Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +0+ῌ+01

LetterRadiocarbon Dating of the Minamidake Debris-Avalanche Deposit,Shiretoko-iozan Volcano, Eastern Hokkaido, Japan
Yoshihiko GOTOῌ, Nobuo GOUCHIῌῌ and Isao MATSUDAῌῌ
(Received March 1, ,*++ ; Accepted August ,0, ,*++)
Mt. Shiretoko-iozan is a Quaternary andesitic composite volcano located on the Shiretoko Peninsula, eastern
Hokkaido, Japan. An amphitheatre at the summit and a debris-avalanche deposit (Minamidake Debris-AvalancheDeposit) at the western foot of the volcano suggest that sector collapse occurred during its eruptive history. Threetrenches (0.2, 1.-, and .., m deep) were dug at the western terminus of the Minamidake Debris-Avalanche Deposit
in order to date the sector collapse event. The stratigraphic section consists of (from lower to upper) : theSashiruidake Lower Lava, a debris-ﬂow deposit, a buried soil, a debris-ﬂow deposit, a buried soil, the Minamidake
Debris-Avalanche Deposit, a buried soil, the Ta-c tephra, and the surface soil. Radiocarbon dating of a buried soil
located immediately beneath the Minamidake Debris-Avalanche Deposit yields an age of -1.*ῌ.* years BP (.,-*ῌ.+3* and .+3*ῌ-32* years cal BP), suggesting that the sector collapse occurred at ca. . ka.
Key words : debris-avalanche deposit, radiocarbon age, sector collapse, Shiretoko-iozan Volcano, trench survey+. Introduction
Mt. Shiretoko-iozan is a Quaternary andesitic composite volcano located on the Shiretoko Peninsula, eastern Hokkaido, Japan (Fig. +). An amphitheatre at thesummit (the Minamidake Crater ; Katsui et al., +32,)
and a debris-avalanche deposit at the western foot of thevolcano (the Minamidake Debris-Avalanche Deposit ;Katsui et al., +32,) suggest that sector collapse occurred during its eruptive history, although the timing of
this event remains unknown. We conducted a trench
survey and performed radiocarbon dating of a buried
soil located immediately beneath the debris-avalanche
deposit to clarify the timing of the sector collapse event.
,. Shiretoko-iozan VolcanoShiretoko-iozan Volcano (elevation of +/0- m above
sea level, base diameter +* km) is the largest Quaternarystratovolcano on the Shiretoko Peninsula (Fig. ,). The
volcanic ediﬁce consists mainly of andesitic lavas (Katsuiet al., +32,) that yield KῌAr ages of *.,.ῌ*.*-, *.+0ῌ*.*,, and *.*/ῌ*.*. Ma, suggesting stratovolcanobuilding activity began at ca. *.,. Ma and continued formore than ,**,*** years (Goto et al., ,***).
An amphitheatre (, km across) occurs just south of
the summit and opens to the west (Fig. ,). The
Minamidake Debris-Avalanche Deposit (Katsui et al.,+32,) extends from the amphitheatre to the western
῍College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1ῌ+, Muroran, Hokkaido */*ῌ2/2/, Japan.
῍῍Shiretoko Museum, Hon-machi .3, Shari-cho, Hokkaido

foot of the volcano, and details of the deposit aredescribed in section -. Katsui et al. (+32,) suggested
that a violent explosion triggered collapse of the northwestern part of the volcano summit and that the subsequent debris avalanche cascaded down the northwest
ﬂank.
The base of the amphitheatre is partly ﬁlled with theNamakoyama Lava Dome and the Nanpo Lava. The
Namakoyama Lava Dome, located in the middle part of
the amphitheatre, is +*** m across, ,,* m thick, andgrades to the west into a lava ﬂow that extends for .***m (Namakoyama Lava, Fig. ,). The Nanpo Lava,
located on the northern rim of the amphitheatre, comprises three lava lobes extending to the northeast, west,
and south, respectively, over distances of 1**ῌ+/** m.
No geochronological data have been reported for theNamakoyama Lava Dome or the Nanpo Lava.
The volcano is active, with recorded eruptions occurring in +2/1, +210, +223, and +3-/ (Katsui et al., +32,).
These historic eruptions took place at a parasitic crateron the northwestern ﬂank of the volcano (Fig. ,) andwere characterized by phreatic explosions with extrusions of large amounts of sulphur (Watanabe, +3.*).
-. Minamidake Debris-Avalanche Deposit
The Minamidake Debris-Avalanche Deposit is 0 km
long (trending EῌW), - km wide (NῌS), up to -* m*33ῌ.++-, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp