Richat Structure

The Richat Structure is a deeply eroded, slightly elliptical dome with a diameter of . The sedimentary rock exposed in this dome ranges in age from Late Proterozoic within the center of the dome to Ordovician sandstone around its edges. The sedimentary rocks composing this structure dip outward at 10–20°. Differential erosion of resistant layers of quartzite has created high-relief circular cuestas. Its center consists of a siliceous breccia covering an area that is at least in diameter.Exposed within the interior of the Richat Structure is a variety of intrusive and extrusive igneous rocks. They include rhyolitic volcanic rocks, gabbros, carbonatites and kimberlites. The rhyolitic rocks consist of lava flows and hydrothermally altered tuffaceous rocks that are part of two distinct eruptive centers, which are interpreted to be the eroded remains of two maars. According to field mapping, aeromagnetic, and gravimetric data, the gabbroic rocks form two concentric ring dikes. The inner ring dike is about in width, from the center of the Richat Structure. The outer ring dike is about in width, from the center of the structure. Spectacular hydrothermal features are a part of the Richat Structure. They include the extensive hydrothermal alteration of rhyolites and gabbros and a central megabreccia created by hydrothermal dissolution and collapse. The siliceous megabreccia is at least thick in its center to only a few meters thick along its edges. The breccia consists of fragments of white to dark gray cherty material, quartz-rich sandstone, diagenetic cherty nodules, and stromatolitic limestone and is intensively silicified. The hydrothermal alteration, which created this breccia, has been dated to have occurred about 98.2 ± 2.6 million years ago using the 40Ar/39Ar method. ==Interpretation==

The structure 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 uplift. 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. It was initially considered to be an impact structure (as is clearly the case with the other three), but a closer study in the 1950s to 1960s suggested that it might instead have been formed by terrestrial processes. After field and laboratory studies in the 1960s, no significant evidence was found for shock metamorphism or other deformation indicative of a hypervelocity extraterrestrial impact. Coesite, an indicator of shock metamorphism, was initially reported as being present in rock samples from the structure, but a further analysis in 1969 concluded that barite had been misidentified as coesite. Work on dating the structure was done in the 1990s. A study of the formation of the structure by Matton, et al. (2005, 2008) concluded it was not an impact structure. concluded that the structure is the result of ring faults which led to gabbroic ring dikes over a large intrusive body of magma, and the uplifting and later erosion of a dome, through intense hydrothermal activity through the fractured substructure. This can form cuestas over time through the differential erosion of the resulting alternating hard and soft rock layers. The underlying alkaline igneous complex exposed through erosion dates to the Cretaceous period. IUGS geological heritage site In respect of it being "a spectacular example of a magmatic concentric alkaline complex", the International Union of Geological Sciences (IUGS) included the Richat Structure in its assemblage of 100 geological heritage sites around the world, in October 2022. The organisation defines an IUGS Geological Heritage Site as "a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history." == Archaeology ==

The Richat Structure is the location of exceptional accumulations of Acheulean artifacts. mapping of artifacts within the area of the structure have found them to be generally absent in its innermost depressions. The local apparent wealth of surface artifacts is the result of the concentration and mixing by deflation over multiple glacial-interglacial cycles. In addition to Acheulean artifacts, the Richat Structure contains Middle Stone Age Aterian stone artifacts produced by modern humans, dating to the latest Middle Pleistocene to Late Pleistocene period, around 145,000-29,000 years ago. The protruding land dikes of the Richat Structure are lined with thousands of stone burial mounds in various styles. These still remain to be excavated in order to establish their age. Several rock art sites have also been identified, which include depictions of horsemen armed with javelins, chariots, bovids, elephants (Tililit, Oued Slil) and Libyco-Berber inscriptions (Tin Labbé, Lemqader). == Fringe theory of Atlantis site ==

The Richat Structure has been the subject of unsubstantiated fringe claims to be the site of Atlantis mentioned in the works of Plato. This claim is primarily based on the concentric nature of the structure, which superficially matches Plato's description of the city. According to archaeologist Sean M. Rafferty, other than superficially matching Plato's Atlantis description in being circular, in its particular details, including being a geologically ancient natural structure, the Richat Structure bears little resemblance to Plato's description of Atlantis, with the Richat Structure's location far inland in a desert contradicting Plato's information regarding the location of Atlantis. Skeptic Steven Novella criticised other inconsistencies in the claim, including the lack of any archaeological evidence for a city having been built there, and the lack of evidence for the canals mentioned in Plato's Atlantis account. ==Notes==