Download Metamorphic Rock 4

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CHAPTER 6
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Rocks
SECTION
4 Metamorphic Rock
KEY IDEAS
As you read this section, keep these questions in mind:
• What happens during metamorphism?
• How are regional metamorphism and contact metamorphism different?
• What are foliated and nonfoliated metamorphic
rocks?
What Is Metamorphism?
Heat, pressure, and hot fluids can change the chemical composition of rock. They can cause minerals to
react with one another and form new minerals. They can
cause mineral crystals to change size or shape. They can
even cause mineral molecules to move through the rock.
Molecules of similar minerals move together and form
bands, or stripes, of mineral crystals.
Metamorphism is the process in which heat, pressure, or hot fluids change the chemical composition of
rock. Rock that has been affected by metamorphism is
called metamorphic rock. All metamorphic rock forms
from sedimentary, igneous, or other metamorphic rock.
What Are the Two Types of Metamorphism?
READING TOOLBOX
Ask Questions Read this
section silently. In your
notebook, write down
questions that you have
about the section. Work with
a partner or a small group
to figure out the answers to
your questions.
READING CHECK
1. Define What is
metamorphism?
Two main types of metamorphism happen in Earth’s
crust. One type is contact metamorphism. Another type is
regional metamorphism.
CONTACT METAMORPHISM
Contact metamorphism can happen when magma
moves through cracks underground. The magma heats
the rock nearby. The heat can cause metamorphism.
Critical Thinking
2. Apply Concepts The rock
that is nearest the magma
may melt. Would this melted
rock become metamorphic
rock? Explain your answer.
Magma
Heat from the magma
can change the chemical
composition of nearby rock.
Contact Metamorphism
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Metamorphic Rock continued
REGIONAL METAMORPHISM
READING CHECK
3. Describe What causes the
heat and pressure that lead
to regional metamorphism?
Earth’s lithosphere is broken up into large pieces
called tectonic plates. The tectonic plates are constantly
moving. In some places, two plates collide, or come
together. When the plates collide, they produce a lot
of heat and pressure. The heat and pressure can cause
metamorphism. This process is known as regional
metamorphism.
Regional metamorphism affects rocks in a much larger
area than contact metamorphism does. However, regional
metamorphism produces so much heat that it can cause
some rock to melt. The magma can cause some contact
metamorphism. Therefore, rocks formed by contact
metamorphism are often found in areas that also contain
rocks formed by regional metamorphism.
How Do Scientists Classify Metamorphic
Rocks?
READING CHECK
4. Identify What are the two
main types of metamorphic
rock?
The main way that scientists classify metamorphic
rocks is by the textures of the rocks. There are two main
types of metamorphic rock: foliated metamorphic rock
and nonfoliated metamorphic rock.
FOLIATED METAMORPHIC ROCK
Most kinds of metamorphic rocks have layers or
bands. These bands form when pressure and temperature
cause mineral molecules in the rock to move. Molecules
of similar minerals move together and form larger
crystals. This type of rock texture is known as foliation.
Metamorphic rocks that show foliation are called foliated
metamorphic rocks.
In many cases, light-colored minerals and dark-colored
minerals separate to form different bands. Therefore,
many foliated metamorphic rocks have light and dark
bands, as shown in the photo below.
LOOKING CLOSER
5. Explain How can you tell
that the rock in the photo is a
foliated metamorphic rock?
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Metamorphic Rock continued
NONFOLIATED METAMORPHIC ROCK
Some kinds of metamorphic rock do not show foliation. Metamorphic rocks that do not show foliation are
called nonfoliated metamorphic rocks. Most nonfoliated metamorphic rocks form from rocks that were made
mainly of only one mineral. Metamorphism causes the
mineral crystals in the original rock to grow larger.
Nonfoliated metamorphic rocks can also form from
rocks that contain mainly square or rounded mineral
crystals. All the sides of those crystals are about the same
length. Therefore, pressure from only one side does not
cause the crystals to move or shift. Because the minerals
do not move, they cannot form bands.
The table below gives some examples of foliated and
nonfoliated metamorphic rocks.
Rock Name
Description
Description
Slate
foliated rock with
very small mineral
crystals that form
thin, flat sheets
Pressure acts on the sedimentary rock
shale. The minerals in the shale are
compressed into thin layers.
Schist
foliated rock with
relatively thick
layers; contains
large amounts of
mica
Large amounts of heat and pressure act
on slate. The heat and pressure cause
minerals to react and form layers of mica
and other minerals.
Gneiss
foliated rock with
very thick, dark
and light layers
Very high heat and pressure act on schist.
Light-colored minerals separate from
dark-colored minerals to form bands.
Quartzite
nonfoliated rock
made up mainly
of the mineral
quartz; very hard
and strong
Heat and pressure cause quartz in
sandstone to recrystallize, or grow new
crystals.
Marble
nonfoliated
rock made up
mainly of calcium
carbonate;
relatively hard,
but breaks down
easily in acid rain
Heat and pressure cause calcite in
limestone to recrystallize.
READING CHECK
6. Describe How does
metamorphism affect a rock
that is made up of only one
mineral?
LOOKING CLOSER
7. Identify Which of the
foliated metamorphic rocks
forms under the highest heat
and pressure?
The texture and composition of a metamorphic rock
give clues to how it formed. Therefore, scientists can use
information from metamorphic rocks to learn about the
geologic history of an area.
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Section 4 Review
SECTION VOCABULARY
contact metamorphism a change in the texture,
structure, or chemical composition of a rock
due to contact with magma
foliation the metamorphic rock texture in which
mineral grains are arranged in planes or bands
metamorphism the process in which one type of
rock changes into metamorphic rock because
of chemical processes or changes in temperature and pressure
nonfoliated the metamorphic rock texture in
which mineral grains are not arranged in
planes or bands
regional metamorphism a change in the texture,
structure, or chemical composition of a rock
due to changes in temperature and pressure
over a large area, generally as a result of
tectonic forces
1. Compare Complete the Venn diagram below to compare regional metamorphism
and contact metamorphism.
Regional
Metamorphism
Contact
Metamorphism
2. Identify What are three ways a rock can change during metamorphism?
3. Infer The Himalaya Mountains are found where two tectonic plates collide. Does
most of the metamorphic rock in that area probably occur in small patches or in
wide regions? Explain your answer.
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