Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 4, pp. 529-541

ArticleThe 18-19 ka Andesitic Explosive Eruption at Usu Volcano,
Hokkaido, Japan
Yoshihiko GOTO＊, Yuko SEKIGUCHI＊, Satomi TAKAHASHI＊, Hayuru ITO＊ and Tohru DANHARA＊＊(Received August 8, 2013 ; Accepted October 23, 2013)
Mount Usu is a Quaternary composite volcano located in southwestern Hokkaido, Japan. Here we report on anandesitic pyroclastic fall deposit (the Usu-Kaminagawa [Us-Ka] tephra) erupted during the initial stage of activity at
Usu Volcano. The tephra extends from the volcano to the east, and comprises a lower andesitic pumice-fall deposit andan upper ash-fall deposit. The tephra overlies the Nj-Os tephra, which was erupted from the Nakajima Islands, and isoverlain by the Usu Somma Lava, which was extruded during the early stages of activity at Usu Volcano. Radiocarbon
dating of buried soils located immediately beneath the Us-Ka tephra yields ages of 18-19 cal ka BP. The distribution,
stratigraphy, and lithology of the tephra, and the radiocarbon ages of the buried soils beneath the tephra, suggest that anandesitic explosive eruption occurred at Usu Volcano at ca. 18-19 ka. This eruption was probably an early
manifestation of activity at Usu Volcano.
Key words : Usu Volcano, tephra, stratigraphy, radiocarbon dating, early volcanic activity.

1．Introduction
Mount Usu is a Quaternary basaltic to rhyolitic composite
volcano located at the southern rim of Toya Caldera insouthwestern Hokkaido, Japan (Fig. 1). Mount Usu is oneof the most active volcanoes in Japan, and has experienced
at least nine major eruptions since AD 1663 (Katsui et al.,
1985; Katsui, 1988; Matsumoto and Nakagawa, 2010;
Mimatsu, 1962; Minakami et al., 1951; Nakagawa et al.,2005; Oba, 1966; Soya et al., 2007; Tomiya et al., 2010;
Ui et al., 2002; Yokoyama et al., 1973). Understanding theeruptive history of this volcano is essential for mitigationof the volcanic hazards in the region.
This paper presents a study of an andesitic pyroclastic fall
deposit (the Usu-Kaminagawa [Us-Ka] tephra; Yamagata
and Machida, 1996), inferred to have been erupted during
the initial stages of activity at Usu Volcano (Kobayashi
and Miyabuchi, 2006). Published geological and geochronological data for the tephra are sparse (Kobayashi and
Miyabuchi, 2006; Machida and Yamagata, 1996; Yamagata
and Machida, 1996), and further detailed study of thetephra may provide invaluable information with which toconstrain the eruptive history of the volcano. Herein, we
describe the distribution, stratigraphy, lithology, and radiocarbon ages of the tephra and discuss the early volcanic
history of Usu Volcano.

＊College of Environmental Technology, Graduate School ofEngineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan＊＊
Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,

2．Usu Volcano
Usu Volcano rises to an elevation 733 m above sea level,
and is a post-caldera cone of Toya Caldera (Fig. 1). The
caldera is 10 km×11 km in size and formed by violent
explosive eruptions associated with pyroclastic flows at ca.
110 ka (Ganzawa et al., 2007; Machida and Arai, 2003;
Okumura and Sangawa, 1984; Takashima et al., 1992;
Yokoyama et al., 1973). The Nakajima Islands, located in
the central part of the caldera, formed by multiple
extrusions of dacitic lavas at 40-45 ka (Takashima et al.,1992). After formation of the Nakajima Islands, Usu
Volcano became active (Soya et al., 2007).
Usu Volcano is a basaltic stratovolcano with a parasitic
scoria cone, and also includes a number of silicic lava
domes and cryptodomes (Fig. 2). According to Yokoyama
et al. (1973) and Soya et al. (2007), the stratovolcano wasconstructed at ca. 10-20 ka by repeated extrusion ofbasaltic lavas (Usu Somma Lava) and scoria (Fig. 2A). A
parasitic scoria cone (Donkoroyama) formed on thenortheastern foot of the stratovolcano during this stage(Fig. 2B). At ca. 7-8 ka, the summit of the stratovolcano
largely collapsed, resulting in generation of a debris
avalanche that travelled down to the southwestern foot ofthe volcano. As a result of this collapse, an amphitheaterthat is 2 km in diameter formed at the summit of thestratovolcano (Fig. 2B). After the summit collapse, Usu
Volcano was dormant for several thousand years, and theKita-ku, Kyoto 603-8832, Japan
Corresponding author: Yoshihiko Gotoe-mail: ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO, Yuko SEKIGUCHI, Satomi TAKAHASHI, Hayuru ITO and Tohru DANHARA

