Roberto Lanza • Antonio Meloni

The Earth's Magnetism

An Introduction for Geologists

With 167 Figures and 6 Tables I

Springer Contents

1 The Earth's Magnetic Field 1 1.1 Observations and Geomagnetic Measurements 2 1.1.1 The Magnetic Dipole 2 1.1.2 Elements of the Earth's Magnetic Field 5 1.1.3 Early Measurements of the Earth's Magnetic Field 7 1.1.4 Modern Magnetic Measurements 11 1.2 Mathematical Description 16 1.2.1 Spherical Harmonic Analysis 16 1.2.2 Methods for g^ and /i™ Computation 21 1.2.3 Results of Spherical Harmonic Analysis 23 1.2.4 A Predominantly Dipolar Field 24 1.2.5 Geomagnetic Coordinates 27 1.2.6 Harmonic Power Spectra of the Geomagnetic Field 31 1.3 Time Variations 33 1.3.1 Secular Variation 34 1.3.2 Magnetic Tomography and Interpretation of Secular Variation 39 1.3.3 Geomagnetic Jerks 40 1.3.4 External Origin Time Variations 42 1.4 Essentials on the Origin of the^Earth's Magnetic Field 48 1.4.1 Toroidal and Poloidal Fields 49 1.4.2 Fundamental Equations of Magnetohydrodynamics 50 1.4.3 Elementary Dynamo*Models 53 1.4.4 Dynamo Energy 56 1.5 Magnetic Observatories, Reference Field Models and Indices 59 1.5.1 Geomagnetic Observatories 59 1.5.2 Geomagnetic Field Reference Models 61 1.5.3 Geomagnetic Indices 62 Suggested Readings and Sources of Figures 66

2 Basic Principles of Rock Magnetism 67 2.1 Magnetic Properties of Solids 69 2.1.1 Diamagnetism 69 2.1.2 Paramagnetism 69 2.1.3 Ferromagnetism 72 2.1.4 Conclusion 76 VIII Contents

2.2 Magnetic Remanence 76 2.2.1 Magnetic Energies 77 2.2.2 Magnetic Hysteresis 80 2.2.3 Remanence Acquisition 82 2.2.4 Magnetic Domains 83 2.2.5 Remanence vs. Time 85 2.3 Magnetic Properties of Minerals 88 2.3.1 Magnetic Susceptibility 88 2.3.2 Fe-Ti Oxides 90 2.3.3 Fe Sulfides 92 2.4 Ferromagnetic Minerals in Rocks 93 2.4.1 Igneous Rocks 93 2.4.2 Sedimentary Rocks 95 2.4.3 Metamorphic Rocks 96 Suggested Readings and Sources of Figures 97

3 Magnetic Prospecting 99 3.1 Instruments and Surveying Procedures 100 3.2 Magnetic Anomalies 102 3.2.1 Data Processing 103 3.2.2 Summary of Operations 104 3.3 Significance of Magnetic Anomalies 105 3.3.1 Forward Modeling 107 3.3.2 Inverse Modeling Ill 3.3.3 Spectral Characteristics 113 3.3.4 Other Frequently Used Techniques 116 3.4 Satellite Magnetic Measurements 118 3.5 Applications of Magnetic Anomalies 119 Suggested Readings and Sources of Figures 126

4 Paleomagnetism 127 4.1 Magnetic Remanence in Rocks 127 4.1.1 Thermal Remaned:' Magnetization (TRM) 127 4.1.2 Chemical Remanent Magnetization (CRM) 130 4.1.3 Detrital and Post-Depositional Remanent MagnetizatW'(DRM, pDRM) 132 4.1.4 Isothermal Remanent Magnetization (IRM) 135 4.1.5 yiscous Remanent Magnetization (VRM) 137 4.1.6 Other Remanent Magnetizations 137 4.1.7' Timing of Remanence Acquisition 138 4.2 Sampling Techniques 139 4.3 Laboratory Techniques 140 4.3.1 Remanence Measurements 140 4.3.2 Demagnetization 141 4.3.3 Identification of Magnetic Minerals 147 4.4 Paleomagnetic Directions and Poles 150 4.4.1 Statistical Analysis 151 Contents IX

4.4.2 Field Tests 154 4.4.3 Tilt Correction and Paleomagnetic Direction 156 4.4.4 Virtual Geomagnetic Pole (VGP) and Paleopole 157 4.5 Paleomagnetic Information 159 4.5.1 Geodynamics and Paleogeography 159 4.5.2 Regional Tectonics 164 4.5.3 Volcanism 167 4.5.4 Prospection 169 4.5.5 Paleofield 170 Suggested Readings and Sources of Figures 175

5 Magnetic Fabric of Rocks 177 5.1 Magnetic Anisotropy 177 5.2 Laboratory Techniques 183 5.3 Magnetic Fabric 186 5.3.1 Sedimentary Rocks 186 5.3.2 Igneous Rocks 188 5.3.3 Magnetic Fabric and Deformation 194 5.4 Anisotropy and Remanence 198 Suggested Readings and Sources of Figures 201

6 Magnetic Signature of the Earth's Crust 203 6.1 Oceanic Crust 205 6.2 Continental Crust 210 6.3 Global Maps of the Earth's Field 213 Suggested Readings and Sources of Figures 216

7 Magnetic Chronology 217 7.1 Geomagnetic Polarity Time Scale (GPTS) 218 7.2 Magnetic Stratigraphy ....,?. 225 7.3 Paleosecular Variation ....' 227 7A Archaeomagnetism ,f 231 Suggested Readings and.Sources of Figures • 234

8 Environmental Geomagnetism 235 8.1 Environmental Prospecting 235 8.2 Enviromagnetic Parameters and Techniques 238 8.3 Magnetic Climatology 241 8.4 Magnetism and Pollution 246 8.5 Seismo- and Volcanomagnetism 248 Suggested Readings and Sources of Figures 252

9 Historical Notes 253 9.1 The Very Ancient Times 253 9.2 A Light in the Middle Ages 255 9.3 The Discovery of Declination and Inclination 256 9.4 Geomagnetism in Gilbert's Epoch 258 X Contents

9.5 Secular Variation 259 9.6 Geomagnetism from Gauss Onwards 261 9.7 Rock Magnetism 262 Suggested Readings and Sources of Figures 264

Appendix - Magnetic Quantities, SI and cgs Units, Conversion Factors 265

Index 267