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Lecture 9

Lecture 9.docx

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David Schneider

Geologic deformationGeologic structures dynamicallyproduced patternsor arrangements of rock or sediment that result fromtectonic forces within the Earth produced as rocks change shape location orientation in response to applied stress structural geology is the study of the shapesarrangement and interrelationships of bedrock unitsand the forces that cause them forcesthat which change motion or orientationof a bodyStress is a force per unit area three basic types of stress in geology1 compressionalshortens2 tensionalelongates3 shearparallel slippage rocks subjected to stresses greater than their own strengthbegin to deform continuum of foldingflowing or fracturingruptureRocks can behave 3 ways1 if deformed materials return to original shape after stressremoval they are behaving elastically however once the stress exceeds the elastic limit of a rockit deforms permanently2 ductile deformation involves bending plastically3 brittle deformation involves fracturingruptureif stress results in irreversible changes in shape or size ofrock strain is a change in size or shape in response to stressstress is the applied forcestrain is the deformationRocks behave as elastic ductile or brittle depending on amountrate stressLow strain rate promotes ductile deformation type of rockMechanically weak rocks tend to deformation throughductile processes temperatureconfining pressureHigh T promote ductile deformationHigh confining P promotes ductile deformationDuctile deformation flowfoldingFolds wavelike undulations in rock layers mmkm scale features result from horizontal or lateralcompressional forces described by the orientation of axialPlanecircular or slightly elongated structures dome upwarped displacement of rocks basin downwarped displacement of rocksBrittle deformation rupturefaultingloss of cohesion of a body underthe influence of deformingstress usually occurs along subplanarsurfaces that separate zones ofcoherent materialJoints are fractures or cracks in bedrock along whichessentially no movement has occurred multiple parallel joints are called joint sets
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