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978-1-107-01752-8 - Mid-Ocean Ridges
Roger Searle
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Mid-Ocean Ridges
The world’s mid-ocean ridges form a single, connected global ridge system that is part of
every ocean, and is the longest mountain range in the world. Geologically active, mid-ocean
ridges are key sites of tectonic movement, intimately involved in sea floor spreading. This
coursebook presents a multi-disciplinary approach to the science of mid-ocean ridges –
essential for a complete understanding of global tectonics and geodynamics. Designed for
graduate and advanced undergraduate students, it will also provide a valuable reference
for professionals in relevant fields. Background chapters provide a historical introduction
and an overview of research techniques, and following chapters cover the structure of the
lithosphere and crust, and volcanic, tectonic and hydrothermal processes. A summary and
synthesis chapter recaps essential points to consolidate new learning. Accessible to students
and professionals working in marine geology, plate tectonics, geophysics, geodynamics,
volcanism and oceanography, this is the ideal introduction to a key global phenomenon.
r Supports students and professionals new to technical aspects or geographic areas with a
full glossary and extensive directory of feature names.
r Avoids jargon and fully introduces and defines technical concepts and terms.
r Richly illustrated, including colour figures and comprehensive data tables.
r Extensive references provide detailed starting points for further study, and a valuable
resource for professional researchers from many different fields.
Roger Searle is Emeritus Professor of Geophysics at Durham University. He has spent
40 years studying mid-ocean ridges, and was a pioneer in the use of side-scan sonar to
study their geodynamic, tectonic and volcanic processes. In his research he also uses topographic analysis and gravity and magnetic modelling to understand ridge structures. He was
awarded the Royal Astronomical Society’s Price Medal in 2011 and elected a Fellow of the
American Geophysical Union in 2012. Professor Searle has worked in many of the world’s
major oceanographic institutions, participated in 37 research cruises and led 18. He was
first full chairman of the international research organisation InterRidge, and has served on
national and international committees, including chairing the International Ocean Drilling
Program’s Site Survey Panel.
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Roger Searle
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‘This volume provides a comprehensive, up-to-date and authoritative account, extensively
illustrated and referenced, of the geology, the morphology, the tectonics and the chemistry
of the ridges, relating these to the underlying mantle movements. It also describes in detail
the techniques used in these studies. Professor Searle has been at the forefront of research
on the mid-ocean ridges throughout his career, and has produced an ideal textbook both for
students and those currently researching the geology of the ocean floor.’
– Sir Anthony Laughton, FRS, formerly Director of the Institute of Oceanographic
Sciences, UK
‘Professor Searle has done a superb job of summarizing and analyzing the history of, and
the latest insights into, mid-ocean ridges, ranging from ultra-slow to fast spreading rates
and including the tectonics, geophysics, geochemistry, volcanism and hydrothermal activity
of this “longest mountain range in the world.” This is an essential volume for any student
or researcher studying mid-ocean ridges, both those in the Earth sciences and those with
backgrounds in marine biology, chemistry oceanography, physical oceanography and other
related fields.’
– Ken C. Macdonald, Emeritus Professor of Marine Geophysics, University of
California at Santa Barbara
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978-1-107-01752-8 - Mid-Ocean Ridges
Roger Searle
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Mid-Ocean Ridges
ROGER SEARLE
Emeritus Professor, Department of Earth Sciences,
Durham University
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Roger Searle
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University Printing House, Cambridge CB2 8BS, United Kingdom
Published in the United States of America by Cambridge University Press, New York
Cambridge University Press is part of the University of Cambridge.
It furthers the University’s mission by disseminating knowledge in the pursuit of
education, learning and research at the highest international levels of excellence.
www.cambridge.org
Information on this title: www.cambridge.org/9781107017528
C Roger Searle 2013
This publication is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without the written
permission of Cambridge University Press.
First published 2013
Printed in the United Kingdom by TJ International Ltd. Padstow Cornwall
A catalogue record for this publication is available from the British Library
Library of Congress Cataloguing in Publication data
Searle, Roger, 1944–
Mid-ocean ridges / Roger Searle, Emeritus Professor, Department of Earth Sciences, Durham University.
pages cm
Includes bibliographical references and index.
