The Omanago volcano group consists of a series of five closely spaced lava domes that were constructed north and NW of Nantai volcano in Nikko National Park. The highest of the dacitic domes is Omanago, which later erupted andesitic lavas. The Mitsudake dome, which overlooks the hot spring resort of Lake Yunoko, was radiocarbon dated at about 5,000 years ago. Mitsudake was constructed on a southerly dipping slope, so that the southern of its two flat-topped domes is lower, and is overlapped by sediments of the Senjogahara plain.