Craton

{{World geologic provinces}}

The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable.{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=craton}} Cratons are composed of two layers: a continental shield, in which the basement rock crops out at the surface,{{sfn|Jackson|1997|loc="shield [tect]"}} and a platform which overlays the shield in some areas with sedimentary rock.{{sfn|Jackson|1997|loc="platform [tect]"}}

The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as {{lang|de|Kratogen}}, referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later Hans Stille shortened the former term to {{lang|de|Kraton}}, from which craton derives.{{cite book |last1=Şengör |first1=A.M.C. |author-link=Celâl Şengör |date=2003 |title=The Large-wavelength Deformations of the Lithosphere: Materials for a history of the evolution of thought from the earliest times to plate tectonics|series=Geological Society of America memoir |volume=196 |page=331}}

Examples of cratons are the Dharwar Craton{{cite journal |last1=Ratheesh-Kumar |first1=R.T. |last2=Windley |first2=B.F. |last3=Xiao |first3=W.J. |last4=Jia |first4=X-L. |last5=Mohanty |first5=D.P. |last6=Zeba-Nezrin |first6=F.K. |title=Early growth of the Indian lithosphere: implications from the assembly of the Dharwar Craton and adjacent granulite blocks, southern India |journal=Precambrian Research |date=October 2019 |volume=336 |pages=105491 |doi=10.1016/j.precamres.2019.105491 |s2cid=210295037 |doi-access= }} in India, North China Craton,{{Cite journal|last1=Kusky|first1=T. M.|last2=Windley|first2=B. F.|last3=Zhai|first3=M.-G.|year=2007|title=Tectonic evolution of the North China Block: from orogen to craton to orogen|journal=Geological Society, London, Special Publications|volume=280|issue=1|pages=1–34|doi=10.1144/sp280.1|bibcode=2007GSLSP.280....1K|s2cid=129902429}} the East European Craton,{{cite journal |last1=Artemieva |first1=Irina M |title=Lithospheric structure, composition, and thermal regime of the East European Craton: implications for the subsidence of the Russian platform |journal=Earth and Planetary Science Letters |date=August 2003 |volume=213 |issue=3–4 |pages=431–446 |doi=10.1016/S0012-821X(03)00327-3|bibcode=2003E&PSL.213..431A |url=https://curis.ku.dk/ws/files/49250885/2003_EPSL_EEC_Art.PDF }} the Amazonian Craton in South America,{{cite journal |last1=Cordani |first1=U.G. |last2=Teixeira |first2=W. |last3=D'Agrella-Filho |first3=M.S. |last4=Trindade |first4=R.I. |title=The position of the Amazonian Craton in supercontinents |journal=Gondwana Research |date=June 2009 |volume=15 |issue=3–4 |pages=396–407 |doi=10.1016/j.gr.2008.12.005|bibcode=2009GondR..15..396C }} the Kaapvaal craton in South Africa,{{cite journal |last1=Nguuri |first1=T. K. |last2=Gore |first2=J. |last3=James |first3=D. E. |last4=Webb |first4=S. J. |last5=Wright |first5=C. |last6=Zengeni |first6=T. G. |last7=Gwavava |first7=O. |last8=Snoke |first8=J. A. |title=Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons |journal=Geophysical Research Letters |date=1 July 2001 |volume=28 |issue=13 |pages=2501–2504 |doi=10.1029/2000GL012587|hdl=10919/24271 |s2cid=15687067 |doi-access=free |hdl-access=free }} the North American Craton (also called the Laurentia Craton),{{cite journal |last1=Hoffman |first1=P F |title=United Plates of America, The Birth of a Craton: Early Proterozoic Assembly and Growth of Laurentia |journal=Annual Review of Earth and Planetary Sciences |date=May 1988 |volume=16 |issue=1 |pages=543–603 |doi=10.1146/annurev.ea.16.050188.002551|bibcode=1988AREPS..16..543H }} and the Gawler craton in South Australia.{{cite journal |last1=Hand |first1=M. |last2=Reid |first2=A. |last3=Jagodzinski |first3=L. |title=Tectonic Framework and Evolution of the Gawler craton, Southern Australia |journal=Economic Geology |date=1 December 2007 |volume=102 |issue=8 |pages=1377–1395 |doi=10.2113/gsecongeo.102.8.1377|bibcode=2007EcGeo.102.1377H }}

Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the typical {{convert|100|km|mi|abbr=on|adj=on|-1}} thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere,{{cite journal

|url=http://www.sciencenews.org/view/feature/id/66927/title/Continental_Hearts

|title=Continental Hearts – Science News

|last=Petit |first=Charles

| journal=Science News | volume=178 | issue=13 |date=18 December 2010

|issn=0036-8423 |publisher=Society for Science & the Public

|doi=10.1002/scin.5591781325

|page=24}} and the low-velocity zone seen elsewhere at these depths is weak or absent beneath stable cratons.{{cite book |last1=Kearey |first1=P. |last2=Klepeis |first2=K.A. |last3=Vine |first3=F.J. |title=Global tectonics. |date=2009 |publisher=Wiley-Blackwell |location=Oxford |isbn=9781405107778 |page=349 |edition=3rd}} 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. The cratonic lithosphere is much older than the oceanic lithosphere—up to 4 billion years versus 180 million years.{{sfn|Petit|2010|p=25}}

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 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.{{sfn|Petit|2010|pp=25-26}} 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 the crystalline residues after extraction of melts of compositions like basalt and komatiite.{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=373, 602–603}}

{{anchor|cratonization}}

File:Lithosphere of Earth - Idealized Cross-section.jpg, including the relationship between cratons, shields and platforms (Abbreviations: cb=cratonic basin, LIP=large igneous province, MOR=mid-ocean ridge)]]

The process by which cratons were formed is called cratonization. Much about this process remains uncertain, with very little consensus in the scientific community.{{cite journal |last1=Hansen |first1=Vicki L. |title=Impact origin of Archean cratons |journal=Lithosphere |date=24 August 2015 |volume=7 |issue=5 |pages=563–578 |doi=10.1130/L371.1|bibcode=2015Lsphe...7..563H |doi-access=free }} However, the first cratonic landmasses likely formed during the Archean eon. This is indicated by the age of diamonds, which originate in the roots of cratons and are almost always over 2 billion years and often over 3 billion years in age.{{sfn|Petit|2010|p=25}} Rock of the 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 per cent of the present continental crust formed during the Archean.{{cite book |last=Stanley |first=Steven M. |author-link=Steven M. Stanley |title=Earth System History |location=New York |publisher=W.H. Freeman and Company |year=1999 |isbn=0-7167-2882-6 |pages=297–302}} Cratonization likely was completed during the Proterozoic. Subsequent growth of continents was by accretion at continental margins.{{sfn|Petit|2010|p=25}}

The origin of the roots of cratons is still debated.{{cite book |last1=Lee |first1=C. |title=Archean Geodynamics and Environments |chapter=Geochemical/Petrologic constraints on the origin of cratonic mantle |editor-last1=Benn |editor-first1=K. |editor-last2=Mareschal |editor-first2=J.C. |editor-last3=Condie |editor-first3=K.C. |year=2006 |series=Geophysical Monograph Series |volume=164 |page=89 |doi=10.1029/164GM08 |bibcode=2006GMS...164...89L |isbn=978-0-87590-429-0 |url=https://static1.squarespace.com/static/54b9bb6fe4b07b4a7d145b55/t/54c85e17e4b0278ebafdc6e9/1422417431292/24-2006LeeAGUcraton.pdf |access-date=20 November 2021}}{{cite journal |last1=Miller |first1=Meghan S. |last2=Eaton |first2=David W. |title=Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions: Formation of cratonic mantle keels |journal=Geophysical Research Letters |date=September 2010 |volume=37 |issue=18 |pages=n/a |doi=10.1029/2010GL044366|s2cid=129901730 |doi-access=free }}{{sfn|Petit|2010|pp=25-26}} 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 per cent of the source rock entering the melt. Such a high degree of melting 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 the 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 the surrounding hotter but more chemically dense mantle.{{cite journal |last1=Jordan |first1=Thomas H. |title=Composition and development of the continental tectosphere |journal=Nature |date=August 1978 |volume=274 |issue=5671 |pages=544–548 |doi=10.1038/274544a0|bibcode=1978Natur.274..544J |s2cid=4286280 }}{{sfn|Petit|2010|p=25}} 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.{{sfn|Jordan|1978|p=546}} 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.{{sfn|Jordan|1978|p=547}}

