Final answer:
Metamorphic grade is the degree to which a parent rock changes during metamorphism, with high-grade rocks being subjected to higher temperatures and pressures than low-grade rocks.
Step-by-step explanation:
The term metamorphic grade refers to the degree to which a parent rock changes during metamorphism. Heat and pressure are the two critical factors that affect metamorphism, with temperature often being the most crucial factor as it allows minerals within the rock to recrystallize and form new minerals. The pressure can also induce foliation, which is the reorientation of minerals within the rock. Metamorphic rocks are classified as high-grade or low-grade based on the conditions they have been subjected to, with high-grade metamorphic rocks experiencing higher pressures and temperatures compared to low-grade metamorphic rocks.
For example, during a continental collision, a pattern known as the Barrovian sequence can be observed where the grade of metamorphism is higher closer and deeper to the collision center. High-grade metamorphic rocks typically form deep within the Earth and may be exposed at the surface through erosive processes.