Emi Koussi (also known as Emi Koussou[3]) is a high pyroclastic shield[1]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 (11,204ft), rising 3km (1.9mi) above the surrounding sandstone plains. The volcano is 60–70 kilometres (37–43mi) wide and has a volume of 2,500 cubic kilometres (600cumi).
Two nested calderas cap the volcano, the outer one being about 15 by 11 kilometres (9.3mi ×6.8mi) in size. Within it on the southeast side is a smaller caldera known as Era Kohor, about 2 kilometres (1.2mi) wide and 350 metres (1,150ft) 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[4] as well as its highest peak[5] and thus of the Sahara;[6] many peaks of the Tibesti exceed 2,000 metres (6,600ft) in height.[5] These mountains are formed by a group of volcanoes that grew on top of a large dome of Earth's surface.[7] Volcanism in this area is poorly studied; as the region is remote and access difficult for political reasons.[8]Dirt roads cross the western and eastern margins of the summit caldera,[9] and according to historical reports the Tibu people lived in its summit caldera[10] and in artificial caves.[11] In the Tedaga language, Emi means "mountains", "massif".[6] The mountain has played an important role in the research and discovery history of the Tibesti.[12]
Emi Koussi rises to a height of 3,415 metres (11,204ft) on the southern side of the volcano,[13][14] towering 3 kilometres (1.9mi) above the surrounding terrain.[15] It is considered by some astronauts to be the most recognizable landmark on Earth, as seen from orbit.[15] Supposedly the entire Tibesti can be seen from the northern summits.[16]
Topographic map of the Emi Koussi
It is a shield volcano with a diameter of 60–70 kilometres (37–43mi)[15] or 60–80 kilometres (37–50mi)[12] and an estimated volume of 2,500 cubic kilometres (600cumi).[17] The summit of Emi Koussi is formed by two overlapping calderas, which together form a[18]15-by-11-kilometre (9.3mi ×6.8mi) wide[15] elliptical caldera that extends from northwest to southeast.[18] The northern caldera formed first, then the southern caldera which is about 50 metres (160ft) deeper below the northern one. 400–300-metre (1,310–980ft) high scarps form the inner margin of the southern caldera,[19] with the floor at an elevation of 2,970 metres (9,740ft).[20] Before the formation of the calderas, the volcano may have reached a height of 4,000 metres (13,000ft). Emi Koussi has been compared to a Martian volcano, Elysium Mons.[5][21] The caldera rim is cut by one outlet, the Porte de Modiounga,[22] from which gorges emanate into the caldera.[23] The highest point of Emi Koussi lies on the southern rim of the caldera.[9]
Nested within this combined caldera is the Era Kohor caldera,[18] which is 300 metres (980ft) deep and 2 kilometres (1.2mi) wide, having the appearance of a giant hole.[19] This caldera is also known as Natron Hole[16] or Trou au Natron.[13]Trachytic lava flows are exposed in its walls,[19] and sodium carbonate has been deposited on its floor,[24] which lies at an elevation of 2,670 metres (8,760ft)[20] and contains a salt lake.[11] The floor of Era Kohor is thus deep white.[22] Three maars and several scoria cones are also nested within the combined caldera,[19] along with lava domes and lava flows.[1] Debris from explosive eruptions fills the calderas.[25]
The Kohor pumices and two sets of ignimbrites cover the flanks of Emi Koussi,[18] which steepen as they approach the summit.[21] Scoria cones on the slopes are accompanied by lava flows.[18] On the upper sectors of the volcano, lavas are crisscrossed by cracks, which are known as "lappiaz".[26]Sandstone crops out on the terrain surrounding the volcano.[18] North of Emi Koussi lie other volcanoes, such as Tarso Ahon and Tarso Emi Chi,[27] the former of which is connected to Emi Koussi by a narrow ridge.[13]
Geology
Tectonic activity appears to have occurred in the Tibesti already between the Carboniferous and the Cretaceous,[5] that is between 358.9 ± 0.4 and 66 million years ago.[28] During the earliest stage of volcanism on Tibesti, alkali basalts formed large plateaus. Later, the central volcanoes developed on top of these plateaus.[15] Volcanism in Tibesti has been explained with a mantle plume, as has been proposed for other African volcanoes,[29] although recently tectonic effects of the collision between Africa and Europe and their effects at a distance have also been advanced as an explanation.[30]
