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39-574: 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 3,415 metres (11,204 ft), rising 3 km (1.9 mi) above

78-441: A lava flow around the cone's base. When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava. If the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent. Basaltic cinder cones are the most characteristic type of volcano associated with intraplate volcanism . They are particularly common in association with alkaline magmatism , in which

117-443: A volcanic vent . The pyroclastic fragments are formed by explosive eruptions or lava fountains from a single, typically cylindrical, vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as either cinders, clinkers, or scoria around the vent to form a cone that often is symmetrical; with slopes between 30 and 40°; and a nearly circular ground plan. Most cinder cones have

156-416: A 15-by-11-kilometre (9.3 mi × 6.8 mi) wide elliptical 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-metre (1,310–980 ft) high scarps form the inner margin of the southern caldera, with the floor at an elevation of 2,970 metres (9,740 ft). Before

195-486: A bowl-shaped crater at the summit. Cinder cones range in size from tens to hundreds of meters tall. They are composed of loose pyroclastic material ( cinder or scoria ), which distinguishes them from spatter cones , which are composed of agglomerated volcanic bombs . The pyroclastic material making up a cinder cone is usually basaltic to andesitic in composition. It is often glassy and contains numerous gas bubbles "frozen" into place as magma exploded into

234-403: 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, but based on cloud cover data, a yearly precipitation of 80–120 millimetres (3.1–4.7 in) has been estimated for Emi Koussi; in the past it was wetter than this. The climate reflects both northerly and southerly influences and

273-454: A group of four young cinder cones NW of Las Pilas volcano. Since its initial eruption in 1850, it has erupted more than 20 times, most recently in 1995 and 1999. Satellite images suggest that cinder cones occur on other terrestrial bodies in the solar system. On Mars, they have been reported on the flanks of Pavonis Mons in Tharsis , in the region of Hydraotes Chaos on the bottom of

312-430: A larger area. Therefore, it seems that erupted amount of material is not sufficient on Mars for the flank slopes to attain the angle of repose and Martian cinder cones seem to be ruled mainly by ballistic distribution and not by material redistribution on flanks as typical on Earth. Cinder cones often are highly symmetric, but strong prevailing winds at the time of eruption can cause a greater accumulation of cinder on

351-468: Is 300 metres (980 ft) deep and 2 kilometres (1.2 mi) 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 2,670 metres (8,760 ft) and contains a salt lake. The floor of Era Kohor is thus deep white. Three maars and several scoria cones are also nested within

390-405: Is characterized by slumping and blasts that destroy the original rim, while the fourth stage is characterized by the buildup of talus beyond the zone where cinder falls to the surface (the ballistic zone ). During the waning stage of a cinder cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain but oozes quietly into the crater or beneath the base of

429-444: 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 60–70 kilometres (37–43 mi) or 60–80 kilometres (37–50 mi) and an estimated volume of 2,500 cubic kilometres (600 cu mi). The summit of Emi Koussi is formed by two overlapping calderas , which together form

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468-512: Is distinct from that of the surrounding deserts. There is no evidence of nivation forms on Emi Koussi and the identification of periglacial landforms is questionable, although periglacial activity likely occurred in the Pleistocene and may continue to the present-day. Even during the Pleistocene, no part of the mountain had sub-freezing temperatures through the entire year. A number of diatom species have been identified in sediments left by

507-717: The Central Andes of South America , as well as in Melanesia (the island of Bougainville alone has two). There are also pyroclastic shields in Africa , such as Emi Koussi in Chad . Categories: This volcanology article is a stub . You can help Misplaced Pages by expanding it . Scoria cone A cinder cone (or scoria cone ) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around

546-728: The Coprates Chasma , or in the volcanic field Ulysses Colles . It is also suggested that domical structures in Marius Hills (on the Moon) might represent lunar cinder cones. The size and shape of cinder cones depend on environmental properties as different gravity and/or atmospheric pressure might change the dispersion of ejected scoria particles. For example, cinder cones on Mars seem to be more than two times wider than terrestrial analogues as lower atmospheric pressure and gravity enable wider dispersion of ejected particles over

585-505: The Tibu people lived in its summit caldera and in artificial caves. In the Tedaga language, Emi 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 3,415 metres (11,204 ft) on the southern side of the volcano, towering 3 kilometres (1.9 mi) above the surrounding terrain. It

624-503: 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 . The diatom fauna appears to be unusual in comparison to other Saharan paleolakes, probably owing to

663-410: The Sahara; many peaks of the Tibesti exceed 2,000 metres (6,600 ft) 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

702-412: The air and then cooled quickly. Lava fragments larger than 64 mm across, known as volcanic bombs , are also a common product of cinder cone eruptions. The growth of a cinder cone may be divided into four stages. In the first stage, a low-rimmed scoria ring forms around the erupting event. During the second stage, the rim is built up and a talus slope begins to form outside the rim. The third stage

