The large Emi Koussi volcano at the SE end of the Tibesti Range, a 60 x 80 km massif constructed over a basement of Cretaceous and Paleozoic sandstones, forms the highest summit of the Sahara. Two nested calderas, 12 x 15 km, truncate the dominantly trachytic volcano, whose high point lies on the southern caldera rim of the low-angle pyroclastic shield volcano. The calderas contain several explosion craters, lava domes, and young scoria cones, along with lava flows of recent and pre-modern age (Geze et al., 1959). The Era Kohor (Trou au Natron du Koussi) caldera on the SE floor of the 2nd caldera is 2-3 km wide and 350 m deep. The latest activity in the calderas produced three maars, and numerous basaltic cinder cones; associated lava flows have been constructed both within the calderas and on their outer flanks. The Yi-Yerra thermal area is located on the S flank.