CHAPTER 7 Metamorphism and Metamorphic Rocks
194
Key Terms
aureole (p. 188)
burial metamorphism (p. 190)
confining pressure (p. 180)
contact metamorphism (p. 188)
differential stress (p. 180)
foliation (p. 182)
gneissic texture (p. 184)
hydrothermal metamorphism
(p. 189)
hydrothermal solutions (p. 189)
impact metamorphism (p. 191)
index minerals (p. 192)
metamorphism (p. 178)
metasomatism (p. 181)
migmatites (p. 193)
nonfoliated (p. 184)
parent rock (p. 178)
porphyroblastic textures (p. 184)
regional metamorphism (p. 190)
rock cleavage (p. 183)
schistosity (p. 184)
shock metamorphism (p. 191)
slaty cleavage (p. 183)
subduction zone metamorphism
(p. 190)
texture (p. 181)
thermal metamorphism (p. 188)
G I V E I T S O M E T H O U G H T
experiencing or may have experienced the metamorphic conditions
necessary to form foliation. Describe the location, and explain what
is happening or has happened that would result in foliated rocks.
Refer to Figure 7.11, the classification chart for common metamorphic rocks, and answer the following:
a. Identify a case in which the metamorphic rock may be stronger
than its parent rock, and explain why.
b. Identify a case in which the metamorphic rock may be weaker
than its parent rock, and explain why.
c. In general, do you think metamorphic rocks are stronger or
weaker than igneous rocks? How about sedimentary rocks?
Explain your answers.
3
Each of the following statements describes one or more characteristics of a particular metamorphic rock. For each statement, identify
the metamorphic rock that is being described:
a. calcite-rich and often nonfoliated
b. loosely coherent and composed of broken fragments that formed
along a fault zone
c. represents a grade of metamorphism between slate and schist
d. very fine-grained and foliated; excellent rock cleavage
e. foliated and composed predominately of platy materials
f. composed of alternating bands of light and dark silicate minerals
g. hard and nonfoliated; resulting from contact metamorphism
Refer to Figure 7.5 showing the formation of foliation in response to
differential stress. Select a location on Earth where rocks may be
2
1
The grade of metamorphism is reflected in the texture and mineral
content of metamorphic rocks. During regional metamorphism,
rocks typically display a preferred orientation called foliation.
Foliation develops as platy or elongated minerals are rotated into
parallel alignment, recrystallize to form new grains that exhibit a
preferred orientation, or are plastically deformed into flattened
grains that exhibit a planar alignment. Rock cleavage is a type of
foliation in which rocks split cleanly into thin slabs along
surfaces where platy minerals are aligned. Schistosity is a type
of foliation defined by the parallel alignment of medium- to
coarse-grained platy minerals. During high-grade metamorphism,
ion migrations can cause minerals to segregate into distinct layers
or bands. Metamorphic rocks with a banded texture are called
gneiss. Metamorphic rocks composed of only one mineral
forming equidimensional crystals often appear nonfoliated. Marble
(metamorphosed limestone) is often nonfoliated. Further,
metamorphism can cause the transformation of low-temperature
minerals into high-temperature minerals and, through the
introduction of ions from hydrothermal solutions, generate new
minerals, some of which form economically important metallic
ore deposits.
Common foliated metamorphic rocks include slate, phyllite,
various types of schists (e.g., garnet-mica schist), and gneiss.
Nonfoliated rocks include marble (parent rock—limestone) and
quartzite (most often formed from quartz sandstone).
The four geologic environments in which metamorphism
commonly occurs are (1) contact or thermal metamorphism,
(2) hydrothermal metamorphism, (3) burial and subduction zone metamorphism, and (4) regional metamorphism. Contact metamorphism
occurs when rocks are in contact with an igneous body, resulting
in the formation of zones of alteration around the magma called
aureoles. Most contact metamorphic rocks are fine-grained, dense,
tough rocks of various chemical compositions. Because directional
pressure is not a major factor, these rocks are not generally foliated. Hydrothermal metamorphism occurs where hot, ion-rich
fluids circulate through rock and cause chemical alteration of the
constituent minerals. Most hydrothermal alteration occurs along
the mid-ocean ridge system, where seawater migrates through hot
oceanic crust and chemically alters newly formed basaltic rocks.
Metallic ions that are removed from the crust are eventually carried to the floor of the ocean, where they precipitate from black
smokers to form metallic deposits, some of which may be economically important. Regional metamorphism takes place at considerable depths over an extensive area and is associated with the
process of mountain building. A gradation in the degree of change
usually exists in association with regional metamorphism, in
which the intensity of metamorphism (low- to high-grade) is
reflected in the texture and mineral content of the rocks. In the
most extreme metamorphic environments, rocks called migmatites
fall into a transition zone somewhere between “true” igneous rocks
and “true” metamorphic rocks.
