This is a video of the formation of volcanoes:
https://www.youtube.com/watch?v=6Z4as_imJfM
Volcanoes form when hot material from below rises and leaks into the crust. This hot material is called magma. Magma comes either from a melt of subducted crustal material, and which is light and buoyant after melting, or it may come from deeper in the interior of a planet and is light and buoyant because it is *very* hot.