The Growth History of the Kuttara Volcanic Group, Hokkaido, Japan

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  • MORIIZUMI Mihoko
    Department of Earth and Planetary Sciences, Graduate school of Science, Hokkaido University

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  • クッタラ火山群の火山発達史
  • クッタラ カザングン ノ カザン ハッタツシ

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Abstract

The Kuttara volcanic group, located in southwestern Hokkaido, is an assembly of small stratovolcanoes, lava domes, pyroclastic cones, and a caldera. The eruptive history of the Kuttara volcanic group is newly classified into following five main eruptive stages: pre-Ayoro (-60ka), Ayoro (ca. 60-49ka), Takeura (ca. 49-45ka), Kuttara (ca. 45-40ka), and Noboribetsu (ca. 40ka-present) stages. During the pre-Ayoro stage, the Kt-8 and Kt-7 eruptions, and Ishiyama lava flow extrusion occurred. During the Ayoro stage, three medium-volume felsic activities, with more than 10km3 (D.R.E.) of eruptive sediments, arose (Kt-6, -4, and -3). The minor pyroclastic eruptions (Kt-5 and -Hy) and lava effusions (472m lava and Kt-Kt) occurred at intervals of the medium-volume eruptions. During the Takeura stage, the basaltic and andesitic magmas were produced and consequently constructed a stratovolcano of 2.5km3 (D.R.E.) in volume. Two medium-volume felsic activities (Kt-2 and -1) began during the Kuttara stage, and the latest medium-volume felsic eruption (Kt-1) formed the Kuttara caldera of 3km in diameter at 40ka. After the Kuttara stage, magmatic activity receded suddenly and suspended until small lava dome formation at 10ka of the Noboribetsu stage. The magma discharge rates were 4.9, 0.8, 4.8, and less than 0.01km3 (D.R.E.)/ka for the Ayoro, Takeura, Kuttara, and Noboribetsu stages, respectively. The vents distributed at the different places possibly provided the each major pyroclastic eruption. The felsic products during the Ayoro and Kuttara stages showed distinctive chemical trends in oxide-oxide diagrams. Although average SiO2 content of the juvenile fragments exhibited the temporal decrease during the Ayoro stage, the content augmented during the Kuttara stage. These facts must reflect the compositional differences in felsic magma systems between the two stages.

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