Calibrating the Geological Time Scale


This activity is part of a collection under development by participants in a June 2006 workshop. Tested versions will be available in Spring 2007.

Calibration of the geological time scale requires numerical age determinations of distinct events in Earth history defined by the rock record. However, relatively few geologic boundaries are dated directly. Thus boundary ages must be extrapolated from other sections with datable material.

This exercise demonstrates the methods of time scale calibration and explores the scientific uses of an accurate and detailed geological time scale. Students will construct geologic time scale calibration curves and use them to assign absolute dates to the geological boundaries observed in the rock record. They will use this time scale to discuss the temporal framework of geological processes and events occuring during that interval.

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Learning Goals

1. Students will access and make use of the CHRONOS website, or other tools for gathering information about core samples.
2. Students will create a composite geologic section to illustrate the discontinuous nature of the rock record.
3. Students will retrieve numerical age dating information from either original sources or secondary compilations.
4. Students will create time scale calibration curves determined from numerical age data and a composite geologic section.
5. Students will identify geological characteristics that make a section of the rock record distinct.

Context for Use

This exercise is intended as a capstone group project for upper-level undergraduate or graduate geochronology, historical geology, stratigraphy and related courses. It can be used in a lower level course provided the instructor prepares and simplifies some material.

The activity may require up to two weeks (4 to 6 one-hour periods) of in-class activity and discussion along with student work outside of class. It requires proficiency with the tools on the CHRONOS website (see Teaching Tips below). Parts of this activity can be adapted for shorter stand-alone class exercises.

Background required for the exercise
- Prior class discussions and readings on biological and geological events throughout Earth's history to permit students to identify an intriguing interval of geologic time for this calibration exercise.
- Students should have completed a basic exercise in graphical correlation that results in the creation of a composite core section.

Description and Teaching Materials

The primary product of this activity is a group poster. The main component of the poster is a calibration diagram for a segment of the geologic time scale. Supporting materials on the poster demonstrate the geological significance of a selected interval of geological time.

We envision a poster with supporting material constructed from CHRONOS and other tools and similar to the K-T boundary poster (from CHRONOS.ORG), but with the calibration diagram as a centerpiece. The poster forms a basis for more detailed class discussion of biological changes and geolgical events that occured during this interval of time.

K-T boundar y poster

Flow of the Activity:
1. Class discussion on how geologic boundaries are defined in the rock record and why the intervals between these boundaries are of interest to geologists. Students break into groups of 2-5 and choose an interval of time to investigate.

2. Instructor provides a basic introduction to the CHRONOS website. Homework: Each student should access the CHRONOS website and learn about its capabilities. A class discussion should follow.

3. Students will meet in their groups to organize their work. Each student in the group will choose one of the tools in CHRONOS (Age-Depth Plot, Age-Range Chart, PSICAT, Timescale Comparison/Timescale Creator), or other sources (other internet sources, primary literature) to find the information about the time interval. Examples of information that could be gathered might include: Age-Depth plot, Diversity Plot, Examples of Microfossils, including descriptions and photographs, image of a representative core/section etc. One student should be assigned the task of learning how to use the CONOP9 tool.

4. Each group will use CHRONOS to search for core samples that cross the time interval being investigated. Information regarding depth and fossil content should be gathered for the compilation of a composite section. The student assigned to learn about CONOP9 could be assigned this task, or the instructor may consider having each student search for appropriate core samples and having the group decide which samples to choose for their composite. When these have been chosen, a composite section should be generated by CONOP9.

5. Each student will construct a calibration diagram using the group composite cross section and the numerical age data for that interval from a published source, (e.g. Gradstein et al.). The student should draw a calibration line and provide a written explanation (one typewritten page) for their curve.

6. When the calibration graphs are completed, students should meet as a group and discuss their results. Each group must produce a graph that they agree upon.

7. Using the calibration line each student should construct the geological timescale for their time interval on the group's calibration graph.

8. Each group will create a poster that includes the calibration diagram and the background information gathered by the group. The CHRONOS KT poster shown below can be used as a model for making posters. Groups will present their poster to the class, explaining the various types of information shown, as well as describing the reasoning behind their calibration graph.

Teaching Notes and Tips

CHRONOS databases are restricted to the Cretaceous and later. Therefore it may be necessary to find additional sources to explore earlier time periods. The general approach outline in this exercise should apply.

When introducing the CHRONOS website, the instructor may want to hold class in a computer lab or a room that is capable of displaying internet webpages. The instructor may want to create a CHRONOS scavenger hunt as a homework assignment.

In the case of a lower level course, the instructor should gather data from the CRONOS website and distribute it to the class when the exercise begins. Also, the creation of the composite core section should be done by the instructor outside of class, or as a class demonstration.

If the exercise is used in a lower level course, the instructor may want to review how graphs are made and interpreted before beginning the exercise.


Students will complete a CHRONOS homework assignment, written assignment describing the reasoning behind their calibration curve, a group poster and class presentation. Grading rubrics can be used for each of these activities. Students may also be graded on their level of participation.

Questions relating to the concepts inherent in the creation of a calibration curve can be used to determine mastery.

References and Resources