Abstract
Hokkaido Komagatake is an andesitic stratovolcano located in northern Japan. Major historic eruptions have occurred four times since the 17th century after 5000 years of dormancy. The 1640 eruption was the first and the largest eruption during historic time. The sequence of the 1640 eruption was as follows: New magma was intruded into the volcanic edifice. A first sector collapse occurred toward the south, followed by a second collapse toward the east. Blast was associated with the second collapse, and emplaced blast deposits on the surface of the (first and second collapse) debris avalanche deposit. Finally, the eruption changed to plinian phase and discharged plinian fall and pyroclastic flow. Estimated volume of the debris avalanche deposits and the pyroclastic deposits is 1.1 km3 and 2.9 km3 respectively. The southern debris avalanche lobe materials are correlated with block-and-ash flow deposits exposed at the southwest-facing amphitheater, and those of the eastern lobe are correlated with massive andesitic lava and agglutinate exposed in the east-facing amphitheater. Juvenile vesiculated andesite is commonly included within the matrix of the eastern lobe and the blast deposits, whereas the matrix of the southern lobe lacks such juvenile material. The processes of the 1640 eruption are generally similar to the 1980 eruption of Mount St. Helens.
