JMA reported that at 2208 on 7 August a small phreatic eruption at Asamayama produced an ash plume that rose higher than 1.8 km above the crater rim and drifted N. Blocks were ejected 200 m from the crater. The eruption lasted about 20 minutes and was the first since 19 June 2015. The Alert Level was raised to 3 (on a scale of 1-5). Ash fell in Tsumagoi Village and Naganohara Town, in the Gunma Prefecture. White plumes rose as high as 700 m above the crater rim during 8-13 August, and the amount of sulfur dioxide released was 90-200 tons per day.

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.

Source: Japan Meteorological Agency (JMA)