Geo-Joint: Hot Springs—Nature’s Soothing Soak

The natural delights of the world range from gorgeous sunsets for the eyes, to powder snow and peeling waves for ecstatic motion, to the music of bird calls for the ears. But for pure sensory immersion, little beats the blissful relaxation of soaking in natural hot springs. Long before the advent of the Jacuzzi, hot springs provided comfort for the sore of back. The ancient artifacts and rock art found around hot springs attest to the idea that people have always been drawn to heated water flowing up from the depths of the Earth. How do we come by this gift?

A view of how hydrothermal convection works at Lassen National Park.

As we all know, the Earth is not a stone cold rock. There is plenty of heat below the surface, enough to melt rock, and quite a lot of that heat is not all that far down. Sometimes it comes right on up to the surface in molten lava, but that’s just a little too hot. Magma bodies found at shallow depths are quite efficient at heating water that has soaked into cracks and fractures in surface rock, which is then heated, and by expansion and pressure rises to the surface again. Volcanic areas are often associated with hot springs and geysers, which perform through a more complicated process involving steam. But there are also plenty of hot springs in non-volcanic regions. These develop in places where rainwater has once again filtered down through fractured rock, deep into the Earth. On a worldwide average, temperatures rise by 3 to 5 degrees Fahrenheit with every 300 feet one descends below the surface, or roughly 90 degrees per mile. This ambient heat arises from the radioactive decay of minerals in the ground, plus the heat caused by the pressure of the overlying material.

Water may percolate down under greater and greater pressure until it finds a fault or some major fracture at a greater depth, which allows it a pathway back to the surface. This cycle of the pressure of meteoric water (i.e. from precipitation) soaking in and bearing down, forcing the heated water back up is called hydrothermal convection. Researchers studying hot springs in Arkansas have determined that water there moves into the Earth at a speed of about a foot a year. It may take something like 4,000 years for rainwater to get to heated depth and then recirculate back to the surface. If the rising water returns to the surface slowly, a cold water spring results. If the water reaches a sizeable fault or if it has been dissolving minerals in its path and opened up larger fissures, it can reach the surface much more quickly—as short as a year—and deliver itself still nicely warmed. Presto—a hot spring! Naturally, such springs come from various depths, with various speeds of movement and volume, so springs can vary from pleasantly warm to scalding. Often such springs cannot be used where they meet the air, but only farther downstream after they have lost some of their heat.

Spring waters may contain an abundance of minerals they have dissolved while underground, and the hot water returning to the surface is even more efficient at this than cold water that may have simply trickled down to an aquifer. If the water’s mineral content is at least 400 parts per million it is considered a mineral spring, and such waters have long been sought to drink for their healthful qualities. Hot springs may also come with an added aspect which may not be quite so desirable—that familiar odor of rotten eggs. Springs that demonstrate such a scent can claim their waters come from very deep, sulphur-rich rock, where anaerobic bacteria have converted dissolved sulfur to H2S, or hydrogen sulfide. The water must then get up to the surface quickly without any contact with oxygen, lest the bacteria be killed off and cease their aromatic production. When conditions are just so, a stinky hot spring will result, which will be very efficient at tarnishing your silver jewelry.

Others prefer their hot soak to be in a less-developed setting.

Taking a dip into the comfortably warm water of natural springs is a tradition of long standing, but the practice grew exceptionally popular between 1880 and 1950, a period sometimes called the Golden Age of Bathing. The curative powers of natural spring water were touted, and people would travel great distances to avail themselves of the healthful benefits of a soak in public baths. In some places, grand buildings and huge pools were constructed to accommodate enthusiasts. The high mineral content of spring waters may or may not enhance the health of those who drink it, though many extol its taste. Whether mere skin contact with spring water improves one’s vigor is debatable, but the relaxing power of even a few minutes in steamy hot water is undeniable. The social aspect of groups relaxing in such a comfortable setting was also a draw. It seems such an extravagant practice as whiling away time in warm water was made more legitimate by the excuse that it was good for your well-being.

The brilliant colors of Prismatic Spring in Yellowstone come from heat-tolerant bacteria.

Animals besides humans also find hot springs to be attractive and useful. Microorganisms that can thrive in very high-temperature spring water also lend their vibrant color to these hot pools. The 24/7, year-round heat provided by hot springs makes life possible for certain fish, amphibian, and reptile species who could not otherwise survive the harsh winters of mountain settings where some hot springs are found. They live in a true oasis of warmth.

So hot springs are a quirky perk of nature that deliver benefits all around. Fortunately, people have not only exploited this resource, but also sought to protect and preserve some of their more scenic locales. In fact, the national parks systems of both the United States and Canada began their illustrious histories with the establishment as parks of two geothermal areas: Yellowstone, the first national park in the US, and the Banff Hot Springs Reserve in Canada, which grew into Rocky Mountains National Park, and later re-named Banff National Park. The US also established another park by the very name, Hot Springs National Park, in Arkansas. What better honors for these surface expressions of the warm heart of the Earth?

Thinking of a trip to Yellowstone National Park to witness geothermal activity in person? Check out this great National Geographic wall map available from



caption: A view of how hydrothermal convection works at Lassen National Park.
source: Wikimedia Commons: USGS (Public domain)

caption: Others prefer their hot soak to be in a less-developed setting.
source: US Air Force: Airman 1st Class Connor J. Marth (Public domain)

caption: The brilliant colors of Prismatic Spring in Yellowstone come from heat-tolerant bacteria.
source: Wikimedia Commons: Brocken Inaglory (CC by SA 3.0)


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