Faults and joints are both structures that form where brittle deformation leads to fracturing of Earth’s crust. A joint is a fracture, whereas a fault is a fracture along which motion has occurred so that the rocks on either side are offset from each other. Figure 1 shows a small fault revealed in a road cut, where the sedimentary beds have been offset by a few meters. Faults of this scale usually occur as single discrete breaks. By contrast, large faults, like the San Andreas Fault in California, have displacements of hundreds of kilometers and consist of many interconnecting fault surfaces. These structures, described as fault zones, can be several kilometers wide and are often easier to identify from aerial photographs than at ground level. Sudden movements along faults cause most earthquakes. However, the vast majority of faults are remnants of past deformation and are inactive.

Dip-Slip Faults

Faults in which movement is primarily parallel to the slope of the fault surface are called dip-slip faults. (In this case, “dip” refers to the angle at which the fault surface is inclined relative to the horizontal.) Geologists identify the rock body that contains the fault’s upper surface as the hanging wall block and the rock body containing the lower surface as the footwall block (Figure 2). These names were first used by prospectors and miners who excavated metallic ore deposits, such as gold, that had precipitated from hydrothermal solutions along inactive fault zones. The miners would walk on the rocks below the mineralized fault zone (the footwall block) and hang their lanterns on the rocks above (the hanging wall block).

Rapid vertical displacements along dip-slip faults tend to produce long, low cliffs called fault scarps, such as the one shown in Figure 2. The movements that form such scarps typically also generate earthquakes. There are two kinds of dip-slip faults: normal faults and reverse faults.

Normal Faults 

When the hanging wall block moves down relative to the footwall block, dip-slip faults are classified as normal faults (Figure 3). Normal faults are associated with tensional stresses that pull rock units apart, thereby lengthening the crust laterally and thinning it vertically. This “pulling apart” can be accomplished either by uplift that causes the surface to stretch and break or by horizontal forces that have opposing orientations. These faults are called normal faults because it is “normal” for gravity to pull a block of rock down an inclined plane (the fault surface).

Normal faults occur in a variety of sizes. Some are small, having displacements of only a meter or so, like the one shown in the road cut in Figure 1. Others, however, extend for tens of kilometers. Most large normal faults have relatively steep dips at the surface but tend to flatten out with depth.

In the western United States, large normal faults are associated with structures called fault-block mountains. Excellent examples of fault-block mountains are found in the Basin and Range Province, a region that encompasses Nevada and portions of the surrounding states (Figure 4). Here the crust has been elongated and broken to create more than 200 relatively small mountain ranges. Averaging about 80 kilometers (about 50 miles) in length, the ranges rise 900 to 1500 meters (about 3000 to 5000 feet) above the adjacent down-faulted topographic basins.

The topography of the Basin and Range Province evolved in association with a system of normal faults trending roughly north–south. Movements along these faults produced alternating uplifted fault blocks called horsts (horst = hill) and down-dropped blocks called grabens (graben = ditch). Horsts form the ranges and are the source of sediments that have accumulated in the topographic basins created by the grabens. As Figure 4 illustrates, structures called half-grabens, which are tilted fault blocks, also contribute to the alternating topographic highs and lows in the Basin and Range Province.

Also notice in Figure 4 that the slopes of many of the large normal faults in the Basin and Range Province decrease with depth and eventually join to form a low-angle, nearly horizontal fault called a detachment fault. These faults represent a major boundary between the rocks below, which exhibit ductile deformation, and the rocks above, which exhibit mainly brittle deformation.

Reverse and Thrust Faults 

Dip-slip faults in which the hanging wall block moves up relative to the footwall block are called reverse faults (Figure 5). A thrust fault is a type of reverse fault in which the fault’s angle is less than 45 degrees. Reverse and thrust faults result from compressional stresses that produce horizontal shortening of the crust.

Most high-angle reverse faults are small and accommodate local displacements in regions dominated by other types of faulting. Thrust faults, on the other hand, exist at all scales, with some large thrust faults having displacements ranging from tens to hundreds of kilometers. Movement along a thrust fault can cause the hanging wall block to be thrust nearly horizontally over the footwall block, as shown in Figure 6.

Thrust faulting is most pronounced along convergent plate boundaries. Compressional forces associated with colliding plates generally create folds as well as thrust faults that thicken and shorten the crust to produce mountainous topography (Figure 7). Examples of mountainous belts produced by this type of compressional tectonics include the Alps, Northern Rockies, Himalayas, and Appalachians.

Strike-Slip Faults

A fault in which the dominant displacement is horizontal and parallel to the trend (direction) of the fault surface is called a strike-slip fault (Figure 8). The earliest scientific records of strike-slip faulting were made following surface ruptures that produced large earthquakes. One of the most noteworthy of these was the great San Francisco earthquake of 1906. During this strong earthquake, structures such as fences and roads that were built across the San Andreas Fault were displaced as much as 4.7 meters (15 feet). Because movement along the San Andreas Fault causes the crustal block on the opposite side of the fault to move to the right as you face the fault, it is called a right-lateral strike-slip fault. The Great Glen Fault in Scotland, which exhibits displacement in the opposite direction, is a well-known example of a left-lateral strike-slip fault.

As discussed earlier this semester, transform faults are strike-slip faults that accommodate motion between two tectonic plates. (Although all transform faults are strike-slip faults, only those strike-slip faults that form plate boundaries are called transform faults.) Numerous transform faults cut the oceanic lithosphere and link spreading oceanic ridges (refer to Figure 4.19). Others accommodate displacement between continental blocks that slip horizontally past each other. Some of the best-known transform faults include California’s San Andreas Fault (refer to Figure 7.20), New Zealand’s Alpine Fault, the Middle East’s Dead Sea Fault, and Turkey’s North Anatolian Fault. Large transform faults like these accommodate relative displacements of up to several hundred kilometers.

