The Vredefort impact structure is one of the largest impact structures on Earth. The crater, which has since been worn away, has been estimated at across when it was formed, The impact structure was formed during the Paleoproterozoic Era, 2.023 billion (± 4 million) years ago. It is among the oldest known impact structures on Earth, after Yarrabubba (2.23 billion years old) and possibly Miralga.

In 2005, the Vredefort Dome was added to the list of UNESCO World Heritage Sites for its geologic interest.

Formation and structure

thumb|right|Map of South Africa showing the location of the Vredefort Dome, the remains of a 2.023-billion-year-old impact structure. The dashed line circle, in diameter, marks the extent of the original crater.

thumb|right|The view from the top of Aasvoëlkop in the heart of the Vredefort impact structure with the [[Vaal River and Venterskroon on its right in the background]]

The asteroid that hit Vredefort is estimated to have been one of the largest ever to strike Earth since the Hadean Eon some four billion years ago, originally thought to have been approximately in diameter with a kinetic energy of 6.32×10<sup>23</sup> Joules. As of 2022, the bolide was estimated at between in diameter, and is estimated to have impacted with a vertical velocity of and an energy of 4.6-6.7×10<sup>24</sup> J, datas apparently more consistent data with the geological evidences of the impact, such as shutter cones and PDFs.

The original impact structure is estimated to have had a diameter of at least , with the impact affecting the structure of the surrounding host rock in a circular region around in diameter. or 2.019/2.020 billion years (± 2-3 million years) old, which places it in the Orosirian Period of the Paleoproterozoic Era. It is among the oldest universally accepted impact structures on Earth. In comparison, it is about 10% older than the Sudbury Basin impact (at 1.849 billion years) and the Yarrabubba impact structure is older than the Vredefort impact structure by about 10%. Other purported older impact structures have either poorly constrained ages (Dhala impact structure, India) or highly contentious impact evidence in the case of the circa 3.023 billion year old Maniitsoq structure, West Greenland and the circa 2.4 billion year old Suavjärvi structure, Russia. Their classification as impact structures remain controversial and unsettled.

The dome in the centre of the impact structure 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, the remains of the central uplift or rebound typical of large impacts, as telltale shatter cones were discovered in the bed of the nearby Vaal River. Shatter cones are found up to ~90 km and PDFs on quartz up to ~45 km, while breccia up to ~150 km, and these distances are very important to estimate the crater size between 250-300 km with a certain degree of reliability. The rocks form partial concentric rings around the impact structure's 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 (e.g. quartzites and banded ironstones), they form the prominent arc of hills that can be seen to the northwest of the impact structure's 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 impact structure's 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. The granting of prospecting rights around the edges of the impact structure has led environmental interests to express fear of destructive mining.

Community

The Vredefort Dome in the centre of the impact structure 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 licence was granted by ICASA to a community radio station to broadcast for the Afrikaans- and English-speaking members of the communities within the impact structure. The Afrikaans name Koepel Stereo (Dome Stereo) refers to the dome and announces its broadcast as KSFM. The station broadcasts on 94.9&nbsp;MHz FM.

See also

  • List of impact structures on Earth
  • List of possible impact structures on Earth

References

  • Parys South Africa
  • Impact Cratering Research Group – University of the Witwatersrand
  • Earth Impact Database
  • Deep Impact – The Vredefort Dome
  • Satellite image of Vredefort impact structure from Google Maps
  • Impact Cratering: an overview of Mineralogical and Geochemical aspects – University of Vienna
  • Google Earth 3d .KMZ of 25 largest craters (requires Google Earth)