The middle shell in the chemical model of the internal structure of the earth's body is called the earth 's mantle . It lies between the earth's crust and the earth's core and, with an average thickness of 2,850 km (depth of the mantle-core boundary: 2,898 km), is the most voluminous and massive of these three shells. While the crust consists to a large extent of relatively aluminum-rich rocks of granitic (continental upper crust) and basaltic (oceanic crust and continental lower crust) composition, the material of the earth's mantle is low in aluminum and relatively rich in iron and magnesium. The corresponding ultramafic rock of the Upper Mantle is called peridotite . The deeper mantle consists of high pressure equivalents of the peridotite. The majority of the earth's mantle, apart from smaller regions in which partial melts occur, is solid , but behaves plastically over geological time periods .
A kind of proto-earth mantle probably formed as early as 4.45 billion years ago, when the highly volatile components such as hydrogen , carbon (in the form of carbon dioxide and methane), nitrogen (including ammonia and nitrogen oxides ) and noble gases were largely released into the primeval -The atmosphere degassed and the siderophilic elements largely sank to the then still completely liquid core of the earth .
Dimensions and temperatures
The mass of the earth's mantle is approx. 4.08 · 10 24 kg and thus around 68% of the total mass of the earth. There are temperatures between at least several 100 ° C at the upper limit of the cladding and over 3500 ° C at the cladding-core boundary.
Although these temperatures far exceed the melting point of the mantle material under atmospheric conditions, especially in deeper areas , the earth's mantle consists almost exclusively of solid rock. The enormous lithostatic pressure in the earth's mantle prevents the formation of melts.
|element||proportion of||connection||proportion of|
|Al||2.2||Al 2 O 3||4.2|
|N / A||0.3||Na 2 O||0.4|
|K||0.03||K 2 O||0.04|
The rock of the upper mantle consists predominantly of ultramafic rocks (primarily peridotites and pyroxenites ). These mainly contain olivine or high-pressure variants of this mineral, various pyroxenes and other mafic minerals . In the depth range between 660 and about 800 km, temperature and pressure conditions are reached at which these minerals are no longer stable and are therefore converted to other minerals by phase transformations (see section Structure of the mantle and phase transitions ). Mantle rock generally shows a higher proportion of iron and magnesium and a lower proportion of silicon and aluminum . The distinction between the earth's crust and the earth's mantle is essentially based on this different chemical composition. Ignorant processes are the cause of this difference : mantle rock partially melts , with the silicon and aluminum-rich rock components in particular liquefying due to their lower melting point , rising as magma and solidifying again at or relatively close to the surface. In this way, today's crust and mantle have differentiated over billions of years.
The chemical composition of the earth's mantle is by no means homogeneous. Heterogeneities probably arose during the formation of the earth's mantle, so that we speak of geochemical earth's mantle reservoirs, with different reservoirs being tapped through different plate tectonic processes. The definition and interpretation of these reservoirs is sometimes highly controversial:
- DM or DMM (Depleted Mantle - predominantly source reservoir for mid-ocean ridge basalts (MORB)) - a mantle that is depleted of incompatible elements
- EM1 (Enriched Mantle 1) - probably re-enriched by subducted oceanic crust and pelagic sediments
- EM2 (Enriched Mantle 2) - probably re-enriched mantle by subducting the upper continental crust
- HIMU (high µ means a high 238 U / 204 Pb ratio) - presumably a mantle changed by subducted oceanic crust and metasomatic processes ; The age of the subducted crust may also play a role (different definitions available)
- FOZO (focal zone) - different definitions available
- PREMA (prevalent mantle reservoir) - the predominant mantle reservoir
Olivine-spinel crystallization temperatures of 1600 ° C, which were determined for samples of Cretaceous basalts from the Galapagos hotspot that have now been accreted to the Pacific continental margin of Central America , suggest that individual very hot archaic mantle reservoirs have survived at least into the late Mesozoic and with plumes got into the upper mantle.
Structure of the mantle and phase transitions in the mantle rock
The earth's mantle is divided into several layers, which differ less in their chemical composition than in the mechanical properties and in the crystal structure and density of the minerals of the mantle rock. A distinction is roughly made between the upper and lower coat.
