Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. . · /, pp. +.1ῌ+/,LetterA 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 the
Us-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 of
the 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

148

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

Fig. +. Location of the Hiyoriyama Cryptodome in theNoboribetsu Geothermal Field, Kuttara Volcano,southwestern Hokkaido, Japan.

Fig. -. Photomicrographs of dacite from the wall of
the summit crater of the Hiyoriyama Cryptodome(A) and from the Hy-a deposit (B). Crossed
nicols. Abbreviations : Qz, quartz ; Pl, plagioclase ;Hy, hypersthene.

major element chemical composition of the dacite(sample numbers Nb-2-a and -2-b), which contains 1*
wt.ῌ SiO,, -./ wt.ῌ Na,O, and +./ wt.ῌ K,O.

Fig. ,. Photographs of the Hiyoriyama Cryptodome.
(A) The Hiyoriyama Cryptodome viewed from thesoutheast. (B) Explosion crater at the summit of
the cryptodome (Hiyoriyama Summit Crater).
Active fumaroles are present within the crater.

-. Phreatic deposit-ῌ+ Nomenclature
The newly identiﬁed phreatic fall deposit occurs at
the rim of the Hiyoriyama Summit Crater and in theadjacent area (Fig. .). It occurs about +* cm beneaththe ground surface and was found within an excavatedtrench (Fig. /). In this paper, the deposit is referred
to as the ‘Hiyoriyama-a phreatic deposit’ (Hy-a deposit). The type locality of the deposit is on the northeastern rim of the crater. The Hy-a deposit di#ers fromthe Shinki-Jigokudani pyroclastic fall deposit reported
by Katsui et al. (+322). The relation between theseunits is discussed below.
-ῌ, Description
The Hy-a deposit is pale brown, massive (nonstratiﬁed), matrix supported, poorly sorted, and com-A Phreatic Explosion after AD +00- at the Hiyoriyama Cryptodome, Kuttara Volcano, Southwestern Hokkaido, Japan

149

Fig. .. Isopach map (A) and maximum-grain-size isopleth map (B) for the Hy-a deposit. The deposit occurs
within ῌ+** m of the Hiyoriyama Summit Crater, and shows an increasing trend in thickness and maximum
grain size toward the crater. The maximum grain size was calculated as the average long-axis diameter of thethree largest lithic clasts. Contour interval is +* m.

Table +. Whole-rock major-element compositions of dacites from the Hiyoriyama Cryptodome(samples Nb-2-a and -2-b) and from the Hy-a deposit (Nb-13B, -2*A, -2*B, -2*C, -2+A,
and -2+B), as determined by X-ray ﬂuorescence (Rigaku RIX-,***) at Shimane University,
Japan, following the analytical method proposed by Kimura and Yamada (+330).

Fe,O-*῍total iron as Fe,O-.

L.O.I.῍loss on ignition.

posed of subangular to subrounded lithic clasts (up to-* cm across) in a ﬁne-grained matrix (Fig. /). The
lithic clasts consist mainly of fresh to altered dacite,with minor altered andesite. No juvenile magmatic
pyroclasts (fresh pumice, scoria, or volcanic glass) are
observed.
The dacite is grey to brownish grey and porphyritic,
containing phenocrysts of plagioclase (῎. mm long, ,,῍,. vol.ῌ), quartz (῎/ mm, /῍2 vol.ῌ), hypersthene(῎, mm, .῍1 vol.ῌ), and trace amounts of augite (῎+ mm) and opaque minerals (῎*./ mm) (Fig. -B).

The groundmass (0-῍0/ vol.ῌ) is granophyric, containing silica minerals, feldspars, and opaque mineralsof ῎*.+ mm in size. Table + lists the whole-rock major
element chemical composition of the dacite (sample
numbers Nb-13B, -2*A, -2*B, -2*C, -2+A, and -2+B),
which contains 1* wt.ῌ SiO,, -./῍-.0 wt.ῌ Na,O, and+./ wt.ῌ K,O. The dacite in the Hy-a deposit isidentical to the dacite lava exposed in the wall of thesummit crater in terms of color, phenocryst assemblage,
phenocryst size, modal abundance of phenocrysts, andchemical composition.

150

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

Fig. 0. Grain-size histogram of the Hy-a deposit,
sampled from the northeastern rim of the
Hiyoriyama Summit Crater (the type locality).
The deposit has a polymodal grain-size distribution
and contains a large proportion of grains less than+/+0 mm in size.

Fig. /. Occurrence of the Hy-a deposit (Hy-a) at thenortheastern rim of the Hiyoriyama Summit
Crater (the type locality). The deposit consists
mainly of dacitic lithic clasts (arrows) in a ﬁnegrained matrix. The deposit overlies the Us-b
tephra (Us-b), which was emplaced in +00-. Scaleruler is + m long (each colored segment is +* cm).

