Introduction to the principles and processes of radiometric dating

John Chadwick (College of Charleston), Susan Zimmerman (Livermore National Laboratory), Gina Szablewski (University of Wisconsin), Erica Crespi (Washington State University)
Initial Publication Date: May 9, 2012


Outline of lecture topics and hands-on activities for introducing radiometric dating.

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Students in an introductory geology or earth history course

Skills and concepts that students must have mastered

Relative dating, different rock types

How the activity is situated in the course

Lecture(s) that can be placed almost at any point during the semester


Content/concepts goals for this activity

1. To understand principles used to measure time
2. To understand the concept of radioactive decay
3. To understand the process of radioactive dating
4. To learn how dates obtained from radioactive dating are verified
5. To gain confidence in using geochronological dating tools to understand the history of the earth

Higher order thinking skills goals for this activity

To apply the concept of time measurement across time scales.
To use radiometric dating methods (half-life and parent-daughter ratio) to generate hypotheses about the ages of rocks.
To use multiple lines of data to assess the certainty of dating estimates.

Other skills goals for this activity

Plotting radiometric data against time and interpreting half lives from plots

Description of the activity/assignment

1. Introduce the concept of time and how we measure it.

Have students think about how a calendar works: Why are months so variable in length? Why is a year 365.25 days? Is there a better, more elegant design for a calendar that doesn't have so much variability in the number of days in a month? This process scaffolds student understanding of how time is measured by beginning with a familiar system of measurement. It's a measure of time that is based on a fundamental physical process: the Earth orbiting the Sun. That can lead to the fact that there is a basic physical process behind radioactive decay as well. Possible discussion questions:

  • What are clocks, and why do we trust them?
  • What is time ultimately based on?
  • Why do we have days, months, and years?

Encourage students to think about measuring time at different scales. For example:

  • How do we know how old you are? (word of mouth)
  • How old was that tree when it died? (tree rings, ice layers)
  • How do we know how old the Parthenon is? (archeology, written history)
  • How do we know when the dinosaurs went extinct? (geochronology)

Bring in an hourglass; demonstrate that time can be measured with a physical process (sand falling). Use multiple hourglasses to illustrate precision in measurements -- preparing for a discussion of the use of multiple radiometric systems.

  • Ask: "What is the most accurate clock in the world?" (an atomic clock!)

2) Describe how radiometric dating works.

Show slides, step-by-step, of the process, from sample collection to mass spectrometry.

Use in-class demonstrations/analogies of the concept of radioactive decay.

3) Directly address the question of the reliability of radiometric dates.

Principles and methods of the radiometric dating process:

  • Do the same test on the same rock multiple times.
  • Do the same test on multiple rock samples from same rock layer.
  • Use different radiogenic systems to date the same rocks.
  • Multiple scientists analyze rock samples in multiple labs.
  • Ask: Does the radiometric date fit with our simpler relative dating systems (e.g. superposition)?

4) Discuss complexities and limitations of radiometric dating.

Be skeptical for your students. Anticipate and address their doubts.

  • Describe the time scales for which each system is useful. Discuss radiocarbon dating, which most of the students will have heard about, and why it is not used on older rocks.
  • Discuss ways scientists validate the quality of the age estimates obtained from radiometric dating. How do we know when a date might be compromised?
  • Discuss the analytical uncertainty on dates and the reasons for uncertainty.

Determining whether students have met the goals

1. Utilize Student Response System, or Clickers during lecture: Inserting questions at the end of each conceptual module within the lecture(s), including a question at the beginning and the end about their comfort level with the age estimates that they've heard about deep time (e.g., dinosaur fossil, a weathered rock)

2. Reflective learning assessment, or metacognition: At the end of the lecture(s), ask students what was 1) the most important, 2) the most interesting, 3) the most confusing, and 4) something they want to know more about.

3. Skills assessment: For an example "rock," students will be asked to count items to calculate parent-daughter ratio, given a plot of the decay, determine the age of the sample, and determine if it makes sense in the geological context, and place the date in its appropriate place in the geological time scale.

4. Concept Map: At the end of the lecture ask students to create concept maps to demonstrate how a list of key words are related.

5. Possible exam questions: 1) Given a number of parent and daughter ratios and a half-life of a particular isotopic system, students should calculate the age of a rock, and place that age on the Geological Time Scale in a period or time boundary; 2) Students should know the appropriate isotopic sytem(s) that could be used for a particular age; e.g. that C-14 cannot be used for K-T boundary rocks.

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