[S-VC32] Active Volcanism

[SVC32-07] Discharge rate, component and chemical composition of ash-fall deposits during the 2019–2020 magmatic activity at Nakadake first crater, Aso Volcano, Japan

*Yasuo Miyabuchi¹, Yoshiyuki Iizuka², Futa Ennyu³, Takahiro Ohkura⁴ (1.Center for Water Cycle, Marine Environment and Disaster Management, Kumamoto University, 2.Institute of Earth Sciences, Academia Sinica, 3.Chosei Junior High School, 4.Aso Volcanological Laboratory, Institute for Geothermal Sciences, Graduate School of Science, Kyoto University)

Keywords:erupted ash mass, ash component, small ash emission, Nakadake first crater

The 2019–2020 magmatic eruption at the Nakadake first crater, Aso Volcano in southwestern Japan was characterized by continuous gentle ash emissions. We present the distribution, discharged mass, and component and chemical composition of the ash-fall deposits to examine the sequence and characteristics of successive small ash emitting activities. We installed 20 ash samplers around the crater (1 km to 9 km of the crater in all directions) and calculated the masses of ash-fall deposits based on 38 isomass maps. From July 26, 2019 to January 2020, the cumulative erupted mass increased at a moderate discharge rate (3.5×10³ tons/day). The rates abruptly increased to 17×10³ tons/day during January 20–February 10, 2020. After late February 2020, it decreased to a low rate (0.5×10³ tons/day). The 2019–2020 ash-fall deposits consisted of glass shards, crystal, and lithic grains. In the July 26, 2019 ash-fall deposit, lithic fragments, which are interpreted to be derived from lavas or pyroclasts of previous eruptions, were dominant (70%). Thereafter, the proportion of glass shards, which are suggestable juvenile materials of newly ascending magma, gradually increased with time, and the August 12–20, 2019 ash contained abundant glass particles (40%). The proportions of glass shards ranged from 7% to 23% until December 6, 2019. Subsequently, it is continuously decreasing, then falling to 2% on February 10, 2020. From May 21, 2020, the proportions increased again and ranged between 29% and 41% until June 15. There is no obvious temporal change in chemical compositions of glass shards included in July 2019–June 2020 ash-fall deposits. The total mass of erupted tephra from the July 2019–June 2020 activity of the Nakadake first crater was 1.1×10⁶ tons (3.5×10³ tons/day), which corresponded to a half of that of the November 2014–May 2015 activity (2.1×10⁶ tons).