[S-VC29] Volcanic and igneous activities, and these long-term forecasting

[SVC29-P03] Holocene tephra ejected from Mitsudake Volcano of Nikko Volcano Group, NE Japan

*Yuki Kusano¹, Takahiro Yamamoto¹, Teruki Oikawa¹, Yoshihiro Ishizuka¹, Yoichi Nakamura² (1.Geological Survey of Japan, AIST, 2.Utsunomiya University)

Keywords:Mitsudake Volcano, Phreatomagmatic eruption, Holocene tephra

Mitsudake Volcano is one of the mountains in Nikko Volcanoes (Yamasaki, 1958), Tochigi prefecture. It is located at northern part of Senjogahara Wetland and east of Lake Yunoko. A topography of Mitsudake lavas is sharp but the age was unknown other than the lavas are covering Ryuzunotaki Pumice Flow Deposit which was erupted 17 ka (Sasaki et al., 1993; Hirayama and Nakamura, 1995). We conducted four trench surveys and observation of a drill core and revealed that Mitsudake Volcano is erupted in Holocene based on the radiocarbon dating of the overlying paleosol and a sliver in the underlying river deposits (Kusano et al., 2020 VSJ Meeting). Also, we found a tephra layer (Mt-1) accompanied with Mitsudake Volcano but the stratigraphic relationship with the lavas is still uncertain.

Mt-1 is a 35 cm-thick whitish gray ash layer with a lapillus part at the top. The lapillus part consists of well-vesiculated gray scoriae and accidental volcanic and plutonic rocks. Phenocryst assemblage and whole-rock composition of these scoriae are similar to a mafic enclave in Mitsudake lavas. The ash part of Mt-1 contains 1 to 10% of vesicular volcanic glass and thick platy to flaky glass particles and the volcanic glass content increases upward. Geochemical composition of these volcanic glass particles suggests that the upper part consists of a fractionated material of Mitsudake lava, and the lower part consists of a mixed material of Mitsudake lava and the mafic enclave. Temporal increase of volcanic glass content and scoria in Mt-1 indicates that Mt-1 was formed by phreatomagmatic eruption and could precede an eruption of some Mitsudake lava.