1990 Volume 42 Issue 6 Pages 35-40
Mt. Tokachi located in central Hokkaido has erupted since December 16, 1988 after 26 years dormancy. The volcanic eruptions continued intermittently, and counted 21 times until March 5, 1989. Pyroclastic flows, pyroclastic surges and small-scale volcanic mudflows initiated by snow-melting occurred with eruptions. As a result of these events, pyroclastic materials were distributed over the hillslopes of Mt. Tokachi. Field researches were carried out to know the area, the configuration and the grain sizes of volcanic deposits. The lower end of the deposits reached 1 km downward from the crater and the volume was estimated 3 x 104m3. The pyroclastic- and mud-flow deposits contain a large number of boulders, the maximum size of which was over 80 cm. In comparison with the eruption in 1926, which initiated a huge mudflow and killed 144 people, the amount of the pyroclastic materials of this event was about 1/70 of that in 1926. Although the depth of the snow that covered the hillslopes of Mt. Tokachi from 1988 to 1989 was up to the annual average, the volume of melted snow was extremely less to develop the mudflow.