{{Short description|High pyroclastic shield volcano in the Sahara}} {{Good article}} {{Infobox mountain | name = Emi Koussi | image = Emi Koussi Volcano, Chad From ISS.JPG | image_caption = Emi Koussi seen from space | location = [[Chad]] | elevation_m = 3447 | prominence_m = 2934 | prominence_ref = <br/><small>[[List of peaks by prominence|Ranked 104th]]</small> | range = [[Tibesti Mountains]] | listing = [[List of countries by highest point|Country high point]]<br />[[Ultra prominent peak|Ultra]]<br>Ribu | map = Chad | map_caption = Location in Chad | map_size = 200 | label_position = right | coordinates= {{coord|19|47|33|N|18|32|47|E|type:mountain_scale:100000|format=dms|display=inline}} | coordinates_ref=<ref name=peakbagger>{{cite peakbagger|pid=11071|name=Emi Koussi|access-date=2021-02-12}}</ref> | topo = | type = [[Pyroclastic shield]] | age = [[Holocene]] | last_eruption = unknown | first_ascent = | easiest_route = Hike }}
'''Emi Koussi''' (also known as Emi Koussou{{sfn|Hellmich|1972|p=10}}) is a high [[pyroclastic shield]]<ref name="gvp"/> [[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 {{convert|3447|m|ft|0}}, rising {{convert|3|km|mi|1|abbr=on}} above the surrounding [[sandstone]] plains. The volcano is {{convert|60|-|70|km}} wide and has a volume of {{convert|2500|km3}}.
Two nested [[caldera]]s cap the volcano, the outer one being about {{convert|15|x|11|km}} in size. Within it on the southeast side is a smaller caldera known as Era Kohor, about {{convert|2|km}} wide and {{convert|350|m|ft}} deep. Numerous [[lava dome]]s, [[cinder cone]]s, [[maar]]s, and [[Lava|lava flow]]s 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 [[Mars|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 [[Fumarole|fumarolic]] and [[hot spring]] activity.
==Geography and geomorphology== Emi Koussi lies in [[Chad]], [[Africa]], and is part of the [[Tibesti Mountains]]{{sfn|Gourgaud|Vincent|2004|p=261}} as well as its highest peak{{sfn|Malin|1977|p=908}} and thus of the Sahara;{{sfn|Pachur|Altmann|2006|p=88}} many peaks of the Tibesti exceed {{convert|2000|m}} in height.{{sfn|Malin|1977|p=908}} These mountains are formed by a group of volcanoes that grew on top of a large [[dome]] of Earth's surface.{{sfn|Gourgaud|Vincent|2004|p=262}} Volcanism in this area is poorly studied; as the region is remote and access difficult for political reasons.{{sfn|Permenter|Oppenheimer|2007|p=609}} [[Dirt road]]s cross the western and eastern margins of the summit caldera,{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=118}} and according to historical reports the [[Toubou|Tibu]] people lived in its summit caldera<ref name="Peel1942" /> and in artificial caves.{{sfn|Pachur|Altmann|2006|p=103}} In the [[Tedaga]] language, {{lang|tuq|Emi}} means "mountains", "[[massif]]".{{sfn|Pachur|Altmann|2006|p=88}} The mountain has played an important role in the research and discovery history of the Tibesti.{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=117}}
Emi Koussi rises to a height of {{convert|3415|m|}} on the southern side of the volcano,{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=137}}{{sfn|Messerli|Winiger|1992|p=327}} rising between <{{convert|1|km}}<ref name="KarátsonThouret2025" /> and {{convert|3|km}} above the surrounding terrain{{sfn|Gourgaud|Vincent|2004|p=264}} and forming the highest summit of the Sahara.{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=1}} It is considered by some astronauts to be the most recognizable landmark on Earth, as seen from [[orbit]].{{sfn|Gourgaud|Vincent|2004|p=264}} Supposedly the entire Tibesti can be seen from the northern summits.{{sfn|Tilho|1920|p=172}}
