Tension gashes with Jell-O

Emily Peterman, Bowdoin College

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Summary

The purpose of this activity is to help students visualize how tensions gashes form.

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Context

Audience

Undergraduate course in structural geology, tectonics or introductory physical geology

Skills and concepts that students must have mastered

None--this is a great way to introduce students to the concepts of extension & compression

How the activity is situated in the course

This activity is situated as the introduction to understanding how to use features like tension gashes to determine the orientation of compression. I use this exercise in the class period before going into the field to examine (gorgeous) tension gashes that have subsequently been dextrally sheared--this provides them with a strong understanding of what things "should" look like in the field before seeing what nature actually looks like.

Goals

Content/concepts goals for this activity

kinematic analysis

Higher order thinking skills goals for this activity

formulation of hypotheses
analog modeling

Other skills goals for this activity

Description and Teaching Materials

The night before class, I prepare Jell-O that is approximately 2-3 cm thick. In class, students first hypothesize about what happens to rocks when they are compressed. After defining a hypothesis (or multiple hypotheses, depending on the course), students sketch a diagram of the experiment (including arrows!). Students then break into groups of 2-3 and deform the Jell-O. They sketch what they see and experiment with a variety of techniques for deforming the Jell-O. For example, what happens if I apply force at an angle to the edge of the Jell-O? Is this similar or dissimilar to the first experiment? What happens if I apply compression and then dextral shear?

Students commonly get the Jell-O to fold, and the Jell-O often records a trace of the fold axis (best for synclines), even when unfolded. This fosters discussion of the variety of ways that rocks can respond to compression.

I wrap-up the exercise by having students describe via annotated drawings what they hypothesized, what happened and what the effects were of composite deformation events.





Teaching Notes and Tips


Assessment

I bring students out into the field to look at and measure the orientation of tension gashes that have been dextrally sheared. I then ask them to use this data to describe the tectonic history as recorded by the tension gashes. Students will have met the goals of this assignment if they state: compression followed by dextral shearing.

On a midterm, I typically ask students to interpret a photo of a sinistrally sheared tension gash.

References and Resources