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).