Rock magnetism

Article By:

Dunlop, David J. Department of Physics, University of Toronto, Ontario, Canada.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.592100

The permanent and induced magnetism of rocks and minerals on scales ranging from the atomic to the global, including applications to magnetic field anomalies and paleomagnetism. Natural compasses, concentrations of magnetite (Fe₃O₄) called lodestones, are one of humankind's oldest devices. W. Gilbert in 1600 discovered that the Earth itself is a giant magnet, and speculated that its magnetism might be due to subterranean lodestone deposits. Observations by B. Brunhes in 1906 that some rocks are magnetized reversely to the present Earth's magnetic field, and by M. Matuyama in 1929 that reversely and normally magnetized rocks correspond to different geological time periods, made it clear that geomagnetism is dynamic, with frequent reversals of north and south poles. Nevertheless, permanent magnetism of rocks remains important because it alone provides a memory of the intensity, direction, and polarity of the Earth's magnetic field in the geological past. From this magnetic record comes much of the evidence for continental drift, seafloor spreading, and plate tectonics. See also: Continental drift; Geomagnetism; Magnet; Magnetism; Paleomagnetism; Plate tectonics

The content above is only an excerpt.