Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +.1ῌ+/,Letter

A Phreatic Explosion after AD +00- at the Hiyoriyama Cryptodome,Kuttara Volcano, Southwestern Hokkaido, Japan
Yoshihiko GOTOῌ, Hirotaka SASAKIῌ, Yoshimasa TORIGUCHIῌ and Akira HATAKEYAMAῌ
(Received October ,1, ,*+* ; Accepted June ,,, ,*++)
Hiyoriyama is a Quaternary dacite cryptodome located in Kuttara Volcano, southwestern Hokkaido, Japan.
The cryptodome is -/*῍//* m across and +-* m high, with an explosion crater at the summit. Here we report ona small-volume, phreatic fall deposit (the Hy-a deposit) erupted from the crater. The deposit consists mainly offresh to altered, dacitic lithic clasts (up to -* cm across) in a ﬁne-grained matrix. The petrological features of thedacite are identical to those of rocks within the wall of the crater. The deposit increases in thickness andmaximum grain size toward the crater, suggesting it was erupted from the crater. The Hy-a deposit overlies theUs-b tephra, which was deposited in AD +00-. The lithology, distribution, and stratigraphy of the Hy-a deposit
suggest that a phreatic eruption occurred after AD +00- at the summit of the cryptodome, resulting in formationof the crater.
Key words : phreatic eruption, explosion crater, Hiyoriyama Cryptodome, Noboribetsu Geothermal Field, Kuttara
Volcano+. Introduction
Hiyoriyama is a Quaternary dacite cryptodome
located in the northern part of the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan (Fig.
+). The cryptodome has a small explosion crater at thesummit, suggesting that a minor eruption occurred
during or after emplacement of the dome (Fig. ,). No
previous study has investigated the timing of the eruption, and the formation age of the crater is unknown.
This paper describes the distribution, lithology, andstratigraphy of a small-volume, phreatic fall deposit
associated with the crater, and discusses the timing ofthe crater-forming eruption.
,. Hiyoriyama Cryptodome
The Hiyoriyama Cryptodome is located in the westernpart of Kuttara Volcano (Fig. +). The volcano consists
mainly of an andesitic stratovolcano (elevation, /.3 m
above sea level) with a small caldera at the summit(Lake Kuttara). The volcano evolved over the period
2*῍./ ka, involving early silicic explosive activity andsubsequent stratovolcano building associated with caldera collapse at .* ka (Katsui et al., +322 ; Yamagata,+33. ; Moriizumi, +332 ; Moriya, ,**-). The Noboribetsu Geothermal Field is inferred to have formed after
the collapse of the caldera (Katsui et al., +322). The
῏College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,Mizumoto-cho ,1῍+, Muroran, Hokkaido */*῍2/2/,Japan.

geothermal ﬁeld is approximately + km wide (northeastsouthwest) and +./ km long (northwest-southeast).
The Hiyoriyama Cryptodome is elliptical in plan view,
ranging in diameter from -/* m (northeast-southwest)
to //* m (northwest-southeast). It rises +-* m above
the surrounding area, with the highest point being -11 m
above sea level (Fig. ,A). The surface of the cryptodome is covered with sediments up to +/ m thick(Katsui et al., +322). Fission-track dating yields ages
for the dome of +/ῌ. ka and +.ῌ. ka (Goto andDanhara, ,*++). An explosion crater at the summit(Hiyoriyama Summit Crater ; Fig. ,B) is .*῍3/ m insize (elongate northwest-southeast) and ,* m deep, andcontains active fumaroles. The crater retains its primary morphological features, including the crater rim
and crater wall.
The Hiyoriyama Cryptodome consists of coherent
dacite that is well exposed in the crater wall. The
dacite is grey and porphyritic, containing phenocrysts ofplagioclase (῎. mm long, ,+῍,/ vol.ῌ), quartz (῎/mm, 0῍2 vol.ῌ), hypersthene (῎, mm, .῍0 vol.ῌ),
trace amounts of augite (῎+ mm) and opaque minerals(῎*./ mm), and rare hornblende (῎*., mm) (Fig. A). The groundmass (0,῍00 vol.ῌ) is granophyric,containing silica minerals, feldspars, and opaque minerals of ῎*.+ mm in size. Table + lists the whole-rockCorresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp