The mantle of the Earth is the most important part of our planet, since it is here that most of the substances are concentrated. It is much thicker than the rest of the components and, in fact, takes up most of the space - about 80%. Scientists have devoted most of their time to studying this particular part of the planet.

Structure

Scientists can only speculate about the structure of the mantle, since there are no methods that would unambiguously answer this question. But, the conducted studies made it possible to assume that this part of our planet consists of the following layers:

• the first, the outer one, occupies from 30 to 400 kilometers of the earth's surface;
• the transition zone, which is located immediately behind the outer layer - according to scientists, it goes deep into about 250 kilometers;
• the lower layer - its length is the largest, about 2900 kilometers. It starts right after the transition zone and goes straight to the core.

It should be noted that in the mantle of the planet there are such rocks that are not in the earth's crust.

Compound

It goes without saying that it is impossible to establish exactly what the mantle of our planet consists of, since it is impossible to get there. Therefore, everything that scientists manage to study happens with the help of fragments of this area, which periodically appear on the surface.

So, after a series of studies, it was possible to find out that this part of the Earth is black and green. The main composition is rocks, which consist of the following chemical elements:

• silicon;
• calcium;
• magnesium;
• iron;
• oxygen.

In appearance, and in some ways even in composition, it is very similar to stone meteorites, which also periodically fall on our planet.

The substances that are in the mantle itself are liquid, viscous, since the temperature in this area exceeds thousands of degrees. Closer to the Earth's crust, the temperature decreases. Thus, a certain circulation occurs - those masses that have already cooled down go down, and those heated to the limit go up, so the process of "mixing" never stops.

Periodically, such heated streams fall into the very crust of the planet, in which they are assisted by active volcanoes.

Ways to study

It goes without saying that layers that are at great depths are quite difficult to study, and not only because there is no such technique. The process is also complicated by the fact that the temperature rises almost constantly, and at the same time, the density also increases. Therefore, we can say that the depth of the layer is the least problem in this case.

However, scientists still managed to advance in the study of this issue. To study this part of our planet, geophysical indicators were chosen as the main source of information. In addition, during the study, scientists use the following data:

• seismic wave speed;
• gravity;
• characteristics and indicators of electrical conductivity;
• the study of igneous rocks and fragments of the mantle, which are rare, but still manage to be found on the surface of the Earth.

As for the latter, it is diamonds that deserve special attention of scientists - in their opinion, by studying the composition and structure of this stone, one can find out a lot of interesting things even about the lower layers of the mantle.

Occasionally, but there are mantle rocks. Their study also allows you to get valuable information, but to one degree or another there will still be distortions. This is due to the fact that various processes occur in the crust, which are somewhat different from those that occur in the depths of our planet.

Separately, we should talk about the technique with which scientists are trying to get the original rocks of the mantle. So, in 2005, a special vessel was built in Japan, which, according to the developers of the project, will be able to make a record deep well. At the moment, work is still underway, and the start of the project is scheduled for 2020 - there is not so much to wait.

Now, all studies of the structure of the mantle are carried out within the framework of the laboratory. Scientists have already precisely established that the lower layer of this part of the planet, almost all consists of silicon.

pressure and temperature

The distribution of pressure within the mantle is ambiguous, in fact, as well as the temperature regime, but first things first. The mantle accounts for more than half of the planet's weight, or more precisely, 67%. In areas under the earth's crust, the pressure is about 1.3-1.4 million atm, while it should be noted that in places where the oceans are located, the pressure level drops significantly.

As for the temperature regime, the data here are completely ambiguous and are based only on theoretical assumptions. So, at the sole of the mantle, a temperature of 1500-10,000 degrees Celsius is assumed. In general, scientists have suggested that the temperature level in this part of the planet is closer to the melting point.

Mantle of the Earth - this is a silicate shell of the Earth, composed mainly of peridotites - rocks consisting of silicates of magnesium, iron, calcium, etc. Partial melting of mantle rocks gives rise to basalt and similar melts, which form the earth's crust when rising to the surface.

