What is this “eye” in the Sahara desert?

You have to gain height to discover this impressive geological structure nestled in the heart of the Mauritanian desert. Known as ” eye of Africa”, or Guelb er Richatthe Richat structure has often been the object of attention ISS astronauts who photographed it many times.

Measuring approximately 40 kilometers in diameter, the origin of this strange concentric structure lost in the middle of a desert landscape has long questioned scientists.

A magmatic origin

First described in 1916, the Richat structure has been the subject of many studies. First interpreted as the remains of a meteorite impact, the study of the rocks composing it shows that its origin is much more terrestrial. The concentric rings are indeed composed of strata oblique sedimentary. The center is formed intrusive (gabbros, carbonatites) and extrusive (rhyolites) igneous rocks. Although eroded, two ancient mouths eruptive can be observed. The gabbros are found in the form of two dikes concentric surrounding the rhyolitic center. Numerous other dyke and sill type intrusions have been identified within the structure. There is therefore no doubt that the eye of Africa actually has a magmatic origin. The dating of the rocks has also made it possible to date the event to around 100 million years.

Older sedimentary rocks in the center

But why this shape in concentric rings? To understand, you have to look at the rings created by the sedimentary strata. Because they provide crucial information: the age of the sediments decreases from the center towards the edges of the structure. This type of architecture is well known to geologists. This is a anticline : the sedimentary strata, flat beforehand, have been folded to form a dome, so that the oldest strata (originally the deepest), are found in the center of the structure. But to obtain the architecture visible today, it is necessary to wait for erosion to do its work. Over millions of years, the windsice and water have planed the dome, gradually removing the superficial envelopes to reveal the heart of the dome and therefore the oldest strata.

But while anticlines are generally the result of compressive tectonic forcesit is necessary here to appeal to magmatism to explain the formation of this structure.

A “bubble” of magma that lifted the sedimentary layers

100 million years ago, an important episode of merger generated an increase of magma which finally crystallized deep within the sediments. During its ascent, however, this magmatic “bubble” deformed the overlying sedimentary strata by lifting them, leading to the creation of a dome-shaped anticline. If some lavas have visibly succeeded in flowing out on the surface, the magmatism has mainly remained intrusive, not giving rise to the formation of a volcano.

Many archaeological remains have been found on the site, including Acheulean stone tools, dating from the early Paleolithic (1.76 – 0.13 million years). It is likely that the levels of quartzitea particularly hard sedimentary rock exposed by the erosion of the dome, were used temporarily for the manufacture of tools for prehistoric men living in the region.