In-class and homework assignment due at the beginning of class on March 27.

This exercise was developed by Eric Hargrave based on Tewksbury, B. J., 1996, Using graphics prgrams to help students undertand strain, in Declan Depaor, editor, Structural Geology and Personal Computers, Tarrytown, NY: Pergamon Press, pp. 60-73.

In this exercise, we will introduce the concepts of pure shear and simple shear using Adobe Photoshop (note that the instructions wre written for version 4.0.1, but hopefully are not significantly different than whatever current version you use). This document includes an explanation of how to use the appropriate parts of photoshop and then what to do to explore deformation.

Where it says to "Open the file containing ______", it also has a hyperlink to that file.  Click on the hyperlink and then right click (if you are on a PC) and save the file to your desktop or workspace.  Then you can open it in Photoshop.

Behavior of Circles during Pure and Simple Shearing

1. Open up the file containing the circles in Photoshop.

2. Use the Marquee Tool to draw a box around the circles. (upper left of floating toolbar).

3. In the Edit menu, choose the Scale option under the Transform command. Small boxes should now appear at the corners and midpoints of the box surrounding the circles.

4. Click on the small upper right-hand box, and pull the box upward and to the left (or downward and to the right) until you have a rectangle (try to keep the same area as the original box). Let go of the mouse button, and the circles will be deformed.

5. To go back to the original circles, choose the Undo command in the Edit menu. If you get to a point where you cannot undo, use the revert command to go back to the original form. If you cannot get the transform to end, click on any tool on the tool bar. Do not save unless you give it a different name!

6. Repeat steps #3-#5 several times using different corners and making different sizes of rectangles. Notice the shapes and orientations of the resulting ellipses.

7. In the Edit menu, choose the Skew option under the Transform command. Small boxes will appear at the corners and midpoints of the box surrounding the circles.

8. Click on the small box at the midpoint of the top line of the box, and pull the box to the right (or left) until you have a parallelogram. Let go of the mouse button, and the circles will be deformed.

9. To go back to the original circles, choose the Undo command in the Edit menu.

10. Repeat steps #7-#9 several times using the midpoints of different edges and making different parallelograms. Notice the changes in shape and orientation during the deformation.

11. In the Edit menu, choose the Distort option under the Transform command. Move the small boxes to various places, and notice what happens to the circles.

Questions:

After the deformation in the first two cases, how are the ellipses similar? How are they different?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

The first two deformations were homogeneous, and the third was inhomogeneous. Based on your observations, give a definition for homogeneous strain.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Behavior of Lines during Pure Shear

1. Use the Line Tool (looks like a pen--click for each end of the line, and then click on another tool to finish that line.  Go back to the pen and repeat.) to draw 10-12 lines of the same length in various orientations (under the file menu, select NEW, and then in the dialogue box that appears, just check to makes sure that RGBcolor or greyscale is the mode, otherwise the transformations will not work). The lines can cross each other, and make sure that a few of them are parallel and perpendicular to the sides of the page. Leave plenty of space around the lines to deform them.

2. Use the Marquee Tool to draw a box around the lines.

3. In the Edit menu, choose the Scale option under the Transform command. Small boxes should now appear at the corners and midpoints of the box surrounding the lines.

4. Click on the small upper right-hand box, and pull the box upward and to the left (or downward and to the right) until you have a rectangle (try to keep the same area as the original box). Let go of the mouse button, and the lines will be deformed.

5. To go back to the original lines, choose the Undo command in the Edit menu.

6. Repeat steps #3-#5 several times using different corners and making different sizes of rectangles. Notice what happens to the lines during the deformation.

Questions:

What happens to the lines perpendicular and parallel to the axes of the original box?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

What happens to the lines oriented at an angle to the axes of the original box?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

What is your definition of pure shear? How does the Scale option fit that definition (if we try to keep the area of the original box and final rectangle the same)?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Behavior of Lines during Simple Shear

1. Use the same set of lines from the previous exercise and draw a box around them using the Marquee Tool.

2. In the Edit menu, choose the Skew option under the Transform command. Small boxes will again appear at the corners and midpoints of the box surrounding the lines.

