Sequence of the 120ka 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(＜Special Section＞Determination of the Construction of an Outcrop Database to Reveal Eruptive History)

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  • 安達太良火山,12万年前噴火(岳(だけ)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(＜特集＞火山噴火史解明のための露頭データベース構築の検討)
  • 安達太良火山,12万年前噴火(岳(だけ)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
  • アダチタイリョウ カザン,12マンネンマエ フンカ(タケ(ダケ)フンカ)ニ オケル フンカ スイイ ノ フクゲン : カコウ キンボウ ロトウ ト サンロク カサイリュウ ト ノ タイヒ ニ ヨル スイサツ

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Abstract

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 the magma system of this volcano that erupted 120ka 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: A to R; from bottom to top. The earlier layers (A to L) are characterized by successive pumice fall deposits, intercalated by thin volcanic sand layers. The later layers (M to R) are rich in scoria fall and are partly welded, with agglutinate found in layers 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 and petrographically 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 flow correlate with layer N, meanwhile the upper part of Motoyama pyroclastic flow correlate with layers O or R. There is an increase in the lithic fraction with decreasing sorting in layer M, which we interpret to indicate increasing enlargement of vent during the phase of this layer. The scoria/pumice volume ratio also increases remarkably in layer M, suggesting that 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 the FeO*/MgO ratios becoming less than 2.1. After this change in chemistry in phases O to R, pyroclastic flows were continuously 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.

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