A bullseye 40 km across in the middle of the Sahara, so regular that it was used as a landmark from orbit long before anyone agreed what it was. The obvious guess was an impact crater and the obvious guess was wrong: there is no shock metamorphism and no deformation of the kind a hypervelocity impact leaves, and the coesite reported in 1969, which would have been the clincher, turned out to be barite misidentified. What it actually is is an eroded dome. Magma pushed up underneath, lifted the sedimentary layers, and erosion has since sliced the top off, so the rings you see are progressively older rock as you walk inward, with rhyolite, gabbro and a megabreccia core. Described in the 1930s and 40s and studied by Théodore Monod in 1952. Adrar, near Ouadane.
Adrar, Mauritania
See it on the map
