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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 Earth’s 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