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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)
What is stress?
force applied over an area
What is strain?
change in size, shape, or volume in response to stress
What are the stress types?
- Shear forces
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
What causes a rock to bend (plastic) rather
than break (brittle)?
- low temp: brittle
- high temp: plastic
- low pressure: brittle
- high pressure: plastic
- Strain rate=deformation
- fast: brittle, breaks
- slow: plastic, flows
- clay, mica, calcite: plastic
- Quartz, feldspar, olivine: brittle
What are tilted rocks?
sedimentary rocks are tilted and folded from their original horizontal orientation
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)
What are folds?
- plastic deformation
- compressive stress
- stress over a long time
- rate of deformation low
Geometry of folds
- symmetric: both limbs same dip
- asymmetric: one limb dips more steeply
- overturned: one limb upside down
- recumbent: axial plane horizontal
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
Erosion along joints creates "fins" which then to erode to arches
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