The Richat Structure, or Guelb er Richât (, ), often called the Eye of Africa
It is an eroded geological dome, in diameter, caused by a subsurface igneous intrusion deforming the overlying sedimentary rock layers, causing the rock to be exposed as concentric rings with the oldest layers exposed at the centre of the structure. Igneous rock is exposed inside and there are and that have undergone hydrothermal alteration, and a central Breccia. The structure is also the location of exceptional accumulations of Acheulean Paleolithic stone tools. It was selected as one of the 100 geological heritage sites identified by the International Union of Geological Sciences (IUGS) to be of the highest scientific value.
Description
The Richat Structure is a deeply
Erosion, slightly
elliptical dome 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
composing this structure dip outward at 10–20°. Differential erosion of resistant layers of
quartzite has created high-relief circular
. Its center consists of a
Silicon dioxide breccia covering an area that is at least in diameter.
within the interior of the Richat Structure is a variety of
Intrusive rock and
Extrusive rock . They include
Rhyolite volcanic rocks,
,
and
. The rhyolitic rocks consist of
lava flows and hydrothermally altered
rocks that are part of two distinct eruptive centers, which are interpreted to be the eroded remains of two
. According to field mapping, aeromagnetic, and
Gravimetry data, the gabbroic rocks form two concentric
. 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.
Thirty-two carbonatite dikes and sills have been mapped within the structure. The dikes are generally about long and typically wide. They consist of massive carbonatites that are mostly devoid of vesicles. The carbonatite rocks have been dated as having cooled between 94 and 104 million years ago. A kimberlitic plug and several sills have been found within the northern part of the structure. The kimberlite plug has been dated to around 99 million years old. These intrusive igneous rocks are interpreted as indicating the presence of a large alkaline igneous intrusion that currently underlies the structure and was created by uplifting the overlying rock.
Spectacular hydrothermal features are a part of the Richat Structure. They include the extensive hydrothermal alteration of rhyolites and gabbros and a central Breccia 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 material, quartz-rich sandstone, diagenetic cherty nodules, and Stromatolite 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
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 crater (south of
Chinguetti),
Temimichat-Ghallaman and
Tenoumer crater.
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.
, 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.
Further analysis of deep structure underneath the surface, including with aeromagnetic and gravimetric mapping,
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.
These Acheulean archaeological sites are located along
that occupy the outermost annular depression of this structure. Pre-Acheulean stone tools also have been found in the same areas. These sites are associated with rubbly
of
quartzite that provided the raw material needed for the manufacture of these artifacts. The most important Acheulean sites and their associated outcrops are found along the northwest of the outer ring, from which Wadi Akerdil heads east and Wadi Bamouere to the west. Sparse and widely scattered
Neolithic spear points and other artifacts have also been found. However, since these sites were first discovered by Théodore Monod in 1974,
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.
Artifacts are found, typically redeposited, deflated, or both, in Late Pleistocene to early Holocene gravelly mud, muddy gravel, sand, and sand. These are often cemented into either masses or beds by calcrete. Ridges typically consist of deeply weathered bedrock representing truncated Cenozoic that formed under tropical environments. The Pleistocene to Middle Holocene sediments occur along wadis as the thin, meter- to less-than-meter-thick accumulations in the interior annular depressions to accumulations along the wadis in the outermost annular depression of the structure. The gravelly deposits consist of a mixture of slope scree, debris flow, and fluvial or even torrential flow deposits. The finer-grained, sandy deposits consist of Aeolian dust and playa lake deposits. The latter contain well-preserved freshwater . Numerous concordant radiocarbon dates indicate that the bulk of these sediments accumulated between 15,000 and 8,000 Before Present during the African humid period. These deposits lie directly upon deeply eroded and weathered bedrock.
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.[Bordes J.G., Mourre V., 2010. Introduction : bilan sur les recherches connues. In : Bordes J.G., Gonzalez-Carballo A., Vernet R., 2010 – La Majâbat al koubrâ. Nord-ouest du bassin de Taoudenni, Mauritanie. Archéologiques 3. MNHA, Luxembourg, 367 p. : 131-163.]
The protruding land dikes of the Richat Structure are lined with thousands of Tumulus in various styles.[Lluch, P.; Philip, S. (2003). "Six stations à gravures du N.E. de l'Adrar (dhar Chinguetti, Mauritanie)". Cahiers de l’AARS. 8: 87–96.][Mauny, R. (1954). Gravures, peintures et inscriptions rupestres de l’ouest saharien. Dakar: IFAN Dakar. p. 92.][Monod, Th. (1938). Contribution à l’étude du Sahara occidental : gravures, peintures, inscriptions rupestres. Paris: Éditions Larose. p. 173.]
Fringe theory of Atlantis site
The Richat Structure has been the subject of unsubstantiated
Fringe theory 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.
Most classicists believe that Atlantis was a fictional
Rhetoric invention by Plato, rather than a real geographic location.
["As Smith discusses in the opening article in this theme issue, the lost island-continent was – in all likelihood – entirely Plato's invention for the purposes of illustrating arguments around Grecian polity. Archaeologists broadly agree with the view that Atlantis is quite simply 'utopia' (Doumas, 2007), a stance also taken by classical philologists, who interpret Atlantis as a metaphorical rather than an actual place (Broadie, 2013; Gill, 1979; Nesselrath, 2002). One might consider the question as being already reasonably solved but despite the general expert consensus on the matter, countless attempts have been made at finding Atlantis." ( Dawson & Hayward, 2016)] 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 being built there, and the lack of evidence for the canals mentioned in Plato's Atlantis account.
Notes
External links
-
Anonymous (nd) Earth’s Bulls-Eye, the Eye of Africa, Landmark for Astronauts. Love These Pics
-
Anonymous (nd) Richat Structure, Mauritania NASA Earth Observatory
-
Discovery - Richat's Enigma (French), a video documentary from Radio Canada.
-
Nemiroff, R., and J. Bonnell (2002) Earth's Richat Structure, Astronomy Picture of the Day, October 28, 2002. Astronomy Picture of the Day, Michigan Technological University, Houghton, Michigan.
-
Documentary film WHAT is the Richat Structure / The Eye of the Sahara
