# A in the earth’s crust. What is

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.

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## 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.

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.

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.

## Horst and Graben

When normal faults occur near each other, they can produce some interesting geological features. When a section of rock gets pushed up between two normal faults we call it a horst. When that section of rock slides downward, we call it a graben.

A horst is shaped like the letter A, while a graben is shaped like the letter V. The terms ‘horst’ and ‘graben’ are German, and mean heap and ditch, respectively. Grabens are common in the normal fault-rich region of Nevada and Utah.

Certain mountain ranges, including the Vosges in France, are horsts.

In the imagehere notice the V-shaped section of rock in the middle is called a graben.

It is between two normal faults. Notice the disruption to the lines of rock on either side of the fault lines.

## Additional Fun Fact

The terms we use today for normal and reverse faults, including hanging wall and footwall, come from English coal miners.

When the miners encountered a fault, it would alter the path of a seam of coal. A normal fault changed the angle of the seam, but not the direction, so the miners could keep moving forward as normal. A reverse fault changed its direction, and the miners had to reverse direction to stay with the seam.

## Lesson Summary

A normal fault is a fault in which the hanging wall moves down relative to the footwall. Reminder: a fault is a rupture in the Earth’s crust; a hanging wall is the side that slides downward and has a shape that makes it look like it is reaching, or hanging, out over the side; and a footwall is the other side of this and is usually shaped like a foot. An example of a normal fault is the infamous San Andreas Fault in California.The opposite is a reverse fault, in which the hanging wall moves up instead of down.

A normal fault is a result of the earth’s crust spreading apart. This often occurs at plate boundaries, but it can happen at faults in the middle of plates also. A horst occurs when two normal faults cause a section of earth to move up, while a graben occurs when two normal faults causes a section of earth to move down.

The terms ‘normal’ and ‘reverse’ fault, as well as ‘hanging wall’ and ‘footwall’, come from English coal miners.

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