Eruptive history of Fuji Volcano from AD 700 to AD 1,000 using stratigraphic correlation of the Kozushima-Tenjosan Tephra

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  • 神津島天上山テフラの同定とその層位に基づく富士火山の西暦700 ～1,000 年の噴火史
  • Eruptive history of Fuji Volcano from AD 700 to AD 1,000 usings stratigraphic correlation of the Kozushima-Tenjosan tephra
  • Eruption history of Fuji volcano from AD 700 to AD 1,000 using stratigraphic correlation of the Kouzushima-Tenjosan Tephra

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Abstract

We analyzed refractive index, microstructure, vesicularity of glass from an alien tephra in the upper weathered volcanic ash soil horizon of Fuji Volcano, Izu peninsula, Miyakejima, and Niijima Volcanoes. The glass size increases toward Izu Islands. Based on the vesiculality of glass, the Kozushima-Tenjosan tephra (AD 838) can be distinguished from the Niijima-Mukaiyama tephra (AD 886). Thus, we determined from where the tephra of low refractive index on the flank of Fuji Volcano comes from. Next, using the stratigraphic correlation of the tephra, the detailed stratigraphy of Fuji Volcano during the period of AD 700-1,000 was clarified. It was difficult to determine exact age of each eruption by 14C dating with 2 sigma error bar (around ±40 years). However, the KozushimaTenjosan tephra horizon can classify Fuji Volcano products those below the tephra and those above the tephra. The Obuchi scoria (OBC), the Kansuyama scoria (KNS), the Nishi-Futatsuzuka scoria on S and SE flanks, the Takamarubi lava flow (Tam) and the Hinokimarubi 2 lava flow (Hnm 2), the scoria associated with the Oniwa-Okuniwa 2 lava flow (Onw 2) on NE and N flanks are below the Kozushima-Tenjosan tephra. The Fudosawa lava flow (Fud), the Obuchimarubi lava flow (Obu), the Higashi-Usuzuka south lava flow (Hum), the Mizugatsuka-hinokimarubi lava flow (Him) on S and SE flanks, the Kenmarubi 2 lava flow (Ken 2), the Tenjin-Igatono scoria (TNG), the Aokigahara lava flow and the Nagaoyama scoria on the NE and N flanks are above the Kozushima-Tenjosan tephra. The Kozushima-Tenjosan tephra was discovered just below the S-24-6 scoria on E flank. The stratigraphy of E flank is rearranged as follows in ascending order: the S-24-5 scoria, the S-24-5-1 scoria (newly defined), the S-24-5-2 scoria (newly defined), the S-24-5-3 scoria (newly defined), the Kozushima-Tenjosan tephra, the S-24-6 scoria, the S-24-7 scoria, the S-24-9 scoria, the S-24-10 scoria.

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