Atmospheric pressure waves generated by the big eruption of Mount St. Helens at 15 32 GMT on May 18, 1980 are recorded with microbarographs at eight stations of Japan Meteorological Agency in Japan of about 7, 000-9, 000 km in great circle distance from the volcano. The wave train is also detected with World Wide Standard Seismograph (long period and vertical component) settled at Matsushiro, Nagano Prefecture. It is the third time since the 1883 Krakatau Eruption and the 1956 Bezymianny Eruption that atmospheric pressure waves caused by volcanic eruptions at long distance are recorded with barograph or micrographs in Japan. The barograms of atmospheric pressure waves begin with increase of atmospheric pressure and distinct waves which give the maximum amplitude of 3-5 minutes of period. The mean of the maximum amplitude of records is 0.10 mb and the mean propagation velocity 308 m/sec. The antipodean wave train (A₂) and the third one (A₃) are not detected. The estimated energy of the eruption by applying the results by HUNT et al. (1960) and WESTON (1961) is at least 10²³ ergs. This estimated energy almost coincides to the energy of the 1956 Bezymianny Eruption estimated by MURAYAMA (1969). It would not be appropriate to directly compare with the kinetic energy of the eruption estimated through the amount of ejecta, but the above-mentioned estimated energy is almost same order of the mechanical energy of the blast according to DECKER R. and DECKER B. (1981). Comparing with records of atmospheric pressure waves detected in the United States, West Germany and the Netherlands, the maximum amplitude of barograms obtained in Japan is less about 10%. This may be caused by prevailing westward winds in the upper atmosphere on the propagation path from Mt. St. Helens to Japan, but the remarkable difference of propagation velocity is not seen.

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A small-scale explosive eruption of Asama volcano lasted from 02 h 25 m to ca. 06 h, April 26, 1982. It produced a thin but far-reaching ash fall bed and very minor pyroclastic flows on the upper slopes. The main axis of ash-fall deposit extended ESE passing Tokyo to reach the Boso Peninsula. Subordinate axis extended toward the southwest to reach Lake Suwa. The wind above 5, 000 m was mainly responsible for the main axis while the low altitude winds produced the subordinate axis as it blew south to southwestwards. The amount of ash fall ranged from more than 300 g/m² to 100 g/m² at the distance of ca. 10 km from the vent and the total mass erupted was estimated to be about 8 million tons. The ash traveled at an average speed of 12 m/s as far as 200 km from the vent along the main axis. Grain size of the ash regularly decreases clockwise at the distance of about 10 km from the crater reflecting the changing wind direction with altitude. No appreciable change in the medium diameter was found for samples taken in Tokyo (130 km away, Md_φ=3.32) and the one 10 km away (Md_φ=3.26). All samples show marked skewness toward fractions finer than 63 μm suggesting that such fine particles descended in aggregates. Field evidence that in some places ash was incorporated in mud droplets strongly supports this mechanism although it was reported that ash fell apparently in a “dry” state. Ash contained several percent of hydrothermally altered older volcanic materials as well as much water-soluble substances, gypsum, alunite, etc. No clay minerals were found by x-ray diffraction, a fact in strong contrast with the ejecta of 1977-1978 eruption of Usu and 1979 eruption of Ontake volcanoes although both materials had a very similar appearance and clayey physical properties as the present ash. No vesicular, juvenile matelials were identified and the bulk consisted of polyhedral grains of hyalopilitic pyroxene andesite very much similar to recent lavas of Maekawa-yama. No liquid magma but a high-temperature steam jet deep out of the vent may have been responsible for this explosive eruption. High temperature of the erupted material was clearly demonstrated by the glowing deposits observed on the upper slopes immediately after the first phase of eruption. These were mainly laid down by the very small scale pyroclastic flows which overflowed the crater rim and descended for a short distance over the northern and southern slopes.

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Mafic-ultramafic xenoliths of alkali basalt from Iki Island were examined to investigate the petrological constitution of the lower crust of southwestern Japan. Mafic and ultramafic rocks as xenoliths from Iki Island can be divided into two groups, I and II, in terms of the texture. Xenoliths of Group I consist of relatively fine-grained granular two-pyroxene gabbro with metamorphic texture. Orthopyroxene is hypersthene with Mg/(Mg+Fe) atomic ratio, 0.60 to 0.70, and clinopyroxene is augite or salite. Plagioclase (An₅₅-An₇₄) is homogeneous and simply twinned. Those of Group II, on the other hand, are the mixtures of various amounts of olivine, clinopyroxene and plagioclase, with subordinate amounts of spinel, magnetite, ilmenite, kaersutite and biotite. Group II xenoliths are relatively coarse-grained and usually still retain igneous textures. Fo content of olivine varies from 67 to 78. Clinopyroxene (augite or salite) is more Mg-rich than that of Group I xenoliths. Plagioclase (An₆₄-An₇₉) is usually zoned and has polysynthetic twins. Plagioclase and olivine often reacted to make spinel-pyroxene symplectite. Two-pyroxene geothermometer denotes the equilibrium temperature of about 1, 000℃ in both groups of xenoliths. Xenoliths of Group I are metamorphic rocks of pyroxene granulite facies, into which basaltic magmas had intruded to have piled mafic and ultramafic rocks of Group II. This petrological constitution of the lower crust may be common to southwestern Japan.

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A small array network for precise leveling was set on Yake-Dake Volcano in 1977. Length of this leveling line is 20-65 m. Leveling has carried out once a year in autumn season. Remarkable tilt changes of inclined northward about 80μ radian was observed during the period from 1977 to 1981, which correspond to a relative upheaval of the summit of Yake-Dake. The tilt rate was increased every year and reached to 31μ radian per year in 1980. On the other hand, number of earthquakes near the volcano was also increased year by year. The tilt movement is very close to the seismic activity of the volcano.

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