Explosive volcanism in Japan and the United States: Gaining an understanding by shock tube experiments

Abstract

On 15 July, 1888, Bandai Volcano in central Japan erupted explosively in a style which, in many aspects, was imitated by Mount. St. Helens nearly 100 years later. The episode began with a violent earthquake, and there was a major debris avalanche and many large explosions that resulted in the death of 461 people. The last and largest explosion was directed laterally across the landscape. The eruption and its aftermath were extremely well documented in the press, by scientists and by eye witnesses. We present some of the early photographs and paintings of the eruption of Bandai-San. The eruption of Mount. St. Helens on 18 May, 1980, documented with modern observational methods, caused a revolution in the understanding of explosive volcanism, and so led to a more detailed understanding of the eruption of Bandai Volcano. The information gathered a century ago serves now as an important resource. Physical processes which cause the destructiveness of explosive eruptions include the energetic fragmentation of liquid magma into fire ash particles and ejection of the particles and liberated gases through the vent at supersonic speed. It is possible to study these processes at laboratory scale by using shock tube techniques. Impulsive, highly accelerating flows, in which the greater effect of earth’s gravity under full scale conditions plays little role, can be generated by rapidly depressurizing volatile liquids or pressurized beds of solid particles. One experiment of each type is described. In the first, superheated refrigerants are used to simulate gas-rich magma, and the explosive vaporization that results (‘evaporation wave’) is studied as a model of magma fragmentation. In the second, it is shown that accelerating dense dusty gases are inherently unstable to density perturbations, and that the resulting flow experience density fluctuations of order 1000. Implications regarding the interpretation of volcanological deposits and blast effects are described.