Name
CHAPTER 8
Class
Date
The Rock Record
1 Determining Relative Age
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What is the principle of uniformitarianism?
• How does the law of superposition help scientists to
determine the relative age of rocks?
• How do the three types of unconformities compare?
• How does the law of crosscutting relationships help
scientists to determine the relative age of rocks?
What Is Uniformitarianism?
At one time, many people thought Earth was only a
few thousand years old. However, scientific evidence
now shows that Earth is much older. Scientists think
Earth is about 4.6 billion years old.
The idea that Earth is billions of years old started in
the 1700s with a Scottish physician and farmer named
James Hutton. The diagram below describes how Hutton
found evidence that Earth is very old.
Hutton saw that his farmland changed slightly each year.
READING TOOLBOX
Color Code Choose four
colored pencils, one color for
each Key Ideas question. As
you read, use the color you
chose for each question to
underline the answer in the
text. You might want to use
the pencils to add notes in
the margin that apply to the
Key Ideas.
He observed that the processes that changed his land worked slowly.
He guessed that those same processes could produce large changes in
Earth’s surface over long periods of time.
He guessed that Earth is very old. It has changed slowly over time by
the same processes that are changing it today.
Hutton thought that people could learn about Earth’s
past by studying the present. His principle of
uniformitarianism states that geologic processes
happened the same way in the past as they do today.
Volcanic eruptions, erosion, and earthquakes are
examples of geologic processes.
Later scientists added to Hutton’s principle of uniformitarianism. They found evidence that the processes of
the past and present are the same. They also learned that
the rates of those processes can vary over time.
READING CHECK
1. Identify How did later
scientists adjust Hutton’s law
of uniformitarianism?
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The Rock Record
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SECTION 1
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Date
Determining Relative Age continued
What Is Relative Age?
READING CHECK
2. Describe What can a scientist learn from the relative
ages of rock layers?
If you look at a group of brothers and sisters, you
might not know the exact age of each person. However,
you might use clues such as height to help you figure out
which child is youngest. Scientists use a similar method
to learn about Earth’s past.
If scientists determine the order in which strata, or
rock layers, formed, they can determine the relative age
of rocks. To date is another way of saying “to determine
the age of.” Relative age is the age of an object or event
compared to another object or event. The relative age
of a rock can tell you that one rock layer is older than
another. However, it cannot tell you the rock’s age in
years.
Although igneous and metamorphic rock may form
layers, scientists generally use the layers in sedimentary
rocks to determine relative ages. The figure below shows
a landform made up of layers of sedimentary rock.
The layers of sedimentary
rock that make up these
rock formations in Utah
were deposited over
millions of years.
LOOKING CLOSER
3. Predict Where do you
think the oldest rock layers
in the rock formations are
found?
Remember that sedimentary rocks form as new sediments are deposited on old layers of sediment. As more
sediments are added, the layers become compressed,
or squeezed. The compressed sediments become stuck
together in sedimentary rock layers called beds. The
boundary between two beds is called a bedding plane.
READING CHECK
4. Identify Under what
conditions can scientists
apply the law of superposition directly?
What Is the Law of Superposition?
Scientists use the law of superposition to determine
the relative ages of layers of sedimentary rocks. This law
states that a layer of rock is older than the layers above
it. It also states that a layer of rock is younger than the
layers below it. Scientists can use the law of superposition only if the rock layers have not been disrupted or
deformed.
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The Rock Record
Name
SECTION 1
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Determining Relative Age continued
Skeletons and shells
of dead marine
organisms settle to
the ocean bottom
and pile up.
Talk About It
Extend Concepts With a
partner, talk about ways you
have found the relative ages
of two or more people,
objects, or events. For
example, you might determine the relative ages of
two trees by comparing the
thicknesses of their trunks.
C
B
A
D
C
Date
The layers of sediment
are buried by other
sediments. Then, the
buried sediments harden
to form limestone.
B
Scientists can apply the law
of superposition to determine the relative ages of the
rock layers.
LOOKING CLOSER
5. Apply Concepts According to the law of superposition, is layer B older or
younger than layer C?
A
Examine the figures above. According to the law of
superposition, layer A was the first layer that was deposited. Thus, it is the oldest layer. Layer D was the last layer
deposited. Thus, it is the youngest layer.
What Is the Principle of Original
Horizontality?
Sedimentary rock generally forms in horizontal
layers. The principle of original horizontality states
that if sedimentary rocks are undisturbed, the horizontal
layers will remain. If scientists find rock layers that
are not horizontal, they can assume that movements of
Earth’s crust have tilted or deformed the layers.
