[S-VC32] Dynamics of volcanic eruptions and their physical and chemical processes

[SVC32-02] The systematics of volcanic ash and transitions in eruption styles - in the 1813 eruption at Suwanosejima volcano -

*Taketo Shimano¹ (1.Graduate School of Environment and Disaster Research, Tokoha University)

Keywords:volcanic ash, micro-spectroscopic colorimetry, Suwanosejima

Volcanic ash is ubiquitus to all eruptions. As well as small explosive eruptions like strombolian and vulcanican types, ash emission usually occurs at the beginning of large ones and is accompanied even by lava effusion sometimes. During some large explosive eruptions, ash may only be available samples due to loss of accessibility into the proximal area for safety reasons. In addition, as some small particles are readily quenched after emission at vent or at fragmentation level of magma, they may preserve original information better on what we concern in contrast to larger particles or lava samples which would have been affected severely by several secondary processes. So far at volcanoes with small eruptions, the changes in the constituent ratio (or componentry) of ash particles have been reported to reflect eruption styles, and thus by ascent processes in conduit due to changes in crystallinity, vesicularity, etc. of magma. Other small particles, on the other hand, are referred to be deeply oriented to shallow vent conditions such as hydrothermal alteration, degassing induced crystallization, etc. Such particle types may provide depth, temperature, or oxidation state as well as indices for the timing of vent opening or widening. Thus, as transitions in eruption sequence can occur both by changes in magmatic and surrounding conditions, the systematics of volcanic ash is really important for monitoring eruption sequence, but the classification standard for the componentry analysis depended on decision of each investigator and has not been shared by each other on quantitative basis.
The 1813 eruption at Suwanosejima volcano, SW Japan, occurred in continuous manner from mild ash emission activity like recent activity, but repetitive injections by mafic magma finally resulted in sub-plinian phase (Shimano and Koyaguchi, 2001; Shimano et al., 2021JpGU). The ash componentry and glass composition changed systematically from the bottom to the upper layers, but both changes are simultaneous so that the crystallinity range within one type also changed temporally. To prepare for the future eruption, quantitative analytical systematics would help understanding temporal changes in magma properties and eruption sequence by catching such complex changes in ash componentry and glass composition. Shimano et al. (2021; VSJ Fall meeting) reported that visible micro spectroscopic colorimetry with some simple morphological analysis has been successful in describing ash particles of the 2018 eruption at Shinmoe-dake, Kirishima volcano. The recommended criteria for discrimination were 1) the average intensity of reflection in the range from 450-700 um of wavelength, 2) the spectral shapes or types which is normalized by the average above, and 3) shape parameter like circularity or solidity. These three values are quantitatively available and can be comparable among investigators in different laboratories. This study is the report of this method adapted for the 1813 ejecta at Suwanosejima volcano.
The result of visible micro spectroscopy on the samples of Suwanosejima volcano showed similar differences in the average intensity and in spectral type with those at Shimnoe-dake, but some of the differences are less significant compared with the range of variation in intensity for each ash type. Shape parameters show similar results, as a whole, indicating the importance of quantitative thresholding for the componentry analysis to monitor eruptive sequence.