Upper crust

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The upper half of the earth's crust , which mainly consists of granitic rocks, is called the upper crust (also known as the continental crust ) . It essentially only exists on the continents , while it is mostly absent under the sea ​​floor or (near the coast) is only very thin. The upper crust, with its locally changing rock and mass distribution, is primarily responsible for local and regional gravity anomalies . 

Beachte:

Die Begriffe Ober- bzw. Unterkruste werden seit längerem auch in einer

etwas anderen Bedeutung verwendet, nämlich als zwei ungefähr gleich dicke

Teile (nur) der kontinentalen Kruste -- siehe die nächsten zwei Absätze.

Typical of the upper crust are its predominantly "acid" minerals ( feldspars and quartz ) with a high content of silica SiO 2 and the mineral-forming aluminum , which is why it was previously also called SiAl . The upper 10–20 km has relatively brittle rock, while the higher temperature makes it ductile (somewhat creepable) below. The separating layer is called Conrad discontinuity , which separates the upper crust from the lower crust in more recent usage . This boundary layer, which cannot always be clearly identified seismically , was only discovered in the 1920s, at which the speed of seismic shock waves increases by around 10%.

Under the upper crust lies the basaltic part of the earth's crust, which mainly consists of basic rocks with a higher magnesium content and is therefore abbreviated to SiMa . In the past it was also known as the lower crust , today mostly as the oceanic crust , as it runs not only under the continents but also under the seabed.

The upper crust has an average rock density of 2.7 g / cm³, which corresponds to the mean value of granites . Other solid rocks are between 2.5 (e.g. compacted sandstone ), 2.7 ( limestone , many metamorphic rocks ) and about 2.9 g / cm³, but in terms of quantity only contribute little to the average value. In contrast, the oceanic crust has an average density of 3.0 g / cm³ and thus an even higher seismic speed of the earthquake waves .

Under these 1-2 layers of solid rock - see lithosphere - the somewhat more plastic upper mantle begins , the density of which rises continuously from about 3.3 to 4 g / cm³. The interface between the earth's crust and the earth's mantle was discovered as early as 1910 and is called the Mohorovičić discontinuity, or Moho surface for short , after its discoverer . The continental plates "float" because of its lower density on the mantle and can slow (a few centimeters per year) move against each other, what research topic of plate tectonics is.

On average, the entire earth's crust is around 30 km thick under the continents and around 50 to 65 km under high mountains . The oceanic crust makes up only about 10 km of this. As continental deep boreholes (e.g. KTB in the Bavarian Forest or on the Kola Peninsula ) show, the top 20 km should be relatively uniform from a petrographic point of view despite Conrad discontinuity .

literature

  • Dieter Richter , General Geology , 3rd edition. Verlag de Gruyter, Berlin - New York 1985