後期更新世の御岳火山 : 火山灰層序学と火山層序学を用いた火山活動史の再検討
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概要
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Through tephrostratigraphic study, I clarify the stratigraphy and volcanic activity of the late Pleistocene Ontake volcano, Central Japan. To understand the stratigraphy of the volcano, I investigated the ejecta stratigraphy first. Then the stratigraphy of the volcanic body, such as lava flow and pyroclastic flow, is revealed with using the pyroclastic marker beds. The study both on the ejecta and on the flow can make clear the exact correlation between the two, which have accumulated from Ontake volcano in the Late Pleistocene. Besides, the correlation clarified that twice of reposes formed paleosoil layers and nonconformities in the ejecta strata and in the volcanic body, respectively. Bounded by the hiatuses, three stratigraphic units can be assigned to Members of the Ohtaki Formation, the total mass of the Late Pleistocene igneous and volcaniclastic rocks. Therefore three times of major volcanic activity has occurred through this period. The Ohtaki-1 (O1), -2 (O2), and -3 (O3) are the recognizable eruption stages of Ontake volcano in the Late Pleistocene. Further, each Stages has two or three sub-stages, respectively. For example, a caldera formed by five pumice emissions occurred in the O1-I and -II Sub-stages. Lava domes formation with four pumice emissions followed in the O1-III Sub-stage. Two times of eruption cycles comprises of the O2 Stage to form a cone volcano beside of the lava domes. At least three tiny cones are formed in the O3 Stage with short repose intervals suggested by fine ash fall layers in the strata. Magma types also differ from a Stage to the other. Medium-K calc-alkaline dacite was in the Stage O1. High-K calc-alkaline andesite followed by High-K tholeiitic andesite were active in the Stage O2, and only High-K calk-alkaline andesite erupted in the Stage O3. As shown above, precise stratigraphic study both on the ejecta and on the volcano can provide a detailed information for volcanic activity. Two phenomena, caldera and lava dome formation, should be comprised within the O1 Stage. The activity continuity and the petrochemical similarity through the Stage support this idea. The different magma types such as tholeiitic and calk-alkaline andesites should be combined in the O2 stage, because the two activities show a little interval. I expect that such the high resolution stratigraphy can lead well understandings either for activity and for magma genesis of a volcano.
- 1993-07-25
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