Richat Structure

File:Richat structure cropped.png

The Richat Structure is a deeply eroded, slightly elliptical dome with a diameter of {{convert|40|km|mi}}. 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 {{convert|30|km|mi}} in diameter.{{Cite web |last=Matton |first=G. |year=2008 |title=The Cretaceous Richat Complex (Mauritania); a peri-Atlantic alkaline |url=http://bibvir.uqac.ca/theses/030084214/030084214.pdf |publisher=Université du Québec à Chicoutimi |place=Chicoutimi, Quebec, Canada |access-date=7 September 2011 |archive-date=2 April 2012 |archive-url=https://web.archive.org/web/20120402073745/http://bibvir.uqac.ca/theses/030084214/030084214.pdf |url-status=dead }}{{Cite journal |last1=Matton |first1=Guillaume |last2=Jébrak |first2=Michel |last3=Lee |first3=James K.W. |year=2005 |title=Resolving the Richat enigma: Doming and hydrothermal karstification above an alkaline complex |url=http://www.archipel.uqam.ca/272/1/Matton_etal05.pdf |journal=Geology |volume=33 |issue=8 |pages=665–68 |doi=10.1130/G21542AR.1 |ref={{sfnref|Matton|2005}}}}{{Cite book |last=Woolley |first=Alan Robert |url=https://www.worldcat.org/oclc/16448519 |title=Alkaline Rocks and Carbonatites of the World |year=1987 |isbn=0-292-70389-9 |publisher=University of Texas Press|location=Austin |oclc=16448519}}Image:ASTER Richat.jpgExposed 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 {{convert|30|m|ft}} in width, {{convert|3|km|mi}} from the center of the Richat Structure. The outer ring dike is about {{convert|70|m|ft}} in width, {{convert|8|km|mi}} from the center of the structure. Thirty-two carbonatite dikes and sills have been mapped within the structure. The dikes are generally about {{convert|300|m|ft}} long and typically {{convert|1 to 4|m|ft}} 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.{{Cite journal |last1=Netto |first1=A.M. |last2=Fabre |first2=J. |last3=Poupeau |first3=G. |last4=Champemmois |first4=M. |year=1992 |title=Datations par traces de fissions de la structure circulaire des Richats |journal=Comptes Rendus de l'Académie des Sciences de Paris |volume=314 |pages=1179–1186}}{{Cite journal |last1=Matton |first1=Guillaume |last2=Jébrak |first2=Michel |year=2014 |title=The 'eye of Africa' (Richat dome, Mauritania): An isolated Cretaceous alkaline–hydrothermal complex |url=https://www.academia.edu/9220436 |journal=Journal of African Earth Sciences |volume=97 |pages=109–124 |bibcode=2014JAfES..97..109M |doi=10.1016/j.jafrearsci.2014.04.006}}

File:GuelbRichatTopo.png

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 {{convert|40|m|ft}} 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.

