Sea-Floor Spreading

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Sea-Floor Spreading
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Mapping the Mid-Ocean Ridge

• Longest chain of mountains in the world
• Mapped using sonar (device that bounces sound
  waves off underwater objects and records echoes
  of these sound waves)
• Curves, extending to all of earths oceans
• Under hundreds of meters of water, some pokes
  above water

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Evidence for Sea-Floor Spreading

• Harry Hess first to study mid-ocean ridge
• Ocean floors move like conveyor belts, carrying
  continents with them
• At the mid-ocean ridge, molten material rises
  from the mantle and erupts. The molten material
  then spreads out, pushing older rock to both
  sides of the ridge.

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Sea-Floor Spreading

• Process that continually adds new material to the
  ocean floor
• Sea floor spreads apart along both sides of the
  mid-ocean ridge as new crust is added
• Evidence molten material, magnetic stripes, and
  drilling samples

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Evidence from Molten Material

• 1960s new material is erupting along mid-ocean
  ridge
• Alvin (small submersible built to withstand
  immense pressure)
• Molten material hardens quickly

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Evidence from Magnetic Stripes

• Magnetic poles reverse themselves
• Last time was 780,000 years ago
• Stripes prove magnetic reversals
• Ocean floor contains iron, as cooled (because
  originated as molten material), lined up with
  magnetic poles

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Evidence of Drilling Samples

• Glomar Challenge drilling ship
• Determined age of rocks
• Youngest rocks were always in the center of the
  ridges.FINAL PROOF!

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Subduction at Deep-Ocean Trenches

• Deep-ocean trenches deep underwater canyons
• Forms where the oceanic crust bends downward
• Subduction process by which the ocean floor
  sinks beneath a deep-ocean trench and back into
  the mantle
• Convection currents under lithosphere push new
  crust that forms at mid-ocean ridge away from the
  ridge and towards the deep-ocean trench
• New oceanic crust is HOT!

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Subduction

• A deep-ocean trenches, subduction allows part of
  the ocean floor to sink back into the mantle,
  over tens of millions of years.
• Sea-floor spreading and subduction together..as
  if ocean floor were moving out from the mid-ocean
  ridge on a giant conveyor belt

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Subduction and Earths Oceans

• Subduction and sea-floor spreading can change
  shape and size of the oceans.
• Pacific Ocean width of ocean is shrinking (deep
  ocean trench swallows more oceanic crust that
  mid-ocean ridge is producing.new crust is not
  added fast enough)
• Atlantic Ocean expanding (few short trenches,
  spreading ocean floor has no where to gomoves
  continents as it expands, ocean gets wider)