Tibesti (departement)

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 is the highest point in both Chad and the Sahara. Bikku Bitti, the highest peak in Libya, is located in the north of the range. The central third of the Tibesti is of volcanic origin and consists of five volcanoes topped by large depressions: Emi Koussi, Tarso Toon, Tarso Voon, Tarso Yega and Toussidé. Major lava flows have formed vast plateaus that overlie Paleozoic sandstone. The volcanic activity was the result of a continental hotspot that arose during the Oligocene and continued in some places until the Holocene, creating fumaroles, hot springs, mud pools and deposits of natron and sulfur. Erosion has shaped volcanic spires and carved an extensive network of canyons through which run rivers subject to highly irregular flows that are rapidly lost to the desert sands.

Tibesti, which means "place where the mountain people live", is the domain of the Toubou people. The Toubou live mainly along the wadis, on rare oases where palm trees and limited grains grow. They harness the water that collects in gueltas, the supply of which is highly variable from year-to-year and decade-to-decade. The plateaus are used to graze livestock in the winter and harvest grain in the summer. Temperatures are high, although the altitude ensures that the range is cooler than the surrounding desert. The Toubou, who were settled in the range by the 5th century BC, adapted to these conditions and turned the range into a large natural fortress. They arrived in several waves, taking refuge in times of conflict and dispersing in times of prosperity, although not without intense internal hostility at times.

The Toubou came into contact with the Carthaginians, Berbers, Tuaregs, Ottomans and the Arabs, as well as the French colonists who first entered the range in 1914 and took control of the area in 1929. The independent spirit of the Toubou and the geopolitics of the region has complicated the exploration of the range as well as the ascent of its peaks. Tensions continued after Chad and Libya gained independence in the mid-20th century, with hostage-taking and armed struggles occurring amid disputes over the allocation of natural resources. The geopolitical situation and the lack of infrastructure has hampered the development of tourism.

The Saharomontane flora and fauna, which include the rhim gazelle and Barbary sheep, have adapted to the mountains, yet the climate has not always been as harsh. Greater biodiversity existed in the past, as evidenced by scenes portrayed in rock and parietal art found throughout the range, which date back several millennia, even before the arrival of the Toubou. The isolation of the Tibesti has sparked the cultural imagination in both art and literature.

Toponymy

The Tibesti Mountains are named for the Toubou people, also written ''Tibu'' or ''Tubu'', that inhabit the area. In the Kanuri language, ''tu'' means "rocks" or "mountain" and ''bu'' means "a person" or "dweller," and thus ''Toubou'' roughly translates to "people of the mountains" and ''Tibesti'' to the "place where the mountain people live".

Most of the mountain names are derived from Arabic as well as the Tedaga and Dazaga languages. The term ''ehi'' precedes the names of peaks and rocky hills, ''emi'' precedes those of larger mountains, and ''tarso'' precedes high plateaus and gently-sloping mountainsides. For example, the Ehi Mousgou is a stratovolcano near Tarso Voon. The name Toussidé means "that which killed the Tou," as in the Toubou, reflecting the danger of the still active volcano. The name of Bardaï, the principal town in the range, means "cold" in Chadian Arabic. In the Tedaga language, the town is known as ''Goumodi'', which means "red pass," signifying the color of the mountains at dusk.

Geography

Location

The mountains lie on the border between Chad and Libya, straddling the Chadian region of Tibesti and the Libyan districts of Murzuq and Kufra, around north of N'djamena and south-southeast of Tripoli. The range is adjacent to Niger and Lake Chad, just south of the Tropic of Cancer. The East African Rift is to the east and the Cameroon line lies to the southwest.

The range is in length, in width, and spans . It draws a large triangle with sides of and vertices facing south, northwest and northeast in the heart of the Sahara, making it the largest mountain range of the desert.

