No. 49: Jan-Feb 1987
A fundamental assumption of paleomagnetism is that the natural remanent magnetism (NRM) of rocks is acquired parallel to the applied magnetic field. There are unsettling exceptions:
"Andesitic pumice, which was hurled several hundred kilometres during the disasterous 1985 eruption of the Nevado del Ruiz volcano (Columbia), carries a stable but reversed NRM with southerly declination and negative inclination. Heating experiments show that this magnetization is due to a self-reversal mechanism which also induces a reversed thermoremanent magnetization (TRM) in the laboratory field."
(Heller, Friedrich, et al; "Reversed Magnetization in Pyroclastics from the 1985 Eruption of Nevado del Ruiz, Columbia," Nature, 324:241, 1986.)
Comment. Much of the evidence for continental drift, especially the paths taken by the continents, is based upon paleomagnetism.