Deformation of the Crust

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Chapter 5

• Deformation of the Crust

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5.1 How the Crust Is Deformed

• Deformation is bending, tilting, or breaking of
  the earths surface
• The crust rides on the mantle (wood on water)
• If you add weight to a block of wood, the wood
  will sink
• As parts of the crust get thicker, it also gets
  heavier and will sink into the mantle
• If you take the weight off the wood, the wood
  will rise
• As parts of the crust become thinner, it also
  gets lighter and will rise higher on the mantle

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How does this occur?

• Two opposing forces the crust down the mantle
  up
• When the forces are balanced there is no movement
• When weight is added to the crust, it sinks until
  a balance of the forces can be reached
• The balance of the forces is called isostacy
• The up and down crustal motions are called
  isostatic adjustments

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Where does this occur?

• Mountain Ranges
• EX Appalachian Mountains as the mountain erodes
  the mountain becomes lighter and the region may
  rise
• Rivers
• EX The mouth of the Mississippi river deposits
  material to the ocean floor which then sinks,
  allowing more material to be deposited
• Glaciers
• EX Glaciers cause an area to sink, as they
  recede the land begins to rise (Canada and
  Northern Europe)

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Stress

• Stress is caused by the isostatic adjustments
• Stress is the amount of force per unit area
• Strain is a change in the shape or volume of
  rocks that results from stress

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Types of Stress

• Compression Occurs when crustal rocks are
  squeezed together
• Pushes rocks higher or deeper into the crust
• Tension A force that pulls rocks apart
• Rocks tend to become thinner
• Shearing Pushes rocks in opposite horizontal
  directions
• Rocks bend, twist, or break apart as they slide
  past one another.

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5.2 The Results of Stress

• When a force is applied to a rock, the rock will
  deform
• When that force is taken away, the rock may go
  back to its original shape
• There is a limit to the amount of force each rock
  type can withstand and still go back to its
  original shape
• If the force exceeds the limit, the rock is
  permanently changed

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Folding

• When a rock responds to stress by becoming
  permanently deformed without breaking, the result
  is called folding
• Three types of folds
• Anticline is an upcurved fold in which the oldest
  layer is in the center of the fold (ridge)
• Oldest Layer

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• Syncline is a downcurved fold in which the
  youngest layer is in the center (valley)
• Youngest Layer
• Monoclineis a fold in which both limbs remain
  horizontal

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Three Types of Faults
Strike-Slip
Thrust
Normal
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Strike-slip Fault Example
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Normal Fault Example
Dixie Valley-Fairview Peaks, Nevada
earthquake December 16, 1954
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Thrust Fault Example
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Thrust Fault Example
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Normal fault
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Reverse fault