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Foto: Agenzia Spaziale Italiana. La struttura di Richat (o Guelb El Richat) è una struttura circolare nel
Deserto del Sahara, in Mauritania. Ha attratto l'attenzione fin dalle
prime missioni spaziali, perché ha la forma di un grosso cerchio in una
parte di deserto altrimenti vuota.
Descritta da alcuni come somigliante ad una grande ammonite, ha un
diametro di quasi 50 chilometri. Inizialmente fu interpretata come il
cratere di un meteorite, a causa della sua elevata circolarità, ma oggi
si pensa che sia un rialzo simmetrico, messo a nudo dall'erosione. Le
parti visibili della struttura sono di quarzite paleozoica.
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The
Richat Structure, also known as the
Eye of the Sahara and
Guelb er Richat, is a prominent circular feature in the Sahara desert of west–central Mauritania near Ouadane. This structure is a deeply eroded, slightly elliptical, 40-km in diameter, dome. The sedimentary rock exposed in this dome range in age from Late Proterozoic within the center of the dome to Ordovician
sandstone around its edges. The sedimentary rocks comprising 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 3 km in diameter.
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This Envisat image, acquired by its MERIS instrument on 1 November 2010,
captures spectacular geological phenomena in the Sahara Desert of
Mauritania in northern Africa: the remarkable circular Richat structure
(right), which resembles an eye from space, and a magnetic mountain, the
Kediet ej Jill Mountain, Mauritania’s highest peak, visible northwest
of
Richat.
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Exposed within the interior of the Richat structure are 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 and aeromagnetic data, the gabbroic rocks form two concentric ring dikes.
The inner ring dike is about 20 m in width and lies about 3 km from the
center of Richat Structure. The outer ring dike is about 50 m in width
and lies about 7 to 8 km from the center of this structure. Thirty-two
carbonatite dikes and sills
have been mapped within the Richat structure. The dikes are generally
about 300 m long and typically 1 to 4 m wide. They consist of massive
carbonatites that are mostly devoid of vesicles.
The carbonatite rocks have been dated as having cooled between 94 to
104 million years ago. A kimberlitic plug and several sills have been
found within the northern part of the Richat structure. The kimberlite
plug has been dated being about 99 million years old. These intrusive
igneous rocks are interpreted as indicating the presence of a large
alkaline igneous intrusion that currently underlies the Richat structure
and created it by uplifting the overlying rock.
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This Envisat image, acquired by its MERIS instrument on 1 November 2010,
captures spectacular geological phenomena in the Sahara Desert of
Mauritania in northern Africa: the remarkable circular Richat structure
(right), which resembles an eye from space, and a magnetic mountain, the
Kediet ej Jill Mountain, Mauritania’s highest peak, visible northwest
of
Richat.
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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 40 m 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.
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Initially interpreted as an asteroid
impact structure
because of its high degree of circularity, it is now argued to be a
highly symmetrical and deeply eroded geologic dome. Despite extensive
field and laboratory studies, geologists have found a lack of any
credible evidence for shock metamorphism or any type of deformation indicative of a hypervelocity extraterrestrial impact.
Coesite,
an indicator of shock metamorphism, had been reported as being present
in rocks samples collected from the Richat structure. As the result of
the further analysis of rock samples from this structure, it was
concluded that barite had been misidentified as coesite.
In addition, the Richat structure lacks the annular depression that
characterizes large extraterrestrial impact structures of this size.
Also, it is quite different from large extraterrestrial impact
structures in that the sedimentary strata comprising this structure is
remarkably intact and "orderly" and lacking in overturned, steeply
dipping strata or disoriented blocks.
A more recent multianalytical study on the Richat megabreccias
concluded that carbonates within the silica-rich megabreccias were
created by low-temperature hydrothermal waters, and that the structure
requires special protection and further investigation of its origin.
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La structure de Richat ou dôme de Richat, surnommé « l'œil de l'Afrique » ou traditionnellement Guelb er Richât, près de Ouadane en Mauritanie en plein désert du Sahara, est une ancienne « énigme scientifique » de 50 km de diamètre, qui ne s'observe pleinement que depuis l'espace. Largement révélé en 1965 par une mission Gémini américaine, ce serait d'après les dernières interprétations scientifiques géologiques1, le résultat d'un phénomène volcanique géant, vieux de 100 millions d'années (Crétacé), totalement effondré à la suite d'une longue érosion différentielle.
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