How can scientists determine the ages of
fossils and rocks?
Agenda & Objectives
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Agenda
 Grab Science Journals
 Answer Questions
about slide picture in
Journal
 Lesson
 Activity
 Journal writing
Objectives
 Explain how a rock’s
relative age is
determined.
 Explain how a rocks
absolute age is
determined.
Activities Procedure
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 Create a data table after the example on the next slide.
 Use the marker to label a sheet of paper radioactive element and another
sheet of paper new element.
 Place all the beans on the sheet marked radioactive element. Each bean
represents 1 gram of a radioactive element in a rock sample. At time zero,
before any decay has occurred, record in the data table the amount of the
radioactive element.
 Assume that the half-life of the beans is 30 seconds. Not your starting time.
 Wait 30 seconds, then remove half of the beans from the radioactive element
paper and place them on the new element paper. Record your data.
 Repeat step 5 two more times. Record your data after each 30 second time
period.
 Graph your data, placing the time in half-lives along the x-axis and the
amount of the radioactive element in grams along the y-axis.
Data Table
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Time
0
1 half-life
2 half-life
3 half-life
Number of grams
radioactive Element new element
Questions
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 1. What do the beans that did not get moved represent?
 2. What do the beans that did get moved represent?
 3. How much of the radioactive element was left after 2
half-lives and after 3 half-lives?
 4. How much of the radioactive element would be left
after 4 half-lives?
What did you learn?
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 : 1.) How does the relative dating of rocks differ from
the absolute dating of rocks?
 2.) Describe the principles of superposition and
crosscutting relationships. Give an example of each.
 3.) How can a rock layer’s absolute age be
determined?
 4.) Why cannot carbon-14 be used to date fossils
older than 50,000 years old?
Lesson Plan Part 1
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Petra Goldberg
EDSE 3275 Secondary Science Methods I
Fa 2012
December 5, 2012
Micro-Teach and Misconceptions
Grade Level: 8th – 9th Grade Earth Science
Lesson Introduction
Overview: In this lesson students learn about the relative and absolute dating of rocks. Then explore the
principles of superposition, crosscutting relationships, and index fossils, as well as radioactive dating methods.
Objectives: To explain how a rock’s relative age is determined. To explain how a rock’s absolute age is
determined.
Questions: 1.) How does the relative dating of rocks differ from the absolute dating of rocks? A: Relative
dating determines which of two rocks is older, while absolute dating determines the actual age of a rock or fossil. 2.) Describe
the principles of superposition and crosscutting relationships. Give an example of each. A: The principle of
superposition stated that in layers of sedimentary rocks the oldest layers are on the bottom and the newest layers are on the top.
For example: a fossil found in a lower layer should be older than one in a higher layer. The principle of crosscutting relationships
states that a feature, such as a rock structure or a fault, that cuts across rock layers is younger than the rock layers, such as when
magma forces its way through cracks in rock layers and hardens to form igneous rock. 3.) How can a rock layer’s absolute
age be determined? A: By comparing the amounts of a radioactive element with the amount of its decay element. 4.) Why
cannot carbon-14 be used to date fossils older than 50,000 years old? A: After about 50,000 years almost all the carbon-14
in an organism has decayed to nitrogen-14.
Vocabulary: relative dating, principles of superposition, principle of crosscutting relationships, index fossil,
absolute dating, half-life.
Lesson Plan Part 2
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Teaching
Relative dating: One way of finding the age of a rock is to compare it to other rocks and finding which is older. This method is called relative
dating; you place rock layers in order from oldest to youngest without using actual dates.
Principle of Superposition: Let’s say you were unpacking a box of books, you can be fairly certain that the books on the bottom were placed in the
box before the books on top. You can use the same principle of thought with layers of sedimentary rock. This is called the principle of superposition and it
states that if sedimentary rock layers have not been overturned, each rock layer is younger than the layers below it and older than those above it. Based on this
principle, a fossil found in the bottom layer is older that a fossil found in a layer above.
Principle of crosscutting relationships: Suppose you saw a nail stuck in a tree trunk. You would realize that the tree grew first and then the nail
was pounded into the tree later. A similar principle is used to determine the relative ages of some rocks. According to the principle of crosscutting
relationships, a rock that cuts through another rock must be younger than the rock it cuts.
Igneous rock is formed when magma forced its way through cracks in the existing rock. The igneous rock is younger than the sedimentary rock
layers it is cutting through. How does this example relate to the principle of superposition?