Fig. 1. Location of Usu Volcano, southwestern Hokkaido, Japan. Solid circles mark the survey locations of the Us-Ka
tephra. Location numbers of the outcrops correspond to those in Figs 4 and 5. The topographic contour interval is 100 m.

erupted magma changed to a silicic composition. Historical activity commenced with a Plinian eruption in AD
1663. Historical eruptions have been recorded in 1663,
1769, 1822, 1853, 1910, 1943-1945, 1977-1978, and2000. These historical eruptions formed silicic lava domes
and cryptodomes within the summit amphitheater and on
the stratovolcano flanks (e. g., Ko-Usu, O-Usu, UsuShinzan, and Showa-Shinzan; Fig. 2B).
3．Usu-Kaminagawa (Us-Ka) tephra
The Us-Ka tephra is inferred to have been erupted
during the initial stages of activity at Usu Volcano, and is
distributed to the east of the volcano (Fig. 1) at depths ofca. 3-10 m beneath the ground surface. In this paper, thetephra is referred to as the Us-Ka tephra following
Yamagata and Machida (1996) and Kobayashi and
Miyabuchi (2006). Machida and Yamagata (1996)
described the tephra as ‘a fallout tephra erupted from Usu
Volcano’. The Us-Ka tephra is presently well exposed in a
gully at Kami-tateyama in Date City (Loc. 1 in Fig. 1;
latitude 42° 3138N and longitude 140° 5245E), where
the tephra is located ca. 3 m beneath the ground surface(Fig. 3A).

At Kami-tateyama (Loc. 1), the Us-Ka tephra comprises
a lower pumice-fall deposit (35 cm thick) and an upper
ash-fall deposit (20 cm thick) (Fig. 3A). The pumice-falldeposit is gray in color and composed of andesitic pumice
clasts (〜71 vol. % of the tephra) and accessory lithic
pyroclasts (〜29 vol.%). The pumice clasts are gray to pale
gray, polyhedral with planar surfaces, poorly vesicular(density 1.3-1.5 g/cm3), and up to 10 cm in size (typically
1-3 cm) (Fig. 3B). The pumice consists of porphyritic
andesite containing phenocrysts of plagioclase (< 2 mmlong), hornblende (< 2 mm), hypersthene (< 0. 5 mm),augite (< 0. 5 mm), and opaque minerals (< 0. 3 mm)(Table 1). The groundmass has a hyalopilitic texture andconsists of fresh volcanic glass and acicular plagioclase
crystals (<0.1 mm). Table 2 lists the whole-rock major
element compositions of the pumice (sample numbers
UKN-1-2A, -14, and -36). The pumice has SiO2 contentsof 61-62 wt.%. The accessory lithic pyroclasts are angular
to subangular (up to 10 cm in size) and comprise various
types of andesite. The andesites are gray to brownish grayin color and vary in their oxidation state (non-oxidized tointensely oxidized) and alteration (fresh to intensely
altered). The pumice-fall deposit grades upwards to theThe 18-19 ka Andesitic Explosive Eruption at Usu Volcano, Hokkaido, Japan

Fig. 2. (A) View of Usu Volcano from the southeast. The
volcano comprises a basaltic stratovolcano (Usu Somma
Lava) and numerous silicic lava domes and cryptodomes(Ko-Usu, O-Usu, Showa-Shinzan, and Usu-Shinzan
domes). (B) Three-dimensional topographic image of
Usu Volcano showing its main geological features.
The stratovolcano has an amphitheater near its summit.
The base map was taken from the Red Relief Image
Map (Chiba et al., 2007) RRIM10 of Asia Air Survey,
using 10 m DEM data of the Geospatial Information
Authority of Japan (GSI).

ash-fall deposit. The ash-fall deposit is brownish gray incolor, massive (non-laminated), and comprises lithic
fragments, mineral fragments (plagioclase, quartz, hypersthene, hornblende, augite, and opaque minerals), andvolcanic glass, which are all up to 0.5 mm in size.
4．Tephra stratigraphy
Stratigraphic sections at representative locations, where
the Us-Ka tephra is exposed, are described below. The
mineral assemblages and volcanic glass refractive indicesof the tephras are listed in Table 1, and their whole-rockmajor element compositions are presented in Table 2.
4-1 Kami-tateyama (Loc. 1)
The stratigraphic section at Kami-tateyama (Loc. 1 inFig. 1; Figs 3 and 4A) consists of a tephra sequencecomprising the following units (from base to top): (1) the

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Fig. 3. Photographs of the Us-Ka tephra at Kamitateyama (Loc. 1 in Fig. 1). (A) Tephra sequence of theToya pyroclastic flow deposit (Toya), Kt-2 tephra (Kt2), Us-Ka tephra (Us-Ka), and a scoria-fall deposit(Sc). The Us-Ka tephra comprises a lower pumice-falland upper ash-fall deposits. (B) Close-up image of thepumice-fall deposit of the Us-Ka tephra, which
comprises andesitic pumice and accessory lithic
pyroclasts. The red segment of the scale ruler is 10 cm
in length.

