Hot Springs and Volcanoes 

The mechanism of the hot springs

Recent studies indicate that most hot springs used for bathing are "circulating waters," that is, they originate from the rain and snow waters that penetrate the soil and absorb its minerals and heat up as they infiltrate and reach deeper geological layers. After a long time in a subterranean environment, these waters reemerge at the surface, since the heated liquid becomes less dense and is often forced to rise by pressure generated by the faults and canals.

Hot springs are classified as volcanic and non-volcanic. Non-volcanic water, in turn, is subdivided between deep groundwater and fossil water.

Volcanic hot springs

In volcanic areas, at kilometers below the surface, the magma chambers, where magma accumulates in cavities, reach temperatures of over 1000℃. Heated by these chambers, the underground water emerges at the surface through geological faults or man-made drilling, giving way to volcanic hot springs. It is believed that the richness and diversity of the thermal waters are the result of the blend of gases and aqueous solutions that come from the magma, as well as the dissolution of minerals present in the rocks during the course of the water back to the surface.

Non-volcanic hot springs: deep groundwater

On planet Earth, the soil temperature increases by an average of 3℃ with every 100 meters of depth. This rate is known as the geothermal gradient. For example, if the surface temperature is 15°C, at 1,000 meters deep the soil will rise to approximately 45°C, while at 1,500 meters the temperature hits 60°C. As such, non-volcanic hot springs that rely on deep groundwater are heated by the hot rocks or by the very action of the geothermal gradient.

Non-volcanic hot springs: fossil water

In certain regions, past geological movements have trapped ancient marine waters underground, forming what is known as “fossil water.” If the fossil water is a few hundred meters below the surface, in areas where there are no volcanoes or high-temperature rocks, the geothermal gradient is not able to heat the water to high temperatures. However, even if the temperature is below 25°C, the water that has a high concentration of salinity is classified as a hot spring, according to Japanese law.

In coastal areas, it is common for current seawater or groundwater in the region to mix with fossil water.