Vesiculation path of ascending magma in the 1983 and the 2000 eruptions of Miyakejima volcano, Japan

The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous in composition (basaltic andesite) and in initial water content (H2O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H2O <0.4 wt%) suggestive of extensive dehydration of magma.