Final answer:
Metamorphic rocks form when a pre-existing rock is altered by changes in temperature, pressure, or contact with fluids.
Step-by-step explanation:
Metamorphic rocks form when a pre-existing rock, called a protolith, is altered by changes in temperature, pressure, or contact with chemically reactive fluids.
This alteration process is known as recrystallization, where the minerals in the rock change in size, shape, and composition.
The end result is a new metamorphic rock that can have a completely different appearance from the original rock.