火山 第 59 巻 ( 2014)
第 4 号 229-239 頁

論 説だけ

安達太良火山，12 万年前噴火（岳噴火）における噴火推移の復元：
火口近傍露頭と山麓火砕流との対比による推察
藤原健一郎＊・長 谷 川 健＊・藤 縄 明 彦＊
（2014 年 8 月 10 日受付，2014 年 10 月 20 日受理）

Sequence of the 120 ka Adatara-Dake Eruption from Adatara Volcano, Fukushima, Japan:
Based on Correlations between the Outcrops of Pyroclastic Fall Deposits at
the Summit and Flow Deposits on the Flank and Foot of the Volcano
Ken-ichiro FUJIWARA＊, Takeshi HASEGAWA＊ and Akihiko FUJINAWA＊
We performed systematic sampling and description of the Adatara-Dake tephra outcrop on the summit of Adatara
volcano, Fukushima Prefecture, Japan in order to reveal the detailed eruption sequence and temporal evolution of themagma system of this volcano that erupted 120 ka BP. Even though there is no recognizable eruption hiatus represented
by a paleosoil layer, pyroclastic fall characteristics at the outcrop permit to divide Adatara-Dake tephra into 19 layers: Ato R; from bottom to top. The earlier layers (A to L) are characterized by successive pumice fall deposits, intercalated bythin volcanic sand layers. The later layers (M to R) are rich in scoria fall and are partly welded, with agglutinate found inlayers O and R. Representative clasts from each layer were analyzed to determine the grain size distribution,componentry, modal composition and whole rock chemistry. Layer M can be correlated petrologically andpetrographically with the lower part of Yugawa pyroclastic flow deposit on the eastern foot and the Motoyama
pyroclastic flow deposit on the western flank of the volcano. Similarly, the upper part of Yugawa pyroclastic flowcorrelate with layer N, meanwhile the upper part of Motoyama pyroclastic flow correlate with layers O or R. There is anincrease in the lithic fraction with decreasing sorting in layer M, which we interpret to indicate increasing enlargementof vent during the phase of this layer. The scoria/pumice volume ratio also increases remarkably in layer M, suggestingthat vent enlargement and sudden increase of mafic magma during phase M caused the column collapse that generated
the Yugawa and Motoyama pyroclastic flows. The mafic magma composition changed after layer O, with theFeO*/MgO ratios becoming less than 2.1. After this change in chemistry in phases O to R, pyroclastic flows werecontinuously generated and agglutinated deposits were formed at the summit. This suggests that the eruption style of the
final phase abruptly changed to relatively low column height.
Key words : Adatara volcano, agglutinate, Pyroclastic flow deposit, Correlation, Eruption sequence, Magma composition
1．は じ め に前あるいはそれよりも古い時期から現在まで，活動を続

福島県中北部に位置する安達太良火山は，東北日本弧けてきた活火山である（藤縄，1980）．藤縄ほか (2001)の火山フロントを構成する第四紀複合火山である (Fig.によれば，その活動期は，休止期の挟在により，大きく1)．本火山は東西約 12 km，南北約 15 km の範囲に，主と三期に区分される．最新の第三期は，三活動期中，最長・して安山岩質の火山錐がほぼ南北に配列した火山列をな最大規模であり，約 25 万年前から現在までに，カルクアし，主なピークは北から鬼面山 (1482 m)，箕輪山 (1728

ルカリ安山岩とデイサイトの噴出を繰り返し，本火山列m)，鉄山 (1709 m)，矢筈ヶ森 (1673 m)，安達太良山 (1700の主体部を形成した．藤縄・鎌田 (2005) では，第三期をm)，和尚山 (1601 m) と称される．本火山は，約 55 万年

さらに，噴火様式や休止期の存在から 3-1，3-2，3-3 期

き めんざんてつざん

や はず

みのわ さんもりおしょうやま＊〒310-8512 茨城県水戸市文京 2-1-1
茨城大学理学部
Department of Earth Sciences, College of Science, Ibaraki
University, 2-1-1 Bunkyo, Mito 310-8512, Japan.

Corresponding author: Akihiko FUJINAWA
e-mail: fujinawa@mx.ibaraki.ac.jp