
volcanism;
e.
all
Which of
nuclear
c.
volcanic eruption;
explosion; d.
differentiation;
meteorite impacting;
d.
22.
How
24.
How
does the solar nebula theory account for the general characteristics of the solar system? 23. What are the three major groups of meteorites?
of these.
its
the following
surface;
flows;
not characteristic of
is
25.
heavy cratering
numerous
scarps; d.
c.
b.
similar to Earth's;
d.
thin, like that of
The
surface of
Mars
Mars;
none of
e.
huge
c.
large craters; d.
Which
these.
smooth
plains;
all
e.
Jupiter; b.
d.
answers
(a)
Saturn;
and
Uranus; answers (a) and
c.
(b); e.
Both Jupiter and Saturn have a core overlain by a zone of: helium;
c.
frozen
e.
carbon dioxide.
b.
relatively small
ammonia;
The only planet whose
hydrogen;
d.
axis of rotation nearly
parallels the plane of the ecliptic
Venus;
rocky
liquid metallic hydrogen;
a.
is:
Uranus; Neptune; e. Pluto. 20. What was the main source of heat for the Earth b.
Saturn;
c.
d.
early in
its
history?
meteor impact; b. radioactivity; c. gravitational compression; d. an initial molten condition; e. spontaneous combustion. a.
how
Earth-Moon system.
the Voyager space probes have changed by.
^
ADDITIONAL
READINGS
American 262, no. 6: 50-59. Grieve, R. A. F. 1990. Impact cratering on the Earth. Scientific American 262, no. 4: 66-73. Horgan, J. 1990. Universal truths. Scientific American 263, no. 4: 108-17. Ingersoll, A. P. 1987. Uranus. Scientific American 256, no. 1: 38-45. Kasting, J. F., O. B. Toon, and J. B. Pollack. 1988. How climate evolved on the terrestrial planets. Scientific American 258, no. 2: 90-97. Kinoshita, J. 1989. Neptune. Scientific American 261, no. 5: 82-91. Kuhn, K. F. 1991. In quest of the universe. St. Paul, Minn.: West Publishing Co. McSween, H. Y., Jr. 1989. Chondritic meteorites and the formation of planets. American Scientist 77, no. 2: 146-53. Saunders, R. S. 1990. The surface of Venus. Scientific American 263, no. 6: 60-65. Taylor, S. R. 1987. The origin of the Moon. American Scientist 75, no. 5: 468-77. Benzel, R. 1990. Pluto. Scientific
a.
a.
30. Discuss
more energy than they
(c).
19.
and history of the four Jovian planets?
our ideas about the planets they have flown
receive?
18.
are the similarities and differences in the
origin
massive volcanoes;
valleys; b.
planets give off
What
into three concentric layers.
of these. 17.
and history of the four terrestrial planets? why Venus, Earth, and Mars currently have
29. Discuss the origin of the
possesses:
a.
are the similarities and differences in the
origin
28. Discuss the origin and differentiation of the Earth
nonexistent;
c.
What
quite different atmospheres.
27.
hydrogen and helium. 15. The atmosphere of Venus is: a. thick and composed of carbon dioxide;
16.
the terrestrial planets differ from the Jovian
26. Discuss
lava
small amounts of atmospheric
e.
do
planets?
a strong magnetic field; b.
of
indicate that the
Big Bang occurred?
Mercury? a.
What two fundamental phenomena
Additional Readings
53
CHAPTER
3
MINERALS *=
OUTLINE
PROLOGUE INTRODUCTION MATTER AND ITS COMPOSITION Elements and Atoms
Bonding and Compounds
MINERALS Naturally Occurring, Inorganic Substances
"^ Guest
Essay: Mineralogy: Diverse Pursuits
The Nature of
A
Career with
Crystals
Chemical Composition Physical Properties
MINERAL DIVERSITY MINERAL GROUPS Silicate
Minerals
Carbonate Minerals r" Perspective 3-1: Quartz— A
'"
Common
Useful Mineral
Other Mineral Groups
PHYSICAL PROPERTIES OF MINERALS Color and Luster Crystal
Form
Cleavage and Fracture
Hardness Specific Gravity
y*
Perspective 3-2:
Diamonds and
Pencil
Leads
Other Properties
IMPORTANT ROCK-FORMING MINERALS MINERAL RESOURCES AND RESERVES CHAPTER SUMMARY "Steamboat"— red and green tourmaline and
From the Tourmaline King mine, near Pala, San Diego County, California. The specimen is about 28 cm high. National Museum of Natural History specimen #R51. (Photo by D. Penland, courtesy of Smithsonian Institution.) colorless quartz crystals.
^^^^^^^^^^^^ ^m>^^^^»jk^
*^«^6
;"
«r-*r
PROLOGUE
the Europeans' lust for gold
fact,
was responsible
for
the ruthless conquest of the natives in those areas. In the United States, gold
Among
the hundreds of minerals used
by humans none is so highly prized and eagerly sought as gold (Fig. 3-1). This deep yellow mineral has been the cause of feuds and wars and was one of the incentives for the exploration of the Americas. Gold has been mined for at least 6,000 years, and archaeological evidence indicates that
North Carolina
was
first
1801 and
profitably
flocked to California to find riches. Unfortunately,
only a few found what they sought. Nevertheless, during the five years from 1848 to 1853, which
people in Spain possessed small quantities of gold
constituted the gold rush proper, million in gold
many
Why in tools
benefits for those is
who
possessed
it is
too soft and pliable to
hold a cutting edge. Furthermore, it is too heavy to be practical for most utilitarian purposes (it weighs about
much
During most of historic time, gold has been used for jewelry, ornaments, and ritual objects and has served as a symbol of wealth and as a monetary standard. Gold is so desired for several twice as
reasons: (1)
which
it
scarcity
its
as lead).
pleasing appearance, (2) the ease with
can be worked, (it is
much
(3) its durability,
and
more than $200
was recovered.
Another gold rush occurred
in
1876 following the
report by Lieutenant Colonel George Armstrong
it.
gold so highly prized? Certainly not for use
or weapons, for
in
in
40,000 years ago. Probably no other substance has caused so much misery, but at the same time provided so
mined
Georgia in 1829, but the truly spectacular finds occurred in California in 1848. This latter discovery culminated in the great gold rush of 1849 when tens of thousands of people in
(4) its
rarer than silver).
Central and South American natives used gold
Custer that "gold in satisfactory quantities can be obtained in the Black Hills [South Dakota]." The flood of miners into the Black Hills, the
War
in the
during which Custer and some 260 of his
were annihilated
Montana
at the Battle of the Little
Indian
men
Bighorn
in
June 1876. Despite this stunning victory, the Sioux could not sustain a war against the U.S. Army, and in September 1876, they were forced to in
relinquish the Black Hills.
For 50 years following the California gold rush, the
United States led the world in gold production, and
it
produces a considerable amount, mostly from
extensively long before the arrival of Europeans. In
still
"^ FIGURE
"^ FIGURE
3-1 Specimen of gold from Grass Valley, California— National Museum of Natural History (NMNH) specimen #R121297. (Photo by D. Penland, courtesy of Smithsonian Institution.)
Holy
Wilderness of the Sioux Indians, resulted
3-2
Homestake Mine headworks is the
The headworks (upper at Lead,
South Dakota,
right) of the in
1900. The
cluster of buildings near the
opening to a
mine.
Prologue
55
mines
Nevada and South Dakota
in
(Fig. 3-2).
Currently, however, the leading producer
is
South
Union a distant second, followed by Canada and the United States. Much gold
Africa with the Soviet
still is
used for jewelry, but in contrast to
uses, gold
=*=
now
its
earlier
has some more practical applications
as well, including the chemical industry, gold plating, electrical circuitry,
and
glass
making. Consequently,
the quest for gold has not ceased or even abated. In
many
industrialized nations, including the United
domestic production cannot meet the demand, and much of the gold used must be imported. States,
INTRODUCTION
The term "mineral" commonly brings to mind dietary substances that are essential for good nutrition such as calcium, iron, potassium, and magnesium. These sub-
mineral quartz, and ore deposits are natural concentra-
stances are actually chemical elements, not minerals in
using mineral resources such as iron, copper, gold, and
is also sometimes used to any substance that is neither animal nor vegetable. Such usage implies that minerals are inorganic substances, which is correct, but not all inorganic substances are minerals. Water, for example, is not a mineral even though it is inorganic and is composed of the same chemical elements as ice, which is a mineral. Ice is, of course, a solid whereas water is a liquid; minerals are
many
the geologic sense. Mineral
tions of economically valuable minerals. Indeed, our in-
dustrialized society depends directly
upon
finding
and
others.
refer to
^ MATTER AND
ITS
COMPOSITION
lme~-means~it has a regular internal structure. Further-
Anything that has mass and occupies space is matter. The atmosphere, water, plants and animals, and minerals and rocks are all composed of matter. Matter occurs in one of three states or phases, all of which are important in geology: solids, liquids, and gases (Table 3-1). Atmospheric gases and liquids such as surface water and groundwater will be discussed later in this book, but here we are concerned chiefly with solids because all
more, a mineral has a narrowly deTmgd~ch eniIcal co m-
minerals are solids.
solids rather than liquids or gases. In fact, geologists
have a very specific definition of the teririmjneral: a naturally occurring,jnorganic crystalline solid. Crystal-
position and characteristic physicaj^ropejrtie^uchas
and hardness. Most rocks are solid agoTone orjnor e minerals, and thus mjneraJs^are
density, color,
"gregates
~~ObviouiIy7 minerals are important to geologists as the constituents of rocks, but they are important for
Many gemstones such as diamond and topaz are actually minerals, and rubies are simply red-colored varieties of the mineral corundum. The sand used in the manufacture of glass is composed of the
other reasons as well.
""»"
TABLE
Characteristics
Solid
Rigid substance that retains
Liquid
Flows a
is
the characteristics of an element. Ninety-one naturally
occurring elements have been discovered, some of which are listed in Table 3-2, and more than a dozen additional
elements have been
its
shape unless distorted by a force
and conforms to the shape of the containing vessel; has well-defined upper surface and greater density than a gas
Flows
Chapter 3
made
in laboratories.
Each naturally
Examples
easily
easily
and expands to
a well-defined upper surface;
56
is made up of chemical elements, each of composed of incredibly small particles called atoms. Atoms are the smallest units of matter that retain
which
Phases or States of Matter
3-1
Phase
Gas
Elements and Atoms All matter
the building blocks of rocks.
Minerals
fill
is
all parts of a containing vessel; lacks compressible
Minerals, rocks, iron,
wood
Water, lava, wine, blood, gasoline
Helium, nitrogen,
air,
water vapor
— TABLE
3-2
^ FIGURE
3-4
Schematic
representation of isotopes of carbon. A carbon atom has an
atomic number of 6 and an atomic mass number of 12, 13, or 14 depending on the number of neutrons in its nucleus.
2
atoms of the same element may have different atomic mass numbers. For example, different carbon (C) atoms have atomic mass numbers of 12, 13, and 14. All of these atoms possess 6 protons, otherwise they would not be carbon, but the number of neutrons varies. Forms of the same element with different atomic mass numbers are isotopes (Fig. 3-4).
isotope but many,
such as uranium and carbon, have several
(Fig. 3-4).
*C(Carbon-14)
but the outermost shell never contains more than eight (Table 3-2).
The
electrons in the outermost shell are
those that are usually involved in chemical bonding.
Two
types of chemical bonds are particularly impor-
and covalent, and many minerals Two other types of chembonds, metallic and van der Waals, are much less
tant in minerals, ionic
contain both types of bonds. ical
A number of elements have a single Some
^C(Carbon-13)
C(Carbon-12)
common, but
are extremely important in determining
the properties of
some very
useful minerals.
isotopes are unstable and spontaneously change to
a stable form. This-proress. c3]icdj^adio active dec ay.
occurs because the forces t hat bind the _nucleus_together
are_not_strong enough. Such decay occurs at
and is the mining age that
rates
known
basis for several techniques for deter-
Chapter 9. Neveran element behave the same chemically. For example, both carbon 12 and carbon 14 are present in carbon dioxide (C0 2 ). will be discussed in
theless, all isotopes of
eight electrons in
complete outer are
known
of
.
The noble
react reacfiTy^with other elements to
because
of
this
electron
gases do not form compounds
configuration.
two or more
elements arebonded, the resulting substance
atoms
is
different is
a
com-
pound. Thus, a chemical substance such as gaseous oxygen, which consists entirely of oxygen atoms, is an element, whereas ice, which consists of hydrogen and oxygen, is a compound. Most minerals are compounds although there are several important exceptions, such as gold and silver. To understand bonding, it is necessary to delve
eight electrons, unless the is
Interactions
tend to produce electron configurations
That
is,
act such that their outermost electron shell
are joined to other
When atoms
contain ing_£Jght electrons; they
similar to those of the noble gases.
Bonding and Compounds called bonding.
s hells
as the noble gases
among atoms
The process whereby atoms
ou tgr most electronjsheU. Some
t heir
elements, however, including neon and argon, have
first shell
(with
atoms
inter-
is filled
two
with
electrons)
also the outermost electron shell as in helium.
One way
in
which the noble gas configuration can be
by the transfer of one or more electrons from one atom to another. Common salt, for example, is attained
is
composed of
sodium (Na) and chlorine when combined chemically, they form the compound sodium chloride (CI),
the elements
each of which
is
poisonous, but
(NaCl), the mineral halite or
common
salt.
Notice
in
Figure 3-5a that sodium has 11 protons and 11 elec-
deeper into the structure of atoms. Recall that negatively
trons; thus, the positive electrical charges of the protons
charged electrons
are exactly balanced by the negative charges of the elec-
in electron shells orbit the nuclei
of
and the atom
atoms. With the exception of hydrogen, which has only
trons,
one proton and one electron, the innermost electron shell of an atom contains no more than two electrons.
chlorine with 17 protons
The other
rine has eight electrons in
58
shells
Chapter 3
contain various numbers of electrons,
Minerals
neutral (Fig. 3-5a).
is
electrically neutral. Likewise,
and 17 electrons is electrically However, neither sodium nor chloits
outermost electron
shell;
sodium has only one whereas chlorine has seven. In order to attain a stable configuration, sodium loses the electron in
its
outermost electron
with eight electrons as the outermost one
shell
sodium ions are bonded to chlorine on all sides, and chlorine ions are surrounded by sodium ions (Fig. 3-5b). neutrality. In halite,
next
ions
(Fig.
However, sodium now has one fewer electron
3-5a).
(negative charge) than
an
it is
shell, leaving its
dimensional framework that results in overall electrical
electrically
it
electron lost by
ermost electron
Such a particle is an + symbolized Na
Covalent bonds form between atom£ when th eir elecmnr-slTeTIs"overlap ancTelectrons are~shared. FoTexarn ple, atoms of the same element, such as oxygen in oxygen gas, cannot bomTBytransferring electrons from o ne atom to another. Carbon (C), which forms the minerals graphite and diamond, has four electrons in its outermost electrqrTshell (Fig. 3-6a). If these four electrons
particle.
ion and, in the case of sodium,
The
Covalent Bonding
has protons (positive charge) so
charged
sodium
is is
shell of chlorine,
.
transferred to the out-
which had seven
elec-
more
trons to begin with. Thus, the addition of one
electron gives chlorine an outermost electron shell of eight electrons, the configuration of a noble gas. Its total
number of electrons, however, is now 18, which exceeds by one the number of protons. Accordingly, chlorine also
An
becomes an
ion, but
bond forms
it is
negatively charged (Cl~
were transferred to another carbon atom, the atom ceiving the electrons
1
would have
ration of eight electrons in
).
between sodium and chlo rine be-
its
re-
the noble gas configu-
outermost electron
shell,
charged sodium ion and the negatively charged chlorine
but the atom contributing the electrons would not. In such situations, adjacent atoms share electrons by overlapping their electron shells. For example, a carbon
ion (Fig. 3-5ay
atom
ionic
cause of th e attrac tive force between the positively
fiT ionic
mineral
compounds, such
halite),
the
ions
as
are
sodium chloride arranged in
a
in
diamond shares
all
four of
its
outermost
elec-
trons with a neighbor to produce a stable noble gas
(the
configuration (Fig. 3-6a).
three-
-^ FIGURE
3-5
{a)
I
onic
bonding The electron in the outermost shell of sodium is transferred to the outermost .
Transfer of electron
: — iztzi. z:—.tz s. z.izz if i zzi.ZJz S ~ e regootfa.
an :
essential source
-r_rr ztztT.T.
onh of ;
:
r.
:
>:_
14"
"^ FIGURE
6-18
Spheroidal weathering,
(a)
are attacked by chemical weathering processes,
The rectangular blocks outlined by (£>)
joints
but the corners and edges are
weathered most rapidly, (c) When a block has been weathered so that it is spherical, entire surface is weathered evenly, and no further change in shape occurs.
and silt-sized mineral grains, especially quartz, but other weathered materials may be present as well. Such solid particles are important because they hold soil particles apart, allowing oxygen and water to circulate more freely. Clay minerals are also important constituents of soils and aid in the retention of water as well as supplying nutrients to plants. Soils with excess clay minerals, however, drain poorly and are sticky when wet and hard
when
dry.
is
(Fig.
6-20a). For example,
if
a
body of granite
weathers, and the weathering residue accumulates over
and
is
converted to
soil,
the soil thus formed
residual. In contrast, transported soils are developed
on weathered material eroded and transported from the weathering site to a new location (Fig. 6-20b). Many fertile transported soils of the Mississippi River valley and the
Pacific
windblown dust
=»
Residual soils are formed where parent material has
weathered
the granite
its
THE
Northwest developed on deposits of called loess (see Chapter 19).
SOIL PROFILE
Soil-forming processes begin at the surface and
downward, so
the upper layer of soil
from the parent material than the
"^ FIGURE
served in vertical cross section, a 6-19
Spheroidal weathering of granite in
Australia.
more
layers below.
soil consists
work
altered
Ob-
of distinct
from one another in and color (Fig. 6-21).
layers or soil horizons that differ
texture, structure, composition,
WWMV^Wg^:
is
Starting
from the top, the horizons
typical of soils are
designated O, A, B, and C, but the boundaries between
horizons are transitional rather than sharp.
The
O
horizon, which
is
generally only a few centi-
meters thick, consists of organic matter. The remains of plant materials are clearly recognizable in the upper part
O
lower part consists of humus. is called top soil (Fig. 6-21). This layer contains more organic matter than those below. It is also characterized by intense biological activity because plant roots, bacteria, fungi, and animals such as worms are abundant. Threadlike soil bacteria give freshly plowed soil its earthy aroma. In
of the
horizon, but
its
Horizon A, lying beneath horizon O,
soils
148
Chapter 6
Weathering, Erosion, and Soil
developed over a long period of time, the
A horizon
(b)
(a)
^p" FIGURE 6-20 (b)
Transported
Residual soil developed on bedrock near Denver, Colorado. developed on windblown dust deposit.
(a)
soil
consists mostly of clays
and chemically
such as quartz. Water percolating
zon
A
dissolves the soluble minerals that
present and carries them
stable minerals
down through
hori-
were originally
away or downward
to lower
by a process called leaching. Horizon B, or subsoil, contains fewer organisms and less organic matter than horizon A (Fig. 6-21). Horizon levels in the soil
B
is
also called the zone of accumulation, because sol-
uble minerals leached from horizon irregular masses.
If
horizon
sion leaving horizon well,
and
stickier
if
A
B exposed,
horizon B
is
when wet than
clayey,
other
Horizon C, the lowest
A
accumulate as away by ero-
stripped
is
do not grow as harder when dry and
plants
it is
soil
in
horizon B
of partially
(Fig.
6-22a).
Pedocals are soils characteristic of arid and semiarid regions and are found in States, especially the
name
rives
its
Such
soils
horizon
horizons.
soil layer, consists
symbols for aluminum (Al) and iron (Fe). Because these soils form where abundant moisture is present, most of the soluble minerals have been leached from horizon A. Although it may be gray, horizon A is generally dark colored because of abundant organic matter, and aluminum-rich clays and iron oxides tend to accumulate
in part
contain
much
southwest
from the
less
of the western United 6-22b). Pedocal de-
(Fig.
three letters of calcite.
first
organic matter than pedalfers, so
A is generally lighter colored
and contains more
unstable minerals because of less intense chemical weath-
altered to unaltered parent material (Fig. 6-21). In horizons
A and B, the composition and texture of the parent material have been so thoroughly altered that the parent material is no longer recognizable. In contrast, rock fragments and
"'''
mineral grains of the parent material retain their identity
or mature
horizon C. Horizo n
C
contains
litt le
in
FIGURE
6-21
soil.
The
soil
horizons in a fully developed
^O
«t
'
J;.. ,
organic matte r.
^ FACTORS CONTROLLING SOIL
Horizons
O
=
thin layer of
organic matter
FORMATION A = zone
of leaching
B = zone
of
Climate It
has long been acknowledged that climate is the single factor in soil origins. A very general
most important
classification recognizes three
major
soil
teristic
of different climatic settings. Soils that develop in
humid
regions such as the eastern United States and
much
of
Canada
are pedalfers, a
Greek word pedon, meaning
soil,
accumulation
types charac-
C =
partially altered to
unaltered parent material
name
derived from the and from the chemical
Factors Controlling Soil Formation
149
-^ FIGURE
Caliche on
6-23
Mormon Mesa
in
southern
Nevada.
soil
water evaporation
intense yields alkali soils that
is
are so alkaline that they cannot support plants. Laterite
a soil
is
weathering
is
formed in the tropics where chemical and leaching of soluble minerals is
intense
complete. Such
soils are red,
commonly extend
to depths
of several tens of meters, and are composed largely of
aluminum hydroxides,
iron oxides, and clay minerals;
even quartz, a chemically stable mineral, leached out
Although not very
is
generally
(Fig. 6-24a).
laterites
fertile.
The
support lush vegetation, they are is sustained by from the surface layer of or-
native vegetation
nutrients derived mostly
ganic matter, but
little
humus
present in the soil
is
because bacterial action destroys
it.
When
such
itself
soils are
cleared of their native vegetation, the existing surface
accumulation of organic matter is rapidly oxidized, and there is little to replace it. Consequently, when societies practicing slash-and-burn agriculture clear these soils,
they can raise crops for only a few years at best. the soil
is
Then
completely depleted of plant nutrients, the
clay-rich laterite bakes brick hard in the tropical sun,
and the farmers move on process
One
"^ FIGURE
6-22
(a)
Pedalfer
is
the type of soil that
develops in humid regions, whereas arid and semiarid regions.
{b)
pedocal
is
typical of
If
is
to another area
aspect of laterites
the parent material
is
is
of great economic importance.
rich in
150
Chapter 6
Weathering, Erosion, and Soil
aluminum, aluminum hy-
may
accumulate in horizon B as bauxite, the ore of aluminum (Fig. 6-24b). Because such intense chemical weathering currently does not occur in North America, we droxides
are almost totally dependent
As soil water evaporates, calcium carbonate leached from above commonly precipitates in horizon B where it forms irregular masses of caliche (Fig. 6-23). Precipitation of sodium salts in some desert areas where ering.
where the
repeated.
num
Some aluminum
on
foreign sources for alumi-
do exist in Arkansas, Alabama, and Georgia, which had a tropical climate about 50 million years ago, but currently it is cheaper to import aluminum ore than to mine these deposits. ores.
ores
(b)
(a)
"^ FIGURE forms
6-24
(a) Laterite,
shown
here in Madagascar,
is
a deep, red soil that
response to intense chemical weathering in the tropics, {b) Bauxite, the ore of in horizon B of laterites derived from aluminum-rich parent materials. (Photo courtesy of Sue Monroe.) in
aluminum, forms
Much humus
Parent Material The same rock type can yield different soils in different climatic regimes, and in the same climatic regime the same soils can develop on different rock types. Thus, it seems that climate is more important than parent material in determining the type of soil that develops. Nevertheless, rock type does exert some control. For example, the metamorphic rock quartzite will have a thin soil because
chemically stable, whereas an adja-
over
it
cent
body of granite
it is
will
have a
much deeper
in soils is
provided by grasses or leaf
decompose to obtain food. In so doing, they break down organic compounds within plants and release nutrients back into the soil. Additionally, organic acids produced by decaying soil organisms litter
that microorganisms
are important in further weathering of parent materials
and soil particles. Burrowing animals constantly churn and mix soils, and their burrows provide avenues for gases and water. Soil organisms, especially
some
types of bacteria, are
soil (Fig.
6-25).
^ SoiHhat develops on basalt will be rich in iron oxides because basalt contains abundant ferromagnesi an min -
buTfocksTacking such minerals will not yield an iron oxide-rich soil no matter how thoroughly they are
erals,
"^ FIGURE 6-25 The influence of parent material on soil development. Quartzite is resistant to chemical weathering, whereas granite is altered quickly.
weathered. Also, weathering of a pure quartz sandstone will yield
no
clay,
whereas weathering of clay
will yield
nqj and. Organic Soils
Activity-
not only depend on organisms for their
fertility,
but
from microscopic, single-celled bacteria to large burrowing animals such as ground squirrels and gophers. Earthworms — as many as one million per acre— ants, sowbugs, termites, centipedes, millipedes, and nematodes, along with various types of fungi, algae, and single-celled animals, make their also provide a suitable habitat for organisms ranging
homes
in the soil.
AUof these
contribute to the formation
and provide humus when they die and are decomposed by bacterial action. of
soils
Quartzite
Factors Controlling Soil Formation
151
STEPHEN
Guest Essay
H.
STOW
TTVTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT»T»TT»TTTTTTTrm
ENVIRONMENTAL GEOLOGY: SUSTAINING THE EARTH
We
can think of Earth as a spaceship, upon which all live. Our existence depends on our learning
The demand
resources are another crucial area.
for
humans
petroleum will continue, as will the need for geologists
about our home, the Earth, about its behavior, its limits, and about how we, as passengers on this spaceship, can most efficiently live with our environment.
in that industry.
Earth science touches almost every aspect of our It
lives.
make
importance
is
mineral, energy, are limited,
we
and
use owes
its
environment sciences.
We
is
human
race
all
fragile
in
cities.
though not yet fully understood, may of humans' release of materials into the
evolved over millions of years. Another problem
and costs of assessing and correcting these are immense, but must be undertaken.
All earth scientists, including geologists, are
much
in
market for geologists was driven by the petroleum industry, but today there is an scientists to
undertake
environmental studies. For instance, hydrology studies dealing with waste disposal issues are needed as are studies of
how
water resources respond to changes
in
global climates. Deciphering the rock record to identify
past fluctuations in climate
may
help us predict future
As populations grow, the proper use of become an increasingly important issue, and earth scientists are becoming fluctuations.
precious land and resources has
intimately involved in the decision process. Energy
many
of the environmental studies.
interest in the Earth goes
days; in high school,
My
field trips
interest
I
Chapter 6
Weathering, Erosion, and Soil
childhood
geology in
was aroused by mineral-hunting
My
who encouraged my
professional interests are no initial
enthusiasms, but that
to be expected because the profession has changed,
It is gratifying to be applying fundamental knowledge to the solution of issues that confront us daily— issues that absolutely must be solved if our
too.
future existience
is
to be ensured.
A
Otephen H. Stow earned a Ph.D. in geochemistry from Rice University. He has worked as a research scientist for Continental Oil
Company and
has served on
the faculty at the University of
Alabama. Currently, he heads the Geosciences Section of the
Environmental Sciences Division at the Oak Ridge National Laboratory in Tennessee.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA AAAAA AAAAAAAAAAAAAAAAAAA AAAAAAAAAAAA 152
my in
with the geology club as well as by an
excellent chemistry teacher,
is
back to
decided to major
longer the same as those
should be
Historically, the job
unprecedented need for earth
My
sites,
field
interests in science.
aware of the fragility of our planet and the impact that we can have on it.
demand.
that guides
college.
is
groundwater contamination due to unrestricted disposal of waste products over the last several decades. The
scientists,
requires sophisticated understanding of
and computer modeling of data and laboratory studies. To function effectively in this area, earth scientists must not only have a sound base in their discipline, but must also be familiar with other sciences, mathematics, and legislation obtained from
become aware of the ozone hole warming with a resultant
Everyone, not just professional earth
it
aspects of disposal
atmosphere, altering the delicate heat balance that
situations
involves the study of the
contaminant transport, structural and stratigraphic
situations,
challenges
company,
Earth processes, such as groundwater flow and
on the Earth's
increase in sea level that could inundate coastal
be the result
My present position
United States;
the atmosphere and global
These
oil
deals almost entirely with the
massive effort being undertaken throughout the entire
another important aspect of the earth
have
work
existence to water,
throughout the world, so shortages often arise, sometimes leading to confrontations between nations. the
current
But most resources
Of equal
processes.
and they are not distributed evenly
The impact of
job after graduate school involved
sites where the Department of Energy (and its predecessor agencies) disposed of nuclear and chemical wastes from nuclear energy and weapons manufacturing. This cleanup is a
its
soil resources.
first
cleanup of historical waste disposal
our dependence on the Earth's resources,
virtually everything
my
studies.
us acutely aware of the dynamics of the
Earth and the need to understand
my
application of the earth sciences to environmental
encompasses natural disasters— volcanoes,
earthquakes, tropical storms, and floods. Such natural events
Although
exploration and geochemistry for a major
extremely important into a
form of
Relief Relief
soil
in
changing atmospheric nitrogen
nitrogen suitable for use by plants.
will
From
of geologic time.
the difference in elevation between high
low points
it
soil-forming process occurs at a rapid rate in the context
and Slope
is
develop faster on unconsolidated sediment than on solid bedrock.* Under optimum conditions of soil formation, the
soil will
and
Because climate changes with
in a region.
formation
soil
is
the
human
perspective, however,
a slow process; consequently, soil
is
regarded as a nonrenewable resource.
elevation, relief affects soil-forming processes largely
through elevation. For example, on the west slope of the Bighorn Mountains in Wyoming, soils change laterally from pedocal at low elevation to pedalfer at the crest of the mountains.
One
Slope affects soils in two ways.
simply slope
is
angle: the steeper the slope, the less opportunity for soil
development because weathered material is eroded faster than soil-forming processes can work. The other slope control
is
the direction the slope faces. In the
Northern Hemisphere, north-facing slopes receive sunlight than south-facing slopes. is
steep,
it
may
receive
If
no sunlight
less
a north-facing slope
at
all.
Consequently,
north-facing slopes have soils with cooler internal temperatures,
may
support different vegetation, and,
if
in a
cold climate, remain frozen longer.
SOIL EROSION
=»
Unquestionably, construction and farming can accelerate the rate of soil erosion,
some
and
soil losses to
erosion are
magnitude of the problem varies. For one thing, a problem in one area may be only a minor inconvenience someplace else; the critical in
areas. Nevertheless, the
two of a thin than the same loss on a deep, fertile loss of a centimeter or
soil soil.
more critical The Soil Con-
is
servation Service of the U.S. Department of Agriculture
has determined that
soil losses
exceeding
tons per
five
acre per year adversely affect the productivity of the
Most
than
less
13%
that
maximum. This same agency
this
of
all
soil.
United States are being eroded at rates
soils in the
estimates
agricultural land accounts for
71%
of
some parts of the world, however, soil much more serious problem. Madagascar,
the erosion. In
erosion
Time Recall our statement that soil-forming processes begin at the surface
and work downward. Thus, the degree of
alteration of parent material in horizon
is
complete
its
soil to
pulverized by plowing, the fine particles are easily
soil is
properties of a soil are determined by the
blown away. The Dust Bowl of the 1930s is a poignant reminder of just how effective wind erosion can be (see the Prologue). Falling rain disrupts soil particles, and
and organisms altering parent matethrough time; the longer these processes have operated, the more fully developed the soil will be. If a soil is weathered for extended periods of time, however, its fertility
decreases as plant nutrients are leached out, un-
new
materials are delivered. For example, agricul-
tural lands adjacent to
major streams such as the Nile
River in Egypt have their
soils
replenished during yearly
floods. In areas of active tectonism, uplift
and erosion
provide fresh materials that are transported to adjacent areas where they contribute to
How much
soils.
needed to develop a centimeter of soil a meter or so deep? No definitive answer can be given because weathering proceeds at vastly different rates depending on climate and time
is
or a fully developed
parent material, but an overall average might be about 2.5
percentage of
practices, overgrazing,
The
rial
soil
lost a large
and deforestation. Most soil erosion occurs by the action of wind and water. When the natural vegetation is removed and a poor farming
has been undergoing change for the longest
factors of climate
less
a
it
because time.
A
is
example, has
for
cm per century. However, a lava flow a few centuries may have a well-developed soil on it,
when is
it
runs off at the surface,
it
carries soil with
it.
This
on steep slopes from which vegetative cover has been removed by overgrazing or
particularly devastating
the
deforestation.
Two
types of erosion by water are recog-
nized: sheet erosion
Sheet erosion the surface
is
and
erosion.
rill
more or
less
and removes thin
evenly distributed over
layers of soil. Rill erosion,
on the other hand, occurs when running water scours small channels. If these rills become too deep to be eliminated by plowing (about 30 cm), they are gullies (Fig. 6-26). Where gullying becomes extensive, croplands can no longer by tilled and must be abandoned. If
the rate of soil erosion
is
less
than
five
tons per year
most parts of the United States — soil-forming processes can keep pace, and the
per acre— as
is
the case in
old in Hawaii
whereas
a flow the
siderably less
soil.
same age in Iceland will have conGiven the same climatic conditions,
"Bedrock
is
a general term for the rock underlying soil or
unconsolidated sediment.
Soil
Erosion
153
"^ FIGURE 6-27 One soil conservation practice is contour plowing, which involves plowing parallel to the contours of the land. The furrows and ridges are perpendicular to the direction that water would otherwise flow downhill and thus inhibit erosion.
ported elsewhere, perhaps onto neighboring cropland, onto roads, or into channels. Sediment accumulates in canals and irrigation ditches, and agricultural fertilizers
and
insecticides are carried into streams and lakes. Problems experienced during the past, particularly during the 1930s, have stimulated the development of methods to minimize soil erosion on agricultural lands.
Various practices including crop rotation, contour plowing,
and the construction of
terraces have
all
proved
helpful (Fig. 6-27). Other practices include no-till plant-
ing in which harvested crop residue to protect the surface
is left on the ground from the ravages of wind and
water.
^ WEATHERING AND MINERAL RESOURCES we
In a preceding section,
discussed intense chemical
and the origin of bauxite, the chief ore of aluminum. Such an accumulation of valuable minerals formed by the selective removal of soluble weathering
""^"
FIGURE
6-26
rainstorm. This gully
is
rill
(a) Rill
was
erosion in a
later
plowed
field
during a
over, (b) This small
too deep to be plowed.
remains productive. If the maximum is exceeded, however, the upper layers of soil— the most productive first,
is
a residual concentration.
It
represents an
insoluble residue of chemical weathering. In addition to
bauxite, a number of other residual concentrations are economically important; for example, ore deposits of
soil
layers— are removed
substances
in the tropics
thus exposing horizon B. Such
iron, manganese, monds, and gold.
Some
clays,
nickel,
phosphate,
tin,
dia-
limestones contain small amounts of iron car-
bonate minerals.
When
the limestone
is
dissolved during
losses are problems, of course, but there are additional
chemical weathering, a residual concentration of insol-
consequences. For one thing, the eroded
uble iron oxides accumulates.
154
Chapter 6
soil is trans-
Weathering, Erosion, and Soil
Some
of the sedimentary
-
iron deposits (see Chapter 7) of the Lake Superior region were enriched by chemical weathering when the soluble constituents that were originally present were carried away. Residual concentrations of insoluble manganese oxides form in a similar fashion from manganese-rich
Country rock
source rocks.
Most commercial clay deposits were formed by hydrothermal alteration of granitic rocks or by sedimentary processes. However, some have formed in place as residual concentrations. For example, a olinite
deposits
in
the
number of ka-
southern United States were
formed by the chemical weathering of feldspars
in peg-
matites and of clay-bearing limestones and dolostones. Kaolinite
is
a type of clay mineral used in the manufac-
ture of paper
Water table
""'
FIGURE
6-28
A
showing a gossan and and the supergene enrichment of
cross section
the origin of oxidized ores ores.
and ceramics.
Gossans, oxidized ores, and supergene enrichment of ores are interrelated, and all result from chemical weathering (Fig. 6-28).
composed
A gossan is
a yellow to reddish deposit
largely of hydrated iron oxides that
formed
by the oxidation and leaching of sulfide minerals such as pyrite (FeS 2 ). The dissolution of such sulfide minerals
forms sulfuric acid, which causes other metallic minerals to dissolve, and these tend to be carried downward toward the groundwater table (Fig. 6-28). Oxidized ores form just above the groundwater table as a result of chemical reactions with these descending solutions.
Some
of the minerals formed in this zone contain cop-
per, zinc,
and
other metals such as lead, zinc, nickel, and copper that
have a greater
affinity for sulfur. Indeed,
)
source of copper than the
latter.
Gossans have been used occasionally as sources of iron, but they are far more important as indicators of underlying ore deposits.
lead.
supergene chal-
(Cu 2 S), an important copper ore, forms as a replacement of primary pyrite (FeS 2 and chalcopyrite (CuFeS 2 ). Notice that both chalcocite and chalcopyrite are copper-bearing minerals, but the former is a richer
cocite
One
of the oldest
known
un-
Supergene enrichment of ores occurs where metalbearing solutions penetrate below the water table (Fig.
derground mines exploited such ores about 3,400 years ago in what is now southern Israel. Supergene enriched
6-28). Such deposits are characterized by the replace-
ore bodies are generally small but extremely rich sources
primary deposit with sulfide minerals introduced by the descending solutions. For example, the iron in iron sulfides may be replaced by
of various metals.
ment of
sulfide minerals of the
^ CHAPTER SUMMARY 1.
4.
that
it is
more nearly
in
soluble salts,
The
can be deposited as sediment, which may become sedimentary rock. Mechanical weathering includes such processes as frost action, pressure release, thermal expansion and contraction, and the activities of organisms. Particles liberated by mechanical weathering retain the chemical composition of the parent material. soil,
or
5.
Ch emical we ather ing p roceeds most
6.
wet environments, but it occurs in all areas, except perhaps where water is permanently frozen. Mechanical weathering aids chemical weathering_ by
in solution.
residue of weathering can be further modified to
form 3.
and ions
various ions in solution, and soluble salts are formed during chemical weathering.
equilibrium
with new physical and chemical conditions. The products of weathering include solid particles,
Solution, oxidation, and hydrolysis are chemical
weathering processes; they result in a chemical change of the weathered products. Clay minerals,
Mechanical and chemical weathering are processes whereby parent material is disintegrated and
decomposed so
2.
The largest copper mine in the world, Bingham, Utah, was originally mined for supergene ores, but currently only primary ores are being mined. at
rapidly inhot,
"breaking parent material intojj maller piec es, thereby
it
7.
exposing more surface a rea. Mechanic al and~ch emical weath eri ng produ ce r egolith , air,
8.
some ofwhich is soil if ft consists^ of solids, humus and supports plant growth.
water, and
Soils are characterized
by horizons that are
designated, in descending order, as O, A, B, and C;
Chapter Summary
155
horizons differ from one another in texture,
soil
structure, composition, 9.
The
and
factors controlling soil formation include
3.
and time.
as the eastern United States Arid and semiarid regions soils are pedocals, many of which contain irregular masses of caliche in
a.
4.
horizon B.
12.
from intense chemical Such soils are deep, red, and sources of aluminum ores if derived from aluminum-rich parent material. Soil erosion, caused mostly by sheet and rill erosion, is a problem in some areas. Human practices such as construction, agriculture, and deforestation can a soil resulting
is
5.
clay. is
activities of
debris produced mostly
e.^_ soil
organisms;
and supergene enrichment of from chemical weathering. 7.
IMPORTANT TERMS
produced
by intense weathering in the tropics. When the ions in a substance become dissociated, the substance has been: weathered mechanically; b. altered to a.
^
c.
oxidized;
dissolved; d.
converted to
soil.
The process whereby hydrogen and hydroxyl water replace ions in minerals is: supergene enrichment; b. a.
14. Gossans, oxidized ores, all result
residual manganese;
d.
an accumulation of: calcium carbonate in horizon B of pedocals; angular rock fragments at the base of a slope; valuable minerals formed by selective removal
Talus
e.
clay.
ores
present.
is
calcium sulfate;
clay:
6.
the mineral calcite
silicon dioxide;
b.
e.
by the
of which
contain valuable minerals such as iron, lead, copper,
if
of soluble substances; d.
responsible for the
many
composed of
is
c.
c.
accelerate losses of soil to erosion. origin of residual concentrations,
pressure
e.
nearly insoluble in pure water but
carbonic acid;
x
b.
in the tropics.
13. Intense chemical weathering
and
X
a.
weathering as
is
dissolves rapidly
humid regions such and much of Canada.
is
Limestone, which
(CaC0 3 ),
and
10. Soils called pedalfers develop in
11. Laterite
oxidation and reduction;
\
release.
climate, parent material, organic activity, relief slope,
d.
color.
c.
laterization; d.
e.
carbonization.
Which of most
X
ions of
oxidation;
hydrolysis;
the minerals in Bowen's reaction series
is
stable chemically?
calcium plagioclase; k s\ quartz; biotite; e olivine. pyroxene; d. Granite weathers more rapidly than quartzite because it contains abundant:
a.
chemical weathering differential weathering
pedocal
erosion
regolith
pressure release
exfoliation
rill
dome
exfoliation
c.
frost action
erosion
minerals;
spheroidal weathering
wedging
soil soil
hydrolysis
solution
laterite
talus
leaching
thermal expansion and
horizon 10.
12.
13.
is:
2.
laterite; b.
d.
bauxite;
e.
A
pedocal;
c.
gossan;
domes? heating and cooling;
exfoliation
contraction;
156
Chapter 6
c.
parent material; top
Y
soil
talus.
e.
known as the: humus layer; c.
also
is
soil; b.
zone of accumulation;
alkali
organic-
e.
The
chief ore of
a.
caliche; b.
d.
gossan;
aluminum
e.
is:
pedalfer;
X^
subsoil;
c.
bauxite.
The removal of thin layers of soil by water over a more or less continuous surface is: a.
gullying; b.
)(
c.
weathering;
d.
sheet erosion; leaching;
e.
exfoliation.
Oxidation and leaching of sulfide minerals yield a yellow to red deposit of hydrated iron oxides known
pedalfer.
Which mechanical weathering process forms a.
d.
Horizon B of a
rich layer.
11.
of soil typical of arid and semiarid regions
a.
a. ~yr
and unconsolidated rock material covering most places are: humus; regolith; b. laterite; c.
soil
a.
transport
weathering zone of accumulation
-V
The
zone; d.
contraction
» REVIEW QUESTIONS The type
9.
carbonate
d.
caliche.
e.
the Earth's surface in
humus
pedalfer
ferromagnesian minerals;
sheet joint
heaving
quartz;
—f- feldspars; b.
c.
frost
1.
a.
sheet erosion
frost
mechanical weathering oxidation parent material
8.
a.
residual deposit; b.
exfoliation
b.
expansion and
the activities of organisms;
Weathering, Erosion, and Soil
dome;
clay deposit. sheet joint; e. )^ gossan; d. 14. Bacterial decay of organic matter yields c.
which
is/are essential to soil fertility.
humus;
sand; b. /\
a
ways in which soil erosion can be minimized on agricultural lands.
30. Discuss several
pedalfer;
c
ferromagnesian minerals. 15. How does mechanical weathering differ from and contribute to chemical weathering? 16. What is differential weathering, and why does it subsoil;
d.
e.
31.
How
^
does frost wedging differ from frost heaving?
18. Explain
how
sheet joints
and exfoliation domes
most minerals not very soluble
are
22.
What
in
pure
role
do hydrogen ions play
why
panicle size
is
Press.
in the hydrolysis
American Planning Association, Planning Advisory Service Report No. 386. Courtney, F. M., and S. T. Trudgill. 1984. The soil: An introduction to soil study. 2d ed. London: Arnold. Gibbons, B. 1984. Do we treat our soil like din? National agricultural erosion.
an important factor
in
chemical weathering. 24. Describe spheroidal weathering. 25.
Draw each
26.
a soil profile
soil
What
is
and
the characteristics of
list
Geographic 166, no. 3:350-89. Loughnan, F. C. 1969. Chemical weathering of the
horizon. the significance of climate
minerals.
and parent
material in the development of soil?
Oilier, C.
How
Parfit,
do organisms contribute to soil formation? 28. Compare and contrast pedalfer, pedocal, and laterite. 29. Explain how plowing, overgrazing, and deforestation 27.
P.
Carroll, D. 1970. Rock weathering. New York: Plenum Press. Coughlin, R. C. 1984. State and local regulations for reducing
process? 23. Explain
1986. Earth: The stuff of life. 2d revised ed. Okla.: University of Oklahoma Press.
W. 1984. Soils and geomorphology. New York: Oxford University Press. Buol, S. W., F. D. rlole, and R. J. McCracken. 1980. Soil genesis and classification. Ames, Iowa: Iowa State University
is an acid solution, and why are acid solutions important in chemical weathering?
What
E.
Birkeland,
water? 21.
F.
Norman,
whereby soluble minerals such
as halite (NaCl) are dissolved.
Why
and how do they
ADDITIONAL READINGS
Bear,
originate.
19. Describe the process
20.
are residual concentrations,
form?
occur? 17.
What
New
silicate
York: Elsevier.
1969. Weathering. New York: Elsevier. dust bowl. Smithsonian 20, no. 3:44-54,
M. 1989. The
56-57.
contribute to soil erosion.
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Additional Readings
157
CHAPTER
7
SEDIMENT AND SEDIMENTARY ROCKS p OUTLINE PROLOGUE INTRODUCTION SEDIMENT TRANSPORT AND DEPOSITION LITHIFICATION: SEDIMENT
TO
SEDIMENTARY ROCK ""•'
Guest Essay: Exploring for Oil and Natural Gas
SEDIMENTARY ROCKS Detrital Sedimentary
Rocks
Conglomerate and Sedimentary Breccia Sandstone
Mudrocks Chemical Sedimentary Rocks Limestone-Dolostone ^-Perspective 7-1: The Mediterranean Desert Evaporites
Chert
Coal
SEDIMENTARY
FACIES
Marine Transgressions and Regressions
ENVIRONMENTAL ANALYSIS Sedimentary Structures Fossils
Environment of Deposition
SEDIMENTS, SEDIMENTARY ROCKS,
AND NATURAL RESOURCES Petroleum and Natural Gas
Uranium Banded Iron Formation **r Perspective 7-2: Persian
Gulf Petroleum
CHAPTER SUMMARY
Sedimentary rocks exposed
in the
Sheep
Rock area of John Day Fossil Beds National Monument, Oregon. This small hill is capped by the remnants of a lava flow.
PROLOGUE
The Green River Formation its
huge deposits of
oil
is
About 50
million years ago,
lakes existed in
what
are
two
now
large
parts of
substance
known
known
for
and an organic
consists of small clay particles
^^pl^|
also well
shale (Fig. 7-2). Oil shale
as kerogen.
When
the appropriate
extraction processes are used, liquid oil and
Wyoming, Utah, and Colorado. Sand, mud, and
combustible gases can be produced from the kerogen
where they accumulated as layers of sediment that were subsequently converted into sedimentary rock. These sedimentary rocks, called the Green River Formation,
of
contain the fossilized remains of millions of
the Green River Formation. During the
dissolved minerals were carried into these lakes
and
fish, plants,
and are a potential source of large quantities of oil, combustible gases, and other substances. Thousands of fossilized fish skeletons are found on single surfaces within the Green River Formation, indicating that mass mortality must have occurred insects
The cause of these events is not with certainty, but some geologists have
repeatedly (Fig. 7-1).
known
suggested that blooms of blue-green algae produced toxic substances that killed the fish. Others propose that rapidly changing water temperatures or excessive salinity at times of increased
evaporation was
Whatever the cause, the fish died by the thousands and settled to the lake bottom where their decomposition was inhibited because the water contained little or no oxygen. One area of the formation in Wyoming where fossil plants are particularly abundant has been designated as Fossil responsible.
Butte National
Monument.
-"^ FIGURE 7-1 Fossil fish from the Green River Formation of Wyoming. (Photo courtesy of Sue Monroe.)
oil shale.
To be designated
as a true oil shale,
however, the rock must yield a gallons of oil per ton of rock.
source of fuel
is
not new, nor
people in Europe used
oil
minimum
The use of is oil
of 10 oil
shale as a
shale restricted to
Middle Ages,
shale as solid fuel for
domestic purposes, and during the 1850s, small
oil
shale industries existed in the eastern United States;
were discontinued, however, when drilling and pumping of oil began in 1859. Oil shales occur on all continents, but the Green River Formation contains the most extensive deposits and has the potential to yield huge quantities of oil. Oil can be produced from oil shale by a process in C which the rock is heated to nearly 500 C in the absence of oxygen, and hydrocarbons are driven off as gases and recovered by condensation. During this process, 25 to 75% of the organic matter of oil shale can be converted to oil and combustible gases. The Green River Formation oil shales yield from 10 to 140 gallons of oil per ton of rock processed, and the total the latter
amount of
oil
recoverable with present processes
is
estimated at 80 billion barrels. Currently, however,
little oil is
produced from
oil
shale in the United
that
would be necessary would have considerable What would be done with
States except at experimental plants, because
environmental impact.
conventional drilling and pumping
billions of tons of processed rock?
Nevertheless, the Green River
is
less
expensive.
shale constitutes one
oil
of the largest untapped sources of oil in the world.
more
effective processes are developed,
more than
eventually yield even
it
If
could
realize,
and sedimentary rocks
(Fig. 7-3).
Any
type of rock
be completely dissolved or chem-
Chapter 6). Such weathered materials are commonly eroded and transported to another location and deposited as sediment. Thus, all sediment is derived from preexisting rocks and ically altered to
form clay minerals
can be characterized
in
is
in
an
already in short supply?
considered by scientists and industry. Perhaps at some future time, the
Green River Formation
some of our energy
any
can weather mechanically to yield small rock fragments and individual mineral grains, and some of a rock's min-
may
huge volumes of water
come from— especially
These and other questions are currently being
Mechanical and chemical weathering disintegrate and decompose rocks yielding the raw materials for both
eral constituents
will the
however, that at the current
INTRODUCTION
soils
Where
necessary for processing area where water
and expected consumption rates of oil in the United States, oil production from oil shale will not solve all of our energy needs. Furthermore, large-scale mining
**
mining be conducted with minimal
disruption of wildlife habitats and groundwater
systems?
the currently
estimated 80 billion barrels.
One should
large-scale
the
Can such
particle, regardless of
1/16 to 2.0
composition, that measures
mm. Gravel- and sand-sized particles are large
enough to be observed with the unaided eye or with lowpower magnification, but silt- and clay-sized particles are too small to be observed except with very high magnification.
Gravel generally consists of rock fragments,
whereas sand,
silt,
and
clay particles are mostly individ-
We
should note, however, that clay
ual mineral grains.
(see
has two meanings: in textural terms, clay refers to sed-
imentary grains
less
than 1/256
mm in size, and in com-
positional terms, clay refers to certain types of sheet icate minerals (see Fig. 3-12).
two ways:
will provide
needs.
sil-
However, most clay-sized
particles in sedimentary rocks are, in fact, clay minerals. 1.
Detrital sediment,
which
consists of rock
fragments and mineral grains. 2.
Chemical sediment, which consists of the minerals precipitated from solution by inorganic chemical processes or extracted from solution by organisms.
In
SEDIMENT TRANSPORT
AND DEPOSITION Detrital sediment can be transported by
any geologic
move
particles of a
agent possessing enough energy to
any case, sediment
is
deposited as an aggregate of
Much accumulated sediment such as mud in a lake, or from
loose solids (Fig. 7-4).
set-
from a fluid, the atmosphere as dust. The term sediment is derived from the Latin sedimentum, meaning settling. Most sedimentary rocks formed from sediment that was transformed into solid rock, but a few sedimentary tled
^
given
size.
Glaciers are very effective agents of transport
and can move any
sized particle.
Wind, on the other
hand, can transport only sand-sized and smaller sediment. Waves and marine currents also transport sediment, but by far the most effective way to erode sediment
rocks skipped the unconsolidated sediment stage. For
^* TABLE
example, coral reefs form as solids when the reef organ-
7-1
Classification of
Sedimentary Particles
isms extract dissolved mineral matter from seawater for their skeletons.
However,
if
a reef
is
broken apart during on
>2
the sea floor are sediment.
One important
mm
Name
Gravel
1/16-2 mm 1/256-1/16
criterion for classifying detrital sedi-
ments and the rocks formed from them is the size of the Gravel refers to any sedimentary particle measuring more than 2.0 mm, whereas sand is
Sediment
Size
a storm, the solid pieces of reef material deposited
Sand
mm
< 1/256 mm
particles (Table 7-1).
160
Chapter 7
Sediment and Sedimentary Rocks
*
Mixtures of
silt
and clay are generally referred to
as
mud.
FIGURE
7-3
The rock
from the weathering
site
cycle,
with emphasis on sediments and sedimentary rocks.
and transport
it
elsewhere
is
by
areas of sand accumulation^Glaciers and mudflows,
streams.
however, are unselective, because their energy allows
During transport, abrasion reduces the size of sedimentary particles. The sharp corners and edges are abraded the most as the particles, especially gravel and sand, collide with one another and become rounded (Fig. 7-5a). Another sediment property modified during
them
transport
is
sorting. Sorting refers to the size distribu-
tion in an aggregate of sediment;
if all
the particles are
approximately the same size, the sediment is characterized as well sorted, but if a wide range of grain sizes occur, the sediment is poorly sorted (Fig. 7-5b). Sorting
from processes that selectively transport and deposit particles by size. Wi ndblown dunes are composed of _well-sorted_ sand, because wind cannot transport gravel effectively and it blows silt and clay beyond_the results
to transport
many
different-sized particles,
and
their deposits tend to be poorly sorted.
Sediment may be transported a considerable distance from its source area, but eventually it is deposited. Some of the sand and mud being deposited at the mouth of the Mississippi River at the present time came from such distant places as Ohio, Minnesota, and Wyoming. Any geographic area in which sediment is deposited is a depositional environment.
Although no completely satisfactory
classification of
depositional environments exists, geologists generally
recognize three major depositional settings: continental, transitional,
depositional
and marine (Fig. 7-6). Major continental environments include stream systems, Sediment Transport and Deposition
161
v^
.
t
^CA^HQp
Desert dunes
Playa lake Alluviarfan
Gi aC a environment j
environment
i
Barrier island
Delta
Beach
Tida
|
f)at
Shallow marine
— environment
Shallow marine environment
Lagoon Continental
Organic reef
shelf
Organic reef
Submarine fan
FIGURE
7-6
Major depositional environments
are
shown
in this generalized
be compacted and/or cemented and thereby converted into
carbonate
sedimentary rock; the process by which sediment
ing a small
is
trans-
formed into sedimentary rock is lithification. When sediment is deposited, it consists of solid particles and pore spaces, which are the voids between particles. The amount of pore space varies depending on the depositional process, the size of the sediment grains, and sorting. When sediment is buried, compaction, resulting from the pressure exerted by the weight of overlying sediments, reduces the amount of pore space, and thus
volume of the deposit (Fig. 7-7b). When deposits of mud, which can have as much as 80% water-filled pore space, are buried and compacted, water is squeezed out, and the volume can be reduced by up to 40%. Sand may have up to 50% pore space, although it is generally somewhat less, and it, too, can be compacted so that the sand grains fit more tightly together. However, once the
sand grains are arranged in a best fit, sand resists further compaction because the rigid mineral-grain framework supports the weight of overlying sediments.
diagram.
(CaC0 3
readily dissolves in water contain-
)
amount of carbonic
acid,
weathering of feldspars and other silica
may
and that chemical
silicate
minerals yields
(Si0 2 ) in solution. These dissolved compounds
pore spaces ot sediments, cement that effectively binds the sediment together (Fig. 7-7c). Calcite cement is easily be pre cipitated
where They
'"•'
FIGURE
in the
act as a
7-7
Lithification of sand, {a)
When
initially
deposited, sand has considerable pore space between grains. (b) Compaction resulting from the weight of overlying sediments reduces the amount of pore space, (c) Sand is converted to sandstone as cement is precipitated in pore spaces from groundwater.
Pore space
Feldspar
Quartz
Compaction alone is generally sufficient for lithificamud, but for sand and gravel deposits cementa-
tion of tion
is
necessary to convert the sediment into sedimen-
tary rock (Fig. 7-7c). Recall
(b)
from Chapter 6 that calcium
Lithification:
and compaction
Burial
(c)
Cementation
Sediment to Sedimentary Rock
163
Guest Essay SUSAN M. LANDON TTTTfTTTTTTTTTTTTTTTTTTrnrTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT
EXPLORING FOR OIL AND NATURAL GAS am
an independent petroleum geologist. I specialize applying geological principles to frontier areas— places where little or no exploration has occurred and few or no hydrocarbons have been I
in
discovered.
It is
very
much
like solving a mystery.
The
earth provides a variety of clues— rock type, organic content, stratigraphic relationships, structure, and the
like— that geologists must piece together to determine the potential for the presence of hydrocarbons.
An example of an exploration frontier is the Precambrian Midcontinent Rift located in the north central portion of the United States. Some rifts, like the Gulf of Suez and the North Sea, are characterized by significant
hydrocarbon
unexplored
rift
reserves,
and the presence of an
basin in the center of North America
is
Rocks deposited in this rift basin are exposed along the shores of Lake Superior where they serve as the host for copper ores. One of the mines in the Upper Peninsula of Michigan, the White Pine Mine, has intriguing.
historically
been plagued by
in the shale.
For
many
oil
bleeding out of fractures
years, this
had been documented
as academically interesting because the rocks are
much
older than those that typically have been associated with
hydrocarbon production. Oil and natural gas are generated from organic material preserved in sediment that is subjected to increased temperature through time.
provided the prospect.
We
final
data necessary to generate a specific
then had to convince management that
this prospect had high enough potential to contain hydrocarbon reserves to offset the significant risks and costs. An economic evaluation was conducted to determine the worth of the project given a probability
of success. In this case,
was
management agreed
offset
authorized.
Amoco
was dry (economically
well
drilling sites in the
My
Midcontinent
Rift.
began very early as a result of collecting rocks and growing up in an oil field in the Midwest. I completed my undergraduate work at a small liberal arts college and earned a master's interest in geology
degree from a larger state university.
well-rounded education provided
me
have contributed to
My career Amoco, and, the company
began after
to
my
petroleum industry with
15 years,
work
I
made them
the organic content.
evaluating a Cretaceous chalk in the
history of the basin
was modeled
oil.
drill
area.
I
is
the decision to leave
independently.
prospects in
adequate organic material to be the source of the
believe that a
with a sound
successful career.
in the
My goal
The thermal
I
geological background and communication skills that
and
that the
to
the well will be used to continue to define prospective
projects.
Mine contained
Iowa
unsuccessful), but the
organisms (algae, fungi, and bacteria) to contribute to
and laboratory work documented
well in
geologic information obtained as a result of drilling
variety of companies, assisting
Field
m
drilled a .5,441
prospect at a cost of nearly $5 million. The
test the
However, the sediments associated with the onebillion-year-old rift had a very limited source of
copper-bearing shale at the White Pine
that the
by the potential for a very large accumulation of hydrocarbons, and a well was
risk
I
to have the opportunity to develop
new
frontier areas.
me
I
am
currently
Rocky Mountain
also teach courses for industry.
provided
consult for a
in exploration
My
career has
with the opportunity to travel to a wide
variety of places.
a
to
determine the timing of hydrocarbon generation.
If
hydrocarbons had been generated prior to deposition of
an effective seal and formation of a trap, the hydrocarbons would have leaked naturally out into the
Jusan M. Landon began
atmosphere.
Further
work
identified sandstones with
enough
porosity to serve as reservoirs for hydrocarbons.
Analogy with other hydrocarbon productive rifts gave the exploration team models for trap types. Seismic data were acquired and interpreted to identify specific traps. Coordination with geophysicists and engineers
career in
her
1974 with Amoco
Company and, in opened her own consulting
Production 1989,
office in
Denver, Colorado. In
1990, she was elected president of the American Institute of Professional Geologists.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA 164
Chapter 7
Sediment and Sedimentary Rocks
.
^ FIGURE
7-8
sedimentary rocks
These in the Valley
of
the Gods, Utah are red because they
contain iron oxide cement. (Photo courtesy of Sue Monroe.)
detected because acid.
canyons of Utah and Arizona are colored by small amounts of iron oxide or hydroxide cement (Fig. 7-8).
effervesces with dilute hydrochloric
it
Rocks cemented by
silica are the
hardest,
most
durable sedimentary rocks.
Calcium carbonate and cements
droxides,
such
[FeO(OH)],
some
silica are
the
most common
=*=
sedimentary rocks, but iron oxides and hy-
in
as
hematite
(Fe 2
and
3)
form a chemical cement in of the iron oxide cement is derived
respectively, also
Much
rocks.
from the oxidation of iron
in
ferromagnesian minerals
present in the original deposit, although
some
is
SEDIMENTARY ROCKS
Even though about 95% of the Earth's crust is composed of igneous and metamorphic rocks, sedimentary rocks are the most common at or near the surface. About 75% of the surface exposures on continents consist of sediments or sedimentary rocks, and they cover
limonite
carried
by circulating groundwater. The yellow, brown, and red sedimentary rocks exposed in the walls of the vast
most of the sea
in
floor.
classified as detrital or
Sedimentary rocks are generally chemical (Tables 7-2 and 7-3). "N>
rc.
^ TABLE
7-2
Classification of Detrital Sedimentary
Rocks -cfcp-
Sediment
and
Size
Gravel (>2
Sand
Mud
7
Name mm)
('/i6-2
(M'<JU.,;-K
\/\A
£,
bed f.
If geologists are to reconstruct Earth history; they must demonstrate the time equivalency of rock units in different areas. This process is known as c orrelat ion
J^CU^vp \OlOL;
Qi
\ ,o)-Cb
J« //ii/^L -A Lfc-OJ*
t
rsTX
i-CLA>n
j.
'JO
i?
C
T~
Correlation
227
/£&
with the lowermost equivalent rocks of another area,
of time during the geologic past. Fossils that are easily
the history of the entire region can be deciphered.
identified, are geographically
Although geologists can match up rocks on the basis of similar rock type and stratigraphic position, correlation of this type can only be done in a limited area where beds can be traced from one site to ano ther. In order to
a rather short geologic time are particularly useful. Such
correlate rock units over a large area or to correlate
guide
fossils are called
fossil
ratnus meet
9-14
Correlation of rock units,
of these criteria and are therefore
all
fossils. In contrast,
identified
the brachiopod Lingula
and widespread, but
Because most
fossils
it
have
of
its
good easily
geologic range of Or-
little
fairly
is
use in correlation.
long geologic ranges,
geologists constructl assemblage range zones to determine
the remains of organisms that lived for a certain length
FIGURE
(
dovician to Recent makes
succession must be used.
Fossils are us eful as time in dic ators because they are
'*'
guide fossils or index fossils Fig. 9-16).
For example, the trilobite Isotelus and the clam Inoce-
age-equivalent units ^>f differenPcornpbsition, fossils
and the principle of
widespread, and existed for
In areas of adequate exposures, rock
(a)
(£>) Correlation by similarities rock type and position in a sequence. The sandstone in section 1 is assumed to intertongue or grade laterally into the shale at section 2. (c) Correlation using a key bed, a distinctive black limestone.
units can be traced laterally even
occasional gaps exist.
if
in
I
I
|.!i
I
|
I
I
i|i|i|
I
I
.
|i
I
I
i|. !
!
I
I
.|
.
I
I
I
iiiiii
|
I
I
i!
.
'
1
'I
I
!i|i|i
i| |
.
I
I
m
'''
I
|i|i|i
i
!
' '
'
PC
ffig
Correlation
229
Precambrian Eon
Fm =
Formation
230
Ss = Sandstone
Chapter 9
Ls = Limestone
Geologic Time
•
Rocks
ol
Ordovician and Silurian age are not present
in
the
Grand Canyon
FIGURE
"**"
9-15
Correlation of rocks within the
(left)
Colorado Plateau. By correlating the rocks from various locations, the history of the entire region can be deciphered.
Atoms, Elements, and Isotopes As we discussed
in
Chapter
3, all
matter
th e age of_thc_sedimentary roclcs-contatBiBfr^he^fossils.
Assemblage range zones are established by plotting the overlapping geologic ranges of different species of
The
first
establish
and
last
fossils.
occurrences of two species are used to
an assemblage zone's boundaries
(Fig. 9-17).
Correlation of assemblage zones generally yields correlation lines that are considered time equivalent. In
is
made up
of
composed of extremely small particles called atoms. The nucleus of an atom is composed of protons and neutrons with electrons encircling it (Fig. 3-3). The number of protons defines an element's atomic number and helps determine its properties and characteristics. The combined number of protons and neutrons in an atom is its atomic mass number. However, not all atoms of the same element have the same number of neutrons in their nuclei. These variable forms of the same element are called isotopes. chemical elements, each of which
is
other words, the strata encompassed by the correlation
thought to be the same age. Geologists are aware, however, that such zones are not exactly the lines are
same age everywhere, because no fossil organism appeared and disappeared simultaneously over its entire geographic range. Even so, first and last appearances do not differ greatly from origins and extinctions in geologic time; thus, correlation of assemblage zones can still
^ FIGURE
The geologic ranges of three marine The brachiopod Lingula is of little use in correlation because of its long geologic range. The trilobite hotelus and the bivalve Inoceramus are good guide fossils 9-16
invertebrates.
because they are geographically widespread, are easily identified, and have short geologic ranges.
be very precise. For example, during the 1840s and
1850s, Albert Oppel was able to subdivide the Jurassic
based on the overlapping ranges of ammonites found in Europe. Most of these
strata into zones fossils called
zones are
less
than a million years in duration
Tertiary
(later
by correlation with radiometrically dated beds) and can be used to correlate Jurassic rocks accurately throughout the world.
verified
Cretaceous
Inoceramus
^ ABSOLUTE DATING METHODS Thus
far,
our discussion has largely concerned the con-
cept of geologic time and the formulation of principles
used to determine relative ages.
It is
somewhat
ironic
that radioactivity, the very process that invalidated Kelvin's calculations,
now
Permian
Lord
serves as the basis for deter-
Pennsylvaman
mining absolute dates. Mississippian
Although most of the isotopes of the 91 naturally occurring elements are stable, some are radioactive and
spontaneously decay to other more stable isotopes of elements, releasing energy in the process. The discovery, in
1903 by
Pierre
and Marie Curie, that radioactive de-
cay produces heat as a by-product meant that geologists finally had a mechanism for explaining the internal heat
Ordovician
of the Earth that did not rely on residual cooling from a
molten origin. Furthermore, geologists and paleontolohad a powerful tool to date geologic events accurately, and thus verify the long time periods postulated by Hutton, Lyell, and Darwin.
gists
Cambrian
Absolute Dating Methods
231
Perspective 9-1
SUBSURFACE CORRELATION AND THE SEARCH FOR OIL AND NATURAL GAS During the early years of the petroleum industry, geologists relied almost exclusively in their search for oil
and
gas.
techniques, they constructed
on surface
Among
studies
other
maps showing rocks and
geologic structures such as folds and faults. Interpretation of such
maps sometimes
interpretation of data regarding geologic features
revealed
subsurface structures, such as those in Figure 7-33,
which oil and natural gas might be trapped. Surface methods are still important in petroleum geology, particularly in unexplored regions, but most exploration is now done using subsurface methods. Subsurface geology is the acquisition and
in
beneath the Earth's surface. Drilling operations have
provided a wealth of data on subsurface geology.
