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KEY CONCEPT
Metamorphic rocks form
as existing rocks change.
BEFORE, you learned
NOW, you will learn
• Igneous rocks form as molten
rock cools
• Sedimentary rocks form from
earlier rocks
• How a rock can change into
another type of rock
• How new minerals can grow
in existing rocks
VOCABULARY
THINK ABOUT
metamorphism p. 96
recrystallization p. 97
foliation p. 100
How does a rock change into another
kind of rock?
Examine a sample of shale and a sample
of schist (shihst). Shale, a sedimentary
rock, can change into schist. Think
about how this change could occur
without the shale’s melting or breaking
apart. Make a prediction about what
process changes shale into schist.
shale
schist
Heat and pressure change rocks.
When you cook popcorn, you use heat to increase the pressure within
small, hard kernels until they explode into a fluffy snack. Cooking
popcorn is just one example of the many ways in which heat and
pressure can change the form of things—even things like rocks.
reading tip
Rocks change into other
rocks by the process of
metamorphism. A similar
word, metamorphosis,
refers to what happens
when a caterpillar changes
into a butterfly.
The process in which an existing rock is changed by heat or
pressure—or both—is called metamorphism (MEHT-uh-MAWR-FIHZuhm). The original sedimentary or igneous rock is called the parent
rock. The resulting rock is a metamorphic rock. Even a metamorphic
rock can be a parent rock for another type of metamorphic rock.
Many of the metamorphic rocks people use were once sedimentary rocks. Limestone is the parent rock of marble, which is used by
builders and artists. Shale can be the parent rock of schist, which can
be a source of the gemstone garnet. Some schists are a source of the
mineral graphite, which is used in pencils.
check your reading
A
96 Unit: Earth’s Surface
Give an example of a way people use metamorphic rocks.
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During metamorphism, rocks undergo many changes. One type of
change occurs when pressure causes a rock’s minerals to flatten out in
one direction. Other changes can occur in a rock’s minerals, but the rock
remains solid. Rocks do not melt when they undergo metamorphism.
If the temperature gets high enough to melt the rock, the end result is
an igneous rock, not a metamorphic rock.
Heat and pressure can break the bonds that join atoms in minerals.
Then the atoms can join together differently as new bonds form. This
process is called recrystallization. It has two main results. First, individual mineral crystals can grow larger as more atoms join their
crystal structures. Second, atoms can combine in different ways, and
new minerals can form in place of older ones. For example, shale is a
sedimentary rock that is formed from silt and clay. During recrystallization, garnet can form from these materials.
How Rocks Change
Because pressure and temperature increase with depth, rocks
change when they are buried deeper in the crust.
1
Shale is a sedimentary rock that forms
near the surface. It can be buried deeper
as blocks of the crust push together.
2
Shale changes to slate as pressure causes
the minerals to line up in layers. Mica starts
to grow as recrystallization begins.
increasing pressure and temperature
3
Slate changes to phyllite (FIHL-YT) deeper
in the crust, where the temperature and
pressure are higher. Phyllite is shiny
because more mica has grown.
4
At even higher temperature and pressure,
phyllite changes to schist. As recrystallization increases, completely new types
of minerals replace older ones.
5
Deep within the crust, schist changes to
gneiss (nys). During recrystallization, light
and dark minerals separate into bands.
Changes are so great that all traces of the
original shale are gone.
Chapter 3: Rocks 97
A
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Metamorphic Changes
How can pressure and temperature change
a solid?
SKILL FOCUS
Modeling
PROCEDURE
1
Use a vegetable peeler to make a handful of wax shavings of three
different colors. Mix the shavings.
2 Use your hands to warm the shavings, and then squeeze them
into a wafer.
MATERIALS
• 3 candles of
different colors
• vegetable peeler
TIME
WHAT DO YOU THINK?
• Describe what happened to the wax shavings.
10 minutes
• How do the changes you observed resemble metamorphic
changes in rocks?
CHALLENGE What changes that occur
in metamorphic rocks were you unable to
model in this experiment?
Metamorphic changes occur over large
and small areas.
The types of metamorphic changes that occur depend on the types of
parent rocks and the conditions of temperature and pressure. When
both high temperature and high pressure are present, metamorphic
changes can occur over very large areas. When only one of these
conditions is present, changes tend to occur over smaller areas.
Change over Large Areas
Most metamorphic changes occur over large areas in which both
temperature and pressure are high. An example is a region where large
blocks of rock are pressing together and pushing up mountain ranges.
This process can affect an area hundreds of kilometers wide and tens
of kilometers deep. In such an area, rocks are buried, pressed together,
bent, and heated. The pressure and heat cause the rocks to undergo
metamorphism. Generally, the deeper below the surface the rocks are,
the greater the metamorphic changes that occur in them. For example,
a sedimentary rock may change to slate near the surface but become
gneiss deep inside a mountain.
check your reading
A
98 Unit: Earth’s Surface
Where can metamorphic changes occur over large areas?
