Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 3, pp. 461-472

ArticleHistory of Phreatic Eruptions in the Noboribetsu Geothermal Field,
Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTO＊, Hirotaka SASAKI＊, Yoshimasa TORIGUCHI＊ and Akira HATAKEYAMA＊(Received December 12, 2012 ; Accepted July 18, 2013)
A 6.3-m-deep trench was dug in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan, to clarify
the history of phreatic eruptions in the field. The stratigraphic section in the trench consists of (from lower to upper): theKt-1 pyroclastic surge deposit, nine phreatic fall deposits (Nb-l to Nb-d), the B-Tm tephra, two phreatic fall deposits(Nb-c, Nb-b), the Us-b tephra, and a phreatic fall deposit (Nb-a). The 12 phreatic fall deposits (Nb-l to Nb-a) are 3-100
cm thick and consist of altered dacitic lithic fragments in a clay-rich fine-grained matrix. These deposits are inferred tohave been erupted from the Noboribetsu Geothermal Field. Buried soil layers occur between the deposits. Radiocarbon
dating of buried soil samples from immediately below each of the phreatic fall deposits suggests that phreatic eruptions
occurred at ca. BC 6450, BC 5370, BC 3980, BC 3440, BC 1990, BC 1710, BC 1280, BC 900, BC 200, AD 980, AD
1480, and after AD 1663, corresponding to an average eruption interval of 〜700 years.
Key words : eruptive history, phreatic explosion, Noboribetsu Geothermal Field, Kuttara Volcano, trench survey

1．Introduction
Geothermal fields are common in active volcanic regions
and have become major sites for tourism and for extracting
geothermal energy. Although there have been few historicalvolcanic eruptions in geothermal fields, geological studies
suggest that phreatic eruptions (here, the term ‘phreatic
eruptions’ is used for steam-driven explosions that do not
involve fresh magma, following Barberi et al., 1992 andMcPhie et al., 1993) in such settings occur at intervals of
100-1000 years (Lloyd, 1972; Hedenquist and Henley, 1985;
Ito, 1999; Kudo et al., 2000; Miyabuchi and Watanabe,
2000; Browne and Lawless, 2001; Okuno, 2002; Ohba andKitade, 2005; Kobayashi et al., 2006; Mannen, 2007).
Therefore, an understanding of the eruptive history of ageothermal field is essential in evaluating the volcanic
hazard in the region.
The Noboribetsu Geothermal Field at Kuttara Volcano,southwestern Hokkaido (Fig. 1), is one of the major geothermal fields in Japan. The existence of small explosion
craters in this field suggests that phreatic eruptions haveoccurred in recent geological time (Yamazaki, 1986;
Katsui et al., 1988; Goto et al., 2011a, 2011b). However,
the history of eruptions remains unknown, because thisfield is generally covered with thick vegetation, and there
are few exposures of phreatic deposits. To address thisshortcoming, we conducted a trench survey and performed
radiocarbon dating of phreatic deposits erupted from thefield. This paper reports on the stratigraphy and radio＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan

carbon ages of the phreatic deposits and discusses theeruptive history of the geothermal field.
2．Noboribetsu Geothermal Field
The Noboribetsu Geothermal Field lies in the westernpart of Kuttara Volcano (Fig. 1), an andesitic to rhyolitic
composite volcano (elevation, 549 m above sea level) witha small caldera at its summit (Lake Kuttara). The volcano
evolved over the period 80-45 ka, involving early silicic
explosive activity and subsequent stratovolcano building
associated with caldera collapse at about 40 ka (Katsui
et al., 1988; Yamagata, 1994; Moriizumi, 1998; Moriya,
2003). The geothermal field, which is inferred to haveformed after the collapse of the caldera (Katsui et al.,1988), is approximately 1 km wide (NE‒SW) and 1.5 kmlong (NW‒SE).
The Noboribetsu Geothermal Field is characterized by adacitic cryptodome (Hiyoriyama Cryptodome), a volcanic
lake (Oyunuma Lake), and a fumarolic valley (Jigokudani
Valley) (Fig. 2). The Hiyoriyama Cryptodome, located in
the northern part of the geothermal field, is 350-550 m indiameter and rises 130 m above the surrounding area (Fig.
3; Goto et al., 2011a). Fission-track analyses indicate thatthe dome formed at 15±3 ka (Goto and Danhara, 2011).
Oyunuma Lake (115 × 210 m in area), located in thecentral part of the geothermal field, is the largest lake inthe field and is filled with hot acidic water (Fig. 3). The
Jigokudani Valley, in the southern part of the geothermal
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp

462

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

140 E
45 N

145 E
N

HOKKAIDO

Ku
40

JAPAN

rile

ar

c

C
FI
CI N
PA CEAO

Fig. 2

Kuttara Volcano
Lake
Kuttara
400 m
300 m
200 m

Noboribetsu
Geothermal Field

100 m

Pacific
Ocean
0

1

2 km

Fig. 1. Location of the Noboribetsu Geothermal Field in
the western part of Kuttara Volcano, southwestern
Hokkaido, Japan. Topographic contour interval is 100 m.

field, extends for 500 m in an ENE‒WSW orientation andhosts active fumaroles. The geothermal manifestations of
the geothermal field (active fumaroles, hot springs, andhydrothermal alteration zones) are distributed in a zone
extending NNW‒SSE from the Hiyoriyama Cryptodome
to the Jigokudani Valley (Goto and Johmori, 2011).
3．Phreatic fall deposits of the Noboribetsu Geothermal
Field
Phreatic fall deposits, extruded from the Noboribetsu
Geothermal Field, are widely distributed over the field(Yamazaki, 1986; Katsui et al., 1988; Goto et al., 2011b).
They occur above the Kt-1 pyroclastic surge deposit(Moriizumi, 1998) or the Kt-1 pyroclastic flow deposit(Moriizumi, 1998), both of which were extruded from
Kuttara Volcano at about 40 ka. In previous studies, thephreatic fall deposits have been named the OyunumaJogokudani explosion deposits (Yamazaki, 1986), theShinki-Jigokudani pyroclastic fall deposit (Katsui et al.,1988), or the Hiyoriyama phreatic deposit (Goto et al.,2011b). Here, these deposits are referred to as theNoboribetsu phreatic fall deposits, to avoid confusion.
These deposits comprise 12 units, ranging from theuppermost Noboribetsu-a phreatic fall deposit (Nb-a) tothe lowermost Noboribetsu-l phreatic fall deposit (Nb-ldeposit), as described below. The relations between theNoboribetsu phreatic fall deposits and the previously
reported phreatic deposits are described in section 7-2.

4．Trench survey
Stratigraphic and geochronological studies of the Noboribetsu phreatic fall deposits are hampered by limited exposure due to thick vegetation cover in the Noboribetsu
Geothermal Field. We conducted a trench survey of thedeposits to determine the stratigraphic record of eruption
events and to collect buried soil samples for radiocarbon
dating. The trench was excavated upon a gentle ridge
located 200 m northeast of Oyunuma Lake (Fig. 2). This
site was chosen because the lake is the largest and mostcentrally located explosion crater in the Noboribetsu
Geothermal Field. The area around the trench site iscovered with tall trees and short bamboo forest (Fig. 3).
The trench was excavated on 24-25 November 2005 using
a backhoe with a 0. 3 m3 bucket. The trench was 3. 5 mwide, 7.0 m long, and 6.3 m deep (Fig. 4). The long axis of
the trench was oriented east‒west.
5．Geological section in the trench
The stratigraphic section in the trench consists of a
subhorizontal sequence comprising (from lower to upper)
the Kt-1 pyroclastic surge deposit, the Nb-l to Nb-d
deposits, the B-Tm tephra (Machida and Arai, 2003), theNb-c deposit, the Nb-b deposit, the Us-b tephra(Yokoyama et al., 1973), and the Nb-a deposit (Figs. 4 and
5). Buried soil layers (0. 5-5. 0 cm thick) occur between
each of these units.
The Kt-1 pyroclastic surge deposit (>71 cm thick) isbrown (Munsell color chart 10YR 4/4), thinly bedded,
coarse grained (1-2 mm), and composed of lithic fragments,
mineral fragments (plagioclase, quartz, hypersthene, augite,
hornblende, opaque minerals), and fresh volcanic glass.
The deposit contains abundant accretionary lapilli (< 5mm across). The refractive index of the volcanic glass
ranges from 1.4961 to 1.5031 (mean, 1.499; mode, 1.499;
as analyzed using RIMS2000 machine at Kyoto FissionTrack Co., Ltd, Japan, following the method of Danhara etal., 1992). This deposit is overlain by a brown (10YR 4/3)
soil layer (5 cm thick).
The Nb-l deposit (70 cm thick) is bright brown (7.5YR5/8), massive (non-stratified), matrix supported, poorly
sorted, and composed of subrounded to subangular, dacitic
lithic clasts (<30 cm across) in a fine-grained matrix (Fig.
4). The clasts are pale brown to pale grey, and intensely
altered. The matrix is clay-rich, cohesive, and composed ofaltered dacite fragments, mineral fragments (plagioclase,quartz, hornblende, hypersthene, augite, opaque minerals),
and weathered volcanic glass. The grain-size distributionof the deposit shows a relatively even spread from −7 to 4
ϕ (Fig. 6 L). X-ray diffraction (XRD) analysis of the <2µm fraction (separated by hydraulic elutriation) indicatesthe presence of gypsum, pyrophyllite, epidote, andvermiculite (Table 1). This deposit is overlain by a brown(10YR 4/3) soil layer (1 cm thick).
The Nb-k deposit (60 cm thick) is light yellow (2.5Y

