all thermal springs in Virginia are located in the Valley and Ridge physiographic province
Source: NOAA National Geophysical Data Center, Thermal Springs
The earth gets hotter at depth. In the core and mantle surrounding the core, there is residual heat from the earth's formation 4.5 billion years ago, and the great pressures so far underground also generate heat.
Much of the heat near the surface is caused by continuing decay of radioactive materials that are concentrated within the Earth's crust, such as uranium. Magma chambers intermittently rise up to the surface and fuel volcanic eruptions. The molten rock cools as it escapes high pressure underground and encounters the atmosphere. Other igneous rock forms when magma cools just below the surface, without erupting.
There were volcanic eruptions 48-35 million years ago in Virginia; Mole Hill west of Harrisonburg is an old volcanic remnant. In Iceland, Yellowstone National Park, and Northern California, magma chambers near the surface still heat underground water and create geysers.
geysers in Yellowstone National Park are generated by water heated at depth, reaching the boiling point, and then surging to the surface
Virginia has no localized deposits of radioactive minerals or shallow magma chambers remaining from an old volcanic pluton, and no geysers now. Virginia does have warm springs, some of which ebb and flow as pressure surges reach the surface. The water in Virginia's thermal springs is heated by the geothermal gradiant, the increasing temperature of all rocks at greater depth.
That gradiant varies across the Earth's crust, but averages 1.5°F for every 100 feet of depth in the portion of Virginia with thermal springs. Virginia has many springs fed by groundwater that has traveled deep underground, but only a few are "warm/hot springs" where the water is warmer than the average temperature at that location.
At the elevation of most thermal springs in Virginia's mountains, groundwater would have to travel almost two miles deep in order for the geothermal gradient alone to heat it about 150°F from the average temperature below the frost line (about 48-54°F) to boiling temperature (about 207°F).1
Generating water that hot without a nearby magma chamber is known to occur at one location on earth. In the Amazon Basin of Peru, groundwater apparently percolates deep enough to create a "boiling river." Water emerging at the surface reaches temperatures up to 196°F, solely from the increasing heat of the earth at greater depth. The closest known volcanic magma chamber to that Peruvian river is 400 miles away.2
the 48-million year old magma chamber that created Mole Hill has cooled, and the spring that feeds Silver Lake provides 52°F water
In Virginia, there are 100 or so springs with water temperatures exceeding mean annual air temperature (48-54°F); the annual average temperature at the frost line is cooler at higher elevations. All thermal springs in Virginia are located in the Valley and Ridge province.
thermal springs in Virginia are concentrated in valleys between Pulaski and Bath counties
Source: Library of Congress, The Virginia Springs, and the Springs of the South and West by Moorman
The water travels deep underground through sandstone and limestone formations, with fractures and solution channels. The 20 thermal springs that are most clearly recognized include some where seepages next to each other are combined to form one named spring:3
Major Thermal Springs in Virginia
|Blue Ridge Springs (Buford's Gap)||66-75°F|
|Healing Springs (Rubino Healing, Sweet Alum)||86°F|
|Hunter's Pulaski Alum Springs||72°F|
|Layton Springs (Keyser's)||63°F, 72°F|
|Lithia Spring (Wilson Thermal)||65°F|
|McHenry Spring||68°F, 65°F, 66°F|
|Mill Mountain Springs||60°F, 65°F, 66°F|
|New River White Sulphur Springs||85°F|
|Rockbridge Baths (Rockbridge Alum, Strickler's)||72°F|
|Sweet Chalybeate Springs||63-76°F|
|Warm Spring (Rockingham County)||64°F|
|Warm Springs (Bath County)||95°F|
The Ordivicial limestone and Cambrian sandstone bedrock formations through which the thermal spring water travels were deposited 450-550 million years ago, but the water in the springs is recent rainfall. It seeps underground, is heated by the warmer rock 2,000-5,000 feet underground, then returns to the surface quickly enough to retain that geothermal heat.
