Download Igneous Rock 2

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CHAPTER 6
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Rocks
SECTION
2 Igneous Rock
KEY IDEAS
As you read this section, keep these questions in mind:
• What three factors affect whether rock melts?
• How does the cooling rate of magma or lava affect
the texture of igneous rocks?
• What are the possible textures and compositions of
igneous rock?
• What structures can igneous rock form?
How Does Magma Form?
Remember that igneous rock forms when melted
rock (magma or lava) cools and hardens. Magma forms
when rock is heated. The heat makes the minerals in
the rock melt. However, not all rocks melt at the same
temperature. There are three main factors that affect
whether a rock will melt at a certain temperature. Those
factors are
• the chemical composition of the rock
• the pressure on the rock
• whether there are any fluids in the rock
READING TOOLBOX
Identify Main Ideas As you
read this section, underline
the main idea in each
paragraph. When you finish
reading, write a summary
of the section using the
underlined ideas.
READING CHECK
Imagine mixing sugar crystals and ice together in a pan
on a hot stove. The ice would melt at a much lower temperature than the sugar would. Just as ice and sugar melt
at different temperatures, different minerals melt at different temperatures. The chemical composition of a mineral affects the temperature at which it melts. Therefore,
the minerals that make up a rock affect how hot the rock
has to become before it will melt.
The pressure on a rock also affects whether it will
melt. In general, the lower the pressure on a rock is, the
lower the temperature the rock will melt at. Therefore,
lowering the pressure on a hot rock can make it melt.
A third factor that affects whether a rock melts is
whether there are any fluids in the rock. Fluids cause a
rock to melt at a lower temperature. Therefore, adding
fluid to a rock can make the rock melt. Also, rocks that
contain fluids tend to melt at lower temperatures than
rocks that do not contain fluids.
1. List What are the three
factors that affect whether
a rock will melt at a certain
temperature?
READING CHECK
2. Describe What may
happen to a hot rock if the
pressure on it decreases?
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Igneous Rock continued
PARTIAL MELTING
Different minerals melt at different temperatures. As
the rock heats up, the minerals with the lowest melting
temperatures will melt first. The higher the temperature
becomes, the more minerals will melt. As each mineral
in the rock melts, the elements in it enter the magma.
As a result, the composition of the magma changes. The
process in which different minerals in a rock melt at
different temperatures is called partial melting.
Mineral
LOOKING CLOSER
Magma
3. Explain What causes the
composition of the magma
to change?
A solid rock is made up of
several different minerals.
In this picture, each mineral is a different color.
When the rock begins
to heat up, some of
the minerals melt.
As the temperature rises,
more minerals melt.
As the minerals melt,
the composition of the
magma changes.
FRACTIONAL CRYSTALLIZATION
Critical Thinking
4. Infer A scientist is
studying the rock that formed
when a magma chamber
cooled completely. Where
in the rock will the scientist probably find minerals
that form early in Bowen’s
reaction series?
Remember that Bowen’s reaction series describes the
order in which minerals form from magma. As magma
cools, different minerals crystallize. The minerals that
were the last to melt generally crystallize first. The
minerals that melted first crystallize last. The formation
of different minerals as magma cools is called fractional
crystallization.
When magma cools, it does not always form the same
minerals that originally melted to form the magma. The
elements in the magma can combine in new ways to form
new minerals.
Most magma is found underground in large holes called
magma chambers. The crystals that form at the beginning
of fractional crystallization can sink to the bottom of the
magma chamber. They can also stick to the sides or top of
the magma chamber.
Crystals take time to grow. The slower the magma in
the chamber cools, the larger the crystals that grow from
it are. Because the composition of the magma changes as
it cools, the composition of the crystal might change as it
grows. A crystal that forms this way will have a different
composition on its outside than on its inside.
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Igneous Rock continued
What Affects the Texture of an Igneous
Rock?
Many igneous rocks contain mineral crystals. The
texture of an igneous rock is a description of the size of
the crystals in it. The main factor that affects the texture
of an igneous rock is how quickly it forms.
Remember that melted rock below Earth’s surface is
called magma, and melted rock above Earth’s surface is
called lava. Igneous rock can form from either magma
or lava. If an igneous rock forms from magma, which
cools underground, it is an intrusive igneous rock. If
an igneous rock forms from lava, which cools on Earth’s
surface, it is an extrusive igneous rock.
Intrusive and extrusive igneous rocks generally
look very different. Because magma cools slowly, large
crystals can form. Therefore, intrusive igneous rocks
generally contain large mineral crystals. Igneous rocks
that contain large crystals have a coarse-grained texture.
Granite and gabbro are examples of coarse-grained
igneous rocks.
In contrast, lava generally cools very quickly. Only
small crystals can form. Therefore, extrusive igneous
rocks generally contain only very small mineral crystals. Igneous rocks that contain small crystals have a
fine-grained texture. Rhyolite and basalt are examples
of fine-grained igneous rocks.
Extrusive igneous rocks may also have porphyritic,
glassy, or vesicular textures. The table below describes
each of these kinds of textures.
Texture
Description
How It Forms
Porphyritic
some large
mineral crystals
mixed with
many small
crystals
Magma begins to cool some kinds of
slowly, and some large granite and basalt
crystals form. The
magma erupts and
finishes cooling quickly,
producing small
crystals.