The oldest rocks below the Tibesti are Precambriandiorites, granites and schists,[27] which are probably of Neoproterozoic age and are differentiated into two units.[30] The volcanic rocks rest on an uplifted basement formed by Cretaceous[5] and Paleozoic sandstone.[15] 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.[27] Older volcanic rocks are exposed in valleys.[31]
During the early Holocene,[34] Era Kohor was filled with a deep, circular lake.[35]Diatom beds have been found 125 metres (410ft) above the floor of Era Kohor[36] and in depressions without outlet elsewhere in the caldera,[37] they reach thicknesses of 4–5 metres (13–16ft).[35] Farther down on the mountain, gullies start appearing at 2,800 metres (9,200ft) elevation and become deep gorges by 2,000–2,500 metres (6,600–8,200ft) elevation.[17] Elleboe wadi originates on Emi Koussi,[38] and several streams on the western flank join into the Enneri Miski which drains southwards and disappears south of the mountains.[39] Small pools of water are found around Emi Koussi.[40]
Emi Koussi erupted between 2.4 and 1.3 million years ago;[41] it is considered to be of Pliocene-Quaternary age.[42] The volcano was constructed mainly during the Miocene[17][43] within about one million years.[44]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;[15][45] formerly Era Kohor was considered to be a Holocene centre.[24]
During the first stage, Emi Koussi erupted trachytes and trachyandesites in the form of ignimbrites and basalticlapilli. This was followed in the second stage by trachytic, green or grey ignimbrites, trachytic and phonolitic lava domes and additional basaltic lava flows.[15] The second stage ignimbrites can be found within the northern caldera and on the southern flank of the volcano.[19] The third stage was dominantly phonolitic, with breccia, tuffs and ignimbrites including the Kohor ignimbrite.[15] The various ignimbrites contain fiamme and often look like lava flows.[46] Each stage was accompanied by the formation of a caldera,[19] and basaltic volcanism continued through all three stages.[15]Phreatic explosions associated with Era Kohor have deposited large blocks within the summit caldera,[19] including beautifully coloured syenite blocks.[47]
Quaternary volcanism gave rise to the cones on the slopes of Emi Koussi,[18] and the three maars are the most recent vents in the combined caldera,[19] while long lava flows have been interpreted to be the youngest volcanism at Emi Koussi.[48] The most recent volcanic vents show little erosion.[47] Early in the Pleistocene, erosion took place on the flanks of Emi Koussi,[5] and carbonates were deposited in Era Kohor until recent times.[49][50]
On the southern foot of the volcano, fumaroles are active[5] such as at Yi Yerra at 850 metres (2,790ft) elevation on the southern flanks.[25] At Yi Yerra, hot springs produce water with temperatures of 37°C (99°F).[51] Emi Koussi is considered to be a Holocene volcano.[49] Lava flows have buried Holocene diatoms beds in its caldera;[11] 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.[52]
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,[39] but based on cloud cover data, a yearly precipitation of 80–120 millimetres (3.1–4.7in) has been estimated for Emi Koussi;[53] in the past it was wetter than this.[14] The climate reflects both northerly and southerly influences and is distinct from that of the surrounding deserts.[52] There is no evidence of nivation forms on Emi Koussi[54] and the identification of periglacial landforms is questionable,[52] although periglacial activity likely occurred in the Pleistocene and may continue to the present-day.[51] Even during the Pleistocene, no part of the mountain had sub-freezing temperatures through the entire year.[55]