History of Phreatic Eruptions in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

Hiyoriyama Cryptodome

N

Noboribetsu
Geothermal Field

463

Trench site

Hiyoriyama
Cryptodome
Trench site

Oyunuma Lake
OkunoyuOyunuma Lake
Jigokudani Valley

Fig. 3. Photograph of the Hiyoriyama Cryptodome andOyunuma Lake in the Noboribetsu Geothermal Field(viewed from the south), showing the trench site.
０5001000 mFig. 2. Topographic map of the Noboribetsu Geothermal
Field, showing the location of the trench site. The
trench was excavated 200 m northeast of Oyunuma
Lake, which is the largest explosion crater in thegeothermal field. The base map is after the 1:25,000
scale topographic map ‘Noboribetsu-Onsen’ issued by
the Geospatial Information Authority of Japan (GSI).
Topographic contour interval is 10 m.

7/4), massive, and composed of subrounded to subangular,
dacitic lithic clasts (<10 cm across) in a fine-grained matrix(Fig. 4). The clasts are pale grey and intensely altered. Someclasts show an impact sag structure onto the underlying
soil layer. The matrix is clay-rich, cohesive, and composedof altered dacite fragments, mineral fragments (plagioclase, quartz, hypersthene, hornblende, augite, opaqueminerals), and weathered volcanic glass. The grain size ofthe deposit is mainly smaller than 4 ϕ (Fig. 6K). XRD
analysis of the < 2 µm fraction (Table 1) indicates thepresence of smectite, gypsum, and epidote. This deposit isoverlain by a brown (10YR 4/3) soil layer (0.5 cm thick).
The Nb-j deposit (65 cm thick) is bright brown (7.5YR5/8), massive, and composed of subrounded to subangular,
dacitic lithic clasts (< 30 cm across) in a fine-grained
matrix (Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(plagioclase, quartz, hypersthene, augite, hornblende, biotite,
opaque minerals), and weathered volcanic glass. Thisdeposit contains charcoal fragments (<4 mm across). The
grain size distribution of the deposit shows a dominance ofgrains smaller than 4 ϕ (Fig. 6 J). XRD analysis of the <2µm fraction (Table 1) indicates the presence of gypsum

and quartz. This deposit is overlain by a dark brown (10YR
2/2) soil layer (0.5 cm thick).
The Nb-i deposit (20 cm thick) is bright brown (7.5YR5/8), massive, and composed of subrounded to subangular,
dacite clasts (< 30 cm across) in a fine-grained matrix(Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(plagioclase, quartz, hypersthene, augite, hornblende,
biotite, opaque minerals), and weathered volcanic glass.
Grain-size analysis shows a dominance of grains smaller
than 4 ϕ (Fig. 6I). XRD analysis of the <2 µm fraction(Table 1) indicates the presence of smectite, gypsum, andquartz. This deposit is overlain by a dark brown (10YR
2/2) soil layer (1 cm thick).
The Nb-h deposit (3 cm thick) is brown (7.5YR 4/6),
massive, and composed of subrounded, dacitic lithic clasts(1-5 cm across) and dacitic pumice clasts (1-2 cm across)in a fine-grained matrix (Fig. 4). All clasts are pale grey
and intensely altered, and their color means they are conspicuous in the brown matrix. The matrix is clay-rich,cohesive, and composed of altered dacite fragments, mineralfragments (plagioclase, quartz, hypersthene, augite, hornblende, opaque minerals), and weathered volcanic glass.
Grain-size analysis shows a bimodal grain-size distribution(Fig. 6H). XRD analysis of the <2 µm fraction (Table 1)
indicates the presence of smectite, kaolinite, quartz, andopal. This deposit is overlain by a dark brown (10YR 2/2)
soil layer (7 cm thick).
The Nb-g deposit (3 cm thick) is brown (10YR 4/6),
massive, and composed of subrounded to subangular,
dacitic lithic clasts (<1 cm across) in a fine-grained matrix(Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(plagioclase, quartz, hypersthene, augite, hornblende,

464

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

Nb-a

Us-b

Nb-b
Nb-c
B-Tm
impact sag structure
Nb-d

Nb-e
Nb-f
Nb-g
Nb-h
Nb-i
Nb-j

Nb-k
Nb-l

Fig. 4. Photograph of the trench, showing the sequence of Noboribetsu phreatic fall deposits (Nb-a to Nb-l), the B-Tm
tephra, and the Us-b tephra (viewed from the east). Buried soil layers occur between adjacent units. Note the impact
sag structure at the bottom of the Nb-c deposit. The striped rod in the trench is 90 cm long (each colored segment is 10
cm long).

