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. As of 2022, the
bolide was estimated at between in diameter, and is estimated to have impacted with a vertical velocity of . 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, as telltale
shatter cones were discovered in the
bed of the nearby
Vaal River. This impact structure is one of the few multiple-ringed impact structures 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 impact structures 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 impact structure. 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. 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 impact structure 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 3.9 billion years ago. This central
peak uplift, or dome, is typical of a complex impact structure, where the liquefied rocks splashed up in the wake of the meteor as it penetrated the surface. File:Earth's geolological timeline.svg|A timeline of the Earth's history indicating when the Vredefort impact structure was formed in relation to some of the other important
South African geological events.
W indicates when the
Witwatersrand Supergroup was laid down,
C the
Cape Supergroup, and
K the
Karoo Supergroup. The graph also indicates the period during which
banded ironstone formations were formed on earth, indicative of an
oxygen-free atmosphere. The Earth's crust was wholly or partially molten during the Hadean Eon. One of the first microcontinents to form was the
Kaapvaal craton, which is exposed at the centre of the Vredefort Dome, and again north of Johannesburg. File:Vredefort crater cross section 2.png|A schematic diagram of a NE (left) to SW (right) cross-section through the 2.020-billion-year-old Vredefort impact structure and how it distorted the contemporary geological structures. The present erosion level is shown. Johannesburg is where the Witwatersrand Basin (the yellow layer) is exposed at the "present surface" line, just inside the impact structure's rim, on the left. Not to scale. ==Conservation==