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List of shields and cratons

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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, 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 a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle.

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43-522: A craton is an ancient part of the Earth's continental crust which has been more or less stable since Precambrian times. Cratons whose ancient rocks are widely exposed at the surface, often with relatively subdued relief, are known as shields . If the ancient rocks are largely overlain by a cover of younger rocks then the 'hidden' craton may be referred to as a platform . Listed by modern continent and Gondwana , include: Craton The term craton

86-400: 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

129-589: A few billion years in age. It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic . Studies by Condie (2000) and Rino et al. (2004) harvp error: no target: CITEREFRinoKomiyaWindleyet_al2004 ( help ) suggest that crust production happened episodically. By isotopically calculating the ages of Proterozoic granitoids it

172-429: A few plausible models that explain tectonics of the early Earth prior to the formation of Columbia, but the current most plausible hypothesis is that prior to Columbia, there were only a few independent cratons scattered around the Earth (not necessarily a supercontinent, like Rodinia or Columbia). The Proterozoic can be roughly divided into seven biostratigraphic zones which correspond to informal time periods. The first

215-526: A period of increasing crustal recycling, suggesting subduction . Evidence for this increased subduction activity comes from the abundance of old granites originating mostly after 2.6  Ga . The occurrence of eclogite (a type of metamorphic rock created by high pressure, > 1 GPa), is explained using a model that incorporates subduction. The lack of eclogites that date to the Archean Eon suggests that conditions at that time did not favor

258-457: 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

301-1310: 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 , 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

344-452: 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

387-553: 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 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

430-535: The Baltic Shield had been eroded into a subdued terrain already during the Late Mesoproterozoic when the rapakivi granites intruded. Proterozoic The Proterozoic ( IPA : / ˌ p r oʊ t ər ə ˈ z oʊ ɪ k , ˌ p r ɒ t -, - ər oʊ -, - t r ə -, - t r oʊ -/ PROH -tər-ə- ZOH -ik, PROT-, -⁠ər-oh-, -⁠trə-, -⁠troh- ) is the third of

473-406: 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

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516-737: The Neoproterozoic Era at the end of the Proterozoic Eon, possibly climaxing with the hypothesized Snowball Earth of the Sturtian and Marinoan glaciations. One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth's atmosphere. Though oxygen is believed to have been released by photosynthesis as far back as the Archean Eon, it could not build up to any significant degree until mineral sinks of unoxidized sulfur and iron had been exhausted. Until roughly 2.3 billion years ago, oxygen

559-592: The Palaeoproterozoic or Mesoproterozoic, according to molecular data. Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids , as well as the animal-like Caveasphaera , appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, particularly in ones from

602-486: The breakup of the supercontinent Columbia and prior to the assemblage of the supercontinent Gondwana (~500 Ma). The defining orogenic event associated with the formation of Gondwana was the collision of Africa, South America, Antarctica and Australia forming the Pan-African orogeny . Columbia was dominant in the early-mid Proterozoic and not much is known about continental assemblages before then. There are

645-502: The craton from sinking into the deep mantle. Cratonic lithosphere 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

688-413: 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

731-399: 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

774-481: 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

817-522: The deciphering of Precambrian Supereon tectonics. It is known that tectonic processes of the Proterozoic Eon resemble greatly the evidence of tectonic activity, such as orogenic belts or ophiolite complexes, we see today. Hence, most geologists would conclude that the Earth was active at that time. It is also commonly accepted that during the Precambrian, the Earth went through several supercontinent breakup and rebuilding cycles ( Wilson cycle ). In

860-426: 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

903-426: The eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity ( orogeny ). There is evidence that the first known glaciations occurred during the Proterozoic. The first began shortly after the beginning of the Proterozoic Eon, and evidence of at least four during

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946-757: The evolution of eukaryotes via symbiogenesis ; several global glaciations , which produced the 300 million years-long Huronian glaciation (during the Siderian and Rhyacian periods of the Paleoproterozoic) and the hypothesized Snowball Earth (during the Cryogenian period in the late Neoproterozoic); and the Ediacaran period (635–538.8  Ma ), which is characterized by the evolution of abundant soft-bodied multicellular organisms such as sponges , algae , cnidarians , bilaterians and