The mantle makes up 67% of the total mass of the Earth and about 83% of the total volume of the Earth. It extends from depths of 5-70 kilometers below the boundary with the earth's crust, to the boundary with the core at a depth of 2900 km. The mantle is located in a huge range of depths, and with increasing pressure in the substance, phase transitions occur, in which minerals acquire an increasingly dense structure. The most significant transformation occurs at a depth of 660 kilometers. The thermodynamics of this phase transition is such that mantle matter below this boundary cannot penetrate it, and vice versa. Above the boundary of 660 kilometers is the upper mantle, and below, respectively, the lower. These two parts of the mantle have different composition and physical properties. Although information on the composition of the lower mantle is limited, and the number of direct data is very small, it can be confidently asserted that its composition has changed much less since the formation of the Earth than the upper mantle, which gave rise to the earth's crust.

Heat transfer in the mantle occurs by slow convection, through plastic deformation of minerals. The rates of movement of matter during mantle convection are on the order of several centimeters per year. This convection drives the lithospheric plates. Convection in the upper mantle occurs separately. There are models that assume an even more complex structure of convection.

Seismic model of the structure of the earth

The composition and structure of the deep shells of the Earth in recent decades continue to be one of the most intriguing problems of modern geology. The number of direct data on the matter of deep zones is very limited. In this regard, a special place is occupied by a mineral aggregate from the Lesotho kimberlite pipe (South Africa), which is considered as a representative of mantle rocks occurring at a depth of ~250 km. The core recovered from the world's deepest well, drilled on the Kola Peninsula and reaching 12,262 m, significantly expanded scientific understanding of the deep horizons of the earth's crust - a thin near-surface film of the globe. At the same time, the latest data of geophysics and experiments related to the study of structural transformations of minerals already now allow modeling many features of the structure, composition and processes occurring in the depths of the Earth, the knowledge of which contributes to the solution of such key problems of modern natural science as the formation and evolution of the planet, dynamics of the earth's crust and mantle, sources of mineral resources, risk assessment of hazardous waste disposal at great depths, energy resources of the Earth, etc.

The widely known model of the internal structure of the Earth (its division into the core, mantle and earth's crust) was developed by seismologists G. Jeffreys and B. Gutenberg back in the first half of the 20th century. The decisive factor in this was the discovery of a sharp decrease in the velocity of seismic waves inside the globe at a depth of 2900 km with a planetary radius of 6371 km. The velocity of propagation of longitudinal seismic waves directly above the specified border is 13.6 km/s, and below it - 8.1 km/s. This is the boundary between the mantle and the core.

Accordingly, the core radius is 3471 km. The upper boundary of the mantle is the seismic section of Mohorovichić (Moho, M), identified by the Yugoslav seismologist A. Mohorovichić (1857-1936) back in 1909. It separates the earth's crust from the mantle. At this boundary, the velocities of longitudinal waves that have passed through the earth's crust increase abruptly from 6.7-7.6 to 7.9-8.2 km/s, but this happens at different depth levels. Under the continents, the depth of the section M (that is, the soles of the earth's crust) is a few tens of kilometers, and under some mountain structures (Pamir, Andes) it can reach 60 km, while under the ocean basins, including the water column, the depth is only 10-12 km . In general, the earth's crust in this scheme appears as a thin shell, while the mantle extends in depth to 45% of the earth's radius.

But in the middle of the 20th century, ideas about a more fractional deep structure of the Earth entered science. Based on new seismological data, it was possible to divide the core into inner and outer, and the mantle into lower and upper. This popular model is still in use today. It was started by the Australian seismologist K.E. Bullen, who proposed in the early 40s a scheme for dividing the Earth into zones, which he designated with letters: A - the earth's crust, B - a zone in the depth interval of 33-413 km, C - a zone of 413-984 km, D - a zone of 984-2898 km , D - 2898-4982 km, F - 4982-5121 km, G - 5121-6371 km (center of the Earth). These zones differ in seismic characteristics. Later, he divided zone D into zones D "(984-2700 km) and D" (2700-2900 km). At present, this scheme has been significantly modified, and only the D "layer is widely used in the literature. Its main characteristic is a decrease in seismic velocity gradients compared to the overlying mantle region.

The inner core, having a radius of 1225 km, is solid and has a high density - 12.5 g/cm 3 . The outer core is liquid, its density is 10 g/cm 3 . At the boundary between the core and the mantle, there is a sharp jump not only in the velocity of longitudinal waves, but also in density. In the mantle, it decreases to 5.5 g/cm 3 . Layer D", which is in direct contact with the outer core, is affected by it, since the temperatures in the core significantly exceed the temperatures of the mantle. In some places, this layer generates huge heat and mass flows directed to the Earth's surface through the mantle heat and mass flows, called plumes. They can manifest themselves on the planet in the form of large volcanic regions, such as in the Hawaiian Islands, Iceland and other regions.