Toya pyroclastic flow deposit (Machida and Arai, 2003);(2) the Kt-2 tephra (Yamagata, 1994); (3) the Us-Ka
tephra; (4) a scoria-fall deposit (‘a scoria deposit erupted
from Usu Volcano’; Machida and Yamagata, 1996); (5)the B-Tm tephra (Machida and Arai, 2003); (6) the Ko-dtephra (Machida and Arai, 2003); and (7) the Us-b tephra(Yokoyama et al., 1973).
The Toya pyroclastic flow deposit (>200 cm thick) is
pale reddish gray in color, and consists of subrounded,highly vesicular pumice clasts (<12 cm in size) set in a

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Yoshihiko GOTO, Yuko SEKIGUCHI, Satomi TAKAHASHI, Hayuru ITO and Tohru DANHARA

Fig. 4. Stratigraphic sections of the Us-Ka tephra at Kami-tateyama (A; Loc. 1 in Fig. 1) and Nuppa-omanai River (B; Loc.
11). Also shown in (A) is the stratigraphic position of the buried soil samples collected for radiocarbon dating. The UsKa tephra lies above the Kt-2 tephra and is covered by the scoria-fall deposit. Note that the Us-Ka tephra is found above
the Nj-Os tephra in (B).

fine-grained matrix (Fig. 3A). The pumice consists of freshvolcanic glass and crystals of plagioclase, quartz, hypersthene, hornblende, minor augite, and opaque minerals(Table 1). The deposit grades upwards into its weatheredequivalent (5 cm thick), which is capped by a 1-cm-thickdark brown humus soil layer.
The Kt-2 tephra (130 cm thick) is pale yellowish gray,and composed of highly vesicular pumice clasts up to 3 cm
in size (Fig. 3A). The pumice consists of fresh volcanic

glass and crystals of plagioclase, hypersthene, augite, andopaque minerals (Table 1). This tephra grades upwards
into its weathered equivalent (15 cm thick), which iscovered by a pale gray loam layer (1-2 cm thick; the term
‘loam’ is used for a humus-poor volcanic soil composed ofa mixture of weathered tephra, clay, silt, sand, and organic
matter; Bates and Jackson, 1983). The pale gray loam layer
is partly overlain by a dark brown humus soil layer that is
up to 0.5 cm thick.

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533

Table 1. Mineral assemblages and refractive indices of volcanic glass in the tephra deposits. The mineral
assemblage of the Usu Somma Lava is also shown.

Pl, plagioclase; Af, alkali feldspar; Qz, quartz; Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Hb, hornblende; Bt,biotite; Opq, opaque minerals; n.d., no data. Trace minerals are given in parentheses. The refractive indices of volcanic glass
were determined by a RIMS2000 instrument at Kyoto Fission-Track Co. Ltd.

The Us-Ka tephra (55 cm thick) overlies either the loam
or soil layer (Fig. 3A) and comprises a lower pumice-falldeposit (35 cm thick) and an upper ash-fall deposit (20 cm
thick). The texture and nature of the tephra are described in
Section 3. The Us-Ka tephrais directly overlain by thescoria-fall deposit.
The scoria-fall deposit (60 cm thick) is reddish black,well bedded, and comprises basaltic scoria clasts that are
up to 12 cm in size (Fig. 3A). At least six fall units (each2-10 cm thick) are identifiable. The scoria contains
crystals of plagioclase, hypersthene, augite, minor olivine,and opaque minerals (Table 1). The scoria has SiO2
contents of 51-52 wt.% (Table 2; samples UKN-1-9A and-9B). The scoria deposit is overlain by a brown loam layer(70 cm thick), which grades upward into a dark brown
humus soil layer (23 cm thick).
The B-Tm tephra (1 cm thick) is pale reddish gray, finegrained, and composed of volcanic glass and crystals ofalkali feldspar, plagioclase, quartz, hypersthene, hornblende,
augite, minor biotite, and opaque minerals (Table 1). The
B-Tm tephra is overlain by a dark brown humus soil layerthat is 3.5 cm thick.
The Ko-d tephra (0.5 cm thick) is pale gray, fine-grained,
and composed of volcanic glass and crystals of plagioclase, hypersthene, augite, hornblende, and opaque minerals(Table 1). The Ko-d tephra is covered by a dark brown
humus soil layer that is 2 cm thick.
The Us-b tephra (90 cm thick) is pale gray, and composed of moderately vesicular pumice clasts that are up to
6 cm in size. The pumice shows an upward increase in
grain size within the tephra layer. The pumice consists offresh volcanic glass and crystals of plagioclase, hypersthene, and opaque minerals (Table 1). The Us-b tephra isoverlain by dark brown surface soil that is 10 cm thick.
4-2 Nuppa-omanai River (Loc. 11)
The stratigraphic section at Nuppa-omanai River (Loc.
11 in Fig. 1; Fig. 4B) consists of a tephra sequence with thefollowing units (from base to top): (1) the Kt-2 tephra; (2)the Nj-Os tephra (Kasugai et al., 1990; Machida and Arai,
2003); (3) the Us-Ka tephra; (4) a scoria-fall deposit; (5)the Us-b tephra; and (6) the Us-Va tephra (Yokoyama etal., 1973).
The Kt-2 tephra (>60 cm thick) is pale yellowish grayand composed of highly vesicular pumice clasts that are up
to 8 cm in size. The pumice consists of fresh volcanic glassand crystals of plagioclase, hypersthene, augite, and opaqueminerals (Table 1). The Kt-2 tephra grades upwards into its
weathered equivalent (100 cm thick), which is partiallycapped by a dark brown humus soil layer (<1 cm thick).
The soil layer is covered by a gray loam layer that is 6 cm

