ECVHS Dating the Geosphere Questions

CA Rev. 8/23/2021General Science 1A
Credit 4
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Name:
CA Rev. 8/23/2021
CREDIT 4A: DATING THE GEOSPHERE
Learning Goal for this Credit
Design an investigation or model using appropriate scientific tools, resources and methods.
Lesson
Title
INTRODUCTION
4.1
The Rock Cycle
4.2
Relative Dating
4.3
Absolute Dating
Assignments

 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 Rock Structures of Joshua Tree National Park
 Review Questions
 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 What’s Up?
 Review Questions
 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 Graphing Half-Life
 Review Questions


PERFORMANCE TASK
QUIZ
Title
Review
Activity
Icon
Student Support Icons
Description
This provides the students with a reminder that they need to answer questions.
Technology
Guides students through the tasks and assignments that require the use of
technology and manipulatives.
Reading
This icon lets the students know they will be completing a reading activity.
Credit Materials
•
•
•
Materials
Pen/Pencil
HMH Earth Science
Textbook (optional)
Packet
•
•
Technology Needs
Internet
Computer
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CREDIT 4A: INTRODUCTION
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Read “What do Rocks Know?” and watch the video “Introduction to Geology: Big History Project” below.
Then answer the essential question.
What do Rocks Know?
Rocks are solid matter, and unlike liquids and gasses, solids remain unchanged for long periods of time. A rock
contains information about its history. The structure of the rock can be used to determine how it formed.
Inclusions such as fossils and radioactive isotopes can be used to determine when it formed. Even if a rock
does not have inclusions, it can be determined if a rock is older or younger than another rock. This is done by
comparing their locations because new sedimentary rock layers form on top of older layers.
Rocks are constantly being created and each rock can contain information about the climate, ecology, and
events from the time that it formed. Everything we know about dinosaurs was contained in the rocks which
were created between 248 million and 65 million years ago. A layer of rocks can be used to show what the
surface of the Earth was like when it formed. Some layers show that there have been periods of time when
Earth’s surface was covered with molten rock. Other rocks show that the Earth was frozen under a layer of ice
and snow. There have been times when the Earth was constantly bombarded by radiation and lacked oxygen in
the atmosphere. Events such as volcanic eruptions, meteor impacts, the building of mountains, the carving of
canyons, and the mass extinctions of organisms can be identified by how these events have also created or
changed layers of rock. The entire history of the planet Earth can be told by taking all of these bits of
information and putting them together.
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An essential question is something that allows you to explore the content of the credit. Before you answer the
question, watch the video. Then answer the essential question to the best of your ability. You will revisit it at
the end of the credit to see if your answer has evolved.
Introduction to Geology: Big History Project (6:44)
https://youtube.com/watch?v=rRFphdMIIvI%3Frel%3D0
“Introduction to Geology | Big History Project.” YouTube. Big History Project, 20 May 2014. Web. 8 Oct. 2014.
Essential Question
How do you think scientists are able to observe, record, study, and predict changes in the Earth?
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LESSON 4.1: THE ROCK CYCLE
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Learning Goal for this Credit
Design an investigation or model using appropriate scientific tools, resources and methods.
Learning Goals for this Lesson
• Identify the three major types of rock, and explain how each type forms.
• Summarize the steps in the rock cycle.
• Summarize the factors that affect the stability of rocks.
Lesson Assignments
 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 Rock Structures of Joshua Tree National Park
 Review Questions
Engage
Connect to Prior Knowledge
List a rock type in which you are familiar. What are the characteristics of this type of rock? What might this
rock be used for?
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Explore
Exploration Activity
The three major rock types are igneous,
metamorphic, and sedimentary rocks.
Igneous rocks form when magna cools and
hardens. When sediment deposited by
water, ice, and wind becomes cemented
together it forms sedimentary rocks.
Metamorphic rocks form when an existing
rock is changed or altered a force such as
extreme heat, pressure, or chemical
processes. Geologic forces and processes
can cause one type of rock to change into
the other. The rock cycle shows the
changes that the three types of rocks may
experience.
Use the rock cycle diagram to answer the
following questions.
1. How are sedimentary rocks able to transform into igneous rocks?
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2. How are sedimentary rocks transform into metamorphic rocks?
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3. How are igneous rocks able to transform into sedimentary rocks?
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4. How are igneous rocks able to transform into metamorphic rocks?
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5. How are metamorphic rocks able to transform into igneous rocks?
