THE OCEAN BASINS' THEIR STRUCTURE AND EVOLUTION THE OCEANOGRAPHYCOURSE TEAM Authors Evelyn Brown (Waves, Tides, etc.; Ocean Chemistry) Angela Coiling (Ocean Circulation; Seawater (2nd edn); Case Studies) Dave Park (Waves, Tides, etc.) John Phillips (Case Studies) Dave Rothery (Ocean Basins) John Wright (Ocean Basins; Seawater; Ocean Chemistr3,; Case Studies) Designer Jane Sheppard Graphic Artist Sue Dobson Cartographer Ray Munns Editor Gerry Bearman This Volume forms part of an Open University course. For general availability of all the Volumes in the Oceanography Series, please contact your regular supplier, or in case of difficulty the appropriate Butterworth- Heinemann office. Further information on Open University courses may be obtained from: The Admissions Office, The Open University, P.O. Box 48, Walton Hall, Milton Keynes MK7 6AA, UK, or from the Open University website: http://w w w. ope n. ac. uk Cover illustration: Satellite image showing distribution of phytoplankton pigments in the North Atlantic off the US coast in the region of the Gulf Stream and the Labrador Current. (NASA, and O. Brown and R. Evans, Universit3, of Miami.) THE OCEAN BASINS' THEIR STRUCTURE AND EVOLUTION PREPARED BY JOHN WRIGHT AND DAVID A. ROTHERY FOR THE COURSE TEAM SECOND EDITION REVISED FOR THE COURSE TEAM BY DAVID A. ROTHERY UTTERWORTH EINEMANN in association with THE OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES, MK7 6AA, ENGLAND Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP A division of Reed Educational and Professional Publishing Ltd -~A member of the Reed Elsevier plc group OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI Copyright 9 1989, 1998 The Open University First edition 1989 Second edition 1998. Reprinted (with corrections) 2001, 2003, 2004 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 without the prior written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence permitting restricted copying issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W IP 0LP. This book may not be lent, resold, hired out or otherwise disposed of by way of trade in any form of binding or cover other than that in which it is published, without the prior consent of the Publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. ISBN 0 7506 3983 0 Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress. Jointly published by the Open University, Walton Hall, Milton Keynes MK7 6AA and Butterworth-Heinemann. Edited, typeset, illustrated and designed by The Open University Printed in Singapore by Kyodo under the supervision of MRM Graphics Ltd., UK s330vli2.4 ABOUT THIS VOLUME 3 ABOUT THIS SERIES 3 CHAPTER 1 INTRODUCTION 1.1 MAPPING THE OCEANS 5 1.1.1 Navigation 9 1.1.2 Depth measurement 11 1.2 MAPPING THE OCEAN FLOORS 13 1.2.1 Bathymetry from satellites 18 1.3 UNDERWATER GEOLOGY 22 1.4 SUMMARY OF CHAPTER 1 25 CHAPTER 2 THE SHAPE OF OCEAN BASINS 2.1 THE MAIN FEATURESOF OCEAN BASINS 29 2.2 CONTINENTAL MARGINS 30 2.2.1 Aseismic continental margins 31 2.2.2 Seismic continental margins and island arcs 33 2.3 OCEAN RIDGES 35 2.3.1 Ridge topography 36 2.3.2 Age-depth relationships across ridges 37 2.4 TRANSFORM FAULTSAND FRACTUREZONES 38 2.5 THE DEEP OCEAN FLOOR 41 2.5.1 Abyssal plains 41 2.5.2 Seamounts 42 2.5.3 The distribution of submarine volcanoes 43 2.5.4 Aseismic ridges 45 2.6 SATELLITE BATHYMETRY- A CASE STUDY 45 2.7 SUMMARY OF CHAPTER2 52 CHAPTER 3 THE EVOLUTION OF OCEAN BASINS 3.1 THE EVOLUTION OF OCEAN BASINS 55 3.2 THE BIRTH OF AN OCEAN 58 3.2.1 The Red Sea 59 3.3 THE MAJOR OCEAN BASINS 62 3.3.1 The Mediterranean 66 3.4 SUMMARY OF CHAPTER 3 67 CHAPTER 4 THE STRUCTURE AND FORMATION OF OCEANIC LITHOSPHERE 4.1 THE FORMATION OF OCEANIC LITHOSPHERE 71 4.1.1 Pillow lavas: the top of the oceanic crust 74 4.1.2 Why a median valley? 76 4.1.3 Formation of the volcanic layer: two case studies 77 4.2 SEGMENTATION OF OCEANIC SPREADINGAXES 83 4.2.1 Second- and third-order segmentation of fast-spreading axes 84 4.2.2 Second- and third-order segmentation of slow-spreading axes 87 4.2.3 A plausible model for lithospheric growth 88 4.2.4 Changes in spreading pattern 89 4.2.5 Crustal abnormalities 89 4.3 SEAMOUNTS AND VOLCANIC ISLANDS 92 4.4 SUMMARY OF CHAPTER4 94 CHAPTER 5 HYDROTHERMAL CIRCULATION IN OCEANIC CRUST 5.1 THE NATURE OF HYDROTHERMALCIRCULATION 98 5.1.1 Heat flow, convection and permeability 99 5.2 CHEMICAL CHANGES DURING HYDROTHERMAL CIRCULATION 100 5.2.1 Changes in the rocks 101 5.2.2 Changes in seawater 103 