1999 Volume 53 Issue 1 Pages 53-70
Numazawa volcano located in the western part of Aizu, Fukushima Prefecture, is a small volcano having a single caldera lake, which has been erupted since the Late Middle Pleistocene (about O.13Ma). The long axis of the caldera lake formed in the last stage of volcanic eruption is similar to that of the old caldera formed during the Pliocene Period. Volcanic successions are divided into the seven units as follows: Sohyama lavas, Mizunuma pyroclastic formation, Mizunuma formation, Ohkuriyama lavas, Hayato pyroclastic formation (51ka), Numazawa pyroclastic formation (before 5000years), and Numazawa formation, in ascending order. Sohyama lavas consist of augite-hypersthene dacite (SiO_2 63-68%). Mizunuma pyroclastic formation consists of pyroclastic flow deposits of biotite rhyolite (SiO_2 75%). Ohkuriyama lavas consist of biotite-hornblende dacite (SiO_2 65%). whereas Hayato pyroclastic formation is composed of lava, welded tuff and pyroclastic flow deposits. They are dacitic (SiO_2 68-70%) in composition containing biotite, hornblende and cummingtonite. Numazawa pyroclastic formation is also dacitic (SiO_2 65%), but it contains hypersthene and hornblende. It is composed of pyroclastic flow deposits with banded pumices formed by mixing of andesite and dacite magmas. Mizunuma formation and Numazawa formation are consisted of fluvial gravel beds. Chemistry of the volcanic rocks from Numazawa volcano is similar to that of the Moriyoshi volcanic chain of the Quaternary volcanoes of the NE Japan.
