Roberto Lanza Antonio Meloni The Earth s Magnetism An Introduction for Geologists

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2 Roberto Lanza Antonio Meloni The Earth s Magnetism An Introduction for Geologists

3 Roberto Lanza Antonio Meloni The Earth s Magnetism An Introduction for Geologists With 167 Figures and 6 Tables

4 Authors Prof. Dr. Roberto Lanza Dipartimento di Scienze della Terra Università di Torino Via Valperga Caluso Torino, Italy Phone: Fax: Dr. Antonio Meloni Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata Roma, Italy Phone: Fax: meloni@ingv.it Library of Congress Control Number: ISBN-10 ISBN Springer Berlin Heidelberg New York Springer Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitations, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media springeronline.com Springer-Verlag Berlin Heidelberg 2006 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Erich Kirchner, Heidelberg Typesetting: Büro Stasch (stasch@stasch.com) Uwe Zimmermann, Bayreuth Production: Agata Oelschläger Printing and Binding: Stürz, Würzburg Printed on acid-free paper 30/2132/AO

5 Preface Geomagnetism has always been at the forefront among the various branches of geophysics. At the end of the 16th century William Gilbert determined that the Earth is a big magnet, implying that it has a magnetic field; in the 1830s Carl Friedrich Gauss was able to formulate a procedure to measure the field completely and analyzed its characteristics with the spherical harmonic analysis, a method still used in the era of satellites and computers. Nevertheless, as recently as in the sixties, geophysics textbooks devoted only a thin chapter to geomagnetism, and limited their discussion mostly to prospecting methods, while many geologists curriculum practically left it out altogether. The essential contribution provided by the study of ocean floor magnetic anomalies and by paleomagnetism in the development of global tectonic models, made geomagnetism popular in the geological community, which nonetheless continued, and still continues, to view it as a highly specialist discipline. The authors of this book, like many of their colleagues, are convinced that geomagnetism is now an essential part of any Earth scientist s education. For this reason the book is meant as a first step, presenting fundamental concepts and their more and more numerous applications in many fields of geology, and stimulating readers interest in further studying the subjects they find most interesting in the many available specialist books. Presenting such a complex, wide-ranging subject as geomagnetism in general terms, requires a drastic choice, both in terms of what to write and of how to write it. A selection of subjects will necessarily be influenced by the authors education; expressing in a simple and thus approximate form physical concepts that should be articulated with due rigor may lead into error. Whatever judgment the readers may ultimately pass on our work, we will deem we have done something useful if, once they are finished reading it, at least some of them will go to a library to consult far more substantial books and browse the vast geomagnetic literature scientific papers. The first four chapters of the book discuss the fundamental subjects of geomagnetism within geology: the Earth s magnetic field, the magnetic properties of rocks, measuring and interpreting magnetic anomalies, and paleomagnetism. The next four chapters briefly go over other fields of application: the magnetic fabric of rocks, the Earth s crust magnetization, magnetic chronology and environmental geomagnetism. A short historical chapter ends the book. First of all we would like to thank those who encouraged us to study geomagnetism: our teachers, who passed their precious experience on to us, and those among our students who asked us the awkward, yet essential, questions that require some sort of answer. As it is impossible to thank each and every one of the persons who helped us along, we will have to restrict ourselves to mentioning the most substantial contributions. Elena

6 VI Preface Zanella prepared the figures of Chaps. 2 and 4 to 8, combining her geomagnetic knowledge and her graphic skills; Roberta Tozzi drew those of Chaps. 1 and 3. Seb De Angelis and Katia Damiani helped to put our concepts in a proper English form. Uwe Zimmermann turned the raw manuscript into a finished book. The various chapters benefited greatly from the comments and suggestions expressed on a preliminary draft by Don Tarling, David Barraclough, Niels Abrahamsen, Paola De Michelis, Ted Evans, Ann Hirt, Frantisek Hrouda and Nicolas Thouveny. Enzo Boschi is thanked for his advice and support. Last, but perhaps foremost in importance, is the Publisher, who had confidence in our idea and gave us the opportunity to make it real. Roberto Lanza Antonio Meloni

