Paleomagnetism refers to the study of the record of the Earth's magnetic field preserved in various magnetic minerals through time. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific The study of paleomagnetism has demonstrated that the Earth's magnetic field varies substantially in both orientation and intensity through time. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges
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. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see
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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. ' Polar wander' is the phenomenon in which the North and South Poles of the Earth move with respect to the continents The magnetic north pole is constantly shifting relative to the axis of rotation of the Earth. The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is subject to the caveats explained below defined as the point in the northern Magnetism is a vector and so magentic field variation is made up of palaeodirectional measurements of magnetic declination and magnetic inclination and palaeointensity measurements. The magnetic declination (also known as grid magnetic angle in military circles at any point on the Earth is the angle between the local magnetic field -- the direction Magnetic dip or magnetic inclination is the angle made by a compass needle with the horizontal at any point on the Earth's surface
- Reversal Magnetostratigraphy examines the periodical polarity reversion of the Earth's magnetic field. A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged The reversals have occurred at irregular intervals throughout the Earth's history. The history of Earth covers approximately 46 billion years (4567000000 years from Earth ’s formation out of the Solar nebula to the present The age and pattern of these reversals is known from the study of sea floor spreading zones and the dating of volcanic rocks. Seafloor spreading occurs at Mid-ocean ridges where new Oceanic crust is formed through volcanic activity and then gradually moves away from the ridge
Principles of remanent magnetization
The study of paleomagnetism is possible because iron-bearing minerals such as magnetite may record past directions of the Earth's magnetic field. Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical Paleomagnetic signatures in rocks can be recorded by three different mechanisms.
Thermal remanent 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. Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock The Curie point ( Tc) or Curie temperature, is a term in Physics and Materials science, named after Pierre Curie (1859-1906 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. Magnetite is not to be confused with Magnesite or Maghemite. Magnetite is a ferrimagnetic Mineral with chemical The spinels are any of a class of Minerals of general formulation XY2 O 4 which Crystallize in the cubic (isometric Altogether there are sixteen known Iron Oxides and oxyhydroxides Gabbro (ˈɡæbrəʊ is a dark coarse-grained intrusive Igneous rock chemically equivalent to Basalt. 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 remanent 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. Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere TRM can also be recorded in pottery kilns, hearths, and burned adobe buildings. Pottery is the Ceramic ware made by potters It also refers to a group of materials that includes Earthenware, Stoneware Kilns are thermally insulated chambers or Ovens in which controlled temperature regimes are produced The discipline based on the study of thermoremanent magnetisation in archaeological materials is called archaeomagnetic dating. Archaeomagnetism dating (adjective 'archaeomagnetic' is the science of how to interpret signatures of the Earth's magnetic field at past times that are recorded in archaeological [1]
Detrital remanent 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 remanent magnetization (DRM). If the magnetization is acquired as the grains are deposited, the result is a depositional detrial remanent magnetization (dDRM); if it is acquired soon after deposition, it is a post-depositional detrital remanent magnetization (pDRM).
Chemical remanent 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 remanent magnetization (CRM). The mineral recording the field commonly is hematite, another iron oxide. Hematite, also spelt hæmatite, is the Mineral form of Iron(III oxide (Fe2O3 one of several Iron oxides Altogether there are sixteen known Iron Oxides and oxyhydroxides 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. Clastic rocks are composed of fragments or clasts, of pre-existing rock. Sandstone is a Sedimentary rock composed mainly of Sand -size Mineral or rock grains. In Geology and Oceanography, diagenesis is any chemical physical or biological change undergone by a Sediment after its initial deposition and during Stratigraphy, a branch of Geology, studies rock layers and layering ( stratification)
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. Continental drift is the movement of the Earth 's Continents relative to each other Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Some applications of paleomagnetic evidence to reconstructing histories of terranes have continued to arouse controversies. A terrane in Geology is a fragment of crustal material formed on or broken off from one Tectonic plate and accreted — " sutured " 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. Hominini is the tribe of Homininae that comprises humans ( Homo) Chimpanzees ( Pan) and their Extinct ancestors [2]
Paleomagnetic studies are combined with geochronological methods to determine absolute ages for rocks in which the magnetic record is preserved. In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils For igneous rocks such as basalt, commonly used methods include potassium-argon and argon-argon geochronology. Igneous rocks (etymology from Latin ignis, fire are rocks formed by solidification of cooled Magma (molten rock Basalt (bəˈsɔːlt ˈbeisɔːlt ˈbæsɔːlt is a common Extrusive Volcanic rock. Potassium-argon dating or K-Ar dating is a Radiometric dating method used in Geochronology and Archeology.
History of paleomagnetic studies
The oldest magnetizations early paleomagnetic studies were able to measure were approximately 250 Ma old (the oldest oceanic crust). Annum is one form of the Latin noun meaning Year, not a form normally used for derivatives in modern languages the accusative singular Oceanic crust is the part of Earth's Lithosphere that surfaces in the Ocean basins Today refined methods can be used to provide field information for dating of rocks as old as 4 Ga. In Astronomy, Geology, and Paleontology, Gya (for Giga years ago is often used as an unit of Time to denote length of
One of the pioneering scientists who studied paleomagnetism was the British physicist P.M.S. Blackett. Patrick Maynard Stuart Blackett Baron Blackett OM CH FRS ( 18 November 1897 &ndash 13 July 1974) was an
Edward A. Irving, a Canadian paleomagnetism specialist, used paleomagnetic studies to support plate tectonics in the 1950s. Edward A "Ted" Irving (born 1927 CM, ScD, FRSC, FRS is a geologist and Emeritus scientist with the Geological The method of identifying polar reversals by examination of oceanic crust was further developed by Frederick John Vine. Frederick John Vine (born June 17, 1939) is a marine geologist and Geophysicist and was a key contributor to the theory of Plate tectonics.
References
- ^ 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.
- ^ 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.
See also
Geophysics, a major discipline of Earth sciences, is the study of the Earth by quantitative physical methods especially by seismic, electromagneticDapyx Software network: MP3 Explorer | Ebook Manager | Zenithic