Topography

The highest peak in the Tibesti Mountains, as well as the highest point in Chad and the Sahara Desert, is the Emi Koussi, located at the southern end of the range. Other prominent peaks include Pic Toussidé at and the Timi on its western side, the Tarso Yega, the Tarso Tieroko, the Ehi Mousgou, the Tarso Voon, the Ehi Sunni, and the Ehi Yéy near the center of the range. The Mouskorbé is a peak notable for its height in the northeastern part of the mountain range. The Bikku Bitti, the highest point in Libya, is nearby, on the other side of the border. The average elevation of the Tibesti Mountains is about ; sixty percent of its area exceeds in elevation.

The range includes five shield volcanoes with broad bases whose diameter can reach : Emi Koussi; Tarso Toon, which rises above sea level; Tarso Voon; Tarso Yega; and Tarso Toussidé, which culminates in the peak of the same name. Several of these peaks are topped with large calderas. Tarso Yega has the largest caldera, with a diameter of and a depth of approximately , while Tarso Voon has the deepest caldera, with a depth of approximately and a diameter of . They are complemented by four large lava dome complexes, high and several km wide, all located in the central part of the mountain range: Tarso Tieroko; Ehi Yéy; Ehi Mousgou; and Tarso Abeki, which rises to above sea level. These volcanic complexes are now considered inactive, but according to the Smithsonian Institution were active during the Holocene. Tarso Toussidé is an active volcano that has spewed lava over the past two millennia. Gases escaping from fumaroles on Toussidé are visible when evaporation is low. The volcano's crater, Trou au Natron, is in diameter and deep. On the northwest side of Tarso Voon is the Soborom geothermal field, which contains mud pools and fumaroles that vent sulfuric acid. The sulfur has stained the surrounding soil bright colors. Fumaroles are also present at the Yi Yerra hot springs on Emi Koussi. Tarso Tôh was an active volcano in the early Holocene. The volcanic area of the Tibesti Mountains is located entirely in Chad; it covers about a third of the total area of the Tibesti Mountains and is responsible for between of rock.

The rest of the Tibesti Mountains consists of volcanic plateaus (''tarsos''), located between elevation, as well as lava fields and ejecta deposits. The plateaus are larger and more numerous in the east: the Tarso Emi Chi, the Tarso Aozi, the Tarso Ahon to the north of Emi Koussi, and the Tarso Mohi. In the center is Tarso Ourari at about . To the west, in the vicinity of Tarso Toussidé, is the aforementioned Tarso Tôh, a small plateau at just , and the even smaller Tarso Tamertiou at . The plateaus are strewn with volcanic spires and are separated by canyons that have been formed by the irregular flow of wadis. After often-violent rains, they see the formation of ephemeral streams and flora. The southern, southwestern and eastern slopes of the mountain range have a gentle rise, while the northern slope of the range is a cliff overlooking the vast Libyan desert pavement known as the ''Serir Tibesti''.

Hydrology

Five rivers in the northern half of the Tibesti Mountains flow to Libya, while the southern half belongs to the endorheic basin of Lake Chad. However, none of the rivers travel long distances, as the water evaporates in the desert heat or seeps into the ground, although the latter may flow great distances through subterranean aquifers.

The wadis in the Tibesti are called ''enneris''. The water mainly originates from the storms that periodically rage over the mountains. Their flow is highly variable. For example, the largest wadi, named Bardagué (or Enneri Zoumeri on its upstream portion) and located in the northern part of the range, recorded a flow of in 1954, yet over the next nine years it experienced four years of total drought, four years of flow less than and one year where three different flow rates were measured: .

Two other major rivers cut into the mountains: the Enneri Yebige flows northward until its riverbed disappears on the ''Serir Tibesti'', while Enneri Touaoul joins the south-flowing Enneri Ke to form Enneri Miski, which then disappears in the plains of Borkou. Their basins are separated by an high watershed that runs from Tarso Tieroko in the west to Tarso Mohi in the east. The Enneri Tijitinga is the longest wadi in the range, flowing some southward. It forms in the west of the range and peters out in the Bodélé Depression, as does Enneri Miski a little further to the east, along with other wadis such as the Enneri Korom and Enneri Aouei. Several rivers flow radially on the southern slopes of the Emi Koussi before seeping into the sands of Borkou and then reemerging at escarpments up to south of the summit, near the Ennedi Plateau.

