The Magnetism of the Earth’s Crust

Abstract The igneous, sedimentary, and metamorphic rocks that compose the Earth’s crust are made up of numerous different minerals, only a few of which (e.g., magnetite and hematite) have comparatively strong magnetic properties. Where rocks with different magnetizations come in contact, as around an orebody or in a geological structure such as a fault or intrusion, the contrast in magnetizations produces a local field, called a magnetic anomaly. By measuring the size and shape of the anomaly, its source can be evaluated. Strong magnetic anomalies on the continents often result from magnetizations induced by the present field in orebodies. By contrast, strong magnetic anomalies over the oceans result from a permanently magnetized lithosphere with alternating polarity, reflecting polarity changes of the magnetic field during seafloor spreading. Rocks lose their magnetization at the Curie temperature of their magnetic minerals, which is reached at a depth in the Earth comparable to the crustal thickness.