Repetitive explosions of silicic volcanoes are thought to be triggered by volcanic gases in the shallow parts of the conduit. To trap the volcanic gases in the conduit, the lava dome or plug must be impermeable. However, silicic magma undergoes brittle fracturing during ascent because its viscosity increases via dehydration and groundmass crystallization. Since fractured magma has high permeability, the reduction of permeability is a key process that allows gases to accumulate, leading to an explosion. We performed uniaxial compression experiments on rhyolite fragments at temperatures of 700-900°C to investigate the permeability reduction of fractured rhyolite. Experimental results show that permeability is controlled by porosity, and permeability reduction can be caused by compaction of fractured magma. Based on the observed relationship between permeability and porosity, we calculate the time scale of permeability reduction, i.e., the transition from permeable to impermeable magma. The calculated time scale (~100-1000 s) is much shorter than the eruption repose (~1-10 h) observed at Sakurajima volcano, Japan. This result implies that an impermeable lava dome and plug formed during the eruption repose. In contrast, the repose time (20-40 min) of Santiaguito volcano, Guatemala, is shorter than the time scale of permeability reduction (>~10 h); this indicates that gases could not have accumulated before volcanic explosions and that other mechanisms could control eruption cyclicity.
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