In contact metamorphism, is the dominant agent of change.

Heat

Explanation:

The agent of the contact metamorphism is the heat, pressure and the differential stress and the chemically active fluids. Where the heat is sufficient to cause the process of the metamorphism and the contracts with the igneous body and thus the rocks that are produced by this process are never foliated. And this changes the geologic structure of the rocks minerals.

I'm pretty sure it's c. extremely gradual but not 100% sure though.

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