The Vredefort crater is the largest verified impact crater on Earth. More than across when it was formed, what remains of it is in the present-day Free State province of South Africa. It is named after the town of Vredefort, which is near its centre. Although the crater itself has long since been eroded away, the remaining geological structures at its centre are known as the Vredefort Dome or Vredefort impact structure. The crater is calculated to be 2.023 billion years old, with impact being in the Paleoproterozoic Era. It is the third-oldest known crater on Earth. 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, thought to have been approximately in diameter. The bolide that created the Sudbury Basin could have been even larger. The original crater is estimated to have had a diameter of roughly, but that has been eroded. 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 in diameter, and is 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, 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. The dome in the centre 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. Earth's Moon has some 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 crater. The rocks form partial concentric rings around the crater centre today, with the oldest, the Witwatersrand rocks, forming a semicircle from the centre. Since the Witwatersrand rocks consist of several layers of very hard, erosion-resistant sediments, they form the prominent arc of hills that can be seen to the northwest of the crater centre in the satellite picture above. The Witwatersrand rocks are followed, in succession, by the Ventersdorp lavas at a distance of about from the centre, and the Transvaal Supergroup, consisting of a narrow band of the Ghaap Dolomite rocks and the Pretoria Subgroup of rocks, which together form a band beyond that. From about halfway through the Pretoria Subgroup of rocks around the crater centre, the order of the rocks is reversed. Moving outwards towards where the crater rim used to be, the Ghaap Dolomite group resurfaces at from the centre, followed by an arc of Ventersdorp lavas, beyond which, at between from the centre, the Witwatersrand rocks re-emerge to form an interrupted arc of outcrops today. The Johannesburg group is the most famous one 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 centre of the Vredefort crater consists of a granite dome which is an exposed part of the Kaapvaal craton, one of the oldest microcontinents which formed on Earth 3.9 billion years ago. This central peak uplift, or dome, is typical of a complex impact crater, where the liquefied rocks splashed up in the wake of the meteor as it penetrated the surface.
Conservation
The Vredefort Dome World Heritage Site is currently subject to property development, and local owners have expressed concern regarding sewage dumping into the Vaal River and the crater site. The granting of prospecting rights around the edges of the crater has led environmental interests to express fear of destructive mining.
Community
The Vredefort Dome in the centre of the crater is home to four towns: Parys, Vredefort, Koppies and Venterskroon. Parys is the largest and a tourist hub; both Vredefort and Koppies mainly depend on an agricultural economy. On 19 December 2011, a broadcasting license was granted by ICASA to a community radio station to broadcast for the Afrikaans- and English-speaking members of the communities within the crater. The Afrikaans name Koepel Stereo refers to the dome and announces its broadcast as KSFM. The station broadcasts on 94.9 MHz FM.
