Here's how it works:
* Heat: Deep within the Earth, temperatures are extremely high. This heat can cause the minerals in existing rocks to recrystallize, changing their texture and sometimes even their chemical composition.
* Pressure: The immense weight of overlying rock layers creates pressure. This pressure can cause existing minerals to deform and rearrange, creating new mineral structures.
The combination of heat and pressure, often in the presence of fluids, transforms the original rock into a metamorphic rock.