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Yoshihiko GOTO, Yuko SEKIGUCHI, Satomi TAKAHASHI, Hayuru ITO and Tohru DANHARA
Table 2. Whole-rock major element compositions of pumice from the Us-Ka tephra (sample numbers UKN-12A, -14, and -36), pumice from the Nj-Os tephra (UKN-11-14A), basaltic scoria from the scoria-fall deposit(UKN-1-9A, -1-9B, and -22-40), and basaltic andesite from the Usu Somma Lava (UGRZ-1).

Compositions were determined by X-ray fluorescence spectrometry (Rigaku RIX-2000) at Shimane University,
Japan, following the analytical methods described by Kimura and Yamada (1996). Fe2O3*=total iron as Fe2O3.
L.O.I.=loss on ignition.

thick.
The Nj-Os tephra (100 cm thick) is pale gray andcomposed of moderately vesicular pumice clasts that are
up to 11 cm in size. The pumice consists of fresh volcanic
glass and crystals of plagioclase, quartz, hornblende,
hypersthene, and opaque minerals (Table 1). The pumice
has SiO2 contents of 61 wt.% (Table 2; sample UKN-1114A). The Nj-Os tephra is covered by a brown, weakly
laminated, ash-fall deposit that is 10 cm thick, which isinterpreted to be a phreatomagmatic fall deposit related to
the Nj-Os tephra. The ash-fall deposit is directly overlain
by the Us-Ka tephra.
The Us-Ka tephra (44 cm thick) comprises a lower
pumice-fall deposit (30 cm thick) and an upper ash-falldeposit (14 cm thick). The pumice-fall deposit is gray andcomposed of andesitic pumice clasts and accessory lithic
pyroclasts. The pumice clasts are gray to pale gray,polyhedral with planar surfaces, poorly vesicular, and up
to 7 cm in size (typically 0.5-2.0 cm). The pumice consistsof porphyritic andesite, containing phenocrysts of plagioclase, hornblende, hypersthene, minor augite, and opaqueminerals (Table 1). The accessory lithic pyroclasts aresubangular and composed of various andesite types (<9cm in size) and minor granites (<4 cm in size). The pumicefall deposit grades upwards to the ash-fall deposit. The
ash-fall deposit is brownish gray, weakly laminated, andcomposed of lithic fragments, mineral fragments(plagioclase, quartz, hypersthene, hornblende, augite, andopaque minerals), and volcanic glass, which are all up to 0.
5 mm in size. The ash-fall deposit is directly overlain by
the scoria-fall deposit.
The scoria-fall deposit (30 cm thick) is reddish black,well bedded, and composed of basaltic scoria clasts thatare up to 3 cm in size. At least nine fall units (each 1-5 cm
thick) are identifiable. The scoria contains crystals ofplagioclase, hypersthene, augite, and opaque minerals(Table 1). The scoria deposit is overlain by a 25-cm-thick,pale yellowish gray, loam layer that contains pumice. Thisloam layer is in turn covered by a brown loam layer that is
30 cm thick, which is capped by a dark brown humus soil
layer that is 5 cm thick.
The Us-b tephra (190 cm thick) comprises a lower
pumice-fall deposit (1 m thick) and an upper ash-falldeposit (90 cm thick, Us-b1; Yokoyama et al., 1973). The
pumice-fall deposit is pale gray and composed ofmoderately vesicular pumice clasts that are up to 5 cm insize. The pumice consists of fresh volcanic glass andcrystals of plagioclase, quartz, hornblende, hypersthene,
and opaque minerals (Table 1). The ash-fall deposit isgreenish gray, massive (non-laminated), and composed oflithic fragments, volcanic glass, and crystals of plagioclase,quartz, hypersthene, augite, and opaque minerals, all ofwhich are up to 2 mm in size (Table 1). The ash-falldeposit is directly overlain by the Us-Va tephra.
The Us-Va tephra (4 cm thick) is pale gray, and composed of moderately vesicular pumice clasts that are up to
4 cm in size. The pumice consists of fresh volcanic glassand crystals of plagioclase, quartz, hypersthene, hornblende,
minor augite, and opaque minerals (Table 1). The tephra isoverlain by a debris deposit (40 cm thick), which iscovered by a dark brown surface soil that is 10 cm thick.
4-3 Kami-nagawa (Loc. 22)
The stratigraphic section at Kami-nagawa (Loc. 22 inFig. 1; Fig. 5) consists of a tephra sequence comprising thefollowing units (from base to top): (1) the Toya pyroclastic
flow deposit; (2) the Kt-2 tephra; (3) the Nj-Os tephra; (4)The 18-19 ka Andesitic Explosive Eruption at Usu Volcano, Hokkaido, Japan

