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Lesson 2
What Causes Changes to Earth’s Landforms?
Fast Fact
Rocky Coast These tall rocks along the Australian coast are sea stacks. They are all
that is left of a rocky cliff that was pounded to pieces by ocean waves. In the
Investigate, you will find out about the forces that shape volcanoes.
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Volcanic Eruptions
Materials
 2-liter plastic bottle
 puffed rice cereal
 small piece of modeling clay
 funnel
 aluminum pie plate
 air pump
Procedure
1. Ask your teacher to make a hole near the bottom of a bottle. Stick the bottom of the
bottle to a pie plate with clay.
2. Use a funnel to add cereal to the bottle until it is one-fourth full.
3. Attach an air pump to the hole in the bottle. Make sure the nozzle points down. Put a
piece of clay around the hole to make it airtight.
4. Pump air into the bottle. Observe what happens.
Draw Conclusions
1. What happened to the cereal when you pumped air into the bottle?
2. Predict how you could model a very large eruption.
3. Inquiry Skill Scientists often make models to help them understand things that
happen in nature. How does the bottle model an erupting volcano?
Investigate Further
Make models using fine sand and gravel to test this hypothesis: A volcano that
forms from thick lava is steeper.
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Reading in Science
VOCABULARY
volcano p. 208
earthquake p. 208
deposition p. 210
glacier p. 211
SCIENCE CONCEPTS
 characteristics of Earth's structure
 what forces change Earth's surface
READING FOCUS SKILL CAUSE AND EFFECT Look for causes of changes to
Earth's surface.
Layers of Earth
Every minute of every day, you are on Earth's surface. If you could cut open Earth and
look inside, you would find the four layers shown in the diagram below.
Earth's thin outer layer is the crust. The crust includes the land that makes up the
continents as well as the land under the oceans.
The mantle is the rock layer below the crust. Deep below Earth's surface, the
temperature rises. The upper parts of the mantle are so hot that the rock can flow. In
some places the rock is melted to form magma.
At Earth's center is the core. The core is made mostly of iron and nickel. The outer core
is liquid. The inner core is solid. The inner core is almost as hot as the surface of
the sun. It is solid because there is so much pressure on it
Earth's crust and upper mantle are broken into large slabs of rock called plates. The
plates move on a layer of the mantle that can flow like taffy.
---see picture
The crust is Earth's thinnest layer. It is solid rock. The crust sits on top of the
mantle, which is Earth's thickest layer. Earth's core is mostly metal, and it's
very hot. Temperatures in the core reach as high as 5000°C (9000°F).
You can't see the movement of these plates. The plates move only a few centimeters per
year. Over a long time, this movement leads to the formation of different
landforms.
Plates move in several ways. Some move toward each other. When two land plates
meet, the edges crush and fold as one is pushed down under the other, forming
mountain chains. Where a land plate and ocean plate or two ocean plates meet,
islands made of volcanic mountains can result.
Some plates travel away from each other. Large cracks can form where the two plates
are moving apart. Magma from the mantle oozes up through these cracks. It
hardens and makes new crust. Often this happens in the oceans.
Plates can also slide past each other. Where this happens, huge cracks appear at Earth's
surface.
CAUSE AND EFFECT What landform can form where two land plates collide?
Insta-Lab
How Mountains Grow
Place both hands flat on a table, with the fingertips facing each other. Keep moving
your hands toward each other until your fingertips are pushing against each other.
What happens to your fingers? How does this model mountain formation?
---see pictures
The Himalayas are Earth's highest mountain chain.
The Himalayas began to form when the Indian plate and the Asian plate pushed
into each other.
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Volcanoes and Earthquakes
On the morning of May 18, 1980, the volcano Mount St. Helens, in the state of
Washington, erupted. A volcano is a mountain that forms as lava flows through a
crack onto Earth's surface. This major eruption threw ash 19 kilometers (12 miles)
into the air. The lava, ash, rock, and hot gases that shoot out of volcanoes change
the land. Hot rock and gas from Mount St. Helens covered the land, filled in
streams, and destroyed forests around the volcano. Since 1980 there have been
many small eruptions of Mount St. Helens.
There are different types of volcanoes. One type is composite volcanoes. They are made
of layers of lava, rock, and ash. They can have steep peaks and are usually
explosive when they erupt. Hawai'i has shield volcanoes. These huge mountains
erupt slowly, and lava flows steadily down their gently sloping sides. Cinder cone
volcanoes are small and have steep sides. They shoot chunks of rock into the air
and down their slopes.
Movement between two plates can cause earthquakes. An earthquake is the shaking of
Earth's surface caused by movement of rock in the crust.
Most earthquakes occur along faults.
---see pictures
When two ocean plates push together, one plate is forced under the other. In the
process, the upper plate melts and forms magma. This magma rises and forms
volcanoes.
The base of this Indonesian volcano is on the sea floor.
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Science Up Close
Strike-Slip Fault
---see pictures
A seismogram shows the movement of Earth's surface during an earthquake.
