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33-638: The Greenland plate is a tectonic microplate bounded to the west by Nares Strait , a probable transform fault ; on the southwest by the Ungava transform underlying Davis Strait ; on the southeast by the Mid-Atlantic Ridge ; and the northeast by the Gakkel Ridge , with its northwest border still being explored. The Greenland craton is made up of some of the oldest rocks on Earth. The Isua greenstone belt in southwestern Greenland contains

66-400: A rising plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons. A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock. A fourth theory presented in a 2015 publication suggests that the origin of

99-462: A separate plate at all. The area between Greenland and Baffin Island is, however, seismically very active, being the location of the epicenter of many earthquakes including a 7.3-magnitude earthquake in 1933 . As of 2009, scientists have been unable to correlate the seismicity with particular geological structures or geophysical anomalies. It has been suggested that seismicity in the region is related to

132-457: A solid residue very close in composition to Archean lithospheric mantle, but continental shields do not contain enough komatiite to match the expected depletion. Either much of the komatiite never reached the surface, or other processes aided craton root formation. There are many competing hypotheses of how cratons have been formed. Jordan's model suggests that further cratonization was a result of repeated continental collisions. The thickening of

165-465: A tectonic plate world map. For purposes of this list, a microplate is any plate with an area less than 1 million km . Some models identify more minor plates within current orogens (events that lead to a large structural deformation of Earth's lithosphere ) like the Apulian, Explorer, Gorda, and Philippine Mobile Belt plates. The latest studies have shown that microplates are the basic elements of which

198-408: A terrane may not contain the full thickness of the lithosphere. Craton A craton ( / ˈ k r eɪ t ɒ n / KRAYT -on , / ˈ k r æ t ɒ n / KRAT -on , or / ˈ k r eɪ t ən / KRAY -tən ; from ‹See Tfd› Greek : κράτος kratos "strength") is an old and stable part of the continental lithosphere , which consists of Earth's two topmost layers,

231-413: A thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle. The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: a continental shield , in which the basement rock crops out at the surface, and a platform which overlays

264-418: Is a list of ancient cratons , microplates , plates , and terranes which no longer exist as separate plates. Cratons are the oldest and most stable parts of the continental lithosphere, and shields are exposed parts of them. Terranes are fragments of crustal material formed on one tectonic plate and accreted to crust lying on another plate, which may or may not have originated as independent microplates:

297-435: Is much older than oceanic lithosphere—up to 4 billion years versus 180 million years. Rock fragments ( xenoliths ) carried up from the mantle by magmas containing peridotite have been delivered to the surface as inclusions in subvolcanic pipes called kimberlites . These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite

330-452: Is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. Harzburgite peridotites represent

363-1126: The East European Craton , the Amazonian Craton in South America, the Kaapvaal Craton in South Africa, the North American Craton (also called the Laurentia Craton), and the Gawler Craton in South Australia. Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into

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396-718: The North Atlantic-Labrador Sea rift system that started forming 140 million years ago in the Early Cretaceous epoch . The Labrador Sea started opening 69 million years ago during the Maastrichtian age but seafloor spreading appears to have ceased by the Oligocene epoch, 30–35 million years ago. Correlations between tectonic units in Canada and Greenland have been proposed; however,

429-423: The asthenosphere , and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons. Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy and a low intrinsic density. This low density offsets density increases from geothermal contraction and prevents the craton from sinking into the deep mantle. Cratonic lithosphere

462-451: The crust and the uppermost mantle . Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates ; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock , which may be covered by younger sedimentary rock . They have

495-406: The "cratonic regime". It involves processes of pediplanation and etchplanation that lead to the formation of flattish surfaces known as peneplains . While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over geological time leads to the formation of so-called polygenetic peneplains of mixed origin. Another result of

528-664: The bulk of the continents and the Pacific Ocean . For purposes of this list, a major plate is any plate with an area greater than 20 million km (7.7 million sq mi) These smaller plates are often not shown on major plate maps, as the majority of them do not comprise significant land area. For purposes of this list, a minor plate is any plate with an area less than 20 million km (7.7 million sq mi) but greater than 1 million km (0.39 million sq mi). These plates are often grouped with an adjacent principal plate on

561-465: The cratons is similar to crustal plateaus observed on Venus, which may have been created by large asteroid impacts. In this model, large impacts on the Earth's early lithosphere penetrated deep into the mantle and created enormous lava ponds. The paper suggests these lava ponds cooled to form the craton's root. The chemistry of xenoliths and seismic tomography both favor the two accretional models over

