Tarso Toussidé, a broad volcanic massif at the western end of the Tibesti Range, is capped by the Toussidé stratovolcano, constructed at the western end of the 14-km-wide ignimbritic Yirrigue caldera of Pleistocene age. The 6000 km2 massif was constructed over a basement of Precambrian schists on the east and Paleozoic sandstones on the west. The summit contains numerous fumaroles and very youthful lava flows that would be of historical age in an inhabited region (Vincent 1992, pers. comm.). Fresh-looking trachytic and trachyandesitic lava flows cover an area of 200 km2 and extend as far as 25 km from the summit on the western side. The steep-sided, 1-km-deep, 8-km-wide Trou au Natron caldera cuts the SE rim of Yirrigue caldera and is one of the youngeset features of the massif. Ehi Timi and Ehi Sosso volcanoes and the smaller 1.5-km-wide explosion crater of Doon Kidimi are located on the NE-to-ESE flanks of the massif.