25 - Rock magnetism and paleomagnetism

Summary

Preamble

Paleomagnetism (ancient magnetism) is the study of the pre-history of the Earth's magnetic field, extending the record from the 400 years of direct observation to almost 4 billion years. It relies on the fact that many rocks retain indefinitely the remanent magnetism that is induced in them during their formation, and so record the direction and, with less certainty, the strength of the inducing field. That rocks have magnetic properties has been known since ancient times. By the late nineteenth century measurement methods had become sensitive enough to observe remanence in a wide range of rock types. However, they were not used to infer the history of the Earth's field. Even the discovery by P. David and B. Brunhes early in the twentieth century of rocks with magnetic remanence opposite to the Earth's field did not attract attention commensurate with its significance for another 50 years. The study of rock magnetism began to develop rapidly in the 1950s, when the revolutionary discoveries of paleomagnetism were appearing and the need for a fundamental understanding had become urgent. The two principal landmark publications in the early development of our understanding are Nagata (1953) and Néel (1955). Dunlop and Özdemir (1997) present a comprehensive statement of physical principles.