Download Lab #21: Igneous Rock Identification

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Name: ____________________________
Date: ______________
Lab #21: Igneous Rock Identification
Class: _________
Introduction: The type of igneous rock formed when molten magma solidifies depends on the mineral composition and the
rate at which the magma cools. Igneous rocks which solidify deep underground are called intrusive or plutonic. They can be
observed when erosion wears away overlying layers. When magma reaches the surface it forms volcanic igneous rocks. The
texture of these extrusive igneous rocks will vary greatly from ones formed by the same magma deep below the surface.
Note: Be sure to use the Igneous Rock identification chart located on the next page to identify each rock and then
record your answers in the data table. Be sure to use the abbreviations under the minerals present column.
Sample
#
Color
(Light
or Dark)
Texture
(Glassy,
Fine,
Coarse)
Env. Of
Formation
(Extrusive
or Intrusive)
Minerals Present
Composition
(Felsic or
Mafic)
Rock Name
Mineral Present Abbreviations (Minerals Present):
Q = Quartz; PotF = Potassium Feldspar; A = Amphibole; B = Biotite; P = Pyroxene; O = Olivine; PlagF = Plagioclase Feldspar
Conclusion: All answers must be in full sentences and should be written on this lab or typed up on a separate piece of paper.
1. How is the size of the mineral grains affected by the rate at which molten rock material cools? Cite evidence to
support your answer.
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2. How can you determine if an igneous rock has had an intrusive or extrusive origin? Cite evidence to support
your answer.
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3. In general, how does the characteristic mineral composition of a light colored igneous rock differ from that of a
dark colored igneous rock? Cite evidence to support your answer.
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4. In general, how does the density of a light colored igneous rock differ for that of a dark colored igneous rock?
Cite evidence to support your answer.
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5. What are the primary differences between rocks formed from lava compared to rocks formed from magma?
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6. Conclusion: Explain how geologists classify and identify igneous rocks.
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