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6. How are metamorphic rocks able to transform into sedimentary rocks?
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Explain
As you complete the reading, answer the questions in the space provided.
Reading
What Are the Three Main Kinds of Rock?
Rock is the material that makes up the solid parts of Earth. Most rocks are made up of one or more minerals.
Some rocks are made up of organic matter. A few types of rock are made up of inorganic matter that is not a
mineral, such as glass. Geologists classify, or group, rocks based on how they form. There are three main types
of rock: igneous rock, sedimentary rock, and metamorphic rock. The table below describes how each type of
rock forms.
Kind of Rock
Igneous
Sedimentary
Metamorphic
How it Formed
Igneous rock forms when melted rock cools
and hardens.
Sedimentary rock forms when pieces of
rocks and organic matter are buried and
pressed together.
Metamorphic rock forms when heat and
pressure change the chemical composition
of a rock without melting it.
Examples
granite, obsidian, basalt
sandstone, shale, limestone
gneiss, slate, quartzite
What Is the Rock Cycle?
Rocks can change from one type to another. For example,
wind and water can break rocks into pieces called sediment.
The sediment can be carried away and laid down at the
bottom of a lake. Over time, the sediment can be buried and
compressed into a sedimentary rock. A sedimentary rock can
be buried and heated until it melts. When the melted rock
cools, an igneous rock forms. The processes that change rock
from one form to another makes up the rock cycle.
1. Describe two paths through the rock cycle that an
igneous rock could follow to become a metamorphic
rock.
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2. How is the way an igneous rock forms different from the way a metamorphic rock forms?
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What Happens When Melted Rock Cools?
Igneous rocks form when melted rock cools and hardens. Melted rock below Earth’s surface is called magma.
Magma that rises to Earth’s surface is called lava. When rock melts to form magma, the minerals in the rock
melt. The different elements in the minerals mix together. As the magma cools, the elements can come together
to form new minerals. Different minerals form at different temperatures. As each mineral forms, it removes
elements from the magma. As the magma cools, different minerals form because different elements are
available. In the early 1900s, a Canadian geologist named N. L. Bowen was studying how minerals form from
cooling magma. He showed that minerals tend to form in a certain order as magma cools. Some minerals, such
as olivine, form early. Other minerals, such as quartz, form later in the process. The order in which minerals
form from cooling magma is called Bowen’s reaction series. Bowen’s reaction series shows that minerals can
form in one of two main ways. In the first way, the composition of the minerals changes slowly over time.
Many feldspars form this way. In the second way, the kind of mineral that forms changes suddenly over time.
Olivine, pyroxene, and amphibole generally form this way.
3. Why do different minerals form as magma cools?
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4. Bowen’s reaction series states that minerals can form from magma in two main ways. What are they?
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What Affects How Strong Rocks Are?
Some rocks, such as granite, are very strong. They do not break down very easily. Other rocks, such as
limestone and sandstone, are softer. They break down quickly. How quickly a rock breaks down depends on
two things: the minerals in the rock and the structure of the rock. Some minerals are very stable on Earth’s
surface. In other words, they do not break down very easily. They do not react with water or air to form softer
substances. Rocks that are made up mainly of such minerals tend to be hard. Most stable minerals, such as
quartz, form at low temperatures. They are some of the last minerals to form in Bowen’s reaction series. Other
minerals are not very stable. They react easily with water or air. Rocks that are made up mainly of such
minerals tend to be soft and break down easily. Most unstable minerals, such as olivine, form at high
temperatures. They are some of the first minerals to form in Bowen’s reaction series. The structure of a rock
also affects how easily it breaks down. Rocks that have cracks or layers are generally soft. The rock is weakest
along the cracks or between the layers. It can easily break in those areas.
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5. What two things determine the strength of a rock?
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Allison, Mead A., et al. “Chapter 6: Rocks/Section 1: Rocks and the Rock Cycle.” Holt McDougal Earth Science Interactive Reader, Holt McDougal, a Division of
Houghton Mifflin Harcourt Publishing Co., 2010, pp. 69-72.
Videos
If you would like to learn more about this topic, watch the videos below for more information. (Optional)
Rock Types and the Rock Cycle: Igneous Sedimentary
Metamorphic (3:45)

“Rock Types and the Rock Cycle: Igneous Sedimentary Metamorphic.” YouTube. Untamed Science, 12 Nov. 2013. Web. 15
July 2015.
