an engineering geological characterisation of tropical clays - GBV

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metamorphic minerals sillimanite, kyanite, staurolite and garnet. This serves to suggest that
some of the volcanic material of the Nairobi region, which on weathering gave rise to Recent
soils, had during their eruption incorporated pieces of the crystalline metamorphic rocks.
3.3 Volcanic rocks
The Nairobi region, including the present study area, are covered by a thick succession of
alkaline volcanic rocks and associated tuffs, derived from the Rift Valley region (Saggerson,
1991).The rocks accumulated in the period of mid-Miocene to atleast Upper Pleistocene,
during which time volcanic activity dominated the geological history of the Nairobi region.
According to Saggerson (1991), the volcanic rocks of Nairobi region are estimated to amount
in volume to more than 1042 km³, and cover an area of about 3000 km². The rocks are a part
of a more extensive late Cainozoic (Pliocene) to Recent petrographic province that extends
outside Kenya, from Ethiopia in the north to Tanzania in the south.
The earliest volcanic activity began with the extrusion of the Kapiti phonolite (dated at 13
Ma) onto a dissected erosion surface, which had developed across nearly the entire East
African region, in pre-Miocene times (Saggerson and Baker, 1965).Weathered pyroclastic
materials are also found associated in the Nairobi area, occurring as interstratifications within
the lava succession and in the deep red soils. They were originally fragmentary deposits, a
result of violent volcanic activity from a number of centres within the Rift region, during
Pleistocene (Saggerson, 1991). The only identified vents include those of the Ngong volcano
and its satellites, Ol Esayeiti and Ngoroi, all of which occur to the south – west of Nairobi
city. Much of the volcanic material must have, therefore, derived from fissure eruptions from
dispersing centres located in the Kikuyu-Limuru area, to the west of Nairobi city and the
present study area. Figure 3.1(a) shows the extent of early lava flows in the Nairobi region,
including the study area; while Table 2.1 (in previous chapter) gives a corresponding
stratigraphic classification. The approximate ages of rock formations immediately underlying
the soils of the area are given in Fig. 3.1(b).
There is a general thinning of the lava flows in an easterly direction across the areas covered.
The eastern limits of the flows are characterised by erosional scarps, which form terminal
walls of the more resistant lava sheets. Studies have also shown the volcanic material to be
mainly phonolitic and trachytic in character; while the pyroclastics are dominantly trachytic
(Saggerson and Baker,1965).