火山 第 58 巻 ( 2013)
第 4 号 499-517 頁

論 説

富士火山北東斜面の滝沢 B 火砕流堆積物の発生・堆積機構
田 島 靖 久＊・吉 本 充 宏＊＊・黒 田 信 子＊,†・瀧尚 子＊＊＊,†・＊＊＊＊＊＊＊＊＊,††・宮 地 直 道・遠 藤 邦 彦＊＊＊＊＊
千葉達朗（2012 年 7 月 4 日受付，2013 年 8 月 23 日受理）

The Formation and Deposition Processes of Takizawa B Pyroclastic Flow Deposits
on the Northeastern Flank of Fuji Volcano, JapanYasuhisa TAJIMA＊, Mitsuhiro YOSHIMOTO＊＊, Nobuko KURODA＊,†, Naoko TAKI＊＊＊,†,Tasturo CHIBA＊＊＊＊, Naomichi MIYAJI＊＊＊＊＊,††・Kunihiko ENDO＊＊＊＊＊
Eruptions of the Fuji volcano during the Younger-Fuji periods generated basaltic scoria and ash pyroclastic flow
deposits. Pyroclastic flows are destructive volcanic hazard because they are high-velocity gravity driven flows andcontain extremely high-temperature pyroclastic materials. On the northeast flank of the Fuji volcano, pyroclastic flows
were triggered by the collapse of scoria cones and lava, which erupted in the last 1,500 years. Takizawa B1 pyroclastic
flow deposit is distributed as far as 5-7 km from the vent. How these pyroclastic flows resulted from basaltic eruptions
and how they reached such distances from the vent are poorly understood. In our field investigations, Takizawa B1
pyroclastic flow and cone deposits were identified. In the middle-lower altitude areas, the lower part of the Takizawa B1
pyroclastic flow deposit consists of a scoria- and lithic lapilli-reduced ash units interbedded with a thin ash-rich units.
The upper part consists of ash-rich flow units. Scoriaceous materials vary vertically in the Takizawa B1 pyroclastic flowdeposit at the middle altitude areas. In the high-altitude area, we found four scoria cone deposits ‒ the Yoshidaguchi
cones, which contained scoriaceous materials. This vertical variation in scoriaceous materials within the Takizawa B1
pyroclastic flow deposit is consistent with the distribution of the Yoshidaguchi cones that contain scoriaceous materials
and lava from 2150 to 3000 m elevation. Therefore, we conclude that the collapse progressed from a low to highelevation in the vent generation area. The Takizawa B2 pyroclastic flow deposit, which has the same depositionalstructure as the Takizawa B1 pyroclastic flow deposit, contains a lower temperature, lithic-rich bottom layer and a hightemperature, ash-rich upper layer. The transition of the depositional textures and temperatures indicates the collapse
occurred from the outer to inner parts of the scoria cones.
Key words : Pyroclastic flow, Basalt, scoria cones collapse, deposition processes, Fuji volcano＊〒812-0007 東京都千代田区麹町 4-2
日本工営株式会社
Nippon Koei CO., LTD., 2 Koujimachi, 4-Chome, Chiyodaku, Tokyo 102-0083, Japan＊＊〒060-0810 札幌市北区北 10 条西 8 丁目北海道大学大学院理学研究院自然史科学部門
Department of Natural History Sciences, Faculty of Science,
Hokkaido University, N10 W8, Kita-ku, Sapporo, 0600810, Japan
＊＊＊〒261-0023 千葉市美浜区中瀬 1-3 幕張テクノガー
デン（株）ウェザーニューズ
Weathernews Inc., 1-3 Nakase, Mihama-ku, Chiba-shi,Chiba, 261-0023, Japan＊＊＊＊〒215-0004 神奈川県川崎市麻生区万福寺 1-2-2
アジア航測（株）
Asia Air Survey Co., Ltd.,〒215-0004 Ltd 1-2-2 Mnpukuji,Aso-ku, Kawasaki-si Kanagawa-ken, Japan＊＊＊＊＊〒156-8550 東京都世田谷区桜上水 3-25-40
日本大学文理学部地球システム科学科
Department of Geosystem Sciences Collage of Humanities
and Sciences Nihon University, 3-25-40 Sakurajosui,
Setagaya-ku, Tokyo 156-8550, Japan†元日本大学††故人
Corresponding author: Yasuhisa Tajima
e-mail: tajima-ys@n-koei.jp