What are the different types of rocks and what is the rock cycle? This video will
explain the rock cycle and how one type of rock can be changed into another.
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Elaborate
Rock Structures of Joshua Tree National Park
Geologists have long studied Earth and its structure.
The development and forms of the three major rock
groups—igneous, sedimentary, and metamorphic—
are well known. Still, there are many examples of rock
structures that do not seem to neatly fit the mold. An
example can be found in Joshua Tree National Park in
the Mojave Desert. Here, visitors can see steep-sided
rock structures that appear to “grow” out of the flat
desert floor. Where did they come from?
About 100 million years ago, magma, super-heated by
Earth’s churning crust, oozed up from the deep mantle
and cooled near the surface. The resulting igneous
intrusions are called monzogranite. Over time, the monzogranite developed three sets of rectangular joints: one
horizontal, one vertical, and another vertical that cut through the first vertical set of joints at a steep angle. The
result was a series of rectangular rock piles called inselbergs.
Gradually, groundwater seeped through the joints, morphing hard minerals into soft clay. The rectangular stones
were weathered down to hard rock blanketed by soft clay and loose mineral grains. With the change from a wet
to arid climate in recent Earth history, the inselbergs were exposed when flash floods carried away the soft and
loose materials. What was left is what we see today: huge rectangular rocks piled one on top of another, sitting
on a flat desert, looking as if they have just sprouted from the ground.
1. What might be considered unusual about the rock structures in Joshua Tree National Park compared
with other desert formations?
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2. How do inselbergs develop?
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3. Might you be likely to see inselbergs in hilly or mountainous areas? Explain your answer.
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Holt McDougal Earth Science Chapter 6: Rocks: Critical Thinking Worksheet. Austin, TX: Houghton Mifflin Harcourt Publishing Company, 2010. PDF.
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Evaluate
Review Questions
Answer the following questions.
1. Give an example of how the rock cycle can change one type of rock into another.
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2. Explain Bowen’s reaction series.
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3. Does every rock go through the complete rock cycle by changing from igneous rock to sedimentary rock
to metamorphic rock, and then back to igneous rock? Explain your answer.
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LESSON 4.2: RELATIVE DATING
Learning Goal for this Credit
Design an investigation or model using appropriate scientific tools, resources and methods.
Learning Goals for this Lesson
• Describe the principle of uniformitarianism.
• Explain how the law of superposition can be used to determine the relative ages of rocks.
• Compare three types of unconformities.
• Apply the law of crosscutting relationships to determine the relative ages of rocks.
Lesson Assignments
 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 What’s Up?
 Review Questions
Engage
Connect to Prior Knowledge
What are some visual observations that tell you someone is older or younger than you are?
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Explore
Exploration Activity
Relative age in Geology indicates that one layer of rock is older or younger than another layer, but does not
indicate the rock’s age in years. The law of superposition states 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. Scientists use this basic
principle to determine the relative age of a layer of sedimentary rock. Use the figures below to answer the
following questions.
1. According to the pictures, how did layer C form?
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2. Which is the oldest layer in this rock section? Explain your reasoning.
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3. Is layer B older or younger than layer C? Explain your reasoning.
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Explain
As you complete the reading, answer the questions in the space provided.
Reading
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.
⇩
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.
1. Explain the principal of uniformitarianism.
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What Is Relative Age?
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
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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. 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.
2. What can a scientist learn from the relative ages of rock layers?
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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.
3. Under what conditions can scientists apply the law of superposition directly?
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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.
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. 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. 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.
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4. What causes ripple marks? How can they be used to indicate the position of a layer of sedimentary
rock?
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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 to the right
describes three types of
unconformities.
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. This can be seen in the figure above.
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5. In the figure on the previous page, which is younger—the fault or the intrusion? Explain your answer.
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Allison, Mead A., et al. “Chapter 8: The Rock Record/Section 1: Determining Relative Age.” Holt McDougal Earth Science Interactive Reader, Holt McDougal, a
Division of Houghton Mifflin Harcourt Publishing Co., 2010, pp. 107-112.
Videos
If you would like to learn more about this topic, watch the videos below for more information. (Optional)
The Basics of Unconformities (9:47)

How do unconformities form? This video will describe the processes that
lead to unconformities.
“The Basics of Geology: Basic Unconformities.” YouTube. Geo Logic, 30 Jul 2011. Web. 26 Apr 2016.