3. Click on the small box at the midpoint of the top line of the box, and pull the box to the right (or left) until you have a parallelogram. Let go of the mouse button.

4. To go back to the original lines, choose the Undo command in the Edit menu.

5. Repeat steps #2-#4 several times using the midpoints of different edges and making different parallelograms. Notice what happens to the lines during the deformation.

Questions:

What happens to the lines perpendicular and parallel to the axes of the original box?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

What happens to the lines oriented at an angle to the axes of the original box?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

What is your definition of simple shear? How does the Skew option fit that definition?
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Principle Axes during Pure Shear

1. Draw two perpendicular lines through the center of one of the circles in the file we gave you. The lines should be parallel and perpendicular to the sides of the page.

2. Use the Marquee Tool to draw a box around the circle.

3. In the Edit menu, choose the Scale option under the Transform command.

4. Click on the small upper right-hand box, and pull the box upward and to the left (or downward and to the right) until you have a rectangle (try to keep the same area as the original box). Let go of the mouse button, and notice what happens to the lines.

5. To go back to the original circle, choose the Undo command in the Edit menu.

6. Repeat steps #3-#5 several times using different corners and making different sizes of rectangles. Notice what happens to the lines during the deformation.

7. Add two lines that are mutually perpendicular but at 45° angles to the sides of the window. Repeat steps #3-#6, and note what happens to these new lines.

Questions:

Does the first set of lines change orientation during deformation? If so, how?

(There are called the principal axes of strain because of this behavior and because they have the maximum and minimum changes of length of all lines in the body. Note that they are not always vertical or horizontal!)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Does the second set of lines change orientation during deformation? If so, how?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Principle Axes during Simple Shear

1. Use two circles from the file we gave you.

2. In the Edit menu, choose the Skew option under the Transform command.

3. Click on the small box at the midpoint of the top line of the box, and pull the box to the right (or left) until you have a parallelogram. Let go of the mouse button.

4. For the left-hand circle (now ellipse), add two lines that represent the principle axes; these lines should be perpendicular to each other, intersect at the center of the circle, and cut the ellipse in halves.

5. Pull the parallelogram to the right (or left) again so that you have further skewed the parallelogram. Now add the principal axes to the right-hand circle (now ellipse). Notice what happened to the principal axes in the left-hand circle.

6. Pull the parallelogram back to its original box shape. Note the orientation of the principal axes in this configuration.

7. Repeat steps #2-#6 several times. Pay attention to the orientations of the principal axes.

Questions:

Do the principal axes change orientation during deformation? If so, how?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Based on your observations, what is the primary difference between pure and simple shearing?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Strain of rock images

This section is designed to let you play with  two other images of real rocks.

1. Open one of the files called granite1.jpeg or granite2.jpeg. This is a picture of a granite.

2. Select a portion of the center of the image and apply a pure shear to it.

How can you make a gneiss (a rock with layered felsic and mafic minerals? At what orientation is the banding to the principal strain directions?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

3. Select an undeformed portion of the image and apply a simple shear.

Can you also make a gneiss? Could you tell the difference from hand sample inspection whether something was subject to pure or simple shear?

What is the relationship of the banding to the principal strain directions?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

4. Open the file called trilob.JPG. This is a picture of some trilobites who might be buried in a shale that is subsequently deformed.

5. Select the image and apply a pure shear to it.

6. Revert of undo and apply a simple shear.

One thing that you will note about the trilobites is that you can define a line along thier back, and one perpendicular to that at the base of their heads. Try to notice those lines and as you apply your deformation, note which pair stays closest to perpendicular. Those two will be near to the directions of the principal strain axes. Try it and see which trilobite is best oriented for different directions of deformation.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Could you tell the difference from hand sample inspection of these fossils whether something was subject to pure or simple shear?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Finally, try some inhomogeneous deformation of the granite and trilobites using the distort transformation.
 
 

How does that deformation differ? Does your original definition of homogenous deformation still hold?