Emi Koussi

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

Emi Koussi rises to a height of on the southern side of the volcano,Template:SfnTemplate:Sfn rising between < and above the surrounding terrainTemplate:Sfn and forming the highest summit of the Sahara.Template:Sfn It is considered by some astronauts to be the most recognizable landmark on Earth, as seen from orbit.Template:Sfn Supposedly the entire Tibesti can be seen from the northern summits.Template:Sfn

File:EmiKoussiTopo.png

Topographic map of the Emi Koussi

It is a shield volcano with a diameter of Template:Sfn or Template:Sfn and an estimated volume of -.Template:Sfn The summit of Emi Koussi is formed by two overlapping calderas, which together form aTemplate:Sfn wideTemplate:Sfn elliptical caldera that extends from northwest to southeast.Template:Sfn 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,Template:Sfn 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.Template:SfnTemplate:Sfn The caldera rim is cut by one outlet, the Porte de Modiounga,Template:Sfn from which gorges emanate into the caldera.Template:Sfn The highest point of Emi Koussi lies on the southern rim of the caldera.Template:Sfn

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

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

Geology

Tectonic activity appears to have occurred in the Tibesti already between the Carboniferous and the Cretaceous,Template:Sfn 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.Template:Sfn Volcanism in Tibesti has been explained with a mantle plume, as has been proposed for other African volcanoes,Template:Sfn although recently tectonic effects of the collision between Africa and Europe and their effects at a distance have also been advanced as an explanation.Template:Sfn

The oldest rocks below the Tibesti are Precambrian diorites, granites and schists,Template:Sfn which are probably of Neoproterozoic age and are differentiated into two units.Template:Sfn The volcanic rocks rest on an uplifted basement formed by CretaceousTemplate:Sfn and Paleozoic sandstone.Template:Sfn 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.Template:Sfn Older volcanic rocks are exposed in valleys.Template:Sfn

Emi Koussi has erupted phonolite, trachyandesite and trachyte,Template:Sfn 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.Template:Sfn Magma genesis mainly involved fractional crystallization processes.Template:Sfn Rocks from Emi Koussi were used as raw material by Neolithic societies of the region.

Until 5,400 years agoTemplate:Sfn during the African humid period (Holocene), Era Kohor was filled with a lake whose water reached a depth of Template:Sfn and a volume of .Template:Sfn It was fed in part by water accumulating in the outer caldera and percolating to Era Kohor.Template:Sfn Diatom beds have been found above the floor of Era Kohor and in depressions without outlet elsewhere in the caldera,Template:Sfn they reach thicknesses of .Template:Sfn Farther down on the mountain, gullies start appearing at elevation and become deep gorges by elevation.Template:Sfn Elleboe wadi originates on Emi Koussi,Template:Sfn and several streams on the western flank join into the Enneri Miski which drains southwards and disappears south of the mountains.Template:Sfn Small pools of water are found around Emi Koussi.Template:Sfn

Eruptive history

File:Caldeira Emi Koussi ASTER.jpg

Emi Koussi caldera, ASTER

Emi Koussi erupted between 2.4 and 1.3 million years ago;Template:Sfn it is considered to be of Pliocene-Quaternary age.Template:Sfn The volcano was constructed mainly during the MioceneTemplate:SfnTemplate:Sfn within about one million years.Template:Sfn 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;Template:SfnTemplate:Sfn formerly Era Kohor was considered to be a Holocene centre.Template:Sfn

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.Template:Sfn The second stage ignimbrites can be found within the northern caldera and on the southern flank of the volcano.Template:Sfn The third stage was dominantly phonolitic, with breccia, tuffs and ignimbrites including the Kohor ignimbrite.Template:Sfn The various ignimbrites contain fiamme and often look like lava flows.Template:Sfn Each stage was accompanied by the formation of a caldera,Template:Sfn and basaltic volcanism continued through all three stages.Template:Sfn Phreatic explosions associated with Era Kohor have deposited large blocks within the summit caldera,Template:Sfn including beautifully coloured syenite blocks.Template:Sfn