ISBN 978-1-107-01752-8 (hardback)
1. Mid-ocean ridges. 2. Plate tectonics. 3. Sea floor spreading. I. Title.
QE511.7.S45 2013
551.1 36 – dc23
2013017281
ISBN 978-1-107-01752-8 Hardback
Cambridge University Press has no responsibility for the persistence or
accuracy of URLs for external or third-party internet websites referred to
in this publication, and does not guarantee that any content on such
websites is, or will remain, accurate or appropriate.
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Roger Searle
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To my family.
‘Could the waters of the Atlantic be drawn off so as to expose to view this great seagash which separates continents, and extends from the Arctic to the Antarctic, it would
present a scene the most rugged, grand and imposing. The very ribs of the solid earth,
with the foundations of the sea, would be brought to light . . . ’
Matthew Fontaine Maury (1860)
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Roger Searle
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Roger Searle
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Contents
Preface
page xi
1 Introduction
1.1
1.2
1.3
1.4
1.5
1.6
The global mid-ocean ridge system
The discovery of MORs
Sea floor spreading and plate tectonics
Oceanographic institutions
Dedicated MOR research programmes
Outline of this book
2 Techniques of MOR study: a brief historical review
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
Introduction
Depth measurement
Magnetic field
Gravity
Heat flow
Earthquake seismology
Seismic refraction
Seismic reflection
Compliance
Side-scan sonar
Electrical methods
Visual imaging
Sampling
Ships and other platforms
Navigation
Summary
3 The oceanic lithosphere
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Crust, mantle, lithosphere and asthenosphere
Oceanic heat flow and the thermal structure of the lithosphere
Thickness of the oceanic lithosphere
Flexure and elastic thickness
Gravity over MORs
Isostatic compensation
Summary
1
1
3
5
8
9
9
11
11
11
14
18
21
22
24
26
29
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31
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vii
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Contents
viii
4 Ridges as plate boundaries
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
Ridges and plate kinematics
Seismicity and focal mechanisms
Spreading centres
Transform faults and fracture zones
Ridge segmentation
The hierarchy of ridge axis discontinuities
Triple junctions
Propagating rifts
Oceanic microplates
Summary
5 Crustal structure and composition
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
Introduction
Crustal thickness
Seismology and the layered model
Melt distribution and magma chambers
Shallow crustal sampling
Deep sampling: ocean drilling
Ophiolites
Departures from the layered crust model
Crustal magnetisation
Summary
6 Volcanism
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Introduction
Mantle melting
Melt delivery to the crust
Lava morphologies
Fast-spreading ridges
Intermediate-spreading ridges
Slow-spreading ridges
Ultra-slow-spreading ridges
Summary
7 Tectonism
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
Introduction
Fissures
Normal faults
Detachment faults and oceanic core complexes
Ultra-slow spreading
Transform and strike-slip faults
Modelling faulting
Summary
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61
63
64
69
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91
92
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106
112
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150
158
162
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185
192
194
196
197
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Roger Searle
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Contents
ix
8 Hydrothermal processes
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
Introduction
Discovery and distribution of hydrothermal vents
Basalt-hosted vent systems
Sub-sea-floor processes
Ultramafic-hosted systems
Hydrothermal alteration of oceanic crust
Hydrothermal plumes
Hydrothermal vent biology
Controls on the distribution of hydrothermal vents
Summary
9 Summary and synthesis
9.1
9.2
9.3
9.4
9.5
9.6
9.7
Common features
Fast-spreading ridges
Intermediate spreading
Slow spreading
Ultra-slow ridges
‘Anomalous’ ridges
Summary
Appendix A Glossary of terms
Appendix B Directory of named features
References
Index
Colour plates section between pages 180–181.
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209
213
216
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222
227
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254
258
309
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Roger Searle
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Preface
Mid-ocean ridges are where the oceanic crust, which covers over 60% of the Earth’s surface
and is renewed every 200 million years or so, is generated. They are thus features of
first-order importance in the Earth system. Mid-ocean ridges were discovered some 150
years ago, and have been studied with increasing intensity and detail since then. We are
now beginning to have an outline level of understanding of their structures and processes.
