Conjugate Fractures form in Clay

Paul Kelso

Lake Superior State University

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This page first made public: Jun 22, 2004

Summary

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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Context

Audience

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.

Goals

Content/concepts goals for this activity

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
Experimentally develop the relationship between the principle stress directions and fracture/fault orientation (strain)
Investigate the theoretical reasons for the observed deformation
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

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

Other skills goals for this activity

Observation
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.