At convergent plate boundaries, oceanic plates are subducted, either beneath a less dense continental plate, or beneath a younger, less dense oceanic plate. At both types of boundary, the process is the same, however the resulting geological structures are different.
As the oceanic plate is subducted (forced underneath) the other plate, the low temperature minerals within the crust begin to melt as the temperature increases, due to the geothermal gradient (the increase in temperature with depth). The melted minerals rise up through the crust as magma, usually with an intermediate composition.
At an oceanic-oceanic plate boundary, where the older, denser plate is subducted beneath the younger, less dense plate, the generated magma rises up through the oceanic crust and forms volcanoes with intermediate and mafic lava, such as basalt.
At an oceanic-continental plate boundary, where the oceanic plate is subducted beneath the less dense continental plate, the generated magma rises up through the continental crust, it is at a high enough temperature to partially melt some of the continental crust, which mixes with the rising magma to form huge volumes of intermediate magma, resulting in huge stratovolcanoes such as those of the Andes. As continental crust is generally granitic in composition, huge granite batholiths (intrusions) may also form at these plate boundaries.