「火山」2004，1，1-12
八ヶ岳火山を起源とする新期テフラ群の層序と噴火史
大石雅之・鈴木毅彦
Tephrostratigraphy and Eruptive History of the Younger Tephra Beds from the Yatsugatake Volcano, Central Japan
Masayuki OISHI and Takehiko SUZUKI
【Abstract】
The north-trending Yatsugatake Volcanic Chain (YVC) spans approximately
20 km on the volcanic front in central Japan. At the eastern foot of the
YVC, five pyroclastic fall deposits derived from the YVC in the younger
period from about 0.2 Ma to the present can be identified. To clarify the
eruptive history of the YVC, this study describes these pyroclastic fall
deposits and infers their ages, the intervals of pyroclastic eruptions,
volumes, eruption rate in the younger period, and the eruption sources
of the tephra beds.
The Yatsugatake Younger Tephra Beds consist of five tephra beds as
follows: Yt-Kw, Yt-Pm1, Yt-Pm2, Yt-Pm3, and Yt-Pm4, in ascending order.
The tephra beds consist of pumiceous lapilli deposited during Plinian eruptions
and, except for Yt-Pm4, are inter-layered with fine gray ash layers, probably
produced by eruption with more strong explosive power (Vulcanian eruption
or phreatmagmatic eruption). Based on the stratigraphic relation of these
deposits to well-dated tephra layers from distant sources, and the thicknesses
of intercalated loess deposits, it is estimated that the eruptions that
created Yt-Kw, Yt-Pm1, Yt-Pm2, Yt-Pm3, and Yt-Pm4 occurred at approximately
171 ka, 86-66 ka, 66 ka, 56 ka, and 29 ka, respectively. Therefore, eruptions
produced pyroclastic fall deposits that occurred with an interval of 85-105
ky between the Yt-Kw and Yt-Pm1 eruptions, and intervals of 10-27 ky, in
general, thereafter. Eruption volumes are estimated as approximately 0.6
km3, 0.2 km3, 0.2 km3, and 0.2 km3 (DRE) for Yt-Kw, Yt-Pm1, Yt-Pm3, and
Yt-Pm4, respectively. Calculations reveal that the YVC has discharged about
0.007 km3 DRE/ ky of pyroclastic fall products from ca. 171 ka to the present.
On the other hand, lava and pyroclastic flow discharges amount to about
0.3 km3 DRE/ ky.
Thickness variations indicate that Yt-Pm4 is derived from Yokodake
Volcano, in the north of the YVC. Since several lava flows cover Yt-Pm4,
the Yokodake Volcano must have had several magmatic eruptions after 29
ka to the present. Yt-Kw, Yt-Pm1, Yt-Pm2, and Yt-Pm3 derived from the center
of the YVC. The stratigraphic relationships of these tephra beds to the
deposits distributed on the summit area are unknown, but since two lava
flows distributed around Tengudake (Tengudake Middle Lava Flow and Tengudake
Upper Lava Flow) are dated to the almost the same time as Yt-Pm2 and Yt-Pm3,
these tephra beds and lava flows may be correlated.
【Key words】
Yatsugatake Volcanic Chain, Younger Tephra Beds, Tephrochronology, Late Quaternary, Eruption History
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