北海道中央部，トムラウシ火山群の地質

Abstract

Tomuraushi volcanic group (TVG) is situated at the middle part of Daisetsu-Tokachi volcanic chain, Central Hokkaido, Japan. The basement rocks of the TVG are composed mainly of Miocene altered volcanics and early Pleistocene Tokachi welded tuff. The volcanic activity of the TVG started in middle Pleistocene and can be divided into Older and Younger stages. The Older stage (1.0∼0.7 Ma) consists of two polygenetic volcanoes, Koganegahara (total volume> 5 km3) and Goshikigahara (>15 km3). Koganegahara volcano, located in the southwestern area of the TVG, is composed of thin lava flows which effused from E-W trending vents. Goshikigahara volcano, located in the northern area of the TVG, consists of three cones, Kaundake, Goshikidake and Ponkaundake. Thin lava flows and voluminous pyroclastics erupted from central conduit of Kaundake cone, form the lower part of Goshikigahara volcano. At the late stage of activity, voluminous lava flows effused from the eastern and western flank of Goshikigahara volcano, built Goshikidake and Ponkaundake cones. After a long dormancy, the volcanic activity of the Younger stage started about 0.3∼0.2 Ma. Both Futamata (>0.08 km3) and Kaunnai (>0.1 km3) volcanoes are monogenetic and effused lava flows from the base of the northern flank of Koganegahara volcano. Tomuraushi volcano (about 5 km3) is composed mainly of thick block lava flows and lava domes with less voluminous pyroclastic fall deposit, which effused from more than 20 eruption centers between Koganegahara and Goshikigahara volcanoes. The primary surface structures of those flows and domes have been preserved. In summary, the volcanism of the TVG changed from polygenetic to monogenetic, and the corresponding differences in erupted volume and mineral assemblage of lavas were found.