At the bottom of many canyons are gueltas, wetlands that accumulate water mainly during storms. Above , ''enneri'' beds sometimes contain sequential pools of water that . The water is replenished several times a year during flooding, and salinity levels are low. The Mare de Zoui is a small permanent body of water above sea level, located in the northern part of the mountains in the wadi of the Enneri Bardagué, east of Bardaï. Supplied by sources upstream of the wadi, in heavy rains it overflows and spills into small wetlands.

The Yi Yerra hot springs is located on the southern flank of Emi Koussi at about elevation. Water emerges from the springs at . A dozen hot springs are also located at the Soborom geothermal field on the northwest side of Tarso Voon, where water emerges at temperatures ranging between .

Geology

The Tibesti Mountains are a large area of tectonic uplift that, according to contemporary theory, resulted from a mantle plume in the craton of the African lithosphere, which is about thick. This tectonic uplift may have been accompanied by the opening, and subsequent closure via subduction, of a rift zone. A system of regional faults, although partially obscured by the volcanic product, has two distinct orientations: a NNE-SSW alignment that could be an extension of Cameroon line, and a NW-SE alignment that could extend to the Great Rift Valley; however, the relationship between these fault systems has not been conclusively demonstrated.

The basement of the Tibesti is composed of granite, diorite and schist, one of six exposures of Precambrian crystalline rock in North Africa. These are overlaid by sandstone of the Paleozoic era, while the peaks consist of volcanic rock. The continental hotspot activity began as early as the Oligocene, although most of the volcanic rock dates from the Lower Miocene to the Pleistocene and, in places, to the Holocene. Due to the comparatively slow movement of the African plate—roughly between per year since the Lower Miocene—there is no relationship between the age of the volcanoes and their dimensions, geographic distribution or alignment, in contrast to hotspots such as the Hawaiian–Emperor and Cook- Austral seamount chains. This phenomenon is also seen in Martian volcanoes, particularly Elysium Mons. Early volcanic activity created trap basalt formations that extend tens of kilometers and stack up to thick. Basanite and andesite are also found in the volcanic layer. More recently in geologic time, volcanic activity has deposited dacite, rhyolite and ignimbrite, as well as trachyte and trachyandesite. This trend towards the production of more felsic, viscous lavas could be a sign of a waning mantle plume.

Geomorphology

Volcanic activity in the Tibesti took place in several phases. In the first phase, uplift and extension of the Precambrian basement occurred in the central area. The first structure to be formed was probably Tarso Abeki, followed by Tarso Tamertiou, Tarso Tieroko, Tarso Yega, Tarso Toon and Ehi Yéy. The product of this early volcanic activity has been completely obscured by later eruptions. In the second phase, the volcanic activity moved north and east, forming Tarso Ourari and the ignimbrite bases of the vast ''tarsos'', as well as Emi Koussi to the southeast. Thereafter, during the third phase, the outpouring of lava and ejecta deposits increased from Tarso Yega, Tarso Toon, Tarso Tieroko and Ehi Yéy; the collapse of these structures formed the first calderas. This phase also saw the formation of the Bounaï lava dome and Tarso Voon. To the east, the lava flows formed the large plateaus of Tarso Emi Chi, Tarso Ahon and Tarso Mohi. Emi Koussi increased in height. The fourth phase saw the formation of Tarso Toussidé and the lava flows of Tarso Tôh in the west, the collapse of the caldera on the summit of Tarso Voon and associated ejecta deposits in the center, and the decline in lava production in the east, with the exception of Emi Koussi, which continued to rise. In the fifth phase, volcanic activity became much more localized and lava production continued to wane. Calderas formed on top of Tarso Toussidé and Emi Koussi, and the lava domes Ehi Sosso and Ehi Mousgou appeared. Finally, in the sixth phase, Pic Toussidé formed on the western rim of several pre-Trou au Natron calderas, along with new lava flows, including Timi on the northern slope of Tarso Toussidé. With scarce time for erosion, these lava flows have a dark, youthful appearance.

