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What is a Normal Fault? - Definition & Example

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  • 0:01 What is Normal and…
  • 0:33 Normal Fault vs Reverse Fault
  • 1:02 Hanging Wall and Footwall
  • 2:36 Horst and Graben
  • 3:20 Additional Fun Fact
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor
Mary Ellen Ellis
Expert Contributor
Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

A normal fault is no more typical, or better, than any other kind of fault. But it is responsible for certain mountain ranges and other interesting geological features in the earth's crust.

What is Normal and What is Not?

The term, 'normal fault' actually comes from coal mining, but more about that later. A fault, which is a rupture in the earth's crust, is described as a normal fault when one side of the fault moves downward with respect to the other side. The opposite of this, in which one side moves up, is called a reverse fault. To remember what a normal fault is, think about it this way: it seems more normal for earth to slide down (because of gravity) than it is for it to go up. Earth moving down is normal; moving up is reverse.

Normal Fault vs. Reverse Fault

Imagine a fault in the earth. It could be a small one in the middle of a continent, or it could be a large plate boundary. Think about what would happen if some force, like the movement of the earth's tectonic plates, pulled the earth apart on either side of the fault. What would happen? One side of the fault would have some wiggle room, causing it to slide downward. This is what happens at a normal fault. Now consider the opposite. When a fault is squeezed together, the earth gets pushed up, which is what happens at a reverse fault.

Hanging Wall and Footwall

We have terminology for the two sides of a fault. In a normal fault, the side that slides downward has a shape that makes it look like it is reaching, or hanging, out over the side, so we call it the hanging wall. The other side is shaped a little bit like a foot. We call that the footwall. The hanging wall slides down the footwall.

normal fault

In this picture of a normal fault, the valley is the hanging wall and the mountain is the footwall.

The motion between the two is not always smooth, and sometimes the walls get caught on each other. Pressure builds up and can be released with a great amount of energy, producing an earthquake. These are less common than earthquakes produced by strike-slip faults, which move past each other horizontally instead of vertically. The San Andreas Fault in California is an example of a strike-slip fault.

normal fault

In the image you can see a normal fault where the white line of rock has been disrupted. The hanging wall is to the left of the fault and the footwall to the right.

This sliding downward of normal faults creates rifts, valleys, and mountains. The rift basin at the bottom of the North Sea is an example of a normal fault in action. The Humboldt Fault in Kansas is another example of a normal fault. It produced a memorable earthquake in Kansas in 1867. There are multiple normal faults in Nevada and Utah that have produced the unique landscape of those states. The Sierra Nevada Fault is also normal and is a spectacular example of the kinds of mountains these faults can create.

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Additional Activities

Modeling a Normal Fault

As you have learned, the word normal in the context of geology doesn't mean typical or ordinary. It has to do with the relative positions of a hanging wall and foot wall at a fault. A fault is a break in a rock bed. We are going to make a fault system with cardboard and show what a normal fault is. We can also use this setup to show a different type of fault.

Steps to Make the Model

  1. Gather 3 unused cardboard boxes with at least 10 inches on one side.
  2. Cut out six 5-inch-wide and 10-inch-long strips of cardboard.
  3. Paint the strips of cardboard different colors and be sure to paint the edges. When the paint dries, move on to the next step.
  4. Glue the strips of cardboard to each other. When the glue dries, move on to the next step.
  5. Draw a diagonal line from top to bottom and from left to right on the side (like \ ) of the glued cardboard pieces. We'll call the glued cardboard pieces the rock bed from now on.
  6. Carefully, with adult help if needed, cut along the diagonal line, resulting in two pieces of the rock bed. Paint the raw edges of the rock bed to match the rest of the colors.

Follow-Up Questions

  1. What is the area between the two rock beds that make up the fault called?
  2. Looking straight onto the rock beds, what is the left side of the fault called?
  3. Looking straight onto the rock beds, what is the right side of the fault called?
  4. Which way do you need to move the right side of the rock bed to make a normal fault?
  5. Referring to question 4, what is the fault called when the reverse of what you did in number 4 is done?
  6. Referring to question 4 again, what type of stress results in a normal fault?

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