741-408: The caldera floor and may have been used as pastures . 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 caldera floor

780-505: 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. Emi Koussi lies in Chad , Africa , and is part of the Tibesti Mountains as well as its highest peak and thus of

819-617: 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

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858-430: 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

897-473: The cone as lava. Lava rarely issues from the top (except as a fountain) because the loose, uncemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent. Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders. Thus, it often burrows out along the bottom of the cinder cone, lifting the less dense cinders like corks on water, and advances outward, creating

936-823: The downwind side of the vent. Some cinder cones are monogenetic , forming from a single short eruptive episode that produces a very small volume of lava. The eruption typically last just weeks or months, but can occasionally last fifteen years or longer. Parícutin in Mexico, Diamond Head , Koko Head , Punchbowl Crater , Mt Le Brun from the Coalstoun Lakes volcanic field , and some cinder cones on Mauna Kea are monogenetic cinder cones. However, not all cinder cones are monogenetic, with some ancient cinder cones showing intervals of soil formation between flows that indicate that eruptions were separated by thousands to tens of thousands of years. Monogenetic cones likely form when

975-589: The erupted lava is enriched in sodium and potassium oxides . Cinder cones are also commonly found on the flanks of shield volcanoes , stratovolcanoes , and calderas . For example, geologists have identified nearly 100 cinder cones on the flanks of Mauna Kea , a shield volcano located on the island of Hawaii . Such cinder cones likely represent the final stages of activity of a mafic volcano. However, most volcanic cones formed in Hawaiian-type eruptions are spatter cones rather than cinder cones, due to

1014-637: The flanks of Emi Koussi, and carbonates were deposited in Era Kohor until recent times. On the southern foot of the volcano, fumaroles are active such as at Yi Yerra at 850 metres (2,790 ft) elevation on the southern flanks. At Yi Yerra, hot springs produce water with temperatures of 37 °C (99 °F). Emi Koussi is considered to be a Holocene volcano. Lava flows have buried Holocene diatoms beds in its caldera; 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. The Tibesti mountains feature

1053-510: The fluid nature of the lava. The most famous cinder cone, Paricutin , grew out of a corn field in Mexico in 1943 from a new vent. Eruptions continued for nine years, built the cone to a height of 424 meters (1,391 ft), and produced lava flows that covered 25 km (9.7 sq mi). The Earth's most historically active cinder cone is Cerro Negro in Nicaragua. It is part of

1092-476: The formation of the calderas, 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 . 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

1131-506: The high elevation of the Emi Koussi lake. Cattail and water milfoil grew around the lake. Generally, in the Tibesti vegetation occurs in five different belts depending on elevation. On Emi Koussi, Erica arborea is found up to elevations of 2,900 metres (9,500 ft), in the past it was probably more widespread. The summit region features montane vegetation, including Artemisia - Ephedra steppes which cover large parts of

1170-502: 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

1209-493: 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

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1248-529: The region. During the early Holocene , Era Kohor was filled with a deep, circular lake. Diatom beds have been found 125 metres (410 ft) above the floor of Era Kohor and in depressions without outlet elsewhere in the caldera, they reach thicknesses of 4–5 metres (13–16 ft). Farther down on the mountain, gullies start appearing at 2,800 metres (9,200 ft) elevation and become deep gorges by 2,000–2,500 metres (6,600–8,200 ft) elevation. Elleboe wadi originates on Emi Koussi, and several streams on

1287-425: 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

1326-465: The summit caldera, including beautifully coloured syenite blocks. 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, while long lava flows have been interpreted to be the youngest volcanism at Emi Koussi. The most recent volcanic vents show little erosion. Early in the Pleistocene , erosion took place on

1365-515: The surrounding sandstone plains. The volcano is 60–70 kilometres (37–43 mi) wide and has a volume of 2,500 cubic kilometres (600 cu mi). Two nested calderas cap the volcano, the outer one being about 15 by 11 kilometres (9.3 mi × 6.8 mi) in size. Within it on the southeast side is a smaller caldera known as Era Kohor, about 2 kilometres (1.2 mi) wide and 350 metres (1,150 ft) deep. Numerous lava domes , cinder cones , maars , and lava flows are found within

1404-575: 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. 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,

1443-502: 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

1482-565: 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. Emi Koussi erupted between 2.4 and 1.3 million years ago; it is considered to be of Pliocene - Quaternary 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

1521-566: Was used as a pasture for camels and goats . 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 , more than 3,800 kilometres (2,400 mi) away from Emi Koussi. [REDACTED] Media related to Emi Koussi at Wikimedia Commons Pyroclastic shield Pyroclastic shields are commonly known to form in

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