The Richat Structure is a geological anomaly visible from orbit, often called the “Eye of the Sahara” due to its distinct, circular shape. This massive formation has captivated scientists and astronauts alike, standing out dramatically against the vast, featureless desert landscape. The structure’s visual prominence from space has historically led to much speculation regarding its origin. Its unique appearance and enormous scale make it a geographical landmark unlike any other on the planet.
Defining the Richat Structure and its Location
The Richat Structure is a deeply eroded geological dome, which geologists categorize as a domed anticline. This means it was formed when rock layers were pushed upward into a dome shape, with the oldest layers at the center. It is not an impact crater caused by a meteorite strike, since it lacks the telltale signs of shock metamorphism and molten rock remnants expected from such an event.
The structure is located in the Adrar Plateau region of Mauritania, a country in West Africa, near the settlement of Ouadane. Its remote placement in the Sahara Desert contributes to its remarkable visibility. The local Arabic name, Guelb er Richât, translates to “Eye of the Richat,” highlighting its striking resemblance to a colossal eye.
The Scale and Measurements of the Structure
The Richat Structure is a colossal feature, with its overall diameter measuring approximately 40 to 50 kilometers (25 to 30 miles) across. This massive size means that the structure is larger than many major metropolitan areas. Early astronauts used the distinct circular pattern as a recognizable landmark while orbiting the Earth.
The concentric rings that give the structure its bullseye appearance are separated by varying elevations. The central part consists of a depression roughly 30 kilometers (19 miles) in diameter. The central rings rise about 80 meters (260 feet) tall, creating a visible topographic relief.
The Geological Origin and Formation
The scientific consensus is that the Richat Structure formed through a combination of geological uplift and subsequent differential erosion. The process began when a large body of magma pushed up from below the Earth’s crust, causing the overlying sedimentary rock layers to dome upward. This doming created an anticline, essentially a giant blister of rock layers.
Once the dome was formed, the relentless forces of wind and water began to strip away the upper layers of rock over millions of years. Because the layers were composed of different materials with varying hardness, they eroded at different rates. The harder, more resistant rock layers remained, forming the raised ridges, while the softer rock layers were worn away, creating the valleys between the concentric rings.
Internal Composition and Ring Features
The Richat Structure is composed of a variety of rock types, with the rings providing a natural cross-section of the region’s geological history. The concentric ridges are largely made of resistant rock, specifically quartzites. Between these hard ridges are valleys formed from the erosion of less-resistant sedimentary rock, such as shale and sandstone.
Within the interior, geologists have identified evidence of igneous rock, which confirms the role of magmatic activity in its formation. These igneous rocks include gabbros and rhyolitic volcanic rocks, which were forced into the sedimentary layers during the uplift. The central depression contains the oldest exposed layers, dating back to the Late Proterozoic era, while the outermost layers consist of Ordovician sandstone.