- Paleomagnetism
Paleomagnetism is the study of the record of the
Earth's magnetic field preserved in variousmagnetic mineral s through time. The study of paleomagnetism has demonstrated that the Earth'smagnetic field varies substantially in both orientation and intensity through time.Paleomagnetists study 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 magentic field. The magneticnorth pole is constantly shifting relative to the axis of rotation of the Earth. Magnetism is a vector and so magentic field variation is made up of palaeodirectional measurements ofmagnetic declination andmagnetic inclination and palaeointensity measurements.* "Reversal magnetostratigraphy" examines the periodical
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 ofsea floor spreading zones and the dating of volcanic rocks.Principles of remnant magnetization
The study of paleomagnetism is possible because
iron -bearing minerals such asmagnetite 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 otherigneous rocks may preserve the direction of the Earth's magnetic field when the rocks cool through theCurie temperature s of those minerals. The Curie temperature ofmagnetite , aspinel -groupiron oxide , is about 580°C, whereas most basalt andgabbro 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 toplate tectonics . TRM can also be recorded inpottery kiln s, hearths, and burned adobe buildings. The discipline based on the study of thermoremanent magnetisation in archaeological materials is calledarchaeomagnetic dating . [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.]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 detrial 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 , anotheriron oxide . Redbeds,clastic sedimentary rocks (such assandstone s) that are red primarily because of hematite formation during or after sedimentarydiagenesis , may have useful CRM signatures, andmagnetostratigraphy can be based on such signatures.Examples
Paleomagnetic evidence, both reversals and polar wandering data, was instrumental in verifying the theories of
continental drift andplate tectonics in the 1960s and 70s. Some applications of paleomagnetic evidence to reconstructing histories ofterrane s 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. [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.]Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved. For
igneous rock s such asbasalt , 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 4 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.ee also
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Geophysics References
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