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A craton (, , or ; from el, κράτος ''kratos'' "strength") is an old and stable part of the continental
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
, which consists of Earth's two topmost layers, the crust and the uppermost
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
. Having often survived cycles of merging and
rift In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...

rift
ing of
continent A continent is any of several large landmass A landmass, or land mass, is a large region In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of th ...

continent
s, cratons are generally found in the interiors of
tectonic plates This is a list of tectonic plates on Earth's surface. Tectonic plates are pieces of Earth's crust 350px, Plates in the crust of Earth Earth's crust is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is ...
; the exceptions occur where geologically recent rifting events have separated cratons and created
passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate Continental margin, margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Contin ...

passive margin
s along their edges. They are characteristically composed of ancient
crystal A crystal or crystalline solid is a solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformatio ...

crystal
line
basement rock upright=1.25, Gneiss outcrop, basement rock, Scotland">outcrop.html" ;"title="Gneiss outcrop">Gneiss outcrop, basement rock, Scotland In geology, basement and crystalline basement are the rocks below a sedimentation, sedimentary platform (geology ...
, which may be covered by younger
sedimentary rock Sedimentary rocks are types of rock Rock most often refers to: * Rock (geology) A rock is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compoun ...

sedimentary rock
. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's
mantle Mantle may refer to: *Mantle (geology) A mantle is a layer inside a planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a n ...
.


Terminology

The term ''craton'' is used to distinguish the stable portion of the
continental crust 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneous rock, igneous, Sedimentary rock, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallo ...
from regions that are more geologically active and unstable. Cratons can be described as ''
shields A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from melee weapon, close-ranged weaponry or projectiles such as arrows, by ...
'', in which the basement rock crops out at the surface, and '' platforms'', in which the basement is overlaid by
sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediments
and sedimentary rock. The word ''craton'' was first proposed by the Austrian geologist
Leopold Kober Leopold Kober (21 September 1883 – 6 September 1970), an Austrian geology, geologist, proposed a number of (subsequently largely discredited) theories of orogeny and coined the term ''craton, kratogen'' to describe stable continental crust, wh ...
in 1921 as , referring to stable continental platforms, and ''orogen'' as a term for
mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. A mountain differs from a plateau in having a limited summit area, and is larger than a hill, typically rising at least ...

mountain
or
orogenic belts An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere A lithosphere ( grc, wikt:λίθος#Ancient Greek, λίθος [] for "rocky", and [] for "sphere") is the rigid, ...
. Later Hans Stille shortened the former term to ''Kraton'' from which ''craton'' derives.


Examples

Examples of cratons are the Dharwar (or Karnataka) Craton in
India India, officially the Republic of India (Hindi Hindi (Devanagari: , हिंदी, ISO 15919, ISO: ), or more precisely Modern Standard Hindi (Devanagari: , ISO 15919, ISO: ), is an Indo-Aryan language spoken chiefly in Hindi Belt, ...

India
,
North China Craton The North China Craton is a continental crustal block with one of Earth's most complete and complex records of , and processes. It is located in northeast China, , , and . The term designates this as a piece of continent that is stable, buoyan ...

North China Craton
, the East European Craton, the Amazonia Craton in
South America South America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continent ...

South America
, the
Kaapvaal Craton Image:Map of Kaapvaal craton.svg, alt=This map shows the outlines of the southern African nations of Namibia, Botswana, Zimbabwe and South Africa. Kaapvaal's outline is superimposed on the countries., The bold outline shows the parameters of the ...
in
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over 60 million people, it is the world's 23rd-most populous nation and covers an area of . South Africa has three capital citie ...

South Africa
, the North American or Laurentia Craton, and the
Gawler Craton The Gawler Craton covers approximately 440,000 square kilometres of central South Australia. Its Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the ...
in
South Australia South Australia (abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Austral ...

South Australia
.


Structure

Cratons have thick lithospheric roots. Mantle
tomography File:Sagittal brain MRI.jpg, thumbnail, Median plane Sagittal plane, sagittal tomography of the head by magnetic resonance imaging. Tomography is image, imaging by sections or sectioning through the use of any kind of penetrating wave. The method i ...
shows that cratons are underlain by anomalously cold mantle corresponding to
lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion o ...
more than twice the typical thickness of mature oceanic or non-cratonic,
continental lithosphere A lithosphere ( grc, wikt:λίθος#Ancient Greek, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geol ...
. At that depth, craton roots extend into the
asthenosphere The asthenosphere ( grc, ἀσθενός 'asthenos''meaning "without strength", and thus "weak", and 'sphaira''meaning "sphere") is the highly viscous The viscosity of a fluid In physics, a fluid is a substance that continually Defor ...
, 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 A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of ...
because cratons have a neutral or positive
buoyancy Buoyancy (), or upthrust, is an upward exerted by a that opposes the of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bo ...

buoyancy
, and a low intrinsic density. This low density offsets density increases due to geothermal contraction and prevents 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 Peridotite ( ) is a dense, coarse-grained igneous rock Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spok ...
have been delivered to the surface as
inclusion Inclusion or Include may refer to: Sociology * Social inclusion, affirmative action to change the circumstances and habits that leads to social exclusion ** Inclusion (disability rights), including people with and without disabilities, people of ...
s in subvolcanic pipes called
kimberlite Cross-section of kimberlite from South Africa. The kimberlite matrix is made up of clay minerals and carbonates, presented in blue, purple and buff colours. Kimberlite is an igneous rock, which sometimes contains diamonds. It is named after the ...

kimberlite
s. 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. 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 Harzburgite, an ultramafic, igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area ar ...
peridotites represent the crystalline residues after extraction of melts of compositions like
basalt Basalt (, ) is a fine-grained extrusive A volcanic rock from Italy with a relatively large six-sided phenocryst (diameter about 1 mm) surrounded by a fine-grained groundmass, as seen in thin section under a petrographic microscope Extrusi ...

basalt
and
komatiite Komatiite () is a type of ultramafic mantle-derived volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igne ...
.