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.{{Cite journal |last=Richard-Molard |first=Jacques |year=1948 |title=The Richât Buttonhole in Mauritanian Adrar |journal=Comptes Rendus de l'Académie des Sciences |volume=227 |pages=142}} 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.{{Cite journal |last=Cailleux |first=A. |year=1962 |title=El Richat: dôme arasé, surbaissé ou cratère bombé? |journal=Notes Africaines |volume=93 |pages=27–29}} 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.{{Cite journal |last1=Dietz |first1=Robert S. |last2=Fudali |first2=Robert |last3=Cassidy |first3=William |year=1969 |title=Richat and Semsiyat Domes (Mauritania): Not Astroblemes |journal=Geological Society of America Bulletin |volume=80 |issue=7 |pages=1367 |bibcode=1969GSAB...80.1367D |doi=10.1130/0016-7606(1969)80[1367:RASDMN]2.0.CO;2}} 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.{{Cite journal |last=Fudali |first=R. F. |date=10 October 1969 |title=Coesite from the Richat Dome, Mauritania: A Misidentification |journal=Science |volume=166 |issue=3902 |pages=228–230 |bibcode=1969Sci...166..228F |doi=10.1126/science.166.3902.228 |pmid=17731489 |s2cid=37249316}} Work on dating the structure was done in the 1990s.{{Cite journal |last1=Netto |first1=A.M. |last2=Fabre |first2=J. |last3=Poupeau |first3=G. |last4=Champemmois |first4=M. |year=1992 |title=Fission Trace Dates of the Richats Circular Structure |journal=Comptes Rendus de l'Académie des Sciences de Paris |volume=314 |pages=1179–86}}
{{Cite journal |last1=Poupeau |first1=G. |last2=Fabre |first2=J. |last3=Labrin |first3=E. |last4=Azdimoussa |first4=A. |last5=Netto |first5=A. M. |last6=Monod |first6=T. |year=1996 |title=Nouvelles datations par traces de fission de la structure circulaire des Richat (Mauritanie) |journal=Mémoires du Service Géologique de l'Algérie |volume=8 |pages=231–36}} 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,{{Cite journal |last1=Abdeina |first1=El Houssein |last2=Bazin |first2=Sara |last3=Chazot |first3=Gilles |last4=Bertrand |first4=Hervé |last5=Le Gall |first5=Bernard |last6=Youbi |first6=Nasrrddine |last7=Sabar |first7=Mohamed Salem |last8=Bensalah |first8=Mohamed Khalil |last9=Boumehdi |first9=Moulay Ahmed |date=2021-11-03 |title=Geophysical modelling of the deep structure of the Richat magmatic intrusion (northern Mauritania): insights into its kinematics of emplacement |url=https://doi.org/10.1007/s12517-021-08734-4 |journal=Arabian Journal of Geosciences |language=en |volume=14 |issue=22 |pages=2315 |doi=10.1007/s12517-021-08734-4 |bibcode=2021ArJG...14.2315A |s2cid=241620488 |issn=1866-7538}} 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.{{efn|The breccia core is genetically related to plutonic activity since doming and the production of hydrothermal fluids were instrumental in creating a favorable setting for dissolution. The resulting fluids were also responsible for subsequent silicification and hydrothermal infilling. To the best of our knowledge, karst collapse phenomena at the summit of an alkaline complex are unique but may be more frequent than previously believed.{{harv|Matton|2005}}}}

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."

Image:Richat Structure - SRTM.jpg

The Richat Structure is the location of exceptional accumulations of Acheulean artifacts. These Acheulean archaeological sites are located along wadis 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 outcrops 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,{{Cite journal |last=Monod |first=T. |year=1975 |title=Three Pebble Deposits in the Mauritanian Adrar (Western Sahara) |journal=Provence Historique |volume=99 |pages=87–97}} 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.{{Cite journal |last1=Sao |first1=Ousmane |last2=Giresse |first2=Pierre |last3=de Lumley |first3=Henry |last4=Faure |first4=Olivier |last5=Perrenoud |first5=Christian |last6=Saos |first6=Thibaud |last7=Rachid |first7=Mouamar Ould |last8=Touré |first8=Ousmane Cherif |year=2008 |title=The Sedimentary Environments of the Pre-Eulean and Aeulean Deposits of the Akerdil and Bamouéré Wadis (Guelb er-Richât, Adrar, Mauritania), A First Approach |journal=L'Anthropologie |volume=112 |issue=1 |pages=1–14 |doi=10.1016/j.anthro.2008.01.001}}

Artifacts are found, typically redeposited, deflated, or both, in Late Pleistocene to early Holocene gravelly mud, muddy gravel, clayey sand, and silty sand. These sediments are often cemented into either concretionary masses or beds by calcrete. Ridges typically consist of deeply weathered bedrock representing truncated Cenozoic paleosols 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 {{convert|3 to 4|m|ft|}}-thick 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 fluviatile or even torrential flow deposits. The finer-grained, sandy deposits consist of eolian and playa lake deposits. The latter contain well-preserved freshwater fossils. Numerous concordant radiocarbon dates indicate that the bulk of these sediments accumulated between 15,000 and 8,000 BP during the African humid period. These deposits lie directly upon deeply eroded and weathered bedrock.{{Cite journal |last1=Giresse |first1=Pierre |last2=Sao |first2=Ousmane |last3=de Lumley |first3=Henry |year=2012 |title=Paleo-environmental Study of the Quaternary Sediments of Guelb and Richât (Adrar of Mauritania) with Respect to Neighboring or Associated Sites of the Lower Paleolithic. Discussion and Perspectives |journal=L'Anthropologie |volume=116 |issue=1 |pages=12–38 |doi=10.1016/j.anthro.2011.12.001}}

Several enclosed structures have been noted within the vicinity of the Richat Structure, which may have served as animal pens. Satellite archaeologist Sarah Parcak suggested that they may be around 8,000 to 5,000 years old.{{cite episode |title=The Curse of Atlantis |url=https://www.natgeotv.com/za/shows/natgeo/atlas-of-cursed-places |series=Atlas of Cursed Places |network=National Geographic |airdate=2020 |season=1 |minutes=40-43 |access-date=2025-05-24 |credits=Sam Sheridan and Sarah Parcak}}

