Eburru

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 complex has a maximum elevation of 980 m above the rift floor (Clarke et al., 1990; Thompson and Dodson, 1963). Three stages of evolution are distinguished by Clarke et al. (1990). Activity commenced in the west but most of the products, except for minor outcrops of pantelleritic lavas, are buried by later formations. The second stage is represented by the rocks forming Waterloo Ridge which is a 19.5 km long, north-south-trending ridge on the eastern side of the complex. The rocks are interpreted as the products of a series of pyroclastic eruptions, that originated in a fault zone, and comprise pantelleritic lavas, unwelded pyroclastics and welded tuffs with obsidian fiamme. The third stage is characterised by various volcanic landforms including craters, small cones, lava flows and domes. More than 50 craters ranging in diameter from 200 m to 1.25 km occupy the summit area. It is the extensive pumice lapilli and ash beds emanating from these centres that cover much of the complex and extend onto the western rift shoulder. There was then eruption of trachyte lava flows and pyroclastic cones and, in a north-south-trending axial zone, pantelleritic obsidian lava flows and cratered pyroclastic cones. The pantellerites of the first and second stages are microcrystalline rocks of alkali feldspar, quartz and sodic amphibole, while the final pantellerites differ from the earlier ones in being predominantly glassy and containing rare phenocrysts of green pyroxene in addition to amphibole. The trachyte lavas contain abundant phenocrysts of alkali feldspar and more rare olivine, sodic hedenbergite to aegirine-augite, aenigmatite, sodic amphibole and an opaque phase. Six analyses of pantellerite are quoted by Macdonald and Bailey (1973).