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What is deformation?
- changes in volume or shape of a material
- applied forces (stress) can deform (strain) rocks so that they become contorted (folds) or fractured (faults and joints)
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What is stress?
force applied over an area
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What is strain?
change in size, shape, or volume in response to stress
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What are the stress types?
- Compressive
- Tensional
- Shear forces
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What are the strain types?
Elastic= deformed object returns to original shape when stress (force) is removed
Plastic = folds in rocks
Brittle = cracks (faults or joints) in rocks
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What causes a rock to bend (plastic) rather
than break (brittle)?
- Temperature:
- low temp: brittle
- high temp: plastic
- Pressure
- low pressure: brittle
- high pressure: plastic
- Strain rate=deformation
- fast: brittle, breaks
- slow: plastic, flows
- Composition
- clay, mica, calcite: plastic
- Quartz, feldspar, olivine: brittle
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What are tilted rocks?
sedimentary rocks are tilted and folded from their original horizontal orientation
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What are strike and dip of tilted rocks?
- Strike: direction exposed end of rock trends on surface (or direction a line
- created by edge of water against the rock points)
Dip: angle tilted bed makes with horizontal plane (how steeply it is tilted)
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What are folds?
- plastic deformation
- compressive stress
- stress over a long time
- rate of deformation low
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Geometry of folds
- symmetric: both limbs same dip
- asymmetric: one limb dips more steeply
- overturned: one limb upside down
- recumbent: axial plane horizontal
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Fractures in Rocks
- Joints: fractures along which no movement has taken place, most rocks are jointed, very little stress is required
- No movement = joint
- Faults: fractures along which differential movement has taken place, faults have a hanging wall and a foot wall
- Movement = fault
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Erosion along joints creates "fins" which then to erode to arches
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