VOlcanoes are fed by magma, which is usually generated in the mantle. THat magma is more buoyant than the crust above so rises up through the crust. THe highly pressured magma forces opn fractures to aid that travel. EVentually it reaches a deth where it is neutrally buoyant, so it hangs around in what you were probably taught was a magma chamber. WHat we've come to realise over the last 30 years or so is that it's really not much of a chamber; the magma is stored in a network of cracks and openings, and as it sits there it starts eroding the host rock around it through melting, as well as crystallising and forming crystal mushes within those spaces. So the magma reservoir is really a semi-porous network which is part filled by liquid melt, and part filled by solid material.
(the full comment is really interesting)