It might look like the site where a massive ancient meteorite once struck the side of the Earth, but this is no impact crater. The Richat Structure – also known as the Eye of the Sahara – is, in fact, a dramatic geological formation that appears like a giant bull’s eye in the otherwise barren desert landscape. Located in the north African country of Mauritania, Richat is a hardened structural dome whose crest has worn away to expose the underlying layers of sedimentary rock and minerals.
All sedimentary rock layers start out horizontal, but due to underground stresses they can get folded – either upwards in a convex shape (anticline) or downwards and concave (syncline). No one knows exactly why, but millions of years ago a circle of rock strata almost 50 kilometres (30 miles) across was uplifted causing an anticline dome to bulge up out of the Earth’s surface. We now know this as the Richat Structure. If it helps, try thinking of the dome as the top half of an onion where each layer represents a different strata of rock.
Extremely slowly, the dome was eroded by the elements to expose a ring of concentric circles on the ground that are now clearly visible from space. Like the onion layers, these circles indicate the different bands of rock radiating out from the central limestone-dolomite shelf. Richat’s most visible bands of rock (or cuestas) are tilted ridges of resistant palaeozoic quartzite that slope away from the centre and gave it the misleadingly crater-like appearance that long confused scientists.