高粘性マグマによるマグマ水蒸気爆発の発生メカニズム：マグマ破壊・吸水モデルの提案

抄録

A phreatomagmatic explosion is a type of eruption observed on the Earth’s surface. This explosion is common because Earth is a water planet and its surface is extensively covered with water. The mechanism of this explosion can be explained as follows: magma and water mix, following which efficient heat conduction occurs and the water evaporates, finally causing the explosion. However, the mechanism of mixing of the high viscosity magma with water during its ascent remains elusive, although it often causes a phreatomagmatic explosion. In this paper, we review the previously proposed mechanisms of phreatomagmatic explosion and then, based on the petrological and geophysical observations for the 2015 eruption of Kuchinoerabujima Volcano, evaluate whether the recognized mechanisms can explain the mixing of high viscosity magma with water. To explain such an explosion, we consider a new model based on the rheological view of the magma. The laboratory experiments have revealed that the high viscosity magma exhibits shear-induced brittle fracturing, resulting in dilatancy and increased permeability. In addition, the fracturing is a common process observed in high viscosity silicic magmas intruded into shallow parts of the upper crust. Based on these observations, we propose that the shear-induced brittle failure of the high viscosity magma in a volcanic conduit causes the decompression of fluid in the magma and water in the crust diffuses into the magma, resulting in heating and pressurisation of water and additional fracturing (magma fracturing and water diffusion model). The feedback between pressurisation, fracturing, and additional thermal interactions results in an explosion. This hypothesis is attractive because of the efficient mixing of high viscosity magma and water, thus facilitating a spontaneous interaction during magma ascent. To strengthen this hypothesis, additional laboratory experiments and field-based observations will be necessary in future studies.