[SVC50-P01] Volcanic activity and magma plumbing system during caldera and post-caldera stage of Mashu volcano, eastern Hokkaido
Keywords:Mashu volcano, caldera, magma plumbing system
Major caldera formation of tephrostratigraphy of Ma-j, Ma-i, Ma-h, Ma-g, Ma-f (Kishimoto et al., 2009) was reviewed. According to the change of lithic fragments and existence of lithic-rich thin layers between pumice fall deposits, the location of the crater may have changed during the plinian eruptions. The most mafic compositions of bulk and minerals in Ma-g tephra has different compositional trend compared with other eruptive products of caldera-forming series. Ma-f large-volume pyroclastic flow deposit shows a wide compositional variations including the range of all other caldera-forming series products. These suggest existence of two different magma chambers of Ma-j, Ma-i, and Ma-h units and of Ma-g unit. It seems that both of the magma chamber eventually reached the catastrophic eruption of Ma-f unit.
During the post-caldera stage (6000?-1000 years ago) magma activity was changed to the new plumbing system based on the bulk chemistry. Kamuishu-island lava dome (SiO2=73.9wt.%) in the center of the caldera erupted at the beginning of a long dormant period after the caldera formation, after that Kamuinupuri small strato-cone was formed in the eastern edge of the caldera. The eruptive sequence of three lavas from Kamuinupuri was deduced by bulk and minerals chemistry. Kamuinupuri northwestern lava (SiO2=68.5wt.%) erupted during the main activity of Kamuinupuri strato-cone formation (Ma-d tephra; SiO2=63.9-69.4wt.%). After the Ma-d tephra activity Kamuinupuri western lava (SiO2=54.5-61.6wt.%) effused. The Kamuinupuri north lava (SiO2=70.6wt.%) can be erupted after the Ma-c tephra layer eruption (2500-1500 years ago). At about 1000 years ago the latest explosive eruptions including plinian fall and pyroclastic flow (Ma-b tephra; SiO2=67.0-69.3wt.%) occurred.