# Using deformed critters to determine angular shear and shear strain

Initial Publication Date: June 18, 2012 | Reviewed: January 19, 2015

#### Summary

This activity provides a step by step example to help students figure out how to determine the angular shear and shear strain, using fossils with bilateral symmetry. Students first complete the work on an idealized critter, and then on a sheared trilobite using simple graphical techniques and basic trigonometry.

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## Context

#### Audience

This activity is used in an undergraduate structural geology course, required for students majoring with a BA or BS in Geology.

#### Skills and concepts that students must have mastered

Use of protractors
Introduction to basic deformation processes and strain markers
Knowledge of very basic trigonometry

#### How the activity is situated in the course

Many of our students struggle with math and many have only had pre-calculus when they take Structural, so this activity is done very early in the semester following a basic trigonometry assignment and discussions/readings on stress and strain.

## Goals

#### Content/concepts goals for this activity

Determination of angular shear and shear strain
Application of basic math skills
Graphical analysis

#### Higher order thinking skills goals for this activity

I don't consider this a higher level activity, this is a remedial icebreaker to help my more math phobic students use simple graphical analysis in preparation for more challenging work later in the semester.

## Description and Teaching Materials

This activity provides a step by step example to help students figure out how to determine the angular shear and shear strain, using fossils with bilateral symmetry. Students first complete the work on an idealized critter using simple graphical techniques and basic trigonometry; the first example can be illustrated step by step if needed with the assistance of an animated Powerpoint file. Students then apply their skills to an image of a sheared trilobite.

Student Handout: Using Deformed Critters to Determine Angular Shear and Shear Strain (Acrobat (PDF) 525kB Jun18 12)
Student Handout: Using Deformed Critters to Determine Angular Shear and Shear Strain (PowerPoint 2007 (.pptx) 2.2MB Jun18 12)
Animated Example: Using Deformed Critters to Determine Angular Shear and Shear Strain (PowerPoint 2007 (.pptx) 95kB Jun18 12)

## Teaching Notes and Tips

Students sometimes have difficulty figuring out how to line up the protractor to measure the acute angle; I've found it better to have them work in pairs, because more adept students can quickly show their partners how to do this (freeing me up to help others). Some students are well beyond this type of remedial approach, so I often do this near the end of class so that those who can fly through it are able to leave a few minutes early and I'm able to assist those that are having more difficulty.

## Assessment

I collect the written responses, and assess whether they obtained a reasonable value for the angular shear, properly identified if it was positive or negative, and correctly computed the shear strain. I provide written feedback in the form of brief comments, and schedule a short meeting with students who are struggling with this type of activity. I also complete one by hand myself and post that as the key in the classroom.