chapter
93
Clues to Earth’s Past
2
section ●
Relative Ages of Rocks
Before You Read
What You’ll Learn
■
how to tell the relative
ages of rock layers
■ how to interpret gaps in
the rock record
Think of two friends. You want to know who is older. What
information do you need to figure out who’s older?
Read to Learn
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Superposition
Imagine that you see an interesting car drive by. Then you
remember seeing a picture of the car in the January edition
of a magazine you have at home. In your room is a pile of
magazines from the past year. As you dig down through the
pile, you find magazines from March, then February.
January must be next. How did you know that the January
issue would be at the bottom?
To find the older magazine under newer ones, you used
the principle of superposition. How does this principle
apply to rocks? The principle of superposition states that in
layers of rock that have not been disturbed, the oldest rocks
are on the bottom and the rocks become younger and
younger toward the top.
Why are rocks in layers?
Sediments build up, forming layers of sedimentary rocks.
The first layer to form is on the bottom. A new layer forms
on top of the first one. A third layer forms on top of the
second layer. The bottom layer is the oldest, because it was
formed first. Sometimes, the layers of rock are disturbed.
When layers have been turned upside down, other clues are
needed to tell which rock layer is oldest.
Study Coach
Make Flash Cards As you
read this section, make flash
cards for each main topic. On
one side of the card, write the
topic. On the other side, write
key information.
●
B Compare Make a
three-tab Foldable to compare
the concept of relative age and
the principle of superposition.
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125
Relative Ages
1.
Identify What term
describes the age of
something compared with
the age of other things?
Remember the old magazine you were trying to find?
What if you want to find another magazine? You don’t
know how old it is, but you know it came after the January
issue. You can find it in the stack by using the principal of
relative age. Relative age is the age of something compared
with the ages of other things.
Scientists figure out the relative ages of rocks by studying
their places in a sequence. For example, if layers of
sedimentary rock have been moved by a fault, or a break in
Earth’s surface, the rock layers had to be there before the
fault cut through them. So, the relative age of the rocks is
older than the relative age of the fault. Relative age doesn’t
tell you how old the rock is in actual years. The rock layer
could be 10,000 years old or one million years old. The
relative age only tells you that the rock layer is younger than
the layers below it and older than the fault cutting through it.
Picture This
2.
Interpret Highlight the
layer of limestone in both
figures. In which rock layer
might the oldest fossils be
found?
It’s easy to figure out relative age if the rocks haven’t been
moved. Look at the figure below on the left showing rock
layers that haven’t been disturbed. Which layer is the oldest?
According to the principle of superposition, the bottom
layer is oldest.
Now look at the figure on the right where the rock layers
have been disturbed. If a fossil is found in the top layer that
is older than a fossil in the lower layer, it shows that the
layers have been turned upside down. This could have been
caused by folding during mountain building.
Limestone
Limestone
Coal
Coal
Sandstone
Sandstone
Undisturbed Layers
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Clues to Earth’s Past
Folded Layers
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do other clues help?
Unconformities
Layers of rock form a record of the past. But the record
may not be whole. Layers or parts of layers might be missing.
These gaps in the rock layers are called unconformities
(un kun FOR muh teez). Unconformities develop when
erosion removes rock layers by washing or scraping them
away. There are three types of unconformities.
What are angular unconformities?
3.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Forces below Earth’s surface can lift and tilt layers of
sedimentary rock as shown in the figure below. Over time,
erosion and weathering wear down the tilted rock layers.
Later, new layers of sedimentary rock are deposited on top
of the tilted and eroded layers. The unconformity that
results when new layers form on tilted layers is called an
angular unconformity.
Sedimentary rocks are deposited
originally as horizontal layers.
The horizontal rock layers are tilted as
forces within Earth deform them.
Picture This
4.
Angular unconformity
The tilted layers erode.
Identify What are gaps
in the rock layers called?
Identify Highlight the
angular unconformity in the
last figure.
An angular unconformity results
when new layers are deposited on the
tilted layers.
Why would a layer of rock be missing?
Now and then, a layer of rock is missing from a stack of
sedimentary rock layers. Careful study reveals an old surface
of erosion. At one time the rocks were exposed and eroded.
Later, younger rocks formed above the erosion surface when
sediments were deposited again. Even though all the layers
are parallel, the rock record still has a gap.
Reading Essentials
127
Disconformity When a rock layer is missing, this type of
unconformity is called a disconformity, shown in the figure
below. A disconformity also forms when a long period of
time passes without any new layers of rock forming.
Picture This
5.
Identify Highlight the
surface where rocks were
exposed and eroded before
new sediments were
deposited over them.
Newer layers formed as sediments were deposited
Old surface
of erosion
disconformity
Parallel rock
layers above
and below
What are nonconformities?
Another type of unconformity is shown in the figure
below. A nonconformity occurs when metamorphic or
igneous rocks are uplifted and eroded. Sedimentary rocks
are then deposited on top of the erosion surface. The
surface between the two rock types is a nonconformity.
Picture This
Determine In the figure
color the rock being
uplifted red. Color the
sedimentary rock being
deposited blue.
Old surface
of erosion
nonconformity
Igneous or
metamorphic
rock
underneath
Matching Up Rock Layers
Suppose scientists are studying a layer of sandstone. Later,
at an area 250 km away, they observe a layer of sandstone
that looks like the sandstone they studied in the first
location. Above the sandstone is a layer of limestone and
then another layer of sandstone. They return to the first
area and find the same sequence—sandstone, limestone,
sandstone. Based on their observations, they theorize that
the same layers of rock are in both locations. Often, layers
of rocks that are far apart can be matched up, or correlated.
7.
Explain What are two
ways to correlate rock
layers?
128
Clues to Earth’s Past
What evidence can correlate rock layers?
One way to correlate exposed rock layer from two places
that are far apart is to walk along the layer from one place
to the next. Walking along a layer can prove it is unbroken.
Layers can also be matched using fossil evidence. If the
same types of fossils are found in the same rock layer in
both places, it shows that the rock layer in each place is the
same age and also that it is from the same deposit.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
6.
Sedimentary rock deposited on top of eroded surface
After You Read
Mini Glossary
principle of superposition: states that in undisturbed rock
layers, the oldest rock is at the bottom and the rocks
become younger and younger toward the top
relative age: age of something compared to the age of
other things
unconformity: a gap in the rock layers due to erosion or a
period without rock deposit
1. Review the terms and their definitions in the Mini Glossary. Choose one term and
explain in your own words what it means.
2. Complete the table about unconformities.
Unconformities
Type
Description
Causes
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Angular
unconformity
Erosion of whole layers or no new
deposition
Sedimentary rock layers over igneous
or metamorphic rock
3. As you read this section, you made flash cards to help you learn. How did the flash cards
help you learn about how layers can be correlated?
Visit blue.msscience.com to access your textbook, interactive
games, and projects to help you learn more about the relative
ages of rocks.
End of
Section
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