There are only a few areas on Earth where continental plate break-up and the attendant magmatism are taking place at the present day. In this context, the East African Rift system takes pride of place as it is the most extensive, presently active, continental extension zone, the extension being accompanied by seismicity, crustal thinning and, in some sectors, magmatism. The reason for this spectacular fracturing of the African Plate has been the subject of much debate but there is general consensus that it is due, at least in part, to the presence of rising thermal plumes in the mantle beneath Africa. The extension is now held to be due to the incipient separation caused by the plume-related eastward drift of the Somalia microplate away from the more stationary Nubian Plate (Fig. 1.1). The attendant fracturing extends from the Afar triple junction in the Red Sea in the north to at least the Zambezi River in the south. It splits into two branches around the Tanzania Craton which forms the topographic high of the East Africa Plateau, which itself is part of the larger highland area that covers much of southern Africa, referred to as the African Superswell (Nyblade & Robinson 1994).
The eastern branch of the fracture system, stretching from the Gulf of Aden, through Ethiopia and Kenya to northern Tanzania, mainly follows the north–south trend of the Mozambique Fold Belt and passes over the localized uplifts of the Ethiopia and Kenya Domes where the sub-parallel rift
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.