Ballistic ejecta from the 1988–1989 andesitic Vulcanian eruptions of Tokachidake volcano, Japan: morphological features and genesis

Abstract

During the 1988–1989 activity of the andesitic stratovolcano, Tokachidake, several eruptions were of the Vulcanian type and ejected morphologically diverse ballistic ejecta. These ejecta are firstly classified into two groups: (1) those derived from juvenile material; and (2) those derived from accessory material. Secondly, the ejecta derived from juvenile material are divided into three types according to the morphological features which dominate, such as jointing, viscosity and vesicularity. Accordingly, these ejecta are termed jointed blocks, plastic bombs, and vesiculated bombs. The ejecta derived from accessory material are termed vent breccia blocks. It is proposed that the morphological differences between these ejecta reflect both the nature of the magma and the processes occurring in the vent prior to Vulcanian eruption and following eruption until shortly after landing. A comparison with other Vulcanian eruptions indicates the eruption energy of Tokachidake Volcano was relatively small.