The upper boundary of the D" layer is indefinite; its level from the surface of the core can vary from 200 to 500 km or more. Thus, we can conclude that this layer reflects an uneven and varying intensity influx of core energy into the mantle region.

The boundary of the lower and upper mantle in the scheme under consideration is the seismic section lying at a depth of 670 km. It has a global distribution and is justified by a jump in seismic velocities towards their increase, as well as an increase in the density of the lower mantle matter. This section is also the boundary of changes in the mineral composition of rocks in the mantle.

Thus, the lower mantle, enclosed between the depths of 670 and 2900 km, extends along the radius of the Earth for 2230 km. The upper mantle has a well-fixed internal seismic section passing at a depth of 410 km. When crossing this boundary from top to bottom, seismic velocities increase sharply. Here, as well as on the lower boundary of the upper mantle, significant mineral transformations take place.

The upper part of the upper mantle and the earth's crust are fused together as the lithosphere, which is the upper solid shell of the Earth, in contrast to the hydro and atmosphere. Thanks to the theory of lithospheric plate tectonics, the term "lithosphere" has become widespread. The theory assumes the movement of plates along the asthenosphere - a softened, partially, possibly, liquid deep layer of reduced viscosity. However, seismology does not show an asthenosphere sustained in space. For many areas, several asthenospheric layers located along the vertical, as well as their discontinuity along the horizontal, have been identified. Their alternation is especially definite within the continents, where the depth of occurrence of asthenospheric layers (lenses) varies from 100 km to many hundreds. Under the oceanic abyssal depressions, the asthenospheric layer lies at depths of 70–80 km or less. Accordingly, the lower boundary of the lithosphere is in fact indefinite, and this creates great difficulties for the theory of the kinematics of lithospheric plates, which is noted by many researchers.

Modern data on seismic boundaries

With the conduct of seismological studies, there are prerequisites for identifying new seismic boundaries. The global boundaries are considered to be 410, 520, 670, 2900 km, where the increase in seismic wave velocities is especially noticeable. Along with them, intermediate boundaries are distinguished: 60, 80, 220, 330, 710, 900, 1050, 2640 km. Additionally, there are indications of geophysicists on the existence of boundaries 800, 1200-1300, 1700, 1900-2000 km. N.I. Pavlenkova recently singled out boundary 100 as a global one, which corresponds to the lower level of the division of the upper mantle into blocks. Intermediate boundaries have a different spatial distribution, which indicates the lateral variability of the physical properties of the mantle, on which they depend. Global boundaries represent a different category of phenomena. They correspond to global changes in the mantle environment along the radius of the Earth.

The marked global seismic boundaries are used in the construction of geological and geodynamic models, while intermediate ones in this sense have so far attracted almost no attention. Meanwhile, differences in the scale and intensity of their manifestations create an empirical basis for hypotheses concerning phenomena and processes in the depths of the planet.

The composition of the upper mantle

The problem of the composition, structure, and mineral associations of deep earth shells or geospheres, of course, is still far from a final solution, but new experimental results and ideas significantly expand and detail the corresponding ideas.

According to modern views, the composition of the mantle is dominated by a relatively small group of chemical elements: Si, Mg, Fe, Al, Ca, and O. The proposed models for the composition of the geospheres are primarily based on the difference in the ratios of these elements (variations Mg/(Mg + Fe) = 0 .8-0.9; (Mg + Fe)/Si = 1.2Р1.9), as well as differences in the content of Al and some other rarer elements for deep rocks. In accordance with the chemical and mineralogical composition, these models received their names: pyrolitic (the main minerals are olivine, pyroxenes and garnet in a ratio of 4: 2: 1), piclogitic (the main minerals are pyroxene and garnet, and the proportion of olivine is reduced to 40%) and eclogitic, which, along with the pyroxene-garnet association characteristic of eclogites, also contains some rarer minerals, in particular Al-bearing kyanite Al 2 SiO 5 (up to 10 wt %). However, all these petrological models refer primarily to upper mantle rocks extending to depths of ~670 km. With regard to the bulk composition of deeper geospheres, it is only assumed that the ratio of oxides of divalent elements (MO) to silica (MO / SiO 2) ~ 2, being closer to olivine (Mg, Fe) 2 SiO 4 than to pyroxene (Mg, Fe) SiO 3 , and among the minerals perovskite phases (Mg, Fe)SiO 3 with various structural distortions, magnesiowustite (Mg, Fe)O with a structure of the NaCl type and some other phases in much smaller quantities predominate.

