6.4: Faults

Why is this lake so long and straight?

The photo is of Crystal Springs Reservoir near the San Francisco Bay Area. The water has collected in a low point in the land. The reason for the low point is a fault, the San Andreas Fault. On one side of the reservoir is the Pacific Plate. On the other side is the North American Plate. Would this be a good place to build a home?

Fractures

A rock under enough stress will fracture. There may or may not be movement along the fracture. If there is not movement, the fracture is a joint. If there is movement, the fracture is a fault.

Joints

If there is no movement on either side of a fracture, the fracture is called a joint. Basalt rocks in Asia show horizontal and vertical joints (Figure below). These joints formed as the lava cooled and contracted.

Columnar joints in basalt in Asia.

Faults

If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure below). Faults can occur alone or in clusters. A cluster of faults creates a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake.

A series of faults offset this outcrop along Highway 191 in Moab, Utah.

Dip-Slip Fault

Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called the fault’s dip. If the fault dips at an angle, the fault is a dip-slip fault (Figure below).

Imagine you are standing on a road looking at the fault. The hanging wall is the rock that overlies the fault. The footwall is beneath the fault. If you are walking along a fault, the hanging wall is above you. The footwall is where your feet would be. Miners often extract mineral resources along faults. They used to hang their lanterns above their heads. That is why these layers were called the hanging wall.

Block A is the hanging wall and block B is the footwall. The left diagram shows a reverse, or thrust, fault and the right diagram shows a normal fault.

Normal Faults

In normal faults, the hanging wall drops down relative to the footwall. Normal faults are caused by tension that pulls the crust apart. This causes the hanging wall to slide down. Normal faults can build huge mountain ranges in regions experiencing tension (Figure below).

The Sierra Nevada Mountains in eastern California rose up along a normal fault.

Thrust Faults

When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault. With a thrust fault the angle of the fault is close to horizontal. Rocks can slip many miles along thrust faults (Figure below).

In this thrust fault, the rock on the right is thrust over the rock on the left. The rock on the right has also started to fold because of this movement.

Strike-Slip Faults

A strike-slip fault is a dip-slip fault where the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. If you stand with one foot on each side of a strike-slip fault, one side will be moving toward you while the other side moves away from you. If your right foot moves toward you, the fault is known as a right-lateral strike-slip fault. If your left foot moves toward you, the fault is a left-lateral strike-slip fault (Figure below).

Diagram of a right lateral strike-slip fault.

San Andreas Fault

The San Andreas Fault in California is a right-lateral strike slip fault (Figure below). It is also a transform fault because the San Andreas is a plate boundary. Unlike what some people think, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northwestward, while the North American plate moves southeast. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco!

The San Andreas Fault runs up through California. This transform fault separates the Pacific plate on the west and the North American plate on the east.

Summary

• A fracture with no movement on either side is a joint.
• Dip-slip faults show vertical movement. In a normal fault, the hanging wall drops down relative to the footwall. The reverse is true of a reverse fault.
• Strike-slip faults have horizontal motions due to shear stress.

Review

1. Compare and contrast normal and reverse faults.
2. Imagine you're looking at a cliff with layers of rock. What features would you see to indicate a fault?
3. What is a strike-slip fault?
4. What type of plate boundary can be a strike-slip fault or fault zone?

Explore More

Use the resource below to answer the questions that follow.

1. In a normal fault, which is the hanging wall and which is the footwall?
2. What is the stress in a normal fault?
3. What is the stress in a reverse fault?
4. How does the hanging wall move relative to the footwall in a reverse fall?
5. Describe a strike-slip fault.