989-735: The first half of the Ediacaran from 0.63–0.55 Ga, and the Belomorian, spanning from 0.55–0.542 Ga. The emergence of advanced single-celled eukaryotes began after the Oxygen Catastrophe . This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria . It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved. By

1032-419: The formation of high grade metamorphism and therefore did not achieve the same levels of subduction as was occurring in the Proterozoic Eon. As a result of remelting of basaltic oceanic crust due to subduction, the cores of the first continents grew large enough to withstand the crustal recycling processes. The long-term tectonic stability of those cratons is why we find continental crust ranging up to

1075-575: The four geologic eons of Earth's history , spanning the time interval from 2500 to 538.8   Mya , the longest eon of the Earth's geologic time scale . It is preceded by the Archean and followed by the Phanerozoic , and is the most recent part of the Precambrian "supereon". The Proterozoic is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic , Mesoproterozoic and Neoproterozoic . It covers

1118-471: 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

1161-665: The late Palaeoproterozoic, eukaryotic organisms had become moderately biodiverse. The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria – in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1.2 billion years ago. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic Era, some 2.4 billion years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis . The Viridiplantae evolved sometime in

1204-625: The late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma). It consisted of a series of continents attached to a central craton that forms the core of the North American Continent called Laurentia . An example of an orogeny (mountain building processes) associated with the construction of Rodinia is the Grenville orogeny located in Eastern North America. Rodinia formed after

1247-468: 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

1290-420: 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

1333-495: 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

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1376-554: The sessile Ediacaran biota (some of which had evolved sexual reproduction ) and provides the first obvious fossil evidence of life on Earth . The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas ; furthermore, many of those rocks are less metamorphosed than Archean rocks, and many are unaltered. Studies of these rocks have shown that

1419-515: 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

1462-606: The time from the appearance of free oxygen in Earth's atmosphere to just before the proliferation of complex life on the Earth during the Cambrian Explosion . The name Proterozoic combines two words of Greek origin: protero- meaning "former, earlier", and -zoic , meaning "of life". Well-identified events of this eon were the transition to an oxygenated atmosphere during the Paleoproterozoic;

1505-567: The time was virtually all obligate anaerobic . A second, later surge in oxygen concentrations is called the Neoproterozoic Oxygenation Event , occurred during the Middle and Late Neoproterozoic and drove the rapid evolution of multicellular life towards the end of the era. The Proterozoic Eon was a very tectonically active period in the Earth's history. The late Archean Eon to Early Proterozoic Eon corresponds to

1548-503: The typical 100 km (60 mi) thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into 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

1591-455: 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

1634-473: 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

1677-444: Was determined that there were several episodes of rapid increase in continental crust production. The reason for these pulses is unknown, but they seemed to have decreased in magnitude after every period. Evidence of collision and rifting between continents raises the question as to what exactly were the movements of the Archean cratons composing Proterozoic continents. Paleomagnetic and geochronological dating mechanisms have allowed

1720-423: 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

1763-511: Was probably due to two factors: Exhaustion of the chemical sinks, and an increase in carbon sequestration , which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. The first surge in atmospheric oxygen at the beginning of the Proterozoic is called the Great Oxygenation Event , or alternately the Oxygen Catastrophe – to reflect the mass extinction of almost all life on Earth, which at

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1806-487: Was probably only 1% to 2% of its current level. The banded iron formations , which provide most of the world's iron ore , are one mark of that mineral sink process. Their accumulation ceased after 1.9 billion years ago, after the iron in the oceans had all been oxidized . Red beds , which are colored by hematite , indicate an increase in atmospheric oxygen 2 billion years ago. Such massive iron oxide formations are not found in older rocks. The oxygen buildup

1849-761: Was the Labradorian, lasting from 2.0–1.65  Ga . It was followed by the Anabarian, which lasted from 1.65–1.2 Ga and was itself followed by the Turukhanian from 1.2–1.03 Ga. The Turukhanian was succeeded by the Uchuromayan, lasting from 1.03–0.85 Ga, which was in turn succeeded by the Yuzhnouralian, lasting from 0.85–0.63 Ga. The final two zones were the Amadeusian, spanning

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