Toussidé

Toussidé is part of the western Tibesti[4] mountains in Chad, Africa.[5] The Tibesti mountains reach elevations of 3 kilometres (1.9 mi) and are surrounded by the Sahara.[6] The towns of Bardai and Zouar lie east-northeast and south of Toussidé, respectively,[2] and a road between the two passes just southeast from Trou au Natron.[7]

Another volcano in Tibesti is Emi Koussi,[2] which is the highest mountain of the Tibesti and of the entire Sahara region.[4] The volcano Tarso Toh lies northwest of Toussidé,[8] the Botoum and Botoudoma/Petit Botoum rhyolite extrusions are located 10 kilometres (6.2 mi) south of Trou au Natron.[9] 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.[10]

Toussidé is a symmetrical[11] 3,265-metre (10,712 ft) high[2] stratovolcano,[5] the second highest peak in Tibesti[2] and the highest peak in the western Tibesti mountains.[12] In the past it was considered to be lower, only 2,500 metres (8,200 ft), before a higher summit height was determined by Jean Tilho and W.G. Tweedale (1920).[13] It rises 1,000 metres (3,300 ft) above the surrounding landscape and covers a base of 8 by 9 kilometres (5.0 mi × 5.6 mi),[12][14] dominating the surroundings.[13] The summit cone is formed by lapilli, pyroclastics and scoria,[15] one half has a white colour and the other half is black;[16] some areas are white from fumarolic alteration.[1] Lava flows emanate from Toussidé in a radial pattern[17] and reach lengths of 25 kilometres (16 mi), covering an area of 200 square kilometres (77 sq mi).[15] These flows descended valleys towards the west[18] and may have buried an older volcanic edifice of Toussidé.[19] The flows have a ropy appearance with a diverse surface texture, including bubbly, glassy and porphyric textures,[15] which are very fresh and free of erosion.[12] The surface of the flows is rough and can be a problem to climbers.[17][20] The relatively steep slopes of Toussidé may reflect the existence of a lava dome underneath the younger lavas;[21] some flows may have originated from parasitic vents.[22]

Toussidé itself lies in part within an even larger caldera, the 14-by-13-kilometre (8.7 mi × 8.1 mi) "pre-Toussidé" (also known as "Yirrigué"[23]) caldera which is in part filled by the lava flows from Toussidé and eruption products from the more recent explosion craters,[24] as well as debris that fell from the steep caldera margin.[25] Toussidé is located on the caldera's western side,[1] and the smaller Trou au Natron caldera cuts into the flanks of Yirrigué.[26] Yirrigué contains a small cinder cone and an associated 1 kilometre (0.62 mi) long lava flow,[27] as well as an alluvial plain.[25] The "Yirrigué" caldera is part of a large rhyolitic shield-shaped volcano[28] that developed on top of a tectonic horst, which in turn may have been formed by intrusion of magmas.[29] The ignimbrites have buried older terrain and filled valleys.[18]

Southeast of Toussidé lies the 8-by-6-kilometre (5.0 mi × 3.7 mi) wide caldera Trou au Natron (also known as Doon[19] or Doon Orei[30]).[2] Its 700–1,000-metre (2,300–3,300 ft) high rim is cut into sequences of lavas[24] and older volcanic cones.[31] In places it is almost vertical.[32] Inside of Trou au Natron are four[21] recent basaltic volcanic cones,[2] the most remarkable of which is the 75-metre (246 ft) high Moussosomi, which has erupted a lava flow.[31] Three of these cones are deeply eroded.[26] Cones are also located outside of the Trou au Natron, and their eruption products have in part flowed into the crater.[32] Like Toussidé, the position of Trou au Natron appears to be controlled by the ring fault of the "pre-Toussidé" caldera.[19] A salty swamp lies within Trou au Natron,[33] whose floor is in part covered by evaporites,[18] mainly sodium sulfate.[34]

Another 1,500-metre (4,900 ft) wide and 300-metre (980 ft) deep crater, Doon Kidimi (also known as Petit Trou[19] or Doon Kinimi[30]), lies northeast of Trou au Natron;[14][17] it is among the most pristine volcanic features in the region.[33] Additional volcanoes in the neighbourhood are the 3,040-metre (9,970 ft) high Ehi Timi northeast and the 2,515-metre (8,251 ft) high Ehi Sosso/Ehi Soso east of Toussidé,[7][1] the former of which features lava domes. The river Enneri Oudingueur originates closely in the area and becomes a tributary of the Enneri Bardagué,[18] which drains the Tibesti northward.[35]

• 
  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 was once filled by a freshwater lake during the last glacial maximum.[36] Fed by meltwater from snow, this lake persisted for several thousands of years. Aside from a lake level drop between 14,900–14,600 years before present,[37] the lake existed[38] until about 12,400 years before present.[39] 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.[40] Charophyte algae (such as Chara globularis and Chara vulgaris[41]), diatoms, gastropods,[2] golden algae, sponges[40] and stromatoliths lived in the lake.[39] Ferns and mosses colonized the margins of the crater.[38]

The lake reached maximum depths of at a minimum 300–350 metres (980–1,150 ft),[2] at least once reaching 500 metres (1,600 ft).[38] Such a large size, relative to its catchment, has raised the question of where this water came from.[42] The formation of such a lake during the glacial maximum was probably dependent on orographic precipitation transported by the subtropical jet stream.[37]