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Classifying Rocks Using a “Key”
Introduction:
Recall that a Rock is a naturally occurring, solid mass of minerals. Geologists (scientists who study rocks!)
classify rocks into three groups depending on how they form.
Igneous Rocks form when magma (melted rock) cools and crystallizes. There are two kinds of Igneous Rocks.
Extrusive rocks form when magma cools quickly at or near the earth’s surface, extrusive rocks have finegrained or glassy texture. Intrusive rocks form as magma cools slowly, farther beneath Earth’s surface.
Intrusive rocks take longer to form, therefore their crystals can grow bigger; this gives them a rough,
coarse-grained texture.
Sedimentary Rocks form when sediments (pieces of broken rocks) are compacted (squished) and cemented
(stuck) together. There are three kinds of Sedimentary Rocks. Clastic Rocks are sedimentary rocks that are
made of fragments (pieces) of weathered Earth materials that may be many different sizes. Chemical Rocks
are sedimentary rocks that form when minerals precipitate (fall) out of a solution (mixture of liquid and
solids). Biochemical Rocks are sedimentary rocks that form due to organic processes (involving living
things)!
Metamorphic Rocks form when existing rocks are changed by extreme pressure or temperature!
Metamorphic rocks can be foliated which means they have parallel “bands” (layers!), or nonfoliated,
without any “bands!”
Purpose:
You will observe rock texture, which is how a rock feels due to its shape, size, and composition (what it’s made
of!), in order to classify rocks into specific groups using a key.
Problem: How can I use a key to classify rocks?
Pre-Lab: (complete sentences!)
1. In your own words, describe what is meant by a rock’s “texture.”___________________
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2. What are two types of Igneous Rocks? What distinguishes these two types of Igneous
Rocks? (How are they different?)_________________________________________
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3. What are the three types of Sedimentary rocks? Which type of sedimentary rock would
be formed if we evaporated water that had reactive minerals mixed in it? ___________
__________________________________________________________________
4. Chalk is made of tiny fragments of marine animals. To which group of rocks does chalk
belong? ____________________________________________________________
5. Suppose you observe a rock with very clear bands (stripes). What type of rock might this
be? What other group might it belong to? Explain._____________________________
__________________________________________________________________
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Materials:
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Igneous Rocks
Sedimentary Rocks
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Metamorphic Rocks
Hand Lens (magnifying glass
Procedure:
1. Observe one rock sample. Examine its 1.)texture, 2.)color, 3.)crystal size, and 4.)composition
with and without the hand lens.
2. Use Data Table 1 (“Key to Rock Classification”) to classify the sample. Start by reading the
first question, answer YES or NO based on your observations.
3. After the words “YES” or “NO” you will find directions to proceed to another question, or
you will discover which group your rock belongs to.
4. If you get to question 8, ask Mr. Jackson for help. We will not be using Hydrochloric
Acid in this lab.
5. Copy Data Table 2, and record the “route” you take through the key, using the numbers of
the questions. For example, your route might be “1-4-5-Extrusive Igneous Rock.”
6. Write the name of each rock group in the last column of Table 2 for each sample.
7. Continue working through the table until you have classified all of your rock samples.
8. Compare your classifications with those provided by your teacher, when everyone is done.
Correct any mistakes.
Observations
DATA TABLE 1:
(USE this table, do NOT copy it!!)
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Data Table 2:
Number of
Sample
COPY & Fill-out this Table!
Route Taken
Group to Which Rock
Belongs
Analysis and Conclusion
1. Using Tables & Graphs: How difficult was it to us the key to classify your rock samples?
What problems did you have? ____________________________________________
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2. Generalizations: How useful was the rock’s color in classifying the samples? Explain your
answer._____________________________________________________________
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3. Generalizations: Describe the overall texture of each group of rocks: igneous,
sedimentary, and metamorphic. ___________________________________________
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4. Analysis: Explain how you can tell the difference between an intrusive igneous rock and
an extrusive igneous rock. _______________________________________________
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5. Classifying: Which two of the rock samples were the easiest to classify? What
properties made them easy to classify?_____________________________________
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