11.1: Rock Deformation
I. Factors Affecting Deformation
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Deformation is a change in a rock’s original shape or size.
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Stress is the force per unit area acting on a solid.
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Stress greater than a rock’s own strength results in deformation usually by folding, flowing, or
fracturing.
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Deformation caused by stress is called strain.
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Factors that influence rock strength & how it deforms are temperature, confining pressure, rock type, and
time.
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Temperature and pressure combine to create rock deformation in two ways:
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Brittle deformation: occurs near to the surface, with lower temperatures and confining pressures;
results in fracturing
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Ductile deformation: occurs deeper, with higher temperatures and confining pressures; results in
flowing
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Rock type: strongly bonded rocks tend to fracture; weaker rocks tend to flow
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Time: forces unable to deform rock when first applied may cause a rock to flow if applied for a long
enough time.
II. Types of Stress
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Compressional stress: rocks are squeezed or shortened (push)
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Tensional stress: rocks are pulled apart in different directions
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Shear stress: distorts rock
III. Folds
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Flat-lying sedimentary and volcanic rock are bent during mountain building into wave-like ripples or
folds:
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Anticline: upfolds or arching of rock layers
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Syncline: downfold or trough or rock layers
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Monocline: large, step-like folds in otherwise horizontal rock layers
IV. Faults
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Fault: fracture in the crust where movement has taken place
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The rock surface above a fault is the hanging wall.
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The rock surface below a fault is the footwall.
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Normal faults: hanging wall block moves down relative to the footwall block
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Steep dips of about 60°
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Mainly vertical movement; some horizontal
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Results in lengthening of the crust
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Tensional stress
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Reverse faults & thrust faults: hanging wall block moves up relative to the footwall block
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Reverse: dip angle greater than 45°; mainly vertical movement
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Thrust: dip angle less than 45°; mainly horizontal movement
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Result in shortening of the crust
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Compressional stress
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Strike-slip faults: movement is horizontal and parallel to the trend of the fault
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Large and linear
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Shear stress
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Joint: fracture where no appreciable movement has occurred