baca01 12/03/2009 10:18 page i beaches and coasts · beaches and coasts richard a. davis jr and...

Beaches and Coasts

Richard A. Davis Jr and Duncan M. FitzGerald

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Beaches and Coasts

Richard A. Davis Jr and Duncan M. FitzGerald

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© 2004 by Blackwell Science Ltda Blackwell Publishing company

350 Main Street, Malden, MA 02148-5020, USA108 Cowley Road, Oxford OX4 1JF, UK550 Swanston Street, Carlton, Victoria 3053, Australia

The right of Richard Davis Jr and Duncan FitzGerald to be identified as the Authors of this Work has been asserted in accordance with the UK Copyright, Designs, and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs, and Patents Act 1988, without the prior permission of the publisher.

First published 2004 by Blackwell Science Ltd

Library of Congress Cataloging-in-Publication DataDavis, Richard.

Beaches and coasts / Richard Davis Jr. and Duncan FitzGerald.p. cm.

Includes bibliographical references (p. ).ISBN 0-632-04308-3 (alk. paper)

1. Beaches. 2. Coasts. 3. Coast changes. I. FitzGerald, Duncan. II.Title.

GB451.2.D385 2003551.45′7—dc21

2002151119

ISBN 0-632-04308-3 (hardback)

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Set in 10/12pt Caslonby Graphicraft Limited, Hong KongPrinted and bound in the United Kingdomby The Bath Press

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Brief contents

Full contents

1 Coastline variability and functions in the global environment 12 The Earth’s mobile crust 113 Sediments and rocks: materials of coastal environments 474 Sea-level change and coastal environments 565 Weather systems, extratropical storms, and hurricanes 756 Waves and the coast 1017 Beach and nearshore environment 1158 Barrier systems 1309 Coastal dunes 167

10 Coastal lagoons 17511 Tides of the ocean 18812 Tidal inlets 20913 Intertidal flats 24514 Coastal wetlands 26315 Estuaries 27816 River deltas: the source of most of our coastal sediments 28917 Glaciated coasts 30718 Rocky coasts 33619 Reef coasts 35320 Coastal erosion 37121 Human interaction with coastal dynamics 380

Glossary 405Index 415

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Acknowledgments x

1 Coastline variability and functions in the global environment 1

1.1 Introduction 21.2 Coastal settings 21.3 Population and the coast 3

1.3.1 History of coastal occupation 41.4 General coastal conditions 61.5 Coastal environments 61.6 Historical trends in coastal research 7Suggested reading 10

2 The Earth’s mobile crust 11

2.1 Introduction 122.2 The Earth’s interior 152.3 Plate boundaries 17

2.3.1 Divergent boundaries 182.3.2 Convergent boundaries 202.3.3 Transform boundaries 242.3.4 Plate movement 25

2.4 Continental margins 272.4.1 Tectonic evolution of

continental margins 292.5 Tectonic coastline classification 31

2.5.1 Collision coasts 322.5.2 Trailing edge coasts 342.5.3 Marginal sea coasts 39

2.6 Tectonic effects on coastal sediment supply 42

2.6.1 Continental drainage 422.6.2 Location of rivers 432.6.3 Continental shelf width 45

2.7 Summary 45Suggested reading 46

3 Sediments and rocks: materials of coastal environments 47

3.1 Introduction 483.2 Rock types 483.3 Sediment texture 49

3.3.1 Grain size 503.3.2 Statistical analysis of grain

size data 503.3.3 Grain shape 51

3.4 Mineralogy 523.4.1 General origin and

distribution of sediments 533.4.2 Composition 533.4.3 Texture 55


4 Sea-level change and coastalenvironments 56

4.1 Introduction 574.2 Changing the size and shape of

the container: tectonic causes 574.2.1 Local and regional changes 574.2.2 Eustatic tectonic changes 58

Contents

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4.3 Climate and sea-level change 584.3.1 Seasonal changes 584.3.2 Nonseasonal cyclic changes 594.3.3 Long-term climatic effects 60

4.4 Sea-level rise due to sediment compaction and fluid withdrawal 62

4.5 Isostasy 634.6 Changes in the volume of the

world ocean 644.6.1 Advance and retreat of ice sheets 644.6.2 Continental rebound 67

4.7 Holocene rise in sea level 684.8 Current and future sea-level changes 694.9 The impact of increasing sea-level

rise on modern coastal environments 744.10 Summary 74Suggested reading 74

5 Weather systems, extratropical storms, and hurricanes 75

5.1 Introduction 765.2 Basic atmospheric circulation and

weather patterns 775.2.1 Wind 775.2.2 Atmospheric circulation 775.2.3 Prevailing winds 795.2.4 Cyclonic and anticyclonic systems 815.2.5 Land breezes and sea breezes 81

5.3 Mid-latitude storms 825.3.1 Frontal weather 825.3.2 Cyclogenesis 845.3.3 Extratropical storms 84

5.4 Hurricanes and tropical storms 875.4.1 Low latitude storms 875.4.2 Origin and movement of

hurricanes 895.4.3 Anatomy of a hurricane 905.4.4 Hurricanes at the coast 92


6 Waves and the coast 101

6.1 Introduction 1026.2 Water motion and wave propagation 104

6.3 Wind wave types 1066.4 Distribution and transfer of wave energy 1086.5 Other types of waves 110

6.5.1 Tsunamis 1106.5.2 Standing waves 111

6.6 Wave-generated currents 1126.6.1 Longshore currents 1126.6.2 Rip currents 1136.6.3 Undertow 114


7 Beach and nearshore environment 115

7.1 Introduction 1167.2 Beach and nearshore morphology 116

7.2.1 Nearshore 1167.2.2 Beach 118

7.3 Beach types 1237.4 Beach materials 1237.5 Beach dynamics 125

7.5.1 Beach processes 1257.5.2 Beach cycles 126


8 Barrier systems 130

8.1 Introduction 1318.2 Physical description 1318.3 Distribution and coastal setting 133

8.3.1 General 1338.3.2 Tectonic controls 1348.3.3 Climatic controls 1358.3.4 Summary 136

8.4 Barrier types 1378.4.1 Barrier spits 1378.4.2 Welded barriers 1438.4.3 Barrier islands 144

8.5 Prograding, retrograding, and aggrading barriers 1488.5.1 Prograding barriers 1488.5.2 Retrograding barriers 1498.5.3 Aggrading barriers 152

8.6 Barrier stratigraphy 153

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8.7 Barrier coast morphology 1568.7.1 Hayes models 1568.7.2 Georgia Bight 161

8.8 Barrier coasts: morphology and evolution 1638.8.1 Eastern shore of Nova Scotia 1638.8.2 Mississippi River delta barriers 165


9 Coastal dunes 167

9.1 Introduction 1689.2 Types of coastal dunes and their

distribution 1689.3 Dune formation 1709.4 Dune dynamics 1729.5 Summary 174Suggested reading 174

10 Coastal lagoons 175

10.1 Introduction 17610.2 Definition 17610.3 Morphology and setting 17610.4 General characteristics 177

10.4.1 Salinity 17710.4.2 Organisms 17710.4.3 Chemical precipitates 17810.4.4 Physical processes 17810.4.5 Lagoonal sediments 180

