大分県立石山火山砕屑岩について

概要

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Tateishi-yama pumice flow covers an area of about 1.5km in the N-S and 3km in the E-W, on the north slope of Tateishi-yama, being covered by the Tateishi-yama lava flow. The pumice flow is similar to the nearby Kuju pumice flow. But the Tateishi-yama pumice flow contains more lithic fragments, less hornblende phenocrysts and shows lighter colour. Banded pumice is frequently found in the upper part of the pumice flow with less felsic bread crust bombs. Dark and pale coloured parts of the banded pumice consist of augite-hypersthene-hornblende dacite. Pale part is richer in SiO_2, Na_2O and K_2O and poorer in FeO, Fe_2O_3, MnO, CaO and TiO_2 as compared with the dark part. The banded structure may have resulted from the mechanical mixing of two different magmas in the process of rising through the narrow volcanic pipe from the reservoir.
特定非営利活動法人日本火山学会の論文
1972-10-01