Report on Asosan (Japan) — February 1990

Bulletin of the Global Volcanism Network, vol. 15, no. 2 (February 1990)
Managing Editor: Lindsay McClelland.

Asosan (Japan) Block and ash ejections increase in late January; daily ash emission in February

Please cite this report as:

Global Volcanism Program, 1990. Report on Asosan (Japan). In: McClelland, L (ed.), Bulletin of the Global Volcanism Network, 15:2. Smithsonian Institution.

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32.884°N, 131.104°E; summit elev. 1592 m

All times are local (unless otherwise noted)

Activity was relatively quiet in the first half of January, but increased in the second half of the month. A 21 January explosion ejected blocks to 300 m above the crater rim. Additional explosions occurred at 1645 on 1 February and 1320 on 7 February, the latter continuously ejecting blocks to 300 m above the crater rim. Minor ash emission was observed almost daily, causing ashfalls around the crater. A total of 30 g/m2 of ash was deposited in January and 3,167 g/m2 in February at AWS. Volcanic tremor amplitude increased from the end of January, but declined toward the end of February.

A pool of water was present in Vent 892 during fieldwork on 15 February. Mud ejection was observed for the first time since September 1989. Vent 892 began to develop in October, and has gradually enlarged to occupy half of the crater floor.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 cu km of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 AD. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.