This demonstration is included in an undergraduate required course in structural geology.

Students must have had a basic introduction to two-dimensional stress, brittle failure, and Mohr circles.

This activity is one of many in-class assignments and demonstrations.

The primary goal of this demonstration is to present a visualization of the brittle deformation process and allow students to practice with Mohr diagrams.

Higher order thinking skills include the ability to analyze fracture data and formulate hypotheses.

Other skills that are developed include the ability to make detailed observations and descriptions of geologic processes.

To prepare for the demonstration students are assigned homework problems dealing with brittle deformation in which they must plot Mohr diagrams and determine shear plane orientations and Coulomb coefficients. In class we examine an undeformed core of Yazoo clay (Eocene) taken from a test site on campus. The core is then uniaxially compressed using a hydraulic press. The students are then asked to observe and describe the development of shear fractures (including conjugate shear surfaces) and measure their orientations. In addition, they are asked to speculate on the development of extension fractures that develop during unloading. The fracture data are then used to estimate the Coulomb coefficient for the Yazoo clay sample and comparisons are made to values obtained from samples of differing lithologies. We wrap up with a class discussion summarizing observations from the fracture demonstration.

The associated homework problems are checked first to make sure the students understand how to plot and evaluate Mohr diagrams. Following the fracturing demonstration, I check the completeness and detail with which the fracture experiment was documented by the student. I also evaluate their analysis of the fracture data and subsequent calculations of the Coulomb coefficient.