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 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. "Trou au Natron" refers to the springs which have deposited white trona in the caldera. "Toussidé" translates as "Which killed the local people (Tou) with fire".
Geography and geomorphology
Regional
Toussidé is part of the western Tibesti mountains in Chad, Africa. The Tibesti mountains reach elevations of and are surrounded by the Sahara. The towns of Bardai and Zouar lie east-northeast and south of Toussidé, respectively, and a road between the two passes just southeast from Trou au Natron.
Another volcano in Tibesti is Emi Koussi, which is the highest mountain of the Tibesti and of the entire Sahara region. The volcano Tarso Toh lies northwest of Toussidé, the Botoum and Botoudoma/Petit Botoum rhyolite extrusions are located south of Trou au Natron. 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.
Local
Toussidé is a symmetrical high stratovolcano, the second highest peak in Tibesti and the highest peak in the western Tibesti mountains. 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 , dominating the surroundings. The summit cone is formed by lapilli, pyroclastics and scoria, one half has a white colour and the other half is black; some areas are white from fumarolic alteration. Lava flows emanate from Toussidé in a radial pattern and reach lengths of , covering an area of . These flows descended valleys towards the west and may have buried an older volcanic edifice of Toussidé. The flows have a ropy appearance with a diverse surface texture, including bubbly, glassy and porphyric textures, which are very fresh and free of erosion. The surface of the flows is rough and can be a problem to climbers. The relatively steep slopes of Toussidé may reflect the existence of a lava dome underneath the younger lavas; some flows may have originated from parasitic vents.
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, as well as debris that fell from the steep caldera margin. Toussidé is located on the caldera's western side, and the smaller Trou au Natron caldera cuts into the flanks of Yirrigué. Yirrigué contains a small cinder cone and an associated long lava flow, as well as an alluvial plain. The "Yirrigué" caldera is part of a large rhyolitic shield-shaped volcano that developed on top of a tectonic horst, which in turn may have been formed by intrusion of magmas. The ignimbrites have buried older terrain and filled valleys.
Southeast of Toussidé lies the wide caldera Trou au Natron (also known as Doon or Doon Orei). Its high rim is cut into sequences of lavas and older volcanic cones. In places it is almost vertical. Inside of Trou au Natron are four recent basaltic volcanic cones, the most remarkable of which is the high Moussosomi, which has erupted a lava flow. Three of these cones are deeply eroded. Cones are also located outside of the Trou au Natron, and their eruption products have in part flowed into the crater. Like Toussidé, the position of Trou au Natron appears to be controlled by the ring fault of the "pre-Toussidé" caldera. A salty swamp lies within Trou au Natron, whose floor is in part covered by evaporites, mainly sodium sulfate.