When """
FIGURE
Core and (b) rock chips are the two types of samples recovered from drill holes. (Photos courtesy of Sue Monroe.) 1
(a)
drilling for oil or natural gas, cores or
rock
chips called well cuttings are usually recovered from 1). These samples are studied under and reveal such important information as rock type, porosity (the amount of pore space) and permeability (the ability to transmit fluids), and the
the drill hole (Fig. the microscope
presence of
oil stains.
In addition, the samples can
also be processed for microfossils that can aid in
determining the geologic age of the sediments
(Fig. 2).
Cores are very useful for correlating rock units from well to well and locating oil- or gas-producing zones. Geophysical instruments may be lowered down a drill hole to record such rock properties as electrical resistivity, density,
and
radioactivity, thus providing a
well log of the rocks penetrated (Fig. 3). (text
"*"""
FIGURE
2
continued on page 234)
Microscopic one-celled animals called
foraminifera can be used to determine the age of the rock they are found in and can be used to correlate rock units between wells. (Scanning electron micrograph by Dee Breger, Lamont-Doherty Geological Observatory.)
232
Chapter 9
Geologic Time
Magnetic recording
Down
hole
logging tool
(a)
"•^ FIGURE 3 {a) A schematic diagram showing how well logs are made. A logging tool is down the drill hole. As the tool is withdrawn, data are transmitted to the surface where they are recorded and printed out as a well log. (b) Electrical logs and correlations of rocks in two wells in Colorado. The curves labeled SP are plots of self-potential (electrical potential caused by different conductors in a solution that conducts electricity) with depth, and the curves labeled R are plots of electrical resistivity with depth. lowered
Absolute Dating Methods
233
Energy source
Satellite
navigation
system
Hydrophones
/^^
^ FIGURE
4 {a) A diagram showing the use of seismic reflections to detect buried rock units at sea. Sound waves are generated at the energy source. Some of the energy of these waves is reflected from various horizons back to the surface where it is detected by hydrophones. Buried rock units can also be detected on land, but here explosive charges are detonated as an energy source, (b) Seismic record and depositional sequences defined in the Beaufort Sea. Boundaries of seismic sequences are shown by solid black lines. The scale on the right shows seismic wave travel time. Notice the sloping lines indicating faults in the right part of the seismic record.
have made it possible to work out problems that could not otherwise have been solved. Such logs have saved oil companies tremendous amounts of money in coring expenses and, by enabling the companies to determine the subsurface fluid content, have helped them discover additional oil that might otherwise have been missed. Electrical logs have also been used for very accurate Electrical logs
structural
correlation, particularly over short distances (Fig. 3).
Subsurface rock units
may
also be detected
and
traced by the study of seismic profiles. Energy pulses,
such as those from explosions, travel through rocks at a velocity determined by rock density, and this
Most
energy
is
reflected
some
of
from various horizons (contacts
isotopes are stabl e, but
s
ome
are unstable
spontane ously cteca~y~to~a more~itirjIe~rbrm.
It
is
and the
between contrasting it is
recorded
continental shelves where
is
to
map
234
Chapter 9
Geologic Time
it is
very expensive to
drill
the structure to see
most well
if it
has the
and gas. Another important use is in predicting where an oil- or gas-producing horizon might occur outside the limits of a known oil field. The choice of subsurface correlation methods depends on the information geologists are seeking, the general geology of the area, and the cost and time
potential for trapping oil
available to run different logs.
atomic nucleus of a different element. radioact ive decay are recognized, the nucleus emits
is the process whereby an unstable atomic nucleuses spontaneously transformed into an
where
is
In petroleum exploration, the purpose of correlations
c hange
Radioactive decay
to the surface,
holes and other techniques have limited use.
j\ o
Radioactive Decay and Half-Lives
back
Seismic stratigraphy
particularly useful in tracing units in areas such as the
^dec ay
rate of u nstable jsotopes tKatgeologi sts meas ure determ ine the absoluteage~oFrocIci^
layers)
(Fig. 4).
all
of
Three
types, of
w hichj-esult in a
o f atomic structure (Fig. 9-18). Injdpjia_decay, two protons and two neutrons with the result that the atomic number decreases by two and the atomic mass number decreases by four. B eta decay is the emission of a fast-moving electron from a neutron in the nucleus; the neutron
is
changed to
consequently the atomic number
is
a proton,
and
increased by one,
"" FIGURE
9-17
Correlation of two
sections by using assemblage range
zones. These zones are established by the overlapping ranges of fossils
A
through E.
with no resultant atomic mass number change. Electron capture results
an electron
when
shell
and
a proton captures an electron is
as a result, the atomic
from
thereby converted to a neutron;
number decreases by one, but
the
atomic mass number does not change. Some elements undergo only one decay step in the conversion from an unstable form to a stable form. For example, rubidium 87 decays to strontium 87 by a sin-
and potassium 40 decays to argon 40 by a single electron capture. Other radioactive elements undergo several decay steps (see Perspective 9-2). Uranium 235 decays to lead 207 by seven alpha and six beta steps, while uranium 238 decays to lead 206 by eight gle beta emission,
alpha and six beta steps
When to
them
discussing decay .rates,
act ive element a"
is
the time
it
it is
convenient to refer
The half-life of
given radioactive element
from
less
is
By measuring the parent-daughter
ratio
and knowing
geologists can calculate the age of a sample containing
The parent-daughter
usually determined by a
mass spectrometer, an
constant
and can be
in the laboratory. Half-lives
active elements range
instruments.
the radioactive element.
t
toms of the original unstable parent element to deca y atoms of a new, more stable daughter elemen t. The
measured
,
he
takes for one-half of
gardless of external conditions
.
a rad io-
to
halt-lite of a
hav e 500,000 parent atom s and 500,000 daugh ter atoms after one half-life After two half-lives, it will have 250,000 parent atoms (one-half of the previous parent atoms "which is equivalent to one-fou rth ot the original parent a toms) and 750,000 daughter atoms. After three half-lives, it will have 125,000 parent atoms (one-half of the previous parent atoms or one-eighth of the original parent atoms) and 875,000 daughter atoms, and so on until the number of parent atoms remaining is so few that they cannot be accurately measured by present-day
the half-life of the parent (determined in the laboratory),
(Fig. 9-19).
in term^oLhalf-Jiyes)
For example, an element with 1.000,000 parent atoms will
ment
that
meas uresjhe proportions
ratio
is
instru-
of_eleme_nts_of dif-
ferent masses.
re-
precisely
of various radio-
than^a-bjllionth of a
second to 49 billion yea rsRadioac tive decay occurs at a geometric rate rath er t han a li negxiatejherefore, a graph of the decay rate produces a curve rather than a straight line (Fig. 9-20).
Sources of Uncertainty
The most accurate radiometric dates are obtained from i gneous rock s. As a magma cools and begins to crystallize, radioactive
parent atoms are separated from previ-
ously formed daughter atoms. Because they are the right size,
some radioactive parent atoms
are incorporated
Absolute Dating Methods
235
Changes in atomic number and atomic mass number
Alpha particle
Atomic number = -2 Atomic mass number = -4
Alpha decay
Beta particle
Atomic number = +1 Atomic mass number = Beta decay
-»-
FIGURE
9-18
radioactive decay,
Three types of Alpha decay,
(a)
Atomic number = -1 Atomic mass number =
which an unstable parent nucleus emits two protons and rwo neutrons, (b) Beta decay, in which an electron is emitted from the in
nucleus,
(c)
Electron capture
Electron capture, in
which a proton captures an electron and is thereby converted to a
Q
Protron
neutron.
into the crystal structure of certain minerals.
daughter atoms, however, are a different
The
size
stable
than the
radioactive parent atoms and consequently cannot into the crystal structure of the
parent atoms. Therefore crystallize, the
when
same mineral the
magma
fit
as the
begins to
mineral will contain radioactive parent
atoms but no stable daughter atoms (Fig. 9-21). Thus, the time that is being measured is the time of crystallization of the mineral containing the radioactive atoms,
not the time of formation of the radioactive atoms.
Exay3t_jnj musual circumstan ces, sedimentary rocks ca nnot be radiometrically dated, be cause one
would be
measuring the age of a particular mineral rather than the time that it was deposited as a sedimentary particle. One of the few instances in which radiometric dates can be obtained on sedimentary rocks is when the mineral glauconite
236
is
present. Glauconite
Chapter 9
is
a greenish mineral cbn-
Geologic Time
#
Neutron
Electron
taining radioactive potassium 40, which decays to argon
40 (Table
marine environments du ring the convers ion from sediments to sedimentary rock. Thus, it forms when the sedimentary rock forms, and a radiometric date indicates the time of the sedimentary rock's origin. However, because the daughter product argon is a gas, it can easily escape from a mineral. Therefore, any date obtained from glauconite, or any other mineral containing the potassium 40— argon 40
~a" s~a
pair,
9-1).
It
forms
in certain
result of chemical reactions with clay minerals
must be c onsidered
a
minimum
To obtain accurate radiometric
ag e.
dates, geologists
must
be sure that they are dealing with a closed system, mean-
atoms have been added or removed from the s ystem since crystallization and that the ratio between them results only from raing that neither parent nor daughter
dioactive decay. Otherwise, an inaccurate date will re-
Magma
^ FIGURE
9-21
(a)
A magma
contains both radioactive and stable atoms, (b) As the magma cools and begins to crystallize,
some
radioactive atoms are incorporated into certain minerals because they
are the right size
and can
fit
into the
crystal structure. Therefore, at the
time of crystallization, the mineral will contain 100% radioactive
parent atoms and 0% stable daughter atoms, (c) After one half-life, 50% of the radioactive parent atoms will have decayed to stable daughter atoms.
daughter ratio of two different radioactive elements
in
same mineral. For example, naturally occurring uranium consists of both uranium 235 and uranium 238 isotopes. Through various decay steps, uranium 235 decays to lead 207, whereas uranium 238 decays to lead 206 (Fig. 9-19). If the minerals containing both uranium the
isotopes have remained closed systems, the ages ob-
tained from each parent-daughter ratio should be in close agreement
and therefore should indicate the time magma. If the ages do not closely agree, other samples must be taken and ratios measured to see which, if either, date is correct. of crystallization of the
Long-Lived Radioactive Isotope Pairs Table 9-1 shows the
five
common,
long-lived parent-
daughter isotope pairs used in radiometric dating. Longlived pairs have half-lives of millions or billions of years. All of these still
were present when the Earth formed and are
present in measurable quantities. Other shorter-lived
radioactive isotope pairs have decayed to the point that
only small quantities near the limit of detection remain.
The most commonly used isotope pairs are the and thorflimjeji^jienes., > which^ are_used prmcTpairyto date ancient igneous intrusives, lunar sam ples, and some meteorites The r ubidium-strontium pa ir tranium-lead
.
is'also
used ~t or very old samples and has been effective
d ating _thiie_Qidest rocks on E artrTas well as meteorites he ggtassium- argor^method is typically used for dating
in 1
.
finegrained v olcanic roc ks from which individual crys-
cannot be separated; hence the whole rock is anaHowever, argon is a gas, so great care must be taken to assure that the sample has not been subjected to heat, which would allow argon to escape; such a sample tals
lyzed.
would
yield
an age that
is
too young. Other long-lived
radioactive isotope pairs exist, but they are rather rare
and
"^ FIGURE
9-22 The effect of metamorphism in driving out daughter atoms from a mineral that crystallized 700 million years ago (M.Y.A.). The mineral is shown immediately after crystallization (a), then at 400 million years (b), when some of the parent atoms had decayed to daughter atoms. Metamorphism at 350 M.Y.A. (c) drives the daughter atoms out of the mineral into the surrounding rock, (d) Assuming the rock has remained a closed chemical system throughout its history, dating the mineral today yields the time of metamorphism, while dating the rock provides the time of its crystallization, 700 M.Y.A.
are used only in special situations.
Radiocarbon Dating Methods
£"/
?^
is an important el ement in nature and is one o fthe ^ba sic elements found in all forms of l ife. It has three isotopes; two of these, carbo n 12 and 13, are stable, where as ?n 14 is radioactive. Carbon 14 has a halt-life of pears plus or minus 30 years. The carbon 14 dating^, ^techniq ue is based on the ratio of carbon 14 to carbon 12 and is generally used to date once-livin g material. The short half-life of carbon 14 makes this dating^ technique pj-gctical only for specimens you nger than abourJZQJDOO years. Consequently, the carbon 14 dating method is especially useful in archaeology and has
,V Carbon
greatly aide d in unraveling the events of the latter por-
p
tion of rh flfl^istocene EpocT
Carbon 14 sphere by the
is
constantly formed in the upper atmo-
bombardment
of cosmic rays, which are
high-energy particles (mostly protons). These high-energy particles strike the atoms of upper-atmospheric gases, splitting their nuclei into protons
When
and neutrons. atom
a neutron strikes the nucleus of a nitrogen
(atomic number 7, atomic mass number 14), it may be absorbed into the nucleus and a proton emitted. Thus, the atomic number of the atom decreases by one,
Absolute Dating Methods
239
Perspective 9-2
RADON: THE SILENT KILLER What
is
radon, what makes
how
so dangerous, and
it
worried should you be about it in your home, school, or business? According to the U.S. National Research Council, approximately 20,000 people die prematurely
home, however, radon can accumulate levels (>4 pCi/L). Continued exposure
to unhealthy
to these
elevated levels over several years can greatly increase the risk of lung cancer.
As one of the natural decay products of uranium
each year from cancers induced by exposure to indoor radon. In fact, radon is the second leading cause of
238, radon
lung cancer in the United States.
elements called radon daughters
Your chances of being adversely affected by radon depend on numerous interrelated factors such as your
time you breathe, these daughter elements become
geographic location, the geology of the area, the
releasing high-energy alpha
climate,
much
how
the building
time you spend
as yet,
no
constructed, and
is
in the building.
how
While there
(Fig.
are,
federal standards defining unacceptable
Environmental Protection Agency (EPA) recommends radon levels not exceed indoor radon
levels, the
four picocuries per
liter
(pCi/L) of air (a curie
is
standard measure of radiation, and a picocurie
the
is
one-trillionth of a curie).
Radon
is
part of the uranium
238—lead 206
series (Fig. 9-19). It
occurs
in
and
the atmosphere
where
it is
harmless levels (0.2 pCi/L
any rock or
level of radon). In
1
Some
of the
diluted is
soil that
Chapter 9
and
dissipates to
the average ambient
an enclosed area such as a
common
radon can enter a house.
240
(Fig. 9-19).
Every
your lungs and eventually break down,
9-18) that
and beta decay particles tissue and can cause lung
damage lung
cancer.
Concern about the health arose during the 1960s
when
risks
the
posed by radon
first
news media revealed
some homes in the West had been built with uranium mine tailings. Since then, geologists have found that high indoor radon levels can be caused by natural uranium in minerals of the rock and soil on
that
is
radioactive decay
contains uranium 238. Outdoors, radon escapes into
"^ FIGURE
in
decays into other radioactive
a colorless, odorless, naturally occurring
radioactive gas that has a three-day half-life
outdoor
trapped
itself
Geologic Time
entry points where
"^"
FIGURE
2
Two
of the most popular commercially
available radon-testing devices are (a) the charcoal canister
and (b) alpha track detectors. Both are left open and exposed to the air and then sent to a laboratory for analysis.
FIGURE 3 Areas in the United States where granite, phosphate-bearing rocks, carbonaceous shales, and uranium occur. These rocks are all potential sources of radon gas.
"'•'
left open and your house and then sent to a
which buildings are constructed. In response to the high cost of energy during the 1970s and 1980s, old buildings were insulated, and new buildings were constructed to be as energy efficient and airtight as
track detectors (Fig. 2). Both devices are
possible. Ironically, these energy-saving measures also
levels of
sealed in radon.
Radon
enters buildings through dirt floors, cracks
in the floor
or walls, joints between floors and walls,
sumps, and utility pipes as well as any cracks or pores in hollow-block walls (Fig. 1). Radon can also be released into a building whenever the water is turned on if the water comes from a private floor drains,
well.
Municipal water
is
generally safe because
it
inexpensive, simple
home
it
gets to
testing devices.
The two
most popular are the charcoal canister and alpha
air in
laboratory for analysis.
radon readings are above the recommended EPA 4 pCi/L, several remedial measures can be taken to reduce your risk. These include sealing up all cracks in the foundation, pouring a concrete slab over If
a dirt floor, increasing the circulation of air
basement and
throughout the house, especially
in the
crawl space, providing
drains and other
utility
filters for
openings, and limiting the time spent in areas
with higher concentrations of radon.
has
your home. To find out if your home has a radon problem, you must test for it with commercially available, relatively usually been aerated before
exposed to the
It is
important to remember that although the radon
hazard covers most of the country, some areas are
more
likely to
radon than others
have higher natural concentrations of (Fig. 3).
For example, such rocks as
uranium-bearing granites, metamorphic rocks of granitic (continued on next page)
Absolute Dating Methods
241
composition, and black shales (high carbon content) are quite likely to cause indoor radon problems. Other
rocks such as marine quartz sandstone, noncarbonaceous shales and siltstones, most volcanic rocks, and igneous and metamorphic rocks rich in iron and magnesium typically do not cause radon
problems. The permeability of the
soil
overlying the
rock can also affect the indoor levels of radon gas.
Some
soils are more permeable than others and allow more radon to escape into the overlying structures. The climate and type of construction affect not only how much radon gets into a structure, but how much
escapes. Concentrations of radon are highest during the
winter
northern climates because houses are sealed as
in
tightly as possible. likely to
Homes
with basements are more
have higher radon
levels
than those built on
homes in Gunderson of the U.S. Geological Survey found that homes with a basement had average radon levels two to three times higher than homes built on a concrete slab. Furthermore, homes that had cracks in their basement walls or that were constructed with hollow-block walls (such blocks are very gas permeable) had higher radon readings than those with solid, poured concrete walls. While research continues into the sources of indoor radon and ways of controlling it, the most important thing people can do is to test their home, school, or business for radon. In this way more data will be available for analysis, some preventive measures can be taken, and a solution to this major problem will be concrete slabs. In a recent study of 3,000 Atlanta, Georgia, Linda
found sooner.
while the atomic mass number stays the same. Because
Tree- ring datingjs^a usefujjriethod forjd atingjecent
number has changed, a new element, carbon 14 (atomic number 6, atomic mass number 14), is formed. The newly formed carbon 14 is rapidly assim-
even ts. The age of a tree can be determined by counting
the atomic
carbon cycle and, along with carbon 12 and 13, is absorbed in a nearly constant ratio by all living organisms (Fig. 9-23). When an organism dies, however, carbon 14 is not replenished, and the ratio of carbon 14 to carbon 12 decreases as carbon 14 decays back to nitrogen by a single beta decay step (Fig. 9-23). The ratio of carbon 14 to carbon 12 is remarkably constant in both the atmosphere and living organisms, and geologists assume that it has also been constant for the past 100,000 years. Comparing ages established by carbon 14 dating of wood samples with ages obtained by counting annual tree rings in the same samples yields slight differences (Fig. 9-24). It appears that the production of carbon 14 and hence the ratio of carbon 14 to carbon 12 has varied slightly over the past several thousand years, in part, because the amount of C0 2 has ilated into the
As a result, corrections in carbon 14 ages have been made to account for such variations in the past.
varied.
Tree-Ring and Fission Track Dating Methods In addition to radiometric dating, various other
ods can yield accurate absolute dates.
common
242
include tree-ring
Chapter 9
and
Two
meth-
of the most
fission track dating.
Geologic Time
the
growth rings
in the
lower part of the trunk. Each
and the pattern of wide and narrow rings can be compared among trees to establish the exact year in which the rings were formed. The procedure of matching ring patterns from numerous trees and wood fragments in a given area is referred to as cross-dating. By correlating distinctive tree-ring sequences from living to nearby dead trees, a time scale has been constructed extending back to about 14,000 years ago (Fig. 9-25). By matching ring patterns to the composite ring scale, wood samples whose ages are not ring represents one year's growth,
known can The ited
be accurately dated.
applicability of tree-ring dating
because
it
is
somewhat
lim-
can only be used where continuous tree
records are found.
It
is
therefore most useful in arid
regions, particularly the southwestern United States. Fissi on
track dating
is
a useful techn ique that can be
applied in dating samples ranging in age from only a tew
hundred to hundreds of millions of years. It is most usetul tor dating samples between about 40,000 and one million years ago, a period for which other dating techniques are not particularly effective.
When
a
uranium isotope
in a
mineral emits an alpha
decay particle, the heavy, rapidly moving alpha particle
damages the
crystal structure.
The damage appears
as
small linear tracks that are visible only under a high-
"^ FIGURE
9-23
The carbon
(right)
cycle
showing the
formation, dispersal, and decay of carbon 14.
powered microscope and only after etching the mineral with hydrofluoric acid. The age of the sample is determined by the number of fission tracks present and the
amount of uranium
number of
One of the problems in when the rocks have been tures. If this
The
the sample contains.
sample, the greater the
older the
Cosmic
tracks (Fig. 9-26).
radiation
fission track dating occurs
subjected to high tempera-
happens, the damaged crystal structures are
Neutron capture
Nitrogen 14
Carbon 14
\
"repaired" by annealing, and consequently, the tracks disappear. In such instances, the calculated age will be
younger than the actual age.
y
C 14 C
^ THE DEVELOPMENT OF scale
is
in
time units of varying duration
absorbed C 12 and
is
(Fig. 9-2).
a
into the tissue
organisms
fairly
constant
ratio.
a hierarchical scale in
the 4.6-billion-year history of the Earth
13
of living
THE GEOLOGIC TIME SCALE The geologic time
is
along with
which
divided into
The geologic
time scale was not developed by any one individual, but rather evolved, primarily during the nineteenth century,
through the efforts of
many
people. By applying relative
dating methods to rock outcrops, geologists in England
and western Europe defined the major geologic time units without the benefit of radiometric dating tech-
niques
and
(Fig. 9-27).
Using the principles of superposition
fossil succession,
When an organism dies, C 14 back to N 14 by beta decay.
they were able to correlate the
converts
various exposures and piece together a composite geoBeta decay
Nitrogen 14
Beta*v particle
"^ FIGURE 9-24 (below) Discrepancies exist between carbon 14 dates and those obtained by counting annual tree rings. Back to about 600 B.C., carbon 14 dates are too old, and those from about 600 b.c to about 5,000 b.c are too young. Consequently, corrections must be made to the carbon 14 dates for this time period.
2.000
1
,000
1
,000
2,000
•
3,000
Proton
4,000
Tree-ring dates
The Development of
the Geologic
Time
Scale
243
H Even as a fossils.
I
Guest Essay MICHAEL L. McKINNEY TTTfTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTI
PALEONTOLOGY
LIFE THROUGH TIME
child,
I
being interested in rocks and
recall
know now
same reasons
the
that
that
I
I
them for enjoy teaching and doing
was
still
attracted to
research in historical geology. For one thing, rocks
and
fossils are a
my
constant reminder that time did not
knowledge leads to a more relaxed view of what I— and the human species for that matter— am doing here. One's self-importance is continually diminished when you work with fossils begin with
existence. This
that are millions of years old.
A
bigger part of
my
motivation, however, comes
from the "detective" work involved
in historical geology.
Like a police detective, the historical geologist trys to reconstruct past events from fragmentary evidence.
Whether an
oil
as a sedimentologist trying to determine
when
basin formed, or a paleontologist trying to find
the ancestors of
modern mammals,
use whatever limited information
be frustrating, but as with
when
many
the challenge
is
is
available. This
puzzles, the
to
can
moment
"come together" is very satisfying. Furthermore, new evidence is always being found so new puzzles always arise and old answers often prove inadequate. Most satisfying of all is the knowledge that the work is more than idle amusementr you are contributing to our understanding of how the Earth and its life came to be what they are today. ideas
Besides being fun, the study of fossils
sedimentary rocks has
many
and
is
Our
built
materials formed. For example,
work
for oil companies,
cores brought
up by
many
on
for a Ph.D. After receiving
paleontologists
examining microfossils
in
rock
this
choice because
projects of
it
my own
number of graduate
allows
me
choosing. students
laboratory, doing research in
United States.
have never worked
offered jobs by
my
two
oil
in industry
submitting grant applications
Some
companies when
master's degree in geology. Instead,
I
I
I
was
completed
chose to go
iiAiiAHiiilititliliilti iilii t iiAil i
244
Chapter 9
Geologic Time
teach I
made
to carry out research
I am helped by a who work in my their own particular
aimed
of
my own
at finding
if
their research
favorite research
is
many
information on the
relevance today,
an alarming
when
over
99%
of
have ever existed have died out, the
amount
contains a vast
we have
species of animals (such as
more
generally
become
likely to
We
extinctions
much
all
at
species that
fossil
record
of useful data about
extinctions. For instance,
some
costly.
becoming extinct
species are
rate. Since
is
currently
seen in the fossil record. This research has
already learned that
mammals)
are
extinct than others
have also discovered that habitat
destruction has been the
main cause of extinction
throughout geologic time, just as it is today. The only difference is that today humans destroy the habitats,
whereas
in the past
changes
impacts, and other natural
in climate, meteorite
phenomena caused
A
destruction.
IVLichael
L.
McKinney
is
an
associate professor in the
Geology and Ecology Programs at the University of Tennessee,
He
has published
books and many technical articles on evolution, paleontology, and environmental three
although
I
one is making highly sophisticated measurements of fossil shapes by using a television camera connected to a computer. Much of this work is supported by grants from agencies such as the National Science Foundation. Funding from these agencies is very competitive, and the grants usually last only a couple of years. Therefore, scientists must often spend a significant amount of time writing and
Knoxville. I
joined the
I
where
areas. For instance,
specializing in sedimentology
fields in the
degree,
undergraduate and graduate courses. I'm glad
drilling rigs. Historical geologists
and stratigraphy are also employed in the search for oil and minerals; they examine the physical characteristics of the rock cores and correlate rock layers. Environmental firms are currently the major employers of geologists, and environmental careers are among the fastest growing
my
faculty at the University of Tennessee
(such as clams).
practical applications.
on ores and energy (such as fossil fuels) that come from the Earth. By studying the history of the Earth, we learn how and, more importantly, where these society
TRACING
:
topics.
the
C. This
beam came
from an old house
VV^^^i
11
ll
1
^ This date obtained by counting back from bark of
A
through B
Specimens taken from
ruins,
when matched and overlapped as indicated, progressively extend
the dating back into prehistoric times.
"^ FIGURE are
9-25
In the cross-dating
matched against each other
method, tree-ring patterns from different woods
to establish a ring-width chronology
logic section. This composite section
is,
in effect, a rel-
ative time scale because the rocks are arranged in their
correct sequential order.
Geologists also recognized that the different fossil as-
semblages, representing distinct time periods in the past, could be used to correlate rock units elsewhere even if the rock types were different.
The names of
these time
backward
in time.
•^ FIGURE
9-26
Each
fission track
length) in this apatite crystal
is
(about 16
p.
in
the result of the radioactive
decay of a uranium atom. In order to make the fission tracks visible, the apatite crystal has been etched with hydrofluoric acid. This apatite crystal comes from one of the dikes of Shiprock, New Mexico, and indicates a calculated age of 27 million years. (Photo courtesy of Charles W. Naeser, U.S. Geological Survey.)
periods were thus based on the areas in which the rock units were originally described. For example, the Camis taken from the Roman word for Wales (Cambria), whereas the Ordovician and Silurian periods are named after the Silures and Ordovices, tribes that
brian Period
Wales during the Roman conquest (Fig. 9-27). By the beginning of the twentieth century, geologists had developed a relative geologic time scale, but did not yet have any absolute dates for the various time unit lived in
boundaries. Following the discovery of radioactivity near the end of the last century, radiometric dates were relative geologic time scale (Fig. 9-2). Because sedimentary rocks, with rare exceptions, cannot be radiometrically dated, geologists have had to
added to the
The Development of
the Geologic
Time
Scale
245
Carboniferous (Coneybeare and Phillips, 1822) ,'
Cambrian (Sedgwick, 1835)
Ordovician (Lapworth, 1879) Silurian
(Murchison, 1835)
^ FIGURE
9-27 The names of the time periods of the geologic time scale were based on areas in England and Europe where the rock units were originally described.
Note
that the
Carboniferous, which is recognized in Europe, is represented by two systems in North America, the Mississippian and Pennsylvanian.
FIGURE 9-28 Absolute ages of sedimentary rocks can be determined by dating associated igneous rocks. In {a) and (b), sedimentary rocks are bracketed by rock bodies for which absolute ages have been determined. "*•*
Nonconformity
150 M.Y.
—
(a)
> 600 to l ^m % i 1
^ CHAPTER SUMMARY 1.
5.
A bsolute
.
da tingr esults
g eologic history of the Eart h. 6.
evidence rather than
While some attempts were quite
ingenious, they yielded a variety of ages that are 3.
known
to be
much
7.
now
too young.
Uniformitarianism as articulated by Charles Lyell, soon becarnet he guiding principle of geology. It holds that~tRelaws of nature have been constant through time and that the same processes operating today have operated in the past, although not .
necessarily at the
same
rates.
is
by correlating all
observations were instrumental in establishing the basis for the principle of uniformitarianism. 4.
Correlation
the stratigraphic practice of
demonstrating equivalency of units in different areas. Time equivalence is most commonly demonstrated
James Hutton believed that present-day processes operating over long periods of time could explain the geologic features of his native Scotland. His
Surfaces of discontinuity that encompass significant
amounts of geologic time are common in the geologic record. Such surfaces are unconformities and result from times of nondeposition, erosion, or both.
During the eighteenth and nineteenth centuries, attempts were made to determine the age of the scientific
Inaddit ion to uniformitari anisnu_rhe prinriples-of
andTossil succe ssion_a_re basic fo r determining relative geologic ages and for interpreting the
in sp ecific
present.
revelation.
.
.
continuity, cros s-cutting relation ships, inclusions,
dates for events, expressed in years before the
Earth based on
.
.
superposition, original horizontality, lateral
Relative dat ing involves placing geologic events in a sequential order as determined from their position in
theT ock record
2.
.-«.
,
-
8.
Radioactivity
strata containing similar fossils.
was discovered during
the late
nineteenth century, and soon thereafter radiometric
dating technique s allowed geologists to determin e ah solute ages jor_g eologic events 9. Absolute age dates for rock samples are usually obtained by determining how many half-lives o f a radioactive parent elerrienTrrave~elapsed since t he sa mple originally crys tallised. A halt-life is the tim e .
it
takes for one-half of the radioactive jjargpt
element to decay to a stable daughter element.
Chapter Summary
247
10.
The most accurate radiometric
dates are obtained
date will be obtained. This date will be actual date.
from long-lived radioactive isotope pairs in igneous rocks. The most reliable dates are those obtained by using at least two different radioactive decay series in the
same rock.
wood
and shells and is effective back to about 70,000_years ago. Carbon 14 ages are determined by the ratio of radio active carbon 14 to stable carbon_12. 12. Through theefforts of many geologists applying the ,
4.
bones.,
scale
was
Most
obtained indirectly by dating associated metamorphic or igneous rocks. fossils are
6.
IMPORTANT TERMS
assemblage range zone beta decay
fission track dating
carbon 14 dating
guide
8.
9.
principle of superposition
radioactive decay relative dating
succession
tree-ring dating
unconformity
principle of inclusions
lateral continuity; b.
c.
original horizontality; d.
e.
cross-cutting relationships.
principle of lateral
which type of radioactive decay are two protons and two neutrons emitted from the nucleus? In
alpha;
beta;
b.
The author
of Principles of Geology and the
and
a
Hutton; b
d.
Smith;
The
era younger than the
Proterozoic; b
d.
Phanerozoic;
Which of
b.
e.
the following
angular unconformity; e. none of
when
d.
the dated mineral
a sedimentary rock;
e.
when
the
was formed.
a radioactive element has a half-life of 4 million
amount?
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3tSUwums3k) At distances greater than 200 km, the deeper, faster seismic waves arrive at seismic stations first, even though seismic station less than
they travel farther.
294
Chapter
1 1
The
Interior of the Earth
Direct
wave
Epicenter
East Pacific Rise
Peru-Chile
South
Mid-Atlantic
Trench
America
Ridge
Oceanic
Oceanic
crust
crust
•^ FIGURE
11-11
The Moho
is
present everywhere except beneath
spreading ridges such as the East Pacific Rise and the Mid-Atlantic Ridge. However, the depth of the
Moho
varies considerably.
ers travel
through the deeper layer and some of their refracted back to the surface (Fig. 11-10). Waves traveling through the deeper layer travel farther to a seismic station but they do so more rapidly than those in the shallower layer. The boundary identified by
averages 35 km, but ranges from 20 to 90
energy
the sea floor
Mohorovicic sepa ratejjh e crust from the mantle and is now called the Mohorovici c discontinuity, or simpl y the Nloho. IFTsTpr esent everywhere except beneath spread-
Although seismic wave velocity
is
ing ridges, but
its
depth varies: beneath the continents
it
it is
5 to 10
km
deep
km; beneath
(Fig. 11-11).
Structure and Composition of the Mantle in the
mantle generally
increases with depth, several discontinuities also exist. Be-
tween depths of 100 and 250 km, both P- and S-wave velocities decrease markedly (Fig. 11-12). This layer be-
"^ FIGURE 11-12 Variations in P-wave velocity in the upper mantle and transition zone.
7^
The Mantle
295
rween 100 and 250
km
deep
is
the low-velocity zone;
it
corresponds closely to the asthenosphere, a layer in which the rocks are close to their melting point and thus are less elastic; this
decrease in elasticity accounts for the observed
The asthenosphere is an important zone because it may be where some magmas are generated. Furthermore, it lacks strength and flows plastically and is thought to be the layer over which the plates decrease in seismic
wave
velocity.
of the outer, rigid lithosphere move.
Even though the low-velocity zone and the asthenosphere closely correspond, they are still distinct. The asthenosphere appears to be present worldwide, but the
'•'
FIGURE
wave
11-13
(a)
Seismic
discontinuities in the mantle
are thought to be caused by structural changes in minerals with
depth,
(b) In olivine,
the
dominant
mineral in peridotite, a silicon atom is surrounded by four oxygen atoms, (c) At greater depth, the olivine structure is rearranged into the denser structure of spinel, which also has four oxygen atoms surrounding a silicon atom, {d) At a depth of about 700 km, another
change occurs, and the spinel structure is converted to that of perovskite,
which has a silicon atom six oxygen atoms.
surrounded by
-i
low-velocity zone
is
not. In fact, the low-velocity zone
appears to be poorly defined or even absent beneath the ancient shields of continents.
Other discontinuities have been detected at deeper levwithin the mantle. However, unlike those between the crust and mantle or between the mantle and core, these probably represent structural changes in minerals rather than compositional changes. In other words, geologists believe the mantle is composed of the same material els
throughout, but the structural states of minerals such as olivine change with depth (Fig. 11-13). At a depth of 400
km, seismic wave
velocity increases slightly as a conse-
Oceanic
Mid-oceanic
Continental
crust
ridge
crust
quence of such changes in mineral structure (Fig. 11-12). Another velocity increase occurs at 640 to 720 km where the minerals break
and
(iron oxide)
dioxide (Si0 2 )
down
MgO
into metal oxides, such as
(magnesium oxide), and
A
11-13).
(Fig.
FeO
silicon
third discontinuity exists
about 1,050 km where P-waves once again increase in velocity. These three discontinuities are within what is called a transition zone separating the upper mantle from the lower mantle (Fig. 11-12). Although the mantle's density, which varies from 3.3 3 to 5.7g/cm can be inferred rather accurately from seisat
,
mic waves,
its
composition
less certain.
is
The igneous
considered the most likely component.
most rocks have densities of 2.0 to 3.0 and the overall density is about 2.70 g/cm 3 (Table 11-2). P-wave velocity in the continental crust is about 6.75 km/sec; at the base of the crust, P-wave velocity abruptly increases to about 8 km/sec. The continental crust varies considerably in thickness. It averages about 35 km thick, but is much thinner in such areas as the Rift Valleys of East Africa and a large area called the Basin and Range Province in the iron ore deposits,
g/cm
3
,
western United States. The crust stretched
and thinned
in
in these areas is
what appear
being
to be the early
stages of rifting. In contrast, continental crust beneath
mountain ranges
much
spars (see Fig. 5-13). Peridotite
pyroxene) with about 10% feldis considered the most
and projects deep into Himalayas of Asia, the continental crust is as much as 90 km thick. Crustal thickening beneath mountain ranges is an im-
likely candidate for three reasons. First, laboratory ex-
portant point that will be discussed in "The Principle of
periments indicate that
Isostasy" later in the chapter.
rock peridotite Peridotite
(60%
is
mostly
contains
olivine
and
30%
would account
that
it
ferromagnesian
minerals
possesses physical properties
for the mantle's density
and ob-
wave transmissions. Second,
is
thicker
the mantle. For example, beneath the
Although variations also occur
in
oceanic crust, they
peri-
are not as distinct as those for the continental crust. For
dotite forms the lower parts of igneous rock sequences
example, oceanic crust varies from 5 to 10 km thick, being thinnest at spreading ridges. It is denser than con-
served rates of seismic
believed to be fragments of the oceanic crust and upper
mantle emplaced on land
(see
Chapter
12).
And
third,
peridotite occurs as inclusions in volcanic rock bodies
known
tinental crust, averaging
about 3.0 g/cm
3 ,
and
it
trans-
mits P-waves at about 7 km/sec. Just as beneath the
come
continental crust, however, P-wave velocity increases at
from great depths. These inclusions are thought to be
the Moho. The P-wave velocity of oceanic crust is what one would expect if it were composed of basalt. Direct observations of oceanic crust from submersibles and deep-sea drilling confirm that its upper part is indeed
such as kimberlite pipes that are
to have
pieces of the mantle (see Perspective 11-1).
^ THE EARTH'S CRUST The of
Earth's crust
its
is
the
most
concentric layers, but
and best studied also the most complex Whereas the core and
accessible
it is
both chemically and physically. mantle seem to vary mostly in a vertical dimension, the
shows considerable vertical and lateral variation. (More lateral variation exists in the mantle than was once believed, however.) The crust along with that part of the upper mantle above the low-velocity zone constitutes the crust
lithosphere of plate tectonic theory.
Two
types of crust are recognized
— continental crust
and oceanic crust— both of which are
less
dense than the
more comwide variety of igneous, sedimentary, and metamorphic rocks. It is generally described as "granitic," meaning that its overall composition is similar to that of granitic rocks. Specifically, its overall composition corresponds closely to that of granodiorite, an igneous rock having a chemical composition between granite and diorite (see Figure 5-13). Continental crust varies in density depending on rock underlying mantle. Continental crust
is
the
plex, consisting of a
type, but with the exception of metal-rich rocks, such as
composed of basalt. The lower part of the oceanic crust is composed of gabbro, the intrusive equivalent of basalt (see Chapter 12 for a more detailed description of the oceanic crust).
^ THE EARTH'S INTERNAL HEAT During the nineteenth century, scientists realized that the Earth's temperature in deep mines increases with depth. Indeed, very deep mines must be air conditioned so that the miners can survive. More recently, the same trend has been observed in deep drill holes, but even in these we can measure temperatures directly down to a depth of only a few kilometers. The temperature increase with depth, or geothermal gradient, near the surface is about 25°C/km, although it varies from area to area. For example, in areas of active or recently active volcanism, the geothermal gradient is greater than in adjacent nonvolcanic areas, and temperature rises faster beneath spreading ridges than elsewhere beneath the sea floor. Unfortunately, the geothermal gradient is not useful for estimating temperatures deep in the Earth. If we were sim-
The
Earth's Internal
Heat
297
Perspective 11-1
KIMBERLITE PIPES-WINDOWS
TO THE MANTLE Diamonds have been economically important throughout history, yet prior to 1870, they had been found only in river gravels, where they occur as the result of weathering, transport,
and deposition.
In
1870, however, the source of diamonds in South Africa was traced to cone-shaped igneous bodies
found near the town of
called kimberlite pipes
Kimberly
(Fig. 1).
Kimberlite pipes are the source
rocks for most diamonds.
The in
greatest concentrations of kimberlite pipes are
southern Africa and Siberia, but they occur in
many
other areas as well. In North America they have been
found
in the
Canadian
Arctic, Colorado,
Wyoming,
Missouri, Montana, Michigan, and Virginia, and one at
Murfreesboro, Arkansas, was
briefly
worked
for
diamonds. Diamonds discovered in glacial deposits in some midwestern states indicate that kimberlite pipes are present farther north. The precise source of these diamonds has not been determined, although some
kimberlite pipes have recently been identified in
A
o
sea
J
A o
°
northern Michigan. Kimberlite pipes are composed of dark gray or blue
igneous rock called kimberlite, which contains olivine, a
potassium- and magnesium-rich mica, serpentines, and calcite
and
silica.
Some
of these rocks contain inclusions
l^^JMMBM
of peridotite that are thought to represent pieces of the Tfr
mantle brought to the surface during the explosive
pipe.
volcanic eruptions that form kimberlite pipes. If
magma
1
Generalized cross section of a kimberlite
kimberlite pipes measure less than
500
m
in
in kimberlite
pipes originated at a depth of at least 30 km. Indeed, the presence of
diamonds and the
structural
form of
the silica in the kimberlite can be used to establish
minimum and maximum
depths for the origin of
ply to extrapolate from the surface
perature at 100 great pressure, for pockets of
km would all
known
magma,
it
downward,
the tem-
be so high that in spite of the
rocks would melt. Yet except
appears that the mantle
is
solid
it transmits S-waves. Accordgeothermal gradient must decrease markedly. Current estimates of the temperature at the base of the crust are 800° to 1,200°C. The latter figure seems to be an upper limit: if it were any higher, melting would
rather than liquid because ingly, the
298
Most
diameter at the surface.
peridotite inclusions are, in fact, pieces of the
mantle, they indicate that the
both
FIGURE
Chapter
1 1
The
Interior of the Earth
the
magma. Diamond and
graphite are different
forms of carbon (see Fig. 3-6), but diamond forms only under high-pressure, high-temperature conditions. The presence of diamond and the absence crystalline
be expected. Furthermore, fragments of mantle rock in kimberlite pipes (see Perspective 11-1), thought to have
come from depths of about 100
to
300 km, appear
to
have reached equilibrium at these depths and at a temperature of about 1,200°C. At the core-mantle boundary, the temperature is probably between 3,500° and
5,000°C; the wide spread of values indicates the uncertainties of such estimates. If these figures are reasonably accurate, however, the geothermal gradient in the man-
Temperature (°C)
600
800
1,000
1,200
1
,400
1
,600
^- FIGURE 2 The forms of carbon silica in kimberlite pipes provide information on the depth at which the magma formed. The presence of and
diamond and
coesite in kimberlite
indicates that the
magma
probably
formed between 100 and 300
shown by
km
as
the intersection of the
calculated continental geotherm with the graphite-diamond and coesite-stishovite inversion curves.
of graphite existed
The
in
kimberlite indicate that such conditions
magma
where the
originated.
calculated geothermal gradient
and the
shown
in
in kimberlite,
is
on
maximum
the other hand,
is
a
form that
depth of about 300 km. Quartz
the form of silica found under low-pressure,
low-temperature conditions. Under great pressure,
pressure increase with depth beneath the continents are
found
indicates a
however, the crystal structure of quartz changes to
Figure 2. Laboratory experiments have
its
established a diamond-graphite inversion curve
high-pressure equivalent called coesite, and at even
showing the pressure-temperature conditions at which graphite is favored over diamond (Fig. 2). According
pipes contain coesite but no stishovite, indicating that
greater pressure
to the data in Figure 2, the intersection of the
the kimberlite
diamond-graphite inversion curve with the geothermal
of
gradient indicates that kimberlite
magma came from
minimum depth of about 100 km. Diamond can establish only a minimum depth kimberlite because
it is
stable at
silica
only about l°C/km. Recently, considerable temper-
new
technique called seismic tomography (see
Perspective 11-2).
Considering that the core uncertainties exist regarding
general estimates of 11-14).
The dashed
its
line
is
its
so remote and so
many
composition, only very
temperature can be made (Fig. in Figure 11-14 is an admittedly
speculative melting point curve for Earth materials
have come from a depth
as indicated by the intersection of
the coesite-stishovite inversion curve with the (Fig. 2).
for
ature variation has been inferred within the mantle by a rather
km
geothermal gradient
any pressure greater
than that occuring at a depth of 100 km. The
tie is
a
changes to stishovite.* Kimberlite
magma must
than 300
less
it
com-
*
Coesite and stishovite are also
known from
environments such as meteorite impact
other high-pressure
sites.
posed mostly of iron. Notice that the melting point curve is above the temperature estimates until the outer core is reached. Recall from earlier discussions that the S-wave shadow zone indicates that the outer core is liquid, whereas P-wave velocities indicate that the inner core
is
solid. Therefore, the postulated
remains within the
field
melting curve
of temperature estimates until
the depth corresponding to the outer core— inner core
boundary
is
reached. According to these considerations,
The
Earrh's Internal
Heat
299
E
— FIGURE
3,000
Outer core
Mantle
11-14
Temperature
estimates for the Earth's interior. The range of estimates increases
with depth indicating greater uncertainties. The dashed line is a speculative melting curve for iron.
Depth (km)
11-15). Higher values are also recorded in areas of con-
maximum temperature at the center of the core is 6,500°C, very close to the estimated temperature for the
tinental volcanism, such as in Yellowstone National
surface of the Sun!
Park
the
in
Wyoming, Lassen National Park
Heat Flow Even though rocks are poor conductors of heat, detectable amounts of heat from the Earth's interior escape at the surface by heat flow. The amount of heat lost from within the Earth is small and can be detected only by Heavy, cylindrical probes are dropped into soft sea-floor sediments, and temperatures are measured at various depths along the cylinder. On sensitive
in California,
Washington. Any area possessing higher than average heat flow values is a potential area for the development of geothermal energy
and near Mount
instruments.
(see
Chapter
Most
St.
Helens
in
17).
of the Earth's internal heat
is
generated by ra-
dioactive decay. Recall from Chapter 3 that isotopes of
some elements spontaneously decay state and, in
doing
to a
so, generate heat.
result of heat flow studies
is
more
One
stable
surprising
that, discounting local vari-
ations, the average values for the continents
and sea
surprising because con-
made
at
in areas
of
oceanic crust. Thus, one would expect the continents to
active or recently active volcanism. For example, greater
have higher heat flow values. Geologists postulate that convection cells and mantle plumes of hot mantle rock beneath the oceanic crust account for the oceanic crust's
the continents, temperature measurements are drill holes and mines. As one would expect, heat flow is greater
heat flow occurs at spreading ridges, and lower than
average values are recorded at subduction zones
Chapter
1 1
The
Interior of the Earth
about the same. This
tinental crust contains
various depths in
300
floor are
(Fig.
is
more radioactive elements than
Perspective 11-2
TOMOGRAPHY
SEISMIC The model of
the Earth's interior consisting of an
iron-rich core
and a rocky mantle
but
is
is
Seismometer
probably accurate
also rather imprecise. Recently, however,
geophysicists have developed a
new technique
called
tomography that allows them to develop three-dimensional models of the Earth's interior. In seismic tomography numerous crossing seismic waves are analyzed in much the same way radiologists analyze CAT (computerized axial tomography) scans. In CAT scans, X-rays penetrate the body, and a two-dimensional image of the inside of a patient is formed. Repeated CAT scans, each from a slightly different angle, are computer analyzed and stacked to
seismic
produce a three-dimensional picture. In a similar fashion geophysicists use seismic to
probe the interior of the Earth. From
its
waves
time of
and distance traveled, the velocity of a seismic computed at a seismic station. Only average
arrival
ray
is
velocity
is
determined, however, rather than variations
tomography numerous wave rays are analyzed so that "slow" and "fast" areas of wave travel can be detected (Fig. 1). Recall that seismic wave velocity is controlled partly by elasticity; cold rocks have greater elasticity and therefore transmit seismic waves faster than hot rocks.
Earthquake
in velocity. In seismic
Using
this technique, geophysicists
"^ FIGURE
1
Numerous earthquake waves
are analyzed
to detect areas within the Earth that transmit seismic waves
than adjacent areas. Areas of fast wave correspond to "cold" regions (blue), whereas "hot"
faster or slower
travel
regions (red) transmit seismic waves
more
slowly.
have detected
areas within the mantle at a depth of about 150
km
where seismic velocities are slower than expected. These anomalously hot regions lie beneath volcanic areas and beneath the mid-oceanic ridges, where convection cells of rising hot mantle rock are thought
several kilometers into the mantle.
Of
course, the base
of the mantle possesses the same features in reverse; geophysicists have termed these features
to exist. In contrast, beneath the older interior parts
"anticontinents" and "antimountains."
of continents, where tectonic activity ceased hundreds
the surface of the core
of millions or billions of years ago, anomalously cold
sinking and rising masses of mantle material.
spots are recognized. In effect, tomographic
three-dimensional diagrams
show heat
maps and
variations
within the Earth. Seismic tomography has also yielded additional and
sometimes surprising information about the core. For example, the core-mantle boundary is not a smooth surface, but has broad depressions and rises extending
As a
is
result of seismic
It
appears tbat
continually deformed by
tomography,
picture of the Earth's interior
is
a
much
emerging.
It
clearer
has
already given us a better understanding of complex convection within the mantle, including upwelling
convection currents thought to be responsible for the
movement Chapter
of the Earth's lithospheric plates (see
13).
The
Earth's Internal
Heat
301
Oceanic ridge (spreading ridge)
3-
CD
X
p
— FIGURE
11-16
(a)
The
gravitational attraction of the Earth pulls
all
objects
mass. Objects
1
toward its center of and 2 are the same
distance from the Earth's center of
mass, but the gravitational
on one is greater because more massive. Objects 2 and 3 have the same mass, but the gravitational attraction on 3 is four times less than on 2 because it is attraction it is
twice as far from the Earth's center of mass, (b) The Earth's rotation generates a centrifugal force that partly counteracts the force of gravity. Centrifugal force
the poles
and maximum
is
zero at
at the
equator.
a
mass deficiency exists over the unconsolidated sediment
because the force of gravity
is
less
than the expected av-
erage (Fig. 11-18). Large negative gravity anomalies also exist over salt
domes
(Fig.
11-19) and at subduction
zones, indicating that the crust
—
"
FIGURE
from a spring
11-17
is
not
The mass suspended shown
in the gravimeter,
diagrammatically, is pulled downward more over the dense body of ore than is
in
in equilibrium.
"" FIGURE gravity
PRINCIPLE OF ISOSTASY
More than 150
years ago, British surveyors in India
m when they compared two measurements between points 600 km
detected a discrepancy of 177 the results of
11-18
anomaly over
structure. it
adjacent areas, indicating a positive
^ THE
A
negative
a buried
-»-
FIGURE
11-19
Rock
salt
is
dense than most other types of rocks. A gravity survey over a salt less
dome shows
a negative gravity
anomaly.
gravity anomaly.
The
Principle of Isostasy
303
^^ Expected \^ plumb
N.
of
"^ FIGURE
deflection
1
1-20
(a)
A plumb
line
is
normally
vertical,
pointing to the Earth's center of gravity. Near a mountain range, one would expect the plumb line to be deflected as shown if the mountains were simply thicker, low-density
line
Himalayas
on denser material, (b) The actual deflection plumb line during the survey in India was less than It was explained by postulating that the
material resting of the
expected.
Himalayas have a low-density
root.
suspended weight) of their surveying instruments from the vertical, thus accounting for the error. Calculations revealed, however, that if the Himalayas were simply thicker crust piled
on denser
material, the error should
have been greater than that observed
(Fig.
11-20).
George Airy proposed that in addition to projecting high above sea level, the Himalayas— and other mountains as well — also project far below the surface and thus have a low-density root (Fig. 11-20). In effect, he was saying that mountains float on denser rock at depth. Their excess mass above sea level is compensated for by a mass deficiency at depth, which would In 1865, Sir
account for the observed deflection of the plumb during the British survey
(Fig.
line
11-20).
Gravity studies have revealed that mountains do indeed have a low-density "root" projecting deep into the mantle. If it were not for this low-density root, a gravity survey across a mountainous area would reveal a huge
The fact that no such anomaly mass excess is not present, so some of the dense mantle at depth must be displaced by
positive gravity anomaly. exists indicates that a
apart.
Even though
this
discrepancy was small,
it
was an
unacceptably large error. The surveyors realized that the gravitational attraction of the nearby tains
probably deflected the plumb
Himalaya Moun-
line (a
cord with a
^
FIGURE 11-21 (a) Gravity measurements along the line shown would indicate a positive gravity anomaly over the excess mass of mountains
if
the
simply thicker crust resting on denser material below, (b) An actual gravity survey across a mountain region shows no departure from the expected and thus no gravity anomaly. Such data indicate that the mass of the mountains above the surface must be compensated for at depth by low-density material displacing denser material.
wave
shown
in
Figure 11-21. (Seismic
studies also confirm the existence of low-density
roots beneath mountains.)
Positive gravity
s~>^
the mountains were
lighter crustal rocks as
anomaly
— FIGURE
An
11-22
iceberg
sinks to an equilibrium position
with about 10% of its mass above water level. The larger iceberg sinks farther
below and
rises
higher above
the water surface than does the
some of
smaller one.
If
above water
level
icebergs will rise
the ice
should melt, the to maintain the
same proportion of ice above and below water level. The Earth's crust floating in more dense material below is analogous to this example.
Airy's proposal is now called the principle of isostasy. According to this principle, the Earth's crust is in floating equilibrium with the more dense mantle below. This phenomenon is easy to understand by an analogy to an iceberg (Fig. 11-22). Ice
and thus
is
slightly less
dense than water,
However, according to Archimedes'* principle of buoyancy, an iceberg will sink in the water until it displaces a volume of water that equals its total weight. When the iceberg has sunk to an equilibrium position, only about 10% of its volume is above water level. If some of the ice above water level should melt, the iceberg will rise in order to maintain the same proportion of ice above and below water (Fig. 11-22).
The in that
Where it
it
floats.
Earth's crust it
is
similar to the iceberg, or a ship,
sinks into the mantle to
the crust
sinks further
is
thickest, as
down
its
equilibrium
level.
ice.
higher above the equilibrium surface (Fig. 11-21). Con-
crust also responds isostatically to widespread (Fig.
11-24).
Unloading of the Earth's crust causes
it
to respond by
upward until equilibrium is again attained. This phenomenon, known as isostatic rebound, occurs in arrising
eas that are deeply eroded
and
in areas that
covered by a vast is still
ice sheet until
rebounding
for-
century
about 10,000 years ago,
isostatically at a rate of
up to
1
m per
ll-25a). Coastal cities in Scandinavia have
(Fig.
been uplifted sufficiently rapidly that docks constructed
now
several centuries ago are
rebound has also occurred land has risen as
much
as
far
from shore. Isostatic Canada where the during the last 6,000
in eastern
100
m
years (Fig. 11 -25 b). If
the principle of isostasy
is
correct,
it
implies that
the mantle behaves as a liquid. In preceding discussions,
however,
we
must be
said that the mantle
transmits S-waves, which will not
solid because
move through
and less dense than oceanic crust stands higher than the ocean basins. Should the crust be loaded, as where widespread glaciers accumulate, it responds by sinking further into the mantle to maintain equilibrium (Fig. 11-23). In Greenland and
When
Antarctica, for example, the surface of the crust has
riods of time,
been depressed below sea level by the weight of glacial
time scales can be considered a viscous liquid.
tinental crust being thicker
were
merly glaciated. Scandinavia, for example, which was
beneath mountain ranges,
into the mantle but also rises
The
erosion and sediment deposition
it
liquid.
How
can
this
considered in terms of the short time necessary
for S-waves to pass through solid.
a
apparent paradox be resolved?
it,
However, when subjected it
will yield
the mantle
is
indeed
to stress over long pe-
by flowage and thus at these
The
A familiar
Principle of Isostasy
305
Crust
Continental crust
(d)
"^ FIGURE 11-23 A diagrammatic representation of the response of the Earth's crust to the added weight of glacial ice. (a) The crust and mantle before glaciation. (b) The weight of glacial ice depresses the crust into the mantle. (c)
When
and the rebound is
the glacier melts, isostatic rebound begins,
crust rises to
its
former position,
(d) Isostatic
complete.
substance that has the properties of a solid or a liquid depending on how rapidly deforming forces are applied is silly
putty.
It
sufficient time,
will flow
under
but shatters as a
its
own
weight
brittle solid if
if
given
struck a
~^ FIGURE
11-24
diagrammatic representation
isostatic
shown in Figure 11-26 is dipolar, meantwo unlike magnetic poles referred to as the north and south poles. The Earth possesses a dipolar magnetic field that resembles, on a large scale, magnetic ing that
field
it
possesses
that of a bar
sharp blow.
A
response of the crust to erosion (unloading) and widespread deposition (loading).
showing the
What
is
magnet
(Fig. 11-27).
the source of this magnetic field?
A number
^ THE EARTH'S MAGNETIC FIELD
of naturally occurring minerals are magnetic, with magnetite being the most common and most magnetic. It is
A
very unlikely, however, that the Earth's magnetic field is generated by a body of buried magnetite because mag-
simple bar magnet has a magnetic field, an area in which magnetic substances are affected by lines of magnetic force radiating from the magnet (Fig. 11-26). The
306
Chapter
1 1
The
Interior of the Earth
netic substances lose their
magnetic properties when
Germany
Poland
(a)
lb)
""'
FIGURE
in centimeters last
11-25
(a) Isostatic
per century,
rebound in Scandinavia. The lines show rates of uplift rebound in eastern Canada in meters during the
(b) Isostatic
6,000 years.
heated above a temperature called the Curie point. The Curie point for magnetite its
is
580°C, which
is
far
below
melting temperature. At a depth of 80 to 100
within the Earth, the temperature
km
high enough that
is
magnetic substances lose their magnetism. The fact that the locations of the magnetic poles vary through time also indicates that buried magnetite
is
not the source of
and Declination
Notice in Figure 11-27 that the lines of magnetic force around the Earth parallel the Earth's surface only near the equator. As the lines of force approach the poles, they are oriented at increasingly large angles with respect to the surface, and the strength of the magnetic
the Earth's magnetic field. Instead, the magnetic field
Inclination
of the Magnetic Field
is
generated within the
Earth by electrical currents (an electrical current
is
a
flow of electrons that always generates a magnetic field). These currents are generated by the different rotation
at the equator and strongest compass needle mounted so can rotate both horizontally and vertically not
field increases;
it is
weakest
at the poles. Accordingly, a
that
it
only points north, but
is
also inclined with respect to the
speeds of the outer core and mantle.
Earth's surface, except at the magnetic equator.
conducting liquid outer core rotates
gree of inclination depends
The electrically more slowly than
and this differential rotation around the Earth's axis generates the electrical currents that create the magnetic field. the surrounding mantle,
on the
along a line of magnetic force
is
field
called magnetic inclination.
The
de-
(Fig. 11-28).
This deviation o f the magnetic zontal
The
needle's location
from the
hori-
To compensate
Earth's Magnetic Field
for
307
"* FIGURE lines of
this,
11-26 Iron filings align themselves along the magnetic force radiating from a magnet.
compasses used
small weight
in the
Northern Hemisphere have a
on the south end of
erty of the Earth's magnetic field
the needle. This propis
important
in deter-
mining the ancient geographic positions of tectonic plates (see Chapter 13). Another important aspect of the magnetic field is that the magnetic poles, where the lines of force leave and enter the Earth, do not coincide with the geographic
— FIGURE inclination.
11-28
The
Magnetic
strength of the
magnetic field changes uniformly from the magnetic equator to the magnetic poles. This change in strength causes a dip needle to parallel the Earth's surface only at
the magnetic equator, whereas
its
inclination with respect to the
surface increases to 90° at the
magnetic poles.
308
Chapter
1 1
The
Interior of the Earth
"^ FIGURE lines
11-27 The magnetic field of the Earth has of force just like those of a bar magnet.
(rotational) poles.
tween the two netic field
At present, an IIV2 angle
(Fig. 11-29). Studies
show
exists be-
of the Earth's mag-
that the locations of the magnetic poles
vary slightly over time, but they
still
correspond closely
on the average with the locations of the geographic poles. A compass points to the north magnetic pole in the Canadian Arctic islands, some 1,290 km away from
Magnetic
Geographic
north pole
north pole
the geographic pole (true north); only along the line
shown
in
Figure 11-29 will a compass needle point to
both the magnetic and geographic north poles. From any other location, an angle called magnemrdeclination exis t s be t we e n
tinesdrawn fromThe iuinpa ss pusi i iorr to
the magnetic pole~aTRLthe~geographic pole (Fig. 11-29).
Magnetic declination must be taken into account during surveying and navigation because, for most places on Earth, compass needles point east or west of true north.
Magnetic Anomalies Variations in the strength of the Earth's magnetic
field
occur on both regional and local scales. Such variations from the normal are called magnetic anomalies. Regional variations are probably related to the complexities
of convection within the outer core where the mag-
netic
field
is
generated.
accounted for by
Local
variations
can be rock
lateral or vertical variations in
types within the crust.
An
instrument called a magnetometer can detect
slight variations in the strength of the
magnetic
""•"
FIGURE
11-29
Magnetic declination.
A
compass
needle points to the magnetic north pole rather than the
geographic pole (true north). The angle formed by the lines from the compass position to the two poles is the magnetic declination.
field,
and deviations from the normal are characterized
as
positive or negative. For example, a positive magnetic
anomaly
exists in areas
iron-bearing
where the rocks contain more
minerals than elsewhere.
In
the
Great
underlain by basalt lava flows, such as the Columbia
River basalts of the northwestern United States
(Fig.
Lakes region of the United States and Canada, huge iron ore deposits containing hematite and magnetite add
4-25), possess positive magnetic anomalies, whereas an
magnetism to that of the Earth's magnetic field; the result is a positive magnetic anomaly (Fig. 11-30). Positive magnetic anomalies also exist where extensive ba-
negative magnetic anomaly (Fig. 11-30).
their
saltic
volcanism has occurred because basalt contains
appreciable quantities of iron-bearing minerals. Areas
Positive
magnetic anomaly
Negative
magnetic anomaly
adjacent area underlain by sedimentary rocks shows a Geologists have used magnetometers for magnetic sur-
veys for decades because iron-bearing rocks can be easily detected by a positive magnetic
anomaly even
if
they are
deeply buried. In addition, magnetometers can defect a
Positive
magnetic
anomaly t
"^ FIGURE
11-32 Magnetic reversals recorded in a shown diagrammatically by red arrows, whereas the record of normal polarity events is shown by black arrows. The lava flows containing a record of such magnetic-polarity events can be radiometrically dated so that a magnetic time scale as in Figure 11-33 can be constructed. succession of lava flows are
"""'
FIGURE
salt
dome.
A
11-31
negative magnetic anomaly over a
domes, which show negative magnetic anomalies (Fig. 11-31); these can be detected by gravity surveys as well. variety of buried geologic structures, such as salt
Magnetic Reversals
When
a
magma
cools through the Curie point,
its
iron-
located roughly at the north and south geographic poles.
However, as early
sals occur, the Earth's
themselves with the Earth's magnetic
that the north~arrow
its
direction
and
strength.
As long
subsequently heated above the Curie point, serve that magnetism. However,
if
recording
field,
as the rock it
the rock
is
not
will preis
heated
above the Curie point, the original magnetism is lost, and when the rock subsequently cools, the iron-bearing minerals will align with the current magnetic field.
The iron-bearing minerals of some sedimentary rocks formed on the deep sea floor) are
were discov-
When these magneti c revermagnetic polarity is reversed, so
geologic past (Fig. 11-32).
bearing minerals gain their magnetization and align
both
as 1906, rocks
showed reversed magnetism. Paleomagnetic studies initially conducted on continental lava flows have clearly shown that the Earth's magnetic field has completely reversed itself numerous times during the ered that
on
a
compass would poinFsouth
rather than north.
Rocks that have
a record of
magnetism the same as the
present magnetic field are describedas jiaving larity ,_whe reas
reversed polarity.
norm al po-
magnetism have The ages ofthlTnormal aricTreversed
rocks with
"th e_opposite
polarity events for the past several million years have been
determined by applying absolute dating techniques to con-
sediments are deposited. These rocks also preserve a
and have been used to construct a magThese same patterns of normal and reversed polarity were soon discovered in
record of the Earth's magnetic
the oceanic crust (see Chapter 13).
(especially those that
also oriented parallel to the Earth's magnetic field as the
the time of their
field at
formation. Such information preserved in lava flows and
some sedimentary rocks can be used
to determine the
directions to the Earth's magnetic poles
of the rock
when
it
Paleomagnetism
and the
latitude
was formed.
is
tinental lava flows
netic reversal time scale (Fig 11-33).
The cause of magnetic reversals is not completely known, although they appear to be related to changes in the intensity of the Earth's magnetic indicate that the magnetic field has
simply the remanent magnetism in
during the
last century. If this
field.
Calculations
weakened about
5%
trend continues, there will
when
ancient rocks that records the direction and strength of
be a period during the next few thousand years
the Earth's magnetic field at the time of their formation.
magnetic
Geologists refer to the Earth's present magnetic
After the reversal occurs, the magnetic field will rebuild
normal, that
310
is,
field as
with the north and south magnetic poles
Chapter 11
The
Interior of the Earth
itself
field will
the
be nonexistent and then will reverse.
with opposite polarity.
^ FIGURE
11-33
(a)
Normal
and reversed polarity events the last 66 million years. Rocks in northern Pakistan
(black) for (b)
correlated with the
magnetic-polarity time scale.
XXX =
Volcanic ash
I
xxxxxxxx
I
xxxxxxxx
xxxxxxxx«xxxxxxxx
1 (b)
60'
The
Earth's Magnetic Field
311
^ CHAPTER SUMMARY
12.
The by
1.
2.
The Earth
is
concentrically layered into an iron-rich
13.
of the information about the Earth's interior has been derived from studies of P- and S-waves that travel through the Earth. Laboratory experiments,
magnetic force
The
lines
of magnetic
phenomenon 14.
of magnetic inclination.
Although the magnetic poles are close
to the
comparisons with meteorites, and studies of inclusions in volcanic rocks provide additional
declination exists between lines
drawn from a compass location to the magnetic and geographic
The
Earth's interior
on the
is
subdivided into concentric
basis of changes in seismic
north poles.
wave
15.
Density and elasticity of Earth materials determine the velocity of seismic waves. Seismic waves are refracted when their direction of travel changes. reflection occurs at boundaries across
The behavior
A
magnetometer can detect departures from the normal magnetic field, which can be either positive or negative.
16.
Although the cause of magnetic reversal understood,
which
shadow zones allow
and composition of and to estimate the size and depth of the core and mantle. The Earth's inner core is thought to be composed of iron and nickel, whereas the outer core is probably composed mostly of iron with 10 to 20% sulfur and the Earth's interior
other substances in lesser quantities. Peridotite most likely component of the mantle.
is
the
and granitic in composition, respectively. The boundary between the crust and the mantle is the Mohorovicic
The oceanic and continental
is
not fully
clear that the polarity of the
magnetic field has completely reversed times during the past.
crusts are basaltic
^
many
itself
IMPORTANT TERMS
The geothermal gradient of 25°C/km cannot continue to great depths, otherwise most of the Earth would be molten. The geothermal gradient for the mantle and core is probably about l°C/km. The temperature at the Earth's center
is
estimated to be
6,500°C. 9. Detectable amounts of heat escape at the Earth's surface by heat flow. Most of the Earth's internal
magnetic field magnetic inclination magnetic reversal mantle Mohorovicic
asthenosphere continental crust
core crust
Curie point
normal polarity
geothermal gradient
anomaly and negative)
gravity
(positive
oceanic crust
paleomagnetism
heat flow isostatic
(Moho)
discontinuity
discontinuity
discontinuity.
peridotite
rebound
principle of isostasy
lithosphere
P-wave shadow zone
low-velocity zone
reflection
magnetic anomaly
refraction
(positive
and negative)
reversed polarity
S-wave shadow zone
magnetic declination
REVIEW QUESTIONS
generated by radioactive decay. 10. According to the principle of isostasy, the Earth's crust is floating in equilibrium with the denser
1.
mantle below. Continental crust stands higher than oceanic crust because it is thicker and less dense. 11. Positive and negative gravity anomalies can be
2.
heat
it is
of P- and S-waves within the Earth and
geologists to estimate the density
is
detected where excesses and deficiencies of mass
312
lines of
geographic poles, they do not coincide exactly. For most places on Earth, an angle called magnetic
the presence of P- and S-wave
8.
surrounded by
except at the equator, thus accounting for the
Much
the properties of rocks change.
