Name _________________________
Date _______________
Regents Earth Science
Period _____
Lab 12: Identifying Igneous Rocks
Question: How can one igneous rock be distinguished from another igneous rock?
Background:
All rocks are made of minerals. Igneous rocks are formed from magma through a process of melting and
solidification. An igneous rock may have inter-grown crystals, glassy appearance, or gas bubbles. When a rock
contains gas bubbles it is said to have a vesicular texture, meaning it is a rock characterized by being pitted
with many cavities (known as vesicles) at its surface and inside. Igneous rocks can be extrusive (volcanic)
which are formed above the Earth’s surface or intrusive (plutonic) formed below the surface of the Earth.
Igneous rocks can be mafic. These rocks are rich in iron and magnesium and have a dark color and are very
dense. Felsic igneous rocks are rich in silicon and aluminum and as a result are light colored and less dense.
Igneous rocks can also be non-crystalline which means no crystals can be seen.
Information about igneous rocks can be found in the ESRT Table # 6 Scheme for Igneous Rock Identification.
form extrusive
igneous rocks.
form intrusive igneous rock.
Procedure
1. Look over the rocks.
2. Describe the color and texture in the data table on page 2.
3. Describe the feature you see in these rocks, using the ESRT Table # 6 Scheme for Igneous Rock
identification and list in column 3 of the data table.
4. Use the ESRT Table # 6 to identify the density as well. Fill this in the column labeled density.
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Data Table
Overall
Color
Texture
Features
Cooling History
Density
Rock Name
Questions
Answer all questions in complete sentences!
1. Which rocks had large crystals? ________________________________________________
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2. What minerals make up these crystals? ___________________________________________
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3. Which of the rocks that you observed are volcanic? __________________________________
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What characteristics indicate this? _____________________________________________
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Refer to textbook page 132:
4. Identify a property that is characteristic of obsidian. ________________________________
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5. What is the archeological significance of obsidian? __________________________________
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6. How does the rate of cooling affect the size of the crystals in a rock? ____________________
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7. Granite, rhyolite and obsidian are essentially made of the same materials. Explain why they look
different. ________________________________________________________________
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Critical Thinking & Application
8. Where would igneous rocks have a change to cool slowly? ______________________________
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9. Where would igneous rocks cool rapidly? __________________________________________
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10. If you saw a rock that contained large, interlocking crystals, what would you say about the way it
formed? __________________________________________________________________
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________________________________________________________________________
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