A hot spring, hydrothermal spring, or geothermal spring is a spring produced by the emergence of geothermally heatedgroundwater that rises from the Earth's crust. While some of these springs contain water that is a safe temperature for bathing, others are hot enough that immersion can be harmful, i.e., lead to scalding and, potentially, death.
There is no universally accepted definition of a hot spring. For example, one can find the phrase hot spring defined as
a type of thermal spring in which hot water is brought to the surface. The water temperature of a hot spring is usually 6.5 °C (12 °F) or more above mean air temperature. Note that by this definition, "thermal spring" is not synonymous with the term "hot spring"
a spring whose hot water is brought to the surface (synonymous with a thermal spring). The water temperature of the spring is usually 8.3 °C (15 °F) or more above the mean air temperature.
a spring with water above average ambient ground temperature.
a spring with water temperatures above 50 °C (122 °F)
The related term "warm spring" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al. (2003) suggest that the phrase "warm spring" is not useful and should be avoided. The US NOAA Geophysical Data Center defines a "warm spring" as a spring with water between 20 and 50 °C (68 and 122 °F).
Sources of heat
Much of the heat is created by decay of naturally radioactive elements. An estimated 45 to 90 percent of the heat escaping from the Earth originates from radioactive decay of elements mainly located in the mantle. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.
Water issuing from a hot spring is heated geothermally, that is, with heat produced from the Earth's mantle. 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 such as Yellowstone National Park, 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 periodically 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 scalded and even killed by accidentally or intentionally entering these springs.
Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a geyser, or fountain.
High-flow hot springs
There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Springs with high flow rates include:
The Dalhousie Springs complex in Australia had a peak total flow of more than 23,000 liters/second in 1915, giving the average spring in the complex an output of more than 325 liters/second. This has been reduced now to a peak total flow of 17,370 liters/second so the average spring has a peak output of about 250 liters/second.
The 2,850 hot springs of Beppu in Japan are the highest flow hot spring complex in Japan. Together the Beppu hot springs produce about 1,592 liters/second, or corresponding to an average hot spring flow of 0.56 liters/second.
The 303 hot springs of Kokonoe in Japan produce 1,028 liters/second, which gives the average hot spring a flow of 3.39 liters/second.
?ita Prefecture has 4,762 hot springs, with a total flow of 4,437 liters/second, so the average hot spring flow is 0.93 liters/second.
The highest flow rate hot spring in Japan is the Tamagawa Hot Spring in Akita Prefecture, which has a flow rate of 150 liters/second. The Tamagawa Hot Spring feeds a 3 m (9.8 ft) wide stream with a temperature of 98 °C (208 °F).
The most famous hot springs of Brazil's Caldas Novas ("New Hot Springs" in Portuguese) are tapped by 86 wells, from which 333 liters/second are pumped for 14 hours per day. This corresponds to a peak average flow rate of 3.89 liters/second per well.
In Florida, there are 33 recognized "magnitude one springs" (having a flow in excess of 2,800 L/s (99 cu ft/s). Silver Springs, Florida has a flow of more than 21,000 L/s (740 cu ft/s).
Evans Plunge in Hot Springs, South Dakota has a flow rate of 5,000 U.S. gal/min (0.32 m3/s) of 87 °F (31 °C) spring water. The Plunge, built in 1890, is the world's largest natural warm water indoor swimming pool.
The hot spring of Saturnia, Italy with around 500 liters a second
There are at least three hot springs in the Nage region 8 km (5.0 mi) south west of Bajawa in Indonesia that collectively produce more than 453.6 liters/second.
There are another three large hot springs (Mengeruda, Wae Bana and Piga) 18 km (11 mi) north east of Bajawa, Indonesia that together produce more than 450 liters/second of hot water.
In Yukon's Boreal Forest, 25 minutes north-west of Whitehorse in northern Canada, Takhini Hot Springs flows out of the Earth's interior at 385 L/min (85 imp gal/min; 102 US gal/min) and 47 °C (118 °F) year-round.
A thermophile is an organism -- a type of extremophile -- that thrives at high temperatures, between 45 and 80 °C (113 and 176 °F). Thermophiles are found in hot springs, as well as deep seahydrothermal vents and decaying plant matter such as peat bogs and compost.
Widely renowned since a chemistry professor's report in 1918 classified them as one of the world's most electrolytic mineral waters, the Rio Hondo Hot Springs in northern Argentina have become among the most visited on earth. The Cacheuta Spa is another famous hot springs in Argentina.
The springs in Europe with the highest temperatures are located in France, in a small village named Chaudes-Aigues. Located at the heart of the French volcanic region Auvergne, the thirty natural hot springs of Chaudes-Aigues have temperatures ranging from 45 °C (113 °F) to more than 80 °C (176 °F). The hottest one, the "Source du Par", has a temperature of 82 °C (180 °F). The hot waters running under the village have provided heat for the houses and for the church since the 14th Century. Chaudes-Aigues (Cantal, France) is a spa town known since the Roman Empire for the treatment of rheumatism.
Carbonate aquifers in foreland tectonic settings can host important thermal springs although located in areas commonly not characterised by regional high heat flow values. In these cases, when thermal springs are located close or along the coastlines, the subaerial and/or submarine thermal springs constitute the outflow of marine groundwater, flowing through localised fractures and karstic rock-volumes. This is the case of springs occurring along the south-easternmost portion of the Apulia region (Southern Italy) where few sulphurous and warm waters (22-33 °C (72-91 °F)) outflow in partially submerged caves located along the Adriatic coast, thus supplying the historical spas of Santa Cesarea Terme. These springs are known from ancient times (Aristotele in III Century BC) and the physical-chemical features of their thermal waters resulted to be partly influenced by the sea level variations.
One of the potential geothermal energy reservoirs in India is the Tattapani thermal springs of Madhya Pradesh.
The customs and practices observed differ depending on the hot spring. It is common practice that bathers should wash before entering the water so as not to contaminate the water (with/without soap). In many countries, like Japan, it is required to enter the hot spring with no clothes on, including swimwear. Typically in these circumstances, there are different facilities or times for men and women. In some countries, if it is a public hot spring, swimwear is required.
^Don L. Leet (1982). Physical Geology (6th ed.). Englewood Cliffs, NJ: Prentice-Hall. ISBN978-0-13-669706-0. A thermal spring is defined as a spring that brings warm or hot water to the surface. Leet states that there are two types of thermal springs; hot springs and warm springs.
^Santaloia, F.; Zuffianò, L. E.; Palladino, G.; Limoni, P. P.; Liotta, D.; Minissale, A.; Brogi, A.; Polemio, M. (2016-11-01). "Coastal thermal springs in a foreland setting: The Santa Cesarea Terme system (Italy)". Geothermics. 64: 344-361. doi:10.1016/j.geothermics.2016.06.013. ISSN0375-6505.
^Ravi Shanker; J.L. Thussu; J.M. Prasad (1987). "Geothermal studies at Tattapani hot spring area, Sarguja district, central India". Geothermics. 16 (1): 61-76. doi:10.1016/0375-6505(87)90079-4.
^D. Chandrasekharam; M.C. Antu (August 1995). "Geochemistry of Tattapani thermal springs, Himachal Pradesh, India--field and experimental investigations". Geothermics. 24 (4): 553-9. doi:10.1016/0375-6505(95)00005-B.