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The Vredefort crater is the largest verified impact crater on Earth, more than 300 km across when it was formed. What remains of it is located in the present-day Free State Province of South Africa and named after the town of Vredefort, which is situated near its centre. Although the crater itself has long since eroded away, the remaining geological structures at its centre are known as the Vredefort Dome or Vredefort impact structure. In 2005, the Vredefort Dome was added to the list of UNESCO World Heritage Sites for its geologic interest. ==Formation and structure== The asteroid that hit Vredefort is estimated to have been one of the largest ever to strike Earth (at least since the Hadean Eon some four billion years ago), thought to have been approximately in diameter. The bolide that created the Sudbury Basin could have been even larger.〔(【引用サイトリンク】 url = http://www.parys.co.za/parys/dome.html )〕 The original crater was estimated to have a diameter of roughly ,〔 although this has been eroded away. It would have been larger than the Sudbury Basin and the Chicxulub Crater. The remaining structure, the "Vredefort Dome", consists of a partial ring of hills 70 km in diameter, and are the remains of a dome created by the rebound of rock below the impact site after the collision. The crater's age is estimated to be 2.023 billion years (± 4 million years),〔 which places it in the Paleoproterozoic Era. It is the second-oldest known crater on Earth, a little less than 300 million years younger than the Suavjärvi Crater in Russia. In comparison, it is about 10% older than the Sudbury Basin impact (at 1.849 billion years). The dome in the center of the crater was originally thought to have been formed by a volcanic explosion, but in the mid-1990s, evidence revealed it was the site of a huge bolide impact, as telltale shatter cones were discovered in the bed of the nearby Vaal River. The crater site is one of the few multiple-ringed impact craters on Earth, although they are more common elsewhere in the Solar System. Perhaps the best-known example is Valhalla Crater on Jupiter's moon Callisto, although Earth's Moon has a number, as well. Geological processes, such as erosion and plate tectonics, have destroyed most multiple-ring craters on Earth. The impact distorted the Witwatersrand Basin which was laid down over a period of 250 million years between 950 and 700 million years before the Vredefort impact. The overlying Ventersdorp lavas and the Transvaal Supergroup which were laid down between 700 and 80 million years before the meteorite strike, were similarly distorted by the formation of the 300 km wide crater.〔McCarthy, T., Rubridge, B. (2005). ‘’The Story of Earth and Life.’’ p. 89-90, 102-107, 134-136. Struik Publishers, Cape Town〕〔Norman, N., Whitfield, G. (2006) ‘’Geological Journeys’’. p. 38-49, 60-61. Struik Publishers, Cape Town.〕 These rocks form partial concentric rings round the crater center today, with the oldest, the Witwatersrand rocks, forming a semicircle 25 km from the center. Since the Witwatersrand rocks consist of several layers of very hard, erosion resistant sediments (e.g. quartzites and banded ironstones),〔〔Truswell, J.F. (1977). ''The Geological Evolution of South Africa''. pp. 23-38. Purnell, Cape Town〕 they form the prominent arc of hills that can be seen to the NW of the crater center in the satellite picture above. The Witwatersrand rocks are followed, in succession, by the Ventersdorp lavas at a distance of about 35 km from the center, and the Transvaal Supergroup, consisting of a narrow band of the Ghaap Dolomite rocks and the Pretoria Subgroup of rocks, which together form a 25–30 km wide band beyond that.〔Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.〕 From about halfway through the Pretoria Subgroup of rocks around the crater center, the order of the rocks is reversed. Moving outwards towards where the crater rim used to be, the Ghaap Dolomite group resurfaces at 60 km from the center, followed by an arc of Ventersdorp lavas, beyond which, at between 80 and 120 km from the center, the Witwatersrand rocks re-emerge to form an interrupted arc of outcrops today, of which the Johannesburg group is the most famous, because it was here that gold was discovered in 1886.〔〔 It is thus possible that if it had not been for the Vredefort impact this gold would never have been discovered.〔 The 40 km diameter center of the Vredefort crater consists of a granite dome (where it is not covered by much younger rocks belonging to the Karoo Supergroup) which is an exposed part of the Kaapvaal craton, one of the oldest microcontinents which formed on earth 3900 million years ago.〔 This central peak uplift, or dome, is typical of a complex impact crater, where the liquefied rocks splashed up in the wake the meteor as it penetrated the surface. 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「Vredefort crater」の詳細全文を読む スポンサード リンク
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