10.5 Example lagoons 18310.5.1 Laguna Madre 18310.5.2 Lake Reeve 18310.5.3 The Coorong 186


11 Tides of the ocean 188

11.1 Introduction 18911.2 Tide-generating forces 189

11.2.1 Gravitational force 18911.2.2 Centrifugal force 19011.2.3 Tide-producing force 191

11.3 Equilibrium tide 19211.3.1 Tidal cycle 19211.3.2 Orbiting moon 19311.3.3 The Moon’s declination 193

11.4 Interaction of the Sun and the Moon 19411.5 Effects of orbital geometry 19511.6 Effects of partitioning oceans 19611.7 Tidal signatures 19811.8 Tides in shallow water 200

11.8.1 Continental shelf effects 20011.8.2 Coriolis effect 20111.8.3 Funnel-shaped embayments 20211.8.4 Tidal bore 20311.8.5 Tidal currents 203


12 Tidal inlets 209

12.1 Introduction 21012.2 What is a tidal inlet? 21012.3 Inlet morphology 212

12.3.1 Tidal deltas 21212.3.2 Ebb-tidal delta morphology 216

12.4 Tidal inlet formation 21712.4.1 Breaching of a barrier 21712.4.2 Spit building across a bay 21712.4.3 Drowned river valleys 22312.4.4 Ephemeral inlets 224

12.5 Tidal inlet migration 22512.6 Tidal inlet relationships 226

12.6.1 Inlet throat area–tidal prism relationship 227

12.6.2 Ebb-tidal delta volume–tidal prism relationship 228

12.7 Sand transport patterns 22912.7.1 General sand dispersal trends 22912.7.2 Inlet sediment bypassing 230

12.8 Tidal inlet effects on adjacent shorelines 23412.8.1 Number and size of tidal inlets 23412.8.2 Tidal inlets as sediment traps 23612.8.3 Changes in ebb-tidal delta

volume 23612.8.4 Wave sheltering 237

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12.8.5 Effects of inlet sediment bypassing 240

12.8.6 Human influences 24112.9 Summary 242Suggested reading 244

13 Intertidal flats 245

13.1 Introduction 24613.2 Morphology of tidal flats 24613.3 Sediments 24713.4 Organisms 248

13.4.1 Vagrant organisms 24813.4.2 Sessile organisms 24813.4.3 Limiting factors 24813.4.4 Bioturbation 249

13.5 Sedimentary structures 25013.6 Tidal flat processes 252

13.6.1 Tides 25313.6.2 Waves 255

13.7 Tidal channels 25513.8 Some examples 256

13.8.1 German Wadden Sea and Jade Bay 257

13.8.2 The Wash 25713.8.3 Bay of Saint Malo 25913.8.4 Bay of Fundy 260


14 Coastal wetlands 263

14.1 Introduction 26414.2 Salt marshes 264

14.2.1 Characteristics of a coastal marsh 264

14.2.2 Marsh plants 26514.2.3 Global distribution 26614.2.4 Marsh characteristics 26614.2.5 Marsh classification 26714.2.6 Marsh sedimentation 26814.2.7 Sediments 27114.2.8 Sea level and marsh

development 27114.2.9 Marsh summary 272

14.3 Mangrove coasts 27314.3.1 Mangrove distribution 27314.3.2 Mangroves and coastal

processes 27514.4 Summary 277Suggested reading 277

15 Estuaries 278

15.1 Introduction 27915.2 Estuarine hydrology 28015.3 Classification of estuaries 28015.4 Estuarine processes 28115.5 Time–velocity relationships 28315.6 Model estuary 28415.7 Estuary types 286

15.7.1 Tide-dominated estuaries 28615.7.2 Wave-dominated estuaries 286


16 River deltas: the source of most of our coastal sediments 289

16.1 Introduction 29016.2 How deltas develop 29116.3 Deltas and sea level 29316.4 Delta environments 295

16.4.1 Delta plain 29516.4.2 Delta front 297

16.5 Delta processes 29816.5.1 River processes 29816.5.2 Human influence 300

16.6 Delta classification 30116.6.1 River-dominated deltas 30216.6.2 Tide-dominated deltas 30316.6.3 Wave-dominated deltas 30316.6.4 Intermediate deltas 304


17 Glaciated coasts 307

17.1 Introduction 30817.2 The world’s glaciers 309

17.2.1 Glacier formation 30917.2.2 Glacier movement 309

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17.2.3 Distribution and types of glaciers 311

17.3 Pleistocene glaciation 31317.3.1 Introduction 31317.3.2 Defining the Pleistocene 31317.3.3 Causes of the Ice Ages 31617.3.4 The late Pleistocene 319

17.4 Glacial effects on coastlines 31917.4.1 General erosional processes 32017.4.2 Fjords 32017.4.3 Rocky coasts 32117.4.4 General depositional processes 32217.4.5 Depositional landforms 323

17.5 Examples of glaciated coastlines 32617.5.1 Cape Cod 32617.5.2 Drumlin coasts 32917.5.3 Sand and gravel beaches 33117.5.4 Uplifted coasts 33217.5.5 Drowned river valleys 333


18 Rocky coasts 336

18.1 Introduction 33718.2 Types and distribution 337

18.2.1 Tectonic settings 33818.2.2 Glaciated regions 33818.2.3 Other bedrock coasts 339

18.3 Erosional processes 34018.3.1 Physical processes 34018.3.2 Biological processes 34218.3.3 Chemical processes 343

18.4 Factors affecting rates of erosion 34318.4.1 Rock type 34318.4.2 Degree of fracturing 34418.4.3 Wave energy 34418.4.4 Tidal range 34518.4.5 Climate 34518.4.6 Relative sea level 345

18.5 Morphology 34518.5.1 Sea cliffs 34518.5.2 Horizontal erosional landforms 34718.5.3 Sea stacks, arches, and erosional

features 349


19 Reef coasts 353

19.1 Introduction 35419.2 General reef characteristics 354

19.2.1 Zonation of reefs 35519.3 Algal stromatolite reefs 35619.4 Oyster reefs 35619.5 Worm reefs 35719.6 Vermitid gastropod reefs 35719.7 Coral–algal reefs 358

19.7.1 Reef classification 35919.7.2 Sea level and reef development 36219.7.3 Reef environments 363

19.8 Coral reef deterioration from human activities 368


20 Coastal erosion 371

20.1 Introduction 37220.2 Causes of erosion 37220.3 Coastal erosion factors 372

20.3.1 Waves 37220.3.2 Wind 37420.3.3 Currents 37420.3.4 Sea-level rise 37520.3.5 Human modification of

the coast 37520.3.6 Bioerosion 376

20.4 Example scenarios of coastal erosion 37620.4.1 Leading edge coastal cells 37820.4.2 Undercutting high cliffs 37820.4.3 Storm erosion of beaches 378


21 Human interaction with coastaldynamics 380

21.1 Introduction 38121.2 Construction on the coast 381

21.2.1 History of coastal construction 38121.2.2 Materials 382

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21.2.3 Hard coastal protection 38521.2.4 Soft coastal protection 39221.2.5 Backbarrier and estuarine

construction 39521.2.6 Dredging 39721.2.7 Dredge and fill construction 399

21.3 Mining the coast 399

21.4 Water quality 40121.4.1 Nutrient loading 402


Glossary 405Index 415

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The authors and publisher gratefully acknowledgethe permission granted to reproduce the copyrightmaterial in this book.

Fig. 2.21: from G. Plafker et al. 1969. Effects of theearthquake of March 27, 1964, on various com-munities. USGS Professional Paper 542-G.

Figs 3.8, 4.19, 8.4, 9.38, and 10.7: from R. A.Davis, Geology of Holocene Barrier Island Systems.© 1994 Springer-Verlag.

