Plate Tectonics

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• Plate Tectonics

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What is plate tectonics?

• Earths lithosphere is broken into plates that
  move on the asthenosphere.
• The movement of these plates is Plate Tectonics.

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The theory of plate tectonics . . .

• The lithosphere is broken into plates that move
  on the asthenosphere.
• Volcanoes and earthquakes occur in specific belts
  along these plate boundaries.
• The age of rocks on the ocean floor give us clues
  to tectonic movement.

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Continental Drift theory . . .

• Alfred Wegner, a German scientist hypothosized
  that the continents have drifted over time.
• Evidence suggesting this theory is the shape of
  coastlines, fossil distribution, and distinctive
  rock formations.

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• Tectonics occurs as the lithosphere of the crust
  floats on the plastic asthenosphere at the
  surface of the mantle.

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How do the plates move?

• There are three types of plate boundaries
• Divergent boundaries -- where new crust is
  generated as the plates pull away from each
  other.
• Convergent boundaries -- where crust is destroyed
  as one plate dives under another.
• Transform boundaries -- where crust is neither
  produced nor destroyed as the plates slide
  horizontally past each other.

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Divergent Boundaries

• Divergent boundaries occur along spreading
  centers where plates are moving apart and new
  crust is created by magma pushing up from the
  mantle.

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Convergent Boundaries

• The size of the earth has not changed
  significantly during the past 600 million years,
  and very likely not since shortly after its
  formation 4.6 billion years ago. The earth's
  unchanging size implies that the crust must be
  destroyed at about the same rate as it is being
  created.

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Two types of converging boundaries

• Subduction Boundary
• When an ocean plate subducts goes under either
  another oceanic plate or a continental plate it
  forms a subduction boundary.

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• Collision Boundary
• Form when the two converging plates are both
  continental.
• The collision causes the crust to be lifted into
  mountain ranges, such as Mt. Everest in Indias
  Himalayan Mountains.

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Transform Boundaries

• The zone between two plates sliding horizontally
  past one another is called a transform boundary.
• Most transform faults are found on the ocean
  floor.

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Review . . .

• Plates move apart at divergent boundaries, toward
  each other at convergent boundaries, and past
  each other at transform boundaries.
• Convergent boundaries can be either collision
  between continents or subduction between
  oceans or between oceans and continents.

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Magnetism helps reveal age of ocean floor

• Evidence for tectonic activity is demon-strated
  by. . .
• Magnetic reversals in rocks on either side of
  mid-ocean ridges
• High heat flow along ridges
• Youngest crust is near ridge center

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We know plates move, but the question is HOW do
they move?

• Well, Im going to tell you . . .
• Three hypotheses describe how plates are moved

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1. Mantle Convection

• The transfer of heat from Earths inner core to
  its outer core.
• These convection currents of heat may drag the
  plate away from its divergent boundary.

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2. Ridge Push

• Ridge push - Gravity acting on the topography of
  ridges tries to spread the ridge outward.
• This pushes the rest of the plate away from the
  ridge.

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3. Slab Pull

• Slab pull The edge of the subducting plate is
  cold and dense therefore, as it sinks into the
  mantle, it carries the rest of the plate with it.

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So, just to review . . .

• The three hypotheses for plate movement are
• 1. Mantle Convection Convection currents move
  the plate.
• 2. Ridge Push Gravity acts on the newly formed
  rock along ocean ridges forcing them to slide
  down divergent
• 3. Slab Pull As the plate subducts, it pulls
  the rest of the plate down with it. convergent

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Plate Movement and Continental Growth

• Plate movements have caused Earths continents to
  change their positions on the globe over time.
• We started with Pangea, from the Greek, pan
  all, gaia Earth

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And the continents grow . . .

• New material continues to be added to the
  continents over time, constantly changing both
  shape and position.

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Before After
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Tectonic activity is constant . . .

• The breakup of Pangea continues today as the
  plates continue to move

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But, what was here first?

• Craton At the core of each continent is an
  ancient rock called the craton
• The North American Craton, containing the oldest
  rocks on the continent, date back about 1 billion
  years.

• The Canadian Shield and platform make up the
  North American craton. The shield is exposed
  rock, while the platform is buried rock.

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A little vocab review . . .

• A terrane is a large block of lithospheric plate
  that has been moved, often a great distance, and
  attached to the edge of a continent.

• Terranes lead to continental growth.
• When two continental plates collide, they can
  weld to each other and fuse, adding to the size
  of the original continent.

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Just a little more on terranes . . .

• Last, but not least, a terrane. . .
• . is completely bounded by major faults.
• has fossils different than those in surrounding
  areas.
• has a different magnetic record than that of the
  surrounding rock.

• There are three characteristics used to identify
  a terrane
• 1. Faults
• 2. Fossils
• 3. Magnetic polarity

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So, now we know . . .

• The lithosphere is broken into plates that move
  on the asthenosphere.
• There are divergent, transform and convergent
  boundaries between those plates.
• Plate movements are thought to be caused by
  mantle convection, ridge push and/or slab pull.
• The position and shape of Earths continents has
  changed over time and is still changing.

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And Plate Tectonics is responsible for . . .

• Volcanoes and the formation of igneous rocks

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• Earthquakes and fissures at plate boundaries

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• The formation of metamorphic rocks at convergent
  plate boundaries, igneous dikes and igneous sills.

Dike
Sill