A GEOCHRONOLOGY PUZZLE – INTEGRATING RELATIVE AND ABSOLUTE AGE STUDIES IN INTRODUCTORY COURSES

We designed a geochronology puzzle homework to facilitate the understanding of the differences between relative and absolute dating and how the two methods combined can yield a more complete history of geologic events. This homework assignment is a two-week exercise with multiple steps that addresses several learning goals, including: 1) using relative dating techniques to establish an order of events; 2) creating data tables and graphs in Excel for generating an exponential decay curve; 3) calculating radiometric ages using isotope concentrations and the generated decay curve; 4) using radiometric geochronology to help resolve ambiguities in the relative order of events; and 5) writing up the results in a summary narrative.

Students first determine the relative ages of rocks and events from a geologic cross section and turn in this list during the first week's laboratory section. Students are then guided in making an exponential curve for the U/Pb system using Excel as a homework assignment with step-by-step instructions. Students first make a spreadsheet of half-lives, number of years corresponding to each half-life, amount of U and Pb, and the U/Pb ratio. From this spreadsheet, students make a scatter plot of the number of years versus the U/Pb ratio. Students then modify the graph, changing both the x and y scales and its overall appearance. Using isotope data for igneous and metamorphic rocks provided, students calculate the corresponding radiometric ages. Finally, students compile their results and summarize the sequence of geologic events in a typewritten narrative that explains their reasoning.

This summary writing exercise reinforces the idea that geologic time problems are not merely abstract puzzles, rather they correspond to historical events. Students who did this exercise performed better on these topics in both lab and lecture exams than previous students.