It’s usually plate tectonics shifting. Massive sheets of rock sliding into one another with enough force and pressure to melt some. It then basically gets squeezed up to the surface through whatever cracks and openings it can find, which are usually volcanoes.
Whether it explodes or just sorta oozes out depends on the magma. There’s a lot of gases that can get trapped in the magma as it melts. If it’s soft and liquidy magma then that’s not a problem, the gases just sorta escape gradually without becoming an issue.
If the magma is thick then it’s hard for the gases to escape and they’ll keep building up until finally there’s just too much gas and they erupt violently. Whether the magma is thick or thin mostly depends on what kind of rock it’s made of.
It’s sorta like shaking a soda bottle. If you open it normally, the gases just fizzle up without an issue. If you shake it though, then a lot of extra gas gets dissolved in the soda and it’ll explode when you open it.
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