Download 2How Do Volcanoes and Earthquakes Change Landforms?

2
How Do Volcanoes and
Earthquakes Change
Landforms?
LESSON GOALS
You will learn
• how volcanoes form.
• how the movement of
rocks In the crust causes
earthquakes.
• how scientists try to
predict earthquakes.
As the plates in the earth's crust move, they change
landforms. Some of these changes take place over
thousands of years. Other changes take place quickly.
Volcanoes and earthquakes can quickly change the
shape of the land.
Two Types of Volcanoes
You can see in the drawing that melted rock-or
magma-lies
deep inside the earth. The magma is
squeezed up through the crust. Sometimes, the
magma breaks through the ground. Magma that
comes above ground is called lava.' As lava cools, it
hardens. This hardened lava can build a mountain
called a volcano.
Volcanoes erupt in different ways. When the
volcano below erupts, it explodes. Lava, rocks, and
ash burst out from the volcano. Sometimes, this kind
of volcano erupts with so much force that it blows
the top off of the volcano.
Lava, rocks, and ash exploding from a volcano
The volcano on the right erupts in a quieter way.
Hot lava bubbles and oozes out of the volcano and
flows down the sides. Rocks and ash do not explode
from this kind of volcano.
The shape of a volcano depends on how the
volcano forms. The volcano on page 246 was formed
by eruptions that are like explosions. The lava, rocks,
and ash that were thrown into the air fell back to the
earth in a pile. Every time the volcano erupted, the
pile grew higher and higher. Volcanoes formed in
this way often have steep sides.
The volcano below formed by quiet eruptions. The
lava that flowed out spread across the land and
hardened into rock. After many eruptions, the lava
formed a wide volcano with gentle slopes.
Volcanoes can be dangerous. Hot lava that flows
from a volcano can burn people's homes. Rocks and
ash can bury people. However, volcanoes are also
helpful to people. After many years, some lava
changes into a dark soil that is good for farming. In
some countries, such as the United States and Japan,
people are learning how to use heat from volcanoes to
make electricity.
Lava flowing from a volcano
(
fault (folt), a crack in the
earth's crust along which
rocks move.
The San Andreas Fault
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Earthquakes
In Lesson 1 you learned that plates of rock make up
the earth's crust. When the plates move, they bend
some of the rocks in the crust. Imagine bending a
stick. If you bend the stick hard enough, the stick will
break. In a similar way, rocks in the crust can break
when they bend. When a rock breaks, it forms a crack
in the crust. If the rock moves along the crack, the
crack is called a fault.
Most faults are deep inside the earth. However,
some faults can be seen on the earth's surface. The
thin line in the picture is the San Andreas Fault in
California. It is more than 1,000 kilometers long.
The drawings show how rocks move along a fault.
Sometimes the rocks move from side to side. Other
times the rocks move up and down.
Movement of rocks along faults
When the rocks at a fault move, the ground shakes.
This shaking of the ground is an earthquake. Most
earthquakes are so weak, people cannot feel them.
Some earthquakes destroy buildings.
Scientists can measure the strength of an
earthquake. They use an instrument-called
a
seismograph (siz/rno graf)-that
measures the
movement of the ground. When an earthquake
happens, the seismograph moves. Notice in the
picture that the seismograph has a pen that records
lines on the paper. Scientists measure the height of
these lines to find the strength of the earthquake. The
higher the lines are, the stronger the earthquake is.
Scientists give earthquakes a number from the
Richter (rik/tor) scale. The stronger the earthquake,
the higher the number the earthquake is given. An
earthquake that is too weak to be felt by people might
be a 1 or 2 on the Richter scale. A strong earthquake
that destroys buildings might be a 6, 7, or 8 on the
Richter scale. Each number represents an earthquake
that is ten times stronger than the number below it.
Dr. Richter observinga
seismograph
SCIENCE IN YOUR LIFE
Many states require that
new buildings be built to
prevent as much damage
from earthquakes as
possible. Some kinds of
buildings are safer than
others. The picture below
shows a test being done by
making an earthquake in a
laboratory. This kind of
test helps people study how
safe different kinds of
buildings are.
A laboratory test
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Predicting Earthquakes
Measuring with laser light
Laser light reflecting
Scientists try to predict earthquakes by looking for
signs that might mean an earthquake is going to
happen. Many weak earthquakes-or
tremorscould mean that a stronger earthquake will happen
soon. Sometimes, the ground changes shape or rocks
along a fault move slightly before an earthquake.
Scientists use different tools to measure these
changes. One of these tools uses laser light. The tool
in the top picture is placed on one side of a fault. This
tool sends a beam of laser light. The light bounces off
a reflector on the opposite side of a fault. Find this
light reflecting in the bottom picture. Any movement
of the ground changes the time the light takes to
travel to the reflector and back.
The water level in a nearby well also might
suddenly change before an earthquake. Many people
have noticed that some animals seem to act strangely
before an earthquake takes place. Scientists use all
these signs to try to predict earthquakes.
Lesson Review
1. What are two ways that volcanoes can form?
2. Why do earthquakes happen near faults?
3. What are two signs that might mean an earthquake
is about to take place?
4. Challenge! Which of the two kinds of volcanoes
described in this lesson is likely to be more
dangerous to people? Explain your answer.
Study on your own, pages 394-395.
On May 18, 1980, Mount St. Helens
erupted. The volcano's explosion, hot gases, and
ash killed millions of animals. It also destroyed
millions of conifers. Use the library to find out
about life returning to Mount St. Helens. Write a
report about what you learn .
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LESSON 2
1. The melted rock that is deep inside the earth is called
pages 246-250
2.
3.
4.
5.
What is lava?
What bursts out of a volcano when it erupts?
What is the shape of a volcano that erupts explosively?
What is the shape of a volcano that formed from quiet
eruptions?
_______
6. In what two ways can volcanoes help people?
7. ~~do
faults form?
~:~~s~~:s~~~t~h~f
~~::~~n~:~r;::
:~~:~ during an
earthquake.
10. The stronger the earthquake, the higher a number an
earthquake is given on the __ scale.
11. If an earthquake measures a 7 on the Richter scale, it is
r@ times stronger than an earthquake that measures a 6 "
on the scale.
12. Sometimes, the %% changes shape before an earthquake.
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