Why do volcanos erupt?

Here's as much as I know about it...
The continental plates are moving away from each other in some places and towards each other in some places. (Where they are moving away from each other, new land forms as magma from beneath the earth's crust cools, but this is just a gradual leak, not a volcano.) Where they are moving together, one plate is forced under the other and is heated as it is forced down. The heated plate melts and pushes up against the crust to form a volcano. This is why volcanoes are found near fault lines. As far as what causes an erruption to happen at a specific time, I'm not sure.

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