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6.1 Brittle Deformation

Deformation, Stress, and Strain

As we learned from our lesson on plate tectonics, the Earth’s crust is constantly in motion. This motion can lead to the collision of mountains, the rifting of valleys, and the shearing of the surface. These processes cause deformation on our surface.

The forces that act over an area of rock, such as pressure from the layers of rocks above them, are called stress. Stress is any force applied over an area.

There are generally two types of stress. The first is confining stress, which is stress that is perfectly applied to all sides of the rock.

Imagine that you have a ball of putty/Play-Doh.

Ball of putty.
Figure 6.1.1 Image by Charlene Estrada, Public Domain.

The second type of stress is called differential stress, which is stress that is applied unevenly to one or different sides of the rock.

After enough stress is applied to a rock, deformation occurs, which is called strain – the resulting displacement or movement of material from stress.

There are two main types of deformation that we can observe on Earth’s surface: brittle deformation and plastic deformation. Although they are both consequences of similar processes, they occur at different temperatures and pressures, and as a result, these two types of deformation result in very different-looking features!

Please watch the video below for a summary on all these features: folds and faults!

Brittle Deformation

Image of a cracked rock outcrop in the City of Rocks National Reserve, Idaho
Figure 6.1.2 Joints in the Almo Pluton, City of Rocks National Reserve, Idaho, Image by wilson44691, CC0, Wikimedia Commons

When enough stress is applied to a rock or layer that is not subjected to significantly high temperatures and pressures, the rock can fracture along a point of weakness. This fracture is called brittle deformation. This type of deformation occurs commonly in the upper crust in the form of fractures, joints, and faulting. Faults involve the physical dislocation of rock layers along a shear plane or plane of weakness. Below are the classifications of faults:

Dip-Slip Faults

A faulted sandstone sample, where layers are abruptly interrupted or disjointed
Figure 6.1.3 Faulted sandstone from the Jurassic of Wyoming, USA. Image by James St. John, Wikimedia Commons, CC BY 2.0

These faults move vertically. Two types of dip-slip faults include:

A normal fault occurs with tension, or pulling apart. The hanging wall is the block that is lower, and the footwall is the block that is higher.
Figure 6.1.4 Diagram of a normal fault. Image by Luis Maria Benitez, Wikimedia Commons, CC BY-SA 3.0, modified by Charlene Estrada

Normal Faults: These faults occur when blocks of rock are pulled apart.

animation shows a block of layers moving atop of the adjacent layers due to compression.
Figure 6.1.5 The process of reverse faulting. Gif by takami torao, Wikimedia Commons, Public Domain.

Reverse or Thrust Faults: These occur when blocks of rock are pushed together.

The hanging wall tends to "hang-over" the footwall. It usually leans over the other block of layers in a fault. The footwall is triangular, in the shape of a shoe or foot.
Figure 6.1.6 The hanging wall tends to “hang-over” the footwall. It usually leans over the other block of layers in a fault. The footwall is triangular, in the shape of a shoe or foot. Image by USGS, Public Domain, Wikimedia Commons
Figure 6.1.7 Look at the shoe and then look at the footwall. Don’t they appear similar in shape? Image by Pharos, Wikimedia Commons, CC0

 

Other terminology related to dip-slip faults is the footwall, which is the underlying block on the fault. The hanging wall is the overlying block on the fault. You can also tell the footwall apart from the hanging wall by the “shoe”-like shape it tends to have.

Strike-Slip Faults

Strike slip faults involve two blocks of rock or land moving past one another horizontally.
Figure 6.1.8 Strike-slip fault motion. Gif by Anynobody, Wikimedia Commons, CC BY SA 3.0

These faults move horizontally. Two types of strike-slip faults include:

Piqiang Fault, China as seen from a bird's eye view. You can tell this is left lateral as the left half of the fault, with rock layers, is moving toward you, or down, and the right half is moving away, or up.
Figure 6.1.9 Piqiang Fault, China as seen from a bird’s eye view. You can tell this is left lateral as the left half of the fault, with rock layers, is moving toward you, or down, and the right half is moving away, or up. Image by NASA from Wikimedia Commons, Public Domain

Left-lateral Faults : If you stand on the fault line of this fault, the left block would move toward you and the right would move away.

In a right-lateral strike-slip fault, the right half of the fault moves toward you, and the left half moves away. Image from USGS, by Steven Dutch, Public Domain
Figure 6.1.10 In a right-lateral strike-slip fault, the right half of the fault moves toward you, and the left half moves away. Image from USGS, by Steven Dutch, Public Domain

Right-Lateral Faults (example also shown in the gif above): If you stand on the fault line of this fault, the right block would move toward you, and the left would move away.

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