Law of Superposition (6:21)

“Law of Superposition.” YouTube. Bozeman Science, 22 May 2011. Web. 16 July 2015.
How does the law of superposition help find the relative age of rock layers? This video
will explain the law of superposition and the principle of horizontality.
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Elaborate
What’s Up?
Background:
Relative dating assumes that the lower layers in any particular cross section are older than the upper layers in
that cross section (“the law of superposition”) and that an object cannot be older than the materials of which it is
composed. Igneous rocks are dated according to whether they caused metamorphism in the surrounding rock
(proof that they intruded into the preexisting rock), whether they cross cut preexisting rocks, or whether
sediments were deposited on them after they were formed.
The profile from one location is then compared with profiles from surrounding sites to determine the geologic
history of a larger area. If fossils are present in the rocks, they may also be used to associate rock layers across
large distances, and now that absolute time has been established, to determine the age of the rocks.
In this activity, you will study the rocks and events in a geologic cross section and put them in the correct order
from oldest to youngest. The easiest way to do relative age dating is to work from oldest to youngest. Try to
find the oldest rock (usually located near the bottom) and work your way up. Your first example is the diagram
below. Review the principle of original horizontality and the principle of superposition and you will see that the
only possible answer to this puzzle is that layer A is the oldest and layer D is the youngest.
Here are some additional hints that will help you with your diagrams.
Sedimentary Rocks:
•
•
If rocks are folded, the folding is younger that the youngest rock affected.
If they are folded into a syncline (a U-shaped fold) the youngest rocks are in the core of the fold (see
figure B). The opposite is true for an anticline (a big dome-shaped fold). Sedimentary rocks that contain
fragments of another rock are younger than the rocks that the fragments came from.
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Igneous Rocks:
• Igneous rocks are formed by the solidification of liquid magma; they therefore can intrude into
preexisting rocks or be poured out onto the surface of the earth.
• If an igneous body crosscuts another rock, the igneous rock is younger than that rock (see
Figures 1 & 3).
• If a body of granite contains unmelted inclusions of another rock, the granite is the younger rock.
• Remember, granites can intrude into other rocks, even though they may be on the bottom of your
geologic diagram. Look carefully for the granitic pattern (see below) and for irregular contacts between
the granite and the preexisting rock (see Figure 2). The granite may also metamorphose the preexisting
rocks.
• Intrusive rocks produce contact metamorphism (shown as a starred pattern within the preexisting rock
pattern, see Figures 2 & 3) along their contacts with the older rocks they intrude into.
• Lava flows may cause contact-metamorphism with the older rocks they lie upon.
Metamorphic Rocks:
• Metamorphic rocks are preexisting rocks that have been metamorphosed (changed into different rocks)
by large amounts of heat and pressure in a region.
• These rocks have usually been deformed by large, mountain forming events, and therefore if they are in
contact with layered or unmetamorphosed rocks, they are usually the oldest rocks in the sequence.
• Always look for the metamorphic pattern (see below) to determine if there is a metamorphic rock in
your sequence. Metamorphic rocks are older than sedimentary rocks deposited above them or with
igneous rocks that may intrude them.
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Now, familiarize yourself with the rock patterns:
Key to Rock Symbols
Sedimentary Rocks
Conglomerate
Sandstone
Siltstone
Igneous Rocks
Metamorphic Rocks
Granite
Gneiss
Granite #2
Schist
Special Features
Contact that is an
unconformity
Fault
Zone of contact
metamorphism
Basalt
Shale
Limestone
Shale #2
Limestone #2
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Practice:
For each of the following cross sections, determine the relative age sequence of the rocks. Remember, always
start by looking for the oldest rock first and working your way from oldest to youngest. Don’t forget to consider
all intrusions and faults! Rank the rock layers from oldest to youngest. Fill in the oldest layer, the youngest
layer, and the relative age in order from oldest to youngest. Figure 1 has been completed for you as an example.
Example: Figure 1
Oldest: C
Youngest: A
Relative age of Figure 1 rocks (oldest
to youngest): C, E, D, B, A
Figure 2:
Oldest: ________________________
Youngest: _____________________
Relative age of Figure 2 rocks (oldest
to youngest): ___________________
_______________________________
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Figure 3:
Oldest: ________________________
Youngest: _____________________
Relative age of Figure 3 rocks (oldest
to youngest): ___________________
______________________________
Figure 4:
Oldest: _______________________
Youngest: _____________________
Relative age of Figure 4 rocks (oldest
to youngest): ___________________
______________________________
McLelland, Christine V. What’s Up? – A Relative Age Dating Activity. Boulder, Colorado: Geological Society of America, n.d. PDF. adapted from Jonathan Bushee
and Raman Singh, Northern Kentucky University.
