マグマ水蒸気爆発の特性とメカニズム : Vapor Explosion Modelによる噴火事例の検討

抄録

A phreatomagmatic explosion is a volcanic eruption caused by the generation of high pressurized steam from interactions between magma and any body of water. Various manners of modes of phreatomagmatic explosions have been known. The author classified the phreatomagmatic explosion modes into two types based on the intensity of eruption; weak phreatomagmatic explosion and strong one. The weak phreatomagmatic explosion is characterized by almost silent tephra finger jets, associated with minor surge activity, reaching several hundred meters high. The strong phreatomagmatic explosion is characterized by a large eruption column reaching 10-20 km high, accompanied by a shock wave and major surge activity. High-pressurized steam generating the phreatomagmatic explosion is caused by the rapid evaporation due to the mechanical mixing of magma and water. This mixing process determines the boundary condition of heat transfer from magma to water. When a lava flow enters the ocean, a frozen glassy crust effectively prevents the hot fluid interior from self-sustained mixing with water. Namely, the initial mechanical mixing of magma and water is the most important factor for triggering the phreatomagmatic explosion. The initial mechanical mixing easily occurs when magma ascends through the vent filled with wet clastic materials, owing to the fluid instability. If pressure pulse produced by the initial mixing is great enough to propagate a shock wave, it promotes the thermal detonation causing further hydrodynamic mixing and subsequent much more energy release. The strong phreatomagmatic explosion takes place in this fashion. On the other hand, the weak phreatomagmatic explosion is accompanied by no shock wave; the pressure pulse is not sufficient for propagating a shock wave. The strong phreatomagmatic explosion is triggered at greater depth of the vent than the weak one. This is mainly because a stronger pressure pulse is required at a higher confining pressure.