The Theory of Plate Tectonics

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The Theory of Plate Tectonics

• Chapter 14 Lesson 3
• p510-519

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Vocabulary

• Plate tectonics (511) Earths surface is made
  of rigid slabs of rock, or plates, that move with
  respect to each other
• Lithosphere (512) the cold and rigid outermost
  rock layer
• Divergent plate boundary (513) forms where two
  plates separate
• Transform plate boundary (513) Forms where two
  plates slide past each other
• Convergent Plate Boundary (513) form where two
  plates collide
• Subduction (513) A process in which the denser
  plate sinks below the more buoyant plate
• Convection (516) the circulation of material
  caused by differences in temperature
• Ridge Push (517) the forces that causes rising
  mantle material at mid-ocean ridges that creates
  the potential for plates to move away from the
  ridges
• Slab Pull (517) As a slab sinks, it pulls on
  the rest of the plate with this force

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The Plate Tectonic Theory

• Earths crust is constantly being created and
  destroyed
• The theory of plate tectonics states that Earths
  surface is made of rigid slabs of rock, or
  plates, that move with respect to each other
• Each plate moves over Earths hot and
  semi-plastic mantle
• The term tectonic describes the forces that shape
  Earths surface and the resulting rock structures
• Plate tectonics is used to explain earthquakes
  and volcanic eruptions

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Tectonic Plates
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Tectonic Plates

• The Pacific Plate is the largest plate
• The Juan de Fuca is one of the smallest
• The boundaries that run through oceans mark the
  positions of the mid-ocean ridges
• Earths outermost layers are cold and rigid
  compared to the layers in Earths interior. It is
  called the lithosphere.
• It is made up of the crust and the solid,
  uppermost mantle
• It is thin below ocean ridges and thick below
  continents
• The tectonic plates are just large pieces of the
  lithosphere
• Just below the lithosphere is called the
  asthenosphere.
• This layer is so hot that it behaves like a
  plastic material
• This enables to lithosphere to move

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Plate Boundaries
Convergent
Convergent
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Divergent Plate Boundaries

• Mid-ocean ridges are located along divergent
  plate boundaries.
• A divergent plate boundary forms where two plates
  separate
• When the seafloor spreads at a mid-ocean ridge,
  lava erupts, cools, and forms new oceanic crust.
• Divergent plate boundaries can also exist in the
  middle of a continent
• They pull continents apart and form rift valleys
• The East African Rift is an example.

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Transform Plate Boundaries

• A transform plate boundary forms where two plates
  slide past each other.
• The San Andrews Fault in California is a well
  known example.
• As the plates move past each other, they can get
  stuck and stop moving,
• Stress builds up where the plates are stuck.
• Eventually the stress is too great and the rocks
  break
• The resulting rapid energy release is an
  earthquake.

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San Andreas Fault
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Convergent Plant Boundaries

• Convergent plate boundaries form where two plates
  collide
• The denser plate sinks below the more buoyant
  plate in a process called subduction
• The area where a denser plate descends into Earth
  is called a subduction zone.

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• When an oceanic plate and continental plate
  collide the denser oceanic plate subducts under
  the edge of the continent
• This creates a deep ocean trench
• A line of volcanoes forms above the subducting
  plate on the edge of the continent
• This process can also happen with two oceanic
  plates.
• Typically the older, denser plate will subduct
  beneath the younger plate.
• This creates a deep ocean trench and a line of
  volcanoes called an island arc

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• When two continental plates collide, neither
  plate is subducted.
• Instead rock is uplifted and create mountains
  like the Himalayas

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Evidence for Plate Tectonics

• Continents move apart or come together at speeds
  of a few centimeters per years
• Today scientists use a network of satellites
  called the Global Positioning System (GPS) to
  measure the movement of the plates
• The theory of plate tectonics explains why
  earthquakes and volcanoes occur in certain places

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• All types of plate boundaries can result in
  earthquakes due to the rapid release of energy
• Diverging and converging plates result in
  volcanoes
• Mountains form when two continental plates
  converge

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Plate Motion

• Convection Currents
• Convection is the circulation of material caused
  by differences in temperature and density
• For example, the upstairs floors of most houses
  often warmer than the lower floor
• This is because warm air rises while denser, cold
  air sinks.

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• Tectonic plate activity is related to convection
  in the mantle.
• Radio active elements heat Earths interior.
• When materials such as solid rock are heated,
  they expand and become less dense
• Hot mantle material rises upward and comes in
  contact with Earths crust
• Thermal energy is transferred to the surface
• As the mantle cools, it becomes denser and then
  sinks, forming a convection current
• These currents in the asthenosphere act like a
  conveyor belt moving the lithosphere

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Forces causing plate motion

• Scientists are still uncertain about which force
  has the greatest influence

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Forces Causing Plate Motion

• Basal Drag
• This is when convection currents in the
  asthenosphere move or drag the lithosphere much
  like how walking sidewalks at the airport move
  people.

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Ridge Push

• Recall that mid-ocean ridges have greater
  elevation than the surrounding seafloor.
• Because they are higher, gravity pulls the
  surrounding rock down and away from the ridge
• Rising mantle material at mid-ocean ridges
  creates the potential for plates to move away
  from the ridge with a force called ridge push.
• This moves the lithosphere away from the
  mid-ocean ridge.

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Slab Pull

• When plates converge, the denser plate will sink
  into the mantel along a subduction zone
• This plate is called a slab. It is usually old
  and cold which makes it denser.
• As a slab sinks, it pulls on the rest of the
  plate with a force called slab pull.

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A Theory in Progress

• Plate tectonics has become the unifying theory of
  geology
• It explains the connection between continental
  drift and the formation and destruction of crust
  along plate boundaries.
• It also helps to explain the occurrence of
  earthquakes, volcanoes, and mountains

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Unanswered Questions

• Several questions remain unanswered
• Why is Earth the only planet in the solar system
  that has plate tectonic activity?
• Why do some earthquakes and volcanoes occur far
  away from plate boundaries?
• Part of the answer has to do with plate
  thickness. The other part is the scientists are
  still learning how active the mantle really is.
• What forces dominate plate motion?