Eruptive history of Adatara volcano, NE Japan, during last 250,000 years based on tephrostratigraphy.

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  • テフラ層序からみた安達太良火山，最近約２５万年間の噴火活動
  • テフラソウ ジョ カラ ミタ アダタラ カザン サイキン ヤク 25マンネンカン ノ フンカ カツドウ

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Abstract

Adatara volcano is one of the active volcanoes at the volcanic front of the northeast Honshu arc. Tephrostratigraphic study and new 14C ages reveal the eruptive history of this volcano as follows. 1) pyroclastic deposits of this volcano are divided into the 250- to 200- ka Adatara-Mizuhara tephra group and the Adatara young stage the 120- to O- ka Adatara young stage tephra group. The volcano was quiescent for about 80 thousands years between the two tephra groups. 2) During the eruptions of the Adatara-Mizuhara tephra group, effusion of large-volume andesite lava took place and formed the main volcanic cones. 3) The young stage activity began with the eruption of the Adatara-Dake tephra. This tephra consists mainly of lower dacite Plinian fall deposits (about 1 km3 in dense rock equivalent, DRE, volume) and upper andesite pyroclastic flow deposits (about 1 km3 in DRE volume). The Numanotaira crater was formed in this eruption. 4) After the Dake eruption, Sub-Plinian or Vulcanian eruptions, forming andesite pyroclastic fall deposits of 10-2 to 10-1 km3 in DRE volume, occurred with 10 to 20 thousands years intervals. 5) During the recent 10 thousands years, six Vulcanian eruptions, forming pyroclastic fall deposits of 10-3 to 10-2 km3 in DRE volume, occurred at the Numanotaira crater with 0.5 to 2 thousands years intervals. The last magmatic eruption took place at 2.4 ka. 6) Since 2.4 ka, three major phreatic explosions, last one of which was the 1900 eruption, occurred depositing tephra layers around the Numanotaira crater. 7) If the magma plumbing system has been stable, Vulcanian eruptions or phreatic explosions will occur at the Numanotira crater again.

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