GEOLOGY AND PETROLOGY OF ESAN VOLCANO, HOKKAIDO, JAPAN

Esan, a volcano of the Nasu volcanic zone, is situated at the eastern end of the Kameda Peninsula, southwestern Hokkaido (Figs. 1 & 2; Table 1). In the Pleistocene time, the activity of Esan volcano begun with eruption of pyroclastic flows (Fig. 3), followed by extrusion of two lava domes, Kaiko-zan and Todo-yama. Then, a viscous lava erupted again, forming a semicircular ridge which has been called as “somma.” After a long quiescence, probably in the Holocene age, the final activity occurred, erupting nuee ardentes, pumice flows, lava flows and dome lavas (Fig. 4). Esan (618m) is the latest lava dome. During the whole history, the center of eruption have migrated from northwest to southeast. Esan volcano is composed of the calc-alkalic rocks which are divided into two types (Table 2). One is augite-hypersthene andesite of the early to middle stages, and the other is quartz-augite-hypersthene andesite of the final stage, in which quartz phenocrysts, bipyramidal or resorbed in form, are included as much as 2-8% by volume. As shown in Table 3, Esan lavas are intermediate in composition, ranging from 57.75 to 62.69% in silica content. They are similar in chemistry to other calc-alkalic rocks from the Nasu volcanic zone in Hokkaido (Figs. 5, 6 & 7). Mode of occurrence of phenocrystic quartz and experimental results by Wyllie (1971) (Fig. 8) suggest that the quartz phenocrysts in Esan lavas have not derived from the basement rocks, but crystallized from an intermediate magma together with large plagioclase, hypersthene, augite and hornblende prior to the crystallization of ordinary phenocrystic minerals under a certain condition of several kb, 1000-1100°C and 1-2% in water contents. Subsequently, the quartz phenocrysts suffered magmatic resorption by decreasing of pressure due to ascent of the magma. Simultaneously, hornblende phenocrysts resolved into the aggregates of minute grains of plagioclase, hypersthene, augite and magnetite (Table 4).