Richat Structure

A 40 km-wide eroded dome of concentric rock rings in the Sahara, so striking from space that it is nicknamed the Eye of the Sahara.

It is not a crater from a space rock, although scientists once thought it might be. It is a dome of layered rock that has been worn down by wind and water, so its different layers show up as rings. It is so large that you cannot see its shape from the ground.

How did it form?

Long ago, hot molten rock pushed up from underground and bent the layers of sedimentary rock above it into a dome. Over a very long time, erosion wore the top away. Harder layers, such as quartzite, stuck out as ridges, while softer rocks were worn into valleys, leaving circles around the center. The oldest rocks are at the middle and younger ones around the edges.

Why do scientists care?

Studies in the 1960s found no signs of the shocked rock that a meteorite impact would leave, so scientists concluded it was formed by Earth's own processes. Inside the structure are volcanic and other igneous rocks. The area also has many stone tools from the Acheulean period of the Old Stone Age. The structure was chosen as one of 100 top geological heritage sites by the International Union of Geological Sciences.

On the globe

The model shows the circular rings of rock in the desert, like an eye.