Fracture Fundamentals: A Cheesy Analog
This activity has students make small cuts (fracture nuclei) in processed cheese food and then apply stresses perpendicular or parallel to the cuts to see how fractures grow. Surprisingly (or not, depending on one’s previous thoughts on cheese), processed cheese food fractures in much the same way as homogeneous rocks. The link below describes 8 short experiments to explore fracture propagation. Have the students make predictions before each experiment and, at the end, have each group explain their results to the rest of the class.
Learning Goals
This exercise:
- Helps students develop an understanding of the complex processes of fracture propagation and fault growth via simple and fun experiments.
- Demonstrates the importance of small cracks in the distribution of stresses in cheese (and rocks) and the growth of larger fractures, and shows the value of analog models.
Context for Use
This activity was designed for a structural geology class, but is also suitable for plate tectonics classes since it simulated the formation of overlapping spreading centers such as those seen along the East Pacific Rise.
It takes about 20 minutes to do (less if you split the eight parts of the activity among the class) with ten minutes for introduction and another 15 to 30 minutes for discussion of the results. So it can be used as a lecture activity or as an introduction to lab.
Teaching Materials
In order to do this activity, each pair or group of students needs
- A package of processed cheese food, as homogenous as possible. Real cheese isn't homogenous. Half-slices fractured more readily than whole ones.
- A knife or razor blade to make simple cuts
- A hole-making device: a drinking straw works well
- A couple of surfaces to place the cheese to cheese on and to apply shearing forces to. Applying force to the cheese directly may not result in simple shear. The best surface turned out to be a pair of pieces of the plastic wrapper.
- Two small pieces of cardboard or index cards should work to induce vertical (mode 3) shear.
The handout contains most of the details about the exercise:
- PDF (Acrobat (PDF) 34kB Jun8 04)
- rich-text (Rich Text File 10kB Jun8 04): can be edited in a word-processor
Teaching Notes and Tips
You can introduce the lab by:
- Reminding students that rocks and beds are not homogenous, they are full of weak spots and even pre-existing fractures that lead to sizeable stress concentrations at the tips.
- Talking about different geometries of stresses relative to the fractures (Mode I: perpendicular to the crack, Mode II: parallel to the crack, Mode III: perpendicular to the bed the crack is in).
- Inviting the students to speculate on how various kinds of applied stresses will act on pre-existing cracks
Have the students work in groups of two, three or four.
Each group should write down their predictions for each step of the activity before they do it and their observations afterward.
Provide the students with an easy way to dispose of used cheese and wrappers. Some students may decide to throw these at one another.
Assessment
At the end of the exercise, choose groups at random and have them present their results for one of the eight parts of the activity to the rest of the class.
References and Resources
More ideas on teaching structural geology will be posted on the site of the Cutting Edge Structural Geology Workshop.