535

Fig. 5. Stratigraphic section and sketch of the Us-Ka tephra at Kami-nagawa (Loc. 22). The
Us-Ka tephra is overlain by a scoria-fall deposit and the Usu Somma Lava.

the Us-Ka tephra; (5) a scoria-fall deposit; (6) the Usu
Somma Lava (Yokoyama et al., 1973); and (7) the Us-b
tephra.
The Toya pyroclastic flow deposit (>13 m thick) is palereddish gray, and comprises a lower lithic-rich layer (>4m thick; lag breccia) and an upper pumice-rich layer (9 mthick). The lithic-rich layer consists of variably altered,subangular andesite clasts that are <30 cm in size, alongwith minor pumice clasts that are <5 cm in size set in afine-grained matrix. The pumice-rich layer consists ofsubrounded pumice clasts that are <6 cm in size and set ina fine-grained matrix. The pumice in both layers consistsof fresh volcanic glass and crystals of plagioclase, quartz,
hypersthene, hornblende, minor augite, and opaque

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Yoshihiko GOTO, Yuko SEKIGUCHI, Satomi TAKAHASHI, Hayuru ITO and Tohru DANHARA

minerals (Table 1). The pumice-rich layer is covered by apale reddish gray loam layer (100 cm thick), and capped bya dark brown soil layer that is 2 cm thick.
The Kt-2 tephra (150 cm thick) is pale yellowish grayand composed of highly vesicular pumice clasts that are up
to 7 cm in size. The pumice consists of fresh volcanic glassand crystals of plagioclase, hypersthene, augite, andopaque minerals (Table 1). The Kt-2 tephra is directly
overlain by the Nj-Os tephra.
The Nj-Os tephra (35 cm thick) is brownish gray,matrix-supported, and composed of pale gray, pumice
clasts that are <4 cm in size, set in a matrix of brownish
gray, fine-grained ash. The pumice consists of freshvolcanic glass and crystals of plagioclase, quartz,
hornblende, hypersthene, and opaque minerals (Table 1).
The tephra is covered by a pale gray loam layer (30-50 cm
thick) or a yellowish gray, reworked pyroclastic deposit(50 cm thick).
The Us-Ka tephra (198 cm thick) consists of a lower
pumice-fall deposit (28 cm thick) and an upper ash-falldeposit (170 cm thick). The pumice-fall deposit is gray,and composed of andesitic pumice clasts (ca. 70 vol.% ofthe deposit) and accessory lithic pyroclasts (ca. 30 vol.%).
The pumice clasts are gray to pale gray, polyhedral withplanar surfaces, poorly vesicular, and up to 21 cm in size
(typically < 12 cm). The pumice consists of porphyritic
andesite containing phenocrysts of plagioclase, hornblende,
hypersthene, augite, and opaque minerals (Table 1). The
accessory lithic pyroclasts are subangular and composed ofvarious andesite types (up to 7 cm in size) and minor
granites (up to 20 cm in size). The ash-fall deposit grades
upwards in color from gray to reddish brown, is weakly
laminated, and is composed of lithic fragments, mineralfragments (plagioclase, quartz, hypersthene, hornblende,
augite, minor biotite, and opaque minerals), and volcanic
glass, which are all up to 0.5 mm in size (Table 1). The
deposit contains abundant accretionary lapilli that are <5mm in size, which suggests it is a phreatomagmatic fall
deposit. The ash-fall deposit is directly overlain by thescoria-fall deposit.
The scoria-fall deposit (40 cm thick) is reddish black,well bedded, and composed of basaltic scoria clasts thatare up to 3 cm in size. Six fall units (each 2-20 cm thick)are identifiable. The scoria contains crystals of plagioclase,hypersthene, augite, and opaque minerals (Table 1). The
scoria has a SiO2 content of 54 wt. % (Table 2; sample
UKN-22-40). The scoria deposit is directly covered by the
Usu Somma Lava.
The Usu Somma Lava (6 m thick) comprises a lower
breccia facies (1 m thick) and an upper massive lava facies(5 m thick). The breccia is gray, clast-supported, andcomposed of angular, vesicular basaltic andesite clasts thatare 5-20 cm in size. The breccia grades upwards into themassive lava characterized by platy joints with 20-30 cm
spacing and columnar joints spaced at 2 m intervals. The

