Abstract
Juvenile materials were found among products of phreatomagmatic eruptions that preceded dacite dome growth at Unzen Volcano in 1991. They give evidence showing that the hydrous magma started degassing with the resultant crystallization around 100 MPa, and was quenched soon thereafter. Ascending at a rate as low as 13 m/d while degassing, however, the still‐molten part inside reacted with water. Phreatomagmatic eruptions started when the magma reached about 1.2 km in depth, and strong ones started at about 0.6 km. Volcanic tremors had occurred at these depths, where the sources of vulcanian explosions and the ground deformation were also located, implying the existence of a possible gas pocket.
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