The Trou au Natron and Doon Kidimi craters have formed even more recently, with the former dissecting the earlier Toussidé calderas. Lava flows, minor pyroclastic deposits, and the appearance of small cinder cones, and the formation of the Era Kohor crater are the most recent volcanic activities on Emi Koussi. there are reports of volcanic activity in various parts of the massif, including hot springs at the Soborom geothermal field and fumaroles on Tarso Voon, Yi Yerra near Emi Koussi and Pic Toussidé. Carbonate deposits in the Trou au Natron and Era Kohor craters are also representative of more recent volcanic activity.

The study of fluvial terraces has revealed coarse sand and gravel alternating with terraces of silt, clay and fine sand. This alternation highlights repeated changes in the dominant fluvial or wind patterns in the valleys of Tibesti during the Quaternary Period. The phases of erosion and sendimentation are indicative of the climate alternating between dry and wet conditions, the latter of which fostered vegetation in the Tibesti that was likely significantly denser than that which exists today. Furthermore, the discovery of calcified charophyta (particularly of the family Characeae) and gastropod fossils in Trou au Natron indicates the presence of a lake at least deep during the Late Pleistocene. These phenomena are associated with various changes in climate, most notably during the last glacial maximum, which increased precipitation and reduced evaporation due to lower temperatures. In fact, the Tibesti supplied a considerable amount of water to the Paleolake Chad until the 5th millennium BC.

Climate

The Tibesti climate is substantially less dry than that of the surrounding Sahara Desert, but rainfall events are highly variable from year to year. In the south of the range, this variation is largely due to oscillations of the Intertropical Convergence Zone (ITCZ), which steadily moves northward toward northern Chad from November until August, accompanied by humid monsoonal air. Normally, the ITCZ repels the Harmattan, a dry trade wind that blows west or southwest from the Sahara Desert, and brings rainfall to southern Tibesti. However, sometimes the front retires early, before reaching the Tibesti, leaving its southern portion dry. In the arid northern Tibesti, where the monsoon has little influence, storms are caused by episodic Sahara-Sudanese weather systems. For example, between 1957 and 1968, Bardaï, on the northern flank of the range, saw an average of of precipitation annually, yet some years were completely dry while others saw of rainfall. In general, the range receives less than of rainfall per year. However, precipitation increases with altitude; for example Trou au Natron receives annually. When the rainfall coincides with low temperatures, it can fall as snow. This occurs, on average, once every seven years.

The average monthly maximum temperature is in the central Tibesti Mountains, while the average monthly minimum is . Lows of are not uncommon. Bardaï, located above sea level, experiences average temperatures ranging between in January, between in April, and between in August. The combination of high temperatures and low humidity results in potentially high evaporation rates, ranging from in January to in May, parching many ''enneris'' before they can exit the mountain range.

Flora and fauna

Like an island surrounded by ocean, the ecology of the Tibesti Mountains is distinguishable from that of the surrounding desert. As such, the mountains lie within their own biome, the Tibesti-Jebel Uweinat montane xeric woodlands ecoregion, along with the Jebel Uweinat, a disjunct mountain range that also rises from the Sahara to the east. Much of the ecoregion remains unexplored due to its remoteness and persistent political instability, yet it is known to contain a number of endemic and endangered species. Indeed, the isolation of the region is a benefit to its flora and fauna, serving as a sort of refuge, allowing plants to grow untrammeled and animals to roam unmolested. Nevertheless, hunting is unregulated in the region, and vegetation has suffered from overgrazing in the past.

