The volcanic field overlies a high Tertiary surface between the Paleozoic to Tertiary Murzuk and Sirte Basins; the Syrte embayment during the Miocene reached into the Haruj mountains. A number of swells and tectonic lineaments, some of which are located at the margins between geologic blocks, characterize the basement beneath Haruj and have influenced the location of volcanic vents. The basement is of Eocene to Oligocene age and consists of conglomerate, dolomite, limestone, marl and sandstone, known as the Bishimah Formation; where the lavas of Haruj are thinner, it often forms white outcrops.