Report on Asamayama (Japan) — 15 September-21 September 2004
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 15 September-21 September 2004
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2004. Report on Asamayama (Japan). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 15 September-21 September 2004. Smithsonian Institution and US Geological Survey.
36.406°N, 138.523°E; summit elev. 2568 m
All times are local (unless otherwise noted)
According to JMA, as reported by the news media, Asama erupted almost continuously for a third straight day on 16 September, causing more than 1,000 earthquakes. Incandescent fragments were ejected ~300 m from the summit and ash columns rose ~1,200 m above the crater. Late that night winds carried ash as far as central Tokyo. The frequency of the eruptions appeared to have tapered off by the afternoon of the 17th, although television footage showed gray smoke mixed with ash billowing over the mountain. By 18 September, JMA was reporting that ash plumes were still rising ~1,200 m, but only about 23 small eruptions and nearly 140 tremors had been recorded that afternoon, a significant change from the nearly consinuous activity of the previous few days. The hazard status remained at 3 on a scale of 5, meaning more small-to-medium eruptions could occur. A radar analysis conducted on 16 September confirmed that there was a lava dome in the crater, the JMA and Geographical Survey Institute reported, the first since 1973. Radar images showed a dome-shaped, layered form several dozen meters high with a radius of about 100 m in the NE part of the crater; it is an estimated 500,000 cubic meters in volume.
Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the horseshoe-shaped remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asama-yama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has an historical record dating back at least to the 11th century CE. Maekake has had several major plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.