Rock Deformation

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 Deformation of the Earth's Crust
Stress acting on rock layers can cause deformation. The results of the past up-and-down and in-and-out movements of the layers are not always apparent from the surface because surface evidence may have worn away over time. Thus, the underlying patterns of\ deformed layers are often evident only when sections of the Earth are cut away, as with the making of roadways.
In this project, you will demonstrate three types of stress that cause rock deformation—compression, tension, and shear—as well as the different types of deformations that result from each type of stress.

Getting Started

Purpose: To model the formation of an anticline.

Materials:

• Permanent marker
• Tap water
• Sponge

Procedure:

1. Use the marker to make a line around the perimeter of the sponge through the center of its outside edge.
2. Moisten the sponge with water to make it pliable, then lay it on a table.
3. Without lifting the sponge, place your hands on its short ends and push the ends toward the center of the sponge (see Figure 38.1). Observe the movement and shape of the sponge.

Results

The center of the sponge bends upward in an arch shape.

Why?

The line drawn on the sponge divides the sponge into layers representing strata (layers of rock material) in the Earth's crust. The force applied to the sponge represents a form of stress, which is a force that acts on rocks in the Earth's crust, causing movement or a change in shape or volume. The type of stress represented in this experiment is compression (squeezing together) of rock. Compression can cause rock to break or bend. The movement of the sponge demonstrated a folding, or bending of rock layers. A fold producing an upward arch shape is called an anticline.

Try New Approaches

A syncline is a fold that curves down, creating a troughlike shape. Hold the sponge from the experiment and apply a compression force to cause it to fold downward. By tilting your hands a little, you should be able to first form an anticline, then a syncline.

Design Your Own Experiment

1.  
   1. Anticlines are not always visible at the surface. They can be eroded or covered with other materials so that the surface is flat instead of bulging upward. A model of a square cut from the Earth can be made to show the folding of the strata beneath the flat surface. Draw a design, such as the one shown in Figure 38.2, on a sheet of typing paper, and color each stratum to indicate different kinds of rocks. (Don't label the tabs or sides.) Cut the diagram out of the paper. Fold the paper along the dashed lines, making all folds in the same direction. Fold the sides over their corresponding tabs—side A over tab A, size B over tab B, and so on. Use tape to secure the tabs to the sides. When standing on its open side, the box will represent an anticline.
   2. Prepare a syncline model with a flat surface using a design such as the one shown in Figure 38.3 and the procedure in the previous experiment. Display the two models with labels.

Get the Facts

1. A rock placed under increasing stress goes through three stages of deformation in succession: elastic deformation, ductile deformation, and fracture. What is an elastic limit? Which deformations are irreversible changes? 
2. The Himalayas are the biggest fold mountains on Earth. They are also the largest mountains and have the twenty-eight tallest peaks. What are the characteristics of fold mountains? How were the Himalayas and other fold mountains formed? At what rate are the Himalayas growing? What are other examples of fold mountains? Where are fold mountains generally found? Prepare a display map showing the locations and names of fold mountains.

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