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Evaluate
Review Questions
Answer the following questions.
1. When did you use the law of superposition to determine the relative age of the rock layers in each of the
figures?
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2. When did you use the law of cross-cutting relationships to determine the age of rock layers?
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3. Which of the figures contain unconformities?
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LESSON 4.3: ABSOLUTE DATING
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Learning Goal for this Credit
Design an investigation or model using appropriate scientific tools, resources and methods.
Learning Goals for this Lesson
• Summarize the limitations of using the rates of erosion and deposition to determine the absolute age of
rock formations.
• Explain how the process of radioactive decay can be used to determine the absolute age of rocks.
• List examples of fossilized traces of organisms.
• Describe how index fossils can be used to determine the age of rocks.
Lesson Assignments
 Connect to Prior Knowledge
 Exploration Activity
 Reading and Questions
 Videos (optional)
 Graphing Half-Life
 Review Questions
Engage
Connect to Prior Knowledge
What are some ways to determine the exact or absolute age of a person or object?
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Explore
Exploration Activity
Fossils — the remains and traces of organisms that have been preserved through time — are a key source of
information for understanding the history and development of life on Earth. Once buried within thick layers of
rock, fossilized remnants of plants and animals help scientists piece together a picture of geologic time. The
fossil record is like a diary of Earth, offering a glimpse into past conditions that have been preserved in
chronologically sequenced rock formations.
Fossilization only happens under particular circumstances. When an organism dies, it is exposed to a variety of
biotic and abiotic factors. Biotic factors, such as scavengers, predators, and decomposers, can break down and
destroy the organism. Abiotic factors, including weathering, erosion, and tectonic processes such as volcanic
activity and earthquakes, can also wear away or obliterate its remains. Thus, organic matter left exposed to air
and microorganisms quickly decomposes, and soft tissue, muscle, and organs do not generally become
fossilized. However, hard, inorganic matter made of minerals, like bones and teeth, stands a better chance of
being preserved, especially once it is buried under sediment. Over a long period of time, as the material slowly
decays, it can be infused with minerals dissolved in ground water and become a fossil. Since the object is now
composed of hard minerals, it maintains its original shape.
Not every type of rock contains fossils, so paleontologists — scientists who study fossils — focus their
discovery efforts on areas with the type of rock most likely to contain them: sedimentary rock. Sedimentary
rock is formed by the deposition of new layers over time. As each new layer is added, the remains of organisms
from previous time periods end up farther and farther from Earth’s surface. However, movement and interaction
of Earth’s lithospheric plates eventually moves fossils around, enabling their discovery. For example, in a
process called uplift, deeply buried geological strata may be forced upward to the surface. Once close to the
surface, fossils in these deposits may be excavated or, as is often the case, uncovered by the processes of
erosion.
1. In what type of rock do paleontologists generally look for fossils?
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2. What must usually happen in order for a dead organism to be preserved as a fossil?
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3. What geologic processes can cause deeply buried fossils to be brought toward the surface?
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4. Why are most fossils found in sedimentary rocks?
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Adapted from University of California Museum of Paleontology. “How a Dinosaur Became a Fossil.” PBS LearningMedia. WGBH, 2006. Web. 23 July 2015.
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Explain
As you complete the reading, answer the questions in the space provided.
Reading
What Is Absolute Age?
Remember that relative dating indicates only that one rock is older or younger than another. To learn more
about Earth’s history, scientists often need to learn a rock’s absolute age, or age in years. Scientists use a
variety of methods to measure absolute age. Some methods require scientists to observe and measure geologic
processes over time. Other methods involve the chemical composition of materials in rocks. Studying rates of
erosion is one way scientists estimate the absolute age of rocks. For example, scientists may measure the rate at
which a stream erodes, or wears away, its stream bed. They can use that measurement to estimate the absolute
age of the stream. Scientists cannot use rates of erosion in all cases. This method is useful only for geologic
features that formed within the past 10,000 to 20,000 years. For features such as the Grand Canyon that formed
over millions of years, the method is less useful. Rates of erosion vary greatly over millions of years. Therefore,
estimates based on recent erosion rates are not dependable.
