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Forming and Transporting Rock Particles
A sandy ocean beach, a gravel bar in a river, and a muddy lake bottom
all consist mainly of rock particles. These particles were broken away
from rocks by the action of water or wind or a combination of both.
Such particles may vary in size from boulders to sand to tiny bits of clay.
Teach from Visuals
To help students interpret the visual
showing sediment particles, ask:
• As water currents slow down, what
size of particles settles first? the
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A
largest, gravel
• What size of particles settles next? the
next smallest, sand, and then silt and
clay
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CLASSZONE.COM
• What does the type of sedimentary
rock that forms depend on? the size
Find information on
sedimentary rocks.
Just as water washes mud off your hands as it runs over them,
rainwater washes away rock particles as it flows downhill. The water
carries these rock particles to streams and rivers, which eventually
empty into lakes or oceans. Strong winds also pick up sand and rock
dust and carry them to distant places.
As winds or water currents slow down, rock particles settle on the
land or at the bottom of rivers, lakes, and oceans. The sediments form
layers as larger particles settle first, followed by smaller ones.
of rock particles from which it forms
Forming Loose Sediments into Rocks
If you have ever watched workers building a road, you know that they
first put down layers of gravel and other materials. Then they press the
layers together, using a huge roller. In a similar way, layers of sediments
Integrate the Sciences
The faster a current flows, the larger
the particles it can carry. Gravity works
against the ability of water to carry particles. A basic law of physics states that all
particles, no matter what their size, fall
to Earth at the same speed in a vacuum.
But in a fluid such as water, larger particles settle faster than smaller ones.
Density is also a factor; however, the
most common minerals in sediments
have similar densities.
Sorting Sediments by Size
Fast-moving water can move large particles of sediment. As the
water slows, the sediment particles settle from it by size.
1
Water in a lake
usually moves
fastest near the
shore or where
a river enters.
In deeper areas,
water moves
slower.
gravel
sand
silt and clay
Ongoing Assessment
Identify the types of materials that
make up sedimentary rocks.
2
Ask: How does most sedimentary rock
develop? Sedimentary rock develops as
Gravel settles near the
shore. Rock containing
large sediment particles,
such as gravel, is known
as conglomerate.
layers of sediments (rock particles, minerals, and plant and animal remains) build
up on land or underwater.
3
4
Sand is carried farther from shore. Rock
that forms from sand-sized particles is
known as sandstone.
Is shale more likely to form near the shore or near the
middle of a big lake or ocean?
Answer: in the middle,
because silt and clay
particles can settle only from very
slow-moving water
A
90 Unit: Earth’s Surface
DIFFERENTIATE INSTRUCTION
?
More Reading Support
A How do rock particles
get into streams and
rivers? Rainwater washes
away rock particles as it
flows downhill, and it
carries these particles
into streams and rivers.
A 90 Unit: Earth’s Surface
Silt and clay are carried
into deep water. Rock
that forms from silt- and
clay-sized particles is
known as shale.
English Learners Point out the introductory clauses beginning with If on pp. 90–91 and p. 93. Tell English learners that
these clauses require them to recall something or imagine a
hypothetical situation. Point out that while the clauses beginning with If are fragments, the main clauses that follow contain
the subject and are complete sentences. English learners might
also lack prior knowledge about the white cliffs of Dover, mentioned on p. 92.
Integrate the Sciences
Both geologists and biologists have used
fossils to learn about Earth’s history. The
British naturalist Charles Darwin based
his theory of evolution partly on his
study of fossil organisms and his understanding of Earth’s geologic history. He
also observed animals and plants in their
native environments.
composed of rock particles may get pressed together to form rock.
One layer gets buried by another, and then another. The overlying
layers apply pressure to, or press down on, the sediments underneath.
Small particles of sediment, such as silt and clay, may be formed
into rock by pressure alone. In other sedimentary rocks the particles
are held together by minerals that have crystallized between them,
acting as cement. Over a long time, these processes transform loose
sediments into sedimentary rocks.
Check Your Reading
What are two processes that can change sediments into rocks?
Teach from Visuals
To help students interpret the photographs of coal:
Some rocks form from plants or shells.
Processes similar to the ones that produce sedimentary rocks from
rock particles also produce rocks from shells or plant remains.
These remains are fossils. A fossil is the remains or trace of an
organism from long ago.
MAIN IDEA WEB
• Ask: What kind of plant appears to be
preserved in the sample of coal? fern
Add a web to your notebook for each main idea.
• Tell students this coal formed in an
ancient swamp. Ask: Does the land
in the photograph appear to be a
swamp? no
Coal
If you look at a piece of coal through a magnifying glass, you may be
able to make out the shapes of bits of wood or leaves. That is because
coal is made up of remains of plants—dead wood, bark, leaves, stems,
and roots. Coal is an unusual sedimentary rock because it forms from
plants instead of earlier rocks.