7 Contents 1 The Earth s Magnetic Field Observations and Geomagnetic Measurements The Magnetic Dipole Elements of the Earth s Magnetic Field Early Measurements of the Earth s Magnetic Field Modern Magnetic Measurements Mathematical Description Spherical Harmonic Analysis Methods for g m n and h m n Computation Results of Spherical Harmonic Analysis A Predominantly Dipolar Field Geomagnetic Coordinates Harmonic Power Spectra of the Geomagnetic Field Time Variations Secular Variation Magnetic Tomography and Interpretation of Secular Variation Geomagnetic Jerks External Origin Time Variations Essentials on the Origin of the Earth s Magnetic Field Toroidal and Poloidal Fields Fundamental Equations of Magnetohydrodynamics Elementary Dynamo Models Dynamo Energy Magnetic Observatories, Reference Field Models and Indices Geomagnetic Observatories Geomagnetic Field Reference Models Geomagnetic Indices Suggested Readings and Sources of Figures Basic Principles of Rock Magnetism Magnetic Properties of Solids Diamagnetism Paramagnetism Ferromagnetism Conclusion

8 VIII Contents 2.2 Magnetic Remanence Magnetic Energies Magnetic Hysteresis Remanence Acquisition Magnetic Domains Remanence vs. Time Magnetic Properties of Minerals Magnetic Susceptibility Fe-Ti Oxides Fe Sulfides Ferromagnetic Minerals in Rocks Igneous Rocks Sedimentary Rocks Metamorphic Rocks Suggested Readings and Sources of Figures Magnetic Prospecting Instruments and Surveying Procedures Magnetic Anomalies Data Processing Summary of Operations Significance of Magnetic Anomalies Forward Modeling Inverse Modeling Spectral Characteristics Other Frequently Used Techniques Satellite Magnetic Measurements Applications of Magnetic Anomalies Suggested Readings and Sources of Figures Paleomagnetism Magnetic Remanence in Rocks Thermal Remanent Magnetization (TRM) Chemical Remanent Magnetization (CRM) Detrital and Post-Depositional Remanent Magnetization (DRM, pdrm) Isothermal Remanent Magnetization (IRM) Viscous Remanent Magnetization (VRM) Other Remanent Magnetizations Timing of Remanence Acquisition Sampling Techniques Laboratory Techniques Remanence Measurements Demagnetization Identification of Magnetic Minerals Paleomagnetic Directions and Poles Statistical Analysis

9 Contents IX Field Tests Tilt Correction and Paleomagnetic Direction Virtual Geomagnetic Pole (VGP) and Paleopole Paleomagnetic Information Geodynamics and Paleogeography Regional Tectonics Volcanism Prospection Paleofield Suggested Readings and Sources of Figures Magnetic Fabric of Rocks Magnetic Anisotropy Laboratory Techniques Magnetic Fabric Sedimentary Rocks Igneous Rocks Magnetic Fabric and Deformation Anisotropy and Remanence Suggested Readings and Sources of Figures Magnetic Signature of the Earth s Crust Oceanic Crust Continental Crust Global Maps of the Earth s Field Suggested Readings and Sources of Figures Magnetic Chronology Geomagnetic Polarity Time Scale (GPTS) Magnetic Stratigraphy Paleosecular Variation Archaeomagnetism Suggested Readings and Sources of Figures Environmental Geomagnetism Environmental Prospecting Enviromagnetic Parameters and Techniques Magnetic Climatology Magnetism and Pollution Seismo- and Volcanomagnetism Suggested Readings and Sources of Figures Historical Notes The Very Ancient Times A Light in the Middle Ages The Discovery of Declination and Inclination Geomagnetism in Gilbert s Epoch

10 X Contents 9.5 Secular Variation Geomagnetism from Gauss Onwards Rock Magnetism Suggested Readings and Sources of Figures Appendix Magnetic Quantities, SI and cgs Units, Conversion Factors Index