Figs 4.2 and 4.3: from P. D. Komar & D. B.Enfield, Short-term sea-level changes andcoastal erosion. In D. Nummedal, O. H. Pilkey& J. D. Howard (eds), Sea-level Fluctuation andCoastal Evolution. SEPM Spec. Publ. No. 41,1987, p. 18. © SEPM.

Fig. 4.7: from B. J. Skinner & S. C. Porter, DynamicEarth. © 1989 Wiley. This material is used bypermission of John Wiley and Sons, Inc.

Fig. 4.13: modified from V. Gornitz & S. J.Lebedeff, Global sea-level changes during thepast century. In D. Nummedal, O. H. Pilkey & J. D. Howard (eds), Sea-level Fluctuation andCoastal Evolution. SEPM Spec. Publ. No. 41,1987, p. 10. © SEPM.

Fig. 7.1: from R. A. Davis, Coastal SedimentaryEnvironments, p. 386, Fig. 6.5. © 1985 Springer-Verlag.

Fig. 7.6: from H. E. Clifton, R. E. Hunter & R. L.Phillips, Depositional structures and processes in the non-barred high-energy nearshore. J. Sed.Petrology, 1971, vol. 41, p. 722. © SEPM.

Fig. 7.15: from L. D. Wright, J. Chappell, B. G.Thom, M. P. Bradshaw & P. J. Cowell,Morphodynamics of reflective and dissipativebeach and inshore systems, SoutheasternAustralia. Mar. Geol., 1979, vol. 32, pp. 105–40,Fig. 6. With permission from Elsevier Science.

Fig. 7.25: modified from R. A. Davis, W. T. Fox,M. O. Hayes & J. C. Boothroyd, Comparison ofridge and runnel systems in tidal and non-tidalenvironments. J. Sed. Petrology, 1972, vol. 42, p. 426. © SEPM.

Fig. 8.3: reprinted from M. O. Hayes, 1979, Barrierisland morphology as a function of tidal and wave regime. In S. P. Leatherman (ed.), BarrierIslands. New York: Academic Press, pp. 1–28.Copyright © 1979, with permission fromElsevier Science.

Fig. 8.27: from D. M. FitzGerald, Geomorphicvariability and morphologic and sedimentologiccontrols on tidal inlets. Journal of CoastalResearch, vol. SI23, pp. 47–71, Fig. 4. © CoastalEducation & Research Foundation. Reproducedwith permission.

Fig. 8.28: from R. Boyd, A. J. Bowen & R. K. Hall,1987, An evolutionary model for transgressivesedimentation on the eastern shore of NovaScotia. In D. M. FitzGerald & P. S. Rosen (eds),Glaciated Coasts. New York: Academic Press, pp. 87–114, Fig. 13. Copyright © 1987. Reprintedwith permission from Elsevier Science.

Fig. 12.32: from P. L. Barnard & R. A. Davis, Jr,Anthropogenic vs. natural influences on inlet

Acknowledgments

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Acknowledgments xi

evolution: west-central Florida. In N. C. Kraus& W. G. McDougal (eds), Coastal Sediments ’99,vol. 2. Proceedings of the 4th InternationalSymposium on Coasting Engineering andScience of Coastal Sediment Processes,Hauppage, New York, June 21–23, 1999.Reston, VA: ASCE, p. 1495. Reproduced bypermission of ASCE.

Figs 13.4, 13.13(a), 14.11, and 15.8: from R. A.Davis, Coastal Sedimentary Environments, Figs3.2, 3.14, 2.6 and 2.48. © 1978 Springer-Verlag.

Fig. 13.23: from H. Reineck & I. B. Singh,Depositional Sedimentary Environments withReference to Terrigenous Clastics, 2 edn, p. 432,Fig. 591. © 1980 Springer-Verlag.

Figs 14.7, 14.9, 14.10, and 14.21: from C. J. Dawes,Marine Botany. © 1981 Wiley. This material is used by permission of John Wiley and Sons,Inc.

Fig. 14.18: from J. M. Edwards & R. W. Frey,Substrate characteristics within a Holocene saltmarsh, Sapelo Island, Georgia. SenckenbergianaMaritima, 1977, vol. 9, pp. 215–59. ©Senckenberg Institute.

Fig. 15.9: from M. Nichols, Effect of increasingdepth on salinity in the James River Estuary. In B. W. Nelson (ed.), EnvironmentalFramework of Coastal Plain Estuaries. Geol. Soc.Amer. Memoir, 1972, 133, p. 209. © GSA.

Fig. 16.2: from J. R. L. Allen, Sediments of themodern Niger Delta: a summary and review. In J. P. Morgan (ed.), Deltaic Sedimentation. SEPMSpec. Publ. 15, 1970, p. 141. © SEPM.

Fig. 16.8: from J. M. Coleman & S. W. Gagliano,Cyclic sedimentation in the Mississippi RiverDeltaic Plain. Trans. Gulf Coast Assoc. Geol. Soc.,1964, vol. 14, p. 69. © Gulf Coast Association ofGeological Societies.

Fig. 16.20: from L. D. Wright & J. M. Coleman,Variations in morphology of major river deltas as a function of ocean wave and river dischargeregimes. Amer. Assoc. Petroleum Geol. Bull., 1973,vol. 57, p. 377. AAPG © 1973. Reprinted by

permission of the AAPG, whose permission isrequired for further use.

Fig. 16.21: modified from J. R. L. Allen, LateQuaternary Niger Delta and adjacent areas: sedimentary environments and lithofacies. Amer.Assoc. Petroleum Geol. Bull., 1965, vol. 49, p. 558.AAPG © 1965. Reprinted by permission of theAAPG, whose permission is required for furtheruse.

Fig. 17.8: from F. K. Lutgens & E. J. Tarbuck,1999, Earth: An Introduction to Physical Geology,6th edn. © Prentice Hall. Reproduced with permission.

Fig. 17.23: from E. Uchupi, G. S. Giese, D. G.Aubrey & D. J. Kim, The Late Quaternary con-struction of Cape Cod, Massachusetts: a recon-sideration of the W. M. Davis model. Boulder,CO: Geological Society of America, SpecialPaper 309, 1996, Figs 19, 20, 21, 22, 23, and 24.Reprinted with the permission of GSA.

Fig. 17.29: from H. E. Wright & D. Frey, The Quaternary of the United States. Copyright © 1965 by Princeton University Press; renewed1993. Reprinted by permission of PrincetonUniversity Press.

Fig. 19.16: from W. G. H. Maxwell, 1968, Atlas ofthe Great Barrier Reef. Elsevier, New York. Withpermission from Elsevier Science.

Fig. 20.11: modified from F. P. Shepard, SubmarineGeology, 2nd edn. 1973, p. 140. Reprinted by permission of Pearson Education, Inc.

Fig. 21.30: modified from M. A. Lynch-Blosse & R. A. Davis, Stabililty of Dunedin andHurricane Passes, Florida. Coastal Sediments ’77.© 1977 ASCE. Reproduced by permission of the ASCE.

Every effort has been made to trace copyright hold-ers and to obtain their permission for the use ofcopyright material. The publisher apologizes forany errors or omissions and would be grateful ifnotified of any corrections that should be incorpor-ated in future reprints or editions of this book.

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1 Coastline variability andfunctions in the globalenvironment

1.1 Introduction1.2 Coastal settings1.3 Population and the coast

1.3.1 History of coastal occupation1.4 General coastal conditions1.5 Coastal environments1.6 Historical trends in coastal researchSuggested reading

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2 Chapter 1

1.1 Introduction

The surface of the Earth is covered by two contrast-ing media: land and sea. They meet at the coast.There are, of course, glaciers that span parts of both the land and sea, such as in Greenland, in partsof the Canadian Arctic, and on Antarctica. Each of the two surfaces may cover millions of squarekilometers over continents and oceans or much lessin the case of small oceanic islands or some lakeswithin continental masses. Nevertheless, a narrowcoastal zone separates these two major parts of theEarth’s surface.