At a strike-slip fault, slabs of rock slide past each other. Movement at the fault can
rip apart roads, bridges, pipelines, and other structures.
A fault is a break in the crust, where rock moves. Sometimes this rock sticks. After
some time, it may move forward suddenly. The movement sends out waves of
energy that move through the crust. This energy causes shaking, rolling, and
cracking in the crust and Earth's surface.
CAUSE AND EFFECT What is the cause of most earthquakes?
The motion of an earthquake tore this Californian highway into several pieces.
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Rivers
Rivers are found all over Earth. Although they aren't as dramatic as volcanoes or
earthquakes, rivers can cause big changes to Earth's surface. Rivers just take longer
to affect the land around them.
Rivers flow through valleys. The shape of a valley depends on the way the river runs
through it. In steep areas, rivers move quickly. The rushing water cuts into the soil
and rock. These valleys are narrow and V-shaped.
As a river gets older, its valley becomes less steep. The floor of the river becomes more
level. The valley walls become farther apart. As a result, older rivers often have
wide valleys with flat floors. They flow through the valleys in wide curves.
As rivers flow, they carry soil and rock. As a river moves, deposition occurs. In
deposition (dep•uh•ZISH•uhn), rivers drop bits of rock and soil along the way. The
slower a river moves, the more deposition occurs. River deposition builds
landforms such as deltas and floodplains.
CAUSE AND EFFECT What causes deposition to increase?
---see pictures
Rivers on wide plains flow in large curves like these.
Rivers that flow down steep slopes can cut deep valleys.
Math in Science Interpret Data
Longest Rivers in the World
In the United States, the longest river is the Missouri, which is 4087 km (2540 mi) long.
There are other rivers in the world that are longer. How much longer are each of
these rivers than the Missouri?
---see chart
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Glaciers
In some places, snowfall is high and temperature is low. Sometimes more snow falls in
winter than melts in summer. The snow piles up year after year. As it thickens, it
turns to ice. If the mass of ice starts to move downhill, it becomes a glacier. A
glacier (GLAY•sher) is a large, moving mass of ice.
There are two main types of glaciers—alpine glaciers and ice sheets. Alpine glaciers
flow down mountain valleys. The ice scrapes the floor and sides of the valley as it
moves. The glacier widens the valley, giving it a U shape. Fiords (FYAWRDZ)
form where these valleys reach the coast. Ice sheets are huge glaciers that cover
large areas, such as Antarctica and Greenland.
Thousands of years ago, ice sheets covered much of Earth. As these ice sheets moved
over the land, they shaped many landforms people see today.
CAUSE AND EFFECT What causes glaciers to form?
---see pictures
Cracks appear in this glacier as it flows slowly down a mountain valley. Glaciers
may move as little was a few centimeters or as much as several meters per
Fiords form where the sea has flooded valleys formed by glaciers.
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Wind and Waves
You have seen trees bend and move on a windy day. Wind can affect the way Earth's
surface looks. In dry areas and along sandy coasts, soil is dry and loose. There
aren't many plants. Wind lifts particles and carries them.
Wind slams sand into rocky surfaces. The wind-blown sand makes pits and grooves in
rock. Wind also carries sand and deposits it in dunes, as you learned in Lesson 1.
Waves break down rocky cliffs. As the cliffs crumble, they move farther inland.
Structures such as stone arches and pillars are left behind. As the sea moves inland,
the structures are left offshore.
Waves also change the shape of sandy coastlines. They remove sand from some areas
and deposit it in other places. This erosion and deposition of sand creates beaches,
sand bars, and barrier islands along the shore.
CAUSE AND EFFECT What conditions are needed for wind erosion?
---see pictures
The pounding of waves carved this sea arch. The hole formed when water wore
away rock at the center of a solid formation sticking out into the sea.
Erosion from a flood has washed away the soil under this house. It will soon topple
into the river below.
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Reading Review
1. CAUSE AND EFFECT Copy and fill in the graphic organizer below.
---see chart
2. SUMMARIZE Briefly describe the cause and effect of an earthquake
3. DRAW CONCLUSIONS Which do you think has a greater effect on landforms—
wind or water? Why?
4. VOCABULARY Use each of the vocabulary terms from this lesson in one
paragraph.
Test Prep
5. Critical Thinking What evidence of wave erosion might be seen in landforms along
the shore?
6. What is magma?
A. hot gases
B. hard rock
C. melted rock
D. hot metals
Writing
Descriptive Writing
Write a report that describes the journey of a piece of rock that erupts from a volcano.
Follow the rock from the mantle until it shoots out of the volcano onto Earth's
surface.
Math
Make a Bar Graph
Use an encyclopedia to identify the world's five most deadly earthquakes during the
past 100 years. Use the number of people who died as the measure. Make a bar
graph to compare the earthquakes.
Art
Illustration
Read in a science book or an encyclopedia about a major earthquake or volcanic
eruption. Make a drawing that shows some part of what happened. Write a caption
to describe it.
For more links and activities, go to www.hspscience.com
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