Paleomagnetism is the study of the record of the
Earth's magnetic fieldpreserved in various magnetic minerals through time. The study of paleomagnetism has demonstrated that the Earth's magnetic fieldvaries 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
geomagnetismto 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 magnetic
north poleis 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 of magnetic declinationand magnetic inclinationand palaeointensity measurements.
* "Reversal magnetostratigraphy" examines the periodical
polarityreversion 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 spreadingzones and the dating of volcanic rocks.
Principles of remnant magnetization
The study of paleomagnetism is possible because
iron-bearing minerals such as magnetitemay 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
basaltand other igneousrocks 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 gabbroare 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. [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, another iron oxide. Redbeds, clasticsedimentary rocks (such as sandstones) that are red primarily because of hematite formation during or after sedimentary diagenesis, may have useful CRM signatures, and magnetostratigraphycan be based on such signatures.
Paleomagnetic evidence, both reversals and polar wandering data, was instrumental in verifying the theories of
continental driftand plate tectonicsin 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
homininbearing 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 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 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.
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paleomagnetism — paleomagnetísm s. n. Trimis de siveco, 10.08.2004. Sursa: Dicţionar ortografic PALEOMAGNETÍSM s.n. Caracteristicile câmpului magnetic terestru din trecutul geologic. [< fr. paléomagnétisme]. Trimis de LauraGellner, 05.07.2005. Sursa: DN … … Dicționar Român
paleomagnetism — [pā΄lē ō mag′nə tiz΄əm] n. 1. the alignment of iron and nickel grains in rock with the earth s magnetic poles, fixed at the time of that rock s formation 2. the study of this alignment, specif. as it relates to the shifting of the earth s… … English World dictionary
paleomagnetism — noun Date: 1854 1. the intensity and direction of residual magnetization in ancient rocks 2. a science that deals with paleomagnetism • paleomagnetic adjective • paleomagnetically adverb • paleomagnetist noun … New Collegiate Dictionary
paleomagnetism — paleomagnetic /pay lee oh mag net ik/ or, esp. Brit., /pal ee /, adj. /pay lee oh mag ni tiz euhm/ or, esp. Brit., /pal ee /, n. Geol. magnetic polarization acquired by the minerals in a rock at the time the rock was deposited or solidified. Cf.… … Universalium
paleomagnetism — noun The study of the strength and direction of the Earths magnetic field as it has changed over geologic time See Also: paleomagnetic, paleomagnetics … Wiktionary
paleomagnetism — Natural remanent magnetization preserved in rock sequences. During rock deposition magnetic minerals are aligned according to the direction and polarity of the earth’s contemporary magnetic field. After movement of the magnetic poles, or… … Lexicon of Cave and Karst Terminology
paleomagnetism — pa·leo·magnetism … English syllables
paleomagnetism — pa•le•o•mag•net•ism [[t]ˌpeɪ li oʊˈmæg nɪˌtɪz əm[/t]] esp. brit. [[t]ˌpæl i [/t]] n. gel magnetic polarization acquired by the minerals in a rock at the time the rock was deposited or solidified • Etymology: 1850–55 … From formal English to slang
paleomagnetism — … Useful english dictionary
Earth's magnetic field — Computer simulation of the Earth s field in a normal period between reversals. The tubes represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and… … Wikipedia