The world’s coastline extends for about 440,000 km, but the coastal zone comprises lessthan 0.05% of the area of the landmasses combined.Because nearly half of the global population liveswithin less than 100 km of the coastline, the coastalzone has become arguably the most critical part of the Earth’s surface in terms of global economy,strategies, and management needs.

1.2 Coastal settings

What do we actually mean by the coast? The coastline or shoreline is simply the contact betweenthe land and the sea; an easy definition. The coastalzone, however, is a bit more difficult to delimit. Forpractical purposes it is any part of the land that isinfluenced by some marine conditions, such astides, winds, biota, or salinity. The coast is global inits distribution but limited in width. We cannotgive an average width, an average character, or anyother average category that adequately typifies thecoast. It is much too varied and complicated in itscharacteristics. In some places the coastal zonemight be only a few hundred meters wide, whereasin others it might be more than 100 km wide. Somecoastal zones include a wide range of environmentsthat separate the true ocean from the terrestrialenvironment. In other situations, a single coastalenvironment may define the land–sea boundary.

In this book we consider the controlling factorsthat determine what type of coast develops. The

processes that develop and maintain coastal envir-onments, as well as those that destroy the coast, arediscussed in order to convey the dynamic nature ofall coastal environments. Each of the major envir-onments is considered in light of these controllingfactors and processes. The impact of human activityalong the coast has been enormous, especially overthe past century. Many examples of this impactappear throughout the book but a special chapterdevoted to the topic is also included. Most of theemphasis in the book is directed toward geologicand physical attributes of the coast, although organ-isms are not overlooked.

Open coasts can be divided into two general categories: those that are dominantly erosional (Fig. 1.1) and those that are primarily depositional(Fig. 1.2) over long periods of time, i.e. thousandsof years or more. Erosional coasts are extensive andhave considerable variety, although they tend to be narrow. They typically are the high relief rockycoasts but also include some bluffs of unconsolid-ated sediments, beaches, and other local deposi-tional features. Although erosional coasts areamong the most beautiful and spectacular of allcoastal types, there is less variation in this generallyrocky type of coast than in those that are character-ized by deposition.

Depositional coasts include a wide spectrum ofsystems, such as river deltas, barrier island systems,strandplain coasts, reef coasts, and glaciated coasts.

Fig. 1.1 Photograph of an erosional coast in Oregon. The bluffshere are composed of a friable Miocene sandstone. Houses ontop are in serious jeopardy.

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Coastline variability and functions 3

Each of these may contain numerous distinct environments. The variety of morphologic featuresand the complex interaction of depositional coastsdeserves extensive attention and is emphasized inthis book.

Climatic differences cause a wide variety of coastaltypes, in that temperature and rainfall exert a majorinfluence on coastal development. Extreme climatessuch as those in the very high latitudes cause coastalareas to be covered with ice; all the time in someplaces, and for only a few months in others. Parts of Greenland and the Antarctic coast are coveredwith ice continually, whereas some of the coasts ofAlaska, Canada, the Scandinavian countries, andRussia have ice cover for at least a few months eachyear. Desert conditions can directly influence coastalenvironments as well. Few significant rivers andtherefore few river deltas are produced from desertareas. Some coastal deserts are dominated by hugesand dunes, such as along Namibia (Fig. 1.3) on thesouthwest coast of Africa. Along the Persian Gulfand in north Africa the arid, low-latitude environ-ment produces extensive coastal environmentscalled sabkhas that are nearly at sea level and havean almost horizontal surface (Fig. 1.4) dominatedby chemically precipated salts and other minerals.

The tectonics of the Earth’s crust also exert amajor influence on the coastal zone. Coasts thatcoincide with or that are near plate boundaries tendto have more relief and are narrow compared to

those that are away from plate boundaries. Collid-ing plates provide for a particularly rugged coast,such as we see along the Pacific Northwest of theUnited States. The relationships between plate tec-tonics and coastal development are treated in detailin Chapter 2.

1.3 Population and the coast

The coast is many things to many people. Depend-ing on where and how we live, work, and recreate,

Fig. 1.2 Photograph of a depositional coast on the Atlanticcoastal plain of the United States. This shows a barrier islandcoast with two large estuaries.

Fig. 1.3 Huge dunes that extend to the shoreline along thecoast of Namibia in southwest Africa. These dunes may be up to 100 m high and the absence of vegetation makes them quitemobile. (Courtesy of N. Lancaster.)

Fig. 1.4 Sabkha along the northern coast of Libya. Theseenvironments are quite flat surfaces that are essentially at meansea level and are located where tidal range is very small.(Courtesy of the US Geological Survey.)

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4 Chapter 1

our perception of it varies greatly. Large populationslive on or near the coast because it is typically verybeautiful and interesting. Many more visit the coastfor the same reasons. A large number of people gaintheir livelihood directly or indirectly from the coast,and some have the task of protecting it from intrud-ers or enemies.

1.3.1 History of coastal occupationThe ancient civilizations of the eastern Mediter-ranean Sea were largely associated with the coast,including the famous Greek, Roman, and Phoeniciansettlements and fortifications of biblical times andbefore. Many of the great cities of the time werelocated on the natural harbors afforded by the geo-logic and physiographic conditions along the coast(Fig. 1.5). These cities provided a setting that wasconducive to trade and that could be defendedagainst enemies.

Far to the north, Viking settlements in theScandanavian countries of Norway, Sweden, andDenmark were typically located along the coast as well. Here the great fjords provided shelter,fortification, and ready access to the sea, which wasa primary food source and the main avenue of trans-portation, and were the sites of many battles. Atabout the same time, the northern coast of what is now Germany and the Netherlands was also

occupied for similar reasons but in a very differentcoastal setting: one of lowlands and barrier islands.

Many centuries later, cities in the New Worldsuch as Boston, New York, Baltimore, and SanFrancisco owe their location to the presence of aprotected harbor. In their early stages of develop-ment many of the major civilizations of the worldwere directly on the coast or had important interac-tion with it.

In the early civilizations, reasons for this exten-sive occupation of coastal areas were strictly prag-matic. Coasts were essential for harboring ships, aprimary means of transporting goods, one of themajor activities of the time. The adjacent sea wasalso a primary source of food. Similar reasons werethe cause for the settlement of many of the greatcities of Europe, such as London, Amsterdam,Venice, Copenhagen, and others. All were settledon the water because their location fostered com-merce that depended on transportation over water.

This pattern of coastal occupation and utilizationcontinued until the latter part of the nineteenthcentury. By that time the interior areas of theUnited States had been settled and large cities werescattered all over the country. Many of these cities,however, are near water as well. In North Americathey are either on the Great Lakes (Chicago,Detroit, Cleveland, and Toronto) or on the banksof large rivers (St Louis, Cincinnati, Pittsburgh,and Montreal).

Since ancient times, the coast has been a strategicsetting for military activity. At first the cities housedmilitary installations. Later it became important asa staging ground for large-scale invasions; examplesinclude the British conquest of France in the four-teenth century and Allied troop landings on thebeaches of Iowa Jima and other islands of the Pacificand on the Normandy coast of France during theSecond World War in the twentieth century.

