In geosciences, the upper mantle is the plastically deformable part of the earth's mantle that supports the earth's crust and extends below it to a depth of 410 km. If you also count (as is usually the case) the so-called transition zone to the upper coat, it extends to a depth of around 750 kilometers (the information in the specialist literature varies between 650 and 900 km). These rock layers comprise almost a third of the entire mantle, the boundary of which to the earth's core was determined by various geoseismic methods with an average depth of 2898 km.
The entire earth's mantle has a mass of about 4.08 · 10 24 kg or around 68% of the total mass of the earth . Its temperature increases from around 300–400 ° C at the upper limit of the mantle (hotter under volcanic chains ) to around 3500 ° C at the beginning of the earth's core. This means that the melting point of many rocks is clearly exceeded and parts of the upper mantle should actually have already liquefied if there were not extremely high lithostatic pressure there . In this way, the rock remains relatively solid in situ . Sometimes it is compared to the viscosity of sealing wax , which - placed over the edge of a table - would bend down after a few days. Nevertheless, the deepest earthquakes reach down to about 600 km, which is theoretically not yet fully understood.
Earthquakes and rock boundaries
Since the earth's crust has a thickness between 10 and 70 km, depending on the geographical location on the continent or the sea, the upper mantle has a locally slightly variable thickness of about 700 km ± 30 km.
The upper boundary surface of the upper mantle - to the earth's crust - is characterized by a change in the rocks : above light granites and other "acid" rocks (high proportion of quartz = SiO 2 ), below dark, " basic " basalts and silicates . Because of the more compact mantle material, the speed of the seismic waves increases from 6½ to almost 8 km / s ( Mohorovičić discontinuity ) below the crust . Despite the increasing temperature of hundreds of degrees, the rocks are still solid and relatively brittle down to depths of 100 to 150 km . Therefore, the top layer of the earth's mantle (on average 100 km; lithospheric mantle ) together with the earth's crust is also called the lithosphere (Greek λίθος, líthos = stone). It is particularly thick and massive among the continents.
The depth of the possible lower boundary of the upper mantle is relatively uniform. It manifests itself in a sudden increase in density from about 4.2 to 4.5 g / cm³ - and a simultaneous change in the speed of the earthquake waves from 10 to 11 km per second. The pressure, which increases sharply with depth (around 30 gigapascals ), causes a phase transition of the hot olivine minerals from spinel to even more compact crystal structures. For some time, it has been possible to generate similarly strong pressures with hydraulic presses (see web links) and thus limit the possible mantle rocks.
Under the lithosphere begins in the not of the Archean Earth dominated regions, the asthenosphere , which some because of the higher temperature (above 500 ° C) plasticity has. This “flexible, weak” layer (Greek asthenos ) extends 200 to 300 km deep. On their upper boundary surface, the crustal plates (with and without continents) can slowly shift ( plate tectonics ), which, according to the measurement data of earth measurements and satellite geodesy, occurs at 2 to 20 cm per year.
A thin zone with lower viscosity , which can be traced back to partial melting along the grain boundaries in the rock, acts as a “lubricant”. For geophysicists , it is noticeable through a local but noticeable decrease in seismic velocities (P and S waves), which has led to the name low-velocity zone .
Transition zone to the deeper mantle
In the transition zone between 400 and about 700 km depth, where most geophysicists allow the upper mantle to end, seismologists have discovered a number of other layers where the earthquake waves are slightly reflected . These discontinuities are usually designated according to their average depth (however, the information can vary by up to 100 km):
- at the sharp 410 km discontinuity , the olivine - which has a density of around 3.3 g / cm³ on the earth's surface - changes into a much denser β phase.
- At a depth of about 520 km ( 520 km discontinuity ) follows the γ phase ( ringwoodite ) with a small proportion of calcium-containing minerals; as Ca perovskite it is also found in the lower mantle .
- From 600–800 km ( 660 km discontinuity ), the rocks known from the surface of the earth disintegrate and take on a new, extremely compact structure. Temperatures around 1000 ° C are assumed there.
On the further 2000 kilometers to the earth's core - where the iron begins and the temperature reaches 3500 ° C - the density can therefore only increase by 1 unit to 5–6 g / cm³.
The dark, ultra-basic rocks of the upper mantle almost certainly consist of different variants of olivine (Mg, Fe) 2 SiO 4 , which is supplemented by peridotite . The latter consist primarily of olivine, ortho- and clinopyroxene in a wide variety of mixtures . Most of these minerals belong to the magnesium-iron- silicates , and the basic chemical form of pyroxene is (Mg, Fe) 2 Si 2 O 6 .
The above phase transitions between 400 and 700 km depth are related to the compressibility of olivine and the very high pressure of the layers of earth above it. At depths of around 700 km, however, the rocks above become unstable and transform into other minerals at temperatures of many hundreds of degrees or at pressures around 25 GPa (250,000 times air pressure ) because their internal structure changes ( phase transformation ). Therefore materials such as perovskite (technically used in crystal lasers ) and ferropericlass should be used from the corresponding depth . Perovskite (whose name has a second meaning) is an iron and magnesium-containing silicate rock with the basic form (Mg, Fe) SiO 3 .
The difference between the shell material and the earth's crust is not only due to the high pressures and temperatures, but also to the different chemical compositions. The mantle rock has less silicon and aluminum content than the earth's crust and more magnesium and iron . This is why the upper areas of the mantle are often called Sifema - in contrast to the Sial of the continent blocks and the Sima of the oceanic crust. These terms, which geologists are reluctant to use (but which are anchored in school and language usage ), can be thought of as grainy, light- colored solid rock such as granite (mean SiAl density ~ 2.7 g / cm³ or 2700 kg / m³), or as dark, basalt or gabbro- like SiMa rock with 3.3 to 4 g / cm³. The material of the upper jacket, however, reaches up to 5 g / cm³ at a greater depth.
- Deep rock, other mantle rocks: garnet , plagioclase
- Plutonism , subduction , volcanism , mantle convection , differential rotation
- Modulus of elasticity
- Lehmann discontinuity
- Mantle peridotites and temperatures - Univ. Cologne (PDF file; 872 kB)
- Characteristics in the Earth's mantle - TH Aachen
- Upper + lower earth mantle ( Memento from December 27, 2005 in the Internet Archive ) (PDF file)
- Geomechanical pressure tests with hydraulic presses
- Peridotites and pyroxenites in the earth's mantle