Mid-ocean Ridges by Roger Searle

8257e457bd77ed9.jpg Author Roger Searle
Isbn 978-1107017528
File size 41.9 MB
Year 2013
Pages 330
Language English
File format PDF
Category physics


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. ‘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 Mid-Ocean Ridges ROGER SEARLE Emeritus Professor, Department of Earth Sciences, Durham University 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 2013017281 551.1 36 – dc23 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. 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) 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 vii 1 1 3 5 8 9 9 11 11 11 14 18 21 22 24 26 29 29 31 33 35 36 39 42 44 44 45 50 53 54 57 59 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 61 61 63 64 69 75 77 82 84 87 91 92 92 92 96 106 112 114 119 120 123 127 129 129 129 135 135 139 145 150 158 162 163 163 163 167 185 192 194 196 197 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. 200 200 200 204 209 213 216 217 222 227 229 231 231 232 233 234 236 237 237 239 254 258 309 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 xii Preface 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 Fr¨uh-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. 1 Introduction 1.1 The global mid-ocean ridge system Mid-ocean ridges (MOR)s are a product of the separation and spreading of tectonic plates, and are a major component of the Earth system. Their role as plate boundaries is discussed in detail in Chapter 4. Back-arc spreading centres are functionally similar, and for many purposes can be grouped together with ridges, although they will not be considered in detail in this book. This chapter introduces MORs, outlines their discovery and the theory of plate tectonics (since ridges form an important class of plate boundaries), and briefly introduces some of the institutions and organisations that have been critical for understanding ridges. The MOR system spans the world, extending to some 65 000 km in length (Figure 1.1). Plate separation rates range from only a few millimetres per year to some 160 mm a−1 , and even faster at some past times (M¨uller et al., 2008; Teagle et al., 2012). MORs are characterised by shallow ocean floor, narrow bands of shallow seismicity and high heat flow. Their crests generally lie some 2.6 km below sea level; their flanks may be thousands of kilometres wide and reach depths in excess of 6 km. They contain arguably the largest array of active volcanoes in the world, and host extensive hydrothermal vent fields where unique ecosystems based on chemosynthesis flourish. They are the places where new oceanic lithosphere is continuously generated, and where this newly created lithosphere is flexed, faulted and chemically altered. This book aims to present an overview of all of these processes. Many aspects of ridges are at least in part related to spreading rate. Strictly speaking, ‘spreading rate’ is the rate at which each tectonic plate grows, or spreads away from the ridge axis. However, it is sometimes also used to mean the rate at which the two plates diverge (which for symmetric spreading is twice the spreading rate). To avoid confusion, the two rates should be identified by different names. Although not entirely accurate, the custom has developed of using ‘half spreading rate’ for the rate of plate growth and ‘full spreading rate’ for the rate of plate separation. Alternative uses would be ‘plate accretion rate’ and ‘plate separation rate’. Because ridge characteristics tend to depend on spreading rate, ridges are often grouped into broad classes as a function of this rate (Table 1.1). Nevertheless, spreading rate varies continuously, and the given class boundaries are approximate and may vary between authors. Other factors, particularly mantle temperature and fertility, may also control ridge morphology, structure and processes. 