Emi Koussi (also known as Emi Koussou) is a high
pyroclastic shield
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 basalti ...
volcano
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
that lies at the southeast end of the
Tibesti Mountains
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 and i ...
in the central
Sahara
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
, map =
, map_image =
, location =
, country =
, country1 =
, ...
, in the northern
Borkou Region
The Borkou region ( ar, بركو) is a region of Chad which was created in 2008 from the Borkou department of the former Borkou-Ennedi-Tibesti region. Its capital is Faya-Largeau.
Geography
Borkou is located in the Sahara Desert, and contains ...
of northern
Chad
Chad (; ar, تشاد , ; french: Tchad, ), officially the Republic of Chad, '; ) is a landlocked country at the crossroads of North and Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic ...
. 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
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
plains. The volcano is wide and has a volume of .
Two nested
caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
s 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 dome
In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on ...
s,
cinder 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 a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...
s,
maar
A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...
s, and
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s are found within the calderas and along the outer flanks of the shield. Era Kohor contains
trona
Trona (trisodium hydrogendicarbonate dihydrate, also sodium sesquicarbonate dihydrate, Na2CO3•2NaHCO3•3H2O) is a non- marine evaporite mineral. It is mined as the primary source of sodium carbonate in the United States, where it has replace ...
deposits, and Emi Koussi has been studied as an analogue of the
Martian
Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. It became the most popular celestial object in fiction in the late 1800s as the Moon was evidently lifeless. At the time, the pred ...
volcano
Elysium Mons
Elysium Mons is a volcano on Mars located in the volcanic province Elysium, at , in the Martian eastern hemisphere. It stands about above its base, and about above the Martian ''datum'', making it the third tallest Martian mountain in terms o ...
. Emi Koussi was active more than one million years ago, but some eruptions may be more recent, and there is ongoing
fumarolic
A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...
and
hot spring
A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circ ...
activity.
Geography and geomorphology
Emi Koussi lies in
Chad
Chad (; ar, تشاد , ; french: Tchad, ), officially the Republic of Chad, '; ) is a landlocked country at the crossroads of North and Central Africa. It is bordered by Libya to the north, Sudan to the east, the Central African Republic ...
,
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, and is part of the
Tibesti Mountains
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 and i ...
as well as its highest peak and thus of the Sahara; many peaks of the Tibesti exceed in height. These mountains are formed by a group of volcanoes that grew on top of a large
dome
A dome () is an architectural element similar to the hollow upper half of a sphere. There is significant overlap with the term cupola, which may also refer to a dome or a structure on top of a dome. The precise definition of a dome has been a m ...
of Earth's surface. Volcanism in this area is poorly studied; as the region is remote and access difficult for political reasons.
Dirt road
A dirt road or track is a type of unpaved road not paved with asphalt, concrete, brick, or stone; made from the native material of the land surface through which it passes, known to highway engineers as subgrade material. Dirt roads are suitable ...
s cross the western and eastern margins of the summit caldera, and according to historical reports the
Tibu people lived in its summit caldera
and in artificial caves. In the
Tedaga
The Teda language, also known as Tedaga, is a Nilo-Saharan language spoken by the Teda, a northern subgroup of the Toubou people that inhabits southern Libya, northern Chad and eastern Niger
)
, official_languages =
, languages_type ...
language, means "mountains", "
massif
In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...
". The mountain has played an important role in the research and discovery history of the Tibesti.
Emi Koussi rises to a height of on the southern side of the volcano, towering above the surrounding terrain. It is considered by some astronauts to be the most recognizable landmark on Earth, as seen from
orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
. Supposedly the entire Tibesti can be seen from the northern summits.
![EmiKoussiTopo](https://upload.wikimedia.org/wikipedia/commons/e/e5/EmiKoussiTopo.png)
It is a
shield volcano
A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...
with a diameter of or and an estimated volume of . The summit of Emi Koussi is formed by two overlapping
caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
s, which together form a wide elliptical caldera that extends from northwest to southeast. 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, 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
Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. It became the most popular celestial object in fiction in the late 1800s as the Moon was evidently lifeless. At the time, the pred ...
volcano,
Elysium Mons
Elysium Mons is a volcano on Mars located in the volcanic province Elysium, at , in the Martian eastern hemisphere. It stands about above its base, and about above the Martian ''datum'', making it the third tallest Martian mountain in terms o ...
