Science Fair Project Encyclopedia
A hot spring is a place where warm or hot groundwater issues from the ground on a regular basis for at least a predictable part of the year, and is significantly above the ambient ground temperature (which is usually around 55~57°F or 13~14°C in the eastern United States).
Sources of heat
The water issuing from a hot spring is heated by geothermal heat, that is heat from the interior of the Earth. 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 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 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; and 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 geothermal areas, such as Warm Springs, Georgia (frequented for its therapeutic effects by polio-stricken U.S. President Franklin D. Roosevelt, who built the Little White House there).
Because heated water can hold more dissolved solids, warm and especially hot springs also often have a very high mineral content, containing everything from simple calcium to lithium, and even radium. Because of both the folklore and the proven medical value some of these springs have, they are often popular tourist destinations, and locations for rehabilitation clinics for those with disabilities.  
North American hot springs
Throughout western North America (including Alaska) there are thousands of hot springs, many of which were created between 20 and 45 million years ago as a result of violent volcanic activity. They range in size from the tiniest seeps to near geysers; from seeps like Fales Hot Ditch north of Bridgeport, California, to subterranean lakes such as the one below Tonopah, Arizona, which provides natural mineral waters to the seven or more hot spring spas that once operated in Tonopah. The ruins of two such spas are still visible.
Native Americans and hot springs
In Tonopah, Arizona, it is probable that water flowed forth from the ground by itself for a few millennia. This led Native Americans to name the area Tonopah, meaning "Hot Water Under The Bush". Native Americans revered the hot springs as a sacred healing place. Though there are no Indian ruins in the immediate vicinity of the hot springs, the presence of grain grinding mortar holes, pottery shards, and other artifacts close by to the west are a clear indication that nomadic hunter-gatherers frequented the area for many years. Every major hot spring in North America, as well as those in South America, has some record of use by Native Americans, some for over 10,000 years.
Additionally, hundreds of very high quality arrowheads have been found at or near existing springs in Tonopah, indicating that Tonopah was a popular hunting ground. This abundance of artifacts is indicative of the importance of the springs to prehistoric peoples.
Native Americans always used these natural shrines in a state of complete nudity. If opposing tribes, even those at war, arrived at the same spring, all conflict ceased because they believed they were walking on sacred ground. Conversations were in hushed tones or, more often, didn't take place at all.
Hot springs in the United States
At least five United States national parks feature hot springs:
- Big Bend National Park in Texas 
- Death Valley National Park in California 
- Hot Springs National Park in Arkansas 
- Lassen Volcanic National Park in California 
- Yellowstone National Park in Wyoming (and Idaho/Montana) 
Other hot or warm springs are located in:
Canadian hot springs
These springs are located in western Canada:
- Province of Alberta: Banff Upper Hot Springs
- Harrison Hot Springs, near Vancouver
- Invermere: Fairmont Hot Springs
- Marjorie Gersh-Young, Hot Springs and Hot Pools of the Southwest: Jayson Loam's Original Guide, Aqua Thermal Access, 2004. ISBN 1-890880-05-1.
- Marjorie Gersh-Young, Hot Springs & Hot Pools Of The Northwest, Aqua Thermal Access, 2003. ISBN 1-890880-04-3.
- G. J Woodsworth, Hot springs of Western Canada: a complete guide, West Vancouver: Gordon Soules Book Publishers. 1999. ISBN 0-919574-03-3.
- Clay Thompson, "Tonopah: It's Water Under The Bush", the Arizona Republic 1-12-03, p. B12.
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