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.

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 of all these features: folds and faults!

Brittle Deformation

When enough stress is applied to a rock or layer 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

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

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

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

Image: InventionSky.com

Another 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.

Strike-Slip Faults

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

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.

Image: InventionSky.com

Right-Lateral Faults: If you stand on the fault line of this fault, the right block will move toward you, and the left will move away.

Image: InventionSky.com

Questions: Brittle Deformation

Image Source: SEG Wiki