Introduction to the Landforms and Geology of Japan

GLGArcs

Goshogake and Tamagawa Hot Springs

Fumaroles with crystals of sulfur, and bathing people
fumaroles Photo 8: Fumaroles []

 

Flowers of sulfur Photo 9: Sulfur crystals [ ]

 

Bathing people Photo 10: Bedrock bathing []

 

Table: Components of geothermal water at the Obuki spring (Yoshiike, 1993)

 

Distribution of hot spring types Fig. 3: Distribution of hot spring types []

There are many active fumaroles ejecting volcanic gas in the crater. Similar to the way minerals precipitate after geothermal water flows out on the surface as mentioned above, abruptly decrease in pressure and temperature of discharged volcanic gas results in deposition of minerals contained in the gas around the fumaroles. Vivid yellow around their mouths is sulfur color. The small needlelike crystals (flowers of sulfur) are seen in closeup (Photo 9). The crystals are formed by sublimation of vapour directly to a solid on cooling. The exhalation has a temperature of slightly over 100°C and strong odor of hydrogen sulfide. Moreover, the volcanic gas alters rocks and prevents vegetation (Photo 8).

A lot of people come to the Tamagawa hot springs to take a bath for their health or recuperation. The ground in the fumarole area is so warm that people take bedrock bathing; they lie down on their mat on the ground (Photo 10). Hotels with a spa using hot spring water from the Obuki spring is near this area.

Tamagawa geothermal water

Geothermal water is usually derived from meteoric water including rainwater, rare from magma as juvenile water (magma contains water). When ground water contacts high-temperature volcanic gas or thermal water, it heats up and takes some components included in the gas and thermal water. The produced geothermal water rises mainly through faults and fracture zones due to lower density and often spouts out at hot springs. Ground water is also heated by conductive heat from magma and high-temperature rock body. The properties of geothermal water, such as pH, temperature, and components, vary depending on chemical and physical conditions. For instance, hot spring water near sea often contains much salt.

The geothermal water discharged from the Obuki spring contains chloride ions (Cl-) and sulfate ions (SO42-), being strong acid (pH 1.2) like mixing hydrochloric acid (HCl) with sulfuric acid (H2SO4). The concentration of Cl- is higher than that of SO42-, although the majority of volcanic acid hot springs is a Cl- < SO42- type in Japan. Such acid geothermal water is produced by which volcanic gas mixes with ground water.

Volcanic gas mainly contains H2O, CO2, SO2, H2S, HCl, and H2. Of those, H2O accounts for 96 to 99% of the volume and CO2 and SO2 for 1 to 4%. Volcanic gas consists of volatile components derived from magma, but it may include volatile materials of shallow sediment rocks and ground water. When the volcanic gas is cooled down and condensed or mixed with ground water at a shallow depth, acid geothermal water containing Cl- and SO42- is produced. The most soluble component of the gas is HCl, followed by SO2 and CO2. Therefore, as geothermal water is produced from volcanic gas, almost all HCl is dissolved in the water.

Hot springs can be classified into the Cl-type, SO4-type, HCO3-type springs by dissolved ions in the water. Hot springs around a volcano are often arranged from the center toward the outside of the volcano in order of Cl, SO4, and HCO3 types. This reason is thought to be a result of differentiation depending on solubility of volcanic gas components as mentioned above, but more complex mechanism affect the actual differentiation. The type of hot springs around Yakeyama changes with increasing distance from Yakeyama as follows: the Cl-SO4 type in Tamagawa, SO4 type, Na-Cl type, and Na-HCO3-Cl type (Figure 3). This arrangement is similar to the order as mentioned above. However, the Tamagawa hot springs are about 3 km away from the summit to the west. Chlorine and sulfate ions of Cl-type and SO4-type springs are derived from the volcanic gas of Yakeyama. However, bicarbonate ions (HCO3-) of HCO3-type springs have not been identified as those from the volcanic gas because the behavior of CO2 in magma and around a volcano is still unclear. Only the Obuki spring is the Cl-type in the Tamagawa hot springs. Other springs are the SO4-type, containing little Cl-, although the pH of their geothermal water is 1 to 2. Vapors ejected from fumaroles are also poor in HCl, consisting of about 99% H2O, 0.2 to 0.5% SO2, 0.4 to 1.0% CO2 by volume.

A geochemical analysis using isotopes of oxygen and hydrogen provided a model of differentiation of the geothermal water derived from the Yakeyama volcanic gas (Matsubaya, 1991). Figure 4 shows this model. Mixing the volcanic gas with ground water initially produces the Cl-SO4 type geothermal water. The water goes up and boils at 250 to 300°C to separate into liquid (hot water) and vapor. Because HCl in the gas dissolves in the separated water, this water becomes the Cl-SO4 type and the water vapor includes SO2 and CO2 only. The Cl-SO4 type water moves to the Tamagawa hot springs and the water vapor to the summit of Yakeyama. The geothermal water going toward the Tamagawa hot springs furthermore differentiates into the Cl-type hot water spouting out at the Obuki spring and vapor by boiling at 150°C. The vapor is ejected from fumaroles or is mixed with ground water or stream water to produce the SO4-type hot water discharged from hot springs other than the Obuki spring. Some of the vapor moving toward the summit of Yakeyama contacts with ground water to become the SO4-type hot water at other springs including the Yunosawa and the Sumikawa hot springs.


Fig. 4: Differentiation model of geothermal water derived from volcanic gas in Yakeyama (Matsubaya, 1991)

The Obuki hot spring water is also rich in Al, Fe, and Ca, although volcanic gas contains a trace of these metallic elements. This reason is that strong acid water reacts with metallic elements of rocks and dissolves them in the water under the ground.

The Obuki spring spouts out a large volume of almost boiling water (about 9000 L/min and 97°C). Therefore, heat is discharged from the spring in large quantity (8.3 × 108 cal/min). The maximum temperature of such spring decreases due to massive amount of discharged heat unless sufficient heat is supplied. A large heat source in addition to conductive heat from magma is needed to keep ejecting nearly 100°C water. It is thought that magmatic emanation directly mixes with ground water in the Tamagawa hot springs. It is unexplained where such large amount of water comes from and passes through.

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