The Richat Structure, also known as The Eye of the Sahara and Guelb er Richat, is a prominent circular feature in the Sahara near Ouadane, west–central Mauritania.
The Richat Structure is a deeply eroded, slightly elliptical dome with a diameter of 40 kilometres (25 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 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 30 kilometres (19 mi) in diameter.
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 and 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 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 Richat Structure and created it 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 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.
Initially interpreted as an asteroid impact structure because of its high degree of circularity, geologists now regard it to be a highly symmetrical and deeply eroded geologic dome. After extensive field and laboratory studies, no credible evidence has been found for shock metamorphism or any type of deformation indicative of a hypervelocity extraterrestrial impact. While coesite, an indicator of shock metamorphism, had initially been reported as being present in rock samples collected from the Richat Structure, further analysis of rock samples 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.
Scientists still have questions about the Eye of the Sahara, but two Canadian geologists have a working theory about its origins. They think that the Eye’s formation began more than 100 million years ago, as the supercontinent Pangaea was ripped apart by plate tectonics and what are now Africa and South America were being torn away from each other. Molten rock pushed up toward the surface but didn’t make it all the way, creating a dome of rock layers, like a very large pimple. This also created fault lines circling and crossing the Eye. The molten rock also dissolved limestone near the center of the Eye, which collapsed to form a special type of rock called breccia. A little after 100 million years ago, the Eye erupted violently. That collapsed the bubble partway, and erosion did the rest of the work to create the Eye of the Sahara that we know today. The rings are made of different types of rock that erode at different speeds. The paler circle near the center of the Eye is volcanic rock created during that explosion.
Modern astronauts are fond of the Eye because so much of the Sahara Desert is an unbroken sea of sand. The Eye is one of the few breaks in the monotony, and now it’s become a key landmark for them.
Some people believe that the Eye of the Sahara is actually the remains of the city of Atlantis, which Plato described as concentric rings of water and land. But if you ask us, the geological history this formation reveals is way more interesting.
Ouadane is the closest town to the Richat Structure, a massive circular landmark visible from space. Ouadane or Wādān (Arabic: وادان) is a small town in the desert region of central Mauritania, situated on the southern edge of the Adrar Plateau, 93 km northeast of Chinguetti. The town was a staging post in the trans-Saharan trade and for caravans transporting slabs of salt from the mines at Idjil. A Portuguese trading post was established in 1487, but was probably soon abandoned. The town declined from the sixteenth century and most of it now lies in ruins. The old town, a World Heritage Site, though in ruins, is still substantially intact, while a small modern settlement lies outside its gate.
The early history of Ouadane is uncertain but it is possible that the town prospered from the trans-Saharan gold trade. In the middle of the 11th century, the Arabic geographer al-Bakri described a trans-Saharan route that ran between Tamdoult near Akka in Morocco to Aoudaghost on the southern edge of the Sahara. This route was used for the transport of gold during the time of the Ghana Empire. In his account al-Bakri mentioned a series of place names but these have not been identified and historians have suggested several possible routes. In 1961 the French historian Raymond Mauny proposed a route that passed through Ouadane but Suzanne Daveau later argued in favour of a more direct route that crossed the Adrar escarpment to the east of the town. The volume of caravan traffic would have declined from the beginning of the 13th century when the oasis town of Oualata located 360 km to the east replaced Aoudaghost as the southern terminus of the trade route.
The first written reference to the town is in Portuguese by Ca’ de Mosto in middle of the 15th century in a muddled account that confused the salt mines of Idjil with those of Taghaza. At about the same date Gomes Eanes de Zurara described Ouadane as the most important town of the Adrar region and the only one with a surrounding wall. Fifty years later Valentim Fernandes wrote a detailed account of the trade in slabs of salt from the Idjil mines and role of Ouadane as an entrepôt. He described Ouadane as a ‘town’ with a population of 400 inhabitants. By contrast Duarte Pacheco Pereira in his Esmeraldo de situ orbis (written in 1505-1508) described the town as having approximately “300 hearths” which would suggest between 1,500 and 1,800 people. The Idjil sebkha lies roughly 240 km northwest of Ouadane, to the west of the town of Fderîck. The date when salt was first extracted from the sebkha is unknown. It is usually assumed that exploitation of the Idjil mines began after the mid 11th century as al-Bakri did not mention them. Instead he described a salt mine at a place that he called ‘Tatantal‘. Historians have usually assumed this corresponds to Tegahza but his description could possibly also apply to the mines at Idjil.
According to Pereira, in 1487 the Portuguese built an entrepôt in Ouadane in an attempt to gain access to the trans-Saharan gold, salt and slave trade. The entrepôt was probably short lived and is not mentioned in the detailed description provided by Fernandes.
In the 16th century the Moroccans made various attempts to take control of the trans-Saharan trade in salt and especially that in gold from the Sudan. They organised military expeditions to occupy Ouadane in 1543-44 and again in 1584. Then in 1585 they occupied Taghaza and finally in 1591 their victory in the battle of Tondibi led to the collapse of the Songhay Empire.
Tegherbeyat, the upper ruined section of the town, is almost certainly the oldest. It would have originally contained a mosque but nothing has survived. The ruins of the lower section of the town include a mosque that was probably built in the 15th century when the town expanded. Some of the horseshoe arches are still standing and some walls still have the remains of clay plaster, suggesting that the mosque was abandoned sometime in the 19th century.
The mosque measured 24 m north-south at its eastern end and 17 m north-south at its western end where the minaret would have stood. From east to west it would have measured 15 m. The terrace was supported by five rows of horseshoe arches. At the eastern end are the remains of an external mirhab and a courtyard measuring 13 by 12 meters that would have been used in hot weather.