What are the three agents of metamorphism?

The three agents of metamorphism are heat, pressure and fluids. Note that only certain fluids can cause metamorphism, typically those with dissolved ions.

All pre-existing rock is known as protoliths. These can be formed into metamorphic rock through any one of the three agents of metamorphism. A metamorphic rock is any rock that becomes altered, usually by becoming denser, having its mineral content changed or by having a different texture formed. Heat often results in certain minerals becoming stable or unstable, resulting in a changed composition. High pressure over time compresses rock into a denser form. Finally, ionized liquids can cause mineral chemical reactions to occur or support changes from the other two agents.