Understanding Radioactivity in Geology: The Basics of Decay

Christina Stringer—University of South Florida, Tampa FL 33620
This activity was developed for Spreadsheets Across the Curriculum. National Science Foundation, DUE 0442629.


One of a collection of PowerPoint/Excel modules designed to reinforce quantitative skills in geologic context. The module explores the exponential decay of radioactive parents through the analogy with popping popcorn. Students build a spreadsheet to calculate the number of unpopped kernels of popcorn as a function of time starting with the probability that a given kernel will pop during a 10-sec interval. The module introduces the mathematics of the exponential function as a modeling function in both analytical and numerical contexts.

Learning Goals

Students will:
  • Consider a familiar occurrence of exponential decay in a quantitative way.
  • Gain experience in forward modeling of an exponential decay phenomenon.
  • Consider the relationship between the decay constant and the probability that a particular parent atom of a radioactive isotope will convert to its radiogenic daughter atom
  • Gain experience in fitting a trendline to a column of data.

Context for Use

This activity was designed for an upper-division math-in-geology course for geology majors but can easily be used in an introductory undergraduate geoscience class, as well as a high school pre-calculus class.

Description and Teaching Materials

module (PowerPoint 703kB Dec24 06)

Teaching Notes and Tips

The module is intended as a stand-alone resource. It can be used as a lab exercise or handwork assignment, or as the basis of an interactive classroom activity with just-in-time teaching of mathematical problem solving. The problem context and the mathematical content are developed within the module.


The module ends with hand-in questions that the students answer by manipulating the spreadsheets that they prepared while working through the module.

References and Resources