Mechanism for geomagnetic polarity reversals

Recent studies of old magnetic observations1and the seismic velocity of the lower mantle2suggest that at least part of the geomagnetic secular variation is driven by temperature anomalies in the solid mantle3. In particular, a patch of flux of opposite sign to that expected for a dipole field occurs beneath southern Africa. It has intensified throughout the twentieth century, by a process that is believed to be flux expulsion associated with a particularly hot part of the lowermost mantle, and is drifting westwards. A similar patch lies beneath South America. Here I show that the present fall in the dipole moment is directly related to the intensification and southward movement of these patches and suggest the fall occasionally leads to polarity reversal. The almost linear increase in the frequency of polarity reversals that has occurred since the Cretaceous quiet interval is attributed to the growth of the hot patch of mantle presently under southern Africa.