paleomagnetism

Paleomagnetism is the study of the record of the Earth's magnetic field preserved in various magnetic minerals through time. The study of paleomagnetism has demonstrated that the Earth's magnetic field varies substantially in both orientation and intensity through time.

A paleomagnetist is a scientist who studies the ancient magnetic field by measuring the orientation of magnetic minerals in rocks and sediments, acquired at the time of their formation (remnant magnetization), then using methods similar to geomagnetism to determine what configuration of the Earth's magnetic field may have resulted in the observed orientation.

Fields of paleomagnetism

Paleomagnetism is studied on a number of scales:

• Secular variation studies look at small-scale changes in the direction and intensity of the Earth's magnetic field. The magnetic north pole is constantly shifting relative to the axis of rotation of the Earth. Magnetism is a vector and so magnetic field variation is made up of palaeodirectional measurements of magnetic declination and magnetic inclination and palaeointensity measurements.

• Reversal magnetostratigraphy examines the periodic polarity reversion of the Earth's magnetic field. The reversals have occurred at irregular intervals throughout the Earth's history. The age and pattern of these reversals is known from the study of sea floor spreading zones and the dating of volcanic rocks.

Principles of remnant magnetization

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.

Thermal remnant magnetization

First, 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 thermal remnant 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.^[1]

Detrital remnant magnetization

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 remnant magnetization (DRM). If the magnetization is acquired as the grains are deposited, the result is a depositional detrital remnant magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional detrital remnant magnetization (pDRM).

Chemical remnant magnetization

In a third process, magnetic grains may be deposited from a circulating solution, or be formed during chemical reactions, and may record the direction of the magnetic field at the time of mineral formation. The field is said to be recorded by chemical remnant magnetization (CRM). The mineral recording the field commonly is hematite, another iron oxide. Redbeds, clastic sedimentary rocks (such as sandstones) that are red primarily because of hematite formation during or after sedimentary diagenesis, may have useful CRM signatures, and magnetostratigraphy can be based on such signatures.

Examples

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.^[2]

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.

History of paleomagnetic studies

The oldest magnetizations early paleomagnetic studies were able to measure were approximately 250 Ma old (the oldest oceanic crust). Today refined methods can be used to provide field information for dating of rocks as old as four Ga.

One of the pioneering scientists who studied paleomagnetism was the British physicist P.M.S. Blackett.

Edward A. Irving, a Canadian paleomagnetism specialist, used paleomagnetic studies to support plate tectonics in the 1950s. The method of identifying polar reversals by examination of oceanic crust was further developed by Frederick John Vine.

See also

• Geophysics

References

1. ^ Herries, A.I.R., Kovacheva, M., Kostadinova, M., Shaw, J., 2007. Archaeo-directional and -intensity data from burnt structures at the Thracian site of Halka Bunar (Bulgaria): The effect of magnetic mineralogy, temperature and atmosphere of heating in antiquity, Physics of the Earth and Planetary Interiors. 162, 199-216.
2. ^ Herries, A.I.R., Adams, J.W., Kuykendall, K.L., Shaw, J., 2006. Speleology and magnetobiostratigraphic chronology of the GD 2 locality of the Gondolin hominin-bearing paleocave deposits, North West Province, South Africa, J. Human Evolution. 51, 617-631.

• Butler, R., 1998, Paleomagnetism: Magnetic Domains to Geologic Terranes

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