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Deformation
General term for the processes of folding, faulting, shearing, compression, or extension of rocks as the result of various natural forces.
Rock Structure
All features created by the processes of deformation from minor fractures in bedrock to a major mountain chain. Also known as geologic structure.
Stress
The force per unit area acting on any surface within a solid.
Confining pressure
Stress that is applied uniformly in all directions.
Differential stress
Forces that are unequal in different directions.
Compressional stress
Differential stress that shortens a rock body and squeezes materials together. Compression causes rocks to fold or fracture
Tensional stress
The type of stress that tends to pull apart a body and involves forces pulling in opposite directions, which results in strain that stretches and thins rock.
Shear
Stress that causes two adjacent parts of a body to slide past one another.
Strain
An irreversible change in the shape and size of a rock body that is caused by stress.
Elastic deformation
Rock deformation in which the rock will return to nearly its original size and shape when the stress is removed.
Brittle deformation
Deformation that involves the fracturing of rock. Associated with rocks near the surface.
Ductile deformation
A type of solid state flow that produces a change in the size and shape of a rock body without fracturing. Occurs at depths where temperatures and confining pressures are high
Fold
A bent rock layer or series of layers that were originally horizontal and subsequently deformed.
Anticline
A fold in sedimentary strata that resembles an arch; the opposite of syncline.
Syncline
A linear downfold in sedimentary strata; the opposite of anticline. A sequence of rock layers, with the youngest at the fold's center or hinge
Dome
A roughly circular upfolded structure similar to an anticline
Basin
A circular downfolded structure
Monocline
A one-limbed flexure in strata. The strata are unusually flat-lying or very gently dipping on both sides of the monocline.
Fault
A break in a rock mass along which movement has occurred.
Dip-slip fault
A fault in which the movement is parallel to the dip of the fault.
Hanging wall block
The rock surface immediately above a fault.
Footwall block
The rock surface below a fault.
Fault scarp
A cliff created by movement along a fault. It represents the exposed surface of the fault prior to modification by weathering and erosion.
Normal fault
A fault in which the rock above the fault plane has moved down relative to the rock below.
Fault block mountain
A mountain formed by the displacement of rock along a fault.
Horst
An elongated, uplifted block of crust bounded by faults.
Graben
A valley formed by the downward displacement of a fault-bounded block.
Half-graben
A tilted fault block in which the higher side is associated with mountainous topography and the lower side is a basin that fills with sediment.
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.
Reverse fault
A fault in which the material above the fault plane moves up in relation to the material below.
Thrust fault
A low-angle reverse fault
Strike-slip fault
A fracture in the rocks of Earth's crust in which the rock masses slip past one another parallel to the strike, the intersection of a rock surface with the surface or another horizontal plane.
Transform fault
A major strike-slip fault that cuts through the lithosphere and accommodates motion between two plates.
Joint
A fracture in rock along which there has been no movement.
Orogenesis
The processes that collectively result in the formation of mountains.
Compressional mountain
Mountains in which great horizontal forces have shortened and thickened the crust. Most major mountain belts are of this type.
Terrane
A crustal block bounded by faults, whose geologic history is distinct from the histories of adjoining crustal blocks.
Microcontinent
Relatively small fragments of continental crust that may lie above sea level, such as the island of Madagascar, or may be submerged, as exemplified by the Campbell Plateau located near New Zealand.
Suture
A zone along which two crustal fragments are jointed together. For example, following a continental collision, the two continental blocks are sutured together.
Isostatic adjustment
Compensation of the lithosphere when weight is added or removed. When weight is added, the lithosphere responds by subsiding, and when weight is removed, there is uplift.
Gravitational collapse
The gradual subsidence of mountains caused by lateral spreading of weak material located deep within these structures.