Craton
A craton is an old and stable part of the
continental crust that has survived the merging and splitting of
continents and supercontinents for at least 500 million years. Cratons are generally found in the interiors of continents and are formed of a crust of lightweight felsic
igneous rock such as
granite attached to a section of the upper
mantle. A craton may extend to depth of 200 km.
Cratons are subdivided geographically into
geologic provinces, each province being classified as an Archon, a Proton or a Tecton according to its age:
Encyclopedia
A
craton is an old and stable part of the
continental crust that has survived the merging and splitting of
continents and supercontinents for at least 500 million years. Cratons are generally found in the interiors of continents and are formed of a crust of lightweight felsic
igneous rock such as
granite attached to a section of the upper
mantle. A craton may extend to depth of 200 km.
Cratons are subdivided geographically into
geologic provinces, each province being classified as an Archon, a Proton or a Tecton according to its age:
- Archons: consist of rocks from the Archaean era, older than 2.5 billion years .
- Protons: consist of rocks from the early to middle Proterozoic era, older than 1.6 Ga.
- Tectons: consist of rocks from the late Proterozoic era, with ages between 1.6 Ga and 800 million years .
As minerals in the Earth's crust tend to become separated with time, the oldest cratons are of the greatest interest to mining companies. Cratons are still being defined by ongoing
geological and geophysical research.
Craton formation
The process by which cratons are formed from early rock is called cratonization. The first large cratonic landmasses formed during the Archean eon. During the Early Archean the 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.
Tectonic and
volcanic activity were considerably more active than they are today; the
mantle was much more fluid and the crust much thinner. This resulted in rapid formation of
oceanic crust at ridges and hot spots, and rapid recycling of oceanic crust at
subduction zones. The 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 probably 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 5-40% of the present
continental crust formed during the Archean. .
A technical but very succulent evolutionary perspective of how the cratonization process probably first began in the Archean is given by 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.
Named cratons
Listed by modern continent, include:
Africa
- Arabian craton
- Congo craton, central southern Africa
- Kaapvaal craton, South Africa
- Kalahari craton
- Saharan craton, Algeria
- Tanzanian craton
- West African craton
- Zaire craton
- Zimbabwe craton
Antarctica
Australia
- Altjawarra craton
- Central craton
- Curnamona craton, South Australia
- Gawler craton, central South Australia
- Pilbara craton, Western Australia
- Yilgarn craton, Western Australia
Eurasia
- Bhandara craton, India
- Bundelkand craton, India
- Dharwar craton, India
- East China craton
- Indian craton
- North China craton
- Sarmatian craton
- Siberian craton
- Singhbhum craton, India
- Sino-Korean craton, Northern China
- Tarim craton, China
- Volgo-Uralian craton, Russia
- Yakutai craton, Eastern Siberia
- Yangtze craton
North America
Northern and Eastern Europe
- East European craton
- Belomorian craton, part of the Fennoscandian Shield, between the Karelian and Kola cratons
- Baltic Shield, part of the East European craton
- Fennoscandian Shield, the exposed Northwestern part of the Baltic Shield in Norway, Sweden and Finland
- Karelian craton, part of the Fennoscandian Shield in Southeast Finland and Karelia Russia,
- Kola craton, part of the Fennoscandian Shield, Kola Peninsula, Northwest Russia
- Midland craton of England and Wales
- North Atlantic Craton
- Ukranian Shield
South America
...
ian craton
- Rio de la Plata craton
- San Francisco craton
References
- Dayton, Gene. "Geological Evolution of Australia." Sr. Lecturer, Geography, School of Humanities, Central Queensland University, Australia.
- Hamilton, Warren B. "How did the Archean Earth Lose Heat?." Department of Geophysics, Colorado School of Mines, Journal of Conference Abstracts, Vol. 4, No. 1, Symposium A08, Early Evolution of the Continental Crust.
- Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0716728826 p. 297-302