594-399: The crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy . Jordan likens this model to "kneading" of the cratons, allowing low density material to move up and higher density to move down, creating stable cratonic roots as deep as 400 km (250 mi). A second model suggests that the surface crust was thickened by

627-420: The crust is composed and that the larger plates are composed of amalgamations of these, and a subdivision of ca. 1200 smaller plates has come forward. In the history of Earth, many tectonic plates have come into existence and have over the intervening years either accreted onto other plates to form larger plates, rifted into smaller plates, or have been crushed by or subducted under other plates. The following

660-481: The crystalline residues after extraction of melts of compositions like basalt and komatiite . The process by which cratons were formed is called cratonization . There is much about this process that remains uncertain, with very little consensus in the scientific community. However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds , which originate in

693-426: The depleted "lid" formed by the first layer. The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either. All these proposed mechanisms rely on buoyant, viscous material separating from a denser residue due to mantle flow, and it is possible that more than one mechanism contributed to craton root formation. The long-term erosion of cratons has been labelled

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726-471: The late Archean, accompanied by voluminous mafic magmatism. However, melt extraction alone cannot explain all the properties of craton roots. Jordan notes in his paper that this mechanism could be effective for constructing craton roots only down to a depth of 200 kilometers (120 mi). The great depths of craton roots required further explanation. The 30 to 40 percent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and

759-468: The longevity of cratons is that they may alternate between periods of high and low relative sea levels . High relative sea level leads to increased oceanicity, while the opposite leads to increased inland conditions . Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn Craton of Western Australia was flattish already by Middle Proterozoic times and

792-657: The oldest known rocks on Earth dated at 3.7–3.8 billion years old. The Precambrian basement of Greenland formed an integral part of the Laurentian Shield that is at the core of the North American continent . Greenland was formed in two rifting stages from the main body of North America. The first, during the Cretaceous period, formed Baffin Bay . Baffin Bay is the northwestern extension and terminus of

825-420: The plume model. However, other geochemical evidence favors mantle plumes. Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than 150 km (93 mi) and may be Archean, while the second is found at depths from 180 to 240 km (110 to 150 mi) and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against

858-569: The pre-spreading fit of Greenland to Canada is still not accurately known. A sinistral transtensive rifting which was proposed with NNE-SSW trending mobile transfer zones fits Greenland to Canada directly in a southward direction. Since the closure of the North Atlantic–Labrador Sea rift, Greenland has moved roughly in conjunction with North America; thus, there are questions as to whether the Greenland plate should still be considered

891-550: The roots of cratons, and which are almost always over 2 billion years and often over 3 billion years in age. Rock of Archean age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, this suggests that only 5 to 40 percent of the present continental crust formed during the Archean. Cratonization likely was completed during the Proterozoic . Subsequent growth of continents

924-602: The shield in some areas with sedimentary rock . The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as Kratogen , referring to stable continental platforms, and orogen as a term for mountain or orogenic belts . Later Hans Stille shortened the former term to Kraton , from which craton derives. Examples of cratons are the Dharwar Craton in India, North China Craton ,

957-452: The stresses associated with post-glacial rebound . List of tectonic plates#Microplates Geologists generally agree that the following tectonic plates currently exist on Earth's surface with roughly definable boundaries. Tectonic plates are sometimes subdivided into three fairly arbitrary categories: major (or primary ) plates , minor (or secondary ) plates , and microplates (or tertiary plates ). These plates comprise

990-515: The surrounding hotter, but more chemically dense, mantle. In addition to cooling the craton roots and lowering their chemical density, the extraction of magma also increased the viscosity and melting temperature of the craton roots and prevented mixing with the surrounding undepleted mantle. The resulting mantle roots have remained stable for billions of years. Jordan suggests that depletion occurred primarily in subduction zones and secondarily as flood basalts . This model of melt extraction from

1023-455: The upper mantle has held up well with subsequent observations. The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans. The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity. Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into

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1056-473: Was by accretion at continental margins. The origin of the roots of cratons is still debated. However, the present understanding of cratonization began with the publication in 1978 of a paper by Thomas H. Jordan in Nature . Jordan proposes that cratons formed from a high degree of partial melting of the upper mantle, with 30 to 40 percent of the source rock entering the melt. Such a high degree of melting

1089-423: Was possible because of the high mantle temperatures of the Archean. The extraction of so much magma left behind a solid peridotite residue that was enriched in lightweight magnesium and thus lower in chemical density than undepleted mantle. This lower chemical density compensated for the effects of thermal contraction as the craton and its roots cooled, so that the physical density of the cratonic roots matched that of

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