During May 14-23, 1978, small steam eruptions of ash occurred from Fumarole “A” at the summit of Tarumai, after 23 years of dormancy since the 1955 activity. The sequence of the present eruptions and the nature of ash are reported here, together with a dry pulverulent flow of ash which was accompanied by this activity. Fine ash, mostly less than 0.1 mm in size, amountaing to 4×104 m3, was produced in this eruption. On the basis of both cooling process of the ash deposit and thermal experiment on the polyvinyl chloride which was used for fence around the fumarole, the initial temperature of the pulverulent flow was estimated to be 220±10℃. The ratio of vertical distance to horizontal distance travelled for the pulverulent flow is 0.36, being similar to that for cold rock avalanches but lower than that for pyroclastic flows.
