JMA reported that at 0757 on 26 July a small eruption at Asosan’s Nakadake Crater generated grayish-white ash plumes that rose 1.6 km above the crater rim and drifted NW. Webcam images showed incandescent material in the vent. Minor ashfall was reported in areas downwind including Minamioguni-cho (Kumamoto Prefecture, N) and Kuze-cho (Oita Prefecture, NE). Plumes continued to be emitted during 0900-1300, rising to 400 m. Activity increased at 0442 on 28 July and remained elevated at least through 1500 on 29 July. Grayish-white plumes rose as high as 1.2 km above the crater rim and drifted NE and N. Sulfur dioxide emissions were very high on 29 July, at 4,300 tons per day. The Alert Level remained at 2 (on a scale of 1-5).

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.

Source: Japan Meteorological Agency (JMA)