Lesson: Absolute and Relative Dating
Author: Melissa Kruse
Overview:
This activity will guide students through an inquiry-based, critical thinking approach to
the study of absolute and relative dating methods. The students will be asked to apply
their knowledge in a similar way to how real scientists approach dating archaeological
and palentological sites. In this lesson, students will interpret an archaeological site and
determine appropriate dating techniques.
Keywords: radiometric dating, absolute dating, half-life, archaeology,
Grade Level: 9-10
Time requirements: 2-3 class periods
Goals:
Students will:
 Learn absolute dating principles
 Learn relative dating principles
 Make observations and reading graphs
 Make decisions about how to analyze their data
 Learn to support their interpretations based on collected evidence
 Analyze, interpret data, and draw conclusions to write the history of an area
Materials:
Phase 1: Students will work in pairs. Each pair should have:
 50 M&Ms
 calculator
 Half Life of M&M Worksheet and graph paper
Phase 2: Students will work in small groups of 4-5 students. Each group will share:
 4 prepared plastic bins filled with sediments and artifacts, labeled 1-4
representing each stratum
 garden trowels
 rulers
Each student will have a copy of:
 School Yard Archaeological Site Worksheet
 Archaeological Excavation Recording Form
 Radiometric and Relative Dating Results Worksheet
National Science Standards:
Content Standard A: Science as Inquiry
Content Standard D: Earth and Space Science
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Arizona State Science Standards (High School):
Strand 1: Inquiry Process
Concept 2, PO 5: Record observations, notes, sketches, questions, and ideas using
tools such as journals, charts, graphs, and computers.
Concept 3, PO 1: Interpret data that show a variety of possible relationships
Concept 4, PO 2: Produce graphs that communicate data
Concept 4, PO 3: Communicate results clearly and logically
Concept 4, PO 4: Support conclusions with logical scientific arguments
Strand 6: Earth and Space Science
Concept 3, PO 4: Interpret a geologic time scale.
Concept 3, PO 5: Distinguish between relative and absolute geologic dating
techniques.
Teacher Overview and Introduction:
This activity has students working as archaeologists. They will excavate a hypothetical
archaeological site from their school yard, make observations, and write an interpretation
of the history of the area based on the evidence they will excavate. To facilitate the
activity, the teacher will provide some introduction to absolute dating principles through
the Half Life of M&M Worksheet. Then in Phase 2 of the project, the teacher will
provide the students with plastic bins each representing a different stratum of the
archaeological site.
Using the absolute dating principles provided in Phase 1 of the lesson, students will make
decisions of which artifacts to send to a lab for absolute dating. Based on this
information, they will learn how to relatively date associated artifacts. They will
interpret their archaeological site by writing an explanation of when each stratum and
artifact was deposited in their site.
Lesson Plan Procedures:
The procedure descriptions have been broken down into two phases with 4 parts each to
structure the steps of the activity more clearly.
Phase 1: Half Life of M&Ms
This activity will introduce the concept of a half life and absolute dating. Students will
likely not have the necessary background to understand radioactive decay and how
absolute dating works which will be necessary for Phase II. If students have this
understanding, the class can proceed with Phase II.
Part A
1. Give each pair of students a copy of the Half Life of M&Ms Worksheet, a bag of
50 M&Ms, and a calculator. Instruct each pair to draw two circles on their desk
(or on 2 pieces of paper) and label one “Original No. of radioactive M&Ms” and
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“No. of decayed M&Ms”. They should place all 50 M&Ms in the circle labeled
original.
2. Instruct the students to determine 10% of their original radioactive M&Ms. These
are the decayed M&Ms and should be placed in the circle labeled decayed. The
10% value is the decay rate of the hypothetical M&M element and each turn
represents a year that has passed and time for 10% to decay. The students should
continue to determine 10% of the No. of radioactive M&Ms remaining and
continue moving M&Ms between the circles as well as completing the data table
on the worksheet. They may need to complete one or two years as a class before
doing it on their own. They should be instructed that they can round up when
moving their M&Ms rather than splitting them. Completing this part of the
activity with the M&Ms and with the data table is an effort to accommodate
different types of student learners. Some will be able to see the patterns in the
data table while others will need to act out the decay in order to understand the
process of decay.
