Teach the Earth > Structural Geology > Teaching Activities > Using play-doh to understand 3D Flinn Plots

# Using play-doh to understand 3D Flinn Plots

Carol Ormand
,
Wittenberg University
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#### Summary

In this activity, students are introduced to the 3D Flinn Plot. They have previous experience with 1+e2 vs. 1+e1 plots of plane strain, but this is their first exposure to 3D strain. Students deform play-doh, then plot the resulting shapes on Flinn Plots. The exercise links the abstract 3D Flinn Plot to concrete strain ellipsoids.

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## Context

#### Audience

Undergraduate required course in structural geology

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

Students must understand 2D strain, strain ellipses and 1+e2 vs. 1+e1 plots of plane strain.

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

This is one of many in-class exercises I use to introduce new concepts and to see immediately where students have difficulty with them.

## Goals

#### Content/concepts goals for this activity

Visualizing 3D strain; plotting 3D strain on a Flinn Plot; interpreting 3D strain ellipse shapes from Flinn Plots; measuring 3D strain in rocks

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

data analysis, 3D visualization

#### Other skills goals for this activity

measuring, calculating, and plotting

## Description of the activity/assignment

In preparation for this exercise, students have studied 2D strain, become familiar with strain ellipses, and have plotted 1+e2 vs. 1+e1 for progressive pure shear and simple shear deformations. They have measured 2D strain using a variety of standard lab methods. And they have read about 3D strain and the strain ellipsoid.

During class, I have each student make a play-doh cube and mark circles on at least three of the (mutually perpendicular) sides. Then I have each student deform their cube (maintaining an overall rectangular prism shape). I request that they make a different shape than their neighbors' as they deform their play-doh. I ask them to describe what happens to the inscribed circles, and therefore what would be happening to an imaginary sphere within their cube.

Next I introduce the idea of a Flinn Plot, as an abstract but elegant means of conveying 3D strain ellipsoid shapes. I describe the axes, point out that the origin is at (1,1), and plot an example, using my own play-doh parallelipiped, deformed like theirs. Each student then calculates (1+e1)/(1+e2) and (1+e2)/(1+e3) for their parallelipiped, and plots their strain ellipsoid on a Flinn Plot on the board. As a class, we examine each deformed block of play-doh and compare it to its corresponding point on the Flinn Plot. I ask the class to generalize about the deformed shapes above the "plane strain" line versus those below the "plane strain" line. Each student thus practices measuring and calculating 3D strain, and plotting that strain on a Flinn Plot. And they have the opportunity to relate some concrete strain shapes to the abstract Flinn Plot.

I follow this activity up by having students measure 3D strain in a rock sample and plotting their results on a Flinn Plot. Then we go on to discuss the element of time, and also the behaviors of various strain markers during deformation.

## Determining whether students have met the goals

On the next exam, I have students measure 3D strain in a rock sample and plot their results on a Flinn Plot.