University of Colorado
Class Note 10
Stress and Strain
Stress is the applied force.
Strain is the resultant deformation.
Stress can be compressional, tensional, shear, or isostatic.
- Compressional stress pushes matter together.
- Tensional stress pulls matter apart.
- Shear stress is rotational.
- Isostatic stress is even pressure as with burial.
All applied stresses cause rock (or any other solid) to deform (strain).
Strain can be elastic or plastic.
- Elastic strain is recovered (disappears) on release of the stress.
- Plastic strain is not recovered (permanent) on release of stress.
If a material undergoes continuous plastic deformation, it is said to be
If it fractures, it is said to be brittle.
Fig 16.1. Split Mountain Anticline, Dinosaur National Monument, Colorado.
This is an example of a plunging, anticlinal fold in Paleozoic and Mesozoic
sedimentary rocks. The river is the Green just below its confluence with the
There are also some pre-Cambrian sediments exposed in
the core of this classic anticline, upstream of this photo.
Sedimentary rocks that deform plastically are said to form folds.
syncline is a concave-upward fold.
anticline is a concave-downward fold.
monocline is a fold connecting two vertically offset sections of
The orientation of the strata (beds) at any point in a fold can be
described by the strike and dip.
strike is the compasss direction of the intersection of the
stratum with the horizontal plane.
dip is angle measured from the horizontal to the stratum
perpendicular to strike.
If no movement has occurred on a fracture it is called a
If movement has occurred on a fracture it is called a
Movement on a fault is described as dip-slip, strike-slip, or oblique-slip.
- Dip-slipmovement is movement of one block up or down the dip of the fault plane.
- A reverse fault is upward dip-slip movement of the upper block
relative to the lower block.
- A thrust fault is a low-angle reverse fault.
- Reverse faults are the result of compressional stress.
- A normal is downward dip-slip movement of the upper block.
- Normal faults are the reult of tensional stress.
- A strike-slip fault is the horizontal movement of one block
relative to the other.
- A left-lateral strike-slip is one in which the opposite block
has moved to the left.
- A right-lateral strike-slip is one in which the opposite block
has moved to the right.
- An oblique-slip fault has a combination of strike-slip and dip-slip movements.
Any break in the sedimentary geologic record is an
- A disconformity is a break in sedimentation with erosion with no deformation so that the upper beds are parallel to the lower.
- An angular unconformity is a break in sedimentation with tilting and erosion of the surface before sedimentation resumes (without metamorphism of the lower beds).
This is the "great unconformity" of the Grand Canyon. Here the Paleozoic sediments lie on top of
the slightly tilted late Proterozoic sediments at the bottom right.
- A non-conformity is an angular unconformity in which the older beds are metamorphosed and deformed.
GEOL 1010 Syllabus
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