Semi-Brittle Deformation Experiments

Alan D. Chapman and Jacqueline Reber
Macalester College and Iowa State University
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This lab uses analog modeling to investigate how earth materials deform within the brittle-ductile transition zone.

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The exercise is intended for students enrolled in an introductory level structural geology course for majors.

Skills and concepts that students must have mastered

I perform this exercise in week 6 of a 15 week semester, at which point students are comfortable describing and interpreting the products of brittle deformation processes and are ready to begin thinking about ductile deformation.

How the activity is situated in the course

This exercise is done as a special lab - special in that my class travels from Macalester College (St. Paul, MN) to Jacqueline Reber's research lab at Iowa State University. It should be noted that the research-grade setup described in the attached file allows the experimenters to construct force-displacement-time plots while visualizing developing semi-brittle structures. This is a major perk, but I could envision doing a version of the lab with a simpler setup. For details of how to construct the shear devices and pressure cell used in the lab, contact Jacqueline.


Content/concepts goals for this activity

Visualizing semi-brittle deformation, understanding the influence of pre-existing weakness on material strength, understanding the influence of pore fluid pressure on material strength, understanding the relationships between principal stress directions and fracture orientations, reporting and analyzing force-displacement-time data, relating analog modeling results to the real world (e.g., rocks and seismic events - slow slip events in particular).

Higher order thinking skills goals for this activity

Formulation of hypotheses, testing hypotheses, data analysis (including the construction of stress-strain plots and Mohr diagrams), data interpretation, scaling from non-geologic to geologic materials and from human to geologic time scales.

Other skills goals for this activity

Working in groups, operating lab equipment, collaborating with graduate students, writing a collaborative lab report.

Description of the activity/assignment

The stage is set for this exercise with a class period introducing semi-brittle deformation: What does it look like? How does it happen? Over what time/length scales does it occur? Distributing candy bars with nuts (brittle) and caramel (ductile) provides a tasty sense of semi-brittle deformation while chewing. The lab is done ideally in groups of 3-4 with each group responsible for performing experiments on one apparatus (distributed shear, localized shear, and pressure cell). Each experiment (described in the attached file) has setup, running, and cleanup phases. During the running phase, students in other groups are encouraged to view key moments in the experiment (e.g., propagation of fractures, modifying pore fluid pressure, seismic events). When the lab is completed, groups are scrambled so each group contains at least one student from each experiment. Students in newly formed groups collaborate to produce a lab report, containing abstract, introduction, methods, results, discussion, and conclusion sections.

Determining whether students have met the goals

Thoughtful lab reports demonstrating a clear understanding of what we did, why we did it, what the results are (with clearly labeled and captioned figures), what the results mean, and how the results extrapolate to the real world (geologic materials, timescales, and spatial scales) will earn full marks.

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