The heat associated with plate boundaries is the source of liquid rock released by volcanos.
At divergent boundaries the crust cracks allowing magma to rise to the surface. The theory is that this is the result of an upward convection current carrying the heat of the mantle upwards. There are volcanos in Iceland that are a result of the divergent plate boundary along the Mid Atlantic ridge.
Near convergent boundaries that result in subduction zones volcanos form. The Pacific Rim of fire is an example of this relationship. The Ocean crust is forced under a continental crust at a subduction zone convergent boundary. As the Ocean Crust is forced under it melts do the heat of friction and the increased temperature with increased depth. The melted crust becomes magma and is pushed toward the surface creating volcanos.