This model of melt extraction from the upper mantle has held up well with subsequent observations.{{sfn|Lee|2006}} The properties of mantle xenoliths confirm that the geothermal gradient is much lower beneath continents than oceans.{{sfn|Jordan|1978|p=544}} The olivine of craton root xenoliths is extremely dry, which would give the roots a very high viscosity.{{sfn|Petit|2010|p=26}} Rhenium–osmium dating of xenoliths indicates that the oldest melting events took place in the early to middle Archean. Significant cratonization continued into the late Archean, accompanied by voluminous mafic magmatism.{{sfn|Kearey|Klepeis|Vine|2009|p=351}}

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 {{convert|200|km||sp=us}}. The great depths of craton roots required further explanation.{{sfn|Jordan|1978|p=547}} The 30 to 40 per cent partial melting of mantle rock at 4 to 10 GPa pressure produces komatiite magma and a solid residue very close in composition to Archean lithospheric mantle. Still, 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.{{sfn|Kearey|Klepeis|Vine|2009|p=351}} There are many competing hypotheses of how cratons have been formed.

Jordan's model suggests that further cratonization resulted from repeated continental collisions. The thickening of the crust associated with these collisions may have been balanced by craton root thickening according to the principle of isostacy.{{sfn|Jordan|1978|p=547}} 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 {{Convert|400|km|mi|abbr=on|sp=us}}.{{sfn|Petit|2010|p=26}}

A second model suggests that the surface crust was thickened by 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.{{sfn|Petit|2010|p=26}}{{sfn|Petit|2010|pp=25-26}}

A fourth theory presented in a 2015 publication suggests that the origin of 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{{sfn|Lee|2006}} and seismic tomography both favor the two accretional models over the plume model.{{sfn|Petit|2010|p=26}}{{sfn|Miller|Eaton|2010}} However, other geochemical evidence favors mantle plumes.{{cite journal |last1=Tomlinson |first1=Kirsty Y. |last2=Condie |first2=Kent C. |title=Archean mantle plumes: Evidence from greenstone belt geochemistry |journal=Mantle Plumes: Their Identification Through Time |date=2001 |doi=10.1130/0-8137-2352-3.341 |isbn=9780813723525 |url=https://www.researchgate.net/publication/245540739 |access-date=21 November 2021}}{{cite journal |last1=Ernst |first1=Richard E. |last2=Buchan |first2=Kenneth L. |last3=Campbell |first3=Ian H. |title=Frontiers in large igneous province research |journal=Lithos |date=February 2005 |volume=79 |issue=3–4 |pages=271–297 |doi=10.1016/j.lithos.2004.09.004|bibcode=2005Litho..79..271E }}{{sfn|Kearey|Klepeis|Vine|2009|p=352}} Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than {{Convert|150|km|mi|abbr=on|sp=us}} and may be Archean, while the second is found at depths from {{Convert|180 to 240|km|mi|abbr=on|sp=us}} and may be younger. The second layer may be a less depleted thermal boundary layer that stagnated against the depleted "lid" formed by the first layer.{{cite journal |last1=Yuan |first1=Huaiyu |last2=Romanowicz |first2=Barbara |title=Lithospheric layering in the North American craton |journal=Nature |date=August 2010 |volume=466 |issue=7310 |pages=1063–1068 |doi=10.1038/nature09332|pmid=20740006 |bibcode=2010Natur.466.1063Y |s2cid=4380594 }} 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.{{sfn|Kearey|Klepeis|Vine|2009|p=351}}