. The caldera rim is cut by one
outlet, the Porte de Modiounga, from which
gorge
A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...
s 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 is deep and wide, having the appearance of a giant hole. This caldera is also known as
Natron
Natron is a naturally occurring mixture of sodium carbonate decahydrate ( Na2CO3·10H2O, a kind of soda ash) and around 17% sodium bicarbonate (also called baking soda, NaHCO3) along with small quantities of sodium chloride and sodium sulfate. N ...
Hole or Trou au Natron.
Trachytic
Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and ...
lava flows are exposed in its walls, and
sodium carbonate
Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...
has been deposited on its floor, which lies at an elevation of
and contains a salt lake. The floor of Era Kohor is thus deep white. Three
maar
A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...
s and several
scoria cone
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...
s are also nested within the combined caldera, along with
lava dome
In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on ...
s and
lava flows
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
.
Debris from
explosive eruption
In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such ...
s fills the calderas.
The Kohor
pumice
Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...
s and two sets of
ignimbrite
Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...
s cover the flanks of Emi Koussi, which steepen as they approach the summit. Scoria cones on the slopes are accompanied by
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s. On the upper sectors of the volcano, lavas are crisscrossed by cracks, which are known as "lappiaz".
Sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
crops out on 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.
Geology
Tectonic activity appears to have occurred in the Tibesti already between the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
and the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
, that is between 358.9 ± 0.4 and 66 million years ago.
During the earliest stage of volcanism on Tibesti,
alkali basalt
Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...
s formed large plateaus. Later, the central volcanoes developed on top of these plateaus. Volcanism in Tibesti has been explained with a
mantle plume
A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...
, 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
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...
diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-silic ...
s,
granite
Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...
s and
schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes o ...
s, which are probably of
Neoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago.
It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...
age and are differentiated into two units. The volcanic rocks rest on an uplifted basement formed by Cretaceous and
Paleozoic
The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon.
The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838
by combining the Greek words ''palaiós'' (, "old") and ' ...
sandstone. 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. Older volcanic rocks are exposed in valleys.
Emi Koussi has erupted
phonolite
Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...
,
trachyandesite
Trachyandesite is an extrusive igneous Rock (geology), rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by Sodium, sodic plagioclase and alkali feldspar. It is formed from the cooling of ...
and trachyte, as well as
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
rocks like
basanite
Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...
and
tephrite
Tephrite is an igneous, volcanic (extrusive) rock, with aphanitic to porphyritic texture. Mineral content is usually abundant feldspathoids (leucite or nepheline), plagioclase, and lesser alkali feldspar. Pyroxenes (clinopyroxenes) are common a ...
. The erupted rocks define two alkaline suites.
Phenocryst
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
chemistry and content varies between the various rocks; among the minerals are alkali
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
,
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...
,
biotite
Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...
,
clinopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
,
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, oxides and
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
. Alkali feldspar,
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
, clinopyroxene, olivine,
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
,
mica
Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
,
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
, oxides, plagioclase,
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
,
sodalite
Sodalite ( ) is a tectosilicate mineral with the formula , with royal blue varieties widely used as an wikt:ornamental, ornamental gemstone. Although massive sodalite samples are opaque, crystals are usually transparent to translucent. Sodalite i ...
,
titanite
Titanite, or sphene (from the Greek ''sphenos'' (σφηνώ), meaning wedge), is a calcium titanium nesosilicate mineral, Ca Ti Si O5. Trace impurities of iron and aluminium are typically present. Also commonly present are rare earth metals in ...
and
zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...
also form the
groundmass
The matrix or groundmass of a rock is the finer-grained mass of material in which larger grains, crystals, or clasts are embedded.
The matrix of an igneous rock consists of finer-grained, often microscopic, crystals in which larger crystals, ca ...
of
microlith
A microlith is a small Rock (geology), stone tool usually made of flint or chert and typically a centimetre or so in length and half a centimetre wide. They were made by humans from around 35,000 to 3,000 years ago, across Europe, Africa, Asia an ...
s in erupted rocks.
Magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
genesis mainly involved
fractional crystallization processes. Rocks from Emi Koussi were used as
raw material
A raw material, also known as a feedstock, unprocessed material, or primary commodity, is a basic material that is used to produce goods, finished goods, energy, or intermediate materials that are feedstock for future finished products. As feedst ...
by
Neolithic
The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...
societies of the region.
