A craton ( , , or ;
Cratons contain the oldest continental crust rocks on Earth. They were formed in the Archean (4 to 2.5 billion years ago) and the Proterozoic (2.5 billion- 538.8 million year ago) . Most were formed in the Archaean.[Frost and Mueller (2024)][Craton, Shield and Platform Explained in Geology (2052) GeologyPoint.com [1]]
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 ."
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 Metamorphism Crystalline rock and and the platform, which is a younger, weakly deformed Sedimentary rock cover which overlies this basement. Continental shields are exposed (they outcrop at the surface) cratonic basement rocks and are thus dominated by crystalline and metamorphic rocks. Shields and platforms are physiographic terms rather than Tectonics entities.[Bleeker and Davis (2004)]
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 orogeny. Later Hans Stille shortened the former term to Kraton, from which craton derives.
Examples
Examples of cratons are the
Dharwar craton in India, North China Craton,
the East European Craton,
the
Amazonian shield 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.
Wyoming Craton
The Wyoming Craton is an exposed piece of Archean crust. The craton covers an area greater than 300,000 km^2.
Science suggests that the craton was formed around the time ca. 3.7 and ca 3.5 Ga.
This dates to be one of the earliest recordings of archeon craton formation.
Archean Wyoming Craton Map is referenced here.
Relevance
North America contains the best and most prominent example of cratons with the Laurentia Craton. This craton is one of the biggest cratons on our planet and is a prime example of how cratons can shape our crust. Small pieces of this craton in the past have had subliminal changes, slowly creating the North American continent to what is seen now.
Another example is The Río de la Plata Craton in South America, which is crucial in the formation of south west South America.
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 Magma containing peridotite have been delivered to the surface as inclusions in subvolcanic rock pipes called . 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 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
, 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.
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.
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 and secondarily as Flood basalt.
]
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 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 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 . The great depths of craton roots required further explanation. 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. There are many competing hypotheses of how cratons have been formed.
Repeated continental collision model
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. 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 .
Molten plume model
A second model suggests that the surface crust was thickened by a Mantle plume of molten material from the deep mantle. This would have built up a thick layer of depleted mantle underneath the cratons.
Subducting ocean slab model
A third model suggests that successive slabs of subducting oceanic lithosphere became lodged beneath a proto-craton, underplating the craton with chemically depleted rock.[
]
Impact origin model
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.
Evidence for each model
The chemistry of xenoliths and seismic tomography both favor the two accretional models over the plume model. However, other geochemical evidence favors mantle plumes.
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 of cratons has been labelled the "cratonic regime". It involves processes of pediplain and etchplain that lead to the formation of flattish surfaces known as .[ While the process of etchplanation is associated to humid climate and pediplanation with arid and semi-arid climate, shifting climate over deep 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 . High relative sea level leads to increased oceanicity, while the opposite leads to increased inland climate.]
Many cratons have had subdued topographies since Precambrian times. For example, the Yilgarn craton of Western Australia was flattish already by Proterozoic times[ and the Baltic Shield had been eroded into a subdued terrain already during the Mesoproterozoic when the intruded.]
See also
-
List of shields and cratons
-
Cratonic sequence
Further reading
-
Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia.
-
. Symposium A08, Early Evolution of the Continental Crust.
External links
