Relative and Absolute Geologic Time with Maps and Spreadsheets

Eileen Herrstrom

University of Illinois at Urbana-Champaign

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This page first made public: Jun 17, 2019

Summary

This activity takes place in a laboratory setting and takes ~1.5-2 hours to complete. Students apply stratigraphic principles for relative dating and basic equations of isotopic systems for absolute dating and bracket the ages of undateable sedimentary rocks.

Historical Geology | College Introductory

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Context

Audience

Undergraduate class on introductory physical geology or quantitative reasoning for non-majors

Skills and concepts that students must have mastered

Must know how to read maps and vertical cross-sections, understand unconformities, and use Microsoft Excel (enter formulas, fill down, make charts)

How the activity is situated in the course

This is a laboratory activity that follows lectures on relative and isotopic dating, and it occurs just past the middle of the course.

Goals

Content/concepts goals for this activity

Define the main stratigraphic principles used to determine relative ages, explain the concept of an unconformity and how it relates to the ages of rocks, and apply these principles to decipher the relative ages of geologic units and features

Higher order thinking skills goals for this activity

Calculate parent and daughter fractions during radioactive decay for three isotopic systems, construct an Excel chart illustrating the decay of parent isotopes, and explain the relation between the decay constant and half-life of a radioactive isotope

Other skills goals for this activity

Determine ages for three samples using isotopic methods, infer the ages of undateable rocks by combining absolute and relative methods, and interpret the Geologic Column/Time Scale in relative/absolute terms

Description of the activity/assignment

Much of what we know about the geologic history of Earth is based on information in rocks now exposed at Earth's surface. Research into Earth history is much like research into human history, but instead of searching through accounts written on sheets of paper as historians do, geologists examine layers of rock. Relative dating of geologic features involves putting geological events in a correct time sequence from oldest to youngest. Absolute dating yields a number (years or Ma = mega-anna = millions of years). In this exercise, students learn and apply the concepts geologists use to determine the relative and absolute ages of rocks.

Student materials for this exercise include a Microsoft Excel spreadsheet with with data for radioactive decay and isotopic dating and an MS Word instruction sheet. The exercise is divided into three parts.

In Part I, students study stratigraphic principles and use them to interpret both a map and a vertical cross section. The Geologic Column is introduced in relative terms.

Part II involves calculating the fractions of parent and daughter isotopes in samples using the equation for radioactive decay and the decay constants for three isotopic systems (40K-40Ar, 238U-206U, and 14C-14N). Students also graph their results and interpret the graphs in terms of half-life and decay constants.

In Part III, students calculate isotopic ages for three rocks, bracket the ages of undateable rocks, and interpret the Geologic Time Scale in absolute terms.

Determining whether students have met the goals

As originally designed for a traditional face-to-face course and later used in the online version, this activity is assessed by the answers to the questions. It is also possible to have students submit their completed spreadsheets, although this option works best in a small class.

More information about assessment tools and techniques.