Hot, and solid anyway
Why Is the Inside of the Earth Not Liquid?
Deep rock is far hotter than the temperature it melts at on the surface, and it is solid rock all the same. Molten rock is the rare exception, and something has to go wrong to make it.
-
Step 1 of 5
It is hot enough to melt, and it does not melt.
Squeeze something hard enough and its atoms cannot move far enough apart to become a liquid, whatever the temperature. Deep rock sits under the weight of everything above it, so it stays solid at temperatures that would have it running freely at the surface.
-
Step 2 of 5
Take the pressure off and it melts on the spot.
Where plates pull apart, hot rock rises and the weight above it drops away. Nothing gets any hotter — the lid simply comes off, and rock that was solid a moment ago becomes liquid. Most of the magma on Earth is made this way, unseen, under the oceans.
-
Step 3 of 5
Adding water melts it too.
Where one plate dives beneath another it carries wet seafloor down with it. The water is squeezed out into the hot rock above, and it lowers the melting point by hundreds of degrees — the same trick as salt thrown onto an icy path.
-
Step 4 of 5
It pools for a long time before it ever gets out.
Magma is lighter than the rock around it, so it rises until it is stopped, and collects in chambers that can sit for centuries. A volcano is only the top of that plumbing, which is why the same mountain erupts again and again rather than once.
-
Step 5 of 5
Most of it never reaches the surface at all.
The majority of magma stalls underground and freezes there, and because it cools over thousands of years its crystals grow big enough to pick out with the naked eye. Granite is magma that never made it out — every speckled kitchen worktop is a failed eruption.
You can read the speed of cooling straight off a rock. Big crystals mean slow and deep; a fine grey stone with none mean it cooled fast in the open air.
The short version
Deep rock is hot enough to melt but held solid by pressure, so magma is made by releasing pressure or adding water — and most of it freezes underground as granite.
Try it yourself
Find a granite worktop or kerbstone and look closely. The pink, white and black specks are separate crystals, each grown slowly underground.