7.
is
crust.
Wave
6.
The Earth
force are inclined with respect to the Earth's surface,
velocities at discontinuities.
5.
thought to be generated
similar to those of a bar magnet.
layers
4.
is
core with a solid inner core and a liquid outer part, a rocky mantle, and an oceanic crust and continental
information. 3.
Earth's magnetic field
electrical currents in the outer core.
The average
line
occur, respectively. Gravity surveys are useful in
c.
exploration for minerals and hydrocarbons.
gradient.
Chapter 11
The
Interior of the Earth
is
6.75; d.
3 - g/cm
.
1.0;
showing the direction of movement of a small wave front is a: P-wave reflection; seismic discontinuity; b. seismic particle beam; e. wave ray; d
part of a a
5.5; c
2.5.
e
A
density of the Earth
12.0; b
a
3.
When
seismic waves travel through materials having
14. Iron-bearing minerals in a
different properties, their direction of travel changes.
phenomenon
This
4.
is
a.
elasticity; b.
c.
refraction; d.
A major seismic km is the:
wave: energy dissipation; deflection;
6.
reflection.
e.
oceanic
b.
crust-continental crust boundary;
5.
field
discontinuity at a depth of 2,900
core-mantle boundary;
a.
reflected.
lithosphere-asthenosphere boundary.
18.
Why
is
sulfur; b.
d.
potassium;
Which
probably composed mostly iron.
e.
a.
inclusions in volcanic rocks; b.
c.
meteorites; d.
zone;
peridotite;
iron-nickel alloy;
spreading ridges;
the:
Moho;
determine that a discontinuity,
less dense than continental crust; primary source of magma.
Most
of the Earth's internal heat
a.
moving
c.
earthquakes;
e.
meteorite impacts.
plates; b.
is
According to the principle of isostasy: a. more heat escapes from oceanic crust than from continental crust; b. the Earth's crust is floating in equilibrium with the more dense mantle below; c. the Earth's crust behaves both as a liquid and a solid; d. much of the asthenosphere is molten; e. magnetic anomalies result when the crust is loaded by glacial ice. 12. The magnetic field is probably generated by: 11
a.
the
b.
the solar wind;
tilt
of the Earth's rotational axis; c.
electrical currents in the
deformation of the asthenosphere; e. a large deposit of magnetite at the North Pole. 13. Except at the magnetic equator, a compass needle in the Northern Hemisphere points to the magnetic north pole and downward from the horizontal. This outer core; d.
phenomenon
is:
magnetic declination; b. magnetic reflection; c. magnetic reversal; d. magnetic polarity; e. magnetic inclination. a.
it
geologists account for the fact that heat
is
the continental crust is deeply eroded in one area and loaded by widespread, thick sedimentary If
how
will
it
respond
isostatically
at each location?
25.
generated by:
volcanism; radioactive decay;
d.
do
deposits in another,
the
in
about the same through oceanic crust and it should be greater through the latter? 24.
e.
How flow
thinnest at
b.
Moho,
continental crust even though
granitic in composition;
c.
called the
decrease within the Earth? 23.
gabbro.
e.
now
between the crust and the mantle. 21. How do oceanic and continental crust differ composition and thickness? 22. What is the geothermal gradient? Why must
high-velocity
d.
10
is
transition zone.
Oceanic crust is: a 20 to 90 km thick;
probably
exists
Continental crust has an overall composition corresponding closely to that of: a. basalt; b. sandstone; c. granodiorite; d.
is
mantle. What accounts for these discontinuities? 20. Explain the reasoning used by Mohorovicic to
diamonds; S-wave
e.
at the base of the crust
magnetic anomaly; b. geothermal gradient; d. e.
the inner core thought to be
19. Several seismic discontinuities exist within the
of the following provides evidence for the
shadow zone. The seismic discontinuity
shadow zone? composed of
the significance of the S-wave
is is
iron and nickel whereas the outer core composed of iron and sulfur?
of:
nickel;
silica; c.
Curie
magnetic-polarity
magnetic declination. determines the velocity of P- and S-waves? 16. Explain how seismic waves are refracted and
e.
Earth's core
isostasy curve; d.
field; e.
What
What
a.
9.
c.
17.
a.
they cool through the:
point;
inner core-outer core boundary;
The
gain their
negative magnetic anomaly; b.
d.
c
8.
when
magma
align themselves with the magnetic
a.
15.
Moho;
c.
composition of the core?
7.
magnetism and
What
is meant by positive and negative gravity anomalies? Give examples of where each type of anomaly might occur.
What
is the magnetic field, and how is it thought to be generated? 27. Explain the phenomenon of magnetic inclination.
26.
28. Illustrate
how
a vertical succession of ancient lava
flows preserves a record of magnetic reversals.
^
ADDITIONAL READINGS
Anderson, D. L., and A. M. Dziewonski. 1984. Seismic tomography. Scientific American 251, no. 4: 60-68. Bolt, B. A. 1982. Inside the Earth: Evidence from earthquakes. San Francisco: W. H. Freeman and Co. Brown, G. C. 1981. The inaccessible Earth. London: George Allen Unwin. Fowler, C. M. R. 1990. The solid Earth. New York: Cambridge
&
University Press.
Heppenheimer, T. A. 1987. Journey to the center of the Earth. Discover 8, no. 10: 86-93. Jeanloz, R. 1983. The Earth's core. Scientific American 249, no. 3: p.
56-65.
McKenzie, D.
P.
1983. The Earth's mantle. Scientific American
249, no. 3: p. 66-78. Monastersky, R. 1988. Inner space. Science
News
136:
266-268.
Additional Readings
313
CHAPTER
12
THE SEA FLOOR ^ OUTLINE PROLOGUE INTRODUCTION OCEANOGRAPHIC RESEARCH CONTINENTAL MARGINS The Continental
Shelf
"^"Perspective 12-1: Lost Continents
The Continental Slope and Rise Turbidity Currents, Submarine Canyons, and
Submarine Fans
TYPES OF CONTINENTAL MARGINS THE DEEP-OCEAN BASIN Abyssal Plains
Oceanic Trenches Oceanic Ridges Fractures in the Sea Floor
Seamounts, Guyots, and Aseismic Ridges "*r Perspective 12-2:
Maurice Ewing and His
Investigation of the Atlantic
Ocean
DEEP-SEA SEDIMENTATION REEFS
COMPOSITION OF THE OCEANIC CRUST RESOURCES FROM THE SEA CHAPTER SUMMARY
Pillow lava on the floor of the Pacific Ocean near the Galapagos Islands.
PROLOGUE |^gJ)lV~||
j
n 1979^ researchers aboard the
submersible Alvin descended about
2,500
m
to the
Galapagos Rift
in the eastern Pacific
Ocean basin and observed hydrothermal vents on sea floor (Fig. 12-1).
the
Such vents occur near spreading
where seawater seeps down into the oceanic and fissures, is heated by the hot rocks, and then rises and is discharged onto the sea floor as hot springs. During the 1960s, hot metal-rich brines apparently derived from hydrothermal vents ridges
crust through cracks
were detected and sampled in the Red Sea. These dense brines were concentrated in pools along the axis of the sea; beneath them thick deposits of metal-rich sediments were found. During the early 1970s, researchers observed hydrothermal vents on the Mid-Atlantic Ridge about 2,900 km east of Miami, Florida, and in 1978 moundlike mineral deposits were sampled from the East Pacific Rise just south of the Gulf of California.
When the submersible Alvin descended to the Galapagos Rift in 1979, mounds of metal-rich sediments were observed. Near these mounds the researchers saw what they
called black
smokers (chimneylike vents)
discharging plumes of hot, black water (Fig. 12-1). Since
1979
similar vents have been observed at or near
spreading ridges in several other areas.
"^ FIGURE 12-1 The submersible Alvin sheds light on hydrothermal vents at the Galapagos Rift, a branch of the East Pacific Rise. Seawater seeps down through the oceanic crust, becomes heated, and then rises and builds chimneys on the sea floor. Communities of organisms, including tubeworms, giant clams, crabs, and several types of fish, live
Submarine hydrothermal vents are interesting for Near the vents live communities of
several reasons.
organisms, including bacteria, crabs, mussels,
starfish,
and tubeworms, many of which had never been seen before (Fig. 12-1). In most biological communities,
near the vents.
"**'
FIGURE
12-2
Formation of a black smoker. The is simply heated water saturated
plume of "black smoke"
with dissolved minerals. Precipitation of anhydrite (CaS0 4 ) and sulfides of iron, copper, and zinc forms the chimney.
months
When
photosynthesizing organisms form the base of the
1979 was
food chain and provide nutrients for the herbivores and carnivores. In vent communities, however, no
activity ceases, the vents eventually collapse
sunlight
is
available for photosynthesis,
inactive six
and the base
The economic is
chemosynthesis; they oxidize sulfur compounds from
Deep of
the
and the nutrients
for other
own
members of
tons of metals, including iron, copper, zinc, the
gold. These deposits are fully as large as the
mined on land.
sulfide deposits
then reacts with the crust and
throughout geologic time.
transformed into a
metal-bearing solution. As the hot solution discharges onto the sea floor, iron, copper,
and zinc
sulfides
it
rises
and
and other minerals that
more common than it is at present because the Earth possessed more heat, and this activity is believed to have been responsible for the formation of the atmosphere and surface water. As we noted in previous chapters, volcanoes emit a variety of gases, the most abundant of
water vapor. The atmosphere and surface wa-
thought to have derived within the Earth and been emitted at the surface by volcanoes in a process called outgassing* (Fig. 12-3). As the Earth cooled, waters are
vapor began condensing and fell as rain, which accumulated to form the surface waters. Geologic evidence clearly indicates that an extensive ocean was present more than 3.5 billion years ago. During most of historic time, people knew little of the oceans and, until fairly recently, believed that the sea floor was flat and featureless. Although the ancient Greeks had determined the size of the Earth rather acter
*The alternate hypothesis— that much of the Earth's surface water was derived from comets — is not yet widely accepted.
316
Chapter 12
The Sea Floor
and major
silver,
of these sulfide
Troodos Massif on have formed on the sea floor
Cyprus, are believed to by hydrothermal vent activity.
Hydrothermal vent
sulfide deposits
None
have formed
are currently being
mined, but the technology to exploit them determined that
exists. In fact,
and Sudanese governments have
it is
feasible to recover such deposits so.
in
Although the oceans are distinct enough to be designated by separate names such as Pacific, Atlantic, and Indian, a single interconnected body of salt water covers more than 70% of the Earth's surface. During its very earliest history, the Earth was probably hot, airless, and lacking in surface water. Volcanic activity, however, was
is
Many
II
million
land, such as the
from the Red Sea and are making plans to do
INTRODUCTION
which
now on
deposits
the Saudi Arabian
cools, precipitating
accumulate to form a chimneylike vent (Fig. 12-2). These vents are ephemeral, however; one observed
^
in the Atlantis
Red Sea contain an estimated 100
food chain. Another interesting aspect of these submarine hydrothermal vents is their economic potential. When seawater circulates downward through the oceanic crust, it is heated to as much as 400°C. The hot water is
and are
potential of hydrothermal vent
tremendous. The deposits
deposits
nutrients
their
incorporated into a moundlike mineral deposit.
of the food chain consists of bacteria that practice the hot vent waters, thus providing their
later.
curately,
Western Europeans were not aware of the vast-
ness of the oceans until the fifteenth and sixteenth cen-
when
turies
various explorers sought
to the Indies.
August
When
new
trade routes
Christopher Columbus set
sail
on
an attempt to find a route to the Indies, he greatly underestimated the width of the Atlantic
3,
1492,
in
Ocean. Contrary to popular
belief,
Columbus was
not attempting to demonstrate that the Earth sphere
is
a
— the Earth's spherical shape was well accepted by
The controversy was over the Earth's circumference and what was the shortest route to China. During these and subsequent voyages, Europeans sailed to the Americas, the Pacific Ocean, Australia, New Zealand, the Hawaiian Islands, and many other islands previously unthen.
known
to them.
Such voyages of discovery added considerably to our knowledge of the oceans, but truly scientific investigations did not begin until the late 1700s. Great Britain was the dominant maritime power, and in order to maintain that dominance, the British sought to increase their knowledge of the oceans. The earliest British scientific voyages were led by Captain James Cook in 1768, 1772, and 1777. In 1872, the converted British warship H.M.S. Challenger began a four-year voyage, during which seawater was sampled and analyzed, oceanic depths were determined at nearly 500 locations, rock and sediment samples were recovered from the sea floor, and more than 4,000 new marine species were classified.
Escapes
Hydrogen Water
h Nitrogen N,
To atmosphere
Carbon dioxide
Erosional debris
—
FIGURE 12-4 The Glomar Challenger 122-m long oceanographic research vessel.
a larger,
is
a 10,500-ton,
more advanced research vessel, the JOIDES* made its first voyage in 1985.
Resolution,
In addition to surface vessels, submersibles, both re-
"
r
motely controlled and manned by
FIGURE
Gases derived from within the Earth by outgassing formed the early atmosphere and surface waters. 12-3
Continuing exploration of the oceans revealed that the sea floor
is
not
flat
and
featureless as formerly be-
lieved. Indeed, scientists discovered that the sea floor
possesses varied topography including oceanic trenches,
submarine ridges, broad plateaus, hills, and vast plains. Some people have suggested that some of these features are remnants of the mythical lost continent of Atlantis (see Perspective 12-1).
Drilling Project,
scientists,
have been
to the research arsenal of oceanographers. In
1985, for example, the Argo, towed by a surface vessel and equipped with sonar and television systems, provided the first views of the British ocean liner R.M.S. Titanic since it sank in 1912. The U.S. Geological Survey is using a towed device to map the sea floor (Fig. 12-5). The system uses sonar to produce images resembling aerial photographs. Researchers aboard the submersible Alvin have observed submarine hydrothermal vents (see the Prologue) and have explored parts of the oceanic ridge system.
The
measurements of the oceanic depths were a weighted line to the sea floor and measuring the length of the line. Now, however, an instrument called an echo sounder is used. Sound waves from a ship are reflected from the sea floor and detected by instruments on the ship, thus yielding a continuous profile of the sea floor. Depth is determined by knowing the velocity of sound waves in water and the time it takes for the waves to reach the sea floor and return to first
made by lowering
^ OCEANOGRAPHIC RESEARCH The Deep Sea
added
an international program
sponsored by several oceanographic institutions and funded by the National Science Foundation, began in 1968. Its first research vessel, the Glomar Challenger, was capable of drilling in water more than 6,000 m deep (Fig. 12-4). It was equipped to drill into and recover long cores of sea-floor sediment and the oceanic crust. During the next 15 years, the Glomar Challenger drilled more than 1,000 holes in the sea floor. The Deep Sea Drilling Project came to an end in 1983 when the Glomar Challenger was retired. However, an international project, the Ocean Drilling Program, continued where the Deep Sea Drilling Project left off, and
the ship.
Seismic profiling
more
similar to echo sounding but even waves are generated at an energy
is
useful. Strong
source, the waves penetrate the layers beneath the sea floor,
and some of the energy
*JOIDES is an acronym Deep Earth Sampling.
for Joint
is
reflected
from various
Oceanographic Institutions for
Oceanographic Research
317
"^ FIGURE
12-6 Diagram showing how seismic profiling used to detect buried layers at sea. Some of the energy generated at the energy source is reflected from various horizons back to the surface where it is detected by hydrophones. is
"^ FIGURE 12-5 The sonar system used by the U.S. Geological Survey for sea-floor mapping.
acquired since World
War
II.
This statement
with respect to the sea
larly true
floor,
is
particu-
because only in
recent decades has instrumentation been available to
The data
geologic horizons back to the surface (Fig. 12-6). Recall
study this largely hidden domain.
from Chapter 11 that seismic waves are reflected from boundaries where the properties of Earth materials
not only important in their own right but also have provided much of the evidence that supports plate tec-
change. Seismic profiling has been particularly useful in mapping the structure of the oceanic crust beneath sea-
tonic theory (see Chapter 13).
^ CONTINENTAL MARGINS
floor sediments.
Oceanographers also use gravity surveys to detect domes beneath the continental margins are recognized by negative gravity anomalies, and oceanic trenches also exhibit negative gravity anomalies. Magnetic surveys have also provided
bounded by continental margins, zones separating the part of a continent above sea level
gravity anomalies. For example, salt
important information regarding the sea floor
All continents are
from the deep-sea
-^ FIGURE
12-7
A
generalized
showing
features of the continental margins.
The
vertical
The
continental margin consists
clined continental slope, and, in
(see
the continental margin
is
'»
_
in-
cases, a deeper,
Seaward of
the deep-ocean basin. Thus,
the continental margin extends to increasingly greater
depths until
it
merges with the deep-sea
floor.
Continental margin
Continental margin
*
some
gently sloping continental rise (Fig. 12-7).
\
Continental shelf
Continental shelf
dimensions of the
/
features in this profile are greatly
Sea
level
exaggerated because the vertical and horizontal scales
floor.
of a gently sloping continental shelf, a more steeply
Chapter 13). Although scientific investigations of the oceans have been yielding important information for more than two hundred years, much of our current knowledge has been
profile of the sea floor
collected are
Oceanic ridge
differ.
Oceanic trench Continental slope Continental slope i
i
i
i
i
I
500
I
I
i
i
i
1,000
i
i
I
i
I
1,500
i
I
I
i
i
i
2,000
i
I
I
i
2,500
i
I
I
3,000
Distance (km)
318
Chapter 12
The Sea Floor
f'ni^^r^^^^^rT 3,500
4,000
4,500
5,000
-^ FIGURE
12-8
The
transition
from continental to oceanic crust, and hence the geological margin of a continent, occurs beneath the
continental slope.
Most people
perceive continents as land areas out-
by sea level. However, the true geologic margin of a continent— that is, where continental crust changes to oceanic crust— is below sea level, generally somewhere lined
beneath the continental slope
(Fig. 12-8).
Accordingly,
marginal parts of continents are submerged.
The Continental Shelf Between the shoreline and continental slope of all continents lies the continental shelf, an area where the sea floor slopes very gently in a seaward direction. Its slope is much less than 1° (Fig. 12-7); it averages about 2 m/km, or 0.1°.
The outer edge of
erally taken to
the continental shelf
is
gen-
correspond to the point at which the
in-
clination of the sea floor increases rather abruptly to several degrees; this shelf-slope
depth of about 135
m
break occurs at an average
(Fig. 12-7).
Continental shelves
eral
hundred kilometers across
along the west coast
it is
in
some
extend well up onto the continental
but some of them shelf.
associated with streams
more As
on
They are discussed
land.
a
consequence of lower sea level during the Pleismuch of the sediment on continental
shelves accumulated in stream channels
much
as sev-
of these
fully in the following section.
meters to more than 1,000 km. For example, the shelf as
Some
canyons lie offshore from the mouths of large streams. At times during the Pleistocene Epoch (1,600,000 to 10,000 years ago), sea level was more than 100 m lower than at present, so much of the continental shelves were above sea level. Streams flowed across these exposed shelves and eroded deep canyons that were subsequently flooded when sea level rose. However, most submarine canyons extend to depths far greater than can be explained by stream erosion during periods of lower sea level. Furthermore, many submarine canyons are not
tocene Epoch,
is
whereas
Deep, steep-sided submarine canyons are most characteristic of the continental slope,
vary considerably in width, ranging from a few tens of
along the east coast of North America
places,
only a few kilometers wide.
(Fig. 12-9). In fact, in areas
and floodplains
such as northern Europe and
-^ FIGURE lower sea
At times of during the
12-9
level
Pleistocene Epoch, large parts of the
continental shelves were exposed. Accordingly, much of the sediment deposited during these times accumulated in various continental
environments such as stream channels and lakes.
Continental Margins
319
Perspective 12-1
LOST CONTINENTS Most people have heard of
the mythical lost continent
True Continent
of Atlantis, but few are aware of the source of the Atlantis legend or the evidence that
former existence of
this continent.
cited for the
is
Only two known
sources of the Atlantis legend exist, both written in
about 350
B.C.
by the Greek philosopher Plato. In two
of his philosophical dialogues, the Timaeus and the Critias, Plato tells of Atlantis, a large island continent
according to him, was located
that,
Ocean west of the call the Strait
in the Atlantic
of Gibraltar (Fig.
now
which we
Pillars of Hercules,
Plato also wrote
1).
that following the conquest of Atlantis by Athens, the
continent disappeared: .
.
day and night came when
disappeared beneath the sea.
now
the sea there has
which the
island
.
And
.
.
Atlantis
.
.
it
is
produced as
by the it
mud
one assumes that the destruction of Atlantis was one conjured up by Plato to a philosophical point, it
was supposed
Critias,
who
he nevertheless lived long
to have occurred.
turn told
in
it
to Plato.
two types of evidence
claim that Atlantis did indeed exist.
supposed cultural Atlantic
Ocean
similarities
to support their First,
on opposite
W. Ramage,
ed., Atlantis: Fact
or
Fiction? (Bloomington, Ind.: Indiana University Press, 1978), p. 13.
320
Chapter 12
The Sea Floor
the Azores,
Bermuda, the Bahamas, and the
Mid-Atlantic Ridge are alleged to be remnants of Atlantis. If a continent
Atlantic, however,
it
had actually sunk
in the
could be easily detected by a
gravity survey. Recall that continental crust has a
and a lower density than oceanic were actually present beneath the Atlantic Ocean, there would be a huge negative gravity anomaly, but no such anomaly has granitic composition
Thus,
if
a continent
been detected. Furthermore, the crust beneath the
Secondly, supporters of the legend assert that remnants
in E.
No "mud
Atlantic has been drilled in
and those of Central and South America. They contend that these similarities are due to cultural diffusion from the highly developed civilization of Atlantis. According to archaeologists, however, few similarities actually exist, and those that do can be explained as the independent development of analogous features by different cultures.
Quoted
call
shallows" exist in the Atlantic as Plato claimed, but
sides of the
basin, such as the similarity in shape of
the Timaeus.
we now
they point to
the pyramids of Egypt
*From
of the sunken continent can be found.
crust.
Present-day proponents of the Atlantis legend generally cite
According to Plato, Atlantis was a large
1
the Strait of Gibraltar.
sank.*
According to Plato, Solon, an Athenian who lived about 200 years before Plato, heard the story from Egyptian priests who claimed the event had occurred 9,000 years before their time. Solon told the story to his grandson, after
"^ FIGURE
continent west of the Pillars of Hercules, which
shallows
a real event, rather than
make
True Continent
.
for this reason even
become unnavigable and
unsearchable, blocked as
If
and floods and one
there were violent earthquakes
.
terrible
many
samples recovered indicate that
same
places,
its
and
all
composition
the
is
the
as that of oceanic crust elsewhere.
In short, there
is
some may be based on a Nevertheless,
no geological evidence
for Atlantis.
archaeologists think that the legend real event.
About 1390
B.C.,
a huge
volcanic eruption destroyed the island of Thera in the
Mediterranean Sea, which was an important center of
Greek civilization. The eruption was one of the most violent during historic time, and much of the island disappeared when it subsided to form a caldera
early
(Fig. 2).
Most
of the island's inhabitants escaped
(Fig. 3),
but the eruption probably contributed to the demise of
km p^j Pre-collapse island
y
—
.]
I
Collapsed material
Possible pre-collapse
shape
ol island
?* FIGURE
2 The island of Thera was destroyed by a huge eruption about 1390 b.c. Ash was carried more than 950 km to the southeast, and tsunami probably devastated nearby coastal areas. The inset shows the possible profile of the island before the eruption and its shape immediately after the caldera
formed.
culture on Crete. At least 10 cm of ash on parts of Crete, and the coastal areas of the island were probably devastated by tsunami. It is possible that Plato used an account of the destruction the
Minoan
fell
of Thera, but fictionalized
it
for his
own
purposes,
thereby giving rise to the Atlantis legend.
"*»" FIGURE 3 (right) An artist's rendition of the volcanic eruption on Thera in about 1390 b.c. that destroyed most of inhabitants escaped the island's island. Most of the the
devastation.
Continental Margins
321
Shelf-slope
break
Submarine fan
"^r_
FIGURE
12-11
Submarine fans formed by the down submarine canyons by
deposition of sediments carried
Much
turbidity currents.
of the continental rise
is
composed
of overlapping submarine fans.
monly descend
directly into
continental rise
is
The
absent
shelf-slope break
an oceanic trench, and a
(Fig. 12-7). is
a very important feature in
terms of sedimentation. Landward from the break, the
"^ FIGURE 12-10 {a) Turbidity currents flow downslope along the sea floor (or lake bottom) because of their density. (b) Graded bedding formed by deposition from a turbidity current.
parts of
North America,
glaciers
extended onto the ex-
posed shelves and deposited gravel, sand, and mud. Since the Pleistocene Epoch, sea level has risen submerging the shelf sediments, which are now being reworked by marine processes. That these sediments were, deposited on land
is
human mammoths and mastodons
indicated by evidence of
settlements and fossils of (extinct
in fact,
members of the elephant
family)
and other land-
by waves and tidal currents. Seaward of bottom sediments are completely unaffected by surface processes, and their transport onto the slope and rise is controlled by gravity. The continental slope and rise system is the area where most of the sediment derived from continents is eventually deposited. shelf
is
affected
the break, the
Much
of this sediment
rents through
Canyons, and Submarine Fans Turbidity currents are sediment-water mixtures denser
than normal seawater that flow downslope to the deep-
An
flows onto the relatively
deposited
The seaward margin of
the continental shelf
by the shelf-slope break
(at
marked an average depth of 135 m) is
relatively steep continental slope begins (Fig.
12-7). Continental slopes average about 4°, but range
from
1° to 25°. In
many
places, especially
around the
margins of the Atlantic, the continental slope merges with the more gently sloping continental rise. In other places, such as
322
around the
Chapter 12
Pacific
The Sea Floor
flat
individual turbidity current sea floor
where
it
slows and
begins depositing sediment; the coarsest particles are
The Continental Slope and Rise
where the
transported by turbidity cur-
Turbidity Currents, Submarine
sea floor (Fig. 12-10).
dwelling animals.
is
submarine canyons.
Ocean, slopes com-
cles,
first,
followed by progressively smaller parti-
thus forming graded bedding (Fig. 12-10). These
deposits accumulate as a series of overlapping submarine fans,
which constitute a large part of the continental At their seaward margins, these fans
rise (Fig. 12-11).
grade into the deposits of the deep-ocean basins.
No
one has ever observed a turbidity current
progress, so for
many
years there
was considerable
in
de-
bate about their existence. In 1971, however, abnor-
mally turbid water was sampled just above the sea floor in the
North
perhaps play some role
Atlantic, indicating that a turbidity current
in their origin.
bidity currents periodically
and are
had occurred recently. Furthermore, sea-floor samples from many areas show a succession of graded beds and the remains of shallow-water organisms that were ap-
now
Furthermore, tur-
move through
these canyons
thought to be the primary agent responsi-
ble for their erosion.
parently displaced into deeper water.
» TYPES OF CONTINENTAL MARGINS
Perhaps the most compelling evidence for the existence of turbidity currents
is
the pattern of trans-Atlantic
Newfoundland on it was asoccurred on that date
cable breaks that occurred south of
November sumed
18,
1929
(Fig.
Two
12-12). Initially,
that an earthquake that
had ruptured several trans-Atlantic telephone and telegraph cables. However, while the breaks on the continental shelf near the epicenter occurred
when
The broke was known, so
which each cable
in succession.
oceanic lithosphere
was
continental margin logically
It
apparently
moved
at
when
it
a simple
is
tion of land-derived sediments. tal
margins are on the
(Fig.
fully understood. It is known that move through submarine canyons and
12-13b).
narrow, and
activity of the conti-
These passive continen-
edge of a continental plate
They possess broad continental shelves and rise; vast, flat abyssal plains
a continental slope
are
commonly
present adjacent to the rises (Fig. 12-
13b). Furthermore, passive continental margins lack the
100
03:03
trailing
and
Southeast
Time intervals between quake and cable breaks
is
The continenwas stretched, thinned, and fractured as rifting proceeded. As plate separation occurred, the newly formed continental margins became the sites of deposi-
• Breaks due to • Breaks due to
-
andesitic volca-
crust
tal
Northwest
5,000
characterized by seismicity, a geo-
young mountain range, and
the rifting of the supercontinent Pangaea.
reached
Breaks due
a
considerably from their western margins. In the east,
fer
not
00:59"
is
the continental margins developed as a consequence of
However, many have no such association, and
strong currents
is
(Fig. 12-13a). The west good example. Here, the
subducted
The configuration and geologic
yons can be traced across the shelf to associated streams their origin
is
nental margins of eastern North and South America dif-
As mentioned previously, submarine canyons occur on the continental shelves, but they are best developed on continental slopes (Fig. 12-11). Some submarine canland.
margin
Chile Trench.
the continental rise.
on
active continental
the continental slope descends directly into the Peru-
about 80 km/hr on the continen-
but slowed to about 27 km/hr
An
nism. Additionally, the continental shelf
matter to calculate the velocity of the turbidity current. tal slope,
active.
coast of South America
the earth-
precise time at it
and
develops at the leading edge of a continental plate where
quake struck, cables farther seaward were broken later and in succession. The last cable to break was 720 km from the source of the earthquake, and it did not snap until 13 hours after the first break occurred (Fig. 12-12). In 1949, geologists realized that the earthquake had generated a turbidity current that moved downslope, breaking the cables
types of continental margins are generally recog-
nized, passive
to turbidity current
shock, slumps turbidity current
'
Continent
Continental shelf
Continental
slope
Oceanic trench
Upper mantle
(a)
Continent
Continental shelf
Abyssal plain
(b)
"•'
FIGURE
12-13
Diagrammatic views of
passive continental margin.
324
Chapter 12
The Sea Floor
(a)
an active continental margin and
(b) a
^ Oceanic ridge system
Rift
| Abyssal
Oceanic trench
"^ FIGURE
plain
12-14
The
valley
distribution of oceanic trenches, abyssal plains,
and the
oceanic ridge system.
(Fig.
the temperature is generally just above 0°C, and the pressure varies from 200 to more than 1,000 atmospheres depending on depth. Submersibles have carried scientists to the greatest oceanic depths, so some of
12-13). Active continental margins obviously lack a
the sea floor has been observed directly. Nevertheless,
continental rise because the slope descends directly into
much
intense seismic
and volcanic
activity characteristic of ac-
margins.
tive continental
Active and passive continental margins share features, but in other respects they differ
markedly
some
an oceanic trench. Just as on passive continental margins, sediment is transported down the slope by turbidity currents, but it simply fills the trench rather than
forming a
rise.
The proximity of
tinent also explains
why
the trench to the con-
the continental shelf
is
so nar-
life exists,
of the deep-ocean basin has been studied only by echo sounding, seismic profiling, and remote devices that have descended in excess of 11,000 m. Although oceanographers know considerably more about the deepocean basins than they did even a few years ago, many questions remain unanswered.
row. In contrast, the continental shelf of a passive continental
margin
is
much wider because
land-derived
sedimentary deposits build outward into the ocean.
^ THE DEEP-OCEAN BASIN Considering that the oceans are an average 3,865
Abyssal Plains Beyond the continental
rises of passive continental
gins are abyssal plains,
flat
of the sea floor. In
m deep,
most of the sea floor lies far below the depth of sunlight penetration, which is rarely more than 100 m. Accordingly, most of the sea floor is completely dark, no plant
some
flattest, flat
osition
areas they are interrupted by
km, but in general they are the most featureless areas on Earth (Fig. 12-14).
peaks rising more than
The
mar-
surfaces covering vast areas
topography
is
1
a consequence of sediment dep-
on the rugged topography of the oceanic
The Deep-Ocean Basin
crust.
325
60
Miles
"*** FIGURE 12-15 Seismic profile showing the burial of rugged sea-floor topography by sediments of the Northern Madeira Abyssal Plain.
Where sediment accumulates rugged sea floor
ment
in sufficient quantities, the
buried beneath thick layers of sedi-
is
Ocean basin
abyssal plains are covered with fine-grained sediment
derived mostly from the continents and deposited by
Some
turbidity currents.
of this sediment
meaning that
it
is
character-
was deposited
far
from
up to 25° sites
12-13). Oceanic trenches are also the
(Fig.
of the greatest oceanic depths; a depth of more than
11,000 m has been recorded in the Challenger Deep of Marianas Trench. Oceanic trenches show anomalously low heat flow
the
the land by the settling of fine particles suspended in
compared
seawater. Abyssal plains are invariably found adjacent
pears that the crust here
to the continental rises,
which are composed mostly of
overlapping submarine fans that
owe
their origin to dep-
Along active continental margins, sediments derived from the shelf and slope are trapped in an oceanic trench, and abyssal osition by turbidity currents (Fig. 12-11).
plains
fail
common Pacific
Pacific
of oce-
anic trenches, the continental slope descends at angles of
(Fig. 12-15).
Seismic profiles and sea-floor samples reveal that the
ized as pelagic,
common around the margins of the (Fig. 12-14). On the landward side
they are
to develop. Accordingly, abyssal plains are
in the Atlantic
Ocean basin
Ocean
basin, but rare in the
to the rest of the oceanic crust; thus, is
it
ap-
cooler and slightly denser
than elsewhere. Furthermore, gravity surveys reveal that trenches
show
a
huge negative gravity anomaly, indicatis held down and is not in isostatic
ing that the crust
equilibrium.
Seismic activity also occurs at or near
trenches. In fact, trenches are characterized by Benioff
zones in which earthquake foci become progressively deeper in a landward direction
(Fig. 10-8).
Most
of the
Earth's intermediate and deep earthquakes occur in such
(Fig. 12-14).
zones. Finally, oceanic trenches are associated with vol-
canoes, either as an arcuate chain of volcanic islands
Oceanic Trenches
(island arc) or as a chain of volcanoes
Although oceanic trenches constitute a small percentage
arc) adjacent to a trench
of the sea floor, they are very important, for
as in western South
it is
consumed by subduction Oceanic trenches are long, narrow
here
that lithospheric plates are
(see
Chapter
fea-
13).
tures* restricted to active continental margins; thus,
326
Chapter 12
The Sea Floor
km
long,
America
(Fig.
12-13).
Oceanic Ridges
A feature called "The Peru-Chile Trench west of South America is 5,900 but only 100 km wide. It is more than 8,000 m deep.
on land (volcanic
along the margin of a continent
the Atlantic
tury
when
the Telegraph Plateau
Ocean basin during
the
first
was discovered
in
the late nineteenth cen-
submarine cable was
laid
between
North America and Europe. Following the 1925-1927 voyage of the German research vessel Meteor, scientists proposed that this plateau was actually a continuous feature extending the length of the Atlantic Ocean basin (see Perspective 12-2). Subsequent investigations revealed that this proposal this feature the
was
correct,
Mid-Atlantic Ridge
and we now
(Fig.
call
rises
about 2.5
is more than 2,000 km wide km above the sea floor adjacent to
terminate where they are offset along major fractures oriented
more or
less at right angles to ridge
much
submarine 65,000 km long. The oceanic ridge system runs from the Arctic Ocean through the middle of the Atlantic, curves around South Africa, and passes into the Indian Ocean, continuing
mountainous topography
from there into the
larger system of
at least
Pacific
Ocean basin
(Fig.
12-14).
This oceanic ridge system's length surpasses that of the
mountain range on land. However, the latter composed of granitic and metamorphic rocks and sedimentary rocks that have been folded and fractured by compressional forces. The oceanic ridges, on the other hand, are composed of volcanic rocks (mostly basalt) and have features produced by tenlargest
ranges are typically
sional forces.
ologists are convinced that
some geologic
Where
these fractures offset oceanic ridges, they are
characterized by shallow seismic activity only in the area
between the displaced ridge segments
earthquakes, basaltic volcanism, and high heat flow. Direct observation of the ridges and their
rift
valleys
began in 1974. As a part of Project FAMOUS (FrenchAmerican Mid-Ocean Undersea Study), submersible craft descended into the rift of the Mid-Atlantic Ridge,
and more recent dives have investigated other rifts. Although no active volcanism was observed, the researchers did see pillow lavas (Fig. 4-14), lava tubes, and sheet lava flows, some of which appear to have formed very recently. In addition, hydrothermal vents such as black smokers have been observed (see the Prologue).
Profile across the
well-developed central
Continental Slope Rise
Fur-
adjacent to them, the offset segments yield vertical relief
on the sea floor. For example, nearly vertical escarpments 3 or 4 km high develop, as illustrated in Figure 12-17. We will have more to say about such fractures, called transform faults, in Chapter 13.
Seamounts, Guyots, and Aseismic Ridges
large
Chapter 13); ridges are characterized by shallow-focus
(Fig. 12-17).
thermore, because ridges are higher than the sea floor
Rise lack such a feature. These rifts are commonly one to two kilometers deep and several kilometers wide. Such rifts open as sea-floor spreading occurs (discussed in
12-16
on
sion of such fractures into continents.
plain, except for the abyssal plains,
its
ge-
the continents can best be accounted for by the exten-
As noted
FIGURE
Many
features
they are buried beneath sea-floor sediments.
forces (Fig. 12-16), although portions of the East Pacific
Ridge with
(Fig.
kilometers, although they are difficult to trace where
Running along the crests of some ridges is a rift that appears to have opened up in response to tensional
"**
axes
it.
part of a
It is, in fact,
Oceanic ridges are not continuous features winding without interruption around the globe. They abruptly
12-17). Such large-scale fractures run for hundreds of
12-14).
The Mid-Atlantic Ridge and
Fractures in the Sea Floor
previously, the sea floor
underlain by rugged topography
number of volcanic
is
not a
flat,
featureless
and even these are
(Fig.
12-15). In fact, a
seamounts, and guyots
hills,
above the sea floor. Such features are present in all ocean basins, but are particularly abundant in the Pacific. All are of volcanic origin and differ from one another mostly in size. Seamounts rise more than one kilometer rise
above the sea
floor;
if
they are
flat
guyots rather than seamounts
topped, they are called
(Fig. 12-18).
volcanoes that originally extended above sea
Guyots are level.
How-
upon which they were situated continued to grow, they were carried away from a spreading ridge, and the oceanic crust cooled and descended to greater oceanic depths. Thus, what was once an island slowly sank beneath the sea, where it was eroded by ever, as the plate
waves, giving
North Atlantic Ocean showing
it
the typical flat-topped appearance.
the Mid-Atlantic
rift.
Shelf
Bermuda
Mid-Atlantic Ridge
Is.
1
1
1,000
itmm+Mmm
UMte
1.500
The Deep-Ocean Basin
327
"^ FIGURE
12-17
Fractures in the sea floor of the Atlantic
line indicates the crest
of the Mid-Atlantic Ridge.
The
inset
is
basin. The dark diagrammatic view of a
Ocean a
fracture offsetting a ridge. Earthquakes occur only in the segments between offset ridge crests.
Other volcanic features are also known to exist on most of these are much smaller than seamounts, but probably originated in the same way. These so-called abyssal hills average only about 250 m high. the sea floor;
328
Chapter 12
The Sea Floor
They
are
common on
the sea floor
and underlie thick
sediments on the abyssal plains.
Other
common
linear ridges
features in the ocean basins are long, and broad plateaulike features rising as
— FIGURE
Submarine up above sea level to form seamounts. As the plate upon which these volcanoes rest moves away from a spreading volcanoes
12-18
may
build
ridge, the volcanoes sink
sea level
much
km
as 2 to 3
They are known seismic activity.
A
above the surrounding sea
floor.
as aseismic ridges because they lack
few of these ridges are thought to be
small fragments separated from continents during ing.
rift-
Such fragments, referred to as microcontinents, are
"^ FIGURE
12-19
Map
represented by such features as the Jan the
North Atlantic
Most
(Fig.
Mayen Ridge
in
12-19).
aseismic ridges form as a linear succession of
hot spot volcanoes. These
may
develop at or near an
oceanic ridge, but each volcano so formed
showing the locations of some of the aseismic
beneath
and become guyots.
is
carried
ridges.
^75
|
Aseismic ridge
Oceanic ridge system
Oceanic trench
The Deep-Ocean Basin
329
Perspective 12-2
MAURICE EWING AND HIS INVESTIGATION OF THE ATLANTIC OCEAN In 1935,
when Maurice Ewing began
his studies of the
continental shelf off Norfolk, Virginia,
known about
little
was
itself
the deep-sea floor. Ewing's analysis of
seismic evidence had indicated that the continental shelf
is
covered by a thin layer of sediments, but the floor
composed of sediment as much as 4,000 m had been deposited on ocean-floor bedrock.
was of
geologically recent origin.
led two more expeditions to the Mid- Atlantic Ridge, and in 1949 he founded the Columbia Lamont Geologic Observatory, whose main In 1948,
Ewing
studying the ocean
thick that
mission
Since these thick sediments probably contained
discovered that the oceanic crust
hydrocarbons, he tried to interest oil companies in supporting further studies of the continental shelf. was told that oil was so easily found on land that
was no reason
there
to look for
it
under the
is
sea.
Undiscouraged, he pursed his ocean-floor research and made many important discoveries. In 1947, the National Geographic Society commissioned Ewing to explore the little-known Mid-Atlantic Ridge and the adjacent sea floor. Using seismic and echo-sounding techniques as well as equipment for sampling seawater, he determined water temperature at various depths and sampled the sea floor itself. His initial samples and seismic investigations produced surprising results. The data
km
thick,
much
thinner than continental crust.
During the early 1950s, Ewing decided to transfer all of the available seismic profiles of the North Atlantic Ocean floor onto a topographic map. He assigned the job to Bruce Heezen, a graduate student who enlisted the help of Marie Tharp, a cartographer (mapmaker) at the observatory. As the profiles were converted into a map, both Heezen and Tharp were surprised to see a deep canyon (or rift valley) running
down
the center of the Mid-Atlantic Ridge. Initially,
they did not believe that such a large-scale so Heezen and
Ewing began
What emerged was
200
million years of
deposition. Furthermore, dredging across the slopes of
the Mid-Atlantic Ridge brought up pieces of pillow lava (see Fig. 4-14).
Not only was
the ocean floor
rift
existed,
plotting the locations of
mid-ocean earthquakes for which they had data. a band of earthquakes running
all
sediment that had accumulated for billions of years, the sediments were only several hundred meters thick to
Early on, he
composed of
he determined that the oceanic crust
indicated that rather than a thick layer of sea-floor
and represented 100
is
sunken continental material. Furthermore, is only 5 to 10
basalt, not
He
floor.
through not only the middle of the
rift
valley
mapped
by Tharp, but through all the world's oceans. In 1959 Ewing, Heezen, and Tharp published a spectacular three-dimensional map of the North Atlantic Ocean. The
map showed
vast plains
and conical
with the plate upon which it originated. The net such activity is a sequence of seamounts/guyots extending from an oceanic ridge (Fig. 12-18); the Walvis
coarse-grained sediment (sand and gravel) far from land.
Ridge in the South Atlantic is a good example (Fig. 1219). Aseismic ridges also form over hot spots unrelated
the ocean basins, but only trivial
laterally
result of
to ridges.
formed
in
The Hawaiian-Emperor chain such a manner (Fig. 12-19).
in the Pacific
Coarse sediment
in icebergs
Deep-sea sediments consist mostly of fine-grained deposits because few mechanisms exist that can transport
330
Chapter 12
The Sea Floor
its
amounts are
way
into
actually
transported by such processes.
Most of the fine-grained sediment in the deep sea is windblown dust and volcanic ash from the continents and oceanic islands and the
^ DEEP-SEA SEDIMENTATION
or trapped in floating veg-
etation, such as the roots of a tree, can find
isms that
live in the
shells of
microscopic organ-
near-surface waters of the oceans.
Other sources of sediment include cosmic dust and defrom chemical reactions in seawater. The manganese nodules that are fairly common in all the posits resulting
*" FIGURE 1 This map of the sea floor resulted from the work of Maurice Ewing, Bruce Heezen, and Marie Tharp.
seamounts, as well as the Mid-Atlantic Ridge with mysterious
still
rift
valley (Fig. 1).
As more of
its
the
world's ocean floors were explored, this original regional
map was expanded
km
to reveal a
long winding through
The recognition of
all
mountain chain 65,000
the world's oceans.
a curving ridge located
midway
ocean basins are a good example of the latter (Fig. 1220). These nodules are composed mostly of manganese and iron oxides, but also contain copper, nickel, and cobalt.
Such nodules may be an important source of
between and parallel to the coasts of South America and Africa forced geologists to reexamine their theories about the Earth. The realization that new crust was forming along the rift valley of the Mid-Atlantic Ridge hastened the acceptance of sea-floor spreading
and plate tectonic theory.
The bulk of the sediments on the deep-sea floor meaning that they settled from suspension
pelagic,
from land.
Two
ognized: pelagic clay and ooze
(Fig.
12-21). Pelagic clay
covers most of the deeper parts of the ocean basins.
interested in this potential resource.
sized particles derived
is
The contribution of cosmic dust negligible. Even though some
to deep-sea sediment
researchers estimate
360,000 metric tons of cosmic dust may fall to Earth each year, this is a trivial quantity compared to the volume of sediments derived from other sources.
that as
much
as
far
categories of pelagic sediment are rec-
some metals in the future; the United States, which imports most of its manganese and cobalt, is particularly
generally
are
brown or reddish and
is
composed of
It is
clay-
from the continents and oceanic Ooze, on the other hand, is composed mostly of shells of microscopic marine animals and plants. It is characterized as calcareous ooze if it contains mostly calcium carbonate (CaC0 3 skeletons of tiny marine organisms such as foraminifera (see Perspective 9-1, Fig. islands.
)
Deep-Sea Sedimentation
331
^ REEFS Reefs are moundlike, wave-resistant structures composed of the skeletons of organisms are called coral reefs, but
(Fig. 12-22).
many
Commonly they
other organisms in addi-
make up reefs. A reef consists of a solid framework of skeletons of corals, clams, and such encrusting organisms as algae and sponges. Reefs grow to a depth of about 45 or 50 m and are restricted to shallow tropical seas where the water is clear, and the temperature does not fall below about 20°C. Three types of reefs are recognized: fringing, barrier, and atoll (Fig. 12-23). Fringing reefs are solidly attached to the margins of an island or continent. They have a rough, tablelike surface, are as much as one kilometer wide, and, on their seaward side, slope steeply down to tion to corals
-»-
FIGURE
12-20
Manganese nodules on
the sea floor
south of Australia.
the sea floor. Barrier reefs are similar to fringing reefs,
except that they are separated from the mainland by a lagoon. Probably the best-known barrier reef in the 2). Siliceous
ooze
composed of the
is
silica
world
(Si0 2 ) skel-
is the Great Barrier Reef of Australia. It is more than 2,000 km long and is separated from the continent by a wide lagoon (Fig. 12-24).
etons of such single-celled organisms as radiolarians (animals) and diatoms (plants) (Fig. 7-16).
""»'
FIGURE
The
12-21
Calcareous ooze
distribution of sediments
Siliceous
|
332
Chapter 12
The Sea Floor
ooze
on the deep-sea
~~\
floor.
Pelagic clay
"•'
FIGURE
12-22
Reefs such as this one fringing an island in the Pacific are composed of the skeletons of organisms.
wave-resistant structures
The
last
type of reef
is
an
atoll,
which
is
shallow water. However, the island eventually subsides
a circular to
oval reef surrounding a lagoon (Fig. 12-23). Such reefs
below sea
form around volcanic islands that subside below sea level as the plate upon which they rest is carried progressively farther from an oceanic ridge (Fig. 12-18). As subsidence occurs, the reef organisms construct the reef
a more-or-less
upward so
"^"
FIGURE
a lagoon.
12-23
Three-stage development of an
As the island disappears beneath the
atoll. In
continuous reef
common in Many of
are particularly
basin (Fig. 12-25). reefs,
that the living part of the reef remains in
reef forms, but as the island sinks, a barrier reef
lagoon surrounded by 12-23). Such reefs the western Pacific Ocean
level, leaving a circular
first
these began as fringing
but as subsidence occurred, they evolved
barrier reefs
the
(Fig.
and
first
to
finally to atolls.
stage, a fringing
becomes separated from the
island by
sea, the barrier reef continues to
grow
upward, thus forming an atoll. An oceanic island carried into deeper water by plate movement can account for this sequence. Fringing reef
Barrier reef
Atoll
Reefs
333
FIGURE
Deep-sea
View of an
12-25
drill
atoll in the Pacific
Ocean.
holes have penetrated through the upper
oceanic crust into a sheeted dike complex, a zone consisting
26).
almost entirely of vertical basaltic dikes
What
lies
below
this sheeted dike
been sampled. Even though the oceanic crust
is
(Fig.
12-
complex has not
km thick and
5 to 10
can be penetrated only about 1 km by drill holes, geologists have a good idea of the composition of the entire
As mentioned previously, oceanic crust is continconsumed at subduction zones, but a tiny amount of this crust is not subducted. Rather it is emplaced in mountain ranges on continents, where it usually arrives by moving along large fractures called thrust faults (thrust faults and mountain building are discussed more fully in Chapter 14). Such slivers of oceanic crust and upper mantle now on continents are called ophiolites (Fig. 12-26). They are crust.
uously
"*"
FIGURE
of Australia.
12-24 It is
Aerial view of the Great Barrier Reef
more than 2,000
from the continent
km
long and separated
the background) by a wide lagoon.
(in
structurally complex, but detailed studies reveal that an ideal ophiolite consists of a layer of deep-sea sedimen-
tary rocks underlain by pillow basalts
This particular scenario for the evolution of reefs from
and a sheeted dike
fringing to barrier to atoll
complex, the same layers as in deep-sea cores. Further downward in an ophiolite is massive gabbro, and below
years ago by Charles
that
naturalist
on
the
has revealed that
was proposed more than 150 Darwin while he was serving as a ship H.M.S. Beagle. Drilling into atolls they do indeed rest upon a basement of
volcanic rocks, thus confirming Darwin's hypothesis.
^ COMPOSITION OF THE OCEANIC CRUST Sampling and direct observations of the oceanic ridges
pillow lavas
334
Much
(Fig. 4-14),
Chapter 12
of this basalt
may
comthe form of
is
in
represent
magma chamber
magma (Fig.
that
12-26).
Beneath the gabbro is peridotite— sometimes altered by metamorphism to assemblages containing serpentine— that probably represents the upper mantle. Thus, a complete ophiolite consists of deep-sea sedimentary rocks, (Fig. 12-26).
^ RESOURCES FROM THE SEA
is
but sheet flows are also present.
The Sea Floor
layered gabbro that
oceanic crust, and upper mantle
reveal that the upper part of the oceanic crust
posed of basalt.
is
cooled at the top of a
Seawater contains
many
which are extracted
elements in solution, some of
for various industrial
and domestic
Oceanic ridge
"•»•
FIGURE
12-26
New
oceanic
Layered
crust consisting of the layers
gabbro
here forms as
Pendotite
Upper mantle
magma
shown
beneath oceanic ridges. The composition of the oceanic crust is known from ophiolites, sequences of rock on land consisting of deep-sea sediments, oceanic crust, and upper rises
mantle.
uses. For
ble salt)
in many places sodium chloride (taproduced by the evaporation of seawater, and
example,
is
a large proportion of the world's
magnesium
is
^ FIGURE
12-27
120°E
extracted from seawater, but for
many, such as gold, the cost
pro-
duced from seawater. Numerous other elements and
to the United States
compounds can be
is
prohibitive.
on the becoming
In addition to substances in seawater, deposits
sea floor or within sea-floor sediments are
The Exclusive Economic Zone (EEZ) includes and its possessions.
a vast area adjacent
150°E
Resources from the Sea
335
"^ FIGURE
12-28
Exclusive Economic
Sedimentary basins within the
Zone
in
which known or potential
reserves of hydrocarbons occur.
336
Chapter 12
The Sea Floor
increasingly
sources
lie
important.
Many
of these potential re-
well beyond the margins of the continents, so
the ownership of such resources is a political and legal problem that has not yet been resolved. Most nations bordering the ocean claim those resources occurring
The United
within their adjacent continental margin.
example, by a presidential proclamation issued on March 10, 1983, claims sovereign rights over an area designated as the Exclusive Economic Zone (EEZ). States, for
The EEZ extends seaward 200 nautical miles (371 km) from the coast, giving the United States jurisdiction over an area about 1.7 times larger than its land area (Fig. 12-27).* Also included within the EEZ are the areas adjacent to U.S. territories, such as Guam, American
Samoa, Wake
and Puerto Rico (Fig. 12-27). In huge area of the sea floor and any resources on or beneath it. Numerous resources occur within the EEZ, some of which have been exploited for many years. For example, sand and gravel for construction are mined from the continental shelf in several areas. About 17% of U.S. oil and natural gas production comes from wells on the continental shelf. Some 30 sedimentary basins occur within the EEZ, several of which are known to contain hydrocarbons whereas others are areas of potential hydrocarbon production (Fig. 12-28). Ancient shelf deposits in the Persian Gulf region contain the world's largest Island,
short, the United States claims a
CALIFORNIA
Mendocino
fracture
zone
~^~
FIGURE 12-29 Massive sulfide deposits formed by submarine hydrothermal activity have been identified on the Gorda Ridge within the Exclusive Economic Zone.
reserves of oil (see Perspective 7-2).
Other resources of
interest include the massive sulfide
deposits that form by submarine hydrothermal activity
spreading ridges (see the Prologue). Such deposits containing iron, copper, zinc, and other metals have at
EEZ at the Gorda and Oregon; similar deposits the Juan de Fuca Ridge within the Canadian
Ridge off
been identified within the the coasts of California
occur at
EEZ
(Fig.
12-29).
Other potential resources nodules discussed previously
manganese 12-20), and metallif-
include the
(Fig.
erous oxide crusts found on seamounts. Manganese nodules contain manganese, cobalt, nickel, and copper; the United States first
also claim sovereign rights to resources
heavily dependent
on imports of
the
EEZ, however, manganese nodules occur near Johnston Island in the Pacific Ocean and on the Blake Plateau off the east coast of South Carolina and Georgia. In addition,
EEZ
seamounts and seamount chains within the
the Pacific are
*A number of other nations
is
three of these elements (see Fig. 3-25). Within the
known
in
to have metalliferous oxide crusts
several centimeters thick
from which cobalt and man-
ganese could be mined.
within 200 nautical miles of their coasts.
J3K>^^*:^--«^«£^g3^^
Ti
Continental margins separate the continents above sea level from the deep ocean basin. They consist of
^ CHAPTER SUMMARY 1.
Scientific investigations of the
oceans began during
equipped to investigate the sea floor by sounding, and seismic profiling.
drilling,
a continental shelf, continental slope,
cases a continental
the late 1700s. Present-day research vessels are
echo
and
in
some
rise.
Continental shelves slope gently in a seaward direction and vary in width from a few tens of
Chapter Summary
337
4.
meters to more than 1,000 km. The continental slope begins at an average depth of 135 m where the inclination of the sea floor increases rather abruptly
from
less
15.
The United
States has claimed rights to all resources occurring within 200 nautical miles (371 km) of its shorelines. Numerous resources including various
metals occur within this Exclusive Economic Zone.
than 1° to several
degrees. 5.
Submarine canyons are characteristic of the some of them extend well up onto the shelf and lie offshore from large streams. Stream erosion of the shelf during the Pleistocene Epoch may account for some submarine canyons, but many have no association with streams on land and were probably eroded by turbidity currents. Turbidity currents commonly move through submarine canyons and deposit an overlapping series of submarine fans that constitutes a large part of the
IMPORTANT TERMS
continental slope, but
6.
continental 7.
rise.
Active continental margins are characterized by a
narrow
and a slope that descends directly into an oceanic trench with no rise present. Such margins are also characterized by seismic activity and shelf
volcanism. 8.
Passive continental margins lack volcanism exhibit
little
seismic activity.
The
and
active continential
aseismic ridges are oriented more-or-less
continental margin
margin
continental rise
pelagic clay
continental shelf
reef
continental slope
seamount
echo sounder
seismic profiling
Exclusive Economic
Zone
guyot
submarine canyon submarine fan
oceanic ridge
turbidity current
oceanic trench
^ REVIEW QUESTIONS 1.
2.
Much
of the continental rise
a.
calcareous ooze; b.
c.
fringing reefs; d.
e.
ophiolite.
The
sheeted dikes;
greatest oceanic depths occur at:
shelf-slope break; d.
guyots;
Abyssal plains are most
common:
a.
around the margins of the Atlantic;
b.
adjacent to the East Pacific Rise;
in the rift
valley of the Mid-Atlantic Ridge;
on
4.
A
circular reef enclosing a lagoon
a.
barrier reef; b.
ridge; d. 5.
guyot;
e.
continental slopes; d. fractures in the sea floor.
Deep-sea drilling and the study of fragments of sea floor in mountain ranges on land reveal that the oceanic crust is composed in descending order of pillow lava, sheeted dikes, and gabbro.
Chapter 12
The Sea Floor
7.
c.
aseismic
Submarine canyons are most characteristic of
e.
composed of
a(n):
atoll.
c.
6.
is
seamount;
Deep-sea sediments consist mostly of fine-grained particles derived from continents and oceanic islands and the microscopic shells of organisms. The primary types of deep-sea sediments are pelagic clay
atoll.
e.
continental shelves.
chain of seamounts and/or guyots.
and
along
c.
the west coast of South America; d.
continental shelves; b.
Reefs are wave-resistant structures
the
c.
oceanic trenches;
passive continental margins.
e.
3.
of:
submarine fans;
aseismic ridges; b.
a
composed
is
a.
reefs are recognized: fringing, barrier,
338
passive continental
perpendicular to oceanic ridges and consist of a
animal skeletons, particularly corals. Three types of 14
ophiolite
aseismic ridge
and ooze. 13
margin
continental shelf
along such margins is broad, and the slope merges with a continental rise. Abyssal plains are commonly present seaward beyond the rise. 9. Oceanic trenches are long, narrow features where oceanic crust is subducted. They are characterized by low heat flow, negative gravity anomalies, and the greatest oceanic depths. 10. Oceanic ridges consisting of mountainous topography are composed of volcanic rocks, and many ridges possess a large rift caused by tensional forces. Basaltic volcanism and shallow-focus earthquakes occur at ridges. Oceanic ridges nearly encircle the globe, but they are interrupted and offset by large fractures in the sea floor. 11. Other important features on the sea floor include seamounts that rise more than a kilometer high and guyots, which are flat-topped seamounts. Many
12
ooze
abyssal plain
the:
abyssal plains; rift
valleys;
The
Earth's surface waters probably originated through the process of: a. dewatering; b. subduction; c.
outgassing; d.
e.
erosion.
crustal fracturing;
Continental shelves: a.
are
composed of
pelagic sediments; b.
lie
between continental slopes and rises; c. descend slope gently to an average depth of 1,500 m; d. from the shoreline to the shelf-slope break; e.
are widest along active continental margins.
8.
9.
The
flattest,
most
c.
continental slopes; d.
e
continental margins. settles
the:
b.
aseismic ridges;
from suspension pelagic;
a.
abyssal; b.
d.
generally coarse grained;
far
from land
volcanic;
c.
a
is
correct?
most of the continental margins around the oceanic ridges are
Atlantic are passive; b.
c.
Summarize the evidence indicating that turbidity currents transport sediment from the continental shelf onto the slope and rise. 21. Where do abyssal plains most commonly develop? Describe their compositon. 22.
the following statements
composed
others.
characterized
e.
by graded bedding.
Which of
largely of
deformed sedimentary rocks;
the deposits of turbidity currents consist of
What
the significance of oceanic trenches,
is
where are they found? 23. How do mid-oceanic ridges ranges on land?
how
24. Describe
differ
their relative importance.
intermediate and deep earthquakes occur at or near oceanic crust is thicker than oceanic ridges; e.
26. Describe the sequence of events leading to the origin
continental crust.
27. Illustrate and label an ideal sequence of rocks in an
of an
atoll.
Massive
28.
12.
as on passive continental margins; b. accumulations of microscopic shells on the sea floor; by precipitation of minerals near c. from sediments derived hydrothermal vents; d. in oceanic trenches. from continents; e. The most useful method of determining the structure
of the oceanic crust beneath continental shelf
Anderson, R. N. 1986. Marine geology.
sulfide deposits form:
ophiolite.
a.
sediments a.
d.
echo sounding;
observations from
b.
What
seismic profiling;
is
25°; b 40°.
e.
How
4°;
c.
rise.
d
0.1°;
is
a characteristic of: turbidity current
pelagic clay; d.
siliceous ooze;
manganese nodules. do sulfide mineral deposits form on the sea
floor?
17.
What
is
an echo sounder, and
how
is it
used to
study the sea floor? 18.
What
are the characteristics of a passive continental
margin?
How
Economic Zone? What types
^
it?
ADDITIONAL READINGS New
York: John Wiley
Bishop,
J.
M.
1984. Applied oceanography.
An
New
York: John
introduction to the
marine environment. Dubuque, Iowa: W. C. Brown. J. M., and K. Von Damm. 1983. Hot springs on the ocean floor. Scientific American 248, no. 4: 78-93. Gass, I. G. 1982. Ophiolites. Scientific American 247, no. 2:
Edmond,
122-31. Kennett,
J.
R
1982. Marine geology. Englewood
Cliffs, N.J.:
Prentice-Hall. reefs, seamounts, and guyots. Sea 143-49. Pinet, P. 1992. Oceanography: An introduction to the planet oceanus. St. Paul, Minn.: West Publishing Co. Rona, P. A. 1986. Mineral deposits from sea-floor hot springs. Scientific American 254, no. 1: 84-93. Ross, D. A. 1988. Introduction to oceanography. Englewood
Mark, K. 1976. Coral Frontiers 22, no. 3:
continental shelves; b.
deposits;
16.
1°; c
Graded bedding a.
continental
the average slope of the continental slope?
a
the Exclusive
Davis, R. A. 1987. Oceanography:
underwater
e.
volcanic arc; e
e
is
of metal deposits occur within
Wiley &c Sons.
dredging;
c.
photography. 13. Which of the following is not characteristic of an active continental margin? oceanic earthquakes; c. volcanism; b. a. trench; d.
What
8c Sons.
is:
submersible research vessels;
15.
from mountain
an aseismic ridge forms.
11.
14.
and
25. List four sources of deep-sea sediments, and explain
most of the Earth's
calcareous ooze; d.
rise
20.
is:
10.
and explain why a occurs at some continental margins and not at
19. Describe the continental rise,
abyssal plains;
oceanic ridges;
Sediment that
on Earth are
featureless areas
a.
Cliffs, N.J.: Prentice-Hall.
Thurman, H. V. 1988. Introductory oceanography. 5th ed. Columbus, Ohio: Merrill Publishing Co. Tolmazin, D. 1985. Elements of dynamic oceanography. Boston, Mass.: Allen & Unwin.
does such a continental margin
originate?
Additional Readings
339
CHAPTER
13
PLATE TECTONICS: A Unifying Theory OUTLINE PROLOGUE INTRODUCTION EARLY IDEAS ABOUT CONTINENTAL DRIFT
ALFRED WEGENER AND THE CONTINENTAL DRIFT HYPOTHESIS THE EVIDENCE FOR CONTINENTAL DRIFT Continental Fit Similarity of
Rock Sequences and Mountain
Ranges Glacial Evidence Fossil
Evidence
PALEOMAGNETISM AND POLAR
WANDERING SEA-FLOOR SPREADING "^
Perspective 13-1: Paleogeographic
Maps
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading
PLATE TECTONIC THEORY PLATE BOUNDARIES Divergent Boundaries
"*
Perspective 13-2: Tectonics of the Terrestrial Planets
Convergent Boundaries
"^ Guest
Essay: Geoscience Careers— The
Diversity
Is
Unparalleled
Transform Boundaries
PLATE
MOVEMENT AND MOTION
Hot Spots and Absolute Motion
THE DRIVING MECHANISM OF PLATE TECTONICS PLATE TECTONICS AND THE DISTRIBUTION OF NATURAL
RESOURCES CHAPTER SUMMARY Vertical
view of the Himalayas, the youngest
and highest mountain system in the world. The Himalayas began forming when India collided with Asia 40 to 50 million years ago.
PROLOGUE
Both of these events occurred along the eastern portion of the Ring of Fire, a chain of intense seismic
and volcanic
activity that encircles the Pacific
basin (Fig. 13-1).
Two
tragic events that occurred
Ocean
of the world's greatest
disasters occur along this ring because of volcanism
during 1985 serve to remind us of the dangers of living near a convergent plate margin. September 19, a magnitude 8.1 earthquake killed
Some
On
and earthquakes generated by plate convergence. For example, the 1989 volcanic eruptions in Alaska, the
1980 eruption of Mount
St.
Helens, and the 1970
more than 9,000 people in Mexico City. Two months later and 3,200 km to the south, a minor eruption of Colombia's Nevado del Ruiz volcano partially melted its summit glacial ice, causing a mudflow that engulfed Armero and several other villages and killed more than 23,000 people. These two tragedies resulted in more than 32,000 deaths, tens of thousands of injuries, and billions of dollars in
earthquake that killed 66,000 people in Peru all occurred as a consequence of plate convergence. Although earthquakes and volcanic eruptions are very different geologic phenomena, both are related to the activities occurring at convergent plate margins. The Mexico City earthquake resulted from subduction of the Cocos plate at the Middle America Trench (Fig. 13-1). Sudden movement of the Cocos plate beneath
property damage.
Central America generated seismic waves that traveled
*•'
FIGURE
13-1
The Ring of
convergence as illustrated
Fire
is
a zone of intense earthquake
Ocean basin. Most of by the two insets.
activity that encircles the Pacific
and volcanic from plate
this activity results
Mexico City
Volcanoes
Earthquakes
Prologue
341
the mountain; the meltwater rushed
down
mixed with the sediment, and turned
it
the valleys,
into a deadly
viscous mudflow.
The
city
of Armero, Colombia,
lies in
the valley of
the Lagunilla River, one of several river valleys inun-
dated by mudflows. Twenty thousand of the city's 23,000 inhabitants died, and most of the city was destroyed (Fig. 13-2). Another 3,000 people were killed in nearby valleys. A geologic hazard assessment study completed one month before the eruption showed that
Armero was in a high-hazard mudflow area! These two examples vividly illustrate some
of the
dangers of living in proximity to a convergent plate
boundary. Subduction of one plate beneath another "•'
FIGURE
The 1985 eruption of Nevado del Ruiz in Colombia melted some of its glacial ice. The meltwater mixed with sediments and formed a huge mudflow that destroyed the city of Armero and killed 20,000 of its 13-2
inhabitants.
outward
in all directions.
The
violent shaking
experienced in Mexico City, 350
km
away, and
elsewhere was caused by these seismic waves.
The
strata underlying
Mexico City
consist of
unconsolidated sediment deposited in a large ancient lake.
Such sediment amplifies the shaking during
earthquakes with the unfortunate consequence that buildings constructed there are heavily
damaged than those
commonly more on
built
solid
bedrock
(see Perspective 10-1, Fig. 5).
