thumb|upright=1.35|Cratons of South America and Africa during the [[Triassic Period when the two continents were joined as part of the Pangea supercontinent]]

A craton ( , , or ; from "strength") is an old and stable part of continental lithosphere (the Earth's two topmost layers, the crust and the lithospheric 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 composed of ancient crystalline basement rocks covered by younger sedimentary rocks. They have a thick crust and deep lithospheric roots extending several hundred kilometres into Earth's mantle. This crust consists of shields and platforms. Shields are made of basement rock and are visible, while platforms are the portion not visible made up of sedimentary rock.

Cratons contain the oldest continental crust rocks on Earth. They were formed in the Archaean (4 to 2.5 billion years ago) and the Proterozoic (2.5 billion- 538.8 million year ago) geologic eons. Most were formed in the Archaean.

Terminology

The term craton is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable.

Bleeker and Davis (2004) define a craton as "a large, coherent domain of Earth's continental crust that has attained and maintained long-term stability, having undergone little internal deformation, except perhaps near its margins due to interaction with neighbouring terranes."

Scott King (2005) define the Archaean cratons as "relatively flat, stable regions of the crust that have remained undeformed since the Precambrian, forming the ancient cores of the continents."

Cratons are composed of two layers: the cratonic basement of metamorphosed crystalline and metamorphic rocks and the platform, which is a younger, weakly deformed sedimentary cover which overlies this basement. Continental shields are exposed (they crop out at the surface) cratonic basement rocks and are thus dominated by crystalline and metamorphic rocks. Shields and platforms are physiographic terms rather than tectonic entities.

The word craton was first proposed by the Austrian geologist Leopold Kober in 1921 as , referring to stable continental platforms, and orogen as a term for mountain or orogenic belts. Later Hans Stille shortened the former term to , from which craton derives.

Examples

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), the Gawler craton in South Australia, the Archean Wyoming Craton, and the Superior Craton in Canada.

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, 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. The cratonic lithosphere is much older than the 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 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

thumb|upright=1.75|Idealized cross-section of Earth's [[lithosphere, 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. 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. 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. The Archeon eon makes approximately around 3% of the Earth's current surface, but all continents on earth contain crust from it. Cratonization likely was completed during the Proterozoic. Subsequent growth of continents was by accretion at continental margins.

Root origin

The origin of the roots of cratons is still debated. 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. Tomography shows two layers in the craton roots beneath North America. One is found at depths shallower than and may be Archean, while the second is found at depths from 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. The impact origin model does not require plumes or accretion; this model is, however, not incompatible with either.

Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn craton of Western Australia was flattish already by Middle Proterozoic times

See also

  • List of shields and cratons
  • Cratonic sequence

References

Further reading

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