"Mashu is a 6-km-wide Holocene caldera that truncates a stratovolcano constructed on the ESE rim of the large Kutcharo caldera. The steep-walled caldera, filled by the remarkably transparent waters of Lake Mashu, is one of the scenic highlights of Hokkaido. Following caldera collapse about 7000 years ago, a small andesitic stratovolcano, Kamuinupuri, was formed beginning about 4000 years ago, creating a reentrant into the SE side of the deep caldera lake. A large explosive eruption about 1000 years ago, the latest dated eruption from Mashu volcano, created a 1.2 x 1.5 km crater at the summit of Kamuinupuri. The small island of Kamuishu in the center of Lake Mashu represents the tip of a mostly submerged dacitic lava dome."  -Smithsonian Volcano Archive
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