The highest layer of the Upper Mantle is the lithospheric mantle . Together with the earth's crust, it forms the lithosphere , which is mechanically decoupled from the rest of the mantle. The rheological behavior of the lithospheric mantle can be described as rigid compared to the rest of the mantle. Plastic deformation does take place, but in contrast to the rest of the jacket, which flows as a whole, it is limited to discrete areas ( shear zones ). The interface between the earth's lower crust and the lithospheric mantle is known as the Mohorovičić discontinuity . The lithospheric mantle itself extends from less than 100 to over 300 km depth. Already in the top 100 km of the mantle, i.e. still within the lithosphere, phase transitions of the aluminum-containing minerals take place due to the increasing lithostatic pressure , through which the plagioclase, which is stable at low pressures of just under 1 GPa, becomes spinel until 3 GPa turns into garnet . This is accompanied by minor changes in the mineral proportions of the mantle rock (see the tables in the article on peridotite ). The mean density of the rocks of the lithospheric mantle is 3.3 g / cm³.
At the bottom of the lithospheric mantle is the relatively low viscosity and in small parts partially melted, approx. 100 to 200 km thick asthenosphere . Because it is characterized by noticeably low speeds of seismic waves , it is also called the Low Velocity Zone (LVZ). The mean density of the asthenospheric rock is 3.3 g / cm³.
The bottom layer of the upper mantle is the so-called mantle transition zone . It is limited in seismic profiles towards the asthenosphere by the so-called 410 km discontinuity , which marks the phase transformation of olivine from the α phase to the denser β phase ( wadsleyite ). At a depth of around 520 km, wadsleyite changes into the again denser γ phase of olivine ( ringwoodite ) ( 520 km discontinuity ). Around this depth, Ca perovskite is also formed from the other calcium-containing minerals , which makes up a few percent by volume and also exists as a separate phase in the lower mantle. From a depth of around 300 km, pyroxene and garnet gradually form a low-aluminum mixed crystal with a garnet structure ( majorite ), which is stable in most of the transition zone between 410 and 660 km and the uppermost part of the lower mantle. The mean density of the mantle rock of the transition zone is 4.2 g / cm³.
At the 660 km discontinuity , olivine or ringwoodite finally breaks down into perovskite and ferropericlase / magnesiowustite - this prominent seismic discontinuity marks the boundary between the upper and lower mantle. The majority of the lower mantle is also called the mesosphere (not to be confused with the layer of the earth's atmosphere of the same name ). There the minerals of the mantle rock, with an average density of 5.0 g / cm³, no longer appear to undergo any phase transformations that lead to global discontinuities.
A possible exception is the transformation from perovskite to post-perovskite, which takes place at pressures above 120 GPa and may be the cause of the so-called D ″ layer at the boundary between the earth's mantle and the outer core.
Due to a difference in density (which probably results from a temperature difference) between the earth's crust and the outer core of the earth, a convective material circulation takes place in the earth's mantle , which is made possible not least by the fluidity of the solid, ductile mantle material over millions of years. Hot material from the vicinity of the core-mantle boundary rises as a diapir to higher areas of the earth's mantle, while cooler (and denser) material sinks to the bottom. During the ascent, the jacket material cools down adiabatically . In the vicinity of the lithosphere , the pressure relief can cause material of the mantle diapir to partially melt (causing volcanism and plutonism ).
The mantle is in terms of fluid mechanics chaotic process and a drive of the plate tectonics , with both long-term stability as unstable Konvektionsmodelle also be discussed. The sinking of the old, cold and heavy oceanic crust at the subduction zones is also significant for this. The movements of the lithospheric plates of the earth's mantle are partially decoupled, because due to the rigidity of the lithosphere, such a plate (most of which comprise both continental and oceanic crust) can only move as a whole. The changes in position of the continents therefore only provide a blurred image of the movements at the upper limit of the earth's mantle. The convection of the earth's mantle has not yet been clarified in detail. There are several theories that the Earth's mantle is divided into different floors of separate convection.
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