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Paleomagnetics: another method of dating and more

Earth’s magnetic field is internally generated by the rotation of the iron-rich core. For reasons that are not fully understood, the position of the magnetic pole with relation to the surface of the Earth is constantly changing. Furthermore, at intervals through Earth history, the polarity of the magnetic field “flips” such that a compass needle that now points north would point south instead. These changes in the polarity of the magnetic field are called “magnetic reversals,” and there have been 171 of them in the last 76 million years. The current polarity of the magnetic field (in which a compass needle points north) is called “normal” and the alternate state is called “reversed.” The study of how Earth’s magnetic field has changed through geologic time is called paleomagnetics.

When the magma from which igneous rocks form is still molten, iron-rich minerals can orient themselves in line with the local magnetic field in the same way that a compass needle does. As the magma cools, the tiny iron-rich crystals are “frozen” in position, recording the orientation of the local magnetic field at that time. Iron minerals in sediments can also align themselves with the magnetic field, then become fixed as the sediment turns to rock. The orientation of the magnetic crystals in rocks can be used to infer two pieces of information—the direction of the magnetic pole from the point where the rock formed, and the polarity (reversed or normal) of the magnetic field at the time the rock formed.

The orientation of the magnetic crystals in a rock can tell us the latitude at which the rock formed because the magnetic field is oriented almost parallel to the surface of the Earth near the equator, and almost perpendicular to it at the poles. By determining the positions of many rocks with respect to the magnetic pole, paleomagnetists can calculate the past positions of the continents. From that, they can deduce the directions in which continents have moved over millions of years. Adding in the age of the rocks, they can also calculate the speed at which continental plates have moved.

The polarity of the magnetic crystals in a rock also tells us if it was formed during a period with a reversed or normal magnetic field. By measuring the magnetic polarity of a succession of rocks, scientists can determine a sequence of polarity intervals of varying durations. Sometimes this sequence of polarity intervals can be distinctive—for example, a long interval of reversed polarity might be followed by a series of five quick alternations between normal and reversed, then another long interval of reversed polarity. Such local sequences can be compared with a globally compiled record of magnetic reversals, at least for the late Mesozoic and Cenozoic. Some magnetic reversal events have been dated radiometrically, so if the local sequence can be matched to the global record of magnetic reversals, the age of the local rocks can be determined.

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