Scientists believe that Beppu hot spring, a volcanic hot spring, was formed about 50,000 years ago. It is characterized by its high temperature and wide variety of chemical water types. The hot spring’s heat and chemical elements originate from the hot chloride water deep underground beneath the Tsurumi volcanoes. The parent chloride water has temperatures ranging from 250 to 300°C and a chloride concentration of 1400-1600 mg/kg. It takes approximately 50 years on average for the subsurface hot water distributing across the Beppu area to be renewed by rain water which is the original source of hot spring water.
The hot water flows along the Asamigawa fault on the southern ridge of Beppu alluvial fan and the Kannawa fault on the northern ridge. The Beppu Hatto (the eight hot spring areas in Beppu) are situated along these two flow channels.
The hot water boils as it flows up along the channel, generating vapor and causing it to distribute to higher grounds. Hot water, that loses its vapor, continues to flow toward the lower ground and is diluted by the groundwater, forming a chloride water. Meanwhile, vapor containing hydrogen sulfide generates a sulfate water and vapor containing carbon dioxide generates a bicarbonate water. In addition to this, even more types of waters are formed through a mixture of these three types of waters with groundwater in various proportions.