Emi Koussi is the highest point of which desert?

Emi Koussi is a volcano that lies at the southeast end of the Tibesti Mountains in the central Sahara, in the northern Borkou Region of northern Chad. The highest mountain of the Sahara, the volcano is one of several in the Tibesti range, and reaches an elevation of 3,415 metres, rising 3 km (1.9 mi) above the surrounding sandstone plains. The volcano is 60–70 kilometres (37–43 mi) wide and has a volume of 2,500 cubic kilometres (600 cu mi).

The volcano was constructed over a basement of Cretaceous and Paleozoic sandstones. Two nested calderas cap the volcano, the outer one being about 15 by 11 kilometres (9.3 mi × 6.8 mi) in size. Within it on the southeast side is a smaller caldera known as Era Kohor, about 2 kilometres (1.2 mi) wide and 350 metres (1,150 ft) deep. A caldera is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber/reservoir in a volcanic eruption.

Numerous lava domes, cinder cones, maars, and lava flows are found within the calderas and along the outer flanks of the shield. A maar is a broad, low-relief volcanic crater caused by a an explosion which occurs when groundwater comes into contact with hot lava or magma. A maar characteristically fills with water to form a relatively shallow crater lake.

There is still ongoing fumarolic and hot spring activity. A fumerole is a vent in or near a volcano from which hot gases, especially steam are emitted.