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Igneous Rock
2
Overview
Where do igneous rocks come from? Here’s a hint: The word
igneous comes from a Latin word that means “fire.”
READI NG WARM-U P
This section discusses the
formation of igneous rock from
the cooling of magma. Students
learn about the difference between
intrusive and extrusive igneous
rock. Students also learn about
the difference between felsic
and mafic igneous rock and
how the rate of cooling affects
the texture of igneous rock.
Objectives
•
•
Describe three ways that igneous
rock forms.
Explain how the cooling rate of
magma affects the texture of
igneous rock.
Distinguish between igneous rock
that cools within Earth’s crust and
igneous rock that cools at Earth’s
surface.
•
Terms to Learn
intrusive igneous rock
extrusive igneous rock
Bellringer
READI NG STRATEGY
Pose the following question to
students: “Do you think rocks
that cooled and solidified from
lava on Earth’s surface would
look different from those that
cooled and solidified from
magma inside the Earth? Why?”
Reading Organizer As you read this
section, make a table comparing intrusive rock and extrusive rock.
Igneous rock forms when hot, liquid rock, or magma, cools
and solidifies. The type of igneous rock that forms depends
on the composition of the magma and the amount of time it
takes the magma to cool.
Origins of Igneous Rock
Igneous rock begins as magma. As shown in Figure 1, there
are three ways magma can form: when rock is heated, when
pressure is released, or when rock changes composition.
When magma cools enough, it solidifies to form igneous
rock. Magma solidifies in much the same way that water freezes.
But there are also differences between the way magma freezes
and the way water freezes. One main difference is that water
freezes at 0°C. Magma freezes between 700°C and 1,250°C.
Also, liquid magma is a complex mixture containing many
melted minerals. Because these minerals have different melting points, some minerals in the magma will freeze or become
solid before other minerals do.
Figure 1
The Formation of Magma
Composition When fluids
such as water combine with
rock, the composition of the
rock changes, which lowers
the melting point of the rock
enough to melt it.
Discussion ----------------------------------g
Volcanoes Ask students to
discuss how volcanoes affect
people. Discuss eruptions, lava
flows, and ash clouds. Then, ask
students about the benefits of
volcanoes. Explain that lava and
magma form land. Explain that
volcanic soil is some of the most
fertile soil in the world, which is
why many populations are willing to live alongside potentially
dangerous volcanoes. l Logical
Pressure The high pressure deep inside the
Earth forces minerals to
remain solid. When hot
rock rises to shallow
depths, the pressure in
the rock is released, and
the minerals can melt.
Temperature A rise in
temperature can cause
the minerals in a rock to
melt. Different melting
points cause some minerals to melt while other
minerals remain solid.
CHAPTER RESOURCES
Chapter Resource File
CRF
• Lesson Plan
• Directed Reading A b
• Directed Reading B s
Technology
Transparencies
• Bellringer
98
Chapter 4 • Rocks: Mineral Mixtures
Surtsey is a volcanic island south of
Iceland that people actually saw being
born! In 1963, fishermen saw jets of
spray, steam, and lava shooting more
than 30 m out of the ocean. One month
later, the volcano broke through the surface to form an island. By the time the
eruptions ended, Surtsey covered an
area of approximately 2.8 km2.
Figure 2
Igneous Rock Texture
Coarse-grained
Using the Figure ---------b
Fine-grained
Felsic
Granite
Rhyolite
Figure 3 The amount of
time it takes for magma or
lava to cool determines the
texture of igneous rock.
Mafic
Gabbro
Basalt
Composition and Texture of
Igneous Rock
Look at the rocks in Figure 2. All of the rocks are
igneous rocks even though they look different from
one another. These rocks differ from one another in
what they are made of and how fast they cooled.
The light-colored rocks are less dense than the
dark-colored rocks are. The light-colored rocks are
rich in elements such as aluminum, potassium,
silicon, and sodium. These rocks are called felsic
rocks. The dark-colored rocks, called mafic rocks,
are rich in calcium, iron, and magnesium, and
poor in silicon.
Figure 3 shows what happens to magma when it
cools at different rates. The longer it takes for the
magma or lava to cool, the more time mineral crystals have to grow. The more time the crystals have
to grow, the larger the crystals are and the coarser
the texture of the resulting igneous rock is.
In contrast, the less time magma takes to cool,
the less time crystals have to grow. Therefore, the
rock that is formed will be fine grained. Fine-grained
igneous rock contains very small crystals, or if the
cooling is very rapid, it contains no crystals.