Index Fossils: Some fossils, called index fossils, can be used to establish the relative ages of rocks that contain the fossils. Index fossils are fossils
that are widespread and lived for a relatively short period of time. Therefore, when scientists find an index fossil they know the relative age of the rock in which
the fossil was found.
Absolute Dating: Relative dating is useful but absolute dating is more specific. Scientists use absolute dating to find out the absolute age, or actual
age, of a rock or fossil. Scientists find the absolute age of a rock by studying certain radioactive elements the rock contains. Radioactive elements decay, or break
apart, to form other elements. This decay happens at a constant rate. The length of time it takes for half the atoms of a radioactive element to decay is the
element’s half-life. By comparing the amounts of different elements in a rock, scientists can determine the absolute age of the rock.
For example, the radioactive element carbon-14, a form of carbon, is used in absolute dating of some fossils. All living things contain carbon-14.
When a plant or an animal dies, the carbon-14 starts to decay, forming nitrogen-14. The half-life of carbon-14 is 5370 years. After 5730 years, half of the carbon14 decays. Every 5730 years, half of the remaining carbon-14 decays. By measuring the amount of carbon-14 in a sample, scientists can determine how many
years ago the organism died. After about 50,000 years, almost all the carbon-14 in and organism decayed to nitrogen-14; therefore, carbon-14 cannot be used to
date fossils older than 50,000 years old. Other radioactive elements are used to determine the absolute age of older fossils and rocks.
Uranium-238 occurs in some igneous rocks and decays to form lead-206. The half-life of uranium-238 is about 4.5 billion years. Scientists compare
the lead content of a sample to its uranium content to determine the absolute age of a rock. Using this method on meteorites, scientists have determined the age
of the earth to be 4.6 billion years old.
Lesson Plan Part 3
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Investigation: Making a half-life model
Purpose: To model and graph the decay of a radioactive element.
Time: 30 minutes
Process Skills: Making models, collecting and interpreting data
Thinking Skills: Interpreting a graph, making analogies
Preparations:
2 sheets of paper
Marking pen
16 beans
Clock or watch
Graph paper
Procedure:
Create a data table after the example on the board.
Use the marker to label a sheet of paper radioactive element and another sheet of paper new element.
Place all the beans on the sheet marked radioactive element. Each bean represents 1 gram of a radioactive element in a rock sample. At time zero, before any
decay has occurred, record in the data table the amount of the radioactive element.
Assume that the half-life of the beans is 30 seconds. Not your starting time.
Wait 30 seconds, then remove half of the beans from the radioactive element paper and place them on the new element paper. Record your data.
Repeat step 5 two more times. Record your data after each 30 second time period.
Graph your data, placing the time in half-lives along the x-axis and the amount of the radioactive element in grams along the y-axis.
Answer the following questions:
What do the beans that did not get moved represent?
What do the beans that did get moved represent?
How much of the radioactive element was left after 2 half-lives and after 3 half-lives?
How much of the radioactive element would be left after 4 half-lives?
Closing
Take this opportunity to go over each of the vocabulary terms in the lesson. Ask for volunteers to define the terms in their own words, write the definitions on
the board, and clarify them when necessary.
Lesson Plan Part 4
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Assessment
Give groups of students four different colors of play dough to represent rock layers. Ask the students to make a
model that illustrates the principles of superposition and crosscutting. Ask the students to explain, either orally or
in writing, how their model demonstrates these principles.
Teaching options (Cross curricular)
Language Arts: Have students write a letter to your state geological survey requesting information about the
geological history of your area. They may even ask for information about safe places to look for fossils in your area.
Science Journal
Ask student to imagine they are scientists living 5 million years in the future. Have them describe the kinds of
fossils they might find from present times.
Misconceptions: There is no way to tell how old a rock is. Man made radioactivity. Radioactivity is dangerous.
There were three misconceptions that could be answered with this lesson, the first one dealing with how
old rocks are and how as scientists we can know their ages through radioactive decay. Scientist understand and
know how uranium-238 decays into lead-238, which can be measured in rock to allow them to be dated. The second
misconception of Man having made radioactivity is also addressed in that radioactive decay is in all organisms and
a process that ages those organisms or rocks. The third misconception that radioactivity is dangerous can also be
addressed by going further into the different types of radioactivity and how it affects organism and rocks. The main
misconception targeted in this particular lesson was the one that there is no way to tell how old a rock is. Through
index fossil records, relative dating and absolute dating, scientists can determine the age of organisms and rocks to a
given age in geologic history.
Works Cited
American Guidance Service, Inc. (1997). Earth Science. Circle Pines, MN: American Guidance Service, Inc. .
All lesson material came from above source.