View source for Earth:Vredefort impact structure

{{Short description|Largest verified impact structure on Earth, about 2 billion years old}}
{{Use South African English|date = July 2022}}
{{Infobox terrestrial impact site
| name = Vredefort impact structure
| other_name = Vredefort Dome
| photo = Vredefort Crater, South Africa, OLI satellite image, 27 June 2018 cropped.png
| photo_size = 
| photo_alt = 
| photo_caption = Vredefort Dome (centre), with the Vaal river running across it; seen from space with the [[Engineering:Operational Land Imager|Operational Land Imager]] on [[Engineering:Landsat 8|Landsat 8]], {{Date|27 June 2018|DMY}}
| map = South Africa
| map_alt = 
| map_caption = Location of Vredefort impact structure
| map_size = 
| location = 
| label =
| label_position = 
| coordinates = {{coord|27|0|0|S|27|30|0|E|type:landmark_scale:5000000_region:SA|display= inline,title}}
| coordinates_ref = 
| confidence = Confirmed
| diameter = {{convert|170-300|km|abbr=on}} (estimated former crater diameter)
| depth =
| rise =
| imp_size =
| age = {{long fossil range|2023}}± 4 Ma<br /> [[Earth:Orosirian|Orosirian]], [[Earth:Paleoproterozoic|Paleoproterozoic]]
| exposed = Yes
| drilled = Yes
| bolide =
| translation =
| language =
| pronunciation =
| topo = 
| access = 
| country = {{wipe|South Africa}}
| state = 
| province = Free State
| district = 
| municipality = 
| module = {{Infobox UNESCO World Heritage Site
| child         = yes
| official_name = Vredefort Dome
| includes      = {{flatlist|
#Vredefort Dome core area
#The [[Earth:Stromatolite|stromatolite]]/basal fault plane site
#The chocolate tablet breccia site
#The [[Astronomy:Pseudotachylite|pseudotachylite]] (quarry) site
}}
| criteria      = {{UNESCO WHS type|(viii)}}(viii)
| ID            = 1162
| year          = 2005
| area          = {{convert|30000|ha|sqmi|abbr=on}}
| buffer_zone   = 
}}
}}

The '''Vredefort impact structure''' is one of the [[Earth:List of impact craters on Earth#10 Ma or more|largest]] verified impact structures on [[Astronomy:Earth|Earth]].<ref name="UR-2022">{{Cite web|author=((University of Rochester)) |date=26 September 2022 |title=The asteroid that formed Vredefort crater was bigger than previously believed |publisher=Science X |url=https://phys.org/news/2022-09-asteroid-vredefort-crater-bigger-previously.html |archive-url=https://web.archive.org/web/20220926202131/https://phys.org/news/2022-09-asteroid-vredefort-crater-bigger-previously.html |archive-date=26 September 2022 |url-status=live }}</ref> The crater, which has since been eroded away, has been estimated at {{convert|170-300|km|mi|||abbr=|sigfig=}} across when it was formed.<ref name=":0">{{Cite journal |last=Huber |first=M. S. |last2=Kovaleva |first2=E. |last3=Rae |first3=A. S. P. |last4=Tisato |first4=N. |last5=Gulick |first5=S. P. S. |date=August 2023 |title=Can Archean Impact Structures Be Discovered? A Case Study From Earth's Largest, Most Deeply Eroded Impact Structure |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JE007721 |journal=Journal of Geophysical Research: Planets |language=en |volume=128 |issue=8 |doi=10.1029/2022JE007721 |issn=2169-9097|doi-access=free }}</ref><ref name=":1" /> The remaining structure, comprising the deformed underlying bedrock, is located in present-day Free State province of {{wipe|South Africa}}. 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 Era, 2,023&nbsp;million  (± 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 [[Organization:UNESCO|UNESCO]] World Heritage Sites for its geologic interest.

==Formation and structure==
[[File:Map of Vredefort dome.jpg|thumb|left|Map of South Africa showing the location of the Vredefort Dome, the remains of a 2.023-billion-year-old impact structure. The interrupted line circle, {{cvt|300|km|||||}} in diameter, marks the extent of the original crater.]]
[[File:Aasvoelkop.jpg|thumb|left|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 [[Astronomy:Asteroid|asteroid]] that hit Vredefort is estimated to have been one of the largest ever to strike Earth since the [[Earth:Hadean|Hadean]] Eon some four billion years ago, originally thought to have been approximately {{convert|10|–|15|km|abbr=on}} in diameter.<ref name="McCarthy">{{cite book |last1=McCarthy |first1=Terence |title=The Story of Earth & Life |date=2005 |publisher=Struik Publishers |isbn=978-1-77007-148-3 |pages=89–90,102–107,134–136 |oclc=883592852 |language=English}}</ref> As of 2022, the [[Astronomy:Bolide|bolide]] was estimated at between {{Convert|20 and 25|km|mi}} in diameter and to have impacted with a vertical velocity of {{Convert|15|-|25|km/s|mph}}.<ref name=":1">{{Cite journal|last1=Allen |first1=Natalie H. |last2=Nakajima |first2=Miki |last3=Wünnemann |first3=Kai |last4=Helhoski |first4=Søren |last5=Trail |first5=Dustin |year=2022 |title=A Revision of the Formation Conditions of the Vredefort Crater |journal=Journal of Geophysical Research: Planets |volume=127 |issue=8 |page=e2022JE007186 |doi=10.1029/2022JE007186 |bibcode=2022JGRE..12707186A |s2cid=251449730 |url=https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2022JE007186 |doi-access=free }}</ref>

