Ten K-Ar ages of the pre-caldera lava fiows from northern (Kajiki and Kokubu) and southern (Ushine) areas of the Aira caldera were determined precisely in order to document the detailed volcanic history before the Aira pyroclastic eruption at 24.5 ka. The K-Ar dating method used is the sensitivity method and mass fractionation correction procedure for the argon analysis. The results combined with the previous data show the following time-space distribution of the pre-caldera volcanism after 3 Ma; (1) 3 to 1 Ma: eruptions of andesitic lava flows (Hokusatsu volcanic rocks) at the northern and southern areas, (2) 1 to 0.5 Ma: eruptions of andesitic lava flows (Yuwandake andesite) at the northern area and basaltic to rhyolitic lava and pyroclastic fiows at the western area, (3) 0.5 to 0.1 Ma: eruptions of basaltic (Ushine basalt) and rhyolitic (Okoga-shima rhyolite) Iava flows at the southern area, (4) 0.1 to 0.025 Ma: eruptions of andesitic lava (Shikine andesite) and pyroclastic fiows at the northern area and rhyolitic lava flows (Shimizu and Ushine rhyolites) both at the northern and southern areas. The most remarkable feature of the pre-caldera volcanism is the eruptions of similar rhyolitic lava flows, which contain characteristic osumilite, at 15 km apart opposite sides of the caldera rim during 0.04 to 0.03 Ma. This might imply that a large magma chamber has been present under the Aira caldera since at least 0.04 Ma.

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Shirataka Volcano (994 m in maximum altitude) is a small Pleistocene volcano situated behind the Quaternary volcanic front of the NE Japan Arc. This volcano lies on the Shirataka hilly land immediately west of the Yamagata Basin, and a much smaller Konpei rhyolite lava dome of 3.4 Ma neighbors on its west side. Shirataka Volcano is divided in ascending order into the lava flows and block-and-ash flow deposits of Kokuzo dacite, Numata andesitic pumice flow deposit, Hagino andesitic block-and-ash flow deposit, Hariu and Ojirafu debris avalanche deposit, Shiratakayama, Kitsunegoe and Nishikuromoriyama andesitic lava domes, Hataya debris avalanche deposit, and the Higashikuromoriyama lava dome. Shirataka Volcano was intermittently active during the past 0.2 million years from 1 to 0.9-0.8 Ma with a total eruption volume of ■ 5 km³. Distributed on the flank and foot of Shirataka Volcano, the Hariu, Ojirafu and Hataya debris avalanche deposits contain mainly variably-shattered blocks of dacite and andesite with a matrix of their finer clasts. They demonstrate repeated sector collapse of Shirataka Volcano. The Hataya debris avalanche deposit, which is the youngest and largest among the three debris avalanche deposits, represents the last catastrophic event of Shirataka Volcano leaving a horseshoe-shaped caldera a few km wide and a large volume of deposit in excess of 2 km³. Extruded within the collapsed caldera, the Higashikuromoriyama lava dome perhaps marks the last activity of Shirataka Volcano.

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Jigoku spa is located west of Ikenokubo basin of Yomineyama volcano, southwestern part of the central cones of Aso volcano. In this basin, “Ikenokubo tuff ring” and two smaller maars were developed. Two layers of ejecta, about 10⁵ m³ in bulk volume, formed by phreatic explosions were found in this area. They are very poorly sorted and mainly composed of altered fine-grained ash including lithic fragments. Kikai Akahoya ash erupted about 6,300 years BP (¹⁴C) is intercalated between two layers of phreatic ejecta in thick ash layers from Nakadake, the only active central cone. Based on ¹⁴C ages of buried Andisols and the succession of the deposits, the eruption ages of the two ejecta are estimated at about 10,000 years BP for older deposit and at about 4,500 years BP for younger one. Distributions of thickness and maximum grain size of the lithic fragments suggest that the both of two phreatic ejecta were exploded not from “Ikenokubo tuff ring” and two maars but from the Jigoku explosion crater in the western area. Ejecta correlative to the “Ikenokubo tuff ring” Iies under the older of the phreatic ejecta layers. Consequently, the eruption age of “kenokubo tuff ring” is probably older than 10,000 years BP. Frequency of phreatic explosions, Iarger than 10⁵ m³ in volume, is considered to be once in about 5,000 years. This fact is noteworthy for volcanic hazards assessment in the surrounding area of Jigoku spa and adjacent Tarutama spa.

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