Emi Koussi

Emi Koussi (also known as Emi Koussou) is a high pyroclastic shield 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 , rising above the surrounding sandstone plains. The volcano is wide and has a volume of .

Two nested calderas cap the volcano, the outer one being about in size. Within it on the southeast side is a smaller caldera known as Era Kohor, about wide and deep. Numerous lava domes, cinder cones, maars, and lava flows are found within the calderas and along the outer flanks of the shield. Era Kohor contains trona deposits, and Emi Koussi has been studied as an analogue of the Martian volcano Elysium Mons. Emi Koussi was active more than one million years ago, but some eruptions may be more recent, and there is ongoing fumarolic and hot spring activity.

Geography and geomorphology

Emi Koussi lies in Chad, Africa, and is part of the Tibesti Mountains as well as its highest peak and thus of the Sahara; many peaks of the Tibesti exceed in height. These mountains are formed by a group of volcanoes that grew on top of a large dome of Earth's surface. Volcanism in this area is poorly studied; as the region is remote and access difficult for political reasons. Dirt roads cross the western and eastern margins of the summit caldera, and according to historical reports the Tibu people lived in its summit caldera and in artificial caves. In the Tedaga language, means "mountains", "massif". The mountain has played an important role in the research and discovery history of the Tibesti.

Emi Koussi rises to a height of on the southern side of the volcano, towering above the surrounding terrain. It is considered by some astronauts to be the most recognizable landmark on Earth, as seen from orbit. Supposedly the entire Tibesti can be seen from the northern summits.

It is a shield volcano with a diameter of or and an estimated volume of . The summit of Emi Koussi is formed by two overlapping calderas, which together form a wide elliptical caldera that extends from northwest to southeast. The northern caldera formed first, then the southern caldera which is about deeper below the northern one. high scarps form the inner margin of the southern caldera, with the floor at an elevation of . Before the formation of the calderas, the volcano may have reached a height of . Emi Koussi has been compared to a Martian volcano, Elysium Mons. The caldera rim is cut by one outlet, the Porte de Modiounga, from which gorges emanate into the caldera. The highest point of Emi Koussi lies on the southern rim of the caldera.

Nested within this combined caldera is the Era Kohor caldera, which is deep and wide, having the appearance of a giant hole. This caldera is also known as Natron Hole or Trou au Natron. Trachytic lava flows are exposed in its walls, and sodium carbonate has been deposited on its floor, which lies at an elevation of and contains a salt lake. The floor of Era Kohor is thus deep white. Three maars and several scoria cones are also nested within the combined caldera, along with lava domes and lava flows. Debris from explosive eruptions fills the calderas.

The Kohor pumices and two sets of ignimbrites cover the flanks of Emi Koussi, which steepen as they approach the summit. Scoria cones on the slopes are accompanied by lava flows. On the upper sectors of the volcano, lavas are crisscrossed by cracks, which are known as "lappiaz". Sandstone crops out on the terrain surrounding the volcano. North of Emi Koussi lie other volcanoes, such as Tarso Ahon and Tarso Emi Chi, the former of which is connected to Emi Koussi by a narrow ridge.

Geology

Tectonic activity appears to have occurred in the Tibesti already between the Carboniferous and the Cretaceous, that is between 358.9 ± 0.4 and 66 million years ago. During the earliest stage of volcanism on Tibesti, alkali basalts formed large plateaus. Later, the central volcanoes developed on top of these plateaus. Volcanism in Tibesti has been explained with a mantle plume, as has been proposed for other African volcanoes, although recently tectonic effects of the collision between Africa and Europe and their effects at a distance have also been advanced as an explanation.

The oldest rocks below the Tibesti are Precambrian diorites, granites and schists, which are probably of Neoproterozoic age and are differentiated into two units. The volcanic rocks rest on an uplifted basement formed by Cretaceous and Paleozoic sandstone. The latter crop out at the southwestern foot of Emi Koussi, while volcanic rocks dominate north of the volcano, and the Tibesti massif is prevalent east and southeast of Emi Koussi. Older volcanic rocks are exposed in valleys.

Emi Koussi has erupted phonolite, trachyandesite and trachyte, as well as mafic rocks like basanite and tephrite. The erupted rocks define two alkaline suites. Phenocryst chemistry and content varies between the various rocks; among the minerals are alkali feldspar, amphibole, biotite, clinopyroxene, olivine, oxides and plagioclase. Alkali feldspar, apatite, clinopyroxene, olivine, magnetite, mica, nepheline, oxides, plagioclase, quartz, sodalite, titanite and zircon also form the groundmass of microliths in erupted rocks. Magma genesis mainly involved fractional crystallization processes. Rocks from Emi Koussi were used as raw material by Neolithic societies of the region.

During the early Holocene, Era Kohor was filled with a deep, circular lake. Diatom beds have been found above the floor of Era Kohor and in depressions without outlet elsewhere in the caldera, they reach thicknesses of . Farther down on the mountain, gullies start appearing at elevation and become deep gorges by elevation. Elleboe wadi originates on Emi Koussi, and several streams on the western flank join into the Enneri Miski which drains southwards and disappears south of the mountains. Small pools of water are found around Emi Koussi.

Eruptive history

Emi Koussi erupted between 2.4 and 1.3 million years ago; it is considered to be of Pliocene age. The volcano was constructed mainly during the Miocene within about one million years. Radiometric dating has yielded ages of 2.42 ± 0.03 - 2.33 ± 0.09 million years ago for the oldest Emi Koussi stage. The third stage has yielded ages of 1.4 ± 0.3 - 1.32 ± 0.2 million years ago; formerly Era Kohor was considered to be a Holocene centre.

During the first stage, Emi Koussi erupted trachytes and trachyandesites in the form of ignimbrites and basaltic lapilli. This was followed in the second stage by trachytic, green or grey ignimbrites, trachytic and phonolitic lava domes and additional basaltic lava flows. The second stage ignimbrites can be found within the northern caldera and on the southern flank of the volcano. The third stage was dominantly phonolitic, with breccia, tuffs and ignimbrites including the Kohor ignimbrite. The various ignimbrites contain fiamme and often look like lava flows. Each stage was accompanied by the formation of a caldera, and basaltic volcanism continued through all three stages. Phreatic explosions associated with Era Kohor have deposited large blocks within the summit caldera, including beautifully coloured syenite

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