Heat Flow: At Convergent Boundaries
Melting beneath the crust occurs for two basic reasons. A close look at a
subduction zone explains the first reason. Subduction zones drive ocean
crust and its blanket of marine sediments down into the mantle.
Volcanoes occur wherever subduction occurs and are found at all convergent plate margins. They are the most common type of volcano aside from mid-ocean ridges.
The subducting slabs are relatively cold. They have been at the cold ocean bottom and have cooled since formation at a mid-ocean ridge. Further, the sediments they carry contain minerals that have low melting points, having been formed at the Earth's surface at cool temperatures.
Subducting slabs are also accompanied by water, which further depresses melting points for the minerals. Consequently, as the slabs are carried into the deeper, hotter mantle, they heat up and partially or completely melt.
This molten rock, or magma, is less dense than the surrounding rock, and rises buoyantly through the lithosphere to the surface where it makes volcanoes.