Quaternary volcanism gave rise to the cones on the slopes of Emi Koussi,Template:Sfn and the three maars are the most recent vents in the combined caldera,Template:Sfn while long lava flows have been interpreted to be the youngest volcanism at Emi Koussi. The most recent volcanic vents show little erosion.Template:Sfn Early in the Pleistocene, erosion took place on the flanks of Emi Koussi,Template:Sfn and carbonates were deposited in Era Kohor until recent times.Template:SfnTemplate:Sfn

On the southern foot of the volcano, fumaroles are activeTemplate:Sfn such as at Yi Yerra at elevation on the southern flanks.Template:Sfn At Yi Yerra, hot springs produce water with temperatures of .Template:Sfn Emi Koussi is considered to be a Holocene volcano.Template:Sfn Lava flows have buried Holocene diatoms beds in its caldera;Template:Sfn dates of 14,790 ± 400 - 12,400 ± 400 years before present have been obtained by radiocarbon dating on diatom beds which have been penetrated or buried by lava flows.

Climate and vegetation

The Tibesti mountains feature a desert mountain climate. There are no weather stations close to Emi Koussi and the stations at lower elevations likely underestimate precipitation at higher elevations,Template:Sfn but based on cloud cover data, a yearly precipitation of has been estimated for Emi Koussi.Template:Sfn Precipitation was higher in the past,it Template:Sfn when the African monsoon expanded over the SaharaTemplate:Sfn and drew moisture from the Mediterranean Sea to the Tibesti,Template:Sfn The climate reflects both northerly and southerly influences and is distinct from that of the surrounding deserts. There is no evidence of nivation forms on Emi KoussiTemplate:Sfn and the identification of periglacial landforms is questionable, although periglacial activity likely occurred in the Pleistocene and may continue to the present-day.Template:Sfn Even during the Pleistocene, no part of the mountain had sub-freezing temperatures through the entire year.Template:Sfn

A number of diatom species have been identified in sediments left by the Era Kohor lake, including Cocconeis placentula, Cyclotella cyclopuncta, Cyclotella ocellata, Cymbella cistula, Cymbella leptoceros, Cymbella muelleri, Epithemia adnata, Fragilaria construens, Fragilaria pinnata, Gomphonema affine, Gomphonema parvulum, Navicula oblonga and Rhopalodia gibba.Template:Sfn The diatom fauna appears to be unusual in comparison to other Saharan paleolakes, probably owing to the high elevation of the Emi Koussi lake.Template:Sfn Cattail and water milfoil grew around the lake.

Generally, in the Tibesti vegetation occurs in five different belts depending on elevation.Template:Sfn On Emi Koussi, Erica arborea is found up to elevations of ,Template:Sfn in the past it was probably more widespread.Template:Sfn The summit region features montane vegetation, including Artemisia-Ephedra steppesTemplate:Sfn which cover large parts of the caldera floor.Template:Sfn The lava flows high on the mountain have characteristic environmental traits which allow the development of a particular vegetation.Template:SfnTemplate:Sfn This vegetation encompasses Dichrocephala, Eragrostis, Erodium, Helichrysum as well as ferns, liverworts and mosses.Template:Sfn Oldenlandia and Selaginella species grow at fumaroles. The caldera floor was used as a pasture for camels and goats.Template:Sfn

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 thereTemplate:Sfn before it was found to be identical to Asplenium daghestanicum, a species also previously considered to be endemic to Dagestan, more than away from Emi Koussi.Template:Sfn

Gallery

• Another view of it from space
  Another view of it from space
• Calderas of Emi Koussi
  Calderas of Emi Koussi
• Emi Koussi inner crater
  Emi Koussi inner crater
• Natron in the inner caldera of Emi Koussi
  Natron in the inner caldera of Emi Koussi
• Emi Koussi seen from Apollo 7 in October 1968
  Emi Koussi seen from Apollo 7 in October 1968

See also

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

References

Sources

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External links

• Recognition of Tibesti by the battalion commander Tilho Head of the Borkou-Ennedi district September 4 - November 12, 1915.
• Template:Cite peakware
• Template:Cite peakbagger