Process of the caldera formation

One of the important character of the Miyakejima 2000 eruption is the formation of a collapsed caldera. Detailed scientific observation about the formation of the collapsed caldera is, may be, the first case in the world. Here the process of the caldera formation on Miyakejima is summarized mainly based on the observations by Joint Universities Research Group (JURG).
Phreatic eruption on July 8, 2000 and the appearance of a collapsed caldera

Activity of the Miyakejima 2000 eruption is opened with the rise of seismic activity in the evening of June 26, 2000. Ground deformation and migration of the earthquake swarm indicated that a dike intruded from the benearth of the volcano toward the northwestern off of the island. A minor submarin eruption was observed in the morning of 27 June, with eruption of 3*106kg of andesitic spatter. Seismic activity in the northwestern off of the island shows the continuous intrusion of the dike and evacuation of magma from the reservoir beneath the summit through the dike.
About 12 days after the start of the intrusion of dike, minor steam eruption occurred from the previous caldera floor at the summit of Miyakejima, and about 108kg of the volcanic ash were ejected. Afer the eruption, a collapsed caldera with 1 km across and 200 m depth was formed in the summit of the volcano.


Continuous growth of the collapsed caldera (July - August, 2000)

After the summit eruption on July 8, the collapsed caldera continuously became larger and, in the middle of the August, the caldera size of the caldera reaced 1.6 km in across, 450m in depth, and 0.6 km3 in volume. During the caldera growth, phreatomagmatic eruption occurred on 14-15 July. Major phreatomagmatic eruption occured after the growth of the caldera and total volume of the erupted materials is less than 2% of the collapsed volume.




Structure and mechanism of caldera collapse
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