11:15 AM - 11:30 AM
[SVC38-32] Identification of submarine volcanic ash around the Kikai caldera with the Koya pyroclastic flow deposit inferred from glass composition
Keywords:Kikai caldera, Koya pyroclastic flow
The ROV conducted four dives to sea floor where MCS data indicate that uppermost unit is distributed near sea floor around the caldera wall and one dive in the south of the Satsuma Peninsula and collected volcanic ash on the sea floor using nozzle. The chemical composition of glass shards (<200 micrometers) from both subaerial and submarine were measured by Electron Probe Micro Analyzer (EPMA).
The glass shards on the sea floor around the caldera wall show bimodal geochemical characteristic which contain a high proportion of high SiO2 glass shards (SiO2=72-78 wt.%) and lesser amount of low SiO2 glass shards (SiO2=64-68 wt.%). The low SiO2 glass was reported by Fujihara and Suzuki-Kamata (2013) and confirmed that it is unique characteristic for the Kikai-Akahoya eruption by glass and whole rock composition of other volcanics of the Kikai caldera. This indicate that the sea floor around the Kikai caldera covered by volcanic ash derived from the Koya PFD. Samples from the sea floor in the south of the Satsuma Peninsula were also confirmed that they contain small amount of glass shard corresponding the Koya PFD. These results correspond to the MCS data showing that candidate layer for the Koya PFD is thickly and widely distributed at uppermost part around the Kikai caldera. The vertical variation for contents of low SiO2 glass on Takeshima Island, which is proximal area of the Koya PHD with a thickness of ~20 m, shows that the content of low SiO2 glass lacks at the basal level and increases upward and reaches >10% at early stage (~10% level of the total thickness). While, the contents of low SiO2 glass at distal areas across the sea tend to increase upward to <5%, which are lesser than value of proximal area. The contents of low SiO2 glass of the sea floor samples around the caldera rim are >10%. Ash size grains are considered to be less under the influence of segregation by difference of density (Nakaoka and Suzuki-Kamata, 2018). Thus, the Koya pyroclastic flow reached distal area across the sea between the start of eruption and the beginning of containment of low SiO2 glass and was waning to not reach distal area and deposit on sea floor after the content of low SiO2 glass increases to >10%.