Japan Geoscience Union Meeting 2016

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Oral

Symbol S (Solid Earth Sciences) » S-VC Volcanology

[S-VC48] Volcanic and igneous activities, and these long-term forecasting

Sun. May 22, 2016 1:45 PM - 3:15 PM Convention Hall A (2F)

Convener:*Teruki Oikawa(Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Takeshi Hasegawa(Department of Earth Sciences, College of Science, Ibaraki University), Daisuke MIURA(Geosphere Sciences, Civil Engineering Research Laboratory, Central Research Institute of Electric Power Industry), Yoshihiro Ishizuka(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Nobuo Geshi(Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Chair:Maya Yasui(College of Humanities and Sciences, Nihon University), Shohei NANRI(Laboratory of Geomorphology and Quaternary Geology, Department of Geography, Tokyo Metropolitan University)

2:00 PM - 2:15 PM

[SVC48-10] Two eruptive events occurred around 40 ka at the Akagi volcano in North Kanto, NE Japan: Eruptions of the Akagi-Kanuma and Akagi-Shimizu Lithic Tephras

*Shohei Nanri1, Takehiko Suzuki1 (1.Faculty of Urban Environmental Sciences, Tokyo Metropolitan University)

Keywords:Mt. Akagi, Plinian eruption, lithic tephra, Ag-KP, Ag-SLT

Mt. Akagi, located in the northern Kanto, Northeast Japan, is a large Quaternary stratovolcano. The Mizunuma Chert Lapilli Pumice (CLP) dominated by accidental lithic fragments had been reported by Moriya (1968). The aim of this study is to clarify (i) distribution, (ii) stratigraphic position, (iii) sedimentary structure, (iv) petrological features, (v) mineralogical features, (vi) volume and (vii) eruption style, for CLP in more detail. We found new features of CLP then, the deposit was re-defined as the “Akagi Shimizu Lithic Tephra (Ag-SLT)”. The Ag-SLT is distributed to eastern region from Mt. Akagi. The Ag-SLT exists on the Akagi Kanuma Pumice (Ag-KP), erupted at 44 ka. The Ag-SLT is composed of four units: 1L, 2P, 3P, 4L, with lithic fragments from Ashio Belt. Unit 1L has accretionary lapilli (φ13 mm) in its bottom layer. The yellow pumice clasts within the unit 2P has high SiO2 (77.5 - 80.0 wt.% : in major elements chemical composition in volcanic glass), compared to that in Ag-KP (76.0 -77.5 wt.%). Unit 3P is Plinian deposits with pumice and lapilli. Unit 4L is Phreatomagmatic eruption deposits. The Volume of Ag-SLT is about 6 km3 (VEI=5) as the same as the Hoei eruption of Mt. Fuji in 1707. The pumice within the Ag-SLT was formed with crystallization differentiation in magma reservoir. The interval of eruptions (dormant period) was suggested by the existence of the volcanic soil deposits (Tephric loess, so-called Loam) between Ag-KP and Ag-SLT.