Editing Hot spring (section)

==Sources of heat==
Water issuing from a hot spring is heated [[Geothermal energy|geothermally]], that is, with [[heat]] produced from the [[Earth's mantle]]. This takes place in two ways. In areas of high volcanic activity, [[magma]] (molten rock) may be present at shallow depths in the [[Earth's crust]]. [[Groundwater]] is heated by these shallow magma bodies and rises to the surface to emerge at a hot spring. However, even in areas that do not experience volcanic activity, 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 rock. This generally takes place along [[Fault (geology)|faults]], where shattered rock beds provide easy paths for water to circulate to greater depths.<ref>{{cite book |last1=Macdonald |first1=Gordon A. |author-link=Gordon A. Macdonald |last2=Abbott |first2=Agatin T. |last3=Peterson |first3=Frank L. |title=Volcanoes in the sea : the geology of Hawaii |date=1983 |publisher=University of Hawaii Press |location=Honolulu |isbn=0-8248-0832-0 |edition=2nd}}</ref>

Much of the heat is created by [[radioactive decay|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.<ref name="turcotte">{{cite book | last=Turcotte | first=DL |author-link=Donald L. Turcotte |author2=Schubert, G | title=Geodynamics | publisher=Cambridge University Press | location=Cambridge, England, UK| date=2002 | edition=2nd | pages=136–7 | chapter=4 | isbn=978-0-521-66624-4 }}</ref><ref name='Anuta2006'>{{cite news | first=Joe | last=Anuta | title=Probing Question: What heats the earth's core? | date=2006-03-30 | publisher=physorg.com | url=http://www.physorg.com/news62952904.html | access-date = 2007-09-19 }}</ref><ref name=physicsworld>{{cite web|last=Johnston|first=Hamish|title=Radioactive decay accounts for half of Earth's heat|url=http://physicsworld.com/cws/article/news/2011/jul/19/radioactive-decay-accounts-for-half-of-earths-heat|work=PhysicsWorld.com|publisher=Institute of Physics|access-date=18 June 2013|date=19 July 2011}}</ref> The major heat-producing isotopes in the Earth are [[potassium-40]], [[uranium-238]], [[uranium-235]], and [[thorium-232]].<ref>{{cite news | first=Robert | last=Sanders | title=Radioactive potassium may be major heat source in Earth's core | publisher=UC Berkeley News | date=2003-12-10 | url=http://www.berkeley.edu/news/media/releases/2003/12/10_heat.shtml | access-date=2007-02-28 }}</ref> In areas with no volcanic activity, this heat flows through the crust by a slow process of [[thermal conduction]], but in volcanic areas, the heat is carried to the surface more rapidly by bodies of magma.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=6–13}}</ref>
[[File:Evolution of Earth's radiogenic heat.svg|thumb|upright=1.25|The [[radiogenic heat]] from the decay of <sup>238</sup>U and <sup>232</sup>Th are now the major contributors to the [[earth's internal heat budget]].]]

A hot spring that periodically jets water and steam is called a [[geyser]]. In active volcanic zones such as [[Yellowstone National Park]], magma may be present at shallow depths. If a hot spring is connected to a large natural cistern close to such a magma body, the magma may [[superheating|superheat]] the water in the cistern, raising its temperature above the normal boiling point. The water will not immediately boil, because the weight of the water column above the cistern pressurizes the cistern and suppresses boiling. However, as the superheated water expands, some of the water will emerge at the surface, reducing pressure in the cistern. This allows some of the water in the cistern to flash into steam, which forces more water out of the hot spring. This leads to a runaway condition in which a sizable amount of water and steam are forcibly ejected from the hot spring as the cistern is emptied. The cistern then refills with cooler water, and the cycle repeats.{{sfn|Macdonald|Abbott|Peterson|1983}}<ref>{{cite web |title=Hot Springs/Geothermal Features |url=https://www.nps.gov/subjects/geology/hot-springs.htm |website=Geology |publisher=National Park Service |access-date=25 June 2021 |date=10 February 2020 |ref={{harvid|National Park Service|2020}}}}</ref>

Geysers require both a natural cistern and an abundant source of cooler water to refill the cistern after each eruption of the geyser. If the water supply is less abundant, so that the water is boiled as fast as it can accumulate and only reaches the surface in the form of [[steam]], the result is a [[fumarole]]. If the water is mixed with mud and [[clay]], the result is a [[mud pot]].{{sfn|Macdonald|Abbott|Peterson|1983}}{{sfn|National Park Service|2020}}

An example of a non-volcanic warm spring is [[Warm Springs, Georgia]] (frequented for its [[physical therapy|therapeutic]] effects by [[paraplegic]] [[President of the United States|U.S. President]] [[Franklin Delano Roosevelt|Franklin D. Roosevelt]], who built the [[Little White House]] there). Here the groundwater originates as rain and snow ([[meteoric water]]) falling on the nearby mountains, which penetrates a particular [[Formation (geology)|formation]] ([[Hollis Quartzite]]) to a depth of {{convert|3000|ft||}} and is heated by the normal geothermal gradient.<ref>{{cite journal |last1=Hewett |first1=D.F. |author-link=Donnel Foster Hewett |last2=Crickmay |first2=G.W. |title=The warm springs of Georgia, their geologic relations and origin, a summary report |journal=United States Geological Survey Water Supply Paper |date=1937 |volume=819 |page=5 |doi=10.3133/wsp819|doi-access=free |bibcode=1937usgs.rept....5H }}</ref>