(also Tibesti Massif), a highland in the central Sahara, in the Republic of Chad. Elevations range to 3,415 m (the volcano Emi Koussi). Tibesti is composed of schists, lavas, and sandstones and is dissected by deep gullies through which water occasionally flows. Precipitation totals less than 100 mm a year, with most of it falling in summer. The date palm, doom palm, and acacia are among the trees that grow along the foot of the mountains and in the valleys to an elevation of 1,200 m. Clumps of wormwood and grasses grow at elevations of 1,600 to 1,800 m, and mountain and semidesert vegetation is found at higher elevations.