Mashu is a 6-km-wide Holocene caldera that truncates a stratovolcano constructed on the ESE rim of the large Kussharo caldera. The steep-walled caldera, filled by Lake Mashu, is one of the scenic highlights of Hokkaido. Following caldera collapse about 7,000 years ago, a small andesitic stratovolcano, Kamuinupuri, was formed beginning about 4,000 years ago, creating a reentrant into the SE side of the deep caldera lake. A large explosive eruption about 1,000 years ago, the latest dated eruption, 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.