massive lava consists of porphyritic basaltic andesite
containing phenocrysts of plagioclase, olivine, augite,
hypersthene, and opaque minerals (Table 1). The lava has
a SiO2 content of 54 wt.% (Table 2; sample UGRZ-1). Thelava is covered by a dark gray, humus soil layer that is 5
cm thick.
The Us-b tephra (100 cm thick, Us-b1; Yokoyama et al.,1973) is greenish gray and comprises lithic fragments,
volcanic glass, and crystals of plagioclase, quartz,
hypersthene, augite, and opaque minerals, all of which are
up to 2 mm in size (Table 1). The tephra is covered by adark brown surface soil that is 15 cm thick.
5．Distribution and volume of the Us-Ka tephra
The thickness distribution and maximum grain size ofthe pumice-fall deposit of the Us-Ka tephra are shown in
Fig. 6A and 6B, respectively. The maximum grain size wascalculated as the average long-axis diameter of the threelargest pumice clasts. These data suggest that the pumicefall deposit increases in thickness and maximum grain size
toward Usu Volcano. The deposit was not found on
Nakajima Islands within Toya Caldera. The volume of thepumice-fall deposit calculated following the method of
Hayakawa (1985) using the 30-cm isopach is 1.8×108 m3.
The ash-fall deposit of the Us-Ka tephra also increases
in thickness towards Usu Volcano (e.g., 170 cm at Loc. 22;
20 cm at Loc. 1; 14 cm at Loc. 11). However, the thickness
distribution of the deposit is not presented here as the ashfall deposit is less well preserved than the pumice-falldeposit and, as such, it was difficult to obtain accurate
thickness data for the ash-fall deposit.
6．Radiocarbon dating
Radiocarbon ages were determined for five samples ofhumus soil collected from the entire thickness of the 5mm-thick soil layer that is found just beneath the pumicefall deposit of the Us-Ka tephra at Kami-tateyama (Fig.
4A; Table 3; samples UKN-4A, -4B, -23, -24, and -4AAAA). This soil layer was found when a new gully formed
in 2011. The soil layer contains no modern plant roots andshows no field evidence of disturbance (e.g., bioturbation
or erosion) during or after deposition. The five samples
were collected from the same stratigraphic position. The
soil samples are dark brown in color, and are fine-grainedorganic sediments (grain size <0.5 mm).
Radiocarbon dating of the samples was performed by
Beta Analytic (Miami, USA). Four samples (UKN-4A, 4B, -23, and -24) were pretreated by acid washes toremove carbonates. In the pretreatment, each soil sample
was: (1) sieved to <180 µm; (2) washed with 2N HCl at
80℃ for 4 h; (3) rinsed with hot (>95℃) distilled water;(4) washed with 2N HCl at 80℃ for 4 h; (5) rinsed with
hot distilled water; and (6) dried in a oven at 70℃ for 18 h.
The remaining carbon representing the bulk organic
fraction was analyzed by accelerator mass spectrometry

The 18-19 ka Andesitic Explosive Eruption at Usu Volcano, Hokkaido, Japan

Fig. 6. Thickness distribution (A) and maximum grain size distribution of pumice (B) of the Us-Ka pumice-falldeposit. The maximum grain size was taken to be the average of the long-axis diameter of the three largest pumice
clasts. The tephra increases in thickness and maximum grain size towards Usu Volcano.

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Yoshihiko GOTO, Yuko SEKIGUCHI, Satomi TAKAHASHI, Hayuru ITO and Tohru DANHARA
Table 3. Radiocarbon ages of buried soils collected from the entire thickness of the 5-mm-thick soil layer that isfound just beneath the pumice-fall deposit of the Us-Ka tephra at Kami-tateyama (Fig. 4A).

*

Based on Libbyʼs half-life (5568 y) and uncorrected for δ13C values. Ages are expressed in BP (years before AD 1950)
with an error range of 1σ. ** Conventional 14C age with a δ13C correction. Ages are expressed in BP with an error range of 1σ.
***
Calibrated ages were calculated from the conventional 14C ages using a program developed by Beta Analytic, based on
the IntCal09 calibration database (Heaton et al., 2009; Oeschger et al., 1975; Reimer et al., 2009; Stuiver and Braziunas,1993) with a spline smoothing function for the calibration curve (Talma and Vogel, 1993). Ages are expressed in cal BP
with an error range of 2 σ (95 % probability). AMS=accelerator mass spectrometry.

