Emi Koussi

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Emi Koussi
Emi Koussi seen from space

Emi Koussi seen from space

height 3415  m
location Chad
Mountains Tibesti
Coordinates 19 ° 47 '37 "  N , 18 ° 33' 7"  E Coordinates: 19 ° 47 '37 "  N , 18 ° 33' 7"  E
Emi Koussi (Chad)
Emi Koussi
Type Pyroclastic shield volcano
Age of the rock Holocene
Last eruption Unknown

Emi Koussi (also known as Emi Koussou) is a tall pyroclastic shield volcano at the southeastern end of the Tibesti Mountains in the central Sahara , in the northern Borkou region of northern Chad . The highest mountain in the Sahara is one of the few volcanoes of the Tibesti and reaches a height of 3415 meters. It rises 3 kilometers above the sandstone plain around the volcano, has a diameter of 60-70 kilometers and has a volume of 2500 cubic kilometers.

Two overlapping calderas form the top of the volcano, the outer one is about 15 by 11 kilometers wide. Inside this caldera, on the southeast side, is a smaller caldera known as Era Kohor; this is 2 kilometers wide and 350 meters deep. In the calderas and on the flanks of the shield there are several ash cones , lava flows , maars and spring crests . Era Kohor contains Trona deposits , and Emi Koussi has been studied as an analogue of the Martian volcano Elysium Mons . Emi Koussi was active over a million years ago, but some eruptions could be more recent, so both fumaroles and hot springs are present.

Geography and geomorphology

Emi Koussi is located in Chad and is part of the Tibesti Mountains, its highest peak and thus also that of the entire Sahara; many peaks in Tibesti are higher than 2000 meters. The volcanism of the Tibesti has hardly been researched because the region is remote and access is difficult for political reasons. According to reports, the Tubu lived in its summit caldera and in man-made caves. In the Tedaga language, "Emi" means "mountains", " massive ".

The Emi Koussi reaches a height of 3415 meters on its southern side, it rises three kilometers above the surrounding area. Some astronauts refer to Emi Koussi as the most striking structure on earth from orbit . It is said that the entire Tibesti can be seen from the northern peaks of Emi Koussi.

It is a shield volcano with a diameter of 60–70 kilometers and an assumed volume of 2500 cubic kilometers. The tip of Emi Koussi is formed by two overlapping calderas, which together form a 15 × 11 kilometer wide elliptical caldera, which extends in a northwest-southeast direction. The northern caldera was formed first, then came the southern caldera, which is 50 meters below the northern caldera. 400-300 meter high rock walls form the inner edge of the southern caldera, the bottom of which is at an altitude of 2970 meters. Before the calderas formed, the volcano may have reached a height of 4,000 meters. A valley drain cuts through the edge of the caldera, the Porte de Modiounga .

Within this pair of caldera lies the Era Kohor caldera, which is 300 meters deep and 2 kilometers wide and looks like a huge hole. This caldera is also known as the Natronloch or Trou au Natron . Trachyte lava flows are exposed in its walls, and sodium carbonate has deposited on its bottom, which is at an altitude of 2670 meters and contains a salt lake. The bottom of Era Kohor is therefore pure white. Three maars and some cinder cones also occur within the caldera complex, accompanied by lava flows and spring crests . Ejection of an explosive eruption fills the calderas.

The Kohor pumice and two ignimbrites cover the slopes of the Emi Koussi, which become steeper towards the summit. Cinder cones on the slopes are accompanied by lava flows. At the top of the volcano, the lava is furrowed by crevices known as "Lappiaz". Sandstone is exposed in the terrain around the volcano . To the north of the Emi Koussi are other volcanoes such as Tarso Ahon and Tarso Emi Chi , the former is connected to Emi Koussi by a narrow ridge.

geology

Tectonic events took place in the Tibesti Gerbirge between the Carboniferous and the Cretaceous , i.e. between 358.9 ± 0.4 and 66 million years ago. In the early phase of the Tibesti volcanism, alkali basalt formed wide plateaus. Central volcanoes later formed on these plateaus. Volcanism in Tibesti has been explained with a mantle plume as in other African volcanoes, but it has recently been suggested that the effects of the collision between Europe and Africa may be responsible for Tibesti volcanism.

The oldest rocks below the Tibesti are Precambrian diorites , mica schists and granites , which probably come from the Neoproterozoic and are divided into two units. The volcanic rocks rest on a raised base made of Cretan and Paleozoic sandstone . The latter are exposed at the southwestern foot of Emi Koussi, while volcanic rock dominates north of the volcano and the Tibesti massif in the east and southeast of Emi Koussi. Older volcanic rocks appear in valleys.

The Emi Koussi mined phonolite , trachyandesite and trachyte , but also mafic rocks such as basanite and tephrite . The extracted rocks define basic volcanic families. The chemistry of the prisoners and their composition varies between the different 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 basic mass of microliths in extracted rocks. Fractional crystallization was primarily involved in the formation of the magma . Emi Koussi stones were used as raw material by regional Neolithic societies .

