Name
Date
Clues to Earth’s Past
What evidence do scientists use to determine the ages
of rocks?
Before You Read
Before you read the chapter, think about what you know about determining the ages of rocks. Record
your thoughts in the first column. Pair with a partner, and discuss his or her thoughts. Then record
what you both would like to share with the class in the third column.
Think
Pair
Share
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Chapter Vocabulary
Lesson 1
Lesson 2
Lesson 3
NEW
NEW
NEW
fossil
catastrophism
uniformitarianism
carbon film
mold
cast
trace fossil
paleontologist
relative age
superposition
inclusion
unconformity
correlation
index fossil
absolute age
isotope
radioactive decay
half-life
REVIEW
mineral
ACADEMIC
uniform
A Lesson Content Vocabulary page for each lesson is provided in the Chapter Resources Files.
Lesson 1
Fossils
Scan Lesson 1. Write three questions that you have about fossils in your Science Journal. Try
to answer your questions as you read.
Define fossil. Include two types of preserved clues in your
Evidence of the
Distant Past
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definition.
.
Fossils are the preserved remains or evidence of ancient
living things.
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.
Summarize the principles of catastrophism and
uniformitarianism.
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Uniformitarianism
Catastrophism credits
changes to Earth to
quick, violent events over
a short time period.
Uniformitarianism states
that geologic processes
that occur today are similar
to those that have
occurred in the past; it
credits changes to Earth
to slower processes over
a longer time.
Identify factors that promote fossilization. Cross out terms
.
that do not support the likelihood of fossil formation.
buried quickly
decay easily
exposed
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.
soft tissue
hard parts
rotting
microscopic
very large
eaten
Sequence three probable steps of fossil fish formation.
A fish dies
and falls to
a river
bottom.
The fish is
rapidly
covered
with
sediment.
The fish’s
body
decomposes,
and hard
parts change
to rock over
time.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Formation of Fossils
Catastrophism
Answer Key
Clues to Earth’s Past
Lesson 1
Before You Read
1. disagree
2. disagree
Read to Learn
1.
2.
3.
4.
the idea that conditions and creatures on Earth change in quick, violent events
He noticed that the landscape on his farm changed over the years and hypothesized that
the same process could change Earth’s surface over a much longer time.
the principle that processes occurring today are similar to those that occurred in Earth’s
past
the presence of organisms with hard parts and burial by layers of sediment soon after death
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
5.
6.
7.
the hard parts
c. a fossil about the size of a speck of dust
Pressure releases the gases and liquids from the organism’s tissues, leaving behind only
carbon containing the organism’s outline.
8. b. A fossil impression is filled with sediment.
9. tracks, footprints, nests
10. that a fossil that is similar to a living organism probably lived in an environment similar to
that of the living organism
11. Students’ Xs should cover the lightest areas on the map.
12. much warmer
After You Read
1.
Possible answer: Things that happen to change Earth today are similar to things that
changed Earth in the past.
2.
Fossil Type
What They Are/How They Form
Preserved
remains
Actual remains that become preserved in ice, tar, or
amber
Carbon films
Fossilized carbon outline of an organism or part of
an organism left behind after pressure released all
gases and liquids from the tissues
Mineralreplacement
fossils
Form when minerals replace organic remains
Molds and casts Molds are impressions of organisms; casts are
copies of organisms that form when molds are filled
with sediment or minerals
Trace fossils
Evidence of the activity of ancient organisms
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Lesson 1 | Fossils (continued)
Summarize the processes of fossil formation. Name and
Types of
Preservation
describe each process.
Fossil Preservation
Process
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.
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.
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.
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.
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.
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.
Preserved
remains
Actual remains of an organism are preserved in a
substance that keeps it from being exposed to air or
bacteria.
Carbon films
Pressure on the buried organism drives
off gases, leaving a thin outline of carbon.
Mineral
replacement
Minerals in groundwater fill in pore spaces
or replace tissues of dead organisms.
Molds
Sediment hardens around a buried organism;
the organism leaves an impression.
Casts
A fossil copy is made when sediment or
mineral deposits fill a mold of an organism.
Trace fossils
Evidence of the activity of an organism is
preserved, such as footprints.
Complete the concept below.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Ancient Environments
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Description
.
If a fossil of an organism resembles a modern organism,
the ancient organism might have lived in a similar
environment.
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.
Classify evidence of past climates.
Climate
Evidence
Warm
fossils of ferns and tropical plants
Cool
fossils of coarse grasses and mammoths
Fossils provide clues to what happened in the ancient past. Identify a
clue about what might have happened in the recent past in your current environment,
and tell how long that clue is likely to last.
Accept all reasonable responses. Sample answer: The stump of a tree is evidence of
where a tree once grew. Depending on effects of the weather and the size of the tree,
the stump might be around for a few decades.
Lesson 2
Relative-Age Dating
Predict three facts that will be discussed in Lesson 2 after reading the headings. Write your
predictions in your Science Journal.
