A strong earthquake occurred at the Ebino-Yoshimatsu district on February 21, 1968. The most damaged area of this earthquake is covered by the Kakuto formation. The damages were much affected by the “Shirasu” or “Shirasu-like sediments”, which were the main parts of the Kakuto formation, representing the lacustrine facies of the Aira pumice flow which invaded into “Kakuto caldera”.

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Around the central cones of Hakone Volcano, there are several solfataric fields and many hot springs (Fig. 1). Among them, Owakudani and Sounzan geothermal areas, including Ubako, Kaminoyu and Gora hot springs, are the main ones. In these areas, we have measured mass and thermal discharges from steam wells, fumaroles, steaming grounds, hot springs and thermal conduction through soil (Table 1). Total mass discharge from these areas amounts to 129 kg/sec and total thermal discharge amounts to 10.64×10⁶ cal/sec which is equivalent to 1×10²² erg/year, corresponding to a mesoscale volcanic eruption per year. In order to estimate a hydrothermal system and a heat origin of the Owakudani and Sounzan geothermal areas, we have proposed a model of the hydrothermal system as shown in Fig. 2. Then we have assumed that the water released from these areas may be mainly of meteoric origin, but a small part of it must be of magmatic origin because a heat transporter in addition to heat conduction is necessary to maintain the large thermal discharge from a limited area. Cooling of the magmatic steam on the way from magma chamber to thermal water reservoir where the steam mixes directly with the meteoric water can be shown approximately by Eq. 1 which is made of adiabatic and conductive terms. Integrating Eq. 1 and substituting proper physical constants, we can calculate the steam temperature at any depth as a function of flow rate and depth of magma chamber. In Fig. 3, curves (1) show the steam temperature at 1 km depth supposed as the thermal water reservoir depth and curves (2) show the heat transported by the magmatic steam. When the thermal discharge is measured and the depth of the magma chamber is assumed, we can estimate from Fig. 3 the magmatic flow rate and the temperature of the steam at 1 km depth. Of the Owakudani and Sounzan geothermal areas, the flow rate was 6.2～6.4 kg/sec and the temperature was 970～920℃. The former is about 5% of the total mass discharge from the said areas, and the other 95% of the discharge is corresponding to 4.7% of annual precipitation of the areas. These thermal and water balances are illustrated in Fig. 4. We have also calculated the volume of magma chamber necessary to maintain the above geothermal activity.

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Small pick-ups were set at the borehole which is about 30 m in depth, and volcanic micro-earthquakes were observed. It was found that long period volcanic micro-tremors (1st and 2nd kind micro-tremor) were also originated from micro-earthquakes. Consequently, the observation of 2nd kind micro-tremors was carried out in a region wide enough to investigate the mechanism of micro-earthquakes. It is likely that micro-tremors of the 2nd kind occur in phase around the epicentre.

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One of the fundamental differences between volcanic cone and non-volcanic mountains such as folded mountains is that the latter itself is a part of the earth’s crust, while the former is taken as a heavy load which is laid upon the pre-existing earth’s crust within a short geological time and is durable for several tens of thousands of years. In this respect, a volcanic cone resembles an ice sheet, a huge building and a large dam. It is, therefore, postulated that volcanic cone settles down by its own weight. From this point of view, characteristics of the subsidence of some strato-volcanic cones in Japan and Indonesia (Table 1) are comprehensively discussed in this paper. The results are summarized as follows. The settlement of volcanic cone causes various deformations at the foot of volcanic cone such as ring fault, thrust and the circular anticlinally uplifted ridge, all of which tend to encircle the volcanic cone settled. Based on the modes of these deformations at the foot, the settlement of volcanic cone is classified into three types ; 1) fault type, 2) fold type, and 3) mixed type. They are schematically shown in Fig. 5. Which type among the three takes place seems to depend on the nature of the basal rocks beneath the volcanic cone (Table 1 and Fig. 5). The fault type occurs in the case where Pliocene sedimentary rocks are thinner than about two hundreds meters in thickness and also most of the basal rocks are composed of Tertiary sedimentary rocks older than Pliocene. On the contrary, in the case where Pliocene sedimentary rocks are thicker, generally several hundreds to thousands meters, the fold type or the mixed type results. Magnitude of settlement is of order of one to two hundreds of meters in the depth settled. Rate of settlement of Iizuna volcano, which belongs to the fold type, is estimated to be of order of about four millimeters per year. Distance from the center of volcanic cone to the circular deformed feature (D), which is thought to show the magnitude of deformation originated by the settlement, is proportional to the relative height of volcanic cone (H), which can be taken as the substitute for the weight of volcanic cone (Fig. 6). Such relationship between D and H is also found in the case of guyot, which is surrounded by circular moat or ridge (Fig. 5), but not found in the case of collapse calderas as shown in Fig. 6.

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The water released on heating powdered volcanic rocks below 340℃ was studied in relation to heating temperature, grain size, atmospheric humidity during sample preservation, etc. Volcanic rocks used in this study can be classified into four types on the basis of the behavior of the water released below 340℃. Rocks of the first type release water linearly with increasing temperature up to 180℃ or 230℃. Above this temperature only a very small amount of water is evolved. The amount of water released below 250℃ is small (less than 0.6%), and regained reversibly on standing under the same condition as that of the sample preservation. In this case the water may be regarded as adsorbed water and corresponds to H₂O(-) in the original sense. The most of usual volcanic rocks belongs to this type. Rocks of the second type release much water already below 50℃. The loss of water continues above 180℃. Loss and regain of water are not reversible in these samples, thus the greater part of water is not adsorbed water. Pichstone and alkali olivine basalts containing “residual magmatic water” belong to this group. Rocks of third type show the properites intermediate between that of the first and the second type. Many obsidians belong to this group. The rock belonging to the fourth type is only one. This rock behaves similarly to the second type. But, the “H₂O(-)” content is seriously affected by the relative humidity in preservation vessel.

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