Discovering the Principles of Relative Age Determination – a Think-Pair-Share In-Class Activity

This page was authored by James Ebert, State University of New York, College at Oneonta. It is used in the course Earth History and the Fossil Record
State University of New York, College at Oneonta, Earth and Atmospheric Sciences
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Initial Publication Date: May 9, 2012 | Reviewed: July 6, 2017


In this in-class activity, students are challenged to identify rock units and geologic features and determine the relative ages of these features without prior instruction in the classical methods of relative age determination. Field photographs and satellite images are the data from which students make their observations and interpretations. Students are introduced to the formal language of relative age interpretation after this experience with employing the concepts.

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Learning Goals

Through this activity, students will
  1. gain experience with "field" observation and recognition of rock units and geologic features.
  2. apply logical reasoning to interpret sequences of events.
  3. derive principles of relative age determination.
  4. begin to develop a sense of "deep time" as they are challenged to imagine how long it would take to develop the sequences of events that they have interpreted.

Methods of Geoscience

Field observation and principles of relative age determination such as Steno's Laws of Superposition, Original Horizontality and Original Lateral Continuity, and Hutton's principles of crosscutting relationships and included fragments are fundamental methods for geological investigation. Principles of relative age determination are commonly taught through description in lecture or with line diagrams (cross sections and block diagrams) in a "lab" or homework assignment. These verbal and diagrammatic presentations may seem abstract to students and they may have difficulty applying these principles in real world situations. This activity builds the fundamental skill of observation and gives students practice in determining relative age relationships in a classroom setting that more closely models field observation and discussion.

Context for Use

This activity was designed for the 200-level course Earth History and the Fossil Record (GEOL 220) at the State University of New York College at Oneonta, a primarily undergraduate, comprehensive public college.

GEOL 220 is typically the second geoscience course that majors take following an introductory course. Students majoring in Geology, Earth Science, Earth Science Education and Elementary Education (concentration in Earth Science) comprise the students in the class. Enrollment ranges from 27 to 48. The activity is conducted in a 50-minute class and extended through a homework assignment. The activity is scheduled during the first two weeks that the class meets. The skills of observation and logical reasoning are necessary for this activity. A PowerPoint presentation that features field photographs and satellite images of outcrops from Earth, Mars and Europa provides the data, from which students observe, discuss and interpret. This activity should be easily adaptable for use in larger classes and at other colleges.

Description and Teaching Materials

The class is provided with a brief introduction in which it is explained that geologic history is recorded in rocks and that geologists must first observe the features and spatial relationships of rock bodies to interpret this history. Following this, the class is presented with a field photograph of a sequence of horizontal sedimentary rocks. Utilizing a think-pair-share approach, students define how many rock units are present and what distinguishes them. Students discuss these aspects with a partner before the entire class is engaged in discussion. From this simple activity, students learn that classifications are human constructs and that some observers are "lumpers" whereas others are "splitters." Following this discussion, the class is asked which rock unit is the oldest and how do they know?

With this foundation, the class is presented with a series of field photographs and satellite images in a PowerPoint presentation. Images include terrestrial outcrops and satellite images from Mars and Europa. For each image, students are instructed to make observations and interpret age relationships. In the whole class discussions that follow each image, students share their observations and interpretations and are asked to explain how they interpreted relative age relationships. Nine to ten images can be discussed in a 50-minute class. Remaining images are assigned as homework and discussed in the following class. It is only after these experiences that the instructor presents the formal language of relative age determination.

Teaching Notes and Tips

The geologic histories represented by the field photographs and satellite images generally increase in complexity as the activity proceeds.


Through class discussion, it becomes readily apparent which principles of relative age determination are understood by the class and which ones present challenges. These can be addressed via mini-lectures. The homework assignment that continues the in-class activity is collected and evaluated to determine areas which need to be addressed in greater detail, but they are not graded.

References and Resources