Rivers can carry sediment, and then deposit it. Calculating rates of deposition is another way scientists can
estimate absolute age. Scientists can estimate the average rate of deposition of common sedimentary rocks.
These common rocks include limestone, shale, and sandstone. Scientists have found that, in general, about 30
cm of sedimentary rock are deposited over a period of 1,000 years. However, a given sediment layer might not
have been deposited at the average rate. For example, a flood may deposit many meters of sediment in just one
day. Therefore, this method for determining absolute age is not always accurate.
You may know that you can estimate a tree’s age by counting the growth rings in the tree’s trunk. Scientists use
a similar method to estimate the age of certain sedimentary rocks. Some sedimentary rocks show layers called
varves. In general, varves are annual, or yearly, layers. They have a light band of coarse particles and a dark
band of fine particles. Most varves form in glacial lakes. During summer, snow and ice melt quickly. The water
carries large amounts of sediment into the lake. Most of the coarser (larger) sediment particles settle quickly to
the bottom and form a layer. When winter comes, the lake starts to freeze. Finer clay particles that stayed mixed
in the water settle slowly on top of the layer of coarse particles. A coarse summer layer and a fine winter layer
make up one varve. By counting the varves, scientists can estimate the age of the sediments in years.
1. How are varves like tree rings?
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What Is Radiometric Dating?
Remember that all atoms of an element have the same number of protons. Atoms of the same element may,
however, have different numbers of neutrons. Atoms of the same element with different numbers of neutrons
are called isotopes. Radioactive isotopes have nuclei that emit, or give off, particles and energy at a constant
rate. This process is called radioactive decay. The figure on the next page shows two forms of radioactive
decay. Scientists can use the rates of decay of radioactive isotopes to measure absolute age. This method of
finding absolute age is called radiometric dating. As a radioactive isotope decays, it may change to a different
isotope of the same element. It may even change to an isotope of a different element. Scientists can measure the
concentrations of the original radioactive isotope and the newly formed isotopes in a sample. The original
radioactive isotopes are called parent isotopes. The newly formed isotopes are called daughter isotopes.
Scientists determine the ratio of parent and daughter isotopes in a sample of rock. Using this ratio and the
known decay rate, scientists can determine the absolute age of the rock.
Radioactive decay happens at a constant rate. Temperature, pressure, and other environmental conditions do not
change the decay rate. Scientists have found that the time needed for a certain radioactive isotope to decay is
always the same. Scientists typically talk about the half-life of a radioactive isotope. Half-life is the time it takes
for half the mass of a parent isotope to decay into daughter isotopes. For example, suppose you begin with 10 g
of a parent isotope. After one half-life, you would have half, or 5 g, of that isotope. At the end of the second
half-life, you would have one-fourth, or 2.5 g, of the original isotope. Three-fourths of the sample would now
be made up of daughter isotopes. The figure below shows the ratios of parent and daughter isotopes through
four half-lives. Scientists can choose from a variety of radioactive isotopes for radiometric dating. The method
they use depends mainly on how long ago the rock probably formed. If too little time has passed since the rock
formed, the amount of daughter isotope will be too small. Scientists will be unable to determine age accurately.
If too much time has passed since a rock formed, the amount of parent isotope will be too small. For example,
uranium-238 has a half-life of 4.5 billion years. Uranium-238 is most useful for dating rock samples that
contain uranium and that are more than 10 million years old. For younger rocks, the sample would have too few
daughter isotopes for scientists to find an accurate age.
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2. In your own words, define half-life.
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How Do Scientists Use Carbon Dating?
Scientists can date some younger sediments indirectly. They can date organic material, such as wood and bones,
found in sediment layers. Scientists can use carbon-14 dating to find the age of organic material that is less
than about 70,000 years old. Another term for carbon-14 dating is radiocarbon dating. Carbon-14, 14C, is a
radioactive isotope. It is far less common than the carbon-12, 12C, isotope. Both 14C and 12C combine with
oxygen to form carbon dioxide (CO2). Plants take CO2 into their bodies during photosynthesis. The carbon
becomes part of the bodies of the plants. When animals eat the plants or other animals, the carbon becomes part
of their bodies. After an organism dies, it stops taking in carbon. Like all radioactive isotopes, 14C decays at a
constant rate. The amount of 14C in a sample decreases as the 14C decays. Therefore, to find the age of a sample
of organic material, scientists find the ratio of 14C to 12C. They compare the ratio with the ratio they know is
found in a living organism. The higher the ratio is, the younger the sample is.
