No. 49: Jan-Feb 1987
R.A. Muller and D.E. Morris review the evidence tying geomagnetic reversals to the impacts of large bodies with the earth: the tektites and microtektites; the climate changes; the biological extinctions, etc. Then they propose a physical mechanism for geomagnetic reversals:
"The impact of a large extraterrestrial object on the Earth can produce a geomagnetic reversal through the following mechanism: dust from the impact crater and soot from fires trigger a climate change and the beginning of a little ice age. The redistribution of water near the equator to ice at high latitudes alters the rotation rate of the crust and mantle of the Earth. If the sea-level change is sufficiently large ( 10 meters) and rapid (in a few hundred years), then the velocity shear in the liquid core disrupts the convective cells that drive the dynamo. The new convective cells that subsequently form distort and tangle the previous field, reducing the dipole component near to zero while increasing the energy in multipole components. Eventually a dipole is rebuilt by dynamo action, and the event is seen either as a geomagnetic reversal or as an excursion."
(Muller, Richard A., and Morris, Donald E.; "Geomagnetic Reversals from Impacts on the Earth," Geophysical Research Letters, 13:1177, 1986.)
Comment. That the earth's field is generated by internal dynamo action is still a theory, although a widely accepted one.