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 welloriented, 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.
Gneissic
Schistose
Slaty
Granoblastic
Hornfelsic
Coarsely foliated with coarse mineral grains
Finely foliated forming thin parallel bands along which the rock
splits easily.
Very Fine foliated producing almost rigidly parallel planes of easy
splitting
Unfoliated or only faintly foliated
Unfoliated 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
Hornfels
Texture
Hornfelsic
Commonly Derived From
Any fine-grained rock
Quartzite
Sandstone
Marble
Granoblastic,
Fine grained
Granoblastic
Amphobolite
Granoblastic
Granulite
Granoblastic
Limestone, dolomite
Basalt, gabbro, tuff (an extrusive igneous rock
formed from volcanic ash)
Shale, igneous rocks
Folicated Metamorphic Rocks
Metamorphic Rock
Slate
Slaty
Commonly Derived From
Shale, Tuff
Phyllite
Slaty
Slate
Schist
Chlorite
Mica
Schistose
Gneiss
Gneissic
Coarsely banded,
Highly variable
Migmatite
Texture
Basalt, andesite, tuff, shale
Shale, tuff, rhyolite
Granite, shale, diorite, mica schist, rhyolite
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?
______________________________________________________________________________