[S-VC43] Volcanic and igneous activities, and these long-term forecasting

[SVC43-P05] Phreatomagmatic explosion history and petrographical characteristics of Izu-Oshima at southeast

*Yuta Shinomiya¹, Hayato Ishizuka¹, Shunsuke Maezawa¹, Sho Yatsuka¹, Izumi Sakamoto¹, Takumi Nakaza², Yoshihiro Murachi³ (1.The Graduate School of Marine Science & Technology, 2.DAIYAKONSARUTANTO Co. , Ltd , 3.KAWASAKICHISHITSU Co. , Ltd )

Keywords:phreatomagmatic eruption, essential material, accessory material, basement, matrix, breccia

Izu-Oshima Volcano is northernmost frontal arc volcano of the Izu-Bonin arc. The basement of Izu-Oshima is made up of three volcanos, named Okata, Gyojanoiwaya and Fudeshima Volcanos. The volcano is divided into three groups; the Senzu Group, the Older Oshima Group and the Younger Oshima Group in ascending order. Large eruption has occurred in about 150 years cycle, which include phreatomagmatic explosion around coast.

We report stratigraphic and petrographic study of the volcanic products of southeastern of Izu-Oshima volcano, especially around northeastern part of Habuminato and southwestern part of Habuminato.
Northeastern part of Habuminato
L3 lava, L2 lava, Kotoshi tuff, Ooyanokubo pyroclastic surge deposits, L1 lava, Kakihara tuff, Sand and Scoria in ascending order.

Kotoshi tuff contain accessory rock fragments of Ryuo-zaki lava (Pl: 3.3 vol.%, Mg#=44). Kakihara tuff is characterized by containing accretionary lapilli. Rock fragments are Fudeshima volcanic rocks in origin (Pl: 19.6-39.1 vol. %, SiO₂=49.45-50.03 wt.%).
Southwestern part of Habuminato
Shimobara lava (the Old Oshima Group), Silt, Scoria, Habu breccia (N3), Scoria & ash, and Imasaki breccia (Y4) in ascending order.

Habu breccia contain accessory rock fragments of Ryuo-zaki lava (Pl: 1.0-10.8 vol.%, Mg#=44) and Fudeshima volcanic rocks (Pl: 23.2-28.0 vol.%, SiO₂=50.8-51.7). Imasaki breccia contain not only accessory rock fragments of Shimobara lava (Pl:0.2-9.3 vol.%, Mg#=39) but essential rock fragments of Y4 stage lava (Pl:0.3-0.9 vol.%, Mg#=40).