Crust (geology)

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Earth cutaway from the core to the exosphere

The crust is the outer hard layer of the Earth, and is less than 1% of Earth's volume. The crust is made up of different types of rocks; igneous, metamorphic, and sedimentary rocks.

Below the crust is the mantle. The upper part of the mantle is made up of peridotite, a rock denser than rocks common in the crust. The crust and the upper mantle make up the lithosphere. The lithosphere is broken up into tectonic plates that can move.

The crust is of two different types. One is the continental crust (under the land) and the other is oceanic crust (under the ocean). The continental crust is thicker, and the oceanic crust is thinner. Thicknesses of the crust varies from 5–70 km.

The temperature of the crust increases with depth. Where the crust meets the mantle the temperatures can be between 200 °C (392 °F) to 400 °C (752 °F).[1]

What the crust is made of[change | change source]

Elements[change | change source]

About 75% of the Earth's crust is composed of two elements, oxygen and silicon. These usually occur in combination with common metals such as aluminium and iron. The figures are:[2]p41

 % by weight
Oxygen 46.6
Silicon 27.7
Aluminium 8.1
Iron 5.0
All others 12.6

Minerals[change | change source]

A single family of silicates, the feldspars, account for about half of the material in the crust (60% by weight). and quartz is a sizeable proportion of the rest. Other common minerals are mica and hornblende.

Only 8% of the Earth's crust is non-silicate minerals, and this includes carbonates, sulfides, chlorides and oxides.

Rock type[change | change source]

Igneous rocks make up over 90% of the Earth's crust by volume.[2]p47 This is not noticed because they are mostly covered by sedimentary and metamorphic rocks.

Forming the crust[change | change source]

The Earth's mantle and crust formed about 100 million years after the formation of the planet, about 4.6 billion years ago. At first the crust was very thin, and was probably changed often as the tectonic plates shifted around a lot more than they do now. The crust would have been destroyed many times by asteroids hitting the Earth, which were much more common in the Late Heavy Bombardment.

The oldest oceanic basalt crust today is only about 200 million years. Most of the continental crust is much older. The oldest continental crustal rocks on Earth are between 3.7 to 4.28 billion years old.[3][4] These have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on the Fennoscandian Shield. A few zircons at least 4.3 billion years old have been found in the Narryer Gneiss Terrane.

The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years.[5] Most crustal rocks formed before 2.5 billion years ago are in cratons. Such old continental crust and the mantle below it are less dense than other places in the earth. These are not easily destroyed when the plates shift. The making of new continental crust is linked to times of major orogeny or mountain building. This happens at the same time as the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by the coming together of island arcs including granite and metamorphic fold belts. They are kept together partly by the using up of the mantle below the crust, which makes a mantle on which the crust can float.

References[change | change source]

  1. http://www.channel4.com/science/microsites/S/science/nature/journey.html Earth
  2. 2.0 2.1 Levin, Harold L. The Earth through time. 5th ed, Saunders.
  3. http://news.bbc.co.uk/1/hi/sci/tech/7639024.stm
  4. P.J. Patchett and S.D. Samson, 2003. Ages and growth ot the continental crust from radiogenic isotopes. In The Crust (ed. R.L. Rudnick) volume 3, pages 321-348 of Treatise on Geochemistry (eds. H.D. Holland and K.K. Turekian), Elsevier-Pergamon, Oxford.
  5. A.I.S. Kemp and C.J. Hawkesworth 2003. Granitic perspectives on the generation and secular evolution of the continental crust. In The Crust (ed. R.L. Rudnick) volume 3, pages 349-410 of Treatise on Geochemistry (eds. H.D. Holland and K.K. Turekian), Elsevier-Pergamon, Oxford.