3
LESSON GOALS
You will learn
• how earthquakes
change the earth's
crust.
• how volcanoes change
the earth's crust.
How Do Earthquakes
and Volcanoes Change the
Earth's Crust?
Weathering and erosion usually change the
earth's crust very slowly. Sometimes,
however, the crust changes very quickly. The
picture shows where a large crack occurs in the
earth's crust. Land on either side of this crack
can suddenly move a couple meters in a
minute.
earthquake
(erth /kwak I), a shaking
or sliding of the earth's
crust.
Earthquakes change
the earth's crust.
Earthquakes
An earthquake is a shaking or sliding of the
earth's crust. Earthquakes happen when rocks
inside the earth move. As the rocks move,
they may form new cracks in the crust. Sometimes, the movement of the ~ocks makes the
land move up, down, or sideways. The picture
shows land after an earthquake. How did the
earthquake change the land?
Most earthquakes are too weak for people to
notice. However, some earthquakes are strong
enough to shake buildings. The walls of
buildings might fall down during a strong
earthquake. The picture shows the kind of
damage an earthquake can do. The movement
of the ground twisted these railroad tracks as if
they were a rubber band. Earthquakes can
injure or kill people. Many people are hurt by
falling objects.
Scientists try to predict earthquakes.
Knowing when an earthquake will happen can
save lives. People would have time to go to a
safe place.
If you are indoors during an earthquake,
stay away from windows and furniture that
could slide or fall over. If outdoors, stay away
from high buildings and other objects that
could fall.
Earthquake damage
A volcano
Kilauea
volcano (vol ka/no), a
mountain with an
opening through which
lava, ashes, rocks, and
other materials come out.
erupt (i rupt/),
to burst
out.
magma (mag/mag),
hot,
melted rock deep inside
the earth.
220
Volcano
in Hawaii
Volcanoes
Hot, melted rock is shooting out from the
volcano in the picture. A volcano is a
mountain with an opening at the top. When a
volcano erupts, melted rock, ashes, and other
materials burst out from its opening.
What causes a volcano to erupt? The picture
shows that hot, melted rock-or
magmalies deep inside the earth. The magma is
squeezed up through the crust. Sometimes, the
magma is forced up through the volcano, and
the volcano erupts.
Magma that comes out of a volcano is called
lava. When the lava cools, it hardens into solid
rock. The rock forms new crust. Like
earthquakes, volcanoes can change the earth's
crust very quickly.
When a volcano erupts, it can hurt people
and damage property. Hot lava that flows
down a volcano can bum homes.
Scientists look for warning signals to predict
when a volcano will erupt. Small earthquakes
near a volcano might mean that magma is
rising inside. A bulge on a volcano also might
mean that the volcano is about to erupt.
SCIENCE IN YOUR LIFE
A volcano can form
new land. The Hawaiian
Islands were formed by
underwater volcanoes.
The islands are the tops
of volcanoes in the
Pacific Ocean.
lava (la/vo), hot, melted
rock that flows from a
volcano.
Lesson Review
1. How can earthquakes change the earth's
crust?
2. What causes a volcano to erupt?
3. Challenge! Why do earthquakes occur
when magma rises in a volcano?
Study on your own, pages 334-335.
Look in an encyclopedia to find out
where earthquakes occur most often on the
earth. Then, write the answers to the
following questions.
1. What part of North America has the
most earthquakes?
2. What parts of the earth have the fewest
earthquakes?
3. Do more earthquakes occur under water
or on land?
FIND OUT
ON YOUR OWN
221
LESSON 3
pages
218-221
1. What makes earthquakes dangerous?
2. Scientists try to m:m earthquakes to help save lives.
3. What should you do if you are indoors during an
earthquake?
4. What should you do if you are outdoors during an
earthquake?
5. What is magma?
6. How does lava form new crust?
7. Why are volcano eruptions dangerous?