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

ArticleA Fallout Tephra from Tenchozan Volcano,
Shiretoko Peninsula, Hokkaido, Japan
Yoshihiko GOTOῌ
(Received March ., ,*++ ; Accepted September 1, ,*++)
Mt. Tenchozan, located on the Shiretoko Peninsula, eastern Hokkaido, Japan, is a Quaternary andesitic
volcano with crater chains on its summit. This paper reports on a fallout tephra (the Ten-a tephra) extruded
from the volcano. The tephra extends from the summit area of the volcano to the eastern shore of the peninsula,and is composed of pyroclastic lithic fragments, minor juvenile pumice and ash. The tephra increases in thickness
and grain size toward the summit of the volcano. Radiocarbon dating of a buried soil located immediately
beneath the tephra yields an age of +3-*῎.* years BP, and a calibrated calendar age of +30*ῌ+2+* cal BP (, s,3/ῌ probability). The distribution, components and radiocarbon age of the tephra suggest that phreatomagmatic eruptions took place at the summit of the volcano at ca. +3** years BP, resulting in the formation of thecrater chains.
Key words : fallout tephra, radiocarbon age, crater chain, Tenchozan Volcano, Shiretoko Peninsula+. Introduction
Mt. Tenchozan is a Quaternary andesitic volcano
located on the Shiretoko Peninsula, eastern Hokkaido,
Japan (Fig. +). The volcano has two crater chains on
its summit (Fig. ,, Moriya, +32. ; Katsui et al., +32/).
The craters display well-preserved, primary morphological features, suggesting a series of eruptions occurred inrecent geological time. However, the timing of theeruptions remains unknown. This paper reports on the
distribution, components and radiocarbon age of a fallout tephra extruded from the volcano, and discusses thetiming of the crater-forming eruptions.

chain trends southwest῍northeast, extends for +2** m,and consists of +* explosion craters. Each crater iscircular to elliptical in plan view, +**῍,/* m in diameter, and -*῍1* m deep. Some craters are connected toeach other. Larger craters are ﬁlled with water (Fig.
-A and -B). The southern chain trends southwest῍
northeast to west῍east, extends for 1** m, and consistsof four explosion craters. Each crater is elliptical inplan view, 1*῍+** m in diameter, and ,*῍-* m deep.
At present, the craters in both chains contain no active
fumaroles. Historical records make no mention of volcanic eruptions from these craters.

,. Tenchozan Volcano
Tenchozan Volcano is located ../ km southwest of
Rausu Volcano (Fig. ,A). It reaches an elevation of+*.0 m above sea level and has a base diameter of ,./῏
. km (Fig. ,A). The volcanic ediﬁce is ﬂat-topped
with steeply sloping sides and is composed of andesitic
lavas (Tenchozan Lavas, Fig. ,). The lavas retain
their primary morphological features, including curved
ﬂow fronts, and consist mainly of hypersthene῍augite
andesite. Table + lists the whole-rock major-element
chemical composition of the andesite (sample numbers
Ten-+-, -+., -+/, -+0, -+2, -,*), which contains /1῍0+
wt.ῌ SiO,.
There are two chains of explosion craters on thesummit (Fig. ,B, Katsui et al., +32/). The northern-. Fallout Tephra extruded from Tenchozan Volcano-ῌ+ Nomenclature
The newly identiﬁed fallout tephra extends from theTenchozan summit to the eastern shore of the Shiretoko
Peninsula. Outcrop locations are shown in Figure ,.
In this paper, the tephra is referred to as the ῌTenchozana tephra῍ (Ten-a tephra) (Goto et al., ,**/ ; Nakamura
et al., ,**2 ; Goto, ,**3). The type locality is in a
gully located +** m east of Shiretoko Pass (Loc. + in
Fig. ,, latitude ..ῐ*-῎+1ῑN, longitude +./ῐ*0῎,.ῑE),
where the tephra is located approximately -* cm beneath the surface (Fig. .).
-ῌ, Description
At the type locality, the Ten-a tephra is a /* cm thick,pale brown, massive (non-stratiﬁed) pyroclastic deposit,

ῒCollege of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1ῌ+, Muroran, Hokkaido */*ῌ2/2/, Japan.

Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

138

Yoshihiko GOTO

Fig. +. Location of Tenchozan Volcano on the Shiretoko
Peninsula, Hokkaido, Japan.

consisting of lithic fragments, pumice and ash (Fig. .).
The lithic fragments make up ,/῍-* vol.ῌ of the tephra
and are up to 2 cm across. They consist of subangular
to subrounded andesites of various compositions. The
andesites are gray to reddish brown, and vary in state ofoxidation (non-oxidized to intensely oxidized) and alteration (fresh to intensely altered). The fresh andesite lithic fragments are identical in petrographical features (phenocryst assemblage and size) to the Tenchozan
Lavas.
The pumice accounts for +*῍+/ vol.ῌ of the Ten-atephra and shows an upward increase in proportion
within the tephra layer. The pumice is pale gray, subrounded, up to 3 cm across (Fig. /), and commonly
thickly coated with a pale brown, ﬁne-grained ash.
The pumice consists of fresh volcanic glass and crystalsof plagioclase, hypersthene, augite, and opaque minerals. Table + lists the whole-rock major-element chemical composition of the pumice (sample numbers Ten-+
A and -+B), which contains 0,῍0- wt.ῌ SiO,.
The ash (ῌ0* vol.ῌ of the tephra) consists of alteredrock fragments, plagioclase, augite, hypersthene, magnetite, and fresh volcanic glass. The altered rock fragments are petrographically identical to the larger lithic
fragments. The volcanic glass is vesicular and shows a
pumice-like habit (cf., Yoshikawa, +310 ; Machida andArai, ,**-). X-ray di#raction (XRD) analysis of the
῏, mm fraction (separated by hydraulic elutriation)
revealed the presence of opal-CT and minor cristobalite.
XRD analysis with ethylene glycol treatment suggests
the absence of clay minerals.
Figure 0 shows the grain-size distribution of the Ten-atephra sampled at the type locality. Because the matrix
is partly cohesive, sieving was performed in a waterbath. The Ten-a tephra has a high proportion of grains
ranging from +/. to , mm in diameter (῍+ to , f).
Median diameter (Mdf) of the sieved sample is *.*, andstandard deviation (sf, Inman, +3/,) is -...
-ῌ- Stratigraphy and radiocarbon ages
Representative stratigraphic logs of the Ten-a tephra
are shown in Figure 1. At location + (the type locality), the geological section consists of (from lower toupper) : the Miocene Rausugawa Formation (Doi etal., +31* ; thickness ῐ/ m), a debris-ﬂow deposit (+.2m), a volcanic ash (+ cm), the Rausudake pyroclastic
ﬂow deposit (Miyaji et al., ,*** ; ++ m), a reworked
pyroclastic deposit (, m), the Ten-a tephra (/* cm), andthe surface soil (-* cm). The lowermost Rausugawa
Formation comprises greenish, intensely altered, tu#acious
breccia, consisting of angular andesite clasts, up to +/cm across, embedded in a ﬁne-grained matrix. The
formation yields a ﬁssion-track age of 1.2῎*.3 Ma(Koshimizu and Kim, +320). The overlying debrisﬂow deposit is pale brown and composed of subangularandesite cobbles, /῍+* cm across, in a sandy matrix.
The volcanic ash is pale gray, ﬁne grained, nonlaminated, and consists of fresh volcanic glass and crystals of plagioclase, hypersthene, augite, and opaqueminerals. The volcanic ash is inferred to be a fallout
tephra derived from Rausu Volcano, because the ash is
positioned just beneath the Rausudake pyroclastic ﬂow
deposit and is identical in mineral assemblage to thedeposit.
The Rausudake pyroclastic ﬂow deposit is gray andcomposed of several ﬂow units (Goto, ,**3). Each
unit is *./῍+ m thick and consists of angular andesitic
lithic clasts (+῍+* cm across) and subrounded andesitic
pumice clasts (῏-* cm across) in a matrix of andesitic
rock fragments (῏/ mm across). The andesitic lithic
clasts and pumice clasts contain phenocrysts of plagioclase, hypersthene, augite, and opaque minerals. The
pyroclastic ﬂow deposit has a ground surge layer (,῍cm thick) at the base, which includes charcoal fragments and partially carbonized wood. Radiocarbon
dating of a partially carbonized wood sample (number
SP-,, Fig. 1), determined by Beta Analytic (Miami,
USA), yields an age of ,-+*῎0* years BP (Table ,).
Calibrated calendar ages, obtained using the IntCal*.
calibration (Reimer et al., ,**. ; Talma and Vogel,+33-), are ,.0*῍,,3* and ,,1*῍,+0* cal BP (, s, 3/ῌprobability). The Rausudake pyroclastic ﬂow depositA Fallout Tephra from Tenchozan Volcano, Shiretoko Peninsula, Hokkaido, Japan

