Joints in a Cornstarch Analog

Juliet Crider

Western Washington University

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Initial Publication Date: June 18, 2004 | Reviewed: June 13, 2012 (see revision history: 2 events)

Summary

Desiccated cornstarch-water mixture provides an interactive introduction to joints and joint sets. Students interpret relative ages, examine intersection angles, use surface textures to determine propagation direction, and evaluate the role of flaws in joint initiation.

Microstructures, Deformation Mechanisms, Fabrics | College Lower (13-14), College Upper (15-16)

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Context

Audience

Undergraduate required course in structural geology. Typical enrollment: 20 students.

Skills and concepts that students must have mastered

This exercise follows a brief introduction to joints, abutting relationships, and joint-surface textures.

How the activity is situated in the course

one of more than a dozen in-class and laboratory exercises

Goals

Content/concepts goals for this activity

Describing and interpreting joints: abutting relationships (relative age), intersection angles, surface textures, propagation direction, and initiation from flaws

Higher order thinking skills goals for this activity

Interpreting sequence of events

Other skills goals for this activity

Sketching and labeling accurate representations of structures

Description of the activity/assignment

Joints are very important to problems in applied geology (fluid flow, slope stability), but three-dimensional exposures of simple joint sets are not readily accessible from my campus. I developed this exercise based on the experiments of Miller (2001) to give students hands-on practice describing and interpreting joints.

For the exercise, I prepare a cornstarch-water mixture a few days in advance and pour it into plastic petri dishes. I add a "flaw" to each dish (typically a small pebble). As the cornstarch dries, vertical joints develop.

In class, each group of 3-4 students is provided a petri dish of desiccated cornstarch. Students are asked to draw a map of the joints, paying particular attention to intersection angles. (The joints curve to intersect at 90 degrees.) They determine relative ages of the joints using abutting relationships. (Typically 3-6 generations of joints.) Students next dissect the sample and describe the surface textures of the larger joints and the location of the flaw. The cornstarch produces beautiful plumose structure (hackles). Students then interpret the joint propagation direction from the surface textures, and note the origin of the joint. (Typically, a first- or second-generation joint initiates at the flaw.) Students discuss the role of flaws in the initiation of joints in their groups.

Determining whether students have met the goals

At the end of the session, we review observations as a class, with each group reporting. I collect and review individual student maps, sketches and written responses.

More information about assessment tools and techniques.