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)
Figures & Tables
The structure and volcanic activity of the northern Tanzania sector of the Gregory Rift Valley have hitherto been described less than those in Ethiopia and Kenya. This book focuses on northern Tanzania where, although the volcanic area is smaller than those to the north, there are major features such as Kilimanjaro, the highest mountain on the African continent, Ngorongoro, one of the largest calderas on Earth, and Oldoinyo Lengai, the world’s only active carbonatite volcano. Following an account of the discovery and early exploration of the Rift Valley, there are descriptions of the individual volcanoes. These are set within the context of the regional geology and geophysics of the rift valley and in relation to the structural evolution of the rift and its associated sedimentary basins which include Olduvai, an important site in the history of human evolution. The volume concludes with a discussion of the volcanism in relation to the plume-related African Superswell.