Temporal variation in volcanic ash texture during a vulcanian eruption at the Sakurajima volcano, Japan

We discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the Sakurajima volcano, Japan. Falling ash samples were collected every 2–50 min for petrological analysis. These ash samples contained juvenile particles, altered rock fragments, and fragments of crystals; here, we subdivide the juvenile particles into blocky, vesicular, and fluidal particles. Because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles, we refer to the vesicular and fluidal particles as low-crystallinity particles (LCPs) and the blocky particles as high-crystallinity particles (HCPs). The temporal variation observed in ash texture reveals that the ratio of LCPs to HCPs (LCP/HCP) was small at the initiation of the eruption and increased as the eruption progressed. The higher microlite crystallinity in the HCPs indicates crystallization under lower pressures for longer durations relative to the LCP magma. The increase of the LCP/HCP ratio during a single vulcanian eruption suggests that the emission of less viscous gas-rich magma under a cap rock increases as the eruption progresses. Such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption.