To clarify the age of the volcanic activity of Nekodake volcano, the authors conducted K-Ar dating on ten samples of volcanic materials, using the sensitivity method. Four samples yielded ages approximately 50 to 82 ka. These newly obtained K-Ar age data indicate that Nekodake volcano was active during the post-caldera stage of Aso volcano. Based on the obtained ages, petrological characteristics, and lithofacies, the volcanism of Nekodake can be summarized as follows. 1) Effusive eruption of basaltic andesite magma produced thick lava flows at about 82 ka, and also formed agglutinates over a wide area at about 66 ka. The main volcanic body was formed by these eruptions. 2) Andesitic magma intruded the central part of the volcano at about 64 ka (late stage or just after the formation of the main part) forming dykes in an ENE-SSW direction. 3) Basaltic andesite magma intruded the central part of the volcano at about 50 ka and formed dykes in a NW-SE direction.

We studied the 40 ka Kp I eruption deposits of Kutcharo volcano to unravel its eruption sequence and generation mechanisms. Previous studies have suggested that Kp I is the youngest caldera-forming eruption in this volcano and is characterized by large-scale phreatomagmatic activity. We divided Kp I eruption deposits into 7 units (Units 1～7, in ascending order). Units 1～6 consist of alternating thin pumice and thick fine ash layers. Units 1, 3, and 5 are pumice falls (totaling 1.6 km³), while Units 2, 4, and 6 are ash falls (totaling 52.2 km³) with abundant accretionary lapilli. Stratigraphically higher ash fall units are larger in volume, finer in grain size, and more widely distributed (e.g., Units 2, 4, and 6 are 0.2 km³, 13 km³, 39 km³ respectively). Unit 7 is a climactic ignimbrite (76 km³) that subdivides into lower (Unit 7-L), and upper (Unit 7-U) parts based on the pumice size and the existence of a lithic concentration zone (LCZ).

Considering its wide dispersion, high fragmentation, and existence of abundant accretionary lapilli, Unit 6 can be considered to have been deposited by a “phreatoplinian style” eruption. Even though the ejected magma volume increased during the eruption of Unit 1 to 6, interaction between ascending magma and ground water caused maximum explosivity during the eruption that deposited Unit 6. Highly fragmentated magmas might have promoted vaporization and mixing with surface (lake) water to form the buoyant eruption column of Unit 6 eruption phase. Unit 7 is the most voluminous and the richest in lithic fragments at the LCZ, suggesting caldera collapse that generated a climactic pyroclastic flow.

In addition to glass shards of bubble wall and pumiceous types, Kp I eruption deposits also commonly contain flake-, and blocky-shaped glass shards produced by phreatomagmatic (quenching) fragmentation. For both types of glass shards to have been generated, part of the ascending magma would have interacted with ground water before and/or during the magmatic fragmentation (vesiculation) that generally occurs below a depth of approximately 1,000 m in felsic H₂O-saturated magma systems. In conclusion, a large and deep (～1,000 m) aquifer in the former caldera basin was sustainably supplied with ground water through the conduit system. Generation of the phreatoplinian eruption seems to have been controlled by a plumbing where conduits penetrated the huge aquifer of a pre-existing caldera structure that preserved/hosted a large amount of external water.

Volcanoes in Hokkaido had vigorous eruption histories in the last 400 years. Especially in the southern Hokkaido, Hokkaido-Komagatake, Usuzan, and Tarumaesan, reawakened in the 17^th century after the long-dormant period and vigorous magmatic eruptions of VEI5 class have been recorded in the historical literature and also in geological layers. Contrary to these volcanoes, we have documented histories only after the 20^th century for Tokachidake and Meakandake. Continuous volcano monitoring has been performed in major active volcanoes in Hokkaido since 1960s. In the 1970s, with the increase in seismic activity of Tokachidake, sponsored research from Hokkaido Government to Hokkaido University began to establish disaster management plans for future eruptions of active volcanoes in Hokkaido, and research reports was edited by Hokkaido University. Although 50 years have passed since the publication of the first report on Tokachidake, the reports of the research on active volcanoes are still one of the first documents to be referred when investigating the past activities of volcanoes in Hokkaido and the results of old scientific surveys. In addition, the reports include interdisciplinary contents for that time such as prediction of future eruptive activities and disaster prevention measures. The sponsored research by Hokkaido Government has continued to the present, and a new report on Tokachidake was published in 2014. The compilation of such research reports is very effective for volcano researchers and for relating field to share their awareness of the problems of volcanoes beyond their individual fields of expertise. Monitoring network around the active volcanoes in Hokkaido has been remarkably improved for these 20 years by Japan Meteorological Agency (JMA), Hokkaido University and other relating institutions. Recent data exchange in real-time among different organizations reduces duplication of monitoring resources and increases multi-parameter monitoring ability. The improved volcano monitoring network is expected to detect precursory activities of future magmatic eruptions concerned at the major volcanoes. Looking back on the eruption in the 20^th century in Hokkaido, small phreatic eruptions preceded magmatic vigorous eruptions in many cases. Not only mountaineers but also tourists and citizens can easily approach the crater area without any special equipment at several active volcanoes, so even a small eruption can lead to severe volcanic disaster.

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