Final answer:
Metamorphism is the process that alters rocks through heat, pressure, and fluid activity without melting, leading to new metamorphic rocks with different properties and structures.
Step-by-step explanation:
The three processes that bring about metamorphism are heat, pressure, and fluid activity. These processes work together to alter pre-existing rocks, also known as the protolith, into new metamorphic rocks through a recrystallization event.
Heat is considered the most critical factor as it allows minerals to 'soften', enabling ions to migrate and form new crystal structures. Pressure causes minerals to re-orientate and arrange into bands, often leading to foliated textures. Lastly, fluid activity plays a role by delivering chemically reactive fluids which can induce changes in mineral content and rock composition.
Different metamorphic environments and combinations of heat, pressure, and fluids result in various metamorphic rock types. These metamorphic rocks can show significant differences in appearance and mineral content compared to the original rock, for example, the transformation of limestone to marble or shale to schist.
This process occurs without the rock melting, which is an important distinction from the formation of igneous rocks.