The 2000 eruption of Usu Volcano, Japan: characteristics of phreatomagmatic explosions and hazard evaluation for pyroclastic density currents.

摘要

The Usu 2000 eruption began with phreatomagmatic explosions on March 31. The first explosion occurred at the northwestern foot of the volcano, sent a buoyant dark gray plume 3,200 m in the maximum height, and ejected about 130 thousands tons tephra containing dacite pumice. A phreatomagmatic explosion is cause by dynamic mixing of fragmented magma and groundwater. It is necessary for ascending of a buoyant plume that magma interact with water less than 0.2 in water/magma mass ratio. The March 31st explosions presumably originated in a volatile-rich head of the ascending magma. Degassing of the head promoted the magma fragmentation and effective mixing with groundwater in a low mass ratio. Since April 1, the eruptions have been getting mild, and the northwestern foot of the volcano has upheaved due to the intrusion of dacite magma. Cock's tail jets and black plumes less than 1,000 m in height frequently occurred in migrating vents on early April. Their ejecta seldom contain essential materials. The stable surfaces of the intruding magma inhibited the dynamic mixing between magma and groundwater. The cock's tail jets activities seem to represent weak phreatomagmatic explosions more than 0.4 in water/magma mass ratio or phreatic explosions in low water/host rocks mass ratio.During the eruptions, hazard maps wee prepared for a pyroclastic density current that could be expected from a phreatomagmatic explosion at the new vents on the northwestern foot, using an energy cone method. When a water/magma mixing mass ratio is 0.2 to 0.4, a resulting plume will be unstable due to heat loss of ejecta. It is assumed that the Heim coefficient is 0.25; this value is the mean of small volume pyroclastic flows. The height of column collapse is varied as a function of magma mass interacting with groundwater. The March 31st explosion could generate a pyroclastic density current traveling 1 to 2 km from the vent, if its plume collapsed.