Formation

The process by which cratons were formed is called cratonization. There is much about this process that remains uncertain. However, the first cratonic landmasses likely formed during the
Archean The Archean Eon ( , also spelled Archaean or Archæan) is one of the four geology, geologic eon (geology), eons of Earth, Earth's history, occurring (4 to 2.5 Billion years, Gya). During the Archean, the Earth's Crust (geology), crust had ...

Archean
Eon. This is indicated by the age of
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. At Standard conditions for temperature and pressure, room temperature and pressure, another solid form of ...

diamond
s, which originate in 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 350px, The thickness of Crust (geology)#Earth's crust, Earth's crust (km) Continental crust is the layer of Igneous rock, igneous, Sedimentary rock, sedimentary, and metamorphic rocks that forms the geological continents and the areas of shallo ...
formed during the Archean. Cratonization likely was completed during the
Proterozoic The Proterozoic () is a geological eon spanning the time interval from 2500 to 541million years ago. It is the most recent part of the Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest par ...
. Subsequent growth of continents 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 Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter an ...
''. Jordan proposed 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 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 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 suggested that depletion occurred primarily in subduction zones and secondarily as flood basalts. This model of melt extraction from the upper mantle has held up well with subsequent observations. The properties of mantle xenoliths confirm that the
geothermal gradient ). Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consi ...
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 Rhenium–osmium dating is a form of radiometric dating based on the beta decay of the isotope 187rhenium, Re to 187Osmium, Os. This normally occurs with a half-life of 41.6 × 109 y, but studies using fully ionised 187rhenium, Re atoms have found ...
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. However, melt extraction alone cannot explain all the properties of craton roots. Jordan noted in his original paper that this mechanism could be effective for constructing craton roots only down to a depth of . 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 Komatiite () is a type of ultramafic mantle-derived volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igne ...
magma and 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 at least three hypotheses of how cratons have been formed. Jordan's suggestion was that further cratonization was a result of repeated continental collisions. The thickening of the crust associated with these collusions must have been balanced by craton root thickening according to the principle of
isostacy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational equilibrium between Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. ...
. Jordan likened 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. A second model suggests that the surface crust was thickened by a rising plume of molten material from the deep mantle. This would have build up a thick layer of depleted mantle underneath the cratons. The third model suggests that successive slabs of
subducting Subduction is a geological process in which the oceanic lithosphere is Geochemical cycle, recycled into the Earth's mantle at convergent boundary, convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less de ...
oceanic lithosphere became lodged beneath a proto-craton,
underplating Magmatic underplating occurs when basaltic Basalt (, ) is a fine-grained extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (''mafic '' lava) exposed at or very near the surface of a te ...
the craton with chemically depleted rock. The chemistry of xenoliths and
seismic tomography Seismic tomography is a technique for imaging the subsurface of the Earth with seismic waves produced by earthquakes or explosions. P-, S-, and surface waves can be used for tomographic models of different resolutions based on seismic wavelength, ...
both favor the two accretional models over 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 and may be Archean, while the second is found at depths from 180 to 240 km 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. 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.


Erosion

The long-term
erosion In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

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
peneplain In geomorphology incised into shale at the foot of the North Caineville Plateau, Utah, within the pass carved by the Fremont River (Utah), Fremont River and known as the Blue Gate. Grove Karl Gilbert, GK Gilbert studied the landscapes of this a ...

peneplain
s. While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over
geological time The geologic time scale (GTS) is a system of chronological datingChronological dating, or simply dating, is the process of attributing to an object or event a date in the past, allowing such object or event to be located in a previously establish ...
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 level Mean sea level (MSL) (often shortened to sea level) is an average In colloquial, ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in th ...

sea level
s. 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 Western Australia (abbreviated as WA) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspape ...

Western Australia
was flattish already by Middle Proterozoic times and the
Baltic Shield The Baltic Shield (or Fennoscandian Shield) is a segment of the Earth's Crust (geology), crust belonging to the East European Craton, representing a large part of Fennoscandia, northwestern Russia and the northern Baltic Sea. It is composed mostly ...
had been eroded into a subdued terrain already during the Late Mesoproterozoic when the
rapakivi granite Image:Rapakiwi-Geschiebe.jpg, Rapakivi from a moraine in Northern Germany. Rapakivi granite is a hornblende-biotite granite containing large round crystals of orthoclase each with a rim of oligoclase (a variety of plagioclase). The name has come to ...

rapakivi granite
s intruded.


See also

*
List of shields and cratonsA craton A craton (, , or ; from el, κράτος ''kratos'' "strength") is an old and stable part of the continental lithosphere A lithosphere ( grc, wikt:λίθος#Ancient Greek, λίθος [] for "rocky", and [] for "sphere") is the rigid ...
* Cratonic sequence


References


Further reading

* Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia. * * * . Symposium A08, Early Evolution of the Continental Crust.


External links

* {{Authority control Historical geology