139

Fig. ,. (A) Distribution of the Tenchozan Lavas and the Rausudake Lavas. Outcrops of the Ten-a tephra are
indicated by solid circles. Location numbers of the outcrops correspond to those in Fig. 1. Contour interval
is +** m. (B) Crater chains on the summit of the Tenchozan Volcano. The northern chain trends southwest῍
northeast, extends for +2** m, and consists of +* explosion craters. The southern chain trends southwest῍
northeast to west῍east, extends for 1** m, and consists of four explosion craters. Contour interval is ,* m.
Locations of Figs. -A and -B are also shown.

is capped by a reworked pyroclastic deposit (reworked
Rausudake pyroclastic ﬂow deposit), , m thick. The
Ten-a tephra directly overlays the reworked pyroclastic
ﬂow deposit. The textures and components of the
Ten-a tephra are described in Section --,. The tephra
is overlain by a dark brown surface soil, -* cm thick.
At locations , and -, the geological sections show
similar sequences to those observed in the upper part ofsection + (Fig. 1). At locations . and /, the Ten-a tephra
covers a debris-ﬂow deposit. At location ., the tephra
contains a higher concentration of pumice (ῌ,* vol.ῌ
of the tephra) than that observed at location +. At
location /, the Ten-a tephra consists mainly of lithic
fragments.
At location 0 (Ichino-ike Pond, , km east of Rausuko Lake), the geological section consists of (from lower
to upper) : a buried soil (῎+* cm thick), a volcanic ash(, cm), a second buried soil (- cm), the Ten-a tephra (/cm), and the surface soil (+* cm). The lowermost
buried soil is dark brown and consists of peat. The
overlying volcanic ash is pale gray, ﬁne-grained, non-laminated, and consists of fresh volcanic glass and crystals of plagioclase, hypersthene, augite, and opaqueminerals. The volcanic ash corresponds to the volcanicash below Rausudake pyroclastic ﬂow deposit at location +, because they are identical in mineral assemblage(plagioclase, hypersthene, augite, and opaque minerals)
and stratigraphic location (below the Ten-a tephra).
The ash is therefore inferred to have derived from
Rausu Volcano (Goto, ,**3). Radiocarbon dating ofa soil sample, collected from the topmost one centimeterof the soil layer, located immediately below the volcanicash, yields a conventional radiocarbon age of ,,0*῍0*years BP and a calibrated calendar age of ,-0*῍,+-* cal
BP (, s, 3/ῌ probability, sample number TEN-/. ;Table ,). The second buried soil is dark brown andalso consists of peat. The Ten-a tephra is pale brown
and composed mainly of fresh to altered andesitic fragments up to + cm across. Radiocarbon dating of a soil
sample, collected from the topmost one centimeter ofthe second soil layer, yields a conventional radiocarbon
age of +3-*῍.* years BP and a calibrated calendar age

140

Yoshihiko GOTO

Fig. .. Ten-a tephra (Ten-a) at the type locality(+** m east of Shiretoko Pass ; Loc. + in Fig. ,).
The tephra is /* cm thick, pale brown, andcomposed of lithic fragments, pumice and ash.
It overlies the Rausudake pyroclastic ﬂow
deposit, which is capped by a reworked pyroclastic deposit. Note the person for scale. Scalebar is /* cm long.

