The distribution of thermal areas in Japan is geologically divided into the following units: (1) Neogene Tertiary sedimentary basins, (2) Neogene Tertiary plutonic rock areas, (3) Neogene Tertiary green-tuff areas, (4) Pliocene-Quaternary sedimentary basins, and (5) Quaternary volcanic areas.

Among these, the Pliocene-Quaternary sedimentary basins are areas of normal terrestrial heat flow. The Tertiary plutonic rock areas have locally abnormal heat flow, but heat flow from the Tertiary sedimentary basins and green-tuff areas is in general higher than that of normal heat-flow areas.

There are about 100 geothermal areas with water temperatures higher than 80°C in the mentioned areas and most of these are distributed in the Quaternary volcanic areas. Some hot springs issuing from the active volcanic areas are characterized by a low pH because of contained hydrochloric acid. These can be called typical volcanic hot springs.

To promote geothermal development, the Geological Survey of Japan is carrying on geothermal investigations in 30 promising areas. These investigations commenced last year and will be continued for a period of three years. Exploratory wells will be drilled in the areas selected by detailed surveys. These wells will be financed by governmental special funds commencing this year. The present investigations are being made to find new geothermal areas consisting of hot-water and/or dry-steam systems. However, if a study on the extraction of heat from hot dry rocks is commenced in the near future, the Quaternary dacite areas as well as the active volcanic areas will be selected as promising.

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