Geology: Plate Tectonics

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Geology Plate Tectonics

• Marine BiologyJanuary 2005

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Key Concepts

• Earths Structure
• Internal geologic processes
• Plate tectonics
• Continental Drift
• Seafloor Spreading
• Formation of mountains, trenches, volcanoes, etc

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Earths Structure

• Core
• Solid (due to pressure) inner part, 4300oC (even
  after 4.5B yrs of cooling) Fe
• Source of earths magnetic field
• Surrounded by liquid core of molten material Fe
  and S
• Mantle
• Surrounds core, most of earths mass, Fe, Mg, Al,
  Si, O
• Most is solid rock but outermost layer of partly
  melted rock that flows like soft plastic (1000oC)
  (flows doesnt fracture)
• This outermost region of mantle is known as the
  ASTHENOSPHERE
• Crust
• Outermost and thinnest layer
• Oxygen (47), Silicon (28), Al (8), Fe (5)
• Consists of continental (25 miles thick) and
  oceanic (7mi) crusts
• Continental rock low density granite oceanic
  high density basalt
• Lithosphere crust and upper most solid mantle

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Lithosphere The Plate in Plate Tectonics
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Earths Internal Structure
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Internal Earth Processes

• Plate Tectonics
• Continental Drift (Wegener)
• Sea Floor Spreading (Hess)
• Subduction Zone
• Convergent Boundary
• Divergent Boundary
• Transform Fault
• Hotspots (Ring of Fire)

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Internal Processes Plate Tectonics

• Build up of the earths surface
• 1960s Theory explaining the movement of the
  earths 13 plates through the molten magma sea
  of the asthenosphere
• Built upon Wegeners Continental Drift Hypothesis
  and Hess Sea Floor Spreading Hypothesis
• Explains concentrations of earthquakes along
  plate margins
• Explain features such as volcanoes, trenches,
  mtns, etc
• The 60 mile thick plates move constantly like
  icebergs on the surface of the ocean
• Typical speed 2 inches per year (fingernail
  fast)
• Recall in-class simulation of these processes

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Wegeners Continental Drift

• 1915, Wegener first proposed the theory of
  continental drift, which states that parts of the
  Earth's crust slowly drift atop a liquid core.
• The fossil record supports and gives credence to
  the theories of continental drift and plate
  tectonics.
• CD theory hypothesizes that there was an
  original, gigantic supercontinent Pangaea 200
  million years ago consisting of all of Earth's
  land masses.
• It existed from the Permian through Jurassic
  periods. It began breaking up during the Jurassic
  period
• Know evidence for this theory
• Know why this theory was not accepted by
  scientists until the 1960s

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Hess Sea Floor Spreading

• Sonar revolutionized the mapping of the seafloor
  following WWII
• Noticed that there were prominent features
• No seafloor rocks over 200my oldwhy?
• According to sedimentation rates, ocean basins
  should have filled with sediments 10 times over,
  they didnt why?
• 1962 Hess hypothesized that magma oozes from the
  Earths surface along oceanic ridges and creates
  new seafloor. The new seafloor spreads away from
  the ridges eventually sinking into the deep ocean
  trenches found across the globe.

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The 13 main plates
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Distribution of Volcanoes and Earthquakes
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Plate Boundaries Divergent
Creates new seafloor
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Divergent Boundary Example

• Plates move apartnew crust is created by magma
  pushing up from asthenosphere (mantle)
• Mid Atlantic Ridge
• Atlantic ocean grew from small inlet to vast
  ocean
• Growing 2.5cm/yr or 25km in million years
• Iceland
• Straddles Mid Atlantic Ridge
• Is being split apart by divergence

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Iceland Divergent Plate Boundary
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Plate Boundaries Convergent
Seafloor is recycled
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Convergent Ocean/Continental
South America Andes Mountains off Peru
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Convergent Oceanic/Oceanic
Marianas Trench in Pacific (Pacific Philippian
plate)
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Convergent Continental/Continental
Indian Subcontinent crashed into Asia 50mya
Himalayas
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Convergent Boundary Himalayas
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Plate Boundaries Transform Fault
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Plate Boundaries and Features
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Ring of Fire
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Hot Spots

• How did the Hawaiian Islands form if they are in
  the middle of the Pacific Plate?
• HOT SPOTS!!
• Small, long-lasting, and exceptionally hot
  regions that exist below the plates
• The Hawaiian Islands resulted from the Pacific
  Plate moving over a deep, stationary hotspot in
  the mantle, located beneath the present-day
  position of the Island of Hawaii.
• Heat from this hotspot produced a persistent
  source of magma by partly melting the overriding
  Pacific Plate.
• The magma then rises through the mantle and crust
  to erupt onto the seafloor, forming an active
  seamount. Over time, countless eruptions cause
  the seamount to grow until it finally emerges
  above sea level to form an island volcano.
• Continuing plate movement eventually carries the
  island beyond the hotspot, cutting it off from
  the magma source, and volcanism ceases. As one
  island volcano becomes extinct, another develops
  over the hotspot, and the cycle is repeated. This
  process of volcano growth and death, over many
  millions of years, has left a long trail of
  volcanic islands and seamounts across the Pacific
  Ocean floor.

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Hot Spots