(AMS). One sample (UKN-4A-AAA) was pretreated bysequential acid-alkali-acid washes to remove carbonatesand humic acid. In the pretreatment, the soil sample was:(1) sieved to <180 µm; (2) washed with 0.5N HCl at 80℃
until carbonates were removed; (3) rinsed with hot (>95℃) distilled water; (4) agitated with 1 %-2 % NaOH at
25℃; (5) left in 1 %-2 % NaOH at room temperature for 8h; (6) rinsed with hot distilled water; (7) processes (4) to(6) were repeated until the supernatant liquid became
clear; (8) washed with 0.5N HCl at 80℃ for 1 h; (9) rinsed
with hot distilled water; and (10) dried in a oven at 70℃
for 12 h. The remaining alkali-insoluble carbon was
analyzed by AMS. δ 13C values of all samples wereanalyzed using a stable isotope mass spectrometer relative
to the Vienna Pee Dee Belemnite (VPDB) standard.
The samples yield conventional radiocarbon ages of 15,
530±60 (UKN-4A), 15,770±60 (UKN-4B), 14,810±60(UKN-23), 15,050±60 (UKN-24), and 14,920±60 BP(UKN-4A-AAA) (1 σ error; Table 1). The samples pretreated with acid washes (UKN-4A, -4B, -23, and -24) andacid-alkali-acid washes (UKN-4A-AAA) yielded identicalages.
Calibrated ages were calculated from the conventional
radiocarbon ages using a program developed by Beta
Analytic and based on the IntCal09 calibration database(Heaton et al., 2009; Oeschger et al., 1975; Reimer et al.,2009; Stuiver and Braziunas, 1993) with a spline smoothing
function for the calibration curve (Talma and Vogel,1993). The calibrated ages are 18,630-18,810 (UKN-4A),
18,810-18,930 (UKN-4B), 17,900-18,430 (UKN-23), 18,050-18,540 (UKN-24), and 17,980-18,490 cal BP (UKN4A-AAA) (2 σ error range; 95 % probability; Table 1).
7．Discussion
The Us-Ka tephra is inferred to have been erupted from

Usu Volcano for the following reasons: (1) the Us-Ka
tephra increases in thickness and maximum grain size
toward Usu Volcano (Fig. 6); (2) the Us-Ka tephra
contains large pumice clasts (up to 21 cm in size) and lithic
pyroclasts (up to 20 cm) at Kami-nagawa (Loc. 22), on thesoutheastern slope of Usu Volcano; and (3) pumice in the
Us-Ka tephra is characterized by low K2O (0.5-0.6 wt.%;
Table 2), which is consistent with chemical composition of
volcanic products from Usu Volcano (Soya et al., 2007).
Although the Nakajima Islands within Toya Caldera arelocated nearby Usu Volcano, we discount the possibility
that the Us-Ka tephra was erupted from the Nakajima
Islands as: (1) the Us-Ka tephra was not found on theislands; and (2) the Us-Ka tephra has lower K2O than the
Nj-Os tephra (0.9 wt.%; Table 2), which was erupted fromthe islands (Kasugai et al., 1990; Machida and Arai, 2003).
The stratigraphy of the Us-Ka tephra at Nuppa-omanaiRiver (Loc. 11; Fig. 4B) and Kami-nagawa (Loc. 22; Fig.
5) suggests that the Us-Ka tephra occurs above the Nj-Os
tephra that was erupted from the Nakajima Islands(Kasugai et al., 1990; Machida and Arai, 2003). Therefore,
the Us-Ka tephra was erupted after formation of the
Nakajima Islands. The stratigraphy of the Us-Ka tephra at
Kami-nagawa (Loc. 22; Fig. 5) suggests that the Us-Ka
tephra is directly overlain by a scoria-fall deposit and the
Usu Somma Lava (Fig. 5), both of which were extruded
during the initial stages of activity at Usu Volcano (Soya etal., 2007; Yokoyama et al., 1973). Thus, we infer that the
Us-Ka tephra was produced during the early stages ofactivity at Usu Volcano. Some studies have already notedthat the Us-Ka tephra was produced during the early stagesof activity at Usu Volcano (Kobayashi and Miyabuchi,
2006; Machida and Yamagata, 1996; Yamagata andMachida, 1996), which is consistent with our results.
The eruption age of the Us-Ka tephra can be inferredThe 18-19 ka Andesitic Explosive Eruption at Usu Volcano, Hokkaido, Japan

