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The Richat Structure

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The Richat Structure - © Attention Deficit Disorder Prosthetic Memory Program

The Richat Structure, also called Guelb er Richât, the Eye of Africa, Eye of Mauritania or Eye of the Sahara is a prominent circular feature in the Sahara’s Adrar Plateau, near Ouadane, west–central Mauritania, Northwest Africa. It is an eroded dome, 40 kilometres in diameter, exposing sedimentary rock in layers which appear as concentric rings.

Igneous rock is exposed inside and there are spectacular rhyolites and gabbros which have undergone hydrothermal alteration, and a central megabreccia. The structure is also the location of exceptional accumulations of Acheulean archaeological artifacts.

The Richat Structure is regarded by geologists as a highly symmetrical and deeply eroded geologic dome. It was first described in the 1930s to 1940s, as Richât Crater or Richât buttonhole (boutonnière du Richât). Richard-Molard (1948) considered it to be the result of a laccolithic thrust. A geological expedition to Mauritania led by Théodore Monod in 1952 recorded four “crateriform or circular irregularities” (accidents cratériformes ou circulaires) in the area, Er Richât, Aouelloul (south of Chinguetti), Temimichat-Ghallaman and Tenoumer. Origin of Er Richât as an impact structure (as is clearly the case with the other three) was briefly considered, but closer study in the 1950s to 1960s suggested that it was formed by terrestrial processes. After extensive field and laboratory studies in the 1960s, no credible evidence has been found for shock metamorphism or any type of deformation indicative of a hypervelocity extraterrestrial impact. While coesite, an indicator of shock metamorphism, had initially been reported as being present in rock samples collected from the Richat Structure, further analysis of rock samples concluded that barite had been misidentified as coesite.

Work on dating the structure was done in the 1990s. Renewed study of the formation of the structure by Matton et al. (2005) and Matton (2008) confirmed the conclusion that it is not an impact structure. The circular distribution of ridges and valleys is explained as the formation of cuestas by the differential erosion of alternating hard and soft rock layers uplifted as a dome by an underlying alkaline igneous complex of Cretaceous age.

A 2011 multianalytical study on the Richat megabreccias concluded that carbonates within the silica-rich megabreccias were created by low-temperature hydrothermal waters, and that the structure requires special protection and further investigation of its origin.

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