箱根火山の噴気活動とその熱源について

抄録

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.