The Kowun volcano forms a low shield overlying basalts and basement gneiss and is itself overlain by Miocene basalts. The shield has a maximum thickness of 300 m and consists principally of trachyte lavas with trachyte pyroclastics, including welded and air fall tuffs, associated with the few eruptive centres. The lavas form flows 30-40 m thick commonly with alkali feldspar phenocrysts up to 1 cm long. The groundmass consists of alkali feldspar laths, interstitial or poikilitic sodic amphibole and aenigmatite, up to 20% modal quartz and rare pyroxene. Two whole rock analyses are in Key (1987b, Table 5).