Generally, in the Tibesti vegetation occurs in five different belts depending on elevation.[58] On Emi Koussi, Erica arborea is found up to elevations of 2,900 metres (9,500ft),[59] in the past it was probably more widespread.[60] The summit region features montane vegetation,[61] including Artemisia-Ephedrasteppes[62] which cover large parts of the caldera floor[47] and may have been used as pastures.[22] The lava flows high on the mountain have characteristic environmental traits which allow the development of a particular vegetation.[26][63] This vegetation encompasses Dichrocephala, Eragrostis, Erodium, Helichrysum as well as ferns, liverworts and mosses.[62]Oldenlandia and Selaginella species grow at fumaroles.[64] The caldera floor was used as a pasture for camels and goats.[23]
The Tibesti Mountains are a mountain range in the central Sahara, primarily located in the extreme north of Chad, with a small portion located in southern Libya. The highest peak in the range, Emi Koussi, lies to the south at a height of 3,415 meters (11,204 ft) and is the highest point in both Chad and the Sahara. Bikku Bitti, the highest peak in Libya, is located in the north of the range. The central third of the Tibesti is of volcanic origin and consists of five volcanoes topped by large depressions: Emi Koussi, Tarso Toon, Tarso Voon, Tarso Yega and Toussidé. Major lava flows have formed vast plateaus that overlie Paleozoic sandstone. The volcanic activity was the result of a continental hotspot that arose during the Oligocene and continued in some places until the Holocene, creating fumaroles, hot springs, mud pools and deposits of natron and sulfur. Erosion has shaped volcanic spires and carved an extensive network of canyons through which run rivers subject to highly irregular flows that are rapidly lost to the desert sands.
Toussidé is a potentially active stratovolcano in Chad. Toussidé lies in the Tibesti Mountains, the large Yirrigué caldera and the smaller Trou au Natron and Doon Kidimi craters are close to it. It has an elevation of 3,265 m (10,712 ft) above sea level. The volcano is the source of a number of lava flows, which have flowed westward away from Toussidé and east into the Yirrigué caldera.
Mount Hasan is a volcano in Anatolia, Turkey. It has two summits, the 3,069 metres (10,069 ft) high eastern Small Hasan Dagi and the 3,253 metres (10,673 ft) high Big Hasan Dagi, and rises about 1 kilometre (0.62 mi) above the surrounding terrain. It consists of various volcanic deposits, including several calderas, and its activity has been related to the presence of several faults in the area and to regional tectonics.
The Tarso Voon is a 3,100-metre (10,200 ft) high stratovolcano in the north of the Republic of Chad. It is located in the western center of the Tibesti Mountains.
Tarso Toh is a volcanic field located in Chad, north of Tarso Toussidé volcano. It fills valleys and plains over an area of 80 km in east–west direction and 20–30 km in north–south direction. It contains 150 scoria cones and two maars.
In volcanology, a pyroclastic shield or ignimbrite shield is an uncommon type of shield volcano. Unlike most shield volcanoes, pyroclastic shields are formed mostly of pyroclastic and highly explosive eruptions rather than relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano. They typically display low-angle flank slopes and often have a central caldera caused by large eruptions. Lava is commonly extruded after explosive activity has ended. The paucity of associated Plinian fall deposits indicates that pyroclastic shields are characterized by low Plinian columns.
Trou au Natron or Doon Orei is a volcanic caldera of the Tibesti Massif in the nation of Chad in Northern Africa. The volcano is extinct. It is unknown when it last erupted. Its volcano number is 0205–01. Trou au Natron is located just south-east of Toussidé, the westernmost volcano of the Tibesti Mountains. Its edge cuts into the nearby Yirrigue caldera.
Sairecabur is a volcano located on the frontier between Bolivia and Chile. It is part of the Andean Central Volcanic Zone. Sairecabur proper is 5,971 m (19,590 ft) high; other mountains in the range are 5,722 m (18,773 ft) high Curiquinca, 5,819 m (19,091 ft) high Escalante and 5,748 m (18,858 ft) high Cerro Colorado, all of which have erupted a number of lava flows. Also in close proximity to Sairecabur lie the volcanic centres Licancabur, Putana and Juriques.
