Beppu Onsen is a typical volcanic hot spring. The vast area from Beppu Bay, through to Kujyu and Aso stretching to the Shimabara Peninsula is called the rift valley, which is a linear-shaped lowland between several highlands or mountain ranges created by the action of a geographic rift or fault. Volcanic activity about 30,000 years ago activated geothermal activity in the area centering on this rift valley.
Two big faults have occurred to the topography of Beppu area by this volcanic activity. A collection of heated water, called “Netsu-damari”, of high temperature and pressure was formed by the heat which was released from magma thousands of meters underground, as a result of the influence of these faults and the volcanic activity in Mt. Tsurumi and Mt. Garan on that rift valley. The rainwater, which permeated underground, becomes hot spring heated by coming into contact with the “Netsud-amari”. It takes various ingredients from the magma and surrounding rocks, while spouting again to the surface of the earth as hot spring water.