Rockmagnetism Today

Abstract

Rockmagnetism provides an insight into the origin and stability of Natural Remanent Magnetization (NRM) in rocks, which enables one to decipher the geomagnetic field history, continental drift, and seafloor spreading. NRM in some rocks is due to cooling through the Curie point (TRM) and for others, either due to volume growth from superparamagnetic to single domain size (CRM) or alignment along the field vector in a wet sediment (DRM). However, CRM has sometimes been mistaken for TRM; it is important to distinguish between the two so as to assign the correct age for NRM. NRM may also acquire an antiparallel direction to the ambient geomagnetic field (self-reversal); thus providing an ancient direction 180° in error. Theoretical models for self-reversal appeal to exchange anisotropy, compensation point, and chemical phase changes. Self-reversal in nature, however, has been proved only for a limited number of cases. Although some minerals have large magnetostriction, a direct effect on NRM is questioned. Interestingly, stable NRM has recently been found in nominally diamagnetic minerals with 1% iron content and in antiferromagnets.