The Richat Structure is a 45km-wide geological oddity that some compare to Jupiter’s Great Red Spot. Its concentric rings provide a spectacular view…from space. Once you’re on the ground, you see nothing but sand, rocks, and more sand.
So it’s not surprising that the structure was only discovered in 1965 from space. Gemini IV’s four-day flight featured not only the first American spacewalk but the first images of our planet.
A meteor impact?
At first glance, it seems as if this is a remnant of a meteor impact. In fact, it’s likely the result of millions of years of erosion right here on Earth. The meteor impact theory fell out of favor because the site lacks the minerals usually found in cases of shock metamorphism. That is where extreme heat and pressure from a high-velocity impact deform the surrounding rock.
Richat Structure in its true colors. Photo: NASA
Long ago, the Sahara was not a desert but rather a temperate region, full of vibrant rivers, lakes, and greenery. For uncertain reasons, this changed. As it transformed into a barren wasteland, great amounts of weathering peeled away layers of hard and soft rock.
The rings show the older igneous rocks concentrating at the centre and younger quartzite and sedimentary rock in the outer rings and ridges. The Structure has three main rings whose perfect shape scientists still don’t understand.
NASA’s images of the Richat Structure use false coloring to show all the layers of rock and to distinguish bedrock, sand, vegetation and sediments heavy in salt. From space, the rings do not actually look blue.
Tectonic and volcanic activity from the cataclysmic Pangea separation 100 million years ago forced molten rocks to rise and create a dome, whose rocks date back to the Proterozoic and Ordovician periods. The later hydrothermal activity caused the centre to collapse and form megabreccia, a special type of large, fragmented rock containing smaller sandstone fragments rich in quartz. By collapsing in on itself, the centre is below the surrounding rings and ridges.
The Structure, which is also known as the Eye of the Sahara, is surrounded by great sand dunes called the Erg Oudane. These dunes host Ksours, medieval towns, and former stopovers on the Saharan trade route.
Richat Structure false coloring. Photo: NASA
The lost city of Atlantis
One particularly out-there theory suggests that the Richat Structure is the lost city of Atlantis. Plato wrote that Atlantis also had concentric rings. As it turns out, the Structure is less mythological and more archaeological. Scientists have unearthed a wide range of Acheulean artifacts, particularly in the outer ring, made of quartzite from the area. These included spearheads, pottery shards, and even anchors for ships. Not only is it a geological prize but its human history is also wonderful.