Mount Erciyes, also known as Argaeus, its etymon, is an inactive volcano in Kayseri Province, Turkey. It is a large stratovolcano surrounded by many monogenetic vents and lava domes, and one maar. The bulk of the volcano is formed by lava flows of andesitic and dacitic composition. At some time in the past, part of the summit collapsed towards the east.
Lake Ptolemy is a former lake in Sudan. This lake formed during the Holocene in the Darfur region, during a time when the monsoon over Africa was stronger. The existence of the lake is dated between about 9,100–2,400 years before present. This lake could have reached a surface area of 30,750 square kilometres (11,870 sq mi), larger than present-day Lake Erie, although estimates of its size vary and it might have been much smaller. The shorelines in some places, insofar as they are recognizable, feature riparian landscapes and reeds. The lake was a freshwater lake replenished by groundwater and runoff from neighbouring mountains and might itself have been the source for the Nubian Sandstone Aquifer System. The lake featured a diverse ecosystem with a number of species, and possibly facilitated the spread of species between the Nile and Lake Chad.
Wheelwright caldera is a caldera in Chile. It is variously described as being between 11 kilometres (6.8 mi) and 22 kilometres (14 mi) wide and lies in the Central Volcanic Zone of the Andes. A lake lies within the caldera, which is among the largest of the Central Andes. The caldera lies in the region of Ojos del Salado, the world's tallest volcano.
Waw an Namus is a volcano in Libya. Of either Pleistocene or Holocene age, it is located within the eastern Fezzan region. The origin of the volcanism there and at Al Haruj farther north is not clear. Radiometric dating has yielded an age of about 200,000 years, but other circumstantial evidence points to a formation of the volcano during Holocene or even historical times.
Tarso Toon is a volcano in the central Tibesti mountains.
Tarso Yega is a stratovolcano in Tibesti, with a summit caldera that is 19 by 20 kilometres wide and 300 metres (980 ft) deep. The summit of the volcano reaches a height of 2,972 metres (9,751 ft), and its caldera is the largest caldera of the Tibesti. Neighbouring volcanoes include Doudriki east, Ehi Dosoatou south and Ehi Fodoboro southwest of the caldera. The Enneri Yega river drains the caldera westward and then south, before joining the Enneri Debarsar; in the past (Holocene) the rivers reached Lake Chad and lakes formed in Tarso Yega.
Meidob volcanic field is a Holocene volcanic field in Darfur, Sudan. It is one of several volcanic fields in Africa whose origin is explained by the activity of mantle plumes and their interaction with crustal structures. Meidob lies at the southern margin of the Sahara.
Deriba is a Pleistocene or Holocene caldera in Darfur, Sudan. Part of the volcanoes of the Marra Mountains, it lies on the Darfur dome and like the Tagabo Hills and Meidob Hills volcanism may be the product of a mantle plume. After the separation of South Sudan, the highest point of Sudan is on the margin of the caldera.
Tarso Emi Chi is a volcano in Chad.
Atakor volcanic field is a volcanic field in Algeria. It lies in the Hoggar mountains and consists of a variety of volcanic features such as lava flows and about 450 individual vents which create a spectacular scenery.
Tarso Ahon is a Pleistocene volcano in Tibesti, north of Emi Koussi from which it is separated by deep gorges. It covers a surface of about 50 by 60 kilometres and is a so-called volcanic plateau with three summits: Arken Ahon, Dondomé, Tarso Ahon and Tarso Mohi. However, contrary to its plateau-like appearance the basaltic surfaces of Tarso Ahon are difficult to access and steep. Tarso Ahon has erupted andesite, rhyolite and similar magmas, and more recently basalts which form lava flows.
Soborom is a group of hot springs with accompanying fumaroles and mud volcanoes in the Tibesti Mountains of Chad. Three dispersed groups of vents are active there, some of which are used as spas by the local population.
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