A rock fractures if it is hard and brittle and subjected to sudden strain that overcomes its internal crystalline bonds. If the rock has been displaced along a fracture, such as having one side that is moved up or down, the fracture is called a fault, and if there is no displacement along the crack, the fracture is called a joint.

Faults. Horizontal or vertical displacement along the fault plane can range from a few centimeters to hundreds of kilometers. The fault can be merely a crack between the two sides of rock, or it can be a fault zone hundreds of meters wide that consists of rock that has been very fractured, brecciated, and pulverized from repeated grinding movements along the fault plane. The broken material within a fault is called fault gouge. The rocks within a fault zone may also be hydrothermally altered or veined from hot solutions that have migrated up the fault zone. A fault is generally considered active if movement has occurred along it during the past 10,000 years.

Fault movements. Three kinds of fault movements are recognized: dip‐slip, strike‐slip, and oblique‐slip. Movement in a dip‐slip fault is parallel to the dip of the fault plane in an “up” or “down” direction between the two blocks. The block that underlies an inclined dip‐slip fault is called the footwall; the block that rests on top of the inclined fault plane is called the hanging wall. A normal dip‐slip fault, or normal fault, is one in which the hanging wall block has slipped down the fault plane relative to the footwall block. A reverse dip‐slip fault is just the opposite: the hanging wall block has moved upward relative to the footwall block (Figure 1).

Figure 1

Dip‐Slip Faults

The blocks on either side of a strike‐slip fault move horizontally in relation to each other, parallel to the strike of the fault. If a person is standing at the fault and looks across to see that a feature has been displaced to the left, it is called a left‐lateral strike‐slip fault. A right‐lateral strike‐slip fault is one in which the displacement appears to the right when looking across the fault (Figure 2).

Figure 2

Strike‐Slip Faults

If the fault blocks show both horizontal and vertical displacement, the fault is termed an oblique‐slip.

A graben is formed when a block that is bounded by normal faults slips downward, usually because of a tensional force, creating a valley-like depression. A horst results when a block that is bounded by normal faults experiences a compressive force that forces the block upward, forming mountainous terrain (Figure 3).

Figure 3

A Graben and Horst

Thrust faults are reverse dip‐slip faults in which the hanging wall block has overridden the footwall block at a very shallow angle for tens of kilometers. The hanging wall block and footwall block of a thrust fault are typically called the upper plate and lower plate, respectively (Figure 4).

Figure 4

A Thrust Fault

Joints. Joints are generally the result of a rock mass adjusting to compressive or tensional stress or cooling. A joint set is composed of a series of roughly parallel joints that occur in one direction. Tensional stress usually results in a single joint orientation that is perpcndicular to the direction of stress. Compressive stress often generates two cross‐cutting joint sets.