Earth's crust

Article

October 17, 2021

The crust is the outermost layer of the solid Earth, and consists mainly of rocks such as granodiorite, gabbro and basalt. The crust forms the upper part of the lithosphere, all rocks and sediments that lie on the surface belong to the crust. The upper boundary of the crust is called the Earth's surface, and the boundary between the crust and the mantle is referred to as the Mohorovičić discontinuity (or Moho for short). However, it is not possible to determine this with drilling, because it is not possible to drill to this depth. To this end, seismological measurements were performed, in which the speed of seismic waves through the rock is determined. Another seismic discontinuity, the Conrad discontinuity, is sometimes found at variable depths in the Earth's crust. The crust is sometimes divided into an upper crust above this discontinuity and a lower crust below. Although this transition, unlike the Moho, does not involve a clear difference in chemical composition, it can generally be said that the lower crust consists of less hydrated and more mafic rocks than the upper crust.

Properties of the Earth's crust

Composition

The crust is different from the Earth's mantle because it generally has a lower density. It is customary to express the composition in percentages by weight of the oxides of the elements. Compared to the mantle, the crust is enriched in lithophilic elements, mainly the alkalis and alkaline earths plus aluminum and silicon. Compared to other parts of the Earth, the crust contains much more rare earths. This ensures that a relatively large amount of heat is produced in the crust by radioactive decay.

Oceanic and continental crust

Roughly speaking, two types of crust can be recognized. The type of crust that is under the oceans is called oceanic crust. It consists mainly of mafic igneous rocks, basalt on the surface, diorite deeper in the crust and finally gabbro. Usually these igneous rocks are covered by a layer of deep marine sediment. Oceanic crust has a density usually around 3100 kg/m3 and a thickness of around 8–10 km. Under the continents and continental platforms lies continental crust. This type of crust consists of felsic and intermediate igneous and metamorphic and sedimentary rocks. On average, continental crust has the composition of andesite. It has a density of around 2800 kg/m3 (significantly smaller than oceanic crust) and a thickness that can range from 30–40 km normally to as much as 80 km in mountain ranges. While most of the Earth's crust consists of one of the two end types, there is also intermediate crust, which is intermediate in composition.

Rocks

There are three main types of rocks that make up the crust: sedimentary rocks (formed by the accumulation of products of erosion and weathering), igneous rocks (formed from liquid magma), and metamorphic rocks (formed by chemical reactions and deformation of other rocks). Sedimentary rocks (along with sediments) make up the top layer of the crust. Sometimes that layer can be more than 5 km deep, but in most places it varies between 0 and a few kilometers. Igneous rocks make up most of the oceanic crust. Also continental crust is made up of several generations of igneous rock. At depths greater than 2-4 kilometers almost all rocks are clearly slightly metamorphic. The metamorphic degree increases at higher depths. Eventually the metamorphosis can become so high that nothing can be seen of originally sedimentary or volcanic structures. The Earth's crust consists of about 95% igneous rocks and 5% sedimentary rock

Processes

The movement of the crust is investigated by the field of tectonics. Movements are caused by mechanical tension

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