Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 2, pp. 241-249

LetterMultiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan
Yoshihiko GOTO＊, Yoshimasa TORIGUCHI＊, Hirotaka SASAKI＊ and Akira HATAKEYAMA＊(Received July 8, 2014 ; Accepted February 2, 2015)
The Noboribetsu geothermal field (area 1×1.5 km) is located in the western part of Kuttara volcano, southwestern
Hokkaido, Japan. Seventy-one shallow trenches (<90 cm deep) were systematically dug within the geothermal field, inorder to study the stratigraphy and distribution of the most recent phreatic fall deposit (the Nb-a deposit) erupted fromthe field. The Nb-a deposit (<68 cm thick) consists of altered dacitic lithic clasts in a fine-grained clay-rich matrix. The
deposit overlies the Us-b tephra that was emplaced in AD 1663. Stratigraphic sections constructed from the 71 trenches
indicate that the deposit is distributed within an elliptical area measuring 850×1250 m across and extending NNWSSE. Isopach and maximum-grain-size isopleth maps indicate that the deposit increases in thickness and maximumgrain-size toward several explosion craters. These data suggest that phreatic eruptions, through multiple vents, occurredin the Noboribetsu geothermal field after AD 1663.
Key words : phreatic eruption, deposit, trench survey, Noboribetsu geothermal field, Kuttara volcano

1．Introduction
The Noboribetsu geothermal field at Kuttara volcano,southwestern Hokkaido is one of major geothermal fields
in Japan (Fig. 1). Geological and geochronological studies(Goto et al., 2011a, b, 2013 ; Katsui et al., 1988 ; Yamazaki,1986) indicate that at least 12 episodes of phreatic eruptions (here, the term ‘phreatic eruptions’ is used for steamdriven explosions that do not involve fresh magma,
following Barberi et al., 1992 and McPhie et al., 1993)
have occurred in this field within the past 8500 years.
However, the details of each episode, such as the mode andmagnitude of eruptions, remain unclear. We performed astratigraphic survey of the most recent phreatic fall deposit
erupted in this field (the Nb-a deposit ; Goto et al., 2013)
in order to clarify the style of the most recent phreatic
eruptions in this field. As this field is generally covered
with thick vegetation, we excavated and examined 71
shallow trenches throughout the field. This paper reports
the results of the stratigraphic survey and discusses thestyle of the most recent eruptions.
2．Noboribetsu geothermal field
The Noboribetsu geothermal field lies in the westernpart of Kuttara volcano (Fig. 1) and is approximately 1 km
wide (ENE-WSW) and 1.5 km long (NNW-SSE) (Katsui
et al., 1988 ; Moriizumi, 1998). This field is inferred tohave formed at ca. 15 ka (Goto and Danhara, 2011 ; Goto
et al., 2013), and is characterized by a dacitic cryptodome＊

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

(Hiyoriyama cryptodome), a volcanic lake (Oyunuma Lake),
and a fumarolic valley (Jigokudani Valley) (Figs. 2 and 3).
The Hiyoriyama cryptodome, in the northern part of thegeothermal field, is 350-550 m in diameter and rises 130 m
above the surrounding areas (Fig. 2A ; Goto and Johmori,2013). Oyunuma Lake (115×210 m in area), located in thecentral part of the geothermal field, is the largest explosion
crater in the field and is filled with hot acidic water (Fig.
2A). The Jigokudani Valley, in the southern part of thegeothermal field, extends for 500 m in an ENE-WSW orientation and hosts active fumaroles (Fig. 2B). The geothermal
manifestations of the geothermal field (active fumaroles,
hot springs, and hydrothermal alteration zones) are distributed in a zone extending NNW-SSE from the Hiyoriyama
cryptodome to the Jigokudani Valley (Goto and Johmori,2011). A number of small explosion craters (diameter
25-210 m) occur in this zone (Goto et al., 2011a).
3．Trench survey
The Noboribetsu geothermal field is generally coveredby forests consisting of tall trees and short bamboo, except
in areas where geothermal manifestations occur (Fig. 2A).
Stratigraphic and descriptive studies of the Nb-a deposit
are thus hampered by limited exposures caused by a thickcover of vegetation. To address this issue, 71 shallow trenches
were systematically dug throughout the Noboribetsu
geothermal field (Fig. 3). The trenches were excavatedusing a hand shovel. Each trench had a horizontal area of
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO, Yoshimasa TORIGUCHI, Hirotaka SASAKI and Akira HATAKEYAMA

Fig. 1. Location of the Noboribetsu geothermal field in thewestern part of Kuttara volcano, Hokkaido, Japan.