The matrix consists mainly of fresh to altered dacite
and mineral fragments. The dacite fragments are petrologically identical to those present as lithic clasts. The
mineral fragments consist of plagioclase, quartz, hypersthene, augite, and opaque minerals, all of which arecomponents of the dacite. X-ray di#raction (XRD)
analysis of the ῍, mm fraction (as separated by hydraulic elutriation) revealed the presence of quartz, plagioclase, opal, alunite, pyrophyllite, and gypsum.
Figure 0 shows the grain-size distribution of the Hy-adeposit sampled from the northeastern rim of the
Hiyoriyama Summit Crater. Because the matrix is cohesive, the sieving for grain-size analysis was performed
in a water bath. The Hy-a deposit has a polymodal
grain-size distribution and contains a high proportion ofgrains less than +/+0 mm in size.
-ῌ- Stratigraphy
The Hy-a deposit overlies a 0*-cm-thick pumice fall
deposit and is covered by a +*-cm-thick surface soil
(Fig. /). The pumice fall deposit is clast supported
and poorly stratiﬁed, and consists of white to pale

brown pumice clasts (+ῌ- cm in size). The pumiceconsists of fresh volcanic glass and crystals of plagioclase, hypersthene, augite, hornblende, opaque minerals,
and apatite. The refractive index of volcanic glass
ranges from +..221 to +..3,+ (mean, +..3*/ ; mode,+..3+ ; as determined by Kyoto Fission-Track Co. Ltd).
The mineral assemblage, refractive index, and locationof the deposit suggest that it is the Us-b tephra(Yokoyama et al., +31- ; Machida and Arai, ,**-)
derived from Usu volcano, southwestern Hokkaido.
There is no palaeosol between the Hy-a deposit and theUs-b tephra, although a small amount of limonite ispresent.
-ῌ. Distribution and volume
Figure .A and .B shows the distributions of the
thickness and maximum grain size of the Hy-a deposit.
These data suggest that the deposit increases in thickness and maximum grain size toward the Hiyoriyama
Summit Crater. The bulk volume of the deposit, calculated following Hayakawa (+32/) using the /*-cm
isopach, is -.,ῌ+*. m-.
.. Discussion.ῌ+ Origin of the Hy-a deposit
The distributions of the thickness and maximum
grain size of the Hy-a deposit suggest it was erupted
from the Hiyoriyama Summit Crater. The deposit
consists of fresh to altered lithic clasts in a ﬁne-grained
matrix, and contains no juvenile magmatic pyroclasts,
indicating it was produced during a phreatic explosion.
The petrological features of the lithic clasts are identicalto those of dacite in the wall of the explosion crater,
indicating that the crater formed during the same erup-

A Phreatic Explosion after AD +00- at the Hiyoriyama Cryptodome, Kuttara Volcano, Southwestern Hokkaido, Japantion as that which produced the deposit. The volumeof the deposit (-.,ῌ+*. m-) is consistent with the geometry of the explosion crater (surface area, .*ῌ3/ m ;depth, ,* m ; volume, .ῌ+*. m-). The altered andesite
lithic clasts may have originated from the overlying
sediment of the cryptodome.
.ῌ, Age of the Hiyoriyama Summit Crater
The Hy-a deposit overlies the Us-b tephra, which isinferred to have been emplaced in AD +00- (Yokoyama
et al., +31-), suggesting the Hy-a deposit was emplaced
after this date. Hence, the phreatic explosion at thesummit of the cryptodome must have occurred after
AD +00-. The lack of soil development between the
Hy-a deposit and the Us-b tephra indicates that the
explosion occurred at some time between the late +1th
and +3th centuries. This inference is consistent with thewell-preserved morphology of the crater. We found no
mention of the explosion in historical records ; consequently, the date of the explosion remains poorly constrained. Considering the emplacement age of the
Hiyoriyama Cryptodome (ca. +/ ka ; Goto and Danhara,,*++), the phreatic explosion at the summit of thecryptodome occurred much later than emplacement of
the dome. We suggest that the Hiyoriyama Cryptodome has an eruption history of dome emplacement at
about +/ ka and a phreatic explosion at the summit after
AD +00-. A similar eruption history has been reported
for other silicic domes (e.g., Atosanupuri Dome,
Hokkaido ; Katsui et al., +320).
.ῌ- Relation between the Hy-a deposit and the ShinkiJigokudani pyroclastic fall deposit
Katsui et al. (+322) reported a phreatic fall deposit(the Shinki-Jigokudani pyroclastic fall deposit) fromthe southern part of the Noboribetsu Geothermal Field.
The deposit occurs above the Us-b tephra and consistsof intensely altered, dacitic lithic clasts in a ﬁne-grained
matrix. The deposit increases in thickness toward the
Jigokudani Valley, 1/* m south of the Hiyoriyama
Cryptodome (Fig. +). Katsui et al. (+322) concluded
that the deposit formed during a small phreatic explosion within the Jigokudani Valley after AD +00-. Our
ﬁeld survey found no evidence of the deposit on the
Hiyoriyama Cryptodome.
The Hy-a deposit di#ers from the Shinki-Jigokudani
pyroclastic fall deposit in terms of vent location(Hiyoriyama Summit Crater versus the Jigokudani
Valley, respectively) and components (fresh to altered
dacite versus intensely altered dacite, respectively).
However, the two deposits occupy the same stratigraphic position (i.e., above the Us-b tephra) and may
have been produced by the same volcanic activity.
Therefore, we infer that the two deposits were produced
by distinct phreatic explosions during an eruptive episode. Similar series of multiple-vent-forming phreatic
explosions within an eruptive episode have been151

reported from Usu volcano, southwestern Hokkaido (e.
g., the +3+* eruption : Yokoyama et al., +31- ; the ,***
eruption : Ui et al., ,**,).
The results of the present study will be of use inevaluating volcanic hazards within the Noboribetsu Geothermal Field.
Acknowledgements
This research was sponsored by the Ministry of Education, Culture, Sports, Science and Technology of Japan(MEXT), and supported ﬁnancially by the MuroranInstitute of Technology. We thank T. Danhara(Kyoto Fission-Track Co. Ltd) for measuring the refractive index of volcanic glass. We are grateful to Y.
Miyabuchi (Kumamoto University) and an anonymousreferee for reviewing the manuscript. M. Yoshimoto(Hokkaido University) is thanked for editing the manuscript.
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