[1oral601-08] T2. From magma source and magma plumbing system to volcano formation and hydrothermal alteration

9:15 AM - 9:30 AM JST(12:15 AM - 12:30 AM UTC)

[T2-O-1] Petrological and mineralogical characteristics of low-resistivity zones at a site of phreatic eruption: a case study from Iwo-yama, Kirishima Volcano

*Yuki Kusano¹, Takumi Imura², Ryuta Furukawa¹, Takahiro Yamamoto¹, Yoshihirp Ishizuka¹, Jun’ichi Itoh¹, Yoshio Matsushima¹, Daisuke Asahina¹, Keiichi Sakaguchi¹ (1. Geological Survey of Japan, AIST, 2. Yamagata University)

Keywords:

Iwo-yama,Kirishima Volcano,impermeable layer,drilling,Phreatic eruption,low-resistivity zone

　Phreatic eruptions are often associated with the development of low-resistivity geological structures, known as cap rocks, within shallow hydrothermal systems. This study investigates such structures at Iwo-yama in the Kirishima volcanic group using electromagnetic resistivity surveys and core drilling. A borehole (GSJ24-KI-1) was drilled to a depth of 275 m at a site 550 m north of Iwo-yama, targeting low-resistivity zones indicative of hydrothermal alteration.The core composed of andesitic lava flows and pyroclastic deposits, which were previous volcanic edifice, lava flows from the Karakunidake volcano and the Karakunidake debris avalanche materials (Imura and Kobayashi, 2001, Geological map of Kirishima Volcano, Geological Survey of Japan), upward. Hydrothermal alteration was categorized into three zones: dark gray sulfuric alteration (above 30 m), gray clay-silicified alteration (30–275 m), and white clay-silicified alteration (below 120 m). Especially gray clay- silicified zones contain minerals such as cristobalite, quartz, pyrite, and smectite.Hydrothermal alteration was most intense in porous pyroclastic deposits, suggesting selective alteration due to acidic hydrothermal fluids. Dense lava flows likely acted as barriers, enhancing alteration efficiency beneath them. The resistivity structure obtained from geophysical surveys aligns well with laboratory measurements of core samples, confirming the presence of cap rock formations. Future work will focus on refining the alteration sequence and correlating it with physical properties to better interpret subsurface volcanic activity.This study was funded by the MEXT FY2024 project.