In many cases, the movement of tectonic plates pushes
older rock layers on top of younger layers. In such cases,
scientists cannot easily apply the law of superposition.
Scientists must first use other clues to figure out the original positions of the layers. Then, they can use the law of
superposition to find the relative ages of the rock layers.
Critical Thinking
6. Identify Relationships In
what way is the law of original horizontality important to
the law of superposition?
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The Rock Record
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Determining Relative Age continued
Graded Bedding Heavy particles settle
to the bottom of a river or lake faster
than smaller particles do. This pattern
of settling produces graded bedding.
Critical Thinking
7. Draw Conclusions Suppose you find layers of rock
in which the top layers have
larger particles than the
lower layers. What is one
conclusion you might make?
Cross-Beds As sand piles up, it forms
slanting layers.
GRADED BEDDING
One clue to the original position of rock layers is the
size of the particles in the layers. In many areas where
sediments are deposited, the largest, heaviest sediment
particles are deposited in the bottom layer. The arrangement of layers in which the largest particles are found in
bottom layers is called graded bedding.
CROSS-BEDS
The shape of the bedding plane is another clue to the
original position of the rock layers. When sandy sediments are deposited, they may form beds at an angle to
the bedding plane. These beds are called cross-beds.
The tops of these layers erode before new layers are
deposited.
RIPPLE MARKS
Wind or moving water can cause small waves called
ripple marks to form on the surface of sand. When the
sand becomes sandstone, the ripple marks may be preserved. If sedimentary rock layers are undisturbed, the
crests, or tops, of the ripple marks point upward. By
looking at the direction the ripple crests point, scientists
can figure out the original positions of disturbed layers.
READING CHECK
8. Describe What does an
unconformity indicate?
What Is an Unconformity?
In many cases, rock layers have been disturbed
from their original positions or changed in some way.
Disturbing rock layers can create an unconformity, or a
break in the geologic record. An unconformity shows that
deposition stopped for a period of time or that erosion
happened before deposition continued. The table on the
next page describes three types of unconformities.
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SECTION 1
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Determining Relative Age continued
Types of Unconformities
Type of Unconformity
Example
Nonconformity Sediments are
deposited on top of layers of igneous
or metamorphic rock. The boundary
between the layers represents a time
gap and is called a nonconformity.
Sedimentary rock
Igneous or
metamorphic rock
Angular unconformity A new
horizontal sediment layer is
deposited on top of sediment layers
that were folded or tilted and then
eroded. The bedding planes of the
new layers are not parallel to the
bedding planes of the older layers.
LOOKING CLOSER
9. Compare How is a nonconformity different from a
disconformity?
Disconformity Layers of new
sediment are deposited on horizontal
layers of old sediment that has
eroded. The boundary between the
eroded layers and new sediment
layers represents a time gap. The
boundary is called a disconformity.
What Are Crosscutting Relationships?
Horizontal rock layers may be disturbed by features
such as faults or intrusions. A fault is a break or crack in
Earth’s crust along which rocks shift their position. An
intrusion is igneous rock that forms when magma flows
between layers of solid rock and then cools and hardens.
When faults or intrusions have disturbed rock layers,
scientists may have a hard time determining relative age.
In such cases, scientists may apply the law of
crosscutting relationships. The law of crosscutting
relationships states that a fault or intrusion is always
younger than the rock it cuts through.
LOOKING CLOSER
10. Analyze Which is
younger—the fault or the
intrusion? Explain your
answer.
D
C
B
D
A
Igneous intrusion
C
Fault
B
A
Scientists use the law of crosscutting relationships to determine the
relative ages of rocks and the faults and intrusions within them.
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The Rock Record
Name
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Section 1 Review
SECTION VOCABULARY
law of crosscutting relationships the principle
that a fault or body of rock is younger than any
other body of rock that it cuts through
law of superposition the principle that a
sedimentary rock layer is older than the layers
above it and younger than the layers below it
if the layers are not disturbed
relative age the age of an object in relation to
the ages of other objects
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
uniformitarianism a principle that geologic
processes that occurred in the past can be
explained by current geologic processes
1. Define In your own words, state the principle of uniformitarianism.
2. Identify Relationships Complete the table below to describe how each concept or
feature relates to determining the relative age of rock layers.
Concept or Feature
How It Applies to Determining Relative Ages of
Rocks
Law of superposition
Scientists know sedimentary rocks form in horizontal
layers and stay in horizontal layers unless disturbed.
If they find layers that are not horizontal, they must
figure out the original position of the layers. Then they
can determine relative ages.
Law of crosscutting relationships
Ripple marks
3. Compare How is an angular unconformity different from a disconformity?
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