31310289-Geology-11-Lecture-Notes-3 - Lecture 11: Rock...

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Lecture 11: Rock deformation The movements of Earth's tectonic plates creates stresses ± rocks subject to stress will deform ( change in volume or shape of a body of rock ) ways in which rocks deform are varied; which style a particular rock mass adopts depends on the nature of the stress applied to it and properties of the rock itself. Types of deformation ELASTIC (temporary deformation) -Small deformations are elastic, meaning the rock will return to its original shape when the stress is removed. DUCTILE & BRITTLE - Larger deformations, the deformations that produce mountains and faults, are permanent. Stress - force acting on a body; not applied uniformly in all directions Types of stress: Compressional - rocks are squeezed along the direction of stress Tensional - rocks are pulled apart Shear - cause slippage and translation within the rock Measuring deformation in rocks Strike is the azimuth (degrees east of north) of the line formed by the intersection of a layer interface or bedding plane with the horizontal Dip is the angle between the layer interface or bedding plane and the horizontal measured perpendicular to the strike direction Plastic deformation Compressional stress may cause rocks to be deformed into a series of wrinkles or folds (from few inches to hundreds of kilometers across) Parts of a fold Axial plane as the imaginary surface that divides a fold as symmetrically as possible, one limb on each side. Fold axis is the line made by the length-wise intersection of the axial plane with beds in the fold Limbs correspond to the two sides of an anticline or syncline Types of folds Symmetrical folds – axial plane is vertical Asymmetrical folds – beds in one limb deep more steeply than those in the other limb Overturned folds – both limbs tilted beyond the vertical in the same direction Recumbent folds – horizontal axial plane
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Assymetrical Fold Recumbent fold (upper right) Fold Nomenclature Monocline – bend in a gently dipping horizontal strata Anticline – an arch in the form of an inverted letter U Syncline – an arch shaped like the letterU (where is the anticline? syncline?) Brittle deformation Rocks under surface conditions also deform plastically but once elastic limit is exceeded, the rocks will behave like a brittle solid and fracture Joint = break in rock mass in which sections on each side of the break DO NOT move relative to each other Fault = break in rock mass in which sections on either side of the break move relative to each other Fault Nomenclature hanging wall, foot wall (Where is the fault? What type of fault is it?
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This note was uploaded on 11/09/2011 for the course GEOL 11 taught by Professor Pellejera during the Summer '11 term at University of the Philippines Diliman.

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31310289-Geology-11-Lecture-Notes-3 - Lecture 11: Rock...

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