It was only in the latter part of the nineteenthcentury that coastal activities expanded into broad-based recreational use, with related support indus-tries. As a result of the Industrial Revolution andoverall prosperity, both in North America and, to alesser extent, in Europe, people began to look to thecoast as a place to take family holidays.

Fig. 1.5 Photograph of the coast at Alexandria, Egypt, anancient city in the eastern Mediterranean. This development has been here for many centuries but this coast has only beenerosional since the construction of the Aswan Dam in thetwentieth century. (Courtesy of D. Stanley.)

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Box 1.1 Venice, Italy: a city waiting to die

A unique city on the Adriatic Sea coast of northernItaly, Venice is a major historic treasure, and a verypopular tourist stop for people from throughout theworld. The city is commonly known for its canals,gondolas, and the absence of automobiles, as well as for its excellent cuisine. It is located within a large,shallow, backbarrier lagoon and is offshore from the mainland. Venice began in the eighth century as a city-state, with trading and fishing as its majorindustries. It became very famous and its merchantsbecame very wealthy as the city developed into a major economic center of the eastern Mediterranean area.

The general setting is a few kilometers offshore,where there are several low islands that have becomedeveloped as residential and commercial centers.These islands have essentially no natural relief andthey rest on thick muddy sediments deposited byrivers that used to flow into the lagoon. The mainisland, Venice, is almost entirely covered withbuildings that are mostly several hundred years old and that have great historic significance to Italy.Included are numerous churches, governmentalbuildings, plazas, old military installations, etc. Theentire city was built within a meter or two of sea level,but with the protection of the barrier islands a fewkilometers seaward of them.

As time passed, three things occurred to jeopardizethe future of the city: (i) the underlying mud began tocompact; (ii) the rate of sea-level rise increased; (iii)the combination of these phenomena has resulted in a significant relative rise in sea level, essentiallyflooding the city. This has been exacerbated by theunfortunate decision to relocate the mouths of tworivers, the Sile River and the Brenta River, whichnaturally emptied into the Venice lagoon. It was feared that the discharge of sediment would fill up the lagoon. In order to prevent this from happening,the lower course was changed so that both rivers nowempty directly into the Adriatic Sea, one north of theVenice lagoon and one to the south. This has resultedin a lack of sediment to nourish the marshes, which are vital to both the ecology of the lagoon and theprotection of the developed islands from wave attackand erosion.

At the present time the city remains vital but thepopulation, which peaked at more than 250,000 acouple of centuries ago, is now down to about 50,000.Much of this decrease is the result of the tremendouscost of renovating properties for residential orcommercial use. Virtually all buildings fall within the historic preservation regulations and thereforemust be restored in order to be occupied, and suchexpense is more than most people can bear. Anotherdeterrent to Venetian residence is the lack of jobs.

The current rate of sea level rise is 2.5 mm yr−1.With a city that is nearly at the level of spring high tide,the future is limited. The most popular location inVenice is Piazza San Marco, which is also one of thelowest elevations in the city. A hundred years ago thisarea was flooded only a few times per year, but with an increase in sea level of about 30 cm over that time,it is now flooded about 50 times each year, a couple ofdays during each spring tide situation plus times whenwinds blow for a sustained period. As sea-level riseincreases and continues, the city will literally drown.

Numerous engineering approaches have been triedand others have been suggested to help to sustain thecity. First, the barrier islands have been stabilized andnourished at great expense. This will: (i) protect thenumerous communities that inhabit them; (ii) keep thebarriers from migrating landward; and (iii) maintainthe size and geometry of the lagoon, and thereforestabilize the tidal flux in and out of the three large inletsthat serve the lagoon. The inlets have been stabilized tokeep their channels fixed and to allow the large volumeof ship traffic to the port of Venice to continue.

At the present time a plan has been developed, butnot yet funded, to construct large floating gates thatwill prevent wind from blowing large amounts of waterinto the lagoon and raising its level beyond that whichthe city of Venice can withstand. These gates rest onthe inlet channel floor during normal times, then when strong winds begin to raise the water level in the lagoon, water is evacuated from them, causing one end to float up above the surface and to act as a dam to water being blown into the lagoon.

In the long term, there seems to be little that can bedone to save this beautiful and historically significantcity other than encasing it in a large dike. It will be avery expensive and time-consuming project, but itmight be the only way.

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1.4 General coastal conditions

Varied geologic conditions provide different set-tings for the coast, and give variety and beauty tothat part of the Earth’s surface. As a consequence,some coasts are quite rugged, with bedrock cliffsand irregular shorelines, whereas others are low-lying, almost featureless areas with long, smoothshorelines. To be sure, with time, any coast canchange extensively, but some important relation-ships continue through geologically significantperiods of time, up to many millions of years.

Changes at a given part of the coast are typicallyslow and continuous, but they may be sporadic andrapid. Rocky cliffs tend to erode slowly but hurri-canes can change beaches or reefs very quickly.Overprinted on this combination of slow and rapidprocesses of change is the very slow fluctuation insea level over time, about 1–2 mm yr−1. In the geo-logic past this rate was both much faster and even abit slower. The point is that as coastal processeswork to shape the substrate and the adjacent land,the position of the shoreline changes as well. Thistranslates the processes and their effects across theshallow continental shelf and the adjacent coastalzone, producing long and slow, but relatively steady,coastal change.

Each specific coastal setting, regardless of scale, is unique yet is quite similar to other coastal settingsof the same type. Although each delta is different, a common set of features characterizes all deltas.The approach of this book is to consider the generalattributes of each of the various types of coastalenvironments. Numerous examples of each environ-mental type provide some idea of the range avail-able. Finally, the overprint of time demonstrates thedynamic nature of all of these coastal elements.

1.5 Coastal environments

The variety of coastal environments is wide. Thissection briefly introduces each of the major environ-ments to demonstrate this variety. All of these andmore are discussed in detail in the following chapters.

Rivers carry tremendous quantities of sedimentto their mouths, where they deposit it. Much of thesediment is then entrained by waves and currents,but commonly there is a net accumulation of sedi-ment at the river mouth: a delta (Fig. 1.6). In fact,most of the sediment along all types of depositionalcoasts owes its presence, at least indirectly, to ariver. Deltas range widely in size and shape. Mostare dominated by mud and sand but some haveabundant gravel. The primary conditions for deltaformation are a supply of sediment, a place for it toaccumulate, and the inability of the open water pro-cesses to rework and remove all of the sedimentfrom the river mouth.

Sea level has risen considerably over the past sev-eral thousand years as the result of glaciers meltingand a combination of other factors. This increase insea level flooded many parts of the land and devel-oped extensive and numerous coastal bays. Streamsfeed most of these bays. These bays are called estu-aries (Fig. 1.7) and are commonly surrounded bysome combination of wetlands (usually either saltmarshes or mangrove swamps) and tidal flats.

Fig. 1.6 Satellite photo of the Mississippi delta showingconsiderable suspended sediment being discharged throughnumerous distributaries. Although it appears that the sedimentload is very large, it has been reduced greatly during the pastcentury as the result of dams on the river. (Image from EROSData Center.)

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Another common type of coastal bay is one thattends to parallel the coast and is protected from theopen ocean by a barrier island. These elongate waterbodies have little influx of fresh water or tidalexchange. They are lagoons. Tidal flats and marshesare uncommon along this type of bay. Other coastalembayments that cannot be considered as either anestuary or a lagoon are simply termed coastal bays.

Barrier islands are another important part of the scheme of coastal complexes. These islands are a protection in front of the mainland, typicallyfronting lagoons and/or estuaries. These barriersinclude beaches, adjacent dunes, and other environ-ments. Wetlands, especially salt marshes, arewidespread on the landward side of barrier islands.Tidal inlets dissect barrier islands and are amongthe most dynamic of all coastal environments. Theynot only separate adjacent barrier islands, but alsoprovide for the exchange of water and nutrientsbetween the open ocean and estuarine systems.