During the early
Holocene
The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...
, Era Kohor was filled with a deep, circular lake.
Diatom
A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
beds have been found above the floor of Era Kohor
and in depressions without outlet elsewhere in the caldera, they reach thicknesses of . Farther down on the mountain, gullies start appearing at elevation and become deep
gorge
A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tenden ...
s by elevation. Elleboe
wadi
Wadi ( ar, وَادِي, wādī), alternatively ''wād'' ( ar, وَاد), North African Arabic Oued, is the Arabic term traditionally referring to a valley. In some instances, it may refer to a wet (ephemeral) riverbed that contains water onl ...
originates on Emi Koussi, and several streams on 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.
Eruptive history
![Caldeira Emi Koussi ASTER](https://upload.wikimedia.org/wikipedia/commons/5/53/Caldeira_Emi_Koussi_ASTER.jpg)
Emi Koussi erupted between 2.4 and 1.3 million years ago; it is considered to be of
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...](_blank)
age. The volcano was constructed mainly during the
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
within about one million years.
Radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
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; 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
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...
lapilli
Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones".
By definition lapilli range f ...
. 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 the northern caldera and on the southern flank of the volcano. The third stage was dominantly phonolitic, with
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
,
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
s and ignimbrites including the Kohor ignimbrite. The various ignimbrites contain
fiamme
Fiamme are lens-shapes, usually millimetres to centimetres in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density cu ...
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
''Phreatic'' is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption.
Hydrology
The term phreatic (the word originates from the Greek , meaning "well" ...
explosions associated with Era Kohor have deposited large blocks within the summit caldera, including beautifully coloured
syenite
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...
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
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
,
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
took place on the flanks of Emi Koussi, and
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
s were deposited in Era Kohor until recent times.
On the southern foot of the volcano,
fumarole
A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...
s are active such as at Yi Yerra at elevation on the southern flanks. At Yi Yerra,
hot spring
A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heated groundwater onto the surface of the Earth. The groundwater is heated either by shallow bodies of magma (molten rock) or by circ ...
s produce water with temperatures of . 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
Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becaus ...
have been obtained by
radiocarbon dating
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.
The method was dev ...
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, but based on
cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
data, a yearly precipitation of has been estimated for Emi Koussi; in the past it was wetter than this. The climate reflects both northerly and southerly influences and is distinct from that of the surrounding deserts.
There is no evidence of
nivation
Nivation is the set of geomorphic processes associated with snow patches. The primary processes are mass wasting and the freeze and thaw cycle, in which fallen snow gets compacted into firn or névé. The importance of the processes covered by the ...
forms on Emi Koussi and the identification of
periglacial
Periglaciation (adjective: "periglacial", also referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing of snow in areas of permafrost, the runoff from which refreezes in ice wedges and ot ...
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 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 the high elevation of the Emi Koussi lake.
Cattail
''Typha'' is a genus of about 30 species of monocotyledonous flowering plants in the family Typhaceae. These plants have a variety of common names, in British English as bulrush or reedmace, in American English as reed, cattail, or punks, in A ...
and
water milfoil
''Myriophyllum'' (water milfoil) is a genus of about 69 species of freshwater aquatic plants, with a cosmopolitan distribution. The center of diversity for ''Myriophyllum'' is Australia with 43 recognized species (37 endemic).
These submersed ...
grew around the lake.
Generally, in the Tibesti vegetation occurs in five different belts depending on elevation. On Emi Koussi, ''
Erica arborea
''Erica arborea'', the tree heath or tree heather, is a species of flowering plant (angiosperms) in the heather family Ericaceae, native to the Mediterranean Basin and Ethiopia, Kenya and Tanzania in East Africa. It is also cultivated as an or ...
'' is found up to elevations of , in the past it was probably more widespread. The summit region features
montane
Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures fall as elevation increases, causing the ecosystem to stratify. This stratification is a crucial f ...
vegetation,
including ''
Artemisia
Artemisia may refer to:
People
* Artemisia I of Caria (fl. 480 BC), queen of Halicarnassus under the First Persian Empire, naval commander during the second Persian invasion of Greece
* Artemisia II of Caria (died 350 BC), queen of Caria under th ...
