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• Crustal deformation


  Deformation of the Earth's crust
• Crustal deformation


  • Shifting lithospheric plates continually change the face of the planet
  • Results in major mountain belts
  • Rocks containing fossils of marine organisms found thousands of meters above sea level
  • Rocks bent, contorted, overturned, and sometimes rife with fractures
• Deformation


  All changes in the original shape, size (volume), or orientation of a rock body
• Stress


  Forces that deform rocks
• Compressional stress


  Stress that squeezes and shortens a rock mass
• Tensional stress


  Stress that pulls apart or elongates a rock body
• Shear stress


  Stress that causes slippage between individual rock layers
• Rock deformation


  • Rocks begin to deform by flowing or fracturing when subjected to stresses greater than their strength
• Elastic deformation


  Deformation where changes are recoverable, like a rubber band
• Brittle deformation


  Deformation where rocks break into smaller pieces
• Ductile deformation


  Solid-state flow that produces a change in shape without fracturing
• Factors affecting rock strength

  • Temperature
  • Confining pressure
  • Rock type
  • Time
• Temperature effect on rock strength

  High temperatures cause rocks to deform ductilely and flow, low temperatures cause brittle fracturing
• Confining pressure effect on rock strength


  Increased confining pressure from depth causes rocks to flow rather than fracture
• Rock type effect on deformation

  Crystalline rocks tend to fail by brittle fracture, sedimentary and metamorphic rocks are more susceptible to ductile deformation
• Time effect on deformation

  Slow application of stress over long time causes ductile deformation, rapid stress causes brittle failure
• Folding and faulting may occur simultaneously in the same rock body
• Folds


  • Common features of deformed sedimentary rocks
  • Result from ductile deformation and gradual slippage along planes of weakness
  • Indicate rocks were deformed at great depth
• Anticline


  Upfolded, or arched, structure in sedimentary layers
• Syncline


  Downfolded, or trough-like, structure in sedimentary layers
• Fold types


  • Symmetrical - limbs are mirror images
  • Asymmetrical - limbs are not mirror images
  • Overturned - one or both limbs tilted beyond vertical
  • Recumbent - fold axis is horizontal
• Folds do not continue forever, their ends die out
• Plunging fold


  Fold axis penetrates the ground, causing the outcrop pattern to "point" in the direction of plunge
• Ridges are not necessarily associated with anticlines, nor are valleys related to synclines
• Ridges and valleys result from differential weathering and erosion, not just folding
• Domal structure


  • Black Hills of South Dakota
• Resistant sandstone beds in the Valley and Ridge Province


  • Remain as imposing ridges
  • Separated by valleys cut into more easily eroded shale or limestone beds
• Winds tend to increase in strength with an increase in altitude, so strong winds are common in high mountain settings
• The highest wind speed recorded at a surface station is 231 miles per hour, measured at Mount Washington, New Hampshire
• Plunging anticline


  In a plunging anticline the outcrop pattern "points" in the direction of plunge, the opposite is true of plunging synclines
• Dome


  Broad upwarps in basement rock that deform the overlying cover of sedimentary strata
• The Black Hills of South Dakota


  • A large domed structure generated by upwarping
• Basin


  Downwarped structures having a circular or slightly elongated shape
• Basins of Michigan and Illinois

  • Have gently sloping beds similar to saucers, thought to be the result of large accumulations of sediment causing the crust to subside
• Monocline


  Broad, step-like folds in otherwise horizontal sedimentary strata, often the result of reactivation of ancient, steep, dipping faults located in basement rocks
• Fault


  Fractures in the crust along which appreciable displacement has taken place
• Dip-slip fault


  Faults in which movement is primarily parallel to the dip (or inclination) of the fault surface
• Normal fault


  Dip-slip faults where the hanging wall block moves down relative to the footwall block
• Reverse fault


  Dip-slip faults where the hanging wall block moves up relative to the footwall block
• Thrust fault


  Reverse faults having dips less than 45°, so the overlying block moves nearly horizontally over the underlying block