Editing Richat Structure

{{Short description|Circular geological feature in the Sahara desert}}
{{Infobox landform
| water             = 
| name              = Richat Structure
| other_name        = ''Tagense''<ref name="Richard-Molard1952a" />
| type              = [[Dome (geology)|Dome]]
| photo             = Richat Structure ISS030-E-12516.jpg
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| photo_caption     = Image from the [[International Space Station]], 17 December 2011
| map               = Mauritania#Africa
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| map_caption       =Location within [[Mauritania]]##Location within [[Africa]] 
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| location          = [[Adrar Plateau]] of the [[Sahara]]
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| part_of           =[[Adrar Region]], Mauritania
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{{Use dmy dates|date=January 2023}}

The '''Richat Structure''', or '''''Guelb er Richât''''' ({{langx|ar|قلب الريشات|Qalb ar-Rīšāt}}, {{IPA|mey|galb er.riːʃaːt|lang|LL-Q56231 (mey)-Tidjani Saleh-گلب الريشات.wav}}), is a prominent circular geological feature in the [[Adrar Plateau]] of the [[Sahara]]. It is located near [[Ouadane]] in the [[Adrar Region]] of [[Mauritania]]. In [[Hassaniya Arabic]], ''rīšāt'' means ''feathers'' and it is also known locally in Arabic as ''tagense'', referring to the circular opening of the leather pouch that is used to draw water from local wells.<ref name="Richard-Molard1952a">{{Cite journal |last=Richard-Molard |first=J. |year=1952 |title=The Pseudo-boutonniers of Richat |journal=Gouvernement Général de l'Afrique Occidentale Française Bulletin de la Direction des Mines |volume=15 |issue=2 |pages=391–401}}</ref>

It is an eroded [[Dome (geology)|geological dome]], {{convert|40|km|mi}} 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 [[rhyolite]]s and [[gabbro]]s that have undergone [[hydrothermal alteration]], and a central [[Breccia|megabreccia]]. 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.<ref name="IUGS2022a">{{Cite web |title=The First 100 IUGS Geological Heritage Sites |url=https://iugs-geoheritage.org/videos-pdfs/iugs_first_100_book_v2.pdf |access-date=3 November 2022 |website=IUGS International Commission on Geoheritage |publisher=IUGS}}</ref>

==Description==
[[File:Richat structure cropped.png|left|thumb|Cross section of the structure. Magenta is the volcanic intrusion, while purple, green and grey represent sedimentary layers]]
The Richat Structure is a deeply [[Erosion|eroded]], slightly [[elliptical dome|elliptical]] [[Dome (geology)|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 rock]]s composing this structure [[Strike and dip|dip]] outward at 10–20°. Differential erosion of resistant layers of [[quartzite]] has created high-relief circular [[cuesta]]s. Its center consists of a [[Silicon dioxide|siliceous]] [[breccia]] covering an area that is at least {{convert|30|km|mi}} in diameter.<ref name="Matton2008a">{{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 }}</ref><ref name="MattonOthers2005a">{{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}}}}</ref><ref name="Wooley2001a">{{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}}</ref>[[Image:ASTER Richat.jpg|thumb|250px|Satellite picture of the Richat Structure (false color)]]Exposed within the interior of the Richat Structure is a variety of [[Intrusive rock|intrusive]] and [[Extrusive rock|extrusive]] [[igneous rock]]s. They include [[Rhyolite|rhyolitic]] volcanic rocks, [[gabbro]]s, [[carbonatite]]s and [[kimberlite]]s. The rhyolitic rocks consist of [[lava]] flows and [[Hydrothermal alteration|hydrothermally]] altered [[tuff]]aceous rocks that are part of two distinct eruptive centers, which are interpreted to be the eroded remains of two [[maar]]s. According to field mapping, [[Aeromagnetic survey|aeromagnetic]], and [[Gravimetry|gravimetric]] data, the gabbroic rocks form two concentric [[ring dike]]s. 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.<ref name=":0" /> Thirty-two carbonatite dikes and [[Sill (geology)|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 [[Vesicular texture|vesicles]]. The carbonatite rocks have been dated as having cooled between 94&nbsp;and 104&nbsp;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&nbsp;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.<ref name="Matton2008a" /><ref name="MattonOthers2005a" /><ref name="NettoOthers1992a">{{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}}</ref><ref name="MattonOthers2014a">{{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}}</ref>

[[File:GuelbRichatTopo.png|thumb|Topographic map of Guelb el Richat. Elevation in meters. 10 m contour interval with major contour line every 50 m|left]]

Spectacular hydrothermal features are a part of the Richat Structure. They include the extensive [[hydrothermal alteration]] of rhyolites and gabbros and a central [[Breccia|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 [[chert]]y material, [[quartz]]-rich sandstone, diagenetic cherty nodules, and [[Stromatolite|stromatolitic]] [[limestone]] and is intensively silicified. The hydrothermal alteration, which created this breccia, has been dated to have occurred about 98.2&nbsp;± 2.6&nbsp;million years ago using the [[Argon–argon dating|<sup>40</sup>Ar/<sup>39</sup>Ar]] method.<ref name="Matton2008a" /><ref name="MattonOthers2005a" /><ref name="MattonOthers2014a" />

