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

A Fallout Tephra from Tenchozan Volcano,
Shiretoko Peninsula, Hokkaido, JapanYoshihiko 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) extrudedfrom 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 thedistribution, 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 agully located +** m east of Shiretoko Pass (Loc. + inFig. ,, 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