[S-VC32] Active Volcanism

[SVC32-P17] Considering the fracture process causing the phreatic eruption from the internal structure of the ejected block from the 2014 eruption of Mt. Ontake

*Atsuko Namiki¹, Yui Kouketsu¹, Takahiro Kunitomo², Takeo Ito¹ (1.Graduate School of Environmental Studies, Nagoya University, 2.Ontake Science Lab)

Keywords:Mt. Ontake, Ejected block

Phreatic eruptions occur when groundwater heated by volcanic gases supplied from magma pressurizes the region, breaking up the surrounding rocks (host rock) (e.g., Stix and de Moor, 2018 ; Mannen et al., 2018). Alteration by hydrothermal activity could seal the voids in the host rock to increase the underground pressure. To understand how these sealing rocks are distributed underground and how they fracture to create a volcanic conduit, we analyzed ejected blocks from the 2014 eruption of Mt. Ontake. Ejected blocks sample the rocks in the deep conduit and bring them to the surface. Unlike volcanic ash, ejected blocks have large volumes, and it is possible to analyze their structure to decipher the underground processes. The ejected block analyzed in this study contains a few centimeter-sized dense blocks within a rough structure. Based on the results of thin-section observation, the rough structure is thought to be a mineral formed by hydrothermal activity, and the dense blocks are from a fractured clay layer. This structure suggests that part of the clay layer fractured before the eruption and was entrained into the region of hydrothermal activity. In that case, the fracture at depth occurs repeatedly before the eruption.