The Yufugawa pyroclastic flow is a large-scale ignimbrite which occurs along Oita River in central Kyushu and is a useful key marker for volcanic stratigraphy among isolated volcanic edifices in the eastern part of Hohi volcanic zone. Eruption age of the Yufugawa flow has been in controversy owing to insufficient stratigraphic and radiometric data for volcanic rocks in this area. We determined both K-Ar ages and paleomagnetism of the volcanic rocks which overlie and underlie the Yufugawa flow: The Yufugawa overlies the Imaichi pyroclastic flow (reversed polarity; 0-8-0.9 Ma) and some other lava flows dated at 0.5-1.30 Ma. The Yufugawa underlies the lava flows dated 0.2-0.58 Ma. Accordingly the Yufugawa is interpreted to have erupted at about 0.6 Ma in middle Pleistocene. Stratigraphy, geochronology, Bouguer gravity anomaly, and level of pyroclastic plateau suggest that the Yufugawa pyroclaslic flow vented either at the south of Mizukuchiyama or at Yufuin Basin.

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Periodic temperature observations have been carried out on the Ghyben-Herzberg type of groundwater at 11 sites in Izu Oshima Volcano since November, 1986; immediately before the 1986 eruption. The results lend us to conclude the followings; (1) The temperature of the Ghyben-Herzberg type of groundwater is about 19℃ in Izu Oshima Island usually. Rather high-temperature groundwater layers exist in some areas where related to ancient volcanic activites. (2) The change of groundwater temperature that has been observed in central and southern part of the Motomachi area since the 1986 eruption of Izu Oshima Volcano, is caused by the followings. 1) The change of the rate of the intrusion of deep-seated hot spring water into a shallow groundwater layer; relating to the increment of fluid pressure in the volcano caused by the rising of magmas (magmatic emanations). 2) The intrusion of relatively high-temperature saline water into a groundwater layer; relating to the increment of the difference of water temperature between a saline water layer and a groundwater layer caused by the heat supply from magmas (magmatic emanations). 3) The intrusion of high-temperature saline water; relating to the increment of the movability of water caused by the formation of groundcracks resulted from the change of pressure in the volcano. (3) The rising of groundwater temperature that has been observed in northern part of the Motomachi area, since about half a year after the 1986 eruption of Izu Oshima Volcano, is caused by the rising of thermal water through groundcracks and the spreading of it over a groundwater layer. This thermal water was formed in the 1986 eruption of Izu Oshima Volcano under the great influence of magmas (magmatic emanations) and volcanic gases.

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Studies on coastal deposits and geomorphology of Oshima island have revealed some uplifts and subsidences associated with the volcanic activity of Oshima volcano. Uplifts and subsidences of the island are inferred by subtracting the sea levels at the time from the heights of the past sea surface recorded on coastal deposits. Two series of large-scale eruptions starting at the ages of O₄₁ (7,780±160 yB.P.) and S₂ (1,360±40 yB.P.) have similar characteristics as follows. (1) At the ages of O₄₂ (8,200-7,800 yE.P.) and O₁(1,500-1,400 yE.P.) just before the two series of large-scale eruptions, lagoons were formed along the west coast of Oshima island, suggesting the subsidence of the island. (2) At the beginning in each series, a large-scale steam explosion occurred at the summit crater to generate a debris flow. (3) In each series, the volume of the eruptive materials rapidly increased at the beginnings and gradually decreased after those times. (4) Volcanic activities were explosive at the beginnings in the series and gradually changed to effusive. (5) Oshima island was remarkably and rapidly uplifted at the beginnings of the two series of the large-scale eruptions and was gradually subsided. (6) In the stage of uplifling of the island, the scale of eruptions had been enlarging, and in that of subsiding, the scale of eruptions had been getting smaller.

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Detailed petrological study was made on the felsic volcanic rocks dredged from the Meiyo-Daini seamount in the Yamato backarc basin, Japan Sea. The Meiyo-Daini seamount was created as a subaqueous volcanic edifice together with the other seamounts in the basin at the latest stage of backarc rifting. The issued felsic volcanic rocks contain mafic inclusions with varying sizes and textures. Mass balance calculation suggests that the host felsic magmas were differentiated from the mafic parent by fractionation of amphibole and plagioclase feldspar. On the other hand, the derivative felsic magmas have phenocryst assemblages which are different from that expected from the fractional crystallization model. These puzzling phenocryst assemblages are probable reflection of the following magma plumbing system; the felsic magmas were produced in deep-seated magma reservoir(s) (＞2 kbar;＞ca. 7 km below surface), and then ascent to crystallize the present phenocryst assemblages consuming the crystals relevant to the differentiation. The mafic inclusions could be less differentiated portion of the deep-seated magma reservoir(s). The inferred magma plumbing system is consistent with the fact that the Yamato backarc basin is underlain by anomalously deep seismic Moho.

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