[[File:EmiKoussiTopo.png|thumb|Topographic map of the Emi Koussi]]
It is a [[shield volcano]] with a diameter of {{convert|60|-|70|km}}{{sfn|Gourgaud|Vincent|2004|p=264}} or {{convert|60|-|80|km}}{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=117}} and an estimated volume of {{convert|150|km3}}<ref name="KarátsonThouret2025" />-{{convert|2500|km3}}.{{sfn|Hellmich|1972|p=21}} The summit of Emi Koussi is formed by two overlapping [[caldera]]s, which together form a{{sfn|Gourgaud|Vincent|2004|p=263}} {{convert|15|x|11|km|adj=on}} wide{{sfn|Gourgaud|Vincent|2004|p=264}} elliptical caldera that extends from northwest to southeast.{{sfn|Gourgaud|Vincent|2004|p=263}} The northern caldera formed first, then the southern caldera which is about {{convert|50|m}} deeper below the northern one. {{convert|400|-|300|m|adj=on}} high [[Escarpment|scarps]] form the inner margin of the southern caldera,{{sfn|Gourgaud|Vincent|2004|p=267}} with the floor at an elevation of {{convert|2970|m}}.<ref name="Zimmermann1919" /> Before the formation of the calderas, the volcano may have reached a height of {{convert|4000|m}}. Emi Koussi has been compared to a [[Martian]] volcano, [[Elysium Mons]].{{sfn|Malin|1977|p=908}}{{sfn|Permenter|Oppenheimer|2007|p=621}} The caldera rim is cut by one [[River|outlet]], the Porte de [[Modiounga]],{{sfn|Hellmich|1972|p=172}} from which [[gorge]]s emanate into the caldera.{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=119}} The highest point of Emi Koussi lies on the southern rim of the caldera.{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=118}}
Nested within this combined caldera is the Era Kohor caldera,{{sfn|Gourgaud|Vincent|2004|p=263}} which is {{convert|300|m}} deep and {{convert|2|km}} wide, having the appearance of a giant hole.{{sfn|Gourgaud|Vincent|2004|p=267}} This caldera is also known as [[Natron]] Hole{{sfn|Tilho|1920|p=172}} or Trou au Natron.{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=137}} [[Trachyte|Trachytic]] lava flows are exposed in its walls,{{sfn|Gourgaud|Vincent|2004|p=267}} and [[sodium carbonate]] has been deposited on its floor,{{sfn|Malin|1977|p=909}} which lies at an elevation of {{convert|2670|m}}<ref name="Zimmermann1919" /> and contains a salt lake.{{sfn|Pachur|Altmann|2006|p=103}} The floor of Era Kohor is thus deep white.{{sfn|Hellmich|1972|p=172}} Three [[maar]]s and several [[scoria cone]]s are also nested within the combined caldera,{{sfn|Gourgaud|Vincent|2004|p=267}} along with [[lava dome]]s and [[lava flows]].<ref name="gvp" /> Debris from [[explosive eruption]]s fills the calderas.{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=139}}
The Kohor [[pumice]]s and two sets of [[ignimbrite]]s cover the flanks of Emi Koussi,{{sfn|Gourgaud|Vincent|2004|p=263}} which steepen as they approach the summit.{{sfn|Permenter|Oppenheimer|2007|p=621}} Scoria cones on the slopes are accompanied by [[lava flow]]s.{{sfn|Gourgaud|Vincent|2004|p=263}} On the upper sectors of the volcano, lavas are crisscrossed by cracks, which are known as "lappiaz".{{sfn|Poli|1974|p=225}} [[Sandstone]] crops out on the terrain surrounding the volcano.{{sfn|Gourgaud|Vincent|2004|p=263}} North of Emi Koussi lie other volcanoes, such as [[Tarso Ahon]] and [[Tarso Emi Chi]],{{sfn|Permenter|Oppenheimer|2007|p=616}} the former of which is connected to Emi Koussi by a narrow ridge.{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=137}}
