桜島1914-1915年噴火噴出物の石基鉱物の結晶サイズ分布とプリニアン降下軽石の気泡サイズ分布

抄録

The 1914-1915 eruption of Sakurajima volcano produced Plinian fall pumice and lava flows. Crystal size distributions (CSDs) and textures of the groundmass minerals of Plinian fall pumice and lava flow and vesicle size distributions (VSDs) and vesicle texture of Plinian fall pumice were analyzed. Plinian fall pumice samples show lower crystallinity and smaller crystal size compared with lava flow samples. Moreover, Plinian fall pumice samples are divided into three types according to their microlite and vesicle textures. Type-1 and type-2 Plinian fall pumice samples have high vesicularities (53-65vol%) and low (9.2*10¹² to 9.3*10¹³m^-3) to middle (1.6*10¹⁴ to 4.0*10¹⁴m^-3) number densities of plagioclase microlite. In these samples, vesiculation and expansion of magmas occurred in nearly closed system, deriving faster uprise velocity and inefficient crystallization of microlite. Type-3 Plinian fall pumice sample has low vesicularity (40-53vol%) and high number density (5.0*10¹⁴ to 1.5*10¹⁵m^-3) of plagioclase microlite, indicating that vesiculation and expansion occurred in a semi-open system. The CSDs analysis of the type-1 to type-3 samples indicate that increase of number density and crystallinity is due to the increase of microlite of small size. Opening of new conduit in relatively shallow level than that of Plinian conduit enhanced effective degassing of magmas and resulted in the change of eruption style from explosive to effusive. Higher crystallinity and larger microlite size of the lava flow samples compared with Plinian fall pumice samples are interpreted as due to their lower ascent and degassing rate of the magma.