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Vaalbara is theorized to be Earth's first supercontinent, beginning its formation about 3,600 million years ago, completing its formation by about 3,100 million years ago and breaking up by 2,500 million years ago. The name Vaalbara is derived from the South African Kaapvaal craton and the West Australian Pilbara craton. These cratons were combined during the time of the Vaalbara supercontinent. Identical radiometric ages of 3,470 ± 2 million years ago have been obtained for the ejecta from the oldest impact events in each craton. Remarkably similar structural sequences between these two cratons have been noted for the period between 3,500 to 2,700 million years ago. Paleomagnetic data from two ultramafic complexes in the cratons showed that at 3,870 million years the two cratons could have been part of the same supercontinent. The reconstructed apparent polar wander path for the two cratons shows marked similarities. Both the Pilbara and Kaapvaal cratons show extensional faults which were active about during felsic volcanism and coeval with the impact layers. Continental plates have periodically collided and assembled in geologic periods of orogenesis (mountain building) to form supercontinents. The cycle of supercontinent formation, breakup, dispersal and reformation by plate tectonics occurs every 450 million years or so.
Vaalbara is the Earth's theorized first supercontinent.[1] It was a complete supercontinent by 3,100 million years ago. Vaalbara formed possibly beginning 3,600 million years ago and was broken apart by about 2,800 million years ago.[1] It is uncertain when Vaalbara began to break up; geochronological and palaeomagnetic evidence show that the two cratons had a rotational 30 ° latitudinal separation in the time period of 2.78 to 2.77 billion years ago, which indicates they were no longer joined after ~2,800 million years ago.[2] Vaalbara consisted of the eastern South African Kaapvaal craton joined with the Pilbara craton in northwest Western Australia.[1] (The name Vaalbara is derived from the last four letters of each craton's name.) Evidence for Vaalbara South Africa's Kaapvaal craton and Western Australia's Pilbara craton are two of the best-preserved existing Archean cratons on earth; they have remarkably similar early Precambrian cover sequences.[3] Kaapvaal's Barberton granite-greenstone terrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3,200 to 3,500 million years ago.[4] (Similar greenstone belts are now found at the margins of the Superior craton of Canada, and across the cratons of the former Gondwana and Laurasia continents.)[2] Identical single-zircon uranium-lead ages of 3,470 ± 2 million years ago have been obtained for the ejecta from the oldest impact events in each craton.[4] The high temperatures created by the impact’s force fused sediments into small glassy spherules.[5]:27 Spherules of 3,500 million years old exist in South Africa and spherules of a similar age have been found in Western Australia,[5]:27 they are the oldest-known terrestrial impact products.[6] The spherules resemble the glassy chondrules (rounded granules) in carbonaceous chondrites, which are found in carbon-rich meteorites and lunar soils.[5]:27 These deposits represent a global fallout layer which is associated with sedimentation by an impact-generated tsunami.[4] Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for the period between 3,500 to 2,700 million years ago.[7] Paleomagnetic data from two ultramafic complexes in the cratons showed that at 3,870 million years the two cratons could have been part of the same supercontinent.[7] The reconstructed apparent polar wander path for the two cratons shows marked similarities and overlap to a large extent.[7] Both the Pilbara and Kaapvaal cratons show extensional faults which were active about 3,470 million years ago during felsic volcanism and coeval with the impact layers.[7] Supercontinent formation Cratons are the masses of rock composing the basic, initial structure of continents; they have remained stable for billions of years throughout the process of the ocean crust being continually created and destroyed.[1] Continental plates, containing the ancient cratons, have periodically collided and assembled in geologic periods of orogenesis (mountain building) to form supercontinents.[1] A supercontinent is a landmass composed of more than one craton. Mechanism for breakup of supercontinents Supercontinents act as thermal lids, blocking the escape of Earth's internal heat, so the asthenosphere overheats.[1] Eventually the lithosphere begins to dome upward, it cracks, magma wells upward and fragments of the supercontinent slide off the overswell. The east African Rift Valley is a modern-day example of this breaking apart.[8] This cycle – the Wilson Cycle – of supercontinent formation, breakup, and dispersal, followed by convergence and patching, through plate tectonics occurs every 450 million years or so.[5]:90 See also * Ur (continent)
1. ^ a b c d e f "Supercontinents". enotes.com Science. pp. 1–2. http://www.enotes.com/earth-science/supercontinents. Retrieved February 28, 2010. * Bailey, R.C., Cruden, A.R., and B. Nitescu. (2006) "Crustal structure and implications for the tectonic evolution of the Archean Western Superior craton from forward and inverse gravity modeling." Tectonics, vol. 25. Online Abstract: http://www.agu.org/pubs/crossref/2006/2004TC001717.shtml
* Palaeos.com: General earth history Retrieved from "http://en.wikipedia.org/" |
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