Less than
two months
Mexico City
after the
earthquake, Colombia experienced
recorded natural disaster.
Nevado
several active volcanoes resulting
magma
Nevado
^
from the
(Fig. 13-1).
A
is
is
one of
rise
of
subducted
minor eruption on
del Ruiz partially melted the glacial ice
felt far
from
their epicenters.
Since 1900, earthquakes have killed
more than
112,000 people in Central and South America alone. While volcanic eruptions in this region have not caused nearly as many casualties as earthquakes, they have, nevertheless, caused tremendous property damage and have the potential for triggering devastating events such as the 1985 Colombian mudflow. Because the Ring of Fire is home to millions of people, can anything be done to decrease the devastation that inevitably results from the earthquake and volcanic activity occurring in that region? Given our present state of knowledge, most of the disasters could not have been accurately predicted, but better planning and advance preparations by the nations bordering the Ring of Fire could have prevented much life. As long as people live near convergent plate margins, there will continue to be
disasters.
However, by studying and understanding
geologic activity along convergent as well as divergent
and transform plate margins, geologists can help minimize the destruction.
tion
that the Earth's geography has changed
and distribution of many important natural
sources,
now
continuously through time has led to a revolution in the
boundaries, and geologists are
tectonic theory into their prospecting efforts.
the way they view the Earth. Although many people have only a vague notion of what plate tectonic theory
continents, ocean basins,
profound effect on all of our lives. It is now realized that most earthquakes and volcanic eruptions occur near plate margins and are not plate tectonics has a
342
Chapter 13
Plate Tectonics:
A
Unifying Theory
re-
such as metallic ores, are related to plate
geological sciences, forcing geologists to greatly modify
is,
to
merely random occurrences. Furthermore, the forma-
INTRODUCTION
The recognition
which are frequently
tragic loss of
greatest
generated where the Nazca plate
beneath South America of
its
del Ruiz
repeatedly triggers large earthquakes, the effects of
The movement of in turn affects the
incorporating plate
plates determines the location of
and mountain systems, which
atmospheric and oceanic circulation
patterns that ultimately determine global climates. Plate
movements have
also profoundly influenced the geo-
graphic distribution, evolution, and extinction of plants
During the ologist
and animals. Since at least the early 1900s, abundant evidence has
late nineteenth century, the
Edward Suess noted
Late Paleozoic plant
fossils
Austrian ge-
the similarities between the
of India, Australia, Africa,
moving through-
Antarctica, and South America as well as evidence of
out geologic time. Nevertheless, most geologists rejected
glaciation in the rock sequences of these southern con-
was no suitable mechanism to explain such movement. By the early 1970s, however, studies of the Earth's magnetic field, its interior, and the ocean basins (see Chapters 11 and 12) convinced most
tinents. In
geologists that continents are parts of plates that are
where, along with evidence of extensive glaciation,
indicated that the continents have been
the idea because there
moving
in
response to some type of heat transfer system
Plate tectonic theory geologists,
and
is
many
as
we
will use here) for a supercontinent
composed of these southern landmasses. The name came from Gondwana, a province in east-central India abundant
fossils
of the Glossopteris flora occur (Fig.
its
and
now almost universally accepted application has led to a greater
understanding of how the Earth has evolved and continues to do so. This powerful, unifying theory accounts for apparently unrelated geologic events, allowing geol-
view such phenomena as part of a continuing
ogists to
1885 he proposed the name Gondwanaland
Gondwana
13-3). Suess believed the distribution of plant fossils
within the Earth.
among
(or
story rather than as a series of isolated incidents.
Before discussing plate tectonic theory, the various hypotheses that preceded
it
we will
review
"•" FIGURE 13-3 Representative members of the Glossopteris flora. Fossils of these plants are found on all five of the Gondwana continents. Glossopteris leaves from (a) the Upper Permian Dunedoo Formation and (b) the Upper Permian Illawarra Coal Measures, Australia. (Photos courtesy of Patricia G. Gensel, University of North
Carolina.)
and examine the
some people to accept the idea of conmovement and others to reject it. Because plate
evidence that led tinental
quiries
from numerous scientific inand observations, only the more important ones
will be
covered
tectonic theory has evolved
in this chapter.
^ EARLY IDEAS ABOUT CONTINENTAL DRIFT The
idea that the Earth's geography
the past
is
was
different during
not new. During the fifteenth century, Leon-
ardo da Vinci observed that "above the plains of Italy where flocks of birds are flying today fishes were once moving in large schools." In 1620, Sir Francis Bacon commented on the similarity of the shorelines of western Africa and eastern South America but did not make the connection that the Old and New Worlds might once have been sutured together. Alexander von Humboldt made the same observation in 1801, although he attributed these similarities to erosion rather than the splitting apart of a larger continent.
One
of the earliest specific references to continental
drift is in
and
Its
that
all
Antonio
Snider-Pellegrini's
1858 book Creation
Mysteries Revealed. Snider-Pellegrini suggested
of the continents were linked together during the
Pennsylvanian Period and later conclusions
on
split apart.
He
based his
the similarities between plant fossils in the
Pennsylvanian-aged coal beds of Europe and North America.
However, he thought that continental separation was
a consequence of the biblical deluge.
Early Ideas About Continental Drift
343
was a consequence of
glacial deposits
extensive land
bridges that once connected the continents
and
later
sank beneath the ocean.
One
of the
continental
first
Frank
B. Taylor
ing his
own
who
propose a mechanism for
in
the American geologist 1910 published a paper present-
theory of continental
the formation of eral
to actually
movement was
drift. In it
mountain ranges as
movement of
continents.
He
he explained
a result of the lat-
also envisioned the
Geological Association in Frankfurt, Germany, Wegener first
presented his ideas for moving continents. His evi-
dence for continental drift and his conclusions were published in 1915 in his monumental book, The Origin of Continents and Oceans. According to Wegener's comprehensive hypothesis, all of the landmasses were originally united into a single supercontinent that he
named Pangaea, from Wegener portrayed
Greek meaning "all land." grand concept of continental of maps showing the breakup of the
his
present-day continents as parts of larger polar conti-
movement
nents that had broken apart and migrated toward the
forces
Pangaea and the movement of the various continents to their present-day locations. Wegener had amassed a tremendous amount of geological, paleontological, and climatological evidence in support of continental drift, but
Moon
the initial reaction of scientists to his then-heretical ideas
equator because of a slowing of the Earth's rotation due to gigantic tidal forces. According to Taylor, these tidal
were generated when the Earth captured the about 100 million years ago. Although we now know that Taylor's mechanism is incorrect, one of his most significant contributions was his suggestion that the Mid-Atlantic Ridge, discoverd by
1872-1876 might mark the
H.M.S. Challenger expeditions, site along which an ancient continent broke apart to form the present-day Atlantic Ocean. the
British
^ ALFRED WEGENER AND THE CONTINENTAL DRIFT HYPOTHESIS Alfred Wegener, a
German
meteorologist
(Fig. 13-4), is
generally credited with developing the hypothesis of
continental
drift. In
a
1912
lecture before the
German
in a series
can best be described as mixed. Opposition to Wegener's ideas became particularly in North America after 1928 when the American Association of Petroleum Geologists held an international symposium to review the hypothesis of continental drift. After each side had presented its arguments, the opponents of continental drift were clearly in the majority, even though the evidence in support of continental drift, most of which came from the Southern Hemisphere, was impressive and difficult to refute. One problem with the hypothesis, however, was its lack of a mechanism to explain how continents, composed of gra-
widespread
nitic rocks,
could seemingly
move through
the denser
basaltic oceanic crust.
Nevertheless, the eminent South African geologist Alexander du Toit further developed Wegener's arguments
— FIGURE
13-4 Alfred Wegener, a German meteorologist, proposed the continental drift hypothesis in 1912 based on a tremendous amount of geological,
paleontological,
and climatological evidence. He
is
shown
here waiting out the Arctic winter in an expedition hut.
and gathered more geological and paleontological evidence in support of continental drift. In 1937, du Toit published Our Wandering Continents, in which he contrasted the glacial deposits of posits of the
same age found
Gondwana with in the
coal de-
continents of the
Northern, Hemisphere. In order to explain the origin and distribution of these rocks, both of which form under different climatic conditions, du Toit
Gondwana continents
to the South Pole
moved
the
and brought the
northern continents together such that the coal deposits at the equator. He named this northern
were located
Jandm ass Laurasia. It consisted -America. Greenland, Europe, and
of present-da y North Asia (except tor India).
In spite of what seemed to be overwhelming evidence, most geologists still refused to accept the idea that continents moved. It was not until the 1960s when ocean-
ographic research provided convincing evidence that the continents had once been joined together and subsequently separated that the hypothesis of continental drift finally
344
Chapter 13
Plate Tectonics:
A
Unifying Theory
became widely accepted.
THE EVIDENCE FOR CONTINENTAL DRIFT =»
The evidence used by Wegener, du support the hypothesis of continental
Continental Fit Wegener, Toit,
and others
drift includes the
to fit
same same age on
of the shorelines of continents; the appearance of the
rock sequences and mountain ranges of the
now widely separated; the matching of glacial and paleoclimatic zones; and the similarities of many extinct plant and animal groups whose fossil remains are found today on widely separated continents.
like
some before him, was impressed by
the
close resemblance
between the coastlines of continents on opposite sides of the Atlantic Ocean, particularly between South America and Africa. He cited these similarities as partial evidence that the continents were at one
continents
time joined together as a supercontinent that subse-
deposits
quently
split apart.
As
his critics pointed out,
however,
the configuration of coastlines results from erosional
depositional processes and therefore
— FIGURE
is
and
continually being
13-5
The
best
fit
between continents occurs along the continental slope at a depth of 2,000 m.
Areas of overlap
Gaps
The Evidence
for Continental Drift
345
modified. Thus, even
if
the continents
had separated
during the Mesozoic Era, as Wegener proposed, likely that the coastlines
A
more
realistic
would
approach
is
fit
exactly.
to
fit
it is
not
the continents to-
gether along the continental slope where erosion
would
be minimal. Recall from Chapter 12 that the true margin of a continent— that
is,
where continental crust
Similarity of
If
the continents were at one time joined together, then
Edward Bullard, an Enand two associates showed that the
slope (see Fig. 12-8). In 1965 Sir glish geophysicist,
best
fit
between the continents occurs along the conti-
nental slope at a depth of about 2,000
m
(Fig. 13-5).
Since then, other reconstructions using the latest ocean
basin data have confirmed the close nents
"•"
when
FIGURE
fit
between conti-
they are reassembled to form Pangaea.
13-6
and mountain ranges of the same age in adon the opposite continents should match. Such is the case for the Gondwana con(Fig. 13-6). Marine, nonmarine, and glacial rock
the rocks
joining locations closely tinents
changes to oceanic crust— is beneath the continental
Rock Sequences
and Mountain Ranges
sequences of Pennsylvanian to Jurassic age are almost identical for all five
is
that of the Glossopteris flora.
J*
continents, strongly in-
The
trends of several major mountain ranges also
These mounone continent only to apparently continue on another continent across the ocean. For example, in a reconstructed support the hypothesis of continental tain ranges seemingly
Marine, nonmarine, and glacial rock sequences of Pennsylvanian to same for all Gondwana continents. Such close similarity strongly suggests that they were at one time joined together. The range indicated by G
Jurassic age are nearly the
Gondwana
dicating that they were at one time joined together.
end
drift.
at the coastline of
(a)
•^ FIGURE
Various mountain ranges of the deformation are currently widely separated by oceans, (b) When the continents are brought together, however, a single continuous mountain range is formed. Such evidence indicates the continents were at one time joined together and were subsequently separated.
same age and
13-7
{a)
style of
Gondwana, the east-west trending mountain range at the Cape of Good Hope in South Africa abruptly terminates at the coast. However, a mountain range of the same age and
style of
gentina.
deformation occurs near Buenos Aires, ArSouth America and Africa are brought
When
two seemingly different mountain ranges continuous structure (Fig. 13-7). In North America, the folded Appalachian Mountains trend northeastward through the eastern United
together, these
form ,
a single
3,000 I
and Canada and terminate abruptly at the Newfoundland coastline. Mountain ranges of the same age
i
i
i
I
km
States
(b)
The Evidence
for Continental Drift
347
"^ FIGURE
13-8
(a) If
the continents did not
move
in the past, then Late Paleozoic
bedrock in Australia, India, and South America indicate that glacial movement for each continent was from the oceans onto land within a subtropical to tropical climate. Such an occurrence is highly unlikely, (b) (right) If the continents are brought together, such that South Africa is located at the South Pole, then the glacial movement indicated by the striations makes sense. In this situation, the glacier, located in a polar climate, moved radially outward from a thick central area toward its periphery. glacial striations preserved in
and deformational
style
occur in eastern Greenland,
Ire-
and Norway. Even though these mountain ranges are currently separated by the Atlantic Ocean, they form an essentially continuous mountain
land, Great Britain,
range
when
the continents are positioned next to each
All of the
Gondwana
tropical climates.
Mapping
of glacial striations in bed-
rock in Australia, India, and South America indicates that the glaciers moved from the areas of the present-
day oceans onto land
other (Fig. 13-7).
continents except Antarctica
are currently located near the equator in subtropical to
(Fig. 13-8a).
However,
this
would
be impossible because large continental glaciers (such as
occurred on the
Glacial Evidence
Gondwana
Massive glaciers covered large continental areas of the Southern Hemisphere during the Late Paleozoic Era. Ev-
accumulation toward the
idence for this glaciation includes layers of
would have
till
(sedi-
ments deposited by glaciers) and striations (scratch marks) in the bedrock beneath the till. Fossils and sedimentary rocks of the same age from the Northern Hemisphere, however, give no indication of glaciation. Fossil plants found in coals indicate that the Northern Hemisphere had a tropical climate during the time that the Southern Hemisphere was glaciated.
348
Chapter 13
continents during the Late
Paleozoic Era) flow outward from their central area of
Plate Tectonics:
A
Unifying Theory
If
move during
the past, one
how glaciers moved from the and how large-scale continental gla-
to explain
oceans onto land ciers
sea.
the continents did not
formed near the equator. But
if
the continents are
reassembled as a single landmass with South Africa located at the south pole, the direction of movement of Late Paleozoic continental glaciers makes sense. Fur-
thermore, this geographic arrangement places the northern continents nearer the tropics, which
is
consistent
Furthermore, even
if
the seeds
had
floated across the
ocean from one continent to another, they probably would not have remained viable for any length of time in salt water.
The present-day
climates of South America, Africa,
and Antarctica range from
India, Australia,
much
polar and are
compose
plants that
tropical to
too diverse to support the type of
Wegener
the Glossopteris flora.
rea-
soned therefore that these continents must once have been joined such that these widely separated localities
were
the
all in
The
same
latitudinal climatic belt (Fig. 13-9).
remains of animals also provide strong ev-
fossil
drift. One of the best examples is Mesosaurus, a freshwater reptile whose fossils are found in Permian-aged rocks in certain regions of Brazil and South Africa and nowhere else in the world (Fig. 13-9).
idence for continental
Because the physiology of freshwater and marine ani-
mals
is
completely different,
it is
freshwater reptile could have
Ocean and found to
tical
its
could have that
how
a
across the Atlantic
a freshwater environment nearly iden-
former habitat. Moreover,
swum
across the ocean,
should be widely dispersed.
sume
hard to imagine
swum
Mesosaurus
It
is
fossil
more
lived in lakes in
Mesosaurus
if
its
remains
logical to as-
what
now
are
adjacent areas of South America and Africa, but were
then united into a single continent.
Cynognathus
and
Lystrosaurus
both
are
land-
dwelling reptiles that lived during the Triassic Period; their fossils are I
I
Glaciated area tal
Arrows indicate the direction of glacial movement based on striations preserved in bedrock.
rus
found only on the present-day continen-
fragments of
Gondwana
(Fig. 13-9).
Since Lystrosau-
and Cynognathus are both land animals, they
tainly could not have
separating the
swum
Gondwana
cer-
across the oceans currently continents. Therefore, the
(b)
continents must once have been connected.
with the
fossil
and climatological evidence from Laur-
The evidence favoring continental drift seemed overwhelming to Wegener and his supporters yet the lack of a suitable mechanism to explain continental movement prevented
asia (Fig. 13-8b).
its
widespread acceptance. Not
until
new
ev-
idence from studies of the Earth's magnetic field and
oceanographic research showed that the ocean basins Fossil
Some
Evidence
were geologically young features did renewed
of the most compelling evidence for continental
comes from the fossil record. Fossils of the Glosfound in equivalent Pennsylvanianand Permian-aged coal deposits on all five Gondwana
drift
sopteris flora are
continents.
The
Glossopteris flora
is
characterized by
the seed fern Glossopteris (Fig. 13-3) as well as by
many
interest in
continental drift occur.
^ PALEOMAGNETISM AND POLAR WANDERING Some
of the most convincing evidence for continental
came from
other distinctive and easily identifiable plants. Pollen
drift
and spores of plants can be dispersed over great distances by wind, but Glossopteris-type plants produced seeds that are too large to have been carried by winds.
tively
new
some
geologists
the study of paleomagnetism, a rela-
During that time, were researching past changes of the
discipline during the 1950s.
Earth's magnetic field in order to better understand the
Paleomagnetism and Polar Wandering
349
Lystrosaurus Glossopteris
^^ FIGURE
Some
13-9
of the animals and plants whose fossils are found today on
the widely separated continents of South America, Africa, India, Australia, and Antarctica. These continents were joined together during the Late Paleozoic to form the southern landmass of Pangaea. Glossopteris and similar plants are Pennsylvanian- and Permian-aged deposits on all five continents. Mesosaurus a freshwater reptile whose fossils are found in Permian-aged rocks in Brazil and South Africa. Cynognathus and Lystrosaurus are land reptiles who lived during the Early Triassic Period. Fossils of Cynognathus are found in South America and Africa, while fossils of Lystrosaurus have been recovered from Africa, India, and Antarctica.
Gondwana, found
in
present-day magnetic
field.
As so often happens
in sci-
ence, these studies led to other discoveries. In this case,
they led to the discovery that the ocean basins are geologically
indeed
young
features,
moved during
and that the continents have Wegener and oth-
the past, just as
mine the location of the Earth's magnetic poles and the latitude of the rock
when
Recall from Chapter 11 that the Earth's magnetic
it
formed.
Research conducted during the 1950s by the English geophysicist
S.
K.
Runcorn and
his associates
that the location of the paleomagnetic pole, as
by the paleomagnetism
had proposed.
ers
is
in
ferent ages, varied widely.
showed
measured
European lava flows of They found that during
dif-
the
recording both the direction and the intensity of the
500 million years, the north magnetic pole has apparently wandered from the Pacific Ocean northward through eastern and then northern Asia to its presentday location near the geographic north pole (Fig. 1310). This paleomagnetic evidence from Europe could be
magnetic
interpreted in three ways: the continent remained fixed
poles correspond closely to the location of the geo-
graphic poles (see Fig. 11-27).
When
a
magma
cools, the
iron-bearing minerals align themselves with the Earth's
magnetic
350
field
field.
when
they reach the Curie point, thus
This information can be used to deter-
Chapter 13
Plate Tectonics:
A
Unifying Theory
past
and the north magnetic pole moved; the north magnetic still and the continent moved; or both the continent and the north magnetic pole moved. When paleomagnetic readings from numerous lava flows of different ages in North America were plotted on
pole stood
a
to different magnetic pole
map, however, they pointed
same ages
locations than did flows of the
in
Europe
13-10). Furthermore, analysis of lava flows from
had
tinents indicated that each continent
of magnetic poles! Does this
had a
mean
its
(Fig.
con-
all
own
series
that each continent
That would be
different north magnetic pole?
highly unlikely and difficult to reconcile with the laws of
physics and netic field
is
what we know about how
the Earth's
,,
mag-
/jl
Path of
v
European paleomagnetic
generated (see Chapter 11).
pole
Therefore, the best explanation for the apparent
wandering of the magnetic poles
is
that they have re-
mained at their present locations near the geographic poles and the continents have moved. When the continents are fitted together so that the paleomagnetic data
point to only one magnetic pole,
we
find, just as
We-
gener did, that the rock sequences, mountain ranges,
and
glacial deposits
matic evidence
leogeography
match, and that the
fossil
and
cli-
consistent with the reconstructed pa-
is
(see Perspective 13-1).
"•'' FIGURE 13-10 The apparent paths of polar wandering for North America and Europe. The apparent
location of the north magnetic pole is shown for different periods on each continent's polar wandering path.
» SEA-FLOOR SPREADING In addition to the paleomagnetic research in the 1950s,
movement. Hess proposed
oceanographic research led to extensive mapping of the world's ocean basins (see Perspective 12-2). Such mapping revealed that the Mid-
move
a
renewed
interest in
Atlantic Ridge
is
part of a worldwide oceanic ridge
system more than 65,000
km
long.
It
was
also discov-
ered that oceanic ridges are characterized by high heat flow, basaltic volcanism,
and
seismicity.
Furthermore,
magnetic reversals, as recorded in oceanic-crust rocks, and the age of deep-sea sediments immediately above the oceanic crust occur in distinct patterns with respect to ridges.
Harry H. Hess of Princeton University conducted
much
of his oceanographic research while serving in the
central Pacific during
World War
II.
His discovery of
guyots (submerged, flat-topped volcanic islands) prois movaway from the oceanic ridges (see Fig. 12-18). As a result of his discovery of guyots and other re-
vided geologists with evidence that the sea floor ing
search conducted during the 1950s, Hess published a
landmark paper
in
1962
in
which he proposed the hy-
pothesis of sea-floor spreading to account for continental
that the continents
do not
across or through oceanic crust, but rather that the
continents and oceanic crust
move
together and are both
parts of large plates. According to Hess, oceanic crust
new
formed by newly formed oceanic crust moves laterally away from the ridge, thus explaining how volcanic islands that formed
separates at oceanic ridges where
upwelling
magma. As
the
at or near ridge crests later
magma
crust
is
cools,
become guyots
the
(Fig. 12-18).
Hess revived the idea (proposed in the 1930s and 1940s by Arthur Holmes and others) of a heat transfer system — or thermal convection cells— within the mantle as a mechanism to move the plates. According to Hess, hot magma rises from the mantle, intrudes along rift zone fractures defining oceanic ridges, and thus forms new crust. Cold crust is subducted back into the mantle at deep-sea trenches where it is heated and recycled.
How crust
is
could Hess's hypothesis be confirmed? If new forming at oceanic ridges and the Earth's mag-
netic field
is
periodically reversing
itself,
then these mag-
netic reversals should be preserved as magnetic lies in
anoma-
the rocks of the oceanic crust (Fig. 13-11).
Sea-Floor Spreading
351
Perspective 13-1
PALEOGEOGRAPHIC MAPS The
to any reconstruction of world paleogeography is the correct positioning of the continents in terms of latitude and longitude and the
and animals provides a on the latitudes determined by paleomagnetism and can provide additional limits on
proper orientation of the paleocontinent relative to the paleonorth pole. The main criteria used for paleogeographic reconstructions are paleomagnetism,
longitudinal separation of continents.
The key
biogeographic patterns indicated by
continents. For the
Paleozoic Era, however, the paleomagnetic data are
Tectonic activity
the effects of
may
be acquired through
ophiolites.
is
fossil
""'' FIGURE 1 Three paleogeographic maps and one modern during the (a) Late Cambrian Period, {b) Early Triassic Period, and (d) Recent.
Uplands and
I
I
Lowlands
mountains
352
Chapter 13
Plate Tectonics:
A
evidence.
indicated by deformed
Such features allow geologists to recognize (text
PyiSil
known
ancient mountain chains and zones of subduction.
metamorphism or weathering.
(a)
well
sediments associated with andesitic volcanics and
often inconsistent and contradictory because
secondary magnetizations
It is
and animals is controlled by both climatic and geographic barriers. Such information can be used to position continents and ocean basins in a way that accounts for the that the distribution of plants
biogeography, tectonic patterns, and climatology. Paleomagnetism provides the only quantitative data
on the orientations of the
distribution of plants
useful check
Unifying Theory
continued on page 354)
map (c)
depicting the Earth Late Cretaceous Period,
I
I
Shallow sea
I
I
Deep sea
Sea-Floor Spreading
353
These mountain chains may subsequently have been separated by plate movement, so the identification of large, continuous mountain chains provides important information about continental positions in the geologic past. Climate-sensitive sedimentary rocks are used to interpret past climatic conditions. Desert dunes are
and cross-bedded on a large and associated with other deposits, they indicate an arid environment. Coals form in freshwater swamps where climatic conditions promote abundant
exceeds precipitation, such as in desert regions or Tillites result from glacial and indicate cold, wet environments. By combining all relevant geologic, paleontologic, and climatologic information, geologists can construct paleogeographic maps (Fig. 1). Such maps are simply interpretations of the geography of an area for a
along hot, dry, shorelines. activity
The majority
typically well sorted
particular time in the geologic past.
scale,
paleogeographic maps show the distribution of land
plant growth. Evaporites result
when evaporation
Around 1960, magnetic data gathered by scientists Institution of Oceanography in Cali-
and
sea,
probable climatic regimes, and such
geographic features as mountain ranges, swamps, and glaciers.
L.
W. Morley, a Canadian geologist, independently armodel that explained this pattern of magnetic
from the Scripps
rived at a
fornia indicated an unusual pattern of alternating posi-
anomalies.
and negative magnetic anomalies for the Pacific ocean floor off the west coast of North America. The
magma
tive
pattern consisted of a series of roughly north-south parallel stripes,
but they were broken and offset by essen-
It was not until 1963 that F. Vine and D. Matthews of Cambridge University and
tially
354
east-west fractures.
Chapter 13
Plate Tectonics:
A
Unifying Theory
of
These three geologists proposed that when basaltic intruded along the crests of oceanic ridges, it would record the magnetic polarity at the time it cooled. As the ocean floor moved away from these oceanic ridges, repeated intrusions would form a symmetrical series of magnetic stripes, recording periods of normal
Oceanic ridge
Normal magnetism
Reversed magnetism
Magnetic profile as recorded by a
Continental
sequence
magnetometer
of
Continental lava flows
magnetic reversals ""'
FIGURE
crust
The sequence of magnetic anomalies preserved within
13-11
on both
the oceanic
an oceanic ridge is identical to the sequence of magnetic reversals continental lava flows. Magnetic anomalies are formed when intrudes into oceanic ridges; when the magma cools below the Curie
sides of
already
known from
basaltic
magma
records the Earth's magnetic polarity at the time. Subsequent intrusions split formed crust in half, so that it moves laterally away from the oceanic ridge. Repeated intrusions produce a symmetrical series of magnetic anomalies that reflect periods of normal and reversed polarity. The magnetic anomalies are recorded by point,
it
the previously
a magnetometer,
which measures the strength of the magnetic
and reverse polarity
(Fig. 13-11).
Shortly thereafter, the
field.
million years old, whereas the oldest continental crust
is
was supported
3.96 billion years old; this difference in age provides
by evidence from magnetic readings across the Reyk-
confirmation that the ocean basins are geologically
janes Ridge, part of the Mid-Atlantic Ridge south of
young
Vine, Matthews, and Morley proposal
A
features
whose openings and
To many
oceanic ridges.
support of continental
Magnetic surveys for most of the ocean floor have been completed (Fig. 13-12). They demonstrate that the youngest oceanic crust is adjacent to the spreading ridges and that the age of the crust increases with distance from the ridge axis, as would be expected ac-
now
cording to the sea-floor spreading hypothesis. Further-
more, the age of the oldest oceanic crust
is
less
than 180
tially
closings are par-
responsible for continental movement.
group from the Lamont-Doherty Geological Observatory at Columbia University found that magnetic anomalies in this area did form stripes that were distributed parallel to and symmetrical about the oceanic ridge. By the end of the 1960s, comparable magnetic anomaly patterns were found surrounding most Iceland.
Deep-Sea Drilling and the Confirmation of Sea-Floor Spreading amassed in and sea-floor spreading was convincing. Results from the Deep-Sea Drilling Project (see Chapter 12) have confirmed the interpretations made by earlier paleomagnetic studies. Cores of deepsea sediments and seismic profiles obtained by the Glomar Challenger and other research vessels have provided
much
geologists, the paleomagnetic data drift
of the data that support the sea-floor spreading
hypothesis.
Sea-Floor Spreading
355
EaSr% | Pleistocene |
|
to
| Paleocene (58-66
Recent (0-2 M.Y.A.)
Pliocene (2-5 M.Y.A.)
^2 Miocene (5-24
|
M.Y.A.)
^| Oligocene (24-37 Eocene (37-58
M.Y.A.)
|
Late Cretaceous (66-88 M.Y.A.)
|
Middle Cretaceous (88-1 18 M.Y.A.;
Cretaceous (118-144 | B Late Jurassic (144-161 Early
M.Y.A.)
M.Y.A.)
M.Y.A.)
M.Y.A.)
"^ FIGURE 13-12 The age of the world's ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age increases away from the ridge axis.
According to
this hypothesis,
oceanic crust
is
contin-
uously forming at mid-oceanic ridges, moving away
distribution.
Sediments
at a rate of less
sumed
basins were as
at
subduction zones.
If this is
the case, oceanic
and become progressively older with increasing distance away from them. Moreover, the age of the oceanic crust should be symmetrically distributed about the ridges. As we have crust should be youngest at the ridges
just
deep-sea sediments to be several kilometers thick.
How-
fossils from sediments overlying and radiometric dating of rocks found
islands both substantiate this predicted age
spreading. Accordingly, at or very close to spreading
noted, paleomagnetic data confirm these state-
the oceanic crust
356
than 0.3
from numerous drill holes indicate that deepsea sediments are at most only a few hundred meters thick and are thin or absent at oceanic ridges. Their near-absence at the ridges should come as no surprise, however, because these are the areas where new crust is continuously produced by volcanism and sea-floor
ments. Furthermore,
on oceanic
open ocean accumulate, on average, cm per 1,000 years. If the ocean old as the continents, we would expect
in the
from these ridges by sea-floor spreading, and being con-
Chapter 13
Plate Tectonics:
A
Unifying Theory
ever, data
.
Oceanic crust "•"
FIGURE
13-13
The
total
thickness of deep-sea sediments
away from oceanic ridges. because oceanic crust
increases
This Total thickness of
increases
sediment
away from
oceanic ridge
Magma
Upper mantle
ridges
Increasing age of crust
where the oceanic crust
have had
little
ness
increases
(Fig.
13-13).
is
young, sediments
time to accumulate, but their thick-
with distance away from the ridges
accumulate.
much as 250 km thick, whereas those of upper mantle and oceanic crust are up to 100 km thick. The lithosphere overlies the hotter and weaker semiare as
plastic asthenosphere. It
ing from
^ PLATE TECTONIC THEORY As
early as 1965,
J. T.
Wilson of the University of Tor-
He
on the nature of large fracand named them transform
also speculated
tures in the oceanic crust faults
(discussed later in this chapter).
Isacks,
J.
Oliver,
and
L. R.
In
1968, B.
Sykes of Columbia University
the concepts of continental drift, seajjioor spreading.
nnw-heerusharrenerl
Most
it
seemingly
is
it
is
overwhelming, and also
a unifying theory that can explain
unrelated
quently, geologists
now
many
phenomena. Conseview many geologic processes,
into the
geological
phenomena occurring
at their boundaries.
» PLATE BOUNDARIES move
relative to
one another such that
their
boundaries can be characterized as divergent, conver-
and transform. Interaction of plates
at
their
volcanic activity and, as will be apparent in the next chapter, the origin of
mountain systems.
Divergent Boundaries Divergent plate boundaries or spreading ridges occur
of the terrestrial planets have had a similar
where p lates are sepaf ating~and new oceanic lit hosphere is forming. Divergent boundaries are placeswKere the cfusi is "b eing extended, thinned, and fractured as magma, derived from the partial melting of the mantle, rises to the surface. The magma is almost entirely basaltic and intrudes into vertical fractures to form dikes and lava flows (Fig. 13-15). As successive injections of magma cool and solidify, they form new oceanic crust and record the intensity and orientation of the Earth's magnetic field (Fig. 13-11). Divergent boundaries most
cause
all
origin
and
early history, geologists are interested in de-
termining whether plate tectonics it
operates in the same
is
unique to Earth or
way on
the other terres-
planets (see Perspective 13-2).
based on a simple model of both oceanic and continental crust, as well as the underlying upper mantle, consists of numerous variable-sized pieces called plates (Fig. 13-14). The plates vary in thickness; those composed of upper mantle and continental crust Plate tectonic theory
the Earth.
such as at oceanic
and are subducted back
tectonics. Furthermore, be-
from the perspective of plate
trial
the asthenosphere, they separate, mostly
geologic
such as mountain building, seismicity, and volcanism,
whether
result-
boundaries accounts for most of the Earth's seismic and
geologists accept plate tectonic theory, in part
because the evidence for because
movement
transfer system within the
mantle. Individual plates are recognized by the types of
gent, t^4>late_iectonics
move over
trenches, they collide
Plates has:
believed that
at oceanic ridges, while in other areas
proposed the term new global tectonics to encompass
and^ansforrn jaults/Ihat rprm
is
some type of heat
asthenosphere causes the overlying plates to move. As plates
onto proposed that the Earth's crust is composed of several large rigid plates that move with respect to one another.
is
becomes older away from oceanic ridges, and thus there has been more time for sediment to
The
is
rigid outer lithosphere, consisting of
commonly occur along
the crests of oceanic ridges, for
Plate Boundaries
357
Perspective 13-2
TECTONICS OF THE TERRESTRIAL PLANETS Recall from Chapter 2 that the four terrestrial planets— Mercury, Venus, Earth, and Mars— all had a similar early history involving accretion,
and silicate mantle and formation of an early atmosphere by outgassing. Their early history was marked by widespread volcanism and meteorite impacts, both of which helped modify their surfaces. The volcanic and tectonic activity and resultant surface features (other differentiation into a metallic core
and
crust,
"^ FIGURE 2 {a) Western Ishtar Terra and mountain belts surrounding Lakshmi Planum. Surrounding Western Ishtar Terra are a transitional zone (blue) and lowlands plains (rust), (b) A radar image of Akna Montes, Freyja Montes, and a portion of Lakshmi Planum illustrating the folded and faulted nature of the Akna and Freyja montes.
than meteorite craters) of these planets are clearly related to the way in which they transport heat from their interiors to their surfaces.
The Earth appears is
broken up into a
to be
unique in that
series of plates.
The
its
surface
creation and
destruction of these plates at spreading ridges
and
subduction zones transfer the majority of the Earth's internally
produced heat. In addition, movement of
the plates, together with life-forms, the formation of
sedimentary rocks, and water,
is
responsible for the
cycling of carbon dioxide between the atmosphere
Sedna
and
Planitia
lithosphere and thus the maintenance of a habitable
climate
on Earth
340°
(see Perspective 2-2).
"^^
FIGURE 1 This radar image of Venus made by the Magellan spacecraft reveals circular and oval-shaped volcanic features. A complex network of cracks and fractures extends outward from the volcanic features. Geologists think these features were created by blobs of magma rising from the interior of Venus with dikes filling some of the cracks.
358
Chapter 13
Plate Tectonics:
A
Unifying Theory
(a)
350° 50°
,
50°
Heat
is
transferred between the interior
and surface of
both Mercury and Mars mainly by lithospheric conduction. This method
is
sufficient for these planets
because both are significandy smaller than Earth or Venus.
Because both Mercury and Mars have a
single, globally
continuous plate, they have exhibited fewer types of volcanic
and
The warming of Mercury and Mars produced
tectonic activity than has the Earth.
initial interior
expansional features such as normal faults (see Chapter 14)
and widespread volcanism, while their subsequent cooling produced folds and faults resulting from compressional forces, as well as a succession of volcanic activity.
Mercury's surface is heavily cratered and shows the way of primary volcanic structures.
little in
However,
it
does have a global system of lobate scarps These have been interpreted as
(see Fig. 2-10).
evidence that Mercury shrank a
little
soon
after its
crust hardened, resulting in crustal cracking.
Mars has numerous
features that indicate
early period of volcanism.
an extensive
These include Olympus Mons,
the solar system's largest volcano (see Fig. 2-12), lava flows,
uplifted regions believed to have resulted
from
convection. In addition to volcanic features,
Mars
and
mande
abundant evidence of tensional tectonics, numerous faults and large fault-produced valley structures. While Mars was tectonically active during the past, there is no evidence that plate tectonics comparable to that on Earth has ever occurred there. Venus underwent essentially the same early history as also displays
including
the other terrestrial planets, including a period of it is more Earth-like in its tectonics than Mercury or Mars. Initial radar mapping in 1990
volcanism, but either
by the Magellan spacecraft revealed a surface of extensive lava flows, volcanic domes, folded mountain ranges, and an extensive and intricate network of faults, all
of which attest to an internally active planet (Fig.
1).
broad plateau area named the Western Ishtar Terra, a series of mountain belts surrounds Lakshmi In a
Planum, a central smooth plain (Fig. 2). On the basis of detailed mapping from radar images and interpretation
FIGURE
movement. It is thought that the Freyja Montes region was the site of large-scale crustal convergence that is continuing as a result of the underthrusting of the North
Block diagram showing the geologic history region, (a) Crustal convergence and compression cause buckling and underthrusting of the crust and lithosphere. (b) Continued convergence, compression, and underthrusting produce crustal thickening, uplift, and the formation of new zones of underthrusting. (c) Continuing convergence, crustal thickening, and underthrusting cause numerous slabs of crust to overlap one another like shingles, producing the present-day
Polar Plains beneath Ishtar Terra (Fig. 3).
configuration of the region.
of the topography and geology of the
Akna and
"*r-
Freyja
montes, geologists believe that these structures represent
mountain
belts.
faults resulting
Features identified include folds and from compressive forces and horizontal
of the Freyja
3
Montes
Plate Boundaries
359
• Hot spot
—»- Direction
"^ FIGURE direction of
13-14
of
movement
A map
of the world showing the plates, their boundaries,
movement, and hot
spots.
'*"' FIGURE 13-15 Pillow lavas forming along the Mid-Atlantic Ridge. Their distinctive bulbous shape result of underwater eruption.
example, the Mid-Atlantic Ridge. Oceanic ridges are thus is
the
characterized by rugged topography with high relief resulting from displacement of rocks along large fractures,
shallow-focus earthquakes, high heat flow, and basaltic flows or pillow lavas.
Divergent b ound aries also occur under continents
during
trie early"
stages of continental breakup (Fig. 13-
When magma
16).
crust
is
wells
initially elevated,
up beneath a continent, the extended, and thinned (Fig.
13-16a). Such stretching eventually produces fractures
an d
rift
v alleys.
During IKIs
stage, magma~~typically in-
trudes into the faults and fractures forming
sills,
and
valley floor
(Fig.
lava flows; the latter often cover the
13-16b).
example of If
The East African rift valleys
this stage
rift
are an excellent
of continental breakup
spreading proceeds, some
rift
dikes,
(Fig. 13-17).
valleys will continue
and deepen until they form a narrow linear two continental blocks (Fig. 13- 16c). The Red Sea separating the Arabian Peninsula from Africa (Fig. 13-17) and the Gulf of California, which separates to lengthen
sea separating
360
Chapter 13
Plate Tectonics:
A
Unifying Theory
Crustal
upwarp
Narrow sea
"^
FIGURE 13-16 History of a divergent plate boundary, {a) Rising magma beneath a continent pushes the crust up, producing numerous cracks and fractures, (b) As the crust and thinned,
is
and lava flows onto the valley floors, (c) Continued spreading further separates the continent until a narrow seaway develops, (d) As spreading continues, an oceanic ridge system forms, and an ocean basin develops and grows. stretched
rift
valleys develop,
Baja California from mainland Mexico, are good exam-
advanced stage of rifting. As a newly created narrow sea continues enlarging, it may eventually become an expansive ocean basin such as the Atlantic, which separates North and South America from Europe and Africa by thousands of kilometers (13-16d). The Mid-Atlantic Ridge is the boundary between these diverging plates; the American plates are
ples of this
moving westward, and the Eurasian and African are moving eastward.
plates
Convergent Boundaries'^ Because new lithosphere
is
formed
at divergent plate
boundaries, older lithosphere must be destroyed and recycled in order for the entire surface area of the Earth to
Plate Boundaries
361
Most
SO°E
of these planes dip from oceanic trenches beneath
adjacent island arcs or continents, marking the surface of Levantine
Rift
slippage between the converging plates. ing plate
moves down
As the subduct-
into the asthenosphere,
and eventually incorporated subduction does not occur
it is
into the mantle.
when both
heated
However,
of the converging
plates are continental because continental crust
is
not
dense enough to be subducted into the mantle.
Convergent boundaries are characterized by deformamountain building, metamorphism, seis-
tion, volcanism,
micity,
and important mineral
convergent plate
boundaries
oceanic, oceanic-continental,
Oc eanic -Oceanic Carlsberg
Ridge
deposits.
Three types of
recognized:
are
oceanic-
and continental-continental.
Boundaries
When-twxLXiceanic plates^conterge, one of them is subducted beneath t he other along an oceanic-oceanic plate
boundary
13-18). The subducting plate bends an angle between 5° to 10° to form the
(Fig.
downward
at
outer wall of an oceanic trench.
The
inner wall of the
trench consists of a subduction complex
composed of
wedge-shaped slices of highly folded and faulted marine sediments and oceanic lithosphere scraped off from the descending plate. This subduction complex is elevated Rift
T
as a result of uplift along faults as subduction continues
I
'
Rift valley
I
I
Oceanic crust
I
I
(Fig. 13-18).
As the subducting plate descends into the asthenosit is heated and partially melted, generating a
Stretched continental
phere,
crust
magma, commonly
magma and
is
less
of
andesitic
This
composition.
dense than the surrounding mantle rocks
rises to the surface
overriding plate where
through the nonsubducting or forms a curved chain of vol-
it
canoes called a volcanic island arc (any plane intersect-
Madagascar
makes an arc). This arc is nearly parallel to and is separated from it by up to hundred kilometers — the distance depends on
ing a sphere
the oceanic trench several Kilometers
•^ FIGURE
13-17
The East African
the angle of dip of the subducting plate (Fig. 13-18).
L
J
being formed by the separation of eastern Africa from the rest of the continent along a divergent plate boundary. The Red Sea represents an advanced stage of rifting, in which two continental blocks are separated by a narrow sea. rift
valley
is
Located between the volcanic island arc and the subduction complex of the oceanic trench (Fig. 13-18). It typically
362
Chapter 13
Plate Tectonics:
A
Unifying Theory
a fore-arc basin
generally flat-lying detrital sediments up to 5 km thick. These sediments are derived from the weathering and erosion of the island arc volcanoes and reflect a progressive shallowing as the basin
remain constant. Otherwise, we would have an expanding Earth. Such plate destruction occurs at convergent plate boundaries where two plates collide. At a convergent boundary, the leading edge of one plate descends beneath the margin of the other_by_sjibdiigtion. A dipping plane of earthquake foci, referred to as a Benioff zone, defines subduction zones (Fig. 10-8).
is
contains a diverse assortment of
In those areas
where the
fills
up.
rate of subduction
is
faster
than the forward movement of the overriding plate, the lithosphere
arc
may
on the landward
and thinned,
resulting in the formation of a back-arc
basin. This back-arc basin
magma
side of the volcanic island
be subjected to tensional stress and stretched
may grow by
spreading
breaks through the thin crust and forms
if
new
Sea
level
—
FIGURE 13-18 Oceanic-oceanic plate boundary. An oceanic trench forms where one oceanic plate is subducted beneath another. As a result of subduction, a complex of highly folded and faulted marine sediment and scraped-off pieces of oceanic lithosphere forms along the inner Magma
Asthenosphere
wall of the trench.
On
the
nonsubducted plate, a volcanic island arc forms from the rising magma generated from the subducting plate.
The
and Antillean (Caribbean)
oceanic crust (Fig. 13-18). In any case, the back-arc ba-
pine Islands.
with a mixture of volcanic rocks and detrital sediments. A good example of a back-arc basin associated with an oceanic-oceanic plate boundary is the Sea
land arcs are present in the Atlantic Ocean basin.
of Japan between the Asian continent and the islands of
When
sin will
fill
Japan.
Most present-day active volcanic island arcs are in Ocean basin and include the Aleutian Islands,
the Pacific the
Kermadec-Tonga
arc,
and the Japanese and
Philip-
Scotia
Oc eanic-Continen ta
l
is-
Boundaries
an oceanic and a continental plate c onverge, the oceanic plate is subducted under the continental plate alo ng an oceanic-continental pla te_boundary (Fig. 1319).
The oceanic
plate
is
subducted because
it is
denser
than continental crust. Just as at oceanic-oceanic plate
— FIGURE
13-19
Oceanic-continental plate boundary.
Continental interior
When
Trench
Sea level
an oceanic plate is subducted beneath a continental plate, an andesitic volcanic mountain range is
formed on the continental plate result of rising
Magma
as a
magma.
Continental crust
Asthenosphere
Plate Boundaries
363
boundaries, the descending oceanic plate forms the
of subduction, and the Andes Mountains are the result-
outer wall of an oceanic trench; a subduction complex
ing volcanic
forms the inner wall of the trench and between continent
is
it
and the
mountain chain on the overriding plate
(see Fig. 4-31).
a fore-arc basin.
The oceanic trenches of oceanic-continental boundaries typically contain
sediments derived from the ero-
Continental-Continental Boundaries
rocks. These
converge ;dong a boundary, one platem av partially slide undg£the other, but neither plate wil l be subductej becausej^Lt heir low and equal de nsities and
well as
great thickness (Fig. 13-20). These continents are
The subduction complex consists of wedge-shaped slices of complexly folded and faulted sion of continents.
wedges contain continental sediments as some of the sediment and pieces of crust that are scraped off by the overriding continental plate. The subduction complex is elevated as new slices are added by the underthrusting of subduction. The fore-arc basin of the
continental
plates
rtinental plate
ini-
separatecTfrom ojiejmojhgr_ by oceanic crust that being subducted under one of the continents. The edge
tially is
of that continent will display the characteristics of an
oceanic-continental boundary contains detrital sediments
oceanic-continental boundary with the development of
derived from the erosion of the continent. These sediments
a deep-sea trench,
are typically flat-lying or only mildly deformed.
and volcanic arc (Fig. 13-19). Eventually, the oceanic crust is totally consumed and the two continents collide; the sediments and portions of sea floor caught between the two plates are deformed and uplifted. A new mountain range is thus formed, composed of deformed sedimentary rocks, scraped-off oceanic crust, and the vol-
As the
cold, wet,
and
slightly denser oceanic plate
descends into the hot asthenosphere, melting occurs and
magma
is
generated. This
riding continental plate
magma
rises
beneath the over-
and can extrude
at the surface,
producing a chain of andesitic volcanoes (also called a volcanic arc), or intrude into the continental margin as plutons, especially batholiths. filled
A
back-arc basin
may
be
with continental detrital sediments, pyroclastic
and lava flows, derived from and thickening toward the volcanic arc. An excellent example of an oceanic-continental plate boundary is the Pacific coast of South America where the oceanic Nazca plate is currently being subducted under South America. The Peru-Chile Trench is the site materials,
*»-
FIGURE
13-20
When two
canic arc of the overriding plate.
The Himalayas, the world's youngest and highest mountain system, resulted from the collision between India and Asia that began about 40 to 50 million years ago and is still continuing (Fig. 14-35). During this collision, the leading margin of the Indian plate was partially forced under the Asian plate, resulting in a thick accumulation of and the uplift of the Himalayas and the Tibetan Plateau. Other examples of mountain continental lithosphere
Deformed and metamorphosed subduction complex
Continental-continental plate
boundary.
subduction complex, fore-arc basin,
continental
is subducted because of their great thickness and low and equal densities. As the two
plates converge, neither
Oceanic crust fragments
continental plates collide, a
mountain range interior
formed in the of a new and larger is
continent.
Continental crust
Magma Asthenosphere -
364
Chapter 13
Plate Tectonics:
A
Unifying Theory
Oceanic crust
NICHOLAS
Guest Essay
B.
CLAUDY
GEOSCIENCE CAREERS-THE IS UNPARALLELED
DIVERSITY
The following essay originally appeared in the January 1991 issue of Geotimes, and has been adapted with permission from the author.
Department of Energy and the Environmental Numerous employment opportunities in energy-related programs will Protection Agency.
show moderateThe geosciences
offer unparalleled career opportunities
that reflect a unique blend of disciplines.
Whether you
many
scientific
are interested in scientific
sector for the next few years.
and development to problem solving, conserving and protecting natural resources, or disseminating geologic knowledge, the geosciences offer rewarding careers. research, applying research
consultants
into the 1990s.
retirements increase.
employment growth than for the labor force as a whole; and potential shortages of workers, due to depressed enrollments, too few new graduates, and the
The following
-
force
in
employed
sources,
on
More
areas.
qualified secondary
result,
preferred credentials for
its
However,
all
requisites.
A
its
list
of
employers seek a few basic
were
far
more
is
highly desirable.
B.A./B.S.
graduates than jobs available, but the situation
emphasis
was
quite the opposite for those with a master's
degree. Diversity of coursework
Domestically, there will be increased
valued, since
it
experience
(
is
Any work
full-time, part-time or
also a valuable asset. Skills in oral
and an energy
communication are
and viable option.
necessity for
Mining/minerals (9%): Worldwide metallicand nonmetallic-mineral exploration and
highly
allows the employee to be more
adaptable to employer needs.
on improved recovery technology
rather than exploration. Shortages of geoscientists
career remains a strong
own
new employees.
master's degree
In 1990, there
global expansion of energy
are likely in the next few years,
and high
placed on
is
markets and improved research and operations. concentration
summer) is and written
also frequently cited as a
new employees, a
the federal sector will probably not
B. Gaudy graduated from Brown University where he majored in Greek studies and earned a master's degree in Greek from the University of North Carolina at Chapel Hill. In 1979, he joined the American Geological Institute where he is responsible for
hiring significantly, although
preparing several publications. In
production will continue as current supplies decrease. Probable growth in nuclear power will increase interest in energy-related minerals,
such as uranium and plutonium. Federal/state (12%): Due to budget constraints,
efforts will require a larger
expand its some regulatory work force. State
agencies will continue to assume a greater role in regulatory activities. -
some
math literacy. Each category of employer has
alternative energy
and conservation. As a
will be placed
on
on
science skills and
in that area):
world's attention has been refocused oil, realistic
increased emphasis
sciences as increased emphasis
Oil/gas (50%): Since the invasion of Kuwait, the
dependence on
The
school teachers will be needed in the earth
force.
are the major geoscience employers
work
predicted for
environmental studies will perhaps allow growth
(the figure in parentheses indicates the percentage of
the geoscience
is
academia as enrollments begin to recover and
greater
work
And, for those
deal with environmental issues,
Academia (14%): Modest growth
demand for lower unemployment rate and far
aging of the current
who
faster-than-average growth should continue well
Several factors will contribute to the geoscientists: a far
to above-average growth.
Consulting (11%): This has been and will continue to be the fastest growing employment
1986, he became the
.
-
-i.
institute's
director of development. Claudy
notes that his general liberal arts
education
Research institutions/Department of Energy labs (4%): This employment category includes energy-related programs funded by the U.S.
AAAAAAAAAAAAAAAAA,AAAAAAAA«
JN icholas
is
an example of
how
geology-related positions are to people from diverse backgrounds.
open
AAA AAAAAAAAAAJ
Hiliit illi tiiti
j
ranges that formed by continent-continent collision are Sea
the Appalachians, Alps,
and Urals
(see
Chapter
14).
level
Transform Boundaries Thej hird ary
type of rjlaiejjoundary is a transform bounda long transform faults where plates
These occur
slide laterall y past
one another roughly parallel to the
directionof_plate
movemen t. Although
lithosphere
is
neither created nor destroyed along a transform boundary, the
Oceanic
movement between
intensely shattered rock
Upper
plates results in a zone of and numerous shallow-focus
earthquakes.
mantle
Transform
(a)
faults are particular types of faults that
'
transform" or change one type~of motion_betjveen plates lntoan otRer type of notion. The majority of transfoFm raultsconnect two oceanic ridge segments, but they '
Transform fault
Trench
Sea
level
/
can also connect ridges to trenches and trenches to trenches (Fig. 13-21). While the majority of transform faults
^,
occur
in
oceanic crust and are marked by distinct
fracture zones, they
One
may
also extend into continents.
of the best-known transform faults
is the San Andreas fault in California. It separates the Pacific plate from the North American plate and connects spreading ridges in the Gulf of California and the ridge separating the Juan de Fuca and Pacific plates off the coast of
northern California
(Fig.
13-22).
The many earthquakes movement along
that affect California are the result of this fault. (b)
Transform
Trench
Sea
fall
|
„ Oceanic
f
ridge
level
^ PLATE MOVEMENT AND MOTION How
and in what direction are the Earth's various moving, and do they all move at the same rate? Rates of movement can be calculated in several ways. The least accurate method is to determine the age of the sediments immediately above any portion of the oceanic crust and divide that age by the distance from the spreading ridge. Such calculations give an average rate fast
plates
of movement.
Magma
Oceanic
Ajnore
/
crust
the magnetic reversals in the crust of the sea floor. Recall
mantle (c)
'"•'
that magnetic reversals are distributed symmetrically
FIGURE
13-21 Horizontal movement between plates occurs along a transform fault, (a) The majority of transform faults connect two oceanic ridge segments. Note that relative motion between the plates only occurs between the two ridges, (b) A transform fault connecting two trenches, (c) A transform fault connecting a ridge and a trench.
366
accura te method of determining both the avmovement and relative motion is by dating
erage rate of
Upper
Chapter 13
Plate Tectonics:
A
Unifying Theory
about and parallel to the oceanic ridges (Fig. 13-12), and that the age of each reversal has been determined. Therefore, the distance from an oceanic ridge axis to any magnetic reversal indicates the width of new sea floor that formed during that time interval. Thus, for a given interval of time, the wider the strip of sea floor, the faster the plate has moved. In this way not only can the
British
Columbia
<s^Xeg'
&f">"*
**
=*
J
>
in\
1 ,
•
that
«*
'„
*
'I*
%
ii
**
*
«•"* * •-"> -" * ' Ductile-brittle transition zone
t\
»,
;
Ductile lower crust
*
Ji'
%
*
//"*
/+ ~~ *"
=*
.
p
* „ xt
IK
J
1
>
and mantle
Deformation
379
such as a rock
layer.
For example,
in
Figure 14-8, the
surface of any of the tilted rock layers constitutes an inclined plane. The intersection of a horizontal plane with any of these inclined planes forms a line, the direction of which is the strike. The strike line's orientation is
determined by using a compass to measure its angle with respect to north. Dip is a measure of the maximum angular deviation of an inclined plane from horizontal, so it
must be measured perpendicular
to the strike direction
(Fig. 14-8).
Geologic maps indicate strike and dip by using a long line oriented in the strike direction
and a short
line per-
pendicular to the strike line and pointing in the dip direction (Fig. 14-9a). "''"
FIGURE
14-7 The principle of original horizontality holds that sediments are deposited in horizontal layers. These sedimentary rocks in Utah are inclined from horizontal, so we can infer that they were tilted after deposition and lithification. (Photo courtesy of David J. Matty.)
The number adjacent
to the strike
and dip symbol indicates the dip angle. A circled cross is used to indicate horizontal strata, and a strike symbol with a short crossbar indicates layers dipping vertically (Fig. 14-9b and c).
Folds cumulate in nearly horizontal layers (see Fig. 9-3). Thus, sedimentary rock layers that are steeply inclined must have been
tilted
following deposition and lithification
Some igneous
rocks, especially ash falls and form nearly horizontal layers. To describe the orientation of deformed rock layers, geol(Fig. 14-7).
many
lava flows, also
ogists use the concept of strike
Strike
is
and
dip.
the direction of a line formed by the inter-
section of a horizontal plane with an inclined plane,
^ FIGURE The
strike
is
14-8 Strike and formed by the
you place your hands on a tablecloth and move them toward one another, the tablecloth is deformed by compression into a series of up- and down-arched folds. SimIf
ilarly,
rock layers within the Earth's crust commonly
that
is,
to
the rocks have been strained plastically.
rocks at or near the surface are
dip.
(the water surface) with the surface of an inclined plane (the surface of the rock layer). Xhe_dip is th e maximum .angular deviation of the inclined plane from horizontal.
Chapter 14
Most
folding probably occurs deep within the crust because
intersection of a horizontal plane
380
re-
compression by folding. As opposed to the tablecloth, however, folding in rock layers is permanent;
spond
Deformation, Mountain Building, and the Evolution of Continents
brittle
and generally de-
-^ FIGURE
14-9
(a)
Strike
and
The long bar is oriented and the short bar points in the dip direction. The number indicates the dip angle. (£>) The symbol used to indicate horizontal rock layers, (c) The dip symbol.
in the strike direction,
symbol for
form by fracturing rather than by folding. The intensity of folding in
many rocks
is
quite impressive (Fig. 14-10).
^ FIGURE
14-10
vertical rock layers.
Intensely folded sedimentary rocks in
California. (Photo courtesy of
David
J.
Matty.)
Monoclines, Anticlines, and Synclines
A
monocline
is
a simple
bend or flexure
in
otherwise
horizontal or uniformily dipping rock layers (Fig. 1411a).
The large monocline in Figure 1 4- 1 1 b formed when Mountains of Wyoming were uplifted along
the Bighorn
a large fault. This fault did not penetrate to the surface, however, so as uplift occurred, the near-surface layers of rock were bent such that they appear to be draped over
the margin of the uplifted block (Fig. 14-1 lb).
An anticline is an up-arched fold, while a syncline is down-arched fold (Fig. 14-12). Both anticlines and synclines are characterized by an axial plane that divides them into halves; the part of a fold on opposite sides of the axial plane is a limb (Fig. 14-13). Because folds most a
commonly occur
as a series of anticlines alternating with
synclines, a limb
is
generally shared by an anticline and
an adjacent syncline.
important to remember that anticlines and synrock lasers arid not by the configuration of the Earth's surface. Thus, folds may or may not correspond to mountains and It is
clines are defined-hy. the oriejrtation of
valleys
surface
and may, is
rather
in fact, underlie areas flat (Fig.
where the Earth's com-
14-14). Indeed, folds are
Deformation
381
(b)
(a)
^ FIGURE
A
monocline. Notice the strike and dip symbols and the symbol for horizontal layers, (b) Uplift of the Bighorn Mountains in Wyoming formed
14-11
the monocline visible
(a)
on the
skyline.
monly exposed to view in areas that have been eroded. Even where the exposed view has been eroded, anticlines and synclines can easily be distinguished from each other by strike and dip and by the relative ages of the folded strata. As Figure 14-15 shows, in an eroded anticline, the strata of each limb dip outward or away from the center, where the oldest strata are located. In eroded synclines, on the other hand, the strata in each
-»t:
FIGURE
14-12
limb dip inward toward the center, and the youngest strata coincide
Thus folds in
far,
we
with the center of the fold. have described symmetrical, or upright,
which the
axial plane
limb dips at the same angle axial plane
is
inclined, the limbs dip at different angles,
Antidine_and
Calico Mountains of southeastern California.
Chapter 14
and each fold However, if the
vertical,
and the fold is characterized as asymmetrical (Fig. 4-16a). In an overturned fold, both limbs dip in the
s ynclinej n_the
382
is
(Fig. 14-13).
Deformation, Mountain Building, and the Evolution of Continents
^" FIGURE 14-14 These folded rocks in Kootenay National Park, British Columbia, Canada, illustrate that anticlines and synclines do not necessarily correspond to mountains and valleys Synclme
"^ FIGURE
14-13
axial plane, axis,
and
respectively.
Anticline
Syncline and anticline showing the fold limbs.
Plunging Folds Folds
may
be further characterized as nonplunging or
plunging. In the former, the fold axis, a line formed by
same
direction. In other
rotated
words, one fold limb has been
more than 90 degrees from
such that
it is
now
upside
down
its
(Fig.
original position
14-16b). Folds in
the intersection of the axial plane with the folded beds, is
horizontal (Fig. 14-13). However,
common
it
is
for the axis to be inclined so that
much more it
appears to
which theaxial_pjane is- horizontal-are, r eierre d_to_as recumbent (Fig. 14- 16c). Overturned and recumbent folds are particularly common in many mountain ranges
plunge beneath the surrounding strata; folds possessing
(discussed later in this chapter).
geologists use exactly the
an inclined axis are plunging folds (Fig. 14-17). To differentiate plunging anticlines from plunging synclines,
same
criteria
used for non-
•^ FIGURE 14-15 Identifying eroded anticlines and synclines.
Deformation
383
MARIE MORISAWA
Guest Essay
STUDYING THE EARTH: REFLECTIONS OF AN ENTHUSIAST on becoming a geologist; in fact, my major was mathematics. But in my junior year, friends convinced me to take an introductory geology course. That did it! I was fascinated by what I learned about the Earth and by how much we still did not know about it. It was too late to change my major, but my I
As
didn't plan
college
senior year
was
with as
filled
many
geology courses as
I
could take.
That
was held
interest
years, after
which
abeyance, however, for 10
in
decided to go back to graduate
I
school and study geology.
geology professor warned
Why? After all, my former me that I probably could not were not
get a position teaching geology because there
very
many geology departments
When
I
received
Wyoming, an not hire
me
in
company
then, did
I
women's
colleges.
geology at the University of
me he would would hire me as a
recruiter told
as a geologist— but
Why,
secretary.
my M.A.
oil
in
go on to obtain a Ph.D.
in
geology from Columbia University? In part because of students
my
and
my own
hold
interest
my
and encouragement of
the accepting attitude
professors. Then, too,
academically,
and enthusiasm
I
I
felt
that
could succeed.
for geology
fellow
if I
And
a geology teacher,
knowledge
felt
I
could do two things:
essential to their understanding of the
of
I could imbue some them with the same love and enthusiasm for
I have. So throughout my career I taught Brooklyn College, Bryn Mawr College, the University of Montana, Antioch College, and, finally,
geology that at
New York at Binghamton from which I recently retired. For a time, both as a student and as a professor, I also did research as a at the State University of
geologist for the U.S. Geological Survey.
As
I
worked
interested in
in geology,
how
I
became more and more humans and
the environment affects
how humans in turn affect the environment. Much of my research and teaching has been in that area. I found that human activity has upset the natural behavior of the Earth systems.
I
became
particularly interested in natural
(geologic) hazards such as
wave and river erosion, and volcanic eruptions
flooding, landslides, earthquakes,
how humans
and
finally
events.
I
came
have handled these catastrophic
to see that in order to cope with these
hazards in an environmentally compatible manner of
need,
What could be more interesting than the Earth on which we live? How was that rock formed? How do we know that a sheet of ice 915 m thick once covered the state of New York? Why did Mount St. Helens erupt? How did all the beautiful scenery that we see around us
processes at work. Only then can
to be? All these questions
answered.
many
And
and more need to be
good thing about geology
the
questions are
still
challenge— and even
I
unanswered. This
(or you) could
answer some of them. The delight these questions
is
is
is
that so
the
answer
the very complexity of the Earth's
and the continual change that
is
taking place in
itself is
the geologist's textbook
and
laboratory. Geomorphologists, such as myself, are the
who
study the landscape and the As an outdoor person, I combine work and recreation. Doing field work, hiking, canoeing, and camping are all part of a day's work. types of geologists
processes that form
it.
we
disasters. If
Chapter 14
we
do not understand the basic components of the Earth systems and how they work together, we increase the danger rather than mitigate the hazard. This is the me— to use our
present challenge of geology to
knowledge about the Earth to enhance the environment and to use it wisely. This makes geology worthwhile, a
JVlarie Morisawa graduated from Hunter College and earned an M.A. from the University of a Ph.D. from
Wyoming and
Her geomorphology and environmental geology. She has taught at several colleges and universities and recently retired from the State University of New York at Binghamton where she is University.
specialties are
professor emeritus.
lAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAJkAAAAAAAAAAAAAAA,
384
and
take suitable
measures to deal successfully with such
Columbia
the systems.
The Earth
all,
have a chance to
in trying to
physical systems, the interaction of one process with
another,
first
we
to understand the geologic setting
doubts about the future.
come
I
Earth environment. And, perhaps,
could
overcame any
I
could introduce a large number of students to the
Deformation, Mountain Building, and the Evolution of Continents
Axial plane
"•*
FIGURE
14-16
(a)
An
asymmetrical fold. The axial
and the fold limbs dip at different angles. (b) Overturned folds. Both fold limbs dip in the same direction, but one limb is inverted. Notice the special strike and dip symbol to indicate overturned beds, (c) Recumbent plane
is
not
vertical,
folds.
away from the fold whereas in plunging synclines all strata dip inward toward the axis. The oldest exposed strata are in the center of an eroded plunging anticline, whereas the youngest exposed strata are in the center of an eroded plunging syncline (Fig. 14-17b). In Chapter 7 we noted that anticlines form one type of structural trap for petroleum and natural gas (see Fig. 7-33). As a matter of fact, most of the world's petroleum plunging folds: that
is, all
strata dip
axis in plunging anticlines,
production comes from anticlinal traps, although several other types are important as well. Accordingly, geologists are particularly interested in correctly identifying the
geologic structures in areas of potential petroleum and natural gas production. Figure 14-18 shows hypothetical examples of how folds are identified from surface rock exposures and how buried folds are located.
Domes and
Basins
and synclines are elongate structures; that is, they tend to be long and narrow. Domes and basins, on Anticlines
the other hand, are the circular to oval equivalents of anticlines
and synclines
the oldest exposed rock the opposite
is
an eroded dome, whereas in a basin
(Fig. 14-19). In is
at the center,
true. All of the strata in a
dome
dip
away
from a central point (as opposed to dipping away from a fold axis, which is a line). By contrast, all the strata in a basin dip inward toward a central point (Fig. 14-19). Many domes and basins are of such large proportions that they can be visualized only on geologic maps or aerial photographs. The Black Hills of South Dakota, for example, are a large oval dome (Fig. 14-19b). One of the best-known large basins in the United States is the Michigan basin (Fig. 14-19d). Most of the Michigan
Deformation
385
Axial
plane
Angle of plunge
**-
FIGURE
14-17 Plunging folds schematic illustration of a plunging fold, (b) A block diagram (a)
A
showing surface and cross-sectional views of plunging folds. The long arrow at the center of each fold
shows the direction of plunge. (c) Surface view of the eroded, plunging Sheep Mountain anticline in
Wyoming.
basin
(c)
buried beneath younger strata so
is
it is
not
rectly observable at the surface. Nevertheless, strike
dip of exposed strata near the basin margin
sands of
drill
holes for oil and gas clearly
di-
and
and thou-
show
that the
deformed into a large structural basin. The Michigan basin was determined by using a combination of the methods shown in Figure 1418. It is a huge structure of overall basinal configuration, but much of its oil and gas production comes from small anticlines and domes.
Joints
which no movement has ocwhere movement has been perpendicular to
Joints are fractures along
curred, or
may
strata are
the fracture surface. In other words, the fracture
structure of the
open up, but no relative movement of the masses of rock on opposite sides of the fracture occurs parallel to the
386
Chapter 14
The term "joint" was originally used by coal miners long ago for cracks in rocks that appeared to be surfaces where adjacent blocks were "joined" together. fracture.
Deformation, Mountain Building, and the Evolution of Continents
•"-"
FIGURE
14-18
Identification of
geologic structures from surface
exposures,
[a)
Valley with rock exposures.
Data from these exposures are used to map and cross sections of the area. Strike and dip would be recorded at many places but only two (£>)
construct a geologic
are
Joints are the
commonest
structures in rocks; almost
near-surface rocks are jointed to
some degree
all
(Fig. 14-
The lack of any movement parallel to joint surfaces what distinguishes them from faults, which do show movement parallel with the fracture surface.
