Emi Koussi is a high pyroclastic shield volcano that lies at the south end of the Tibesti Mountains in the central Sahara of northern Chad. It is the highest mountain in Chad, and the highest in the Sahara. The volcano is one of several in the Tibesti massif, and reaches 3445 m in altitude, rising 2.3 km above the surrounding sandstone plains. The volcano is 60 by 80 km wide.
Two nested calderas cap the volcano, the outer one being about 12 by 15 km in size. Within it on the southeast side is a smaller caldera, about 2–3 km wide and 350 m deep. Numerous lava domes, cinder cones, maars, and lava flows are found both within the calderas and along the outer flanks of the shield. The inner caldera contains large natron deposits which see some harvesting for domestic animal salt lick use by the local people.
Emi Koussi has been studied as an analog of the Martian volcano Elysium Mons. One of the most important morphological differences between volcanoes on Mars and Earth is the widespread furrowing of the surface due to flowing water on terrestrial volcanoes. The furrows are shallow valleys. Larger channels have a different origin. Major channels can be seen on volcanoes on both planets and indicate low points in caldera rims where lava spilled out of pre-collapse craters.