Mechanism generating the volcanic tremor of the 2nd kind (characterized by the period of 3.5～7 sec and vertically and radially polarized orbit, as firstly pointed out by SASSA (1935) from observation at Aso Volcano, is deduced from volcanic gas issue exciting magma chamber. This process is completely connected with the gas stream through volcanic vent, by which, as shown in the paper (I) (WADA, 1980), the tremor of the 1st kind is generated. Thus both tremors are deduced from the unified time-stationary phenomena as the gas motion. The dimensions of chamber are estimated to be vertically 10～12 km and horizontally 6～8 km, based on the simplest model of chamber shape and the period of tremor.

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“Thickness-isopach area” curves are drawn on the basis of data from 49 recent eruptions and 10 Usu 1977-1978 eruptions. 1) “Thickness-isopatch area” relation of tephras follows the equation (1) ; Y=AXⁿ, (X=log₁₀(h_max/h), Y=log₁₀(S/S_o). S : the area enclosed by isopach of thickness h. S_o : that of maximum thickness h_max). 2) Volume of tephra is calculated using the equation (1). 3) S_e (0.1 h_amx isopach area) or S_e (characteristic area) is more significant than Walker’s D (0.01 h_max isopach area) as a dispersal index of tephra and either of them relates with A and n as follows : A=log₁₀(S_e/S_o), n=log₁₀(h_max/hh_e)/log₁₀(S_c/S_o), and h_c×S_c=(h×S)_max. 4) Volume percentage of deposits inside S_c is about 35%, and M_d size of deposits on h_c (characteristic thickness) isopach indicates the size index of tephra. 5) S_e (0.1 h_max isopach area) is named an equi-distribution area since S_e×h_max indicates volume of tephra.

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Annual discharge of several chemical components of fumarolic gases and of hot spring waters are calculated on the basis of the remote sensing measurement of fumarolic SO₂-discharge by OHKITA et al., (1977) and of the estimation of hot-spring water discharges by KAMADA (1964). An examination is made if the present material discharge could be maintained for 1, 000 years only from feeding sources of near surface such as the volcanic body of Iwo-dake, sea water etc. Chlorine may be supplied from sea water. Fluorine content of sea water is very small, and almost all the fluorine in the whole rocks of Iwo-dake must be given off in order to continue the present fluorine discharge for 1, 000 years. Neither the amount of sulfur in the Iwo-dake rocks nor that in sea water is sufficient to maintain the present discharge only for a few years. Aluminum and iron can be supplied if 5% of the Iwo-dake rocks are completely leached by acid waters. Alkali and alkaline earth metal ions can be supplied either from sea water or from the rocks. MATSUO et al., (1974) reported that fumarolic gases of the Iwo-jima volcano have very high δ¹⁸O values (～+7‰). If these high δ¹⁸O values were the result of oxygen isotopic exchange between surface water (including sea water) and near surface rocks, δ¹⁸O value of the whole Iwo-dake rocks should be lowered to 0‰ within 250 years. The result of the calculations strongly suggests that not only heat but also material supply from magma is necessary to maintain the fumarolic and hot spring activities of Iwo-jima volcano.

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The Kuju volcano group is situated at central Kyushu, Japan. Kuju-iwoyama is an explosive crater of Mt. Hosshoyama which is one of the Kuju volcano group and shows the most intense geothermal (mainly fumarolic) activity in the Kuju area. In the Iwoyama area, we have measured heat and water discharges from fumaroles, steaming grounds, hot springs and thermal conduction through soil. Total heat discharge from the area amounts to 2.38×10⁷ cal/sec (99.2 MW) and total water discharge amounts to 65.2 kg/sec. Also total volcanic gas emissions were estimated, based on chemical analyses of volcanic gases and the total water discharge. As a result, daily volcanic gas emissions were estimated as follows ; CO₂ 166 tons/day, H₂S 53 tons/day, SO₂ 26 tons/day, S 3.2 tons/day, HCl 3.2 tons/day and HF 0.07 tons/day. A quantitative hydrothermal system was proposed, based on the total heat and water discharge. We assume that the hydrothermal system is composed of the surface zone, the shallow thermal water reservoir and the deep heat source (the magma reservoir). From the heat source, the heat and steam are transferred upward by magmatic gas and thermal conduction. Cooling of the magmatic steam on the way from the magma reservoir to the thermal water reservoir is calculated by using the method developed by YUHARA (1968). We also assume that the depth of the thermal water reservoir is 2 km depth. At the depth, the magmatic steam mixes directly with the meteoric water and forms there the thermal water reservoir. Thermal water discharged from the reservoir rises up and it ejects from the ground surface. When the thermal discharge is measured and the depth and the temperature of the heat source are assumed, we can estimate the magmatic steam flow rate and the temperature of the steam at 2 km depth. The steam flow rate and the temperature are estimated to be about 20.3 kg/sec and 990℃, respectively, assuming that the depth of the magmatic reservoir is 5 km and the temperature is 1, 000℃. The temperature of the thermal water reservoir is estimated to be about 370℃, from the mean enthalpy of the reservoir, that is, the thermal state of the reservoir is near the critical point of water. The estimated temperature of the reservoir is nearly equal to the observed maximum temperature of fumaroles. The percentage of the magmatic steam flow to the total water discharge is about 41% and that of the total water discharge to annual precipitation of the area concerned is about 47%. The above mentioned three features are clearly different from those of the ordinary geothermal areas. These may be characteristic features of the hydrothermal system which exists below the active volcano having high temperature fumaroles. Chemical compositions of fluid at the thermal water reservoir and at the magma reservoir are estimated, based on the above mentioned hydrothermal system and the chemical equilibrium of the gas. As a result, it is clarified that the estimated chemical composition at the magma reservoir is very similar to that of the gas in chemical equilibrium with the granitic magma. An important characteristic feature of volcanic gases from Iwoyama is very low content of HCl in comparison with that of sulphide. This fact shows that much HCl remains in the underground or was eliminated on the way from the magma reservoir to the ground surface. Otake and Hatchobaru geothermal area where the geothermal power plants are operated at present, is situated at the western direction of Iwoyama. The distance between them is about 5 km. At Otake and Hatchobaru, much thermal water of sodium chloride type are drawn from production wells. The existence of thermal water of sodium chloride type below the Otake and Hatchobaru may be closely related with the very low content of HCl in volcanic gases from Iwoyama. In this case, we must consider a larger scale of the hydrothermal system.

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Microscopic textures showing flow lineation were measured on oriented thin sections of the welded, late Quaternary Ata pyroclastic flow deposit, southern Kyushu, Japan. Obtained results indicate three factors which control the flow lineation. The radial orientation away from a source as mentioned by ELSTON and SMITH (1970) was obtained only at the sites where basement relief was completely filled up with the pyroclastic flow deposit. The lineations which are obtained from the samples collected in the floor of narrow valleys tends to be parallel to the direction of the valley. The samples which were collected from the wall of valley show the lineation parallel to the slope of the wall. These results suggest that the flow direction of the pyroclastic flow is strongly affected by the existing topographic relief. The earlier surges tend to flow along the valley floor or settle towards the bottom of the valley. After the basement relief was filled up with the earlier flow units, the later fluidized layer preserves its original radial movement until its final settlement.

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