"The Mysterious Eye of Africa"
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 This prominent circular feature, known as the Richat Structure, in the Sahara desert of Mauritania has been noted by astronauts because it is a nearly 50-kilometer-wide (30-mile-wide) bull's-eye on the otherwise rather featureless expanse of the desert. It was first observed from space by Gemini 4 astronauts McDivitt and White in June 1965. Initially mistaken for a possible impact crater, it is now thought to be an erosion of layered sedimentary rocks. However, why the structure is circular remains a mystery.
See it here on a Google Satellite Map
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. It is as large as Uchiura Bay in Hokkaido, Japan.
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.
The Richat Structure was previously thought to have been formed by metorite impact or volcanic activity, but field surveys have demonstrated that neither are correct. The current thinking is that these features were formed by an uplift and subsequent erosion from wind and water. |