Joints can
shown
here.
form under a variety of conditions. For ex-
ample, anticlines are produced by compression, but the
20).
rock layers are arched such that tension occurs perpen-
is
dicular to fold crests,
and
joints
form
parallel to the long
axis of the fold in the upper part of a folded layer (Fig.
Deformation
387
I
14-19 (a) A block diagram of a dome. (b) A satellite view of an elongated dome, the Black Hills in western South Dakota, (c) A block diagram of a basin, [d) A map view of the Michigan basin.
14-21a). Joints also form in response to tension when rock layers are simply stretched (Fig. 14-21 b). Compressive stresses
can also produce joints as shown
in Figure
14-21c. Joints vary
388
from minute fractures to those of regional
Chapter 14
I
I
~~|
| Middle Devonian
Pennsylvanian
| Upper
"^ FIGURE
_H Upper Devonian
Jurassic
I
Mississippian
I
Lower Mississippian
|
H
Silurian
Ordovician
Mississippian and/or Devonian
(d)
extent (Fig. 14-20). Furthermore, they are often ar-
ranged
sets, and it is comtwo or perhaps three promiRegional mapping reveals that joints and joint
in parallel
or nearly parallel
mon
for a region to have
nent
sets.
sets are usually related to
Deformation, Mountain Building, and the Evolution of Continents
other geologic structures such
Weathering and erosion of jointed rocks Utah has produced the spectacular scenery of Arches
as large folds. in
National Park
One
(see Perspective 14-1).
type of joint pattern that
we have
already dis-
cussed consists of columnar joints that form in lava flows
and
in
some
some
intrusive igneous bodies. Recall
from Chapters 4 and 5 that as cooling lava contracts, it develops tensional stresses that form polygonal fracture patterns (see Figs. 4-13 and 5-1). Another type of jointing previously discussed is sheet jointing that forms in response to unloading (see Fig. 6-9).
Faults Faults are fractures along
which movement has occurred
parallel to the fracture surface.
A
tault plane
is
the frac -
"^ FIGURE
14-20
Jointed strata on the northeast flank of
the Salt Valley anticline, Arches National Park, Utah.
ture surface along which blocks of rock on opposite
"^ FIGURE anticline.
(£>)
14-21 Joints
{a) Folding and the formation of joints parallel to the crest of an produced by tension, (c) Joints formed in response to compression.
^-r^
(b)
(a)
(c)
Deformation
389
y
Perspective 14-1
FOLDING, JOINTS, AND ARCHES Arches National Park
in eastern Utah is noted for its which include such landforms as Delicate Arch, Double Arch, Landscape Arch, and many others (Fig. 1). Unfortunately, the term arch is
structures play a significant role in the origin of
panoramic
arches.
used for a variety of geologic features of different
vigorously along joints because these processes can
vistas,
we will restrict the term to mean an opening through a wall of rock that is formed by weathering and erosion. The arches of Arches National Park continue to origin, but here
form
as a result of
weathering and erosion of the
folded and jointed Entrada Sandstone, the rock
underlying
much
of the park. Accordingly, geologic
Where the Entrada Sandstone was folded into it was stretched so that parallel, vertical
anticlines, joints
formed. Weathering and erosion occur most
attack the exposed rock from both the top and the sides,
whereas only the top
adjacent joints. Figure 14-20.
^" FIGURE
sedimentary rocks, as shown
an arch.
2
Many
Some
sides have
such
fins
of rock between
fins are clearly visible in
parts of these fins are
Baby Arch shows the
early
more
development of
-'4
I-
TFault dip angle
attacked in unjointed
Erosion along joints causes them to enlarge, thereby forming long slender
"** FIGURE 1 Delicate Arch in Arches National Park, Utah formed by weathering and erosion of jointed in Figure 3.
is
strata (Fig. 14-20).
mov ed
relative to
one another. Notice
in Fig-
ure 14-22 that the blocks adjacent to the fault plane are labeled banging wall block and footwall block. The
11
hanging wall block is the block that overlies the fault, whereas the footwall block lies beneath the fault plane.
Hanging wall and footwall blocks can be defined with respect to any fault plane except those that are vertical. Understanding the concept of hanging wall and footwall
blocks
is
ment of
important because geologists use the move-
the hanging wall block relative to the footwall
Hanging Arrows
show
directions
of relative
390
movement
Chapter 14
w^tt-btock
-» FIGURE
14-22
Deformation, Mountain Building, and the Evolution of Continents
Fault terminology.
and erosion than others, and
susceptible to weathering
may
as the sides are attacked, a recess
form.
If it
does,
eventually pieces of the unsupported rock above the recess will fall away, forming an arch as the original recess
is
enlarged (Figs. 2 and
remnants of along
fins
joints.
Historical observations to
Thus, arches are
3).
formed by weathering and erosion
show
form today. For example,
enlarged
The park
when
a large block
in
that arches continue
1940, Skyline Arch was
fell
from
collapsed during prehistoric time.
* FIGURE
Arches National Park
recess, (b)
arches,
The arches continue
pinnacles, spires,
(c)
to enlarge until they finally
is
underside.
of arches that
When
collapse, they leave isolated pinnacles
(a) Weathering and erosion of a fin form a 3 These recesses expand and eventually develop into
its
many examples
also contains
arches
and
spires.
well worth visiting; the
and arches are impressive features
indeed.
collapse.
block to distinguish between two different types of
do wn
faults.
fault.
Like sedimentary beds, fault planes can be characterand dip (Fig. 14-22). Two basic types
ized by their strike
of faults are distinguished on the basis of whether the
blocks on opposite sides of the fault plane have
moved
relative to the block on the opposite side of th e Although it is not possible to tell how the blocks actually moved, it is usually easy to determine which block appears to have moved up or down in relation to the other. Thus, geologists refer to relative movement on faults. For example, in Figure 14-23a one cannot tell if
parallel to the direction of dip or along the direction of
the hanging wall block
strike.
block
moved down,
or
if
the footwall
both blocks moved. Nevertheless, the hanging wall block app ears to hav e moved dow nward "relative to the footwall bloc kT Such faults are
Dip-Slip Faults Dip-slipfaults are those on
moved
wh ich
all
movemen t
is
p ar-
called
up, or
normal
if
faults ,
whereas those where the hanging
alieTwithThe
wall block movedLug^relative to the footwall block are
words,
reverse faults (Fig. 14-23b).
all
dip of the fault p lane (Fig. 14-2j).~In other movement is such that one block moves up or
A
type of reverse fault in-
Deformation
391
Normal
Reverse
fault
fault
Rift
zone
Offset
stream
Strike-slip fault
Thrust
fault
0Wft£>, "'-w
^ FIGURE
Oblique-slip fault
14-23 Types of faults, (a), (b), and (c) are dip-slip faults, {a) Normal fault— hanging wall block down relative to footwall block. \b) and (c) Reverse and thrust faults— hanging wall block up. (d) Strike-slip fault— all movement parallel to strike of fault, (e) Oblique-slip fault— combination of dip-slip and strike-slip.
392
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
"•r FIGURE 14-24 east in
Owens
View of
uplifted along a large
normal
Nevada from the The mountains have been
the Sierra
Valley, California.
fault.
yojving a fault plane with a dip of
less
than 45°
is
a
thrust fauI t~(Fig~ 14-23c).
Normal
faults are
caused by tensional forces, s uch as
when the Earth's crust is stretched and by rifting. The mountain ranges of a large area
those that occur
thinned
called the Basin
and Range Province
in the
western
United States are bounded on one or both sides by major normal faults. A large normal fault is present along the east side of the Sierra
Nevada
in California; these
moun-
have been uplifted along this normal fault so that above the lowlands they now stand more than 3,000 tains
m
Continued normal faulting is also found along the eastern margin of the Teton Range
to the east (Fig. 14-24).
in
Wyoming
(Fig. 14-1).
Unlike normal jaults, reverse (and thrust) faults ar e
by compressio n (Fig. 14-25). Many large reverse and thrusfTauTti are present in mountain ranges that form at convergent plate margins (discussed later in the chapter). A well-known thrust fault is the Lewis overthrust of Montana. A large slab of Precambrian-aged rocks moved at least 75 km eastward on this fault and now rests upon much younger rocks of Cretaceous age c aused
(Fig.
14-26).
Strike-Slip Faults
Shearing forces are responsible for strike-slip faulting, a type~oTfau1tingTnvolving horizontal movement in which
(b)
opp osite sides of a^a^iltj^kne_sli de~siclewa vs past one~ano ther (Fig. 14-23d). In other words, all movement islrTthe direction of the fault plane's strike.
Mojave Desert, California, (b) Thrust fault in Sumter County, Alabama. The fault plane dips at 8°.
blocks o n
^ FIGURE
14-25
{a)
Reverse fault
in
welded
tuff,
Deformation
393
Precambrian rocks Chief Mountain
Cretaceous rocks (a)
(c)
(b)
"^ FIGURE
14-26
mountain,
Chief Mountain.
The Lewis overthrust fault in Glacier National Park, Montana. (a) Cross section showing the fault. As the slab of Precambrian rocks moved east along the fault, it deformed the rocks below. Chief Mountain is an erosional remnant of a more extensive slab of rock, (b) The trace of the fault is the light line on the side of the (c)
One of the best-known strike-slip faults is the San An dreas fau lt of California.* Recent movement on this fault caused the October zy, 1989 earthquake that damaged so much of Oakland, San Francisco, and several communities to the south and resulted in a 10-day delay
of the
World
Series (see the Prologue to
Chapter
10).
can be characterized as right-lateral depending on the apparent direction of
Strike-slip faults
or left-lateral, offset. In
Figure 14-23d, for example, an observer look-
ing at the block
mines whether to the
left.
on the opposite
it
side of the fault deter-
moved to the example, movement appears
appears to have
In this
'Recall from Chapter 13 that the San Andreas fault
transform fault in plate tectonics terminology.
394
Chapter 14
is
been to the
left,
so the fault
lateral strike-slip fault. strike-slip fault, the
is
Had
characterized as a
this
left-
been a right-lateral
block across the fault from the ob-
to have moved to the right. The San Andreas fault is a right-lateral strike-slip fault (see Figs. 10-3b and 14-27), whereas the Great Glen fault in Scot-
server
land
would appear
is
left-lateral (Fig. 14-28).
Oblique-Slip Faults It is
possible for
movement on
a fault to
show compo-
right or
nents of both dip-slip and strike-slip. For example,
to have
movement may be accompanied by a dip-slip component giving rise to a combined movement that includes left-lateral and reverse, or right-lateral and normal (Fig. 14-23e). Faults having components of both dip-slip and strike-slip movement are oblique-slip faults.
also called a
strike-slip
Deformation, Mountain Building, and the Evolution of Continents
-»-
FIGURE
14-27 Right-lateral by the San Andreas southern California, the offset about 21 m.
offset of a gully fault in
gully
is
^ MOUNTAINS any area of land that stands
The term mountain
refers to
significantly higher
than the surrounding country. but
Some
much more
mountains are
single, isolated peaks,
commonly they
are parts of a linear association of peaks
FIGURE 14-28 Map view of the left-lateral offset along the Great Glen fault of Scotland. The body of granite has been displaced by about 105 km.
and/or ridges called mountain ranges that are related in age and origin.
A
mountain system
is
a
tainous region consisting of several or ranges; the
Porky Mountains and
complex mounmany mountain
A ppalachians
are ex-
amples of mountain system s. Major mountain systems are indeed impressive features
and represent the
effects of
erating within the Earth.
The
dynamic processes op-
forces necessary to elevate
Himalayas of Asia to nearly 9 km above sea level are comprehend, yet when compared with the size of the Earth, even the loftiest mountains are very the
difficult to
small features. In fact, the greatest difference in elevation
on 2
on Earth
a globe 1
is
m
mm. From
about 20 km;
if
we
depicted this to scale
in diameter, its relief
the
human
would be
less
than
perspective, however, major
mountain systems are large-scale manifestations of tremendous forces that have produced folded, faulted, and thickened parts of the crust. Furthermore, in some mountain systems, such as the Andes of South America
Mountains
395
can develop over a hot spot, but more commonly a
series
of volcanoes develops as a plate moves over the hot spot,
Hawaiian Islands (see Fig. 13-24). also forms where the crust has been intruded by batholiths that are subsequently uplifted and eroded (Fig. 14-29). The Sweetgrass Hills as in the case of the
Mountainous topography
of northern
Montana
consist of resistant plutonic rocks
exposed following uplift and erosion of the softer overlying sedimentary rocks. Yet another way to form mountains — block-faulting— involves considerable deformation (Fig. faulting involves
or
more blocks
classic
example
movement on normal
14-30). Block-
faults so that
one
are elevated relative to adjacent areas. is
A
the large-scale block-faulting currently
occurring in the Basin and Range Province of the western
United States, a large area centered on Nevada but extend-
and northern Mexico. This numerous north-south trending mountain ranges, each of which is separated from the next range by a valley (Fig. 14-31). In the Basin and Range Proving into several adjacent states
region
^ FIGURE
14-29
(a)
Pluton overlain by sedimentary
is
characterized by
ince, the Earth's crust
is
being stretched in an east-west
rocks, (b) Erosion of the softer overlying rocks reveals the
pluton and forms small mountains.
direction; thus, tensional stresses
produce north-south
ented, range-bounding faults. Differ ential
and down-dropped blocks called grabens (Fig. 14-30). Horsts and grabens are bounded on both sides by parallel normal faults. Erosion of the horsts has yielded the mountainous topography now present, and the grabens have filled with sediments eroded from the horsts (Fig. 14-30). The processes discussed above can certainly yield mountains. However, the truly large mountain systems of the continents, such as the Alps of Europe and the Appalachians in North America, were produced by compression along convergent plate margins.
these faultsjias yielded uplifted blocks called horsts
and the Himalayas of Asia, the mountain-building processes remain active today.
Types of Mountains Mountainous topography can develop in a variety of ways, some of which involve little or no deformation of the Earth's crust. For example, a single volcanic mountain
'"•'"
FIGURE
14-30
Block-faulting and the origin of a horst and a graben.
^ Graben Horst
396
Chapter 14
ori-
movement n n
Deformation, Mountain Building, and the Evolution of Continents
"^ FIGURE
14-31
and Range Province bounded by normal in Nevada.
(a)
Cross section of part of the Basin
Nevada. The ranges and valleys are faults, (b) View of the Humboldt Range in
* MOUNTAIN BUILDING: OROGENESIS An orogeny
is an episode of mountain building du ring which ntense deformation occurs, generally accom pan ied bymetamo rp hism and the emplacement of pluton s, i
especially batholiths. esis', is still
Mountain
building, called orogen-
not completely understood, but
to be related to plate
movements. In
it is
fact, the
known
advent of
changed the way mountain systems. Any theory accounting for orogenesis must adequately explain the characteristics of mountain systems such as their long, narrow geometry and their location at or near plate tectonic theory has completely
geologists view the origin of
The intensity of deformation increases from the continental interior into mountain systems whereToverturned and recumbent folds and reverse and thfusTTaults indica ting compression are common Furthermore, both shallow and deep marine sedimentary rocks in mountain systems have been elevated far above sea level — in some cases as high as 9,000 m! plate margins.
.
Plate Boundaries
and Orogenesis
of the Earth's geologically recent and present-day orogenic activity is concentrated in two major zones or
Most belts:
the
Alpine-Himalayan orogenic belt and the
circum-Pacific orogenic belt (Fig. 14-32).
Most of
the
number of
these orogens, such as the
Himalayan oro-
gen, are active today. Older orogenic belt s include the
areas of the present-day Appalachia n frJ
Mountains of
orth America and the Ural Mountains in the So viet
Union.
Most orogenies occur
at
convergent plate boundaries
where one plate is subducted beneath another or where two continents collide. Subduction-related orogenies are t hose involving oceanic-oceanic and oceamc^ontinental plate boundaries.
Orogenesis at Oceanic-Oceanic Plate Boundaries Orogenies occurring where oceanic lithosphere is subducted beneath oceanic lithosphere are characterized by the formation of a volcanic island arc and by deformation
and igneous
activity.
Deformation occurs when sed-
iments derived from the volcanic island arc are compressed
along
a
convergent plate
boundary.
These
Earth's volcanic
sediments are deposited on the adjacent sea floor and in
two
and seismic activity also occurs in these Figs. 4-28 and 10-7). Both belts are com-
the back-arc basin.
posed of a number of smaller segments called orogens; each orogen is a zone of deformed rocks, many of which have been metamorphosed and intruded by plutons. A
sediments deposited in the oceanic trench, are deformed
belts (see
Those on the sea
floor,
including
and scraped off against the landward side of the trench (Fig. 14-33), thus forming a subduction complex, or
Mountain
Building: Orogenesis
397
"^ FIGURE activity
is
14-32
Most of
concentrated
and present-day orogenic and Alpine-Himalayan orogenic belts.
the Earth's geologically recent
in the circum-Pacific
-»-
FIGURE 14-33 Orogenesis and the origin of a volcanic island arc at an oceanic-oceanic plate
boundary.
Volcanic island arc
Asthenosphere
398
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
— accretionary wedge, of intricately folded rocks cut by
Orogenesis at Continental-Continental
numerous compression-induced thrust
Plate Boundaries
tion, orogenesis in
faults. In addi-
generated by plate convergence results
low-temperature, high-pressure metamorphism char-
acteristic of the blueschist facies (see Fig. 8-22).
Deformation of sedimentary rocks also occurs in the where it is caused largely by the emplacement of plutons, and many rocks show evidence of high-temperature, low-pressure metamorphism. The
In contrast to the Andes, the
when to
India
first
Himalayas of Asia formed 40
collided with Asia beginning about
50 million years ago. Prior
to that time, India
was
far
island arc system
overall effect of island arc orogenesis
is
the origin of
two
more-or-less parallel orogenic belts consisting of a land-
ward volcanic
island arc underlain by batholiths
seaward belt of deformed trench rocks
and a
(Fig. 14-33).
Orogenesis at Oceanic-Continental
"***
FIGURE
Generalized diagrams showing three Andes of South America. (a) Prior to 200 million years ago, the west coast of South America was a passive continental margin, (b) Orogenesis began when the west coast of South America became an active continental margin, (c) Continued deformation, volcanism, and plutonism.
Plate Boundaries
Passive continental margin
Sea
Many major mountain
systems including the Alps of
Europe and the Andes of South America formed
at
The— Ande s
of
oceanic-continental
western South
plate
Amer ica
boundaries.
are perhaps the best
such continuing orogeny of the
(Fig. 14-32).
example of
Among the ranges
Andes are the highest mountain peaks
Americas and
many
in the
active volcanoe s. Furthermore, the
west coast of South America
ment of the
cir cum-Pacific
is
an extremely active seg-
earth quake belt.
One
of the
Earth's great ocea nic trenchsysteTnp, the Peru-Chile
Trench,
lies just
14-34
stages in the development of the
orTlhe west coast ^Fig. 12-14).
200 million years ago, the western margin of South America was a passive continental margin, where sediments accumulated on the continental shelf, slope, and rise much as they currently do along the east coast of North America. However, when Pangaea split apart in response to rifting along what is now the MidAtlantic Ridge, the South American plate moved westward. As a consequence, the oceanic lithosphere west of South America began subducting beneath the continent (Fig. 14-34). As subduction proceeded, sedimentary rocks of the passive continental margin were folded and faulted and are now part of the accretionary wedge Prior to
along the west coast of South America. Accretionary wedges here and elsewhere commonly contain fragments of oceanic crust and upper mantle called ophiolites (see Fig. 12-26). Subduction also resulted in partial melting of the descending plate prod ucing a~v ofcanic arc, and numerous large plutons were emplaced beneath the arc (Fig. 14-34t: The Rocky Mountains of North America also formed as a consequence of pl ate convergence and subdu ction. However, they differ from other mountain systems in several important aspects (see Perspective 14-2).
level
v K
Perspective 14-2
THE ORIGIN OF THE
ROCKY MOUNTAINS
are part of a complex mountainous region known as the North American Cordillera, which extends from Alaska into central
The Rocky Mountains
-"-FIGURE
1
Map
of the
North American Cordillera United States.
Mexico. In the western United States, the Cordillera widens to about 1,200 km and is one of the most complex parts of the circum-Pacific orogenic belt
in the
Cenozoic basins Coast
of Pacific
Pliocene-
Pleistocene volcanics
Oceanic
Forearc
Arc volcanoes
trench
seismicity
\
Backarc Continental crust
seismicity
Base
of
lithosphere
(a)
Block
uplift
and rupture
"^*
FIGURE 2 Orogenies resulting (a) steep and [b) shallow subduction at oceanic-continental plate boundaries. In the shallowsubduction model, the subducted slab moves nearly horizontally beneath the continent, and volcanism ceases. from
Subhorizontal seismic zone (b)
(Fig. 1).
Although the Cordillera has a long history of
much
less steep
angle and moves nearly horizontally
deformation, the most recent episode of large-scale
beneath the continental lithosphere, deforming
deformation was the Laramide orogeny, which began 85 to 90 million years ago. Like many other
continental crust far inland from the continental
orogenies,
it
occurred along an oceanic-continental
However, deformation in the area of present-day Wyoming and Colorado occurred much farther inland from the continental margin than is typical (Fig. 1). Furthermore, mountain building was not accompanied by significant intrusions of granitic plate boundary.
batholiths.
To account for these observations, geologists have modified the classic model for orogenies along convergent plate margins. Geologists think that when is subducted beneath continental descends at a steep angle (30° or more),
oceanic lithosphere lithosphere,
it
from the trench, and on the continental the Laramide style of
a volcanic arc develops inland
the thick sediments deposited
margin are deformed. In orogeny, the subducted oceanic slab descends at a
margin
(Fig. 2).
occur only
Furthermore, magmatism seems to
when
the descending plate penetrates as
deep as the asthenosphere, so orogeny,
magmatism
is
in the
Laramide type of
suppressed.
Another consequence of shallow subduction seems produced large-scale fracturing of the crust and uplift of fault-bounded blocks; such deformation differs from the intense folding and to be deformation that
thrust faulting that characterizes a typical
oceanic-continental plate boundary orogeny. the ranges in the present-day as large blocks that
The Laramide
Many
of
Rocky Mountains began
were elevated along such faults. deformation ceased about 40
style of
million years ago, but since that time the Rocky Mountains have continued to evolve. For example, the mountain ranges that formed during the orogeny were (continued on next page)
Older sedimentary rocks
Thrust
Volcanic ash
fault
falls
Younger sedimentary rocks
Older sedimentary rocks Valleys
filled
to overflowing
Normal
"^ FIGURE
3
(a)
through
fault
(c)
Sediments eroded from the
blocks uplifted during the Laramide orogeny (d)
filled
the
were nearly covered. The sediment-filled valleys are eroded, and deep canyons
valleys
between ranges
until the ranges
are cut into the uplifted blocks by streams.
eroded, and the valleys between ranges
sediments
buried in their
402
rilled
with
Many of the ranges were nearly own erosional debris, and their
(Fig. 3).
Chapter 14
present-day elevations are the result of renewed uplift that continues to the present in
Prologue).
Deformation, Mountain Building, and the Evolution of Continents
some
areas (see the
south of Asia and separated from (Fig. 14-35a). As the Indian plate
it
by an ocean basin
moved northward,
"""
a
FIGURE
14-35
subduction zone formed along the southern margin of
{a)
was consumed (Fig. 1435a). Partial melting generated magma, which rose to form a volcanic arc, and large granite plutons were emplaced into what is now Tibet. At this stage, the activity along Asia's southern margin was similar to what is now Asia where oceanic lithosphere
showing the and the origin of the Himalayas.
Simplified cross sections
collision of India with Asia
The northern margin of
India before
its
collision
with
Asia. Subduction of oceanic lithosphere beneath southern
Tibet as India approached Asia, (b) About 40 to 50 million years ago, India collided with Asia, but since India was too light to be subducted, it was underthrust beneath Asia. (c) Continued convergence accompanied by thrusting of rocks of Asian origin onto the Indian Subcontinent. (d) Since about 10 million years ago, India has moved
occurring along the west coast of South America.
beneath Asia along the main boundary fault. Shallow marine sedimentary rocks that were deposited along India's northern margin now form the higher parts of the Himalayas. Sediment eroded from the Himalayas has been deposited on the Ganges Plain.
Crust
Volcano
Main Central Thrust
(c)
20-40
m.y.
Main Boundary Fault
Main Central Thrust -
(d)
20-0
m.y.
Main Boundary Fault
Mountain
Building: Orogenesis
403
The ocean separating
India from Asia continued to and India eventually collided with Asia (Fig. 1435b). As a result, two continental plates became welded, or sutured, together. Thus, the Himalayas are now loclose,
northward, and two major thrust faults carried rocks of Asian origin onto the Indian plate (Fig. 14-35c and d).
Rocks deposited ern margin
14-32 and 14-35b). The exact time of India's collision with Asia is uncertain, but between 40 and 50 million years ago, India's rate of northward drift decreased abruptly— from 15 to 20 cm per year to about 5
cm
(Figs.
per year. Because continental lithosphere
dense enough to be subducted,
this
is
not
decrease in rate
seems to mark the time of collision and India's resistance to subduction. Consequently, the leading margin of India
was
thrust beneath Asia, causing crustal thick-
ening, thrusting, and uplift. Sedimentary rocks that
been deposited
in
had
the sea south of Asia were thrust
Chapter 14
uplifted,
they were also
eroded, but at a rate insufficient to match the
Much
uplift.
of the debris shed from the rising mountains
was
transported to the south and deposited as a vast blanket
of sediment on the Ganges Plain and as huge submarine fans in the Arabian Sea
14-36). Since
its
and the Bay of Bengal
(Fig.
collision with Asia, India has been un-
derthrust about 2,000
km beneath Asia.
Currently, India
moving north at a rate of about 5 cm per year. A number of other mountain systems also formed as a result of collisions between two continental plates. The Urals in the Soviet Union and the Appalachians of is
"•" FIGURE 14-36 Sediment eroded from the Himalayas has been deposited as a vast blanket on the Ganges Plain and as large submarine fans in the Arabian Sea and the Bay of Bengal.
404
shallow seas along India's north-
the higher parts of the Himalayas.
As the Himalayas were
cated within a continent rather than along a continental
margin
in the
now form
Deformation, Mountain Building, and the Evolution of Continents
North America both formed by such
collisions (see Per-
platforms are collectively called cratons, so shields are
simply the exposed parts of cratons. Cratons are con-
spective 14-3).
sidered to be the stable interior parts of continents.
^ THE ORIGIN AND EVOLUTION
In
much
OF CONTINENTS Rocks 3.8
billion years old that are
continental crust are
known from
ing Minnesota, Greenland,
North America, the Canadian Shield includes of Canada; a large part of Greenland; parts of the
thought to represent
several areas, includ-
and South
ologists agree that even older crust
Africa.
Most
ge-
probably existed,
and, in fact, rocks dated at 3.96 billion years were re-
Canada. According to one model for the origin of continents,
cently discovered in
the earliest crust
was
thin
and unstable and was com-
posed of ultramafic igneous rock. This early ultramafic crust was disrupted by upwelling basaltic magmas at
and was consumed at subduction zones (Fig. 14would therefore have been destroyed because its density was great enough to make recycling by subduction very likely. Apparently, only crust of a more granitic composition, which has a lower density, is resistant to destruction by subduction. A second stage in crustal evolution began when partial melting of earlier formed basaltic crust resulted in the formation of andesitic island arcs, and partial melting of ridges
37a). Ultramafic crust
lower crustal andesites yielded granitic
were emplaced
in the crust that
magmas
had formed
that
earlier (Fig.
14-37b). By 3.96 to 3.8 billion years ago, plate motions accompanied by subduction and collisions of island arcs had formed several granitic continental nuclei.
Shields, Cratons,
and the
Evolution of Continents Each continent is characterized by one or more areas of exposed ancient rocks called a shield (see Fig. 8-4). Extending outward from these shields are broad platforms of ancient rocks that are buried beneath younger sediments and sedimentary rocks. The shields and buried
^ FIGURE
14-37
continental crust.
The
Model
for the origin of granitic
earliest crust
may have been
composed of ultramafic rock but was disrupted by rising magmas, {a) Basaltic crust is generated at spreading ridges its high density, subduction zones and is form at convergent plate margins. Granitic continental crust forms by collisions of
underlain by mantle plumes. Because of basaltic crust
is
consumed
at
recycled, (b) Andesitic island arcs
island arcs
and intrusions of
granitic
Subduction zone
magmas.
The Origin and Evolution of Continents
405
Perspective 14-3
PLATE TECTONIC HISTORY OF THE APPALACHIANS (Fig. 1) of eastern North America have a long and complex history that includes continental rifting, opening and closure of the same ocean basin, continental collision, and finally renewed continental rifting. The relationship between mountain building and the opening and closing of ocean basins is known as the Wilson cycle in honor of the Canadian geologist J. T. Wilson. Wilson was the first to suggest that an ancient ocean had closed to form the Appalachians and then reopened and widened to form the present-day Atlantic Ocean. During the Late Proterozoic Eon, a large rift
The Appalachian Mountains
developed
in a
supercontinent consisting of what are
now North America and
As rifting proceeded, an ocean basin formed and continued to widen along a divergent plate boundary (Fig. 2a and b). During this time, the east coast of North America and the west coast of Europe were passive continental margins,
much
Eurasia.
central Massachusetts,
and Vermont, was the
first
of
several orogenies to affect the Appalachian region.
Radiometric age dating of igneous rocks from Georgia Newfoundland indicates that the Taconic orogeny
to
occurred 480 to 440 million years ago. Continuing closure of the ocean basin resulted
in the
Acadian orogeny during the Silurian and Devonian periods (Fig. 2d). It affected the Appalachian region
from Newfoundland to Pennsylvania as continental margin sedimentary rocks were deformed and thrust northward and westward. Like the Taconic orogeny, the Acadian orogeny occurred along an oceanic-continental plate boundary, but collision occurred
it
culminated
when
continental
during the Devonian Period.
The Acadian orogeny was of
greater magnitude
than the Taconic orogeny, as indicated by more
widespread regional metamorphism and granitic intrusions. Radiometric dates from these rocks cluster
between 350 and 400 million years ago, indicating
as they are at the present. Plate
was the time of maximum deformation.
separation continued until the Early Paleozoic Era, at
that
which time the plate motions reversed, forming oceanic-continental plate boundaries on both sides of the ocean basin (Fig. 2c).
During the Late Paleozoic Era, the southern parts of the Appalachian region from New York to Alabama
The
resulting Taconic orogeny,
named
for the
present-day Taconic Mountains of eastern
New
York,
were further deformed. This event, the Alleghenian orogeny,
was
the last in a succession of orogenies
beginning during the Early Paleozoic, and
it
coincides
with the amalgamation of the supercontinent Pangaea.
^ FIGURE
1
The folded Appalachian Mountains
eastern United States.
in the
During the Late Triassic Period, the first stage in the breakup of Pangaea began, with North America separating from Eurasia and North Africa. Along the
North America, from Nova Scotia to North Carolina, block-faulting occurred and formed numerous ranges with intervening valleys much like those of the present-day Basin and Range Province of east coast of
the western United States (Fig. 3). Great quantities of
poorly sorted red-colored nonmarine detrital sediments were deposited in the valleys, some of which are well-known for dinosaur footprints. Rifting was accompanied by widespread volcanism, which resulted in extensive lava flows and numerous dikes and sills (see Fig. 5-22).
Erosion of the block-fault mountains during the and Cretaceous periods produced a broad,
Jurassic
low-lying erosion surface.
Renewed
uplift
and erosion
during the Cenozoic Era account for the present-day
topography of the Appalachian Mountains.
406
Chapter 14
Deformation, Mountain Building, and the Evolution of Continents
(a)
Continental crust
Caledonian
AcadianCaledonian
Continental-
Tacontc Highlands
continental plate
bOL'
*- FIGURE 2 Early history of the Appalachian region. [a\ Opening of the Iapetus Ocean basin during the Late Proterozoic Eon. \b) The ocean continues to widen during the Early Paleozoic Era. (c) The ocean begins closing, and subducnon occurs on both sides, id) Final closure
'Oceanic-cc^' nenta (c)
plate
Ocean during
boundary
the
of the Iapetus
Devonian Period.
"• r FIGURE 3 Rifting of Pangaea during the Tnassic Period resulted in block-faulting in eastern North America. (j) Location of basins formed by block-faulting. [b-c\ Thick sedimentary deposits and dikes and sills filled the basins,
which were themselves broken by faults
Albany .
during
a
complex of normal
rifting.
^Connecticut Valley -'area
The Origin and Evolution of Continents
407
is not directly observable except in the Canadian where one can easily see the remnants of ancient mountains and early small cratons. Many of the exposed rocks are plutonic and metamorphic, and many of them show the structural complexities associated with
cretion
Shield
orogenesis.
^ MICROPLATE TECTONICS AND MOUNTAIN BUILDING In the preceding sections,
we
discussed orogenies along
convergent plate boundaries resulting cretion.
Much
during such events crust,
in continental ac-
of the material accreted to continents is
simply eroded older continental
but a significant amount of
to continents as well
— igneous
new
material
is
added
rocks that formed as a
consequence of subduction and partial melting, for example. While subduction is the predominant influence I
I I
on the tectonic history
Canadian Shield
I
in
many
regions of orogenesis,
other processes are also involved in mountain building Other exposed Precambrian rocks
and continental accretion,
Covered Precambrian rocks
I
"^ FIGURE
The North American
14-38
craton.
The
exposed Precambrianaged rocks. Extending from the shield are platforms of buried Precambrian rocks. The shield and platforms collectively make up the craton.
Canadian Shield
is
especially the accretion of mi-
croplates.
a large area of
During the
late
1970s and 1980s, geologists discovmany mountain systems are com-
ered that portions of
posed of small accreted lithospheric blocks that are clearly of foreign origin. These microplates differ completely in their fossil content, stratigraphy, structural
and paleomagnetic properties from the rocks of mountain system and adjacent craton. In fact, these microplates are so different from adjacent rocks that most geologists think that they formed elsewhere and were carried great distances as parts of other trends,
the surrounding
Lake Superior region in Minnesota, Wisconsin, and Michigan; and parts of the Adirondack Mountains of
New is
York
(Fig. 14-38). In general, the
a vast area of subdued topography,
Canadian Shield numerous lakes,
plates until they collided with other microplates or con-
and exposed ancient metamorphic, volcanic, plutonic, and sedimentary rocks. By about 2.5 billion years ago, the Canadian Shield area formed by the amalgamation of smaller cratons
tinents.
that collided along belts of deformation called orogens,
croplates are
thereby forming a larger craton
(Fig.
14-39a). Several
additional episodes of orogenesis resulted in further ac-
and eastern margins of the 570 million years ago, North America had a size and shape approximating that in Figure 14-39c. Further orogeny and accretion during the last 570 million years occurred mostly along the eastern, southern, and western margins cretion along the southern
craton as
shown
in
Figure 14-39b, so that by
Geologic evidence indicates that more than
25%
of
the entire Pacific coast from Alaska to Baja California
The accreting micomposed of volcanic island arcs, oceanic ridges, seamounts, and small fragments of continents that were scraped off and accreted to the continent's consists
of accreted microplates.
margin as the oceanic plate with which they were carwas subducted under the continent. It is estimated that more than 100 different-sized microplates have been added to the western margin of North America
ried
during the
The
last
200 million years
(Fig.
14-40).
basic plate tectonic reconstruction of orogenies
of the craton, giving rise to the present configuration of
and continental accretion remains unchanged, but the
North America.
details of such reconstructions are decidedly different in
Much younger
408
of the North American craton
is
covered by
strata, so the evidence for early continental ac-
Chapter 14
view of microplate tectonics. For example, growth along active continental margins is faster than along passive
Deformation, Mountain Building, and the Evolution of Continents
billions of
years
"*"
FIGURE
EZS3 >2.5
14-39
Hi 1.9-1.8
I
I
1.8-1.7
Three stages
I
1
1.7-1.6
I
1
1.2-1.0
in the early evolution
of the North American craton. (a) By about 2.5 billion years ago, North America consisted of the elements shown here, {b) and (c) Continental accretion along the southern and eastern margins of North America. By the
end of the Proterozoic Eon, 570 million years ago, North America had the size and shape shown diagrammatically in (c).
Microplate Tectonics and Mountain Building
409
FIGURE
""•*"
Some
14-40
of the accreted lithospheric
blocks called microplates that form the western margin of the North American craton. The light brown blocks
probably originated as parts of continents other than North America. The reddish brown blocks are possibly displaced parts of North America.
continental margins because of the accretion of microplates.
new
Furthermore, these accreted microplates are often
additions to a continent, rather than reworked older
continental material.
So far, most microplates have been identified in mountains of the North American Pacific coast region, but a number of such plates are suspected to be present in other ficult to
mountain systems as well. They are more difrecognize in older mountain systems, such as
the Appalachians, however, because of greater deforma-
and erosion. Nevertheless, about a dozen mi-
tion
croplates have been identified in the Appalachians, but their
boundaries are hard to
tectonics provides a
new way
identify.
Thus, microplate
of viewing the Earth and
of gaining a better understanding of the geologic history of the continents.
SUMMARY
CHAPTER 1.
Contorted and fractured rocks have been deformed or strained by applied stresses.
2.
Stresses are characterized as compressional,
tensional, or shear. Elastic strain
is not permanent, removed, the rocks return to their original shape or volume. Plastic strain and fracture are both permanent types of
meaning that when the
stress
is
deformation. 3.
The
orientation of deformed layers of rock
is
described by strike and dip. 4.
Rock layers that have been buckled into up- and down-arched folds are anticlines and synclines, respectively. They can be identified by the strike and dip of the folded rocks and by the relative age of the rocks
5.
in the center
Domes and
of eroded folds.
basins are the circular to oval
equivalents of anticlines and synclines, but are
commonly much 6.
Two
larger structures.
recognized: joints are fractures along which the only
410
7.
types of structures resulting from fracturing are
Joints,
form 8.
On
which are the commonest geologic
in
structures,
response to compression, tension, and shear.
dip-slip faults, all
movement
Two
is
in the dip
movement, if any, is perpendicular to the fracture surface, and faults are fractures along which the blocks on opposite sides of the fracture move
to tension, while reverse faults are caused by
parallel to the fracture surface.
compression.
Chapter 14
direction of the fault plane. faults are recognized:
Deformation, Mountain Building, and the Evolution of Continents
normal
varieties of dip-slip
faults
form
in
response
Strike-slip faults are those
9.
in the direction
on which
movement
all
is
*F
characterized as right-lateral or left-lateral depending
on the apparent direction of
offset of
1.
one block
Some
faults
strike-slip;
11.
12.
13.
dip-slip
and
they are called oblique-slip faults. 2.
continental plates collide.
4.
3.
volcanic island arc, deformation, igneous activity,
oceanic lithosphere at an oceanic-continental plate
15.
boundary also results in orogeny. Some mountain systems, such as the Himalayas, are within continents far from a present-day plate boundary. Such mountains formed when two continental plates collided and became sutured. A craton is the stable core of a continent. Broad areas in which the cratons of continents are exposed are called shields; each continent has at least one
17.
characterized as
compression;
d.
plastic; e.
as a result of accretion, a process
b.
brittle; b.
sheared;
fractured;
a.
d.
Most
fracturing; b.
d.
convection;
An
syncline;
An
An
a central point
fault
down
d.
reverse;
Faults
on which both
normal
basin
oblique-slip fault
compressional stress
orogeny
craton
plastic strain
dip
plunging fold
dip-slip fault
reverse fault
dome
shear stress
elastic strain
shield
fault
strain
plane footwall block
stress
fracture
strike-slip fault
hanging wall block
syncline
joint
tensional stress
microplate
thrust fault
fault
normal;
strike-slip; c. joint.
e.
dip-slip
and
strike-slip
are referred to as:
recumbent; c. obliqueb. normal-slip. nonplunging; e. The range-bounding faults in the Basin and Range Province of the western United States plunging;
slip; d.
9.
fault
are
10.
faults.
a.
normal;
d.
strike-slip; e.
A a.
strike
to
is
fault.
a.
^ IMPORTANT
dome; recumbent
relative to the footwall block
movement has occurred
anticline
strata dipping a(n):
is
basin.
e.
thrust; b.
TERMS
is
on which the hanging wall block appears
a.
8.
all
plunging anticline; b. overturned syncline; d.
a. c.
a
microplates collide with
the axis
vertical; c.
oval to circular fold with
and igneous rocks to the margin of a craton during
when
is
the strata in one limb are horizontal;
outward from
continents.
basin;
c.
anticline.
the strata are faulted as well as folded.
e.
realize that continental accretion
the strata dip in
monocline;
e.
inclined; d.
A
all
a(n):
the axial plain
b.
7.
is
rifting;
overturned fold is one in which: both limbs dip in the same direction;
a.
6.
compaction; c. compression.
e.
elongate fold in which
d. 5.
ductile;
c.
folding results from:
a.
orogenesis. also occurs
plastic strain are
of these.
all
e.
have moved
now
tensional;
elastic; c.
shear.
involving the addition of eroded continental material
Geologists
deformed rocks
if
they are no longer subjected
Rocks that show a large amount of
syncline;
formed
when
a.
toward the center a. dome; b.
shield area. 16. Cratons
is
said to be:
and metamorphism characterize orogenies occurring at oceanic-oceanic plate boundaries. Subduction of
14.
Strain
to stress.
show components of both
Mountains can form in a variety of ways, some of which involve little or no folding or faulting. Mountain systems consisting of several mountain ranges result from deformation related to plate movements. Most orogenies occur where plates converge and one plate is subducted beneath another or where two
A
QUESTIONS
regain their shape
relative to the other.
10.
REVIEW
of strike of the fault plane. They are
graben
reverse;
b.
c.
thrust;
oblique-slip.
is a:
fold with a horizontal axial plane; b.
of reverse fault with a very low dip;
c.
type fracture
along which no movement has occurred; down-dropped block bounded by normal d. faults; e.
type of structure resulting from
compression. 11. In
which of the following
is
an orogeny currently
taking place? a.
east coast of
North America;
coast of South America;
d
central Africa;
e.
c.
b.
west
the Appalachians;
western Europe.
monocline
Review Questions
411
have have mainly
mainly vertical displacement;
c.
horizontal movement; d
are faults
movement has by
yet occurred;
Which of
What
c.
normal
are recumbent and overturned folds?
How
do
30.
Draw
subjected to
overturned.
from
joints differ
faults?
a simple cross section
showing the
displacement on a normal fault. 31. What type of stress is responsible for reverse 32. Explain
strike-slip fault;
basin;
fault; d.
recumbent
e.
33.
Draw on
fold.
which no movement has occurred monoclines;
joints; b.
axial planes;
transform
c.
fold limbs.
e.
intersection of an inclined plane with a
horizontal plane
is
the definition of:
a.
horizontal strata; b.
c
folded strata; d
movement;
dip-slip strike; e
mountain systems that form
joint.
at continental
is
meant by an oblique-slip fault. map showing the displacement
a left-lateral strike-slip fault.
two ways
in
which mountains can form with
or no folding and faulting.
little
faults; d.
what
a simple sketch
34. Discuss
are:
two examples of mountain systems in which mountain-building processes remain active. 36. Explain why two roughly parallel orogenic belts develop where oceanic lithosphere is subducted beneath continental lithosphere. 37. How do geologists account for mountain systems within continents, such as the Urals in the Soviet 35. Cite
Union?
margins: the Earth's crust
a.
between
faulting?
anticline; b.
17. In
criteria for distinguishing
What
folded; c
a
The
two
29.
the following might result from tensional
15. Fractures along
16.
are the
same patterns on two important ways.
28.
stresses?
a
the
have been:
elastically strained; e.
tension; d.
show
basins
deformed by movement along
sheared; b
Assume
them?
closely spaced slippage planes are said to
a
Domes and
geologic maps, but differ in
uplift of the footwall block.
13. Solids that have been
14.
27.
on which no
are characterized
e.
syncline.
that these folds plunge to the east.
are low-angle reverse faults; b.
a.
and an adjacent plunging
anticline
12. Strike-slip faults:
is
thicker than average;
model
38. Briefly outline the
most deformation is caused by tensional little or no volcanic activity occurs; stresses; c. stretching and thinning of the continental d. crust occur; e. most deformation results from
that
b.
was presented
39. Explain
40.
What
is
how
for the origin of continents
in this chapter.
continents
"grow" by
accretion.
the difference between a reverse fault and a
thrust fault?
rifting.
18
The
circular equivalent of a syncline
is
a(n):
joint; c. basin; monocline; b. overturned fault. asymmetric anticline; e. d. 19 Sediments deposited in an oceanic trench and then deformed and scraped off against the landward side of the trench during an orogeny form a(n): divergent margin complex; b. accretionary a. island arc wedge; c. back-arc basin facies; d. orogenic continental margin complex. system; e. 20. An excellent example of a mountain system forming a.
as a result of a continent-continent collision
is
the:
^ ADDITIONAL
READINGS
Davis, G. H. 1984. Structural geology of rocks
and
regions.
&
New
York: John Wiley Sons. J. G. 1987. Structural geology:
Dennis,
Dubuque, Iowa: Hatcher, R. D.,
Jr.
Wm.
An
introduction.
C. Brown.
1990. Structural geology: Principles, concepts,
and problems. Columbus, Ohio: Merrill Publishing Co. Howell, D. G. 1985. Terranes. Scientific American v. 253, no. 5:
116-125. 1989. Tectonics of suspect terranes: Mountain building and continental growth. London: Chapman and Hall. Jones, D. L., A. Cox, P. Coney, and M. Beck. 1982. The growth of western North America. Scientific American v. 247, no. 5: .
21
Rocky Mountains;
c.
Andes; b. Himalayas;
What
types of evidence indicate that stress remains
a.
d.
Alps;
e.
Appalachians.
70-84.
active within the Earth?
22
How
do compression, tension, and shear
differ
from
How
is it
possible for rocks to behave both
and plastically? meant by the elastic
elastically
24.
What
is
25. Explain
how
limit of rocks?
the factors of rock type, time,
temperature, and pressure influence the type of strain in rocks.
26.
412
Draw
R.
a simple geologic
Chapter 14
map showing
a plunging
J.
1988. Geological structures and maps:
A
practical
New
York: Pergamon Press. Miyashiro, A., K. Aki, and A. M. C. Segnor. 1982. Orogeny. guide.
one another? 23.
Lisle,
&
New York: John Wiley Sons. Molnar, P. 1986. The geologic history and structure of the Himalaya. American Scientist 74, no. 2: 144-154. 1986. The structure of mountain ranges. Scientific American v. 255, no. 1: 70-79. Spencer, E. W. 1988. Introduction to the structure of the Earth. New York: McGraw-Hill Book Company.
Deformation, Mountain Building, and the Evolution of Continents
CHAPTER
15
MASS WA STING ^OUTLINE PROLOGUE INTRODUCTION FACTORS INFLUENCING MASS WASTING Slope Gradient
Weathering and Climate
Water Content Vegetation
Overloading
Geology and Slope
Stability
Triggering Mechanisms
^"Perspective 15-1: The Tragedy at Aberfan, Wales
TYPES OF MASS WASTING Falls
Slides -^-
Guest Essay: Cleansing the Earth— Waste
Management Flows
Complex Movements
RECOGNIZING AND MINIMIZING THE EFFECTS OF MASS MOVEMENTS ""T Perspective 15-2: The Vaiont Dam Disaster
CHAPTER SUMMARY
Hong Kong's most
destructive landslide
occurred on Po Shan road on June 18, 1972. Sixty-seven people were killed when a 68-m wide portion of this steep hillside failed, destroying a four-story building and a 13-story apartment block.
^'» * TK^ric-'«r^3E^K^aEC .-^^•^-^^•^^.^TK.^.-Kr* -
:
>
PROLOGUE
.
more than 50,000,000 m3 mud, rock, and water, flowed over ridges 140 m
the avalanche, consisting of
of
high obliterating everything in
|||||IlV|j
On May
31, 1970, a devastating
earthquake occurred about 25 km in the Peruvian Andes, about 65 km to the east, the violent shaking from the earthquake tore loose a huge block of snow, ice, and west of Chimbote, Peru. High
rock from the north peak of
Nevado Huascaran
(6,654 m), setting in motion one of this century's
worst landslides. Free-falling for about 1,000 m, this block of material smashed to the ground, displacing
thousands of tons of rock and generating a gigantic debris flow (Fig. 15-1). Hurtling down the mountain's steep glacial valley at speeds
up to 320
km
per hour,
its
path.
About 3 km east of the town of Yungay, where the valley makes a sharp bend, part of the debris flow overrode the valley walls and within seconds buried Yungay, instantly killing more than 20,000 of its residents (Fig. 15-1).
down
The main mass of
the flow
overwhelming the town of Ranrahirca and several other villages and burying about 5,000 more people. By the time the flow reached the bottom of the valley, its momentum carried it across the Rio Santa and some 60 m up the continued
the valley,
opposite bank. In a span of roughly four minutes
from the time of the
initial
ground shaking,
"»»" FIGURE 15-1 An earthquake 65 km away triggered a landslide on Nevado Huascaran, Peru, that destroyed the towns of Yungay and Ranrahirca and killed more than 25,000 people.
Pacific
Ocean
Prologue
415
^ FIGURE part of
15-2
Yungay
Cemetery Hill was the only 1970 landslide that of the town. Only 92 people
to escape the
destroyed the rest survived the destruction by running to the top of the hill.
approximately 25,000 people died, and most of the area's transportation, power, and communication
network was destroyed. Ironically, the
only part of Yungay that was not
buried was Cemetery Hill, where 92 people survived
by running to geophysicist
its
top
who was
Yungay provided
(Fig. 15-2).
A
Peruvian
giving a French couple a tour of
a vivid eyewitness account of the
disaster:
breaker coming in from the ocean.
one-half to three-quarters of a minute
when
the
earthquake shaking began to subside. At that time I heard a great roar coming from Huascaran. Looking
saw what appeared to be a cloud of dust and it looked as though a large mass of rock and ice was breaking loose from the north peak. My immediate reaction was to run for the high ground of Cemetery Hill, situated about 150 to 200 m away. I began running and noticed that there were many others in Yungay who were also running toward Cemetery Hill. About half to three-quarters of the way up the hill, the wife of my friend stumbled and fell and I turned up,
down
hill
who was
carrying
two small
children
toward the hilltop. The debris flow caught him and he threw the two children toward the hilltop, out of the path of the flow, to
swept him
down
safety,
although the debris flow
the valley, never to be seen again.
I
remember two women who were no more than a few meters behind me and I never did see them again. Looking around, I counted 92 persons who had also
also
saved themselves by running to the top of the
was and
the most horrible thing I
I
hill. It
have ever experienced
will never forget it.*
I
to help her
The
416
estimated the
to be at least
meters
As we drove past the cemetery the car began to shake. It was not until I had stopped the car that I realized that we were experiencing an earthquake. We immediately got out of the car and observed the effects of the earthquake around us. I saw several homes as well as a small bridge crossing a creek near Cemetery Hill collapse. It was, I suppose, after about
I
80 m high. I observed hundreds of people in Yungay running in all directions and many of them toward Cemetery Hill. All the while, there was a continuous loud roar and rumble. I reached the upper level of the cemetery near the top just as the debris flow struck the base of the hill and I was probably only 10 seconds ahead of it. At about the same time, I saw a man just a few
wave
back to her
crest of the
Chapter 15
feet.
wave had
As was,
and devastating as was not the first time a
tragic it
had swept down
Mass Wasting
huge
avalanche
the Rio Shacsha valley. In January
1962, another large chunk of snow,
ice,
and rock
broke off from the main glacier and generated a large debris avalanche that buried several villages and killed
about 4,000 people. *B. A. Bolt et
a curl, like a
this debris
destructive landslide
al.,
Geological Hazards
1977), pp. 37-39.
(New York:
Springer-Verlag,
Mass wasting
^ INTRODUCTION Geologists use the term landslide in a general sense to
cover a wide variety of mass movements that loss of life,
(also called mass movement) is defined downslope movement of material under the direct influence of gravity. Most types of mass wasting are aided by weathering and usually involve surficial material. The material moves at rates ranging from almost
as the
may
cause
property damage, or a general disruption of
human
imperceptible, as in the case of creep, to extremely fast
the
as in a rockfall or slide.
activities. For example, in 218 B.C., avalanches in European Alps buried 18,000 people; an earthquake-generated landslide in Hsian, China, killed an estimated 1,000,000 people in 1556; another 200,000 people died when the side of a hill collapsed due to an earthquake in Kansu, China, in 1920; and 7,000 people died when mudflows and avalanches destroyed Huaraz, Peru, in 1941. What makes these mass movements so terrifying, and yet so fascinating, is that they almost always occur with little or no warning and are over in a very short time, leaving behind a legacy of death and
Mass wasting is an important geologic process that can occur at any time and almost any place. While most people associate mass wasting with steep and unstable
destruction (Table 15-1).
ceptible types, such as creep, usually
Every year about 25 people are killed by landslides
in
the United States alone, while the total annual cost of
damages from them exceeds $1 billion. Almost all of the major landslides have natural causes, yet many of the smaller ones are the result of human activity and could have been prevented or their damage minimized.
"^ TABLE
15-1
Selected Landslides, Their Cause,
While water can play an imporis the major force
tant role, the relentless pull of gravity
behind mass wasting.
slopes,
it
can also occur on near-level land, given the
right geologic conditions. Furthermore, while the rapid
types of mass wasting, such as avalanches flows, typically get the
most
and mud-
publicity, the slow, imper-
do the greatest
amount of property damage.
A
basic
knowledge of mass wasting
some
is
important to
have been knowledge can help one avoid selecting an unsafe building site for a house or business or can be useful in making decisions about land use. avoid a recurrence of mistakes,
made during
the past. Such
and the Number of People Killed
tragic, that
GRAVITATIONAL FORCE
-•'
FIGURE
on
material's strength
the
amount of
A
15-3
strength depends
slope's shear
the slope
and cohesiveness,
internal friction
between grains, and any external support of the slope. These factors
promote slope
stability.
The
force
of gravity operates perpendicular to the horizontal but has a component acting parallel to the slope. force,
which promotes
When
this
instability,
Component
exceeds a slope's shear strength, slope
* FACTORS INFLUENCING MASS WASTING When its
the gravitational force acting
on
ternal support of the slope (Fig.
resisting forces helping to
Opposing
a slope exceeds
maintain slope
ity.
a slope's shear strength
causing instability gle, the greater the
between grains, and any ex-
is
the force of grav-
but has a component acting parallel to the slope, thereby
include the slope material's strength and cohesion, the internal friction
These factors
Gravity operates perpendicular to the horizontal
stability
amount of
15-3).
collectively define a slope's shear strength.
resisting force, slope failure (mass wasting) occurs.
The
of gravitational
force acting parallel to slope
failure occurs.
the slope,
The
and the
(Fig. 15-3). The greater a slope's ancomponent of force acting parallel to greater the chance for mass wasting.
steepest angle that a slope can maintain without
collapsing
is its
angle of repose. At this angle, the shear
strength of the slope's material exactly counterbalances the force of gravity. For unconsolidated material, the angle
of repose normally ranges from 25° to 40°. Slopes steeper
than 40° usually consist of unweathered solid rock.
"^ FIGURE
15-4 Undercutting by stream erosion removes a slope's base, which increases the slope angle and (b) can lead to slope failure, (c) Undercutting by stream erosion caused slumping along this stream near Weidman, (a)
Michigan.
418
Chapter 15
Mass Wasting
All slopes are in a state of
means
dynamic equilibrium, which
that they constantly adjust in response to
new
Slope Gradient
con-
While we tend to view mass wasting as a disrupand usually destructive event, it is one of the ways that
ditions.
Slope gradient
tive
ing.
a slope adjusts to
new
conditions.
Whenever
a building or
is
probably the major cause of mass wast-
Generally speaking, the steeper the slope, the
stable
it
is.
Therefore, steep slopes are
more
on a hillside, the equilibrium of that The slope must then adjust, perhaps by mass wasting, to this new set of conditions. Many factors can cause mass wasting: slope gradient,
experience mass wasting than gentle ones.
weakening of material by weathering, increased water content, changes in the vegetation cover, and overloading. Although most of these are interrelated, we will examine them separately for ease of discussion, but will also show how they individually and collectively affect a
the slope angle,
slope's equilibrium.
are another
road
slope
is
is
constructed affected.
less
likely to
A number of processes can oversteepen a slope. One of the
most
common
is
undercutting by stream or wave ac-
tion (Fig. 15-4). This removes the slope's base, increases
and thereby increases the gravitational
force acting parallel to the slope.
Wave
action, especially
during storms, often results in mass movements along the shores of oceans or large lakes.
Excavations for road cuts and hillside building
major cause of slope
failure (Fig.
sites
15-5).
""' FIGURE 15-5 {a) Highway excavations disturb the equilibrium of a slope by [b) removing a portion of its support as well as oversteepening it at the point of excavation, (c) Such action can result in frequent landslides. (d) Cutting into the hillside to construct this portion of the
Pan American Highway in Mexico resulted in a rockfall that completely blocked the road. (Photo courtesy of R. V. Dietrich.)
Factors Influencing
Mass Wasting
419
30
—
"•"
FIGURE
15-7
A
California
Highway Patrol officer stands on top of a 2-m high wall of mud that rolled over a patrol car near the
Golden
State
Freeway on October
23, 1987. Flooding and mudslides also trapped other vehicles and closed the freeway.
up (Fig. 15-7). The soils of many hillZealand are sliding because deep-rooted native bushes have been replaced by shallow-rooted dollars to clean sides in
New
grasses used for sheep grazing.
When
heavy rains satucannot hold the
rate the soil, the shallow-rooted grasses
and parts of
slope in place,
it
rection as the slope, water can percolate along the var-
friction
particularly true
when
there are interbedded clay layers
when
because clay becomes very slippery
Even
slide downhill.
and decrease the cohesiveness and between adjacent rock units (Fig. 15-8a). This is
ious bedding planes
if
wet.
the rocks are horizontal or dip in a direction
may dip in the same Water migrating through them weathers the rock and expands these openings until the opposite to that of the slope, joints direction as the slope.
Overloading is almost always the result of human acand typically results from dumping, filling, or piling up of material. Under natural conditions, a material's load is carried by its grain-to-grain contacts, and a slope is thus maintained by the friction between the grains. The additional weight created by overloading, however, increases the water pressure within the material, which in turn decreases its shear strength, thereby weakening the slope material. If enough material is added, the slope will eventually fail, sometimes with
Overloading
weight of the overlying rock causes
it
to
fall (Fig.
15-8b).
tivity
tragic consequences.
Geology and Slope The
relationship between topography
of an area (Fig.
Stability
is
important
in
and the geology
determining slope stability
15-8). If the rocks underlying a slope dip in the
same direction to occur
than
as the slope, if
mass wasting
is
more
likely
the rocks are horizontal or dip in the
opposite direction.
When
the rocks dip in the
same
di-
Triggering Mechanisms While the factors previously discussed all contribute to slope instability, most— though not all — rapid mass movements are triggered by a force that temporarily disturbs slope equilibrium. The most common triggering mechanisms are strong vibrations from earthquakes and excessive amounts of water from a winter snow melt or a heavy rainstorm. Earthquakes are the most common type of strong vibrations and thus trigger many mass movements (see the Prologue and the Chapter 13 Prologue). In many cases, the resulting landslide causes far more damage and poses a greater threat to people than the earthquake
itself.
Volcanic eruptions, explosions, and even loud claps of thunder slope
is
may
be enough to trigger a landslide
sufficiently unstable.
Many
Factors Influencing
if
the
avalanches, which
Mass Wasting
421
Perspective 15-1
THE TRAGEDY AT ABERFAN, WALES debris brought out of underground coal mines in southern Wales typically consists of a wet mixture of
The
various sedimentary rock fragments. This material usually builds
dumped along
up
is
the nearest valley slope where
into large waste piles called tips.
A
it
tip is
long as the material composing it is and its sides are not oversteepened. Between 1918 and 1966, seven large tips composed of mine debris had been built at various elevations on the valley slopes above the small coal-mining village of Aberfan (Fig. 1). Shortly after 9:00 a.m. on October 21, 1966, the 250 m high, rain-soaked Tip No. 7 collapsed, and a black sludge flowed down the fairly stable as
relatively dry
it came 800 m from its starting place, the flow had destroyed two farm cottages, crossed a canal, and
valley with a loud train roar (Fig. 2). Before to a halt
buried Pantglas Junior School, suffocating virtually
A
all
144 people died in the flow, among them 116 children who had gathered for morning assembly in the school. the children of Aberfan.
total of
After the disaster, everyone asked,
tragedy occur and could
it
"Why
did this
have been prevented?" The
subsequent investigation revealed that no stability
•^ FIGURE 1 Aberfan, Wales, and a cross section showing the various tips built along the valley walls above Aberfan.
422
Chapter 15
Mass Wasting
could result from a combination of various geologic features including springs In 1939, 8
km
and seeps from the
tip.
to the south, a tip constructed
under
conditions almost identical to those of Tip No. 7
no one was injured, but was soon forgotten and the Aberfan tips continued to grow. In 1944 Tip No. 4 failed, and again no one was injured. By the time Tip No. 5 was closed in 1956, it had a large, ominous bulge growing on its lower side, but fortunately it collapsed. Luckily
unfortunately the failure
never
slid.
1958 Tip No. 7 was sited solely on the basis of available space, with no regard to the area's geology. The springs and seeps, though they were visible and well known, were completely ignored. In spite of previous tip failures and warnings of slope failure by tip workers and others, mine debris was being piled onto Tip No. 7 until the day of the disaster. What exactly caused Tip No. 7 and the others to In
fail?
The
official investigation
revealed that the
had become saturated with water from the springs over which they were built. In the case of the collapsed tips, pore pressure from the water exceeded the friction between grains, and the entire mass liquefied like a "quicksand." Behaving as a liquid, the mass quickly moved downhill spreading out laterally. As it flowed, water escaped from the mass, and the sedimentary particles regained their foundation of the
tips
cohesion.
Following the inquiry,
"^ FIGURE
2 Aerial view of the Aberfan which 144 people died.
tip disaster in
had ever been made on the
tips
and that
repeated warnings about potential failure of the as well as previous slides,
that a
new
tip sites.
and advise on the
Unfortunately, six years
Aberfan disaster, a similar incident occurred West Virginia, where a water-saturated, coal-mining refuse dam collapsed. The resulting mudflow swept down the valley killing 118 people. after the
tips,
had all been ignored. As warned that tip failures
early as 1927, a publication
was recommended
assess the dangers of existing tips
construction of studies
it
National Tip Safety Committee be established to
in
Factors Influencing
Mass Wasting
423
Water percolates through soil and sandstone, wetting the clay layer,
which swells and
becomes
"•"
FIGURE
dipping hill's
slippery
(a) Rocks same direction as a
15-8
in the
slope are particularly
susceptible to
mass wasting.
Undercutting of the base of the slope by a stream removes support
and steepens the slope at the base. Water percolating through the soil and into the underlying rock increases
its
weight and,
if
clay
layers are present, wets the clay
making them
slippery, (b) Fractures dipping in the same direction as a slope are enlarged by chemical weathering, which can remove enough material to cause mass
wasting.
are rapid
movements of snow and ice down steep mounby the sound of a loud gunshot
tain slopes, are triggered or, in rare cases,
even a person's shout.
^ TYPES OF MASS WASTING Geologists recognize a variety of mass ble 15-2).
Some
a combination of different types.
424
movements
(Ta-
are of one distinct type, while others are
Chapter 15
Mass Wasting
It is
not
uncommon
for
one type of mass movement to change into another along its course. For example, a landslide may start out as a slump at its head and, with the addition of water, become an earthflow at its base. Even though many slope failures are combinations of different materials and movements, it is still convenient to classify them according to their dominant behavior. Mass movements are generally classified on the basis of three major criteria (Table 15-2): (1) rate of move-
"^ TABLE
15-2
Classification of
Mass Movements and Their
Characteristics
"^ FIGURE
15-10 Numerous rockfalls have resulted from wedging of these bedded and fractured rocks at Alberta Falls, Rocky Mountain National Park, Colorado. Accumulations of talus can be seen at the base of these frost
outcrops.
Rockfalls range in size from small rocks falling from a cliff to massive falls involving millions of cubic meters
of debris that destroy buildings, block highways (Fig. 15-11),
and even bury towns. When
large blocks of rock
into restricted bodies of water, they
fall
may
generate
huge waves capable of tremendous damage. One of the largest of these occurred on July 9, 1958, in Lituya Bay, Alaska. An earthquake dislodged an estimated 30.5 mil-
m3
lion
of rock that
fell
on
level
its
opposite side (see Perspective 20-1, Fig.
Rockfalls are a eas
m into the bay, m above the bay's
more than 900
causing a surge of water that rose 524
common
where roads have been
built
2).
mountainous arby blasting and grading
hazard
in
through steep hillsides of bedrock. Anyone who has ever driven through the Appalachian Mountains, the Rocky Mountains, or the Sierra Nevada is familiar with the
"Watch
for Falling
Rocks"
warn
signs posted to
drivers
of the danger. Slopes particularly prone to rockfalls are
sometimes covered with wire mesh in an effort to prevent dislodged rocks from falling to the road below. Another tactic is to put up wire mesh fences along the base of the slope to catch or slow down bouncing or rolling rocks.
Slides
A
slide involves
more soil,
rock, or a combination of the two, and
apart during
426
movement of material along one or The type of material may be
surfaces of failure.
movement or remain
Chapter 15
Mass Wasting
intact.
it
A
may
break
slide's rate
•**- FIGURE 15-11 Rockfall in Jefferson County, Colorado. All eastbound traffic and part of the westbound lane of Interstate 70 were blocked by the rockfall. Heavy rainfall and failure along joints and foliation planes in Precambrian gneiss caused this rockfall.
of
movement can vary from extremely slow
to very
rapid (Table 15-2).
Two
types of slides are generally recognized:
(1)
slumps or rotational slides, in which movement occurs along a curved surface; and (2) rock or block glides, which move along a more-or-less planar surface. A slump involves the downward movement of material along a curved surface of rupture and is characterized by the backward rotation of the slump block (Fig. 15-12). Slumps occur most commonly in unconsolidated or weakly consolidated material and range in size from small individual sets, such as occur along stream banks, to massive, multiple sets that affect large areas
and cause considerable damage. Slumps can be caused by a variety of factors, but the most common is erosion along the base of a slope, which removes support for the overlying material. This local steepening may be caused naturally by stream erosion along its banks (Fig. 15-12) or by wave action at the base of a coastal cliff. Slope oversteepening can also be caused by human activity, such as the construction of highways and housing developments. Slumps are particularly prevalent along highway cuts and fills where they are generally the most frequent type of slope failure observed. While many slumps are merely a nuisance, large-scale slumps involving populated areas and highways can cause extensive damage. Such is the case in coastal southern California where slumping and sliding have been a constant problem. Many areas along the coast are underlain by poorly to weakly consolidated silts,
BONNIE ROBINSON
Guest Essay
rTTTTTTTTTTTTTTTTTTTTTTTTTTfTTTTTTTTyTT'TTTTT T TTTY T TTTTTTT
CLEANSING THE EARTHWASTE MANAGEMENT*
I
remember the moment when
in geology.
My
I
theory of continental
drift;
became interested was discussing the
first
fifth-grade teacher
using a
map
of the world,
showed us how North and South America could against Europe and Africa to form a single giant she
continent! This intriguing concept
made
so
fit
much
sense— it was like putting together the pieces of a giant jigsaw puzzle— and that is how I still view geology. From that time, I knew that the sciences were my
was an unusual pursuit
I
for an Africanan urban environment. always enjoyed being outdoors and examining maps.
