Paleomagnetism

Even when the effect of reversal and change of location of the earth's magnetic poles are taken into consideration, deviations of magnetic minerals in rocks from true north are still observed. In some cases, this deviation is very great. Since the 1960s, scientists have believed that the reason for these variations is that large chunks of Earth's surface have moved significant distances across the planet's face over millions of years.

In accord with plate tectonic theory, Earth's crust and upper mantle, or lithosphere, consists of about 20 large segments, known as plates, that are about 60 mi (100 km) thick and thousands of miles wide. These plates slide back and forth on top of a lower layer of material known as the asthenosphere. The plates collide with each other head on, slide back and forth against each other, and pull apart from each other. Significant geological events, such as volcanoes and earthquakes, are produced.

One of the strongest pieces of evidence for plate tectonics has been paleomagnetism. Evidence has shown, for example, that some rocks in Alaska have magnetic minerals oriented in such a way that they must have been laid down at or near the equator. The fact that they are now at 70° north latitude suggests strongly that the plate on which they are riding must have migrated a very long distance during Earth history.

Paleomagnetism can also be used to match up land masses that are now separated from each other, but which must once have been joined. For example, the orientation of magnetic minerals along the eastern coast of South America very closely matches that of similar minerals on the western coast of Africa. This correlation, taken with other evidence, provides strong support for the notion that South America and Africa were once joined together as a single land mass.

One of the most remarkable successes of paleomagnetism has been in the study of sea floor spreading. Mid-oceanic ridge-rift systems are areas in the oceans where the edges of two plates, and any continents that may be on them, are being forced away from each other by currents in the underlying asthenosphere. Magma from the asthenosphere is pushed up from below the rift to fill in the void created by spreading and to create new ocean floor.

Strong evidence for this theory has come from the study of paleomagnetism on either side of these rifts. Magnetometers dragged by survey ships sailing above the rifts have found that the patterns of orientation of magnetic minerals on either side of a rift are mirror images of each other. Patterns of high and low intensity and specific inclination and declination running parallel to the rift on one side are exactly matched by similar patterns on the opposite side. This pattern could exist only if new rock were being formed simultaneously on either side of the rift, as suggested by the above theory.