Fast-cooling lava
Fine-grained
igneous rock
Magma
Slow-cooling
magma
Coarse-grained
igneous rock
✓
Reading Check Explain the difference between
felsic rock and mafic rock. (See the Appendix for answers
to Reading Checks.)
Answer to Reading Check
Felsic rocks are light-colored igneous rocks
rich in aluminum, potassium, silicon, and
sodium. Mafic rocks are dark-colored igneous
rocks rich in calcium, iron, and magnesium.
Making Inferences Have students rank the rocks shown in
Figure 2 by how fast they
cooled. Tell students to pay careful attention to the grain size of
each rock. (From fastest cooled to
slowest cooled, the rocks are
basalt, rhyolite, gabbro, and
granite.) l Visual/Logical
CONNECTION to
Life Science -----------------------------------a
Life Along a Rift Until 1977,
biologists thought few lifeforms lived at ocean depths
where sunlight does not reach.
When scientists in the submersible Alvin explored the bottom
of a deep ocean trench called
the Galápagos Rift, they discovered structures called black
smokers that release dissolved
mineral compounds and heat
the water. Scientists were
amazed to discover an entire
ecosystem that did not depend
on photosynthesis for energy.
This discovery has led some
scientists to speculate that life
may also have originated in the
outer solar system—particularly
in the oceans that may exist
under the surface of Europa,
one of Jupiter’s moons. Have
students research the bizarre
life-forms that scientists found
living around black smokers.
l Logical
Is That a Fact!
The Deccan Traps of India Between
68 and 64 million years ago, a hot
spot under western India erupted over
1 million km3 of basaltic lava. These
lava flows, called the Deccan Traps, are
more than 2 km thick in some places.
Mathematicians calculate that if the
lava from the eruptions in western
India were spread evenly over the
entire Earth, it would cover the Earth
with a layer more than 2 m thick!
Section 2 • Igneous Rock
99
Igneous Rock Formations
Igneous rock formations are located above and below the
surface of the Earth. You may be familiar with igneous rock
formations that were caused by lava cooling on the Earth’s
surface, such as volcanoes. But not all magma reaches the
surface. Some magma cools and solidifies deep within the
Earth’s crust.
For another activity related
to this chapter, go to
go.hrw.com and type in
the keyword HZ5RCKW.
Reteaching -------------------------------------b
Word Meanings Compare the
words intrusive and extrusive
with the words interior and
exterior. Have students brainstorm other words that use the
prefixes in- and ex- to help them
remember the meanings of the
terms intrusive and
extrusive. l Logical e
Intrusive Igneous Rock
When magma intrudes, or pushes, into surrounding rock
below the Earth’s surface and cools, the rock that forms is
called intrusive igneous rock. Intrusive igneous rock usually has
a coarse-grained texture because it is well insulated by surrounding rock and cools very slowly. The minerals that form
are large, visible crystals.
Masses of intrusive igneous rock are named for their size and
shape. Common intrusive shapes are shown in Figure 4. Plutons
are large, irregular-shaped intrusive bodies. The largest of all
igneous intrusions are batholiths. Stocks are intrusive bodies
that are exposed over smaller areas than batholiths. Sheetlike
intrusions that cut across previous rock units are called dikes,
whereas sills are sheetlike intrusions that are oriented parallel
to previous rock units.
intrusive igneous rock rock
formed from the cooling and
solidification of magma beneath
the Earth’s surface
Quiz ---------------------------------------------------------------------g
1. Name five types of bodies
of intrusive igneous rock.
(batholiths, stocks, dikes, sills,
and volcanic necks)
2. What is a fissure? (A fissure is
Figure 4 Igneous intrusive
bodies have different shapes
and sizes.
a long crack in the Earth’s crust
through which lava erupts and
flows.)
Volcanic neck
Dike
Alternative
Assessment ---------------------------g
Modeling Igneous Rock
Bodies Have students create a
model cross section that shows
the formation of both intrusive
and extrusive igneous rock.
Students can use Figure 4 as a
basis for their models. Supply
students with several different
colors of clay so that they can
color-code different bodies, such
as the magma source, dikes, sills,
plutons, and the lava that forms
extrusive rock.
l Kinesthetic/Visual
Dike
Stock
Sill
e
Batholith
h -----------------------------a
Volcanic Necks A volcanic neck
is the hardened core of a volcano
that is left behind after the volcano
erodes away. Ship Rock, a volcanic neck
on a Navajo reservation in New Mexico,
soars 518 m above the desert. Devils Tower
National Monument, in Wyoming, rises
386 m. Have students research one of these
formations to learn how it formed.
