倶多楽火山の外輪山期噴出物の地質と岩石

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Kuttara Volcano belonging to Northeast Honsyu arc, is situated at southwestern Hokkaido. The eruptive history of Kuttara Volcano has the following three stages; Somma Stage, Caldera Stage and Post-caldera Stage. During the Somma Stage, this volcano erupted basaltic and andesitic lavas and scoriaceous ejecta. In the later of the stage, when the volcano grew up to a stratovolcano, andesitic lavas were erupted to form Mt. Kita-yama. In the Caldera Stage, a large amount of dacitic pumice flows were erupted, resulted into forming of the caldera. Finally, a dacitic lava dome was built and two explosion craters were opend in the Post-caldera Stage. The Kuttara Volcano Effusive Rocks vary in composition from basalt to dacite, depend ing on the stage. The Somma Stage Effusives are mostly basaltic rocks associated with andesitic rocks. These rocks have the following five; Lower Deposit, Middle Deposit, Upper Deposit with Takeura Scoria Deposit, Uppermost Deposit and Kita-yama Lava in ascending other. These deposit show conformable relationship with each other, excepting for the relationship between the Uppermost Deposit and Kitayama lava. Microscopic examination has revealed that the Kita-yama Lava is classified as hypersthenic rock series, and the others are pigeonitic ones. On the Si0_2-Σ FeO/MgO diagram, most of the basaltic lavas, are plotted in the field of tholeiitic series, and some of the andesitic lavas are plotted in the field of calc-alkalic series. The cotent of Na_2O and K_2O of the Quaternary volcanic rocks from southwestern Hokkaido, increases from the Pacific Ocean side to the Japan Sea side. In particular, the K_2O content of the Kuttara Volcanic Rocks shows the lowest values.
地学団体研究会の論文
1991-01-25

倶多楽火山の外輪山期噴出物の地質と岩石

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