JMA reported that explosive eruptions at Asosan’s Nakadake Crater occurred at 2152 on 7 October and 0146 on 8 October, after a period of deformation was detected. Inclement weather prevented visual observations, although satellite images showed the 8 October ash plume rising to an altitude of 11 km (36,100 ft) a.s.l. The Alert Level was raised to 3 (on a scale of 1-5). During an overflight scientists observed high temperatures in the crater, and white plumes rising 300 m above the rim. Ash deposits extended as far as 1.6 km on the NW flank and 1 km on the SE flank, and were abundant on the NE flank. Ashfall 3 cm thick was reported at the Aso police station 6 km NE. According to news articles, ashfall was reported as far away as 320 km, and some farmers 6-8 km away reported damage to their greenhouses, and a window was cracked from tephra at a youth center 5 km away. Samples analyzed by The National Institute of Advanced Industrial Science and Technology (AIST) and the National Research Institute for Earth Science and Disaster Prevention (NIED) revealed a 10% juvenile magma component, and that the explosions were possibly phreatomagmatic.

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), Reuters