9-15. a. What does the phrase “metamorphic grade” mean? What…

9-15. a. What does the phrase “metamorphic grade” mean? What are the differences between low-grade metamorphism and high-grade metamorphism? b. Low grade metamorphic rocks are often slate or phyllite, medium grade metamorphic rocks are commonly schists, and high grade metamorphic rocks are commonly gneiss. Why?