Richat Structure, Mauritania, satellite image. North is at top. The Richat Structure (just left of centre) is a geological formation in the Sahara's Maur Adrar Desert in Mauritania. Although it resembles an impact crater, the Richat Structure formed when a volcanic dome hardened and gradually eroded, exposing the onion-like layers of rock. The structure's diameter is about 50 kilometres. Image data obtained on 13 January 1987.