O-sima rises from a submarine ridge extending from the Izu Peninsula, central Honsyu, to Mariana Arc. In the vicinity of O-sima, the ridge is probably made up of older Miocene volcanic rocks (Yugasima Group). Basalt lava and pyroclastic rocks probably of Pliocene age lie on the Miocene volcanic rocks and form the direct basement of O-sima Volcano. These rocks are now exposed in two separate areas, one on the northern coast of the island and another on the southeastern, and are named the Okata Basalt Group and the Hude-sima Basalt Group respectively. The Pliocene rocks are tholeiitic olivine basalt having compositions more mafic than the lavas of the volcano. O-sima is a flat stratovolcano with a summit caldera within which lies an active cone Mihara-yama. The main body is composed of thin flows of olivine-bearing tholeiitic basalt and pyroclastic layers (Somma lava). There are more than ten parasitic spatter cones, scoria cones, and pit craters arranged roughly in three narrow zones radiating from the center of the main body; namely from the center to NW, NE, and SE. Probably they represent rift zones comparable to those found in Huzi Volcano and Hawaiian volcanoes. The eruption of the Somma lava started probably in late Pleistocene or early Holocene and closed with the formation of the caldera which took place within the last one or two thousand years. Mihara-yama is also a stratovolcano with a summit crater which has been active during the historic time. The Mihara-yama lava (Central Cone lava) is tholeiitic pyroxene basalt free from olivine, representing a more advanced stage of fractionation of basaltic magma than the Somma lava.

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Recorded volcanic activity of O-sima volcano in the historic age [table]

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The results of geophysical researches on Volcano O-sima by pioneers and active investigaters are reviewed in the general discussion from the standpoint of the following items: §1. Earthquakes and volcanic tremors. §2. Geomagnetism. §3. Gravity. §4. Physical properties of rocks. §5. Topography and geothermotics. In the special discussion, the reviewer describes energetics in Volcano Mihara during 1950-54 summarizing the results of various observations and the internal constitution of the central part of that volcano which has a caldera of Glen Coe type: §6. Energetics. §7. Caldera formation.

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Geochemical studies of volcanic activity of Volcano O-sima, Izu during the period from 1933 to 1957 are reported in this paper. The scope of geochemistry of volcanoes, the definition of magma, and the natures of magmatic substances, magmatic water, magmatic gas and the primary hot spring are discussed. The natures of some of them obtained from the lava lake of Volcano O-sima in 1951 are reported. Radioactivity of volcanic gas (Rn = 3～10 × 10^-10c/l) and volcanic sublimates (radioactivity of a bluish green copper compound is about 14 times larger than natural leakage) was found at the first time in the real magmatic substances. Magmatic water obtained directly from the lava lake is of acidic nature and can be distinguished from the residual magmatic water obtained from cavities of volcanic rocks at Hamada, Imari and Imazu districts. The formation of the various kinds of the primary hot springs is discussed from the definition of hot springs and the various kinds of the magmatic waters. The formation of the radioactive hot spring, the radioactive volcanic gases and the radioactive volcanic sublimates is discussed from the sublimation of the radioactive elements. Many magmas which have almost the same chemical composition of major constituents can also be derived from different magmas, but by the contents of minor constituents they can be distinguished each other. Vanadium and copper contents of the volcanic rocks from Volcano O-sima are very large, but chromium and nickel contents are very small. Distribution of vanadium in volcanic rocks is reported.

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