マグマ水蒸気爆発のメカニズム  [in Japanese] Phreatomagmatic explosions : basic problems of dynamic interactions between magma and water  [in Japanese]

マグマ水蒸気爆発は, マグマと間隙水に富む母岩との安定した界面がトリガーにより壊れる過程で, マグマから間隙水への高速伝熱により高圧水蒸気が発生して起こる噴火現象である。爆発のトリガーとしては間隙水の急加熱による母岩の破壊, マグマと湿った母岩との流体境界の不安定, マグマの発泡破砕, 火道内の減圧による地下水の流入などがある。これらの諸過程は母岩の含水率・浸透率・破壊強度やマグマの粘性・揮発性成分含有量に依存するが, 実際の噴火での定量的な見積は十分でない。特にマグマの発泡破砕はマグマ全体の伝熱面積の増加をもたらすが, 一方で高温の火山ガスの混入がマグマと水の液-液接触を阻害するため, 発生するマグマ水蒸気爆発の強度は低く抑えられるらしい。

Phreatomagmatic explosions are caused by generation of high-pressurized steam due to rapid heat transfer from magma to ground water. In general, magma must break up into small particles during self-sustained mixing of magma and water for the rapid heat transfer. However, quenched crusts and vapor films shield fluidal interiors of magma from violent mixing, because the initial contact temperature between magma and water is lower than the glass transition one of magma and higher than the homogeneous nucleation one of water. In fact, no self-triggering explosion occurs without entrapments of water close to magma, when lava flows enter the sea. Within volcanic conduits there are several triggering precesses as follow; (1) fragmentation of low-permeable host rocks around intruding magma due to explosive expansion of heated pore water, (2) unstable intrusion of magma into wet unconsolidated sediments filling the conduits, resulting in entrapments of ground water, (3) capture of ground water from the conduit wall caused by explosive ascent of mixtures of volcanic gases and fragmented magma particles, and (4) water in flow caused by decrease of magmatic pressure below the hydrostatic pressure of the surrounding aquifer and fracturing of the conduit wall. These processes are quite different from unstable collapse of vapor films leading to industrial vapor explosions between molten fuels and vaporizable coolants; the behavior of heated ground water is highly controlied by porosity, permeability, and tensile strength of host rocks, especially in the first process. The second process promotes hydrodynamic mixing of magma and wet sediments resulting in the rapid heat transfer, when the viscosity of intruding magma is lower than the one of unconsolidated sediments. Volatile fragmentation of magma increases greatly the contact area between magma and ground water, however, the hot volcanic gases inhibit liquid-liquid-contact to encourage much more energy release in the third process. Geological data suggests that strong phreatomagmatic explosions are triggered at greater depth of the vents than weak ones; this is mainly because the strong explosions are required at higher confining pressure to suppress degassing of magma. The fourth process occasionally takes place with rapid withdrawal of fluid magma or the end of Plinian eruptions.