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Vredefort Crater – The Largest Impact Crater in the World

The Vredefort crater is the largest verified impact crater in the world. It was more than 300 km across when it was formed. It’s also one of the oldest impact craters in the world at around 2 Billion Years old. What remains of it is located in the present-day Free State Province of South Africa. This crater was named after the town of Vredefort, which is situated near its centre. The Vredefort crater has long since eroded, and the remaining geological structures at its centre are known as Vredefort Dome. It is also known as the Vredefort impact structure. In 2005, the Vredefort Dome was added to the list of UNESCO World Heritage Sites for its geologic interest.

Map of Vredefort dome - Oldest & Largest Impact Crate in the world - Vredefort Crater
Map of South Africa showing the location of the Vredefort Dome, the remains of a 2.020 billion year-old impact crater. The interrupted line circle, 300 km in diameter, marks the extent of the original crater.

The asteroid that hit Vredefort is estimated to have been one of the largest ever to strike planet Earth. It is thought to have been approximately 15–20 km in diameter, much as distance between Joburg and Sandton.

 

The original crater was estimated to have a diameter of roughly 300km, although this has been eroded away. It would have been larger than the 250 km Sudbury Basin in Ontario, Canada. It would have also been larger than the 180 km Chicxulub Crater on the Yucatan Peninsula in Mexico. The “Vredefort Dome” consists of partial ring of hills 70 km in diameter. These 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,  which places it in the Paleoproterozoic Era. It is the second-oldest known crater on Earth. This crater is 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 centre of the crater was originally thought to have been formed by a volcanic explosion. In the mid-1990s though, evidence revealed that it was the site of a huge bolide impact.  This was confirmed by the discovery of telltale shatter cones in the bed of the nearby Vaal River.

 

The crater site is one of the few multiple-ringed impact craters on Earth. They are more common elsewhere in the Solar System though. 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.

"Vredefort

The Vredefort impact was so great such that it 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. These rocks form partial concentric rings round the crater centre today, with the oldest, the Witwatersrand rocks, forming a semicircle 25 km from the centre. Since the Witwatersrand rocks consist of several layers of very hard, erosion resistant sediments (e.g. quartzites and banded ironstones), they form the prominent arc of hills that can be seen to the NW of the crater centre.

The Witwatersrand rocks are followed, in succession, by the Ventersdorp lavas at a distance of about 35 km 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 25–30 km wide 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 60 km from the centre. That is followed by an arc of Ventersdorp lavas, beyond which, at between 80 and 120 km from the centre. 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 centre 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.

 

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 center of the crater is home to three towns, Parys, Vredefort and Koppies & 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 (Dome Stereo) refers to the dome and announces its broadcast as KSFM. The station broadcasts on 94.9 MHz FM.

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