Passage I
Metamorphic rocks form when temperature and/or pressure cause changes in preexisting rock. Figure 1 shows the temperature and pressure conditions in which certain facies (categories of metamorphic rocks) are formed.
A rock’s metamorphic grade (a measure of the intensity of metamorphism) is classified on a scale of low (very similar to the original rock) to high (very different from the original rock). Table 1 lists the grades of Facies A–G from Figure 1. Figure 2 shows characteristic minerals that may be present in rocks of a given grade.
4. According to Figure 2, the presence of which of the following minerals in a metamorphic rock would be least helpful in determining that rock’s grade?
Your Answer is
Correct Answer is J
Explanation
Looking at figure 2, plagioclase is found in low grade, medium grade and high grade, so the presence of plagioclase cannot tell which metamorphic grade the rock is