Thursday, July 2, 2009

Richat Structure - Eye of Africa, Mauritania

From space this mysterious depression in the Sahara Desert of Mauritania really does look like a human eye. The image to the left is the "pupil" but a visit to Google Earth zoomed out a little will reveal the cliffs that make up the rest of the eye. This natural phenomenon is actually a richat structure caused by the dome shaped symmetrical uplifting of underlying geology now made visible by millennia of erosion. Please note that this explanation is not wholly accepted by the scientific community. There still remain academics that believe it is the sight of a meteor impact and yet others still that believe it resembles the formations caused by underground nuclear blasts. By the way, we estimate that the detonation would have had to be in the gigaton range. Currently no country in the world has a weapon even close to this destructive yield.

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.

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.
However, why the structure is circular remains a mystery.

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