3. Suppose you have a shark’s tooth that you think is about 15,000 years old. Would you use uranium-238
or carbon-14 to date the tooth? Explain your answer.
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What Are Fossils?
Fossils are the body remains or traces of organisms that lived in the past. The study of fossils is called
paleontology. Scientists can learn many things from fossils, including the ages of rock layers, information
about past climates, how life on Earth has changed over time. Almost all fossils that scientists find are in
sedimentary rock. Fossils are rare in igneous and metamorphic rock because those rocks form under conditions
of high heat and pressure. Such conditions generally destroy existing fossils or material that could become
fossils. The fossil record is made up of all fossils that scientists have found so far. The fossil record shows how
organisms have changed over time. These changes give scientists information about Earth’s past environments
and how these environments have changed. For example, scientists have found fossils of marine plants and
animals in areas far from present oceans. The fossils tell us that these areas were once covered by ocean.
Normally, the bodies of dead plants and animals are eaten by animals or decomposed by bacteria. An organism
can become a fossil only if it is preserved before it is eaten or decays. In most cases, the only parts of an
organism that are preserved are hard parts. Trace fossils are any evidence other than body parts that an
organism once existed. Examples of trace fossils include tracks, footprints, and burrows. These traces can
fossilize when sediments cover them and harden. Trace fossils can give scientists clues about how an animal
looked and how it lived.
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4. Why are fossils uncommon in igneous and metamorphic rock?
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5. Most organisms never become fossils. Why?
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What Are Index Fossils?
Index fossils are fossils found only in rock layers of a certain geologic age. To be an index fossil, a fossil must
be present in rocks found over a large region, have features that make it clearly different from other fossils, be
from an organism that lived during a short span of geologic time, and be found with many other fossils of the
same organism. Because index fossils meet very specific requirements, scientists can use them to determine the
age of rocks. For example, suppose scientists find the same index fossils in different parts of the world. They
can conclude that the rock in these areas formed at about the same time. Recall that the original organisms of
index fossils lived during short spans of time. Scientists can use this information to determine the absolute age
of the rock.
6. Suppose a rock layer in Mexico and a rock layer in Australia contain the same index fossil. What do you
know about the absolute age of the layer in both places? Explain your answer.
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Allison, Mead A., et al. “Chapter 8: The Rock Record/Section 2: Determining Absolute Age.” Holt McDougal Earth Science Interactive Reader, Holt McDougal, a
Division of Houghton Mifflin Harcourt Publishing Co., 2010, pp. 113-118.
Allison, Mead A., et al. “Chapter 8: The Rock Record/Section 3: The Fossil Record.” Holt McDougal Earth Science Interactive Reader, Holt McDougal, a Division of
Houghton Mifflin Harcourt Publishing Co., 2010, pp. 119-122.
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Videos
If you would like to learn more about this topic, watch the videos below for more information. (Optional)
Radiocarbon Dating (9:28)

“Radiocarbon Dating.” YouTube. Bozeman Science, 29 Dec. 2010. Web. 17 July 2015.
How is radiocarbon dating used to date rocks and other ancient materials? This video
will describe the process of radioactive decay in carbon and how it can be used in
absolute dating.
Becoming a Fossil (2:34)
http://www.pbslearningmedia.org/resource/tdc02.sci.life.evo.becfossil/becoming-a-fossil/
“Becoming a Fossil.” PBS LearningMedia. NOVA, 2001. Web. 23 July 2015.
How do bones become fossils? This video will explain the process of fossilization to
show how remains of organisms can be preserved for millions of years.
Relative Dating vs Absolute Dating (7:38)

How do these types of dating differ? What are index fossils? This video will explain
what these concepts are, and how they are used to date rocks.
“Relative Dating vs Absolute Dating (updated).” Color Me Scientifically. 23 Feb 2021 Web. 26 Apr. 2021
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Elaborate
Graphing Half-Life
Scientists use line graphs as tools to communicate data, specifically to show how data change over time. Line
graphs make it easy to compare different sets of data over time. For example, in radiometric dating, the absolute
age of rock is determined by comparing the relative percentages of a radioactive (parent) isotope and a stable
(daughter) isotope.
Half-life is the time it takes for half the mass of a radioactive isotope to decay into its daughter isotope. To
make a line graph of the half-life of any radioactive isotope, plot the amount of the parent isotope against the
amount of the daughter isotope over time. Even if you do not know the time periods for each half-life of a
radioactive isotope, the relationship can still be plotted on a line graph.
