Deformation of Rocks: Rocks Under Stress

The Earth's crust and its rigid upper mantle, the lithosphere, are broken into sections or blocks called tectonic plates. As the lithosphere moves, rocks in the crust are squeezed, stretched, and twisted. The force put on the rocks by these actions is called stress. There are three different types of stress in the rocks of the lithosphere: tension, compression, and shear. During tension stress, rocks are pulled in opposite directions. Compression stress squeezes and shortens rock as they are pushed together. Shear stress distorts the shape of rocks by pushing certain sections in opposite directions. In this activity you will model the three types of rock stresses using large marshmallows.

Materials

Three large marshmallows

Paper towel

Activity

1. Model tension stress by holding a marshmallow between the fingers of both hands. Slowly pull the edges of the marshmallow in opposite directions as if you were attempting to pull it apart. Observe what happens.



2. Model compression stress by holding a second marshmallow between your thumbs and forefingers. Push the edges of the marshmallow toward each other as if you were trying to make them meet. Observe what happens.



3. Model shear force by holding the last marshmallow between your thumbs and forefingers. Push one edge of the marshmallow downward while you push the other edge upward. Observe what happens.

Follow-Up Questions

1. Does the shape of the marshmallow lengthen during compression or tension stress?
2. What happens to the shape of the marshmallow during shear stress?
3. Describe the differences between the three types of stresses that can occur to rocks as the lithosphere moves.

Answers

1. Tension stress.
2. Shear stress pulls the marshmallow in more than one direction.
3. Compression stress squeezes and shortens rocks, tension stress pulls rocks apart, and shear stress distorts the shapes of rocks.

Extension

Do some research and find out what happens to different types of rocks when put under these three types of stresses.