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.
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.
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?
Where this comes from
- The explanation
- U.S. National Park Service, Geology — Hot Springs / Geothermal Features, read . 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.