Glassy
looks like
glass; contains
no crystals
Lava cools so quickly
that crystals cannot
form.
obsidian
fine-grained
or glassy rock
with tiny holes
in it
Gas in lava bubbles
out as it cools, like the
bubbles in a soda. As
the lava hardens, the
bubbles are frozen in
place.
pumice
Vesicular
READING CHECK
5. Identify What is the main
factor that affects the texture
of an igneous rock?
READING CHECK
6. Explain Why do intrusive
igneous rocks generally contain large mineral crystals?
Example
Critical Thinking
7. Apply Concepts Do
the minerals that form the
large crystals in porphyritic
rocks probably form at
high temperatures or low
temperatures?
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Igneous Rock continued
What Compositions Can Igneous Rock Have?
READING CHECK
8. Identify What are the
three groups of igneous rock
compositions?
Talk About It
Make Connections With a
partner, discuss the meaning
of the word intermediate. If
you wish, look it up in a dictionary. Together, talk about
why some igneous rocks are
said to have an intermediate
composition.
LOOKING CLOSER
9. Compare Which type of
rock has the largest amount
of silica: diorite, gabbro, or
granite?
Different igneous rocks contain different minerals.
The chemical composition of the magma or lava affects
the minerals that form. Scientists have defined three main
groups of igneous rock compositions: felsic, mafic, and
intermediate.
The main difference between the three types of rock
is the amount of silica they contain. Silica refers to the
ratio of silicon-oxygen tetrahedrons to other elements
in a mineral. Minerals that are high in silica have large
ratios of silicon-oxygen tetrahedrons to other elements.
Minerals that are low in silica have low ratios of siliconoxygen tetrahedrons to other elements.
Felsic igneous rocks contain large amounts of silica.
Mafic igneous rocks contain smaller amounts of silica.
They also contain large amounts of iron and magnesium.
Intermediate igneous rocks contain less silica than felsic
rocks, but more silica than mafic rocks. The table below
summarizes the three main igneous rock compositions.
Composition
Description
Common
Minerals
Examples
Felsic
high in silica;
generally
light-colored
quartz, some
kinds of feldspar
and mica
granite, rhyolite,
obsidian, pumice
Mafic
low in silica;
high in iron and
magnesium;
generally
dark-colored
olivine, some
kinds of feldspar,
amphibole
basalt, gabbro
Intermediate
higher in silica
than mafic rocks;
lower in silica
than felsic rocks;
may be lightcolored or darkcolored
some kinds of
feldspar and
mica, pyroxene,
amphibole
andesite, diorite
What Structures Can Intrusive Rock Form?
Remember that intrusive igneous rocks form when
magma cools underground. Magma can move through
cracks and gaps in rock underground. Therefore, intrusive igneous rocks can be found in many different places.
A body of intrusive igneous rock is called an intrusion.
Batholiths, stocks, laccoliths, sills, dikes, and volcanic
necks are examples of intrusions.
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Igneous Rock continued
Volcanic necks form
when magma inside
a volcano cools
and hardens.
A dike forms when
magma cuts across
several rock layers.
A laccolith forms when
magma flows between
two rocks layers and
pushes one of them
up to form a dome.
A sill forms when
magma flows between
two rock layers.
Talk About It
Use Logic The word root lith
comes from a Greek word
that means “rock.” With a
partner, discuss the words
you have learned so far that
include the root lith. Talk
about why that word root is
appropriate for each of the
words.
Dike
LOOKING CLOSER
A stock is simialar
to a batholith, but has
an area less than 100 km2.
A batholith is an intrusion
that has an area greater
than 100 km2.
10. Describe What is the
main difference between a
stock and a batholith?
What Structures Can Extrusive Rock Form?
A body of extrusive igneous rock is called an
extrusion. The most common kind of extrusion is a
lava flow. A lava flow forms when lava moves out of a
volcano, cools, and hardens. The lava flow forms flat
masses of rock.
In some areas, a volcano has erupted many times.
The lava flows have built up to form a lava plateau. A
lava plateau is made up of many thick layers of extrusive
igneous rock.
Some volcanoes do not give off large amounts of lava.
Instead, they mainly give off volcanic ash and dust. The
ash and dust can build up to form a type of rock called
tuff. Because ash and dust are light, wind can carry them
long distances. They can form thick layers of tuff that
cover very large areas. These layers of tuff are called tuff
deposits.
READING CHECK
11. Describe How does a
lava flow form?
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Section 2 Review
SECTION VOCABULARY
extrusive igneous rock rock that forms from
the cooling and solidification of lava at Earth’s
surface
felsic describes magma or igneous rock that is
rich in feldspars and silica and that is generally
light in color
igneous rock rock that forms when magma
cools and solidifies
intrusive igneous rock rock formed from the
cooling and solidification of magma beneath
Earth’s surface
mafic describes magma or igneous rock that is
rich in magnesium and iron and that is generally dark in color
1. Describe Complete the concept map below to describe how scientists classify
igneous rocks. Use the terms composition, texture, fine grained, coarse grained,
mafic, felsic, and intermediate.
can be
Igneous
rocks
classified by
can be
2. Apply Concepts Which will probably melt at the lower temperature, a rock that
contains fluids and is under low pressure or a rock that does not contain fluids
and is under high pressure? Explain your answer.
3. Compare Describe two differences between intrusive igneous rocks and extrusive
igneous rocks.
4. Infer If there were a laccolith below the ground in an area, what might the area
look like? Explain your answer.
Copyright © Holt McDougal. All rights reserved.
Holt McDougal Earth Science
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Rocks