Paleomagnetism

The phenomena observed by Delesse and Brunhes can be explained because of the fact that certain common iron-containing minerals are affected by any magnetic field, including that of Earth. Two of the most important of these minerals are the oxides of iron, magnetite (Fe₃O₄) and hematite (Fe₂O₃). When these minerals occur in molten rock, their atoms are free to move in such a way as to align themselves with Earth's magnetic field. When the rocks cool, the minerals are then frozen in position, oriented along the earth's magnetic north-south axis.

Magnetic minerals found in rocks today, however, are not necessarily oriented along Earth's present magnetic north-south axis. They may have shifted slightly in a vertical direction (their inclination or dip) or in a horizontal direction (their declination). The deviation of a mineral's orientation to the present magnetic field is of value in determining changes in Earth's structure in the past.

For example, a magnetic mineral originally laid down along the equator would have an inclination of 0°, while one laid down at one or the other of the poles would have an inclination of 90°. Suppose one finds a rock lying at 40° north latitude with minerals that have an inclination of 0°. Then, one might conclude that this rock originally was laid down along the equator and, by some means, it was transported northward by a distance of 40° of latitude.