Melt caused by the addition of volatiles. This type of melt generation is associated with subduction zone boundaries. 

Melt caused by the increase in temperature. This type of melt generation can be associated with many settings that have magma present or also with mantle plumes and hotspots.

Melt caused by lowering the pressure, without much change to temperature. This type of melt generation is associated with divergent boundaries and with hotspots-mantle plumes.