Rather than being a single fracture, most continental transform faults consist of a zone of roughly parallel fractures. While this zone may be up to several kilometers wide, the most recent movement is often along a strand only a few meters wide, which may offset land features, such as stream channels. Crushed and broken rocks produced during faulting are more easily eroded, so depressions, such as linear valleys, often mark the locations of large strike-slip faults. The crushed-up rock along the Great Glen Fault, Scotland, traces numerous lakes, including the legendary Loch Ness.

Joints

Joints are among the most common geologic structures and can be found in nearly all rock outcrops. As mentioned earlier, joints differ from faults in that no appreciable displacement has occurred along the fracture. Although some joints have random orientations, most occur in roughly parallel groups (Figure 9).

Most joints are produced when rocks in Earth’s outermost crust are deformed by tensional stresses that stretch the rock layer and cause it to fail by brittle fracture. One way in which rock layers are stretched is in response to relatively subtle regional upwarping and downwarping of Earth’s crust. This is illustrated in Figure 10, which shows how regional upwarping of the Entrada Sandstone in Arches National Park, Utah, generated a set of nearly parallel joints.

Not all joints are produced by regional tensional stresses, however. Recall from earlier this semester that columnar joints form when igneous rocks cool and develop shrinkage fractures that produce elongated, pillar-like columns (Figure 11). Another example of jointing can be seen in Figure 13, which illustrates sheeting, a process that produces joints that form parallel to the surface of large igneous masses.

Joint patterns profoundly affect the weathering and erosion of bedrock. In particular, joints allow ion-rich water to penetrate to depth and start the weathering process long before the rock is exposed. As a result, joints strongly influence how landforms develop. The iconic landscape in Figure 10 formed as weathering and erosion enlarged joints, creating long, narrow walls called fins. These narrow rock walls, in turn, are the setting in which differential weathering created the park’s famed arches (Figure 13).

Because jointing weakens rocks, highly jointed rocks present a risk to construction projects, including bridges, highways, and dams. On June 5, 1976, the Teton Dam in Idaho failed, taking 11 lives, drowning more than 15,000 head of livestock, and causing nearly $1 billion in property damage. This earthen dam, constructed of easily eroded clays and silts, was situated on highly fractured volcanic rocks. Although attempts were made to fill the voids in the jointed rock, water gradually penetrated the subsurface fractures and undermined the dam’s foundation. Eventually the moving water cut a tunnel into the easily erodible clays and silts. Within minutes the dam failed, sending a 20-meter- (65-foot-) high wall of water down the Teton and Snake Rivers.

Jointed rocks also provide physical and economic benefits. For example, highly jointed rocks are often a significant source of groundwater. In addition, some of the world’s largest and most important mineral deposits are located along joint systems. Hydrothermal solutions (mineralized fluids) can migrate into fractured host rocks and precipitate economically significant amounts of copper, silver, gold, zinc, lead, and uranium.

• Faults and joints are fractures in rock that form through brittle deformation.

• A fault is a fracture along which motion occurs, offsetting the rocks on either side. If the movement is in the direction of the fault’s dip (or inclination), the rock above the fault plane is the hanging wall block, and the rock below the fault is the footwall block. If the hanging wall moves down relative to the footwall, the fault is a normal fault. If the hanging wall moves up relative to the footwall, the fault is a reverse fault. Large normal faults with low dip angles are called detachment faults. Large reverse faults with low dip angles are thrust faults.

• Faults that intersect Earth’s surface may produce a “step” in the land known as a fault scarp. Areas of tectonic extension, such as the Basin and Range Province, produce fault-block mountains—horsts separated by neighboring grabens or half-grabens.

• Areas of tectonic compression, such as mountain belts, are dominated by reverse faults that shorten the crust horizontally while thickening it vertically.

• Strike-slip faults have most of their movement in a horizontal direction along the trend of the fault trace. Transform faults are strike-slip faults that serve as tectonic boundaries between lithospheric plates.

• Joints form in the shallow crust when rocks are stressed under brittle conditions. They facilitate groundwater movement and mineralization of economic resources, and they may result in hazards to humans.

• detachment fault: A nearly horizontal fault that may extend hundreds of kilometers below the surface. Such a fault represents a boundary between rocks that exhibit ductile deformation and rocks that exhibit brittle deformation.

• dip-slip faults: Faults in which the movement is parallel to the dip of the fault. Dip refers to the angle the fault is inclined relative to the horizontal.

• fault scarps: Cliffs created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion.

• fault-block mountains: Mountains formed by the displacement of rock along a fault.

• footwall block: The rock surface below a fault.

• grabens: Valleys formed by the downward displacement of fault-bounded blocks.

• half-grabens: Tilted fault blocks in which the higher side is associated with mountainous topography and the lower side is a basin that fills with sediment.

• hanging wall block: The rock surface immediately above a fault.

• horsts: Elongated, uplifted blocks of crust bounded by faults.

• joints: Fractures in rock along which there has been no movement.

• normal faults: Faults in which the rock above the fault plane has moved down relative to the rock below.

• reverse fault: Fault in which the material above the fault plane moves up in relation to the material below.

• strike-slip fault: A fault along which the movement is horizontal.

• thrust fault: A low-angle reverse fault.

• transform faults: Major strike-slip faults that cut through the lithosphere and accommodate motion between two plates. Equivalent to a transform plate boundary.