火山 第 59 巻 ( 2014)
第 3 号 185-206 頁

論 説

男体火山の最近 17,000 年間の噴火史
石 崎 泰 男＊・森 田 考 美＊＊・岡 村 裕 子＊＊,＊＊＊・小 池 一 馬＊＊＊＊・
宮本亜里沙＊＊＊＊・及 川 輝 樹＊＊＊＊＊（2014 年 5 月 1 日受付，2014 年 8 月 11 日受理）

Eruption History of Nantai Volcano During the Last 17,000 Years
Yasuo ISHIZAKI＊, Takami MORITA＊＊, Yuko OKAMURA＊＊,＊＊＊, Kazuma KOIKE＊＊＊＊,
Arisa MIYAMOTO＊＊＊＊ and Teruki OIKAWA＊＊＊＊＊
Nantai volcano (2,486 m a.s.l.), a near-conical stratovolcano with a summit crater ~1 km wide, is located along thevolcanic front of NE Japan. To date, the eruptive history and characteristics of this volcano have been poorly studied,except for an explosive eruption that occurred at ~17 cal. ka BP (Stage 2 eruption). In this paper, we present the results ofinvestigation of the stratigraphy of recent proximal eruption products, the tephrostratigraphy of the northeastern foot ofthe volcano, and new radiocarbon ages. The results show that at least six eruptions of Nantai volcano have occurred afterStage 2 eruption, and we refer to these as Stage 3 eruptions.
We identify four tephra layers and one pyroclastic flow deposit in the soil sections above the Stage 2 pumice flow
deposit, at the northeastern foot of the volcano. These are classified in the ascending order as: (1) Nantai-Bentengawara
Tephra 4 (Nt-Bt4), (2) Bentengawara Pyroclastic Flow Deposit, (3) Nt-Bt3, (4) Nt-Bt2, and (5) Nt-Bt1. The Nt-Bt2 isphreatic fallout with no juvenile material; the other tephra layers are phreatomagmatic fallouts containing juvenile
pyroclasts together with ash aggregates. Six Stage 3 eruption products are identified within and around the summit
crater: (1) a blocky lava flow (Osawa Lava) exposed on the northern crater wall, (2) a partly dissected scoria cone and(3) a poorly-preserved tuff ring (and its resedimented deposits) on the steep crater floor, (4) a subaqueous lava andassociated lava fragments sandwiched by lacustrine deposits, (5) a tuff breccia containing hydrothermally-altered lava
block and clayey matrix, and (6) a stratigraphically uppermost phreatomagmatic tephra (Nantai-Yudonoyama Tephra).
All the proximal eruption products, except for the subaqueous lava, can be correlated with the tephra layers and thepyroclastic flow deposit on the northeastern foot based on stratigraphic positions, lithologies, and geochemical affinities.
Our study reveals that five tephra-forming eruptions (ca. 14, 12, 8, 7. 5, and 7 cal. ka BP) and one non-explosive
subaqueous eruption (between 12 and 8 cal. ka BP) occurred during Stage 3, from a discrete eruption center inside thesummit crater. Moreover, the tephra-forming eruptions were diverse in style, with strombolian (12 cal. ka BP),phreatomagmatic (14, 8, and 7 cal. ka BP), and phreatic (7. 5 cal. ka BP) eruptions. Eruption style was determinedprimarily by vent position and spatiotemporal variations in local hydrological factors (e.g., the presence or absence of acrater lake, wet lacustrine deposits, and streams).
Key words : Nantai volcano, proximal eruptives, tephra, eruption history＊〒930-8555 富山市五福 3190
富山大学大学院理工学研究部
Graduate School of Science and Engineering, Universityof Toyama, 3190 Gofuku, Toyama 930-8555, Japan.
＊＊〒930-8555 富山市五福 3190
富山大学大学院理工学教育部
Graduate School of Science and Engineering for Education, University of Toyama, 3190 Gofuku, Toyama
930-8555, Japan.
＊＊＊〒315-0035 茨城県石岡市南台 3-1-11
岡村地質
Okamura Chishitsu Co. Ltd., 3-1-11 Minamidai, Ishioka,

Ibaraki 315-0035, Japan.
〒930-8555 富山市五福 3190
富山大学理学部地球科学科
Department of Earth Science, Faculty of Sciences, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan.
＊＊＊＊＊〒305-8567 茨城県つくば市東 1-1-1 中央第 7独立行政法人産業技術総合研究所地質情報研究部門
AIST, Geological Survey of Japan, Tsukuba Central 7,1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan.
＊＊＊＊

Corresponding author: Yasuo Ishizaki
e-mail: ishizaki@sci.u-toyama.ac.jp