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0  ,, The Owakidani Tephra Group: A Newly Discovered Post-magmaticEruption Product of Hakone Volcano, Japan
Makoto KOBAYASHIῌ, Kazutaka MANNENῌῌ, Mitsuru OKUNOῌῌῌ,Toshio NAKAMURAῌῌῌῌ and Kazuo HAKAMATAῌῌῌῌῌ
We discovered a set of phreatic explosion deposits, herein referred to as the Owakidani tephra group, on thenorthern slope of Mt. Kamiyama and in the Owakidani fumarolic area of the Hakone Volcano. The tephra groupis the product of the volcanic activities since the latest magmatic eruption of Hakone Volcano at around ,.3 ka.
It comprises ﬁve units named Hk-Ow+ to Hk-Ow/ in the ascending order. Both Hk-Ow+ and Hk-Ow, comprisetephra fall deposits and secondary debris ﬂow deposits. In addition to these deposits, Hk-Ow, is also associatedwith surge deposits. Hk-Ow-, Hk-Ow. and Hk-Ow/ consist of tephra fall deposits. The ash of these tephra falldeposits and the matrix of the secondary debris ﬂows are principally composed of clay, altered lithics andsecondary minerals supposed to be of fumarolic area origin.
It is possible that Hk-Ow+ and Hk-Ow, erupted from a ﬁssure on the northeastern ridge of Mt. Kamiyama,
while Hk-Ow-, Hk-Ow. and Hk-Ow/ erupted at Owakidani.
No juvenile material was found within the deposits of these eruptions except for Hk-Ow,, while the surge
deposit of Hk-Ow, contained trace amounts of volcanic glass fragment. Although it is considered that theprincipal nature of the eruptions of the Owakidani tephra group is phreatic, the deformation of the ediﬁce aroundthe source area implies the possibility of magma intrusion to shallow levels.
Based on the calendar ages of the Owakidani tephra group and the stratigraphic position of the KozushimaTenjosan tephra, we estimated that Hk-Ow-, Hk-Ow. and Hk-Ow/ erupted in relatively short intervals between
the latter half of the +,th and +-th centuries. On the other hand, Hk-Ow+ and Hk-Ow, erupted at around - kyr
BP and , kyr BP, respectively.
The eruption ages of the Owakidani tephra group generally correspond to the seismic events that occurred in
the Kozu-Matsuda Faults and the Tanna-Hirayama tectonic line. It is suggested that the activity of the Hakone
Volcano may be closely related to the tectonic events in this region.
Key words : Hakone volcano, Owakidani tephras, phreatic eruption, Kozushima-Tenjosan tephra, radiometriccarbon age!--+ῌ20-2 "#$%&'()*+,- ,ῌ,1,ῌ./01234567896:
Dia Consultant Co., Ltd., ,ῌ,1,ῌ- Yoshino-cho, Kitaku, Saitama, Saitama --+ῌ20-2, Japan.
!,/*ῌ**-+ ;<=$>)?@ /20;<=$ABCDEFGHot Springs Research Institute of Kanagawa Prefecture, /20 Iriuda, Odawara, Kanagawa ,/*ῌ**-+, Japan.
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Department of Earth System Science, Faculty of Science, Fukuoka University, 2ῌ+3ῌ+ Nanakuma, Jonan-ku, Fukuoka, Fukuoka 2+.ῌ*+2*, Japan.
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Center for Chronological Research, Nagoya University,Furo-cho, Chikusa-ku, Nagoya, Aichi .0.ῌ20*,, Japan.
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Hakone Town O$ce, ,/0 Yumoto, Hakone, AshigaraShimo, Kanagawa ,/*ῌ*-32, Japan.
Corresponding author : Makoto Kobayashi
e-mail : M.Kobayashi@diaconsult.co.jp