The most northwesterly of the volcanic fields of the Hoggar Eggere covers about 100x60 km. It consists essentially of a basalt plateau across which are scattered numerous vents.
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Hoggar (Ahaggar) Recent Volcanism
The volcanism of the Hoggar extends for over 500 km from southwest to northeast. The area is located on a large lithosphere swell of some 1000 km diameter and one kilometre amplitude (Brown and Girdler, 1980). There are five principal volcanic districts namely, Adrar n'Ajjer, Eggere, Anahef, Atakor and Tahalra. All but Anahef are dominated by alkali basaltic and trachytic volcanism; each of these centres is described separately (Nos 003-00-004 to 003-00-008). Anahef includes a number of intrusive centres, which are described individually. A general description of the deep structure of the Hoggar dome, based on geophysical and petrological data, is given by Lesquer et al. (1988), and Dautria and Girod (1991) discuss the relationship between the regional volcanism and mantle heterogeneity.
The Anahef region, referred to as Amadror by Remy (1967), lies between the volcanic fields of Adrar n'Ajjer and Atakor and differs from the rest of the Hoggar volcanic province in containing not only extensive areas of extrusive rocks but also numerous intrusions.
The Atakor volcanic field extends over 2150 km2 and has an estimated volume of 250 km3 (Girod, 1971). It lies on a 1 km high basement dome which is 80 km in diameter and culminates in Mont Tahat, the highest point in Algeria. The lavas lie on Precambrian granitic and gneissic rocks.
Tahalra is the most southerly of the volcanic fields of the Hoggar region (Fig. 4) and extends over an area of 80x30 km. The volcanic rocks lie on Precambrian metamorphic and igneous rocks and locally their Palaeozoic cover which are cut by a north-south-trending shear zone.
The most northeasterly of the Hoggar volcanic fields Adrar n'Ajjer extends over about 90x40 km. No detailed accounts have been located but Fabre (1976) describes the field as consisting of basalts and phonolites with remarkably well preserved volcanoes.