An illustrated volcanic coastline at sunrise, black rock and a turquoise hot spring at the sea's edge, white steam lit gold, a distant volcanic cone with a faint plume.

How are hot springs formed?

Rain falls, soaks into the ground, sinks until it meets rock that is hot, and comes back up warm. That is the whole of it. The interesting part is that there are two different reasons the rock down there is hot, and only one of them needs a volcano.

AI reconstruction — not a photograph · Shutterstock AI/Shutterstock.com · Shutterstock Standard License

Four-panel schematic: rain falling from a cloud; drops soaking down into the ground; the drops sinking to a layer of hot rock; the water coming back up to a warm pool at the surface. — Diagram — a drawing that explains how something worksRain falls, soaks into the ground, sinks until it meets rock that is hot, and comes back up warm — the same sentence, drawn in four steps. Schematic — not to scale.A picture of the sentence, not of any real spring. It shows no depth, no temperature and no reason the rock is hot; the page gives two reasons, and this drawing takes neither.triassicwater.com · CC BY 4.0 · Drawn for this site

The first way: a volcano heats it

“Subsurface magma heats groundwater, creating steam and hot water. When it reaches the surface, features such as geysers, fumaroles, hot springs, and mud pits are created.”

U.S. National Park Service, Geology — Hot Springs / Geothermal Features.

Magma is melted rock, and it does not have to erupt to matter. Sitting underground and cooling slowly, it warms everything around it — including rock with water in it. In those places NPS warns the water can be superheated, which is a mild way of saying that some hot springs will kill you.

The second way: no volcano needed at all

“In non-volcanic areas, the temperature of rocks within the Earth also increases with depth—this temperature increase is known as the Geothermal Gradient. If water percolates deeply enough into the crust, it comes into contact with hot rocks and can circulate to the surface to form hot springs.”

U.S. National Park Service, Geology — Hot Springs / Geothermal Features.

This is the one people find surprising. You do not need a volcano anywhere near you. The deeper you go into the Earth, the hotter the rock gets, everywhere, all the time — that is the geothermal gradient. Water that sinks far enough will find hot rock no matter where it starts, and if there is a way back up it will come back up hot.

So a hot spring is not really a thing the Earth makes. It is rain that took a long, deep, slow route home.

Which kind is Bath?

This site has a page about Bath, a city famous for hot springs, and that page says it will not explain them because it has no source. That is still true. What you have just read is the U.S. National Park Service describing how hot springs form in general — it is not about Bath, does not mention Bath, and we are not going to quietly apply it to Bath and hope you do not notice.

You can probably guess which of the two descriptions above fits a city in southern England with no volcano in sight. Guessing is allowed — for you. It is the one thing this site will not do, because the moment we start filling gaps with what seems obvious, you have no way of telling which sentences we actually checked.

Steam rising from a geothermal hot spring pool in an open landscape.

Real photograph · NorthSky Films/Shutterstock.com · Shutterstock Standard License

And it is the same water

The water coming out of a hot spring is rain that fell — sometimes a very long time ago — soaked in, went down, got warm and came back. It was not created underground. It is the same water that has been going round this planet since long before there was anyone to bathe in it.

Which is the whole idea this site is built on: is the water we drink dinosaur water?

For teachers

For school

Rain falls, soaks into the ground, sinks until it meets rock that is hot, and comes back up warm.
Magma is melted rock, and it does not have to erupt to matter.
The deeper you go into the Earth, the hotter the rock gets, everywhere, all the time — that is the geothermal gradient.

Classroom prompt

Draw the journey of one raindrop that ends up in a hot spring. Draw it twice: once heated by magma, once by the geothermal gradient. What is different about the two pictures, and what stays the same?

Extension: this page will not say which of the two ways feeds the springs at Bath. What would a source need to tell us before that sentence could be written?

Ages: the raindrop journey works from about seven; the two heat sources and the word “gradient” suit pupils of about ten upwards. That range is our teaching choice for this page, not a curriculum claim.

Where this comes from

The explanation: U.S. National Park Service, Geology — Hot Springs / Geothermal Features, read 1 September 2026. Material created by the National Park Service is generally in the public domain. Both quotations above are verbatim; everything around them is our explanation of them, not extra facts.
How sure is this?: This is a general description of how hot springs form, not a reconstruction of any particular spring. It does not tell you how deep the water went, how long it took, or how hot it got at any named place — and where this site has a page about such a place, it says so rather than borrowing this page’s answer.

Where next

The rain on this page spends its time underground before it comes back warm, and how much water is underground counts the store it joins. The city this page refuses to explain has its own page on Bath, and the rest of this family — volcanoes and the water that comes out of them — starts on the volcano water page.