The Atakor massif of the central Hoggar region of the Sahara is a complex of late Tertiary-Quaternary trachyte, phonolite, basalt, and basanite occurring chiefly as protrusions resembling eroded volcanic necks, and resting upon a basement of Suggarian (Precambrian) formations. Four types of protrusions are recognized among the nearly four hundred in the area: those with accompanying lava resting upon a bed of pyroclastics, circular or elliptical in plan; those in which the lava took the form of an oriented coulee, also resting on pyroclastics; cupola-shaped masses with some pyroclastics and little lava; and those without lava. Factors governing the form of individual protrusions include chemical composition, temperature of emission, and content of dissolved gases, as well as peculiarities of shape of the vent.

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