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A Newly Found Pyroclastic Flow Deposit around +* kaat Nikko Nantai Volcano, Northeast Japan
Yasuyuki MIYAKEῌ, Miyuki SAITOῌῌ, Yoshihiro TAKESHITAῌῌῌ,Teruki OIKAWAῌῌῌῌ and Takeshi SAITOῌῌῌῌῌ
Nantai Volcano is a symmetrical stratovolcano, situated in the southern part of the Northeast Japan arc.
Many geologic studies hitherto have suggested that the stratovolcano was formed during the Main stage, and theoverlying pyroclastic materials and a lava ﬂow were formed in the Later stage. Because no sedimentary gap isfound between any deposits of the Later stage, it is inferred that all of the activity in the Later stage took place
successively around +, ky BP (+/ῌ+. cal ka BP) and went dormant until now. However, we found a pyroclastic
ﬂow deposit named Bentengawara Pyroclastic Flow Deposit (BPFD) at the northeastern ﬂank of the Nantai
volcano about , km from the summit crater. This deposit overlies an 2* cm thick deposit of weathered ashy
sediments that in turn overlies the Arasawa Pumice Flow Deposit, a member of the Later stage.
The lower half of the BPFD consists of volcanic lapilli and ash that is remarkably ﬁne-depleted while theupper half contains abundant scoria of mainly lapilli-block sized clasts. The deposit also includes a small numberof breadcrust blocks and occasional accessory lava blocks and fragments of charred wood. The breadcrust blocks
consist of a dense outer crust that is signiﬁcantly fractured and a vesiculated interior. It is noteworthy that theedges of the cracks are sharp and never rounded, suggesting that the vaporization of the inner magma that
produced these cracks took place just before or immediately following the settlement of the blocks.
Paleomagnetic data from three breadcrust block samples indicate that the magnetic vectors of high temperaturecomponents are aligned with our present-day poles. Two pieces of charred wood were measured for their +.C ages
with results of +,ῌ++ cal ka BP. The whole rock chemistry of scoria and breadcrust blocks are determined to besigniﬁcantly di#erent from any of the rocks of the Later stage, but the accessory block in the BPFD has the similar
chemistry to the Osawa Lava, the last product of the Later stage.
We therefore suggest that the BPFD was deposited after the Later stage with a short ("- ka) dormant period
between them. Since the age is possibly around +* ka, the Nantai volcano should be counted as active volcano
based on the deﬁnition provided by the Meteorological Agency of Japan.
Key words : Nantai volcano, pyroclastic ﬂow, breadcrust block, active volcano
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Department of Geology, Faculty of Science, Shinshu
University, Asahi -ῌ+ῌ+ Matsumoto, -3*ῌ20,+, Japan.
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am/pm Japan, Roppongi +ῌ2ῌ1, Tokyo, +*0ῌ**-,, Japan.
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Togakushi Museum of Natural History, -.** Tochiwara,
Togakushi, Nagano -2+ῌ.+*., Japan.
 #-*/ῌ2/01 UVW&2 +ῌ+ῌ+XY 1 Z[\]^_`/0ab.cde()*+,National Institute of Advanced Industrial Scienceand Technology (AIST), GSJ, Institute of Geology
and Geoinformation +ῌ+ῌ+ Higashi, Tsukuba, -*/ῌ
2/01, Japan.
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International Young Researchers Empowerment Center,
Shinshu University, Asahi -ῌ+ῌ+ Matsumoto,-3*ῌ20,+,
Japan.
Corresponding author : Yasuyuki Miyake
e-mail : ymiyake@shinshu-u.ac.jp