Seafloor Spreading

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Seafloor Spreading

• 5.4
• p. 141-147

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Seafloor Spreading

• Until the 1950s little was known about the ocean
  floor, but after sonar was invented, a complex
  ocean floor with mountains, valleys, trenches,
  and ridges.
• The most complex mountain range was found in the
  center of the Atlantic. Scientists wondered what
  could have caused these mountain ranges.

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Marianas Trench

• Deepest part of the earth
• Has barely been explored
• 36,000 feet deep at the Challenger Deep
• Can fit Mt. Everest in the hole

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Mid Ocean Ridges

• A ridge of mountains that are very deep under the
  oceans and wind along the Earths like seams on a
  baseball.
• At these ridges new crust is being made from
  molten material that reaches the surface.

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Sea-floor spreading

• The process of continually adding new material to
  the sea floor while other material was being
  recycled back into the mantle.
• In sea-floor spreading the sea floor spreads
  apart along both sides of a mid ocean ridge as
  new crust is added. The ocean floors move like
  conveyor belts carrying the continents along with
  them.

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Seafloor Spreading at Mariana
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Harry Hess

• In 1960 scientist Harry Hess suggested an
  explanation.
• His theory called seafloor spreading stated that
  hot molten material is forced upward to the
  surface at the mid-ocean ridge. It flows
  sideways carrying the seafloor away from the
  ridge.
• New magma then fills in the empty spots, then
  solidifies and forms the new seafloor.
• Three pieces of evidence showed this theory to be
  correct.

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Evidence from Molten Material

• Scientists dived 4 km to the ocean floor aboard
  the submarine ALVIN where at the mid ocean ridge
  they found strange rocks shaped like pillows or
  toothpaste squeezed from a tube.
• These rocks form only from rapidly cooling magma
  under water.

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Underwater lava
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Evidence from Magnetic Stripes

• The magnetic field on the Earth has reversed
  itself many times over the course of history
• When that occurs the atoms within the rocks that
  have magnetic properties align themselves with
  whichever way is North at the time.
• Over time there are stripes that align with the
  magnetic field.

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Evidence from Drilling samples

• In 1968 the Glomar Challenger drilled 6 km down
  in the ocean floor. Samples were brought up and
  they determined the ages of the rocks.
• The younger rocks were closest to the mid ocean
  ridge and the oldest were closest to the
  continental shores.

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Subduction at Trenches

• If the seafloor is spreading- dose it keep
  getting wider and wider??????
• NO- As rock reaches the continents the ocean
  floor plunges under the continents at sites
  called deep ocean trenches. This is where the
  ocean crust bends downward and the ocean crust
  sinks back into the mantle.

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The Process of Subduction

• Subduction is the process of ocean crust
  returning to the mantle by sinking over millions
  of years.
• The reason the oceanic crust can sink is because
  it is more dense that the continental crust.

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Subduction and the Earths Oceans

• The ocean floor is renewed about every 200
  million years
• The Pacific ocean is shrinking in size because
  the mid-ocean ridge is producing less new rock
  than is returning to the mantle while the
  Atlantic Ocean is expanding because it only has a
  few trenches but many ridges.

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The Theory of Plate Tectonics

• 5.5
• p. 150-154

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Lithospheres Eggshell texture

• In 1965, Wilson observed cracks in the continents
  similar to those on the ocean floor and proposed
  that the lithosphere is broken into separate
  plates.
• These plates carry pieces of continents and ocean
  floor, or both.
• This combined the theory of continental drift and
  seafloor spreading into one theory called plate
  tectonics.

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How plates MOVE

• The plates are in constant motion moved by
  convection currents deep in the mantle.
• As the plates move they collide, pull apart, and
  grind into one another which causes volcanoes,
  mountain building, and trenches.

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Plate Boundaries

• Three types of boundaries
• Convergent
• Divergent
• Transform

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Divergent Boundaries

• The boundary between two plates that are pulling
  apart is called a divergent boundary. Two
  examples of divergent boundaries are the
  Mid-Atlantic Ridge and the Great Rift Valley in
  Africa.

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Convergent Boundaries

• When the seafloor cools, becomes dense, and
  sinks, or where two plates collide it is called a
  convergent boundary.
• There are three types of convergent boundaries

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Ocean-Continent Collision

• When an ocean floor plate collides with a
  continental plate, the ocean floor plate is
  denser and goes under the continental plate.
• This is called a subduction zone.
• Volcanoes and trenches occur at subduction zones
  because the ocean floor plate is melting as it
  descends under the continental plate.
• The Andes Mountains in South America is an
  example.

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Ocean-Ocean Collisions

• Ocean-Ocean Collisions occur when two ocean
  plates collide, or when one plate cools and
  begins to sink.
• A deep sea trench is formed as one plate goes
  under the other and islands and volcanoes are
  also formed as new magma is forced to the
  surface.
• The islands of Japan were formed this way

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Di-Continental Plate Collide

• When 2 continental plates collide neither is less
  dense than the other so no subduction occurs,
  therefore the two plates crumple upward forming
  mountain ranges.

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Appalachian Mountains

• A look at rocks exposed in today's Appalachian
  mountains reveals elongated belts of folded and
  thrust faulted marine sedimentary rocks, volcanic
  rocks and slivers of ancient ocean floor, which
  provides strong evidence that these rocks were
  deformed during plate collision.
• The birth of the Appalachian ranges, some 480
  million years ago, marks the first of several
  mountain building plate collisions that
  culminated in the construction of the
  supercontinent Pangaea with the Appalachians near
  the center.
• Because North America and Africa were connected,
  the Appalachians formed part of the same mountain
  chain as the Anti-Atlas in Morocco. To the
  northeast, the same mountain chain continued into
  Scotland, from the North America/Europe
  collision.

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Transform Fault Boundaries

• Transform fault boundaries occur when two plates
  slide past each other and are moving either in
  opposite directions or in the same direction at
  different rates.
• When 2 plate slides past each other, an
  earthquake will occur. The San Andreas fault in
  California is the most famous transform fault
  boundary.

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Effects of Plate Tectonics

• Two of the main effects caused by plate tectonics
  are volcanoes and earthquakes. Plate tectonics
  also form all of the rifts and mountains to form.
  
• Tension forces cause the plates to stretch and
  tilt blocks forming a fault-block mountain.
• If two blocks separate a rift valley can occur.

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Compression Forces

• Compression causes massive forces creating
  mountains as they smash together the plates.