Strandplain coasts are low-relief coastal areas of amainland that have many characteristics of the sea-ward side of a barrier island. They contain beachesand dunes but lack the coastal bay (Fig. 1.8).Examples include Myrtle Beach, South Carolina,and the Nayarit Coast of western Mexico.

Rocky or headland coasts can be present as shortisolated sections within extensive sandy depositional

coasts, such as along parts of the east coast ofAustralia or the Pacific Northwest coast. Othergeomorphically similar coasts may have their originin glacial deposits, with New England being a goodexample. In both cases, the coast is characterized aserosional and they may provide sediments fornearby depositional beaches.

Reef coasts (Fig. 1.9) owe their origin to the con-struction of a framework by organisms; both plantand animal. Not only are these coastal environ-ments very beautiful, but they also protect the adja-cent mainland or island from erosion and waveattack from severe storms.

1.6 Historical trends in coastal research

Scientists have only recently undertaken compre-hensive investigations of the coast. Some nineteenth-century publications considered the origin of barrierislands, scientists speculated on the development

Fig. 1.7 Headland on the Oregon coast with an associatedbarrier spit protecting the small estuary. Even though this coasthas a high tidal range, it is eroded at the headlands, with thesediment carried along the coast to form these barrier spits.(Photograph courtesy of W. T. Fox.)

Fig. 1.8 Aerial photograph showing a strandplain coast wherethere is no significant estuary and no barrier island. Such a coasttypically develops where there are no coastal bays and thegradient offshore is relatively steep, thus letting large wavesreach the surf zone.

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of coral reef coasts, including Charles Darwin onhis famous voyage on the Beagle, and observationsdetailed characteristics of the cliffed coasts of theBritish Isles and Brittany in France.

The first systematic efforts at studying the coastin the early twentieth century were made by geo-morphologists, those who study the morphology or landforms of the Earth. Geomorphologists alsoinvestigate mountains, deserts, rivers, and otherEarth features. Their studies produced variousclassifications, maps, and reports on coastal land-forms. Some scientists focused on the evolution of coasts and the processes responsible for moldingthem. For example, Douglas W. Johnson, a professorat Columbia University, wrote a classic and pioneer-ing book in 1919 entitled Shore Processes and ShorelineDevelopment, a monograph that is still commonlyreferenced.

Engineers have also given special attention to the coast over several centuries. Their interest wasdirected toward construction of dikes, harbors,docks, and bridges on the one hand, and stabiliza-tion of the open coast on the other. Although thetwo groups of professionals directed their effortstoward different aspects of the coast, their interestsoverlapped in many circumstances.

Ancient people recognized that the coast is pot-entially dangerous during storms and is continually

changing due to processes associated with wind wavesand storms. They understood that the shoreline isone of the most dynamic areas on the Earth. Erosionwas a particularly important problem (Fig. 1.10) andsettlements were lost or threatened as the shorelineretreated. For centuries dikes have been constructedalong the North Sea coast of the Netherlands andGermany, both for protection (Fig. 1.11) and forland reclamation. In many other areas, however,

Fig. 1.10 Photograph of a seawall designed to protect the coastfrom erosion. These features are designed to stop erosion alongthe shoreline and to protect buildings or infrastructure that isclose to the shoreline

Fig. 1.11 Photograph of a dike along the Netherlands coast on the North Sea. Such structures protect the adjacent land,which is below sea level, from flooding. Like many of the dikes,this one supports a roadway. Some are used to graze sheep and cattle.

Fig. 1.9 Oblique aerial photograph showing a well developedreef along the Florida Keys coast. The portion of the reef thathas the linear features is the seaward and deeper area, while theremainder of the reef is the flat, shallow, upper portion.

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construction on the open coast was designed to slowor prevent erosion. As a result, various types ofstructures were emplaced at critical locations alongdensely inhabited areas of the coast in attempts tostabilize the beach and prevent erosion.

For decades these activities represented the majorefforts of science and technology to understand and,in some respects, to control the response of the coastto natural processes. The Second World War wasalso an important period in furthering our under-standing of the coast. Major war efforts took placealong the coast, particularly the landing of troops,supplies, and equipment, whether on the Europeanmainland or on Pacific islands. All branches of themilitary were involved in studying coastal geomor-phology, coastal processes including waves, tides,and currents, and the analysis of weather patternsalong the coast. Much of the world’s coast wasmapped in detail during this period. The BeachErosion Board, a research branch of the US Armyaffiliated with the Corps of Engineers, made manyvery important contributions to our knowledge ofcoasts. This group conducted extensive research onbeaches, waves, erosion, and other important aspectsof the coast, using both their own staff and academicresearchers from many of the best universities.Francis P. Shepard and Douglas L. Inman of theScripps Institution of Oceanography (Fig. 1.12)were prominent contributors to the research pro-grams of this group and later became among themost prominent coastal researchers in the world.

This coastal research effort continued after theSecond World War, but with a distinctly engineer-ing emphasis. The name of the original researchorganization was changed to Coastal EngineeringResearch Center, originally housed at Fort Belvoir,Virginia, and now located in Vicksburg, Mississippi.At about this time the Office of Naval Research(ONR) became heavily involved in basic researchon the coast. Its first major efforts in this endeavorwere through the Coastal Studies Institute of

Louisiana State University. Although this organiza-tion conducted a variety of coastal research projects,its major effort was a global study of river deltas,beginning near home with the Mississippi delta. Astime passed, the ONR expanded its coastal researchsupport to emphasize beaches, inlets, and deltas –places where military activity could potentially takeplace. During the 1960s and 1970s this agency,through the leadership of Dr Evelyn Pruitt, sup-ported most of the research on modern open coastalenvironments. This period began the modern era of coastal research, which emphasizes process-response systems. In other words, it is no longerenough to observe, describe, and classify coastal features and environments. The focus is nowdirected at determining the origin and develop-ment of these features, which necessitates the studyof the physical and biological processes that operateon the coast and then the integration of these datawith the resulting landforms. Thus began the process-response approach to coastal research. Coastalresearch of this type is only a few decades old.

Fig. 1.12 The old Scripps Pier at Scripps Institution ofOceanography at La Jolla, California. Considerable data onnearshore processes have been collected from this structure over several decades. It has now been replaced by a new version.(Photograph courtesy of Scripps Institution of Oceanography.)

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Suggested reading

Box 1.2 Beach Erosion Board of the USArmy, Corps of Engineers

The Corps of Engineers is the part of the US Army that typically deals with construction: generallystructures like dams, jetties at inlets, and seawalls to protect from erosion. It is also responsible formaintaining waterways, commonly through dredgingthe channels to permit shipping traffic, and for harborsfor berthing the ships. During the Second World Warthere were some special needs associated with thedeployment of ships and personnel in coastal waters.Landing troops on the beaches of Europe and in thePacific were important military activities, and had to be done with careful planning and understanding of the coast and its various conditions.

As a consequence, the military, through the USArmy, Corps of Engineers, enlisted a peacetimeorganization, the Beach Erosion Board, to assist in the task of learning as much as possible about coastal dynamics and geomorphology in order to aidthe military effort in both the Eurpoean and Pacifictheaters of war. The Beach Erosion Board, which had been a group of experts that advised the Corps on matters relating to ports, harbors, beach erosion,and navigation, now became an important aspect ofthe war effort. In order to carry out these activitiesseveral projects to be conducted at universities weredeveloped and funded by the Army.