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Change over Small Areas
Some metamorphic changes occur over small areas. For example,
magma can push into rocks underground, or surface rock can be covered
by a lava flow. The magma or lava heats the rock it is in contact with,
causing recrystallization. These changes are mainly due to high
temperature, not pressure. The rocks get roasted but not squeezed.
The thickness of rock changed by the heat can range from less than
one meter to several hundred meters, depending on the amount and
temperature of the molten rock.
Small areas of metamorphic rock can also be formed by high
pressure alone. At or near Earth’s surface, rocks move and grind past
one another during earthquakes. Rocks that grind together in this way
can be subjected to high pressures that cause metamorphic changes.
RESOURCE CENTER
CLASSZONE.COM
Find information on
metamorphic rocks.
Metamorphic Changes
Changes can occur over hundreds of kilometers or over just a
few centimeters.
Changes over Large Areas
Forces within Earth start to press rock layers
together over hundreds of kilometers.
Heat and pressure change the rock
layers that make up the mountains
into metamorphic rocks.
Changes over Small Areas
Magma can push into rock layers and
cause changes over areas ranging from
a few centimeters to tens of meters.
The magma is hot enough to bake the
surrounding rocks into metamorphic rocks.
Compare how heat and pressure cause changes over the large and small areas
shown above.
Chapter 3: Rocks 99
A
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Most metamorphic rocks develop bands
of minerals.
VOCABULARY
Add a magnet word
diagram for foliation
to your notebook.
Some buildings have floors covered with tiles of the metamorphic
rock slate. This rock is especially useful for tiles because it displays
foliation, a common property of metamorphic rocks. Foliation is
an arrangement of minerals in flat or wavy parallel bands. Slate can
be split into thin sheets along the boundaries between its flat bands
of minerals.
You may be familiar with the word foliage. Both foliage and foliation
come from the Latin word folium, meaning “leaf.” Foliated rocks either
split easily into leaflike sheets or have bands of minerals that are lined
up and easy to see.
Foliated Rocks
Foliation develops when rocks are under pressure. Foliation is common
in rocks produced by metamorphic changes that affect large areas.
However, as you will see, a metamorphic rock that consists almost
entirely of one type of mineral does not show foliation.
Foliation in Metamorphic Rocks
Foliated
Metamorphic rocks that contain several minerals
develop foliation under pressure.
phyllite
Using a microscope, you can
see that the minerals are lined
up in bands.
Phyllite is a foliated metamorphic rock that
contains several types of minerals.
Nonfoliated
marble
Marble is a nonfoliated metamorphic rock that
consists almost entirely of only one mineral.
The mineral crystals in this rock
are not lined up.
Compare the pictures of the minerals in the foliated rock and the
nonfoliated rock. What is different about their arrangements?
A
100 Unit: Earth’s Surface
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Foliation develops when minerals flatten out or line up in bands.
At low levels of metamorphism, the bands are extremely thin, as in slate.
With higher pressure and temperature, the mineral mica can grow and
make the rock look shiny, as is common in phyllite and schist. At even
higher levels of metamorphism, the minerals in the rock tend to
separate into light and dark bands, like those in gneiss.
check your reading
How do rocks change as foliation develops?
Nonfoliated Rocks
Metamorphic rocks that do not show foliation are called nonfoliated
rocks. One reason a metamorphic rock may not display foliation is that
it is made up mainly of one type of mineral, so that different minerals
cannot separate and line up in layers. One common nonfoliated metamorphic rock is marble, which develops from limestone. Marble is used
as a decorative stone. It is good for carving and sculpting. Because
marble is nonfoliated, it does not split into layers as an artist is working
with it. Another example of a nonfoliated rock is quartzite. It forms
from sandstone that is made up almost entirely of pieces of quartz.
Another reason that a metamorphic rock may lack foliation is that
it has not been subjected to high pressure. Hornfels is a metamorphic
rock that can form when a rock is subjected to high temperatures.
Hornfels, which often forms when magma or lava touches other rock,
is nonfoliated.
check your reading
What are two reasons a metamorphic rock might not
show foliation?
KEY CONCEPTS
CRITICAL THINKING
1. What conditions can cause
a sedimentary or igneous
rock to change into a
metamorphic rock?
4. Draw Conclusions Would
gneiss be more likely to form
at shallow depths or at great
depths where mountains are
being pushed up? Why?
2. How do new minerals grow
within existing rocks?
3. Why do bands of minerals
develop in most metamorphic
rocks?
5. Infer Would you expect to
find foliated or nonfoliated
metamorphic rocks next to a
lava flow? Why?
CHALLENGE
6. Synthesize What features of
sedimentary rocks are unlikely
to be found in metamorphic
rocks? What features of metamorphic rocks do not occur in
sedimentary rocks?
Chapter 3: Rocks 101
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