Name
CHAPTER 6
Class
Date
The Rock and Fossil Record
SECTION
2 Relative Dating: Which Came First?
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 2b
• What is relative dating?
• How can rock layers be disturbed?
What Is Relative Dating?
Imagine that you get a newspaper every day. At the end
of the day, you stack the day’s paper on top of the paper
from yesterday. In time, you build up a large stack of newspapers. Where are the oldest newspapers in the pile? Where
are the newest ones? The oldest papers are at the bottom of
the pile, and the newest ones are at the top.
Layers of rock are similar to your stack of newspapers.
In most cases, the oldest layers of rock are found below the
youngest layers. The idea that younger rocks lie above older
rocks is called superposition.
The idea of superposition can help geologists learn the
order in which different rock layers formed. In general, rock
layers near the top of a rock sequence formed after layers of
rock lower in the sequence. Therefore, the layers at the top
of the sequence are younger than the layers lower down.
Figuring out whether a rock layer is older or younger than
the layers around it is called relative dating.
STUDY TIP
Compare In your notebook,
make a chart explaining
different ways that rock
layers can be changed after
they form.
READING CHECK
1. Define What is relative
dating?
4HESELAYERSOF
SEDIMENTARYROCK
WERELAIDDOWNIN
ORDERJUSTASTHE
NEWSPAPERSWERE
4HEYOUNGESTROCK
LAYERSLIKETHE
YOUNGESTNEWSPAPERS
AREONTHETOP
4HEOLDESTLAYERSLIKE
THEOLDEST
NEWSPAPERSAREON
THEBOTTOM
The idea of
says that rock layers at the bottom of a body of
rock are older than layers at the top. Geologists can use this idea to determine the
relative age of different rock layers.
TAKE A LOOK
2. Identify Fill in the blank
line in the figure.
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The Rock and Fossil Record
Name
SECTION 2
Class
Date
Relative Dating: Which Came First? continued
THE GEOLOGIC COLUMN
Say It
Share Experiences Have
you ever been to a place
where you could see many
rock layers stacked up? In a
group, talk about what you
observed.
The idea of superposition only applies to rock layers
that have not been changed after they formed. However,
not all rock layers are undisturbed. Forces from inside
the Earth and processes on the Earth’s surface can affect
rock layers. These forces and processes can break rock
layers apart or cause them to bend or tilt. Sometimes, the
forces can even turn the rock layers upside down!
These disruptions can make it difficult for a geologist to determine the relative ages of different rocks.
However, geologists have an important tool that can help
them in relative dating: the geologic column.
The geologic column is a detailed series of rock layers. It contains all the known fossils and rock formations
on Earth, ordered from oldest to youngest. Geologists
have created the geologic column by combining information from all over the world.
'EOLOGICCOLUMN
2OCKSEQUENCES
#
"
!
TAKE A LOOK
3. Explain How do
geologists construct the
geologic column?
The three rock sequences (A, B, and C) are from three different places on Earth.
Some of the rock layers are found in more than one rock sequence. Geologists
construct the geologic column by combining information from rock sequences
around the world.
Geologists use the geologic column to help them interpret rock sequences. They also use it to help them identify rock layers in complicated rock sequences.
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Name
SECTION 2
Class
Date
Relative Dating: Which Came First? continued
How Can Rock Layers Be Disturbed?
Gravity causes sediment to be deposited in flat, horizontal layers. Flat, horizontal layers of sediment should
form flat, horizontal layers of rock. If rock layers are not
horizontal, then some force must have disturbed them
after they were formed.
CHANGED ROCK LAYERS
Folding and tilting are two ways that rock layers can
be disturbed. Folding happens when rock layers are bent
because of pressure. Tilting happens when forces from
inside Earth move rock layers so that they are slanted.
READING CHECK
4. Describe What is one
thing that tells a geologist
that rock layers have been
disturbed?
TAKE A LOOK
5. Compare How is folding
different from tilting?
Folding happens when rock layers
bend and buckle under pressure.
Tilting happens when forces from
inside Earth cause rock layers to
become slanted.
Faults and intrusions can cut across many rock layers.
A fault is a break or crack in Earth’s crust. Large pieces
of rock can move or slide along a fault. An intrusion
forms when melted rock moves into cracks in rock layers
and then cools.
READING CHECK
6. Identify What kind of
rock is an intrusion made of?
TAKE A LOOK
A fault is a break in Earth’s crust. Rock
can slide along a fault and disturb rock
layers.
Intrusions form when melted rock
moves through cracks in rock layers.
The melted rock cools and hardens to
form igneous rock.
7. Define What is a fault?
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Name
SECTION 2
Class
Date
Relative Dating: Which Came First? continued
MISSING ROCK LAYERS
Critical Thinking
8. Infer Imagine that you
are a geologist and you find
an unconformity between
two rock layers. What can you
guess about the environment
at the time the unconformity
was forming?
Think back to your stack of newspapers. Imagine
that you want to read something in the paper from
Valentine’s Day, February 14. However, when you look,
the paper from February 14 is not there. The papers go
from February 13 to February 15. What happened? Maybe
you didn’t put that day’s newspaper on the stack. Maybe
someone took that paper out of the stack.
