Toussidé

Toussidé (also known as Tarso 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 Template:Cvt 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.

Trou au Natron, the depression southeast of the volcano, measures approximately in diameter and in depth. During the last glacial maximum or the early-middle Holocene, it was filled with a lake. A number of volcanic cones have developed within Trou au Natron. Fumarolic activity on the peak of Toussidé and geothermal manifestations within Trou au Natron represent signs of volcanic activity at Toussidé.

Names

Toussidé is also known as Tarso Toussidé. Another transliteration is "Tusside". "Trou au Natron" refers to the springs which have deposited white trona in the caldera.Template:Sfn "Toussidé" translates as "Which killed the local people (Tou) with fire".Template:SfnTemplate:Sfn

Geography and geomorphology

Regional

Toussidé is part of the western TibestiTemplate:Sfn mountains in Chad, Africa.Template:Sfn The Tibesti mountains reach elevations of and are surrounded by the Sahara.Template:Sfn The towns of Bardai and Zouar lie east-northeast and south of Toussidé, respectively,Template:Sfn and a road between the two passes just southeast from Trou au Natron.Template:Sfn

Another volcano in Tibesti is Emi Koussi,Template:Sfn which is the highest mountain of the Tibesti and of the entire Sahara region.Template:Sfn The volcano Tarso Toh lies northwest of Toussidé,Template:Sfn and the Botoum and Botoudoma/Petit Botoum rhyolite extrusions are located south of Trou au Natron.Template:Sfn The origin of volcanism in Tibesti is unclear; both a hotspot-related mechanism and tectonic effects of the collision between the African Plate and the Eurasian Plate have been proposed.Template:Sfn

Local

Toussidé is a symmetricalTemplate:Sfn Template:Sfn stratovolcano,Template:Sfn the second-highest peak in TibestiTemplate:Sfn and the highest peak in the western Tibesti mountains.Template:Sfn In the past it was considered to be lower, only , before a higher summit height was determined by Jean Tilho and W.G. Tweedale (1920). It rises above the surrounding landscape and covers a base of ,Template:SfnTemplate:Sfn dominating the surroundings. The summit cone is formed by lapilli, pyroclastics, and scoria.Template:Sfn One half has a white colour and the other half is black;Template:Sfn some areas are white from fumarolic alteration. Lava flows emanate from Toussidé in a radial patternTemplate:Sfn and reach lengths of , covering an area of .Template:Sfn These flows descended valleys towards the westTemplate:Sfn and may have buried an older volcanic edifice of Toussidé.Template:Sfn The flows have a ropy appearance with a diverse surface texture, including bubbly, glassy, and porphyric textures,Template:Sfn which are very fresh and free of erosion.Template:Sfn The surface of the flows is rough and can be a problem to climbers.Template:SfnTemplate:Sfn The relatively steep slopes of Toussidé may reflect the existence of a lava dome underneath the younger lavas;Template:Sfn some flows may have originated from parasitic vents.Template:Sfn

Toussidé itself lies in part within an even larger caldera, the "pre-Toussidé" (also known as "Yirrigué") caldera which is in part filled by the lava flows from Toussidé and eruption products from the more recent explosion craters,Template:Sfn as well as debris that fell from the steep caldera margin.Template:Sfn Toussidé is located on the caldera's western side, and the smaller Trou au Natron caldera cuts into the flanks of Yirrigué.Template:Sfn Yirrigué contains a small cinder cone and an associated lava flow,Template:Sfn as well as an alluvial plain.Template:Sfn The "Yirrigué" caldera is part of a large, rhyolitic, shield-shaped volcanoTemplate:Sfn that developed atop a tectonic horst, which in turn may have been formed by intrusion of magmas.Template:Sfn The ignimbrites have buried older terrain and filled valleys.Template:Sfn

Southeast of Toussidé lies the caldera Trou au Natron (also known as DoonTemplate:Sfn or Doon OreiTemplate:Sfn).Template:Sfn Its rim is cut into sequences of lavasTemplate:Sfn and older volcanic cones.Template:Sfn In places it is almost vertical.Template:Sfn Inside of Trou au Natron are fourTemplate:Sfn recent basaltic volcanic cones,Template:Sfn the most remarkable of which is the Moussosomi, which has erupted a lava flow.Template:Sfn Three of these cones are deeply eroded.Template:Sfn Cones are also located outside of the Trou au Natron, and their eruption products have in part flowed into the crater.Template:Sfn Like Toussidé, the position of Trou au Natron appears to be controlled by the ring fault of the "pre-Toussidé" caldera.Template:Sfn A salty swamp lies within Trou au Natron,Template:Sfn whose floor is in part covered by evaporites,Template:Sfn mainly sodium sulfate.Template:Sfn

Another and crater, Doon Kidimi (also known as Petit TrouTemplate:Sfn or Doon KinimiTemplate:Sfn), lies northeast of Trou au Natron;Template:SfnTemplate:Sfn it is among the most pristine volcanic features in the region.Template:Sfn Additional volcanoes in the neighbourhood are the Ehi Timi to the northeast and the Ehi Sosso/Ehi Soso east of Toussidé,Template:Sfn the former of which features lava domes. The river Enneri Oudingueur originates closely in the area and becomes a tributary of the Enneri Bardagué,Template:Sfn which drains the Tibesti northward.Template:Sfn

• Trou au Natron (centre down) and Doon Kidimi (upper right corner)
  Trou au Natron (centre down) and Doon Kidimi (upper right corner)
• Trou au Natron seen from northeast; Toussidé lava flows upper right corner and Doon Kidimi lower right corner
  Trou au Natron seen from northeast; Toussidé lava flows upper right corner and Doon Kidimi lower right corner

