Joints in a Cornstarch Analog
Juliet Crider
, Western Washington University
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.
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 Müller (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°.) 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.
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°.) 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.Download teaching materials and tips
- Activity Description/Assignment (Acrobat (PDF) 21kB Jun10 04)
- Instructors Notes (Acrobat (PDF) 7kB Jun10 04)
Other Materials
Supporting references/URLs
Müller, G., 2001, Experimental simulation of joint morphology, Journal of Structural Geology, v. 23, p. 45-49.
