The Giant Eye in the Sahara Is Not a Crater

Imagine floating past a window of the International Space Station and looking down at the Sahara. For a long while there is only sand: pale orange dunes combed into lines by the wind, and flat, dark plateaus. Then, in Mauritania, something seems to look back at you. A huge set of rings, about 40 kilometres across, sits in the desert like a giant eye, or an archery target left out by someone very large. French geographers described it in the 1930s and called it the Richat buttonhole. It became famous around the world after the astronauts of the Gemini IV mission photographed it from orbit, and the name that stuck is the Eye of the Sahara.

What people often get wrong

Anything that round and that big in a desert looks as if it must have been punched out by something from space. For years that was a serious idea. Large meteorites really do leave circular marks on the Earth's surface, and the Richat Structure was first thought to be an impact crater. The idea still turns up online today. But geologists who studied its rocks found that it does not hold up, and the world's geological heritage experts now state plainly that the impact theory has been disproved.

The truth runs the other way. Instead of something crashing down, something pushed up. Deep underground, molten rock forced its way into the layers above and lifted them into a broad dome, a bit like a cake rising in the oven and swelling in the middle. Some of the rocks linked to that activity have been dated to about 85 and 99 million years old, which places the making of the dome in the Cretaceous period, when dinosaurs still walked the Earth.

How a dome became an eye

A dome on its own would not look like an eye. What carved the rings was erosion. Over an immense span of time, wind and water wore the top of the dome away, slicing through its layers the way a knife slices the top off a layered cake. Some layers are hard and resist; others are soft and wear down quickly. The hard ones were left standing as ridges, and because the layers had been bent into a dome, the ridges come out as circles, one inside the next. Geologists call ridges like these cuestas, and the Richat has three nested rings of them, tilting gently outward.

• ≈ 40 km across the outermost rings
• 3 nested rings of ridges, tilting outward
• 85–99 million years the age of some of the rocks that pushed the dome up

The colours help tell the story. In satellite pictures the rings show up in orange and grey, because they are made of different kinds of rock: sedimentary layers laid down long ago, and igneous rock that cooled from the molten material below.

Why the sand stays away

The strangest part may be that the rings can be seen at all. The Richat sits among great seas of dunes, some running in long straight lines and others lying across the wind. Yet steady winds from the northeast keep much of the dome swept clear of sand, so its rock layers stay on show, as if the desert had agreed to keep the eye open.

So a slow bulge underground, an unimaginably long spell of weather and a helpful wind came together to make one of the most recognisable shapes on the planet. It is not a mark left by a visitor from space. It is a piece of the Earth's inside, gently pushed up and patiently polished, and it has been watching the dunes drift past for a very long time.