Question: How does the rock of a volcano hold the magma without it melting through?

Matthew Pankhurst answered on 17 Jun 2013:

Hello, good question! I answered a question very similar to this earlier – here is the answer I wrote 🙂

The difference between a liquid rock and a solid rock and how they behave with each other is very important to understand how volcanoes work.
Earth is a basically a big lump of rock traveling through space. But, it’s not a cold rock like asteroids – it’s hot. The reason it’s hot is that some elements produce heat by something called radioactive decay. This heat wants to escape to space, but rocks don’t make this easy for the heat. If you think of a castle, they are very cold because the thick stone walls keep the heat out. If you go on holiday and dig into the sand at the beach, its cold only about a foot under the ground. Rocks are good “insulators”. Most of the earth is solid rock, it’s not all sloshy magma down there like we are sometimes taught. Now, all that heat has to go somewhere, and when the heat slowly builds in a certain area to finally melt a bit of rock, the rock turns to magma, and travels upwards breaking the solid rock as it goes. Underneath a volcano this magma gathers together, and the surrounding rocks slowly heat up. They can withstand the extreme heat for some time, but eventually they melt too, and mix together with the magma. If you think of the magma like a hot soup in a bowl, putting an ice cube in there will melt the ice -but this means the soup cools down a little bit. Add more and more ice, and the soup will cool down more and more. Add heaps of ice, and the soup will freeze. This is what happens under a volcano – if you keep adding cold rock in – the magma will freeze.

So lfcdansk, it’s a balancing act betwen melting through and the magma cooling down because the action of melting other things takes energy away from the magma itself 🙂