I
was
I
went.
re. It
erican girl
I
a
growing up
in
rockhound, collecting rocks and
majored
broadened
in
my
wherever
fossils
geology at Oberlin College and
understanding of the
field
during
summer
Geology was fascinating because it linked all of the natural and physical sciences together with engineering and applied them to the study of the Earth. I learned that geology internships at the Smithsonian Institution.
human health and the environment, and and administrative controls on the generation, handling and disposal of the wastes. A national E&P waste management program would have far-reaching implications due to the complexity of the oil and gas industry, the wide range of environmental settings affected, and the variety of state regulatory programs. Oil and gas production is scattered throughout more than 30 states, where over 26,000 companies are involved in the exploration and production of oil and gas. Each year thousands of new wells are drilled and thousands of well sites are abandoned. The major wastes generated at these locations consist of water extracted with the oil and gas, drilling fluids, and a variety of lesser wastes. These wastes often contain varying amounts of potentially hazardous constituents. impacts on
legal
One
of the key issues facing the
determine the most
E&P
impacts on
land-use planning requires knowledge of geology, social
domestic production
and other
After college
I
but
skills.
worked
in
environmental geology at
oil
I
how
to
improving
and gas production. Continued is
vital to the nation's interest,
must be balanced with adequate environmental
protection.
Knowledge of
the U.S. Geological Survey, followed by graduate studies at the University of California, Santa Cruz.
it
is
waste management without significant adverse
influenced other fields of endeavor. For example, proper
sciences,
EPA
efficient alternatives for
literacy,
is
science
and technology, or science making
essential for intelligent decision
spent the next 13 years as a petroleum geologist,
regarding critical national issues. Opportunities exist
working on oil and gas exploration and development projects throughout the western United States. My
for full participation by minorities
interest in
environmental issues affecting the
petroleum industry led to field
In
my
desire to
work
technology. in
the
and women, who and that we encourage, develop, and
are severely underrepresented in science
utilize this
It is vital
pool of
talent.
A
of waste management.
my
position at the Environmental Protection
Agency (EPA), I am involved in the development of the program for improved management of wastes generated by crude oil and natural gas exploration and production (E&P) activities. The EPA's Office of Solid Waste is conducting studies of the characteristics of the wastes, waste handling methods and their "Opinions expressed in this paper are solely those of the author and do not necessarily represent those of the U.S. Environmental Protection Agency.
Oonnie Robinson earned an A.B. degree
in
geology from
Oberlin College in 1974, followed by graduate studies at the University of California at
Santa Cruz. She worked as a petroleum geologist in Denver,
Colorado, for 13 years and recently joined the U.S.
Environmental Protection Agency in Washington, D.C.
AAAAAAAAAAAAAAA AAAAAAAAAAAAAAAAAAAAAAAAAA AAAAAAAAA AAA AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Types of Mass Wasting
427
-~-
FIGURE
material
15-12
In a slump,
moves downward along
the
curved surface of a rupture, causing the slump block to rotate backward. Most slumps involve unconsolidated or weakly consolidated material and are typically caused by erosion along the slope's base.
Surface
of rupture
some of
sands, and gravels interbedded with clay layers,
which are weathered ash California
is
falls.
In addition, southern
tectonically active so that
many
of these
deposits are cut by faults and joints, which allow the
infrequent rains to percolate
and lubricating the clay
downward
rapidly, wetting
Southern California dry most of the year.
tween November fall in
lies in
When
a semiarid climate
a short time.
,
canyon walls
Pacific Palisades '
Santa Monica
Los Angeles
"»»
FIGURE
wave action
15-13
Undercutting of steep sea
cliffs
by
resulted in massive slumping in the Pacific
on March 31 and April 3, 1958. Highway 1 was completely blocked. Note the heavy earth-moving equipment for scale. Palisades area of southern California
428
Chapter 15
Mass Wasting
Pacific
is
Thus, the ground quickly becomes
saturated, leading to landslides along steep as well as along coastal cliffs (Fig. 15-13).
layers.
and
does rain, typically beand March, large amounts of rain can it
Ocean
Most
of the
•^ FIGURE
15-14
Rock
glides occur
when
material
moves downslope along
a
generally planar surface.
slope failures along the southern California coast are the
about 21
These slumps have destroyed many expensive homes and forced numerous roads to be closed and relocated. A rock or block glide occurs when rocks move downslope along a more-or-less planar surface. Most rock glides occur because the local slopes and rock layers dip in the same direction, although they can also
tons.
result of slumping.
occur along fractures parallel to a slope
(Fig. 15-14).
known rock glide in the world is the preSaidmarreh landslide in southwestern Iran (Fig. 15-15). A slab of limestone 305 m thick, 14 km long, and 5 km wide became detached from the Kabir Kuh ridge and slid down and across the adjacent 8 km wide The
largest
historic
Saidmarreh Valley with enough momentum to climb over a ridge 460 m high before stopping nearly 18 km
from
its
source!
The volume of
the slipped material
was
it
km 3
When
,
and
it
weighed approximately 50
billion
the debris from the rock glide finally settled,
covered an area of 166
The causes of factors: (1)
km 2
.
rock glide probably involved three the massive limestone dipped in the same this
was un-
direction as the local slope; (2) the limestone derlain by a
weak
claystone;
and
(3) its
base was un-
dercut by the Karkheh River. In addition, the area seismically active,
and
it is
is
believed an earthquake prob-
ably triggered the slide. In addition to slumping, rock glides are also
common
occurrences along the southern California coast. At Point Fermin, seaward-dipping rocks with interbedded slippery clay layers are undercut by
merous
waves causing nu-
glides (Fig. 15-16a).
Farther south in the residents
watched as
a
town of Laguna Beach,
startled
rock glide destroyed or damaged
Types of Mass Wasting
429
"^ FIGURE 15-15 The world's largest known rock glide occurred in the Saidmarreh Valley, some 96 km northwest of Dizful, Iran. An earthquake is believed to have triggered this massive prehistoric slide that covered an area of 166 km*.
5
Rubble following rock glide
Karkheh River
^-
430
Chapter 15
Mass Wasting
/
/
/
km
(b)
(a)
ir FIGURE
A
combination of interbedded clay beds that become slippery in the same direction as the slope of the sea cliffs, and undercutting of the sea cliffs by wave action has caused numerous rock glides and slumps at Point Fermin, California, (b) The same combination of factors apparently activated a rock glide farther south at Laguna Beach that destroyed numerous homes and cars on October 2, 1978. (Photo (a) courtesy of Eleanora I. Robbins, U. S.
15-16 (a) when wet, rocks dipping
Geological Survey.)
50 homes on October
2,
1978
(Fig.
15-16b). Just as at
previous winter's heavy rains wet a subsurface clayey
Point Fermin, the rocks at Laguna Beach dip about 25° in the same direction as the slope of the canyon walls
siltstone, thus
and contain clay beds that "lubricate" the overlying rock layers, causing the rocks and the houses built on them to glide. In addition, percolating water from the
about five acres, it was part of a larger ancient slide complex. Not all rock glides are the result of rocks dipping in
reducing
activate the glide.
its
shear strength and helping to
Although the 1978
glide covered only
Types of Mass Wasting
431
^V
v
the
same direction
The rock
as a hill's slope.
glide at
Frank, Alberta, Canada, on April 29, 1903, illustrates
how
nature and
human
activity
can combine to create a
situation with tragic results (Fig. 15-17). It would appear at town of Frank, lying was in no danger from
many
first
at least
50%
silt-
and
clay-sized particles, (b)
Mudflow
in
Estes Park, Colorado.
glance that the coal-mining
at the base of Turtle
Mountain,
a landslide (Fig. 15-17). After
of the rocks dipped
"»" FIGURE 15-18 [a) Mudflows are the most fluid of flows and consist of large amounts of water combined with
away from
all,
the mining valley,
unlike the situations at Saidmarreh, Point Fermin, and
Laguna Beach. The
joints in the massive limestone
com-
posing Turtle Mountain, however, dip steeply toward
and are essentially parallel with the slope of mountain itself. Furthermore, Turtle Mountain is supported by weak limestones, shales, and coal layers that underwent slow plastic deformation from the weight of the overlying massive limestone. Coal mining the valley the
along the base of the valley also contributed to the stress
on the rocks by removing some of the underlying support. All of these factors, as well as frost action and chemical weathering that widened the joints, finally re3 sulted in a massive rock glide. Almost 40 million m of rock slid down Turtle Mountain along joint planes, killing 70 people and partially burying the town of Frank.
(a)
Flows Mass movements
which material flows as a viscous movement are termed flows. Their rate of movement ranges from extremely slow to extremely rapid (Table 15-2). In many cases, mass movements may begin as falls, slumps, or slides and fluid
in
or displays plastic
change into flows further downslope.
Mudflows are the most fluid of the major mass movement types (Fig. 15-18). They consist of at least 50% silt- and clay-sized material combined with a significant amount of water (up to 30%). Mudflows are common in arid and semiarid environments where they are triggered by heavy rainstorms that quickly saturate the regolith, turning it into a raging flow of mud that engulfs everything in
its
path.
Mudflows can
also occur in
mountain regions and in areas covered by volcanic ash where they can be particularly destructive (see Chapter 4). Because mudflows are so fluid, they generally follow preexisting channels until the slope decreases or the
channel widens, at which point they fan out. Mudflows are very dangerous types of mass move-
ments because they typically form quickly, usually move very rapidly (at speeds up to 80
capable of transporting
all
km
per hour), and are
different sizes of objects.
As
urban areas in arid and semiarid climates continue to expand, mudflows and the damage they create are beTypes of Mass Wasting
433
•^ FIGURE
15-19
Debris flows
contain larger-sized particles than mudflows and are not as fluid. Debris flows can be very destructive in
mountainous regions because of and
the steep slopes, loose material,
water available from melting snow.
coming problems. For example, mudflows are very common in the steep hillsides around Los Angeles where they have damaged or destroyed many homes. In addition to the damage they cause on hillsides, mudflows are also a hazard to structures built along the bases of steep mountain fronts. This danger arises because mudflows forming in the mountains follow valleys down the mountainside until they reach the base where they fan out onto the
flat
highway, or railroad tracks will be quickly
flow tive
moved or
valley floor. in the
Any
building,
path of the mudflow
buried. For example, a
mud-
Cajon Pass near Los Angeles carried a locomoa distance of more than 600 m before burying it. in
Debris flows are composed of larger-sized particles
much water. Conmore viscous than mudflows,
than mudflows and do not contain as sequently, they are usually typically
do not move
as rapidly,
and
rarely are confined
to preexisting channels. Debris flows can, just as
however, be large ob-
damaging because they can transport
jects (Fig. 15-19).
In semiarid regions, debris flows, like mudflows, are
quite destructive,
and depending on the amount of water
commonly
wet regolith
mudflows and debris any size, and are frequently destructive. They occur, however, most commonly in (Fig.
15-20). Like
flows, earthflows can be of
humid heavy
climates
on grassy soil-covered slopes following
rains.
Some clays spontaneously liquefy and flow like water when they are disturbed. Such quick clays have caused serious damage and loss of lives in Sweden, Norway, eastern Canada, are
composed of
and Alaska (Table 15-1). Quick clays fine silt and clay particles made by the
grinding action of glaciers. Geologists believe these fine
sediments were originally deposited
in a
marine envi-
ronment where their pore space was filled with salt water. The ions in the salt water helped establish strong bonds between the clay particles, thus stabilizing and strengthening the clay. However, when the clays were subsequently uplifted above sea level, the salt water was flushed out by fresh groundwater, reducing the effectiveness of the ionic bonds between the clay particles and thereby reducing the overall strength and cohesiveness of the clay. Consequently,
when
a sudden shock or shaking,
it
the clay
is
disturbed by
essentially turns to a liquid
part of a hillside, leaving a scarp, and flows slowly
and flows. An example of the damage that can be done by quick clays occurred in the Turnagain Heights area of Anchorage, Alaska, in 1964 (Fig. 15-21). Underlying most of the Anchorage area is the Bootlegger Cove Clay, a massive clay unit of poor permeability. Because the Bootlegger Cove Clay forms a barrier preventing groundwater from
downslope
flowing through the adjacent glacial deposits to the sea,
present, they
intergrade. Debris flows are also
mountainous regions because of the combination of steep slopes, great amounts of loose debris, and large volumes of water from melting snow. particularly destructive in
Earthflows
move more slowly than
or debris flows.
434
An
either
mudflows
earthflow slumps from the upper
as a thick, viscous, tongue-shaped
Chapter 15
Mass Wasting
mass of
considerable hydraulic pressure builds up behind the clay.
Some of this water has
the clay
and
flushed out the salt water in
also has saturated the lenses of sand
associated with the clay beds.
Good
When
Friday earthquake struck on
and
silt
the 8.5-magnitude
March
27, 1964, the
shaking turned parts of the Bootlegger Cove Clay into a quick clay and precipitated a series of massive slides in the coastal bluffs that destroyed
most of the homes
in the
Turnagain Heights subdivision. Solifluction is the slow downslope movement of water-saturated surface sediment. Solifluction can occur in
any climate where the ground becomes saturated with
water, but
is
most common
in cold climates
where the
upper surface periodically thaws and freezes. Permafrost is ground that remains permanently frozen. It
covers nearly
20%
of the world's land surface
(Fig.
15-
During the warmer season when the upper portion of the permafrost thaws, water and surface sediment form a soggy mass that flows by solifluction and produces a 22a).
topography (Fig. 15-22b). As might be expected, many problems are associated
characteristic lobate
A good what happens when an uninsulated building is constructed directly on permafrost. In this instance, heat escapes through the floor, thaws the ground below,
Construction of the Alaska pipeline from the oil fields Prudhoe Bay to the ice-free port of Valdez raised numerous concerns over the effect it might have on the permafrost and the potential for solifluction. Some in
thought that
oil
warm enough
flowing through the pipeline would be
to melt the permafrost, causing the pipe-
ground and possibly rupture. were conducted, scientists concompleted in 1977, could safely
line to sink further into the
numerous
After
studies
cluded that the pipeline,
be buried for more than half of its 1,280 km length; where melting of the permafrost might cause structural problems to the pipe, it was insulated and installed above ground. Creep is the slowest type of flow. It is also the most widespread and significant mass wasting process in terms of the total amount of material moved downslope and the monetary damage that it does annually. Creep involves extremely slow downhill rock. Although
mate,
it
is
most
it
movement of
soil
can occur anywhere and in any
effective
and
or cli-
significant as a geologic
agent in humid rather than arid or semiarid climates. In the most
common form
of mass wasting in the
with construction in a permafrost environment.
fact,
example
southeastern United States and the southern Appala-
is
and turns
ground into the
is
it
into a soggy, unstable
no longer
mush. Because the
solid, the building settles
unevenly
ground, and numerous structural problems
15-20
chian Mountains.
Because the rate of movement
is
essentially impercep-
we are frequently unaware of creep's existence unwe notice its effects: tilted trees and power poles,
tible, til
broken
streets
foundations
sult (Fig. 15-23).
"^ FIGURE
re-
it is
and sidewalks, cracked retaining walls or 15-24). Creep usually involves the
(Fig.
Earthflows form tongue-shaped masses of wet regolith that in humid climates on grassy An earthflow near L'Anse, Michigan.
{a)
move slowly downslope. They occur most commonly soil-covered slopes, {b)
Types of Mass Wasting
435
"^ FIGURE (a)
15-21
Groundshaking by the 1964
Alaska earthquake turned parts of Cove Clay into a quick clay, causing numerous slides the Bootlegger (b)
that destroyed
many homes
in
the Turnagain Heights subdivision
of Anchorage.
436
Chapter 15
Mass Wasting
•
ii&
)
horizontal layer
is
ward. During the Cenozoic Era, however, regional uplift commenced, and as a consequence of the uplift, the streams began eroding
downward and were superposed
on
forming water gaps
resistant strata, thus
(Fig. 16-38).
day floodplain
some
(Fig. 16-39). In
several steplike surfaces
above
its
cases, a stream has
present-day floodplain,
indicating that stream terraces formed several times.
Although
all
stream terraces result from erosion, they
are preceded by an episode of floodplain formation
^ STREAM TERRACES Adjacent to
many
floodplains formed
stream to cut
They
downward
Once
until
is
it
the stream again
once again graded
becomes graded,
streams are erosional remnants of
(Fig. 16-40).
when
begins eroding laterally and establishes a
the streams were flowing at a
higher level. These erosional remnants are stream terraces.
consist of a fairly flat upper surface
and
deposition of sediment. Subsequent erosion causes the
and
a
steep slope descending to the level of the lower, present-
at a
lower
level.
it
floodplain
Several such episodes account for the
multiple terrace levels seen adjacent to (Figs.
new
some streams
16-39 and 16-40).
-•-
FIGURE
16-39
Stream
terraces adjacent to the
River
in
Madison
Montana.
Stream Terraces
477
many stream
Floodplain.
terrace*, greater runoff in a stream's drain-
age basin can also result in the formation of terraces. Recall that one of the variables controlling velocity discharge. Thus, a stream can erode
change
in
is
downward with no
base level and form terraces.
^ INCISED MEANDERS Some streams
are restricted to deep, meandering can-
yons cut into solid bedrock, where they form features called incised meanders. For example, the San Juan River in Utah occupies a meandering canyon more than 390 meters deep (Fig. 16-41). Such streams, being reby solid rock walls, are generally ineffective
stricted
in
eroding laterally; thus, they lack a floodplain and oc-
cupy the entire width of the canyon floor. Some incised meandering streams do erode laterally, thereby cutting off meanders and producing natural bridges (see Perspective 16-2). It is
not
difficult to
downward
understand
how
a stream can cut
into solid rock, but forming a
pattern in bedrock
is
meandering
another matter. Because lateral
one must meandering course was established when the stream flowed across an area covered by alluvium. For example, suppose that a stream near base level has established a meandering pattern. If the land over which the stream flows is uplifted, erosion is initiated, and the
erosion
is
inhibited once downcutting begins,
infer that the
meanders become incised into the underlying bedrock.
-^ FIGURE
Uplift does not account for
Origin of stream terraces, {a) A stream has a broad floodplain adjacent to its channel, (b) The stream erodes downward and establishes a new floodplain at a lower level. Remnants of its old floodplain are stream terraces, (c) Another level of terraces originates as the
16-40
downward
stream erodes
again.
Where they
some
are cut into solid bedrock.
are cut into bedrock, the terrace surface
generally covered by a thin veneer of sediment. In
is
many
stream valleys, terraces are paired, meaning that they
occur at the same elevation on opposite sides of the channel
(Fig.
16-40b and
Renewed erosion and
c).
the formation of stream ter-
races are usually attributed to a change in base level.
which a stream flows or gradient and increased flow velocity, thus initiating an episode of downcutting. When the stream reaches a level at which it is once again graded, downcutting ceases. Although changes in base level no doubt account for Either uplift of the land over
lowering of sea
478
level yields a steeper
Chapter 16
Running Water
pattern provided that face.
tern
As is
level it
all
incised meanders.
A
can establish a meandering
flows over a gently sloping sur-
in the last case,
however, the meandering pat-
already established before erosion into bedrock
occurs.
Stream terraces are commonly cut into previously deposited sediment, but
stream far above base
^ FIGURE
16-41
Goose Necks of
the San Juan River.
Perspective 16-2
Af*.
-
NATURAL BRIDGES
The term natural bridge has been used
to describe a
variety of features including spans of rock resulting
from wave erosion, the partial collapse of cavern roofs, and weathering and erosion along closely spaced, parallel joints as in Arches National Park in Utah (see Perspective 14-1). Here, however, we are concerned only with natural bridges that span a valley eroded by running water.
The is
in
best place to observe this type of natural bridge
Natural Bridges National
Monument
in
southwestern Utah. Three natural bridges are present within the
way. it
Of
monument, and
all
originated in the
these three, Sipapu Bridge
stands 67
m
is
same
the largest (Fig. 1);
above White Canyon and has
a
span of
The process by which these natural bridges were formed is well understood, and, as a matter of fact, is still going on. In the first stage, a meandering stream was incised into solid bedrock (Fig. 2). In Natural Bridges National Monument,
^ FIGURE
1
Sipapu Bridge
Monument, Utah. (Photo
81.5 m.
this
it
rock unit
which consists of sandstone formed from windblown sand deposited during the Permian Period. When local meandering streams
the Cutler Formation,
became incised, lateral erosion created a thin wall of rock between adjacent meanders that was eventually breached (Fig. 2). As the breach was subsequently enlarged, the stream abandoned its old meander and
the stream flow previously,
process is
was
oxbow
Natural Bridges National
As we discussed formed by a similar
diverted.
lakes are
(Fig. 16-21).
in
courtesy of Sue Monroe.)
The only
significant difference
is
form natural bridges are incised. Natural bridges are temporary features. Once formed, they are destroyed by other processes. For example, rocks fall from the undersides of bridges, their surfaces are weathered and eroded, and that the streams that
eventually they collapse.
The monument contains
several examples of such collapsed bridges, but
ones are
in the process of forming.
-*r FIGURE 2 Origin of a natural bridge, (a) A meandering stream flows across a gently sloping surface, (b) Incised meanders develop as the stream erodes down into solid rock. (c) A thin wall of rock between meanders is eventually breached, forming a natural bridge.
new
CHAPTER SUMMARY
large
marine deltas are more complex. Marine
deltas are characterized as stream-, wave-, or
Water
is
rises as
continually evaporated from the oceans,
water vapor, condenses, and
20%
About
precipitation.
of
falls
as
tide-dominated.
land and eventually returns to
precipitation falls
surface runoff.
consist mostly of sand arid regions 15. Sea level
Running water moves by
either laminar or turbulent
which streams can erode. However, streams
one another, complexly intertwined.
streams, or the points where they flow across
in
streams
is
turbulent.
particularly resistant rocks.
Gradient generally varies from steep to gentle along
channels so that they develop a smooth, concave profile of equilibrium. Such streams are graded. In a graded stream, a balance exists between gradient,
the course of a stream, being steep in upper reaches
discharge, flow velocity, channel characteristics, and
and gentle in lower reaches. Flow velocity and discharge are related. A change in one of these parameters causes the other to change
within the channel.
sediment load so that
or no deposition occurs
processes including downcutting, lateral erosion,
stream and its tributaries carry runoff from its drainage basin. Drainage basins are separated from one another by divides.
Many
19.
meaning that they once flowed on a higher surface and eroded downward into resistant rocks. Renewed downcutting by a stream possessing a
ridges directly in their paths are superposed,
dissolution of soluble rocks.
The coarser part of
a stream's
sediment load
is
transported as bed load, and the finer part as
suspended load. Streams also transport a dissolved load of ions in solution.
measure of the maximum-sized and is related to velocity. Capacity is a function of discharge and is a measure of the total load transported by a stream. is
a
particles that a stream can carry
mass wasting, sheet wash, and headward erosion. streams flowing through valleys cut into
18.
Streams erode by hydraulic action, abrasion, and
Competence
little
17. Stream valleys develop by a combination of
A
commonly results in the formation of stream terraces, which are remnants of an older floodplain at a higher level. floodplain
20. Incised meanders are generally attributed to renewed
downcutting by a meandering stream such that occupies a deep, meandering valley.
now
Braided streams are characterized by a complex of dividing and rejoining channels. Braiding occurs
when sediment
transported by the stream
IMPORTANT
TERMS
is
deposited within channels as sand and gravel bars.
Meandering streams have a single, sinuous channel with broad looping curves. Meanders migrate laterally as the cut bank is eroded and point bars form on the inner bank. Oxbow lakes are cutoff meanders in which fine-grained sediments and
abrasion
hydraulic action
alluvial fan
hydrologic cycle
alluvium
incised
base level bed load braided stream
infiltration capacity
organic matter accumulate.
delta
oxbow
Floodplains are rather flat areas paralleling stream channels. They may be composed mostly of point
discharge
point bar
dissolved load
runoff
divide
floodplain
stream stream terrace superposed stream suspended load
graded stream
velocity
bar deposits formed by lateral accretion or
accumulated by
vertical accretion
mud
during numerous
floods.
drainage basin
drainage pattern
Deltas are alluvial deposits at a stream's mouth.
Many
small deltas in lakes conform to the three-part
division of bottomset, foreset,
480
local base levels such as lakes, other
16. Streams tend to eliminate irregularities in their
as well.
13.
rates are high.
ultimate base level, the lowest level to
commonly have
in the latter they are
streams.
12.
where erosion
gravel.
flow. In the former, streamlines parallel
Runoff can be characterized as either sheet flow or channel flow. Channels of all sizes are called
11.
is
and
whereas
Most flow
10.
on land that They form best in
14. Alluvial fans are lobate alluvial deposits
on the oceans, mostly by
all
Chapter 16
Running Water
and topset beds, but
gradient
meander
meandering stream natural levee lake
it
QUESTIONS
REVIEW
c.
Trellis
drainage develops on:
a.
natural levees; b.
granite;
fractured
c.
14
sedimentary rock layers; horizontal layers of volcanic rocks. e. Mounds of sediment deposited on the margin of a 15
stream are: a.
natural levees; b.
c.
bottomset beds;
e.
alluvial fans.
The
direct
saltation;
b.
cutoff;
atmosphere;
The
vertical
distance
the
d.
base
drainage pattern.
level; e.
velocity;
c.
rectangular; b.
d.
deranged;
dendritic;
trellis; c.
18.
radial.
e.
saltation; b.
dissolved load;
c.
capacity; d.
suspended load;
e.
alluvium. capacity of a stream
volume of water; d.
a
is
10.
measure of
discharge; e
its:
total
c.
a single, sinuous channel;
alluvial fans; b.
floodplains; d
Which of
the following
a.
lake; b.
d
point bar;
c.
channel;
a broad,
21.
22.
and
(a)
and
channel
c.
(b); e.
all
of
A
is
sediment carried by saltation and rolling bed is the:
sliding along a stream
natural levee;
divide; b.
d
valley;
drainage pattern
alluvial
c.
point bar.
e.
in
which streams flow
longitudinal;
a
radial; b.
d.
rectangular;
is
deranged;
graded.
e.
would you expect
to find
deposits?
point bar;
delta; b.
Why
c.
and out
in
is:
incised
c.
floodplain.
alluvial fan; e.
d.
the Earth the only planet that has
abundant
How
do solar radiation, the changing phases of and runoff cause the recycling of water from the oceans to the atmosphere and back to the
What
the difference between laminar and why is flow in streams usually
is
turbulent flow, and turbulent?
floodplain
alluvial fan.
from
a(an):
oceans?
a local base level? c.
is
water,
natural
Erosional remnants of floodplains that are higher than the current level of a stream are: stream cut banks; c. oxbow lakes; b a natural incised meanders; e. terraces; d. All of the
gradient;
b.
answers
d.
liquid water?
24. Explain
what
important 25.
A
infiltration capacity
km
is
and why
it is
in considering runoff.
stream 2,000
1,500
m
above sea
to the sea.
What
level at its source flows
is its
gradient?
Do you
think the gradient that you calculated would be accurate for all segments of this stream?
bridges. 13.
1,000;
feature separating one drainage basin
mudflow
point bars; deltas; e
ocean; e.
375; d
125; c
of the following controls flow velocity in
meanders; in its
a deep, narrow valley; shallow channel; d. e. long, straight reaches and waterfalls. In which of the following do foreset beds occur?
c
m" and
/sec.
20. In which of the following
ability to
23.
12.
The
a.
a.
stream can
variation in flow
of lakes with irregular flow directions
levees.
11
Which
fan;
19.
velocity;
b.
meandering stream is one having: numerous sand and gravel bars
b.
3
500; b 200.
a.
erode.
a.
vertical distance a level; e.
stream with a cross-sectional area of 250
anether
a.
load of sediment;
the:
these.
Sediment transport by intermittent bouncing and skipping along a stream bed is:
A
A
roughness;
tree.
a.
is
hydraulic action;
rate at
streams? channel shape; a.
drainage pattern resembles the
a
Infiltration capacity
m 17
gradient;
The
downcutting.
a flow velocity of 1.5 m/sec has a discharge of
is its:
discharge; b.
branching of a
vertical accretion; d.
e.
given horizontal
in a
a.
The
c.
a
drop of a stream
channel by:
its
headward erosion;
runoff; b.
streams;
lakes; d.
c.
16.
glaciers.
e.
channel
velocity across a stream channel.
level.
is in:
the groundwater system; b.
a.
stream can lengthen
a.
absorb water; d. erode below sea
hydraulic
base
e.
A
a.
is:
c.
on Earth
of the fresh water
capacity; e.
distance which a stream erodes; b. a stream flows from its source to the ocean; c. maximum rate at which surface materials can
lakes;
incised meanders;
d.
meander
action; d.
Most
oxbow
impact of running water
bed load;
a.
bed load;
pattern.
tilted
basalt; d.
_ drainage
suspended load; b. stream profile; d. _
a.
26.
How
do channel shape and roughness control flow
velocity?
Review Questions
481
27.
Is
the statement "the steeper the gradient, the greater
what can about the underlying rocks of the region? 29. How do streams erode and acquire a sediment load? 30. Explain the concepts of stream competence and 28.
If
How
braided streams look
like,
and what do
is it
maintain a more or
less
constant
do oxbow lakes and meander scars form? how floodplains can develop by both lateral
vertical accretion.
How
does a stream-dominated delta differ from a wave-dominated delta? 36. What are alluvial fans and where are they best developed?
is
ultimate base level for most streams.
sea level drops with respect to the land,
If
how would
a stream respond?
do streams tend
to eliminate irregularities in
their channels?
40.
What
is
a graded stream,
and why are streams
How
do headward erosion and stream piracy
Illustrate
a
482
York: John
J.,
ed. 1971. Introduction to fluvial processes.
Edward Arnold. Leopold, L.
B.,
M. G. Wolman, and
J. P.
Miller. 1964. Fluvial
how
a stream can be superposed
water gap.
Chapter 16
&Co.
Running Water
Straus,
J.
&
1989. The control of nature. Giroux.
and form
New
York. Farrar,
Morisawa, M. 1968. Streams: Their dynamics and morphology. New York: McGraw-Hill. Petts, G., and I. Foster. 1985. Rivers and landscape. London:
Edward Arnold.
New
York: John Wiley
&
Sons.
Schumm,
lengthen a stream channel?
42
New
London: Methuen. Crickmay, C. H. 1974. The work of the river. London: Macmillan. Frater, A., ed. 1984. Great rivers of the world. Boston: Little, Brown. Knighton, D. 1984. Fluvial forms and processes. London:
Rachocki, A. 1981. Alluvial fans. rarely
graded except temporarily? 41
The channels of Mars. Austin, Texas:
Carling, eds. 1989. Floods.
Wiley &c Sons.
McPhee,
alluvial fans?
38. Sea level
P.
processes in geomorphology. San Francisco: W. H. Freeman
37. What two depositional processes predominate on
Why
ADDITIONAL READINGS
Chorley, R.
Explain
39.
terraces form?
possible for a stream near base level to
University of Texas Press.
possible for a meandering stream to erode
laterally yet
How
^
Beven, K., and
channel width?
and
is it
Baker, V. R. 1982.
What do
they transport and deposit?
35
do paired "Stream
infer
capacity.
32
How How
erode a deep meandering valley?
a stream possesses rectangular drainage,
you
31
43. 44.
the flow velocity" correct? Explain.
&
S.
Sons.
A. 1977. The fluvial system.
New
York: John Wiley
CHAPTER
17
GROUND WAT E R ^ OUTLINE PROLOGUE INTRODUCTION
GROUNDWATER AND THE HYDROLOGIC CYCLE POROSITY AND PERMEABILITY THE WATER TABLE GROUNDWATER MOVEMENT SPRINGS, WATER WELLS, AND ARTESIAN SYSTEMS Springs
Water Wells
"^
Perspective 17-1:
Mammoth
Cave
National Park, Kentucky Artesian Systems
GROUNDWATER EROSION AND DEPOSITION Sinkholes and Karst Topography
Caves and Cave Deposits
MODIFICATIONS OF THE GROUNDWATER SYSTEM AND THEIR EFFECTS Lowering of the Water Table Saltwater Incursion
Subsidence
Groundwater Contamination "**r
Perspective 17-2: Radioactive Waste
Disposal
HOT
SPRINGS
AND GEYSERS
Geothermal Energy
CHAPTER SUMMARY
The Leaning Tower of is
Pisa, Italy.
partly the result of subsidence
removal of groundwater.
The
tilting
due to the
gT
K^^«CTE3KJg«r»^nr»rTK3*3Ka^^
»m
PROLOGUE For more than two weeks in February 1925, Floyd Collins, an unknown farmer and cave explorer, became a household word (Fig. 17-1). News about the attempts to rescue him
from a narrow subsurface
fissure
near
.
^ K^C^'yrrv information booths redirected unsuspecting tourists away from Mammoth Cave. It was in this
environment that Floyd Collins grew up. Seven years before his tragic death, Collins had discovered Crystal Cave on the family farm and opened it up for visitors. But like most of the caves
in
Mammoth
Cave, Kentucky, captured the attention of the nation.
The saga of Floyd
Collins is rooted in what is Cave War of Kentucky. The western region of Kentucky is riddled with caves formed by groundwater weathering and erosion. Many of them were developed as tourist attractions to help supplement meager farm earnings. The largest and best known is Mammoth Cave (see Perspective 17-1). So spectacular is Mammoth, with its numerous caverns, underground rivers, and dramatic cave deposits, that it soon became the standard by which all other caves were measured. As Mammoth Cave drew more and more tourists, rival cave owners became increasingly bold in attempting to lure visitors to their caves and curio shops. Signs pointing the way to Mammoth Cave
known
as the Great
frequently disappeared, while "official" cave
"^ FIGURE
17-1 {a) Location of the cave in which Floyd was trapped, (b) Collins looking out of a fissure near cave where he ultimately died, (c) Cross section showing fissure where Collins was trapped, the rescue shaft that
Collins the the
was sunk, and the
lateral tunnel that finally
reached him.
(O
Prologue
485
the area, Crystal visited
Cave attracted few tourists — they Cave instead. Perhaps it was the
attempts led by Floyd's brother
Mammoth
thought of discovering a cave rivaling Mammoth or even connecting to it that drove Collins to his fateful exploration of Sand Cave on January 30, 1925. As Collins inched his way back up through the narrow fissure he had crawled down, he dislodged a small oblong piece of limestone from the ceiling that immediately pinned his left ankle. Try as he might, he
Homer
continued.
Three days after he had become trapped, a harness was put around Collins's chest and rescuers tried to
numerous attempts to yank him abandon that plan because Collins was unable to bear the pain. Meanwhile at the surface, a carnival-like atmosphere had developed as hordes of up to 20,000 people converged on the scene, and the National Guard had to be called out to
pull
him
free.
out, workers
After
had
to
was trapped in total darkness 17 m below ground. As he lay half on his left side, Collins's left arm was partially wedged under him, while his right arm was
maintain order.
held fast by an overhanging ledge. During his
rescuers collapsed, sealing Collins's fate.
and further immobilizing him
struggles to free himself, he dislodged
small rocks to bury his legs,
and adding
enough
silt
several neighbors reached Collins
and
were able to talk to him, feed him, encourage him, and try to make him more comfortable, but they could not get him out.
Word
of his plight quickly
spread and the area soon swarmed with reporters. Eventually, volunteers were able to excavate an area
around Collins's upper body, but could not free his legs. While an anxious country waited, rescue
days after the attempt to pull Collins out of
hope now was
The only
to dig a vertical relief shaft
a lateral tunnel could be
dug
from which
to reach Collins. For 12
dug the on February 16, rescuers reached the chamber where Collins lay entombed. There was no sign of life. With the news of his death, Floyd Collins's place in American folklore was secured. His body was finally brought out and buried near Crystal Cave, where it is appropriately marked by a beautiful stalagmite and pink granite headstone.
more
to his anguish.
The next day
Two
the fissure failed, part of the passageway used by
days, volunteers using picks and shovels
shaft. Finally
pinned
^
INTRODUCTION
stored in the open spaces within underground rocks and unconsolidated material— is a
Groundwater— the water
valuable natural resource that
is
essential to the lives of all
importance to humans is not new. Groundwahave always been important in the western United States, and many legal battles have been fought over them. Groundwater also played a crucial role in the
people. ter
Its
rights
development of the U.S. railway system during the nineteenth century when railroads had to have a reliable source of water for their steam locomotives. Much of the water used by the locomotives came from groundwater tapped
by wells.
Today, the study of groundwater and its movement has become increasingly important as the demand for fresh water by agricultural, industrial, and domestic us-
an all-time high. More than 65% of the groundwater used in the United States each year goes for irrigation, with industrial use second, followed by do-
ers has reached
mestic needs. Such
demands have
severely depleted the
groundwater supply in many areas and led to such problems as ground subsidence and saltwater contamination. In other areas, pollution from landfills, toxic waste, and agriculture has rendered the groundwater supply unsafe. 486
Chapter 17
Groundwater
As the world's population and industrial development expand, the demand for water, particularly groundwater, will increase. Not only is it important to locate new groundwater sources, but, once found, these sources must be protected from pollution and managed properly to ensure that users do not withdraw more water than can be replenished. Consequently, geologists trained in groundwater exploration and management are in great demand. If we wish to maintain adequate supplies of clean groundwater in the future, we must ensure that the
groundwater supply is intelligently managed. To do this, a knowledge of where groundwater occurs, how it moves, and how it becomes polluted is essential.
^ GROUNDWATER AND THE HYDROLOGIC CYCLE Groundwater represents approximately
km 3
22%
(8.4 mil-
of the world's supply of fresh water (see Fig. 16-3). This amount is about 36 times greater than the total for all of the streams and lakes of the world (see lion
)
Chapter 16) and equals about one-third the amount in the world's ice caps (see Chapter 18). If the
locked up world's
groundwater were spread evenly over the it would be about 10 m deep.
Earth's surface,
Pore space
rocks, other types of porosity can include cracks, fractures, faults,
and
vesicles in volcanic rocks (Fig. 17-2).
Porosity varies
pendent on the
among
size,
different rock types
and
de-
is
shape, and arrangement of the
ma-
composing the rock (Table 17-1). Most igneous and metamorphic rocks as well as many limestones and dolostones have very low porosity because they are composed of tightly interlocking crystals. However, their poterial
rosity can be increased if they have been fractured or weathered by groundwater. This is particularly true for massive limestone and dolostone whose fractures can be
enlarged by acidic groundwater.
By
contrast, detrital sedimentary rocks
composed of
well-sorted and well-rounded grains can have very high
two grains touch only at a single open spaces between the grains (Fig. 17-2a). Poorly sorted sedimentary rocks, on the other hand, typically have low porosity because finer porosity because any
point, leaving relatively large
grains "^^
FIGURE
17-2
A
is
dependent on the
shape, and arrangement of the material composing the rock, {a) A well-sorted sedimentary rock has high porosity size,
while (b) a poorly sorted one has low porosity,
(c) In soluble rocks such as carbonates, porosity can be increased by
solution, while (d) crystalline rocks can be rendered
porous
by fracturing.
Groundwater is one reservoir of the hydrologic cycle. The major source of groundwater is precipitation that infiltrates the ground and moves through the soil and pore spaces of rocks (see Fig. 16-6). Other sources include water infiltrating from lakes and streams, recharge ponds, and wastewater treatment systems. As the groundwater moves through soil, sediment, and rocks, many of its impurities, like disease-causing
out.
Not
some
all soils
microorganisms, are
and rocks are good
filters,
serious pollutants are not removed.
eventually returns to the surface reservoir
filtered
however, and Groundwater
when
it
enters
lakes, streams, or the ocean.
* POROSITY AND PERMEABILITY Porosity and permeability are important physical properties
of rocks, sediment, and soil and are largely respon-
sible
for the
amount,
availability,
and movement of
groundwater. Water soaks into the ground because the soil, sediment, or rock has open spaces or pores. Porosity
volume that is pore While porosity most often consists of the spaces between particles in soil, sediments, and sedimentary is
the percentage of a material's total
space.
fill
in the
space between the larger grains, reduc-
ing the porosity (Fig. 17-2b). In addition, the rock's porosity
amount of
cement between grains can also decrease porosity. Although porosity determines the amount of groundwater a rock can hold, it does not guarantee that the water can be extracted. The capacity of a material for transmitting fluids
is its
permeability. Permeability
is
de-
pendent not only on porosity, but also on the size of the pores or fractures and their interconnections. For example, deposits of silt or clay are typically more porous than sand or gravel. Nevertheless, shale has low permeability because the pores between its clay particles are very small,
and the molecular attraction between the clay and
the water
"•-
is
great, thereby preventing
TABLE
17-1
Porosity
movement of
the
water. In contrast, the pore spaces between grains in sand-
stone and conglomerate are attraction
on the water
is
much larger, and the molecular
therefore low. Chemical
and
bio-
chemical sedimentary rocks, such as limestone and dolostone,
and many igneous and metamorphic rocks
that are
highly fractured can also be very permeable provided that the fractures are interconnected. In fact, as northern Georgia, for their
many
depend on fractured
areas, such
crystalline rocks
groundwater supply.
A permeable layer transporting groundwater is called an aquifer, from the Latin aqua meaning water. The
material that
ward its
it is
mof ing through and
progress. This region
water
is
called
is
halts
suspended water
(Fig. 17-3).
spaces in this zone contain both water and ing irregularly
upward
its
down-
the zone of aeration, and
The pore
air.
Extend-
a few centimeters to several
meters from the base of the zone of aeration
is
the cap-
Water moves upward in this region from the zone of saturation below because of surface tension. Such movement is analogous to the upward movement
illary fringe.
of water through a paper towel.
When
precipitation occurs over land,
Beneath the zone of aeration lies the zone of saturation which all of the pore spaces are filled with groundwater (Fig. 17-3). The base of the zone of saturation varies from place to place, but usually extends to a depth where an impermeable layer is encountered or to a depth where confining pressure closes all open space. The surface separating the zone of aeration from the underlying zone of saturation is the water table (Fig. 17-3). In general, the configuration of the water table is a subdued replica of the overlying land surface; that is, it has its highest elevations beneath hills and its lowest elevations in valleys. In most arid and semiarid regions, however, the water table is quite flat and is below the
rates,
some of
level of river valleys.
most
effective aquifers are deposits of well-sorted
and
well-rounded sand and gravel. Limestones in which fractures and bedding planes have been enlarged by solution are also good aquifers. Shales and many igneous and
metamorphic rocks, however, are typically impermeable. Rocks such as these and any other materials that prevent the movement of groundwater are called aquicludes.
^ THE WATER TABLE some of it evapoaway by runoff in streams, and the remainder seeps into the ground. As this water moves down from the surface, some of it adheres to the
^ FIGURE
it is
17-3
carried
The zone of
aeration contains both air and water within its open space, while all of the open space in the zone of is filled with groundwater. The water table is the surface separating the zones of aeration and saturation. Within the capillary fringe, water rises upward by surface tension from the zone of saturation into the zone of aeration.
saturation
488
Chapter 17
Groundwater
in
Several factors contribute to the surface configuration of a region's water table.
These include regional
** FIGURE 17-4 Groundwater moves downward due to the force of gravity. It moves through the zone of aeration to the zone of saturation where
some of
it
moves
along the slope of the water table and the rest of it moves through the zone of saturation from areas of high pressure toward areas of low pressure.
amount of rainfall, permeability, and groundwater movement. During periods of high rainfall, groundwater tends to rise beneath hills because it cannot flow fast enough into the adjacent valleys to maintain a level surface. During droughts, however, the water table falls and tends to flatten out
has been demonstrated that groundwater ve-
differences in the
methods,
the rate of
locity varies greatly
because
it is
not being replenished.
^ GROUNDWATER MOVEMENT Groundwater moves very slowly through the pore spaces It moves fastest in the central area of the pore space and decreases in velocity to zero along the edges because of friction and the molecular attraction between the water molecules and the material through which it moves. Gravity provides the energy for the downward movement of groundwater. Water entering the ground moves of Earth materials.
it
and depends on many factors. Vem per day in some extremely permeable material to less than a few centimeters per year in nearly impermeable material have been measured. In most ordinary aquifers, however, the average velocity of groundwater is a few centimeters per day. locities
ranging from 250
^ SPRINGS, WATER WELLS, AND ARTESIAN SYSTEMS Adding water to the zone of saturation is called recharge, and it causes the water table to rise. Water may be added by natural means, such as rainfall or melting snow, or artificially at recharge basins or wastewater treatment plants (Fig. 17-5). If groundwater is discharged without sufficient replenishment, the water table drops.
Groundwater discharges naturally whenever
through the zone of aeration to the zone of saturation (Fig. 17-4). When water reaches the water table, it con-
move through the zone of saturation from arwhere the water table is high toward areas where it
tinues to
"•»
eas
New
is
lower, such as streams, lakes, or
swamps
FIGURE
17-5
A
recharge basin in Nassau County"
York.
(Fig. 17-4).
Only some of the water follows the direct route along the slope of the water table. Most of it takes longer curving paths downward and then enters a stream, lake, or swamp from below. This occurs because groundwater moves from areas of high pressure toward areas of lower pressure within the saturated zone. Below the wagroundwater is under greater pressure beneath than at the same elevation beneath a valley. The rate at which groundwater flows can be deter-
ter table,
a
hill
mined in several ways. The most common method is to add dye to the groundwater in a well and measure how long the dye takes to appear in the groundwater at another well a known distance away. Using this and other Springs,
Water Wells, and Artesian Systems
489
the water table intersects the
ground surface as at a swamp. Groundwater
way
by pumping water
ally,
spring or along a stream, lake, or
can also be discharged
from
artificially
(Fig. 17-6).
Where percolating groundwater reaches
the water table or an impermeable layer,
and
if
this
it
flows later-
flow intersects the Earth's surface, the
water discharges onto the surface as a spring (Fig. 17-7). in Kentucky, for example, is underlain by fractured limestones that have been en-
wells.
The Mammoth Cave area Springs
A
larged into caves by solution activity (see Perspective
where groundwater flows or seeps out of the ground. Springs have always fascinated people because the water flows out of the ground for no apparent reason and from no readily identifiable source. spring
It is
is
a place
not surprising that springs have long been regarded
with superstition and revered for their supposed medic-
and healing powers. Nevertheless, there is nothing mystical or mysterious about springs. Although springs can occur under a wide variety of geologic conditions, they all form in basically the same inal value
where and caves intersect the ground surface allowing groundwater to exit onto the surface. Springs most commonly occur along valley walls where streams have cut valleys below the regional water table. 17-1). In this geologic environment, springs occur
the fractures
Springs can also develop wherever a perched water table intersects the Earth's surface (Fig. 17-8).
water table
may
Most commonly,
sandstone.
As water migrates through
Springs
they
form when percolating water reaches an impermeable layer and migrates laterally until
it seeps out Springs also can occur in areas underlain by fractured soluble rocks such as limestones where groundwater
at the surface.
moves
freely
cavities until
and flows
(£>)
through underground it
reaches the surface
out.
Water table
490
Chapter 17
Groundwater
perched
within a larger aquifer, such as a lens of shale within a
"»- FIGURE 17-6 Springs form wherever laterally moving groundwater intersects the Earth's surface, (a)
A
occur wherever a local aquiclude occurs
Springs
the zone of aera-
tion,
stopped by the local aquiclude, and a localized
it is
zone of saturation "perched" above the main water table is created. Water moving laterally along the perched water table
may
produce a spring.
intersect the Earth's surface to
Water Wells
A water well
is
made by digging or
drilling into the
zone
most water wells today are dug, particularly in areas where the
of saturation. Although drilled,
some
water table saturation
are
is
is
still
very close to the surface.
Once
the zone of
reached, water percolates into the well and
water table. Most wells must be groundwater to the surface. When a well is pumped, the water table in the area around the well is lowered, because water is removed from the aquifer faster than it can be replenished. A cone of depression thus forms around the well, varying in size according to the rate and amount of water being withdrawn (Fig. 17-9). If water is pumped out of a well faster than it can be replaced, the cone of depression grows until the well goes dry. This lowering of the water table normally does not pose a problem for the average fills it
to the level of the
pumped
to bring the
domestic well provided that the well ciently
is
drilled suffi-
deep into the zone of saturation. The tremendous
amounts of water used by industry and
irrigation,
how-
"^ FIGURE
ever,
may
17-7
Periodic Spring, near Afton,
Wyoming.
create a large cone of depression that lowers
the water table sufficiently to cause shallow wells in the
immediate area to go dry (Fig. 17-9). Such a situation is uncommon and frequently results in lawsuits by the owners of the shallow dry wells. Furthermore, lowering of the regional water table is becoming a serious problem in many areas, particularly in the southwestern United States where rapid growth has placed tremennot
"•-
FIGURE
17-8
If
a localized
aquiclude, such as a shale layer,
occurs within an aquifer, a perched water table may result with springs
Localized aquiclude
occurring where the perched water table intersects the Earth's surface.
Springs
Zone
of saturation
Springs,
Water Wells, and Artesian Systems
491
Perspective 17-1
MAMMOTH PARK,
CAVE NATIONAL
KENTUCKY
Within the limestone region of western Kentucky largest cave system in the world. In 1941,
lies
the
approximately
set aside and designated as Mammoth Cave National Park. In 1981 it became a World
51,000 acres were Heritage
Site.
Recently, the National Park Service has
been considering closing health hazard created by
groundwater
Mammoth
Cave because of the raw sewage and contaminated
in the area.
From ground
level,
the topography of the area
is
unimposing with numerous sinkholes, lakes, valleys, and disappearing streams. Beneath the surface, however, are
more than 230 km of interconnecting passageways whose spectacular geologic features have been enjoyed by numerous cave explorers and tourists alike. Based on carbon 14 dates from some of the many artifacts found in the cave (such as woven cord and wooden bowls), Mammoth Cave had been explored and used by Native Americans for more than 3,000 years prior to its rediscovery in 1799 by a bear hunter named Robert Houchins. During the War of 1812, approximately 180 metric tons of saltpeter (a potassium nitrate mineral), used in the manufacture of gunpowder, were mined from Mammoth Cave. At the end of the war, the saltpeter market collapsed, and
Mammoth
Cave was developed as a
overshadowing the other caves in the area. Over 150 years, the discovery of new passageways and caverns helped establish Mammoth Cave as the world's premier cave and the standard against which all others were measured (see the Prologue). Mammoth Cave formed in much the same way as all other caves (Fig. 17-18). Groundwater flowing through the St. Genevieve Limestone eroded a complex network of openings, passageways, and caverns. Flowing through the various caverns is the Echo River, a system of subsurface streams that eventually joins the Green River at the surface. The colorful cave deposits are the primary reason millions of tourists have visited Mammoth Cave. Here can be seen numerous stalactites, stalagmites, and easily
the next
columns, as well as spectacular travertine flowstone deposits (Fig. 1). Other attractions include the Giant's
m
and giant about 58 m high (Fig. 2). The cave is also home to more than 200 species of insects and other animals, including about 45 blind species; some of these can be seen on the Echo River Tour, which conveys visitors 5 km along the underground stream. Coffin, a 15
rooms such
as
collapse block of limestone,
Mammoth Dome,
which
is
tourist attraction,
FIGURE 1 Frozen Niagara is a spectacular example massive travertine flowstone deposits.
FIGURE 2 Looking up Mammoth Dome, in Mammoth Cave, Kentucky.
"••"
^r*
:>f
room
the largest
—
FIGURE 17-9 A cone of depression forms whenever water withdrawn from a well. If water withdrawn faster than it can be
is
is
replenished, the cone of depression will
grow
in
depth and
circumference, lowering the water
and causing nearby shallow wells to go dry. table in the area
Cone of depression
dous demands on the groundwater system. Unrestricted withdrawal of groundwater cannot continue indefinitely, and the rising costs and decreasing supply of groundwater should soon limit the growth of this region
well was drilled in a.d. 1126 and is still flowing today. The term artesian, however, can be applied to any sys-
of the United States.
able to rise above the level of the aquifer
People in rural areas and those without access to a
municipal water system are well aware of the problems of locating an adequate
groundwater supply. The
distri-
bution and type of rocks present, their porosity and permeability, fracture patterns, that determine (Fig.
whether
a
and so on are
all
factors
water well will be successful
17-10).
Artesian Systems
The word
artesian
province of Artois times) near Calais,
comes from the French town and (called Artesium during Roman where the first European artesian
tem
in
which groundwater
high hydrostatic
drilled
is
confined and builds up
(fluid) pressure.
through the confining
Water
layer,
in
such a well if
a well
is is
thereby reducing the
upward (Fig. 17-11). For an artesian system to develop, three geologic conditions pressure and forcing the water
must be present
(Fig.
17-12): (1) the aquifer must be
confined above and below by aquicludes to prevent wa-
from escaping; (2) the rock sequence is usually tilted and exposed at the surface, enabling the aquifer to be recharged; and (3) there is sufficient precipitation in the recharge area to keep the aquifer filled. ter
The elevation of the water table in the recharge area and the distance of the well from the recharge area determine the height to which artesian water rises in a well. The surface defined by the water table in the re-
•*r
FIGURE
17-10
Many
factors
determine whether a water well will be successful. Wells A and E were drilled to the same depth. Well A was successful because it tapped a perched water table, whereas well E did not. To be successful, it will have to be drilled below the water table like well C. Well B tapped a fracture below the water table and
Perched water
was
successful,
whereas well
D
missed the fractures and was dry.
ei^ Fractured crystalline
basement rock
Springs,
Water Wells, and Artesian Systems
493
artesian-pressure surfece. Friction, however, slightly re-
duces the pressure of the aquifer water and consequently the level to which artesian water rises. This is why the pressure surface slopes.
An only
artesian well will flow freely at the
if
the wellhead
is
at
pressure surface. In this
ground surface
an elevation below the artesiansituation, the water flows out of
it rises toward the artesian-pressure which is at a higher elevation than the wellhead. In a nonflowing artesian well, the wellhead is above the artesian-pressure surface, and thus the water will rise in
the well because surface,
the well only as high as the artesian-pressure surface. In addition to artesian wells,
many
also exist. Such springs can occur
if
artesian springs
a fault or fracture
intersects the confined aquifer allowing
water to
rise
commonly
arte-
Because the geologic conditions necessary for
arte-
above the
aquifer.
Oases
in deserts are
sian springs.
sian water can occur in a variety of ways, artesian sys-
^
FIGURE 17-11 Artesian well at Deep Well Ranch, South Fork of the Madison River, Gallatin County,
tems are quite
Montana.
in
many areas of the world unOne of the best-known
artesian systems in the United States underlies South
charge area, called the artesian-pressure surface, cated by the sloping dashed line in Figure 17-12.
were no
common
derlain by sedimentary rocks.
friction in the aquifer, well
tesian aquifer
would
is
indi-
If
there
water from an
rise exactly to the elevation
ar-
of the
Dakota and extends southward to central Texas. The majority of the artesian water from this system is used for irrigation. The aquifer of this artesian system, the Dakota Sandstone, is recharged where it is exposed along the margins of the Black Hills of South Dakota. in this system was originally
The hydrostatic pressure
—
FIGURE 17-12 An artesian system must have an aquifer confined above and below by aquicludes, the aquifer must be exposed at the surface, and there must be sufficient precipitation in the recharge area to keep the aquifer
filled.
The
elevation of the
water table
in the
which is dashed line
(the artesian-pressure
recharge area, indicated by a sloping
surface), defines the highest level to
which well water can
rise. If
elevation of a wellhead
is
the
below the
elevation of the artesian-pressure surface, the well will be free-flowing
because the water will
rise
toward which
the artesian-pressure surface, is
at a higher elevation than the
wellhead.
wellhead
If is
the elevation of a at or
above that of the
artesian-pressure surface, the well will be nonflowing.
494
Chapter 17
Groundwater
Artesian-pressure surface
~^~
FIGURE
17-13
The
distribution of the major limestone
produce free-flowing wells and to opThe extensive use of water for irrigation over the years, however, has reduced the pressure in many of the wells so that they are no longer freegreat
enough
to
erate waterwheels.
flowing and the water must be pumped.
These carbonates are exposed at the surface
in the
northwestern and central parts of the state where they are recharged, and they dip toward both the Atlantic and Gulf coasts
where they are covered by younger sediments. The
carbonates are interbedded with shales forming a series of confined aquifers and aquicludes. This artesian system
is
tapped in the southern part of the state where it is an important source of fresh water and one that is being rapidly depleted.
^ GROUNDWATER EROSION AND DEPOSITION When
soluble rock, groundwater sion and thus
is
is
the principal agent of ero-
responsible for the formation of
many
major features of the landscape.
common
sedimentary rock composed
primarily of the mineral calcite
(CaC0 3 ),
underlies large
areas of the Earth's surface (Fig. 17-13). Although lime-
stone
is
practically insoluble in pure water,
amount of weak acid
it
readily
Carbonic that forms when carbon acid (H 2 C0 3 is a + C0 2 -» H 2 C0 3 dioxide combines with water (H 2 (see Chapter 6). Because the atmosphere contains a small amount of carbon dioxide (0.03%), and carbon dioxide is also produced in soil by the decay of organic matter, most groundwater is slightly acidic. When groundwater percolates through the various openings in limestone, the slightly acidic water readily reacts with the calcite to dissolve the rock by forming soluble calcium bicarbonate, which is carried away in solution (see Chapter 6). dissolves
if
a small )
acid
is
present.
)
Sinkholes and Karst Topography
rainwater begins seeping into the ground,
mediately starts to react with the minerals
weathering them chemically. In an area underlain by
Limestone, a
Another example of an important artesian system is the Floridan aquifer system. Here Tertiary-aged carbonate rocks are riddled with fractures, caves, and other openings that have been enlarged and interconnected by solution activity.
and karst areas of the world.
it
it
im-
contacts,
In regions underlain
may
by soluble rock, the ground surface
be pitted with numerous depressions that vary in
Groundwater Erosion and Deposition
495
in this
way
are a serious hazard, particularly in
lated areas. In regions
popuprone to sinkhole formation, the
depth and extent of underlying cave systems must be mapped before any development to ensure that the underlying rocks are thick enough to support planned structures.
A
karst topography
is
by groundwater erosion
one that has developed
The name
(Fig. 17-15).
largely
karst
is
derived from the plateau region of the border area be-
tween Yugoslavia and northeastern of topography
is
Italy
where
this type
well developed. In the United States,
regions of karst topography include large areas of south-
western
Illinois,
southern Indiana, Kentucky, Tennessee,
northern Missouri, Alabama, and central and northern Florida (Fig. 17-13).
Karst topography
is
numerous caves, and disappearing
characterized by
springs, sinkholes, solution valleys,
streams
(Fig.
17-15).
When
adjacent sinkholes merge,
they form a network of larger, irregular, closed depressions called solution valleys. Disappearing streams are
another feature of areas of karst topography. They are so
named because
they typically flow only a short distance
and then disappear into a sinkhole. The water continues flowing underground through various at the surface
fractures or caves until
it
surfaces again at a spring or
other stream.
Karst topography can range from the spectacular high relief
landscapes of China to the subdued and pock-
marked landforms of Kentucky
common
(Fig.
17-16).
to all karst topography, however,
is
What
is
that thick-
(b)
*w FIGURE and
9,
1981,
17-14 (a) This sinkhole formed on May 8 Winter Park, Florida, due to a drop in the
in
water table after prior dissolution of the underlying limestone. The sinkhole destroyed a house, numerous cars, and the municipal swimming pool. It has a diameter of 100 m and a depth of 35 m. {b) This sinkhole in a rural area near Montevallo, central Alabama, formed on December 2, 1972. Its diameter is 130 m, and its depth is 45 m.
and shape. These depressions, called sinkholes or merely sinks, mark areas where the underlying rock is
bedded, readily soluble rock
is
present at the surface or
and enough water is present for solution activity to occur. Karst topography is, therefore, typically restricted to humid and temperate climates. At the present, however, some of the best karst topography can be found in arid and semiarid regions such as Bexar County, Texas, and the Carlsbad Caverns region in New Mexico. The examples of karst topography in these regions are relicts that originally formed when the climate was more humid. just
below the
soil,
size
soluble (Fig. 17-14). Sinkholes usually form in one of
two ways. The
first is
when
the soluble rock
below the
by seeping water. Natural openings in and filled in by the overlying soil. As the groundwater continues to dissolve the rock, the soil is eventually removed, leaving depressions that are typically shallow with gently sloping sides. soil is dissolved
the rock are enlarged
Sinkholes also form
when
a cave's roof collapses,
usually producing a steep-sided crater. Sinkholes formed
496
Chapter 17
Groundwater
Caves and Cave Deposits Caves are some of the most spectacular examples of the combined effects of weathering and erosion by groundwater. As groundwater percolates through carbonate rocks (limestone and dolostone), larges original fractures
and enform a complex caves, caverns, and
it
and openings
interconnecting system of crevices,
dissolves
to
underground streams. A cave is usually defined as a naturally formed subsurface opening that is generally con-
Solution valleys
Springs
Karst valley
Disappearing streams
Deeply intrenched permanent stream
•^ FIGURE
nected to the surface and enter.
A
cavern
is
is
large
enough
a very large cave or a
for a person to
system of
inter-
connected caves.
More than 17,000
17-15
Some
of the
features of karst topography.
Cave
caves are
known
in the
United
Most of them are small, but some are quite large and spectacular. Some of the more famous caves in the United States are Mammoth Cave, Kentucky (see Perspective 17-1); Carlsbad Caverns, New Mexico; Lewis States.
"^ FIGURE
17-16 (a) The Stone Forest, 126 km southeast of Kunming, People's Republic of China, is a high relief karst landscape formed by the dissolution of carbonate rocks, (b) Solution valleys, sinkholes, and sinkhole lakes dominate the subdued karst topography east of Bowling Green, Kentucky.
'"
M
Groundwater Erosion and Deposition
497
•^ FIGURE
17-17
Some
of the
spectacular cave deposits of
Meramec
Caverns, Missouri.
and Clark Caverns, Montana; Wind Cave and Jewel Cave, South Dakota; Lehman Cave, Nevada; and Meramec Caverns, Missouri, which Jesse James and his outlaw band often used as a hideout (Fig. 17-17). Caves and caverns form as a result of the dissolution of carbonate rocks (limestone, dolostone, and occasionally marble) by weakly acidic groundwater (Fig. 17-18). Groundwater percolating through the zone of aeration slowly dissolves the carbonate rock and enlarges its fractures and bedding planes. Upon reaching the water table, the groundwater migrates toward the region's surface streams (Fig. 17-4). As the groundwater moves through the zone of saturation,
it
same manner and are collectively known as dripstone. As water seeps through a cave, some of the dissolved carbon dioxide in the water escapes, and a small amount of calcite
is
precipitated. In this manner, the various
dripstone deposits are formed. Stalactites are icicle-shaped structures
dripping water
(Fig. 17-19).
thin layer of calcite
The water
continues to dissolve
a
from a cave's
ceiling also pre-
amount of calcite when it hits the floor. calcite is deposited, an upward growing
passageways through which the dissolved rock is carried to the streams. As the surface streams erode deeper valleys, the water table drops in response to the lower elevation of the streams. The water that flowed through the system of horizontal passageways now percolates down to the lower water table where a new system of passageways begins to form. The abandoned channelways now form an interconnecting system of caves and caverns that may continue to enlarge as groundwater percolates through them and dissolves the surrounding rock. As the caves increase in size, they may become unstable and collapse, littering the floor with fallen debris. When most people think of caves, they think of the seemingly endless variety of colorful and bizarre-shaped deposits found in them. Although a great many different types of cave deposits exist, most form in essentially the
As additional
Groundwater
With each drop of water,
deposited over the previous layer,
that drips
cipitates a small
Chapter 17
is
forming a cone-shaped projection that grows downward from the ceiling. While many stalactites are solid, some are hollow and are appropriately called soda straws.
the rock and gradually forms a system of horizontal
498
hanging from
cave ceilings that form as a result of precipitation from
projection called a stalagmite forms (Fig. 17-19).
If
a
and stalagmite meet, they form a column. Groundwater seeping from a crack in a cave's ceiling may form a vertical sheet of rock called a drip curtain, while water flowing across a cave's floor may produce stalactite
travertine terraces (Fig. 17-18).
»
MODIFICATIONS OF THE
GROUNDWATER SYSTEM AND THEIR EFFECTS Groundwater
is
a valuable natural resource that
idly being exploited
with
little
is
rap-
regard to the effects of
overuse and misuse. Currently, about
20%
of
all
water
^ FIGURE
The formation of caves, (a) As groundwater percolates through and flows through the zone of saturation, it dissolves the carbonate rocks and gradually forms a system of passageways, (b) Groundwater moves along the surface of the water table, forming a system of horizontal passageways through which dissolved rock is carried to the surface streams and thus enlarging the passageways. (c) As the surface streams erode deeper valleys, the water table drops, and the abandoned channelways form an interconnecting system of caves and caverns. 17-18
the zone of aeration
Modifications of the Groundwater System and Their Effects
499
1
•""
FIGURE
17-19
Stalactites are
the icicle-shaped structures seen
hanging from the ceiling, while the upward-pointing structures on the cave floor are stalagmites. Several columns are present where the stalactites and stalagmites have met in this chamber of Luray Caves, Virginia.
used in the United States age
is
groundwater. This percent-
is
and unless this resource is sufficient amounts of clean ground-
increasing, however,
used more wisely, water will not be available in the future. Modifications of the groundwater system may have many conse-
quences including
(1)
lowering of the water table, which
causes wells to dry up;
(2) loss
of hydrostatic pressure,
which causes once free-flowing wells to require pumping; (3) saltwater encroachment; (4) subsidence; and (5) contamination of the groundwater supply.
from
irrigated lands can be triple
viding the quantities of water that
some
parts of the
water
is
being
High
pumped
Consequently, water faster
than
it
is
Plains,
will
it
has
in the past. In
from 2 to 100 times more
annually than
is
is
being recharged.
being removed from the aquifer
being replenished, causing the water
table to drop significantly in
What
happen
many
areas (Fig. 17-20).
to this region's
economy
if
long-
term withdrawal of water from the High Plains aquifer
Lowering of the Water Table Withdrawing groundwater
what they would be
without irrigation. While the High Plains aquifer has contributed to the high productivity of the region, it cannot continue pro-
greatly exceeds
at a significantly greater rate
its
recharge rate such that
it
can no
longer supply the quantities of water necessary for
irri-
recharge
gation? Solutions range from going back to farming
effects. For example, the High Plains one of the most important aquifers in the United States. Underlying most of Nebraska, large parts of Colorado and Kansas, portions of South Dakota, Wyoming, and New Mexico, as well as the panhandle regions of Oklahoma and Texas, it accounts for approximately 30% of the groundwater used for irrigation in the United States (Fig. 17-20). Irrigation from the High
without irrigation to diverting water from other regions such as the Great Lakes. Farming without irrigation
than
it is
replaced by either natural or
artificial
can have serious aquifer
is
Plains aquifer
is
largely responsible for the high agricul-
tural productivity of this region.
A
significant percent-
age of the nation's corn, cotton, and wheat
is
grown
and half of our beef cattle are raised in this region. Large areas of land (more than 14 million acres) are
here,
currently irrigated with water Plains aquifer. Irrigation
500
Chapter 17
is
pumped from
the
High
so popular because yields
Groundwater
would result in greatly decreased yields and higher costs and prices for agricultural products, while the diversion of water from elsewhere would cost billions of dollars and the price of agricultural products would still rise.
Saltwater Incursion
The
excessive
can result
Long lines
pumping of groundwater
in saltwater
in coastal areas
incursion such as occurred on
Island, New York, during the 1960s. Along coastwhere permeable rocks or sediments are in contact
with the ocean, the fresh groundwater, being
less
dense
than seawater, forms a lens-shaped body above the un-
^
FIGURE 17-20 Areal extent of the High Plains aquifer and " changes in the water table, predevelopment to 1980.
