Geology, volcanic history and eruptive style of the Yakedake Volcano Group, Central Japan.

Abstract

Geology, volcanic history and eruptive style of the Yakedake Volcano Group in the southern part of the Hida Mountain Range, were clarified using volcano-stratigraphy, K-Ar dating and ^14C dating. The Yakedake Volcano Group consists of six volcanoes Odana, Iwatsuboyama, Warudaniyama, Shiratani-yama, Akandana, and Yakedake volcanoes. These volcanoes are grouped into the older volcanoes (120 to 70ka; Warudaniyama, Iwatsuboyama, and Odana) and the younger ones (26 ka to present; Yakedake, Akandana, and Shirataniyama). The youngest magmatic products are the Yakedake Dome Lava and the simultaneous Nakao pyroclastic flow deposit of the Yakedake Volcano at 2.3 ka (cal BP). All of these volcanoes are composed of lava and/or block-and-ash flow of andesite-dacite (SiO_2 59-65 wt.%) with varying phenocrystal compositions. There is no evidence of explosive products such as pumice or scoria fall ever erupting throughout the history of these volcanoes. The long-term eruption rate of the Younger Yakedake Volcano Group is calculated to be 0.1-0.35km^3/ky, and this rate is within the range of estimated average eruption rates of the late Quaternary volcanoes in Japan. But the volume of the Yakedake Volcano Group is still as small as about 8km^3, smaller than the average volume of the late Quaternary volcanoes in Japan. Because the activity period and the total volume of the volcano group is short and small, it can be said that will continue in the future, taking the fact into consideration that the recent eruption rate has not lowered. Eruption style of these volcanoes in the near future is expected to be non-explosive, such as lava dome effusion and accompanied block-and-ash flow similar to that which has occurred in the past.