History of Phreatic Eruptions in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

465

0
Nb-a

Us-b

1
Nb-b
370 ± 40 BP (Ot-14), 310–510 cal BP

Nb-c1080 ± 40 BP (Ot-20), 930–1060 cal BP

B-Tm
2

Depth below the ground surface

Nb-d2170 ± 40 BP (Ot-36), 2060–2320 cal BP

3
Nb-e
Nb-f
Nb-g
Nb-h

2750 ± 40 BP (Ot-40), 2760–2950 cal BP
3020 ± 50 BP (Ot-42), 3070–3360 cal BP
3420 ± 50 BP (Ot-44), 3560–3830 cal BP
3640 ± 40 BP (Ot-47), 3850–4080 cal BP

Nb-i
4670 ± 40 BP (Ot-300), 5310–5580 cal BP

4

Nb-j

5200 ± 40 BP (NB-2C-375), 5910–6000 cal BP

Nb-k
6400 ± 60 BP (Ot-61b), 7180–7430 cal BP

5

Nb-l

7600 ± 60 BP (Ot-110), 8340–8540 cal BP

6

Kt-1 pyroclastic
surge deposit
LEGEND
phreatic fall deposit
pumice fall deposit
ash fall deposit
pyroclastic surge deposit

7

soil

m

Fig. 5. Stratigraphic section in the trench. The section
comprises the Noboribetsu phreatic fall deposits(Nb-a to Nb-l), the Kt-1 pyroclastic surge deposit(Kt-1), the B-Tm tephra, and the Us-b tephra. Buried
soil layers occur between adjacent units. Radiocarbon
ages (conventional ages) and calibrated ages (error
range 2 σ) are also shown.

opaque minerals), and weathered volcanic glass. Thisdeposit contains many charcoal fragments (<3 mm across).
Grain-size analysis (Fig. 6G) shows a dominance of grains
smaller than 4 ϕ. XRD analysis of the < 2 µm fraction

Fig. 6. Grain-size histograms of the Noboribetsu phreaticfall deposits (Nb-a to Nb-l) sampled from the trench.
The deposits contain a large proportion of grains
smaller than 4 ϕ. The samples were sieved at meshintervals of 1 ϕ (where ϕ=−log2d, with d being thegrain size in millimeters), using a set of sieves
ranging from −7 to 4 ϕ (128 mm to 1/16 mm). The
sieving was carried out in a water bath because thesamples were cohesive. The sieved samples were dried
and weighed to an accuracy of 0.01 g on a laboratory
balance.

(Table 1) indicates the presence of smectite, kaolinite,
quartz, and opal. This deposit is overlain by a dark brown466

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA
Table 1. Results of X-ray diffraction (XRD) analyses of the <2 mm fraction of the Noboribetsu phreatic fall deposits(Nb-a to Nb-l), sampled from the trench. The modal abundance of each mineral is classified as abundant (+++),
moderate (++), or trace (+).

The <2 mm fraction sample was separated by hydraulic elutriation and prepared as oriented samples on glass slides.
The XRD analyses were carried out at Okayama University of Science, using a Geigerflex RAD-B diffractometer(Rigaku Corporation) with settings of 40 kV, 60 mA, and scan speed of 2.0° per minute. Ethylene glycol treatment wasused to identify smectite.

