Report on Asosan (Japan) — 9 July-15 July 2003

Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 9 July-15 July 2003
Managing Editor: Gari Mayberry

Please cite this report as:

Global Volcanism Program, 2003. Report on Asosan (Japan). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 9 July-15 July 2003. Smithsonian Institution and US Geological Survey.

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32.884°N, 131.104°E; summit elev. 1592 m

All times are local (unless otherwise noted)

A tremor event with a moderate amplitude was recorded at Aso on 10 July at 1718. Aso weather station personnel inspected the area around Nakadake crater and found a small amount of tephra newly deposited at Hakoishi-Toge about 6 km ENE of the crater. Dr. Yasuaki Sudo of Aso Volcanological Laboratory, Kyoto University, inspected the crater area and determined that a phreatic eruption had occurred. Mud emitted during the eruption reached as far as 10 km from the crater. The color of the crater lake surface changed to dark gray from green, its color on 8 July.

Geologic Background. 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 cu km 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 AD. 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.

Source: Japan Meteorological Agency (JMA)