Fig. -. Crater chains on the summit of Tenchozan
Volcano. (A) An explosion crater of the northern
chain viewed from the southwest. The diameterof the crater is ,/* m. This crater is ﬁlled withcold water, forming a /0-m-wide lake. Note theperson (left) for scale. The mountain behind thecrater is Rausu Volcano. (B) An explosion craterof the northern chain viewed from the northeast.
This crater is ﬁlled with water in springtime,
forming a shallow ,*-m-wide lake, but is dry fromsummer until autumn.

of +30*ῌ+2+* cal BP (, s, 3/ῌ probability, sample
number TEN-/+ ; Table ,). The Ten-a tephra is overlain by the dark brown surface soil, +* cm thick.
-ῌ. Distribution and volume
The thickness distribution and maximum grain size of
the Ten-a tephra are shown in Figures 2A and 2B,
respectively. The maximum grain size was calculated
as the average long-axis diameter of the three largest
lithic clasts. These data suggest that the tephra increases in thickness and maximum grain size toward thesummit of the Tenchozan Volcano. The bulk volumeof the tephra, calculated following Hayakawa (+32/)
and using the /-cm isopach, is ..,ῌ+*1 m-.

Fig. /. Pumice in the Ten-a tephra. The photograph
was taken after washing the sample in the laboratory. Before washing, the pumice had beencoated by a ﬁne-grained ash.

.. Discussion.ῌ+ Origin of the Ten-a tephra
The Ten-a tephra increases in thickness and maximum grain size toward the summit of Tenchozan Volcano, suggesting the tephra was erupted from the volcano.
The tephra is inferred to have formed by phreatomagmatic explosions because the tephra is composed ofnon-juvenile lithic fragments that are interpreted tohave formed by fragmentation of the pre-existing volcano surface, and fresh subrounded pumice, interpreted tobe juvenile pyroclasts formed by the fragmentation andejection of magma. The volume (..,ῌ+*1 m-) andgrain-size distributions of the Ten-a tephra (high pro-A Fallout Tephra from Tenchozan Volcano, Shiretoko Peninsula, Hokkaido, Japan

141

Table +. Whole-rock major-element compositions of andesites from the Tenchozan
Lavas (samples Ten-+-, -+., -+/, -+0, -+2, -,*) and pumice from the Ten-a tephra(Ten-+A and -+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. Sample locations (latitude, longitude) : Ten-+- (..῏*,῍.2ῐN, +./῏*/῍.,ῐE), Ten-+. (..῏*,῍.2ῐN, +./῏*/῍.,ῐE), Ten+/ (..῏*,῍.3ῐN, +./῏*/῍-0ῐE), Ten-+0 (..῏*,῍.2ῐN, +./῏*/῍--ῐE), Ten-+2 (..῏*,῍.+ῐN, +./῏*0῍+0ῐE), Ten-,* (..῏*,῍..ῐN, +./῏*/῍,-ῐE), Ten-+A and -+B (..῏*-῍+1ῐ
N, +./῏*0῍,.ῐE).

Fig. 0. Grain-size histogram of the Ten-a tephra,sampled from +** m east of the Shiretoko Pass(the type locality ; Loc. + in Fig. ,). The sample
was sieved at intervals of + f (where f῎ῌlog,d,with d being the grain size in millimeters), using a
set of sieves ranging from ῌ0 to . f (0. mm to +/+0 mm). The sieving was carried out in a water
bath because the samples were partly cohesive.
The sieved samples were dried and weighed to *.*+
g on a laboratory balance.

portion of grains smaller than , mm ; Fig. 0) are consistent with phreatomagmatic explosions (see Morrissey etal., ,***). The pumice in the tephra is thickly coatedwith a pale brown, ﬁne-grained ash, implying steam-richeruptions. The crater chains at the summit of theTenchozan Volcano are composed of a number of small
explosion craters (diameter : +**ῌ,/* m in the northern
chain, 1*ῌ+** m in the southern chain) and are consistent with steam-driven, multiple-vent-forming, explosive eruptions (cf., the +3+* eruption at Usu Volcano :Yokoyama et al., +31- ; the ,*** eruption at Usu Volcano : Ui et al., ,**,). The crater chains are thus inferred to have formed during the same eruptions as
those that produced the Ten-a tephra. Volcanic
Explosivity Index (VEI, Newhall and Self, +32,) of theeruptions is -.
.ῌ, Eruption age
Previous geochronological studies (Orlova andPanychev, +33- ; Okuno et al., +331 ; Xu et al., ,**.)
suggested that the radiocarbon age of a buried soil
located immediately below a mass-ﬂow or pyroclastic
deposit represents the emplacement age of the deposit.
The radiocarbon ages of the buried soil immediately
below the Ten-a tephra (sample TEN-/+) and a volcanic ash (sample TEN-/.) may represent the emplacement
ages of the tephra and ash. The TEN-/+ age for the
Ten-a tephra (+3-*῍.* years BP) and the TEN-/. age
for the volcanic ash (,,0*῍0* years BP) are consistent
with the stratigraphic positions of these deposits (Fig.
1). The TEN-/+ age for the Ten-a tephra (+3-*῍.*142