1 Introduction 2 Table 1.1 Spreading rate categories of mid-ocean ridges Class Superfast Fast Intermediate Slow Ultra-slow Figure 1.1 Total opening rate, km Ma−1 >130 –150 90–130 50–90 20–50 <20 Reference Example Sinton et al. (1991) Lonsdale (1977) Lonsdale (1977) Lonsdale (1977) Grindlay et al. (1998) East Pacific Rise 20° S East Pacific Rise 13° N Juan de Fuca Ridge Northern Mid-Atlantic Ridge Southwest Indian Ridge, Gakkel Ridge World topography with mid-ocean ridges superimposed (heavy black lines). Back-arc spreading centres have been omitted for clarity. AAR, American–Antarctic Ridge; ASC, Azores Spreading Centre; CaR, Carlsberg Ridge; ChR, Chile Rise; CIR, Central Indian Ridge; CN, Cocos–Nazca Spreading Centre; EPR, East Pacific Rise (north and south); Ex, Explorer Ridge; GA, Gulf of Aden; GoR, Gorda Ridge; GaR, Gakkel Ridge; JF, Juan de Fuca Ridge; KR, Kolbeinsey Ridge; LS, Laptev Sea Rift; MAR, Mid-Atlantic Ridge (north and south); MR, Mohns Ridge; PAR, Pacific–Antarctic Rise; RR, Reykjanes Ridge; RS, Red Sea; SEIR, Southeast Indian Ridge; SWIR, Southwest Indian Ridge. For colour version, see plates section. Each ocean contains an MOR (Figure 1.1). Most are named for the ocean they are in (e.g., Mid-Atlantic Ridge – MAR), but some have specific regional names (e.g., Gakkel Ridge in the Arctic Ocean), and some are named for the plates they separate (e.g., Cocos– Nazca Spreading Centre). The fast-spreading East Pacific Rise (EPR) and Pacific–Antarctic Rise are called ‘rise’ because their morphology is gentler than that of the slower-spreading 3 Figure 1.2 1.2 The discovery of MORs The first map of the North Atlantic, showing the position of the MAR after Murray and Hjort (1912), reproduced from Maury (1860). ‘ridges’. Some ridges are not centred in their ocean, having come about by rifting of a pre-existing oceanic basin rather than of the bounding continent, so are not ‘mid-ocean’ in the strict sense; however, their structures and processes are common to the MORs sensu stricto. 1.2 The discovery of MORs Mid-ocean ridges were discovered in the nineteenth century. The American hydrographer Matthew Fontaine Maury developed improved methods of deep-sea sounding, and produced the first map showing the varying depth of the North Atlantic Ocean (Maury, 1860). His Plate VII shows a broad, shallow region in the position of the MAR between 20° N and 52° N (Figure 1.2). The northernmost part of this was called Telegraphic Plateau, and was discovered by surveys for the first trans-Atlantic telephone cables. The rest of this broad rise, south of 45° N, was known as ‘Middle Ground’. The first systematic oceanographic expedition was the circum-global cruise of HMS Challenger from 1872 to 1876 (Thomson, 1877). The Challenger expedition’s studies Introduction 4 Figure 1.3 Map of the MAR, after Tizard (1876). included both depth soundings and measurements of water temperature. On the basis of a difference between the deep-water temperatures between the western and eastern Atlantic, Tizard (1876) inferred the existence of a topographic ridge separating the two basins. His Plate 6 shows the existence of this ridge, named ‘Dolphin Ridge’ in the North Atlantic and ‘Challenger Ridge’ in the south (Figure 1.3). They are joined by ‘Connecting Ridge’ which comprises what are now known as the equatorial Atlantic fracture zones. Early in the twentieth century, Murray and Hjort (1912, p. 135) produced a map showing many of the world’s MORs, although only one, the MAR, was yet named as such. In the Pacific, the US ship Albatross had discovered the Albatross Plateau (now known to be part of the EPR) between 1880 and 1905. Thousands of soundings had been made world-wide by 1923, when electronic echosounding began to be introduced (Shepard, 1959). The German ship Meteor ran 14 echosounding lines across the south Atlantic in 1925–1927 (Marmer, 1933), clearly revealing the southern MAR in detail (Shepard, 1959, pp. 162–163). These were the only detailed profiles available prior to World War II (Heezen and Menard, 1963). At about the same time, the Danish vessel Dana (1928–1930) discovered the Carlsberg Ridge in the Indian Ocean (Tharp, 1982). Following World War II, continuously recording precision echosounders came into widespread use (Section 2.2.2), and by the middle of the twentieth century the general outline of the global MOR system was well established (Shepard, 1948; Figure 1.4). 5 Figure 1.4 1.3 Sea floor spreading and plate tectonics Bathymetric chart of the world’s oceans, after Shepard (1948). 