''-''
Ephedra Ephedra may refer to:
* Ephedra (medicine), a medicinal preparation from the plant ''Ephedra sinica''
* ''Ephedra'' (plant), genus of gymnosperm shrubs
See also
* Ephedrine
Ephedrine is a central nervous system (CNS) stimulant that is of ...
''
steppe
In physical geography, a steppe () is an ecoregion characterized by grassland plains without trees apart from those near rivers and lakes.
Steppe biomes may include:
* the montane grasslands and shrublands biome
* the temperate grasslands, ...
s which cover large parts of the caldera floor and may have been used as
pasture
Pasture (from the Latin ''pastus'', past participle of ''pascere'', "to feed") is land used for grazing. Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep, or swine ...
s. The lava flows high on the mountain have characteristic environmental traits which allow the development of a particular vegetation. This vegetation encompasses ''
Dichrocephala
''Dichrocephala'' is a genus of flowering plants in the family Asteraceae, native to tropical Africa and southern Asia.
; Species
* ''Dichrocephala benthamii'' C.B.Clarke - China, Indian Subcontinent, Indochina
* ''Dichrocephala chrysanthemifoli ...
'', ''
Eragrostis
''Eragrostis'' is a large and widespread genus of plants in the grass family, found in many countries on all inhabited continents and many islands.
''Eragrostis'' is commonly known as lovegrass or canegrass. The name of the genus is derived fr ...
'', ''
Erodium
''Erodium'' is a genus of flowering plants in the botanical family Geraniaceae. The genus includes about 60 species, native to North Africa, Indomalaya, the Middle East, and Australia. They are perennials, annuals, or subshrubs, with five-petall ...
'', ''
Helichrysum
The genus ''Helichrysum'' consists of an estimated 600 species of flowering plants in the sunflower family (Asteraceae). The type species is ''Helichrysum orientale''. They often go by the names everlasting, immortelle, and strawflower. The name ...
'' as well as
fern
A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes except t ...
s,
liverwort
The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of g ...
s and
mosses
Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and horn ...
. ''
Oldenlandia
''Oldenlandia'' is a genus of flowering plants in the family Rubiaceae. It is pantropical in distribution and has about 240 species.Inge Groeninckx, Steven Dessein, Helga Ochoterena, Claes Persson, Timothy J. Motley, Jesper Kårehed, Birgitta Bre ...
'' and ''
Selaginella
''Selaginella'' is the sole genus of vascular plants in the family Selaginellaceae, the spikemosses or lesser clubmosses.
This family is distinguished from Lycopodiaceae (the clubmosses) by having scale-leaves bearing a ligule and by having ...
'' species grow at fumaroles.
The caldera floor was used as a pasture for
camel
A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
s and
goat
The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
s.
The
wood rush ''
Luzula tibestica'' is
endemic
Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...
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
Dagestan ( ; rus, Дагеста́н, , dəɡʲɪˈstan, links=yes), officially the Republic of Dagestan (russian: Респу́блика Дагеста́н, Respúblika Dagestán, links=no), is a republic of Russia situated in the North C ...
, more than away from Emi Koussi.
Gallery
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
Apollo 7 (October 1122, 1968) was the first crewed flight in NASA's Apollo program, and saw the resumption of human spaceflight by the agency after the fire that killed the three Apollo 1 astronauts during a launch rehearsal test on Ja ...
in October 1968
See also
*
Geography of Chad
Chad is one of the 47 landlocked countries in the world and is located in North Central Africa, measuring , nearly twice the size of France and slightly more than three times the size of California. Most of its ethnically and linguistically di ...
*
List of volcanoes in Chad
This is a list of active and extinct volcanoes in Chad.
References
Notes
{{Reflist
Chad
Volcanoes
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases ...
*
List of Ultras of Africa
This is a list of all the Ultra prominent peaks (with topographic prominence greater than 1,500 metres) in Africa. Also shown is Mount Catherine in Sinai, Egypt which is not geographically part of Africa. Not listed here are the 5 Ultras of the Can ...
References
Sources
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External links
*
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{{Authority control
Tibesti Mountains
Pyroclastic shields
Calderas of Chad
Hotspot volcanoes
Volcanoes of Chad
Maars of Chad
Mountains of Chad
Pleistocene shield volcanoes
Holocene shield volcanoes
Inactive volcanoes
Highest points of countries