==Geology== Tectonic activity appears to have occurred in the Tibesti already between the [[Carboniferous]] and the [[Cretaceous]],{{sfn|Malin|1977|p=908}} that is between 358.9 ± 0.4 and 66 million years ago.<ref name="ICC2018" /> During the earliest stage of volcanism on Tibesti, [[alkali basalt]]s formed large plateaus. Later, the central volcanoes developed on top of these plateaus.{{sfn|Gourgaud|Vincent|2004|p=264}} Volcanism in Tibesti has been explained with a [[mantle plume]], as has been proposed for other African volcanoes,{{sfn|Gourgaud|Vincent|2004|p=288}} although recently tectonic effects of the collision between Africa and Europe and their effects at a distance have also been advanced as an explanation.{{sfn|Deniel|Vincent|Beauvilain|Gourgaud|2015|p=3}}
The oldest rocks below the Tibesti are [[Precambrian]] [[diorite]]s, [[granite]]s and [[schist]]s,{{sfn|Permenter|Oppenheimer|2007|p=616}} which are probably of [[Neoproterozoic]] age and are differentiated into two units.{{sfn|Deniel|Vincent|Beauvilain|Gourgaud|2015|p=3}} The volcanic rocks rest on an uplifted basement formed by Cretaceous{{sfn|Malin|1977|p=908}} and [[Paleozoic]] sandstone.{{sfn|Gourgaud|Vincent|2004|p=264}} 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.{{sfn|Permenter|Oppenheimer|2007|p=616}} Older volcanic rocks are exposed in valleys.{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=138}}
Emi Koussi has erupted [[phonolite]], [[trachyandesite]] and trachyte,{{sfn|Gourgaud|Vincent|2004|p=264}} 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 [[microlith]]s in erupted rocks.{{sfn|Gourgaud|Vincent|2004|pp=268-269}} [[Magma]] genesis mainly involved [[fractional crystallization (geology)|fractional crystallization]] processes.{{sfn|Gourgaud|Vincent|2004|p=288}} Rocks from Emi Koussi were used as [[raw material]] by [[Neolithic]] societies of the region.<ref name="Smith2001" />
Until 5,400 years ago{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=4}} during the [[African humid period]] ([[Holocene]]), Era Kohor was filled with a {{convert|4.1|km2}} lake whose water reached a depth of {{convert|130|m}}{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=1}} and a volume of {{convert|0.45|km3}}.{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=4}} It was fed in part by water accumulating in the outer caldera and percolating to Era Kohor.{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=6}} [[Diatom]] beds have been found {{convert|125|m}} above the floor of Era Kohor<ref name="Hoelzmann2016" /> and in depressions without outlet elsewhere in the caldera,{{sfn|Hellmich|1972|p=56}} they reach thicknesses of {{convert|4|-|5|m}}.{{sfn|Pachur|Altmann|2006|p=119}} Farther down on the mountain, gullies start appearing at {{convert|2800|m}} elevation and become deep [[gorge]]s by {{convert|2000|-|2500|m}} elevation.{{sfn|Hellmich|1972|p=21}} Elleboe [[wadi]] originates on Emi Koussi,{{sfn|Tilho|1920|p=171}} and several streams on the western flank join into the Enneri Miski which drains southwards and disappears south of the mountains.{{sfn|Dumont|1987|p=135}} Small pools of water are found around Emi Koussi.{{sfn|Dumont|1987|p=141}}
==Eruptive history== [[File:Caldeira Emi Koussi ASTER.jpg|Emi Koussi caldera, [[Advanced Spaceborne Thermal Emission and Reflection Radiometer|ASTER]]|thumb|250px]] Emi Koussi erupted between 2.4 and 1.3 million years ago;{{sfn|Gourgaud|Vincent|2004|p=266}} it is considered to be of [[Pliocene]]-[[Quaternary]] age.{{sfn|Gourgaud|Vincent|2004|p=268}} The volcano was constructed mainly during the [[Miocene]]{{sfn|Hellmich|1972|p=21}}{{sfn|Bruneau de Miré|Quézel|1959|p=135}} within about one million years.{{sfn|Deniel|Vincent|Beauvilain|Gourgaud|2015|p=17}} [[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;{{sfn|Gourgaud|Vincent|2004|p=264}}{{sfn|Deniel|Vincent|Beauvilain|Gourgaud|2015|p=18}} formerly Era Kohor was considered to be a Holocene centre.{{sfn|Malin|1977|p=909}}