When
become con-
derlying salt water (Fig. 17-21a). The weight of the fresh water exerts pressure on the underlying salt water. As long as rates of recharge equal rates of withdrawal, the contact between the fresh groundwater and the seawater
tained fresh water.
remain the same. If excessive pumping occurs, howdeep cone of depression forms in the fresh groundwater (Fig. 17-21b). Because some of the pressure from the overlying fresh water has been removed, salt water
is a major problem in many rapgrowing coastal communities. As the population in these areas grows, greater demand for groundwater creates an even greater imbalance between recharge and withdrawal. Natural recharge of the groundwater sys-
will
ever, a
migrates
upward
to
fill
the pore space that formerly con-
this occurs, wells
water and remain contaminated until recharge by fresh water restores the former level of the fresh groundwater water table. taminated with
salt
Saltwater incursion
idly
Modifications of the Groundwater System and Their Effects
501
Ocean
filtrate
the groundwater supply
may
also be constructed
Both of these methods are successfully used on Long Island, which has had a saltwater incursion problem for several decades. (Fig. 17-5).
Subsidence
Fresh groundwater
Salty
As excessive amounts of groundwater are withdrawn from poorly consolidated sediments and sedimentary rocks, the water pressure between grains is reduced, and
groundwater
(a)
the weight of the overlying materials causes the grains to pack closer together, resulting in subsidence of the ground. Subsidence is becoming a major hazard in many areas and can cause damage to buildings, water lines, utility lines, and roads. As more and more groundwater is pumped to meet the increasing needs of agriculture and population growth, subsidence is becoming more prevalent. The San Joaquin Valley of California is a major agricultural region that relies largely on groundwater for irrigation. Between 1925 and 1975, groundwater withdrawals in parts of the
Ocean
Fresh groundwater
Salty
groundwater
(b)
m
valley caused subsidence of almost 9
Other examples of subsidence
Ocean
clude
New
in the
(Fig.
17-22).
United States
in-
Orleans, Louisiana, and Houston, Texas,
both of which have subsided more than 2 m, and Las Vegas, Nevada, which has subsided 8.5
Elsewhere
"^ FIGURE
17-21 Saltwater incursion, (a) Because fresh not as dense as salt water, it forms a lens-shaped body above the underlying salt water, (b) If excessive pumping occurs, a cone of depression develops in the fresh groundwater, and a cone of ascension forms in the underlying salty groundwater that may result in saltwater contamination of the well, (c) Pumping water back into the groundwater system through recharge wells can help lower is
the interface between the fresh groundwater and the salty groundwater and reduce saltwater incursion.
is further decreased as large areas of the ground are covered by roads and buildings, which prevent water
tem
from
infiltrating the soil.
To counteract
the effects of saltwater incursion, re-
charge wells are often drilled to
pump
water back into
the groundwater system (Fig. 17-21c). Recharge
ponds
that allow large quantities of fresh surface water to in-
502
Chapter 17
Groundwater
world, the
tilt
m
(Table 17-2).
of the Leaning
Tower
groundwater withdrawal. The tower started tilting soon after construction began in 1173 because of differential compaction of the foundation. During the 1960s, the city of Pisa withdrew everlarger amounts of groundwater, causing the ground to subside further; as a result, the tilt of the tower increased until it was considered in danger of falling over. However, strict control of groundwater withdrawal and
of Pisa
water
in the
is
partly due to
stabilization of the foundation have reduced the
of tilting to about
1
mm
amount
per year, ensuring that the
tower should stand for several more centuries. A spectacular example of subsidence occurred
in
which is built on a former lake bed. As groundwater is removed for the ever-increasing needs of
Mexico
City,
the
the fine-grained lake sediments are compacting,
city,
and Mexico City is slowly and unevenly subsiding. Its opera house has settled more than 3 m, and half of the first floor is now below ground level. Other parts of the city have subsided more than 6 m, creating similar problems for other structures (Fig. 17-23). Withdrawal of groundwater is not the only cause of surface subsidence. The extraction of oil can also cause subsidence. a result of
Long Beach,
34 years of
California, has subsided 9
oil
production.
More
m as
than $100
1955
"^"
FIGURE
The dates on this power pole amount of subsidence the San Joaquin Valley has undergone since 1925. Due to withdrawal of groundwater for agricultural needs and the ensuing compaction of sediment, the ground subsided almost 9 m between 1925 and 1975. 17-22
dramatically illustrate the
damage was done to the pumping, transporand harbor facilities in this area because of subsidence and encroachment of the sea (Fig. 17-24). Once secondary recovery wells began pumping water back into the oil reservoir and stabilizing it, subsidence virmillion of tation,
tually stopped.
~^~
TABLE
17-2
Subsidence of Cities and Regions
1963
Groundwater Cofttamination
A
major problem facing our society is the safe disposal numerous pollutant by-products of an industrialized economy. We are becoming increasingly aware that our streams, lakes, and oceans are not unlimited reservoirs for waste, and that we must find new safe ways to of the
dispose of pollutants.
The most common sources of contamination
are sew-
age, landfills, toxic waste disposal sites (see Perspective
17-2), and agriculture. Once pollutants get into the groundwater system, they will spread wherever groundwater travels, which can make containment of the contamination difficult. Furthermore, because groundwater
moves very
slowly,
it
takes a very long time to cleanse a
groundwater reservoir once
many
In
way
it
of disposing of sewage.
leases
has become contaminated.
areas, septic tanks are the
A
sewage into the ground where
oxidation and microorganisms and
ment most
as
it
most common
septic tank slowly reit is
decomposed by
filtered
by the sedi-
percolates through the zone of aeration. In
situations,
by the time the water from the sewage it has been cleansed of
reaches the zone of saturation,
any impurities and is safe to use (Fig. 17-25a). If, howwater table is very close to the surface or if the rocks are very permeable, water entering the zone of saturation may still be contaminated and unfit to use. Landfills are also potential sources of groundwater contamination (Fig. 17-25b). Not only does liquid waste
ever, the
^ FIGURE
17-23
Lady of Guadalupe)
The right Mexico
in
side of this church
(Our
City has settled slightly
than a meter. (Photo courtesy of R. V. Dietrich.)
more
seep into the ground, but rainwater also carries dis-
^
FIGURE 17-24 The withdrawal of petroleum from the oil field in Long Beach, California,
m
of ground up to 9 subsidence because of sediment compaction. It was not until secondary recovery wells began resulted in
pumping water back
into the
reservoir to replace the petroleum
that
ground subsidence essentially 29 feet = 0.6 to 8.8
ceased. (2 to
meters)
504
Chapter 17
Groundwater
Drain pipes
Septic tank
•*r
Zone
of aeration
Average water table
Zone
of saturation
17-25
(a)
A
septic
supply.
(b)
solved chemicals and other pollutants
downward
into
groundwater reservoir. Unless the landfill is carefully designed and lined below by an impermeable layer such as clay, many toxic and cancer-causing compounds will find their way into the groundwater system. For example, paints, solvents, cleansers, pesticides, and battery acid are just a few of the toxic household items that end up in landfills and can pollute the groundwater supply. Toxic waste sites in which dangerous chemicals are either buried or pumped underground are an increasing the
source of groundwater contamination.
The United
States
alone must dispose of several thousand metric tons of
hazardous chemical waste per year. Unfortunately, much of this waste has been, and still is being, improperly
dumped and
FIGURE
system slowly releases sewage into the zone of aeration. Oxidation, bacterial degradation, and filtering by the sediments usually remove all of the natural impurities before they reach the water table. If, however, the rocks are very permeable or the water table is too close to the septic system, contamination of the groundwater can result, (b) Unless there is an impermeable barrier between a landfill and the water table, pollutants can be carried into the zone of saturation and contaminate the groundwater
is
contaminating the surface water,
soil,
and
groundwater.
Examples of indiscriminate dumping of dangerous and toxic chemicals can be found in every state. Perhaps the most famous is the Love Canal, near Niagara Falls, New York. During the 1940s, the Hooker Chemical
Company dumped approximately 19,000
tons of chem-
waste into the Love Canal. In 1953 it covered one of the dump sites with dirt and sold it for one dollar to the Niagara Falls Board of Education, which built an elementary school and playground on the site. Heavy rains and snow during the winter of 1976-1977 raised- the water table and turned the area into a muddy swamp in the spring of 1977. Mixed with the mud were thousands of different toxic, noxious chemicals that formed puddles in the playground, oozed into people's basements, ical
and covered gardens and lawns. Trees, lawns, and gardens began to die, and many of the residents of the area suffered from serious illnesses. The cost of cleaning up the Love Canal site and relocating its residents will eventually exceed $100 million, and the site and neighborhood are now vacant. Toxic wastes are also disposed of by injecting them into deep wells. These wells extend below all fresh water aquifers and are completely isolated from them to ensure that existing or potential water supplies are not
Modifications of the Groundwater System and Their Effects
505
Perspective 17-2
RADIOACTIVE WASTE DISPOSAL One
of the problems of the nuclear age
is
finding safe
until
around the year 2030, at which time and backfilled.
entrance
its
storage sites for the radioactive waste from nuclear
shafts will be sealed
power
The canisters holding the waste are designed to remain leakproof for at least 300 years, so there is
plants, the manufacture of nuclear weapons, and the radioactive by-products of nuclear medicine. Radioactive waste can be grouped into two categories: low-level and high-level waste. Low-level wastes are low enough in radioactivity that, when properly handled, they do not pose a significant environmental threat.
Most
fuel assemblies
used
in
Currently,
dump
first
it
high-level
Such a facility must be able to isolate high-level waste from the environment for at least 10,000 years, which is the minimum time such waste will remain dangerous. The Yucca Mountain site will have a capacity of 70,000 metric tons of waste and will not be completely filled radioactive waste
Under
dump
repository will be buried in a volcanic tuff at a depth
more than 15,000 metric tons of spent
southern Nevada as the nation's
isotopes from entering the groundwater system.
of about 300 m.
uranium fuel are awaiting disposal, and the Department of Energy (DOE) estimates that by the year 2000 the nation will have produced almost 50,000 metric tons of highly radioactive waste that must be disposed of safely. Near the end of 1987, Congress authorized the DOE to study the feasibility of using Yucca Mountain in
however, that
site must be located so that the groundwater from the site to the outside environment is at least 1,000 years. The radioactive waste at the Yucca Mountain
extremely
dangerous because of high amounts of radioactivity; therefore presents a major environmental problem.
believes,
travel time for
nuclear reactors and is
DOE
the geology of the area will prevent radioactive
a radioactive
High-level radioactive waste, such as the spent the material used in nuclear weapons,
possibility that leakage could occur over the
next 10,000 years. The
an Environmental Protection Agency (EPA) regulation,
low-level wastes can be safely buried in
controlled dump sites where the geology and groundwater system are well known and careful monitoring is provided.
uranium
some
(Fig. 1).
The water
table in the area will be
an additional 200 to 420 m below the dump site. Thus, the canisters will be stored in the zone of
which was one of the reasons Yucca selected. Only about 15 cm of rain fall in this area per year, and only a small amount of this percolates into the ground. Most of the water that does seep into the ground evaporates before it migrates very far. Thus, the rock at the depth the canisters are buried will be very dry, helping prolong aeration,
Mountain was
the lives of the canisters.
Geologists believe that the radioactive waste at
Yucca Mountain environment if it
is is
most
likely to
in liquid
contaminate the
form;
if
liquid,
it
could
seep into the zone of saturation and enter the
groundwater supply. But because of the low moisture in the zone of aeration, there is little water to carry the waste downward, and it will take well over 1,000
way
them
contaminated. Monitoring wells are usually drilled into
must
the aquifers to ensure that the waste
the contamination of our groundwater supply.
is
not migrating
find a
to dispose of
safely
and prevent
upward. One of the problems associated with deep well disposal, however, tential to initiate
is
that such injections have the po-
earthquakes (see Chapter 10).
Other sources of groundwater pollution include toxic chemicals from fertilizers, pesticides, and herbicides that are sprayed on fields and eventually percolate downward into the groundwater supply. As more chemicals come into industrial, agricultural, and domestic use, we
506
Chapter 17
Groundwater
^HOT SPRINGS AND The subsurface rocks
in regions
GEYSERS of recent volcanic ac-
hot for thousands of years. Groundwater percolating through these rocks is heated and, if returned to the surface, forms hot springs or geysers. Yellowstone National Park in the United States, Rotivity usually stay
Interior
view of Yucca Mountain
Volcanic rock
Storage tunnels
300
m
deep Emplacement ramp
truck
Exhaust
Excavation
equipment
ramp
Storage pile of rock removed during excavation I
High
(not to
/
level
m
300
.
/
/
Metal alloy
tunnels
sca e i
lining
radioactive Stainless-
waste
steel
container
Volcanic rock
Water
table
-
"^FIGURE
1 The location of Nevada's Yucca Mountain and a schematic diagram of the proposed high-level radioactive waste dump.
years to reach the zone of saturation. In fact, the
DOE
estimates that the waste will take longer than 10,000
years to
One
move from
the repository to the water table.
of the concerns of
some
geologists
is
that the
climate will change during the next 10,000 years. the region should will percolate
become more humid, more water
through the zone of aeration. This
increase the corrosion rate of the canisters
cause the water table to travel
If
rise,
will
and could
thereby decreasing the
saturation. This area of the country
humid between 2
was much more
million and 10,000 years ago (see
Chapter 18). While it appears that Yucca Mountain meets
all
of
the requirements for a safe high-level radioactive
waste dump, the
site is still controversial, and further must be conducted to ensure that the groundwater supply in this area is not rendered
studies
unusable by nuclear waste.
time between the repository and the zone of
New Zealand, and Iceland are all famous for hot springs and geysers. They are all sites of recent volcanism, and consequently their subsurface rocks and
more than 1,000
torua,
springs in the United States,
their
Far West, while the rest are in the Black Hills of South
groundwater are very hot.
Dakota, the Ouachita region of Arkansas, Georgia, and the Appalachian region (Fig. 17-27).
A
hot spring (also called a thermal spring or warm is a spring in which the water temperature is
spring)
warmer than the temperature of
human body (37°C) however, are much hotthe
17-26). Some hot springs, with temperatures ranging up to the boiling point in many instances. Of the approximately 1,100 known hot (Fig.
ter,
Hot world.
common
in
other parts of the
of the most famous
is
at Bath,
springs are also
One
are in the
England,
where shortly after the Roman conquest of Britain in a.d. 43, numerous bathhouses and a temple were built around the hot springs (Fig. 17-28). The heat for most hot springs comes from magma or
Hot Springs and Geysers
507
some hot springs, h6*wever, is circulated deep into the Earth, where it is warmed by the normal increase in temperature, the geothermal gradient. For example, the
Warm Springs, Georgia, is heated in this manner. This hot spring was a health and bathing resort long before the Civil War; later with the establishment of the Georgia Warm Springs Foundation, it was used to spring water of
help treat polio victims.
Geysers are hot springs that intermittently eject hot water and steam with tremendous force. The word comes from the Icelandic geysir which means to gush or
One of the most famous geysers in the world Old Faithful in Yellowstone National Park in Wyoming (Fig. 17-29). With a thunderous roar, it erupts a column of hot water and steam every 30 to 90 minutes.
rush forth. is
"" FIGURE 17-26 Hot springs are springs with a water temperature greater than 37°C. This hot spring is in West Thumb Geyser Basin, Yellowstone National Park, Wyoming.
cooling igneous rocks. activity in the
large
number
-»-FI
The
geologically recent igneous
western United States accounts for the
of hot springs in that region.
The water
in
Other well known geyser areas are found
New
in Iceland
and
Zealand.
Geysers are the surface expression of an extensive underground system of interconnected fractures within hot igneous rocks (Fig. 17-30). Groundwater percolating down into the network of fractures is heated as it comes into contact with the hot rocks. Since the water
dissolve
Due
more
rapidly in
warm
water than
in cold water.
to this high mineral content, the waters of
springs are believed by
some
many hot
to have medicinal proper-
Numerous spas and bathhouses have been built throughout the world at hot springs to take advantage of these supposed healing properties. ties.
When
the highly mineralized water of hot springs or
geysers cools at the surface,
solution
is
some of
the material in
precipitated, forming various types of depos-
its. The amount and type of precipitated mineral depend on the solubility and composition of the material through which the groundwater flows. If the groundwa-
ter
contains dissolved calcium carbonate
(CaC0 3 ),
then
travertine or calcareous tufa (both of
which are varieties of limestone) are precipitated. Spectacular examples of hot spring travertine deposits are found at Mammoth Hot Springs in Yellowstone National Park and at Pamukhale in Turkey (Fig. 17-31). Groundwater containing dissolved silica will, upon reaching the surface, precipitate a soft, white,
ter or geyserite,
opening
hydrated mineral called siliceous
sin-
which can accumulate around a geyser's
(Fig. 17-32).
Geothermal Energy Energy that is harnessed from steam and hot water trapped within the Earth's crust is called geothermal It is a desirable and relatively nonpolluting alform of energy. Approximately 1 to 2% of the world's current energy needs could be met by geothermal energy. In those areas where it is plentiful, however,
energy. ternate
^
FIGURE 17-28 One of the many bathhouses in Bath, England, that were built around hot springs shortly after the Roman conquest in a.d. 43.
near the bottom of the fracture system pressure than that near the top,
higher temperature before
it
it
is
under greater
must be heated
will boil.
geothermal energy can supply most,
if
not
all,
of the
•^ FIGURE 17-29 Old Faithful Geyser in Yellowstone National Park, Wyoming, is one of the world's most famous geysers, erupting approximately every 30 to 90 minutes. _
to a
Thus, when the
deeper water
is heated to very near the boiling point, a temperature or a drop in pressure, such as from escaping gas, will cause it to instantly change to
slight rise in
The expanding steam quickly pushes the water above it out of the ground and into the air, thereby producing a geyser eruption. After the eruption, relatively
steam.
cool groundwater starts to seep back into the fracture it is heated to near its boiling temperature and the eruption cycle begins again. Such a process explains how geysers can erupt with some regularity. Hot spring and geyser water typically contains large quantities of dissolved minerals because most minerals
system where
Hot Springs and Geysers
509
FIGURE 17-30 The formation of a geyser. Groundwater percolates downward into a network of interconnected openings and is heated by the hot igneous '**'
(a)
The water near the bottom of the fracture system is under greater pressure than that near the top and consequently must be heated to a higher temperature before it will boil, {b) Any rise in temperature of the water above its boiling point or a drop in pressure will cause the water to change to steam, which quickly pushes the water above it upward and out of the ground, producing a geyser eruption. rocks.
heated from geothermal wells. Direct heating
manner
heating and
The for
its
fields.
city
much
this
cleaner.
New
of Rotorua in
Zealand
is
world famous
volcanoes, hot springs, geysers, and geothermal Since the
more than 800
first
well
own
was sunk by hand
in the
1930s,
wells have been drilled to tap the hot
water and steam below. their
in
significantly cheaper than fuel oil or electrical
is
Many homes
in
Rotorua have
well for heating, hot water, and even steam
barbecuing. Geothermal energy in Rotorua variety of ways:
is
used
in a
home, commercial, and greenhouse
heating; powering refrigeration plants for air conditioning;
water
ture;
and
Research
distillation; the
commercial geothermal 1960 at The Geyabout 120 km north of San Francisco, California 17-33). Here, wells were drilled into the numerous
electrical generating plant
(Fig.
first
was
built in
near-vertical fractures underlying the region.
"***"
FIGURE
17-31
Minerva Terrace
Springs in Yellowstone National Park,
gneous
when calcium
other types of energy.
Some
of the countries currently
using geothermal energy in one form or another include Iceland, the United States,
Mexico,
Italy,
New
Zealand,
Japan, the Philippines, and Indonesia.
Geothermal energy has been successfully used
in Ice-
land since 1928. In Reykjavik, Iceland's capital, steam
and hot water from wells
pumped
drilled in
geothermal areas are
into buildings for heating
and hot water. Fruits
and vegetables are grown year-round
510
Chapter 17
Groundwater
in
hot houses
As
pres-
at Mammoth Hot Wyoming, formed
carbonate-rich hot spring water cooled,
precipitating travertine deposits.
energy needs, sometimes at a fraction of the cost of
furni-
Institute.
In the United States, the
sers,
manufacture of cane
for various research activities at the Forest
^ FIGURE
17-32
is
"^ FIGURE
Cap in Wyoming,
Liberty
Yellowstone National Park,
California.
mound produced by
a geyserite
17-33 The Geysers, Sonoma County, Plumes of steam can be seen rising from several
steam-generating plants.
repeated geyser eruptions. Each
eruption of hot silica-rich water
amount of
precipitated a small
geyserite, eventually building
large
mound.
sure
on the
up
this
groundwater decreases, the water
rising
changes to steam that
is
piped directly to
electricity-
The present electrical generating caThe Geysers is about 2,000 megawatts, which
ment have begun. While geothermally generated generally clean source of power,
ity is a
it
electric-
can also be
generating turbines.
expensive because most geothermal waters are acidic and
pacity at
very corrosive. Consequently, the turbines must either be
is
enough
to supply
about two-thirds of the
electrical
needs of the San Francisco Bay area.
As
built of expensive corrosion-resistant alloy metals or fre-
quently replaced. Furthermore, geothermal power
becoming
not
is
west-
The steam and hot water removed for geothermal power cannot be easily replaced, and eventually
ern United States, such as the Salton Sea area of southern
pressure in the wells drops to the point at which the geo-
oil
reserves decline, geothermal energy
an attractive alternative, particularly California,
is
in parts of the
where geothermal exploration and develop-
The water stored
in the
pore spaces of subsurface
rocks and unconsolidated material
is
called
groundwater. 2.
Groundwater
is
part of the hydrologic cycle and
represents approximately
22%
of the world's supply
Porosity soil
is
the percentage of a rock, sediment, or
consisting of pore space. Permeability
ability of a rock,
field
must be abandoned.
material that transmits groundwater is an aquifer and one that prevents the movement of groundwater is an aquiclude. The water table is the surface that separates the zone of aeration (in which pore spaces are filled with both air and water) from the zone of saturation (in which all
pore spaces are
filled
with water).
Groundwater moves very slowly through the pore
of fresh water. 3.
thermal
A
^CHAPTER SUMMARY 1.
inexhaustible.
sediment, or
soil to
is
the
transmit
fluids.
spaces of rocks, sediment, or soil (zone of aeration)
and moves through the zone of saturation to
outlets
such as streams, lakes, and swamps.
Chapter Summary
511
6.
A
spring occurs wherever the water table intersects
the Earth's surface.
Some
springs are the result of a
perched water table, that is, a localized aquiclude within an aquifer and above the regional water
water well zone of aeration zone of saturation
spring stalactite
stalagmite
water table
table. 7.
8.
Water wells are made by digging or drilling into the zone of saturation. When water is pumped out of a well, a cone of depression forms. If water is pumped out faster than it can be recharged, the cone of depression deepens and enlarges and may locally drop to the base of the well, resulting in a dry well. Artesian systems are those in which confined groundwater builds up high hydrostatic pressure. Three conditions must generally be met before an artesian system can form: the aquifer must be confined above and below by aquicludes; the aquifer is usually tilted and exposed at the Earth's surface so it can be recharged; and precipitation must be
keep the aquifer filled. 9. Karst topography results from groundwater, weathering, and erosion and is characterized by sinkholes, solution valleys, and disappearing streams. 10. Caves form when groundwater in the zone of saturation weathers and erodes soluble rock such as
^ 1.
2.
3.
4.
the correct order, from highest to lowest, of in the
United States?
b.
industrial, domestic, agricultural;
c.
domestic, agricultural, industrial;
d.
agricultural, domestic, industrial;
e.
industrial, agricultural, domestic.
What
percentage of the world's supply of fresh
water
is
a
5; b
The
represented by groundwater? 22; d
18; c
43;
porosity; b.
c.
solubility; d.
e.
saturation. table
is
is:
permeability;
a.
The water
50.
e
capacity of a material to transmit fluids aeration quotient;
a surface separating the:
zone of porosity from the underlying zone of
a.
permeability; b.
capillary fringe
underlying zone of aeration;
11. Modifications of the
from the capillary fringe
c.
from the underlying zone of saturation;
zone
d.
of aeration from the underlying zone of saturation;
zone of saturation from the underlying zone
e.
of aeration. 5.
Groundwater:
moves slowly through the pore spaces of b. moves fastest through the
a.
Earth materials;
central area of a material's pore space;
move upward
areas of low pressure;
eject
6.
A
7.
An
can
c.
against the force of gravity;
moves from areas of high pressure toward
d.
rocks. Geysers are hot springs that intermittently
IMPORTANT TERMS
is
agricultural, industrial, domestic;
a.
limestone. Cave deposits, called dripstone, result
hot water and steam. 14. Geothermal energy comes from the steam and hot water trapped within the Earth's crust. It is a relatively nonpolluting form of energy that is used as a source of heat and to generate electricity.
What
groundwater usage
sufficient to
from the precipitation of calcite. groundwater system can cause serious problems. Excessive withdrawal of groundwater can result in dry wells, loss of hydrostatic pressure, saltwater encroachment, and ground subsidence. 12. Groundwater contamination is becoming a serious problem and can result from sewage, landfills, toxic waste, and agriculture. 13. Hot springs and geysers may occur where groundwater is heated by hot subsurface volcanic
REVIEW QUESTIONS
all
e.
of these.
perched water table: a. occurs wherever there is a localized aquiclude within an aquifer; b. is frequently the site of springs; c. lacks a zone of aeration; d. answers (a) and (b); e. answers (b) and artesian system
water
a.
is
is
one
in
which:
water can
confined; b.
when
rise
aquiclude
groundwater
the level of the aquifer
aquifer
hot spring
artesian system
karst topography
capillary fringe
perched water table
water must be pumped; d. answers answers (a) and (b). Which of the following is not an example of
cave
permeability
column
porosity
a.
karst topography; b.
cone of depression
recharge
c.
sinkholes; d.
dripstone
saltwater
geothermal energy
512
incursion
sinkhole
geyser
Chapter 17
Groundwater
a well
is
c.
and
8.
above
drilled; (a)
(c); e.
groundwater erosion?
9.
caves;
stalactites; e.
caverns.
What
percentage of the water used in the United
States
is
a
50; b
provided by groundwater? 40; c 30; d
20; e
10.
10.
Rapid withdrawal of groundwater can result a cone of depression; b. ground
23.
in:
subsidence;
saltwater incursion; d.
c.
hydrostatic pressure; 11. In
which area are you
loss of
of these.
all
e.
least likely to find
hot springs
or geysers?
24.
eastern Canada; b.
c.
Iceland; d.
New
western United States; Zealand; e. none of
The water
in
hot springs and geysers:
is
believed to have curative properties;
b.
is
noncorrosive;
contains large
c.
(b); e.
answers
(a)
and
(a)
groundwater removal may have on
14.
The Geysers, California; Wyoming; d. Omaha, Nebraska. e. Which of the following is not a cave deposit?
the following
stalagmite; b. stalactite; e.
Discuss the role
make good
types of materials
^ADDITIONAL READINGS
subdued
What
does groundwater surface water?
20.
Where
21.
How What
ed.
American
Columbus,
Ohio: Merrill Publishing Co. J. 1990. Dreams of riches led Floyd Collins to a nightmarish end. Smithsonian 21, no. 2: 137-49. Freeze, R. A., and J. A. Cherry. 1979. Groundwater. Englewood Fincher,
Cliffs, N.J.: Prentice-Hall.
aquifers
and
replica of the
causes the water table
J.
N. 1983. Karst landforms. American
Scientist 71,
578-86.
no. 6:
1985. Karst geomorphology. 2d ed. Oxford, England:
Monastersky, R. 1988. The 10,000-year
so
much slower than
does a perched water table differ from a
is a cone of depression and important?
Science
News
133:
M. 1985. Introducing groundwater. London: Allen &c
Unwin. Rinehart,
are springs likely to occur?
test.
139-41. Price,
move
J. S.
1980. Geysers and geothermal energy.
York: Springer-Verlag. Sloan, B., ed. 1977. Caverns, caves, and caving.
New
New
Brunswick, N.J.: Rutgers University Press.
regional water table? 22.
cities.
38-47. W. 1988. Applied hydrogeology. 2d
Jennings,
level to fluctuate?
Why
and
Basil Blackwell.
the water table a
surface topography?
a thermal spring
what ways has geothermal energy been used?
.
is
groundwater system
Scientist 74, no. 1:
aquicludes?
19.
Give
a geyser?
Fetter, C.
How can a rock be porous and yet not be permeable? Why
a
Dolan, R., and H. G. Goodell. 1986. Sinking
cycle.
18.
a region.
is
room; c. dripstone; none of these. of groundwater in the hydrologic
a.
d.
ways that
may become contaminated. What is the difference between
30. In
(c).
not a geothermal site? Rotarua, New Zealand; b. Reykjavik, a. Yellowstone National Park; Iceland; c.
What
pumped?
does groundwater weather and erode?
How do caves and their various features form? 27. Discuss the various effects that excessive
29.
answers
Which of
17.
How
28. Discuss the various
a.
13.
16.
artesian wells free-flowing while
some examples.
quantities of dissolved minerals; d.
15.
some
26.
a.
and
are
25. List the surface features of karst topography and explain how they form.
these.
12.
Why
others must be
a.
why
is it
so
Additional Readings
513
CHAPTER
18
GLACIERS AND G
L
AC
I
AT O N I
^ OUTLINE PROLOGUE INTRODUCTION GLACIERS AND THE HYDROLOGIC CYCLE THE ORIGIN OF GLACIAL ICE TYPES OF GLACIERS THE GLACIAL BUDGET RATES OF GLACIAL MOVEMENT GLACIAL EROSION AND TRANSPORT Erosional Landforms of Valley Glaciers
U-Sbaped Glacial Troughs
Hanging
Valleys
Cirques, Aretes,
and Horns
Erosional Landforms of Continental Glaciers
GLACIAL DEPOSITS Landforms Composed of
Till
End Moraines Lateral
and Medial Moraines
Drumlins
Landforms Composed of
Outwash Plains and Karnes and Eskers Glacial
Stratified Drift
Valley Trains
Lake Deposits
PLEISTOCENE GLACIATION "^
Perspective 18-1: Glacial Lake Missoula
and the Channeled Scablands Pleistocene Climates Pluvial
"^
and Proglacial Lakes
Perspective 18-2:
A
Brief History of the
Great Lakes
Changes
in
Sea Level
GLACIERS AND ISOSTASY CAUSES OF GLACIATION The Milankovitch Theory Short-Term Climatic Events
CHAPTER SUMMARY Climbers ascending Ingraham Glacier on Mount Rainier, Washington.
^^ ^>ra^^3aagg^^
PROLOGUE Following the Great Ice Age, which ended about 10,000 years ago, a
warming trend occurred
general
that
was
periodically
interrupted by short relatively cool periods. cool period, from about a.d.
1500
One
such
to the mid- to
was characterized by the expansion of small glaciers in mountain valleys and the persistence of sea ice at high latitudes for longer periods than had late- 1800s,
occurred previously. This interval of nearly four centuries
The
is
known
most of the problems. Particularly hard hit were Iceland and the Scandinavian countries, but at times much of northern Europe was affected (Fig. 18-1). Growing seasons were shorter during many years, resulting in food shortages and a number of famines.
as the Little Ice Age.
climatic changes leading to the Little Ice
Age
began by about a.d. 1300. During the preceding centuries, Europe had experienced rather mild temperatures, and the North Atlantic Ocean was warmer and more storm-free than it is at the present. During this time, the Vikings discovered and settled Iceland, and by a.d. 1200, about 80,000 people resided there. They also discovered Greenland and North America and established two colonies on the former and one on the latter. As the climate deteriorated, however, the North Atlantic became stormier, and sea ice occurred further south and persisted longer each year. As a consequence of poor sea conditions and political problems in Norway, all shipping across the North Atlantic ceased, and the colonies in Greenland and North America eventually actually
"^ FIGURE 18-1 (a) During the Little Ice Age, many of the glaciers in Europe, such as this one in Switzerland, much farther down their valleys than they do at The Unterer Grindelwald painted in 1826 by Samuel Birmann (1793-1847). (b) This mid-1600s painting by Jan-Abrahamsz Beerstraten titled The Village of Nieukoop in Winter shows the canals of Holland frozen. These canals rarely freeze today. extended present.
disappeared.
During the Little Ice Age, many of the small Europe and Iceland expanded and moved
glaciers in far
down
their valleys, reaching their greatest historic
A small ice cap formed in where none had existed previously, and glaciers in Alaska and the mountains of the western United States and Canada also expanded to their greatest limits during historic time. Although glaciers caused some problems in Europe where they advanced across roadways and pastures, destroying some villages in Scandinavia and threatening villages elsewhere, their overall impact on humans was minimal. Far more important from the human perspective was that during much of the Little Ice Age the summers in northern latitudes were cooler and wetter. Although worldwide temperatures were a little lower during this time, the change in summer extent by the early 1800s. Iceland
conditions rather than cold winters or glaciers caused
Prologue
515
from its high of 80,000 40,000 by 1700. Between 1610 and
Age ended is debatable. end at 1880, whereas others ended as early as 1850. In any case, during 1800s, the sea ice was retreating northward, were retreating back up their valleys, and
when
Exactly
Iceland's population declined
the*Little Ice
Some
authorities put the
1870, sea
ice was observed near Iceland for as much months a year, and each time the sea ice persisted for long periods, poor growing seasons and
think
it
as three
the late
food shortages followed.
summer weather became more
in
1200
to about
m.^^ i^^.^ -g
m. -
-
g.^ -ic^g^^^ m ^L T
^ INTRODUCTION Most people have some idea of what a glacier is, but many confuse glaciers with other masses of snow and ice. A glacier is a mass of ice composed of compacted and recrystallized snow that flows under its own weight on
land. Accordingly, sea ice as in, for example, the
north polar region
is
not glacial
ice,
icebergs glaciers even though they
from
glaciers that flowed into the
high mountains
may
nor are drifting
may have derived sea. Snow fields in
persist in protected areas for years,
but these are not glaciers either because they are not
moving. At the present time, glaciers cover nearly 15 million km 2 or about one-tenth of the Earth's land surface (Table 18-1). Numerous glaciers exist in the mountains of actively
,
the western United States, especially Alaska, western
Canada, the Andes in South America, the Alps of Europe, the Himalayas of Asia, and other high mountains.
^ TABLE
18-1
glaciers
Present-Day Ice-Covered Areas
-
.
^
fc
.
^-
stable.
^ ^ ^'SK-^^-^^ ^ ^^g^i
•
'
-
-
"^ FIGURE
18-2
Glacier in Glacier
=»
Iceberg calving from the Margerie Bay National Park, Alaska.
THE ORIGIN OF GLACIAL
Ice is
crystalline structure cal
ICE
a mineral in every sense of the word;
and possesses
it
has a
characteristic physi-
and chemical properties. Accordingly, geologists
consider glacial ice to be rock, although
rock that
is
easily
forward manner
deformed. (Fig.
It
forms
When
18-3).
it is
a type of
in a fairly straight-
an area receives
more winter snow than can melt during the spring and
summer seasons, a fallen snow consists but
it
compacts
as
net accumulation occurs. Freshly
of about it
80%
air
refreezes; in the process, the original
verted to a granular type of ice called firn is cial
further
ice,
and
20%
solids,
accumulates, partly thaws, and
compacted and
consisting of about
is
snow
finally
90%
layer
is
con-
Deeply buried converted to gla-
firn.
solids
(Fig.
18-3).
When
accumulated snow and
ice
reach a
critical thick-
40 m, the pressure on the ice at depth is sufficient to cause deformation and flow, even though it remains solid. Once the critical thickness is reached and
ness of about
^ FIGURE
The conversion snow to firn and
18-3
of freshly fallen glacial ice.
The Origin of
Glacial Ice
517
'**'
FIGURE
18-5
Movement
of a glacier by a
combination of plastic flow and basal
slip. If
solidly frozen to the underlying surface,
it
a glacier
is
moves only by
plastic flow.
» TYPES OF GLACIERS Geologists generally recognize two basic types of gla-
and continental.
ciers:
valley
name
implies,
is
A
valley glacier, as
its
confined to a mountain valley or per-
haps to an interconnected system of mountain valleys (Fig. 18-6). Large valley glaciers commonly have several
(b) "•'"
FIGURE
The Margerie Glacier in Alaska can At lower latitudes glaciers exist only at high elevations as this one on Mount Cook, New Zealand. 18-4
(a)
exist at sea level, (b)
(Photo courtesy of R.
V. Dietrich.)
flow begins, the moving mass of polar regions where
little
ice
becomes
summer melting
a glacier. In
of
snow
oc-
curs, glaciers can exist at or very near sea level, but at
lower latitudes they are found only at higher elevations (Fig. 18-4).
which causes permanent deformation, is the primary way move. They may also move by basal slip,
Plastic flow,
occurs in response to pressure and that glaciers
which occurs when a glacier surface (Fig. 18-5). Basal slip
slides is
over the underlying
facilitated
by the pres-
ence of meltwater that reduces frictional resistance be-
tween the underlying surface and the
518
Chapter 18
Glaciers
glacier.
and Glaciation
much
smaller tributary glaciers,
as large streams have
from higher to lower elevations and are invariably small in comparison to continental glaciers, even though some may be more than 100 km long, several kilometers wide, and several hundred meters thick. tributaries. Valley glaciers flow
Continental glaciers, also called areas (at least 50,000
km 2
)
ice sheets,
cover vast
and are unconfined by
to-
pography (Fig. 18-7). In contrast to valley glaciers, which flow downhill within the confines of a valley, continental glaciers flow outward in all directions from a central area of accumulation. Valley glaciers flow in
the direction of an existing slope, whereas the direction a continental glacier flows ice thickness. Currently,
is
determined by variations
in
only two continental glaciers
one in Greenland and the other in Antarctica. Both are more than 3,000 m thick in their central areas, become thinner toward their margins, and cover all but exist,
"•*
FIGURE
18- T
The Antarctic
ice sheet,
one of two
continental glaciers existing at present.
»
THE GLACIAL BUDGET
Just as a savings account
grows and shrinks
as funds are
deposited and withdrawn, glaciers expand and contract in response to accumulation and wastage. Their behavior can be described in terms of a glacial budget, which is essentially a balance sheet of accumulation and wastage.
The upper pan of lation
surface
lower losses "**
FIGURE
A
18-6
is
perennially covered by snow. In contrast, the
pan of the same glacier is
a zone of wastage, where from melting, sublimation, and calving of icebergs
At the end of winter, a
(Fig.
18-8).
During the
Pleis-
with
covered
tocene Epoch, such glaciers covered large pans of the
snow recedes during
Northern Hemisphere continents. Many of the erosional and depositional landforms in much of Canada and the northern tier of the United States formed as a consequence of Pleistocene glaciation. Although valley and continental glaciers are easily differentiated by their size and location, an intermediate va-
limit (Fig. 18-9).
riety called ilar to,
an
ice
cap
is
also recognized. Ice caps are sim-
but smaller than, continental glaciers and cover
than 50,000
less
km 2 Some ice caps form when valley glaciers .
grow and overtop the divides and passes between adjacent valleys and coalesce to form a continuous ice cap. They also form on fairly flat terrain including some of the islands of the Canadian Arctic and Iceland.
(Fig. 18-9).
glacier's surface
is
usually
accumulated seasonal snowfall. During spring and summer, however, the snow begins to melt, first at lower elevations and then progressively higher up the glacier. The elevation to which completely
mountains
zone of accumuand the glacier's
a
is
losses,
exceed the rate of accumulation large valley glacier in Alaska. Notice
the tributaries to the large glacier.
the highest
a valley glacier
where additions exceed
the
a wastage season
One can
is
called the'firn
zones of accumulation and wastage by noting the position of the easily identify the
firn limit.
Observations of a single glacier reveal that the posifrom year to year.
tion of the firn limit usually changes If it
does not change or shows only minor fluctuations, is said to have a balanced budget; that is,
the glacier
additions in the zone of accumulation are exactly bal-
anced by losses in the zone of wastage, and the end or terminus of the glacier remains stationary. the firn limit
moves down
positive budget;
its
terminus advances
the glacier, the glacier has a
additions exceed (Fig.
distal
When
18-10b).
The
If
its
losses,
the budget
Glacial Budget
and is
its
nega-
519
i
70°
L H7S«_^grE'!sworth «5° 60° '
Mts.
#po(e
2000
Mirny
^ FIGURE
18-8
The two
existing continental glaciers. {a)
almost completely averaging thick and reaching thickness of about
Antarctica
covered by an about 2,160
is
Ungiaciated surface
ice sheet
m
a
maximum
4,000 m.
{b)
sheet has a
The Greenland
maximum
Land ice Ice shelf
ice
thickness
of approximately 3,350 m.
(a)
the glacier recedes— its terminus retreats
tive,
glacial valley (Fig. 18-10c).
But even though a
up the glacier's
may be receding, the glacial ice continues to move toward the terminus by plastic flow and basal slip. terminus
If
a negative budget persists long enough, however, a
glacier recedes
and
which
thins to the point at
it
no
longer flows, thus becoming a stagnant glacier.
Although we used a valley glacier as our example, the the flow of conti-
same budget considerations control
nental glaciers as well. For example, the entire Antarctic ice sheet
»
in the
is
the ocean
zone of accumulation, but
it
flows into
where wastage occurs.
RATES OF GLACIAL
In general, valley glaciers
MOVEMENT
move more
rapidly than con-
tinental glaciers, but the rates for both vary, ranging
from centimeters to tens of meters per day. Valley ciers
moving down
glaciers of
that
all
steep slopes flow
comparable
size
on
more
gla-
rapidly than
gentle slopes, assuming
other variables are the same.
The main glacier in volume of ice
a valley glacier system contains a greater
and thus has a greater discharge and flow 520
Chapter 18
Glaciers
and Glaciation
velocity than
"^ FIGURE 18-9 The glacial budget is the annual balance between additions in the zone of accumulation and losses in the zone of wastage. Ice and rock debris are progressively buried by newly formed ice in the zone of accumulation, but eventually reach the surface in the zone of wastage as the
Zone
of
accumulation Annual snow
line
\
(firn limit)
overlying ice melts.
Zone
of
wastage its
tributaries (Fig. 18-6).
Temperature exerts a seasonal
control on valley glaciers because although plastic flow
remains rather constant year-round, basal
important during warmer months
more abundant. Flow rates also vary within the
slip is
more
when meltwater
ice itself.
is
For example,
flow velocity generally increases in the zone of accumulation until the firn limit
is
reached; from that point, the
Zone of wastage
accumulation
-^ FIGURE 18-10 Response of a hypothetical glacier to changes in its budget, {a) If the losses in the zone of wastage, shown by stippling, equal additions in the zone of
accumulation,
shown by
crosshatching, the terminus of the
Gains exceed losses, and the glacier's terminus advances, (c) Losses exceed gains, and the glacier's terminus retreats, although the glacier continues to flow. glacier remains stationary, (b)
Rates of Glacial
Movement
521
•^ FIGURE
18-12
Crevasses and an
ice fall in a glacier in
Alaska.
FIGURE 18-11 Flow velocity in a valley glacier varies both horizontally and vertically. Velocity is greatest at the top-center of the glacier. Friction with the walls and floor of the glacial trough causes the flow to be slower adjacent to these boundaries. The length of the arrows in the figure is "•"
proportional to the velocity.
velocity
becomes progressively slower toward the
gla-
through a glacier at a velocity several times faster than the normal flow. Although surges are best documented in valley glaciers, they occur in ice caps and continental glaciers as well. During a surge, a glacier's terminus may
advance several kilometers during a year. The causes of surges are not fully understood, but some of them have occurred following a period of unusually heavy precipitation in the zone of accumulation. Others developed when excessive amounts of snow and ice were dislodged from mountain peaks and fell onto the upper parts of glaciers.
Continental glaciers ordinarily flow at a rate of cen-
cier's terminus. Valley glaciers are similar to streams, in
that the valley walls
and
floor cause frictional resistance
to flow. Thus, the ice in contact with the walls
moves more slowly than
the ice
some
and
floor
away
distance
Notice
in
Figure 18-11
upward
until the
that the flow velocity in-
top few tens of meters of
ice are
or no additional increase occurs after that point. This upper ice constitutes the rigid part of the glacier that is moving as a consequence of basal slip and reached, but
little
plastic flow below.
The
fact that this
of ice behaves as a brittle solid
is
m
upper 40 or so demonstrated
clearly
by large fractures called crevasses that develop when a valley glacier flows over a step in its valley floor where the slope increases or where it flows around a corner (Fig.
18-12). In either case, the glacial ice
is
rate of a meter or so per
Chapter 18
Glaciers
and Glaciation
move comparatively
day has a great cumu-
One
slowly
reason continenis
that they exist
and are frozen to the underlying surface most of the time, which limits the amount of basal
at higher latitudes
slip.
Some
basal slip does occur even beneath the Ant-
most of its movement is by plastic some parts of continental glaciers achieve extremely high flow rates. For exam-
arctic ice sheet, but
flow. Nevertheless,
manage ple, is
to
near the margins of the Greenland
forced between mountains in
glaciers. In
ing
100
m
some of
what
ice sheet, the ice
are called outlet
these outlets, flow velocities exceed-
per day have been recorded.
stretched
and large crevasses develop, but they extend downward only to the zone of plastic flow. In some cases, a valley glacier descends over such a steep precipice that crevasses break up the ice into a jumble of blocks and spires, and an ice fall develops (Fig. 18-12). The flow rates of valley glaciers are also complicated by glacial surges, which are bulges of ice that move (subjected to tension),
522
modest
lative effect after several decades. tal glaciers
(Fig. 18-11).
creases
timeters to meters per day. Nevertheless, even a rather
^ GLACIAL EROSION AND TRANSPORT Glaciers are currently limited in areal extent, but during the Pleistocene Epoch, they covered
much
larger areas
and were thus more important than their present distribution would indicate. Glaciers are moving solids that
»" FIGURE
18-14 Origin of a roche moutonnee. As the moves over a hill, it smooths the "upstream" side by abrasion and shapes the "downstream" side by plucking. ice
^
FIGURE 18-13 A glacial erratic near York. (Photo courtesy of R. V. Dietrich.)
Hammond, New
can erode and transport huge quantities of materials, especially unconsolidated sediment
areas of
Canada and
and
In
soil.
many
the northern United States, glaciers
transported boulders,
some of huge proportions,
for
form called a roche moutonnee, which is French for "rock sheep." As shown in Figure 18-14, a glacier smooths the "upstream" side of an obstacle, such as a small hill, and plucks pieces of rock from the "downstream" side by repeatedly freezing and pulling away from the obstacle. Sediment-laden glacial ice can effectively erode by abrasion. For example, bedrock over which sediment-
long distances before depositing them. Such boulders
laden glacial
are called glacial erratics (Fig. 18-13).
polish, a
Important erosional processes associated with glaciers include bulldozing, plucking,
and abrasion.
dozing, although not a formal geologic term,
is
Bullfairly
(Fig.
ice
has
moved commonly develops
smooth surface that
18-15a). Abrasion also yields glacial striations,
consisting of rather straight scratches (Fig.
a glacial
glistens in reflected light
on rock surfaces more than a
18-15b). Glacial striations are rarely
glacial ice freezes in the cracks
few millimeters deep, whereas glacial grooves are simibut much larger and deeper (Fig. 18-16). Abrasion also thoroughly pulverizes rocks so that they yield an
and crevices of a bedrock projection and eventually
aggregate of clay- and silt-sized particles having the con-
self-explanatory: a glacier simply shoves or pushes un-
consolidated materials in quarrying, occurs
pulls
it
loose.
W FIGURE (b)
when
One
18-15
its
path. Plucking, also called
manifestation of plucking
(a)
Glacial polish
on
is
a land-
lar
sistency of flour, hence the
name rock
flour.
Rock
flour
quartzite near Marquette, Michigan. Monument, California.
Glacial striations in basalt at Devil's Postpile National
Glacial Erosion and Transport
523
-~- FIGURE 18-16 Glacial grooves on Kelly's Island in Lake Erie.
is
so
common
in
streams discharging from glaciers that
Continental glaciers can derive sediment from
moun-
through them, and windblown dust seton their surfaces. Otherwise, most of their sediment
tains projecting tles
» FIGURE
18-17
derived from the surface over which they
move and
is
trast, valley glaciers
but
it is
(Fig.
carry sediment in
all
parts of the ice,
concentrated at the base and along the margins
18-17).
Some
of the marginal sediment
is
derived
by abrasion and plucking, but much of it is supplied by mass wasting processes. The sediments carried along the margins and center become lateral and medial moraine Sediment
is
transported in
all
parts of
The sediment carried along the margins is moraine; where two lateral moraines coalesce, they
a valley glacier. lateral
is
transported in the lower part of the ice sheet. In con-
the water generally has a milky appearance.
deposits, respectively, as discussed later in this chapter (Fig. 18-17).
form a medial moraine.
Erosional Landforms of Valley Glaciers
Some
of the world's most inspiring scenery
by valley
begin with, but
is
produced
Many mountain
ranges are scenic to
when modified by
valley glaciers, they
glaciers.
take on a unique aspect of jagged, angular peaks and ridges in the midst of
broad valleys
(Fig. 18-18).
Many
landforms resulting from valley glaciation are easily ognized. Such features enable us to appreciate the
mendous
erosive
power of moving
rectre-
ice.
U-Shaped Glacial Troughs
A U-shaped
glacial
trough
is
one of the most
features of valley glaciation (Fig 18-18c).
distinctive
Mountain
val-
eroded by running water are typically V-shaped in cross section; that is, they have valley walls that descend leys
steeply to a
narrow
trast, valleys
valley
bottom
(Fig.
18-18a). In con-
scoured by glaciers are deepened, widened,
and straightened such that they possess very steep or
524
Chapter 18
Glaciers
and Glaciation
U-shaped glacial trough
•^ FIGURE
18-18
Erosional landforms produced by valley glaciers,
area before glaciation. (b)
The same
area during the
maximum
(a)
A
mountain
extent of the valley
glaciers, (c) After glaciation.
vertical walls,
but have broad, rather
thus, they exhibit a
Many
glacial
U-shaped
contain
troughs
flat
valley floors;
profile (Fig. 18-19).
—
FIGURE 18-19 A U-shaped glacial trough northwestern Montana.
in
triangular-shaped
truncated spurs, which are cutoff or truncated ridges that extend
Another
into the preglacial valley
common
feature
basins in the valley floor
of varying resistance;
is
where the
many
(Fig.
18-18c).
a series of steps or rock glacier eroded rocks
of the basins
now
contain
small lakes.
During the Pleistocene, when glaciers were extensive, was about 130 m lower than at present, so glaciers flowing into the sea eroded their valleys to much greater depths than they do now. When the glaciers melted at the end of the Pleistocene, sea level rose, and the ocean filled the lower ends of the glacial troughs so sea level
that
now
they are long, steep-walled embayments called
fiords (Fig. 18-20).
Glacial Erosion and Transport
525
-^ FIGURE
18-20 Milford Sound, a fiord in New Zealand. (Photo courtesy of George and Linda Lohse.
Fiords are restricted to high latitudes where glaciers can be maintained even at low elevations, such as Alaska, western Canada, Scandinavia, Greenland, southern New Zealand, and southern Chile. Lower sea level during the Pleistocene was not entirely responsible for the formation of all fiords. Unlike running water,
can erode a considerable distance below sea 500 m thick can stay in contact with the sea floor and effectively erode it to a depth of about 450 m before the buoyant effects of water cause glaciers
level. In fact, a glacier
the glacial ice to float! pressive;
some
m
deep.
1,300
Hanging
in
The depth of some
Norway and
fiords
is
im-
southern Chile are about
which
is
a tributary valley
valleys meet, the
perched far above the
whose
floor
is
at a
mouth of the hanging main valley's floor (Fig.
valley
is
18-18c).
Accordingly, streams flowing through hanging valleys
plunge over vertical or very steep precipices. Although not all hanging valleys form by glacial erosion, many do. As Figure 18-18 shows, the large glacier in the
main valley vigorously erodes, whereas
the smaller
glaciers in tributary valleys are less capable of large-scale
erosion.
When
tary valleys
526
Yosemite
Falls in
Yosemite National
courtesy of Sue Monroe.)
higher level than that of the main valley. Thus, where the
two
18-21
Valleys
Although waterfalls can form in several ways, some of the world's highest and most spectacular are found in recently glaciated areas. For example, Yosemite Falls in Yosemite National Park, California, plunge 435 m vertically, cascade down a steep slope for another 205 m, and then fall vertically 97 m, for a total descent of 737 m (Fig. 18-21). The falls plunge from a hanging valley,
"^ FIGURE
Park, California plunge from a hanging valley. (Photo
the glaciers disappear, the smaller tribu-
remain as hanging
Chapter 18
valleys.
Glaciers and Glaciation
Cirques, Aretes,
and Horns
Perhaps the most spectacular erosional landforms in areas of valley glaciation occur at the upper ends of glacial troughs and along the divides separating adjacent glacial troughs. Valley glaciers form and move out from steepwalled, bowl-shaped depressions called cirques at the upper end of their troughs (Fig. 18-1 8c). Cirques are
on three sides, but one side is open and leads into the glacial trough. Some cirques typically steep-walled
slope continuously into the glacial trough, but many have a lip or threshold at their lower end (Fig. 18-22).
Although the details of cirque origin are not fully understood, they apparently form by erosion of a preexisting depression
accumulate
on
a
mountain
As snow and ice wedging and plucking
side.
in the depression, frost
takes on the typical cirque shape. In or threshold, the glacial ice apparently not only moves outward but rotates as well, scouring out
enlarge
it
until
cirques with a
it
lip
rimmed by rock. Such depressions commonly contain a small lake known as a tarn (Fig. 18-22). Cirques become wider and are cut deeper into mountain sides by headward erosion as a consequence of abrasion, plucking, and several mass wasting processes. a depression
For example, part of a steep cirque wall
may
collapse,
while frost wedging continues to pry loose other rocks
tumble downslope. Thus, a combination of promountain side depression into a large cirque; the largest one known is the Walcott Cirque in Antarctica, which is 16 km wide and 3 km deep. that
cesses can erode a small
The fact that cirques expand laterally and by headward erosion accounts for the origin of two other distinctive erosional features, aretes and horns. Aretes— narrow, serrated ridges — can form in two ways. In many cases, cirques form on opposite sides of a ridge, and headward erosion reduces the ridge until only a thin partition of rock remains (Fig. 18-18c). The same effect occurs when erosion in two parallel glacial troughs reduces the
^" FIGURE
18-22
Many
called tarns such as these
cirques contain small lakes
on Mount Whitney
in California.
intervening ridge to a thin spine of rock (Fig. 18-23).
The most majestic of these
steep-walled,
all mountain peaks are horns; pyramidal peaks are formed by
headward erosion of cirques. In order for a horn to form, a mountain peak must have at least three cirques on its flanks, all of which erode headward (Fig. 18-18c).
Excellent examples of horns include
Mount Assiniboine
Canadian Rockies, the Grand Teton in Wyoming 14-1), and the most famous of all, the Matterhorn
in the (Fig.
in
Switzerland
(Fig.
18-24).
—- FIGURE 18-23
The
knifelike
ridges adjacent to these glaciers in
the
North Cascades of Washington
are aretes.
Glacial Erosion and Transport
527
In a large part of Canada, particularly the vast Canadian Shield region, continental glaciation has stripped off the soil and unconsolidated surface sediment, revealing extensive exposures of striated and polished bedrock (Fig. 18-25). Similar though smaller bedrock exposures
are also widespread in the northern United States from
Maine through Minnesota. Farther south, however, one sees the deposits of these same glaciers. Another consequence of erosion in these areas is the complete disruption of drainage that has not yet become reestablished. Thus, much of the area is characterized by deranged drainage (Fig. 16-29e), numerous lakes and
swamps, low relief, extensive bedrock exposures, and little or no soil. Such areas are generally referred to as ice-scoured plains (Fig. 18-25).
^ GLACIAL DEPOSITS consequence of
All sediment deposited as a tivity is called glacial drift.
aged glacial -~-
FIGURE
18-24
The Matterhorn
in
Switzerland
is
a
well-known horn.
States
A
drift exists in the
glacial ac-
vast sheet of Pleistocene-
northern
and adjacent parts of Canada
tier
of the United
(Fig. 18-26).
Smaller
accumulations of similar material are found where valley
remain active. Glacial deposits in sevupper midwestern states are important sources of groundwater and rich soils, and in several states they are exploited for their sand and gravel.
glaciers existed or
Erosional Landforms of Continental Glaciers Areas eroded by continental glaciers tend to be smooth and rounded because such glaciers bevel and abrade high areas that projected into the ice. Rather than yielding the sharp, angular landforms typical of valley glaci-
produce a landscape of rather nous topography interrupted by rounded ation, they
flat,
monoto-
hills.
eral
Geologists generally recognize two distinct types of glacial drift,
till
and
stratified drift. Till consists of sed-
iment deposited directly by glacial stratified; that
or density, and
it
ice. It is
not sorted or
by
size
does not exhibit any layering.
Till
is, its
particles are not separated
deposited by valley glaciers looks
much
like the
till
of
continental glaciers except that the latter's deposits are
^ FIGURE
18-25
Territories of
Canada.
An
ice-scoured plain in the Northwest
much more extensive and have much farther.
generally been trans-
ported
Stratified drift
name
implies,
is
is
sorted by size and density and, as
layered. In fact,
its
most of the sediments
recognized as stratified drift are braided stream deposits;
which they were deposited received water and sediment load directly from melting gla-
the streams in
their
cial ice.
Landforms Composed of
Till
Landforms composed of till include several types of moraines and elongated hills called drumlins.
End Moraines The terminus of either may become stabilized
528
Chapter 18
Glaciers and Glaciation
a valley or a continental glacier in
one position for some period
"^ FIGURE drift
18-26
Exposure of Pleistocene-aged
glacial
of time, perhaps a few years or even decades. Such stabilization of the ice front does
has ceased flowing, only that
When
an
not mean that the glacier it
ice front is stationary,
dumped
is
terminus
An end moraine
18-27
which continue
in the
middle distance
to
grow as long as the ice front is staEnd moraines of valley glaciers are
bilized (Fig. 18-28).
commonly
flow within the glacier
valley occupied by the glacier.
upon
as a pile of rubble at the glacier's
(Fig. 18-27).
FIGURE
has a balanced budget.
continues, and the sediment transported within or the ice
"•*'
spans the valley of the Casement Glacier in Alaska.
near Plymouth, Massachusetts.
Such deposits are end moraines,
crescent-shaped ridges of
ciers similarly parallel the ice
till
spanning the
Those of continental glafront, but are much more
extensive.
Following a period of stabilization, a glacier
may
ad-
^ FIGURE as terminal
18-28 (a) The origin of an end moraine, (b) End moraines are described moraines or recessional moraines depending on their relative positions with produced them.
respect to the glacier that
Valley train
(a)
During glaciation
(b)
After glaciation
Glacial Deposits
529
vance or it
retreat,
depending on changes
in its
budget.
advances, the ice front overrides and modifies
If its
former moraine. Should a negative budget occur, howtoward the zone of accumu-
ever, the ice front retreats
As the ice front recedes, till is deposited as it is from the melting ice and forms a layer of ground moraine (Fig. 18-28b). Ground moraine has an lation.
liberated
irregular, rolling sists
topography, whereas end moraine con-
raines.
of long ridgelike accumulations of sediment.
After a glacier has retreated for
nus
Illinois. Their outermost end momarking the greatest extent of the glaciers, go by the special name terminal moraine (valley glaciers also deposit terminal moraines). As the glaciers retreated from the positions at which their terminal moraines were deposited, they temporarily ceased retreating numerous times and deposited dozens of recessional mo-
Ohio, Indiana, and
raines,
may once
again stabilize, and
it
some
time,
its
will deposit
termi-
another
end moraine. Because the ice front has receded, such moraines are called recessional moraines (Fig. 18-28b). During the Pleistocene Epoch, continental glaciers in the mid-continent region extended as far south as southern
"^ FIGURE
18-29
Lateral
and medial moraines on
a
and Medial Moraines
Lateral
As we previously
discussed, valley glaciers transport
considerable sediment along their margins.
Much
of
this
abraded and plucked from the valley walls, but a significant amount falls or slides onto the glacier's surface by mass wasting processes. In any case, when a glacier melts, this sediment is deposited as long ridges of till called lateral moraines along the margin of the glasediment
is
cier (Fig. 18-29).
glacier in Alaska.
Where two
lateral
moraines merge, as when a tribumoraine
tary glacier flows into a larger glacier, a medial
forms (Fig. 18-29). In fact, a large glacier often has sevdark stripes of sediment on its surface, each of which is a medial moraine. Thus, although medial mo-
eral
raines are identified by their position
on a
valley glacier,
they are, in fact, formed from the coalescence of two
moraines.
lateral
many tributaries
One can
generally determine
a valley glacier has by the
how
number of its
medial moraines.
Drumlins In
many
till,
the
areas where continental glaciers have deposited
till
has been reshaped into elongated
hills called
Some drumlins measure as much as 50 m high km long, but most are much smaller. From the
drumlins.
and
1
drumlin looks like an inverted spoon with the end on the side from which the glacial ice advanced, and the gently sloping end pointing in the diside, a
steep
rection of ice
movement
(Fig.
18-30). Thus, drumlins
ice movement. Drumlins are most often found in areas of ground moraine that were overridden by an advancing ice sheet. Although no one has fully explained the origin of drumlins, it appears that they form in the zone of plastic flow
can be used to determine the direction of
as glacial ice modifies preexisting
till
into streamlined
Drumlins rarely occur as single, isolated hills; instead they occur in drumlin fields in which hundreds or thousands of drumlins are present. Drumlin fields are found in several states and Ontario, Canada, but perhaps the finest example is near Palmyra, New York.
hills.
530
Chapter 18
Glaciers and Glaciation
"*" FIGURE 18-30 These elongated hills in Antrim County, Michigan are drumlins. (Photo courtesy of B.
(a)
M.
C.
Pape.)
Landforms Composed of As already noted,
Stratified Drift
stratified drift
posit that exhibits sorting
and
is
a type of glacial de-
layering, an indication
was deposited by running water. Stratified drift is and continental glaciers, but one would expect, it is more extensive in areas of
that
it
associated with both valley as
continental glaciation.
Outwash
Plains
and
Valley Trains
Glaciers discharge meltwater laden with sediment
most
of the time, except perhaps during the coldest months.
Such meltwater forms a
series of
braided streams that
from the front of continental glaciers over a wide region. So much sediment is supplied to these radiate out
much
streams that as
so
of
it is
deposited within the channels
sand and gravel bars. The vast blankets of sediments
formed are called outwash plains (Fig. 18-3 la). amounts of meltwater
Valley glaciers discharge huge
and, like continental glaciers, have braided streams ex-
tending from them. However, these streams are generally
confined to the lower parts of glacial troughs, and
their long,
narrow deposits of
stratified drift are
known
as valley trains (Fig. 18-31b).
Outwash numerous
plains
and
valley trains
commonly contain many of which
circular to oval depressions,
contain small lakes. These depressions are kettles; they
form when a retreating block of ice that (Fig.
18-32).
is
When
a depression;
if
ice sheet
or valley glacier leaves a
subsequently partly or wholly buried the ice block eventually melts,
it
leaves
the depression extends below the water
Sediment-filled
depressions
End moraine
(b)
"^ FIGURE
18-33
(a)
An
area of ground moraine and an
esker. (b) This small, conical hill
of B.
M.
a
is
kame. (Photo courtesy
C. Pape.;
"^ FIGURE 18-32 Two stages in the origin of kettles, kames, and eskers. (a) During glaciation. (£>) After glaciation.
they form in tunnels beneath stagnant ice and in meltwater channels on the surface of glaciers (Fig. 18-32).
Long sinuous ridges of stratified drift, many of which meander and have tributaries, are called eskers (Figs. 18-32 and 18-33a). Most eskers have sharp crests and about 30°. Some are quite high, as 100 m, and can be traced for more than 100 km. Eskers occur most commonly in areas once covered by continental glaciers, but they are also associated with large valley glaciers. The sorting and stratification of the sediments within eskers clearly indicate deposition by sides that slope at
much
as
Glacial
Lake Deposits
Numerous
consequence of glaciers scouring out depressions; others occur where a stream's drainage was as a
blocked
(see Perspective 18-1);
and others are the
Regardless of
how
they formed, glacial lakes, like
lakes, are areas of deposition.
into
them and deposited
Sediment
and observations of present-day
glacial lakes are
Chapter 18
Glaciers and Glaciation
may
all
be carried
as small deltas, but of special
interest are the fine-grained deposits.
532
result
of water accumulating behind moraines or in kettles.
running water. The physical properties of ancient eskers glaciers indicate that
Some have
lakes exist in areas of glaciation.
formed
commonly
Mud
deposits in
finely laminated, consisting
"•" FIGURE 18-34 with a dropstone.
of alternating light and dark layers. Each light-dark cou-
Each varve represents light layers form during the spring and summer and consist of silt and clay; the dark layers form during the winter when the smallest particles of clay and organic matter settle from suspen-
plet
is
called a varve (Fig. 18-34).
an annual episode of deposition; the
sion as the lake freezes over. dicates
how many
Another
The number of varves
in-
years a glacial lake has existed.
distinctive feature of glacial lakes containing
varved deposits
is
the presence of dropstones (Fig. 18-
some of boulder size, in otherwise very fine-grained deposits. The presence of varves indicates that currents and turbulence in such lakes was minimal, otherwise clay and organic matter would not have settled from suspension. How then can 34).
These are pieces of
we account ment? Most
gravel,
for dropstones in a low-energy environ-
of them were probably carried into the
lakes by icebergs that eventually melted
sediment contained
in the ice.
and released
Glacial varves
^ PLEISTOCENE GLACIATION In hindsight,
it is
hard to believe that so many compewere skeptical that
tent naturalists of the last century
widespread glaciers existed on the northern continents during the not-too-distant past. Many naturalists invoked the biblical flood to account for the large boulders throughout Europe that occur far from their sources. Others believed that the boulders were rafted to their present positions ters. It
was not
until
by icebergs floating
1837
in
floodwa-
that the Swiss naturalist Louis
Agassiz argued convincingly that the displaced boulders,
many
coarse-grained sedimentary deposits, polished and
and many of the valleys of Europe from huge ice masses moving over the land. We know today that the Pleistocene Ice Age began about 1.6 million years ago and consisted of several intervals of glacial expansion separated by warmer interglacial periods. At least four major episodes of Pleisstriated bedrock,
resulted
Pleistocene Glaciation
533
Perspective 18-1
GLACIAL LAKE MISSOULA AND THE CHANNELED SCABLANDS The term scabland
is
used in the Pacific Northwest to
interpretation based
on normal stream erosion over
describe areas from which the surface deposits have
long period of time. In contrast, Bretz held that the
been scoured, thus exposing the underlying rock. Such
scablands were formed rapidly during a flood of
an area exists in a large part of eastern Washington where numerous deep and generally dry channels are
glacial
present.
Some
flows, are
more than 70
m
deep, and their floors are
high and 70 to 100 of high
hills in
m
much
apart. Additionally, a
as
10
meltwater that lasted only a few days.
The problem with
Bretz's hypothesis
was
that he
could not identify an adequate source for his
of these channels, cut into basalt lava
covered by gigantic "ripple marks" as
a
m
number
the area are arranged such that they
appear to have been islands in a large braided stream. In 1923, J Harlan Bretz proposed that the
floodwater.
