Total Fe and ten minor elements, Li, Mn, Co, Ni, Cu, Zn, Rb, Sr, Cd and Pb, were determined by atomic absorption spectrometry for 17 fall units of the “B scoria fall deposit” formed by the 1108 AD eruptive activity of Asama volcano. Distribution of these elements was observed to be quite uniform throughout the air fall sedimentation layers with the average concentrations as follows : Li, 11; Mn, 916; Co, 22; Ni, 28; Cu, 54; Zn, 72; Rb, 28; Sr, 324; Cd, 0.27; Pb, 8 (ppm) and Fe, 5.39 (%). As far as the present observation concerns, homogeneity of these elements is suggested for the portion of the magma having erupted as scoria fall in this activity.

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Thick acid tuff layers are widely distributed in the Southern Shimanto terrane of the western Kii Peninsula, Southwest Japan (YANAI, 1980). They are interstratified within the Late Cretaceous strata (Coniacian-Santonian∿Maastrichtian?) of sandy mudstone, lithic sandstone rich in volcanic rock fragments, and submarine slump and sliding breccia. Those strata were accumulated in a shallow-sea environment (YANAI, 1982). The acid tuff layers tend to decrease in thickness from south to north. The grain size also decreases in the same direction. The acid tuff layers gradually change their lithofacies toward north from coarse-grained pumice tuff and volcaniclastic sandstone rich in pumice fragments to finegrained vitric tuff with sedimentary structures characteristic of turbidites. These lines of evidence suggest that the source area of these pyroclastic materials presumably lay to and/or on the south of the depositional area, the Shimanto geosynclinal basin sensu stricto. The Late Cretaceous volcanic activities are also known in the Eo-Nippon Cordillera, to the north of the geosynclinal basin (eg. KOITO, 1975). Therefore, the violent acid volcanic activities were found at least in two rows on both north and south sides of the geosynclinal basin. The ancient subduction axis was situated farther to the south of the geosynclinal basin during Late Cretaceous time.

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The Hachinohe ash is a widespread pyroclastic-fall deposit erupted from the Towada caldera about 13, 000 years B. P. Along the dispersal axis, the thickness is 150cm 50km away from source, and the estimated volume is 14km³ It is composed of alternating beds of fine ash (65%) and pumice lapilli (35%). No erosional break is observed and the contacts between beds are gradational. The fine ash beds have two components : a dominate component of grain-supported accretionary lapilli and a subordinate component of fine ash-coated pumice; an ash matrix is lacking. The maximum grain size of accretionary lapilli does not decrease systematically away from source. The size population of constituent ash particles shows a small degree of fractionation with distance from source; the grain size class 1mm to 1/4mm increases while the class finer than 1/16mm decreases. Pumice beds are composed primarily of sub-angular to sub-rounded pumice fragments coated with fine ash and a subordinate amount of lithic fragments and accretionary lapilli. Maximum pumice size and maximum lithic size systematically decrease away from source. The beds show bimodal grain size distributions and contain more than 10 weight percent fine ash. An individual fine ash particle has too low a terminal velocity to fall out as a separate grain near the source area. It is certain that, throughout the Hachinohe ash eruption, fine ash continued to fall in the form of accretionary lapilli and/or attached to pumice fragments. The fine grained nature and wide dispersal indicate that the Hachinohe ash is representative of the phreatoplinian deposit formed by the interaction of water and silicic magma during explosive eruptions. At times when the proportion of erupted magma to lake water gaining access to the vent became sufficiently high, violet eruptions took place and deposited pumice fragments and accretionary lapilli simultaneously at the same place. Examples of phreatoplinian deposits are also reported from the Kutcharo and Hakone calderas, in addition to two other deposits from the Towada caldera. Such deposits are used as a possible indicator of source environment.

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During the cruise of IPOD Legs 56 ＆ 57, many cores were successfully recovered around the Japan Trench region off the Tohoku Arc. Two different temporal patterns of the explosive volcanism were recorded at two different sites which represent the reference stratigraphic sections of landward and seaward of the trench. The landward site is located on the Eurasian Plate and the seaward site on the Pacific Plate. Both landward and seaward referencs sites, 438 & 439 and 436 show good continuous stratigraphy attaining as deep as Cretaceous. Landward reference site shows two distinct maxima around 16-15 Ma and 5-2 Ma in the frequency versus age diagram of the volcanic ash layers, whereas seaward reference site represents gradual increase from 11 Ma toward present. Sedimentation rates of each site are also relevant. Landward site has two definite fast sedimentation periods between 16-15 Ma and 5-2 Ma, whereas seaward site shows the monotonously increasing sedimentation rate from 14 Ma to present. Ash frequency patterns and sedimentation rates are identical with each other at the same site. The disagreement between landward and seaward records of explosive volcanism may be explained as follows : landward pattern with two maxima is common to the one of the Tohoku Arc. Earlier maximum corresponds to the acidic volcanic activity in Daijima and Nishikurosawa stages which may be related to the formation of the Kuroko deposits. The later maximum corresponds to Funakawa and Kitaura stages when a large scale of the acidic volcanic activity including the intrusion of granites (the so-called Tertiary granites) related to the Dewa disturbance. On the other hand, the apparent increase in the frequency of volcanic ash layer observed in the seaward reference site is governed by the movement of the Pacific Plate with time, approaching gradually towards the volcanic source, the Japanese Islands. The result of the presnt study implies that the increase in global volcanicity during Quaternary proposed by KENNETT and THUNELL (1975), might be only apparent and nearly reflecting the approach to the volcanic sources.

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Kiso-Ontake Volcano erupted suddenly on the 28th of October, 1979. The eruption started forming ten new craters on the southern flank of the volcano’s summit. This paper deals with geochemical study on its volcanic activity after the eruption. The ratio of Cl to S was found to be high in the water-soluble components of the volcanic ash. The content of SO₂ was larger than that of H₂S in the volcanic gas. According to these observations and considering the sulfur isotopic ratio, the under ground temperature was estimated to be higher than 250℃, while that at the orifice was measured to be as low as 90℃. These facts and seismic observation indicate that magma was not elavated to a sarrow part, but that only high temperature-gas, which was separated from the magma, came up rapidly along the crack, and then rushed into the mud reservoir near the surface, where a large amount of water was evaporated, following the eruption caused by prompt increase of pressure. At an early stage of the eruption, the content of Cl^- was larger than that of SO²₄ in spring and pond waters, which were in contact with the fumarolic gases at the summit, but its relation was reversed later. This has been explained by the absorption of HCl, which had been contained in the volcanic gas at the early stage, into the water phase near the summit. From 1980 to 1982, the outlet temperature of fumarole increased to 108-145℃. However, the ratio of SO₂ to H₂S reduced rapidly, which indicates the decrease of temperature at depth. Therefore, the increase of the temperature of the fumarole is thought to be caused due to the decrease of cooling effect by ground water, because water was evaporated by the continuation of the eruptive activity. Recently no significant changes have been observed in temperature and chemical compositions of volcanic gas, suggesting that the activity will remain to be low for the time being.

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