The Nakuru area, together with the Thomson’s Falls and Lake Hannington (now Lake Bogoria) areas to the north (part of the Baringo-Laikipia area. No. 085-00-032), have been described by McCall (1967) and illustrated by accompanying quarter degree geological maps. The western part of the Nakuru area lies within the rift valley in which are situated Lakes Nakuru and Elmenteita and the large caldera of Menengai volcano (No 085-00-052). East of the rift is a high shoulder zone the western half of which forms a broad faulted region. The whole area is blanketed by Tertiary to Quaternary volcanic rocks with only minor superficial sediments in parts of the rift. The stratigraphy and geological history are exceptionally complicated and for detail reference needs to be made to Table 1 of McCall (1967) and the accompanying geological map. The earliest volcanic rocks were thin basalt and picrite lava flows of the Samburu series that are overlain unconformably by thick flows of ‘plateau’ phonolite. After a long period of erosion a new cycle commenced with outpouring of thin flows of basalt followed by phonolites and trachytes from fissures and centres such as Menengai, which comprise lava flows and tuffs, including ignimbrites, some of them welded. There was then a period of faulting, probably the main period of rift formation, which was followed by eruption of small volumes of basalt succeeded by trachyte and phonolite in the area of Lakes Nakuru and Elmenteita. After further faulting a final, and minor, volcanic episode is represented by basaltic cinder cones, tuff rings and lavas at Elmenteita and trachytic cinder cones and lavas at Menengai, the latter probably being only a few 100s of years old and younger than the Menengai pumice tuffs which mantle much of the area. Numerous subdivisions have been recognised amongst the plateau phonolites which are tabulated by McCall (1967, p. 25). They form flows, generally 17-35 m thick, with few tuffs. No eruptive sources have been identified but McCall (1967) suggests that they probably came from fissures in a zone running along the margin of the rift. The flows are chemically similar but texturally generally distinctive. Most are holocrystalline rocks of alkali feldspar, euhedral nepheline, aegirine-augite and various combinations of aenigmatite and amphiboles variously described as katophorite, riebeckite and barkevikite; analcime may be present. There are some flows with glass-rich bases or tops. A few flows of trachyte occur in the series these lacking nepheline but having similar mafic minerals. The north-south-trending Sirrkon Ridge immediately east of Lake Nakuru is probably a separate central volcano. It comprises phonolitic lavas and trachytes that are very similar to those of Menengai. Phonolitic and trachytic tuffs of the next cycle cover a huge area east of the rift and in the west underlie products of the Menengai volcano. The succession varies throughout the area but is detailed for many localities by McCall (1967). It includes pumice, lapilli and crystal tuffs and ignimbrites some of which are welded. Although some of these rocks are phonolitic the ignimbritic rocks are all trachytes. There are a number of lesser areas of younger phonolite and trachyte, notably within the rift in the vicinity of Lake Nakuru, and the most recent volcanic activity, excluding Menengai, is represented by various tuff cones in the vicinity of Lake Elmenteita. McCall (1967) gives 15 analyses of whole rocks, though some come from the area north of the Nakuru map sheet.
MCCALL, G.J.H. 1967. Geology of the Nakuru - Thomson’s Falls - Lake Hannington area. Report, Geological Survey of Kenya, 78: 1-122.