(10YR 2/2) soil layer (2 cm thick).
The Nb-f deposit (5 cm thick) is yellowish brown (10YR
5/6), massive, and composed of subrounded to subangular,
dacitic lithic clasts (<1 cm across) in a fine-grained matrix(Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(plagioclase, quartz, hypersthene, augite, hornblende, opaqueminerals), and weathered volcanic glass. This deposit
contains many charcoal fragments (<3 mm across). Grainsize analysis (Fig. 6F) shows a dominance of grains
smaller than 4 ϕ. XRD analysis of the < 2 µm fraction(Table 1) indicates the presence of smectite, kaolinite,
quartz, and opal. This deposit is overlain by a dark brown(10YR 2/2) soil layer (3 cm thick).
The Nb-e deposit (20 cm thick) is bright brown (7.5YR5/8), massive, and composed of subrounded to subangular,
dacitic lithic clasts (< 10 cm across) in a fine-grained
matrix (Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(plagioclase, quartz, hypersthene, augite, hornblende,opaque minerals), and weathered volcanic glass. Someclasts show an impact sag structure onto the underlying
soil layer and also onto the Nb-f deposit beneath the soil
layer. Grain-size analysis shows a dominance of grains
smaller than 4 ϕ (Fig. 6E). XRD analysis of the <2 µm
fraction (Table 1) indicates the presence of smectite,kaolinite, and jarosite. This deposit is overlain by a darkbrown (10YR 2/2) soil layer (6 cm thick).
The Nb-d deposit (100 cm thick) is light yellow (2.5Y7/4), massive, and composed of subrounded to subangular,
dacitic lithic clasts and minor andesitic lithic clasts (<80cm across) in a fine-grained matrix (Fig. 4). The dacitic

lithic clasts are pale grey and intensely altered, whereas theandesitic lithic clasts are pale brownish grey and slightly
altered. The matrix is clay-rich, cohesive, and composed ofaltered dacite fragments, mineral fragments (quartz, plagioclase, hypersthene, augite, hornblende, opaque minerals),
and weathered volcanic glass. Some clasts show an impact
sag structure onto the underlying soil layer and also onto
the Nb-e deposit beneath the soil layer (Fig. 4). Grain-size
analysis shows a bimodal grain-size distribution, with adominance of grains smaller than 4 ϕ (Fig. 6D). XRD
analysis of the < 2 µm fraction (Table 1) indicates thepresence of smectite, kaolinite, quartz, tridymite, and opal.
This deposit is overlain by a dark brown (10YR 2/2) soil
layer (5 cm thick).
The B-Tm tephra (1 cm thick) is white (N9), finegrained, and composed of fresh volcanic glass and crystalsof K-feldspar, plagioclase, quartz, orthopyroxene, clinopyroxene, and opaque minerals (Fig. 4). The refractive
index of the glass (as analyzed using RIMS2000 machine)shows a bimodal distribution, with one mode ranging from1.5082 to 1.5145 (mean, 1.510; mode, 1.510; proportion ofanalyses, 58 %) and the other from 1. 5167 to 1. 5217
(mean, 1.520; mode, 1.520; proportion of analyses, 42 %).
The refractive index is consistent with that of the B-Tm
tephra reported by Machida and Arai (2003). The tephra isoverlain by a dark brown (10YR 2/2) soil layer (4 cm
thick).
The Nb-c deposit (37 cm thick) is reddish brown (5YR
4/8), massive, and composed of subrounded to subangular,
dacitic lithic clasts (< 20 cm across) in a fine-grained
matrix (Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(quartz, plagioclase, hypersthene, augite, hornblende,

History of Phreatic Eruptions in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

opaque minerals), and weathered volcanic glass. Someclasts show an impact sag structure onto the underlying
soil layer and also onto the Nb-d deposit beneath the soil
layer (Fig. 4). The grain-size distribution of the deposit
shows a relatively even spread (Fig. 6C). XRD analysis of
the < 2 µm fraction (Table 1) indicates the presence ofsmectite, kaolinite, and gypsum. This deposit is overlain
by a dark brown (10YR 2/2) soil layer (1 cm thick).
The Nb-b deposit (40 cm thick) is light yellow (2.5Y7/3), massive, and composed of subrounded to subangular,
dacitic lithic clasts (< 20 cm across) in a fine-grained
matrix (Fig. 4). The clasts are pale brown to pale grey, andintensely altered. The matrix is clay-rich, cohesive, andcomposed of altered dacite fragments, mineral fragments(quartz, plagioclase, hypersthene, hornblende, biotite, augite,
opaque minerals), and weathered volcanic glass. Someclasts show an impact sag structure onto the underlying
soil layer. Grain-size analysis shows a bimodal grain-size
distribution, with a dominance of grains smaller than 4 ϕ(Fig. 6B). XRD analysis of the <2 µm fraction (Table 1)
indicates the presence of smectite, kaolinite, pyrophyllite,
jarosite, illite, and quartz. This deposit is directly overlain
by the Us-b tephra.
The Us-b tephra (80 cm thick) is clast supported andpoorly stratified, and consists of rhyolite pumice (1-3 cm
across; Fig. 4). The pumice is white (N9) and consists offresh volcanic glass and crystals of plagioclase, hypersthene, augite, hornblende, and opaque minerals. The
refractive index of the volcanic glass ranges from 1.4887
to 1. 4921 (mean, 1. 491; mode, 1. 491; analyzed using
RIMS2000 machine). This tephra is directly overlain by
the Nb-a deposit.
The Nb-a deposit (10 cm thick) is olive brown (2.5Y4/3), massive, and composed of subrounded to subangular,
dacitic lithic clasts (<1 cm across) in a fine-grained matrix(Fig. 4). The clasts are pale grey to grey, and intensely
altered. The matrix is clay-rich, cohesive, and composed ofaltered dacite fragments, mineral fragments (quartz, plagioclase, hypersthene, hornblende, biotite, augite, opaqueminerals), and weathered volcanic glass. Grain-size analysis
shows a dominance of grains smaller than 4 ϕ (Fig. 6A).
XRD analysis of the <2 µm fraction (Table 1) indicatesthe presence of smectite, kaolinite, and quartz. This deposit
is overlain by dark brown (10YR 2/2) surface soil (10 cm
thick).
6．Radiocarbon dating
Radiocarbon ages were determined for 11 soil samples
collected from buried soil layers located immediately
below 11 of the phreatic fall deposits exposed in the trench(see Fig. 5). The soil layers contain no modern plant roots,
and show no sign of disturbance (e.g., bioturbation, erosion)
during or after deposition. Each sample was collected fromthe uppermost 1 cm of the soil layer. In the case that a soil
layer was less than 1 cm thick, the entire thickness of the467

