Atakor volcanic field

The field lies in the Hoggar[1] and the terrain approaches elevations of 3,000 metres (9,800 ft)[3] although the volcanics form an only superficial cover.[4] The ground has a desert-like appearance.[5] The scenery of the volcanic field is considered to be spectacular,[1] with the lava domes and volcanic necks rising above the surrounding terrain.[3]

The field consists of lava domes, lava flows,[1] maars,[6] scoria cones and volcanic necks[1] which cover an area of about 2,500 square kilometres (970 sq mi)[6] with a volume of about 250 cubic kilometres (60 cu mi) of volcanic rock.[7] Basalts form a 400 metres (1,300 ft) thick plateau,[3] and deep gorges lead up to the volcanic field[8] and split the Hoggar Mountains in a number of segments.[9] Wadis diverge from the Atakor volcanic field;[10] some of them reached Lake Chad in the past,[11] others continued through the Grand Erg Oriental towards Chott Melrhir.[12]

Tahat in the Atakor

There are about 450 recent volcanic centres, of which about 400 are lava domes and 50 small stratovolcanoes,[13] the latter of which include many recent cones[6] which are accompanied by lava flows whose length reaches 20 kilometres (12 mi).[14] Among the older volcanoes of Atakor are the peaks Assekrem and Tahat,[7] the latter of which is the highest summit in the Hoggar.[15] Some of these volcanoes have craters, including double craters, and others are eroded to the point that only volcanic necks remain,[16] while lava domes include shapes from steep pillars to short stubby lava flows[17] and are responsible for much of the field's scenery.[18] Some of these lava domes and necks penetrated older basaltic layers.[19] Among the stratovolcanoes is the Oued Temorte cone, which is 300 metres (980 ft) high, 800 metres (2,600 ft) wide and has erupted an over 10 kilometres (6.2 mi) long lava flow. It has also erupted volcanic ash, lapilli and slag.[20]

Atakor is one in a group of volcanic fields of the Hoggar around Tamanrasset which include Adrar N' Ajjer, Eg'ere, Manzaz and Tahalra,[21] and is considered to be part of the Hoggar volcanic province which since 34 million years has covered an area of 11,700 square kilometres (4,500 sq mi) with 1,650 square kilometres (640 sq mi) of volcanic rocks.[22] A low-seismic velocity anomaly underpins the Atakor volcanic field in the mantle[23] but does not appear to reflect the existence of a hotspot.[24]

The basement is formed by Precambrian rocks that form a 1 kilometre (0.62 mi) high swell[21] known as the Hoggar swell,[25] and is further part of the Neoproterozoic Tuareg shield[26] and a metacraton formed during the Eburnean orogeny.[27] The basement crops out in deeply incised valleys,[28] which in general appear to be younger than the Hoggar volcanism.[29] Active faults occur throughout the region.[30]

Atakor has erupted basalts, phonolite and trachyte,[1] the latter two form lava domes.[22] The basalts are characterized by alkali basalts and basanite and form about 80% of all volcanic rocks in Atakor, with less important occurrences of benmoreite, hawaiite, mugearite and rhyolite.[6] Phenocrysts in some volcanic rocks include amphibole, clinopyroxene, olivine and zircon.[20] The Taessa lavas from this volcanic complex[22] have a porphyritic texture.[31] The volcanic rocks appear to ultimately derive from mantle plume melts,[32] although a tectonic origin resulting from the convergence between African and Europe has also been suggested.[33]

Atakor lies within the tropics (south of the Tropic of Cancer) and at high elevation. Precipitation is more common than in the surrounding desert and during winter it can occur in the form of snow;[3] at Assekrem annual precipitation is about 100–150 millimetres (3.9–5.9 in).[8] In the past, precipitation was considerably higher than today,[39] such as during the Villafranchian and the Paleolithic when nivation landforms developed above 2,000 metres (6,600 ft) elevation,[40] as well as moraines[41] such as at Tahat[42] and rock glaciers.[43] A last wet period occurred during the Neolithic.[36]

Vegetation in Atakor is subdivided into several belts, a lower Sudanian belt at 1,700–1,800 metres (5,600–5,900 ft) elevation with bushes and trees,[3] a sub-Mediterranean between 1,800–2,400 metres (5,900–7,900 ft) which includes the olive and a high Mediterranean belt which includes Clematis flammula.[5] The volcanic field is used as a pasture.[8]

• Girod, Michel (1971). Le massif volcanique de l'Atakor (Hoggar, Sahara algérien): étude pétrographique, structurale et volcanologique (in French). Éditions du Centre national de la recherche scientifique.
• Rognon, Pierre (1967). Le Massif de l'Atakor et ses bordures (Sahara central), étude géomorphologique . (in French). Éditions du centre national de la recherche scientifique.