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 ‘phreaticeruptions’ 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 (Katsuiet 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 volcaniclake (Oyunuma Lake), and a fumarolic valley (JigokudaniValley) (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