The same ideas that apply to a missing newspaper apply
to a missing rock layer. An unconformity is a break in, or a
missing part of, the geologic record. Unconformities can form
when sediment is not deposited in an area for a long time.
If sediment is not deposited, no new layer of rock can form.
This is like your forgetting to put a newspaper onto the stack.
Unconformities can also form when erosion removes a
layer of rock after it formed. This is like someone taking
a paper out of the stack.
2. If erosion stops, no
more sediment moves
downhill. Deposition
stops. No sediment layers
1. Sediment is eroded
from hills or mountains. form for a long time.
It moves downhill. The
sediment is deposited in
layers in a low area.
3. When erosion begins again,
more sediment moves downhill.
It is deposited in the low area.
An unconformity has formed
where no sediment was
deposited for a long time.
Unconformity
TAKE A LOOK
9. Identify Give two ways
that an unconformity can
form.
Unconformity
Uplift
2. Erosion removes some of the
mountain and sediment layers. The
material is moved farther away.
3. More sediment begins
to be deposited. An unconformity has formed where
material was eroded.
Geologists put all unconformities into three main
groups: disconformities, nonconformities, and angular
unconformities.
DISCONFORMITIES
A disconformity is a place where part of a sequence of
parallel rock layers is missing. Disconformities can form
when a sequence of rock layers is pushed up because
of forces inside the Earth. Erosion removes the younger
layers of rock at the top of the sequence, forming an
erosion surface. Later, deposition starts happening again,
and sediment buries the erosion surface.
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The Rock and Fossil Record
Name
Class
SECTION 2
Date
Relative Dating: Which Came First? continued
Disconformities are
the most common
kind of unconformity.
The rock layers above
the disconformity
may be thousands to
many millions of years
younger than those
below it.
Disconformity
TAKE A LOOK
10. Define What is a
disconformity?
NONCONFORMITIES
A nonconformity is a place where sedimentary rocks
are found on top of eroded igneous or metamorphic
rocks. The igneous or metamorphic rocks can be pushed
up by forces inside the Earth. Then, erosion can remove
some of the rock. Later, sediment may be deposited on
top of the eroded rock.
READING CHECK
11. Explain How do
nonconformities form?
Nonconformity
Sedimentary rock
Igneous rock
The rock layers above
a nonconformity may
be millions of years
younger than those
below it.
ANGULAR UNCONFORMITIES
An angular unconformity is a place where horizontal
rock layers lie on top of tilted or folded rock layers. The
tilted or folded layers were eroded before the horizontal
layers formed above them.
TAKE A LOOK
12. Compare How is a
nonconformity different from
an angular unconformity?
Angular unconformity
The rock layers above
an angular unconformity
may be millions of years
younger than those
below it.
Folding, tilting, faults, intrusions, and unconformities
all disturb rock layers. Sometimes, a single rock body
may have been disturbed many times. Geologists must
use their knowledge of the things that disturb rock layers
to piece together the Earth’s history.
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The Rock and Fossil Record
Name
Class
Date
Section 2 Review
NSES
ES 2b
SECTION VOCABULARY
geologic column an ordered arrangement of
rock layers that is based on the relative ages of
the rocks and in which the oldest rocks are at
the bottom
relative dating any method of determining
whether an event or object is older or younger
than other events or objects
superposition a principle that states that
younger rocks lie above older rocks if the
layers have not been disturbed
unconformity a break in the geologic record
created when rock layers are eroded or when
sediment is not deposited for a long period of
time
1. Identify Give two ways that an unconformity can form.
2. Apply Concepts If the tops of the rock layers in the figure below were eroded and then
deposition started happening again, what kind of unconformity would have formed?
3. Describe What does the idea of superposition say about rock layers that have not
been disturbed?
4. Identify Give two ways in which geologists use the geologic column.
5. Explain How does a disconformity form?
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The Rock and Fossil Record
Earth Science Answer Key continued
Chapter 6 The Rock and
Fossil Record
3. Burning coal contributes to acid precipitation.
Surface mining of coal removes soil and can
destroy habitats. Petroleum spills can poison
the oceans. Burning petroleum contributes to
smog. Natural gas is highly flammable.
SECTION 1 EARTH’S STORY AND THOSE
WHO FIRST LISTENED
1. deposits of sediment
2. Catastrophism: Earth changes only because
SECTION 3 ALTERNATIVE RESOURCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
fission and fusion
barium-142 and krypton-91
Nuclear energy does not cause air pollution.
two protons and two neutrons
People haven’t figured out how to control
fusion reactions on Earth.
Many windmills can get more energy from
the wind than just one or two can.
Windmills take up a lot of space.
Fuel cells produce electricity, and water is
the waste product.
renewable, nonpolluting, inexpensive
Habitats might be destroyed or species
might become extinct.
The hot water can be used to create electricity
and heat buildings.
Arrows follow the path of the numbered steps.
of sudden events.
Uniformitarianism: Earth changes only
because of slow processes over time.
3. Some changes on Earth happen quickly, but
most happen slowly.
