Conjugate Fractures form in Clay

Paul Kelso
Lake Superior State University
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Pottery clay deformed by uniaxial compression, with a standard hydraulic rock trimmer, produces conjugate fractures at approximately 30 degrees to sigma 1. This activity allows students to observe fracturing and its orientation relative to sigma 1. It also provides insight into the formation of conjugate fractures and their relationship to Mohr-Coulomb diagrams.

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This conjugate fracture experiment is used in a sophomore level required structural geology course. I usually devote one class period to the activity and associated discussions.

Skills and concepts that students must have mastered

Students must have an introductory understanding of structural concepts such as fracture, stress and strain.

How the activity is situated in the course

I use this as either an introductory, or as a culminating activity for discussions related to Mohr-Coulomb failure or conjugate fracture/fault systems. Thus, the activity can be used as an entry point or as a final exercise for discussions of fractures or at some appropriate point during discussions. I typically undertake this activity prior to a field trip where we observe and measure a conjugate fracture system.


Content/concepts goals for this activity

  1. Use a physical experiment to examine Coulomb failure and its relationship to Mohr-Coulomb diagrams
    Tie together observations and theoretical development of the fracturing of rocks
    Provide students a physical understanding of conjugate fracture/fault systems
  2. Experimentally develop the relationship between the principle stress directions and fracture/fault orientation (strain)
  3. Investigate the theoretical reasons for the observed deformation
  4. Students determine the principle stress directions for this experiment
The following week we measure the orientations of conjugate fractures at an outcrop and then determine the associated principle stress directions

Higher order thinking skills goals for this activity

  1. Student develop hypothesis (for how deformation likely proceeds)
  2. Students interpret the experimental results
  3. Students examine how different variables may influence deformation processes
  4. Students revise their hypothesis and discuss the reasons for the differences between their original and final hypothesis
  5. Students discuss/defend their interpretations citing supporting evidence

Other skills goals for this activity

  1. Observation
  2. Brainstorming

Description of the activity/assignment

Determining whether students have met the goals

The students initial individual, partner and class sketches and descriptions plus their final sketches and interpretations of this experiment may be collected and evaluated.
We also visit an outcrop with great conjugate fractures and students are required to relate the above analog experiment to their field observations to interpret the likely stress orientations in the rock that produced the observed conjugate fractures. The activities related to this outcrop are written up and handed in.

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