Much of the work was done in California at ScrippsInstitution of Oceanography and at the University ofCalifornia at Berkeley. The Scripps researchers had

long been noted for their marine research in physicaloceanography and marine geology. The pier there was one of the first research piers equipped withinstruments to record wave data, meteorology, andother coastal environmental phenomena. Suchprominent scientists as Francis Shepard and DouglasInman were in the group at Scripps that conducted thisresearch. The coastal engineering group at Berkeleywas also a leader in the field and was composed ofexperts at designing temporary facilities for militaryoperations, including harbors, breakwaters, andbridges. Among them were Murrough P. O’Brien and Robert Weigel, two of the pioneers in coastalengineering.

Several prominent scientists and engineers wereemployed full-time as researchers for the military and some joined the military as scientific advisors.Although it is impossible to tell precisely, the efforts of both the academics and the military in aiding thesewar efforts saved many lives and countless dollars.Most of their work was unnoticed by the general publicbut is well known to the coastal research community.

Eventually, the Beach Erosion Board became theCoastal Engineering Research Center, the researchbranch of the US Army, Corps of Engineers. It hasaided and continues to aid both civilian and militarycoastal and navigational operations. Originally locatedin Arlington, Virginia, near Washington DC, it is nowcombined with the Waterways Experiment Station ofthe Corps in Vicksburg, Mississippi. It is the premiercoastal engineering research unit of the federalgovernment of the United States.

Barnes, R. S. K. (ed.) (1977) The Coastline. London: John Wiley & Sons.Bird, E. C. F. & Schwartz, M. L. (eds) (1985) The World’s Coastline. New York: Van

Nostrand Reinhold.Carter, R. W. F. (1988) Coastal Environments. New York: Academic Press.Davies, J. L. (1980) Geographic Variation in Coastal Development. New York: Longman.Fox, W. T. (1983) At the Sea’s Edge. Englewood Cliffs, NJ: Prentice Hall.Johnson, D. W. (1919) Shorelines and Shoreline Development. New York: John Wiley &

Sons.Shepard, F. P. & Wanless, H. R. (1971) Our Changing Coastlines. New York: McGraw-

Hill.

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2The Earth’s mobile crust

2.1 Introduction2.2 The Earth’s interior2.3 Plate boundaries

2.3.1 Divergent boundaries2.3.2 Convergent boundaries2.3.3 Transform boundaries2.3.4 Plate movement

2.4 Continental margins2.4.1 Tectonic evolution of continental margins

2.5 Tectonic coastline classification2.5.1 Collision coasts2.5.2 Trailing edge coasts2.5.3 Marginal sea coasts

2.6 Tectonic effects on coastal sediment supply2.6.1 Continental drainage2.6.2 Location of rivers2.6.3 Continental shelf width

2.7 SummarySuggested reading

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2.1 Introduction

Coastlines of the world exhibit a wide range of mor-phologies and compositions in a variety of physicalsettings (Fig. 2.1). There are sandy barrier islandcoasts such as those of the East and Gulf coasts ofthe United States, deltaic coasts built by majorrivers including those at the mouths of the Nile andNiger rivers, glacial alluvial fan coasts of the CopperRiver in Alaska and the Skiedarsar Sandar coast ofsoutheast Iceland, coastlines fronted by expansivetidal flats such as the southeast corner of the NorthSea in Germany, volcanic coasts of the HawaiianIslands, carbonate coasts of the Bahamas and the

South Pacific atolls, mangrove coasts of Malaysiaand southwestern Florida, bedrock cliff and wave-cut platform coasts of the Alaskan peninsula andsouthwest Victoria in Australia, and many othertypes of coastlines. The diversity of the world’scoastlines is largely a product of the Earth’s mobilecrust. The eruptions of Mount Saint Helens inWashington state (1980) and Mount Pinatubo inthe Philippines (1991), and the devastating earth-quakes in Mexico City (1985) and in Kobe, Japan(1995), are dramatic expressions of this mobility.The formation of pillow basalts and new oceaniccrust at mid-ocean ridges and the presence of hydro-thermal vents at these sites are also a manifestationof crustal movement. The theory that explains themobility of the Earth’s crust and the large-scale features of continents and ocean basins, includingthe overall geological character of coastlines, is knownas plate tectonics.

Plate tectonics theory has done for geology whatthe theory of evolution did for biology, the big bangtheory for astronomy, the theory of relativity forphysics, and the establishment of the periodic tablefor chemistry. Each of these advancements revolu-tionized its respective field, explaining seeminglyunrelated features and processes. For example, ingeology the cause and distribution of earthquakes,the construction of mountain systems, the existenceof deep ocean trenches, and the formation of oceanbasins are all consequences of the unifying theory of plate tectonics. The germination of this theorybegan many centuries ago with scientists’ and worldexplorers’ interest in the distribution of continentsand ocean basins. As early as the 1620s, Sir FrancisBacon recognized the jigsaw puzzle fit of the easternoutline of South America and the western outline ofAfrica. By 1858, Antonio Snider had published twomaps illustrating how North and South Americawere joined with Africa and Europe during Car-boniferous time (~300 million years ago) and howthe continents had split apart to form the AtlanticOcean. He reconstructed the positions of continents300 million years ago to show why plant remainspreserved in coal deposits of Europe are identical tothose found in coal seams of eastern North America.Snider’s maps were an important step in promoting

Fig. 2.1 Dissimilar tectonic settings produce very differenttypes of coastlines. (a) The coastal plain setting of SouthCarolina fronted by barriers and tidal inlets (Murrells Inlet,1977) is in sharp contrast to (b) the mountainous fjord coast of Kenai, Alaska.

(a)

(b)

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The Earth’s mobile crust 13

a theory that later became known as continentaldrift (the theory that envisions continents movingslowly across the surface of the Earth).

In the early twentieth century, the idea of con-tinental drift was popularized by Alfred Wegener, a German scientist at the University of Marburg(Fig. 2.2). Wegener was a meteorologist, astronomer,geologist, and polar explorer, and led several expedi-tions to Greenland. He was the first to present asophisticated and well researched theory of con-tinental drift, which he did in a series of lectures toEuropean scientific societies in 1912. Three yearslater he published his ideas in a book entitled DieEntstehung der Kontinente und Ozeane (The Origin ofContinents and Oceans). Wegener believed that allthe continents were once joined together in a supercontinent he called Pangaea (Greek for “all Earth”)which was surrounded by a single world ocean henamed Panthalassa (all ocean) (Fig. 2.3). The north-ern portion of this supercontinent, encompassingNorth America and Eurasia, was called Laurasia,and the southern portion, consisting of all the othercontinents, was Gondwanaland. Partially separatingthese two landmasses was the Tethys Sea.

Wegener used a variety of supporting evidence tobolster his theory of a single continent, includingthe continuity of ancient mountain fold belts andother geological structures that extend across con-tinents now separated by wide oceans. He notedthat the coal deposits in frigid Antarctica and glacial

Fig. 2.2 This picture of Alfred Wegener was taken when he was 30 years old during one of his expeditions to Greenland. It was during this period of his life that he proposed his theory of “continental drift.”

EURASIA

NORTHAMERICA

SOUTHAMERICA AFRICA

INDIAAUSTRALIA

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160° 140° 120° 100° 80° 0° 20° 40° 60° 80° 100° 120° 140° 160°

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Fig. 2.3 One of the compelling pieces of evidence that Wegener used to argue for his continental drift theory was the geometric fit of South America and Africa. In fact, he believed that about 200 million years ago all the continents were joined together in asupercontinent he called Pangaea.

