Lecture

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Lecture 05- Continents vs.Oceans
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Continents vs. Oceans

• Although the Earth is well described by radial
  models, significant lateral variations exist.
• The lithosphere is the most heterogeneous layer
  of the Earth. This is due to the difference
  between continental and oceanic material.

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Continents vs. Oceans

• It is not only continental and oceanic crust that
  is so different. It is also the upper mantle
  beneath the crust. The entire lithosphere is
  characterized as either continental or oceanic.
• The differences can exist to depths of 300-400 km.

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Continents vs. Oceans
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Continents vs. Oceans

• The boundaries between oceans and continents are
  different than the boundaries between tectonic
  plates.
• A tectonic plate can consist of both oceanic and
  continental material.

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Continents vs. Oceans

• Continents can be thought of as rafts which ride
  along on tectonic plates.
• Continental material is virtually permanent once
  it has been formed
• Oceanic material is continuously recycled and
  only temporarily exists near the surface

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Convection
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Continents vs. Oceans (Age)

• Continental crust can be very old, over 4 billion
  years old.
• Oceanic crust is much younger. The oldest oceanic
  crust on Earth is about 200 million years old -
  this is 20 times younger than the oldest
  continental crust.

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Continents vs. Oceans (Age)

• Oceanic crust, and lithosphere, is so young
  because it is constantly being recycled.
• Over a 200 million year time span, oceanic
  lithosphere is
• produced at a mid-ocean ridge
• moved across the asthenosphere
• buried at a subduction zone

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Continents vs. Oceans (thickness)

• Continental areas are much thicker than oceanic
  areas.
• Continental crust can be 70-80 km thick oceanic
  crust is at most 10-15 km thick
• Continental lithosphere can extend to 300-400 km
  in depth oceanic lithosphere extends only to
  100-150 km in depth.

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Continents vs. Oceans (buoyancy)

• Continental lithosphere is less dense than
  oceanic lithosphere.
• Because they are lighter, continents never get
  buried beneath one another in other words they
  do not subduct.
• Continental material can remain at the Earths
  surface indefinitely

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Continents vs. Oceans (buoyancy)

• Oceanic lithosphere is very hot when it first
  forms at mid-ocean ridges
• As it cools it thickens and gets heavier
• This causes the oceanic lithosphere to fall
  away from the ridge
• The older the oceanic lithosphere, the colder and
  heavier it becomes

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Continents vs. Oceans (chemistry)

• Oceanic crust and lithosphere has a fundamentally
  different composition from the continents
• Oceanic crust is primarily basalt, while
  continental crust is more silica rich
• Relative to the continents, oceanic crust is
  enriched in FeO and MgO at the expense of SiO2.

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Summary of Differences

• Continent Material
• thick
• crust up to 70-80 km
• lithosphere up to 300-400 km
• old
• over 4 billion years
• light
• never (rarely) subducts
• enriched in silica (SiO2)

• Oceanic Material
• thin
• crust 10-20 km
• lithosphere 100-150 km
• young
• younger than 200 million years
• heavy
• always subducts eventually
• enriched in FeO and MgO