Name: ______Block: ______Date: ______

Classifying Common Metamorphic Rocks

In the classification of rocks, a geologist usually uses what is called a field classification. Field classification represents a broad frame work into which the majority of rocks can be placed. It is based primarily on the texture of the rock and upon its mineral composition. These are the two main characteristics of a rock that one should look for in a specimen.

Metamorphic rocks are “changed rocks.” They were once other types of rock---sedimentary, igneous, or metamorphic---but have had their textures, structures, and composition changed by heat, pressure, and/or chemical reactions. They may still possess some of the characteristics of the rocks from which they were made. As a result, it is easy to make a mistake in the rock’s classification.

Texture

The differences in the orientation, or alignment, of crystals and the size of the crystals determine the texture of a metamorphic rock. There are two general texture groups.

Foliated textures are those in which platy or leaflike minerals such as mica or chlorite are nearly all aligned parallel to one another. As a result, the rock splits readily along the well-oriented, nearly parallel cleavages of its mineral particles.

Nonfoliated textures are composed of either randomly oriented platy minerals or minerals that are not foliated. Metamorphic rocks that are nonfoliated will break into agular pieces. Many times metamorphic rocks that are nonfoliated are called massive. Marble and quartzite are the best examples.

There are five basic textures found in metamorphic rocks.

GneissicCoarsely foliated with coarse mineral grains

SchistoseFinely foliated forming thin parallel bands along which the rock

splits easily.

SlatyVery Fine foliated producing almost rigidly parallel planes of easy

splitting

GranoblasticUnfoliated or only faintly foliated

HornfelsicUnfoliated with mineral grains that are commonly microscopic;

breaks sharply into angular pieces

On the next page is a simplified identification table for metamorphic rocks. Study it carefully and then answer the questions.

Unfoliated or lightly Folicated Metamorphic Rocks
Metamorphic Rock / Texture / Commonly Derived From
Hornfels / Hornfelsic / Any fine-grained rock
Quartzite / Granoblastic,
Fine grained / Sandstone
Marble / Granoblastic / Limestone, dolomite
Amphobolite / Granoblastic / Basalt, gabbro, tuff (an extrusive igneous rock formed from volcanic ash)
Granulite / Granoblastic / Shale, igneous rocks
Folicated Metamorphic Rocks
Metamorphic Rock / Texture / Commonly Derived From
Slate / Slaty / Shale, Tuff
Phyllite / Slaty / Slate
Schist
Chlorite
Mica / Schistose / Basalt, andesite, tuff, shale
Shale, tuff, rhyolite
Gneiss / Gneissic / Granite, shale, diorite, mica schist, rhyolite
Migmatite / Coarsely banded,
Highly variable / Mixtures of igneous and other metamorphic rocks

Refer to the information on the front and the table above to answer the following questions.

1. The classification of rocks is based primarily on ______and

______composition.

2. Name the five basic textures of metamorphic rocks.

a. ______

b. ______

c. ______

d. ______

e. ______

3. Name the five most common types of metamorphic rocks. (Hint: check your notes and/or

textbook)

a. ______

b. ______

c. ______

d. ______

e. ______

4. What type or types of rocks would result from the metamorphism of the following rocks?

a. limestone ______

b. sandstone ______

c. granite ______

d. shale ______

5. For each of the following description, indicate the type of metamorphic rock.

a. Granoblastic, fine to coarse-grained, composed mainly of calcite, dolomite or both

______

b. Very fine-grained, exceptionally well-foliated rock

______

c. Coarse-grained rock with distinct layers or lenses of different minerals; among the most

plentiful of metamorphic rocks

______

d. Very hard, sugary-texture granoblastic rock; very widely distributed; made of interlocking

quartz grains.

______

e. Hard unfoliated, very fine-grained rock that breaks into sharp angular pieces

______

Metamorphic Rocks Enrichment Activity

Metamorphic Process

Use the graph to answer questions about the formation of metamorphic rocks.

1. What is the relationship between rock temperature and depth beneath the surface?

______

2. At a depth of 10 kilometers, what would the temperature be? ______

3. Between 0 and 10 kilometers, how many degrees per kilometers does the temperature increase?

4. Which metamorphic rock shown forms at the highest temperature? ______

5. Through what general temperature range would you expect schist to form? ______

6. How many kilometers beneath the surface does gneiss occur? ______

7. As depth increases, what happens to the temperature at which rock melts? ______

______

8. Suppose you knew that a certain metamorphic rock begins to form at about 300oC. How many kilometers beneath the surface might this rock temperature occur? ______

9. How far beneath the surface does shale metamorphose to slate? ______

10. Metasomatism is described as a process by which metamorphic rocks may gradually change into granite (an igneous rock). At which temperature does metasomatism begin?

______