On 18 September JMA reported that an eruption from Asosan’s Nakadake Crater continued; ash plumes rose 900 m that same day. During an overflight scientists observed that pyroclastic-flow deposits from the 14 September explosion extended down the SE flank as far as 3 km; scientists from Kumamoto University estimated that about [40,000 tons] of ash were ejected that day. On 21 September an off-white plume rose 900 m. The Alert Level remained at 3 (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)