Paleomagnetism is the study of the magnetic properties of rocks. Paleomagnetism aids in our understanding of plate tectonics, minerology, petrogenesis, geochronology, and the history of the Earth's magnetic (the "geomagnetic") field.
Paleomagnetism is possible because some magnetic minerals common in igneous rocks, such as magnetite, will "acquire" the magnetic field of their surroundings as the rock cools. At very high temperatures, magnetic minerals are highly susceptible to the magnetic field of their surroundings. When the magnetic mineral cools through its "Curie point" temperature (which varies from mineral to mineral), the surrounding magnetic field becomes "frozen," and the magnetic mineral is magnetized according to the surrounding field at the time it cooled below its Curie point. Paleomagnetism can also be studied in some water-born sedimentary rocks, as magnetically charged detrital particles will tend to become aligned with the geomagnetic field when settling in an aqueous environment.
The magnetization of a rock when it is sampled is called its natural remanent magnetization (NRM). The NRM of a rock will be the sum of the various forces which have acted upon a rock throughout its history. Usually the primary influence upon NRM will be the intensity and direction of the geomagnetic field at the location of the rock at the time it formed. Other forces can include heating events, tectonic events, and lightning strikes.
The geomagnetic field approximates a magnetic dipole. The first order shape of the geomagnetic field is thus what would be expected if there were a giant bar magnet in the center of the Earth, aligned with the spin axis. Currently, the north pole of the bar magnet would be in the arctic region. However, the field is thought to be generated by convection currents in the liquid outer core.
Periodically, the geomagnetic field shifts its "polarity," that is, there have been times in the past where magnetic north on a compass would point in the direction of the current position of Antarctica. The exact causes of geomagnetic polarity "reversals" are not well-understood, however their presence has aided greatly in constructing the "geomagnetic timescale."
The geomagnetic timescale is a record of the periods of "normal" (where magnetic north is located approximately where it is today), and "reversed" (where magnetic north is located somewhere around the present position of Antarctica) polarity. Paleomagnetic studies of rocks can determine whether a rock cooled during a period of normal or reversed polarity, which can then help date the rock to the proper period of the geomagnetic timescale.
Paleomagnetic studies generally sample the magnetic properties of rocks in one of two ways:
1. Take direct samples in the field by drilling rock cores. The spatial orientation of the core samples is carefully measured while in the field. Samples can then be prepared in the lab and measured in highly sensitive magnetometers. Measuring techniques can then determine both the direction and intensity of the geomagnetic field at the position the rock was located at the time that it cooled.
2. Paleomagnetics can also be studied via remote sampling, typically using a magnetometer towed behind a ship. When large zones of the Earth's crust are uniformly magnetized, as is the case when oceanic crust cools as it moves away from a spreading center, the crustal NRM can then create a detectable anomaly in the geomagnetic field at the ocean surface. Ship-towed magnetometers can then measure this crustal magnetic anomaly and determine the polarity of the geomagnetic field at the time the crust cooled. This technique can also been used for interplanetary paleomagnetic studies, such as those using magnetometers on satellites orbiting Mars.
Magnetic inclination also changes with latitude and the thus paleolatitude of the rock at the time it formed can also be reconstructed by measuring the NRM. This can aid greatly in reconstructing plate tectonic history. On the other hand, if the position of the rock at the time it formed is already known, paleomagnetic studies can aid in constructing a "paleopole," or the position of magnetic north at the time the rock cooled. Such studies can help elucidate the shape and orientation of the geomagnetic field throughout time.
Web Resources On Paleomagnetism
Paleomagnetism and the Privileged Planet by Jay W. Richards and Guillermo Gonzalez.
Book Resources On Paleomagnetism
Paleomagnetic Principles and Practice by Lisa Tauxe