[S-VC36] Volcanic and igneous activities, and these long-term forecasting

[SVC36-P10] Volcaniclastic deposits distributed at the southern foot of Garan-dake Volcano, Japan

*Hayato Oji¹, Takeshi Saito² (1.Department of Science Graduate school of Science and Technology, Shinshu University , 2.Institute of Science, Academic Assembly school of science and technology, Shinshu University)

Keywords:Garandake, Radioactive dating, Pyroclastic material

Garan-dake volcano in central Kyushu, Japan, is an active volcano located at the northernmost part of the Yufu-Tsurumi volcano group (Kobayashi, 1984). Although its body is not so large (1045 m above sea level), strong fumarolic activities have continued (Ohsawa et al., 1996) and Garan-dake is the most active volcano in the Yufu-Tsurumi volcano group. Garan-dake volcano consists of mostly Garan-dake lava, which was effused at slightly older than 10.5 cal ka BP (Fujisawa et al., 2002). Several volcaniclastic deposits and widespread tephras were distributed at the southern slope of the volcano (Tsutsui et al., 2019). More than ten tephra layers (T-Ga2~T-Ga0) derived from Garan-dake were deposited at the southern slope in addition to Kikai-Akahoya tephra (K-Ah), Nakazuru tephra (T-Nzf) and Yufu-dake tephra (YA1). Eruption age of T-Ga2 was estimated to be about 1300 yBP and Garan-dake tephras were considered to be erupted during the last two thousand years. There were no volcanic activities during the effusion of the Garan-dake lava and T-Ga2 tephra and there was no report about pyroclastic flow eruption. In this study, we carried out geological surveys in order to reevaluate the activity of Garan-dake volcano and identified unreported volcaniclastic deposits. At the 500 m south of Garan-dake summit, one massive volcaniclastic deposits (T-GaP) with a thickness of about 150 cm was observed below the T-Nzf. At the base of the deposits, small carbonized wood fragments were found. The radiocarbon age of the wood fragments is 12.1±0.15 cal ka BP. Lava clasts in T-GaP contain abundant amphibole phenocrysts which did not suffer decomposition or breakdown. Amphibole in Garan-dake lava does not suffer decomposition, while that in deposits from Tsurumi volcano suffer partial or complete decomposition (Nagasaki et al., 2017). These infer that pyroclastic flow eruption occurred at about twelve thousand years ago at Garan-dake volcano. In addition, Garan-dake lava seems to be located below T-GaP, suggesting the eruption of Garan-dake lava is older than twelve thousand years ago, this value is older than previous reported age. White colored volcanic ash layers (T-GaW) with a thickness of about 30 cm were found at another outcrop. T-GaW is unconformably cover K-Ah and consists of seven thin tephra layers, which suffered strong alteration. The radiocarbon age of paleosol immediately below and above T-GaW is 6700±50 and 6250±50 cal BP, respectively. It is suggested explosive eruption occurred at about 6500 years ago.