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Explore the origins of one of the coolest geologic formations in the world, West Africa's Richat Structure.

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It's been less than a hundred years since we got our first really good look at the geological marvel known as the Eye of the Sahara, despite the fact that people have been walking across it right there in the middle of Mauritania for thousands of years.  But at 50km across, it helps to have a wider view of one of the world's coolest geological features.

So the best way to see the formation, formally known as the Richat structure, is from space.  It's so big that early NASA missions actually used it as a landmark.  Given the structure's concentric circles and uncanny symmetry, it's hard to fault scientists for initially suspecting that it was a crater from some enormous impact.  But it is not the product of a meteor, comet, or even an ancient volcano as scientists once theorized.

While there's still disagreement about how this almost perfectly circular shape came to be, most scientists will tell you that the Richat structure is actually a deeply eroded geologic dome.  This dome formed more than 100 million years ago when the churning landmasses that make up Africa caused the lithosphere, that's the crust and the upper mantle of the earth's surface, to weaken.

This weakness allowed the dome to rise up as magma swelled below the surface.  Geologists call this kind of uplift an anticline, basically an enormous fold of rock sticking up from its surroundings.  And this one was nearly symmetrical, a circular anticline.

The fold contained alternating layers of the three most common types of rocks, sedimentary, metamorphic, and igneous.  And because of how an anticline forms, the oldest rocks in the formation are at the center of the fold while the younger rocks are on the outer layers.  Over millions of years the dome eroded away, likely sped along by all the hydrothermal water in the area, another symptom of the magma lurking near the surface.

The different types of rocks erode at much different rates, so the layers of sedimentary rock, which erode more easily, were worn away to form the valleys within the structure.  Meanwhile, the harder metamorphic rocks, like quartzite, and igneous rocks, which are more resistant to erosion, remained.  This left the oldest rocks exposed as cliffs separated by valleys where the younger, softer rock used to be, creating the Richat structure's weird concentric landscape.

So, if you're ever in Mauritania or at low Earth orbit, keep a big ol' eye out for it.

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