50×50 cm and was <90 cm deep (Fig. 4).
4．Stratigraphic sections
Figure 5 shows stratigraphic sections obtained from the
71 trenches. The sections indicate that the Nb-a deposit
overlies the Us-b tephra (> 60 cm thick ; Machida andArai, 2003 ; Yokoyama et al., 1973) and is covered bysurface soil (5-20 cm thick). The maximum thickness of
the Nb-a deposit is 68 cm (Loc. 16). A thin (<5 mm) soil
layer occurs at several locations between the Nb-a deposit
and the Us-b tephra (as observed at trench Locs. 1, 7, 8,
10-14, 17-20, 25, 38-43, 57, 60). The Us-b tephra waserupted from Usu volcano of southwestern Hokkaido in
AD 1663 (Yokoyama et al., 1973).
The Nb-a deposit occurring in the Jigokudani Valley,observed in trench location 1 (Fig. 3), is representative of
the 71 trench sites and is therefore described in detailbelow. At this location, the Nb-a deposit (40 cm thick) islight yellow, massive (non-stratified), matrix supported,
poorly sorted, and composed of subangular to subroundedlithic clasts (up to 30 cm across) within a fine-grained
matrix (Fig. 4). The lithic clasts consist mainly of intensely
altered dacite. No juvenile magmatic pyroclasts (fresh
pumice or scoria) are observed. The matrix of the deposit
is clay-rich, cohesive, and composed of altered dacitic
lithic fragments, altered pumice fragments, mineral fragments (plagioclase, quartz, hypersthene, hornblende, augite,
and opaque minerals), and weathered volcanic glass.
The Us-b tephra at this location comprises a pumice fallFig. 2. Photographs of the Noboribetsu geothermal field.
(A) The Hiyoriyama cryptodome and Oyunuma Lake,
viewed from the south. Road for scale. (B) The
Jigokudani Valley viewed from the east. The valley
contains active fumaroles.

deposit, > 40 cm thick (Fig. 4). It is pale brown, clastsupported, poorly stratified, and consists of white to pale
brown pumice clasts (1-3 cm in size). The pumice consistsof fresh volcanic glass and crystals of plagioclase, hypersthene, augite, hornblende, opaque minerals, and apatite.
The refractive index of volcanic glass (as analyzed using
an RIMS2000 apparatus at Kyoto Fission-Track Co. Ltd. ;Danhara et al., 1992) ranges from 1.4887 to 1.4921 (mean,
1.4905 ; mode, 1.491).
5．Lateral variations in the Nb-a deposit
The Nb-a deposit at all 71 trench sites is generally
identical in texture. In detail, however, the Nb-a deposit
shows slightly variable textures among the trench sites. For
example, in the Jigokudani Valley (Loc. 1) and at most
locations, the deposit consists of one fall unit, whereas atsome locations (e.g., Loc. 21) the deposit comprises two
fall units that can be distinguished by slight changes incolor. These fall units are not easily identifiable in the
trenches, and are difficult to correlate from one trench toMultiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan

243

Fig. 3. Topographic map showing the locations of trench sites (numbered 1 to 71) excavated for stratigraphic studies.
Stratigraphic sections of each trench site are provided in Fig. 5. KN, Kasayama North crater. Contour intervals are 2 m.

another ; consequently, they are not presented in the stratigraphic sections given in Fig. 5.
The Nb-a deposit also varies slightly in component
among the trench sites. At the Jigokudani Valley (Loc. 1)
and at most other locations, the deposit consists ofintensely altered dacitic lithic clasts in a fine-grained clayrich matrix, whereas at the Hiyoriyama cryptodome (Locs.
30-32) it consists of fresh to altered dacitic lithic clasts andminor andesitic lithic clasts, embedded in a fresh toaltered, fine-grained matrix. Such variations are alsodifficult to correlate between trenches.
6．Grain-size distributions and alteration minerals of
the Nb-a deposit
Figure 6 shows grain-size distributions of the Nb-a
deposit sampled from various trench sites (Locs. 1, 6, 16,
and 31). The samples (each 1-2 kg) were sieved at intervals of 1ϕ (where ϕ=−log2 d, with d being the grain sizein millimeters), using a set of sieves ranging from−6ϕ to
4ϕ (64 mm to 1/16 mm). The sieving was carried out in a

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Yoshihiko GOTO, Yoshimasa TORIGUCHI, Hirotaka SASAKI and Akira HATAKEYAMA

7．Distribution and volume of the Nb-a deposit
Figures 7 and 8 show the distributions of the thickness
and maximum grain size of the Nb-a deposit, respectively.
The maximum grain size at each location was calculatedusing the average long-axis diameter of the three largest
lithic clasts in each trench. These data indicate that the Nba deposit is distributed in an ellipsoidal area, measuring
850×1250 m across and elongate NNW-SSE. The deposit
increases in thickness and grain size toward several explosion craters : Hiyoriyama Summit, Oyunuma, Kasayama
North, Okunoyu, and Tessen-ike (Figs. 7 and 8 ; crater names
are after Goto et al., 2011a). The isopachs of the deposit
around Oyunuma, Kasayama North and Okunoyu arecomplicated (Fig. 7). The bulk volume of the Nb-a deposit
is 4.6×105 m3, as calculated using the method proposed
by Hayakawa (1985), based on a 10 cm isopach.

