Fig. 10

A schematic illustration of an inferred mechanism of phreatic eruptions accompanied by precursors in Ioto. a Before the eruption, hydrothermal reservoirs were sealed beneath the lava layer and subjected to lithostatic pressure, and existing in a stable manner even at a relatively high temperature and high pressure. b Intermittent uplift and seismic activity cause a partial pressure decrease and a partial boiling point exceeded by supply of high temperature fluid from the deeper section due to a pressure increase in a magma chamber. c The horizontal planar hydrothermal reservoir inflates and leads to further pressure reduction and boiling. After that, if the tip of the planar reservoir hits an active fault or a crack, hot water is ejected and a phreatic eruption occurs. d After the beginning of the phreatic eruption, the planar hydrothermal reservoir rapidly contracts