from the radiocarbon ages of buried soils just beneath thetephra (Table 3). In general, radiocarbon ages of buriedsoils located immediately below a mass flow or pyroclastic
deposit represent the emplacement age of the deposit(Okuno et al., 1997; Orlova and Panychev, 1993; Xu et al.,2004). However, this requires that the deposit overlies thesoil with no disturbance and that the soil was a closed
system after emplacement of the deposit. In the case of the
Us-Ka tephra, the soil shows no sign of disturbance,suggesting that the tephra passively mantled the soil layer.
Soil samples pretreated by acid washes (UKN-4A, -4B, 23, and -24) and pretreated by acid-alkali-acid washes(UKN-4A-AAA) yield identical ages, indicating that aclosed system was maintained in the soil layer after
emplacement of the tephra. We therefore infer thatradiocarbon ages of the buried soil immediately below the
Us-Ka tephra represent the emplacement age of the Us-Ka
tephra. Calibrated ages for the samples are 18-19 cal ka
BP (Table 1), suggesting that the Us-Ka tephra wasemplaced at this time. We therefore conclude that an
explosive andesitic eruption responsible for the Us-Ka
tephra occurred at ca. 18-19 ka at Usu Volcano. This age
is consistent with the timing of initiation of activity at Usu
Volcano estimated by previous studies (10-20 ka;
Yokoyama et al., 1973; Soya et al., 2007).
The Us-Ka tephra comprises a lower pumice-fall depositthat consists mainly of andesitic pumice, and an upper ashfall deposit that includes accretionary lapilli, suggestingthat the tephra was formed by early, explosive andesitic
magmatism and later, phreatomagmatic activity. The
presence of phreatomagmatic activity implies a water-richenvironment. Such an environment is to be expected
because Usu Volcano is located at the southern rim ofToya Caldera, which is filled with water. The eruption that
produced the Us-Ka tephra most likely produced a
volcanic crater, although we did not locate such a crater in
the area of Usu Volcano. The source crater of the Us-Ka
tephra may have been buried by emplacement of the Usu
Somma Lava (Fig. 2B).
Most previous studies (e.g., Katsui et al., 1985; Katsui,
1988; Soya et al., 2007; Yokoyama et al., 1973) havesuggested that the eruptive history of Usu Volcano ischaracterized by early basaltic activity that produced the
Usu Somma Lava (SiO2=49-54 wt.%; Yokoyama et al.,1973) and later silicic activity that produced lava domes(SiO2 = 68-73 wt. %; Yokoyama et al., 1973), and that
andesitic activity has not taken place at Usu Volcano.
However, our study indicates that Usu Volcano began
activity with the extrusion of andesitic magma (SiO2 =61-62 wt.%; Table 2), and this necessitates a revision of
the eruptive history of the volcano. The chemicalcomposition of the Us-Ka tephra (SiO2 =61-62 wt.%) isintermediate to that of the Usu Somma Lava and lava
domes and, as such, our study indicates that further
detailed petrological study of the Us-Ka tephra would

539

provide invaluable information on the magmatic evolutionof Usu Volcano.
Acknowledgements
This research was financially supported by the MuroranInstitute of Technology. T. Yamashita (Kyoto FissionTrack Co. Ltd) is thanked for technical support indetermining the refractive indices of volcanic glass. S.
Matsuyama (Geoscience Laboratory) is thanked forconstructive discussions about the radiocarbon analyses. T.
Kobayashi (Kagoshima University) and Y. Miyabuchi(Kumamoto University) provided helpful discussions in
the field. Comments by anonymous referees and N.
Hasebe (Kanazawa University) significantly improved themanuscript.
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北海道有珠火山，18-19 ka の安山岩質爆発的噴火
後藤芳彦・関口優子・高橋里実・伊藤

映・檀原

徹

北海道南西部の有珠山東方に分布する有珠上長和テフラ (Us‒Ka) は，下部の軽石層と，上部の細粒火山灰層からなる．軽石層は安山岩質の軽石と石質岩片からなり，軽石の SiO2量は 61-62 wt.% である．火山灰層
は火山豆石を含み，マグマ水蒸気噴火により形成されたことを示す．Us‒Ka テフラは，洞爺湖中島起源のNj‒Os テフラを覆い，有珠外輪山溶岩に覆われる．テフラ直下の土壌層は，18-19 cal ka BP の放射性炭素年
代値を示す．テフラの層厚と最大粒径は有珠山に向かって増大し，有珠山から噴出したことを示す．Us‒Ka
テフラは，約 18-19 ka の爆発的噴火により形成され，この噴火は有珠火山の先駆的な活動であると考えられ
る．有珠山は，安山岩質の爆発的噴火により活動を開始し，玄武岩質溶岩（有珠外輪山溶岩）を噴出して成層火山を形成した．この成層火山の山体崩壊後，珪長質マグマの噴出により溶岩ドーム群を形成した．

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