Chapter 3 Identifying and Classifying Rocks
Introduction
Recall that a rock is a naturally occurring, solid mass of minerals or mineral-like matter. Geologists
classify rocks into three major groups based on how 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.
Sedimentary rocks form when pieces of rocks, minerals, or organic matter – all of which are called
sediments – are compacted and cemented. Clastic rocks are sedimentary rocks that are made of
fragments of weathered Erath 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 a 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 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.
Problem
How can you use a key to classify rocks?
Pre-Lab Discussions
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Inferring What is the purpose of this investigation?
Forming Operational Definitions In your own words, describe what is meant by a rock’s texture.
Observing What distinguishes the two main types of igneous rocks? Explain your answer.
Drawing Conclusions Chalk is made of tiny fragments of marine organisms. To which group of
rocks does chalk belong to?
5. Classifying Suppose you observe a rock with distinct bands. What type of rock might this be?
Can the rock also belong to another group of rocks? Explain your answer.
Materials
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Igneous rocks
Sedimentary rocks
Metamorphic rocks
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Hand lens
Pen or pencil
Procedure
1. Make a Data Table to record your observations.
2. Examine each rock specimen with and without a hand lens. Determine and record the overall
color, texture, crystal size and composition of each rock in your Data Table.
3. In the Data Table make and color a detailed sketch of each sample.
4. Use the Key to Rock Classification (Data Table 1) to classify the sample. Begin by reading the first
question. Answer Yes or No based on your observations.
5. After the words Yes and No, you will find directions to proceed to another question, or you will
discover 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.
6. Continue working through the key in this way until you come up with a statement that allows
you to classify your rock sample.
7. In your Data Table record the route that you take through the key, using the numbers of the
questions. For example, your route could be “1 – 4 – 5 – extrusive igneous rock.” In the last
column of the table, write the name of the rock group to which each sample belongs. Then
specify which rock you think it is using the pictures in your book and the description from your
teacher.
8. Compare your classifications with those provided by your teacher. If you made a mistake in
classifying any of the samples, put the correct answer in red next to your answer.
Analyze and Conclude
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 of each of the major groups of rocks –
intrusive, extrusive, clastic, chemical, 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.
Data Table
Rock Sample #___
Sketch
Color
Composition
Texture
Route Taken
Rock Group
Rock Name