The general geology of the Naivasha area has been described by Thompson and Dodson (1963) but the more recent description of Clarke et al. (1990) is the principal source of the following account.
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Kenya probably contains a greater volume of alkaline igneous rocks than any country in the world. From the northern boundary with Ethiopia to the Tanzanian border Kenya is traversed by the East African Rift along the floor and shoulders of which is a continuous cover of volcanic rocks, about half of which are basalts (Williams, 1972a) the rest comprising a broad spectrum of alkaline rocks including nephelinites, phonolites, peralkaline trachytes and rhyolites. The unbroken nature of these presents the difficulty of subdividing them into discrete localities, which is the style adopted for these volumes. There are many individual volcanoes, which are given their own accounts, but there are also huge areas of ‘plateau’ lavas and pyroclastic rocks that are not so readily subdivided. The system adopted has been to describe the rocks in terms of ‘degree’ and ‘quarter degree’ squares, generally corresponding to the areas covered in the Reports of the Geological Survey of Kenya. These areas are outlined on Fig. ?? Although it might be preferable to describe the geology within a full degree square, only in the northern half of the rift zone have recent reports been so organised. Most of the earlier reports, and all those for the southern half of the rift, consist of descriptions of quarter degree squares and it generally proved difficult to write a coherent account combining adjacent reports, because of inconsistent correlation between the sheets. In the southern half of the rift zone, therefore, many accounts are based on quarter degree squares only.
A full interpretation of the general geology, petrology, geochemistry, structure and geochronology of the volcanism along the Kenya rift is not within the remit of this chapter, but a number of review papers are available. Several comprehensive accounts have been published by Williams (1972a, 1978a and 1978b) that include consideration of the continuation of the volcanic rocks southwards into Tanzania, their volumes and temporal and spatial variations of petrography and chemistry. A description of the tectonic and magmatic evolution of the southern part of the rift zone is given by Baker et al. (1978), while Baker (1987) considers the petrology of the volcanic rocks exploring the variation of rock types and chemistry with time. A useful review of the stratigraphy, tectonic history, petrography and petrochemistry of the volcanism of southwest Kenya is that of Crossley and Knight (1981). The sequence and geochronology of the volcanic rocks are covered in considerable detail in Baker et al. (1971). Further dates are given in many subsequent papers including that by Jones and Lippard (1979). A general account of the major late Quaternary volcanoes within the rift will be found in Williams et al. (1984). For a broad overview of the structural and volcanic evolution of the Gregory Rift Valley see King (1978). Two special issues of Tectonophysics on the themes “Crustal and upper mantle structure of the Kenya Rift” (1994, vol. 236) and “Structure and dynamic processes in the lithosphere of the Afro-Arabian Rift System” (1997, vol. 278) contain many reviews of the structural and magmatic evolution of the rift system.
The alkaline rocks and carbonatites around Homa Bay on the Nyanza (Kavirondo) Gulf of Lake Victoria in western Kenya are fully and comprehensively described in the book by Le Bas (1977).
Located immediately northwest of Lake Naivasha, the Eburru volcanic complex lies within the rift valley and covers some 470 km2, but tuffs that emanated from this centre blanket much of the rift shoulder to the west.
The Greater Olkaria Volcanic Complex (Clarke et al., 1990), formerly referred to as the Naivasha Complex, lies immediately south and west of Lake Naivasha and is a multi-centred volcanic field covering 240 km2. The Longonot volcano (No.
Longonot volcano has a well-developed cone with a 2 km diameter summit crater defined by very steep to vertical walls between 75 and 350 m high.
These two quarter degree areas (Shackleton, 1945; Thompson, 1964) encompass the whole of the Aberdare Range, that reaches heights of over 4000 m and lies to the east and parallel with the eastern margin of the East African Rift, which occupies the western part of the area.
At 5200 m (17,058 feet) the Mount Kenya volcano is the second highest mountain in Africa, although Baker (1967) estimates that originally there must have been a crater at over 6000 m and hence that it was higher than Kilimanjaro.
This area represents the most southwesterly manifestation within Kenya of volcanic rocks directly associated with the Gregory Rift Valley.
The Suswa ‘quarter degree area’ (Randel and Johnson, 1991) is dominated by the Suswa volcano which occurs in the northeastern part and is described separately (No. 085-00-061).
Suswa is the southernmost caldera volcano in the Kenya Rift and its products occupy much of the eastern part of the Suswa quarter degree area (No. 085-00-060). It rises from the rift floor, at a height of 1525-1615 m, to 2537 m. The products of the volcano extend over some 1000 km2.
The rocks of the Nairobi area consist of a succession of lavas and pyroclastics of Cainozoic age overlying folded Precambrian schists and gneisses of the Mozambique belt (Saggerson, 1991).