Richat Structure

From orbit the Richat Structure looks like a target painted on the Sahara: a set of concentric rings some 40 kilometres across, laid out on the Adrar plateau of Mauritania with a precision that seems too perfect to be natural. Known in Hassaniya Arabic as Guelb er Richât and nicknamed the Eye of Africa, it has fascinated geologists since it was first described in the 1930s and 1940s. On the ground the rings are much harder to read. What you walk across are low quartzite ridges, the cuestas, rising above broad sandy depressions, one after another, each ridge a hard layer of rock that has resisted the wind while the softer beds between them wore away.

For decades the obvious explanation was a meteorite impact, and Théodore Monod's 1952 expedition listed it alongside three genuine craters in the region. Closer study found none of the signs of shock that an impact leaves, and a mineral once reported as the shock indicator coesite turned out in 1969 to be barite. The accepted picture today is quite different: a large body of alkaline magma pushed up from below during the Cretaceous, around 100 million years ago, arching the overlying sediments into a dome. Hot fluids then dissolved and collapsed the summit into a thick siliceous breccia, and erosion planed the whole dome flat, exposing the layers in rings, with late Proterozoic rock at the centre and Ordovician sandstone at the rim. Two concentric ring dikes of gabbro, a kimberlite plug and some thirty carbonatite dikes complete the picture. In 2022 the International Union of Geological Sciences included it among its first 100 geological heritage sites as a spectacular example of a magmatic concentric alkaline complex.

The Richat is also an archaeological landscape. Its outermost ring holds exceptional concentrations of Acheulean hand axes and other stone tools, made from the quartzite outcrops along the wadis of the northwest, and Middle Stone Age Aterian tools show that modern humans were here too. The lake and river deposits in which many of them lie date largely from the African Humid Period, between about 15,000 and 8,000 years ago, when this corner of the Sahara was far wetter than today. Thousands of stone burial mounds line the ridges, and rock art nearby shows horsemen, chariots, cattle and elephants. The structure's ring shape has also attracted popular claims that it was Plato's Atlantis, which geologists and archaeologists reject.

The appeal of a visit lies in the scale and silence of the place and in the long approach across the plateau, not in any single viewpoint: the rings reveal themselves gradually, ridge by ridge, and the full circle belongs to aerial images alone.

Getting there

The nearest town is Ouadane, an old caravan town whose ruined ksar is a World Heritage site; the outer ring of the structure lies about 30 km to the north-east, reached by 4x4 over sandy desert. Atar is about 180 km from Ouadane, and its airport (ATR) has flights from Nouakchott; the airport at Zouérat (Tazadit, OUZ), about 210 km away as the crow flies, is not a practical gateway. The usual route runs from Atar via Chinguetti to Ouadane with a local driver and guide.