Abstract The present-day Gregory Rift Valley in northern Tanzania is an elongate north–south half graben flanked on its western side by a high, eastwards-facing escarpment; this differs from Kenya where there are also major faults on the eastern side of the rift valley. The words ‘present-day’ are used advisedly, as the present morphology is imposed upon an earlier, wider, volcano-infilled tectonic depression. As in Kenya, within and to either side of the rift valley there are extensive areas of Neogene and Quaternary volcanic rocks (Fig. 3.1 ). To the SW of the main volcanic area is the Eyasi half graben and, to the SE, the Pangani graben; these major features are infilled by volcanoes at their northeastern and northern ends, respectively. The northern Tanzania volcanic province is younger overall than the Ethiopian, where magmatism was initiated at around 40 Ma, and Kenya provinces ( Baker et al . 1972 ; George et al . 1998 ) suggesting that more recent mantle perturbations are responsible for the intra-plate magmatism and crustal fracturing in this part of Africa. The province stands astride the surface interface between the Archaean rocks of the Tanzania Craton and the north–south-trending Mozambique orogenic fold belt (Fig. 3.1 ). The rocks of the craton have been divided into three major formations, the Dodoman (oldest), the Nyanzian and the Kavirondian ( Quennell et al . 1956 ; Schlüter 1997 ) but, in brief, the craton is an amalgamation of several terranes comprising Archaean metasediments, including the so-called greenstone belts (some as old as >3 Ga)