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B
• Ask: What does this tell you about the
area? The environment has changed
over time.
The coal people use today started forming millions of years ago
in swamps. As plants died, their remains fell upon the remains of
earlier plants. Then layers of other sediments buried the layers of
plant remains. The weight of the sediments above pressed the plant
material into coal.
The dark layer in
these rocks is coal.
Ongoing Assessment
Answer: Sediments can
be put under pressure or
cemented together by new minerals to
form rock.
Here, you can see fossils of
ancient plants preserved
in coal.
Chapter 3: Rocks 91
A
DIFFERENTIATE INSTRUCTION
?
More Reading Support
B How is a rock like coal
formed? Plant remains
are buried under layers
of sediment. The weight
of the sediments above
presses the plant material
into coal.
Below Level
Have students draw a diagram of the process
by which coal forms. The diagram should show layers of plant
remains, with the layers becoming more and more compacted
as the depth increases.
Chapter 3 91 A
Teach from Visuals
To help students interpret the photographs showing limestone formation, ask:
Limestone
Limestone is made up of carbonate minerals, such as calcite. The shells
and skeletons of ocean organisms are formed of these minerals.
When the organisms die, the shells and skeletons settle on the ocean
floor as layers of sediment. Over time, the layers become buried,
pressed together, and cemented to form limestone. The photographs
below show how loose shells can become limestone.
• In the three photographs, what overall
change do you see in the rock? The
shells become harder to see.
• What caused this change? The
minerals in the shells dissolved and
re-formed to produce limestone.
• How does the limestone in the coral
reef pictured at the bottom of the
page differ from the limestone in the
top photographs? The limestone in
the coral reefs is added directly to
the reef. It is not loose sediment first.
These shells were made
by ocean organisms.
Integrate the Sciences
Limestone is a biochemical sedimentary
rock. Many marine organisms take in
dissolved calcium carbonate from
seawater and then secrete it to form
their shells. Coral organisms, called
polyps, do the same thing. Most coral
polyps live in colonies, and the ones
that form coral reefs attach themselves
to one another. Coral reefs contain the
skeletons of millions of coral polyps.
reading tip
Notice that limestone made
up of cemented shells and
the limestone in coral reefs
were both formed by
ocean organisms.
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1
2
3
The shells get cemented
together into limestone
as some of their minerals dissolve and re-form.
Individual shells become
harder to see as minerals
in the limestone continue
to dissolve and re-form.
Over time, what was
once loose sediment
becomes limestone with
no recognizable shells.
The famous white cliffs of Dover, England, consist of a type of
limestone called chalk. The limestone began to form millions of years
ago, when the land was under the ocean. The rock developed from shells
of tiny organisms that float in the ocean. Most limestone comes from
shells and skeletons of ocean organisms. The materials the organisms
use to build their shells and skeletons are present in ocean water because
they were dissolved from earlier rocks. Like almost all sedimentary rock,
limestone forms from material that came from older rocks.
Coral reefs also consist of limestone that comes from organisms.
However, in the case of reefs, the limestone is produced directly as coral
organisms build their skeletons one on top of another. In the formation
of coral, the rock does not go through a loose-sediment stage.
EXPLORE
Revisit “How Can Rocks Disappear?” on
p. 73. Have students relate their results
to the formation of limestone.
The skeletons of these tiny
coral organisms eventually
make huge coral reefs.
A
92 Unit: Earth’s Surface
DIFFERENTIATE INSTRUCTION
?
More Reading Support
C Where does most
limestone come from? the
shells and skeletons of
ocean organisms
Advanced Have students explain how the formation of
limestone demonstrates the rock cycle. Minerals from existing
rocks dissolve in rainwater and are carried by rivers to the
oceans. Ocean organisms use dissolved carbonate minerals to
form their shells. When the organisms die, their shells build
up on the ocean floor and form layers of sediment. The layers
get buried and then pressed and cemented together to form
limestone.
R
A 92 Unit: Earth’s Surface
Challenge and Extension, p. 175
Teach from Visuals
To help students interpret the photographs of limestone formations, ask:
Some rocks form when dissolved
minerals re-form from water.
• How do the stalactites pictured in the
top right photograph form? Water that
If you have grown crystals in a container, you know that
some substances can dissolve in water and then re-form
as the water evaporates. The same process happens in
nature. Some sedimentary rocks are made up of minerals
that crystallized as water dried up.
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D
drips from the roof of a cave leaves
behind minerals to form stalactites.
• What do the towers in the photograph
at the bottom of the page have in
common with these stalactites? They
The water in oceans, lakes, rivers, and streams
contains minerals that came from rocks. Some of these
minerals are in solid form. As rainwater washes over
rocks, it picks up pieces of minerals and rock particles
and carries them into streams and rivers, where many of
them settle to the bottom. However, some of the minerals
dissolve in the water and are carried along with it.
both consist of limestone that had
been dissolved in water.
