Emi Koussi

Emi Koussi
Natron in the Era Kohor Crater at the summit of Emi Koussi
Highest point
Elevation	3,445 m (11,302 ft)
Prominence	2,934 m (9,626 ft) Ranked 103rd
Listing	Country high point Ultra
Coordinates	19°47′37″N 18°33′7″E
Geography
Emi Koussi Location in Chad
Location	Chad
Parent range	Tibesti Mountains
Geology
Age of rock	Holocene
Mountain type	Pyroclastic shield
Last eruption	unknown
Climbing
Easiest route	Hike

Emi Koussi is a high pyroclastic shield volcano that lies at the southeast end of the Tibesti Mountains in the central Sahara of the northern Borkou Region 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 Mountains, and reaches 3,445 metres (11,302 ft) in elevation, rising 2.3 km (1.4 mi) 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 analogue 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.

It was climbed in September 1938 by Wilfred Thesiger and Idris Daud as recorded in the former's autobiography A Life of My Choice.

Geography and geomorphology

Emi Koussi lies in Chad, Africa, and is part of the Tibesti Mountains which often exceed 2,000 metres (6,600 ft) height; Emi Koussi is the highest of these mountains. These mountains are formed by a group of volcanoes that grew on top of a large dome of Earth's surface. Volcanism there is poorly studied, as the region is remote and access difficult for political reasons.

Emi Koussi rises to a height of 3,415 metres (11,204 ft), towering 3 kilometres (1.9 mi) above the surrounding terrain and is considered the most recognizable landmark on Earth, as seen from orbit. Supposedly the entire Tibesti can be seen from the northern summits, while the main summit lies on the southern side of the volcano.

It is a shield volcano with a diameter of 60–70 kilometres (37–43 mi). The summit of Emi Koussi is formed by two overlapping calderas, which together form a 15 by 11 kilometres (9.3 mi × 6.8 mi) wide ellipse-shaped caldera that extends from northwest to southeast. The northern caldera formed first, then the southern caldera which is about 50 metres (160 ft) deeper below the northern one. 400–300 metres (1,310–980 ft) high scarps form the inner border of the southern caldera. Before the formation of the caldera, the volcano may have reached a height of 4,000 metres (13,000 ft). Emi Koussi has been compared to a Martian volcano, Elysium Mons.

Nested within this combined caldera is the Era Kohor caldera (also known as Natron Hole or Trou au Natron), which is 300 metres (980 ft) deep and 2 kilometres (1.2 mi) wide, like a giant hole. Trachytic lava flows are exposed in its walls, and sodium carbonate has been deposited on its floor. Three maars and several scoria cones are also nested within the combined caldera.

The Kohor pumices and two sets of ignimbrites cover the flanks of Emi Koussi, which steepen as they approach the summit. Scoria cones are accompanied by lava flows. Sandstone crops out on the terrain surrounding the volcano. Lava flows on the upper sectors of the volcano are crisscrossed by cracks, which are known as "lappiaz". 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. During the earliest stage of volcanism on Tibesti, alkali basalts formed large plateaus. Later, the central volcanoes developed on top of this plateau. The occurrence of volcanism in Tibesti has been explained with a mantle plume, as has been proposed for other African volcanoes, although recently far-field tectonic effects of the collision between African and Europe 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 east and southeast thereof. Older volcanic rocks are exposed in the 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 or 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.

Era Kohor may have been filled with a lake in the past. Diatom beds have been found 125 metres (410 ft) above the floor of the caldera. Elleboe wadi originates on Emi Koussi.

Climate and vegetation

Based on cloud cover data, a yearly precipitation of 80–120 millimetres (3.1–4.7 in) has been estimated for Emi Koussi. There is no evidence of nivation forms on Emi Koussi.

Cattail and water milfoil grew around the Era Kohor lake.

On Emi Koussi, Erica arborea is found to elevations of 2,900 metres (9,500 ft). The summit region features montane vegetation, including Artemisia-Ephedra steppes. The lava flows high on the mountain have characteristic environmental traits which allow the development of a particular vegetation. This vegetation encompasses Dichrocephala, Eragrostis, Erodium, Helichrysum as well as ferns, liverworts and mosses. Oldenlandia and Selaginella species grow at fumaroles.

The wood rush Luzula tibestica is endemic on Emi Koussi. The fern Asplenium quezelii was discovered on Emi Koussi and at first thought to be endemic there before it was found to be identical to Asplenium daghestanicum, a species also previously considered to be endemic to Dagestan, over 3,800 kilometres (2,400 mi) away from Emi Koussi.