5.2.3 Variability in hydrothermal systems 105 5.3 BLACK SMOKERS- AN EXERCISE IN PREDICTION 106 5.3.1 Black smokers, white smokers and warm-water vents 107 5.3.2 The lifetimes of hydrothermal systems 110 5.3.3 Anatomy of a vent field 111 5.4 THE EXTENT OF HYDROTHERMALACTIVITY 112 5.4.1 The biological significance of hydrothermal vent systems 115 5.4.2 Hydrothermal plumes 115 5.4.3 Event plumes 117 5.4.4 Off-axis hydrothermal circulation 118 5.4.5 The extent of hydrothermal metamorphism 120 5.5 MASS TRANSFER BY HYDROTHERMAL CIRCULATION 120 5.6 SUMMARY OF CHAPTER 5 122 CHAPTER 6 PALAEOCEANOGRAPHYAND SEA-LEVEL CHANGES 6.1 THE DISTRIBUTION OF SEDIMENTS 124 6.1.1 Sediments and palaeoceanography 127 6.2 CHANGES IN SEA-LEVEL 129 6.2.1 Different time-scales in sea-level changes 130 6.2.2 Using satellites to monitor sea-level changes 131 6.2.3 The post-glacial rise in sea-level 135 6.2.4 Measuring Quaternary changes in sea-level 136 6.2.5 The growth of an ice-sheet: Antarctica 140 6.2.6 The salinity crisis in the Mediterranean 142 6.2.7 The migration of climatic belts 146 6.2.8 The effect of plate-tectonic processes on sea-level 147 6.2.9 Major transgressions and regressions 148 6.3 SUMMARY OF CHAPTER 6 149 CHAPTER 7 THE BROADER PICTURE 7.1 THE GLOBAL CYCLE 151 7.1.1 Changes in components of the cycle 154 7.1.2 Some effects of short-term changes 155 7.1.3 The steady-state ocean 156 7.2 SOME RATES COMPARED 157 7.3 SUMMARY OF CHAPTER 7 158 APPENDIX THE STRATIGRAPHIC COLUMN 160 SUGGESTED FURTHER READING 161 ANSWERS AND COMMENTS TO QUESTIONS 162 ACKNOWLEDGEMENTS 179 INDEX 181 This is one of a Series of Volumes on Oceanography. It is designed so that it can be read on its own, like any other textbook, or studied as part of $330 Oceanography, a third level course for Open University students. The science of oceanography as a whole is multidisciplinary. However, different aspects fall naturally within the scope of one or other of the major 'traditional' disciplines. Thus, you will get the most out of this Volume if you have some previous experience of studying geology, geochemistry or geophysics. Other Volumes in this Series lie more within the fields of physics, chemistry and biology (and their associated sub-branches). Chapters l to 4 describe the processes that shape the ocean basins, determine the structure and composition of oceanic crust and control the major features of continental margins. Today's ocean basins are geologically ephemeral features, and these Chapters show why. Chapter 5 deals with the 'hot springs' of the deep oceans that result from the circulation of heated seawater through oceanic crust. This phenomenon was not even suspected until the mid-1960s and was not confirmed by observation until some years later. Since then, many people have seen the striking photographs of 'black smokers' at ocean ridges. Chapter 6 summarizes the main patterns of sediment distribution in the ocean basins and shows how sediments can preserve a record of past climatic and sea- level changes. Finally, Chapter 7 considers the role of the oceans as an integral part of global chemical cycles. You will find questions designed to help you to develop arguments and/or test your own understanding as you read, with answers provided at the back of the Volume. Important technical terms are printed in bold type where they are first introduced or defined. The Volumes in this Series are all presented in the same style and format, and together provide a comprehensive introduction to marine science. Ocean Basins deals with the structure and formation of oceanic crust, hydrothermal circulation, and factors affecting sea-level. Seawater considers the seawater solution and leads naturally into Ocean Circulation, which is the 'core' of the Series. It provides a largely non-mathematical treatment of ocean-atmosphere interaction and the dynamics of wind- driven surface current systems, and of density-driven circulation in the deep oceans. Waves, Tides and Shallow-Water Processes introduces the physical processes which control water movement and sediment transport in the nearshore environment (beaches, estuaries, deltas, shelves). Ocean Chemistry and Deep-Sea Sediments is concerned with biogeochemical cycling of elements within the seawater solution and with water-sediment interaction on the ocean floor. Case Studies in Oceanography and Marine Affairs examines the effect of human intervention in the marine environment and introduces the essentials of Law of the Sea.