Where does volcanic water come from?
Steam comes out of the ground near volcanoes, sometimes in enormous quantities. It has to come from somewhere, and the sources this site holds describe two different somewheres.
Route one: it is rain that went down and came back
“Water vapor is generally the main component of fumarolic emissions and is usually from meteoric groundwater.”
U.S. National Park Service, “Fumaroles”. Courtesy of the National Park Service.
Unpacked: the steam is mostly water, and the water is mostly rain. “Meteoric” is the word geologists use for water that fell out of the sky — nothing to do with meteors. The same page is blunter still about one particular case:
“Most of the steam released in the fumaroles came from buried snow fields, precipitation and groundwater.”
U.S. National Park Service, “Fumaroles”. Courtesy of the National Park Service.
Snow. The steam roaring out of the ground was snow, not long ago.
Route two: it came down with the sea floor
The second route is stranger and much slower. Where one slab of Earth’s surface slides beneath another, it takes water down with it — and does not keep it.
“Increased temperature and pressure at that depth cause the rocks to metamorphose and dehydrate”
“A by-product of the metamorphism is the release of hot fluids, especially water.”
“The water rises and melts rock in its path.”
U.S. National Park Service, “Convergent Plate Boundaries—Subduction Zones”. Courtesy of the National Park Service.
Unpacked, in order: rock going down gets hot and squeezed, and the water in it is driven out — the page’s own word is that the rock “sweats”. That water rises. And on the way up it does something a child would not guess: it makes solid rock melt. Water is not just a passenger in a volcano. It is part of what makes one.
How deep this happens is a question with an answer for exactly one place in the sources this site holds, and the page on water inside Earth quotes it — carefully, because it is about one plate and not about the world.
Four questions, four different kinds of evidence
The pages below are not four versions of one page. What separates them is what sits underneath each answer.
How are hot springs formed?
Route one, followed all the way to the surface: rain taking a deep detour past hot rock and coming back warm. Its evidence is two sentences from a national park service, and it covers the case where there is no volcano involved at all.
Can volcanoes create water?
The one with a genuine argument in it. Its evidence is a geological survey on what volcanic gas is made of and a space agency on where the oceans’ water came from — and it refuses to settle whether releasing water counts as creating it.
What is groundwater?
The water route one is made of, before anything heats it. Its evidence is numbers rather than sentences: twelve published figures for where all Earth’s water sits.
Is there water inside Earth?
The page about the edge of the evidence. Its subject is where the counting stops, and it is the only one of the four whose main finding is something nobody here can measure.
Where this comes from
- The quotations
- U.S. National Park Service: “Fumaroles” (last updated April 18, 2023) and “Convergent Plate Boundaries—Subduction Zones” (last updated February 11, 2020), both read . Material created by the National Park Service is in the public domain. Every quotation on this page is checked against the page it came from, word for word, each time this site is built.
- How sure is this?
- These are two routes a source describes, not a complete list of every way water reaches a volcano, and this page does not claim otherwise. Nothing here rests on a reconstruction or a model. Where the sources give a depth or a place, it stays attached to that place rather than being turned into a fact about the planet.