The Eye of the Sahara is a 25-mile wide geologic feature in the desert of Mauritania. Officially known as the Richat structure, it appears from the air as a series of concentric circles on the earth. No one ever saw it until it was photographed by the Gemini IV mission in 1965, but now it is a landmark for astronauts orbiting the earth. The Gemini mission was looking for impact craters, and the Eye of the Sahara was thought to be one for quite some time, but scientists now think there’s a more complex origin for the circles.
They think that the Eye's formation began more than 100 million years ago, as the supercontinent Pangaea was ripped apart by plate tectonics and what are now Africa and South America were being torn away from each other.
Molten rock pushed up toward the surface but didn't make it all the way, creating a dome of rock layers, like a very large pimple. This also created fault lines circling and crossing the Eye. The molten rock also dissolved limestone near the center of the Eye, which collapsed to form a special type of rock called breccia.
A little after 100 million years ago, the Eye erupted violently. That collapsed the bubble partway, and erosion did the rest of the work to create the Eye of the Sahara that we know today. The rings are made of different types of rock that erode at different speeds. The paler circle near the center of the Eye is volcanic rock created during that explosion.
So the colored circles are the result of the wind and sand shaving off and leveling the dome that was created millions of years before. Read more about the Eye of the Sahara at Business Insider. -via the Presurfer
(Image credit: SRTM Team NASA/JPL/NIMA)