Vaalbara started to form about 3,600 million years ago (mya). It formed by about 3,100 mya and broke up by 2,500 mya. The name Vaalbara comes from the South African Kaapvaal craton and the West Australian Pilbara craton. These cratons were combined in the Vaalbara supercontinent.
South Africa's Kaapvaal craton and Western Australia's Pilbara craton are two of the best-preserved Archaean cratons on Earth. They have remarkably similar early Precambrian rocks. Identical radiometric ages of 3,470 ±2 mya have been got from rocky material thrown out by meteorite strikes. Similar structures from these two cratons have been dated between 3,500 to 2,700 mya. Similar greenstone belts are found at the edges of the Superior craton of Canada, and across the cratons of the former Gondwana and Laurasia continents.
Palaeomagnetic data from rocks in the cratons showed that at 3,870 mya the two cratons could have been part of the same supercontinent. Both the Pilbara and Kaapvaal cratons show extensional faults which were active about the same time during volcanism at the same time as the impact layers were formed.
Continental plates have periodically collided and assembled in periods of orogeny (mountain building) to form supercontinents. The cycle of supercontinent formation, breakup, dispersal and reformation by plate tectonics occurs every 450 million years or so.
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References[change | change source]
- Byerly, Gary R; Lowe, Donald R; Wooden, Joseph L; Xie, Xiaogang (2002). "An Archean impact layer from the Pilbara and Kaapvaal cratons". Science 297 (5585): 1325–7. . . http://www.sciencemag.org/cgi/content/abstract/297/5585/1325. Retrieved March 27, 2010.
- "Paleogeography: paleogeology, paleoclimate, in relation to evolution of life on Earth". Posted 12/30/2008 at 11:58:00 PM. p. 2. http://paleogeology.blogspot.com/2008/12/archaean-supercontinents.html. Retrieved February 28, 2010.