All proposed models are very generalized and hypothetical. The pyrolitic model of the olivine-dominated upper mantle suggests its chemical composition to be much closer to that of the entire deeper mantle. On the contrary, the piclogitic model assumes the existence of a certain chemical contrast between the upper and the rest of the mantle. A more particular eclogitic model allows for the presence of separate eclogitic lenses and blocks in the upper mantle.

Of great interest is the attempt to harmonize the structural-mineralogical and geophysical data related to the upper mantle. It has been assumed for about 20 years that the increase in seismic wave velocities at a depth of ~410 km is mainly associated with the structural rearrangement of olivine a-(Mg, Fe) 2 SiO 4 into wadsleyite b-(Mg, Fe) 2 SiO 4 , accompanied by the formation of a denser phase with large values ​​of elasticity coefficients. According to geophysical data, at such depths in the Earth's interior, seismic wave velocities increase by 3–5%, while the structural rearrangement of olivine into wadsleyite (in accordance with the values ​​of their elastic moduli) should be accompanied by an increase in seismic wave velocities by about 13%. At the same time, the results of experimental studies of olivine and olivine-pyroxene mixture at high temperatures and pressures revealed a complete agreement between the calculated and experimental increase in seismic wave velocities in the depth interval of 200-400 km. Since olivine has approximately the same elasticity as high-density monoclinic pyroxenes, these data should indicate the absence of a highly elastic garnet in the underlying zone, the presence of which in the mantle would inevitably cause a more significant increase in seismic wave velocities. However, these ideas about the garnetless mantle came into conflict with the petrological models of its composition.

Thus, the idea arose that the jump in seismic wave velocities at a depth of 410 km is associated mainly with the structural rearrangement of pyroxene garnets inside Na-enriched parts of the upper mantle. Such a model assumes an almost complete absence of convection in the upper mantle, which contradicts modern geodynamic concepts. The overcoming of these contradictions can be associated with the recently proposed more complete model of the upper mantle, which allows the incorporation of iron and hydrogen atoms into the wadsleyite structure.

While the polymorphic transition of olivine to wadsleyite is not accompanied by a change in the chemical composition, in the presence of garnet, a reaction occurs that leads to the formation of wadsleyite enriched in Fe compared to the initial olivine. Moreover, wadsleyite can contain significantly more hydrogen atoms than olivine. The participation of Fe and H atoms in the wadsleyite structure leads to a decrease in its rigidity and, accordingly, a decrease in the propagation velocities of seismic waves passing through this mineral.

In addition, the formation of Fe-enriched wadsleyite suggests the involvement of a larger amount of olivine in the corresponding reaction, which should be accompanied by a change in the chemical composition of rocks near section 410. Ideas about these transformations are confirmed by modern global seismic data. On the whole, the mineralogical composition of this part of the upper mantle seems to be more or less clear. As regards the pyrolitic mineral association, its transformation down to depths of ~800 km has been studied in sufficient detail. In this case, the global seismic boundary at a depth of 520 km corresponds to the rearrangement of wadsleyite b-(Mg, Fe) 2 SiO 4 into ringwoodite - g-modification of (Mg, Fe) 2 SiO 4 with a spinel structure. The transformation of pyroxene (Mg, Fe)SiO 3 garnet Mg 3 (Fe, Al, Si) 2 Si 3 O 12 occurs in the upper mantle over a wider depth range. Thus, the entire relatively homogeneous shell in the interval of 400-600 km of the upper mantle mainly contains phases with garnet and spinel structural types.

