[S-VC32] Active Volcanism

[SVC32-16] Hydrothermal activity around the Ioyama after the 2018 eruption in Kirishima Volcano

*Yasuhisa Tajima¹, Jun-ichiro Ishibashi², Keiko Suzuki-Kamata², Takeshi Matsushima³, Tomoharu Miyamoto³, Kazuhiko Shimada³, Jun Oikawa⁴, Ken T. Murata⁵ (1.Research and Development Center, Nippon Koei Co.,LTD., 2.KOBEC, Kobe University, 3.Kyushu University, 4.ERI, Tokyo University, 5.Institute of Information and Communications Technology)

Keywords:Kirishima Volcano, Ioyama, Hydrothermal activity, Minor events

The geothermal activity around Ebinokogen-Ioyama resumed in late December 2015, and a minor hydrothermal (phreatic) eruption occurred in April 2018, which formed the Ioyama south craters and Ioyama west crater (Tajima et al., 2020). Following this eruption, hydrothermal water production started in the pools within the south craters of Ioyama. However, those hydrothermal water pools disappeared once in May 2019. The chemical composition ratios of the hydrothermal pools changed after their disappearance from the south craters. Hydrothermal and fumarolic activities also ceased at the west crater of Ioyama in July 2021. Subsequently, an increase in the fumarole-H temperature began in the summit area after the cessation of activity in the west crater. In addition, five small hydrothermal blowout events and two small molten sulfur blowout events occurred around the Ioyama south craters from late 2022 to 2023. The temperature of fumarole-H dropped several months before the blowouts, and rainfall was observed immediately before the blowouts in some cases. The sulfur blowout events may have been caused by a blockage in the pathway that solidified sulfur due to rainfall, which elevated the molten sulfur. In other cases, gypsum minerals were observed in many of the events. Therefore, blowout events might have occurred because of progressive blockage by gypsum. Altered rock blocks and lapilli from the hydrothermal blowout events reached a distance of 20 m from the vents, and a sulfuric surge from the sulfuric blowout event flowed for a distance of approximately 100 m.