At Hot Springs and Warm Springs, the water has been underground at least 20 years before re-emerging at the surface. At the many, many other springs in Virginia, groundwater moves to the surface slowly enough that the water temperature adjusts to match the average temperature at that location. As a result, in the summertime most springs offer cool water. Springhouses were built by early settlers and used until electricity reached rural areas, so the cool spring water could be used to chill milk/butter as frontier refrigerators.
possible movement of ground water through a multilayered folded/faulted/fractured aquifer, or a faulted/fractured anticlinal ridgeh
Source: US Geological Survey (USGS), Hydrology and Geochemistry of Thermal Springs of the Appalachians (Professional Paper 1044-E, Figure 9 and Figure 11)
At Warm Springs, the surface water percolates one mile below the surface. It flows down through the sedimentary rock layers until it reaches a resistant layer and is pushed upwards, emerging in a valley quickly enough to retain some of its geothermal heat aquired at depth.
Warm Springs was a gathering spot for the Virginia wealthy, prior to the Civil War
Source: Harper's New Monthly Magazine, Virginia Illustrated (February 1855)
Cool groundwater closer to the surface may mix with warmer water from greater depth. The temperature of Bolar Spring drops as the flow increases, suggesting that warm water from greater depth dominates during dry periods but cooler water from near the surface is mixed in after rains.4
the temperature at Bolar Spring drops when flow increases, as cooler water from nearer the surface mixes with warmer water from greater depth
Source: US Geological Survey (USGS), Hydrology and Geochemistry of Thermal Springs of the Appalachians (Professional Paper 1044-E, Figure 18)
thermal and cool springs are intermixed in western Virginia; in most cases, groundwater does not circulate deep enough or return to the surface fast enough to retain geothermal heat
Source: Virginia Minerals, Ancient warm springs deposits in Bath and Rockingham Counties, Virginia (May 1997)
The hot springs at the modern Homestead Resort have been used for 9,000 years. On a cold winter day, Native Americans must have luxuriated in the 106°F natural hot tub - and when no humans were hunting in the area, wildlife would have used the thermal springs as well.
The Gentleman's Pool House there was constructed in 1761, and the separate structure for women was built in 1836. They are named the Jefferson Pools because former president Thomas Jefferson visited there in 1818. George Washington beat him to a thermal spring by 40 years, enjoying the Berkeley Springs (now in West Virginia) while surveying that area in 1748.5
Prior to the Civil War, mountain springs were developed as entertainment resorts for the wealthy, especially those living in Tidewater who desired to escape the heat and humidity in the days before air conditioning. The chemicals in the water, including alum and sulfur, were thought to help invalids recover their health. David Hunter Strother, under the pen name Porte Crayon, described the Hot Springs in 1855:6
nationally, most thermal springs are located in the western United States
Source: NOAA National Geophysical Data Center Thermal Springs
Today, the water from the Hot Springs is piped to The Homestead spa, where it fills bathtubs (for single-person use) and indoor/outdoor swimming pools.7
Springs enriched with alum would purge the bowels, while sulfur-enriched and iron-enriched (chalybeate) springs were thought to offer a cure for various illnesses. Even without claims of chemical impacts, thermal springs provided comfort and distraction from aching bones and itching skin. The healing power of springs was once expected to trigger a population increase and economic boom in western Virginia:8
As medical knowledge expanded, the public lost faith in the capacity of mineral springs to cure rheumatism and other ailments. The invention of air conditioning also lowered demand for summer vacations in the mountains. Except for The Homestead in Hot Springs, none of the resorts in Virginia have survived as an overnight tourist destination. The Greenbrier Resort in nearby White Sulfur Springs, West Virginia went through bankruptcy in 2009 and is still operating today. (Both resorts hosted interned Japanese diplomates at the start of World War II.)
Homestead Resort in Bath County, sometime between 1890-1910
Source: Library of Congress, Virginia Hot Springs, Va., the Homestead
in the Eastern United States, warm springs extend from Saratoga Springs in New York to Warm Springs in Georgia
Source: US Geological Survey (USGS), Thermal Springs of the United States and Other Countries of the World - A Summary (Professional Paper 492)