Science
How Do Hot Springs Form? The Science in Plain English
Last updated October 8, 2026 · 3 min read
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A hot spring is simply groundwater that reaches the surface warmer than it went in. What makes it hot, and why springs cluster in certain places, comes down to three things: a source of heat, a supply of water and a fast path back to the surface.
The heat: Earth gets hotter with depth
Rock temperature rises the deeper you go. In most continental areas the increase is roughly 25°C to 30°C per kilometer, or about 70°F to 90°F per mile. Water that sinks two miles down can heat well above 150°F without any volcano nearby.
In volcanic regions the heat is much closer to the surface. Magma or recently cooled igneous rock can sit only a few miles down, heating water to boiling and beyond. This is why places like Yellowstone, Iceland and New Zealand’s Taupō Volcanic Zone have springs, geysers and steaming ground.
The water: rain and snow on a long journey
Most hot spring water began as rain or snowmelt. It soaks into the ground in mountains or uplands and moves slowly through cracks and porous rock. Some of it travels for decades or thousands of years before it emerges. At Hot Springs National Park in Arkansas, for example, the National Park Service estimates the water fell as rain roughly 4,000 years before it flows from the springs.
The path: faults and fractures
Hot water is lighter than cold water, so it wants to rise. What it needs is a quick route up, so it does not cool on the way. Faults and fractures provide that route. That is why many hot springs line up along valley edges and mountain fronts, where faults cut through the rock.
The basin-and-range country of Nevada, Idaho, Oregon and Utah is full of these faults, and it is full of hot springs too.
Why the American West has most US hot springs
- Thin, stretched crust in the Great Basin brings hot rock closer to the surface.
- Active faults give water deep paths down and fast paths up.
- Young volcanic areas such as Yellowstone, the Cascades and the Long Valley Caldera in California add extra heat.
- Mountains catch rain and snow that feed the system.
The eastern US has older, colder crust, so springs there are rarer and usually warm rather than hot.
Minerals: what the water carries
Hot water dissolves more minerals than cold water. As it rises and cools, some of those minerals come out again:
- Travertine is calcium carbonate that builds terraces and mounds, like those at Thermopolis in Wyoming or Mystic Hot Springs in Utah.
- Siliceous sinter forms around very hot springs and geysers in volcanic areas.
- Sulfur compounds give the classic rotten-egg smell and sometimes yellow crusts.
- Iron stains rocks orange and red.
Colorful mats around springs are not minerals but microbes. Different heat-loving bacteria and algae live at different temperatures, which is why pools often show rings of color.
Hot spring, warm spring, thermal spring
There is no single definition. A common scientific rule is that a thermal spring is noticeably warmer than the average air temperature of its region. In Japan, the Hot Spring Act counts water of 25°C (77°F) or more at the source as onsen water. Our listings simply show the recorded temperature and a comfort band so you can judge for yourself.
Geysers and fumaroles
A geyser is a hot spring with a constricted plumbing system. Water deep in the vent is heated past its surface boiling point under pressure, until it flashes to steam and erupts. A fumarole is a vent that releases mostly steam and gas, with little liquid water. Both are signs of very hot, shallow heat and are never safe to enter.
Explore the hottest readings in the directory on our hottest hot springs page.
Frequently asked questions
What heats the water in a hot spring?
Heat from inside the Earth. In volcanic areas, magma a few miles down heats groundwater directly. Elsewhere, water seeps deep along faults, warms up because rock gets hotter with depth, then rises quickly back to the surface.
Why do some hot springs smell like rotten eggs?
The smell is hydrogen sulfide gas, which forms when water passes through rocks containing sulfur or meets volcanic gases. In open air it is usually harmless at hot spring levels, but avoid enclosed hollows where gas can collect.
Is hot spring water rainwater?
Mostly, yes. Studies of hot spring water usually show it started as rain or snow that soaked into the ground, sometimes thousands of years before it reached the surface again.