==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 [[laccolith]]ic uplift.<ref>{{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}}</ref> 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|Aouelloul]]'' (south of [[Chinguetti]]), ''[[Temimichat Crater|Temimichat-Ghallaman]]'' and ''[[Tenoumer crater|Tenoumer]]''.<ref>{{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}}</ref> 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.<ref name="DietzOthers1969a">{{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}}</ref> [[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.<ref name="Fudali1969a">{{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}}</ref> Work on dating the structure was done in the 1990s.<ref>{{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}}<br/>{{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}}</ref> A study of the formation of the structure by Matton, et al. (2005, 2008) concluded it was not an impact structure.<ref name="Matton2008a"/><ref name="MattonOthers2005a"/>

Further analysis of deep structure underneath the surface, including with aeromagnetic and gravimetric mapping,<ref name=":0">{{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}}</ref> concluded that the structure is the result of ring faults which led to [[Gabbro|gabbroic]] [[Ring dike|ring dikes]] over a large intrusive body of [[magma]], and the uplifting and later erosion of a dome, through intense [[Hydrothermal circulation|hydrothermal activity]] through the fractured substructure. This can form [[Cuesta|cuestas]] over time through the differential erosion of the resulting alternating hard and soft rock layers.<ref name=":0" /> 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}}}}

===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."<ref name="IUGS2022a"/>

== Archaeology ==
[[Image:Richat Structure - SRTM.jpg|thumb|right|250px|A topographic reconstruction (scaled 6:1 on the vertical axis) from satellite photos. False colouring as follows: <br />• Brown: bedrock <br />• Yellow/white: sand <br />• Green: vegetation <br />• Blue: salty sediments]]
The Richat Structure is the location of exceptional accumulations of [[Acheulean]] [[Artifact (archaeology)|artifact]]s.<ref name="Monod1975a" /><ref name="SaoOthers2008a" /> These Acheulean archaeological sites are located along [[wadi]]s 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 [[outcrop]]s 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,<ref name="Monod1975a">{{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}}</ref> mapping of artifacts within the area of the structure have found them to be generally absent in its innermost depressions. So far, neither recognizable midden deposits nor manmade structures have been recognized and reported from the structure. This is interpreted as indicating that the area of the Richat Structure was used for only short-term hunting and stone tool manufacturing. The local apparent wealth of surface artifacts is the result of the concentration and mixing by deflation over multiple [[glacial]]-[[interglacial]] cycles.<ref name="SaoOthers2008a">{{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}}</ref><ref name="GiresseOthers2012a" />

Artifacts are found, typically redeposited, deflated, or both, in [[Late Pleistocene]] to early [[Holocene]] gravelly [[mud]], muddy [[gravel]], [[clay]]ey [[sand]], and [[silt]]y sand. These [[sediment]]s are often cemented into either [[concretion]]ary masses or beds by [[calcrete]]. Ridges typically consist of deeply weathered [[bedrock]] representing truncated [[Cenozoic]] [[paleosol]]s 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 [[fluvial|fluviatile]] or even torrential flow deposits. The finer-grained, sandy deposits consist of [[Aeolian dust|eolian]] and [[playa lake]] deposits. The latter contain well-preserved [[freshwater]] [[fossil]]s. Numerous concordant [[radiocarbon]] dates indicate that the bulk of these sediments accumulated between 15,000 and 8,000 [[Before Present|BP]] during the [[African humid period]]. These deposits lie directly upon deeply eroded and weathered bedrock.<ref name="GiresseOthers2012a">{{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}}</ref>

== Fringe theory of Atlantis site ==
The Richat structure has been the subject of [[Fringe theory|fringe claims]] to be the site of [[Atlantis]] mentioned in the works of [[Plato]].<ref>{{cite book |author=Mark Adams |url=https://books.google.com/books?id=4hTUCwAAQBAJ&pg=PA203 |title=Meet Me in Atlantis: Across Three Continents in Search of the Legendary Sunken City |date=26 April 2016 |publisher=Penguin Publishing Group |isbn=978-1-101-98393-5 |page=203}}

{{cite book |author=Rob Shelsky |url=https://books.google.com/books?id=rYaVCwAAQBAJ&pg=PA75 |title=Invader Moon |date=23 February 2016 |publisher=Simon and Schuster |isbn=978-1-61868-666-4 |page=75}}</ref><ref>{{Cite news |title=Is this Atlantis, hiding in plain sight in the Sahara? |language=en |work=Newshub |url=https://www.newshub.co.nz/home/world/2018/09/is-this-atlantis-hiding-in-plain-sight-in-the-sahara.html |archive-url=https://web.archive.org/web/20180907043527/https://www.newshub.co.nz/home/world/2018/09/is-this-atlantis-hiding-in-plain-sight-in-the-sahara.html |url-status=dead |archive-date=7 September 2018 |access-date=2023-11-08}}</ref> This claim is primarily based on the concentric nature of the structure, which superficially matches Plato's description of the city.<ref name=":1" /> Most classicists believe that Atlantis was a fictional rhetorical invention by Plato, rather than a real geographic location.<ref>{{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}}}}</ref><ref>"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])</ref> Skeptic [[Steven Novella]] criticised the claim, stating that the structure is inconsistent with Plato's description of Atlantis, and that the site shows no evidence of a city ever being built at the location.<ref name=":1">{{Cite web |last=Novella |first=Stephen |date=2018-11-19 |title=No – Atlantis Has Not Been Discovered in North Africa |url=https://theness.com/neurologicablog/index.php/no-atlantis-has-not-been-discovered-in-north-africa/ |access-date=2023-11-08 |website=NeuroLogica}}</ref>

==Notes==
{{Notelist}}

==References==
{{Reflist}}

==External links==
{{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] 

{{Authority control}}

[[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]]