Name ___________________________ Section ________________ Date _____________
Classifying Rocks
Introduction
Recall that a rock is a naturally occurring, solid mass of minerals or minerallike matter. All rocks are mixtures of one or more minerals. Geologists classify
rocks into three major groups based on how the rocks form. Igneous rocks form
when molten material—lava or magma—cools either on Earth’s surface or
underground. Extrusive rocks form when lava cools quickly at or near Earth’s
surface. Extrusive rocks have either a fine-grained texture or a glassy texture.
Intrusive rocks form as magma cools slowly farther beneath Earth’s surface. This
slow rate of cooling allows mineral grains to grow large, and such a rock is said to
have a coarse-grained texture. Porphyritic rocks cool at varying rates and have a
mix of crystal sizes. Sedimentary rocks form when pieces of rocks, minerals, or
organic matter—all of which are called sediment—are compacted and cemented.
Clastic rocks are sedimentary rocks that are made of fragments of weathered
Earth materials. The fragments might be fairly large, such as pebbles; somewhat
smaller, such as grains of sand; or very small, such as grains of clay. Chemical
rocks are sedimentary rocks that form when minerals settle out of solution.
Biochemical rocks are sedimentary rocks that form as the result of organic
processes. Metamorphic rocks are rocks that form when existing rocks are
subjected to changes in pressure or temperature. They can also form when they
are subjected to chemical solutions. Metamorphic rocks may be foliated, which
means that the components are arranged in parallel bands, or nonfoliated, which
means that the rock’s components are not arranged in bands.
In this investigation, you will observe rock texture, which is the shape, size,
and arrangement of a rock’s components. You will use rock texture and other
properties to classify rocks using a key.
Prelab Summary: Fill in the flow chart on the next page.
Materials (per group)
Known igneous rocks
Known sedimentary rocks
Known metamorphic rocks
Unknown samples
Bottle of dilute (1M) hydrochloric acid (HCl) with dropper
Paper towels
Brick for hardness testing
Safety
Take care when using chemicals such as hydrochloric acid as they may irritate the skin or stain
skin or clothing. Never touch or taste a chemical unless instructed to do so. Wash your hands
thoroughly after completing this investigation.
Part 1 Observing Known Samples Procedure:
1. Choose one of the known rock samples provided by your teacher.
2. Observe its texture, color, crystal size, and hardness.
3. Practice using the key (Table 3), book, and reference cards. You may find that some are
easier to use than others, and that the effective tool may change with the sample! Record
your observations in the Table 1 below. You may use any tool to start with.
a. To use the key, begin by reading the first question. Answer Yes or No based on
your observations.
b. After the words Yes and No, you will find directions to proceed to another
question, or you will discover to which group of rocks your specimen belongs. If
you find directions to proceed to another question, go to that question, answer it,
and follow the directions.
c. Continue working through the key in this way until you come to a statement that
allows you to classify your rock sample.
d. As you work through the known samples, the key should lead you to the rock
type that you chose your sample from.
4. Repeat this procedure until you feel familiar with igneous, sedimentary & metamorphic
rocks as well as the key, book, and reference cards. Students should observe a minimum
of nine samples, 3 samples from each type.
Table 1
Known
Sample
Rock
Type
Texture(M/I)
or Type (S)
Color
Quartzite
M
Nonfoliated
Pink/white
Crystal
Size
Some large
crystals,
glittery
Other Observations
Rough to the touch, one
edge has a “crust,”
scratches brick
Part 2 Observing Unknown Types Procedure:
1.
2.
3.
4.
5.
6.
Get an unknown rock bag from your instructor.
Each bag contains rocks of the same type: igneous, metamorphic, OR sedimentary.
Use the key, book, and reference cards to identify the type of rocks that are in the bag.
Record in the table below. Have your instructor check your work.
If you are correct, get another unknown rock bag and repeat.
You must correctly identify two rock types before you can continue.
Table 2
Sample
Observations/Reasoning
Identification
Part 3 Unknown Rocks Procedure:
5. Choose an unknown rock sample.
6. Use the Key to Rock Classification (Table 3), and the reference cards provided to classify
the sample. You may use either source to start with.
a. To use the key, begin by reading the first question. Answer Yes or No based on
your observations.
b. After the words Yes and No, you will find directions to proceed to another
question, or you will discover to which group of rocks your specimen belongs. If
you find directions to proceed to another question, go to that question, answer it,
and follow the directions.
c. Continue working through the key in this way until you come to a statement that
allows you to classify your rock sample. To answer Question 8 in the key (Table 3),
put the rock on a paper towel and place a single drop of HCl on the rock. Wipe the
rock off once you have completed your observations; return the HCl to your instructor, and
WASH YOUR HANDS.
7. In Data Table 4, record your observations and the reasoning that you used to classify the
rock. In the last column of the table, write the name of the rock group to which each
sample belongs.
8. When you have classified as many samples as time allows, thoroughly wash your hands
with soap and water.
9. Compare your classifications with those provided by your teacher.
TABLE 3
Key to Rock Classification
1.Does the rock contain visible
connecting crystals?
Yes: Go to Question 2
No: Go to Question 4.
.
2.Are all of the crystals the same color
Yes: The rock is a nonfoliated
and shape?
metamorphic rock (possibly marble or
quartzite).
No: Go to Question 3.
3.Are all of the crystals in a mixed “salt- Yes: The rock is an intrusive igneous
and-pepper” pattern?
rock (possibly granite or diorite).
No: The rock is a foliated metamorphic
rock (possibly schist or gneiss).
4.Does the rock contain many small
Yes: The rock is an extrusive igneous
holes or have a uniform dark color?
rock (possibly pumice or basalt).
No: Go to Question 5.
5.Is the rock glassy (does it resemble
Yes: The rock is an extrusive igneous
broken glass)?
rock (obsidian). No: Go to Question 6.
6.Does the rock have flat, thin layers
Yes: The rock is a foliated metamorphic
That can be broken apart?
rock (slate). No: Go to Question 7.
7.Does the rock contain pebbles, are
Yes: The rock is a clastic sedimentary
cemented together?
rock (possibly conglomerate, sand, or
smaller particles that sandstone, or
shale).
No: Go to Question 8.
8.Does the rock fizz when dilute HCl is
Yes: The rock is chemical or organic
dropped on it?
sedimentary rock (limestone or chalk).
No: Ask your teacher for assistance.
Observations
DATA TABLE 4
Number
of
Sample
Observations and Reasoning
Group to Which
Rock Belongs
(Igneous, Sedimentary,
Metamorphic)
Analysis and Conclusions
1. Using Tables and Graphs How difficult was it to use the key to classify your rock
samples? What problems did you encounter?
2. Making Generalizations How useful was rock color in classifying the rock samples?
Explain your answer.
3. Making Generalizations Describe the overall texture/type of each of the major groups of
rocks—intrusive, extrusive, clastic, chemical/biochemical, foliated, and nonfoliated.
4. Comparing and Contrasting Which two of the rock samples were the easiest to classify?
What properties made them easy to classify?
5. Comparing and Contrasting Which two rock samples were the hardest to classify?
Explain your answer.