Page 223 - Laboratory Manual in Physical Geology

Description and Interpretation of ACTIVITY
Metamorphic Rock Samples
7.4 Metamorphic Grades and
The complete classification of a metamorphic rock requires
knowledge of its composition, texture(s), and other distinc- Facies
tive properties. Follow these steps to analyze and classify
a metamorphic rock: THINK What can metamorphic rocks tell
Step 1: Determine and record the rock’s textural features. About It us about Earth’s history and the
environments in which the rocks
Determine and record if the rock is foliated or nonfoliated,
formed?
and what other specific kinds of textural features are pres-
ent. Use this information to work from left to right across

OBJECTIVE Infer regional geologic history and the
the three columns of Step 1 in FIGURE 7.16 , and match the

relationship of metamorphic facies to plate tectonics
rock texture to one of the specific categories there.
using index minerals, pressure-temperature diagrams,
Step 2: Determine and record the rock’s mineralogical com- and geologic maps.
position and/or other distinctive features. List the minerals
PROCEDURES
in order of increasing abundance, and distinguish between
porphyroblasts and mineralogy of the groundmass making 1. Before you begin , read about Using Index
up most of the rock. Use this information and any other Minerals to Interpret Metamorphic Grades and
distinctive features to match the rock to one of the catego- Facies. Also, this is what you will need :
ries in Step 2 of FIGURE 7.16 . ___ Activity 7.4 Worksheets (pp. 205–206 ) and pencil

Step 3: Recall how you categorized the rock in Steps 1 2. Then follow your instructor’s directions for
and 2. Use this information to work from left to right completing the worksheets.

across FIGURE 7.16 and determine the name of the rock.

You can also modify the rock name by adding the names
of minerals present in the rock in order of their increasing
chart in FIGURE 7.16 , notice that the rock name is gneiss.

abundance. If the rock is porphyroblastic, then you can
Step 4—The parent rock for gneiss is mudstone, shale,
add this to the name as well (e.g., FIGURE 7.15 ).

slate, phyllite, schist, granite, or diorite. Step 5—Gneiss is
Step 4: After you have determined the metamorphic rock commonly used as a construction stone, decorative stone,
name in Step 3, look to the right along the same row of or as the source of some gemstones.
FIGURE 7.16 and find the name of a parent rock (protolith)

for that kind of metamophic rock. Using Index Minerals to Interpret
Step 5: After you have determined the parent rock in Step Metamorphic Grades and Facies
4, look to the right along the same row of FIGURE 7.16 and

The changes in metamorphic grade that are indicated by
find out what the rock is commonly used for.
textural changes, like schist to gneiss, are also accompanied
Example: by mineralogical changes. Minerals that indicate specific
Study the metamorphic rock in FIGURE 7.4 . Step 1—The grades of metamorphism are called index minerals
rock has obvious layering, so it is foliated. Step 2—The ( FIGURE 7.6 ). Assemblages of index minerals make up

rock has alternating layers of light and dark colored metamorphic facies, which can be interpreted using
minerals. Step 3—By moving from left to right across the pressure-temperature diagrams.
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