Hot Spots
The other reason melting occurs beneath the crust is mantle convection.
Mantle convection brings heat to the surface causing hot spots to occur.
Heat is delivered to the base of the lithosphere by mantle convection that brings hot material to shallower depths, thus releasing pressure, enabling melting of both the rising material, and the lithospheric rocks that it contacts. The sites where volcanoes form from this sort of deep-seated heat delivery are called hot spots.
Modern seismic imaging has made it possible to construct the distribution of mantle convection cells globally. They are roughly, but not completely uniformly distributed around the Earth.
Hot spots can and do occur anywhere on the Earth's surface. However, because the lithosphere is thicker in some places than others, and because lithospheric plates moves laterally relative to the mantle convection system at various rates, some plates and parts of plates have more hot spots than others. As a result, the slow moving plates that include continents such as Africa and Antarctica have numerous hot spots, as do thin plates, such as the Pacific plate, which is entirely composed of thin ocean lithosphere.