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Rock Deformation
- A.When rocks are stressed beyond their elastic limit, permanent deformation results
- B. Types of stress
- 1. Tensional Stress-divergent plate boundaries
- 2.Compressional stress-convergent plate boundaries
- 3.Shear stress- transform boundaries
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Altitude of Beds
- A.Dip-angle of inclination of beds measured from the horizontal
- B.Strike-line formed by the intersection of a dipping bed and the earths surface
- C.Dip Directinon-direction into which a bed descends(slopes)
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Folds
- A. Compressional stress causes shortening and thickening of the crust
- B.Most folds form during mountain building events
- C. Types of of folds
- 1. Anticline-
- a.Upward fold in rocks
- b.Limbs dip away from the center of fold
- c. Oldest rocks exposed in center
- 2.Syncline
- a. downward fold in rocks
- b. Limbs dip toward center of fold
- c. Youngest rocks exposed in the center
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FOLDS
- D.SYMMETRY OF NONPLUNGING FOLDS
- 1. Fold is axis horizontal
- 2.Symmetrical antincline or syncline
- a. Limbs dip at about the same angle
- 3. Asymmetrical anticline or syncline
- a. One limb is inclined more steeply than the other limb
- 4. Overturned anticline or syncline
- a. Limbs are inclined in the same direction
- 5.Recumbent anticline or syncline-axial plane is horizontal
- 6. Erosion of nonplunging folds-repetition of beds
- E. PLUNGING FOLDS
- 1. Axis is inclined
- 2.Eroded plunging anticlines
- a. Oldest rocks exposed in center
- b. Rocks dip away from centre
- c. "V" points in direction of plunge
- 3. Eroded plunging synclines
- a. Youngest rocks exposed in center
- b.Rocks dip toward from center
- c. "V"points away from the direction of plunge
- F.DOMES
- 1. Circular or oval equivalents of anticlines
- 2.Oldest beds exposed in center
- 3. Black Hills, South Dakota
- G. BASINS
- 1. Circular or oval equivalents of synclines
- 2.Youngest beds exposed in center
- 3. Michigan Basin
- 4.Both domes and basins normally form inside continents (not at plate boundaries)
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Faults
- A. Fractures in crust along which displacement has occurred
- B.DIP SLIP FAULTS
- 1. NORMAL FAULTS
- a. Hanging wall block moves down relative to the footwall block
- b.Result from tensional stress
- c. Grabens- central blocks that dropped down along normal faults
- d. Horsts-uplifted blocks that bound grabens
- 2. Reverse faults-
- a. Hanging wall block moves up relative to the footwall block
- b. Result from compressional stress
- 3.Thrust faults-low angle reverse faults
- B. STIRKE SLIP FAULTS
- 1. Displacement is horizontal
- a. Right lateral strike slip faults
- b. Left-lateral strike slip faults
- 2. Transform fault-type of strike -slip fault found at plate boundary
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Joints
- A. Fractures in rocks along which no appreciable displacement has occurred
- B. SHEET JOINTS
- 1. Form in igneous rocks as erosion removes the overburden
- 2. Exfoliation domes form as a result of rocks breaking off along sheet joints
- C. COLUMNAR JOINTS- elongate, pillarlike columns form as basalt cools and shrinks
- D. TECTONIC JOINTS-forces associated with crustal movement cause rocks to fracture
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