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.{{Citation |last=Garcea |first=Elena A.A. |title=Aterian |date=2021-11-29 |work=Oxford Research Encyclopedia of Anthropology |url=https://oxfordre.com/anthropology/view/10.1093/acrefore/9780190854584.001.0001/acrefore-9780190854584-e-327 |access-date=2025-06-02 |publisher=Oxford University Press |language=en |doi=10.1093/acrefore/9780190854584.013.327 |isbn=978-0-19-085458-4}}

The protruding land dikes of the Richat Structure are lined with thousands of stone burial mounds in various styles.{{Cite journal |last=Vernet |first=R. |date=2025 |title=Funerary Monuments of Central Mauritania: Inventory and Distribution |url=https://prehistoireouestsaharienne.wordpress.com/2025/05/10/inventaire-des-peintures-rupestres-du-plateau-du-tagant/ |journal=Arid Zone Archeology Monographs |volume=10 |pages=129-151}} 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).{{Cite web |last=Vernet |first=Auteur Robert |date=2020-06-27 |title=Les gravures rupestres de l’Adrar de Mauritanie – III. Autres stations |url=https://prehistoireouestsaharienne.wordpress.com/2020/06/27/les-gravures-rupestres-de-ladrar-de-mauritanie-iii-autres-stations/ |access-date=2025-06-02 |website=Préhistoire de l'Ouest Saharien |language=fr-FR}}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.

The Richat Structure has been the subject of 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. Most classicists believe that Atlantis was a fictional rhetorical invention by Plato, rather than a real geographic location.{{cite book |last=Clay |first=Diskin |title=Proceedings of the Boston Area Colloquium in Ancient Philosophy |publisher=E. J. Brill |year=2000 |isbn=978-90-04-11704-4 |editor-last=Cleary |editor-first=John J. |volume=15 |location=Leiden |pages=1–21 |chapter=The Invention of Atlantis: The Anatomy of a Fiction |author-link=Diskin Clay |editor2-last=Gurtler |editor2-first=Gary M. |chapter-url={{Google books |plainurl=yes |id=AMRl67uqD9wC |page=1}}}}"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." ([http://shimajournal.org/issues/v10n2/c.-Dawson-Hayward-Introduction-Shima-v10n2.pdf 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 inland location in a desert contradicting Plato's information regarding the location of Atlantis.{{Cite book |last=Rafferty |first=Sean M. |title=Mythologizing the Past: Archaeology, History, and Ideology |date=2025 |publisher=ROUTLEDGE |isbn=978-1-032-69020-9 |location=S.l. |pages=26, 46 |quote=Then there is the Richat Structure, a 30 km wide circular formation in the Sahara Desert. A dome of sedimentary rock formed millions of years ago has eroded at the surface, leaving a circular formation of exposed strata. Proponents of an Atlantean connection have argued that this circular structure exactly duplicates the descriptions of Plato, and so it must be the remains of the actual city. The facts that it is actually not a very good match for Plato’s description (aside from being circular), is vastly too old, and is located in a desert, are data that again miss being cherry-picked.}}

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{{Commons category|Richat Structure}}

• Anonymous (nd) [http://www.lovethesepics.com/2011/04/earths-bulls-eye-the-eye-of-africa-landmark-for-astronauts-14-pics/ Earth’s Bulls-Eye, the Eye of Africa, Landmark for Astronauts]. [http://www.lovethesepics.com/2011/04/earths-bulls-eye-the-eye-of-africa-landmark-for-astronauts-14-pics// Love These Pics]
• Anonymous (nd) [https://earthobservatory.nasa.gov/images/2561/richat-structure-mauritania Richat Structure, Mauritania] NASA Earth Observatory
• [https://www.youtube.com/watch?v=pTTnYRaZ55A Discovery - Richat's Enigma (French)], a video documentary from Radio Canada.
• Nemiroff, R., and J. Bonnell (2002) [http://antwrp.gsfc.nasa.gov/apod/ap021028.html Earth's Richat Structure, Astronomy Picture of the Day, October 28, 2002]. Astronomy Picture of the Day, Michigan Technological University, Houghton, Michigan.
• Documentary film [https://www.youtube.com/watch?v=mdmODR6JmmU WHAT is the Richat Structure / The Eye of the Sahara]

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Category:Adrar region

Category:Geology of Mauritania

Category:Geography of Mauritania

Category:Regional geology

Category:Structural geology

Category:Tiris Zemmour region

Category:Geologic domes

Category:Sahara

Category:First 100 IUGS Geological Heritage Sites

Category:Ring dikes