Rocks as high-fidelity tape recorders

Abstract

Ferromagnetic and ferrimagnetic particles in igneous rocks, when they cool from their Curie points, record thermoremanent magnetization (TRM). TRM is a high-fidelity signal: (1) it parallels the Earth's (recording) field H that acted at the blocking temperature; (2) its strength is proportional to H for H \lsim 2 Oe, the usual range in nature; and (3) it is for the most part impervious to overprinting by later fields, since most particles have switching times \tau \gsim 10^{6} yr at ordinary temperatures. Only the finest particles are overprinted by viscous remanent magnetization (VRM) produced by later fields, and this noise is usually easily erased by AC or thermal treatment. Much of theoretical rock magnetism is directly applicable to DC, AC or thermal recording in fine-particle or thin-film media over ordinary time scales. Topics considered in this review include the thermal recording process, TRM signal strength and its field dependence, thermoviscous overprinting of TRM, erasing VRM overprints, novel methods of magnetic granulometry, and domain structure in particles just above critical single-domain size.