soil layer was used for dating. Each sample for dating
consisted of dark brown or brown, organic sediment (grain
size < 0.5 mm).
Radiocarbon dating was performed by Beta Analytic(Miami, USA). All samples were pretreated with acidwashes, and the remaining carbon (bulk organic fraction)
was analyzed for dating (Table 2). Eight samples (Ot-14,
Ot-20, Ot-36, Ot-40, Ot-42, Ot-44, Ot-61b, Ot-110) wereanalyzed using a standard radiometric method, and threesamples (Ot-47, Ot-300, NB-2C-375) were analyzed using
an accelerator mass spectrometry (AMS) method (Table2). The soil samples yield conventional radiocarbon agesof (from the lowermost to uppermost samples) 7600±60
BP, 6400±60 BP, 5200±40 BP, 4670±40 BP, 3640±40
BP, 3420±50 BP, 3020±50 BP, 2750±40 BP, 2170±40
BP, 1080±40 BP, and 370±40 BP (BP=years before
AD 1950; error range 1 σ; Table 2). These ages are consistent with the stratigraphic positions of the soils (Fig. 5).
Calibrated ages were calculated from the conventional
radiocarbon ages, using a program developed by Beta
Analytic and based on the IntCal09 calibration database(Oeschger et al., 1975; Stuiver and Braziunas, 1993;
Heaton et al., 2009; Reimer et al., 2009) with a splinesmoothing function for the calibration curve (Talma andVogel, 1993). The calibrated ages are (from the lowermost
to uppermost samples) 8340-8540 cal BP (BC 63906590), 7180-7430 cal BP (BC 5230-5480), 5910-6000 cal
BP (BC 3960-4050), 5310-5580 cal BP (BC 3360-3630),
3850-4080 cal BP (BC 1900-2140), 3560-3830 cal BP(BC 1610-1880), 3070-3360 cal BP (BC 1120-1410),
2760-2950 cal BP (BC 810-1000), 2060-2320 cal BP (BC110-370), 930-1060 cal BP (AD 890-1020), and 310-510
cal BP (AD 1440-1640) (error range 2 σ, 95 % probability;
Table 2).
The calibrated ages are consistent with ages reported
previously for the Kt-1 pyroclastic surge deposit (〜40 ka;
Moriizumi, 1998), the B-Tm tephra (〜10th century; Machida
and Arai, 2003), and the Us-b tephra (AD 1663; Yokoyama
et al., 1973; Machida and Arai, 2003) (Fig. 5). The Nb-a
deposit occurs above the Us-b tephra, suggesting it was
deposited after AD 1663.
7．Discussion
7-1 Origin of the Noboribetsu phreatic fall deposits
The Noboribetsu phreatic fall deposits (Nb-l to Nb-a)
consist of altered dacitic lithic clasts in an altered finegrained matrix, and contain no juvenile magmatic pyroclasts, suggesting they were produced during phreatic
explosions. The grain-size distributions of the deposits(which generally show a dominance of grains smaller than4 ϕ) and the presence of clay minerals (e.g., smectite andkaolinite; Table 1) are consistent with the deposits
originating during a phreatic explosion. The presence oflarge lithic clasts (up to 80 cm across) and impact sag
structures suggests that the deposits were erupted from

Radiocarbon ages of buried soils collected from the trench.

