[S-VC48] Volcanic and igneous activities, and these long-term forecasting

[SVC48-P18] Foaming temperature and textural classification of vesicular substance by heating experiments of obsidians

*Keiji Wada¹, Shunya Tsurumaki¹, Ryo Ikeyauchi¹, Kyohei Sano², Eiichi Sato³ (1.Earth Science Laboratory, Hokkaido University of Education at Asahikawa, 2.Shirataki-Geopark Promotion Department, Engaru town, 3.Institute for Promotion of Higher Education, Kobe University)

Keywords:obsidian, perlite, heating experiment, foaming temperature, volcanic glass

Obsidian includes H₂O component in the glass. The H₂O component is vesiculated when heated at a high temperature, and their foams surrounded by dense obsidian glass expand to porous material, referred to as “perlite”. We have carried out heating experiments with an electric furnace for 11 obsidian samples from each different locality to determine the foaming temperature (Tf) and perlite-formation temperature (Tp) of their obsidians. The results are as follows; Akaigawa obsidian Tf=780℃; Tp=830℃, Okushiri obsidian Tf=790℃; Tp=850℃, Kozushima obsidian Tf=890℃; Tp=950℃, Shirataki obsidian (IK outcrop) Tf=900℃; Tp=1030℃, Tokachi-Mitsumata obsidian Tf=930℃; Tp=1060℃, Oketo obsidian (Tokoroyama) Tf=990℃; Tp=1100℃, Oketo obsidian (Kita-Tokoroyama) Tf=1010℃; Tp=1090℃, Shirataki obsidian (Tokachi-Ishizawa outcrop) Tf=1030℃; Tp=1160℃, Shirataki obsidian (Kyukasawa outcrop) Tf=1060℃; Tp=1150℃, Shirataki obsidian (Nishi atelier) Tf=1070℃; Tp=1190℃, Shirataki obsidian (Ajisainotaki outcrop) Tf=1070℃; Tp=1190℃.
Perlite texture was classified into three types (type-A, -B and -C) based on the diameter, morphology and number density of vesicles. Type-A has discrete spherical forms with about 1 mm in diameter, which belongs to Tf>990℃, Tp>1060℃ group. Type-B, Tf=900-930℃, Tp=1030-1060℃ group, has distorted spherical forms with 1.5 to 5.0 mm in diameter. Type C is characterized by high number density and small size of vesicles (< 0.5 mm) connecting with each other, belonging to low Tf (<890℃) and Tp (<950℃) group.
The texture of perlite made by obsidian heat experiment is different from that of natural vesicle substance such as pumice and vesiculated obsidian. Lower number density of vesicles in the experimental product perlite is due to low H₂O content in quenched obsidian after degassing of H₂O in obsidian magma. The difference in vesicle morphology between the spherical form of perlite and reticulated irregular-shaped vesicle of natural vesicle substance is related to the difference in the static or dynamic condition of formation environment.