Lecture-5 1 Lecture #05- Continents vs.Oceans

Lecture-5 2 Continents vs. Oceans F Although the Earth is well described by radial models, significant lateral variations exist. F The lithosphere is the most heterogeneous layer of the Earth. This is due to the difference between continental and oceanic material.

Lecture-5 3 Continents vs. Oceans F 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. F The differences can exist to depths of km.

Lecture-5 4 Continents vs. Oceans

Lecture-5 5 Continents vs. Oceans F The boundaries between oceans and continents are different than the boundaries between tectonic plates. F A tectonic plate can consist of both oceanic and continental material.

Lecture-5 6

Lecture-5 7 Continents vs. Oceans F Continents can be thought of as rafts which ride along on tectonic plates. F Continental material is virtually permanent once it has been formed F Oceanic material is continuously recycled and only temporarily exists near the surface

Lecture-5 8 Convection

Lecture-5 9

Lecture-5 10 Continents vs. Oceans (Age) F Continental crust can be very old, over 4 billion years old. F 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.

Lecture-5 11 Continents vs. Oceans (Age) F Oceanic crust, and lithosphere, is so young because it is constantly being recycled. F 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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Lecture-5 13 Continents vs. Oceans (thickness) F Continental areas are much thicker than oceanic areas. F Continental crust can be km thick; oceanic crust is at most km thick F Continental lithosphere can extend to km in depth; oceanic lithosphere extends only to km in depth.

Lecture-5 14 Continents vs. Oceans (buoyancy) F Continental lithosphere is less dense than oceanic lithosphere. F Because they are lighter, continents never get buried beneath one another; in other words they do not subduct. F Continental material can remain at the Earth’s surface indefinitely

Lecture-5 15 Continents vs. Oceans (buoyancy) F Oceanic lithosphere is very hot when it first forms at mid-ocean ridges F As it cools it thickens and gets heavier F This causes the oceanic lithosphere to “fall” away from the ridge F The older the oceanic lithosphere, the colder and heavier it becomes

Lecture-5 16 Continents vs. Oceans (chemistry) F Oceanic crust and lithosphere has a fundamentally different composition from the continents F Oceanic crust is primarily basalt, while continental crust is more silica rich F Relative to the continents, oceanic crust is enriched in FeO and MgO at the expense of SiO 2.

Lecture-5 17 Summary of Differences F Continent Material –thick u crust up to km u lithosphere up to km –old u over 4 billion years –light u never (rarely) subducts –enriched in silica (SiO2) F Oceanic Material –thin u crust km u lithosphere km –young u younger than 200 million years –heavy u always subducts eventually –enriched in FeO and MgO