*
Based on Libbyʼs half-life (5568 years), uncorrected by δ13C values. Ages are expressed in BP (years before AD 1950) with an error range of 1 σ. ** The conventional 14C age includes
δ13C correction. Ages are expressed in BP with an error range of 1 σ. Vienna Pee Dee belemnite (VPDB) standard was used for δ13C analysis. *** Calibrated ages were calculated from theconventional 14C ages. Ages are expressed in cal BP and cal BC/AD with an error range of 2 σ (95 % probability). Intercept ages with the calibration curve are also shown. AMS=
accelerator mass spectrometry. The stratigraphic positions of the samples are shown in Fig. 5.

Table 2.

468
Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMAHistory of Phreatic Eruptions in the Noboribetsu Geothermal Field, Kuttara Volcano, Hokkaido, Japan

469

Nb-a
Nb-b

Phreatic fall deposit

Nb-c
Nb-d
Nb-e
Nb-f
Nb-g
Nb-h
Nb-i
Nb-j
Nb-k
Nb-l
BC
7000

BC
6000

BC
5000

BC
4000

BC
3000

BC
2000

BC
1000

0

AD
1000

AD
2000

Calibrated age
Fig. 7. Timing of phreatic eruptions in the Noboribetsu Geothermal Field. The width of each bar represents 2 σ(95 % probability).

within the Noboribetsu Geothermal Field, which isconsistent with the presence of many small explosion
craters in the field (Goto et al., 2011a).
The 12 phreatic deposits (Nb-l to Nb-a) and intervening
soil layers indicate the occurrence of 12 phreatic eruptions
separated by periods of dormancy. The trench survey was
performed at only one location, and it is possible that morethan 12 phreatic fall deposits are present in theNoboribetsu Geothermal Field. We therefore conclude thatat least 12 episodes of phreatic eruptions have occurred in
the Noboribetsu Geothermal Field.
Since the trench survey gives only a one-dimensional
perspective, the spatial distribution of each deposit (Nb-l toNb-a) remains unknown; consequently, it is not possible toaccurately estimate the volume of each deposit. The
thicknesses of the deposits (maximum 100 cm, Nb-d)suggest that each phreatic eruption was relatively large inmagnitude. We roughly estimate that the volume of eachdeposit is on the order of 105 m3.
7-2 Relations between the Noboribetsu phreaticfall deposits and previously reported phreatic
deposits
Some of the Noboribetsu phreatic fall deposits (Nb-l toNb-a) have been reported previously (Yamazaki, 1986;
Katsui et al., 1988; Goto et al., 2011b). Yamazaki (1986)
noted a set of phreatic fall deposits (the OyunumaJogokudani explosion deposits) from 200 m east of Oyunuma
Lake. The deposits are 5.6 m thick in total and comprise 10
units. Each unit is 5-150 cm thick and consists of dacitic

lithic clasts in a clay-rich, fine-grained matrix. Threeburied soil layers occur between the units, and one soil
layer was dated as 1880±100 BP by radiocarbon methods.
Yamazaki (1986) inferred that the deposits formed byphreatic eruptions within the Noboribetsu Geothermal
Field. The 10 units may correspond to most of the Nb-l to
Nb-a deposits, but detailed correlations between them aredifficult, because the 10 units were poorly described andthe radiocarbon age (1880±100 BP) does not match ourage data. The outcrop examined by Yamazaki (1986) is nolonger accessible because it is covered by cement.
Katsui et al. (1988) described a phreatic fall deposit (theShinki-Jigokudani pyroclastic fall deposit) from the southernpart of the Noboribetsu Geothermal Field. The deposit
occurs above the Us-b tephra, and consists of altered,dacitic lithic clasts in a fine-grained matrix. Katsui et al.
(1988) inferred that the deposit formed during a small
phreatic eruption within the Jigokudani Valley after AD
1663. Goto et al. (2011b) reported a phreatic fall deposit(the Hiyoriyama phreatic deposit) from the northern part of
the Noboribetsu Geothermal Field. The deposit also occurs
above the Us-b tephra and consists of fresh to altered,dacitic lithic clasts in a fine-grained matrix. Goto et al.
(2011b) inferred that the deposit formed during a small
phreatic eruption at the summit of the Hiyoriyama Cryptodome after AD 1663. The stratigraphic location (i.e., above
the Us-b tephra) of these two deposits suggests that theycorrespond to the Nb-a deposit. The two deposits (i.e., theNb-a deposit) are inferred to have been produced by

