Why do volcanoes occur

"Because it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually some of the magma pushes through the vents and fissures in the earth's surface ".

And then a volcanic eruption occurs

-Quora

Volcanoes occur because of the position of the crust over a plate boundary or a 'hot spot' in the Earth's mantle. Continental volcanoes form when subducted crust melts to magma against the mantle, then is forced upward under pressure through cracks in the rocks above.