八ヶ岳火山の最新軽石噴火によるYt-Pm4テフラの放射性炭素年代

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  • <sup>14</sup>C Dating of the Yt-Pm4 Tephra, the Youngest Pumice Fall from the Yatsugatake Volcano, Japan
  • ヤツガタケ カザン ノ サイシン カルイシ フンカ ニ ヨル Yt-Pm4 テフラ ノ ホウシャセイ タンソ ネンダイ

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The Yatsugatake volcano located on the volcanic front in central Japan, has been active since the Middle Pleistocene, with many N-S trending eruptive centers. Little work has been previously performed on the age determination for the Late Pleistocene and/or Holocene age. Therefore, it is necessary to reconstruct the activity during that time duration. In this paper, I report a new AMS dating result of the Yt-Pm4 tephra, which is the youngest pumice fall from the Yatsugatake volcano. The pumice fall is actually from the Yokodake, one of the volcano located on the northernmost part of Yatsugatake. A 14C age of 30667±130 yr BP (32951-32333 cal BP: 2σ) was obtained from the charred wood collected at the uppermost part of the silty soil which is covered directly by the Yt-Pm4 tephra, suggesting that the eruption of the Yt-Pm4 tephra occurred at ca. 32 cal ka BP. The Yokodake consists of nine lava flows (Y1 to Y9, from oldest to newest). The stratigraphic position of Yt-Pm4 tephra is between Y5 and Y6. In addition, Okuno and Kobayashi (2010) estimated the eruptive age of Y9 lava flow to be 2.35-2.15 cal ka BP. This means that the younger four lava flows (Y6-Y9) erupted in 29.85-29.65 ky. The magma discharge rate at Yokodake, including the volume of Yt-Pm4 tephra, is calculated to be ca. 0.0102 km^3/ky. It is emphasized, based on the age determination, that small-scale eruptive activities have been continuous in the northern part of Yatsugatake volcano even during Late Pleistocene and Holocene.

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  • 火山

    火山 60 (4), 477-481, 2015

    特定非営利活動法人 日本火山学会

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