Plate Tectonics

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Chapter 17

• Plate Tectonics

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Plate Tectonics

• Alfred Wagener
• Found coasts of SA and Africa match
• Fossils also matched
• Rocks matched
• Climatic change evidence
• His idea At one time, all continents connected
  as Pangaea (supercontinent) and moved to their
  current position through a process called
  continental drift

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Wageners Opposition

• He was opposed by the scientific community
  (especially geologists and physicists) because he
  could not give evidence of a force or mechanism
  that could move continents across a sea

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Sea-floor Spreading

• The evidence is found later at the Mid-Atlantic
  Ridge
• Rocks at the ridge (the middle of the ocean) were
  younger than those near shore- HOW???
• Oldest oceanic rock- 150 MILLION years old
• Oldest continental rock- 4 BILLION years old
• Ridge is a rift (or break) in the crust where
  molten rock can erupt
• Ocean floor moves away from rift (like a conveyer
  belt)
• New rock is deposited at rift
• Old rock gets pushed further out from the rift
• This process is called Seafloor Spreading

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Evidence of Sea-floor Spreading

• The evidence of Seafloor Spreading was the
  paleomagnetism of rocks deposited at rift
• When rocks solidify, the iron inside records the
  magnetic orientation (polarity) of the earth
• N and S switch regularly (Magnetic Reversal)
• If you look at the rocks going away from the
  rift, you can see these reversals by looking at
  the direction of iron
• Rock layers on both sides of the rift match in
  distance from rift, composition, and polarityso
  rocks must be moving in opposite directions from
  rift

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Sea-floor Spreading Steps

• Seafloor Spreading Process steps
• Magma rises to fill rift
• Magma hardens into rock layer (recording the
  polarity)
• New magma pushes from underneath rift causing
  layer to fracture down the middle
• As new material rises, each half of the layer is
  sent in opposite directions away from the ridge
• The rift now acts as a mirror with similar layers
  at equal distance on either side
• See picture
• This process (Seafloor Spreading) also moves the
  continents.
• This process is called Continental Drift

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Divergent Plate Boundary
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Earths Interior and Plate Tectonics

• Interior
• Crust
• Outermost layer
• Thinnest layer
• Solid
• Cool
• 2 types
• Oceanic- thin, dense
• Continental- thick, less dense

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Interior cont

• Mantle
• Between crust and core
• 80 of Es volume
• 2 layers
• Outer is solid
• Inner is plastic

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Interior cont

• Core
• Center of E
• Made of Ni, Fe
• 2 layers
• Inner is solid metal
• Outer is liquid metal
• As travel toward core, temps increase
• Rocks melt due to radioactive decay

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Layers
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Geology layers of Earth that are used to discuss
Plate Tectonics /Continental Drift

• Lithosphere
• Thin outer shell of E made of crust outer
  mantle
• Broken into pieces called tectonic plates
• These move in relation to one another through a
  process called plate tectonics which changes
  landforms on Es surface
• Lithospheric plates float and move on
  asthenosphere
• Zone of inner mantle beneath the lithosphere
• Convection currents in asthenosphere move the
  plates

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Earths Plates
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Convection Currents
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Plate Boundaries

• Plate boundaries are places where lithospheric
  plates meet/touch
• Divergent
• Boundary where plates move away from each other
• As they move apart, a rift forms
• Magma rises through rift to create new crust
• Mid-ocean ridges

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Divergent Plate Boundary
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Convergent Plate Boundaries

• Plates collide
• Usually, denser one subducts- the process in
  which a tectonic plate dives beneath another into
  the asthenosphere
• It melts and its material rises as magma to make
  new crust
• 3 kinds of convergent plate boundaries
• Oc-Oc- older subducts
• Oc- Cont- oc subducts
• Cont-Cont- none subducts

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Convergent Oc-Oc
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Convergent Oc-Cont
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Convergent Cont-Cont
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Transform Boundary

• Plates slide/grind past each other
• Fault- crack in Es surface created when rocks on
  either side of crack move
• Commonly, this movement creates EQ
• San Andreas Fault

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