Fig. 4. Photograph of the Nb‒a deposit (Nb‒a) on thenorthern slope of the Jigokudani Valley (Loc. 1 in Fig.
3 ; latitude 42° 29 55 N, longitude 141° 08 56 E).
The Nb‒a deposit is located 10 cm beneath the groundsurface. The deposit consists of altered dacitic lithic
clasts (indicated by an arrow) within a fine-grained
clay-rich matrix. The deposit overlies the Us‒b tephra(Us‒b), which was emplaced in AD 1663.

water bath because the samples were cohesive. The sieved
samples were dried and weighed to an accuracy of 0.01 g.
The grain-size distributions indicate that the Nb-a deposit
generally contains a large proportion of grains smaller than
4ϕ (1/16 mm).
Table 1 lists the results of X-ray diffraction (XRD)
analyses of the Nb-a deposit from various trench sites(Locs. 1, 6, 16, and 31). The XRD analyses were performed
for the <2 μm fraction of the matrix of the Nb-a deposit.
The <2 μm fraction sample was separated by hydraulicelutriation 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 a scan speed of 2.0° per minute. An ethylene glycol
treatment was used to identify smectite. The analyses
indicate a variable assemblage of alteration minerals (e.g.,smectite, kaolinite, pyrophyllite) among the samples.

8．Discussion
The Nb-a deposit consists of altered lithic clasts in aclay-rich, fine-grained matrix, and contains no juvenile
magmatic pyroclasts (Fig. 4), suggesting it was producedby phreatic explosions. The grain-size distribution of thedeposit, which generally show a dominance of grains
smaller than 4ϕ (Fig. 6), and the presence of clay minerals(e.g., smectite and kaolinite ; Table 1) are consistent withthe deposit originating from phreatic explosions. The
presence of large lithic clasts (up to 30 cm in length)strongly suggests that the deposit was erupted from within
the Noboribetsu geothermal field. This interpretation isconsistent with the presence of many small explosion
craters in the field (Goto et al., 2011a).
The distributions of the thickness and maximum grain
size of the Nb-a deposit (Figs. 7 and 8) suggest that thedeposit was erupted from several explosion craters (Hiyoriyama Summit, Oyunuma, Kasayama North, Okunoyu, andTessen-ike). The complicated isopachs around Oyunuma,
Kasayama North and Okunoyu (Fig. 7) may be attributed
to overlapping of phreatic deposits erupted from thesecraters. This inference is consistent with the wellpreserved morphology of these explosion craters (Goto etal., 2011a). The explosion craters are aligned NNW-SSE,
extending from the Hiyoriyama cryptodome to the
Jigokudani Valley, and their distribution suggests thatphreatic eruptions, through multiple vents, occurred within
this zone. Lateral variations in the texture and componentof the Nb-a deposit may be attributed to the different
locations of the many source vents that were active during
the phreatic eruption. The variable assemblage of alteration minerals of the Nb-a deposit (Table 1) may also be
attributed to the many source vents. Similar phreatic
eruptions with multiple vents have been reported from the1910 and 2000 eruptions of Usu volcano, southwestern
Hokkaido, Japan (Ui et al., 2002 ; Yokoyama et al., 1973).
The isopach and maximum-grain-size isopleth maps of
the Nb-a deposit (Figs. 7 and 8) also suggest that the deposit

Multiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan

245

Fig. 5. Stratigraphic sections of the Nb‒a deposit obtained from the trenches. Location numbers of the sections correspondto those in Fig. 3.

Table 1. Results of X-ray diffraction (XRD) analyses of the <2 μm fraction of the Nb-a deposit, sampled from locations 1,
6, 16, and 31. The modal abundance of each mineral is classified as abundant (+++), moderate (++), or trace (+).