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sediments in what are now the tropical regions ofSouth Africa, India, and Australia could only belogically explained by moving the continents to different latitudinal settings. He also demonstratedthat identical fossils and rocks of similar age couldbe found on widely separated continents but actu-ally plot side by side when Pangaea is reconstructed.Using additional fossil evidence, he theorized thatPangaea separated into a number of pieces approx-imately 200 million years ago, forming the AtlanticOcean among other features. He reasoned that ifcontinents could move, then the presence of glacialdeposits in India meant that India had once beenclose to Antarctica and that following the breakupof Gondwanaland, its northward movement andultimate collision with Asia caused the formation ofthe Himalayan Mountains (Fig. 2.4).

Although Wegener’s book and ideas were initi-ally very popular, the geological community of thattime ultimately scorned his new theory. His failureto convince the scientific community stemmed fromhis weak argument that the continents plowed theirway through or slid over the oceanic crust. He

believed that this movement was caused by thegravitational pull of the sun and moon (tidal force)acting with a differential force on the surface crustrelative to the underlying mantle. Contemporaryscientists showed that the tidal forces were muchtoo weak to account for the drifting of continentsand thus his theory was abandoned. Wegener did notlive long enough to witness the revival and accept-ance of many aspects of his theory, as he perished in 1930 at age 50 during a fourth expedition toGreenland while on a rescue mission. Indeed, it wasnot until the late 1950s and early 1960s that scient-ists solved the puzzle of the moving continents andWegener’s theory was revived.

In this chapter we demonstrate that the overallphysical character of the edge of continents for severalthousands of kilometers of coast, such as the westcoasts of North and South America, is a function of plate tectonic processes. This includes whether acoast is seismically and volcanically active, whetherit is bordered by mountain ranges, a broad coastalplain, or something in between, and whether it isfronted by a narrow or wide continental shelf. Plate

Ocean ridge

Lithosphere

Asthenosphere

Ocean ridge

Lithosphere

Asthenosphere

INDIAASIA

INDIA

N

N

Himalayas

Fig. 2.4 According to Wegener theIndian subcontinent traveled northwardfollowing the breakup of Gondwanalandand eventually collided with Asia,forming the Himalayan Mountains.

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tectonics are also shown to have an importantinfluence on the supply of sediment to a coastlineand the extent of depositional landforms such asdeltas, barrier chains, marshes, and tidal flats.

2.2 The Earth’s interior

In order to grasp why continents have “drifted” tonew positions, why ocean basins have opened andclosed, how mountain systems and ocean trencheshave formed, as well as the dimensions of these sys-tems, it is necessary to understand the composition,layering, and processes that occur within the Earth’sinterior. In its early beginning, the Earth was essenti-ally a homogeneous mass consisting of an aggregateof material that was captured through gravitationalcollapse and meteoric bombardment. The heat gen-erated by these processes, together with the decay of radioactive elements, produced at least a partialmelting of the Earth’s interior. This melting allowedthe heavier elements, especially the metals of nickeland iron, to migrate toward the Earth’s center, whileat the same time the lighter rocky constituents rosetoward the surface. These processes, which havedecreased considerably as the Earth has cooled, ulti-mately led to a layered Earth containing a core,mantle, and crust. The layers differ from one anotherboth chemically and physically (Fig. 2.5). Most ofour knowledge of the Earth’s interior comes fromthe study of seismic waves that pass through theEarth and are modified by its different layers.

The center of the Earth is divided into two zones:a solid inner core (radius 1250 km) with a densityalmost six times that of the crust and with a temper-ature comparable to the surface of the sun; and a vis-cous molten outer core (width 2220 km) some fourtimes as dense as the crust. The mantle is 2880 kmthick and contains over 80% of the Earth’s volumeand over 60% of its mass. It is composed of iron andmagnesium-rich silicate (silica–oxygen structure)minerals, which we have observed at locations wherematerial from the mantle has been intruded into the overlying crust and subsequently exposed at theEarth’s surface. The chemical composition is con-sistent throughout the mantle, but due to the increas-

ing temperature and pressure the physical propertiesof the rocks change with increasing depth.

Compared to the inner layers, the outer skin ofthe Earth is cool, very thin, much less dense, andrigid. The crust is rich in the lighter elements thatrose to the surface during the Earth’s melting anddifferentiation stage, forming minerals such asquartz and feldspar. There are two types of crust:the relatively old (up to 4.0 billion years) and thickgranitic crust of the continents (20–40 km thick)and the thin and geologically young (less than 200 million years) basaltic crust beneath the oceanbasins (5–10 km thick). Continental crust can be upto 70 km thick at major mountain systems. The lessdense and greater thickness of the continental crust(2.7 g cm−3) as compared to the thinner more dense(2.9 g cm−3) ocean crust has important implicationsfor how these two crusts have formed and the stabil-ity of the crusts.

The divisions described above are based mostlyon the different chemical character of the layers.However, in terms of plate tectonic processes, espe-cially in understanding the movement of plates, thecrust and mantle are reconfigured into three layerson the basis of physical changes in the nature of therocks:1 Lithosphere. This is the outer shell of the Earthand is composed of oceanic and continental crustand the underlying cooler, uppermost portion of themantle (Fig. 2.5). The lithosphere is approximately100 km thick and behaves as a solid, rigid slab. Incomparison to the diameter of the Earth, the litho-sphere is very thin and would represent the shell ofan egg. The outer shell is broken up into eight majorplates and numerous smaller plates. The Pacific Plateis the largest plate, encompassing a large portion ofthe Pacific Ocean, whereas the Juan de Fuca Plateoff the coasts of Washington and Oregon is one ofthe smallest. The tectonic or lithospheric plates, asthey are called, are dynamic and are continuouslymoving, although very slowly, with an average ratevarying from a few to several centimeters per year.Whereas once this movement was calculated throughindirect means, such as age determinations of oceaniccrust, rates can now be measured directly fromsatellites orbiting the Earth.

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16 Chapter 2

2 Asthenosphere. The lithospheric plates float ontop of a semi-plastic region of the mantle called theasthenosphere, which extends to a depth of about350 km. In this part of the mantle the high temper-ature and pressure causes the rock to partially melt,resulting in about 1–2% liquid. The partially meltedrock allows the asthenosphere to be deformed plastically when stress is applied. Geologists havecompared this plasticity to cold taffy, hot tar, andred hot steel. One way of illustrating this concept to yourself is by putting a chunk-sized piece of ice between your back teeth and applying slow

constant pressure. You will see that the ice willdeform without breaking in a manner similar tohow the asthenosphere behaves when stressed. Interms of plate tectonics, the semi-plastic nature ofthis layer allows the lithospheric plates to move.3 Mesosphere. Below the asthenosphere is themesosphere, which extends to the mantle–coreboundary. Although this layer has higher temper-atures than the asthenosphere, the greater pressureproduces a rock with a different, more compactmineralogy. This portion of the mantle is mechan-ically strong.

Lowermantle

Outercore

Innercore

Crust 5–70 km

Upper mantle

Oceaniccrust

Continentalcrust

Lithosphere(rigid solid)

Asthenosphere(capable of flow)

100

200

300

660

Uppermantle

Dep

th (

km)

12162270 km

2900 km

km

Fig. 2.5 The layered Earth reflects gross chemical and physical changes of the rocks with depth. These changes (which are reallygradual changes) are responsible for the major boundaries in the Earth, including the inner and outer core, lower and upper mantle,and thin crust.

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