Richat Structure

The Richat Structure, nicknamed the Eye of the Sahara, is a circular geological dome on the Adrar Plateau of Mauritania, near the town of Ouadane, roughly 40 kilometers in diameter and striking enough in its concentric rock rings to be a recognizable landmark from orbit. It formed when a subsurface igneous intrusion pushed up and deformed the overlying layers of sedimentary rock, with ring faults producing gabbroic ring dikes above the magma body; as the dome was later eroded, the different rock layers were stripped down to different depths, exposing the rings visible today. Its center exposes rhyolite and gabbro altered by hydrothermal fluids around a siliceous megabreccia core at least 30 kilometers across, cut by at least 32 mapped carbonatite dikes and sills. The carbonatite rock has been dated to between about 104 and 94 million years old, placing the intrusion in the Cretaceous period, with hydrothermal alteration dated to roughly 98 million years ago. The structure was once suspected to be an impact crater, but geologists found no shock metamorphism or other evidence of a hypervelocity impact, and an early report of the impact mineral coesite was later found to be a misidentified sample of barite. This entity documents the Richat Structure specifically as a geological formation produced by igneous intrusion and structural doming, not by impact.