Bull. Volcanol. Soc. Japan
Vol. /- (,**2) No. /, pp. +/+ῌ+/1

LetterThe Tephra Layers Distributed around the Eastern Foot of the Zao Volcano
ῌAges and Volumes of the Za-To + to . Tephrasῌ
Kotaro MIURA῍, Masao BAN῍῍ and Hiroshi YAGI῍῍῍(Received, February ,3, ,**2 ; Accepted, October 3, ,**2)
Ages of four scoriaceous volcanic sand layers (Za-To + to .) distributed around the eastern foot of the Zao
volcano are determined to be ca. --ῌ-,, -*.1, ,1.+, and +,.3 cal kyr BP, respectively, based on four newly obtained
radiocarbon dates and loess chronometry. Besides, we detect To-H (To-HP) glasses from the loess below theZa-To . tephra. The estimated age of the loess horizon is consistent with the previously reported one for To-H(To-HP). Volumes of Za-To + to . tephras are estimated to be ca. -῍+*ῌ,, -῍+*ῌ+, +῍+*ῌ+ and +῍+*ῌ+ DREkm-, respectively, and the averaged discharge rate from Za-To + to . is calculated to be ca. *.*,ῌ*.*- km-/ky.
Key words : Zao volcano, tephra layers, radiocarbon dating, volcanic glass, discharge rate+. Introduction
The Zao volcano is a stratovolcano which is locatedin the central part of the volcanic front of the northeast
Japan arc (Fig. +). This volcano started its activity at
about + Ma (Takaoka, et al., +323), and has continued
to the present day. Geologic and petrologic studies of
the whole of the eruptive products were performed byvarious authors (e.g. Chiba, +30+ ; Oba and Konda,+323 ; Sakayori, +33,), and according to these studies,the newest stage of the Zao volcano began at ca. -* ka,
and numerous small to medium sized eruptions havecontinued for the past -*-ky. At about -* ka, the horseshoe-shaped Umanose caldera (+.1 km in diameter),which is located in the central part of the Zao, wasformed by explosive eruptions (Sakayori, +33,). The
newest cone Goshikidake is situated in the inner part of
the Umanose caldera and the newest crater lake Okama
is located in the western part of the Goshikidake.
Tephro-stratigraphy is very useful to establish thedetailed eruptive history of volcanoes (e.g. Thorarinsson,+32+). In the case of the Zao newest stage, the tephrostratigraphic studies were performed by various authors(e.g., Imura, +330, +333 ; Itagaki et al., +32+ ; Saigusa
and Shoji, +32.). Imura (+333) recognized ten volcanic
sand layers (Za-To + to +*). Za-To + to . tephras aremostly distributed around the eastern foot of the Zao
volcano, while Za-To / to +* are mainly near the summit

area. Imura (+333) also estimated that the ages ofZa-To + to . tephras are older than ca. ,* ka, based ona radiocarbon age (Itagaki et al., +32+), and the estimated AT (Aira-Tn tephra) horizon. AT is a widespread tephra ejected from the Aira caldera about ,3 cal
kyr BP (Okuno, ,**,), widely distributed in Japan. In
the Zao, AT volcanic glasses which is the bubble wall
type (Machida and Arai, ,**-) are found in the upper
part of the loess below the Za-To . tephra (Imura,+330). In this study, we determined the ages of Za-To+ to . tephras layers based on newly obtained fourradiocarbon dates, coupled with the loess chronometry(Hayakawa, +33/) and tephrochronology. Using thisnew age data and the magma discharge volume estimated from the isopach maps, we have calculated themagma discharge rates during the Za-To + to . activities.

῎Division of Interactive Symbiosphere Sciences, Grad-῎῎῎Department of Information, Environmental and Fooduate School of Science and Engineering, Yamagata
University, +ῌ.ῌ+,, Kojirakawa-machi, Yamagata, 33*ῌ
2/0*, Japan.
῎῎Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, +ῌ.ῌ+,, Kojirakawa-machi, Yamagata, 33*ῌ2/0*, Japan.

Sciences, Faculty of Education, Art and Science, Yamagata University, +ῌ.ῌ+,, Kojirakawa-machi, Yamagata,33*ῌ2/0*, Japan.

,. Stratigraphic description of the Za-To + to . tephralayers
Za-To + to . tephra layers consist of scoriaceous
volcanic sand, and are well sorted and laminated. In
an outcrop at Haraobi (Loc. 1 ; Fig. +), all of the Za-To+ to . tehpra layers can be observed and they areintercalated in the brown colored loess (partly reddishdark brown colored paleosol), observed at the easternfoot of Zao.

Corresponding author : Masao Ban
e-mail : ban@sci.kj.yamagata-u.ac.jp