Volcanism in the Afar depression: Its tectonic and magmatic significance

Abstract

Three main types of volcanism, viz. oceanic, continental and stratoid, reflect the tectonic evolution of Afar. Stratoid volcanism (flood basalts and ignimbrites) is believed to be an early manifestation of continental rifting, as exemplified by the Ethiopian Rift. It probably developed during the early stages of Red Sea and Gulf of Aden rifting. Continental volcanism is located on either side of the rift's axis; it is mainly silicic with minor proportions of basalts. The large, essentially silicic, central volcanoes present along the margins of the depression north from 11° 00 'N are believed to be due to interactions between subcrustal magma and sialic crust. Volcanism, of oceanic type, essentially basaltic with differentiates reaching to peralkaline silicics, is related to crustal separation. It occurs only along the rift-in-rift axes of the Red Sea-Gulf of Aden megastructures and is partly superimposed on the stratoid volcanics. The southern edge of this oceanic megastructure marks the northern limit of the continental rift (main Ethiopian rift). Tectonic and thermal evidence suggests the rise of upper mantle material beneath the active axes of the Afar depression.