Many volcanoes in the Japanese archipelago are considered to have been formed by lava and pyroclastic flows spewed out repeatedly from magma chambers in the shallow part of the crust formed by the magma rising from the upper mantle and the bottom crust. The calc-alkaline series magma is believed to form a magma chamber in the shallow part, a few kilometers underground, due to its high buoyancy. Many hot springs with a temperature of 90ºC or higher, near boiling point, are found around volcanoes originated from calc-alkaline rocks because the magma chamber in the shallow part heats up groundwater and forms a high-temperature hydrothermal convection system.

When drilling is conducted on a hydrothermal convection system around a volcano, even hot reservoir water will vaporize and become steam with a temperature of 160-330ºC due to reduced pressure during the ascent in a well and will flush out to the surface by itself. It is possible to generate electricity without consuming fossil fuels if this steam is used to rotate a turbine. This is the mechanism of geothermal power generation.

Geothermal power production is advantageous not only in terms of energy security because it is a purely domestic energy, but also as a countermeasure against global warming because it is a clean energy that generates only a very small amount of carbon dioxide. However the capacity of the geothermal power generation facilities in Japan is approximately 550,000 kW, which places Japan only sixth in the world.

Recently, even nations with few volcanoes are working toward the development of geothermal power production by drilling to great depths and utilizing non-volcanic crustal heat flow. Needless to say, a volcanic nation like Japan has advantages regarding geothermal power production, thus Japan is aiming to improve its systems for more extensive use.

Japan is leading the world in the development of geothermal technology. We have already mentioned imagining actually observing a magma chamber in the shallow part of the crust forming a high-temperature hydrothermal convection system. This image was in fact physically proved for the first time in the world by the Deep Geothermal Resources Survey in Kakkonda, Shizukuishi-cho, Iwate Prefecture in Japan. In the survey, the 3729 meter-deep well, called WD-1a was drilled in 1995. As shown in Figure 1, the well passes through the hydrothermal convection system and also the Kakkonda Granite, a solidified magma chamber, the heat source of a hydrothermal convection system. The complex can be referred to as a newly-solidified magma chamber because the temperature at the boundary of granitic rock was 370ºC and the temperature at the bottom of the well dug to the depth of 3729 meters was more than 500ºC, as shown in Figure 2. These records lead the world in terms of scientific achievement in geothermal drillings and thus have caught the world’s attention.