All currently proposed models for the composition of mantle rocks admit that they contain Al 2 O 3 in an amount of ~4 wt. %, which also affects the specifics of structural transformations. At the same time, it is noted that in some areas of the compositionally heterogeneous upper mantle, Al can be concentrated in such minerals as corundum Al 2 O 3 or kyanite Al 2 SiO 5 , which, at pressures and temperatures corresponding to depths of ~450 km, transforms into corundum and stishovite is a modification of SiO 2 whose structure contains a framework of SiO 6 octahedra. Both of these minerals are preserved not only in the lower mantle, but also deeper.

The most important component of the chemical composition of the 400-670 km zone is water, the content of which, according to some estimates, is ~0.1 wt. % and the presence of which is primarily associated with Mg-silicates. The amount of water stored in this shell is so significant that on the surface of the Earth it would make up a layer with a thickness of 800 m.

Composition of the mantle below the boundary of 670 km

The studies of structural transitions of minerals carried out in the last two or three decades using high-pressure X-ray chambers made it possible to model some features of the composition and structure of the geospheres deeper than the 670 km boundary.

In these experiments, the crystal under study is placed between two diamond pyramids (anvils), which, when compressed, create pressures commensurate with the pressures inside the mantle and the Earth's core. Nevertheless, there are still many questions about this part of the mantle, which accounts for more than half of the entire interior of the Earth. Currently, most researchers agree with the idea that all this deep (lower in the traditional sense) mantle mainly consists of a perovskite-like phase (Mg,Fe)SiO 3 , which accounts for about 70% of its volume (40% of the volume of the entire Earth), and magnesiowiustite (Mg, Fe)O (~20%). The remaining 10% are stishovite and oxide phases containing Ca, Na, K, Al and Fe, the crystallization of which is allowed in the structural types of ilmenite-corundum (solid solution (Mg, Fe)SiO 3 -Al 2 O 3), cubic perovskite (CaSiO 3) and Ca-ferrite (NaAlSiO 4). The formation of these compounds is associated with various structural transformations of minerals in the upper mantle. At the same time, one of the main mineral phases of a relatively homogeneous shell lying in the depth interval of 410–670 km, spinel-like ringwoodite, transforms into an association of (Mg, Fe)-perovskite and Mg-wustite at the boundary of 670 km, where the pressure is ~24 GPa. Another important component of the transition zone, a representative of the garnet family, pyrope Mg 3 Al 2 Si 3 O 12, undergoes a transformation with the formation of rhombic perovskite (Mg, Fe) SiO 3 and a solid solution of corundum-ilmenite (Mg, Fe) SiO 3 - Al 2 O 3 at several high pressures. This transition is associated with a change in the velocities of seismic waves at the turn of 850-900 km, corresponding to one of the intermediate seismic boundaries. The transformation of andradite sagarnet at lower pressures of ~21 GPa leads to the formation of another important Ca 3 Fe 2 3+ Si 3 O 12 component mentioned above in the lower mantle, cubic Saperovskite CaSiO 3 . The polar ratio between the main minerals of this zone (Mg,Fe) - perovskite (Mg,Fe)SiO 3 and Mg-wustite (Mg, Fe)O varies over a fairly wide range and at a depth of ~1170 km at a pressure of ~29 GPa and temperatures of 2000 -2800 0 C changes from 2:1 to 3:1.

The exceptional stability of MgSiO 3 with a rhombic perovskite structure in a wide range of pressures corresponding to the depths of the lower mantle allows us to consider it one of the main components of this geosphere. The basis for this conclusion was the experiments, during which samples of Mg-perovskite MgSiO 3 were subjected to a pressure 1.3 million times higher than atmospheric pressure, and at the same time, a laser beam with a temperature of about 2000 0 C was exposed to a sample placed between diamond anvils. Thus, we simulated the conditions that exist at depths of ~2800 km, i.e., near the lower boundary of the lower mantle. It turned out that neither during nor after the experiment did the mineral change its structure and composition. Thus, L. Liu, as well as E. Nittle and E. Zhanloz came to the conclusion that the stability of Mg-perovskite allows us to consider it as the most common mineral on Earth, constituting, apparently, almost half of its mass.

