Ring I: outer limit of the melt sheet + meteorite fragments Ring II: outer limit of melt and/or suevite/allochthonous breccia Ring III: outer limit of shock metamorphism, central uplift, and intense brecciation Ring IV: zone of groundwater-bearing fractures, faulting, and the limit of post-impact doming
Artistic representation of a brachiosaurid, with the Morokweng impactor in the background, moments before impact
Estimates of the diameter of the structure vary widely with some studies suggesting a smaller size of 75–80km (47–50mi) in diameter while others suggests a much larger size of around 240km (150mi) or more. Its age is estimated to be 146.06 ± 0.16 million years, placing it within the Tithonian stage of the Late Jurassic, several million years before the Jurassic–Cretaceous boundary.[2][3][4] Discovered in 1994, it is not exposed at the surface, but has been mapped by magnetic and gravimetric surveys. Core samples have shown it to have been formed by the impact of an L chondrite asteroid estimated to have been 5 to 10km (3.1 to 6.2mi) in diameter.[5]
In May, 2006, a group of scientists drilling into the site announced the discovery of a 25-centimetre-diameter (9.8in) fragment of the original asteroid at a depth of 770m (2,530ft) below the surface, along with several much smaller pieces a few millimetres across at other depths. This discovery was unexpected, since previous drillings on large impact structures had not produced such fragments, and it was thought that the asteroid had been almost entirely vaporised.[6] Some of the fragments can be seen in the Antenna Wing of London's Science Museum and in the "Museo del cielo e della Terra" sited in San Giovanni in Persiceto (Bologna - Italy).
A 2007 paper by Andreoli and others compiled microgravity, aeromagnetic, satellite, and borehole data to present the case that the structure is up to 240 km diameter with five "rings" representing differing subsurface conditions.[4] Four of the five rings are shown in the figure in upper right.
↑Kenny, G.G., Harrigan, C.O., Schmitz, M.D., Crowley, J.L., Wall, C.J., Andreoli, M.A., Gibson, R.L. and Maier, W.D., 2021. Timescales of impact melt sheet crystallization and the precise age of the Morokweng impact structure, South Africa. Earth and Planetary Science Letters, 567, no. 117013, 13 p.
↑Schmieder, M. and Kring, D.A., 2020. Earth's impact events through geologic time: a list of recommended ages for terrestrial impact structures and deposits. Astrobiology, 20(1), pp.91-141.
12Andreoli, M. A. G., et al. "The 144 Ma Morokweng impact crater, South Africa: geophysical and borehole evidence for a~ 240 km structure." 10th SAGA Bienniak Technical Meeting and Exhibition. 2007.
↑Mcdonald, I., Andreoli, M.A.G., Hart, R.J. and Tredoux, M., 2001. Platinum-group elements in the Morokweng impact structure, South Africa: Evidence for the impact of a large ordinary chondrite projectile at the Jurassic-Cretaceous boundary. Geochimica et Cosmochimica Acta, 65(2), pp.299-309.
↑Maier, WD; Andreoli MA; McDonald I; Higgins MD; Boyce AJ; Shukolyukov A; Lugmair GW; Ashwal LD; Graser P; Ripley EM; Hart RJ (11 May 2006). "Discovery of a 25-cm asteroid clast in the giant Morokweng impact crater, South Africa". Nature. 441 (7090): 203–6. Bibcode:2006Natur.441..203M. doi:10.1038/nature04751. PMID16688173. S2CID4373614.
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