Electric currents in the outer core of the Earth generate a magnetic field that, when time-averaged, is essentially dipolar. The geodynamo reversed itself in the course of geologic history several times with a reversal frequency that varied from zero to about 4-5 reversals per million years. Today we live in a period of normal polarity - the Brunhes Normal Chron - that was preceded, from about 0.8 to about 2.6 Ma, by a period of reverse polarity called Matuyama Reverse Chron. Polarity transitions are relatively rapid over geologic time.
Some rocks can register intensity and orientation of the magnetic field of the Earth during or shortly after their formation, retaining this information through geologic time. Paleomagnetists can therefore reconstruct the sequence of polarity reversals that occurred in the past and use this sequence as a tool for global correlations. Magnetostratigraphy is at the foundation of all modern time scales. It contributed to demonstrate, for example, that the mass extintion caused by an asteroid impact at the end of the Cretaceous was a global event that occurred in both the marine and continental environments.
Paleomagnetism is also used to detect and describe the motion of lithospheric plates. Magnetized rock particles behave like natural compasses. When sufficiently time-averaged, a rock net magnetization points in the direction of the geographic North Pole. Paleomagnetic poles of ancient rocks have been shown to disagree with the present-day geographic North Pole. This was taken in the early days of the plate tectonic revolution as evidence that indeed plates move with respect to the spin axis.
Paleomagnetism has a wide variety of geologic applications. Researchers at ALP apply Paleomagnetism to the resolution of geologic problems in the fields of Stratigraphy, Paleogeography, Paleoceanography, Volcanology, and Archeology. For further information about Paleomagnetism, see this online publication. Additional information can be also found at the AGU Geomagnetism and Paleomagnetism Section which includes a list of international paleomagnetic laboratories. ALP welcomes visiting scientists and students upon contact with one of the lab members.