A kettle with a narrow neck
Why Does the Ground Shoot Boiling Water?
A geyser is water heated underground and held from boiling by pressure until it suddenly gives way.
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Step 1 of 5
Rainwater seeps down to hot rock.
Water works its way through cracks until it reaches rock heated by magma below, sometimes a couple of kilometres down.
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Step 2 of 5
Pressure stops it boiling at 100 degrees.
Water only boils when it can push the weight above it aside. Under a deep column of water it stays liquid far past its usual boiling point.
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Step 3 of 5
So the water becomes superheated.
It sits there hotter than boiling but unable to boil, storing an enormous amount of energy and waiting for the pressure to drop.
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Step 4 of 5
A small boil starts a chain reaction.
Some water near the top finally boils and the steam pushes water out. That lightens the column, dropping the pressure below.
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Step 5 of 5
The whole column flashes to steam.
With the pressure gone, the superheated water boils all at once and expands violently, blasting the column skyward as an eruption.
Old Faithful erupts every 90 minutes or so because its plumbing takes about that long to refill and reheat.
The short version
A geyser erupts because pressure keeps underground water liquid far past boiling, until a small boil drops that pressure and the rest flashes to steam.
Try it yourself
Watch a pressure cooker release its valve. The sudden rush is the same physics: pressure dropping lets hot water boil at once.
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