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Rocks
Igneous 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 igneous rocks form?
• What are the common
types of igneous rocks?
After
3. Large crystals form when lava cools quickly on
Earth’s surface.
4. Igneous rocks form when cooling magma
crystallizes.
3TUDY#OACH
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Igneous Rock Formation
Recall that lava is magma that has reached Earth’s surface
during a volcanic eruption. What do you think of when you
hear the word lava? Perhaps you picture hot, gooey liquid
that flows easily. When lava cools and crystallizes, it becomes
igneous rock. Lava cools quickly when it comes in contact
with the cooler air.
Make Flash Cards For each
head in this lesson, write a
question on one side of a flash
card and the answer on the
other side. Quiz yourself until
you know all of the answers.
Not all magma makes it to Earth’s surface. Large amounts
of magma cool and crystallize beneath Earth’s surface. No
cool air comes in contact with the magma. As a result, the
process of cooling and crystallizing takes much longer. Igneous
rock that formed beneath Earth’s surface looks much different
from rock that formed from lava cooling on Earth’s surface.
Over time, wind, rain, and other factors wear down rocks
on Earth’s surface. Igneous rock that was once underground
can become exposed on Earth’s surface.
Extrusive Rocks
There are two types of igneous rocks. When volcanic material
erupts and cools and crystallizes on Earth’s surface, it forms a type of
igneous rock called extrusive rock. Extrusive igneous rocks form
as lava and ash become solid.
Reading Essentials
Key Concept Check
1. Explain How do igneous
rocks form?
Rocks
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Fine-Grained Textures Lava that erupts onto Earth’s surface
Use a sheet of paper to make
a horizontal two-tab book.
Collect information on
extrusive and intrusive
igneous rocks.
cools rapidly. Rapid cooling keeps large crystals from growing.
Some extrusive igneous rocks have fine-grained textures.
Volcanic glass is rock that forms when lava cools too quickly to form
crystals. Obsidian is an example of volcanic glass. It has a
shiny, glasslike surface with small, fine grains.
Holes in Extrusive Rocks Magma stored underground can
Extrusive Intrusive
Rocks Rocks
Reading Check
2. Relate Why are there
holes in some igneous rocks?
Reading Check
3. Locate Where do
Intrusive Rocks
Igneous rocks that form as magma cools underground are called
intrusive rocks. Magma cools slowly underground. As a result,
large crystals form. The crystals are arranged randomly in
intrusive rocks. These large crystals lock together like pieces
of a jigsaw puzzle. The random arrangement and large size
of the crystals are features of intrusive igneous rocks.
Igneous Rock Identification
As you learned, geologists use texture and composition to
help identify all rocks. Geologists identify igneous rocks by
the arrangement and size of the mineral crystals in the rock.
Chemical composition can also help to identify igneous rocks.
Texture
Geologists decide whether an igneous rock is extrusive
or intrusive by studying the rock’s texture. If the crystals are
small or impossible to see with the unaided eye, the rock is
extrusive. If all the crystals are large enough to see and have
a texture that locks together, the rock is intrusive.
Composition
Geologists also study the mineral composition of igneous
rocks. Igneous rocks are classified, in part, based on their silica
content. Light-colored minerals contain greater amounts
of silica. Dark-colored minerals contain less silica and greater
amounts of elements such as magnesium and iron.
You can sometimes estimate a rock’s mineral composition
by its color. Lighter-colored rocks are similar to granite in
mineral composition. Darker-colored rocks are similar to
basalt in mineral content.
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Rocks
Reading Essentials
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
intrusive rocks form?
contain dissolved gases. As magma moves slowly toward the
surface, pressure decreases and the gases separate from the
molten mixture. When gas-rich lava erupts from a volcano,
the gases escape. This process is similar to carbon dioxide
escaping when a carbonated beverage is opened. The rocks
that form have holes left by the escaping gases. Pumice
(PUH mus) is one of the extrusive igneous rocks that forms
in this way.
Types of Igneous Rocks The type of igneous rock that forms
is determined by
• magma composition,
• the environment in which the lava or magma cools
and crystallizes, and
• the cooling rate.
Granite is an intrusive igneous rock. It has a high silica
content and cools slowly beneath Earth’s surface. Basalt is an
extrusive igneous rock that has a low silica content. It forms
as lava cools quickly on Earth’s surface.
Common Igneous Rocks The table below lists common
igneous rocks by their texture and composition. Notice that an
extrusive igneous rock can have the same mineral composition
as an intrusive igneous rock. Their compositions might be the
same, but their textures are different.
Key Concept Check
4. Contrast How are
extrusive and intrusive rocks
different?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Common Igneous Rocks
Main Rock-Forming
Minerals Present
Intrusive Texture
(all crystals can be
seen with the
unaided eye)
Extrusive Texture
(some or no crystals
can be seen with the
unaided eye)
quartz, feldspar, mica,
amphibole
granite
rhyolite
pyroxene, feldspar,
mica, amphibole, some
quartz
diorite
andesite
olivine, pyroxene,
feldspar, mica,
amphibole, little or
no quartz
gabbro
basalt
Visual Check
5. Identify Which minerals
are common to these igneous
rocks?
Reading Essentials
Rocks
53
Mini Glossary
extrusive rock: a type of igneous rock that forms when
volcanic glass: an igneous rock that forms when lava cools
material that erupts from a volcano cools and crystallizes
on Earth’s surface
too quickly to form crystals
intrusive rock: an igneous rock that forms as magma cools
underground
1. Review the terms and their definitions in the Mini Glossary. Using the terms, explain in one
or two sentences why the crystals are larger in intrusive rocks than in extrusive rocks.
2. Classify each description as a feature of either extrusive rocks or intrusive rocks. Place
a check mark in the correct column in the chart.
Description
Extrusive Rocks
Intrusive Rocks
(a) form underground
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
(b) coarse grains
(c) form above ground
(d) cool quickly
(e) form from lava
(f ) volcanic glass
3. Describe the texture of an intrusive rock.
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?
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Rocks
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Reading Essentials