Teaching about Geochronology: Absolute (Numerical) Ages
Geochronology - the process of determining numerical ages and dates for Earth materials and events - is fundamental to understanding geologic time and geologic history. Although this topic is essential to understanding and appreciating geoscience, it is routinely overlooked and superficially addressed in introductory textbooks, many of which omit the mathematical aspects of radiometric dating (Shea, 2001). In addition, many students arrive in college classrooms with misconceptions about basic chemistry that interfere with their ability to understand radioactive decay and its use in geochronology (Prather, 2005).
Recommendations for Teaching
Based on our collective experiences as geoscience educators and/or geochronologists, here are our recommendations for teaching about this essential topic in geoscience.
Articulate learning goals (to yourself and to your students)
The first step in teaching effectively about any topic is determining what your learning goals are for your students. What is it that you want your students to know, understand, and to be able to do, related to geochronology? Here are a few examples of learning goals related to geochronology; you may wish to revise, select from, or expand on these for your own classes.
A student that has successfully learned about geochronology will be able to:
- describe the structure of an atom.
- define what an isotope is.
- describe the relationship between radioactive decay, half-life, and parent and daughter isotopes.
- explain why the choice of which radiometric system to use in calculating an age or date depends on the approximate age of the material being dated and on the minerals present in the material.
- explain several sources of geological and analytical uncertainties, and how we account for them in our age/date estimates.
- explain how geochronologists decide whether a radiometric age/date is reliable.
- interpret absolute ages for sediment grains, igneous rocks, and metamorphic rocks.
Review basic chemistry to address student misconceptions
- The structure of atoms, including the locations and charges of protons, neutrons, electrons (Prather, 2005)
- Isotopes and how they relate to radioactivity (what they are and why they are important to geochronology) (Prather, 2005)
Use analogies, particularly for things we cannot observe directly
- Although we cannot see time, we use clocks to measure its passage and calendars to record important dates in history. In fact, many different cultures have independently developed some means of measuring the passage of time. You and your students can explore the analogy that radiometric systems are geoscientists' clocks. You might also have them consider the questions, "What makes a good clock? How do you know whether it is accurate?" in the context of that analogy.
- Have students simulate radioactive decay using an activity such as M&M decay or coin flipping or counting beads.
Use "scaffolding": build from simpler, familiar ideas to more complex, less familiar ones
When people learn, we build on what we already know. As teachers, we can enhance this process by explicitly linking new ideas to familiar concepts. For example:
- Radioactive decay follows an exponential decay function. Many students struggle to understand logarithms and exponents (Shea, 2001). Remind them of familiar exponential functions, such as population growth, before exploring graphs of radioactive decay. It is also helpful to use multiple representations to accommodate different learning styles.
- Geoscientists use many methods to establish absolute ages and dates, from counting tree rings, varves, or laminae in ice cores to radiocarbon dating to other radiometric dating systems. Most students are familiar with dendrochronology (though they may not know that term) and have heard of radiocarbon dating (though they may not know and understand the details of how it works). These provide a foundation for discussion of other geochronological methods.
Have students solve real problems using real data
Having students solve real geologic problems gives them the authentic experience of doing science. Here are a couple of examples of teaching activities that use real data to teach students about radiometric dating:
Review the process of science as you address students' questions
Thoughtful students are likely to ask thought-provoking questions about geochronology, such as
- How do we know there was no radiogenic daughter in the crystal when it formed?
- How do we know that the decay rates have been constant over time?
- Why do different radiometric systems give different "ages" for the same rock sample?
Answering these questions is critically important. If we ignore them, we run the risk of reinforcing students' misconceptions that radiometric dates are unreliable. Acknowledging the challenges of radiometric dating, while explaining how scientists know what we know, deepens our students' understanding both of geochronology and of science. Mark Schmitz and Karen Viskupic at Boise State University have put together several slide sets that explain and show, in detail, how U-Pb geochronology is done.
Prather, Edward (2005), Students' Beliefs About the Role of Atoms in Radioactive Decay and Half-life. Journal of Geoscience Education, v. 53, n. 4, pp. 345-354.
Shea, James (2001), Teaching the Mathematics of Radiometric Dating. Journal of Geoscience Education, v. 49, n. 1, pp. 22-24.