Flora

The flora in the Tibesti is Saharomontane, mixing Mediterranean, Sahara, Sahel and Afromontane vegetation. Biodiversity and endemism levels are higher in the Tibesti than in the Aïr Mountains or the Ennedi Plateau, although the vegetation's coverage is highly dependent on rainfall. Oases lie along the courses of the ''enneris'', such as Enneri Yebige. These oases, which are more numerous to the north and west of the range, are scattered with acacia, figs, palms and tamarisks. Most gueltas are lined with macrophytes—including smooth flatsedge (''Cyperus laevigatus'') and branched horsetail (''Equisetum ramosissimum'')—and bryophytes—including '' Oxyrrhynchium speciosum'' and species of ''Bryum''. Egyptian acacia (''Vachellia nilotica'' syn. ''Acacia nilotica'') grows near these water basins. Saharan myrtle (''Myrtus nivellei'') and oleander (''Nerium oleander'') grow between elevations of in the western part of the range, while Nile tamarisk (''Tamarix nilotica'') grows at similar elevations in its northern part. Downstream, where the current of the ''enneris'' is slower and the riverbed is deeper, there are dense thickets of Athel tamarisk (''Tamarix aphylla'' syn. ''Tamarix articulata'') and arak (''Salvadora persica'').

Around the edge of the Tibesti, where the canyons exit the range, are doum palms (''Hyphaene thebaica''). The banks of Mare de Zoui are home to dense stands of reeds (''Phragmites australis'' and ''Typha capensis''), along with sedges (''Scirpoides holoschoenus''), sea rush (''Juncus maritimus''), toad rush (''Juncus bufonius'') and branched horsetail (''E. ramosissimum''), while pondweed (''Potamogeton'' spp.) grows in the open water. Although the lake appears rich in phytoplankton, it has not been thoroughly studied. To the south and southwest of the range, between elevation, the wadis support woody species characteristic of the Sahel, such as Egyptian balsam (''Balanites aegyptiaca''), grey-leaved cordia (''Cordia sinensis''), red-leaved fig (''Ficus ingens''), sycamore fig (''F. sycomorus''), wonderboom (''F. salicifolia'') and gay acacia (''Senegalia laeta'' syn. ''Acacia laeta''). '' Chrysopogon plumulosus'' is the most common grass in the area. Other plants have more Mediterranean characteristics, such as globularia (''Globularia alypum'') and lavender (''Lavandula pubescens'') or the more tropical sweet Indian mallow (''Abutilon fruticosum'') and least snout-bean (''Rhynchosia minima'' syn. ''Rhynchosia memnonia''). The liverwort '' Plagiochasma rupestre'' is found around the wadis at these elevations, as are mosses of the genera ''Fissidens'', ''Gymnostomum'' and '' Timmiella''.

Saharomontane grasslands are found on the slopes, plateaus and the upper portions of the wadis at elevations between . They are dominated by '' Stipagrostis obtusa'' and '' Aristida caerulescens'', as well some '' Eragrostis papposa'' locally. In addition, shrubs represented by jointed anabis (''Anabasis articulata''), '' Fagonia flamandii'' and ''Zilla spinosa'' dot this environment. On the sheltered upper slopes of Emi Koussi is the endemic grass '' Eragrostis kohorica'', named after the volcano's crater.

The vegetation above consists of dwarf shrubs, which are generally limited to in height and do not exceed . The shrubbery consists of the species '' Pentzia monodiana'', ''Artemisia tilhoana'' and ''Ephedra tilhoana''. At the highest elevations of the Tibesti, tree heath (''Erica arborea'') grows from moist crevices formed by early lava flows, while 24 different species of moss provide substrate for the tree heath. Various genera of mosses also grow around fumaroles, including ''Fissidens'', ''Campylopus'', ''Gymnostomum'' and ''Trichostomum''. Lichen

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