The Kuttara volcanic group, located in southwestern Hokkaido, is an assembly of small stratovolcanoes, lava domes, pyroclastic cones, and a caldera. The eruptive history of the Kuttara volcanic group is newly classified into following five main eruptive stages: pre-Ayoro (-60ka), Ayoro (ca. 60-49ka), Takeura (ca. 49-45ka), Kuttara (ca. 45-40ka), and Noboribetsu (ca. 40ka-present) stages. During the pre-Ayoro stage, the Kt-8 and Kt-7 eruptions, and Ishiyama lava flow extrusion occurred. During the Ayoro stage, three medium-volume felsic activities, with more than 10km³ (D.R.E.) of eruptive sediments, arose (Kt-6, -4, and -3). The minor pyroclastic eruptions (Kt-5 and -Hy) and lava effusions (472m lava and Kt-Kt) occurred at intervals of the medium-volume eruptions. During the Takeura stage, the basaltic and andesitic magmas were produced and consequently constructed a stratovolcano of 2.5km³ (D.R.E.) in volume. Two medium-volume felsic activities (Kt-2 and -1) began during the Kuttara stage, and the latest medium-volume felsic eruption (Kt-1) formed the Kuttara caldera of 3km in diameter at 40ka. After the Kuttara stage, magmatic activity receded suddenly and suspended until small lava dome formation at 10ka of the Noboribetsu stage. The magma discharge rates were 4.9, 0.8, 4.8, and less than 0.01km³ (D.R.E.)/ka for the Ayoro, Takeura, Kuttara, and Noboribetsu stages, respectively. The vents distributed at the different places possibly provided the each major pyroclastic eruption. The felsic products during the Ayoro and Kuttara stages showed distinctive chemical trends in oxide-oxide diagrams. Although average SiO₂ content of the juvenile fragments exhibited the temporal decrease during the Ayoro stage, the content augmented during the Kuttara stage. These facts must reflect the compositional differences in felsic magma systems between the two stages.

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A steam explosion occurred at about 14:30 JST, February 11th, 1995, in the hot-spring area near Yakedake volcano, central Japan. More than six workers were near the site of the explosion for the road construction, and four of them were buried by the ejected material and killed. A small initial explosion began at the bottom of a 4m deep moat dug by a backhoe and it was followed by the maximum explosion, which ejected about 6,000m³ of blocks (maximum length is more than 2m) and mud, with steam and volcanic gas. The ejecta contain gravels of welded tuff, granite and mesozoic sedimentary rocks, which are the components of a pyroclastic dike of Pliocene age, and pumiceous lapilli tuff derived from the terrace sediments covering the pyroclastic dike. The explosion caused a landslide from the western cliff and the vent was buried by the slid debris, most of which was blown away by the second explosion. All of these processes took place within a few minutes. A small depression (20×5m²) on the west of the mound of the ejecta may represent part of the vent; its depth is estimated to be about 60m or more. Gaseous S0₂(＜30ppm) and H₂S(＜90ppm) were detected at the explosion site for three days after the explosion. The chemical composition of gas collected from the holes drilled after the explosion were nearly same as the gas from the summit crater of the Yakedake volcano. Because a wall-like Low-Q zone is suggested by seismologists beneath Yakedake volcano and the explosion site, it is most probable that there existed a magma beneath the explosion site and that the heat for the explosion was supplied by the magma and gas exsolved from the magma.

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