CHAPTER 3 Igneous Rocks and Intrusive Activity
88
Key Terms
andesitic composition (p. 67)
aphanitic texture (p. 68)
assimilation (p. 79)
basaltic composition (p. 67)
batholiths (p. 83)
Bowen’ s reaction series (p. 78)
coarse-grained texture (p. 69)
columnar joints (p. 82)
concordant (p. 82)
crystallization (p. 65)
crystal settling (p. 78)
decompression melting (p. 76)
dikes (p. 82)
discordant (p. 82)
extrusive igneous rocks (p. 65)
felsic (p. 66)
fine-grained texture (p. 68)
fragmental texture (p. 70)
geothermal gradient (p. 76)
glass (p. 68)
glassy texture (p. 69)
groundmass (p. 69)
granitic composition (p. 66)
hydrothermal solutions (p. 86)
igneous rocks (p. 64)
intermediate composition
(p. 67)
intrusions (p. 81)
intrusive igneous rocks (p. 65)
laccoliths (p. 83)
lava (p. 64)
mafic (p. 67)
magma (p. 64)
magma mixing (p. 79)
magmatic differentiation (p. 78)
massive (p. 82)
melt (p. 64)
partial melting (p. 80)
pegmatites (p. 70)
pegmatitic texture (p. 71)
phaneritic texture (p. 69)
phenocrysts (p. 69)
plutonic rocks (p. 65)
plutons (p. 81)
porphyritic texture (p. 69)
porphyry (p. 69)
pyroclastic texture (p. 70)
sills (p. 82)
stocks (p. 83)
tabular (p. 82)
texture (p. 67)
ultramafic (p. 67)
vesicular texture (p. 69)
volatiles (p. 64)
volcanic rocks (p. 65)
G I V E I T S O M E T H O U G H T
Would you expect all of the crystals in an intrusive igneous rock to
be the same size? Explain why or why not.
Apply your understanding of igneous rock textures to describe the
cooling history of each of the igneous rocks pictured here.
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1
b. A phaneritic rock containing about 20 percent quartz, 40 percent
potassium feldspar, 20 percent sodium-rich plagioclase feldspar,
a few percent muscovite, and the remainder dark-colored silicate.
c. An aphanitic rock containing about 50 percent plagioclase
feldspar, 35 percent amphibole, 10 percent pyroxene, and minor
amounts of other light-colored silicates.
d. A phaneritic rock made mainly of olivine and pyroxene, with
lesser amounts of calcium-rich plagioclase feldspar.
During a hike you pick up an interesting looking igneous rock. It is
dark in color and has a fine, granular appearance with small, fairly
round crystals of a glassy, deep green mineral scattered throughout.
Answer the following questions about this rock:
a. What mineral is the small, rounded, glassy green crystals likely
to be?
b. Did the magma from which this rock formed likely originate in
the mantle or in the crust? Explain.
c. Was the magma likely a very high temperature magma or a fairly
low temperature magma? Why?
d. What other minerals might make up the rest of this rock?
A common misconception about Earth’s mantle is that it is a thick
layer of molten rock. Explain, in your own words, why Earth’s mantle
is actually solid under normal conditions.
Describe two situations in which mantle rock can melt without an
increase in temperature. How do these magma-generating mechanisms relate to plate tectonics?
7
6
5
A. A.
B. B.
C. C.
D. D.
Is it possible for two igneous rocks to have the same mineral
composition but be different rocks? Use an example to explain
your answer.
Using Figure 3.3 and Figure 3.10, classify the following igneous
rocks:
a. An aphanitic rock containing about 30 percent calcium-rich plagioclase feldspar, 55 percent pyroxene, and 15 percent olivine.
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3
88
Key Terms
andesitic composition (p. 67)
aphanitic texture (p. 68)
assimilation (p. 79)
basaltic composition (p. 67)
batholiths (p. 83)
Bowen’ s reaction series (p. 78)
coarse-grained texture (p. 69)
columnar joints (p. 82)
concordant (p. 82)
crystallization (p. 65)
crystal settling (p. 78)
decompression melting (p. 76)
dikes (p. 82)
discordant (p. 82)
extrusive igneous rocks (p. 65)
felsic (p. 66)
fine-grained texture (p. 68)
fragmental texture (p. 70)
geothermal gradient (p. 76)
glass (p. 68)
glassy texture (p. 69)
groundmass (p. 69)
granitic composition (p. 66)
hydrothermal solutions (p. 86)
igneous rocks (p. 64)
intermediate composition
(p. 67)
intrusions (p. 81)
intrusive igneous rocks (p. 65)
laccoliths (p. 83)
lava (p. 64)
mafic (p. 67)
magma (p. 64)
magma mixing (p. 79)
magmatic differentiation (p. 78)
massive (p. 82)
melt (p. 64)
partial melting (p. 80)
pegmatites (p. 70)
pegmatitic texture (p. 71)
phaneritic texture (p. 69)
phenocrysts (p. 69)
plutonic rocks (p. 65)
plutons (p. 81)
porphyritic texture (p. 69)
porphyry (p. 69)
pyroclastic texture (p. 70)
sills (p. 82)
stocks (p. 83)
tabular (p. 82)
texture (p. 67)
ultramafic (p. 67)
vesicular texture (p. 69)
volatiles (p. 64)
volcanic rocks (p. 65)
G I V E I T S O M E T H O U G H T
Would you expect all of the crystals in an intrusive igneous rock to
be the same size? Explain why or why not.
Apply your understanding of igneous rock textures to describe the
cooling history of each of the igneous rocks pictured here.
2
1
b. A phaneritic rock containing about 20 percent quartz, 40 percent
potassium feldspar, 20 percent sodium-rich plagioclase feldspar,
a few percent muscovite, and the remainder dark-colored silicate.
c. An aphanitic rock containing about 50 percent plagioclase
feldspar, 35 percent amphibole, 10 percent pyroxene, and minor
amounts of other light-colored silicates.
d. A phaneritic rock made mainly of olivine and pyroxene, with
lesser amounts of calcium-rich plagioclase feldspar.
During a hike you pick up an interesting looking igneous rock. It is
dark in color and has a fine, granular appearance with small, fairly
round crystals of a glassy, deep green mineral scattered throughout.
Answer the following questions about this rock:
a. What mineral is the small, rounded, glassy green crystals likely
to be?
b. Did the magma from which this rock formed likely originate in
the mantle or in the crust? Explain.
c. Was the magma likely a very high temperature magma or a fairly
low temperature magma? Why?
d. What other minerals might make up the rest of this rock?
A common misconception about Earth’s mantle is that it is a thick
layer of molten rock. Explain, in your own words, why Earth’s mantle
is actually solid under normal conditions.
Describe two situations in which mantle rock can melt without an
increase in temperature. How do these magma-generating mechanisms relate to plate tectonics?
7
6
5
A. A.
B. B.
C. C.
D. D.
Is it possible for two igneous rocks to have the same mineral
composition but be different rocks? Use an example to explain
your answer.
Using Figure 3.3 and Figure 3.10, classify the following igneous
rocks:
a. An aphanitic rock containing about 30 percent calcium-rich plagioclase feldspar, 55 percent pyroxene, and 15 percent olivine.
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3