In the early Holocene , the Era Kohor was filled by a circular, deep lake. Diatomeenablagerungen were 125 meters above the bottom of the Era Kohor found and in sinks without drainage elsewhere in the caldera, they reach thicknesses of 4-5 meters. Further down the mountain, erosion channels appear from an altitude of 2,800 meters and deep gorges become deep gorges from an altitude of 2,000–2500 meters . The Wadi Elleboe has its origin on the Emi Koussi, and some episodic streams on the western flank lead into the Enneri Miski, which drains south and disappears south of the mountains. Small lakes can be found around Emi Koussi.

Activity chronology

The Emi Koussi erupted between 2.4 and 1.3 million years ago; he is considered to be a Pliocene to Quaternary age. The volcano formed in advance in the Miocene in a time frame of about a million years. Radiometric dating has shown ages of 2.42 ± 0.03 - 2.33 ± 0.09 million years for the oldest stage of Emi Koussi. The third stage gave ages of 1.4 ± 0.3 - 1.32 ± 0.2 million years; previously the Era Kohor was understood as a Holocene volcano.

In the first stage, trachyte and trachyandesite broke out of the Emi Koussi in the form of ignimbrites and basaltic lapilli . This was followed in the second stage by trachytic, gray or green ignimbrites, spring domes made of trachyte or phonolite and additional lava flows made of basalt. The second stage ignimbrites are found in the northern caldera and on the southern flank of the volcano. In the third stage, phonolite was mined in advance, in the form of breccias , tuffs and ignimbrite including Kohor ignimbrite. The various ignimbrites contain flames and often look like lava flows. A caldera was formed during each stage, and all three stages were accompanied by basaltic volcanism. Phreatic explosions of the Era Kohor left large rocks in the summit caldera, including splendidly colored syenite blocks.

Quaternary volcanism has formed cones on the slopes of Emi Koussi, and within the summit caldera the three maars represent the most recent volcanism, while long lava flows are considered to be the youngest volcanic products of Emi Koussi. The youngest volcanic formations are little eroded. Erosion occurred on the flanks of the Emi Koussi in the early Pleistocene , and carbonates have been deposited in the Era Kohor until recently .

At the southern foot of the volcano there are fumarolic phenomena like at Yi Yerra at 850 meters altitude on the south side. At Yi Yerra, 37 ° C warm water flows from thermal springs . The Emi Koussi is considered to be a volcano of the Holocene Age. In its caldera, diatom deposits from the Holocene have been overflowed by lava flows; the radiocarbon method determined ages of 14,790 ± 400 - 12,400 ± 400 years for the diatom deposits buried or affected by lava flows.

Climate and vegetation

In the Tibesti Mountains there is a mountain desert climate. Near the Emi Koussi there are no weather stations and stations at lower altitudes most likely underestimate the precipitation at higher altitudes, but an annual precipitation of 80–120 millimeters per year for the Emi Koussi has been derived from the degree of cloudiness ; in the past it was even higher than it is today. The climate is influenced by both the north and the south and differs from that of the deserts around Emi Koussi. There is no evidence of nivation on the Emi Koussi and the presence of periglacial landforms is questionable, but periglacial activity likely occurred in the Pleistocene; they could last until today. Even in the Pleistocene, the temperature at no point on the mountain was below freezing all year round.

Several species of diatoms have been identified in the sediments of 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 . Compared to other lakes in the Sahara, the diatom fauna is unusual; this is probably due to the higher elevation of Lake Emi-Koussi. Cattails and milfoil grew around the lake .

In general, vegetation in Tibesti occurs in five different height belts. On Emi Koussi, Erica arborea occurs at an altitude of up to 2900 meters, but was probably even more widespread in the past. Mountain vegetation predominates in the summit region , including artemisia - seaweed - steppes which cover much of the caldera's floor and were probably used as pasture . On the lava flows at high altitudes, there are special environmental conditions that enable the development of special vegetation. This vegetation includes Dichrocephala , Eragrostis , Erodium , Helichrysum and club mosses , ferns and mosses one. Oldenlandia and Selaginella species grow near fumaroles.

The woodrush Luzula tibestica is on the Emi Koussi endemic . The fern Asplenium quezelii was discovered on the Emi Koussi and was initially assumed to be endemic there, before it was found that it is identical to Asplenium daghestanicum , a species previously understood to a limited extent than in Dagestan , i.e. over 3800 kilometers from Emi Koussi .

gallery

  • The calderas by Emi Koussi

  • Emi Koussi-Caldera, ASTER

  • Emi Koussi, inner crater

  • Soda in the inner caldera of Emi Koussi

See also

supporting documents

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  2. ↑ a b Emi Koussi in the Global Volcanism Program of the Smithsonian Institution (English)
  3. ^ Gourgaud and Vincent, 2004. p. 261
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  9. ↑ Pachur and Altmann 2006. p. 103
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Web links

Commons : Emi Koussi  - collection of images, videos and audio files