1.3 Sea floor spreading and plate tectonics The theory of sea floor spreading views tectonic plates as being created at MORs and then ‘spreading’ away from them on either side. The theory developed from the ideas on continental drift proposed by Wegener (1912, 1966), based on the matching coastlines and geological features, and the divergence of evolutionary trends, on either side of the Atlantic. Dietz (1961) proposed that oceanic seismic layer 3 (the lower crust, see Chapter 5) and the uppermost mantle are chemically the same, and introduced the term ‘lithosphere’ to describe the outermost, rigid part of the Earth (Chapter 3). Importantly, he suggested that the sea floor represents the tops of convection cells; the MORs mark the up-welling sites and the trenches are associated with down-welling. This contains the essence of sea floor spreading and plate tectonics as now understood. Hess (1962) also accepted mantle convection, and suggested that the MAR is spreading at about 1 cm a−1 half-rate. He proposed that ridges’ elevations reflect their thermal expansion, and that their low seismic velocities are partly due to raised temperatures. Crucially, he suggested that continents ride passively on the convecting mantle rather than having to plough through oceanic crust, providing a more acceptable mechanism for continental drift. Critical evidence for sea floor spreading came from detailed marine magnetic surveys. Detailed surveys by Scripps Institution of Oceanography in conjunction with the United States Coast and Geodetic Survey off the west coast of the USA and Canada (Mason, 1958; Mason and Raff, 1961; Raff and Mason, 1961) revealed extensive, parallel, linear magnetic anomalies (Figure 1.5) in the region of the Gorda, Juan de Fuca and Explorer Ridges (though these ridges were not recognised at that time). Subsequent studies elsewhere 6 Figure 1.5 Introduction Magnetic lineations in the northwestern Pacific, after Raff and Mason (1961). showed the remarkable symmetry of magnetic lineations about ridge axes (e.g., Heirtzler et al., 1966; Figure 1.6). Vine and Matthews (1963) of Cambridge University conducted a small but very detailed survey over part of the Carlsberg Ridge. They demonstrated that the observed magnetic lineations could be explained if approximately 50% of the sea floor was underlain by normally magnetised material and 50% by reversely magnetised material. By combining this observation with the recognition of periodic reversals of the Earth’s magnetic field (Cox et al., 1963) they were able to explain the magnetic lineations in terms of sea floor spreading. A much greater body of supporting evidence was later published by Vine (1966). Essentially the same interpretation had been put forward more or less simultaneously by Lawrence Morley, who was unable to get his ideas published immediately (Glen, 1982) but did so later (Morley and Larochelle, 1964). In summary, the theory of sea floor spreading states that tectonic plates diverge from MOR axes, at separation rates ranging from <10 km Ma−1 (Dick et al., 2003) to >160 km Ma−1 (Naar and Hey, 1989). This causes the underlying ductile mantle to rise and, at higher spreading rates, to partially melt producing magma that solidifies to form a volcanic crust (Figure 1.7); at slower spreading rates, the mantle is extruded directly onto the sea floor (Chapter 7).

Author Roger Searle Isbn 978-1107017528 File size 41.9 MB Year 2013 Pages 330 Language English File format PDF Category Physics Book Description: FacebookTwitterGoogle+TumblrDiggMySpaceShare 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 seafloor spreading. This coursebook presents a multidisciplinary 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, with succeeding chapters covering 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.     Download (41.9 MB) The Galapagos: A Natural Laboratory for the Earth Sciences Soil Mechanics: A One-Dimensional Introduction Geology of the Alps (2nd Revised edition) Encyclopedia of Marine Geosciences Global Tsunami Science: Past and Future, Volume I Load more posts

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