The Richat Structure

Popularly known as the Eye of the Sahara, and Guelb er Richat, it is a prominent circular feature in the Sahara desert near Ouadane, west–central Mauritania. From above it appears to be a giant bulls-eye, or a giant ammonite. This massive structure is a deeply eroded, slightly elliptical, 40 km in diameter dome of concentric circles. A variety of rock structures are revealed in various sections of the "Eye", including rhyolitic volcanic rocks, gabbros, carbonatites and kimberlites. 

Exposed to some scraping on the central dome Late Proterozoic to Ordovician age rocks and sand is located around ujunggnya. some sedimentary rocks derived from these structures dip outward at 10 ° -20 °. Differential erosion-resistant layer of quartzite has created a high-relief circular cuestas. Eye center of Africa consists of siliceous breccia area having a diameter of at least 3 km.

"These concentric circles are actually alternating layers of sedimentary, metamorphic,  and igneous rocks that were pushed upward in a  geologic dome, a fractureless upwarping of rock strata from below due to a small incursion of magma.  The Richat dome happened to be pushed upward in a rather neat circle thanks to lithospheric weakness during the final separation of West Africa from South America around 100 million years ago.  Over time, the top of the dome was eroded away, exposed the pushed-up inner layers of the dome and producing the current circular formation."