A Discussion on volcanism and the structure of the Earth - Volcanism of the Kenya rift valley

Abstract

The Kenya rift valley is a sector of the rift system of eastern Africa which is marked by volcanic activity throughout its history from Miocene times to the present day. Activity is not confined to the rift zone but extends for distances of 200 km or more both to the west and east and is broadly centred on the Kenya ‘dome’, a topographic culmination in the course of the rift. The volcanic rocks show a considerable diversity of compositions ranging from basic to acid, but all are characteristically alkaline varying, however, from a mildly alkaline, alkali basalt-trachyte series, to strongly alkaline and undersaturated nephelinites and phonolites. The mode of extrusion and form of the volcanic accumulations are also very varied, evidently dependent in part on composition. There are thus the widespread ‘plateau’ phonolites of central and southern Kenya, possibly fissure eruptions; the large nephelinite central volcanoes of eastern Uganda, including Mt Elgon, and western Kenya; and the giant phonolite-trachyte or basalt-phonolite-trachyte volcanoes of Mts Kenya and Kilimanjaro. Extensive basalt fields were variously the products of fissure eruption, such as those of Samburu, or derived from numerous small centres as in the Nyambeni area or the Chyulu Hills. Large low-angle cones in the northern part of the rift are formed mostly of trachyte flows, whereas the axis of the rift is marked by a series of conspicuous trachyte-basalt volcanoes, often with spectacular calderas. The composition of the volcanic rocks shows variations with time, possibly indicating a dependence on the structural evolution of the rift, but sequences are not simple and cannot be easily defined. The nephelinite volcanoes of eastern Uganda are of Miocene age, but this composition also characterizes recent volcanoes of northern Tanzania. The basalt-basanite association dominates the earliest volcanic rocks of the rift zone itself, but has been repeatedly represented to the present. The flood phonolites were, however, largely confined to the upper Miocene; the Pliocene and earlier Pleistocene were marked by great eruptions of trachyte lavas and ignimbrite, whereas acid volcanic rocks, comendites and pantellarites, of Quaternary age are limited to a small area in the central part of the rift. The total volume of volcanic rocks cannot be estimated with any accuracy, but may be of the order of several 100 000 km 3 . The second part of this account presents in preliminary form the results of field mapping and chemical analytical programmes on the Cainozoic volcanics of the northern Kenya rift. It is shown that in this sector there is a distinct petrochemical evolution from the Miocene to the Pleistocene, the general trend being a decrease in silica undersaturation in both mafic and felsic rocks. The succession of lavas and sediments has a maximum thickness of 3 km and the main unconformities, indicating the major faulting episodes, coincide with the petrochemical changes.