Ridges are primarily studied by geophysicists and geologists. But chemists are interested
because ridge crest hydrothermal systems exchange chemical elements between the rock of
the oceanic crust and the overlying ocean waters; physical oceanographers are concerned
with how ridge topography and geothermal heat influence ocean waters and currents, and
biologists study the unique ecosystems that inhabit hydrothermal vents, which may hold
clues to the origins of life and the nature of the ‘deep biosphere’ of microbes that live deep
in crustal rocks.
This book attempts to set out an overview of the current understanding of mid-ocean
ridges across most of the scientific disciplines involved. I have tried to make it reasonably
comprehensive, while admitting that an encyclopaedic coverage is certainly beyond my
ability. I intend the book to be suitable for a wide audience, in terms of both their level
of prior knowledge and the nature of their disciplines. Thus I hope it can be used as
a general introduction and reference by senior undergraduates and starting postgraduate
students taking courses in, for example, geodynamics, Earth systems or oceanography, by
doctoral students as a starting point for their researches, and by both academic and other
professionals who may need an introduction or reference to areas outside their immediate
specialties.
My aim has been to highlight at least some of the milestone papers that have influenced
our understanding of ridges, and to use illustrations from them. The bibliography is by no
means comprehensive, but I hope it contains enough key references to serve as a useful
starting point for further research. I have provided a brief historical background to ridge
studies, and have included two appendices to aid the reader new to this field. Appendix A is
a glossary of technical terms used, and Appendix B is a directory of feature names, briefly
giving the nature of each feature referred to in the book and its geographical location.
The book includes brief mathematics, including some critical equations where appropriate, but is largely non-mathematical. However, I have tried to make clear the physical
principles involved in the various processes described. There is no detailed discussion
of petrology or biology, although I have tried to give an outline of key petrological and
biological issues where required.
The book starts with an introduction followed by a brief historical review of techniques.
It then follows a logical path through the lithosphere, ridges as plate boundaries, crustal
xi
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Preface
xii
structure, volcanism, tectonism and hydrothermal systems. Each chapter has a brief summary of the main topics covered. Each can be read more-or-less independently, especially
with the help of the appendices, and ample cross-references are provided. The final chapter
summarises the descriptions given in the earlier chapters, and attempts to set them in a
unified conceptual model that synthesises current thinking. The reader seeking a quick
introduction to mid-ocean ridges might start at this final chapter, before referring to earlier
chapters for details.
SI units are used throughout, and temperatures are given in degrees Celsius (°C). Years
are indicated by ‘a’, with thousand years and million years denoted ‘ka’ and ‘Ma’. Figures
generally have scale bars or, if not, latitude scales. A useful guide is that one degree of
latitude is approximately 111 km, and one minute of latitude (1/60th of a degree) is about
1 mile or 1.8 km. North is to the top unless otherwise indicated.
I am greatly indebted to the many colleagues and students who over the years have
informed, encouraged, argued and generally contributed to my nevertheless sadly limited
understanding of mid-ocean ridges. There are too many individuals to name, and it would
be invidious to list just a few, but thanks for your friendship on this exciting journey. I must,
however, particularly thank Suzanne Carbotte, Colin Devey, Gretchen Früh-Green, Rachel
Haymon, Marvin Lilly and Ken Macdonald for reading and providing invaluable comments
on draft chapters, thereby saving me from a number of howlers. Any remaining errors and
omissions are, of course, my responsibility alone. I am also indebted to John Gould, Mark
Holmes, Dave Sandwell, Martin Sinha and Adam Soule, who supplied original versions of
illustrations. The writing was supported in part by a Leverhulme Trust Emeritus Fellowship.
I am grateful to all rightsholders who kindly gave permissions for re-use of figures in
this book. In every case, reasonable effort was made to establish the correct rightsholder,
and credit to the source is given for all figures, but in the case of any unfortunate omission,
the rightsholder should contact the publisher to arrange correction.
Finally, I must thank my family for their forbearance during the writing of this book;
until I began it I did not realise how appropriate this traditional acknowledgement is! They
have suffered many weeks of my self-imposed isolation with computer, books and reprints,
not to mention the four years of my life spent on research expeditions at sea. Thank you to
them for their enduring support and encouragement.
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