Abstract
The Kirishima Volcano Group is a volcanic field ideal for studying the mechanism of steam-driven eruptions because many eruptions of this type occurred in the historical era and geophysical observation networks have been installed in this volcano. We made regular geothermal observations to understand the hydrothermal activity in Ebinokogen Ioyama Volcano. Geothermal activity resumed around the Ioyama from December 2015. A steam blowout occurred in April 2017, and a hydrothermal eruption occurred in April 2018. Geothermal activity had gradually increased before these events, suggesting intrusion of the magmatic component fluids in the hydrothermal system under the volcano. The April 2018 eruption was a magmatic hydrothermal eruption caused by the injection of magmatic fluids into a very-shallow hydrothermal system as a bottom–up fluid pressurization, although juvenile materials were not identifiable. Additionally, the upwelling of mixed magma–meteoric fluids to the surface as a kick was observed just before the eruption to cause the top–down flashing of April 2018. A series of events was generated in the shallower hydrothermal regime consisting of multiple systems divided by conductive caprock layers.
Highlights
Explosive volcanic eruptions that do not contain juvenile materials have been called by several names, e.g., steam, phreatic or hydrothermal eruption
Steam-driven eruptions have been dominant at Ebinokogen Ioyama Volcano over the long term, and there has been geothermal activity around there in recent times [32,34,35]
Tremor and low-frequency earthquake (LFE) based on the Japan Meteorological Agency (JMA) web site [50]; (b) Temperature observed at point A, before turning into a jet and the jet fumarole A; (c) Temperature observed at point H, before turning into a jet and the jet fumarole H; (d) Water temperature observed at the spring K3; (e) pH of water observed at the spring K3; (f) Geothermal anomaly (>50 ◦ C) area in the summit of Ioyama
Summary
Explosive volcanic eruptions that do not contain juvenile materials have been called by several names, e.g., steam, phreatic or hydrothermal eruption. One of the triggers of eruption in general may be pressurization of a hydrothermal system heated by input of magmatic fluids from the depths (bottom–up gas/fluid pressurization) [12] In this case, an eruption is caused by the fluid pressure exceeding the lithostatic pressure [3,13]. Steam-driven eruptions have been dominant at Ebinokogen Ioyama Volcano over the long term, and there has been geothermal activity around there in recent times [32,34,35]. The time description in this study is the Japan Standard Time (JST), UTC+9
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