The original impact structure is estimated to have had a diameter of at least {{cvt|170|km|||||}}, with the impact affecting the structure of the surrounding host rock in a circular region around {{cvt|300|km|||||}} in diameter.<ref name=":0" /> Other estimates have placed the original crater diameter closer to {{convert|300|km|mi|||abbr=on|sigfig=}}.<ref name=":1" /> The landscape has since been eroded to a depth of around {{convert|7-11|km|mi|abbr=on}} since formation, obliterating the original crater. The remaining structure, the "Vredefort Dome", consists of a partial ring of hills {{cvt|70|km|||||}} in diameter, and is the remains of the central uplift created by the rebound of rock below the impact site after the collision.<ref name=":0" />

Estimates have placed the structure’s age to be 2.023 billion years (± 4 million years)<ref>{{Cite journal |last=Kamo |first=S.L. |last2=Reimold |first2=W.U. |last3=Krogh |first3=T.E. |last4=Colliston |first4=W.P. |date=November 1996 |title=A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre |url=https://linkinghub.elsevier.com/retrieve/pii/S0012821X9600180X |journal=Earth and Planetary Science Letters |language=en |volume=144 |issue=3-4 |pages=369–387 |doi=10.1016/S0012-821X(96)00180-X}}</ref> or 2.019/2.020 billion years (± 2-3 million years) old.<ref>{{Cite journal |last=Moser |first=D. E. |date=1997 |title=Dating the shock wave and thermal imprint of the giant Vredefort impact, South Africa |url=https://pubs.geoscienceworld.org/geology/article/25/1/7-10/190190 |journal=Geology |language=en |volume=25 |issue=1 |pages=7 |doi=10.1130/0091-7613(1997)025<0007:DTSWAT>2.3.CO;2 |issn=0091-7613}}</ref> which places it in the [[Earth:Orosirian|Orosirian]] 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 [[Earth:Sudbury Basin|Sudbury Basin impact]] (at 1.849 billion years) and the Yarrabubba impact structure is older than the Vredefort impact structure by about 0.2 billion years.<ref name="EricksonOthers2020a">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. {{doi|10.1038/s41467-019-13985-7}}</ref> Other purported older impact structures have either poorly constrained ages (Dhala impact structure, India)<ref name="PatiOthers2020a">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. {{doi|10.1111/maps.12826}}</ref> or highly contentious impact evidence in case of the circa 3.023 billion year old [[Earth:Maniitsoq structure|Maniitsoq structure]], West Greenland<ref name="GardeOthers2012a">Garde, 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. {{doi|10.1016/j.epsl.2012.04.026}}</ref> and the circa 2.4 billion year old [[Earth:Suavjärvi|Suavjärvi structure]], Russia.<ref name="MashchakOthers2012a">Mashchak, M.S. and Naumov, M.V., 2012. ''The Suavjärvi impact structure, NW Russia.'' ''Meteoritics & Planetary Science'', 47(10), pp.1644-1658. {{doi|10.1111/j.1945-5100.2012.01428.x}}</ref> Their classification as impact structures remain controversial and unsettled.<ref name="EricksonOthers2020a"/><ref name="ReimoldOthers2014a">{{cite journal |last1=Reimold|first1=W.U.|last2=Ferrière|first2=L.|last3=Deutsch|first3=A.|last4=Koeberl|first4=C.|author4-link=Christian Koeberl|date=2014 |title=Impact controversies: impact recognition criteria and related issues |url=https://onlinelibrary.wiley.com/doi/pdf/10.1111/maps.12284 |journal=Meteoritics & Planetary Science |volume=49 |issue=5 |pages=723–731 |doi=10.1111/maps.12284|bibcode=2014M&PS...49..723R |access-date=12 Oct 2023|doi-access=free}}</ref>

