Deformation of the Crust

					 Deformation of
   the Crust

By: Mrs. Severe
Earth Science
Section 1 – How the Crust
       is Deformed

Objectives:
1. Predict isostatic adjustments that
   will result from changes in the
   thickness of the earth’s crust.
2. Identify sources of stress in
   crustal rock.
          Deformation


• Bending, tilting, and breaking of
  the earth's crust
• Plate tectonics is the major cause
  of crustal deformation, but is not
  the only force that shapes the
  earth’s crust.
          Other Forces

• Changes in the weight of some
  parts of the crust
  – Thicker and heavier = sink more
    deeply into the mantle
  – Thinner and lighter = rise higher on
    the mantle
    Isostatic Adjustment
• Up-and-down movements of the
  crust occur because of two opposing
  forces.
• Crust presses down on the mantle
• Mantle presses up on the crust
• When the two forces balance, the
  crust moves neither up nor down
    Isostatic Adjustment
         Continued…

• When weight is added to the crust,
  it sinks until a balance of the
  forces is reached again
• Balancing of the two forces is
  called:
    • ISOSTASY
    Isostatic Adjustment
         Continued…

• Up-and-down movements of the
  crust to reach isostasy is called
    • ISOSTATIC ADJUSTMENTS
• As the adjustments occur, areas of
  the crust are bent up and down
• Pressure from this causes rocks in
  that area of the crust to deform
 Isostatic Adjustment Still
         Continued…
• Isostatic adjustments occur
  constantly:
  – Mountain ranges
    • *As the crust becomes lighter, the region
      may rise
  – Rivers flow into large bodies of water
    carrying large amounts of mud, sand,
    and gravel
    • *Added weight cause the floor to sink
    Isostatic Adjustment
     STILL Continued…
• Isostatic adjustments occur
  constantly:
  – Glaciers once covered the land
    • *Weight of the ice caused the crust
      underneath it to sink
    • Glacial ice retreats
    • Land slowly begins rising again in response
      to its reduced weight
Isostatic Adjustment
       FINAL!
                   Stress
    (Not the kind you cause your teacher to have)




• Isostatic adjustment and plate
  movement cause stress in rocks
  that make up the earth’s crust
• Amount of force per unit area that
  is placed on any given material
                     Strain
• Crustal stress occurs
  when lithospheric plates
  collide, separate, or
  rub together
• A change in the shape
  or volume of rocks that
  results from the stress
  of being squeezed,
  twisted, or pulled apart
       Types of Stress
• Compression - occurs when crustal
  rocks are squeezed together
  – often reduces the volume of the rocks
• Tension - the force that pulls rocks
  apart
  – rocks tend to become thinner
• Shearing - sliding rocks past each
  other in opposite horizontal
  directions
Types of Stress
                Review

1. Explain isostatic adjustment…
2. Define Stress and Strain…
3. Draw a diagram of each of the
   following:
  –   Compression
  –   Tension
  –   Shearing
Section 2 – The Results of
          Stress

Objectives:
• Compare folding and faulting as
  responses to stress
• Describe four types of faults
    The Results of Stress
        Introduction
• High pressure and temperatures caused
  by stress deform rocks
• Stress applied slowly = rock may return
  to its original shape as the force is
  removed
• If the force exceeds an acceptable
  amount, the shape of the rock changes
  permanently
• Rock may also break because of extreme
  stress
              Folding
• Rock responding to stress by
  becoming permanently deformed
  without breaking
• Usually occurs deep in the crust
  where overlying rocks create great
  pressure so behavior is more plastic
3 Types of Folds

   • Anticline
   • Syncline
   • Monoclin
              Anticline
• Up-curved fold in
  which the oldest
  layer is in the
  center of the fold
• Generally forms a
  ridge
• Can you think of
  examples…
              Syncline
• Down-curved fold
  in which the
  youngest layer is
  in the center
• Generally forms a
  valley
• Can you think of
  examples…
                Monocline

