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Faults: The Earth's Crustal Breaking Point

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Author: Janice VanCleave

Faulting results when the Earth's crust not only breaks but moves. Stress produces motion in different directions and causes the separate pieces to move in relation to each other. The motion of the crust after breaking is used to classify faults.

In this project, you will study and model the different types of faults and the types of stress that cause them. You will also use models to show the Earth's surface features as a result of faulting.

Getting Started

Purpose:   To determine the distinguishing characteristics of a normal fault.

Materials

    • 2 lemon-size pieces of clay of different colors
    • Table knife
    • 2 round toothpicks

Procedure

Faulting: The Earth's Crustal Breaking Point

  1. Break each piece of clay in half.
  2. Shape each piece of clay into a roll about 4 inches (10 cm) long.
  3. Lay the clay rolls together, one on top of the other, alternating the colors.
  4. Press the rolls together into one large clay piece. Flatten the sides of the clay piece by tapping them against a hard surface, such as a table.
  5. Use the table knife to cut the clay piece into two parts diagonally.
  6. Secure the layers in each part together by inserting a toothpick through the layers, top to bottom.
  7. Hold the parts together so that the colored layers match up, then move the left part up and the right part down, as shown in Figure 18.1.

Results

The clay is cut and shifted so that the layers of colored clay in the two parts no longer form continuous horizontal lines.

Why?

Each clay color represents a stratum of one kind of rock material. Cutting the clay represents the stress that causes the Earth's crust to fracture. If there is no movement along the fracture, the fracture is called a joint. But if there is movement, the fracture is called a fault. The fracture line of a fault is called the fault plane. If the fault plane shows vertical displacement (up-and-down movement), the fault block (rock that bounds a fault plane) above the fault plane is called the hanging wall and the fault block below the fault plane is called the footwall. In this experiment, the hanging wall of the fault moves down in relation to the footwall. The stress represented is tension (the stretching, or pulling apart, of rocks), and the type of fault modeled is a normal fault. The angle formed by the fault plane and the top of the hanging wall, measured from the horizontal, is called the dip (see Figure 18.1).

While the hanging wall and footwall of the model both moved, it is not always possible to determine whether two fault blocks of the Earth move or whether one stands still while the other moves past it. Fault types are classified by relative displacement, that is, how one side of a fault is pushed out of place in a given direction relative to the other side. With a normal fault, the hanging wall moves down in relation to the footwall.

Try New Approaches

  1. Compression causes a reverse fault. This fault is similar to a normal fault, except the hanging wall moves upward in relation to the footwall. Repeat the experiment moving the hanging wall up and the footwall down to model a reverse fault.
  2. Shearing (stress that twists, tears, or pushes rocks past each other) produces a lateral fault, also called a strike-slip fault. The movement of a lateral fault along a vertical fault plane is mainly horizontal, with little or no up-and-down movement. The left or right direction is determined by an observer standing on either fault block: the movement of the other block is a left lateral fault if it is to the left or a right lateral fault if it is to the right. Repeat the original experiment twice, making two clay models. Use one model to represent a right lateral fault and the other a left lateral fault. Science Fair Hint: Display the models for each type of fault. Display before-and-after photographs of the position of the clay model representing the different types of faulting.
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