Paleomagnetism

The natural magnetization of a rock is parallel to the ambient magnetic field. It is carried by minute amounts of ferrimagnetic minerals and can be stable over geological time scales.

Minerals can be magnetized and oriented with Earth's magnetic field in a variety of ways. One of these methods was described above. Igneous rocks are formed when molten rock escapes from beneath Earth's surface and cools sufficiently to form new rocky material. As long as the original rock is molten, minerals are too hot to hold a magnetic field or to stay in a permanent position. As the rock cools, however, it reaches a point where it can retain a magnetic field and assume a fixed position. At this point, the minerals are frozen into place as compass-like indicators of the direction of Earth's magnetic field.

Magnetic minerals can also be found in sedimentary rocks. As sand, silt, clay, and other such materials are moved from place to place by wind, water, waves, and other forces, the magnetic minerals are constantly reoriented. However, when these materials finally settle out and form permanent accumulations, the minerals orient themselves with Earth's magnetic axis as they settle. Therefore, these sediments, which may eventually become sedimentary rocks, preserve the orientation of Earth's magnetic field just as igneous rocks do.

Magnetization of minerals also occurs within rocky material during the chemical changes that result from metamorphism, or exposure to highly elevated temperature and pressures, which produces metamorphic rocks. Again, freedom of movement allows the minerals to become magnetized along Earth's existing magnetic lines of force.

The study of the orientation of magnetic minerals laid down in the past is, however, further complicated by the fact that more than one episode of magnetization may have affected a sample. For example, an igneous rock might be worn away by erosion and then re-deposited as a sedimentary rock. Then this sedimentary rock may be metamorphosed to produce a metamorphic rock, and then this rock may be exposed to another episode of metamorphism. Each of the metamorphic episodes has the potential to reorient the original sediments, or it may leave them relatively undisturbed. Recognizing the changes in the magnetic materials that occurred over millions of years within such a rock can be difficult.