Writing
l Logical
100
Chapter 4 • Rocks: Mineral Mixtures
Batholith
Extrusive Igneous Rock
Answers to Section Review
Igneous rock that forms from magma that erupts,
or extrudes, onto the Earth’s surface is called
extrusive igneous rock. Extrusive rock is common
around volcanoes. It cools quickly on the surface
and contains very small crystals or no crystals.
When lava erupts from a volcano, a lava flow
forms. Figure 5 shows an active lava flow. Lava does
not always flow from volcanoes. Sometimes lava
erupts and flows from long cracks in the Earth’s
crust called fissures. Lava flows from fissures on
the ocean floor at places where tension is causing
the ocean floor to be pulled apart. This lava cools
to form new ocean floor. When a large amount of
lava flows out of fissures onto land, the lava can
cover a large area and form a plain called a lava
plateau. Pre-existing landforms are often buried by
these lava flows.
1. Sample answer: Intrusive
igneous rock forms from
magma that solidifies underground. Extrusive igneous rock
forms from magma that solidifies
after it has reached the surface.
2. c
3. Temperature, pressure, and a
change in the composition of a
rock can cause magma to form.
A rise in temperature can cause
minerals in a rock to melt, forming magma. When pressure in a
rock that is hot is released, the
minerals in that rock can melt,
forming magma. When fluids
such as water combine with
rock, the composition of the
rock changes. This change in
composition lowers the melting
point of the rock enough to
melt it, forming magma.
4. When magma cools slowly,
crystals have a longer time to
grow, so the igneous rock that
forms is coarse grained. When
magma cools quickly, crystals
have a short time to grow, so
the igneous rock that forms is
fine grained.
5. 1,825 ft 3.28 ft/m ! 556.4 ft/m
6. A sill intrudes rock parallel to the
surrounding rock layers. A dike
cuts across the surrounding
rock layers.
7. Because the rock formed from
slowly cooling magma deep
inside the Earth, the crystals had
more time to grow. Therefore,
the texture of the rock would
most likely be coarse grained.
✓Reading Check
How does new ocean floor form?
Review
Summary
•
•
•
Igneous rock forms
when magma cools and
hardens.
The texture of igneous
rock is determined by
the rate at which the
rock cools.
Igneous rock that solidifies at Earth’s surface is
extrusive. Igneous rock
that solidifies within
Earth’s surface is
intrusive.
of common
• Shapes
igneous intrusive bodies include batholiths,
stocks, sills, and dikes.
Figure 5 An active lava flow is shown in
this photo. When exposed to Earth’s surface
conditions, lava quickly cools and solidifies
to form a fine-grained igneous rock.
extrusive igneous rock rock that forms as a
result of volcanic activity at or near the Earth’s
surface
Using Key Terms
Critical Thinking
1. In your own words, write a definition for each of the following
terms: intrusive igneous rock and
extrusive igneous rock.
Understanding Key Ideas
2.
7. Predicting Consequences An
igneous rock forms from slowcooling magma deep beneath
the surface of the Earth. What
type of texture is this rock most
likely to have? Explain.
is an example of a coarsegrained, felsic, igneous rock.
a.
b.
c.
d.
6. Making Comparisons Dikes and
sills are both types of igneous
intrusive bodies. What is the
difference between a dike and
a sill?
Basalt
Gabbro
Granite
Rhyolite
3. Explain three ways in which
magma can form.
4. What determines the texture
of igneous rocks?
Developed and maintained by the
National Science Teachers Association
For a variety of links related to this
chapter, go to www.scilinks.org
Math Skills
5. The summit of a granite batholith has an elevation of 1,825 ft.
What is the height of the batholith in meters?
Answer to Reading Check
New sea floor forms when lava that flows from
fissures on the ocean floor cools and hardens.
Topic: Igneous Rock
SciLinks code: HSM0783
CHAPTER RESOURCES
Chapter Resource File
CRF
• Section Quiz g
• Section Review g
• Vocabulary and Section Summary g
Technology
Transparencies
• Intrusive Igneous Rock Bodies
Section 2 • Igneous Rock 101