A plain definition
A hot spring is a place where groundwater that has been heated inside the earth flows out at the surface. The U.S. Geological Survey (USGS) explains the basic cycle: water from rain and snow seeps down through cracks in the rock to hot zones underground, is heated there, becomes less dense, and rises back to the surface along fissures and cracks. USGS adds that hot springs occur in thermal areas where the surface of the earth intersects the water table.
The water that feeds most springs is called meteoric water, meaning it started as precipitation. A hot spring is not a separate source of water. It is the same groundwater found in ordinary wells, except that it has traveled deep enough, or close enough to a heat source, to warm up before returning to daylight.
Where the heat comes from
There are two main heat sources. In volcanic areas such as Yellowstone, USGS describes water percolating down to high-temperature rock around a magma body, active or recently solidified but still hot. The National Park Service (NPS) describes Yellowstone's more than 10,000 hydrothermal features as the visible expression of the Yellowstone volcano.
Outside volcanic areas, the heat comes from the normal rise in temperature with depth, called the geothermal gradient. A USGS study by Nathenson and Guffanti (1988) mapped gradients across the conterminous United States and found that most of the country falls between 15 and 35 degrees C per kilometer of depth. The eastern United States averaged 25 degrees C per kilometer and the western United States 34 degrees C per kilometer. Water that circulates a few kilometers down along faults can warm well above surface temperatures and come back up still hot.
Thermal spring or hot spring: the threshold
There is no single legal temperature that separates a warm spring from a hot one. The most widely used U.S. dataset sets its own cutoff. NOAA's Thermal Springs List for the United States (Key to Geophysical Records Documentation No. 12, June 1980), compiled by George W. Berry, Paul J. Grim and Joy A. Ikelman, includes natural surface hydrothermal features at temperatures of 20 degrees C (68 degrees F) or greater. Anything at or above that line counts as a thermal spring in the list, whether it measures 70 degrees F or boils.
The printed list records 1,702 thermal spring locations in 23 of the 50 states; the digital copy that NOAA archives today has 1,661 entries, and that is the figure used on the maps and tables of this site. It covers springs, pools, mud pots, mud volcanoes, geysers, fumaroles and steam vents, and it groups features within half a mile of each other as one site. For each site it gives the most recently reported maximum surface temperature. Where only a description existed, the compilers used the codes B, H and W for boiling, hot and warm. NOAA built the list mainly from Waring's 1965 USGS compilation, the USGS GEOTHERM file and state agency data, and it has not been revised since publication.
Hot springs, geysers and fumaroles
The same underground system can produce different surface features. NPS explains the difference at Yellowstone this way:
- Hot spring: an open plumbing system lets heated water reach the surface without restriction, and convection keeps the water circulating, so pressure does not build up.
- Geyser: a constriction in the plumbing, usually near the surface, traps water until it flashes to steam and erupts. Yellowstone has more than 500 geysers.
- Fumarole: a vent that releases steam and other gases, reaching the surface through conduits that pass through the water table (USGS). NPS reports steam vent temperatures in Yellowstone as high as 280 degrees F.
Travertine and sinter
Hot water dissolves minerals underground and drops them at the surface as it cools. Two deposits are common, and they reveal what kind of rock the water passed through.
Travertine is calcium carbonate. NPS explains that at Mammoth Hot Springs in Yellowstone, hot water dissolves limestone underground and deposits calcite as it reaches the surface, building stepped terraces. Travertine also forms the tub at Umpqua Hot Springs in Oregon, which the U.S. Forest Service describes as hewn out of the travertine deposits around the springs.
Sinter is silica. NPS describes it as silica dissolved from rhyolite, a volcanic rock, and deposited by hydrothermal water, forming the rims, pools and cones in Yellowstone's geyser basins. Splashing silica-rich water forms a bumpy variety called geyserite.
Where U.S. hot springs cluster and why
Thermal springs in the NOAA list are concentrated in the West. The pattern follows geology:
- Yellowstone and young volcanic areas. A shallow magma body supplies heat directly, which is why Yellowstone holds the largest concentration of hydrothermal features in the country.
- The Basin and Range. Nevada and parts of the neighboring states sit in a region that USGS describes as shaped by east-west crustal extension and normal faulting. The faults give water a path down to hot rock and back up, the same fissure-and-crack route USGS describes for hot springs in general, and many springs there have no active volcano nearby.
The western United States also has the steeper average geothermal gradient in the USGS study, 34 degrees C per kilometer against 25 in the East, so water reaches a given temperature at a shallower depth in the West. The NOAA table includes eastern states such as Arkansas, Virginia, West Virginia and North Carolina, but only a handful of entries each, while California and Nevada carry the longest lists of sites.