V22A-03

Kuju volcano is a mountains composite volcano which consists of 16 hornblende-andesite lava domes and associated lava flows, 5 pyroxene-andesite cone, and 1 olivine basalt cone. The domes and cone are surrounded by pyroclastic flow deposits, block-and-ash-flow deposits, and debris-avalanche deposit (Fig. 1 from Kamata & Kobayashi, 1997; Photo 1). Several explosion craters (and crater-like landform) were observed around summit area and flank of lava domes. The history and frequency of explosive phreatic activity of Kuju volcano are, however, poorly known.
To confirm the history of explosive phreatic activity of Kuju volcano, the field investigation and trench survey were performed around summit area and the foot of the volcano (Fig. 2).
Several of argillaceous ash layers are observed in the studied area (Fig.3, Photo 2 - 4). These ash are mainly composed of hyrothermal alternated lithic fragments and clay. It is not confirm the presence of essential material. It appears from above that these argillaceous ash were erupted by phreatic explosions (and/or hydrothermal explosion). The age of explosion was determined by radiocarbon dating of soil organic matter, which are overlain by argillaceous ash layers. As the result, phreatic explosion occured at sevelal times in the last 5 ka (Fig. 4).
The argillaceous ash which was erupted at 3.3-3.6 ka, has an extensive distribution around the volcano. The grain size of lithic fragment and thickness of the 3.3-3.6 ka argillaceous ash increase to hydrothermal alternated area of western area of the volcanic group. In the eastern area, the 3.3-3.6 ka argillaceous ash was overlain by andesitic ash, which was erupted from the Komekubo volcano (Fig. 5).
The eruption process at 3.3-3.6 ka is follows: Phreatic explosion ocuured at hydrothermal area which spread around the western part of Kuju volcanic group. Continuously, magmatic eruption ocuured at Komekubo Volcano which is 4-6 km away from the eruption center of phreatic explosion.