Possible underground heat sources and their cooling processes of the Nishinoshima-shinto are analized semi-quantitatively and discussed, based on the various thermal data summarized in our previous paper (Ehara et al. 1977). Rapid cooling of the Nishinoshima-shinto is interpreted as follows: The mass of the heat source which may exist in the underground at the present time is not greater than 1×10¹²g (or if the shape of the heat source is spherical, its radius does not exceed 50m). Furthermore, volcanic products empted not only below but also above the sea level are cooled by sea water. This is another main reason of the rapid cooling of the Nishinoshima-shinto. Of cource the underground heat source mentioned above is also being cooled by sea water at the present time. The rapid cooling of the Nishinoshima-shinto is caused mainly by the sea water around the island.

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Kozu-shima, which belongs to the northern subzone of the Fuji Volcanic Zone, is one of the Seven Izu Islands and is situated about 160km south of Tokyo. In this report, the author intends to clarify the volcanic geology of the island. The main results are as follows: 1) The rhyolitic volcanic activity (soda-rhyolite) in the latest Pleistocene to the Holocene age in the island can be divided into three stages. The activity of the first stage occurred under shallow submarine enviroment and formed several islets. The second stage is characterized by extrusions of a large amount of pyroclastic rocks and several lava domes. Some activities might have occurred on land. The channels between the islets made in the first stage were filled with the pyroclastic rocks of this stage. The third stage activity occurred on land and made five lava domes. 2) The eruptive centers of biotite rhyolite are arranged in a NW direction. 3) Many fractures and lineaments are obsered in the island. The fractures can be divided into three kinds as follows: a. normal fault: NS (strike), ca 60°W (dip) b. fault: ca NW, ca 90°c. fault: ca NE, ca 90°4) Alignment of domes of biotite rhyolite agrees with the NW-trending fault in direction. The relation between the volcanic alignment and the fractures is discussed at the last part.

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Heat discharge through the terrain surface at the volcanic and geothermal area has been so far estimated mainly by the in situ field measurements. In volcanic area, direct measurements are not applicable to inaccessible area, such as crater floor. In recent years, however, remote measurement technique by means of infrared detectors has been developed and handy IR radiometers or thermo cameras are commercially available. Application of these equipments to the desired area yields the detailed surface temperature distributions very easily and safely. Hence, it is highly desirable to establish formulae which can evaluate the amount of heat discharge from the desired area. based on the surface temperature measurement using infrared equipments. This paper deals with a preliminary experiment on the heat discharge from the surface of the porous medium heated from below. Fine glass beads were used as a porous medium and the experiments were carried out under dry and water saturated conditions. The vertical temperature gradient of the medium, the surface temperature and the amount of heat discharge were measured by thermocouples. The surface of the medium was subjected to the free air and evaporation was allowed for the water saturated condition.

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Gravity surveys have been carried out over Asama and Kusatsu-shirane volcanos, the central part of Japan, for the purposes of obtaining a distribution map of Bouguer anomaly and establishing a precise gravity net for detecting gravity changes associated with volcanic activities. The Bouguer anomaly map over an area of about 6,300km² centring Volcano Asama is obtained by compiling the data of the present survey together with the gravity survey in the past. The distribution of the Bouguer anomalies reflects well the geological feature in the surveyed area. The high and low anomaly areas correspond to plutonic or volcanic rocks and sedimentary areas respectively. The gravity high anomaly corresponds to the central belt of uplift running south-westwards from the northerb part of Volcano Kusatsu-shirane. Bisecting the low anomaly zone, another high anomaly zone, a branch of the central belt of uplift, runs along the mountain chain of Mt. Eboshidake, Mt. Takamine and Volcano Asama. Mt. Arafune is also occupied by a high anomaly. 0n the other hand, there is no remarkable anomaly around Volcano Kusatsu-shirane. In order to calculate the subterranean structure around Volcano Asama from the obtained gravity distribution, the well-known sinx/x method was used for analyzing the gravity data. With reference to the explosion seismic data, a density contrast of two layers that have a compressional wave velocity of 4.4km/sec and 6.Okm/sec respectively is chosen as 0.3g/cm³, and the depth of the boundary between both the layers was taken as 3km beneath ground surface at the eastern part of Ueda. In the underground structure computed in such a way, we see the low anomalies in the northern part of Volcano Asama and the southern part of Komoro. They may indicate thick sediments about 5km deep. The latter shows a caldera-like structure.

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Mechanisms of the formation of pyroclastic flow deposits of Japan have been investigated on the basis of their grain-size dize distribution. The pyroclastic flow deposits of Japan can be classified into A-, B- and C-type on the basis of differences of their grain-size distributions, and the pyroclastic flow deposits of these types correspond to Murai's (1961) nuee ardente deposits and intermediate-type pyroclastic flow deposits; St. Vincent-type ash flow deposits; and Krakatoa-type ash flow deposits respectively. 0n a Rosin’s law paper, cumulative curves of the A-type pyroclastic flow deposits resemble those of ideal pyroclastic flow deposits which are composed of particles concentrated in the lower part of the eruptive column. The pyroclastic flow deposits of this type, there-fore, was probably formed when the particles of the lower part of the eruptive column dashed down the mountain slope. It can be considered that the B-type pyroclastic flow deposits were formed in the same way as the A-type deposits were formed, because the grain-size distribution of this type on a probability scale is very similar to that of the A-type. Whereas, on the probability scale, the grain-size distribution of the C-type pyroclastic flow deposits does not resemble that of the A-and B-type deposits, but resemble that of the turbidity current deposits stated by Visher(1969). It seems that the pyroclastic flow of C-type traveled as the turbidity current did.

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