Vredefort impact structure

(Redirected from Vredefort crater)

The Vredefort impact structure /ˈfrɪərdəfɔːrt/ is the largest verified impact structure on Earth.[1] The crater, which has since been eroded away, was around 160–300 km (100–200 mi) across when it was formed.[2][3] The remaining structure, comprising the deformed underlying bedrock, is located in present-day Free State province of 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 billion years (± 4 million years) ago. It is the second-oldest known impact structure on Earth, after Yarrabubba.

Vredefort impact structure
Vredefort Dome
Vredefort Dome STS51I-33-56AA.jpg
Vredefort Dome (centre), with the Vaal river running across it; seen from space with the Operational Land Imager on Landsat 8, 27 June 2018
Impact crater/structure
ConfidenceConfirmed
Diameter300 km (190 mi)
Age2,023 ± 4 Ma
Orosirian, Paleoproterozoic
ExposedYes
DrilledYes
Location
Coordinates27°0′0″S 27°30′0″E / 27.00000°S 27.50000°E / -27.00000; 27.50000Coordinates: 27°0′0″S 27°30′0″E / 27.00000°S 27.50000°E / -27.00000; 27.50000
CountrySouth Africa
ProvinceFree State
Vredefort impact structure is located in South Africa
Vredefort impact structure
Location of Vredefort impact structure
Official nameVredefort Dome
Includes
  1. Vredefort Dome core area
  2. The stromatolite/basal fault plane site
  3. The chocolate tablet breccia site
  4. The pseudotachylite (quarry) site
CriteriaNatural: (viii)
Reference1162
Inscription2005 (29th Session)
Area30,000 ha (120 sq mi)

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

Formation and structureEdit

 
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, 300 km (190 mi) in diameter, marks the extent of the original crater.
 
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 10–15 km (6.2–9.3 mi) in diameter.[4] The bolide that created the Sudbury Basin was larger.[1][5] As of 2022, the bolide was estimated at between 20 and 25 kilometers in diameter.[6]

The original impact structure is estimated to have had a diameter of roughly 300 km (190 mi),[3] but that has been eroded. It would have been larger than the 130 km (81 mi) Sudbury Basin and the 150 km (93 mi) Chicxulub crater. The remaining structure, the "Vredefort Dome", consists of a partial ring of hills 70 km (43 mi) 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),[2] which places it in the 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 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.[7] Other purported older impact structures have either poorly constrained ages (Dhala impact structure, India)[8] or highly contentious impact evidence in case of the circa 3.023 billion year old Maniitsoq structure, West Greenland[9] and the circa 2.4 billion year old Suavjärvi structure, Russia.[10] Their classification as impact structures remain controversial and unsettled.[7][11]

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 300-kilometre-wide (190 mi) impact structure.[4][12] The rocks form partial concentric rings around the impact structure's centre today, with the oldest, the Witwatersrand rocks, forming a semicircle 25 km (16 mi) from the centre. Since the Witwatersrand rocks consist of several layers of very hard, erosion-resistant sediments (e.g. quartzites and banded ironstones),[4][13] 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 35 km (22 mi) 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-to-30-kilometre-wide (16 to 19 mi) band beyond that.[14]

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 60 km (37 mi) from the centre, followed by an arc of Ventersdorp lavas, beyond which, at between 80 and 120 km (50 and 75 mi) 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.[4][14] It is thus possible that if it had not been for the Vredefort impact this gold would never have been discovered.[4]

The 40-kilometre-diameter (25 mi) 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.[4] 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.

ConservationEdit

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.[15] The granting of prospecting rights around the edges of the impact structure has led environmental interests to express fear of destructive mining.

CommunityEdit

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 MHz FM.

See alsoEdit

ReferencesEdit

  1. ^ a b University of Rochester (26 September 2022). "The asteroid that formed Vredefort crater was bigger than previously believed". Science X. Archived from the original on 26 September 2022.
  2. ^ a b "Vredefort". Earth Impact Database. Planetary and Space Science Centre University of New Brunswick Fredericton. Retrieved 30 December 2008.
  3. ^ a b "Deep Impact - The Vredefort Dome". Hartebeesthoek Radio Astronomy Observatory. 1 August 2006. Retrieved 19 September 2007.
  4. ^ a b c d e f McCarthy, Terence (2005). The Story of Earth & Life. Struik Publishers. pp. 89–90, 102–107, 134–136. ISBN 978-1-77007-148-3. OCLC 883592852.
  5. ^ "The Vredefort Dome: Centre of the World's Largest Meteorite Impact Structure!". Archived from the original on 31 December 2011. Retrieved 31 December 2012.
  6. ^ Allen, Natalie H.; Nakajima, Miki; Wünnemann, Kai; Helhoski, Søren; Trail, Dustin (2022). "A Revision of the Formation Conditions of the Vredefort Crater". Journal of Geophysical Research: Planets. 127 (8): e2022JE007186. doi:10.1029/2022JE007186.
  7. ^ a b 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
  8. ^ 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
  9. ^ 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
  10. ^ 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
  11. ^ 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. doi:10.1111/maps.12284
  12. ^ Norman, Nick; Whitfield, Gavin (2006). Geological Journeys. Struik Publishers. pp. 38–49, 60–61. ISBN 978-1-77007-062-2. OCLC 974035410.
  13. ^ Truswell, J. F (1977). The Geological Evolution of South Africa. Purnell. pp. 23–38. OCLC 488347575.
  14. ^ a b Geological map of South Africa, Lesotho and Swaziland (1970). Council for Geoscience, Geological Survey of South Africa.
  15. ^ Momberg, Eleanor (23 August 2009). "River heading for the rocks". Retrieved 22 March 2011.

External linksEdit