Explain why a single rock cannot be described in terms of
Relative Ages of Rocks
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.
relative age.
Relative age is the age of rocks and geologic features with
respect to other nearby rocks and features. Other rocks
must be included in the comparison to describe a rock’s
relative age.
Model the principles of relative age dating below in
drawings and descriptions.
Concept
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.
.
.
Description
Superposition
Drawings should
show multiple layers
arranged oldest to
youngest from
bottom to top.
Layers of rock
are arranged
oldest to
youngest from
bottom to top.
Original
horizontality
Drawings should
show sediments
deposited in flat
layers. Students
might also show the
same layers tilted.
Layers of rock
can be tilted or
folded, but they
originated as flat,
horizontal layers.
Lateral
continuity
Drawings should
show same layers as
above with the
addition of a river
cutting through
the layers.
Layers of rock are
deposited as flat
sheets in all
directions. Erosion
can cut into the
rock, but the order
of layers does not
change.
Inclusion
Drawings should
show any rock
shape with smaller
fragments in the
rock.
A piece of an older
rock becomes part
of a newer rock
mass.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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.
Drawing
Lesson 2
Before You Read
3. disagree
4. disagree
Read to Learn
1.
2.
3.
4.
5.
6.
7.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
8.
Possible answer: Students might put their age in context with other members of their
families.
Rock layers can be tilted or folded.
the rock layer on the bottom
pieces of older rock that become part of a new rock
older, because the fault cuts across the dike
superposition, original horizontality, lateral continuity, inclusions, and cross-cutting
relationships
Erosion has worn away part of the rock record in an unconformity, producing a
gap in time.
A disconformity occurs between sedimentary layers; a nonconformity occurs
between sedimentary layers and igneous or metamorphic layers.
9. superposition, original horizontality, and lateral continuity
10. the Moenkopi formation
11. They can use fossils. If two rock formations contain similar fossils, the formations are
about the same age.
12. Index fossils of a known age indicate that the rocks that contain them are similar in
age.
13. Some students may circle a tail on each species; others may circle only the most
“obvious” tails.
After You Read
1.
Possible answer: If rocks are not disturbed, younger rocks are on top of older rocks.
2.
Superposition
In undisturbed
rock layers,
the oldest
rocks are on
the bottom.
3.
Lateral
Continuity
Sediment is
deposited in
large,
continuous
sheets.
Original
Horizontality
Cross-Cutting
Relationships
Most rockforming material
was originally
deposited in
horizontal
layers.
If one geologic
feature cuts
across another,
the feature that
it cuts across is
older.
According to the principle of inclusion, if one rock contains pieces of another rock, the
rock containing the pieces must be younger than the pieces.
Lesson 2 | Relative-Age Dating (continued)
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Order the features in the illustration from youngest to
.
oldest.
dike
fault
inclusion
sedimentary layers
youngest
fault
dike
inclusion
sedimentary layer
oldest
Define unconformity, and identify and describe 3 types.
Unconformities
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.
Unconformity: a surface where rock has worn away,
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
producing a gap in the rock record
Type:
disconformity
Type:
angular
unconformity
Type:
nonconformity
Description:
Description:
Description:
Younger
sedimentary
layers are
deposited on
top of older,
horizontal
sedimentary
layers that
have eroded.
Sedimentary
layers are
deposited
on top of tilted
or folded
sedimentary
layers that
have eroded.
Younger
sedimentary
layers are
deposited on
older igneous
or metamorphic
rock layers that
have eroded.
Lesson 2 | Relative-Age Dating (continued)
Complete the rock-dating concept in the diagram below.
Correlation
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.
matching
rocks and fossils
+
separate
locations
=
correlation
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.
Characterize organisms that form index fossils.
Organisms that
form index fossils
lived on
Earth for
short length
of time
.
lived in many
locations
Analyze the usefulness of index fossils. Write the correct terms.
Index fossils allow scientists to learn
rock formations
continents
index fossils
are of similar
that are very
the relative ages
of
far apart
or on different
infer
that layers with
. Scientists
different locations
found in
age
.
Museums all over the world collect samples of rocks and fossils. What
is the benefit to scientists of these collections?
Accept all reasonable responses. Sample answer: Because a great deal about Earth’s
past is learned from comparisons, access to many samples collected from many
locations is helpful to scientists.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
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were
abundant
Lesson 3
Absolute-Age Dating
Scan Lesson 3. Read the lesson titles and bold words. Look at the pictures. Identify three facts
that you discover about absolute-age dating. Write these facts in your Science Journal.
Absolute Ages of Rocks
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Define absolute age.
.
the numerical age, in years, of a rock or
Absolute age:
other object
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.
Summarize absolute age and relative age.
Ways to describe the ages of objects
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Relative Age
age described with
respect to another
object or person
Describe the makeup of an atom.
Atoms
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Absolute Age
numerical age
determined using
radioactivity
.
smallest
An atom is the
part of an element that has all
the properties of the element. Each atom contains smaller
particles called protons , neutrons , and electrons .
neutrons
Protons
and
are located in an atom’s
nucleus
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.