For example, suppose the original mass of a radioactive isotope is 100,000 grams. A table and a line graph
showing the half-lives for this amount of radioactive isotope would look like the following:
Number of half-lives
0
1
2
3
4
5
Parent isotope
100,000 g
50,000 g
25,000 g
12,500 g
6,250 g
3,125 g
Daughter isotope
0g
50,000 g
75,000 g
87,500 g
93,750 g
96,875 g
Use the line graph to answer the following questions.
1. How many half-lives have passed when there are three times as much daughter isotope as parent
isotope?
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2. How many grams of the parent isotope are left in the sample after three half-lives?
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3. Why is the line graph a curve instead of a straight line?
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4. If a sample contained 94,000 g of the daughter isotopes, where on the line graph would the sample be
shown?
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The table below shows the radioactive decay of a 10 kg sample of carbon-14. Create a graph using the data
below. Label the x-axis “Number of half-lives.” Label the y-axis “Amount of isotope (g).” Plot the decay of
carbon-14 in terms of half-lives. Each half-life for carbon-14 is about 5,700 years.
Number of Half-Lives
0
1
2
3
4
5
6
7
8
9
Years passed
0
5,700
11,400
17,100
22,800
28,500
34,200
39,900
45,600
51,300
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Carbon-14 (g)
10,000
5,000
2,500
1,250
625
312
156
78
39
20
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5. About how old is a sample of bone that contains 900 g of carbon-14? Mark its position on the line graph.
How many half-lives have passed?
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Holt McDougal. Holt McDougal Earth Science Chapter 8 Graphing Skills Worksheet. Austin, TX: Houghton Mifflin Harcourt Publishing Company, 2010. PDF.
Evaluate
Review Questions
Answer the following questions.
1. Explain the difference between relative and absolute age.
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2. Explain how radiometric dating is used to estimate absolute age.
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3. How can half-life be used to determine an object’s absolute age?
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4. Explain how index fossils are used to date rocks.
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5. If a rock layer in Mexico and a rock layer in Australia contain the same index fossil, what do you know
about the ages of the layers in both places? Explain your answer.
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Revisit the essential question. Did your answer change? Why or why not?
Essential Question
How do you think scientists are able to observe, record, study, and predict changes in the Earth?
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Learning Goal for this Credit
Design an investigation or model using appropriate scientific tools, resources and methods.
Lesson
4.1
Title
The Rock Cycle
4.2
Relative Dating
4.3
Absolute Dating
Learning Goals For Each Lesson
•
Identify the three major types of rock, and explain how each type forms.
•
Summarize the steps in the rock cycle.
•
Summarize the factors that affect the stability of rocks.
•
Describe the principle of uniformitarianism.
•
Explain how the law of superposition can be used to determine the relative ages of rocks.
•
Compare three types of unconformities.
•
Apply the law of crosscutting relationships to determine the relative ages of rocks.
•
Summarize the limitations of using the rates of erosion and deposition to determine the absolute age
of rock formations.
•
Explain how the process of radioactive decay can be used to determine the absolute age of rocks.
•
List examples of fossilized traces of organisms.
•
Describe how index fossils can be used to determine the age of rocks.
General Science Rubric
Credit Grading
Responses to Packet
and Questions
40 pts.
Performance Task
40 pts.
Quiz
20 pts.
4
• My responses in
my packet show
clear reasoning and
use of evidence.
3
• My responses in
my packet show
basic reasoning and
use of evidence.
• I made connections
to other ideas
within and across
science credits.
• I completed all of
the expectations of
the assignment to
thoroughly show
what I know.
• My explanation is
clear and supported
by valid scientific
evidence.
• I mainly completed
the expectations of
the assignment to
show what I know.
• My explanation is
supported by
scientific evidence.
2
• My responses in
my packet show
basic reasoning but
limited evidence to
support it.
1
• My answers to the
questions in my
packet are either
unscientific or
overly simplistic,
and have limited
evidence.
• I completed some
of the expectations
of the assignment
to show what I
know.
• I completed some
of the expectations
but struggled to
show what I know.
• My explanation is
simplistic or basic
and supported by
limited scientific
evidence.
• My explanation is
not supported by
scientific evidence.
Students receive 2 points per correct response.
___x 10 = ___/40
___x2 = ___/20
Total:
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___x 10 = ___/40
___/100
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