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Geologic Age: Activity

By Educational Resources

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Activity (Allow 2 class periods)

INSTRUCTIONS:

  1. Have students work in pairs or small groups.
  2. Any small cube-shaped objects will suffice - sugar cubes, math cubes, homemade cubes of wood or styrofoam, or dice.
  3. Students should already have been exposed to the concepts of relative time, absolute time, radioactivity and rates of radioactive decay.

EXPLORATION PHASE:

  1. You may wish to introduce this activity by having the students participate in the following introductory activity:
    • Have one student list on the chalkboard the names of everyone in her/his immediate family (parents, siblings) in order from the oldest to the youngest. Tell the student not to list the ages of any member.
    • Ask the remainder of the class if there is any way to know from the available information the exact age of any member of the family. They will realize it is not possible. The information they have can only be used to determine relative age, i.e., father is older than son or daughter, etc.
    • Explain that relative age is often used in the study of rocks. As scientists recognized that layers of rock had been deposited in sequence, one on top of another, they derived the principle of stratigraphic superposition, which says that in any sequence of strata, not later disturbed, the order in which they were deposited is from bottom to top. Therefore, rocks on the bottom of a sequence of undisturbed strata are older than rocks on the top of the sequence.
    • Have students identify the type of information they would need to determine the absolute age of any family member.
  2. Explain to students that they are going to participate in an activity that will demonstrate to them how geologists determine the absolute age of rocks or minerals. Instructions to students:
    1. The cubes you have been given represent the imaginary chemical element "Zorkium".
    2. Mark only one side of each cube with a felt-tip pen.
    3. Hold lid tightly and turn the box over twice. Remove lid.
    4. Take out all cubes that have the marked-side up. These cubes represent atoms that have decayed into the daughter element DOZ (Daughter of Zorkium). In the data table beside Trial 1, record the number of cubes removed and the number of cubes remaining.
    5. Repeat the procedures "c" and "d" until you have completed twelve trials or until all the cubes have been removed.

    CONCEPT DEVELOPMENT PHASE:

    1. Have students use the data collected in the chart to construct a graph. On the vertical axis, plot the number of cubes remaining each time. On the horizontal axis, plot the trial numbers.
    2. Connect the points you have plotted. Draw a best fit line for these points.
    3. Explain radioactive decay of an element.
    4. Define half-life of a radioactive element and how it can be used as an "atomic clock."
    5. Compare relative and absolute dating. Discuss differences in these two ways of dating materials.

    APPLICATION PHASE:

    1. Ask the following questions:
      • How many trials did it take for half of the Zorkium atoms to decay?
      • Suppose each trial equals 1000 years, what is the half-life of Zorkium?
      • After half (50) of the Zorkium cubes were removed from the box, how long did it take for half of the remaining cubes to decay? (Keep in mind that each trial represents 1000 years.) This amount of time represents the half-life of Zorkium.
      • Does your graph look like your neighbor's graph? Why or why not?
    2. Imagine that you have a radioactive sample containing both Zorkium and DOZ atoms. After analysis, you find that it contains 25 atoms of Zorkium and 75 atoms of DOZ. How old is your sample? (Hint: you must use the half-life of Zorkium determined earlier in this activity.)