Wustite F x O is no less stable, the composition of which under conditions of the lower mantle is characterized by the value of the stoichiometric coefficient x< 0,98, что означает одновременное присутствие в его составе Fe 2+ и Fe 3+ . При этом, согласно экспериментальным данным, температура плавления вюстита на границе нижней мантии и слоя D", по данным Р. Болера (1996), оценивается в ~5000 K, что намного выше 3800 0 С, предполагаемой для этого уровня (при средних температурах мантии ~2500 0 С в основании нижней мантии допускается повышение температуры приблизительно на 1300 0 С). Таким образом, вюстит должен сохраниться на этом рубеже в твердом состоянии, а признание фазового контраста между твердой нижней мантией и жидким внешним ядром требует более гибкого подхода и уж во всяком случае не означает четко очерченной границы между ними.

It should be noted that the perovskite-like phases prevailing at great depths can contain a very limited amount of Fe, and elevated concentrations of Fe among the minerals of the deep association are characteristic only of magnesiowustite. At the same time, for magnesiowiustite, the possibility of the transition under the influence of high pressures of a part of the ferrous iron contained in it into ferric iron, remaining in the structure of the mineral, with the simultaneous release of the corresponding amount of neutral iron, has been proved. Based on these data, H. Mao, P. Bell, and T. Yagi, employees of the geophysical laboratory of the Carnegie Institute, put forward new ideas about the differentiation of matter in the depths of the Earth. At the first stage, due to the gravitational instability, magnesiowustite sinks to a depth, where, under the influence of pressure, some of the iron in a neutral form is released from it. Residual magnesiowustite, characterized by a lower density, rises to the upper layers, where it mixes again with perovskite-like phases. Contact with them is accompanied by the restoration of the stoichiometry (that is, the integer ratio of the elements in the chemical formula) of magnesiowiustite and leads to the possibility of repeating the described process. The new data make it possible to somewhat expand the set of chemical elements probable for the deep mantle. For example, the stability of magnesite at pressures corresponding to depths of ~900 km, substantiated by N. Ross (1997), indicates the possible presence of carbon in its composition.

The identification of individual intermediate seismic boundaries located below the 670 line correlates with data on structural transformations of mantle minerals, the forms of which can be very diverse. An illustration of the change in many properties of various crystals at high values ​​of physicochemical parameters corresponding to the deep mantle can be, according to R. Jeanlose and R. Hazen, the rearrangement of the ion-covalent bonds of wuestite recorded during experiments at pressures of 70 gigapascals (GPa) (~1700 km). in connection with the metallic type of interatomic interactions. The 1200 milestone can correspond to the rearrangement of SiO 2 with the stishovite structure into the structural type CaCl 2 (rhombic analogue of rutile TiO 2), and 2000 km - its subsequent transformation into a phase with a structure intermediate between a-PbO 2 and ZrO 2 , characterized by a denser packing of silicon-oxygen octahedra (data from L.S. Dubrovinsky et al.). Also, starting from these depths (~2000 km), at pressures of 80–90 GPa, the decomposition of perovskite-like MgSiO 3 is allowed, accompanied by an increase in the content of periclase MgO and free silica. At a slightly higher pressure (~96 GPa) and a temperature of 800 0 С, a manifestation of polytypy in FeO was established, associated with the formation of structural fragments of the nickeline NiAs type, alternating with anti-nickel domains, in which Fe atoms are located in the positions of As atoms, and O atoms - in positions Ni atoms. Near the D" boundary, the transformation of Al 2 O 3 with the corundum structure into a phase with the Rh 2 O 3 structure occurs, which is experimentally modeled at pressures of ~100 GPa, i.e., at a depth of ~2200–2300 km. Using the Mössbauer spectroscopy method at the same pressure, the transition from the high-spin (HS) to the low-spin (LS) state of Fe atoms in the structure of magnesiowustite, that is, a change in their electronic structure.In this regard, it should be emphasized that the structure of wuestite FeO at high pressure is characterized by compositional nonstoichiometry, atomic packing defects, polytypia, and also a change in the magnetic ordering associated with a change in the electronic structure (HS => LS - transition) of Fe atoms. The noted features allow us to consider wustite as one of the most complex minerals with unusual properties that determine the specifics of the deep zones of the Earth enriched with it near the D boundary.

