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Richat Structure
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==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"/>
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