Section 2: The Theory of Plate Tectonics

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Section 2 The Theory of Plate Tectonics

• Preview
• Key Ideas
• How Continents Move
• Tectonic Plates
• Types of Plate Boundaries
• Causes of Plate Motion
• Plate Tectonics

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Key Ideas

• Summarize the theory of plate tectonics.
• Identify and describe the three types of plate
  boundaries.
• List and describe three causes of plate movement.

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How Continents Move

• plate tectonics the theory that explains how
  large pieces of the lithosphere, called plates,
  move and change shape.
• lithosphere the solid, outer layer of Earth that
  consists of the crust and the rigid upper part of
  the mantle.
• asthenosphere the solid, plastic layer of the
  mantle beneath the lithosphere made of mantle
  rock that flows very slowly, which allows
  tectonic plates to move on top of it.

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How Continents Move, continued

• The lithosphere forms the thinouter shell of
  Earth and isbroken into several blocks,
  ortectonic plates.
• The tectonic plates ride onthe asthenosphere in
  much the same way that blocks of wood float on
  water.
• Tectonic plates can include continental crust,
  oceanic crust, or both.
• Continents and oceans are carried along on the
  moving tectonic plates in the same way that
  passengers are carried by a bus.

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Tectonic Plates

• Scientists have identified about 15 major
  tectonic plates and many smaller plates.
• Plates are often bordered by major surface
  features, such as mountain ranges or deep
  trenches in the oceans.
• Scientists identify plate boundaries primarily by
  studying data from earthquakes.
• The locations of volcanoes can also help identify
  the locations of plate boundaries.

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Tectonic Plates, continued

• The boundaries of tectonic plates do not always
  match the outlines of continents.

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Tectonic Plates, continued

• Reading Check
• How do scientists identify locations of plate
  boundaries?
• Scientists use the locations of earthquakes,
  volcanoes, trenches, and mid-ocean ridges to
  outline tectonic plates.

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Types of Plate Boundaries

• Tectonic plate boundaries may be in the middle of
  the ocean floor, around the edges of continents,
  or even within continents.
• The three types of plate boundaries are divergent
  boundaries, convergent boundaries, and transform
  boundaries.
• Each plate boundary is associated with a
  characteristic type of geologic activity.

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Types of Plate Boundaries, continued

• Divergent Boundaries
• divergent boundary the boundary between tectonic
  plates that are moving away from each other
• Magma rises to the surface and cools to form the
  warm, light rock that sits higher than the
  surrounding sea floor because it is less dense.
• The narrow valley that forms where the plates
  separate is a rift valley.

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Types of Plate Boundaries, continued

• Convergent Boundaries
• convergent boundary the boundary between tectonic
  plates that are colliding
• When oceanic lithosphere collides with
  continental lithosphere, the denser oceanic
  lithosphere subducts, or sinks under the less
  dense continental lithosphere.
• The region along this plate boundary is called a
  subduction zone.

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Types of Plate Boundaries, continued

• Reading Check
• Describe the three types of collisions that
  happen at convergent boundaries.
• Collisions at convergent boundaries can happen
  between two oceanic plates, between two
  continental plates, or between one oceanic plate
  and one continental plate.

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Types of Plate Boundaries, continued

• Convergent Boundaries, continued
• When two plates that are made of continental
  lithosphere collide, the colliding edges crumple
  and thicken, which cause uplift that forms large
  mountain ranges.
• When two plates that are made of oceanic
  lithosphere collide, one plate subducts under the
  other plate, and a deep-ocean trench forms. A
  chain of volcanic islands, called an island arc,
  may form.

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Types of Plate Boundaries, continued

• Transform Boundaries
• transform boundary the boundary between tectonic
  plates that are sliding past each other
  horizontally
• Plate edges at a transform boundary scrape
  against each other in a series of sudden spurts
  of motions that are felt as earthquakes.
• Short segments of a mid-ocean ridge are connected
  by transform boundaries called fracture zones.

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Causes of Plate Motion

• The movement of tectonic plates is part of the
  mantle convection system.
• Convection is the movement of heated material due
  to differences in density that are caused by
  differences in temperatures.
• The convection process can be modeled by boiling
  water in a pot on the stove. As the water at the
  bottom of the pot is heated, the water at the
  bottom expands and becomes less dense than the
  cooler water above it. The cooler, denser water
  sinks, and the warmer water rises to the surface
  to create a cycle called a convection cell.

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Causes of Plate Motion, continued

• Mantle Convection
• Scientists think that Earth is also a convecting
  system. Energy generated by Earths core and
  radioactivity within the mantle heat the mantle
  material. This heated material rises through the
  cooler, denser material around it.
• As the hot material rises, the cooler, denser
  material flows away from the hot material and
  sinks into the mantle to replace the rising
  material.
• As the mantle material moves, the overlying
  tectonic plates move along with it.

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Causes of Plate Motion, continued

• Ridge Push
• Newly formed rock at a mid-ocean ridge is warmer
  and less dense than older rock nearby.
• As the newer rock cools and becomes denser, it
  begins to slide down the slope between the
  lithosphere and asthenosphere.
• The sliding rock exerts a force called a ridge
  push, which pushes the rest of the plate away
  from the mid-ocean ridge.

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Causes of Plate Motion, continued
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Causes of Plate Motion, continued

• Reading Check
• How may density differences in the rock at a
  mid-ocean ridge help to drive plate motions?
• When denser lithosphere sinks into the
  asthenosphere, the asthenosphere must move out of
  the way. As the asthenosphere moves, it drags or
  pushes on other parts of the lithosphere, which
  causes movement.

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Causes of Plate Motion, continued

• Slab Pull
• Where the lithosphere cools and becomes dense
  enough, it begins to subduct into the
  asthenosphere.
• As the leading edge of the plate sinks, it pulls
  the rest of the plate along behind it. The force
  exerted by the sinking plate is called slab pull.
• In general, plates that are subducting move
  faster than plates that are not subducting. This
  evidence indicates that the downward pull of the
  subducting lithosphere is a strong driving force
  for tectonic plate motion.

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Causes of Plate Motion, continued
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Plate Tectonics
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