[S-VC31] Active Volcanism

[SVC31-P01] Magma Ascent process of caldera-forming eruption based on the textural analysis for pumice deposits from 7.6ka Mashu eruption

*Kyohei Sano¹, Keiji Wada², Eiichi Sato² (1.Graduate School of Regional Resource Management, University of Hyogo, 2.Earth Science Laboratory, Course of Subjects Education, Graduate School of Education, Hokkaido University of Education at Asahikawa)

Keywords:Masyu volcano, cardera forming eruption, textural analysis, pumice

Caldera forming eruption has a huge influence on human society and it is important to reveal the sequence of magma ascent process during the eruption. Approximately 7.5 ka, caldera-forming activity of Mashu Volcano started as phreatomagmatic eruption (Ma-j) and followed plinian falls (Ma-i, -h, -g) and pyroclastic flow (Ma-f). In this study, we performed the textural analysis of pumice in Mashu caldera-forming activity and discuss the magma ascent process during the caldera-forming eruption.
We conducted the field survey and collected samples for textural analysis. We sieved the samples and picked up the pumices larger than 2φ for the textural analysis. Samples were washed by a sonic wave washer and dried at 110℃ and were used for the thin section with a polished surface. The scanning electron microscope images were captured by a JEOL JXA-8530FPlus FE-EPMA at the University of Hyogo. The accelerating voltage of 15 keV with a 10 nA beam current was used for the analysis. Vesicularity of pumice in the Ma-i layer ranges from 60 % to 90 % and bubble number density ranges from 2.8×10 to 1.4×10⁴ [no./ mm²]. Especially the increase of vesicularity and decrease of bubble number density at the end of the Ma-i eruption were observed. We discuss the magma ascent process of caldera-forming eruption based on the textural analysis for pumice.