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At convergent plate boundaries, two tectonic plates move towards each other. There are three types of convergent boundaries: oceanic-oceanic, oceanic-continental, and continental-continental. In the case of oceanic-continental and oceanic-oceanic boundaries, one of the plates is forced beneath the other, a process known as subduction. As the subducting plate descends into the Earth's mantle, it experiences increasing temperature and pressure conditions, which eventually lead to partial melting.

Partial melting is the process in which some minerals in a rock melt while others remain solid, creating a mixture of molten and solid materials. This happens because different minerals have different melting points, and the temperature reached inside the Earth's mantle during subduction is typically not high enough to melt the entire rock.

The primary mechanism for partial melting at a convergent plate boundary is water-induced melting. When oceanic lithosphere subducts beneath another plate, it carries water and other volatile components that were trapped in the sediments and minerals of the descending plate. As the slab descends into the mantle, these volatiles (mainly water) are released, lowering the melting point of the surrounding mantle rocks, leading to partial melting. The result is the formation of magma, which is less dense than the surrounding solid mantle material and consequently rises towards the surface to generate volcanic activity above the subduction zone. To summarize, the primary mechanism for partial melting at a convergent plate boundary is water-induced melting, triggered by the release of water and other volatiles from the subducting oceanic plate, causing a decrease in melting points of mantle rocks and leading to the formation and ascent of magma.