geomagnetism

The magnetic field of the earth, also known as terrestrial magnetism. The axis of this field emerges from the earth's surface at the magnetic poles. The position of these poles varies over time, and sometimes the positions of the north and south magnetic poles switch places. The pattern of the magnetic field at any one time will be preserved in any contemporary extrusions of volcanic rock. The study of past magnetic fields, palaeomagnetism, can yield information about the creation of new material at the oceanic ridges, about continental drift, and about the dating of certain deposits.