Measuring Geologic Time: The Case of Bloody Canyon

Susan Zimmerman
Center for AMS, Lawrence Livermore National Lab
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This activity introduces students to the fundamental ideas of relative versus radiometric dating, using the students themselves as a sample population. In the first half, the students attempt to order the people in the group in a relative sense, using physical observations and their memory-record of historical events. In the second half, the students use increasingly precise estimates of their age (from their birthdays) to check their relative order against their absolute age order. The discussion should include concepts of precision and accuracy, estimation, observation versus inference, and drawing conclusions based on incomplete, inaccurate, or updated information.

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This exercise will work best with a small group (~20), and is aimed at undergraduates in an introductory course in physical or historical earth science, but may provoke more sophisticated discussions in upper-level courses; a wide-range of student ages is ideal (e.g., community college) but not necessary.

Skills and concepts that students must have mastered

Students should have a some introduction to earth science, including the differences between observations and inferences, concepts of weathering and erosion, and the basics of radiometric dating, but the preliminary presentation and discussions can be tuned to include review or presentation of these concepts.

How the activity is situated in the course

The activity is intended to be a dynamic illustration of the power and pitfalls of measuring geologic time, to get students thinking about how geologists order and date events in earth history, give them a sense of the potential problems, and why dates in earth's history sometimes change.


Content/concepts goals for this activity

  • developing physical clues to estimate relative age, and refining sequences based on new information;
  • basic understanding of how radiometric dating works;

Higher order thinking skills goals for this activity

  • identifying the observations that led to interpretations, and recognizing the difference between them;
  • understanding the difference between accuracy and precision in estimating ages;
  • dealing with less-than-complete, conflicting, or confusing information;
  • recognzing sources of error and assessing their importance

Other skills goals for this activity

Description of the activity/assignment

In this lab, students are introduced to the difference between relative and absolute dating, using the students themselves as the material to be ordered. Initially, the students are asked to develop physical clues to put themselves in order from youngest to oldest (exposing the inferences we make unconsciously about people's ages), and this will be refined/modified using a list of current events from an appropriate historical period that more and more of the students will remember, depending on their age (among other variables). Absolute age is introduced by having the students order themselves by birth decade, year, month, and day, and comparing the absolute age order to the order worked out in the relative-dating exercise, with a discussion of dating precision and accuracy.

Determining whether students have met the goals

More information about assessment tools and techniques.

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