火山 第 60 巻 ( 2015)
第 1 号 1-16 頁

論 説

鳥海火山北麓に分布するラハール堆積物の運搬・堆積過程と構成物質の時間変化
南裕 介＊・大 場司＊＊・林信太郎＊＊＊・片 岡 香 子＊＊＊＊（2014 年 6 月 3 日受付，2014 年 12 月 29 日受理）

Depositional Processes and Temporal Component-change of Lahar Deposits
at the Northern Foot of Chokai Volcano, NE Japan
Yusuke MINAMI＊, Tsukasa OHBA＊＊, Shintaro HAYASHI＊＊＊ and Kyoko S. KATAOKA＊＊＊＊
Chokai volcano is an andesitic stratovolcano in northeast Japan. The collapse of parts of the volcano occurred about
2500 years ago, and formed the Kisakata debris avalanche deposits. The post-2.5 ka fan (partly volcaniclastic apron)deposits are largely distributed in north to northwestern foot of the volcano. The fan deposits consist of several
lithological/sedimentological units, however, most of their geological, sedimentological, and geomorphic characteristics
have not been examined in previous studies. This paper, therefore, aims at understanding of geomorphology anddepositional processes of these lahar deposits and mineralogical component changes in relations with different sources.
Field survey in the upstream and observation of a trench in the downstream area revealed that the fan deposits
consist of more than 16 units, which are composed of debris flow, hyperconcentrated flow and streamflow facies; mostof them were deposited by lahar events. The lahar deposits have total thickness of 30 m, and overlie the Kisakata debris
avalanche deposit. In proximal areas (with steep to moderate slopes), lahars flowed down as debris flows. Whereas in
the distal area (with very gentle slopes), the lahars were transformed into hyperconcentrated flow and/or stream flows
commonly, although some of them reached to the distal area with a debris flow nature.
Clasts of the lahar deposits comprise altered and fresh andesites, mudstones and sandstones. Proportions of alteredandesite clasts to total clasts decrease upwards in stratigraphic sequences. Matrices of the lower eight units arecomposed of grayish-blue clay and are different from those of the upper eight units, which are composed of brownish
yellow volcanic sand. The stratigraphic variation in the matrix component is consistent with the change in matrix
mineral assemblage, possibly reflecting changes in the source materials from Chokai volcano.
Key words : Chokai volcano, Lahar, Volcanic fan
1．は じ め にし，前世紀においてはおよそ 30,000 人の死者をもたら

ラハールは火山周辺で発生する火山砕屑物と水の混合し，火山災害の中で火砕流に次ぐ最も危険な現象の一つ

物の急速な流動現象の総称である (Smith and Fritz, 1989)．

である（Schmincke, 2004 など）．ラハールによる影響・ラハールはこれまでも世界中で多くの火山において発生被害は甚大で，例えば，フィリピン，ピナトゥボ火山で＊〒010-8502 秋田市手形学園町 1-1
秋田大学大学院工学資源学研究科
Graduate School of Engineering and Resource Science,
Akita University, 1-1, Tegata Gakuenmachi, Akita 0108502, Japan.
＊＊〒010-8502 秋田市手形学園町 1-1
秋田大学国際資源学部
Faculty of International Resource Science, Akita University, 1-1, Tegata Gakuenmachi, Akita 010-8502, Japan.
＊＊＊〒010-8502 秋田市手形学園町 1-1
秋田大学教育文化学部

Faculty of Education and Human Studies, Akita University, 1-1, Tegata Gakuenmachi, Akita, Japan, 010-8502＊＊＊＊〒950-2181 新潟市西区五十嵐 2 の町 8050 番地
新潟大学災害・復興科学研究所
Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Ikarashi 2-cho 8050, Nishiku, Niigata 950-2181, Japan.
Corresponding author : Yusuke Minami
e-mail : d9514007@wm.akita-u.ac.jp