Part B
1. After completing the data table, instruct the students to complete a graph of their
data. They will need to decide which axis to plot the number of years of decay
and which to plot the number of radioactive atoms remaining, which will require
some thinking. This part of the activity provides an opportunity for them to
understand exponential graphs.
Part C
1. After completing the graph, the students should complete the first half of the
worksheet questions. This worksheet asks students to think about the graph they
just created and determine how long it took for the M&Ms to decay.
2. Once they have completed the activity, they can eat the M&Ms.
Part D
1. Discuss as a class the answers to the questions to assess their understanding of
these concepts thus far. Prepare a lecture to more formally introduce the concepts
associated with radiometric dating and half-life. Use the activity with the M&Ms
as reference to tie the concepts to their experiences. After taking notes, the
students should now complete the second half of the questions which asks them to
define the term “half-life” and discuss the half-life of their radioactive M&Ms.
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Phase 2: School Yard Archaeological Site
Part A
1. Introduce the activity and pass out the School Yard Archaeological Site
Worksheet and the Archaeological Excavation Recording Form. Students should
work in small groups with 4 students. After reading the introduction and setting
up the background to Schoolyard Archaeological Site, students should be given
instructions on how to excavate the bins representing each stratum. It is best if
each group could have a set of all four of the stratum but it is more likely that
there will be 2-3 sets per class and the groups will have to share. They should use
the garden trowels to remove the dirt and find all of the artifacts. They should be
instructed that they need to make at least 3 qualitative and 3 quantitative
observations about each stratum and draw the objects on the profile map. They
should be shown an example of a completed Profile Map if you think they will
have difficulty wit this part of the activity. Inform them that they can use the
back of their Recording Form to make drawings of artifacts or a plan map of their
excavation. Instruct students to make observations that seem to be the most
important for interpreting the site.
2. Once they are finished excavating each bin they should put the artifacts back in
the same location they found them to exchange with another group. Encourage
them to come up with their own system to keep track of the locations, provide
flags or other suggestions. Stress the importance of where the objects are found
by reviewing the concept of superposition. In archaeology, it is important not just
what you find but what you find out. Therefore, the location of objects is
important in the interpretation of the site as they will discover in Part C.
3. Materials that can be buried in the excavation units include fruit pits, glass beads,
coins or other metal fragments, pottery, bones (think about using the plastic casts
from the Anatomy teacher), burnt wood or charcoal, burnt corn cobs, shells,
fossils, igneous rocks, etc. Think about things that can be dated using C14 and
P40 methods (burnt corn cobs or igneous rocks) as well as things that cannot be
dated using these methods (coins, pottery, fossils). This will give some variety to
their choices in Part B.
4. Instruct the students to begin their excavations. You should make sure they are
making good observations and accurately updating their profile maps. Excavation
and recording each stratum should take about 10 minutes.
Part B
1. Put away all excavation materials and pass out the Radiometric and Relative
Dating Results Worksheet. Students should now determine which artifact to send
to the lab for absolute dating. They will need to record why they choose to date
certain artifacts over others and discuss with their group to be sure that they will
make good decisions to interpret the history of their site. A group that chooses
wisely will select an artifact from each stratum.
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2. After they have made their selection they can bring their Worksheet up to the
front of the classroom and receive the information from the instructor about the
number of radioactive atoms remaining for the artifact selected. An index card
for every possible datable item in the excavations should be determined by the
instructor. The students will use the provided tables and graphs to determine how
old their sample is based on the information provided.
3. Instruct the students to share and discuss the date they determined for their object
with the other members of their group.
Part C
1. Instruct students to fill out the Interpretation section of the School Yard
Archaeological Site Worksheet. They should apply what they learned about the
absolute dates of some artifacts to determine the estimate the dates of other
artifacts. Encourage them to use statements such as, “the burnt log in stratum 3
must be older than ______ years ago but younger than ______ years ago because
it is located below _______ dated to ________but above ________ dated to
________. Students should discuss ideas with others in their group and support
their interpretations with the evidence they discovered.
Part D
1. Have the students make a class presentation to report their group’s interpretation
of when different parts of the archaeological site were formed and when the
different artifacts were deposited.
Assessment:
Students will be assessed on the answers they provide on each worksheet and, especially,
on their interpretation write-up of the School Yard Archaeological Site. This section
reflects critical thinking and use of data to support their explanations in a logical fashion
using clear language.
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