軽石の密度 : 熔結凝灰岩生成機巧についての一考察

抄録

The bulk density of pumice was measured on about 150 samples collected from the pumice flow deposit of Towada caldera. It varied between 0.211 and 0.777, the average being 0.51, and the porosity varied between 67 and 90%. In spite of these wide differences in the bulk density, mineralogical as well as chemical compositions of these samples are essentially similar, and the true density of homogeneous glasses obtained by melting these samples is also similar. Therefore, the difference in the bulk density of pumice is probably due to the difference in physical conditions, such as temperature and relief of hydrostatic pressure. It is apparent that peculiar mode of grain size distribution of pumice flow deposit of Towada caldera is not genetically related to the difference in bulk density. The results of thermal experiment carried out on these pumice samples at temperatures between 700℃ and 1370℃, are compared with the natural welded tuffs. The obsidian patches in the latter can be correlated to the result obtained at 1200℃, but this gives only the upper limit of formation of welded tuffs, because the experiments do not take into account the hydrostatic pressure or volatile components, both of which play important roles in the formation of welded tuffs. As the bulk density of pumice flows is generally estimated to be less than 1.0, they are not accumulated firmly on the bottom of the ocean, if they are erupted by submarine volcanic activity. Therefore, the welded tuffs may be taken as the feature of terrestrial formation.