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The Vredefort impact structure is the largest verified impact structure on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. The crater, which has since been eroded away, was around across when it was formed. The remaining structure, comprising the deformed underlying bedrock, is located in present-day
Free State province The Free State, known as Orange Free State until the 28th of June 1995 when its name was changed, is a province of South Africa. Its capital is Bloemfontein, which is also South Africa's judicial capital. Its historical origins lie in the Boer ...
of
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. It is bounded to the south by of coastline that stretch along the Atlantic Ocean, South Atlantic and Indian Oceans; to the ...
. It is named after the town of Vredefort, which is near its centre. The structure's central uplift is known as the Vredefort Dome. The impact structure was formed during the
Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
Era, 2.023 billion years (± 4 million years) ago. It is the second-oldest known impact structure on Earth, after Yarrabubba. In 2005, the Vredefort Dome was added to the list of
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s 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 since the Hadean Eon some four billion years ago, originally thought to have been approximately in diameter. The
bolide A bolide is normally taken to mean an exceptionally bright meteor, but the term is subject to more than one definition, according to context. It may refer to any large crater-forming body, or to one that explodes in the atmosphere. It can be a ...
that created the Sudbury Basin was larger. As of 2022, the bolide was estimated at between in diameter and to have impacted with a vertical velocity of . The original impact structure 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 Chicxulub crater () is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore near the community of Chicxulub, after which it is named. It was formed slightly over 66 million years ago when a large a ...
. 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 impact structure's age is estimated to be 2.023 billion years (± 4 million years), which places it in the
Orosirian The Orosirian Period (; grc, ὀροσειρά, translit=oroseirá, meaning "mountain range") is the third geologic period in the Paleoproterozoic era (geology), Era and lasted from annum, Mya to Mya (million years ago). Instead of being based ...
Period of the Paleoproterozoic Era. It is the second oldest universally accepted impact structure on Earth. In comparison, it is about 10% older than the Sudbury Basin impact (at 1.849 billion years) and the Yarrabubba impact crater is older than the Vredefort impact structure by about 0.2 billion years.Erickson, T.M., Kirkland, C.L., Timms, N.E., Cavosie, A.J. and Davison, T.M., 2020. ''Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure.'' ''Nature communications'', 11(1), pp.1-8. Other purported older impact structures have either poorly constrained ages ( Dhala impact structure, India)Pati, J.K., Qu, W.J., Koeberl, C., Reimold, W.U., Chakarvorty, M. and Schmitt, R.T., 2017. ''Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure, India.'' ''Meteoritics & Planetary Science'', 52(4), pp.722-736. or highly contentious impact evidence in case of the circa 3.023 billion year old Maniitsoq structure, West GreenlandGarde, A.A., McDonald, I., Dyck, B. and Keulen, N., 2012. ''Searching for giant, ancient impact structures on Earth: the Mesoarchaean Maniitsoq structure, West Greenland.'' ''Earth and Planetary Science Letters'', 337, pp.197-210. and the circa 2.4 billion year old Suavjärvi structure, Russia.Mashchak, M.S. and Naumov, M.V., 2012. ''The Suavjärvi impact structure, NW Russia.'' ''Meteoritics & Planetary Science'', 47(10), pp.1644-1658. Their classification as impact structures remain controversial and unsettled.Reimold, W.U., Ferrière, L., Deutsch, A. and Koeberl, C., 2014. ''Impact controversies: impact recognition criteria and related issues.'' ''Meteoritics & Planetary Science'', 49(5), pp.723-731. The dome in the centre of the impact structure was originally thought to have been formed by a
volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...
explosion, but in the mid-1990s, evidence revealed it was the site of a huge bolide impact, as telltale
shatter cone Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the rang ...
s were discovered in the
bed A bed is an item of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many beds ...
of the nearby
Vaal River The Vaal River ( ; Khoemana: ) is the largest tributary of the Orange River in South Africa. The river has its source near Breyten in Mpumalanga province, east of Johannesburg and about north of Ermelo and only about from the Indian Ocean. ...
. 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
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's moon
Callisto Callisto most commonly refers to: *Callisto (mythology), a nymph *Callisto (moon), a moon of Jupiter Callisto may also refer to: Art and entertainment *''Callisto series'', a sequence of novels by Lin Carter *''Callisto'', a novel by Torsten Kro ...
. Earth's
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
has some as well. Geological processes, such as erosion and
plate tectonics Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
, have destroyed most multiple-ring impact structures on Earth. The impact distorted the
Witwatersrand Basin The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which ...
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 The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa. It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which ...
rocks, forming a semicircle from the centre. Since the Witwatersrand rocks consist of several layers of very hard, erosion-resistant sediments (e.g.
quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...
s 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.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 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 The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga (billion years ago) crust on Earth. Similarities of rock records fr ...
, one of the oldest microcontinents which formed on Earth 3.9 billion years ago. This central
peak Peak or The Peak may refer to: Basic meanings Geology * Mountain peak ** Pyramidal peak, a mountaintop that has been sculpted by erosion to form a point Mathematics * Peak hour or rush hour, in traffic congestion * Peak (geometry), an (''n''-3)-di ...
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 The Kaapvaal Craton (centred on Limpopo Province in South Africa), along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga (billion years ago) crust on Earth. Similarities of rock records fr ...
, 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

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 impact structure. 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 Parys (pronounced ) is a town situated on the banks of the Vaal River in the Free State province of South Africa. The name of the town is the Afrikaans translation of Paris. The name was given by a German surveyor named Schilbach who had partic ...
, 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 MHz FM.


See also

*
List of impact craters on Earth This list of impact craters on Earth contains a selection of the 190 confirmed craters given in the Earth Impact Database as of 2017. To keep the lists manageable, only the largest craters within a time period are included. Alphabetical lists f ...
*
List of possible impact structures on Earth This is a list of possible impact structures on Earth. More than 130 geophysical features on the surface of the Earth have been proposed as candidate sites for impact events by appearing several times in the literature and/or being endorsed by the ...


References


External links


Parys South Africa

Impact Cratering Research Group – University of the Witwatersrand





Satellite image of Vredefort impact structure
from
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Impact Cratering: an overview of Mineralogical and Geochemical aspects – University of Vienna

Google Earth 3d .KMZ of 25 largest craters
(requires Google Earth) {{Impact cratering on Earth Extinction events Impact craters of South Africa Landforms of the Free State (province) Proterozoic impact craters World Heritage Sites in South Africa