Teach Difficult Concepts
Water is shaping this limestone cavern.
Water dissolves and transports minerals,
then leaves the minerals behind as it
evaporates.
Students may not understand how the
limestone towers in Mono Lake formed.
Streams from the Eastern Sierras flow into
Mono Lake, carrying salts and dissolved
minerals. As a result, the lake water contains large amounts of carbonates. The
lake is also fed by underground springs
that carry calcium. In the lake, the calcium
combines with carbonate, and the compound precipitates out as limestone. Over
a long period of time, the limestone builds
up into underwater towers around the
mouths of the springs.
Water often flows through cracks in rock that is near
Earth’s surface. As water moves through limestone,
some of the rock dissolves. A large open space, or cave,
can be left in the rock. As the water flows and drips
through the cave, some of it evaporates. The new limestone that forms
can take many odd and beautiful shapes.
Sometimes minerals crystallize along the edges of lakes and oceans
where the climate is dry and a lot of water evaporates quickly. Over time,
the minerals build up and form layers of sedimentary rock. Rock salt
and gypsum form in this way. Under the city of Detroit, for example,
is a large bed of rock salt that developed when part of an ancient
ocean dried up.
check your reading
How are the origins of rock salt and some limestone similar?
These limestone towers
in Mono Lake, California,
formed underwater.
They are now above the
surface because the lake
level has dropped.
Real World Example
The deposits that form on a water
faucet are an everyday example of
minerals precipitating out of water.
Ongoing Assessment
Describe the three groups of
sedimentary rocks.
Ask: How do the three groups of sedimentary rocks form? from rock particles
Chapter 3: Rocks 93
A
DIFFERENTIATE INSTRUCTION
?
More Reading Support
D Where do the
minerals dissolved in
water come from? from
rainwater washing over
rocks
Alternative Assessment Have students draw and label
diagrams showing the three ways sedimentary rock is formed:
rock particles that build up and get pressed together, shells or
plant remains that build up and get pressed together, dissolved
minerals that re-form from water.
that build up in layers and get pressed
together; from shells or plant remains
that build up in layers and get pressed
together; from dissolved minerals that
re-form from water
Answer: Both can dissolve
in water, be carried to a
new location, and re-form as the water
evaporates.
Chapter 3 93 A
INVESTIGATE Rock Layers
PURPOSE Make a model to show how
sedimentary rocks form in layers
Rock Layers
TIP 20 min. Divide the class into small
groups for this activity.
How do sedimentary rocks form in layers?
SKILL FOCUS
Modeling
PROCEDURE
WHAT DO YOU THINK? Layers of sediment are deposited at different times
and contain sediments of different
sizes. Layers of sediment made up
mainly of pebbles or gravel would be
deposited when water is moving very
quickly, such as during a flood or
where a mountain river spreads out
and water slows down. The middle
layer of sand-size sediment would be
deposited when water has slowed
down but is still moving fairly quickly.
The plaster of Paris is similar to minerals that grow between the particles of
sediment and cement them together.
1
Prepare the plaster of Paris by
mixing it with the water.
4 Mix the rest of the gravel with
the rest of the plaster of Paris.
Add the mixture to the paper
cup, on top of the sand mixture.
2 Mix 2 tablespoons of the gravel
with 2 tablespoons of the plaster
of Paris and pour the mixture
into the paper cup.
5 After the mixtures harden for
3 Mix the sand with 2 tablespoons
about 5 minutes, tear apart the
paper cup and observe the layers.
of the plaster of Paris and the
food coloring. Add the mixture
to the paper cup, on top of the
gravel mixture.
MATERIALS
• 1 paper cup
• 3 mixing cups
• 6 tbs plaster
of Paris
• 3 tbs water
• 4 tbs gravel
• 2 tbs sand
• 3 drops food
coloring
TIME
20 minutes
WHAT DO YOU THINK?
• How is the procedure you used to make your
model similar to the way sedimentary rock forms?
• Describe how similar layers of real rock
could form.
CHALLENGE A model of how limeCHALLENGE How would you create
stone forms could be made in a similar
way, using shells in place of sand or
pebbles.
R
a model to show the formation of fossil-rich
limestone?
Datasheet, Rock Layers, p. 176
Sedimentary rocks show the action of wind
and water.
Technology Resources
Customize this student lab as needed
or look for an alternative. Print rubrics
to assess student lab reports.
reading tip
Notice that sedimentary
rocks are laid down in
layers. As conditions in an
area change, so do the
characteristics of the layers.