4. plants
5. Fossils can show how environments and life
on Earth have changed with time.
Review
1. Sudden processes can cause large changes
2.
Review
1. The sun cannot be used up.
2. Because biomass grows back very quickly,
3.
4.
it can be replaced. However, if people use
biomass more quickly than it can grow back,
it is no longer considered renewable.
3. Geothermal energy is useful only if hot rock is
near the surface. The city near the volcano is
more likely to have hot rock near the surface,
so it is more likely to be able use geothermal
energy.
4. Wind energy is useful only in places that
have strong, regular winds. Also, wind
energy collection requires large areas of
empty land for the windmills, so it is usually
not usable in cities or other crowded areas.
5. Answers will vary. Students should show
an understanding of the requirements of
their chosen kind of energy (e.g., a student
living in the desert should not choose
hydroelectric power).
5.
or small changes. The changes can be longterm or short-term. For example, a hurricane can affect a small part of Earth for a
few days. A comet impact on Earth could
cause Earth’s climate to change significantly.
Rocks are broken down into smaller pieces.
The pieces are carried over the Earth’s surface and deposited in layers.
A fossil is evidence that life once existed in
a place.
Some fossils are signs, such as footprints,
that organisms once existed.
Because the clams lived in shallow ocean
water, the rock in which the fossils were
found must have been part of a shallow ocean
5 million years ago. This probably means that
the ocean in this area once reached much
farther inland than it does today.
SECTION 2 RELATIVE DATING: WHICH
CAME FIRST?
1. determining the age of a rock relative to
2.
3.
4.
5.
6.
7.
other rocks
superposition
They combine information from rock
sequences around the world.
They are not horizontal.
Folded rocks are bent or buckled. The shape
of the rock layers has changed. Tilted rocks
are simply moved so that they are no longer
horizontal.
igneous rock
a break in Earth’s crust
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Earth Science
Earth Science Answer Key continued
2. After 1 million years, there would be (1/2) 8. Possible answers: There was no deposition
9.
10.
11.
12.
(20 mg) 10 mg of parent isotope remaining.
After 2 million years, (1/2) (1/2) (20 mg) 5 mg of parent isotope would remain.
3. uranium-238, rubidium-87
4. Potassium-40, because it can be used to date
rocks that are older than about 1 million years.
5. C-14 dating can be used only on the remains
of living organisms. Igneous rocks do not
contain these remains. C-14 dating can be
used only on remains that are less than
50,000 years old. Dinosaur bones are older
than this.
happening at that time; there was a lot of
erosion happening at that time.
erosion, nondeposition
a place where part of a sequence of parallel
rocks is missing
Rocks are pushed up and eroded. Later, sediment is deposited on top of the eroded rock.
The rock layers below an angular unconformity are tilted and may be any kind of rock.
Review
1. An unconformity can form if no sediment is
2.
3.
4.
5.
deposited for a long time. An unconformity can
also form if layers of rock are eroded away.
an angular unconformity
The youngest rock layers are at the top, and
the oldest rock layers are at the bottom.
to interpret rock sequences and to identify
rock layers
A sequence of rock layers is uplifted and
eroded. Then, sediment is deposited on the
exposed rock layers. After a while, the sediment turns into rock.
SECTION 4 LOOKING AT FOSSILS
1. Body fossils are fossilized parts of an
2.
3.
4.
5.
6.
SECTION 3 ABSOLUTE DATING: A
MEASURE OF TIME
7.
1. atoms of an element with different numbers
2.
3.
4.
5.
6.
7.
of neutrons
Radioactive isotopes can break down. Stable
isotopes do not break down.
The amount of parent isotope decreases, and
the amount of daughter isotope increases.
8 mg
No, because K-Ar dating can be used only for
rocks that are older than about 1 million years.
by eating plants
Parent isotope
Daughter
isotope
8.
9.
10.
Review
1. tracks, burrows, coprolites
2. Only a small fraction of the organisms that
Half-life
Potassium-40
argon-40
1.3 billion years
Uranium-238
lead-206
4.5 billion years
Rubidium-87
strontium-87
48 billion years
Carbon-14
nitrogen-14
5,730 years
organism. Trace fossils are signs that an
organism once existed.
shells, teeth, bones
when minerals replace an organism’s tissue
They are not made of parts of an organism,
but they show that an organism once existed.
an impression left in sediment
the kinds of organisms that lived in the past;
how the environment has changed; how
organisms have changed
Organisms that had hard parts or lived in
certain environments were more likely to be
fossilized when they died.
They compare fossils. They also compare
fossils to living organisms.
Answers include: according to their age, by
absolute and relative dating methods
their shells
have existed in Earth’s history have been
fossilized. Many fossils have not yet been
discovered.
3. A beetle, because fossils in amber are made
when an organism gets stuck in tree sap.
Smaller organisms that can be found on
trees are more likely than other organisms
to become fossils in amber.
4. The climate was probably much warmer
when the plant was alive.
5. It must be common throughout the world. It
must have existed for a relatively short geologic time. It must be easy to identify.
Review
1. Radiometric dating uses known rates of
radioactive decay to determine the age of a
rock sample.
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Earth Science