Regional comparisons, petrochemistry and petrogenesis
In a short review of the structure and volcanism of the southern part of the Gregory Rift, Dawson (1992) drew attention to differences in the contemporaneous volcanism in southern Kenya and northern Tanzania, the differences taking place abruptly at a latitude of c. 3°S. In Chapter 3 (Dawson 2008), differences were noted in the relationship of the volcanism to their setting relative to the buried cratonic margin and to the thermal structure of the crust and the upper mantle in the two areas. The composition of the mantle and its effect on the nature of the volcanicity are discussed in greater detail here.
Table 8.1 summarizes the volcanic stratigraphy of southern Kenya and northern Tanzania, mainly from c. 8 Ma, i.e. the age of the oldest known activity in northern Tanzania at Essimingor. The period 8–c. 1.3 Ma (the time of the Older Extrusives) is dominated by central-type volcanoes characterized by effusion of basanites, basalts and minor trachytes, the last being reasonably interpreted as fractionates of the more voluminous basic rocks. There were some peralkaline volcanoes such as Essimingor, Sadiman and Mosonik, which, particularly in the case of Essimingor, the oldest of the northern Tanzania volcanoes, are similar to the peralkaline volcanoes (Mt Elgon, Napak, Yelele) that erupted prior to the onset of basaltic activity in Kenya (Baker et al. 1972).
In southern Kenya during this period, volcanic rocks were erupted from both major central volcanoes or from fissures and very minor volcanoes, the latter including the 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.