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5.b. Students know natural processes,
including freezing and thawing and
the growth of roots, cause rocks to
break down into smaller pieces.
Standard Set 5. Earth Sciences
5. Waves, wind, water, and ice shape
and reshape Earth’s land surface. As a
basis for understanding this concept:
5.c. Students know moving water
erodes landforms, reshaping the land
by taking it away from some places
and depositing it as pebbles, sand,
silt, and mud in other places (weather,
transport, and deposition).
5.a. Students know some changes in
the earth are due to slow processes,
such as erosion, and some are due to
rapid processes, such as landslides,
volcanic eruptions, and earthquakes.
by Lillian Duggan
Genre
Nonfiction
Comprehension Skill
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Glossary
Science Content
Earth’s Surface
Scott Foresman Science 4.7
ISBN 0-328-23559-8
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Vocabulary
deposition
earthquake
erosion
fault
landform
landslide
soil
transport
volcano
weathering
Changing
Earth
by Lillian Duggan
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ISBN: 0-328-23559-8
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How does Earth’s
surface slowly
change?
Weathering
Earth’s Crust
Chemical Weathering
Earth’s outer surface is called the crust. The crust is
a layer of rock that covers all of Earth. In some places
the crust is beneath the ocean.
Earth’s surface has many natural features, or
landforms. Mountains, flat plains, and peninsulas
are landforms. Peninsulas stick out into the water from
coasts. Landforms have different shapes and sizes.
It can take millions of years for a landform such as
a mountain to form. But what if many rocks roll down
its side? That can change the mountain quickly.
Chemicals cause rocks to change into different
materials and break down. This is chemical
weathering. Acid in rain can cause chemical
weathering. When the acid touches rock, it combines
with the rock. This forms a new substance. It causes the
rock to break down.
Landforms can change only when their rocks break
apart. Weathering is the process that breaks rocks in
Earth’s crust into smaller pieces. Weathering is caused
by water, ice, temperature changes, chemicals, and
living things. It can change the land quickly or slowly.
Acid rain has worn away the
stone from Cleopatra’s Needle.
This mountain is a landform. How do
you think it has changed over time?
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What causes
physical weathering?
Physical Weathering
Physical weathering causes rock to break down
into smaller pieces. During chemical weathering rock
changes into another material. This does not happen
during physical weathering. Physical weathering is
caused by plants, ice, and water.
Plants cause physical weathering by growing in
cracks in rocks. Their growing roots break up the rocks.
Glaciers are huge sheets of ice. They move slowly
over land. As glaciers move, they cause physical
weathering. They drag rocks along the ground. The
rocks scrape the ground. The scraping forms valleys
and ridges. Glaciers also grind against rocks. They
break the rocks into smaller pieces.
Freezing and Thawing
Ice can also break rocks. Water from rain or melted
snow gets into cracks in rocks. The water expands
when it freezes into ice. This makes the ice push
against the sides of the crack. The crack becomes
bigger. It keeps growing bigger as more ice forms.
Eventually the rock splits.
Plant roots caused
cracks to form in this rock.
This rock split apart
because of water
freezing and thawing.
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Water and Weathering
Soil
The landforms along coasts are being reshaped
all the time. Waves and flowing water cause these
changes. Crashing waves contain pieces of sand and
gravel that wear away rocks. Beach sand also gets
carried away by crashing waves.
Water shapes landforms in other ways. Rivers carry
pebbles, sand, mud, and other materials. The materials
rub against rocks on the bottom of the river. As they
rub, they slowly wear away the rocks. This causes
valleys and canyons to form.
Weathering breaks down rocks into tiny pieces.
These tiny pieces are part of soil. Soil is the thin layer
of loose, weathered material that covers most of the
land surface of Earth. Scientists have named the rocks
in soil based on their sizes.
Sand, silt, and clay make up most soil. Soil also
contains humus. Humus is decaying plant and animal
material. Different soils have different kinds of rocks
and humus. The spaces between the bits of sand, silt,
clay, and humus fill with water and air. Different
plants do best in different kinds of soil.
