The heat from the core causes the mantle to rise towards the crust, but as it continues upwards it loses its heat making it fall back down. This cycle will continue in a circular motion called a convection current, this causes the tectonic plates to move. Most volcanoes are created by the movement of these tectonic plates which are basically huge pieces of rock that float on the mantle. Tectonic plates are always moving, though so slow that we cant feel it, its about a few centimetres every year.
If the plates are moving apart, volcanoes are produced as molten magma is allowed to escape.
If the plates are moving towards each other, the edges of the plates crumple, and one plate slides under the other. It produces mountains, like the Himalayas. The friction of the movement can also melt rocks and produce volcanoes.