A 25-mile bull’s-eye in the Sahara is so perfect astronauts use it as a landmark

From orbit, a giant circular pattern of concentric rings stares up out of the Mauritanian desert like a target painted on the Sahara. Roughly 40 kilometers across, the formation is so distinct against the flat surrounding terrain that it became a recognizable orientation point for astronauts looking down on West Africa. Its symmetry is unusual enough that, for a time, its origin was genuinely disputed.

The feature is often nicknamed the Eye of the Sahara, or the Eye of Africa, and its striking bull’s-eye appearance long invited comparison to an impact crater gouged by a falling object. Careful geological work has since replaced that idea with an explanation rooted in slow processes deep underground and millions of years of erosion, though the result is no less remarkable to look at from space.

Not an impact crater

Early observers reasonably guessed that so round a structure marked where a meteorite had struck. That interpretation did not survive closer study. The rings lack the shocked minerals, melt rock, and other telltale signs of a high-energy impact, and the layering does not match what a crater would produce. According to the geological description of the feature, it is instead a deeply eroded dome, its concentric rings formed as differential erosion stripped away softer layers and left resistant bands of rock standing in relief.

How the rings formed

The formation sits at the northwestern edge of the Taoudeni Basin on the Adrar Plateau. Beneath it, a body of molten rock pushed upward long ago, doming the overlying sedimentary layers into a broad blister without breaking through to the surface in a single eruption. Over time the top of that dome eroded away, exposing the folded rock as a series of nested circles. The oldest layers are now revealed at the center, with progressively younger sedimentary rock toward the edges. Bands of hard quartzite resisted erosion better than the surrounding stone, producing the ridged, ringed relief that reads so clearly from above. The mechanism is the same one that shapes eroded domes elsewhere, but the unusual regularity of the layering here, combined with the arid climate that keeps vegetation from masking the rock, leaves the pattern exceptionally crisp. Differential erosion of the resistant quartzite created high-relief circular ridges, known as cuestas, that trace the concentric outline of the buried dome.

A window into deep rock

Because erosion has sliced down through the dome, the interior exposes a variety of igneous rocks that ordinarily stay hidden far below ground. Mapped within the structure are rhyolitic volcanic rocks, gabbros, carbonatites, and even kimberlite, the rock type associated with diamonds. Two concentric ring dikes of gabbro encircle the center, and dozens of carbonatite dikes and sills cut through the formation. The carbonatite rocks have been dated to a cooling age of roughly 94 to 104 million years, and a kimberlite plug in the northern part of the structure has been dated to about 99 million years old, anchoring the igneous activity to the Cretaceous period.

Dimensions and scale

The dome is slightly elliptical, with a diameter of about 40 kilometers, which translates to roughly 25 miles across. Its central zone consists of a siliceous breccia covering an area at least 30 kilometers wide. The sedimentary rocks exposed in the structure range in age from the Late Proterozoic at the core to Ordovician sandstone around the rim, and they dip gently outward at angles of 10 to 20 degrees. The combination of enormous size, near-perfect symmetry, and flat desert surroundings is what makes the formation stand out so sharply in satellite imagery and from the vantage of low Earth orbit.

A record of ancient toolmakers

The Richat Structure is not only a geological curiosity. It is also the site of exceptional accumulations of Acheulean stone tools, artifacts left by early humans during the Paleolithic. Their concentration here reflects a time when the region was far less arid and could support the animals and water sources that drew toolmaking populations. During earlier, wetter phases the Sahara held lakes, rivers, and grassland, and the ringed depression may have channeled or held water in ways that made it attractive to both wildlife and the humans who hunted them. The formation therefore preserves both a deep-time record of the Earth’s interior and a much more recent record of human presence in a Sahara that once looked very different.

A recognized heritage site

The scientific value of the formation has earned it formal recognition. It was selected as one of the 100 geological heritage sites identified by the International Union of Geological Sciences as being of the highest scientific importance, a designation reserved for locations that are especially informative about the planet’s history. That status underscores how a feature first mistaken for a scar from the sky turned out to be a superb natural cross-section of processes that unfold over tens of millions of years. What began as a puzzle visible from orbit has become a textbook example of how doming and erosion together can etch order into stone on a scale that dwarfs anything built by human hands.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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