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# Faults: Definition & Types

Lesson Transcript
Instructor: Charles Spencer

Charles teaches college courses in geology and environmental science, and holds a Ph.D. in Interdisciplinary Studies (geology and physics).

Faults can be both active and inactive. Understand the definition of faults, discover the different types of faults such as dip-slip, strike-slip, and oblique-slip faults, and compare active vs. inactive faults. Updated: 09/12/2021

## Definition of Faults

The word 'fault' comes to us from Old Scottish and means 'be deficient.' In the case of the rocks that make up Earth's lithosphere, the deficiencies in question are cracks in an otherwise solid rock mass.

The lithosphere is constantly pushed and pulled, squeezed and stretched. Quite often, the forces exceed the strength of the rocks, and they break (geologists call cracks in rocks fractures). Once fracturing occurs, application of additional force can cause the rocks on either side of a break to slide.

A geologic fault is a more-or-less planar (flat) fracture along which the rocks on either side have slid past each other. The sliding surface is also known as the fault plane, or the fault surface. The amount of sliding that occurs is known as the slip.

Rarely, however, does the shifting motion occur along a single, simple plane. As the rocks grind past each other, they get crushed and broken, and movement can happen anywhere within that volume of disturbed rocks. Due to this, geologists often refer to the crushed volume of rock as a fault zone.

Many faults (and fault zones) are not vertical. They make an angle with a horizontal plane (or the ground surface). That angle is known as the dip of the fault. The line of intersection between the fault and that horizontal plane is called the strike of the fault. There's a reason you need to know those two terms, which we'll discuss shortly.

The rock masses on either side of a dipping fault are also given names that are derived from the angle of the fault: the hanging wall and the footwall. The names come from the mining industry. When a miner was extracting the minerals along a dipping fault, he would be standing on the footwall and the hanging wall was hanging above his head. How practical.

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## Types of Faults

You now have all of the fault geometry and terminology you need to classify different types of faults.

Faults are classified based on how the rocks on either side moved relative to each other. In other words, the name tells you something about which side moved up or down or to the left or right, with respect to the rocks on the opposite side.

### Dip-Slip Faults

If rocks slide directly up or down the fault plane, the movement is called dip-slip. That kind of movement results in the same rock layers being higher on one side than the other (in other words, there has been vertical movement of the rocks).

A normal fault is one where rocks on the hanging wall side slide down an inclined fault surface, just like if you slid down a playground slide. Normal faults form when rocks of the lithosphere are pulled in opposite directions (called extensional forces).

A reverse fault is - as you might guess - the opposite case; the hanging wall rocks were pushed up the inclined fault plane. Reverse faults are the result of compressional forces - when rocks are squeezed from opposite directions.

If the fault plane has a dip of less than 45 degrees, the reverse fault is called a thrust fault.

Here's a puzzle for you: what kind of fault would you call it if the fault plane was vertical and either side could be the hanging wall or the footwall? The answer is in the forces at work. In most cases, that situation is the result of extensional forces, so it would be a normal fault; however, if compression was the cause, it would be a reverse fault.

### Strike-Slip Faults

Sideways-sliding along the fault plane is referred to as strike-slip. There are two types of strike-slip faults.

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