Fault

Fault (geology) :- crack in the crust of the earth along which there has been movement of the rocks on either side of the crack. A crack without movement is called a joint. Faults occur on a wide scale, ranging in length from millimeters to thousands of kilometers. Large-scale faults result from the movement of tectonic plates, continent-sized slabs of the crust that move as coherent pieces .

How Faults Are Created: Faults are created by stress in the earth’s crust. Stress is a force, such as squeezing or stretching, that changes the shape of an object. When a material is stressed, the material may respond in three different ways. It can deform (stretch or compress) elastically, which means that when the stress is removed, the material goes back to its original shape. An elastic deformation is, therefore, reversible. Alternatively, a stressed material can deform inelastically, which means that when the stress is removed, the material stays in its new, deformed shape. An inelastic deformation is irreversible. Lastly, a stressed material can fracture, or break into pieces. Most solid materials, including rocks, deform elastically under small stress. Ductile materials are materials that will deform inelastically under moderate stress and will fracture under higher stress. Brittle materials fracture with little or no inelastic deformation. Rocks tend to be brittle when they are cold and become more ductile when they are hot. Rocks also tend to become more ductile when they are under pressure. For these reasons, most rocks are brittle near the surface of the earth where there is less heat and pressure, and they become more ductile with depth. Most

faults occur in the top 10 km (6 mi) of the crust. Below this depth, most rocks bend and fold in response to stress. Rocks can experience three different kinds of stress: tension, compression, and shearing. Tension pulls rocks apart, pushing opposite sides away from each other. Compression squeezes rocks, pushing opposite sides toward each other. Shearing pushes opposite sides past each other in opposite but parallel directions. These different kinds of stress create different classes of faults, as

discussed below.

Terminology and Classification : The two sides of a fault are separated by a fault plane. Two different terms are used to describe a fault plane’s orientation, or position in the crust. These terms are strike and dip. The strike describes the orientation of a fault plane in terms of compass directions. The dip describes how steeply a fault plane dips into the ground. Dip varies between 0° for a horizontal fault and 90° for a vertical one. Geologists are also interested in how far the two sides of a fault have moved along that fault. The total distance that the two sides have moved relative to each other is called the net slip. The net slip is made up of slip measured along the direction of strike and along the direction of dip of the fault plane. The strike-slip distance is the horizontal motion measured in the direction of the strike. The dip-slip distance is measured in the direction of the dip. The dip-slip distance is similar to the throw, which is the vertical movement along the fault. Unless the dip is exactly 90°, one side of a fault will hang over the other. The side overhanging the fault plane is called the hanging wall and the side underlying the fault plane is called the footwall. When the hanging wall has moved downward relative to the footwall, the fault is known as a normal fault. Such faults are associated with crustal tension and represent areas where the crust is being stretched. They are common at divergent-plate boundaries where two crustal plates move away from each other. When the hanging wall has moved upward relative to the footwall the fault is called a reverse fault unless the dip is nearly horizontal, in which case it is called a thrust fault. Both of these kinds of faults are associated with crustal compression and

represent areas where the crust is being shortened. They are common at convergent-plate boundaries where two crustal plates are colliding. Thrust faults can push old rocks over younger rocks, reversing the normal pattern of younger rocks lying on top of older rocks. When the net slip is entirely horizontal (with no vertical component), the fault is known as a strike-slip fault because the net displacement is parallel to the strike. Such faults are associated with crustal shearing. They are common at transform-plate boundaries where two crustal plates are moving past each other. The San Andreas Fault in California is an example of a strike-slip fault that occurs at a transform boundary where the North America plate is sliding past the Pacific plate. If, when facing a strike-slip fault, the far block is displaced to the right, then the fault is known as a right-lateral fault. If the far block is displaced to the left, then the fault is known as a left-lateral fault. A fault that combines some motion along the strike and along the dip is known as an oblique-slip fault.

Earthquakes and Faults: Earthquakes are sudden movements in the earth’s crust. They occur along faults when stress building up in the crust is suddenly released. Most earthquakes occur along plate boundaries. Landforms Associated with Faults Faults create several unique landforms. One of the most common is a fault scarp, a cliff produced when the earth’s surface on one side of the fault plane rises relative to the surface on the other side of the fault. Another common landform produced by faults, especially normal faults, is a graben. A graben is a low block of rock surrounded on two sides by parallel fault scarps

leading to higher land. The valley comprised of the graben and the fault scarps on either side is called a rift valley. The Great Rift Valley in Africa is an extensive example of a rift valley that stretches more than 4800 km (more than 3000 mi) from Syria to Mozambique. Sometimes, large sets of parallel normal faults produce an alternating pattern of raised and sunken blocks. The Basin and Range region in the southwestern United States consists of hundreds of sunken blocks (basins) and raised blocks (ranges). See Basin: Basin and Range Effects on Rocks Faults affect the rocks in which they occur. Movement on the fault surface can produce grooves or scratches, called striae, and the polished surface on which striae occur is termed a slickenside surface. Movement that is more extensive can result in the crushing of rock along the fault surface. This produces a zone of crushed rock called either a fault breccia (if the material is coarse grained) or a fault gouge (if the material is fine grained). Because breccia is porous, zones of breccia are extremely important as pathways for the flow of groundwater or hydrothermal fluids. The presence of fluids can lubricate the fault surface, promoting further movement. If the fluids that flow along fracture surfaces and brecciated zones carry dissolved metal, they can leave behind significant ore deposits along these zones.1