The long-term erosion of cratons has been labelled 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 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.{{cite journal |last1=Fairbridge |first1=Rhodes W. |last2=Finkl Jr. |first2=Charles W.|author-link=Rhodes Fairbridge |date=1980 |title=Cratonic erosion unconformities and peneplains|journal=The Journal of Geology |volume=88 |issue=1 |pages=69–86|doi=10.1086/628474 |bibcode=1980JG.....88...69F |s2cid=129231129 }}

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 the Baltic Shield had been eroded into a subdued terrain already during the Late Mesoproterozoic when the rapakivi granites intruded.{{cite web |url=http://uppslagsverket.fi/sv/sok/view-103684-BerggrundOchYtformer |title=berggrund och ytformer |last=Lindberg |first=Johan |date=April 4, 2016 |website=Uppslagsverket Finland |language=sv |access-date=February 13, 2018 |archive-date=January 6, 2018 |archive-url=https://web.archive.org/web/20180106174856/http://www.uppslagsverket.fi/sv/sok/view-103684-BerggrundOchYtformer |url-status=live }}{{cite journal |last1=Lundmark |first1=Anders Mattias |last2=Lamminen |first2=Jarkko |date=2016 |title=The provenance and setting of the Mesoproterozoic Dala Sandstone, western Sweden, and paleogeographic implications for southwestern Fennoscandia|journal=Precambrian Research|volume=275|pages=197–208|doi=10.1016/j.precamres.2016.01.003 |bibcode=2016PreR..275..197L }}

• List of shields and cratons
• Cratonic sequence

{{Reflist}}

• {{cite book |last=Dayton |first=Gene |year=2006 |title=Geological Evolution of Australia |url=http://courseprofile.cqu.edu.au/CourseProfile/deliverPDF.jsp?docId=33717&courseCode=GEOG11023}} Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia.
• {{Citation

|last1=Grotzinger |first1=John P. |author1-link=John P. Grotzinger

|last2=Jordan |first2=Thomas H.|author-link2=Thomas H. Jordan

|title=Understanding Earth

|edition=Sixth |date=4 February 2010 |publisher=W. H. Freeman

|isbn= 978-1429219518

}}

• {{cite journal |last=Hamilton |first=Warren B. |title=Archean magmatism and deformation were not products of plate tectonics |journal=Precambrian Research |volume=91 |issue=1–2 |date=August 1998 |pages=143–179 |doi=10.1016/S0301-9268(98)00042-4|bibcode=1998PreR...91..143H }}
• {{cite journal |last=Hamilton |first=Warren B. |year=1999 |title=How did the Archean Earth Lose Heat? |journal=Department of Geophysics, Colorado School of Mines, Journal of Conference Abstracts |volume=4 |issue=1 |url=http://www.the-conference.com/JConfAbs/4/140.html |url-status=dead |archive-url=https://web.archive.org/web/20060514104020/http://www.the-conference.com/JConfAbs/4/140.html |archive-date=2006-05-14 }}. Symposium A08, Early Evolution of the Continental Crust.

• {{cite web

|url=http://www.mnh.si.edu/earth/text/4_1_3_1.html

|title=The Dynamic Earth @ National Museum of Natural History

|author=Smithsonian

|publisher=Smithsonian National Museum of Natural History

|access-date=2011-01-09

|archive-date=2005-03-06

|archive-url=https://web.archive.org/web/20050306191515/http://www.mnh.si.edu/earth/text/4_1_3_1.html

|url-status=dead

}}

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Category:Historical geology