Increased activity from [Crater 1] of Naka-dake continued through April. The vapor cloud contained little or no ash during March, but on 4 April the emissions increased in volume and a small ashfall was observed. Continuous emission of a grayish-white cloud began 7 April and lasted through the end of the month. Small quantities of mud and fine particles were ejected from vents in Naka-dake crater, but none of this material rose more than 20 m above the crater bottom. None of the ejecta contained evidence of fresh magma. Short-period volcanic tremor recorded by seismographs near the crater continued through April.

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; D. Shackelford, CA.