Question

How do geologists use "index minerals"?

a. Index minerals are specific minerals that form only at specific geographic locations. The presence of these minerals within a metamorphic rock indicates the region in which it was found.
b. Index minerals are specific minerals that form at specific temperatures. The presence of these minerals within a metamorphic rock indicates the rock's metamorphic grade.
c. Index minerals are specific minerals that dissolve in chemically active fluids in a certain sequence. The presence of these minerals within a metamorphic rock indicates if chemically active fluids were an agent in its metamorphism.
d. Index minerals are specific minerals that have formed only at specific times in Earth's history. The presence of these minerals within a metamorphic rock indicates when it formed.
e. Index minerals are minerals that occur only in rocks showing specific foliations. The presence of these minerals within a metamorphic rock indicates the rock's metamorphic grade.

Answer 1

Geologists use index minerals to determine the metamorphic grade of a rock, which reflects the temperature and pressure conditions during its transformation. The correct option is D.

Step-by-step explanation:

Geologists use index minerals as a vital tool in studying metamorphic rocks. These minerals, which include chlorite, biotite, garnet, kyanite, and sillimanite, form only at specific temperatures and pressures.

The presence of these minerals within a metamorphic rock indicates the rock's metamorphic grade, which refers to the conditions of temperature and pressure during the rock's metamorphosis. This relationship allows geologists to infer the metamorphic history of the rock and the geological processes that have shaped a region.