Geomagnetism In Marine Geology

Elsevier Oceanography Series, 6 GEOMAGNETISM IN MARINE GEOLOGY VICTOR VACQUIER Professor of Geophysics University of California, San Diego, California; Marine Physical Laboratory, Scripps Institution of Oceanography, Sun Diego, California ELSEVIER PUBLISHING COMPANY Amsterdam - London - New York I972 ELSEVIER PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211, Amsterdam, The Netherlands AMERICAN ELSEVIER PUBLISHING COMPANY, INC. 52 Vanderbilt Avenue New York, New York 10017 Library of Congress Card Number: 78-190683 ISBN: 0-444-41001-5 With 149 illustrations and 6 tables Copyright @ 1972 by Elsevier Publishing Company, Amsterdam 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 publisher, Elsevier Publishing Company, Jan van Galenstraat 335, Amsterdam Printed in The Netherlands PREFACE In the past decade magnetic surveys of the oceans, combined with results from other branches of geophysics and geology have provided data for the construction of a plausible scheme for the evolution of the present distribution of land and oceans on the earth. According to this scheme, the surface of the globe is divided into roughly ten major rigid plates presently moving with respect to each other in response to unknown forces deep in the earth. Plates may consist of both continental and oceanic parts, their present boundaries being outlined by earthquakes. Where two plates move away from each other new crust is formed at the rate of 1 to 12 cm/year, while when they move toward each other crust is being consumed often by underthrusting of a cold plate thus causing deep and shallow earthquakes. The most conspicuous spreading occurs in the ocean floor at the crest of oceanic ridges like the Mid-Atlantic Ridge where only shallow earthquakes occur because hot rock is close to the surface. As it cools, the new crust becomes magnetized in the direction of the magnetic field. When the field reverses its direction the rock is magnetized in the opposite way. Thus strips of positively and negatively magnetized rock form symmetrical bands on either side of the spreading ridges, which cause identical sequences of magnetic anomalies in widely different parts of the world. These properties of symmetry about the ridge axis and world-wide identity of the sequence of magnetic anomalies parallel to oceanic ridges establish beyond the shadow of a doubt that during the last 80 million years (m.y.) at least, the geomagnetic field can be regarded as due to a geocentric dipole lying along the earth’s rotational axis, which from time to time rapidly changes from one polarity to the other, and that the banded magnetic anomalies constitute a record of sea-floor spreading at fairly uniform rate in the manner of magnetic tape in a sound recorder. The chronology of the geomagnetic reversals has been tied to radiometric ages of lava flows on land, paleomagnetic measurements on oceanic sediment cores and with paleontological ages from the JOIDES Deep Sea Drilling Program. Areas of the ocean floor several thousand kilometers in width have thus been dated to about 80 m.y. by magnetic anomalies and about 200 m.y. by the deep drilling. Magnetic anomalies in large areas of the oceans thus have recorded the motions of crustal plates in the geologic past, which along with paleomagnetic data lets us make likely paleogeographic reconstructions with respect to geographic north. Although the relative movement of large crustal blocks was obviously accepted by Wegener (1929), Du Toit (1937) and other advocates of continental drift it was not until the delineation of the world-wide mid-ocean ridge system, that a reasonable place for VI PREFACE