Tokachi-dake, an active volcano situated in the central highland of Hokkaido, Japan, was in eruptive state three times in the last 100 years: 1857, 1887-1889, and 1926-1928. The last activity was famous for its disastrous mud flow that killed 144 people, caused by a hot volcanic avalanche. After a quiescence of 34 years from 1926-1928 activity, on June 29, 1962, Tokachi-dake was again in activity. Some abnormal phenomena were noted during the last several years prior to this activity, viz. increasing of temperature and HF, HCl, H2S and SO2 contents of fumarolic gas, activity of volcanic tremor and earthquakes, and opening of minor fissures near the central cone. The first explosion started from about 22hl5m and reached a climax at 22h45m-55m, ejecting volcanic blocks and ash derived from the surface. Five sulfur-mine workers, sleeping near the crater, deceased under the falling volcanic blocks. After a pause for about three hours, the second eruption occurred. This activity was manifested by forcible Strombolian eruption, ejecting volcanic bombs, scoriae, lapilli and ash, most of which were originated from new molten magma. The volcanic ash was blown up 12, 000m high, and fell over wide areas from eastern Hokkaido to Kurile Islands (Figs. 2-6, and Tables l&5). Several new craters and fissures opened on the southern side of the central cone. They are arranged parallel to the southwestern wall of Ground-kakô which is a somma of the central cone (Figs. 1 and 3). Total volume of ejecta is estimated as 7.1×107m3, most of which is essential volcanic ash. In both 1926 and 1962 activities, essential ejecta are olivine-bearing hypersthene augite andesitic, and they have quite similar mineral and chemical compositions (Tables 2-4).
Japan Association of Mineralogical Sciences