The 13 x 15 km Shikotsu caldera, largely filled by the waters of Lake Shikotsu, was formed during one of Hokkaido's largest Quaternary eruptions about 31-34,000 years ago. The small andesitic Tarumai stratovolcano was then constructed on its SE rim and has been Hokkaido's most active volcano in historical time. Pyroclastic-flow deposits from Tarumai extend nearly to the Pacific coast. Two other Holocene post-caldera volcanoes, Fuppushi (adjacent to Tarumai) and Eniwa (on the opposite side of the caldera), occur on a line trending NW from Tarumai, and were constructed just inside the caldera rim. Minor eruptions took place from the summit of Eniwa volcano as late as the 17th century. The summit of Tarumai contains a small 1.5-km-wide caldera formed during two of Hokkaido's largest historical eruptions, in 1667 and 1739. Tarumai is now capped by a flat-topped summit lava dome that formed in 1909.