Why do some volcanoes explode?

Why do some volcanoes explode?

some volcanoes are different then others. 

example:

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this is a Cone volcano. it it tall and it will EXPLODE!!!

example:

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this is a Shield volcano. it is short, and the lava that comes out is runny and moves faster then the cone volcano.

this is another example:

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this is another way of explaining the Cone volcano.

example:

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this is another way of explaining the Shield volcano.