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Yoshihiko GOTO, Yoshimasa TORIGUCHI, Hirotaka SASAKI and Akira HATAKEYAMA

consists of several fall units that originated from different
craters. Previous workers have described two such fallunits. Katsui et al. (1988) described a phreatic fall deposit(the Shinki-Jigokudani pyroclastic fall deposit) from the
Jigokudani Valley. Goto et al. (2011b) reported a phreaticfall deposit (the Hiyoriyama-a phreatic deposit) from the
Hiyoriyama cryptodome. Both units occur above the Us-b
tephra, indicating they correspond to the Nb-a deposit. Our
results suggest that both the Shinki-Jigokudani and the
Hiyoriyama-a deposits represent subunits of the Nb-a
deposit that originated from the Tessen-ike Crater and the
Hiyoriyama Summit Crater, respectively.
The Nb-a deposit overlies, and is therefore younger
than, the Us-b tephra inferred to have been emplaced in
AD 1663 (Fig. 4 ; Yokoyama et al., 1973). Hence, thephreatic eruptions associated with the Nb-a deposit musthave occurred after AD 1663 (Katsui et al., 1988). The
presence of a soil layer between the Nb-a deposit and theUs-b tephra at several locations indicates that the explosive
events occurred after the late 17th century. An examinationof historical records (from AD 1845 to the present)
revealed no mention of the eruptions. The phreatic
eruptions are thus inferred to have occurred between thelate 17th century and AD 1845.
9．Conclusions
The most recent phreatic fall deposit that erupted fromthe Noboribetsu geothermal field (the Nb-a deposit) is
distributed in an elliptical area, measuring 850×1250 m
across. The deposit overlies the Us-b tephra, which was
emplaced in AD 1663. The isopach and maximum-grainsize isopleth maps of the Nb-a deposit suggest that phreatic
eruptions, through multiple vents, occurred in the Noboribetsu geothermal field after AD 1663. These eruptions
resulted in the formation of several explosion cratersaligned NNW-SSE. The present results are of value inevaluating the volcanic hazards in the Noboribetsu geothermal field.

Fig. 6. Grain-size histograms of the Nb-a deposit for
samples from locations 1 (A), 6 (B), 16 (C), and 31 (D).

Acknowledgements
This research was sponsored by the Ministry of
Education, Culture, Sports, Science and Technology ofJapan (MEXT), and was supported financially by the
Muroran Institute of Technology. We thank T. Danhara(Kyoto Fission-Track Co. Ltd.) for measuring the refractive index of volcanic glass. Comments by Y. Miyabuchi(Kumamoto University), an anonymous referee, and Y.
Suzuki (Waseda University) significantly improved themanuscript. A component of this research was performed
as part of compiling a hazard map of Kuttara volcano,issued by the City of Noboribetsu.

Multiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan

247

Fig. 7. Isopach map for the Nb-a deposit. The deposit is distributed in an elliptical area (850×1250 m across) that iselongate NNW-SSE. The thickness of the deposit increases toward the following explosion craters : Hiyoriyama Summit(HS), Oyunuma (OY), Kasayama North (KN), Okunoyu (OK), and Tessen-ike (TI) (red areas in the figure).
Topographic contour intervals are 2 m.

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Yoshihiko GOTO, Yoshimasa TORIGUCHI, Hirotaka SASAKI and Akira HATAKEYAMA

Fig. 8. Maximum-grain-size isopleth map for the Nb-a deposit. The deposit increases in maximum-grain-size toward thefollowing explosion craters : Hiyoriyama Summit (HS), Oyunuma (OY), Kasayama North (KN), Okunoyu (OK), andTessen-ike (TI) (red areas in the figure). Topographic contour intervals are 2 m.

Multiple Vent-forming Phreatic Eruptions after AD 1663 in the Noboribetsu
Geothermal Field, Kuttara Volcano, Hokkaido, Japan
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北海道クッタラ火山登別地熱地域の水蒸気噴火 : 1663 年以降の活動
後藤芳彦・鳥口能誠・佐々木央岳・畠山信

北海道クッタラ火山，登別地熱地域の最も新しい水蒸気噴火堆積物 (Nb-a) の調査を行い，その分布域と噴火様式を解明した．調査は登別地熱地域全域の 71 地点で手掘りトレンチ（深度 <90 cm）を掘削して行った．水蒸気噴火堆積物は，最大層厚 68 cm で，変質した岩石片と細粒なマトリクスからなり，有珠山 b 降下
軽石（Us-b，西暦 1663 年）を覆う．この水蒸気噴火堆積物は，北北西-南南東に伸長する 850×1250 m の範囲に分布し，その層厚と最大粒径は，複数の爆裂火口（日和山山頂，大湯沼，笠山北，奥の湯，鉄泉池）に向かって増大する．登別地熱地域では，西暦 1663 年以降（17 世紀末〜19 世紀ごろ），複数の地点で水蒸気噴火が起き，北北西-南南東方向に配列する複数の火口が形成されたと考えられる．