Mantle (geology)

As the most powerful mantle, middle shell is signified in the internal structure of the Earth. It is located directly under the earth's crust and is an average of 2850 km thick ( depth of the mantle -core boundary 2898 km ). The mantle is like the earth's crust firmly, but differs significantly in its mechanical properties and chemical composition of this top, often made ​​of basalt ( seabed and under continents ) and existing granite (especially on continents ) " crust ":

Formation

The mantle probably exists already since 4.45 billion years after the volatile elements degassed into the atmosphere and the siderophile elements formed the then still completely liquid core. Since then depleted the mantle in incompatible elements, which preferentially go into melt and incorporated into the crust, as well as siderophile elements, which are continuous at the core- mantle boundary in the liquid outer core.

Dimensions and temperatures

The mass of the Earth's mantle is approximately 4.08 · 1024kg, which is about 68 % of the total mass of the Earth. It's such temperatures between some 100 ° C on the jacket cap and about 3500 ° C at the mantle - core boundary. Although these temperatures exceed particular deeper parts of the melting point of the shell material at atmospheric conditions, by far, is the mantle almost exclusively of solid rock. The enormous lithostatic pressure in the mantle prevents the formation of melts.

Chemical composition

Total composition

The rock of the upper mantle consists mainly of ultramafic rocks ( peridotite and pyroxenite in the first place ). In these mainly olivine or high-pressure variants of this mineral, various pyroxenes and other mafic minerals are included. In the depth range between 660 and 800 km Temperature and pressure conditions are reached at which these minerals are not stable and are therefore converted to other minerals by phase transformations; this form perovskite and ferropericlase. Mantle rock shows a higher content of iron and magnesium and a minor amount of silicon and aluminum. The distinction between crust and mantle is essentially based on this different chemistry.

Mantle reservoirs

However, the chemical composition of the mantle is by no means homogeneous. Probably already been incurred in the development of the mantle heterogeneities, so that is spoken by geochemical Erdmantelreservoirs using different reservoirs are tapped by different plate tectonic processes. The definition and interpretation of these reservoirs is partially highly controversial:

• DM and DMM ( depleted ( MORB ) Mantle ) - in incompatible elements depleted mantle
• EM1 ( Enriched Mantle 1) - presumably by subducted / recycled oceanic crust and pelagic sediments again enriched mantle
• EM2 ( Enriched Mantle 2) - probably again enriched by subducted upper continental crust mantle
• HIMU (high μ is meant a high 238U/204Pb ratio) - presumably by subducted oceanic Kruse and metasomatic processes modified mantle; possibly also the age of the crust subducting plays a role ( different definitions available)
• Fozo ( focal zone ) - different definitions exist
• PREMA ( prevalent mantle reservoir ) - the predominant mantle reservoir

See also Dupal anomalies

Phase transitions in the mantle rock

The above-mentioned phase transformations are not the only ones in the Earth's mantle. Already found in the upper 100 km instead of phase transitions of the aluminum-containing minerals, the particular at low pressures up to just under 1 GPa stable plagioclase to spinel, which merges from 2.5 to 3 GPa in garnet; hereby go smaller changes in the mineral proportions of the mantle rock associated (see the tables in the article about peridotite ). With increasing pressure form from about 300 km depth pyroxene and garnet gradually an aluminum- poor mixed crystal having a garnet structure that is stable in most of the transition zone 410-660 km away, and the uppermost part of the lower mantle.

The upper limit of the mantle transition zone is marked by a relatively sharply defined phase transformation of olivine, in that of the α - phase to the β phase ( wadsleyite ) passes; it is known from seismic observations than 410 - km discontinuity. In about 520 km depth, wadsleyite transforms into the γ - phase of olivine ( ringwoodite ) to (520 - km discontinuity ). About this depth range is also formed from the other calcium- bearing minerals Ca - perovskite, which accounts for some volume percent and exists as a separate phase in the lower mantle. At the 660 - km discontinuity eventually disintegrates in olivine and perovskite ferropericlase; these prominent seismic discontinuity marks the boundary between upper and lower mantle. In the lower mantle, the mantle minerals appear to undergo no phase transformations that lead to global discontinuities; A possible exception is a transformation from perovskite to post- perovskite, which takes place at pressures above 120 GPa and possibly the cause of the D "layer at the boundary between Earth's mantle and core of the Earth is.

Pressure and temperature conditions in the mantle cause the cladding material is flowing even in the solid state. Mantle rock is therefore not more brittle (as opposed to crustal rocks ), but plastically deformable (such as clay ), and also does not break because of it. Although one might therefore assume that there are no more earthquakes below 300 km depth, but can be deep earthquakes between 400 km and 670 km below the earth's surface register.

The boundary between the mantle and the outer plastic, brittle lithosphere also called shell does not coincide with the boundary between the ( chemically defined ) earth's crust and mantle. It runs contrary within the Earth's mantle. The lithosphere includes not only the brittle crust and the outermost, also brittle, areas of ( chemically defined ) mantle. The transition region between the crust and the lower part of the lithosphere or the mantle therefore is called Mohorovičić discontinuity. The boundary between the lithosphere and mantle is a thin, that is some 10 km powerful area, which is characterized by a relatively high proportion of molten material, but mainly consists of solid material. This area is called the asthenosphere, or even because he is distinguished by conspicuously low velocities of seismic waves than the low velocity zone designated.

Mantle convection

Due to a difference in density (which is probably the result of a temperature difference ) between the crust and the outer core of the Earth is in the mantle convective material circulation instead, which is made possible not least due to the fluidity of the solid, ductile cladding material over millions of years. This hot material rises from near the core - mantle boundary as a diapir in higher areas of the mantle, while cooler ( and denser ) material sinks downward. During the ascent from the mantle material cools adiabatically. Near the lithosphere, the pressure relief cause of Manteldiapirs material melts partially ( and thus volcanism and plutonism causes ).

The mantle is a fluid mechanics in the sense of chaotic process and a drive of continental drift, where both long-term stable and unstable Konvektionsmodelle be discussed, but also the sinking of old cold and heavier oceanic crust at plate edges is important. The movements of the continents and the mantle are decoupled partial, since due to the rigidity of the earth's crust, a crustal plate (most include both continental and oceanic crust ) can only move as a whole. The changes in position of the continents only provide a blurred image of the movements at the upper limit of the mantle. The convection in the mantle is not yet clarified in detail. There are several theories, according to which the Earth's mantle is divided into several separate floors convection.