Paleolake

Trou Au Natron was once filled by a freshwater lake during the last glacial maximum.Template:Sfn Fed by meltwater from snow, this lake persisted for several thousands of years. Aside from a lake level drop 14,900

1. REDIRECT Template:En dash

Template:R protected14,600 years before present,Template:Sfn the lake existedTemplate:Sfn until about 12,400 years before present.Template:Sfn Later analysis suggested that there was no late Pleistocene lake stage and that Trou au Natron was filled with water between 8,645 calibrated radiocarbon years ago to about 4,425 calibrated radiocarbon years ago, thus at the same time as lakes in the lowland. Charophyte algae (such as Chara globularis and Chara vulgarisTemplate:Sfn), diatoms, gastropods,Template:Sfn golden algae, sponges, and stromatoliths lived in the lake.Template:Sfn Ferns and mosses colonized the margins of the crater.Template:Sfn

The lake reached maximum depths of at a minimum ,Template:Sfn at least once reaching .Template:Sfn Such a large size, relative to its catchment, has raised the question of where this water came from. The formation of such a lake during the glacial maximum was probably dependent on orographic precipitation transported by the subtropical jet stream.Template:Sfn

Geology

The Tibesti mountains are part of a volcanic province that reaches from Libya into Chad and covers a total surface area of about .Template:Sfn The Tibesti Mountains have been volcanically active since the late Cenozoic, with one old volcanic unit being dated to 17 million years ago.Template:Sfn While all the higher peaks are volcanoes, not all of Tibesti is formed by volcanic material; the volcanoes have developed on top of a basement uplift.Template:Sfn

The terrain beneath Toussidé is formed by Precambrian schists and sandstones, including the Nubian Sandstone. A thick layer of ash has covered much of the terrain and, with the exception of the younger volcanoes, only few parts of the terrain crop out.Template:Sfn Several of these outcrops can be found east of Trou au Natron.Template:Sfn The volcanism of the Tibesti has been subdivided into several series.Template:Sfn

The volcano has erupted rhyolite,Template:Sfn trachybasaltTemplate:Sfn bordering on trachyandesite, which define a subalkaline/Template:Sfnhyperalkaline suite.Template:Sfn The lavas contain phenocrysts of augite, olivine, plagioclase, and sanidine.Template:Sfn The cones in Trou au Natron are andesitic.Template:Sfn Yirrigué conversely has erupted peralkaline rhyolite,Template:Sfn while Ehi Timi erupted rhyolite and trachyte, and Ehi Sosso only rhyolite.Template:Sfn The formation of Trou au Natron has been accompanied by the eruption of bedrock material.Template:Sfn

Climate and vegetation

Annual temperatures at Trou au Natron fluctuate between , with a daily temperature variation of ; this is less than in the lowlands.Template:Sfn At higher altitudes frost can be expected.Template:Sfn Precipitation amounts to at Trou au Natron. Most of it falls as frontal precipitation during summer,Template:Sfn and it is more copious than in the lowlands; the Tibesti mountains are the sources for wadis.Template:Sfn Other, more indirect estimates yield precipitation of per year at Toussidé.

A characteristic vegetation has been discovered on the fumaroles of Toussidé. It ranges from cyanophyceae, ferns, mosses, Oldenlandia and Selaginella within the fumarole vents to small meadows consisting of mosses and Campanula monodiana, Fimbristylis minutissima, Lavandula antineae, Mollugo nudicaulis, Oxalis corniculata, Satureja biflora, and other species.Template:Sfn The growth of these plants is favoured by the water emanating from the fumaroles.Template:Sfn The plant Erodium toussidanum is endemic at the fumaroles of Toussidé, and the mountain is the type locality of Salvia tibestiensis. Trou au Natron also has its own unique flora, including many Sahelian species.Template:Sfn

Eruption history

Volcanic activity at Toussidé appears to be of Quaternary age.Template:Sfn The "pre-Toussidé" caldera is considered to be the source of numerous local ignimbrites,Template:Sfn including the 430,000 ± 110,000-year-old Yirrigué ignimbrite. This ignimbrite covers a surface area of with about of rock.Template:Sfn The eruption that generated this ignimbrite also led to the formation of the Yirrigué caldera.Template:Sfn

Trou au Natron probably formed through two or three separate phreatic eruptionsTemplate:Sfn that deposited large blocks around the crater,Template:Sfn while an alternative proposal that considers it a collapse calderaTemplate:Sfn appears to not be consistent with field evidence.Template:Sfn It formed after the "pre-Toussidé" caldera considering that its caldera rim is cut by Trou au Natron, at a time where part of the Toussidé volcano already existed,Template:Sfn but before the Würm glaciation. Doon Kidimi on the other hand may have formed during the Neolithic Subpluvial, and after Trou au Natron.Template:Sfn

Toussidé is among the youngest volcanoes in Tibesti,Template:Sfn and may have erupted in historical time. A large number of fumaroles are active on its summit, exhaling mainly water vapourTemplate:Sfn at temperatures of ;Template:Sfn thus it is considered to be the only active Tibesti volcano.Template:Sfn Likewise, the volcanic cones in Trou au Natron are considered to be recent;Template:Sfn lake deposits underneath the cones have been radiocarbon-dated to be between 15,000 and 12,500 years old.Template:Sfn Finally, in Trou au Natron there are hot springs that deposit tronaTemplate:Sfn and fumarolic activity has been reported there.Template:Sfn

See also

• List of volcanoes in Chad
• List of Ultras of Africa

Other sources

• National Aeronautics and Space Administration
• High-resolution NASA photograph