The Nasaken volcano covers about 350 km2 but the eastern parts are concealed under younger volcanic rocks. Some 95% of the volcano comprises trachytic rocks including lavas, with individual flows up to 80 m thick, pumice, pumice-lapilli, crystal and welded tuffs and agglomerates. The pyroclastic rocks form about 30 % of the total. Basalt, hawaiite and mugearite make up less than 5% of the volcano. The trachytes contain anorthoclase phenocrysts with or without minor phenocrysts of hedenbergite, magnetite, fayalite, arfvedsonite and aenigmatite in a matrix of alkali feldspar laths, quartz, sodic amphibole, aenigmatite and minor pyroxene (hedenbergite, aegirine-augite or aegirine). Rock analyses indicate that the trachytes contain up to 20% normative quartz; four analyses are given by Weaver et al. (1972).
KEY, R.M. 1987b. Geology of the Maralal area. Report, Geological Survey of Kenya, 105: 1-93. WEAVER, S.D., SCEAL, J.S.C. and GIBSON, I.L. 1972. Trace-element data relevant to the origin of trachytic and pantelleritic lavas in the East African rift system. Contributions to Mineralogy and Petrology, 36: 181-94.