Crust (geology)

The earth's crust or the earth's crust is the outermost solid shell of the earth. The crust is 35 km thick on average and in the internal structure of the earth the thinnest shell. It is classified according to their chemical composition in the still existing oceanic and continental crust.

The Earth's crust generally has a lower density than the mantle ( the middle shell ), so it is on this. The crust together with the outermost region of the upper mantle ( the lithospheric mantle ) lithosphere.

Oceanic and continental crust

There are two types of crustal material:

  • The continental crust - SiAl also called because it is composed mainly of silicon and aluminum ( in addition to oxygen ).

The two types of crust differ in their origin, their composition, density and thickness. The data obtained so far on other planets and their moons suggest that the Earth in our solar system is the only celestial body that has two different types of crust.

Oceanic crust

Oceanic crust is formed at the boundaries of divergent tectonic plates on the ocean floor, where leaking constantly basic magma from the mantle, which solidifies and the global system, the mid-ocean ridge (MOR ) forms. This " young " crustal rocks composed mainly of basalt and gabbro -like has a relatively high density ( about 3 g / cm ³). Oceanic crust has a small thickness of five to seven kilometers, only occasionally it is more than ten kilometer thick.

The process of seafloor spreading is driven according to current knowledge by convection currents in the mantle. The respective adjacent lithospheric plates typically strive to achieve a speed of several millimeters to centimeters a year apart ( spreading rate ).

Continental crust

Continental crustal rocks, however, is lighter ( density 2.7 g / cm ³). The chemical composition is similar in the middle of a granite ( "sour", percentage of silica SiO2 about 66 percent) and his twin metamorphic rock gneiss. It is the end product of a process that could ascend the less dense minerals throughout the earth to the surface. Isostasy and volcanism have played the main role, but also metamorphosis and chemical-physical processes of weathering, leading to the deposition of granular soils ( sediments ).

The continental crust has a thickness of 30 to 60 km, where they mountain countries much further down enough (especially with long mountain chains). The average is just under 40 km, local extreme values ​​are 25 km ( flat coasts ) and 70 km (Himalayas ). " Swim " Because of their lower density, the continents higher in the mantle than oceanic crust, but appear simultaneously ( analogous to a high iceberg ) deeper down. Since rocks behave plastically at geologically slow movements, in the course of millions of years a far-reaching equilibrium is reached.

Composition of the earth's crust

Presumably, the simatische, oceanic crust layer (or petrographically related material ) continues partly under the continental sialic crust and forms its lower third ( see picture and Kertz S. 209 ff.) From the analysis of seismic waves Andrija Mohorovičić 1909 could thus indirectly demonstrate the speed jump and the increase in density at the Earth's mantle back, which is 0.5 g / cm ³, or nearly 20%. This so-called Mohorovičić discontinuity ( Moho short ) runs in varying depth under the continents (after leather Steger means 33 km deep, according to modern seismic 38 km). Around 1920, Victor Conrad discovered a second density jump, which was named after him Conrad discontinuity about 20 km depth. It corresponds to the ( not entirely consistent ) interface between Sial and Sima - crustal rocks. Usually today between upper crust and lower crust.

Almost all chemical elements - namely the 93 currently (2012 ) 118 elements of the periodic table - can be found in the earth's crust along with oceans and atmosphere. Here, the oxygen with 46.6 weight percent accounts for the largest part, followed by silicon at 27.7 % and aluminum 8.1%. Other important components are iron (4.7% ), calcium (3.6%), sodium ( 2.8% ), potassium (2.6%) and magnesium ( 2.1%). The remaining 85 elements are each less than one percent, most are present only in trace amounts.

Origin of the earth's crust in the Earth's

With a few tiny Zirkonminerale can prove the existence of a crust before 4.4 billion years. The composition of this crust in the earliest stage of the creation of Earth is controversial: both the possibility of a mafic and felsic crust of an already being considered. Both presupposes the existence of a mantle. For the existence of a shell structure existing before the crust there is no evidence. Was this it can be concluded that either there was no solid crust, or an existing anorthositische ( KREEP -like ) or komatiitische ( ultramafic ) crust within geologically short time completely mixed with the mantle, which is why repeatedly postulated in the lower mantle reservoirs be, which should contain the remains of such crusts. The dimensions of this at this early stage of the earth's crust already solidified section are highly controversial.

Research

While the Earth's surface has been researched for time immemorial, mapped and their landforms are interpreted, the geology of the subjacent layers of the Earth's crust is being investigated only since the 18th century.

That downward rises to the temperature, is known for millennia by mines and evidenced by volcanism. Earthquakes were some early conclusions. Therefore, it has been suggested in earlier antiquity that deeper layers of the earth are in a molten state. Even the Greek natural philosophers were concerned about the exact structure and origin of the earth.

Isaac Newton calculated the Earth flattening with a simple physical model. The detailed investigation of the Earth's interior began about 200 years ago. By 1900, eventually became the modern measurement methods of gravimetry, seismology and geomagnetism.

The deepest hole to explore the Earth's crust is the Kola hole; reached on the Russian Kola Peninsula the record depth of 12,262 meters. In Germany the continental deep drilling ( KTB ) reached a depth of 9101 meters. The hole was terminated at this depth because the temperatures were higher than expected (see geothermal depth level ).

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