|
Vaalbara is Earth's theorized first supercontinent.〔 There is some dispute as to Vaalbara's supremacy as the oldest supercontinent as the Australian continent also contains remnants of Ur. Vaalbara is understood to have been a complete supercontinent by 3.1 billion years ago. Vaalbara possibly formed beginning 3.6 billion years ago and was broken apart by about 2.8 billion years ago.〔 Vaalbara consisted of the eastern South African Kaapvaal craton joined with the Pilbara craton in northwest Western Australia.〔 (The name Vaalbara is derived from the last four letters of each craton's name.) 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 ~2.8 billion years ago.〔 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. ==Evidence for Vaalbara== South Africa's Kaapvaal craton and Western Australia's Pilbara craton have similar early Precambrian cover sequences. Kaapvaal's Barberton granite-greenstone terrane and Pilbara's eastern block show evidence of four large meteorite impacts between 3.2 to 3.5 billion years ago. (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.) The high temperatures created by the impact’s force fused sediments into small glassy spherules.〔 Spherules of 3.5 billion years old exist in South Africa and spherules of a similar age have been found in Western Australia,〔 they are the oldest-known terrestrial impact products. The spherules resemble the glassy chondrules (rounded granules) in carbonaceous chondrites, which are found in carbon-rich meteorites and lunar soils.〔 Remarkably similar lithostratigraphic and chronostratigraphic structural sequences between these two cratons have been noted for the period between 3.5 to 2.7 billion years ago.〔 Paleomagnetic data from two ultramafic complexes in the cratons showed that at the two cratons could have been part of the same supercontinent. Both the Pilbara and Kaapvaal cratons show extensional faults which were active about during felsic volcanism and coeval with the impact layers.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vaalbara」の詳細全文を読む スポンサード リンク
|