Concentric rocky rings adorned with ancient artwork wear a magma 'hat' in the Sahara — Earth from space

This intriguing astronaut photo shows a series of rocky rings and their distinctive "hat" towering above a dune sea in the Sahara. The concentric, shadow-filled walls are home to ancient artworks and, occasionally, herds of cattle.

The colossal structure, known as Mount Arkanu (sometimes spelled Arkenu), is a massif, or group of mountains, that stands up to 2,600 feet (800 meters) above the surrounding sands and stretches up to 15.5 miles (25 kilometers) across at its widest point. The peaks are located in southeast Libya, near the border with Egypt.

Due to the mountains' shape, scientists previously assumed that the massif was an ancient impact crater that formed when a giant space rock slammed into the northeast Sahara. But more recent research has shown that the rocky rings are not of extraterrestrial origin.

Instead, the mountains were created when "magma [repeatedly] rose toward the surface and intruded into the surrounding rock," according to NASA's Earth Observatory. This formation process likely ended hundreds of millions of years ago but has not been properly dated.

"Repeated intrusion events produced a series of overlapping rings, their centers roughly aligned toward the southwest," Earth Observatory representatives wrote. "The resulting ring complex — composed of igneous basalt and granite — is bordered to the north by a hat-shaped formation made of sandstone, limestone, and quartz layers."

The massif has two large gaps in its outermost southern wall (bottom right in the photo). These have been slowly carved out by a pair of wadis, or ghost rivers, which sporadically fill with water when it rains. The phantom waterways can also be seen snaking through the dark plateau surrounding the complex.

This region of the Sahara is hyperarid; it gets only 1 to 5 millimeters (0.04 to 0.2 inches) of rain each year, according to the Earth Observatory. However, the massif receives between 5 and 10 mm (0.2 to 0.4 inches) of rain annually, due to a phenomenon known as "orographic precipitation," which occurs when clouds form over raised landforms, according to the U.K. Met Office.

As a result of this additional water, the rings are partially filled with grasses, bushes and trees that the shadows from the towering peaks shelter from the desert's scorching temperatures.

Cattle herding

The combination of shade, vegetation and occasional water has historically made the massif a popular destination for nomads traversing the surrounding dune seas.

Another massif, Mount Awaynat (sometimes spelled Uwaynat or Uweinat), is located roughly 12 miles (20 km) southeast and has provided similar benefits to desert wanderers for generations.

Some of the canyon-like walls of both Mount Arkanu and Mount Awaynat are covered with rock carvings, or petroglyphs, that were first discovered in 2003. These ancient artworks depict human fugures, along with cattle and other tethered animals, such as giraffes.

Early-20th-century explorers noted that members of the local bedouin tribe trekked their cattle herds to the massifs and left them inside to graze for up to three months at a time — and blocked up the holes created by the wadis to stop the animals from wandering off.

The cattle carvings within both structures suggest that this practice has been going on for thousands of years.

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