Constructive Forces - PowerPoint

							Constructive Forces

   Building the Crust
          I. Plate Tectonics
A. Moving Plates
- current theory is that Earth’s surface is
   composed of sections of the lithosphere
   called plates
- Lithosphere is composed of the crust and
   the rigid mantle
- The study of the movement of these plates
   is called plate tectonics
- original concept was proposed by Alfred
   Wegener as part of his theory of continental
   drift
     Plate Tectonics (contd.)
- Pangaea - original
  supercontinent
- Concept that all of
  the continents were
  combined at one
  time to make one,
  giant landmass
- Geologic forces
  caused Pangaea to
  split up
     Plate Tectonics (contd.)
B. Plate Thickness
- lithospheric plates
  are approximately
  100 km thick
- ocean basin is
  mostly basalt
- continental crust is
  mostly granite
     Plate Tectonics (contd.)
C. Plate Movement
- asthenosphere -
  partially melted layer
  of the mantle that
  the lithosphere
  floats on
- convection currents
  occur inside the
  asthenosphere
      Plate Tectonics (contd.)
- hot material moves
  toward the crust
  because it is less dense
- As material rises, ir
  cools and becomes
  more dense
- More dense material
  sinks back towards the
  bottom of the
  asthenosphere
- Movement of mantle
  material pushes the
  lithospheric plates that
  float on top of it
     Plate Tectonics (cpnyd.)
D. Evidence of Plate
    Tectonics
1. Africa and S. America
    fit together like the
    pieces of a jigsaw
    puzzle
2. Earthquakes and
    volcanoes occur along
    plate boundaries
      Plate Tectonics (contd.)
3. Paleomagnetism -
   oceanic bedrock
   shows patterns of
   magnetic reversal that
   could only be caused
   by seafloor spreading
4. Seafloor spreading -
   oceanic crust is
   growing at the mid-
   ocean ridges - crust
   increases in age as
   you move away from
   the ridges
     Plate Tectonics (contd.)
II. Types of Plate
     Boundaries
A. Divergent - where two
     or more plates are
     moving away from one
     another
- occurs primarily at the
     mid-ocean ridges
- ex.: Mid-Atlantic Ridge
- Great Rift Valley
     (Africa)
      Plate Tectonics (contd.)
B. Convergent - where two
    or more plates are
    colliding
1. Subduction Boundary
    - where continental
    and oceanic plates
    collide
- oceanic crust is more
    dense than continental
    crust, so it sinks
    underneath the
    continental crust and
    is pushed into the
    mantle
      Plate Tectonics (contd.)
- as the oceanic crust
  melts, pockets of magma
  are created
- Magma rises through the
  crust at the boundaries
  and forms a volcanic
  mountain on the
  overriding continental
  plate
- Also forms deep sea
  trenches where the
  oceanic crust plunges into
  the mantle
- Ex.: Pacific NW - Cascade
  Mtns.
      Plate Tectonics (contd.)
C. Transform Boundary
- where plates slide past
   one another
- Can be right-moving or
   left-moving
- frequently create faults -
   cracks in the crust where
   movement occurs
- May see faults further
   inland from the plate
   boundary
- Ex.: San Andreas fault,
   CA
     Plate Tectonics (contd.)
III. Moving Plates
- plate movement is
   evidenced from the
   occurence of
   earthquakes and
   volcanoes
- different hypotheses
   about the causes of
   plate movement
      Plate Tectonics (contd.)
A. Mantle Convection
- asthenosphere is
   semi-solid, like slush
   or putty
- as heat radiates from
   the mantle, it causes
   movement in the
   asthenosphere
- Creates convection
   currents
- Movement is caused
   by changes in density
     Plate Tectonics (contd.)
- Hotter material is
   less dense - floats
   upward
- rises, cools, and
   sinks back down
   towards the bottom
   of the
   asthenosphere
       Plate Tectonics (contd.)
B. Ridge Push
- occurs at mid-ocean
   ridges
- As new rock is formed, it
   is less dense and hotter
   than older rock
- New rock slides down
   the side of the ridge
- Cools and hardens -
   becomes more dense
- Creates force pushing on
   lithosphere from the
   force of gravity pulling on
   the denser rock
       Plate Tectonics (contd.)
C. Slab Pull
- at subduction boundaries,
   one plate is colder and
   more dense than the other
- One plate sinks into the
   mantle
- The edge of the sinking
   plate is still colder and
   more dense than the
   mantle
- As the plate sinks into the
   mantle, it pulls the rest of
   the plate behind it
- Considered to be stronger
   than ridge push
      Plate Tectonics (contd.)
IV. Continental Growth
A. Craton - ancient
    continental cores
- Continents were
    originally much                     QuickTime™ and a
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- large amounts of
    crustal material has
    been added to the
    continental plates over
    time
     Plate Tectonics (contd.)
B. Sources of Crustal
   Material
1. Deep sea
   sediments
2. River sediments
   (both make
   sedimentary rock)
3. Volcanic eruptions
     Plate Tectonics (contd.)
C. Thin-skinned
   thrusting
- pushing of thin,
   horizontal sheets of
   rock from
   continental margins
   over great distances
   along level fault
   surfaces
    Plate Tectonics (contd.)
D. Terranes - large
  block of a
  lithospheric plate
  that has been
  moved and attached
  to the edge of a
  continent