470

Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

distinct phreatic explosions during a series of multiplevent-forming phreatic explosions within an eruptive
episode (Goto et al., 2011b).
7-3 History of phreatic eruptions in the Noboribetsu
Geothermal Field
The radiocarbon age of a buried soil located immediately below a mass-flow or pyroclastic deposit may represent
the emplacement age of the deposit, in the case that thedeposit overlies the soil without any disturbance (Orlova
and Panychev, 1993; Okuno et al., 1997; Xu et al., 2004).
In the present study, the soil layers show no sign ofdisturbance, suggesting the radiocarbon age of each soil
represents the emplacement age of the overlying deposit.
The calibrated ages (intercept ages; Table 2) suggest thatphreatic eruptions occurred in the Noboribetsu Geothermal
Field at ca. BC 6450 (Nb-l), BC 5370 (Nb-k), BC 3980
(Nb-j), BC 3440 (Nb-i), BC 1990 (Nb-h), BC 1710 (Nb-g),
BC 1280 (Nb-f), BC 900 (Nb-e), BC 200 (Nb-d), AD 980(Nb-c), AD 1480 (Nb-b), and after AD 1663 (Nb-a) (Fig.
7). The average interval between eruptions is 〜700 years(i.e., 12 eruptions in the past 8500 years), and the intervals
range from 300 to 1400 years.
Considering the emplacement age of the Hiyoriyama
Cryptodome (ca. 15 ka; Goto and Danhara, 2011), the 12
phreatic eruptions occurred after the emplacement of thedome. We suggest that the Noboribetsu Geothermal Field
has an eruption history of dome emplacement at about 15
ka and phreatic eruptions after BC 6450 (or 8400 cal BP).
The formation of the Hiyoriyama Cryptodome is the only
magmatic event during the evolution of the Noboribetsu
Geothermal Field (Katsui et al., 1988), and the eruption
history in this field is characterized by an early magmatic
activity and later phreatic activity. Such an eruption history
could be explained by early magma ascent followed by therepeated ascent of high-temperature volcanic fluid from
depth.
A similar eruption history (i. e., early dome-forming
magmatic activity and later phreatic activity) has beenreported for other geothermal fields (e.g., the Owakudaniarea, Hakone Volcano, central Japan; Kobayashi et al.,2006; Mannen, 2007). In the Owakudani area, a lava dome
(Mt. Kanmurigatake) formed at about 3000 years ago,followed by at least five phreatic eruptions (Kobayashi etal., 2006; Mannen, 2007). The frequency of phreatic
eruptions in the Owakudani area (i.e., 5 eruptions in thepast 3000 years; Kobayashi et al., 2006; Mannen, 2007) isalso similar to that in the Noboribetsu Geothermal Field(12 eruptions in the past 8500 years; this study). The
present results are therefore of value in evaluating the
volcanic hazard in 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 the Ministry of theEnvironment, Japan, for granting permission for the trench
survey. We thank T. Danhara (Kyoto Fission-Track Co.
Ltd) for measuring the refractive index of volcanic glass.
We are grateful to K. Mannen (Hot Springs ResearchInstitute of Kanagawa Prefecture) and T. Ohba (Akita
University) for reviewing the manuscript. Part of this work
was performed as a component of compiling a hazard map
for the City of Noboribetsu.
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Yoshihiko GOTO, Hirotaka SASAKI, Yoshimasa TORIGUCHI and Akira HATAKEYAMA

北海道クッタラ火山，登別地熱地域の水蒸気噴火史
後藤芳彦・佐々木央岳・鳥口能誠・畠山信

北海道クッタラ火山登別地熱地域，大湯沼の北東 200 m 地点において，トレンチ調査（深さ 6.3 m）を行い，登別地熱地域の噴火史を解明した。トレンチ断面の層序は，12 層の水蒸気噴火堆積物（厚さ 3-100 cm）と，それらに挟在する B-Tm テフラ，Us-b テフラからなる。12 層の水蒸気噴火堆積物は，変質したデイサイ
ト質石質岩片（最大粒径 80 cm）と粘土質のマトリクスから構成され，サグ構造を示すことから，登別地熱地域から噴出したと考えられる。各々の水蒸気噴火堆積物は土壌層を挟在し，12 回の噴火が休止期を挟んで繰り返し起きたことを示す。水蒸気噴火堆積物の直下土壌層の放射性炭素年代測定値，および広域テフラとの対比により，水蒸気噴火は，約 BC6450 年，BC5370 年，BC3980 年，BC3440 年，BC1990 年，BC1710 年，
BC1280 年，BC900 年，BC200 年，AD980 年，AD1480 年，および AD1663 年以降に起きたと推定される。登別地熱地域では，過去 8500 年間に 12 回以上の水蒸気噴火が起こり，噴火の頻度は 700 年に 1 回程度であると考えられる。