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CHAPTER 4
LESSON 4
Rocks
Metamorphic Rocks
Key Concepts
What do you think? Read the two statements below and decide
whether you agree or disagree with them. Place an A in the Before column
if you agree with the statement or a D if you disagree. After you’ve read
this lesson, reread the statements to see if you have changed your mind.
Before
Statement
• How do metamorphic
rocks form?
• How do types of
metamorphic rock differ?
After
7. With the right pressure and temperature
conditions, minerals in a rock can change
shape without breaking or melting.
8. Metamorphic rocks have layers that form as
minerals melt and then recrystallize.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Metamorphic Rock Formation
Imagine that you left home with a cheese sandwich in your
backpack. You threw your backpack into your hot locker at
school. Would the sandwich look the same by lunchtime? The
cheese would likely be soft because of changes in temperature.
Also, your heavy backpack could squish the sandwich. Like the
sandwich, metamorphic rocks are affected by changes in
temperature and pressure. Metamorphism is any process that
affects the structure or composition of a rock in a solid state as a result
of changes in temperature, pressure, or the addition of chemical fluids.
Most metamorphic rocks form far below Earth’s surface.
Deep within Earth’s crust and mantle, temperatures and
pressures are high. Metamorphic rocks, like igneous rocks,
form under high temperature and pressure conditions.
Unlike igneous rocks, metamorphic rocks do not crystallize
from magma.
If you compare metamorphic rocks to sedimentary rocks,
you will notice differences. Sedimentary rocks form on Earth’s
surface, while metamorphic rocks form beneath Earth’s
surface. Erosion and deposition contribute to the formation
of sedimentary rocks. These processes are not part of the
formation of metamorphic rocks.
Reading Essentials
Identify Main Ideas Write
a phrase beside each
paragraph to summarize the
main point of the paragraph.
After you have read the
lesson, review the phrases
you have written.
Reading Check
1. Define What is
metamorphism?
Rocks
59
Temperature and Pressure
When rocks undergo high temperatures and intense
pressure, they have properties similar to plastic. The rocks
can bend and fold without melting. A permanent change in
shape by bending and folding is called plastic deformation. It is one
way that a rock’s texture changes during metamorphism.
Plastic deformation is common during uplifting events when
tectonic plates collide and form mountains. Changes in
composition and structure are clues that a rock has
metamorphosed.
Key Concept Check
The parent rock is the rock that changes during
metamorphism. The parent rock’s composition determines
the temperature at which the rock will metamorphose. Some
metamorphic rock forms at temperatures between 150°C and
200°C. Like temperature, pressure increases with depth. The
relationship between pressure and depth in Earth’s crust
and mantle is shown in the graph below. Pressure is
measured in kilobars (kb).
2. Describe Under what
conditions do metamorphic
rocks form?
Pressure v. Depth
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10
20
Math Skills
The line graph represents
pressure below Earth’s
surface. What is the pressure
at a depth of 50 km?
a. Read the title of the
graph to determine what
data are represented.
b. Read the labels on the
x-axis and y-axis to
determine the units.
c. Move horizontally from
50 km to the diagonal line.
Move vertically from the
diagonal line to the x-axis.
The pressure is 14 kb.
3. Use Graphs At what
depth is the pressure 20 kb?
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Rocks
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50
60
70
80
90
100
4
8
12 16 20
Pressure (kb)
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Metamorphic Rock Identification
Changes in temperature, pressure, or the addition of
chemical fluids can result in the rearrangement of minerals
or the formation of new minerals in a metamorphic rock.
Geologists study the texture and composition of minerals to
identify metamorphic rocks.
Metamorphic rocks are classified into two groups based
on texture. Pressure can cause minerals to align and form
layers in metamorphic rocks. The layers look similar to the
layers in clastic sedimentary rock. The mineral content helps
geologists tell the difference between metamorphic and
sedimentary rock.
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Depth (km)
30
Foliated Rocks
The metamorphic rock schist is an example of a foliated
rock. Foliated rocks contain parallel layers of flat and elongated
minerals. Often, the layers are different colors. The layers
develop from an uneven distribution of pressure during
metamorphism. Foliation is a common feature of
metamorphic rock.
Nonfoliated Rocks
Nonfoliated rocks are metamorphic rocks that have mineral
grains with a random, interlocking texture. In nonfoliated rocks,
the minerals do not line up. Instead, the crystals are shaped
like blocks and are all about the same size. The minerals are
generally the same color, which helps distinguish them from
igneous rocks such as granite.
Reading Check
4. Name What type of
metamorphic rock has layers?
Contact and Regional Metamorphism
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
One way that a nonfoliated rock forms is through
contact with magma. During contact metamorphism, magma
comes in contact with existing rock, and its thermal energy and
gases interact with the surrounding rock to make new metamorphic
rock. Contact metamorphism can increase crystal size. It can
also form new minerals and change the rock. Marble is a
common example of a nonfoliated rock. Marble has a
uniform color and crystal size.
Regional metamorphism is the formation of metamorphic rock
bodies that are hundreds of square kilometers in size. This process
can create an entire mountain range of metamorphic rock.
Changes in temperature and pressure and the presence of
chemical fluids act on large volumes of rock and produce
metamorphic textures. Geologists study these textures to
understand more about how mountains formed. Regional
metamorphism is associated with the formation of the
Himalayas in Asia and the Appalachian Mountains in the
eastern United States.
Make a vertical two-tab book
to organize your notes on
contact and regional
metamorphism.
Contact
Metamorphism
Regional
Metamorphism
Key Concept Check
5. Compare and contrast
contact metamorphism and
regional metamorphism.
Reading Essentials
Rocks
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Mini Glossary
contact metamorphism: a process in which magma comes
in contact with existing rock, and its thermal energy and
gases interact with the surrounding rock to make new
metamorphic rock
nonfoliated rock: a metamorphic rock that has mineral
grains with a random, interlocking texture
plastic deformation: a permanent change in shape by
bending and folding
foliated rock: a rock that contains parallel layers of flat and
elongated minerals
regional metamorphism: the formation of metamorphic
rock bodies that are hundreds of square kilometers in size
metamorphism: any process that affects the structure or
composition of a rock in a solid state as a result of changes
in temperature, pressure, or the addition of chemical fluids
1. Review the terms and their definitions in the Mini Glossary. Write two sentences that
describe the difference between a foliated rock and a nonfoliated rock.
2. Identify three forces that can change rock into new metamorphic rock.
3. Why is the pressure greater on a rock that is deep underground than on a rock that is
close to Earth’s surface?
What do you think
Reread the statements at the beginning of the
lesson. Fill in the After column with an A if you
agree with the statement or a D if you disagree.
Did you change your mind?
62
Rocks
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END OF
LESSON
Reading Essentials
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Metamorphic
Rock