Eruptive history

Emi Koussi developed between 2.4 and 1.3 million years ago, it is considered to be of Pliocene-Quaternary age. The volcano was constructed within about one million years. Radiometric dating has yielded ages of 2.42 ± 0.03 million years ago to 2.33 ± 0.09 million years ago for the oldest Emi Koussi stage. The third stage has yielded ages of 1.4 ± 0.3 to 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 more trachytic, green or grey ignimbrites, trachytic and phonolitic lava domes and more 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.

Quaternary volcanism gave rise to the cones on the slopes of Emi Koussi, and the three maars are the most recent vents in the combined caldera. The explosive eruptions filled the calderas with debris. Early in the Pleistocene, erosion took place on the flanks of Emi Koussi, and trona was deposited in Era Kohor until recent times.

On the southern foot of the volcano, fumaroles are active such as at Yi Yierra at 850 metres (2,790 ft) elevation on the southern flanks. Emi Koussi is considered to be a Holocene volcano.

See also

• Geography of Chad
• List of volcanoes in Chad
• List of Ultras of Africa

References

Sources

• Deniel, C.; Vincent, P. M.; Beauvilain, A.; Gourgaud, A. (1 September 2015). "The Cenozoic volcanic province of Tibesti (Sahara of Chad): major units, chronology, and structural features". Bulletin of Volcanology. 77 (9): 74. doi:10.1007/s00445-015-0955-6. ISSN 0258-8900. {{cite journal}}: Invalid |ref=harv (help)
• Gèze, B.; Hudeley, H.; Vincent, P.; Wacrenier, Ph (1 December 1959). "Les volcans du Tibesti (Sahara du Tchad)". Bulletin Volcanologique (in French). 22 (1): 135–172. doi:10.1007/BF02596582. ISSN 0366-483x. {{cite journal}}: Check |issn= value (help); Invalid |ref=harv (help)
• Gourgaud, A; Vincent, P.M (January 2004). "Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad". Journal of Volcanology and Geothermal Research. 129 (4): 261–290. doi:10.1016/S0377-0273(03)00277-4. ISSN 0377-0273. {{cite journal}}: Invalid |ref=harv (help)
• Malin, Michael C. (1 July 1977). "Comparison of volcanic features of Elysium (Mars) and Tibesti (Earth)". GSA Bulletin. 88 (7). doi:10.1130/0016-7606(1977)88<908:COVFOE>2.0.CO;2. ISSN 0016-7606. {{cite journal}}: Invalid |ref=harv (help)
• Messerli, B. (1973). "Problems of Vertical and Horizontal Arrangement in the High Mountains of the Extreme Arid Zone (Central Sahara)". Arctic and Alpine Research. 5 (3). {{cite journal}}: Invalid |ref=harv (help); Unknown parameter |subscription= ignored (|url-access= suggested) (help)
• Permenter, Jason L.; Oppenheimer, Clive (1 April 2007). "Volcanoes of the Tibesti massif (Chad, northern Africa)". Bulletin of Volcanology. 69 (6): 609–626. doi:10.1007/s00445-006-0098-x. ISSN 0258-8900. {{cite journal}}: Invalid |ref=harv (help)
• Poli, Emilia (1974). Tatsachen und Probleme der Grenzen in der Vegetation. Bericht über das Internationale Symposion der Internationalen Vereinigung für Vegetationskunde in Rinteln 8.–11. April 1968 (in German). Springer, Dordrecht. pp. 223–251. doi:10.1007/978-94-011-7595-1_21. ISBN 9789401175968. {{cite book}}: Invalid |ref=harv (help)
• Tilho, Jean (1920). "The Exploration of Tibesti, Erdi, Borkou, and Ennedi in 1912-1917: A Mission Entrusted to the Author by the French Institute (Continued)". The Geographical Journal. 56 (3): 161–183. doi:10.2307/1781532. {{cite journal}}: Invalid |ref=harv (help)
• Viane, R.L.L (1987). "NOTES ABOUT ASPLEN/UM I. ASPLEN/UM QUEZEL/1, A PSEUDO-ENDEMIC SPECIES IDENTICAL WITH A. DAGHESTANICUM (ASPLENIACEAE : PTERIDOPHYTA)" (PDF). The Fern Gazette. 13 (3). Retrieved 13 April 2018. {{cite journal}}: Invalid |ref=harv (help)

Sources

• Original entry was from the NASA Earth Observatory; [1]
• NASA Earth Explorer page
• Gourgaud, A.; Vincent, P. M. (2004). "Petrology of two continental alkaline intraplate series at Emi Koussi volcano, Tibesti, Chad". Journal of Volcanology and Geothermal Research. 129 (4): 261–290. Bibcode:2004JVGR..129..261G. doi:10.1016/S0377-0273(03)00277-4. {{cite journal}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
• "Emi Koussi". Peakware.com.
• "Emi Koussi, Chad" on Peakbagger

External links

• Media related to Emi Koussi at Wikimedia Commons