He knew
that the glaciers
had advanced
as
Spokane, Washington, but he could not explain how so much ice melted so rapidly. The answer to Bretz's dilemma came from western Montana where an enormous ice-dammed lake (Lake far south as
Missoula) had formed. Lake Missoula formed
when
channeled scablands of eastern Washington were
an advancing glacier plugged the Clark Fork Valley at
formed during a single, gigantic flood. Bretz's unorthodox explanation was rejected by most
western
geologists
~^»"
who
FIGURE
1
preferred a
more
Ice
Cork, Idaho, causing the water to
Montana
fill
the valleys of
At its highest level, Lake 2 Missoula covered about 7,800 km and contained an
traditional
(Fig. 1).
Location of glacial Lake Missoula and the channeled scablands
of eastern Washington.
Canada
Glacial Lake Clark
Montana Flathead
Lobe Alpine glaciers
534
Chapter 18
Glaciers and Glaciation
*^~
FIGURE
at Missoula,
2 The horizontal lines on Sentinel Mountain Montana are wave-cut shorelines of glacial
Lake Missoula.
estimated 2,090
km 3
of water (about
42%
into Washington.
The maximum
of the
rate of flow
estimated to have been nearly 11 million
m
3
is
/sec,
about 55 times greater than the average discharge of
Amazon
River.
When
these raging floodwaters
These gravel ridges are the so-called giant glacial Lake Missoula
this area
near
Camas Hot
Springs,
Montana.
Bretz originally believed that one massive flood formed the channeled scablands, but geologists now know that Lake Missoula formed, flooded, and re-formed at least four times and perhaps as many as seven times. The largest lake formed 18,000 to 20,000 years ago, and its draining produced the last great flood. How long did the flood last and did humans witness it? It has been estimated that approximately one month passed from the time the ice dam first broke and water
rushed out onto the scablands to the time the scabland streams returned to normal flow.
anyone witnessed the
reached eastern Washington, they stripped away the
if
and most of the surface sediment, carving out huge valleys in solid bedrock. The currents were so powerful and turbulent they plucked out and moved pieces of basalt measuring 10 m across. Within the channels, sand and gravel was shaped into huge ridges, the so-called giant ripple marks (Fig. 3).
evidence of
soil
3
marks that formed when
drained across
volume of present-day Lake Michigan). The shorelines of Lake Missoula are still clearly visible on the mountainsides around Missoula, Montana (Fig. 2). When the ice dam impounding Lake Missoula failed, the water rushed out at tremendous velocity and drained south and southwest across Idaho and
the
"^ FIGURE ripple
flood.
No
The
one knows for sure
oldest
known
from the Marmes Man site in southeastern Washington dated at 10,130 years ago, nearly 2,000 years after the last flood from
humans
in the region
is
Lake Missoula. However, it is now generally accepted that Native Americans were present in North America least
at
15,000 years ago.
Pleistocene Glaciation
535
•^ FIGURE 18-35 (a) Standard terminology for Pleistocene glacial and
interglacial stages in
America,
[b)
A
North
reconstruction
showing an idealized succession of deposits and soils developed during the glacial and interglacial stages.
tocene glaciation have been recognized in North America (Fig. 18-35),
and
and
six or seven
major
glacial
advances
now
appears,
retreats are recognized in Europe. It
Pleistocene Climates
As one would expect, Pleistocene
however, that at least 20 warm-cold cycles can be de-
popular
tected in deep-sea cores. In view of these data, the tra-
is
subdivision
four-part
ditional
of the
Pleistocene
of
the climatic effects responsible for
glaciation
belief,
were worldwide. Contrary to
however, the world was not as
commonly portrayed
in
vicinity of the glaciers experienced short
know
climates.
initely,
the present interglacial period will persist indef-
or whether
we
will enter
another glacial interval.
The onset of glacial conditions really began about 40 million years ago when surface ocean waters at high southern latitudes suddenly cooled. By about 38 million years ago, glaciers had formed in Antarctica, but a con-
tinuous ice sheet did not develop there until 15 million years ago. Following a brief
warming trend during
the
Late Tertiary Period, ice sheets began forming in the
Northern Hemisphere about 2 to 3 million years ago, and the Pleistocene Ice Age was under way. At their greatest extent, Pleistocene glaciers covered about three times as much of the Earth's surface as they do now and were up to 3 km thick (Fig. 18-36). Large areas of North America were covered by glacial ice as were Greenland, Scandinavia, Great Britain, Ireland, and a large area in the northern Soviet Union. Mountainous areas also experienced an expansion of valley glaciers and the devel-
opment of
536
ice caps.
Chapter 18
Glaciers and Glaciation
it
times of glacier growth, those areas in the immediate
North America must be modified. Based on the best available evidence, it appears that the Pleistocene ended about 10,000 years ago. However, geologists do not if
frigid as
cartoons and movies. During
long,
summers and
wet winters.
Areas outside the glaciated regions experienced varied During times of glacial growth, lower ocean temperatures reduced evaporation so that most of the world was drier than it is today. However, some areas that are arid today were
much
wetter. For example, since
the cold belts at high latitudes expanded, the temperate,
and tropical zones were compressed toward and the rain that now falls on the Mediterranean shifted so that it fell on the Sahara of North Africa enabling lush forests to grow in what is now desert. California and the arid southwestern United States were also wetter because a high-pressure zone over the northern ice subtropical,
the equator,
sheet deflected Pacific winter storms southward.
Following the Pleistocene, mild temperatures pre-
and 6,000 years ago. After this became cooler and moister favoring the growth of valley glaciers on the Northern Hemisphere continents. Careful studies of the deposits at the margins of present-day glaciers reveal that during the last 6,000 years (a time called the Neo-
vailed between 8,000
warm
period, conditions gradually
(b)
(a)
Centers of ice accumulation and maximum extent of Pleistocene glaciation in North America, (b) Centers of ice accumulation and directions of ice movement in Europe during the maximum extent of Pleistocene glaciation. "^"
FIGURE
18-36
(a)
glaciation), glaciers
expanded
The
several times.
last ex-
pansion, which occurred between 1500 and the mid- to late- 1800s,
Pluvial
was
Age
the Little Ice
(see the Prologue).
test, driest
North America. During the Pleisenough rainfall to lake 145 km long and 178 m deep. When the place in
tocene, however, that area received
maintain a
and Proglacial Lakes
During the Pleistocene, many of the basins in the western United States contained large lakes that formed as a result of greater precipitation and overall cooler temperatures (especially during the summer), which lowered
The largest of these was Lake Bonneville,
the evaporation rate (Fig. 18-37). pluvial lakes, as they are called,
which attained a
maximum
depth of at least 335
m
size of
(Fig.
50,000
18-37).
The
km
posits of the Bonneville Salt Flats west of Salt
Utah formed Great Salt Lake
in
and a
vast salt de-
Lake City
as parts of this ancient lake dried up: is
simply the remnant of this once great
lake.
Another large pluvial lake existed
in
California (see Perspective 19-2), which
is
Death
now
Valley,
the hot-
Arizona
"•"
FIGURE
18-37
Pleistocene pluvial lakes in the western
United States.
Pleistocene Glaciation
537
Perspective 18-2
BRIEF HISTORY OF THE GREAT LAKES A
Before the Pleistocene, no large lakes existed in the
of the
Great Lakes region, which was then an area of generally flat lowlands with broad stream valleys
level.
draining to the north (Fig.
1).
As
the glaciers
advanced southward, they eroded the stream valleys more deeply, forming what were to become the basins of the Great Lakes. During these glacial advances, the ice front moved forward as a series of lobes, some of which flowed into the preexisting lowlands where the ice
became thicker and moved more
rapidly.
As
a
consequence, the lowlands were deeply eroded— four
-^ FIGURE 1 Theoretical preglacial drainage in the Great Lakes region. The divide separating the preglacial Mississippi and St. Lawrence drainage basins was probably near its present location. The future sites of the Great Lakes are outlined by dotted lines.
At
Great Lakes basins were eroded below sea
five
their greatest extent, the glaciers
covered the
entire
Great Lakes region and extended
south
(Fig.
far to the
18-36a). As the ice sheet retreated
northward during the periodically stabilized,
late Pleistocene, the ice front
and numerous recessional
moraines were deposited. By about 14,000 years ago, parts of the Lake Michigan and Lake Erie basins were ice-free, and glacial meltwater began forming
As the retreat of the ice sheet continued— although periodically interrupted by minor readvances of the ice front— the Great Lakes basins were uncovered, and the lakes expanded until they eventually reached their present size and configuration proglacial lakes (Fig. 2).
(Fig. 2). Currently, the Great Lakes contain nearly 3 23,000 km of water, about 18% of the water in all fresh water lakes. Although the history of the Great Lakes just
presented
is
generally correct,
it is
oversimplified. For
and depths of the evolving Great Lakes fluctuated widely in response to minor instance, the areas
readvances of the filled,
ice front.
Furthermore, as the lakes
they spilled over the lowest parts of their
margins, thus cutting outlets that partly drained them.
And
finally, as
the glaciers retreated northward,
rebound raised the southern patts of the Great Lakes region, greatly altering their drainage systems. We shall have more to say about isostatic rebound in this region in a later section. The present-day Great Lakes and their St. Lawrence River drainage constitute one of the great commercial waterways of the world. Oceangoing vessels can sail into the interior of North America as far west as Duluth, Minnesota. To do so, however, isostatic
lake evaporated, the dissolved salts were precipitated
the other shorelines consist of moraines.
on the
named
valley floor;
some of
these evaporite deposits,
especially borax, are important mineral resources.
which form far from glaproglacial lakes are formed by the meltwater ac-
In contrast to pluvial lakes, ciers,
cumulating along the margins of glaciers. In fact, in many proglacial lakes, one shoreline is the ice front itself, while 538
Chapter 18
Glaciers and Glaciation
in
honor of the French
Lake Agassiz,
naturalist Louis Agassiz,
was a large proglacial lake covering about 250,000 km' of North Dakota and Manitoba, Saskatchewan, and Ontario,
Canada.
It
persisted until the glacial ice along
its
northern margin melted, at which time the lake was able to drain
northward into Hudson Bay.
^•>>^ Laurentide Ice Sheet
covered with vegetation. Indeed, a land bridge existed across the Bering Straits from Alaska to Siberia. Native
Americans crossed the Bering land bridge, and various animals migrated between the continents; the American bison, for example, migrated from Asia. The British Isles were connected to Europe during the glacial intervals because the shallow floor of the North Sea was above
When
sea level.
the glaciers disappeared, these areas
were again flooded, drowning the plants and forcing the animals to migrate farther inland. San Francisco
Lowering of sea
level
during the Pleistocene also
af-
most major streams. When sea level dropped, streams downcut as they sought to adjust to a new lower base level (see Chapter 16). Stream channels in coastal areas were extended and deepened along the emergent continental shelves. When sea level rose at the end of the Pleistocene, the lower ends of river valleys along the east coast of North America were flooded and are now important harbors (see Chapter 20). A tremendous quantity of water is still stored on land fected the base level of
in
present-day glaciers
(Fig.
should completely melt, sea flooding
many
16-3).
level
these
If
would
rise
Los Angeles
-*r
FIGURE
Large parts of North America— and
18-38
other continents— would be flooded by the (70 m) that
would
result
if all
all
rise in sea level
the Earth's glacial ice melted.
glaciers
about 70 m,
of the coastal areas of the world where
all
of the world's large population centers are located
the greatest crustal depression, occurred farther north in
(Fig. 18-38).
Canada
^ GLACIERS AND ISOSTASY
rebound has not been evenly distributed over the entire glaciated area: it increases in magnitude from south to north (see Fig. 11 -25b). As a result of this uneven isos-
In
Chapter
1 1
we
discussed the concept of isostasy and
noted that loading or unloading of the Earth's crust causes
it
to respond isostatically to
an increased or de-
creased load by subsiding and rising, respectively. There is
no question that
isostatic
rebound has occurred
as a
features in such areas can be explained only
consequence of
isostatic
adjustments of the Earth's
crust.
When
the Pleistocene ice sheets
in size, the
weight of the
ice
zones of accumulation. For these reasons,
rebound, coastal features
formed and increased
caused the crust to respond
above
their
former
levels in the
far we have examined the effects of glaciation, but have not addressed the central questions of what causes
large-scale glaciation
and why so few episodes of wide-
spread glaciation have occurred. For more than a cenprehensive theory explaining
at a rate of
about
1
m
per century (see Fig. ll-25a). In Perspective 18-2
we noted
that the Great Lakes
evolved as the glaciers retreated to the north. As one
would expect,
isostatic
retreated north.
rebound began as the
Rebound began
first
part of the region because that area
in the
was
ice front
southern
free of ice first.
Furthermore, the greatest loading by glaciers, and hence
540
Chapter 18
Glaciers and Glaciation
north and thus slope to
Thus
tury, scientists
rebounding
re-
elevated higher
^ CAUSES OF GLACIATION
was depressed as much as 300 m below preglacial elevations. As the ice sheets disappeared, the downwarped areas gradually rebounded to their former positions. As noted in Chapter 11, parts of still
Great Lakes
now
the south.
by slowly subsiding deeper into the mantle. In some places, the Earth's surface
Scandinavia are
in the
gion, such as old shorelines, are
in the
areas formerly covered by continental glaciers. In fact, a
number of
tatic
in the
have been attempting to develop a comall
aspects of ice ages, but
have not yet been completely successful. their lack of success sible
is
for glaciation,
One
reason for
that the climatic changes respon-
the cyclic occurrence of glacial-
and short-term events such as the Little Ice Age operate on vastly different time scales. Only a few periods of glaciation are recognized in the geologic record, each separated from the others by long intervals of mild climate. Such long-term climatic changes probably result from slow geographic changes interglacial episodes,
related to plate tectonic activity.
carry continents to high latitudes
Moving
where
plates can
glaciers
can ex-
— FIGURE
18-39
{a)
The
Earth's orbit varies from
nearly a circle (dashed line) to an ellipse (solid line)
and
back again in about 100,000 years, [b) The Earth moves around its orbit while spinning about its axis, which is tilted to the plane of the ecliptic at 23.5° and points toward the North Star. The Earth's axis of rotation slowly moves and traces out the path of a cone in space, (c) At present, the Earth is closest to the Sun in January when the Northern Hemisphere experiences winter, (d) In about 11,000 years, as a result of precession, the Earth will be closer to the Sun in July, when summer occurs in the Northern Hemisphere.
ist,
provided that they receive enough precipitation as
snow. Plate collisions, the subsequent uplift of vast areas
(a)
and the changing atmospheric and oceanic circulation patterns caused by the changing shapes and positions of plates also contribute to longfar
above sea
level,
Axis
in 1 1
approximately ,000 years
term climatic change. Intermediate-term climatic events, such as the glacial-
occur on time hundreds of thousands of years. The cyclic nature of this most recent episode of glaciation has long been a problem in formulating a compreheninterglacial episodes of the Pleistocene,
scales of tens to
sive theory of climatic change.
The Milankovitch Theory A
particularly interesting hypothesis for intermediate-
term climatic events was put forth by the Yugoslavian astronomer Milutin Milankovitch during the 1920s. He
proposed that minor irregularities in the Earth's rotation and orbit are sufficient to alter the amount of solar radiation that the Earth receives at any given latitude
and hence can
affect climatic changes.
Milankovitch theory,
it
was
initially
Now
(b)
called the
Conditions
received renewed interest during the last
20
years. January
Milankovitch attributed the onset of the Pleistocene Ice Age to variations in three parameters of the Earth's orbit (Fig. 18-39).
which
is
now
ignored, but has
The first of these is orbital eccentricity,
the degree to
(c)
which the orbit departs from a
perfect circle. Calculations indicate a roughly 100,000-
year cycle between times of
maximum
eccentricity.
Conditions
in
about
1
1.000 years
This
corresponds closely to 20 warm-cold climatic cycles that occurred during the Pleistocene. The second parameter is the angle between the Earth's axis and a line perpendic-
)
January
July
ular to the plane of the ecliptic (Fig. 18-39). This angle
i
(d)
about 1.5° from its current value of 23.5° during a 41,000-year cycle. The third parameter is the precession shifts
of the equinoxes, which causes the position of the equinoxes and solstices to shift slowly around the Earth's elliptical orbit in a
23,000-year cycle (Fig. 18-39). in these three parameters cause the
Continuous changes
amount of slightly
solar heat received at
however, remains
and
any
latitude to vary
over time. The total heat received by the planet, little
changed. Milankovitch proposed,
now many scientists agree, that the interaction of these Causes of Glaciation
541
three parameters provides the triggering
mechanism
for
space. Records kept over the past dicate that during this time the
the glacial-interglacial episodes of the Pleistocene.
has varied only energy
Short-Term Climatic Events
may
slightly.
75 years, however,
amount of
in-
solar radiation
Thus, although variations
in solar
influence short-term climatic events, such a
correlation has not been demonstrated.
Climatic events having durations of several centuries,
During large volcanic eruptions, tremendous amounts
Age, are too short to be accounted for by plate tectonics or Milankovitch cycles. Several hypotheses have been proposed, including variations in
of ash and gases are spewed into the atmosphere where
such as the
Little Ice
they reflect incoming solar radiation and thus reduce
at-
Variations in solar energy could result from changes
mospheric temperatures. Recall from Perspective 4-2 that small droplets of sulfur gases remain in the atmosphere for years and can have a significant effect on the
or from anything that would reduce
climate. Several such large-scale volcanic events have
The
been recorded, such as the 1815 eruption of Tambora, and are known to have had climatic effects. However, no
solar energy
and volcanism.
within the Sun the
itself
amount of energy
latter
the Earth receives from the Sun.
could result from the solar system passing through
clouds of interstellar dust and gas or from substances in
relationship between periods of volcanic activity
the Earth's atmosphere reflecting solar radiation back into
riods of glaciation has yet been established.
^ CHAPTER SUMMARY
and pe-
hanging valleys are also products of valley glaciation.
1.
Glaciers are masses of ice plastic flow
and basal
on land
slip.
that
move by
Glaciers currently cover
about 10% of the land surface and contain all water on Earth. 2.
2%
of
Valley glaciers are confined to mountain valleys and
flow from higher to lower elevations, whereas continental glaciers cover vast areas and flow
outward
from a zone of
in all directions
abrade and bevel high areas, producing a smooth, rounded landscape. 10. Depositional landforms include moraines, which are ridgelike accumulations of till. Several types of moraines are recognized, including terminal, recessional, lateral, and medial moraines. 11. Drumlins are composed of till that was apparently reshaped into streamlined hills by continental 9. Continental glaciers
accumulation. 3.
A
glaciers.
forms when winter snowfall in an area exceeds summer melt and therefore accumulates year after year. Snow is compacted and converted to glacial ice, and when the ice is about 40 m thick, glacier
pressure causes 4.
The behavior which
is
it
composed of on
its
budget,
13.
respectively.
move
depending on the and season. Valley glaciers tend to
at varying rates
slope, discharge,
Glaciers are powerful agents of erosion are particularly effective at eroding soil
7.
542
arid regions,
They
lower part of the ice, whereas valley glaciers may carry sediment in all parts of the ice. Erosion of mountains by valley glaciers yields several sharp, angular landforms including cirques, aretes, and horns. U-shaped glacial troughs, fiords, and Chapter 18
Glaciers
and Glaciation
and
sea level
was
as
are
now
130
m
what
much
as
lower than at present. 15.
Loading of the Earth's crust by Pleistocene
glaciers
caused isostatic subsidence. When the glaciers disappeared, isostatic rebound began and continues
unconsolidated sediment, and they can transport any size sediment supplied to them. Continental glaciers transport most of their sediment in the
8.
equator, large pluvial lakes existed in
and
about
widespread glaciation, separated by interglacial North America. The other Northern Hemisphere continents were also affected by widespread Pleistocene glaciation. 14. Areas far beyond the ice were affected by Pleistocene glaciation; climate belts were compressed toward the
and
transport because they are solids in motion.
glaciers covered
of the land surface. Several intervals of
periods, occurred in
flow more rapidly than continental glaciers. 6.
stratified drift.
During the Pleistocene Epoch,
30%
the relationship between accumulation and
If a glacier possesses a balanced budget, its terminus remains stationary; a positive or negative budget results in advance or retreat of the terminus,
Glaciers
by meltwater streams issuing from glaciers; it is found in outwash plains and valley trains. Ridges called eskers and conical hills called kames are also
to flow.
of a glacier depends
wastage.
5.
12. Stratified drift consists of sediments deposited in or
16.
in some areas. Major glacial intervals separated by
tens or
hundreds of millions of years probably occur as a consequence of the changing positions of tectonic plates, which in turn cause changes in oceanic and atmospheric circulation patterns.
17.
Currently, the Milankovitch theory is widely accepted as the explanation for glacial-interglacial
6.
intervals.
18.
Rocks abraded by is
The reasons
for short-term climatic changes, such as
Two
the Little Ice Age, are not understood.
proposed causes for such events are changes in the amount of solar energy received by the Earth and
may
glaciers
develop a smooth
surface that shines in reflected light. Such a surface
7.
volcanism.
called glacial:
a.
grooves;
d.
striations; e.
A
small lake
polish;
b.
cirque
in a
flour;
c.
till.
a.
pluvial lake; b.
c.
tarn; d.
is a:
proglacial lake;
salt lake; e.
trough
glacial
lake. 8.
IMPORTANT
TERMS
The most
recent ice age occurred during the:
c.
Archean Eon; b. Mesozoic Era; d.
e.
Tertiary Period.
a
abrasion
glacier
arete
drumlin
ground moraine hanging valley horn lateral moraine medial moraine
end moraine
Milankovitch theory
move
esker
outwash plain
a.
rock creep;
fiord
plastic flow
d.
surging;
firn
recessional moraine
firn limit
stratified drift
is a:
terminal moraine
a.
basal slip
cirque
continental glacier
glacial
budget
glacial drift
till
glacial erratic
U-shaped
glacial
groove
9.
Firn
the zone of wastage;
glacial trough
on
10. Pressure
e.
depth
causes
in a glacier
it
to
by: fracture;
b.
glacial erosion
medial moraine;
fiord; b.
basal slip;
c.
plastic flow.
e.
pyramid-shaped peak formed by
horn;
c.
hanging valley. 12. Glacial drift is a general term for: a the erosional landforms of continental cirque;
glacial ice
valley train
glaciers; b.
glacial polish
zone of accumulation zone of wastage
c.
glacial striation
ice at
a granular type of another name for a type of glacial groove.
b.
a valley train; d.
d.
valley glacier
snow;
freshly fallen
ice; c.
A
Cambrian Period;
is:
a.
11.
Pleistocene Epoch;
e.
all
the deposits of glaciers; the
icebergs floating at sea; d.
of glaciers by plastic flow and basal
movement the
slip; e.
annual wastage rate of a glacier. 13. The number of medial moraines on a glacier
^ REVIEW QUESTIONS 1.
Crevasses in glaciers extend
down
generally indicates the
to:
the base of the glacier; about 300 m; b. variable the zone of plastic flow; d. c. the depths depending on how thick the ice is; e.
2.
If
increases;
The bowl-shaped depression glacial trough a.
Which
is
cirque;
of the following
is
e.
16.
till.
U-shaped moutonnee.
d.
5.
lateral
glacial trough; e
is
a(an):
horn; moraine.
a.
fiord; b.
e.
lateral
Which
of the following
erosion of a group of cirques on the
flanks of a
mountain may produce
tarn; b.
d.
kettle; e
a glacial erratic?
a.
deposit of unsorted, unstratified
b.
glacially transported c.
e.
varve;
horn.
c.
18.
U-shaped
its
glacial
deposits consisting of light and dark
How
does glacial
ice
form, and
why
is it
how do
considered
valley glaciers differ
What
is
from
the relative importance of plastic flow
and low
19. Explain in terms of the glacial budget active glacier
a(an):
drumlin;
till;
boulder far from sand and gravel deposited in a
basal slip for glaciers at high
Headward a.
is
cirque;
arete; d.
c.
continental glaciers?
moraine; roche
valley
e.
to be a rock?
not an erosional
arete; c
plains;
knifelike ridge separating glaciers in adjacent
17. Other than size,
horn; b
outwash
layers.
lateral
c.
its
terminal moraines;
depression on a glacier; d.
upper end of a
landform? a
A
source;
no longer form.
at the
eskers; d.
trough;
drumlin;
d.
15.
a(an):
inselberg; b.
moraine; 4.
crevasses will
e.
c.
valleys
a glacier has a negative budget:
its the terminus will retreat; b. accumulation rate is greater than its wastage rate; the glacier's length all flow ceases; d c.
3.
14.
layer.
a.
tributary glaciers; b.
trains.
a
outwash
number of
a.
20.
What
is
becomes
a glacial surge
and
latitudes?
how
a once
a stagnant glacier.
and what are the probable
causes of surges?
Review Questions
543
21. Explain
how
glaciers erode
by abrasion and
ADDITIONAL
READINGS
plucking.
22.
Why
are glaciers
more
effective agents of erosion
and transport than running water? 23. Describe the processes responsible for the origin of a cirque, U-shaped glacial trough, and hanging valley. is an arete and how does one form? do the erosional landforms of continental glaciers differ from those of valley glaciers? 26. Discuss the processes whereby terminal, recessional, and lateral moraines form. 27. How does a medial moraine form, and how can one
24.
What
25.
How
determine the number of tributaries a valley glacier has by its medial moraines? 28. Describe drumlins, and explain how they form.
What
outwash plains and valley trains? 30. In a roadside outcrop, you observe a deposit of alternating light and dark laminated mud containing a few large boulders. Explain the sequence of events
29.
are
responsible for 31.
How
32
We
544
its
deposition.
do pluvial lakes differ from proglacial lakes? Give an example of each of these types of lakes. can be sure that the ancient shorelines of the Great Lakes were horizontal when they were formed, yet now they are not only elevated above their former level but they also tilt toward the south. How can you account for these observations?
Chapter 18
Glaciers and Glaciation
and G. H. Denton. 1990. What drives glacial cycles? Scientific American 262, no. 1: 49-56. Carozzi, A. V. 1984. Glaciology and the ice age. Journal of Geological Education 32: 158-70. Covey, C. 1984. The Earth's orbit and the ice ages. Scientific American 250, no. 2: 58-66. Drewry, D. J. 1986. Glacial geologic processes. London: Edward Arnold. Grove, J. M. 1988. The Little Ice Age. London: Methuen. Imbrie, J., and K. P. Imbrie. 1979. Ice ages: Solving the mystery. New Jersey: Enslow Press. John, B. S. 1977. The ice age: Past and present. London:
W.
Broecker,
S.,
Collins. .
1979. The winters of the world. London: David
&
Charles.
Kurten, B. 1988. Before the Indians.
New
York: Columbia
University Press.
— McClean, D. M. 1978. A lessons from the past. Science 201: 401-406. Schneider, S. H. 1990. Global warming: Are we entering the greenhouse century? San Francisco, Calif.: Sierra Club Books. Sharp, R. P. 1988. Living ice: Understanding glaciers and glaciation. New York: Cambridge University Press. terminal Mesozoic "greenhouse"
S., Jr. 1983. Glaciers: Clues to future climate? United States Geological Survey. Wright, A. E., and F. Moseley, eds. 1975. Ice ages: Ancient and modern. Liverpool, Great Britain: Seel House Press.
Williams, R.
CHAPTER
19
THE WORK OF WIND AND DESERTS * OUTLINE PROLOGUE INTRODUCTION SEDIMENT TRANSPORT BY WIND Bed Load Suspended Load
WIND EROSION Abrasion Deflation
^f
Perspective 19-1: Evidence of Activity
Wind
on Mars
WIND DEPOSITS The Formation and Migration of Dunes
Dune Types Loess
AND GLOBAL WIND PATTERNS THE DISTRIBUTION OF DESERTS AIR PRESSURE BELTS
CHARACTERISTICS OF DESERTS Temperature, Precipitation, and Vegetation "^Perspective 19-2: Death Valley National
Monument Weathering and
Soils
Mass Wasting, Streams, and Groundwater Wind
DESERT LANDFORMS
CHAPTER SUMMARY
Racetrack Playa, Death Valley, California,
famous
for
its
is
"sliding rocks." Geologists
winds push the rocks across a lake's exposed wet, slippery bed after a rainstorm. This limestone block was believe that strong
moved 24
m
by the wind.
PROLOGUE
fringe areas include large regions in several parts of
world (Fig. 19-1). While natural processes such as climatic change result in gradual expansion and contraction of desert the
During the last few decades, deserts have been advancing across millions of
regions,
much
recent desertification has been greatly
human
acres of productive land, destroying rangelands,
accelerated by
croplands, and even villages. Such expansion,
natural vegetation has been cleared as crop cultivation
estimated at 70,000
km
human
2
per year, has exacted a
activities. In
many
areas, the
has expanded into increasingly drier fringes to support
Because of the relentless advance of deserts, hundreds of thousands
the growing population. Because these areas are
of people have died of starvation or been forced to
common
migrate as "environmental refugees" from their
susceptible to increased
terrible toll in
homelands
to
suffering.
camps where
the majority are severely
especially
prone to droughts, crop
failures are
occurrences, leaving the land bare and
wind and water erosion. Because grasses constitute the dominant natural
malnourished. This expansion of deserts into formerly
vegetation in most fringe areas, raising livestock
productive lands
common economic
Most
is
called desertification.
regions undergoing desertification
the margins of existing deserts. delicately
lie
along
These margins have a
balanced ecosystem that serves as a buffer
between the desert on one side and a more humid environment on the other. Their potential to adjust to increasing environmental pressures from natural causes or
"^"
human
FIGURE
19-1
activity
is
limited. Currently, such
is
a
activity. Usually, these areas
achieve a natural balance between vegetation and livestock as
nomadic herders graze
the available grasses. In
many
their
animals on
fringe areas, however,
numbers have been greatly increasing in recent and they now far exceed the land's capacity to support them. As a result, the vegetation cover that livestock
years,
protects the soil has diminished, causing the soil to
Desert areas of the world and areas threatened by desertification.
Prologue
547
-*"
FIGURE
19-2
A
sharp line
marks the boundary between pasture and an encroaching dune in Niger, Africa. As the goats eat the remaining bushes, the dune will continue to advance, and more land will be lost to desertification.
crumble. This leads to further drying of the accelerated soil erosion by
wind and water
soil
desertification because important nutrients in the
and
are not returned to the
(Fig. 19-2).
Desertification captured the world's attention
Drilling water wells also contributes to desertification because
around a well
human and
site strips
away
during the Sahelian drought of
livestock activity
the vegetation.
With
its
The Sahel averages between 10 and 60 cm
starvation.
merge with the surrounding desert. In addition, the water used for irrigation from these wells sometimes contributes to desertification by increasing the salt content of the soil. As the water
of rainfall per year,
resultant bare areas
amount of
salt
is
deposited in the
1968-1973 when
nearly 250,000 people and 3.5 million cattle died of
vegetation gone, the topsoil blows away, and the
evaporates, a small
dung
soil.
falls.
90%
Because drought
is
of which evaporates
common
when
it
in the Sahel, the
region can support only a limited population of livestock
and humans. Traditionally, herders and
livestock existed in a natural balance with the
it would be in an area more rain. Over time, the salt concentration becomes so high that plants can no
vegetation, following the rains north during the rainy
that receives
season and returning south to greener rangeland
longer grow. Desertification resulting from soil
planted and
soil
and
is
not flushed out as
salinization
Middle
is
during the dry seasons.
a major problem in North Africa, the
East, southwest Asia,
and the western United
Collecting firewood for heating and cooking
is
another major cause of desertification, particularly
many
less-developed countries where
wood
is
major
fuel source. In the Sahel of Africa (a belt
1,100
km
wide that
lies
in
the
300
to
south of the Sahara), the
expanding population has completely removed all trees and shrubs in the areas surrounding many towns and cities. Journeys of several days on foot to collect firewood are common there. The use of dried animal dung to supplement firewood has exacerbated
548
Chapter 19
The Work of Wind and Deserts
Some
areas were alternately
fallow to help regenerate the
soil.
During fallow periods, livestock fed off the stubble of the previous year's planting, and their dung helped fertilize
States.
left
the
soil.
With the emergence of new nations and increased foreign aid to the Sahel during the 1950s and 1960s, nomads and their herds were restricted, and large areas of grazing land were converted to cash crops such as peanuts and cotton that have a short growing season. Expanding human and animal populations and more intensive agriculture put increasing demands on the land until the worst drought of the century brought untold misery to the people of the Sahel.
Without
rains, the crops failed
and the
livestock
denuded the land of what little vegetation remained. As a result, the adjacent Sahara expanded southward as much as 150 km. The tragedy of the Sahel and prolonged droughts in other desert fringe areas serve to remind us of the
delicate equilibrium of ecosystems in such regions.
Once
the fragile soil cover has been
erosion,
it
Chapter
6).
will take centuries for
3t3t3Eg3K^Tg^^rym^Cg^^
» INTRODUCTION Most people
Wind it
is
associate the
deserts.
an effective geologic agent in desert regions, but an important role wherever loose sediment
can be eroded, transported, and deposited, such as along shorelines or the plains (see the Prologue to Chapter 6).
we
will first consider the
work of wind
in
general and then will turn to the distribution, charac-
and landforms of
teristics,
deserts.
^ SEDIMENT TRANSPORT BY WIND ment wind
is
in
and therefore transports sedimuch the same way as running water. Although a turbulent fluid
typically flows at a greater velocity than water,
silt-size particles
as
suspended load. Sand and larger the ground as bed load.
moved along
Bed Load Sediments too large or heavy to be carried in suspension by water or wind are moved as bed load either by saltation or
by rolling and
ter 16, saltation is the
sliding.
As we discussed
in
Chap-
process by which a portion of the
bed load moves by intermittent bouncing along a stream
"^ FIGURE
.
Tfc.
**.
«.«».
VI
lifts
descending sand grains grains causing 19-3).
Wind
them
to
hit the surface, they strike other
bounce along by saltation
tunnel experiments have
shown
(Fig.
that once
sand grains begin moving, they will continue to move, if the wind drops below the speed necessary to start them moving! This happens because once saltation be-
even
it
sets off a
chain reaction of collisions between
grains that keeps the sand grains in constant motion. Saltating sand usually
even
when winds
moves near the
surface,
and
are strong, grains are rarely lifted
If the winds are very strong, wind-whipped grains can cause extensive abrasion (Fig. 19-4). A car's paint can be removed by sandblasting in a short time, and its windshield will become completely frosted and translucent from pitting.
higher than about a meter. these
it
has a lower density and, thus, can carry only clay- and particles are
TE
The wind starts sand and carries some grains short distances before they fall back to the surface. As the grains rolling and
gins,
Wind
TE.
(see
bed. Saltation also occurs on land.
work of wind with
also plays
Therefore,
removed by soil to form
new
Particles larger than sand can also be moved along the ground by the process of surface creep. This type of movement occurs when saltating sand grains strike the larger particles and push them forward along the ground.
"• r
FIGURE
The effects of wind abrasion can be Dunes National Recreation Area, Florence, Oregon. The glass is frosted as a result of pitting by windblown sand. 19-4
seen on this bottle at
is moved near the ground Sand grains are picked up by the wind falling back to the before and carried a short distance ground where they usually hit other grains, causing them to bounce and move in the direction of the wind.
19-3
Most sand
surface by saltation.
Sediment Transport by
Wind
549
— FIGURE Death
19-5
A
dust storm in
Valley, California.
Suspended Load
originated in the Sahara of Africa has been collected
on
Silt-
and clay-sized particles constitute most of a wind's suspended load. Even though these particles are much smaller and lighter than sand-sized particles, wind usually starts the latter moving first. The reason for this
the Caribbean island of Barbados.
phenomenon
that a very thin layer of motionless air
Recall that streams and glaciers are effective agents of
silt and clay remain undisturbed. The larger sand grains, however, stick up into the turbulent air zone where they can be moved. Unless the stationary air layer is disrupted, the silt and clay particles remain on the ground providing a smooth surface. This phenomenon can be
erosion, much more so than wind. Even in deserts, where wind is most effective, running water is still responsible for most erosional landforms, although stream channels are typically dry (Fig. 16-4). Nevertheless, wind action can still produce many distinctive erosional features and
lies
is
next to the ground where the small
particles
observed on a
road on a windy day. Unless a vehicle travels over the road, little dust is raised even though it is windy. When a vehicle moves over the road, it breaks
^ WIND EROSION
is
an extremely
Abrasion
the calm
Wind
layer of dust,
tion.
boundary layer of air and disturbs the smooth which is picked up by the wind and forms a dust cloud in the vehicle's wake. In a similar manner,
turbed,
silt-
and carried
when
and clay-sized in
a sediment layer
is
dis-
particles are easily picked
up
suspension by the wind, creating clouds
of dust or even dust storms (Fig. 19-5).
Once
these fine
particles are lifted into the atmosphere, they
may
be
from their source. For example, large quantities of fine dust from the southwestern United States were blown eastward and fell on New England during the Dust Bowl of the 1930s (see carried thousands of kilometers
the Prologue to Chapter 6). In addition, fine dust that
550
efficient sorting agent.
dirt
Chapter 19
The Work of Wind and Deserts
erodes material in two ways: abrasion and deflaAbrasion involves the impact of saltating sand grains on an object and is analogous to sandblasting (Fig. 19-4). The effects of abrasion, however, are usually
minor because sand, the most sion,
is
rarely carried
common
more than
1
m
agent of abra-
above the surface.
Rather than creating major erosional features, wind abrasion merely modifies existing features by etching, pitting, smoothing, or polishing. Thus, wind abrasion is most effective on soft sedimentary rocks. Ventifacts are a these are stones ted,
common
whose
product of wind abrasion;
surfaces have been polished, pit-
grooved, or faceted by the wind
(Fig. 19-6). If the
-^ -^
^^
(a)
"^ FIGURE
19-6 (a) A ventifact forms when wind-borne abrade the surface of a rock (2) forming a flat surface. If the rock is moved, (3) additional flat surfaces are formed, (b) A granite ventifact in the dune corridor along the Michigan shore, Lake Michigan. (Photo courtesy of particles (1)
Marion A. Whitney.)
wind blows from different directions, or if the stone is moved, the ventifact will have multiple facets. Ventifacts are most common in deserts, yet they can also form wherever stones are exposed to saltating sand grains, as on beaches in humid regions and some outwash plains in
New
England.
Yardangs are larger features than ventifacts and also result from wind erosion (Fig. 19-7). They are elongated and streamlined ridges that look like an overturned ship's hull. They are typically found grouped in clusters aligned parallel to the prevailing winds. They probably
^ FIGURE
19-7 Profile view of a streamlined yardang in the Roman playa deposits of the Kharga Depression, Egypt. (Photo courtesy of Marion A. Whitney.)
(b)
form by allel to
differential erosion in
which depressions, par-
the direction of wind, are carved out of a rock
body, leaving sharp, elongated ridges. These ridges
may
then be further modified by wind abrasion into their
Although yardangs are fairly comthem was renewed when images radioed back from Mars showed that they are also widespread features on the Martian surface (see
characteristic shape.
mon
desert features, interest in
Perspective 19-1).
Deflation Another important mechanism of wind erosion is deflation, which is the removal of loose surface sediment by the wind.
Among
the characteristic features of deflation in
many
and semiarid regions are deflation hollows (also called blowouts). These shallow depressions of variable dimensions result from differential erosion of surface maarid
Wind
Erosion
551
~*ir
FIGURE
3
Large dune
fields
surrounding the north
polar ice cap are testimony to the incessant wind action
occurring on Mars.
'
.'.J**'
particles
have been discovered surrounding the north (Fig. 3). The origin of these dunes is still
polar ice cap 2 A planetary dust storm obscured Mariner view of the Martian surface for the first few weeks after went into orbit around Mars in 1971.
-^FIGURE 9's it
most of the debris on the northern plains and the dunes themselves consist of material eroded from the polar deposits. When the deposits of dust-sized particles were removed by the wind, the sand-sized particles were left behind and were transported by saltation to form controversial. Geologists think that
dunes.
clay that are deposited over large areas
commonly
far
from
downwind and
their source.
The Formation and Migration of Dunes The most
characteristic features associated with sand-
covered regions are dunes, which are
mounds
or ridges
Dunes form when the wind must flow over and around an obstruction. This results of wind-deposited sand.
*» FIGURE
19-8
A
deflation hollow in
Death
Valley,
California.
Wind
Deposits
553
Desert pavement ends)
(deflation
res
i1^"5^e»
*o
"o
„'"5=£»
^r^
5
2k* ^^X^ m. % ^^ ^ U « '»3 ,
l
.
» CHAPTER SUMMARY 1.
2.
3.
4.
The waves become oversteepened and plunge forward onto the shoreline, thus expending
Shorelines are continually modified by the energy of waves and longshore currents and, to a lesser degree,
Waves approaching
by
a longshore current.
Such currents are capable of
Rip currents are narrow surface currents that carry water from the nearshore zone seaward through the
length.
breaker zone. Beaches are the most
Little
or no net forward motion of water occurs in waves in the open sea. When waves enter shallow
They processes, and
water, they are transformed into waves in which
seasonal changes.
water does move in the direction of wave advance. Wind-generated waves, especially storm waves, are
bars, and tombolos all form and consequence of longshore current transport and deposition. Barrier islands are nearshore sediment deposits of uncertain origin. They parallel the mainland but are separated from it by a lagoon. The volume of sediment in a nearshore system
work on
features.
shorelines,
but waves can also be generated by faulting, volcanic
10.
explosions, and rockfalls.
596
a shoreline at an angle generate
considerable erosion, transport, and deposition.
on water surfaces that transmit energy in the direction of wave movement. Surface waves affect the water and sea floor only to wave base, which is equal to one-half the wave oscillations
responsible for most geologic
5.
their
kinetic energy.
tidal currents.
Waves are
.
Breakers form where waves enter shallow water and the orbital motion of water particles is disrupted.
Chapter 20
Shorelines and Shoreline Processes
11
common
shoreline depositional
are continually modified by nearshore their profiles generally exhibit
Spits,
baymouth
grow
as a
remains rather constant unless the system
somehow 12.
when dams
Erosion of a sea
is
are built across
cliff
produces a gently sloping
surface called a(an):
the streams supplying sand to the system.
a.
submergent coast; b
Many
c.
beach;
shorelines are characterized by erosion rather
than deposition. Such shorelines have sea cliffs and wave-cut platforms. Other features commonly
coast.
present include sea caves, sea arches, and sea stacks.
mainland by
Submergent and emergent coasts are defined on the
a
basis of their relationships to changes in sea level.
bars; d.
13.
14.
disrupted as
The
gravitational attraction of the
Moon
causes the ocean surface to rise and
twice daily in most shoreline areas. currents have
on
effect
little
fall
and Sun
Most
tidal
shorelines.
lagoon
a
sea stacks;
force of
waves impacting on shorelines
c.
hydraulic action; d.
e.
translation.
distance the
terracing;
berm;
fetch; b.
a.
a water surface
is
marine terrace rip current
paths but with
baymouth bar
shoreline
of
beach beach face
spit
a.
breakers; b.
submergent coast
c.
swells; d.
berm
tide
e.
rip currents.
breaker
tombolo
crest (wave)
trough (wave) wave base wave-cut platform
10.
11.
deep-water waves, the water moves
In
little
wave wave wave
e.
12.
drift
waves;
more nearly
that they
is:
wave oscillation; wave refraction;
translation; b. deflection; d. reflection.
The excess water the
are:
longshore
The bending of waves so
c.
in orbital
in the direction
refracted waves;
parallel the shoreline
wave height wave length wave period wave refraction
movement
net
wave advance. Such waves
a.
wave
spit; d.
c.
wave trough.
e.
barrier island
headland longshore current longshore drift
is:
oscillation;
wind blows over
backshore
foreshore
wave
corrosion; b
period;
fetch
sea arches.
the:
IMPORTANT TERMS
emergent coast
baymouth
atolls; c.
e.
a
The
the
are:
barrier islands; b.
The
wave-cut platform; emergent
e.
composed of sand and separated from
Islands
as tides
backshore;
d.
in the
nearshore zone returns to
b.
longshore currents;
open sea by:
a.
tombolos;
c.
wave
emergence;
refraction; d.
rip
e.
currents.
^ REVIEW QUESTIONS
13.
A
sand deposit extending into the mouth of a bay
is a:
1.
Which
of the following
is
not a depositional
2.
d.
a.
spit; b.
d.
beach;
The speed
at
water surface
3.
tombolo;
c.
baymouth
bar;
which a wave form advances over a
celerity; b.
d.
wave base;
wave
length;
sea stack.
c.
of sea
erosion of
cliffs; b.
streams; d.
breakers;
coastal submergence.
e.
15.
Erosional remnants of a shoreline
now
rising
above
a wave-cut platform are:
is:
a.
the distance offshore that waves break;
b.
the width of a longshore current;
c.
the
Waves approaching
a shoreline obliquely generate:
a
flood tides; b.
c.
tidal currents;
d
longshore currents; marine berms; e
16.
a.
barrier islands; b.
c.
beaches; d.
Which
Most beach sand
is
composed of what mineral?
a
basalt;
b
calcite; c
d
quartz; e
feldspar.
gravel;
drowned
c.
range;
How
sea stacks;
marine terraces;
of the following
emergent coasts? a. marine terraces;
What
terraces. 5.
wave erosion
a.
refraction;
c.
fetch.
e.
spit;
c.
Although there are exceptions, most beaches receive most of their sediment from: offshore reefs;
is:
depth at which the orbital motion in surface waves dies out; d. the distance wind blows over a the height of storm waves. water surface; e 4.
14.
sea stack.
e.
a.
Wave base
headland; b. beach; wave-cut platform; e.
a.
landform?
is
b.
e.
spits.
a distinctive feature of
estuaries;
very high tidal
river valleys; d.
fiords.
e.
do deep- and shallow-water waves differ? is wave base and how does it affect waves
as
they enter shallow water?
Explain
how
What
longshore drift?
is
a longshore current
is
generated.
Review Questions
597
21.
What and
is
the relationship between longshore currents
22. Sketch a north-south shoreline along which several groins have been constructed.
Assume
approach from the northwest. 23. Explain why quartz is the most composing beach sands.
25.
How
why
they
common
mineral a winter
differ.
does a tombolo form?
26. Explain the concept of a nearshore sediment budget. 27.
How
does a wave-cut platform develop? how an initially irregular shoreline
28. Explain 29.
Why
30.
What
A
is
may
be helpful. does an observer at a shoreline experience two
straightened.
sketch
Fox,
W.
Prentice-Hall. J. 1988. America in peril from the sea. New Scientist 118:54-59. Komar, P. D. 1976. Beach processes and sedimentation. Englewood Cliffs, N.J.: Prentice-Hall. 1983. CRC handbook of coastal processes and erosion. Boca Raton, Fla.: CRC Press. Pethick, J. 1984. An introduction to coastal geomorphology. London: Edward Arnold. Schneider, S. H. 1990. Global warming: Are we entering the greenhouse century? San Francisco, Calif.: Sierra Club Books.
Hecht,
summer beach and
24. Sketch the profiles of a
beach, and explain
that waves
F., and M. L. Schwartz. 1985. The world's coastline. York: Van Nostrand Reinhold Co. T. 1983. At the sea's edge. Englewood Cliffs, N.J.:
Bird, E. C.
New
rip currents?
high and two low tides each day? are the characteristics of a submergent coast?
.
Snead, R. 1982. Coastal landforms and surface features. Stroudsburg, Pa.: Hutchinson Ross Publishing Co.
Walden, D. 1990. Raising Galveston. American Heritage of Invention Technology 5:8-18.
&
Williams,
crisis. U.S.
^
ADDITIONAL
Abrahamson, D.
E., ed.
Washington, D.C.: Island Bird, E. C.
F.
1984. Coasts:
geomorphology.
598
New
Chapter 20
READINGS
1989. The challenge of global warming. Press.
An
introduction to coastal
York: Blackwell.
Shorelines and Shoreline Processes
K. Dodd, and K. K. Gohn. 1990. Coasts in Geological Survey Circular 1075.
S. J.,
•^ **- *«• "^
•*-
-^T^gr
ANSWERS TO MULTIPLE-CHOICE AND FILL-IN-THE-BLANK
REVIEW QUESTIONS CHAPTER 1. c; 2. e; 3.
CHAPTER
1
b; 4. c; 5. d; 6. e; 7. a; 8. d; 9. c; 10. b; 11. a;
11
b; 2. c; 3. c; 4. a; 5. e; 6. c; 7. b; 8. c; 9. b; 10. d; 11. b;
12. c; 13. d; 14. a; 15. a; 16. e; 17. b.
12. c; 13. e; 14. b.
CHAPTER
CHAPTER
1. a; 2.
2
d; 3. e; 4. c; 5. b; 6. a; 7. c; 8. e; 9. d; 10. a; 11. c;
12. e; 13. e; 14. a; 15. a; 16. e; 17. d; 18. b; 19. c;
20. b.
y CHAPTER 1.
b; 2. d; 3. a; 4. e; 5. c; 6. c; 7. d; 8. b; 9. b; 10. a; 11.
c;
12. d; 13. e; 14. c; 15. b.
CHAPTER 3
1.
b; 2. e; 3. c; 4. d; 5. b; 6. c; 7. b; 8. a; 9. c; 10. b; 11.
12
1.
13
d; 2. a; 3. e; 4. c; 5. e; 6. b; 7. c; 8. d; 9. b; 10. c; 11. a; 12.
b; 13. c; 14. b; 15. divergent; 16. oceanic-oceanic convergent;
a; 12. a; 13. b; 14. e; 15. c.
17. transform; 18. oceanic-continental convergent.
CHAPTER
CHAPTER
1. a; 2. c; 3.
4
a; 4. e; 5. b; 6. b; 7. c; 8. b; 9. e; 10. b; 11.
1.
14
b; 2. c; 3. e; 4. d; 5. a; 6. b; 7. c; 8. c; 9. a; 10. d; 11. b;
a; 12. c; 13. a; 14. c; 15. d; 16. a; 17. d; 18. e; 19. d.
12. c; 13. a; 14. c; 15. a; 16. d; 17. a; 18. c; 19. b; 20. c.
CHAPTER
CHAPTER
1.
^
1.
5
b; 2. a; 3. d; 4. a; 5. c; 6. d; 7. d; 8. e; 9. b; 10. d; 11.
1. e; 2. e; 3.
a; 12. a; 13. d.
12. d.
CHAPTER
CHAPTER
6
15
b; 4. d; 5. c; 6. a; 7. e; 8. e; 9. c; 10. a; 11. e;
16
1.
b; 2. e; 3. a; 4. b; 5. c; 6. d; 7. b; 8. a; 9. a; 10. d; 11.
1.
d; 2. a; 3. c; 4. e; 5. b; 6. c; 7. a; 8. c; 9. b; 10. d; 11. a; 12.
e;
12. b; 13. c; 14. b.
c;
13. d; 14. b; 15. c; 16. c; 17. e; 18. a; 19. c; 20. d.
CHAPTER 1. c; 2.
CHAPTER
7
d; 3. a; 4. e; 5. a; 6. d; 7. b; 8. c; 9. a; 10. e; 11. c;
12. b; 13. c; 14.
CHAPTER
"
b";
15. d; 16.
17
b; 4. d; 5. e; 6. d; 7. e; 8. b; 9. d; 10. e; 11.
a; 12. e; 13. e; 14. b.
e.
CHAPTER
8
1. c; 2. e; 3. a; 4. c; 5. a; 6. c; 7.
1. a; 2. c; 3.
d; 8. c; 9. d; 10. b; 11. e;
18
1. c; 2. a; 3. b; 4. c; 5. e; 6. b; 7. c; 8. b; 9. b;
12. b; 13. d; 14. b; 15. a; 16. e; 17. b; 18. d.
12. b; 13. a; 14. c; 15. b.
CHAPTER
CHAPTER
1. c; 2. c; 3.
9
a; 4. e; 5. d; 6. a; 7. c; 8. e; 9. d; 10. b; 11. c;
12. e; 13. b.
CHAPTER 1. c; 2.
10
b; 3. a; 4. e; 5. a; 6. d; 7. e; 8. a; 9. b; 10. c; 11. d;
12. e; 13. c; 14. b.
10. e; 11. c;
19
1.
d; 2. b; 3. a; 4. c; 5. e; 6. d; 7. c; 8. a; 9. b; 10. e; 11.
c;
12. b; 13. d; 14. a; 15.
CHAPTER
e.
20
1. e; 2. a; 3. c; 4.
b; 5. d; 6. b; 7. a; 8. c; 9. a; 10. c; 11. d;
12. e; 13. c; 14. c; 15. b; 16. a.
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GLOSSARY
mainly of hornblende and
aa
A
lava flow with a surface of
plagioclase.
rough, jagged angular blocks and fragments.
angular unconformity An unconformity below which older
abrasion The process by which exposed rock is worn and scraped by the impact of solid particles.
strata dip at a different angle
absolute dating The process of assigning actual ages to geologic events. Various dating techniques
based on radioactive decay are used to determine absolute ages.
The
abyssal plain
flat
rises
of
margin
A
continental margin that develops at the leading edge of a continental plate
where oceanic lithosphere
is
subducted. alluvial fan A lobate deposit of sand and gravel deposited by a stream on lowlands adjacent to
highlands, usually in an arid or
A
general term for
detrital material deposited
by a
black, lustrous, hard
up-arched fold characterized by an axial plane that in half.
it
aphanitic A fine-grained texture in igneous rocks in which the individual mineral grains are too small to be seen without magnification. An aphanitic texture results from rapid
cooling of
magma.
aquiclude prevents the
Any material that movement of
groundwater.
A
allows the
permeable layer that
movement of
groundwater.
A
particle consisting of
two protons and two neutrons from the nucleus
artesian system
of an atom; emission of an alpha
groundwater is up high hydrostatic
foliated
A
dark-colored
metamorphic rock composed
assemblage range zone A type of biozone established by plotting the overlapping ranges of fossils that have different geologic ranges; the first
and
last
occurrences of
fossils
are used to establish assemblage
range zone boundaries.
A
assimilation
process in which a
reacts with preexisting rock it
comes
in contact.
asthenosphere The part of the mantle that lies below the lithosphere; behaves plastically and flows.
atom
The
smallest unit of matter
that retains the characteristics of an element.
atomic mass number The total of protons and neutrons in the nucleus of an atom,
number
atomic number protons
in the
The number of nucleus of an atom,
aureole A zone surrounding an igneous intrusion in which contact metamorphism has taken place.
narrow, serrated ridge
arete
two
glacial valleys or
B
adjacent cirques.
back-arc basin
A
system in which confined and builds (fluid) pressure.
aseismic ridge A long, linear ridge or broad plateaulike feature rising as
much
mm
erupted by a volcano.
with which
An
anticline
separating
amphibolite
is
lower-grade coals.
alpha decay A type of radioactive decay involving the emission of a
atomic number by two and the atomic mass number by four.
that
magma
stream.
particle decreases the
Uncemented pyroclastic material measuring less than 2
of volatile matter. Anthracite usually forms from the metamorphism of
aquifer
semiarid region.
alluvium
A
anthracite
coal that contains a high percentage of fixed carbon and a low percentage
divides
passive continental margins. active continental
strata.
surface of
the sea floor, covering vast areas
beyond the continental
(usually steeper) than the overlying
younger
ash
km
above the surrounding sea floor and lacking as 2 to 3
seismic activity.
A
basin formed on
the continent side of a volcanic island arc; thought to
form by
back-arc spreading; the site of a marginal sea, e.g., the Sea of Japan.
backshore is
The area of
a beach that
usually dry, being covered by
water only by storm waves or exceptionally high tides.
Glossary
601
bajada A broad alluvial apron formed at the base of a mountain range by coalescing alluvial fans.
barchan dune A crescent-shaped dune whose tips point downwind; found in areas with generally flat dry surfaces with
little
vegetation, limited
supply of sand, and nearly constant
wind
direction.
barchanoid dune A dune intermediate between transverse and barchan dunes; typically forms along the edges of a
dune
A
field.
narrow island composed of sand and separated from the mainland by
barrier island
long,
a
lagoon.
A
basal slip
type of glacial
that occurs when a glacier over the underlying surface.
movement slides
A
basalt plateau
large plateau built
fissure eruptions.
circular equivalent of a
dip toward a central point.
The
largest of intrusive
bodies, having at least 100 surface area.
Most
km 2
of
batholiths are
discordant and are composed chiefly of granitic rocks.
baymouth bar grown
until
it
A
A
spit that
has
completely cuts off a
bay from the open
beach
sea.
deposit of unconsolidated
sediment extending landward from low tide to a change in topography or where permanent vegetation begins.
beach face The sloping area below the berm that is exposed to wave swash.
The coarser part of
a
or slope gently in a landward
formed
direction.
resulting
beta decay A type of radioactive decay during which a fast-moving electron is emitted from a neutron and thus is converted to a proton; results in an increase of one atomic
number, but does not change atomic mass number.
A
Big Bang
model
for the evolution
state
is
followed by expansion,
and a
less
dense
state.
chemical processes of organisms; a subcategory of chemical sedimentary
sedimentary rocks.
The bounding
surface that separates one layer of strata
602
from another.
Glossary
large, steep-sided,
by summit collapse from the underlying magma chamber being partly drained, or by a large explosion in which the summit is blown away. either
The area extending upward a few centimeters
capillary fringe irregularly
to several meters
from the base of
the zone of aeration.
carbon 14 dating technique An absolute dating method that relies upon determining the ratio of C 14 C 12 in a sample; useful back to about 70,000 years ago; can be
to
applied only to organic substances.
A
carbonate mineral
mineral that
rocks.
bonding The process whereby atoms are joined to other atoms.
carbonate rock A rock containing predominately carbonate minerals,
Bowen's reaction series A mechanism that accounts for
cave A naturally formed subsurface opening that is generally connected
.
the
and
derivation of intermediate and felsic
to the surface
magmas from
for a person to enter.
a mafic
magma.
It
is
The
large
enough
consists of a discontinuous branch of
cementation
ferromagnesian minerals that change from one mineral to another over specific temperature ranges and a continuous branch of plagioclase feldspars whose composition changes as the temperature decreases.
binding material between and
precipitation of
around the grains of sediment, thus converting
it
to sedimentary rock.
chemical sedimentary rock Originates by precipitation of minerals derived from the ions and
braided stream A stream possessing an intricate network of
salts
dividing and rejoining channels.
chemical weathering The process whereby rock materials are decomposed by chemical alteration
when sediment
transported by the stream
is
and gravel
bedding plane
A
circular or oval volcanic depression
contains the negatively charged -2 carbonate ion (C0 3 )
sand and gravel.
in
underlying rocks.
caldera
deposited within channels as sand
Another name for layering
rapid erosion of the less resistant
The backshore area of a beach consisting of a platform composed of sediment deposited by waves; berms are nearly horizontal
stream's sediment load; consists of
bedding
found in arid and semiarid regions; formed by the breaching of a resistant cap rock, which allows
berm
Braiding occurs
bed load
feature of
biochemical sedimentary rock A sedimentary rock resulting from the
syncline. All of the strata in a basin
batholith
dipping seismic
island arcs and deep ocean trenches; such zones indicate the angle of plate descent along a convergent plate boundary.
cooling,
base level The lowest limit to which a stream can erode.
The
A
common
of the universe in which a dense, hot
up by numerous lava flows from
basin
Benioff zone zone that is a
breaker as
it
bars.
A
wave
enters shallow water until the
An
of the parent material.
cinder cone that oversteepens
crest plunges forward.
butte
taken into solution in the weathering environment.
isolated, steep-sided,
pinnacle-like erosional structure
A
small steep-sided
volcano that forms from the accumulation of pyroclastic material
around a
vent.
circum-Pacific belt
A
zone of
seismic and volcanic activity that
nearly encircles the margins of the Pacific
Ocean
basin; the majority of
the world's earthquakes
and volcanic
eruptions occur within this
cirque
A
belt.
steep-walled, bowl-shaped
concordant Refers to plutons whose boundaries are parallel to the layering in the country rock.
cone of depression The lowering of the water table around a well in
depression formed by erosion by a
the shape of a cone; results
valley glacier.
water
is
faster
than
clastic texture
A
texture of
when
removed from an aquifer it
can be replenished.
metamorphism Metamorphism in which
convergent plate boundary The boundary between two plates that are moving toward one another; three types of convergent plate
boundaries are recognized.
core
The
interior part of the Earth
which begins
at a depth of about 2,900 km; probably composed mostly of iron and nickel; divided into an outer liquid core and an
sedimentary rocks consisting of the broken particles of preexisting rocks or organic structures such as shells.
contact
cleavage The ability to break or split along a smooth plane of weakness. Cleavage is determined by the strength of the bonds within
rock.
Coriolis effect
continental-continental plate
winds to the right of their direction of motion (clockwise) in the Northern Hemisphere and to the left of their direction of motion (counterclockwise) in the Southern Hemisphere due to the
body
alters the
boundary plate
A
a
magma
surrounding country
type of convergent
boundary along which two
minerals.
continental lithospheric plates collide
column A cave deposit formed when stalagmites and stalactites
Asia).
the collision of India with
A
columnar jointing jointing that forms
The
igneous rocks.
rocks overlying the
type of
columns joints
in
commonly
form a polygonal (usually hexagonal) Columnar joints are most
pattern.
in
compaction lithification
mafic lava flows.
A method
correlation
The demonstration of
time equivalency of rock units in different areas.
igneous, sedimentary, and
country rock The rock that is invaded by and surrounds an igneous
metamorphic rocks. It has an overall composition corresponding closely to granodiorite and an overall density 3 of about 2.70 g/cm
intrusion.
covalent
bond
A bond
formed by
.
whereby the pressure
amount of pore space and thus volume of a deposit.
that
a single landmass that broke apart
the
A
combination of different types of mass movements in which one type is not dominant; most complex sliding
The theory
continental drift
the sharing of electrons between
atoms.
the continents were once joined into
overlying sediment reduces the
movements involve
deflection of
consisting of a wide variety of
of
exerted by the weight of the
complex movement
The
Earth's rotation.
The continental upper mantle and
continental crust
join.
common
(e.g.,
inner solid core.
and
flowing.
with the various fragments (continents) moving with respect to one another; proposed by Alfred Wegener in 1912. continental glacier covering a vast area
km 2
A
large glacier
(at least
50,000
and unconfined by topography. Also called an ice sheet.
crater
A
circular depression at the
summit of
a volcano resulting
the extrusion of gases
and
connected by a conduit to a
chamber below the Earth's craton
The name applied
from
lava;
magma
surface. to the
relatively stable part of a continent;
consists of a shield
and
a platform,
)
a buried extension of a shield;
the ancient nucleus of a
The area
composite volcano A volcano composed of both pyroclastic layers and lava flows typically of intermediate composition. Composite
continental margin
volcanoes, also called
continental rise
stratovolcanoes, are steep-sided near
the base of the continental slope
crest
summits (up to 30°), but decrease in slope toward their base where they are generally less than 5°.
with a gentle slope.
cross-bedding Beds that are deposited at an angle to the surface upon which they are accumulating.
their
A
above sea
level
from the deep-sea
floor.
slowest type of flow.
The area between and continental slope
continental shelf the shoreline
where the sea floor slopes very gently in a seaward direction.
different elements.
continental slope
substance resulting
compressional stress resulting
when
Stress
rocks are squeezed by
The imperceptible downslope movement of soil or rock; it is the
creep
The area beyond
from the bonding of two or more
compound
continent.
separating the part of a continent
The
relatively
steep area between the shelf-slope
break
(at
an average depth of 135 m)
crust
The
highest part of a wave.
The outermost
layer of the
Earth; the upper part of the lithosphere,
which
the mantle by the into continental
is
separated from
Moho;
divided
and oceanic
The
external forces directed toward one
and the more gently sloping
crystal settling
another.
continental rise or oceanic trench.
separation of minerals by
crust.
physical
Glossary
603
The expansion of
desertification
and gravitational
crystallization
A topographicaly high
divide •' i-
deserts into formerly productive
settling.
.'tZi'i'r:'.
'
-
J:'
'-
"
".''.
a
T.k'iS:
lands.
A solid in which atoms are arranged
crystalline solid
dome
A
an a dome dip
circular equivalent of
a regular, three-dimensional
detntal sedimentary rock Sedimentary rock consisting of
anticline. All strata in
framework.
detritus, the solid panic.
away from a
preexisting rocks. Such rocks have a
drainage basin The area occupied by a drainage system that contributes water to a given stream.
the constituent
A
crystalline texture
in
texture of
clastic texture.
rocks consisting of an interlocking
mosaic of mineral
Pressure that not applied equally to all sides of a rock body; results in distortion of the body. differential pressure
crystals.
is
Curie point The temperature at which iron-bearing minerals in a
magma
cooling
attain their
weathering
differential
magnetism.
of rock at different rates,
Weathering producing
an uneven surface.
A
dike
daughter element An element formed by the radioactive decay of another element, e.g., argon 40 is the daughter element of potassium 40.
A
debris avalanche
movement steep
mountain ranges;
starts
out as a rockfall.
A
movement
to
dip
in
high pressures.
A measure
of the
maximum
movement
is
A
':;
;:•:.-
Various cave deposits
from the deposition of
fault
drumlin An elongated hill of till measuring as much as 50 m high and 1 km long; formed by the movement
on which
A
dry climate
climate that occurs
low and middle
where the potential
perpendicular to the strike direction. dip-slip fault
resulting
in the
plane from horizontal; measured
water than a
:.-a..-.5i-e
dripstone
of a continental glacier.
rocks subjected
angular deviation of an inclined
typically
type of mass
less
changes occurring rj
that contains larger-sized
and
particles
A model u.sed on
predict earthquakes based
complex
that often occurs in very-
debris flow
discordant pluton.
model
drainage partem The regional arrangement of channels in a
calche.
tabular or sheetlike
dilatancy
central point.
all
parallel with the dip of
latitudes
loss of
water by
evaporation exceeds the yearly precipitation; covers 30% of the Earth's land surface and is divided
and arid
into semiarid
dune
A mound
regions.
or ridge of
the fault plane.
wind-deposited sand.
discharge The total volume of water in a stream moving past a particular point in a given period of
dynamic metamorphism Metamorphism associated with
zones where rocks are subjected to
depression of variable dimensions
time.
high differential pressures.
that results from the differential
disconformity An unconformity above and below which the strata
mudflow.
The removal of
deflation
loose
surface sediment by the wind,
deflation
hollow
A
shallow
erosion of surface materials by wind, delta the
An
mouth
alluvial deposit
formed
at
depositional environment An area in which sediment is deposited; a depositional
site differs in
aspects, chemistry,
physical
and biology from
adjacent environments, desert
Any
than 25
cm
are parallel.
discontinuity
of a stream.
area that receives
less
of rain per year.
A marked
change
in
scarp,
Earth materials or their properties,
as a thick, viscous, tongue-shaped
discordant Refers to plutons whose boundaries cut across the layering of country rock.
mass of wet
dissolved load
That part of a
taken into solution by chemical weathering.
divergent plate boundary
sand-sized and smaller panicles by
The boundary between two plates that are moving apart; new oceanic lithosphere forms at the boundary; characterized by volcanism and
wind.
seismicity.
A
surface mosaic
boulders found
in
many
dry regions
and formed by the removal of
604
Glossary'
flow that moves from
indicating a significant change in
developed soil and is mostly or completely devoid of vegetation,
pavement
A
the upper part of a hillside, leaving a
stream's load that consists of ions
of close-fitting pebbles, cobbles, and
earthflow
the velocity of seismic waves
Typically, a desert has poorly
desert
fault
and flows slowly downslope regolith.
The vibration of the Earth caused by the sudden release of energy, usually as a result of the earthquake
displacement of rocks along
faults.
echo sounder
An
sound signal to and return.
travel to the sea floor
instrument that determines the depth of the sea floor by measuring the time it takes for a
elastic
A theory earthquakes occur. rocks are deformed, they store rebound theory
that explains
When
how
dasr>.
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dasnc
Ho.\«1n.
A
electron
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.