Three “black mesas”, remnants of the Paleozoic era, create unusual sand dunes in the Sahara Desert — Earth seen from space

This interesting astronaut photo shows three ancient “black mesas” sitting side by side in the Sahara Desert. The dark structure created a series of unusual dunes around it, as well as a surprising “dune-free zone.”

The three mesas, or flat-topped hills, are located about 8 miles (13 km) northwest of the town of Gerou in southern Mauritania, home to about 22,000 people. Mesas are made of sandstone and rise steeply from the surrounding plains, reaching heights of 300 to 400 meters (1,000 to 1,300 feet) above the ground. The largest of the trio is about 6 miles (9.5 km) in diameter at its widest point, while a fourth mesa lies just north of the trio, but just outside the frame.

According to NASA Earth Observatory, the dark color of these circular hills is the result of “rock varnish.” This is a black, clay-based coating rich in manganese and iron oxides that forms over thousands of years on exposed, dry rock. The coating is likely partially fixed by microorganisms and consists of multiple micrometer-thick layers, according to Science Direct.

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To the west of the mesa (to the left of the photo) is a barren, rocky plain, surprisingly devoid of sand dunes. But to the east of the flattened hills, several large sand dunes can be seen that appear to flow out of the black rock like undulating tails.

Two main types of dunes are visible in the images. The first type is the unusual “climbing dune,” a larger ridge-like pile of sand built up along the east wall of the mesa. The second type is the “Balchan dune”, which is more common and forms the striped tail of the mesa. In both cases, the dunes have a distinctive reddish-yellow hue.

Dunes form only on the eastern side of the mesa. This is because the wind mainly blows from that direction, carrying the sand and causing it to wind up on the slopes surrounding the black rock.

According to the Earth Observatory, the lack of sand accumulation on the mesa’s west side is due to a phenomenon known as a “wind scar.” The phenomenon is caused by superfast vortices in the wind that are forced between mesas, blowing sand away from flattened hills.

Another astronaut photo taken in 2014 shows this strange effect extensively (see below). In this shot, you can see the Vulcan Dunes stretching far away from the mesa in the photo, as well as another large mesa further east.

According to the Earth Observatory, all of these mesas were likely part of a single giant rock formation during the Paleozoic Era, which lasted from 541 million years ago to 251.9 million years ago, before being destroyed by thousands of years of water and wind erosion.

The formation may have been similar to the Richa Structure, a giant concentric rock formation also known as the “Eye of the Sahara” located about 460 km (285 miles) north of Gerou in Mauritania.

According to the National Park Service, mesas are found all over the world, but are particularly concentrated in the Sahara Desert and other parts of the United States, including Colorado, New Mexico, Utah, and Arizona.

Elsewhere in the solar system, mesas are prominent geological features on Mars, carved out of the red planet by billions of years of wind erosion, according to Live Science’s sister site Space.com.

For more amazing satellite photos and astronaut images, check out Earth from the Space Archives.