Seismological measurements indicate that both the inner (solid) and outer (liquid) cores of the Earth are characterized by a lower density compared to the value obtained on the basis of a core model consisting only of metallic iron with the same physicochemical parameters. Most researchers attribute this decrease in density to the presence in the core of elements such as Si, O, S, and even O, which form alloys with iron. Among the phases likely for such "Faustian" physicochemical conditions (pressures ~250 GPa and temperatures 4000-6500 0 C), Fe 3 S with a well-known structural type of Cu 3 Au and Fe 7 S are called. Another phase assumed in the core is b-Fe, whose structure is characterized by a four-layer close packing of Fe atoms. The melting temperature of this phase is estimated at 5000 0 C at a pressure of 360 GPa. The presence of hydrogen in the core has long been controversial due to its low solubility in iron at atmospheric pressure. However, recent experiments (data by J. Badding, H. Mao and R. Hamley (1992)) made it possible to establish that iron hydride FeH can form at high temperatures and pressures and is stable at pressures exceeding 62 GPa, which corresponds to depths of ~1600 km . In this regard, the presence of significant amounts (up to 40 mol.%) of hydrogen in the core is quite acceptable and reduces its density to values ​​consistent with seismological data.

It can be predicted that new data on structural changes in mineral phases at great depths will make it possible to find an adequate interpretation of other important geophysical boundaries fixed in the bowels of the Earth. The general conclusion is that at such global seismic boundaries as 410 and 670 km, there are significant changes in the mineral composition of mantle rocks. Mineral transformations are also noted at depths of ~850, 1200, 1700, 2000 and 2200-2300 km, that is, within the lower mantle. This is a very important circumstance that makes it possible to abandon the idea of ​​its homogeneous structure.

What is the Earth's mantle made of?

For a long time, olivine was considered the main material of the mantle - a well-known yellowish-green, olive, and even brown mineral, which is part of almost all of the heaviest rocks of the Earth that have ever erupted from the bowels of the earth with molten magma. Olivine is also mainly composed of stone meteorites that come to us on Earth from outer space.

Some scientists believe that these are the remains of the building material from which the planets including our earth. If it were so... How many problems and mysteries would be solved... But so far, only by indirect evidence can one discuss the possible composition and structure of the mantle matter.

In 1936, the famous English physicist and prominent public figure John Bernall suggested that in the depths of the earth's interior in conditions high temperatures and pressures, olivine crystals are compressed, the atoms are repacked, and crystals of another, larger density.

A similar idea was expressed at the same time by Vladimir (Vartan) Nikitovich Lodochnikov. He believed that all the physical properties of matter located in the depths of the Earth must change.

Scientists began to test olivine in laboratories. Cubes of the yellow-green mineral were squeezed and heated, heated again and squeezed again. Olivine under pressure was very suitable in its seismic characteristics to the substance of the mantle, but ... At pressures corresponding to a depth of about 400 kilometers, it collapsed. This means that only the upper and partially middle mantle could consist of it. And what is included in the composition of the bottom? ..

The Earth's mantle is the part of the geosphere located between the crust and the core. It contains a large proportion of the entire substance of the planet. The study of the mantle is important not only from the point of view of understanding the inner mantle. It can shed light on the formation of the planet, give access to rare compounds and rocks, help understand the mechanism of earthquakes, etc. However, obtaining information about the composition and features of the mantle is not easy. People do not yet know how to drill wells so deep. The Earth's mantle is now mainly studied using seismic waves. And also by modeling in the laboratory.

Structure of the Earth: mantle, core and crust

According to modern concepts, the internal structure of our planet is divided into several layers. The top layer is the crust, followed by the mantle and core of the Earth. The crust is a hard shell divided into oceanic and continental. The Earth's mantle is separated from it by the so-called Mohorovicic boundary (named after the Croatian seismologist who established its location), which is characterized by an abrupt increase in the velocities of longitudinal seismic waves.

The mantle makes up about 67% of the planet's mass. According to modern data, it can be divided into two layers: upper and lower. In the first, the Golitsyn layer or the middle mantle is also distinguished, which is a transition zone from the upper to the lower. In general, the mantle extends at a depth of 30 to 2900 km.

The core of the planet, according to modern scientists, consists mainly of iron-nickel alloys. It is also divided into two parts. The inner core is solid, its radius is estimated at 1300 km. External - liquid, has a radius of 2200 km. Between these parts, a transition zone is distinguished.

Lithosphere

The crust and upper mantle of the Earth are united by the concept of "lithosphere". It is a hard shell with stable and mobile areas. The solid shell of the planet consists of which, as expected, move through the asthenosphere - a rather plastic layer, probably a viscous and highly heated liquid. It is part of the upper mantle. It should be noted that the existence of the asthenosphere as a continuous viscous shell is not confirmed by seismological studies. The study of the structure of the planet allows us to identify several similar layers located vertically. In the horizontal direction, the asthenosphere, apparently, is constantly interrupted.