• Fold in which both
  limbs remain
  horizontal
• Gently dipping one
  way or the other
• Can you think of
  examples…
             Faulting
• Breaks in rocks when the rocks on
  either side of the break move is
  faulting
• Breaks in rocks when the rocks on
  either side of the break do not
  move is fracture
• Near the crust’s surface rocks are
  more brittle and tend to break, not
  bend
       Fault Vocabulary
• Fault plane - surface of a fault
  along which any motion occurs
• Hanging wall - in a non-vertical
  fault, the rock above the fault
  plane
• Footwall - rock below the fault
  plane
4 Types of Faulting


  •   Normal Fault
  •   Reverse Fault
  •   Thrust Fault
  •   Strike-slip Fault
             Normal Faulting
• Hanging wall moves
  down relative to
  the footwall
• Form along
  divergent
  boundaries
• Usually occurs in a
  series of parallel
  fault lines
  Reverse and Thrust Fault
• Reverse - Forms
  when compression
  causes the hanging
  wall to move up
  relative to the
  footwall
• Thrust - Fault plane
  is at a low angle or
  nearly horizontal.
• Common in steep
  mountains such as the
  Rockies and Alps
        Strike-slip Fault

• Rock on either
  side of the fault
  plane slides
  horizontally
• Often occur in
  transform
  boundaries
• Example: San
  Andreas Fault
              Review

1. What results when rock responds
   to stress by permanently
   deforming without breaking?
2. Explain why faulting is more likely
   to occur near the surface than
   deep within the earth…
3. Draw and describe four types of
   faults
    Section 3 – Mountain
         Formation

Objectives:
• Identify the types of plate
  collisions that build mountains.
• Identify four types of mountains
  and discuss the forces that
  shaped them.
      Mountain Formation
         Introduction
• A mountain range is a group of adjacent
  mountains with the same general shape
  and structure
• A group of adjacent mountain ranges
  make up a mountain system
• Largest mountain systems are part of
  still larger systems called mountain belts
  – Circum-Pacific Belt
  – Eurasian-Melanesian Belt
     Plate Tectonics and
          Mountains
• Circum-Pacific and Eurasian-
  Melanesian mountain belts are
  located along convergent plate
  boundaries
• Scientists think this is evidence
  that most mountains were formed
  when lithospheric plates collided
 How Plate Collisions Form
        Mountains
• Collisions between Continental and
  Oceanic Crust = Subduction zones form
  coastal volcanoes
• Collisions between Oceanic Crust and
  Oceanic Crust = Volcanic island arcs
  form on ocean floor
• Collisions between Continents = Crust
  crumples and rises (Example are the
  Himalayas)
     Types of Mountains

• Mountains are complicated
  structures with rock formations
  that yield evidence of the forces
  that created them.
• Classified by: deformation and
  shape
• 4 Types
     Folded Mountains and
           Plateaus
• Continental Crust
  is pushed
  together and up
• Highest mountain
  ranges in the
  world
• Plateaus are large
  uplifted flats are
  also formed near
  folded mountains
    Fault-Block Mountains

• Range fronts rise
  along normal faults
  as a result of
  crustal stretching
• Examples: Lost
  River Range, most
  of Nevada
      Volcanic Mountains

• Mountains that
  form when molten
  rock erupts onto
  the earth’s
  surface
• Hot Spots –
  formed on the
  ocean floor
          Dome Mountains
• Formed when molten
  rock rises through
  the crust and pushes
  up the rock layers
  above it
• When pushed up,
  rock layers are worn
  away exposing
  hardened rock
• Where the rock
  wears away and
  leaves separate high
  peaks is dome
  mountains
                Review

1. Describe the types of lithospheric plate
   collisions that build mountains.
2. Name the four types of mountains and
   explain how each is formed.
3. How do volcanic mountains grow?