The chlorine in the samples from an outcrop of simple cooling unit of the Aso-III A pyroclastic flow deposit was determined as a mean of studying the behavior of chlorine during the cooling process of pyroclastic flow. Content of the insoluble chlorine only is discussed in this paper, since the water-solubule chlorine in rocks may not be magmatic in origin but have come from by contamination after cooling. The results obtained can be summarized as follows : 1. The insoluble chlorine content of the glassy phases in the Aso-II, Aso-III A and Aso-III B pyroclastic flow deposits ranges from 0.06 to 0.08% Cl. 2. The chlorine content in a simple cooling unit depends on crystallinity of the matrix of rocks, and glassy rocks contain higher amounts of chlorine than rocks of crystalline matrix. 3. The chlorine was expelled from melts as minerals crystallized out, and then, escaped to the air through secondary fumaroles, or other interspaces in the deposit. 4. The chlorine content of the fresh glassy rock, not hydrated, may give the most reliable information on the chlorine content of magma.

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From August, 1965, earthquake swarms wepe felt at Kozu-sima, one of the seven Izu-Island, 170km SSW of Tokyo. Earthquake swarms continued in the following year and some of them had Intensity of III and IV. Kozu-sima is one of the volcanic island having lave dome of liparite called "Tenzyo-san" which is believed to have been formed by violent eruption in 838. Since then, no eruption has been recorded. We carried out seismic observations at Kozu-sima in May and October, 1966 and determined epicenters to be located at off-shore of east of the island. No small earthquakes have been observed originated from the volcano. We concluded that the recent seismic activity at Kozu-sima is not directly related to the future eruption, and any feature of coming eruption could not be expected. We further studied in Chapter 3 and 4 the spatial relation between volcanism and seismicity of Huzi Volcanic Belt extending from Asama-yama in the north to Tori-sima in south. Most of the concerned part includes Izu Island arc. The main results are as follows. 1) Seismic activity is quite different beneath the region of Volcano Asama, in Honshu, from beneath the volcanoes in Izu Islands. From the depth of several tens Kilometers to 150km, we have evidences of lack of seismic foci and also the marked absorption of seismic wave energy which imply high temperature or even the existence of molten pockets beneath the volcanic region in Honshu. In contrast, beneath the Izu Islands, seismic activity is highly concentrated at the depth from the surface to about 100 km. This is essential difference in relation between volcanism and seismicity in Honshu and in Izu Island. 2) Between Miyake-zima and Hatizyo-zima, submarine topography shows discontinuous feature of the ridge, at this very point, seismicity also shows discontinuous change. South part shows very low seismic activity having a trend of deeper seismic foci as goes to the south. The relation between the volcanism and marked difference in seismicity beneath the Izu Island arc is worthy to be studied in future. 3) Seismic energy liberated per year has been calculated for the divided region including Izu Island arc as shown in Figs. 10 and 11 using the material of forty years. Most of the seismic energy is liberated at depth of 40 km beneath volcanic islands of Kozu-sima, Miyake-zima and neighbouring islands. This is worthy to note concerning the depth of magmatic foci beneath these volcanoes.

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The present author analysed statistically explosion smokes, explosion-earthquakes and explosion-air-vibrations of the volcano Sakurazima. Sakurazima is one of the most active volcanos in Japan, and explosive eruptions at the summit crater have been taking place frequently since 1955. Features explosion activity are shown in Fig. 4 or Table 2. There were close relations between frequency of explosion-earthquakes and total energy of them, but, generally speaking, larg explosions did not occur during any period of swarm of small explosions. Energy of explosion-earthquakes gradually increased until a new crater was born in 1964 at the rim of the old crater, and after that, it decreased. Energy of each explosion-earthquake which is classified according to the quantity of explosion smokes has an upper limit value. On the other hand, frequency distribution of maximum amplitudes of explosion-earthquakes or that of explosion-air-vibrations which examined by Ishimoto-Iida’s method is not expressed by a straight line on a diagram with both logarithmic scales, but by an upward convex curve. These phenomena are explained by assuming that explosive destructions occur only in a narrow vent.

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Present status of heat flow data is reviewed briefly. Topics such as the chondritecoincidence, upward concentration of heat source in the earth’s interior, possible differences between continental mantle and oceanic mantle, mechanism of heat transfer in the mantle, correlation between heat flow and geologic features, and heat flow in the volcanic areas are discussed. Finally, energetics of volcanic activity is treated with special reference to the Rikitake-Yokoyama’s energy.

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