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