194
Key Terms
aureole (p. 188)
burial metamorphism (p. 190)
confining pressure (p. 180)
contact metamorphism (p. 188)
differential stress (p. 180)
foliation (p. 182)
gneissic texture (p. 184)
hydrothermal metamorphism
(p. 189)
hydrothermal solutions (p. 189)
impact metamorphism (p. 191)
index minerals (p. 192)
metamorphism (p. 178)
metasomatism (p. 181)
migmatites (p. 193)
nonfoliated (p. 184)
parent rock (p. 178)
porphyroblastic textures (p. 184)
regional metamorphism (p. 190)
rock cleavage (p. 183)
schistosity (p. 184)
shock metamorphism (p. 191)
slaty cleavage (p. 183)
subduction zone metamorphism
(p. 190)
texture (p. 181)
thermal metamorphism (p. 188)
G I V E I T S O M E T H O U G H T
experiencing or may have experienced the metamorphic conditions
necessary to form foliation. Describe the location, and explain what
is happening or has happened that would result in foliated rocks.
Refer to Figure 7.11, the classification chart for common metamorphic rocks, and answer the following:
a. Identify a case in which the metamorphic rock may be stronger
than its parent rock, and explain why.
b. Identify a case in which the metamorphic rock may be weaker
than its parent rock, and explain why.
c. In general, do you think metamorphic rocks are stronger or
weaker than igneous rocks? How about sedimentary rocks?
Explain your answers.
3
Each of the following statements describes one or more characteristics of a particular metamorphic rock. For each statement, identify
the metamorphic rock that is being described:
a. calcite-rich and often nonfoliated
b. loosely coherent and composed of broken fragments that formed
along a fault zone
c. represents a grade of metamorphism between slate and schist
d. very fine-grained and foliated; excellent rock cleavage
e. foliated and composed predominately of platy materials
f. composed of alternating bands of light and dark silicate minerals
g. hard and nonfoliated; resulting from contact metamorphism
Refer to Figure 7.5 showing the formation of foliation in response to
differential stress. Select a location on Earth where rocks may be
2
1
The grade of metamorphism is reflected in the texture and mineral
content of metamorphic rocks. During regional metamorphism,
rocks typically display a preferred orientation called foliation.
Foliation develops as platy or elongated minerals are rotated into
parallel alignment, recrystallize to form new grains that exhibit a
preferred orientation, or are plastically deformed into flattened
grains that exhibit a planar alignment. Rock cleavage is a type of
foliation in which rocks split cleanly into thin slabs along
surfaces where platy minerals are aligned. Schistosity is a type
of foliation defined by the parallel alignment of medium- to
coarse-grained platy minerals. During high-grade metamorphism,
ion migrations can cause minerals to segregate into distinct layers
or bands. Metamorphic rocks with a banded texture are called
gneiss. Metamorphic rocks composed of only one mineral
forming equidimensional crystals often appear nonfoliated. Marble
(metamorphosed limestone) is often nonfoliated. Further,
metamorphism can cause the transformation of low-temperature
minerals into high-temperature minerals and, through the
introduction of ions from hydrothermal solutions, generate new
minerals, some of which form economically important metallic
ore deposits.
Common foliated metamorphic rocks include slate, phyllite,
various types of schists (e.g., garnet-mica schist), and gneiss.
Nonfoliated rocks include marble (parent rock—limestone) and
quartzite (most often formed from quartz sandstone).
The four geologic environments in which metamorphism
commonly occurs are (1) contact or thermal metamorphism,
(2) hydrothermal metamorphism, (3) burial and subduction zone metamorphism, and (4) regional metamorphism. Contact metamorphism
occurs when rocks are in contact with an igneous body, resulting
in the formation of zones of alteration around the magma called
aureoles. Most contact metamorphic rocks are fine-grained, dense,
tough rocks of various chemical compositions. Because directional
pressure is not a major factor, these rocks are not generally foliated. Hydrothermal metamorphism occurs where hot, ion-rich
fluids circulate through rock and cause chemical alteration of the
constituent minerals. Most hydrothermal alteration occurs along
the mid-ocean ridge system, where seawater migrates through hot
oceanic crust and chemically alters newly formed basaltic rocks.
Metallic ions that are removed from the crust are eventually carried to the floor of the ocean, where they precipitate from black
smokers to form metallic deposits, some of which may be economically important. Regional metamorphism takes place at considerable depths over an extensive area and is associated with the
process of mountain building. A gradation in the degree of change
usually exists in association with regional metamorphism, in
which the intensity of metamorphism (low- to high-grade) is
reflected in the texture and mineral content of the rocks. In the
most extreme metamorphic environments, rocks called migmatites
fall into a transition zone somewhere between “true” igneous rocks
and “true” metamorphic rocks.