Waves carrying sand and pebbles
formed this cut in the rocks.
Sizes of Rocks
Cobble 100 mm
Pebble 30 mm
Clay You cannot see clay
particles with your eyes.
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Sand 1 mm
Silt Silt looks
like tiny specks.
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More Weathering
Weathering breaks some rocks into tiny pieces.
Larger rocks are affected by weathering too. The
weathering of large rocks can form huge arches,
towers, sand dunes, and other shapes.
Some small pieces of weathered rock are moved
by wind or other forces. Blowing winds can pick up
tiny pieces of sand, soil, or dust. When the pieces blow
against a rock, they scrape it. Tiny bits of the rock can
break off and then blow away.
These weathered rocks are
in California’s Death Valley.
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Pieces of rock and soil in open areas are constantly
weathering. The top layer of soil is called topsoil. Crops
grow best in it. Topsoil gets weathered in farm fields.
Waves
Moving water and waves can break down large
rocks. Pieces of rock, soil, and sand can be carried away
by water and wind. They gradually become smaller
and smaller. Later these pieces can end up in other
places. The water or wind may bring them there.
Weathering shaped these rocks in
California’s Alabama Hills.
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How does weathered
material move?
Erosion and Transport
Small pieces of rock sometimes are picked up and
carried to other places. This movement of weathered
materials is called erosion. Erosion is caused by wind,
water, glaciers, living things, and gravity.
Water carries, or transports, weathered materials.
First, rain picks up small pieces of rock. It carries them
into rivers and streams.
The rivers move the pieces to other places. The
pieces might settle on the rivers’ bottoms. Or they
might be carried to the ocean. There they might make
a new landform.
Weathered materials are also transported by wind
and gravity. Wind blows dry sand and soil to new
places. Rocks and soil get pulled downhill by gravity.
Soil is moved by living things as well. Worms carry
soil as they move underground. They mix and move it.
As tree roots grow down, they push soil away.
Rivers carry pieces of rocks to new places.
These valleys were made by erosion.
Photo A. was taken
one minute before
Photo B. Waves caused
the rock to collapse.
Ocean waves may
transport the pieces of
broken-up rock from
place to place.
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Transport and Deposition
Pieces of eroded rock and soil are transported and
left in new places. The laying down of pieces of rock
and soil is called deposition.
Fast-moving water can transport particles a long
distance. When the water slows down, the particles
begin to settle out. Larger pieces of rocks are deposited
first. Then sand particles will sink. The clay and silt are
last to be deposited.
This water is transporting silt.
The silt makes it muddy.
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Deltas can form where rivers deposit sediment.
Ocean waves carry around sediment. They create
sandy islands near coasts when they deposit the
sediment. Erosion moves these islands around too!
Wind deposits a layer of silt and clay on top of
soil. Sand dunes are formed by wind depositing grains
of sand in deserts and other places. Blowing winds
constantly change the shape of sand dunes.
Glaciers deposit rocks and soil when they melt.
Ridges of broken rocks and soil are evidence of glaciers.
Rivers have transported and deposited silt
here. They have created a new landform.
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What causes rapid
changes to landforms?
Rapid Changes
The side of a hill slides down a cliff wall. Suddenly
the landforms have changed. Changes to Earth’s
landforms can happen in an instant. Landslides,
earthquakes, and volcanoes cause rapid changes.
A landslide changed
the shape of this
coastline.
Gravity and Landslides
Objects are pulled from higher places to lower
places by gravity. Gravity can cause loose rocks and
dirt to roll slowly or quickly downhill. Heavy rain can
loosen a steep hill’s materials. Gravity pulls down the
materials. They land in piles at the bottom.
The rapid downhill movement of a large amount
of rock and soil is a landslide. Freezing and thawing
can loosen rock. Sometimes rock gets loosened during
winter. It may slide downhill in spring.
