JMA reported that during 17-19 April white plumes from Asosan’s Nakadake Crater rose 1 km above the crater rim. A very small eruption was recorded at 0824 on 19 April, producing a plume that rose 500 m above the crater rim and drifted S. During an overflight that same day scientists observed a pool of hot water on part of the crater floor and a weak volcanic gas odor. According to the Tokyo VAAC a plume rose 500 m and drifted E at 1842. A weak volcanic gas odor was also evident. 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.

Sources: Japan Meteorological Agency (JMA), Tokyo Volcanic Ash Advisory Center (VAAC)