The [[Earth:Dome (geology)|dome]] in the centre of the impact structure was originally thought to have been formed by a [[Earth:Volcano|volcanic]] explosion, but in the mid-1990s, evidence revealed it was the site of a huge bolide impact, as telltale [[Astronomy:Shatter cone|shatter cone]]s were discovered in the [[Earth:Stream bed|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 [[Astronomy:Jupiter|Jupiter]]'s moon [[Astronomy:Callisto (moon)|Callisto]]. Earth's [[Astronomy:Moon|Moon]] has some as well. Geological processes, such as erosion and [[Earth:Plate tectonics|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&nbsp;million years between 950 and 700&nbsp;million years before the Vredefort impact. The overlying Ventersdorp lavas and the [[Earth:Transvaal Supergroup|Transvaal Supergroup]] which were laid down between 700 and 80&nbsp;million years before the meteorite strike, were similarly distorted by the formation of the {{convert|300|km|mi|adj=mid|-wide}} impact structure.<ref name="McCarthy" /><ref name="Norman">{{cite book |last1=Norman |first1=Nick |last2=Whitfield |first2=Gavin |title=Geological Journeys |date=2006 |publisher=Struik Publishers |isbn=978-1-77007-062-2 |oclc=974035410 |pages=38–49, 60–61|language=English}}</ref> The rocks form partial concentric rings around the impact structure's centre today, with the oldest, the Witwatersrand rocks, forming a semicircle {{cvt|25|km|||||}} from the centre. Since the Witwatersrand rocks consist of several layers of very hard, erosion-resistant sediments (e.g. [[Earth:Quartzite|quartzite]]s and banded ironstones),<ref name="McCarthy" /><ref>{{cite book |last1=Truswell |first1=J. F |title=The Geological Evolution of South Africa |date=1977 |publisher=Purnell |oclc=488347575 |language=English|pages=23–38}}</ref> 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 {{cvt|35|km|||||}} from the centre, and the Transvaal Supergroup, consisting of a narrow band of the Ghaap [[Earth:Dolomite (rock)|Dolomite]] rocks and the Pretoria Subgroup of rocks, which together form a {{convert|25|to|30|km|mi|adj=mid|-wide|}} band beyond that.<ref name="geological map">Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.</ref>

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 {{cvt|60|km|||||}} from the centre, followed by an arc of Ventersdorp lavas, beyond which, at between {{cvt|80|and|120|km|||}} 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.<ref name="McCarthy" /><ref name="geological map" /> It is thus possible that if it had not been for the Vredefort impact this gold would never have been discovered.<ref name="McCarthy" />

The {{convert|40|km|mi|adj=mid|-diameter}} 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&nbsp;billion years ago.<ref name="McCarthy" /> This central [[Astronomy:Peak ring (crater)|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.

<gallery widths="500px" heights="170px">
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 [[Earth:Banded iron formation|banded ironstone formations]] were formed on earth, indicative of an [[Earth:Great Oxygenation Event|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.
</gallery>

==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.<ref>{{cite web
 | title = River heading for the rocks
 | date = 23 August 2009
 | author = Momberg, Eleanor
 | url = http://www.iol.co.za/news/south-africa/river-heading-for-the-rocks-1.456012
 | access-date = 22 March 2011
 }}</ref> 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==
* [[Earth:List of impact craters on Earth|List of impact craters on Earth]]
* [[Earth:List of possible impact structures on Earth|List of possible impact structures on Earth]]
*Deniliquin multiple-ring feature
{{clear}}

== References ==
{{reflist | 30em}}

==External links==
* [http://www.parys.co.za/ Parys South Africa]
* [https://web.archive.org/web/20160601234203/https://www.wits.ac.za/science/schools/geosciences/research/impact-crater-research/ Impact Cratering Research Group – University of the Witwatersrand]
* [http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/Vredefort.html Earth Impact Database]
* [http://www.hartrao.ac.za/other/vredefort/vredefort.html Deep Impact – The Vredefort Dome]
* [https://maps.google.com/maps?ll=-27.016754,27.435608&spn=0.632744,0.962814&t=k&hl=en Satellite image of Vredefort impact structure] from [[Software:Google Maps|Google Maps]]
* [http://lithosphere.univie.ac.at/impactresearch/impact-cratering/ Impact Cratering: an overview of Mineralogical and Geochemical aspects – University of Vienna] {{Webarchive|url=https://web.archive.org/web/20110706085435/http://lithosphere.univie.ac.at/impactresearch/impact-cratering/ |date=2011-07-06 }}
* [http://www.thinklemon.com/ge/impact-top25.kmz Google Earth 3d .KMZ of 25 largest craters] (requires [[Software:Google Earth|Google Earth]])
{{Impact cratering on Earth}}

[[Category:Extinction events]]

{{Sourceattribution|Vredefort impact structure}}