During the last decade there has been considerable activity in geophysical investigations in the East African Rift under the auspices of the Upper Mantle Project. Studies in the fields of explosion and earthquake seismology, gravity, magnetism, heat flow and electrical conductivity have considerably increased our understanding of the deep structure and processes beneath the Rift. The gravity data also throw light on the near-surface geology. A gravity map of Kenya at 1:2000000 has just been compiled at the University of Leicester using data from over 6500 stations, most of which have been recently established in the Rift area. The regional field can be represented by a seventh order trend surface with a ‘saddle’ centred at Marsabit in the north. From the centre of this, the regional field rises north-westwards towards Lake Turkana and falls south-westwards towards a pronounced bowl-shaped minimum centred in the Narok-Naivasha region near the culmination of the Kenya dome. The trend surface correlates inversely with the topography and may be attributed to deep structure. The residual gravity field of the Rift Valley region contains a number of features which are clearly related to the visible geology.
The axial region is associated with an undulating high along which lie a number of prominent caldera volcanoes—Suswa, Menengai, Silali, Emuruangogolak and Lomi. Further north the most spectacular high occurs at the syenite plug, El Moiti, which is probably the remnant of a trachyte volcano. The flanking negative anomalies can usually be correlated with low density lavas and sediments—notably at Lake Turkana, near Lokichar, the Kerio Valley and further south at Londiani and Ol Kalou, but lows are notably absent over the Baringo and Suguta basins. Quantitative interpretation requires an estimate of the true regional, as opposed to the synthetic regional of trend surface analysis, and is often extremely difficult to make, due to lack of geologic data. Nevertheless, several east-west profiles have been modelled, yielding, amongst other parameters, estimates of the thickness of sediments and volcanics.
