[SVC43-03] Role of crystallization and conduit structure on small transitions of Strombolian eruption at Suwanosejima volcano, SW Japan
Keywords:crystallization, conduit environment, Strombolian eruption
Ash samples are mainly consists of glassy vesicular and crystalline denser particles with some amount of isolated crystals. The classification threshold of the two types is the crystallinity of about 45vol.% which is calculated from K2O content of the matrix glass. The ratio of the two main components change with eruptive activity. Vesicular types are up to 80% during Strombolian stage whereas they decreases down to about 50% during intermittent stage and then to ca. 30% during explosion stage. In addition, the crystallinity of the glassy particles gradually increased and eventually indistinguishable with crystalline types when number of explosion increased. During each stage, the products of larger size had characteristic shapes respectively. Fluidal or vesicular spatters are common in Strombolian stage with large amount of flaky scorias. Vesicular blocky bombs are dominant during intermittent stage, some of which are fragments of conduit breccia. Then, blocky dense bombs are common in explosion stage.
From these findings, it is concluded that transition into explosive activities from strombolian is due to the change in magma property by crystallization up to more than 45%. Then, intermittent ash emitting activity begins when the crystalline part of ascending magma in a conduit exceeds ca 50vol%. Strong explosion seems to begin when crystalline part exceeds 70% with highly crystalline glassy part. Such temporal changes of short interval in the ratio of crystalline part of magma could be explained only by crystallization at shallow level with small conduit radius that upcoming fluidal magma can seal the vent breccia with small amount to raise pressure of gas pocket just beneath the vent.