What Are the Causes of Volcanoes?

At convergent plate boundaries, so like pacific ring of fire and many of the many land based volcanoes, you've got one plate subducting onto another. And as that happens, water is dragged down by the subducted plate. This leads to the melting temperature of the mantle turning into a liquid - forming a magma which then rises up because it's less dense than solid. But when it reaches the surface of the earth, it can form a volcano. So that's the cause of many of these sorts of volcanoes.