Richat Structure, large geographic feature with a diameter of about 50 km (30 miles), consisting of a series of concentric rings located in the Sahara near the town of Ouadane, Mauritania.
Named for a Berber term for “feather,” the Richat Structure was originally thought to be an impact crater because of its raised outer perimeter and sunken center. However, the structure lacks key features of impact craters, such as a central peak and evidence of molten rock from the heat of impact. It has since been determined to be an uplifted geological dome, or domed anticline, originally formed by molten rock during the Cretaceous Period (about 145 million to 66 million years ago).
The Richat Structure’s concentric rings, which from above resemble an eye, are a combination of several different types of sedimentary and igneous rock that have been eroded by wind, sand, and water over millions of years. The shorter rings are softer, sedimentary rock, which is more susceptible to erosion, whereas the higher rings are composed of harder, igneous rock. Some of the rocks that form the structure are basalt, carbonatites, kimberlites, sandstone, shale, and quartzite. The central rings of the structure are about 80 meters (260 feet) tall.
Although age estimates vary, geologists agree that the structure is at least 100 million years old. There is evidence that the Richat Structure was inhabited by early humans. Acheulean and pre-Acheulean tools found there, such as hand axes, suggest the presence of hunting and manufacturing activities.
The Richat Structure was used as a landmark by pilots in World War II, and it was first described scientifically in 1948, by French geographer Jacques Richard-Molard, who called it the “Richat boutonniere.” It has often been photographed from space since astronauts James McDivitt and Edward White captured a spectacular image of it during the Gemini 4 mission in June 1965.
Because of its location and appearance, some have claimed that the Richat Structure is evidence of the legendary lost city of Atlantis, which was written of by Plato about 360 bce. However, classicists agree that Atlantis is fictional, being simply a metaphor used by Plato.