Activity at Aso has gradually increased since October 1984. Moderate ash emissions from Crater 1 of Naka-dake were observed almost daily from the beginning of May (10:6) until 20 June. White steam vapor dominated for the last 10 days of the month. Total June ashfall at JMA's [Asosan Weather Station] was 1429 g/m²2.

Activity declined in July, when neither ash plumes nor deposits were observed at the [Weather Station]. On the morning of 1 July, [Weather Station] personnel found that the vent on the floor of the crater had been covered by water from heavy late-June rains. The level of water within the crater gradually rose in early July. About 70% of the crater floor has been covered by hot (60°C) water since mid-July. There were many fumaroles along the margin of the pool, and sand and water were ejected at many points on the surface. Seismic activity remained low throughout June and July.

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.

Information Contacts: JMA.