Origins of geothermal springs
A spring is the natural flow of water from the ground which can occur when geologic, hydrologic or human forces cut into the underground layers of soil and rock where water is circulating, thus allowing the water to rise to the surface under pressure.
The amount of water that flows from springs depends on several factors, including the size of the spaces within rocks (volume of water), the water pressure in the aquifer, the size of the reservoir basin and the amount of precipitation that is necessary to replenish the aquifer.
Human activities can greatly influence the volume of water that discharges from a spring through excessive groundwater withdrawals, eventually reducing the pressure in the aquifer and ultimately decreasing the flow from all natural springs in the area.
Geothermal and mineral spring water can originate from several groundwater sources:
natural groundwater (meteoric water),
rain or lake water (seepage and replenishment,
infiltrated seawater (seepage),
artesian water (water in a confined aquifer and rising under pressure in wells),
water trapped in sediments (connate water),
water introduced by magmatic processes (juvenile water),
water reinjected in the ground.
Several sources underground can act as a heat source:
direct volcanic activity,
the geothermal temperature gradient as water passes through subterranean rocks,
fractures and fissures in the rock formations resulting in pressure build-up that heats the water as it passes.
Warm water, regardless of its origin, is lighter than cold water and readily rises to the surface if not confined. Hot spring water is generally more highly mineralised than cold springs, because as the temperature increases while the water is circulating underground it dissolves the surrounding host rocks, which release minerals into the hot groundwater.
There are no restrictions where geothermal springs can surface - they are widely distributed all-over the world although clusters are more likely to be found in regions with volcanic activity such as for example in Japan, Iceland or New Zealand. But also areas overlying large groundwater aquifers such as the Great Artesian Basin in Australia or the Guarani Aquifer in South America have an abundance of geothermal springs which are used for various purposes.