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Two types of lithosphere

There are two types of lithosphere: the oceanic lithosphere, or oceanic crust, and the continental lithosphere, or continental crust. The oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. The continental crust is the layer of rocks that form the continents and areas of shallow seabed close to their shores, known as continental shelves. The two types of crust differ in composition, density, and thickness. As a whole, the oceanic crust is thinner but denser than the continental crust.

The oceanic crust is generally less than 10 kilometers (km) thick, and its mean density is about 3.3 grams per cubic centimeter (g/cm3). The thickness of the continental crust ranges from 20 to 80 km, and its density is less than 3 g/cm3.

The oceanic crust. The increase in estimated age of the material is indicated by a progression of colors, from red to yellow to green to blue. Dark red represents newly formed material; dark blue represents crust that is 180 million years old. (Dark gray areas represent landmasses; light gray areas indicate sediment-covered continental shelves.)

As a consequence of the density difference, when active margins of continental crust meet oceanic crust in regions known as subduction zones, the oceanic crust typically sinks beneath the continental crust and is recycled back into the mantle. At the same time, new oceanic crust is continually being produced at mid-ocean ridges from mantle material. In addition, as the oceanic lithosphere grows older, it gets cooler and denser, with the result that if two oceanic plates converge, the older one will subduct below the younger one. As a consequence of these processes, most of the present-day oceanic crust is less than 200 million years old.

By contrast, the continental crust is rarely subducted or recycled back into the mantle. For this reason, the oldest rocks on Earth are within the stable "cratons" of the continents, rather than in repeatedly recycled oceanic crust. (A craton is a stable part of the continental crust that has survived continental merging and splitting for 500 million years or more.) The oldest continental rock is the Acasta Gneiss, with an estimated age of 4.01 billion (4.01x109) years.

Composition of oceanic crust

The oceanic crust is composed mainly of mafic rocks. The term mafic is applied to silicate minerals and rocks that have high concentrations of relatively heavy elements, particularly magnesium and iron. The word "mafic" is derived by combining letters from magnesium and ferrum, the Latin word for iron 1.

Mafic minerals are usually dark in color. Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite and other micas, augite and calcium-rich plagioclase feldspars. Common mafic rocks include basalt and gabbro.

Composition of continental crust

The North American craton.

The continental crust consists predominantly of felsic rocks. The term felsic is used in referring to silicate minerals, magmas, and rocks that are enriched in silica and light elements such as oxygen, aluminium, sodium, and potassium. The word "felsic" combines letters from the words feldspar and silica. Felsic minerals are usually light in color. Common felsic minerals include quartz, biotite, muscovite, hornblende, orthoclase, and sodium-rich plagioclase feldspars. The most common felsic rock is granite.

It is a matter of debate whether the amount of continental crust has been increasing, decreasing, or remaining constant over geological time. One model suggests that prior to 3.7 billion years ago, the continental crust constituted less than 10 percent of the present amount. By 3.0 billion years ago, that figure rose to about 25 percent, and by about 2.6 billion years ago, it was about 60 percent of the current amount (Taylor and McLennan 1995). The growth of continental crust is thought to have occurred in "spurts" of activity, corresponding to five episodes of increased production through geologic time (see graphic at Butler).

See also

  • Asthenosphere
  • Earth
  • Earth's atmosphere
  • Biosphere
  • Cryosphere
  • Hydrosphere
  • Plate tectonics

References

  • Butler, Rob. Making new continents. //earth.leeds.ac.uk/assyntgeology/extra_info/ehistory.htm Accessed 01/29/2006
  • Earth's Crust, Lithosphere and Asthenosphere
  • Crust and Lithosphere
  • Stanley Chernicoff and Donna Whitney. Geology. An Introduction to Physical Geology, 4th ed., Pearson 2007
  • Saal, A.L., Rudnick R.L., Ravizza G.E. & Hart S.R. 1998. Re-Os isotope evidence for the composition, formation and age of the lower crust. Nature 39317, 1998.
  • Taylor and McLennan. 1995. Model of growth of continental crust through time in John Victor Walther 2005, Essentials Of Geochemistry. ones & Bartlett. ISBN 0763726427
  • von Huene, R. and D.W. Scholl, 1991. "Observations at convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust." Reviews of Geophysics 29: 279-316.

External links

All links retrieved July 24, 2018.

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