The Richat Structure is one of those geological features that are more clearly observed from space than from down on the ground. It was first observed from space by Gemini 4 astronauts McDivitt and White in June 1965.
Located in the center of Mauritania, the western end of the Sahara desert this prominent circular feature has attracted attention since the earliest space missions because it forms a conspicuous bull's-eye in the otherwise rather featureless expanse of the desert. Described by some as looking like an outsized ammonite in the desert, this 'eye of Africa' , which has a diameter of almost 50 kilometers (30 miles) has become a landmark astronauts since the earliest manned missions.
Most of the image looks yellowish, indicating sand desert. The dark brown part is bare sedimentary rocks, and within that you can see the Richat Structure, a gigantic ring structure of some 40 km in diameter.
The onion-like formation is formed by concentric bands of resistant paleozoic quartzite rocks form ridges, and between with valleys of less-resistant rock between them.
The part of the sedimentary rock corresponding to the white of the eye is a plateau standing some 200 m above the sand desert. The Richat Structure corresponding to the iris of the eye lies in a depression, and the peak of the outer rim is 485 m above sea level. The Richat Structure consists of Early Paleozoic rocks, some 600 million years old. Around the center, rocks resistant to weathering and erosion (purple and blue-green part) make 100 m high ridges, and nonresistant rocks (yellow and brown part) form valleys. These features alternate and are concentric.
However, why the structure is circular remains a mystery.