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.{{sfn|Gourgaud|Vincent|2004|p=264}} The second stage ignimbrites can be found within the northern caldera and on the southern flank of the volcano.{{sfn|Gourgaud|Vincent|2004|p=267}} The third stage was dominantly phonolitic, with [[breccia]], [[tuff]]s and ignimbrites including the Kohor ignimbrite.{{sfn|Gourgaud|Vincent|2004|p=264}} The various ignimbrites contain [[fiamme]] and often look like lava flows.{{sfn|Deniel|Vincent|Beauvilain|Gourgaud|2015|p=8}} Each stage was accompanied by the formation of a caldera,{{sfn|Gourgaud|Vincent|2004|p=267}} and basaltic volcanism continued through all three stages.{{sfn|Gourgaud|Vincent|2004|p=264}} [[Phreatic]] explosions associated with Era Kohor have deposited large blocks within the summit caldera,{{sfn|Gourgaud|Vincent|2004|p=267}} including beautifully coloured [[syenite]] blocks.{{sfn|Hellmich|1972|p=22}}
Quaternary volcanism gave rise to the cones on the slopes of Emi Koussi,{{sfn|Gourgaud|Vincent|2004|p=263}} and the three maars are the most recent vents in the combined caldera,{{sfn|Gourgaud|Vincent|2004|p=267}} while long lava flows have been interpreted to be the youngest volcanism at Emi Koussi.<ref name="Oppenheimer1997" /> The most recent volcanic vents show little erosion.{{sfn|Hellmich|1972|p=22}} Early in the [[Pleistocene]], [[erosion]] took place on the flanks of Emi Koussi,{{sfn|Malin|1977|p=908}} and [[carbonate]]s were deposited in Era Kohor until recent times.{{sfn|Permenter|Oppenheimer|2007|p=619}}{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=165}}
On the southern foot of the volcano, [[fumarole]]s are active{{sfn|Malin|1977|p=908}} such as at Yi Yerra at {{convert|850|m}} elevation on the southern flanks.{{sfn|Gèze|Hudeley|Vincent|Wacrenier|1959|p=139}} At Yi Yerra, [[hot spring]]s produce water with temperatures of {{convert|37|C}}.{{sfn|Dumont|1987|p=134}} Emi Koussi is considered to be a Holocene volcano.{{sfn|Permenter|Oppenheimer|2007|p=619}} Lava flows have buried Holocene diatoms beds in its caldera;{{sfn|Pachur|Altmann|2006|p=103}} 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.<ref name="Hellmich1972" />
== 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,{{sfn|Dumont|1987|p=135}} but based on [[cloud cover]] data, a yearly precipitation of {{convert|80|-|120|mm}} has been estimated for Emi Koussi.{{sfn|Messerli|1973|p=146}} Precipitation was higher in the past,it {{sfn|Messerli|Winiger|1992|p=327}} when the [[African monsoon]] expanded over the Sahara{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=1}} and drew moisture from the [[Mediterranean Sea]] to the Tibesti,{{sfn|Hoelzmann|Claussen|Dallmeyer|Darius|2025|p=5}} The climate reflects both northerly and southerly influences and is distinct from that of the surrounding deserts.<ref name="Hellmich1972" /> There is no evidence of [[nivation]] forms on Emi Koussi{{sfn|Messerli|1973|p=141}} and the identification of [[periglacial]] landforms is questionable,<ref name="Hellmich1972" /> although periglacial activity likely occurred in the Pleistocene and may continue to the present-day.{{sfn|Dumont|1987|p=134}} Even during the Pleistocene, no part of the mountain had sub-freezing temperatures through the entire year.{{sfn|Messerli|Winiger|1992|p=317}}
