Adam Cuthbert Dear Adam, Volcanoes form by the accumulation of lava at the surface. For most volcanoes the magma that supplies them originates by melting in the mantle . Along the edge of spreading plates volcanologists think the mantle is flowing upward. As the hot rock gets closer to the surface the pressure decreases and the rock begins to melt, producing magma .
Subduction zones are a little different.
The mantle above the subducting plate and below the over riding plate is relatively hot, but not hot enough to melt rock. Remember that the oceanic plate, before being subducted, was on the ocean floor for tens of millions of years. It incorporated a lot water and was buried in sediment. As the plate subducts it comes in contact with the overlying hot mantle. This causes the water and sediment to melt and migrate into the overlying mantle.