Yoshihiko GOTO

Fig. 1. Stratigraphic sections of the Ten-a tephra. Location numbers of the sections correspond to those in
Figure ,. The section at location + is shown at small and large scales. Radiocarbon ages are also shown.

Table ,.

Results of radiocarbon dating.

+.

C age* is based on Libby’s half-life (//02 years), uncorrected by d+-C values. Ages are expressed in BP (years
before AD +3/*) with an error range of +s. **The conventional +.C age includes d+-C correction. Ages are
expressed in BP with an error range of +s. ***Calibrated calendar ages were calculated from the conventional +.C
ages, using the program developed by Talma and Vogel (+33-), based on the IntCal*. calibration database (Reimer etal., ,**.). Ages are expressed in cal BP with an error range of ,s (3/ῌ probability) and +s (02ῌ probability).
Sample TEN-/+ is a dark brown soil (grain size ῎*./ mm), collected from the uppermost + cm of the --cm-thickburied soil layer located immediately below the Ten-a tephra at Ichino-ike Lake (Loc. 0 in Figs. , and 1, latitude..῏*+῎.2ῐN, longitude +./῏*0῎+2ῐE). Acid wash pretreatment was done before radiocarbon dating. Sample TEN/. is a dark brown soil (grain size ῎*./ mm), collected from the uppermost + cm of the +*-cm-thick buried soil layer
located immediately below a volcanic ash at Ichino-ike Lake (Loc. 0). Acid wash pretreatment. Sample SP-, is apartly carbonized wood (size .῍.῍0 cm), collected from a ground surge layer at the base of the Rausudake
pyroclastic ﬂow deposit, located +** m east of Shiretoko Pass (Loc. +, ..῏*-῎+1ῐN, +./῏*0῎,.ῐE). Acid-alkali-acidwash pretreatment.

years BP) is also consistent with the stratigraphy at
location +, where the tephra overlies the Rausudake
pyroclastic ﬂow deposit, which yields a radiocarbon ageof ,-+*ῌ0* years BP (Fig. 1, sample SP-,). The
TEN-/+ sample (+3-*ῌ.* years BP) yields a calibrated
calendar age of +30*῍+2+* cal BP, suggesting the Ten-atephra was emplaced at ca. +.3 ka. This age is consistent with the well-preserved morphology of the crater
chains on the summit of the volcano. Therefore, it isinferred that Tenchozan Volcano erupted at ca. +.3 ka,
resulting in the formation of the crater chains.

A Fallout Tephra from Tenchozan Volcano, Shiretoko Peninsula, Hokkaido, Japan

Fig. 2. (A) Thickness distribution of the Ten-a tephra (in cm). (B) Grain-size distribution of
the Ten-a tephra (in cm). The maximum grain size is determined by the average of thelong-axis diameters of the three largest lithics. The tephra increases in thickness andmaximum grain-size toward the summit of Tenchozan Volcano.

143

144

Yoshihiko GOTO

Acknowledgements
This research was supported ﬁnancially by the
Muroran Institute of Technology. The author thanks
H. Takizawa (Shiretoko Sanko-sha), Y. Yokoyama, A.
Sawada (Muroran Institute of Technology), N. Gouchi(Shiretoko Museum) and all the sta# at the Shiretoko
Museum for their help in the ﬁeld. H. Sasaki (Kokusai
Kogyo) and M. Nakagawa (Hokkaido University) are
thanked for constructive discussion. We are gratefulto I. Miyabuchi (Kumamoto University) and an
anonymous referee for reviewing the manuscript. I.
Miyagi (AIST) is thanked for editing the manuscript.
Part of this work was performed as part of compiling ageological guidebook, Geology of the Shiretoko Peninsula, issued by the Hokkaido Shinbun, Shari Town, andthe Shiretoko Museum.
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(Editional handling Isoji Miyagi)

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