MAGNETISM IN ROCKS
Earth is a giant magnet. Like other magnets that you are familiar with, Earth has two magnetic poles, which we call north and south, and a magnetic field that attracts charged or magnetic objects. At present, these magnetic poles are close to, but not identical with, the geographical poles. Thus, we can use a magnetic compass, adjusted for the difference between magnetic and true north, to tell us which way north is. The magnetic field has not always been in its present configuration. At times in the past, the poles have reversed or changed direction so that the north magnetic pole becomes south and vice versa. There does not appear to be any regularity to these magnetic reversals, as they are called, nor do we know how reversals occur. Speculation is that the magnetic field weakens; becoming virtually nil, then builds up again. When the magnetic field re-builds, there is an equal probability that it will be either in the same orientation or in the reversed direction. If the field has the same orientation as previously, there is no reversal. A reversal occurs only when the magnetic field orientation changes to the opposite direction.
Past reversals of the magnetic field are recorded in the rocks. Many rocks contain iron-bearing minerals that act as tiny magnets. As magma or lava cool, these minerals begin to form. At this point the molten rock has not completely solidified, so the magnetic minerals floating in the molten mass, become aligned to the magnetic field. When the rock finally solidifies, these minerals lock in the magnetic field as so many tiny compasses. Sedimentary rocks also have a magnetic record. As iron bearing sedimentary minerals are deposited from the water column, they also become aligned with the existing magnetic field. The magnetism remains locked in the rock unless the rock is subsequently heated above the Curie Point, the temperature at which all magnets lose their magnetism. When the heated rock again cools below the Curie Point, it will record the magnetic field at this later time, and the old magnetic field will be lost. It is important, therefore, to establish that a rocks magnetism is primary and has not been re-set at a later time.