Subduction
What happens to the material that subducts? Where does it go?

Where two tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle. (The mantle is the hotter layer under the crust.) ... At a subduction zone, the oceanic crust usually sinks into the mantle beneath lighter continental crust.

Explanation:

metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). the change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. i hoped i !

"d.) carbon dioxide and hydrogen sulfide"