Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmgp51a0283g&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #GP51A-0283
Mathematics
Logic
1510 Dynamo Theories, 1535 Reversals (Process, Timescale, Magnetostratigraphy), 1599 General Or Miscellaneous
Scientific paper
The destabilization of San Carlos olivine in the presence of carbon produces a carbonate melt. This melt could separate through geological times and become trapped in higher concentration at the bottom of the lithosphere and perhaps at the bottom of the 660 km boundary, as suggested by seismic wave attenuation. Carbonate melt could reduce the viscosity of the olivine rich solid mantle by more than ten orders of magnitude. This low viscosity would allow the development of a significant differential rotation within the mantle at the 660 km boundary, thereby decoupling the lower mantle from the upper. The increasing temperature with depth in the mantle reduces the electron retaining capacity of its atoms resulting in an overall positive charge in the lower mantle and an overall negative charge in the upper mantle and the lithosphere. The differential rotation of these two oppositely charged layers will generate a dipole magnetic field. The direction of the dipole field depends on the relative angular velocities of the two sections of the mantle. If the 660 km outer shell is the faster, then the field is normal with its dipole moment pointing north. The last two R-N reversals coincided with major impacts, suggesting that the impacts triggered a process which slowed the rotation of the lower mantle or sped up the rotation of the 660 km outer shell or both. The rotation of the 660 km outer shell could be speeded up by global cooling, triggered by impacts. This would occur if glaciation reduces the moment of inertia of the outer shell by shifting ocean water from the equator to ice at the poles. If only this glaciation process was active at the time of the last reversal then a 140 m reduction of the global sea level would have been needed to reverse the sign of the differential rotation and generate the observed normal field. Alternatively, the rotation of the lower mantle could have been slowed down if the projectile reached the lower mantle, thereby increasing its moment of inertia. The existence of plateau basalt formations at the impact sites coinciding in age with the last R-N reversal gives credibility to this scenario. Assuming that the differential rotation between the inner and outer shells is equal to the observed westward movement of the dipole field, and using a simplified spherical model, it can be shown that a 7x10-7 C/cm3 uniform charge of the outer shell is required to generate the contemporary dipole field. This uniform charge falls within the range of the friction generated charges, therefore it is reasonable. The proposed model is consistent with all the features of both the contemporary field and, within reasonable uncertainty, the paleomagnetic field. It also explains: -the correlation between the gravitational and magnetic field, -the correlation determined between the length of the day (LOD) and the strength of the magnetic field, -the inverse correlation between the westward drift of the field and LOD, -the lunar secular variation of the field, -the preferred longitudes of the virtual geomagnetic poles for N-R and R-N reversals, -the correlation between climate and magnetic field, -the inverse correlation between the frequency of the magnetic reversals and plume and volcanic activity and -the stronger geomagnetic field for the superchrons.
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