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Rock deformation

Deformation occurs due to stress applied to the crust. The stress arises due to forces acting on a body (force per unit area)

Some stresses:

pressure = a compressive vertical or normal stress.

pressure = force [(gm*cm)/sec²] per unit area [cm²] = gm/(cm*sec²)

tensile stress – an extensional stress
shear stress - stress on a plane oriented obliquely to applied compressive stress

When stress is applied to a material that material deforms. Brittle vs. ductile materials defined by how much deformation they can tolerate without fracture

- If there is little deformation at low to moderate stress – the material is RIGID. In this case if enough stress is applied and failure occurs, the material is also BRITTLE.

- If there is considerable deformation at low to moderate stress and when the stress is released, the material bounces back the material is ELASTIC. This deformation (also known as strain) is reversible.

- If the material initially resists deformation but then yields and deforms in proportion to the applied stress the material is PLASTIC. When the yield stress is exceeded then get permanent STRAIN.

The ultimate strength is the greatest stress difference sample can tolerate.

– The absolute amount stresses not as important as differences between them. For example, if compressive stresses are large but equal, the sample is compressed but will not rotate or fracture.

Strain (or deformation), the response to stress, depends on the temperature (T), the pressure (P), whether fluids are present, and time.

Faulting – is localized displacement along a plane. It occurs when the rocks break.

The nomenclature depends on the attitude of the fault plane and the relative movement of the blocks on either side of the break - see text for definitions.

Folds are the response of rocks to deformation by flow. Must have some marker to define folds - usually layers.

Flow is the normal mode of deformation at depth. The flow occurs by several mechanisms:
- cataclasis or crushing
- intracrystalline glide
- recrystallization by solid diffusion
- by local melting
- by solution and deposition of minerals

There are various types of folds depending on fold shape. The nomenclature is given in your text.

Folding effects depend on nature of rock the competency or stiffness of the layers. If stiff, gentle open folds.
If weak or incompetent - layers may become very contorted

- at the Earth’s surface folds probably tend to be worn away almost as fast as they form because the process is slow. When this occurs, one is left with ridges formed by the harder units and valleys where the units are softer.

Reconstructions of folded layers that do not take account of the fact that erosion probably keeps up with folding lead to pictures of folds that probably never occurred.