Lab Generator CD-ROM
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E
Sedimentary rocks are laid down in layers, with the oldest layers on the
bottom. A geologist studying layers of sedimentary rocks can tell something about what conditions were like in the past. For instance, fossils
of fish or shells in a layer of rock show that the area was covered by a
lake or an ocean long ago.
Fossils are not the only way to tell something about what past
conditions were like. The sediments themselves contain a great deal of
information. For example, a layer of sedimentary rock may contain
sediment particles of different sizes. The largest particles are at the very
bottom of the layer. Particles higher in the layer become increasingly
smaller. A layer like this shows that the water carrying the sediment
was slowing down. The largest particles dropped out when the water
was moving quickly. Then smaller and smaller particles dropped out
Metacognitive Strategy
Ask students to write a few sentences
at the end of their lab reports describing
how making this model helped them
understand the process of sedimentary
rock formation.
A
94 Unit: Earth’s Surface
Teaching with Technology
If you have access to a video camera,
you might tape groups of students
showing the models they created and
explaining the answers to the Investigate
questions. You could show the videotape
at an open house or play it during the
wait time at parent conferences.
A 94 Unit: Earth’s Surface
DIFFERENTIATE INSTRUCTION
?
More Reading Support
E What do fossils of fish
in a layer of rock indicate?
that the area was covered by a lake or ocean
long ago
Below Level
After completing the Investigate in roughly 20
minutes, some students may think that actual sedimentary rock
layers are also formed over short time periods. Stress that the
layers of rock are formed over thousands to millions of years
and continue to change.
Ongoing Assessment
Crossbeds
Ripples
Explain how sedimentary rocks
record past conditions.
Mud Cracks
Ask: What can geologists learn about
past conditions by examining dunes,
bars, and ripples? the direction that
wind or water currents were traveling
and carrying sediments
The tilted layers in these sandstone rocks
are called crossbeds. The layers were once
moving sand dunes.
The surface of this sandstone preserves ancient
sand ripples.
Answer: Ripples preserved
in rocks show that sand
was once moving along that surface.
Mud cracks show that an area was
once wet and then became very dry.
As wet silt and clay dry
out, cracks develop on the
surface of the sediment.
as the water slowed. This type of layer is often created by a flood,
when a large amount of water is at first moving quickly.
Reinforce
Sedimentary rocks can give information about the directions in
which long-ago wind or water currents were traveling when sediments
settled from them. Sand can be laid down in tilted layers on the slopes
of sand dunes or sandbars. Sand can also form ripples as water or
wind moves over its surface. If the sand has been buried and cemented
into sandstone, a geologist can examine it and tell the direction in
which the water or wind was moving.
Have students relate the section to the
Big Idea.
R
Reinforcing Key Concepts, p. 177
ASSESS &
RETEACH
Some rocks made of clay or silt have cracks that developed when
the mud from which they formed dried out. Mud cracks show that the
rocks formed in areas where wet periods were followed by dry periods.
check your reading
What could a geologist learn by finding rocks that have ripples
or mud cracks?
Assess
A
Section 3.3 Quiz, p. 44
Reteach
Copy the following information on the
board and use it for review.
KEY CONCEPTS
CRITICAL THINKING
1. What types of material can
make up sediments?
4. Infer Why is coal called a
fossil fuel?
2. Describe the three processes by
which sedimentary rocks form.
5. Analyze How could the
speed of flowing water change
to lay down alternating layers
of sand and mud?
3. Describe how a sedimentary
rock can show how fast water
was flowing when its sediments
were laid down.
How sedimentary rock forms
CHALLENGE
6. Synthesize How is it possible
for a single sedimentary rock to
contain rock particles, animal
shells, and minerals that
crystallized from water?
• From layers of rock particles that are
pressed and cemented together
• From layers of shells or plants that
pressed and cemented together
• From dissolved minerals that are left
behind as water evaporates
What sedimentary rocks can tell us
about past conditions
Chapter 3: Rocks 95
2. pieces of minerals and rocks
are cemented together; shells
and skeletons of ocean animals are cemented together;
dissolved minerals are left
behind as water evaporates
• Fossils of fish indicate an area was
once covered with water.
• Layering with the largest particles first
might indicate that a flood occurred.
ANSWERS
1. pieces of minerals, rocks,
plant and animal remains
A
3. A rock made of large
pieces shows that water
was moving quickly.
silt and clay that make up
mud settle from slowermoving water.
4. It is made of the remains
of ancient plants.
6. A rock that forms from
sediment on a beach could
have pieces of shells and
rocky sand cemented
together by minerals that
crystallized from water.
5. Weather patterns cause
flow rates of rivers to vary.
Sand settles from faster-moving water; smaller pieces of
Technology Resources
Have students visit ClassZone.com
for reteaching of Key Concepts.
CONTENT REVIEW
CONTENT REVIEW CD-ROM
Chapter 3 95 A