Decompression rate of magma at fragmentation: Inference from broken crystals in pumice of vulcanian eruption

Decompression rate of magma at fragmentation (immediately before or after fragmentation) was determined using broken crystals found in the pumices ejected during the vulcanian explosion of Sakurajima volcano, Japan. The combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7.0 × 103–7.8 × 104 Pa/s for the ejecta produced during the later phase of vulcanian explosion. This result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent. An understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation.

► We model formation of broken crystals in pumice of vulcanian eruption at Sakurajima. ► Magma decompression rate at fragmentation is estimated using the model. ► The decompression rate is estimated to be 7.0 × 103–7.8 × 104 Pa/s. ► Estimated decompression rate is consistent with that in other explosive eruptions. ► Decompression rate of magma drops to one-tenth from initial to late stage.