Menengai

The caldera of the Menengai volcano lies within the rift valley immediately north of Lake Nakuru, but ignimbrites and air-fall tuffs from the volcano cover some 1350 km2 (Leat, 1984) and extend into the Molo area (No. 085-00-049) (Jennings, 1971), the quarter degree sheet area lying west of the Nakuru sheet (McCall, 1967). Menengai is a central volcano comprising an initial low-angle trachyte lava shield of about 22x15 km the central part of which foundered to form a 77 km2 caldera which was accompanied by eruption of air-fall tuffs and then two large ash-flow tuff units. Activity concluded with the eruption of lava flows onto the caldera floor, the building of cinder cones and the emplacement of plinian-type tephra sheets (Leat, 1984). All the rocks of the volcano are silica oversaturated trachytes with no rocks of mafic or intermediate composition (Leat et al., 1984). Rocks of the early shield, which had a maximum thickness of 300 m, are exposed in the caldera walls and consist predominantly of 11 to 60 m thick trachyte flows but there are also air-fall tuffs, some of which are welded, that become progressively more important westwards. The petrography of these trachytic rocks, under the name ‘Lower Menengai volcanics’, is given in detail by McCall (1967) and they variably include aegirine-augite, aegirine, riebeckite, katophorite and aenigmatite. The two ash-flow tuff units that were erupted at the time of caldera formation are each underlain by air-fall pumices, have a combined volume of some 50 km3, about half of which was ponded within the caldera, and extend over about 1000 km2 (Leat, 1984). They are non-welded, except in the vicinity of the caldera, and display wide facies and chemical variation, as illustrated by Leat (1985) and Leat et al. (1984). They are generally strongly peralkaline but the part of the second unit erupted last is peraluminous. The stratigraphy and mineral and rock chemistry of the second ash-flow tuff unit have been investigated in detail by Macdonald et al. (1994) who interpret compositional variations in terms of a compositionally zoned magma chamber. The dominant post-caldera rocks are trachyte lava flows, about 70 of which cover the caldera floor, but there are also air-fall deposits interstratified with the lavas, which mantle much of the country around Menengai and 18 separate units of which can be distinguished on the western caldera wall (Leat, 1984). The trachytes vary from obsidians through glassy rocks with sparse anorthoclase phenocrysts to coarser types of alkali feldspar, aegirine-augite, sodic amphiboles (katophorite, riebeckite) and aenigmatite (McCall, 1967). Numerous chemical analyses, including many trace elements, are given by Leat et al. (1984) who demonstrate the strong chemical zonation in the main ash-flow units and the highly peralkaline nature of the rocks.