10. A Few Characteristic Features of the Geomagnetic Field During Reversals

Volcanic records of reversals are mostly exempt of complications linked to their magnetization process and thus potentially tell us the most significant story about the field variations prevailing during these periods. We have found no convincing indication supporting the presence of long-term non-zonal features governing the transitional field. A few VGP paths seem to be controlled by flux patches of the present non-axial dipole field lying immediately below the sites, but the detailed reversal records are characterized by scattered VGPs that are not related to anomalies of the present non axial dipole field. Assuming that clusters of VGPs over Australia would be associated with an hypothetical time persistence of the present anomaly in this area, then the geometry of the transitional field would have to be controlled by the equatorial dipole, since it is responsible for the present Australian patch. This is difficult to reconcile with our present knowledge of the variability of the equatorial dipole as well as with the structure of most detailed VGP paths. In fact, the existence of complex directional changes with rebounds and precursors in the detailed volcanic records reflect the persistence and the amplification of secular variation following the collapse of the axial dipole. We have now learned much more about the evolution of the axial dipole from studies of relative paleointensity in sediments but also from the records of absolute paleointensity that have been obtained for a few volcanic records. The data converge to indicate asymmetrical pre- and post-reversal phases but also a systematic overshoot marking the end of the recovery phase. These features can be explained by a dynamical model assuming a coupling of the Earth’s dipole with the quadrupolar mode during reversals.