Paleomagnetism

Paleomagnetism is the study of ancient magnetism in rocks. The phenomenon was first discovered by the French physicist Achilles Delesse in 1849. Delesse observed that certain magnetic minerals in rocks were lined up along the earth's magnetic field, just as if they were tiny compasses that had been set in place in the rocks. A related discovery that was even more startling was made by the French physicist Bernard Brunhes in 1906. Brunhes observed that the magnetic minerals in some rocks are oriented in exactly the reverse position than would be expected if they were simply tiny compasses. That is, some of these minerals were oriented with their north poles pointing to Earth's north magnetic pole, and their south poles to Earth's south magnetic poles.

The first ever treatise on experimental science by thirteenth century scholar Petrus Peregrinus of Marincourt dealt with magnetism ("Epistola de Magnete"). However, direct observations of the geomagnetic field were not recorded until the late sixteenth century, when the magnetic compass became a widespread tool for navigation. In order to understand nature and origin of Earth's magnetic field, however, much longer records are necessary. Paleomagnetic research draws this information from rocks that acquire a remanent magnetization upon formation.