Landslides can cause a lot of damage. As they slide
downhill, they can carry very heavy objects. Sliding
mud can bury anything in the landslide’s path. Homes,
roads, and bridges can be destroyed by landslides.
This boulder fell during a landslide.
It completely blocked the road.
A landslide caused
rocks and soil to slide
down this hill.
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Volcanoes
A volcano is a cone-shaped landform that forms
at a weak spot in Earth’s crust where magma reaches
the surface. Deep underground, hot molten rock called
magma mixes with gases. The gases force it toward the
surface. If magma reaches a weak spot in Earth’s crust,
a volcano will form. As more magma comes to the
surface, the volcano erupts. Lava is magma that has
reached the surface.
Some eruptions are more violent than others.
Sometimes the magma’s gases explode. Hot rocks,
gases, and ash burst from the volcano’s openings.
Other times, magma oozes out slowly.
Mount St. Helens was once
almost 3,000 meters tall.
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When a volcano erupts, lava and ash can spread
over a large area. This changes the volcano’s shape. It
also changes the surrounding land.
Dormant and Active Volcanoes
An active volcano is one that erupts often or shows
signs that it will soon erupt. Kilauea is a volcano in
Hawaii. It has not stopped erupting since 1983.
A dormant volcano is one that has not erupted
for a long time. Mount Lassen is a dormant volcano.
Located in California’s Cascade Mountains, it has not
erupted since 1921. Scientists do not know when it will
erupt again. A volcano is extinct if scientists believe it
can never erupt again. Africa’s Mount Kenya is extinct.
The 1980 eruption of Mount St. Helens reduced
the height of the mountain by 400 meters.
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Earthquakes
Earth’s upper mantle lies beneath the crust. The
crust and upper mantle move around as plates.
Plates are very large. They move constantly. Many
earthquakes and volcanic eruptions happen where
plates meet.
A fault is a break or crack in rocks where Earth’s
crust can move. Rocks sometimes get stuck in faults.
But the plates continue to move. As they move, they
push on the rocks. If they push hard enough, the rocks
will break. This causes the plates to move suddenly.
The sudden movement that causes Earth’s crust to
shake is an earthquake.
The San Andreas fault (below) runs for hundreds
of kilometers through California.
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Earthquakes can change Earth’s surface quickly.
They cause vibrations which move through Earth as
waves. The waves can cause cracks in Earth’s surface.
Plates Meeting
The San Andreas fault is mostly in California. It is
where two plates meet. Many towns and cities are built
along it. Earthquakes happen frequently near the fault.
Some earthquakes are very powerful. In 1906, a
strong earthquake broke gas lines in San Francisco.
Another strong earthquake did the same thing to San
Francisco in 1989. The broken gas lines caused fires
each time. Earthquakes can also cause landslides.
The San Andreas
fault runs through
San Francisco. It
caused earthquakes
there in 1906 and
1989.
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Glossary
What did you learn?
1. What is a peninsula?
deposition
the laying down of pieces of rock and
soil
earthquake
a sudden movement that causes
Earth’s crust to shake
erosion
the movement of weathered materials
fault
a break or crack where Earth’s crust
can move
landform
natural feature of Earth
landslide
the rapid downhill movement of a
large amount of rock and soil
soil
the thin layer of loose, weathered
material that covers most of the land
surface of Earth
transport
to carry from one place to another
volcano
place on Earth’s crust where magma
reaches the surface
weathering
the process that breaks down rocks in
Earth’s crust into smaller pieces
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2. What kinds of materials are deposited from water first?
3. What kind of volcano is Mount Lassen? When did it last erupt?
4.
You learned about three things that
cause rapid changes to landforms: landslides, earthquakes, and
volcanoes. Write about all three in a couple of paragraphs.
Include an introductory paragraph, supporting paragraphs,
and a concluding paragraph. Use correct indentation.
5.
Sequence Describe the sequence of events when water
physically weathers rock.