Report on Asamayama (Japan) — 4 August-10 August 2021
Smithsonian Institution / US Geological Survey
Weekly Volcanic Activity Report, 4 August-10 August 2021
Managing Editor: Sally Sennert.
Please cite this report as:
Global Volcanism Program, 2021. Report on Asamayama (Japan) (Sennert, S, ed.). Weekly Volcanic Activity Report, 4 August-10 August 2021. Smithsonian Institution and US Geological Survey.
36.406°N, 138.523°E; summit elev. 2568 m
All times are local (unless otherwise noted)
On 6 August JMA lowered the Alert Level for Asamayama to 1 (on a scale of 1-5), noting that the number of shallow volcanic earthquakes had been decreasing, sulfur dioxide gas emissions had been low, and deformation on the W flank had stabilized. JMA reminded the public to stay 500 m away from the crater.
Geological Summary. 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 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-Asamayama 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 observed activity 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.