Craton


A craton is an old and stable part of the continental lithosphere, which consists of the 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. They 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 the 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 can be described as shields, in which the basement rock crops out at the surface, and platforms, in which the basement is overlaid by sediments and 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 North China Craton, the Sarmatian Craton in Russia and Ukraine, the Amazonia Craton in South America, the Kaapvaal Craton in South Africa, the North American or Laurentia Craton, and the Gawler Craton in South Australia.

Structure

Cratons have thick lithospheric roots. Mantle tomography shows that cratons are underlain by anomalously cold mantle corresponding to lithosphere more than twice the typical thickness of mature oceanic or non-cratonic, continental lithosphere. At that depth, craton roots extend into the asthenosphere. Craton lithosphere is distinctly different from oceanic lithosphere because cratons have a neutral or positive buoyancy, and a low intrinsic isopycnic 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 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. 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.

Formation

The process by which cratons were formed from early rock is called cratonization. The first large cratonic landmasses formed during the Archean eon. During the early Archean, Earth's heat flow was nearly three times higher than it is today because of the greater concentration of radioactive isotopes and the residual heat from the Earth's accretion. There was considerably greater tectonic and volcanic activity; the mantle was less viscous and the crust thinner. This resulted in rapid formation of oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at subduction zones. There are at least three hypotheses of how cratons have been formed: 1) surface crust was thickened by a rising plume of deep molten material, 2) successive subducting plates of oceanic lithosphere became lodged beneath a proto-craton in an under-plating process, 3) accretion from island arcs or continental fragments rafting together to thicken into a craton.
Earth's surface was probably broken up into many small plates with volcanic islands and arcs in great abundance. Small protocontinents formed as crustal rock was melted and remelted by hot spots and recycled in subduction zones.
There were no large continents in the early Archean, and small protocontinents were probably the norm in the Mesoarchean because they were prevented from coalescing into larger units by the high rate of geologic activity. These felsic protocontinents probably formed at hot spots from a variety of sources: mafic magma melting more felsic rocks, partial melting of mafic rock, and from the metamorphic alteration of felsic sedimentary rocks. Although the first continents formed during the Archean, rock of this age makes up only 7% of the world's current cratons; even allowing for erosion and destruction of past formations, evidence suggests that only 5 to 40 percent of the present continental crust formed during the Archean.
One perspective of how the cratonization process might have first begun in the Archean is given by Warren B. Hamilton:
Very thick sections of mostly submarine mafic, and subordinate ultramafic, volcanic rocks, and mostly younger subaerial and submarine felsic volcanic rocks and sediments were oppressed into complex synforms between rising young domiform felsic batholiths mobilized by hydrous partial melting in the lower crust. Upper-crust granite-and-greenstone terrains underwent moderate regional shortening, decoupled from the lower crust, during compositional inversion accompanying doming, but cratonization soon followed. Tonalitic basement is preserved beneath some greenstone sections but supracrustal rocks commonly give way downward to correlative or younger plutonic rocks... Mantle plumes probably did not yet exist, and developing continents were concentrated in cool regions. Hot-region upper mantle was partly molten, and voluminous magmas, mostly ultramafic, erupted through many ephemeral submarine vents and rifts focussed at the thinnest crust.... Surviving Archean crust is from regions of cooler, and more depleted, mantle, wherein greater stability permitted uncommonly thick volcanic accumulations from which voluminous partial-melt, low-density felsic rocks could be generated.

Erosion

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.
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.