# Absolute and Relative Dating

Grade Level:

9-10

overview:

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:**

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The procedure descriptions have been broken down into two phases with 4 parts each to structure the
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`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

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necessary background to understand radioactive decay and how absolute dating works which will be
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`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

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calculator. Instruct each pair to draw two circles on their desk (or on 2 pieces of paper) and label one
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“Original No. of radioactive M&Ms” and “No. of decayed M&Ms”. They should place all 50 M&Ms in the
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`circle labeled original. `

2.Instruct the students to determine 10% of their original radioactive M&Ms. These are the decayed

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M&Ms and should be placed in the circle labeled decayed. The 10% value is the decay rate of the
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hypothetical M&M element and each turn represents a year that has passed and time for 10% to decay.
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The students should continue to determine 10% of the No. of radioactive M&Ms remaining and continue
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moving M&Ms between the circles as well as completing the data table on the worksheet. They may need
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to complete one or two years as a class before doing it on their own. They should be instructed that they
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can round up when moving their M&Ms rather than splitting them. Completing this part of the activity with
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the M&Ms and with the data table is an effort to accommodate different types of student learners. Some
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will be able to see the patterns in the data table while others will need to act out the decay in order to
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`understand the process of decay. `

Part B

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1.After completing the data table, instruct the students to complete a graph of their data. They will need
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to decide which axis to plot the number of years of decay and which to plot the number of radioactive
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atoms remaining, which will require some thinking. This part of the activity provides an opportunity for
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`them to understand exponential graphs. `

Part C

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1.After completing the graph, the students should complete the first half of the worksheet questions. This
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worksheet asks students to think about the graph they just created and determine how long it took for the
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`M&Ms to decay.`

2.Once they have completed the activity, they can eat the M&Ms.

Part D

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1.Discuss as a class the answers to the questions to assess their understanding of these concepts thus far.
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Prepare a lecture to more formally introduce the concepts associated with radiometric dating and half-life.
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Use the activity with the M&Ms as reference to tie the concepts to their experiences. After taking notes,
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the students should now complete the second half of the questions which asks them to define the term
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`“half-life” and discuss the half-life of their radioactive M&Ms. `

**Phase 2: School Yard Archaeological Site**

Part A

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1.Introduce the activity and pass out the School Yard Archaeological Site Worksheet and the Archaeological
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Excavation Recording Form. Students should work in small groups with 4 students. After reading the
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introduction and setting up the background to Schoolyard Archaeological Site, students should be given
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instructions on how to excavate the bins representing each stratum. It is best if each group could have a
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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
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have to share. They should use the garden trowels to remove the dirt and find all of the artifacts. They
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should be instructed that they need to make at least 3 qualitative and 3 quantitative observations about
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each stratum and draw the objects on the profile map. They should be shown an example of a completed
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Profile Map if you think they will have difficulty wit this part of the activity. Inform them that they can use
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the back of their Recording Form to make drawings of artifacts or a plan map of their excavation. Instruct
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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
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found them to exchange with another group. Encourage them to come up with their own system to keep
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track of the locations, provide flags or other suggestions. Stress the importance of where the objects are
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found by reviewing the concept of superposition. In archaeology, it is important not just what you find but
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what you find out. Therefore, the location of objects is important in the interpretation of the site as they
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will discover in Part C.
3.Materials that can be buried in the excavation units include fruit pits, glass beads, coins or other metal
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fragments, pottery, bones (think about using the plastic casts from the Anatomy teacher), burnt wood or
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charcoal, burnt corn cobs, shells, fossils, igneous rocks, etc. Think about things that can be dated using
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C14 and P40 methods (burnt corn cobs or igneous rocks) as well as things that cannot be dated using these
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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
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observations and accurately updating their profile maps. Excavation and recording each stratum should take
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`about 10 minutes. `

Part B

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1.Put away all excavation materials and pass out the Radiometric and Relative Dating Results Worksheet.
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Students should now determine which artifact to send to the lab for absolute dating. They will need to
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record why they choose to date certain artifacts over others and discuss with their group to be sure that
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they will make good decisions to interpret the history of their site. A group that chooses wisely will select an
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artifact from each stratum.
2.After they have made their selection they can bring their Worksheet up to the front of the classroom and
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receive the information from the instructor about the number of radioactive atoms remaining for the artifact
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selected. An index card for every possible datable item in the excavations should be determined by the
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instructor. The students will use the provided tables and graphs to determine how old their sample is based
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on the information provided.
3.Instruct the students to share and discuss the date they determined for their object with the other
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`members of their group. `

Part C

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1.Instruct students to fill out the Interpretation section of the School Yard Archaeological Site Worksheet.
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They should apply what they learned about the absolute dates of some artifacts to determine the estimate
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the dates of other artifacts. Encourage them to use statements such as, “The burnt log in stratum 3 must be
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older than ______ years ago but younger than ______ years ago because it is located below _______ dated
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to ________but above ________ dated to ________.” Students should discuss ideas with others in their
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`group and support their interpretations with the evidence they discovered. `

Part D

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1.Have the students make a class presentation to report their group’s interpretation of when different parts
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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

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interpretation write-up of the School Yard Archaeological Site. This section reflects critical thinking and use
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of data to support their explanations in a logical fashion using clear language.
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keywords:

radiometric dating, absolute dating, half-life, archaeology

Attachment | Size |
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Archaeological Excavation Recording Form.doc | 174.5 KB |

Half Life of M & M.doc | 61.5 KB |

Kruse School Yard Archaeological Site Worksheets.doc | 212.5 KB |