Lecture

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Lecture 08- Subduction Zones
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Subduction Zones

• When two tectonic plates converge often one will
  get buried or subducted beneath the other
• The plate boundary regions where this occurs are
  called subduction zones

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Subduction Zones

• There are two types of lithosphere, oceanic and
  continental, so there are three possibilities at
  a convergent boundary
• oceanic and oceanic
• oceanic and continental
• continental and continental
• In which of these cases can subduction occur ?

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Subduction Zones

• Subduction zones only occur at convergent
  boundaries between oceans and continents, and
  oceans and oceans
• When oceanic lithosphere converges with
  continental lithosphere it is the oceanic
  material that is always subducted beneath the
  continental material.
• When the convergent boundary is between two
  oceans it the older (heavier) plate which usually
  subducts.

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Subduction Zones

• Examples of an oceanic lithosphere subducting
  beneath a continental lithosphere
• South America subduction zone Nazca plate
  (oceanic) subducting beneath South American plate
  (continental)
• Aleutian subduction zone Pacific plate (oceanic)
  subducting beneath North American plate
  (continental) in Alaska

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Subduction Zones

• Examples of oceanic lithosphere subducting
  beneath oceanic lithosphere of another plate
• Marianas subduction zone Pacific plate
  subducting beneath Phillipine Sea plate in
  western Pacific
• Tonga subduction zone Pacific plate subducting
  beneath Australian plate in western Pacific

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General Picture of Subduction
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General Picture of Ocean-Ocean Convergence
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General Picture of Ocean-Continent Subduction
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Second General Example of Ocean-Continent
Subduction
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The Termination of a Subduction Zone
Indian-Eurasian Boundary
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Subduction Zones

• Two dominant features associated with subduction
  zones are
• deep earthquakes
• volcanoes

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Subduction Zones and Deep Earthquakes

• Earthquakes can only occur in brittle material
  (high viscosity)
• It follows that earthquakes happen only in the
  lithosphere, which is usually 100-200 km thick
• However, we observe earthquakes down to a depth
  of 700 km ???

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Subduction Zones andDeep Earthquakes

• It turns out the the deep earthquakes we observe
  (depth gt 200 km) are occurring in lithosphere
  that has been subducted.
• Deep earthquakes do not occur in any place except
  for subduction zones since this is the only place
  where brittle material (lithosphere) exists below
  its normal depth.

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Subduction Zones andDeep Earthquakes

• Deep earthquakes occur in planar (2D)
  arrangements called Wadati-Benioff Zones
• Seismologists use the locations of deep
  earthquakes to map out the geometry of subducting
  lithosphere.

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Sometimes Slab Geometry is Simple
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Often it is Complicated (South America)
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Often it is Complicated (Tonga)
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Subduction Zones and Volcanoes

• Volcanic activity is associated with all active
  subduction zones
• We see dormant and fossil volcanoes at places
  where subduction used to occur
• This type of volcanic activity is fundamentally
  different than volcanoes at mid-ocean ridges and
  hot-spots

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Subduction Zones and Volcanoes

• As oceanic crust ages and moves away from the
  ridge where it was formed it accumulates
  sediments which are rich is water
• Water also reacts with the newly formed crust and
  becomes chemically bound to it

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Subduction Zones and Volcanoes

• Some sediment layers get scraped off the
  oceanic crust when it subducts at a trench
  however a large amount of water is retained in
  the subductiong slab of oceanic material.
• Thus, some water gets transported into the mantle
  while chemically bound to the rocks.

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Subduction Zones and Volcanoes

• At about a depth of 100 km the temperature
  becomes hot enough that a chemical reaction takes
  place and the water is liberated from the
  material which carried it down into the mantle.
• This is called a dehydration reaction.

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Subduction Zones and Volcanoes

• The free water that has just been liberated
  immediately starts to percolate upwards and
  begins to partially melt the asthenosphere above
  it.
• This partially molten material, and water, is
  much lighter than the surrounding material and
  begins rising

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Subduction Zones and Volcanoes

• When the partially molten material nears the
  surface it often becomes fully molten because of
  decreasing pressure now we call it magma.
• The outermost crust at the Earths surface is
  cold, brittle and strong so it is difficult for
  the magma to break-through
• Thus magma will often pond beneath volcanoes in a
  magma chamber until the pressure becomes high
  enough for it to break though the outermost crust
  and erupt

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Subduction Zone Summary

• Subduction zones occur at convergent plate
  boundaries they are burial grounds
• Oceanic material can subduct beneath oceanic
  material on another plate or beneath continental
  material on another plate
• Continental lithosphere never subducts

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Subduction Zone Summary

• Subduction zones are the only place where deep (gt
  200 km) earthquakes occur
• The deep earthquakes line up on planar
  structure that delineate the subducting oceanic
  plates
• These seismicity patterns are called
  Wadati-Benioff zones

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Subduction Zone Summary

• Volcanoes are also prevalent at subduction zones.
• They are formed from water that dehydrates from
  the subducting slab (at about 100 km) and the
  percolates upward causing magma formation.
• This volcanic material is chemically distinct
  from MOR volcanoes and hot-spots