Fault (geology)

Rocks can crack and move. This crack is called a fault.

Sometimes the rocks get stuck. They cannot slide past each other easily. This happens because of friction.

Stress builds up when they are stuck. Then the rocks break and move fast. This sudden move causes an earthquake.

One side of a fault is the hanging wall. This part is above the crack. The other side is the footwall.

Faults can move up or down. They can also move side to side. This is how the ground shifts.

A fault is a crack in a large volume of rock.

This crack happens when rocks move. Most big faults come from plate tectonic forces. These forces move the giant plates of Earth's crust.

Sometimes, the two sides of a fault get stuck. This happens because of friction. Friction is a force that stops things from sliding easily. When they are stuck, stress builds up. Eventually, the stress becomes too strong. The rock breaks and moves fast. This sudden move lets out energy as waves. These waves cause an earthquake.

We can name the sides of a fault. The hanging wall is the side above the crack. The footwall is the side below it. Faults move in different ways. In a normal fault, the hanging wall moves down.

In a reverse fault, the hanging wall moves up.

Some faults move side to side. We call these strike-slip faults.

If a fault moves both up and side to side, it is an oblique-slip fault.

A fault is a crack or a break in a large volume of rock.

This break happens when rocks move past each other. Large faults in the Earth's crust come from plate tectonic forces. These forces move the giant plates that make up our planet. Most big faults form at the boundaries between these plates.

Some faults move very slowly through a process called aseismic creep. Other faults move quickly and release energy. This energy travels as seismic waves, which we feel as earthquakes.

Faults work through a build-up of stress. The two sides of a fault cannot always slide easily. This is because of friction and the stiffness of the rocks. High friction areas can become locked together. These locked spots are called asperities. When a fault is locked, stress builds up over time. Eventually, the stress becomes stronger than the rock can hold. The fault then ruptures, and the stored energy is released.

Geologists use special names to describe how faults move. They look at the hanging wall and the footwall. The hanging wall is the side above the fault plane. The footwall is the side below the fault plane. In a normal fault, the hanging wall moves downward. In a reverse fault, the hanging wall moves upward.

If the hanging wall moves up at a shallow angle, it is a thrust fault.

There are many different ways to classify these breaks. Strike-slip faults move mostly side-to-side. A transform fault is a special strike-slip fault at a plate boundary. These faults are called conservative because they do not create or destroy rock.

Some faults move in both vertical and horizontal ways. These are called oblique-slip faults. You might also see a fault zone, which is a cluster of many parallel faults.

Faults can change the shape of the land we see. Normal faults can create a landscape called basin and range topography. This happens when blocks of land move to form hills called horsts and valleys called grabens.

Faults can also create circular ring faults in volcanic calderas. Even the rock itself changes during this process. The movement can crush the rock into something called fault gouge.

This shows how much power moves through the Earth.

In geology, a fault is a planar fracture or discontinuity within a volume of rock. This break occurs when there is significant displacement caused by rock-mass movements. Faults are essential to understanding how the Earth's crust changes over time. Large faults within the crust result from plate tectonic forces. These forces often create the boundaries between tectonic plates. For example, megathrust faults occur in subduction zones. Transform faults also form at these plate boundaries.

Faults operate through a complex process of stress and release. Because of friction and rock rigidity, the two sides of a fault cannot always glide past each other easily. Movement sometimes stops entirely when the sides become locked. The specific regions of higher friction along a fault plane are called asperities. When a fault is locked, stress builds up at these points. If the stress exceeds the strength threshold of the rock, the fault ruptures. This sudden rupture releases accumulated strain energy as seismic waves, which we experience as an earthquake.

Geologists use specific terms to describe the parts and movements of a fault. The fault plane is the actual surface of the fracture. A fault trace is the place where the fault can be seen on the surface. When a fault is not vertical, the two sides are called the hanging wall and the footwall. The hanging wall is the block located above the fault plane. The footwall is the block located below the fault plane. This terminology originated from mining practices.

Faults are classified by their direction of slip and the angle of their dip. The dip is the angle the fault plane makes with the Earth's surface. Strike-slip faults involve predominantly horizontal movement parallel to the fault trace. These can be sinistral, or left-lateral, or dextral, or right-lateral. Transform faults are a special class of strike-slip faults found at plate boundaries. They are called conservative boundaries because the lithosphere is neither created nor destroyed.

Dip-slip faults involve vertical movement and are divided into normal and reverse types. In a normal fault, the hanging wall moves downward relative to the footwall. These often occur in extensional environments. A reverse fault occurs when the hanging wall moves upward. This indicates compressional shortening of the crust. If the dip of a reverse fault is less than 45 degrees, it is called a thrust fault. Thrust faults often form ramps and flats as they move through rock layers.

Specific fault types can create unique geological structures. Normal faults can create a landscape known as basin and range topography. This occurs when blocks of land move to form grabens and horsts. A graben is a downthrown block between two normal faults. A horst is a block stranded between two grabens.

Some faults have specialized shapes or locations. A listric fault is a normal fault with a concave-upward shape. It is steeper near the surface and becomes more horizontal at depth. Detachment faults are low-angle normal faults that can reach great dimensions. In volcanic areas, ring faults may form a circular outline within a caldera. Finally, the movement itself changes the rock texture. This can create fault rock, such as salmon-colored fault gouge, which is formed from crushed rock.