Geochronology in the San Juan Mountains (Advanced)

Kéyah Math Project development team: Nancy Zumoff, Christopher Schaufele, Steven Semken, Tracy Perkins, Lynn Onken, Philippe Laval, David Gonzales, and Andrew Becenti (deceased).
Kéyah Math Project directed by Steven Semken , Arizona State University; and Christopher Schaufele and Nancy Zumoff, Professors of Mathematics, Emeritus.
Archived at Arizona State University School of Earth and Space Exploration.

This activity was selected for the On the Cutting Edge Reviewed Teaching Collection

This activity has received positive reviews in a peer review process involving five review categories. The five categories included in the process are

  • Scientific Accuracy
  • Alignment of Learning Goals, Activities, and Assessments
  • Pedagogic Effectiveness
  • Robustness (usability and dependability of all components)
  • Completeness of the ActivitySheet web page

For more information about the peer review process itself, please see

This page first made public: Jun 30, 2008


This is a mathematically more advanced version of the Ages of Rocks and the Earth activity that introduces students to the mathematics of radiometric dating. Students derive the decay equation for the rubidium-strontium isotopic system, then apply it to date rock samples from the San Juan Mountains of southwest Colorado and a meteorite assumed to be approximately coeval with the Earth.

Click here to view the full activity on the Kéyah Math Project website.

Learning Goals

  • Review the use of radioactive decay as a "natural clock" for dating rocks.
  • Derive the decay equation for the rubidium-strontium isotope system.
  • Apply this decay equation to obtain ages for rock samples from the San Juan Mountains in southwest Colorado.
  • Apply this equation to data from a meteorite to estimate the age of the Earth

    Mathematical Skills
  • Use basic algebra.
  • Solve exponential equations.
  • Use logarithms.
  • Perform linear regression analysis.
  • Context for Use

    This activity consists of a set of quantitative problem-solving exercises that can be used as an in-class activity or an assignment in any introductory course with a unit on geologic time or geochronology, such as:
    -Physical geology or physical geography
    -Historical geology or Earth history
    -Earth materials, mineralogy, or petrology

    Description and Teaching Materials

    Teaching Notes and Tips

    An Instructor's Guide to all Kéyah Math activities is available online from the Instructor Resources page on the Kéyah Math website.


    Students record their work and answers in a word-processor document or a notebook, which can be submitted to the instructor for assessment. Solutions to these problems are available online from the Instructor Resources page on the Kéyah Math website.

    References and Resources