The water issuing from a hot spring is heated by geothermal heat, i.e., heat from the Earth’s interior. In general, the temperature of rocks within the earth increases with depth. The rate of temperature increase with depth is known as the geothermal gradient. If water percolates deeply enough into the crust, it will be heated as it comes into contact with hot rocks. The water from hot springs in non-volcanic areas is heated in this manner.
In active volcanic zones , water may be heated by coming into contact with magma (molten rock). The high temperature gradient near magma may cause water to be heated enough that it boils or becomes superheated. If the water becomes so hot that it builds steam pressure and erupts in a jet above the surface of the Earth, it is called a geyser. If the water only reaches the surface in the form of steam, it is called a fumarole. If the water is mixed with mud and clay, it is called a mud pot.
Note that hot springs in volcanic areas are often at or near the boiling point. People have been seriously burned and even killed by accidentally or intentionally entering these springs.
Warm springs are sometimes the result of hot and cold springs mixing but may also occur outside of volcanic areas.
Infections from hot springs
Unfortunately, hot springs can create ideal conditions to spread infections. For example:
- Naegleria fowleri, an amoeba, lives in warm waters and soils worldwide and can be a cause of meningitis. Several deaths have been attributed to this amoeba, which enters the brain through the nasal passages.
- Acanthamoeba also can spread through hot springs, according to the US Centers for Disease Control.
- Legionella bacteria have been spread through hot springs.