Self-reversal of natural remanent magnetization in andesitic pumice

The natural remanent magnetization of andesitic pumice erupted from the Nevado del Ruiz volcano (Colombia) in 1985 is reversed with respect to the present geomagnetic field because of self-reversal. Laboratory thermoremanent magnetization acquired in fields of < 1 mT fully re-establishes the self-reversal mechanism. Hysteresis experiments indicate that the mechanism is caused by antiparallel coupling as a result of exchange interaction in two haemoilmenite phases of different composition. The weak critical field needed to suppress this self-reversal effect (≈ 1.2 mT) can be explained by a domain wall model. © 1990.