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]]''.{{sfn|Pachur|Altmann|2006|p=137}} The diatom fauna appears to be unusual in comparison to other Saharan paleolakes, probably owing to the high elevation of the Emi Koussi lake.{{sfn|Pachur|Altmann|2006|p=120}} [[Cattail]] and [[Water Milfoil|water milfoil]] grew around the lake.<ref name="Hoelzmann2016" />
Generally, in the Tibesti vegetation occurs in five different belts depending on elevation.{{sfn|Lézine|Zheng|Braconnot|Krinner|2011|p=1353}} On Emi Koussi, ''[[Erica arborea]]'' is found up to elevations of {{convert|2900|m}},{{sfn|Messerli|1973|p=142}} in the past it was probably more widespread.{{sfn|Lézine|Zheng|Braconnot|Krinner|2011|p=1354}} The summit region features [[montane]] vegetation,<ref name="Romo2011" /> including ''[[Artemisia (plant)|Artemisia]]''-''[[Ephedra (plant)|Ephedra]]'' [[steppe]]s{{sfn|Poli|1974|p=226}} which cover large parts of the caldera floor.{{sfn|Hellmich|1972|p=22}} The lava flows high on the mountain have characteristic environmental traits which allow the development of a particular vegetation.{{sfn|Poli|1974|p=225}}{{sfn|Bruneau de Miré|Quézel|1959|p=136}} This vegetation encompasses ''[[Dichrocephala]]'', ''[[Eragrostis]]'', ''[[Erodium]]'', ''[[Helichrysum]]'' as well as [[fern]]s, [[liverwort]]s and [[mosses]].{{sfn|Poli|1974|p=226}} ''[[Oldenlandia]]'' and ''[[Selaginella]]'' species grow at fumaroles.<ref name="Schnell2014" /> The caldera floor was used as a pasture for [[camel]]s and [[goat]]s.{{sfn|Jäkel|Hövermann|Jensch|Valentin|1972|p=119}}
The [[wood rush]] ''[[Luzula tibestica]]'' is [[endemic]] on Emi Koussi.<ref name="Romo2011" /> The fern ''[[Asplenium quezelii]]'' was discovered on Emi Koussi and at first thought to be endemic there{{sfn|Viane|1987|p=143}} before it was found to be identical to ''[[Asplenium daghestanicum]]'', a species also previously considered to be endemic to [[Dagestan]], more than {{convert|3800|km}} away from Emi Koussi.{{sfn|Viane|1987|p=147}}
==Gallery== <gallery mode="packed" heights="150"> Emi Koussi.JPG|Another view of it from space Emi Koussi Volcano, Chad.JPG|Calderas of Emi Koussi Emi Koussi inner crater.jpg|Emi Koussi inner crater Emi Koussi crater natron.jpg|Natron in the inner caldera of Emi Koussi AS07-05-1621 - Apollo 7 - Apollo 7 Mission, Chad, Tibesti Mountains, Emi Koussi volcano - NARA - 16665747.jpg|Emi Koussi seen from [[Apollo 7]] in October 1968 </gallery>
==See also== *[[Geography of Chad]] *[[List of volcanoes in Chad]] *[[List of Ultras of Africa]]
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==External links== * [https://sites.google.com/view/racines-tchadiennes/reconnaissance-du-tibesti-1915-jean-tilho?authuser=0 Recognition of Tibesti by the battalion commander Tilho Head of the Borkou-Ennedi district September 4 - November 12, 1915.] * {{cite web | url = https://earthobservatory.nasa.gov/images/3023/emi-koussi-volcano-chad-north-africa |title = Emi Koussi Volcano, Chad, North Africa |date = 8 December 2002 | publisher = NASA Earth Observatory}} * {{cite peakware|id=147|name=Emi Koussi}} * {{cite peakbagger|pid=11071|name=Emi Koussi, Chad}}
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{{Authority control}}{{Highest points of Africa}}
[[Category:Tibesti Mountains]] [[Category:Pyroclastic shields]] [[Category:Calderas of Chad]] [[Category:Hotspot volcanoes]] [[Category:Volcanoes of Chad]] [[Category:Maars of Chad]] [[Category:Mountains of Chad]] [[Category:Pleistocene shield volcanoes]] [[Category:Holocene shield volcanoes]] [[Category:Inactive volcanoes]] [[Category:Highest points of countries]] [[Category:Shield volcanoes of Africa]] [[Category:Ultra-prominent peaks of Africa]]