According to the Japanese Meteorological Agency, a "mud eruption" occurred at Aso's Crater 1 on 14 January at 1541. The eruption was accompanied by volcanic tremor and ash emissions that rose to low levels above the crater. Small amounts of very fine ash fell in Takamori Town about 10 km ESE of the crater. The level of thermal activity at Aso had risen during the previous year, with the last "mud eruption" occurring in July 2003. The Alert Level at Aso was raised from 2 to 3, and no tourists were permitted entrance within 1 km of the crater.

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: Reuters, Volcano Research Center-Earthquake Research Institute (University of Tokyo)