[S-VC28] International volcanology

[SVC28-P07] Temporal micro-texture change of volcanic ashes from the 2018–2019 activity at Kuchinoerabujima Volcano, Japan

*Yusuke Minami¹, Keiko Matsumoto¹, Nobuo Geshi¹, Hiroshi Shinohara¹ (1.Geological Survey of Japan, AIST)

Keywords:Eruptive sequence, phreatomagmatic eruption, Kuchinoerabujuma Volcano

The 2018–2019 eruption of Kuchinoerabujima Volcano in Japan shifted from continuous ejection of ash-laden plumes from October 21 to the middle of December, to intermittent explosive activity accompanied by several pyroclastic density currents until January 2019. To understand the behaviors of magma and hydrothermal fluid that controlled the eruptive sequence, we carried out a series of analysis of the particle component, mineral assemblage, and chemistry of the leachate of ash samples throughout the eruption. The proportion of non-altered particles is ~15 % in the earlier phase, then it decreased to less than 10 % in the later explosive phase. Accordingly, the mineral assemblage of the volcanic ash changed from plagioclase-dominant to sulfate minerals-dominant. Concentration of SO₄^2- and Cl/SO₄ values of the ash-leachates decreased toward the later activity. These results indicate that the proportion of fresh volcanic rocks decreased, and sulfuric acid fluid-derived sulfate minerals increased toward the later activities. Consequently, the 2018–2019 eruption at Kuchinoerabujima Volcano shifted from magmatic activity to phreatomagmatic activity. Weak glowing of the crater was observed during the magmatic activity, indicating the volcanic conduit was hot enough to dry up the subvolcanic hydrothermal system. The following phreatomagmatic activity indicates that the hydrothermal fluid recharged after the magmatic eruption phase. Recharge of the hydrothermal fluid likely caused the variation of the eruption style, and is a process that may control the evolution of hazards during future eruption scenarios at similar active volcanoes in Japan and worldwide.