Preface 1. Origin and history of the Solar System 2. Composition of the Earth 3. Radioactivity, isotopes and dating 4. Isotopic clues to the age and origin of the Solar System 5. Evidence of the Earth's evolutionary history 6. Rotation, figure of the Earth and gravity 7. Precession, wobble and rotational irregularities 8. Tides and the evolution of the lunar orbit 9. The satellite geoid, isostasy and post-glacial rebound 10. Elastic and inelastic properties 11. Deformation of the crust: rock mechanics 12. Tectonics 13. Convective and tectonic stresses 14. Kinematics of the earthquake process 15. Earthquake dynamics 16. Seismic wave propagation 17. Seismological determination of Earth structure 18. Finite strain and high pressure equations of state 19. Thermal properties 20. The surface heat flux 21. The global energy budget 22. Thermodynamics of convection 23. Thermal history 24. The geomagnetic field 25. Rock magnetism and paleomagnetism 26. Alternative energy sources and natural climate variations: some geophysical background Appendix A. General reference data Appendix B. Orbital dynamics (Kepler's laws) Appendix C. Spherical harmonic functions Appendix D. Relationships between elastic moduli of an isotropic solid Appendix E. Thermodynamic parameters and derivative properties Appendix F. An Earth model: mechanical properties Appendix G. A thermal model of the Earth Appendix H. Radioactive isotopes Appendix I. A geological time scale 2004 Appendix J. Problems References Index.

Physics of the Earth

Radiocarbon ages have been published for nine basaltic lava flows on the island of Hawaii; the ages range from 2600 to somewhat older than 17,900 years B.P. By using the Thelliers' method in vacuum, geomagnetic paleointensity values were obtained from eight of the lavas; the ninth proved unsuitable. The paleointensities for the four youngest flows (2600–4600 years B.P.) yield virtual dipole moments (VDM's) that are 20% greater to more than twice the worldwide values for those times obtained by V. Bucha from archeomagnetic data. The dispersion of virtual geomagnetic poles for the eight lavas is 15.5°, appreciably larger than the average for older lava flows on Hawaii. These results contrast with the historic magnetic field in the region of Hawaii, in which both secular variation and nondipole components are very low. At about 10,000 years B.P. the measured VDM is not very different from the long-term worldwide average but differs considerably from a smooth extrapolation of Bucha's average curve. At about 18,000 years B.P. the measured VDM is very low and is associated with an unusually shallow paleomagnetic inclination for the latitude of Hawaii. These new paleointensity and paleodirectional data strongly suggest that sizable nondipole geomagnetic fields have existed in the vicinity of Hawaii at various times during the Holocene epoch and perhaps earlier.

Geomagnetic paleointensities from radiocarbon‐dated lava flows on Hawaii and the question of the Pacific nondipole low

Sediments have proved irresistible targets for attempts at determining the relative variations in the Earth's magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution. The assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited. Early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short, no dependence on magnetic field was expected. Nonetheless, a number of experiments showed that sedimentary magnetizations varied in accordance with the field, albeit not always in a simple, linear fashion. Experiments in which the sediments were stirred in the presence of a field (to simulate bioturbation) showed a reasonably linear relationship with the applied field, and these results spurred the hope that variations in the Earth's magnetic field might indeed be recoverable from appropriate sedimentary sequences. Examination of existing paleointensity data sets allows a few general conclusions to be drawn. It appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the Earth's magnetic field. The dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences. Moreover, when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences, there is considerable agreement among the various records. When viewed over timescales of hundreds to thousands of years, relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another, suggesting that they are dominated by nondipole field behavior. When viewed over timescales of a few tens of thousands to hundreds of thousands of years, however, the records show coherence over large distances (at least thousands of kilometers) and may reflect changes in the dipole field. On the basis of a sequence spanning the Brunhes and Matuyama chrons, the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years, but these oscillations are not clear in the record prior to about 300 ka; thus they are probably not an inherent feature in the geomagnetic field, and the correspondence of the period of oscillation to that of obliquity is probably coincidence.

Sedimentary records of relative paleointensity of the geomagnetic field: Theory and practice

A variety of rock types from eighteen volcanic units of the western United States were studied by Thellier's method. Ninety-five NRM-TRM curves were determined, and paleo-intensities are estimated from twelve of the units. Each paleo-intensity, on the average, represents the mean of values derived from five separate samples. Two paleo-intensities have standard deviations of the mean less than 5%, two between 5 and 10% and eight between 10 and 20%; no estimate is made for the remaining six units because of great internal inconsistency of the data or insufficient work. The largest ratio of paleo-intensity to the present field intensity at the same location is 1.1, the lowest 0.2. The 0.2 value is for a Miocene transition zone, supporting the hypothesis that the intensity decreases during a field reversal. Low ratios were found for a few other units. One such Pliocene unit also has an anomalous direction of magnetization, suggesting the field may have been in the act of reversing when the unit was magnetized 7.2±0.3 m.y. ago. The direction is normal for the other units with low ratios; one of these is probably less than 10,000 years old and thus cannot be associated with a reversal. Samples of dacite from the 1915 eruption of Mt. Lassen behave in a suprising and probably atypical manner; simple comparison of the NRM to the total TRM yields an experimental value much closer to the known intensity than that determined by Thellier's method.

E. Thellier

Papers

Sur l'intensite du champ magnetique terrestre dans le passe historique et geologique