Electrons
.
surround the nucleus.
Define isotopes.
Isotopes: atoms of the same element that have different
numbers of neutrons
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.
Explain how radioactive decay releases energy from
unstable atoms.
Radioactive
isotopes decay, releasing
new, more stable atoms
and forming
The element that decays is
called the
isotope.
energy
parent
.
The new element that forms
is called the
isotope.
daughter
Lesson 3
Before You Read
5. disagree
6. agree
Read to Learn
1.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
2.
3.
4.
5.
Relative age is an age in relation to other objects. Absolute age is a numerical age given
in years.
in the number of neutrons they contain
different numbers of neutrons
Hydrogen is the parent; helium is the daughter.
the time required for half of the parent isotopes to decay into daughter isotopes
6.
7.
8.
9.
6.25% parent and 93.75% daughter
the ratio of the amount of parent isotope to the amount of daughter product
C-14 stays the same in a living organism and decays in a dead organism.
Sedimentary rocks formed from grains of igneous or metamorphic rock. Dating these grains
would give the ages of the original materials, not of the sedimentary rock.
10.
11.
12.
13.
They would have enough parent isotope to measure.
rubidium-87
between 4.03 billion and 4.28 billion years old
3 × 48.8 billion years = 146.4 billion years
After You Read
1.
Possible answer: Isotopes are atoms of the same element that have different numbers of
neutrons. Carbon-14, uranium-235, and potassium-40 are radioactive isotopes.
2.
3.
constant; decays; stays the same; C-14; C-12
Students should describe how vocabulary cards helped them remember the meanings of
important words.
Lesson 3 | Absolute-Age Dating (continued)
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.
Calculate the change in isotopes during radioactive
decay.
Percent
Parent
Percent
Daughter
100
0
One half-life
50
50
Two half-lives
25
75
12.5
87.5
close to
close to
0
0
Original materials
Three half-lives
After many more
half-lives
Describe why radiometric dating can be used to determine
Radiometric Ages
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.
an object’s age.
Radiometric Dating
constant rate
,
so they can be used to measure
age
.
parent
The ratio of
used as a measure.
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.
daughter
isotope to
product is
Explain how radiocarbon dating uses decay to help
determine age.
Organism
Description
Alive
C-14
• The organism takes in
• The ratio of radioactive carbon, or
C-14
Dead
•
to
C-12
C-14
• The ratio of C-14
.
, remains constant.
begins to decay.
to C-12
changes.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Radioactive isotopes decay at a
Lesson 3 | Absolute-Age Dating (continued)
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Identify two reasons that radiocarbon dating can be used to
.
measure the ages of once-living things accurately.
1. The ratio of C-14 to C-12 is used to determine how long
the organisms have been dead.
2. With a half-life of 5,730 years, C-14 is useful for
measuring the age of remains up to 50,000 years old.
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.
Explain why radiometric dating is not useful for determining the
age of sedimentary rock.
Sample answer: Radioactive isotopes would probably
measure the ages of the grains that make up the rock, not
the time when the sediments were deposited.
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.
Identify five radioactive isotopes that can be used for dating
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rocks. Circle the two isotopes with the longest half-lives.
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.
1.
uranium-235
4.
potassium-40
2.
uranium-238
5.
thorium-232
3.
rubidium-87
Summarize the conclusions that scientists have made about
Earth’s age.
Earth, the Moon, and
meteorites formed at
about the same time .
Radiometric dating of Moon
rocks indicates that Earth is
4.5 billion
years old.
You find a piece of petrified wood. Explain whether radiocarbon dating
could be used to date your find. If not, what could be used?
Sample answer: Petrified wood was once a living organism, and radiocarbon dating
works on once-living objects. However, during the fossilization process, the organic
material of the wood was replaced with rock-forming minerals, so radiocarbon dating
would not work. Instead, radiometric dating with any of the other radioactive isotopes
could be used.
Review
Clues to Earth’s Past
Chapter Wrap-Up
Now that you have read the chapter, think about what you have learned.
Use this checklist to help you study.
Complete your Foldables® Chapter Project.
Study your Science Notebook on this chapter.
Study the definitions of vocabulary words.
Reread the chapter, and review the charts, graphs, and illustrations.
Review the Understanding Key Concepts at the end of each lesson.
Look over the Chapter Review at the end of the chapter.
Reread the chapter Big Idea and the lesson Key Concepts. To
illustrate how geology is a type of detective work, write a summary of the kinds of
changes that have affected Earth’s surface according to geological clues. Identify at
least three types of changes.
Accept all reasonable responses. Sample answers: Layers of rock are bent upward
to form mountains. Large cuts erode into rock and form canyons. Earth’s climate
has changed over time.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Challenge Build a three-dimensional model representing geological layers. Include features such
as unconformities, inclusions, faults, and fossil clues in your model. Show and explain your model
to your class.