Plate Tectonics

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Plate Tectonics

• How the earth moves under your feet!

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Continental Drift Theory

• There have been many theories about how the
  continents once fit together.
• In 1620 Francis Bacon looked at a map and made an
  observation that Africa and South America fit
  together like jigsaw puzzle pieces.
• In 1912 a scientist named Alfred Wegener made the
  same observation and published a theory on why
  this was true.

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Alfred Wegener- 1880- 1930

• In 1915 published the theory of continental
  drift. Used as evidence
• 1) Continents fitted like a jigsaw puzzle.
• 2) Identical fossil remains on now widely
  separated continents.
• 3) Evidence of common paleo-climates- glacial
  striations.
• 4) Similar rock (mountain) formations.
• Problem Could not explain how large land masses
  could move so far!

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Fossil Evidence
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Ancient Ice Ages

• Striations or scratches from moving glacial
  sheets left uniform marks on adjacent land masses
  during the Permian and Carboniferous periods
• Also, coal has been found in Antarctica. Coal
  only warms under warm, wet conditions. It must at
  one point been closer to the equator.

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Similarity in Rock

• Similarities in rock formation can be found in
  now widely separated land masses.

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Plate Tectonic Theory

• Wegners theory was intriguing but he could not
  explain how or why his theory was true.
• In 1948 a scientist named Ewing was visiting a
  group of island in the middle of the Atlantic
  Ocean. He discovered that the islands were
  actually the top of a mountain range in the
  ocean. He also found that the mountain range was
  very young, not ancient. He named this mountain
  range, the Mid-Atlantic Ridge.

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Hess

• In 1962 Hess discovered that new rocks by the
  Mid-Atlantic Ridge were very young and that rocks
  next to the USA coast line were much older.
• Concluded that the sea floor was slowly moving
  outwards from the Ridge.
• It turns out the sea floor in the Atlantic Ocean
  is spreading by up to 5cm every year.

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Magnetic Reversal

• Ewing also discovered that the Earth reverses its
  magnetic field every few thousand years.
• The volcanic rock that make up Earths ocean
  crust has iron particles in them that line up
  with the magnetic field.
• Scientists discovered that the rocks showed bands
  of where the magnetic fields reversed back and
  forth.
• This meant that rock had been flowing out of the
  Atlantic Ridge on either side towards Europe and
  North America.

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Magnetic Reversal
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Plate Tectonic Theory-1960

•   The lithosphere is made of the Earth's crust
  and part of the upper mantle.  It is rigid, but
  the underlying asthenosphere is like a very thick
  liquid.
•      There are 7 very large plates and about a
  dozen small plates that make up the crust of our
  Earth.  Plates move because they are sliding over
  the more mobile asthenosphere.  Plates generally
  move at a rate of 1 cm to 10 cm each year.

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Crustal Plates Fill in your plate diagram now.
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Convection Cells How Plates Move

• The time scale involved for rocks to rise from
  the lower mantle to the surface, get cooled, and
  return to the interior is estimated to be around
  200 million years .
• Done Part 1, look at Questions on CD and PT.

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Convection Cells

• The movement of the plastic matter of the
  aesthenosphere tugs on the rigid lithosphere,
  causing either the plates to move apart , crash
  together, or slide past one another.

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Types of Boundaries

• Convergent- area where opposing plates smash
  together Ex.- west coast of North America
• Divergent- Area where plates are moving apart and
  spreading Ex Mid-Atlantic rift
• Transform- area where two plates are sliding past
  one another in opposite directions Ex- San
  Andreas Fault

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Divergent

• The lithosphere) is pulled apart, it typically
  breaks along parallel faults.
• The block between the faults cracks and drops
  down into the soft, plastic interior (the
  asthenosphere).
• The sinking of the block forms a central valley
  called a rift. Magma (liquid rock) seeps upward
  to fill the cracks.
• New crust is formed along the boundary.
• Earthquakes occur along the faults, and
  volcanoes form where the magma reaches the
  surface.

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Divergent Zones
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Convergent Boundary

• Collisions are very slow and last millions of
  years.
• The edge of the continental plate has folded into
  a huge mountain range
• The edge of the oceanic plate has bent downward
  and dug deep into the Earth and melts
• The melted rock rises up through the continental
  plate, causing more earthquakes and forming
  volcanic eruptions where it finally reaches the
  surface.
• An oceanic trench is a consistent feature.
• Most dangerous place to live

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Transform Boundaries

• Plates on either side of a transform boundary are
  merely sliding past each other and not tearing or
  crunching each other
• Transform boundaries lack the spectacular
  features found at convergent and divergent
  boundaries.
• Transform boundaries are marked in some places by
  linear valleys along the boundary where rock has
  been ground up by the sliding.

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Break-up of Pangaea- 210 million years ago
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Cobiquid-Chedabucto Fault N.S.