Report on Asosan (Japan) — August 1977
Natural Science Event Bulletin, vol. 2, no. 8 (August 1977)
Managing Editor: David Squires.
Asosan (Japan) Activity declines after 22 July
Please cite this report as:
Global Volcanism Program, 1977. Report on Asosan (Japan) (Squires, D., ed.). Natural Science Event Bulletin, 2:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.NSEB197708-282110.
32.884°N, 131.104°E; summit elev. 1592 m
All times are local (unless otherwise noted)
The increased activity on 31 March was accompanied by continuous large amplitude tremor. A small vent formed on 8 May and emitted ash. During the night, a 2-10 m "flame" was observed. In late May, more fist-sized ejecta fell on the crater floor. An earthquake swarm occurred 3 June, and large-amplitude tremor, some of which could be felt near the crater, was recorded 18 June. Small scale eruptions deposited ash in early June. These eruptions increased in strength 20-23 June, depositing fist-sized ejecta inside the crater.
Activity then declined until 20 July, when explosions at 1321, accompanied by airshocks, projected black and gray ash clouds 1,500 m above the crater. Ash fell up to 500 m from the crater, reaching a maximum depth of 30 cm, and blocks up to 80 cm across fell on the rim. Similar explosions occurred at 1341 on 22 July, producing a 1,300 m cloud and depositing considerable ash around the crater. Activity then declined. No fresh magma was ejected at any time during the eruption.
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 km3 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 CE. 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; D. Shackelford, CA.