Permalink : http://hdl.handle.net/2115/20431

JaLCDOI : 10.14943/gbhu.70.153

The 1977 eruption of Mt.Usu generated four high-altitude eruption columns which produce BIGⅠ,BIGⅡ,BIGⅢ and BIGⅣ tephra-fall deposits respectively. Expressing the initial eruption velocity in terms of the solid mass eruption rate, the initial temperature, the initial radius and the initial gas fraction from the conservation of solid particles, we apply a steady plume model to the four eruption columns. The initial velocity increases with initial gas mass fraction and initial temperature for the solid mass eruption rate estimated from the volume of deposits, the apparent density of tephra and the eruption duration. For three eruption columns depositing BIGⅠ,BIGⅡ and BIGⅢ, the initial temperature and initial gas mass fraction are estimated at 600K and >0.11. The eruption column depositing BIGⅣ is formed at 800K and >0.15. The estimated initial temperature are low and initial gas mass fraction are high in comparison with general physical properties of dacitic magma. These inadequate values suggest that the growth of eruption columns take the influence of pyroclasts fallout from the column and thermal disequilibrium between pyroclasts and gases.