What does a metamorphic grade refer to, and how can it be determined? How does grade differ from facies? Identify the axes of the graph, and the areas representing conditions in which low-, intermediate-, and high-grade metamorphic rocks form. (G)

Metamorphic grade refers to the intensity of metamorphism a rock has undergone, determined through the identification of index minerals. Metamorphic facies, on the other hand, are sets of metamorphic rocks formed under similar temperature and pressure conditions, characterized by specific index minerals. The difference between the two lies in the aspects of metamorphism they indicate: grade refers to the degree of metamorphism, while facies represent specific environmental conditions. On a graph displaying metamorphic rock formation, the y-axis represents pressure (in kbar) and the x-axis represents temperature (in °C). Low-grade metamorphic rocks form in the lower-left part of the graph, under low temperature and pressure conditions. Intermediate-grade rocks form under intermediate conditions, while high-grade rocks form in the upper-right part of the graph, under high temperature and pressure conditions.