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Report on Asosan (Japan) — January 1985


Scientific Event Alert Network Bulletin, vol. 10, no. 1 (January 1985)
Managing Editor: Lindsay McClelland.

Asosan (Japan) Moderate ash emissions continue; volcanic flame on two nights

Please cite this report as:

Global Volcanism Program, 1985. Report on Asosan (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 10:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198501-282110



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

All times are local (unless otherwise noted)

Ash was ejected a little more vigorously on 31 October, rising to the crater rim (about 100 m above the vent). On 2 November, ashfall was again confined to the crater. On 5-6 November weak rumbling was heard at the JMA's Aso Weather Station, 1.2 km SW of the crater.

Occasional weak ash emission that continued after 2 November was interrupted by water flowing into the active vent on 12 November. From then until 30 December muddy water and small rocks were ejected almost continuously, at varying intensity. During the strongest activity, on 9 December, this mixture fountained to an average height of 10 m above the crater floor, sometimes to more than 20 m.

Moderate ejections of ash to 150 m (as high as the crater rim) resumed 30 December and ended 2 January. On 30 December at 0815 an ash-laden plume rose 200 m above the rim. Ash fell on the S slope of the crater . . . . A prohibited zone within 1 km of Crater 1 was established 28 November.

On 11 January a grayish plume was observed rising 400-500 m above the crater rim, where ashfall was 1 cm thick. A little ash also fell on the S flank. Since the 11th an ash-laden plume was observed almost every day in January. Activity increased slightly on 18 January, when a gray plume rose 600 m above the rim. Volcanic flame [from a pit on the crater floor] was observed [during visits] the nights of 21 and 25 January.

Geological Summary. 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; A. Izumo, Yokohama Science Center; Japan Times.