Which is the primary process involved in the formation of magma at a subduction boundary?

a) water released by rocks plunging into the asthenosphere lowers melting temperatures.
b) a decrease in pressure lowers the melting temperatures of rock materials.
c) a plume of heated material rises from deep within the core.
d) heat from the core melts the plates at subduction boundaries.

a) Water released by rocks plunging into the asthenosphere lowers melting temperatures.

Explanation:

Subduction zone is a convergent plate margin in which a denser plates goes beneath a lighter one.

When lithospheric plates subducts, rock slabs goes into the subsurface and are subjected to intense heat and pressure as they pull further inside the earth.

The primary process which helps in the formation of magma in this zone is dewatering of rocks. When plates subducts, the conditions they are subjected to forces water and other fluids from them. As this water rises up to the crust, it causes reflux melting of nearby rocks. The water generally acts as a mobile phase for dissolution of ions and most importantly, it lowers the melting temperature significantly. When this occurs, the rocks melts and magma forms.