Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of tectonic plates. The record of geomagnetic reversals preserved in volcanic and sedimentary rock sequences (magnetostratigraphy) provides a time-scale that is used as a geochronologic tool. Geophysicists who specialize in paleomagnetism are called paleomagnetists.
Paleomagnetists led the revival of the theory of continental drift and its transformation into plate tectonics.[1] Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading. Paleomagnetism continues to extend the history of plate tectonics back in time and are applied to the movement of continental fragments, or terranes.[2]
Paleomagnetism relied heavily on new developments in rock magnetism, which in turn has provided the foundation for new applications of magnetism. These include biomagnetism, magnetic fabrics (used as strain indicators in rocks and soils), and environmental magnetism.
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As early as the 18th century it was noticed that compass needles deviated near strongly magnetized outcrops. In 1797, Von Humboldt attributed this magnetization to lightning strikes (and lightning strikes do often magnetize surface rocks).[1][2] In the 19th century studies of the direction of magnetization in rocks showed that some recent lavas were magnetized parallel to the Earth's magnetic field. Early in the 20th century, work by David, Brunhes and Mercanton showed that many rocks were magnetized antiparallel to the field. Motonori Matuyama showed that the Earth's magnetic field reversed in the mid-Quaternary, a reversal now known as the Brunhes-Matuyama reversal.[1]
The British physicist P.M.S. Blackett provided a major impetus to paleomagnetism by inventing a sensitive astatic magnetometer in 1956. His intent was to test his theory that the geomagnetic field was related to the Earth's rotation, a theory that he ultimately rejected; but the astatic magnetometer became the basic tool of paleomagnetism and led to a revival of the theory of continental drift. Alfred Wegener first proposed in 1915 that continents had once been joined together and had since moved apart. Although he produced an abundance of circumstantial evidence, his theory met with little acceptance for two reasons: (1) no mechanism for continental drift was known, and (2) there was no way to reconstruct the movements of the continents over time. Keith Runcorn[3] and Edward A. Irving[4] constructed apparent polar wander paths for Europe and North America. These curves diverged, but could be reconciled if it was assumed that the continents had been in contact up to 200 million years ago. This provided the first clear geophysical evidence for continental drift. Then in 1963, Morley, Vine and Matthews showed that marine magnetic anomalies provided evidence for seafloor spreading.
Paleomagnetism is studied on a number of scales:
The study of paleomagnetism is possible because iron-bearing minerals such as magnetite may record past directions of the Earth's magnetic field. Paleomagnetic signatures in rocks can be recorded by three different mechanisms.
Iron-titanium oxide minerals in basalt and other igneous rocks may preserve the direction of the Earth's magnetic field when the rocks cool through the Curie temperatures of those minerals. The Curie temperature of magnetite, a spinel-group iron oxide, is about 580°C, whereas most basalt and gabbro are completely crystallized at temperatures above 900°C. Hence, the mineral grains are not rotated physically to align with the Earth's field, but rather they may record the orientation of that field. The record so preserved is called a thermoremanent magnetization (TRM). Because complex oxidation reactions may occur as igneous rocks cool after crystallization, the orientations of the Earth's magnetic field are not always accurately recorded, nor is the record necessarily maintained. Nonetheless, the record has been preserved well enough in basalts of the ocean crust to have been critical in the development of theories of sea floor spreading related to plate tectonics. TRM can also be recorded in pottery kilns, hearths, and burned adobe buildings. The discipline based on the study of thermoremanent magnetisation in archaeological materials is called archaeomagnetic dating.[5]
In a completely different process, magnetic grains in sediments may align with the magnetic field during or soon after deposition; this is known as detrital remanent magnetization (DRM). If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remanent magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional detrital remanent magnetization (pDRM).[6]
In a third process, magnetic grains grow during chemical reactions, and record the direction of the magnetic field at the time of their formation. The field is said to be recorded by chemical remanent magnetization (CRM). A common form of chemical remanent magnetization is held by the mineral hematite, another iron oxide. Hematite forms through chemical oxidation reactions of other minerals in the rock including magnetite. Redbeds, clastic sedimentary rocks (such as sandstones) are red because of hematite that formed during sedimentary diagenesis. The CRM signatures in redbeds can be quite useful and they are common targets in magnetostratigraphy studies.[7]
Remanence that is acquired at a fixed temperature is called isothermal remanent magnetization (IRM). Remanence of this sort is not useful for paleomagnetism, but it can be acquired as a result of lightning strikes. Lightning-induced remanent magnetization can be distinguished by its high intensity and rapid variation in direction over scales of centimeters.[8][9]
IRM is often induced in drill cores by the magnetic field of the steel core barrel. This contaminant is generally parallel to the barrel, and most of it can be removed by heating up to about 400 ℃ or demagnetizing in a small alternating field.
In the laboratory, IRM is induced by applying fields of various strengths and is used for many purposes in rock magnetism.
Paleomagnetists, like many geologists, gravitate towards outcrops because layers of rock are exposed. Road cuts are a convenient man-made source of outcrops.
There are two main goals of sampling:
One way to achieve the first goal is to use a rock coring drill that has a pipe tipped with diamond bits. The drill cuts a cylindrical space around some rock. This can be messy - the drill must be cooled with water, and the result is mud spewing out of the hole. Into this space is inserted another pipe with compass and inclinometer attached. These provide the orientations. Before this device is removed, a mark is scratched on the sample. After the sample is broken off, the mark can be augmented for clarity.[11]
Paleomagnetic evidence, both reversals and polar wandering data, was instrumental in verifying the theories of continental drift and plate tectonics in the 1960s and 70s. Some applications of paleomagnetic evidence to reconstructing histories of terranes have continued to arouse controversies. Paleomagnetic evidence also is used in constraining possible ages for rocks and processes and in reconstructions of the deformational histories of parts of the crust.
Reversal magnetostratigraphy is often used to estimate the age of fossil and hominin bearing sites.[12] Conversely, for a fossil of known age, the paleomagnetic data can fix the latitude at which the fossil was laid down. Such a paleolatitude provides information about the geological environment at the time of deposition.
Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved. For igneous rocks such as basalt, commonly used methods include potassium-argon and argon-argon geochronology.
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