Ways to study the mantle

The layers lying below the crust are inaccessible for study. The enormous depth, the constant increase in temperature and the increase in density are a serious problem for obtaining information about the composition of the mantle and core. However, it is still possible to imagine the structure of the planet. When studying the mantle, geophysical data become the main sources of information. The speed of seismic waves, the features of electrical conductivity and gravity allow scientists to make assumptions about the composition and other features of the underlying layers.

In addition, some information can be obtained from fragments of mantle rocks. The latter include diamonds, which can tell a lot even about the lower mantle. Mantle rocks are also found in the earth's crust. Their study helps to understand the composition of the mantle. However, they will not replace samples obtained directly from deep layers, since as a result of various processes occurring in the crust, their composition differs from that of the mantle.

Earth's mantle: composition

Another source of information about what the mantle is like is meteorites. According to modern concepts, chondrites (the most common group of meteorites on the planet) are close in composition to the earth's mantle.

It is assumed that it contains elements that were in a solid state or entered into a solid compound during the formation of the planet. These include silicon, iron, magnesium, oxygen and some others. In the mantle, they combine with form silicates. Magnesium silicates are located in the upper layer, the amount of iron silicate increases with depth. In the lower mantle, these compounds decompose into oxides (SiO 2 , MgO, FeO).

Of particular interest to scientists are rocks that are not found in the earth's crust. It is assumed that there are many such compounds (grospidites, carbonatites, and so on) in the mantle.

Layers

Let us dwell in more detail on the extent of the layers of the mantle. According to scientists, the upper of them occupies a range of about 30 to 400 km from there. Then there is a transition zone, which goes deeper into another 250 km. The next layer is the bottom. Its boundary is located at a depth of about 2900 km and is in contact with the outer core of the planet.

pressure and temperature

As you move deeper into the planet, the temperature rises. The Earth's mantle is under extremely high pressure. In the asthenosphere zone, the effect of temperature outweighs, so here the substance is in the so-called amorphous or semi-molten state. Deeper under pressure, it becomes solid.

Studies of the mantle and the Mohorovicic boundary

The Earth's mantle haunts scientists for quite a long time. In laboratories, experiments are being carried out on rocks that are presumably part of the upper and lower layers, allowing us to understand the composition and features of the mantle. Thus, Japanese scientists found that the lower layer contains a large amount of silicon. The upper mantle contains water reserves. It comes from the earth's crust, and also penetrates from here to the surface.

Of particular interest is the Mohorovichic surface, the nature of which is not fully understood. Seismological studies suggest that at a level of 410 km below the surface, a metamorphic change of rocks occurs (they become denser), which manifests itself in a sharp increase in the speed of waves. It is assumed that the basalt rocks in the area are transformed into eclogite. In this case, the density of the mantle increases by about 30%. There is another version, according to which, the reason for the change in the speed of seismic waves lies in the change in the composition of the rocks.

Chikyu Hakken

In 2005, a specially equipped ship Chikyu was built in Japan. His mission is to make a record deep well at the bottom of the Pacific Ocean. Scientists propose to take samples of the rocks of the upper mantle and the Mohorovichic boundary in order to get answers to many questions related to the structure of the planet. The implementation of the project is scheduled for 2020.

It should be noted that scientists have not just turned their attention to the oceanic bowels. According to studies, the thickness of the crust at the bottom of the seas is much less than on the continents. The difference is significant: under the water column in the ocean, it is necessary to overcome only 5 km to magma in some areas, while on land this figure increases to 30 km.

Now the ship is already working: samples of deep coal seams have been obtained. The implementation of the main goal of the project will make it possible to understand how the Earth's mantle is arranged, what substances and elements make up its transition zone, and also to find out the lower limit of the spread of life on the planet.

Our understanding of the structure of the Earth is still far from complete. The reason for this is the difficulty of penetrating into the bowels. However, technological progress does not stand still. Advances in science suggest that in the near future we will know much more about the characteristics of the mantle.

Scientists have no doubt that our planet consists of at least three structures: the outer shell is the crust, the inner core is the core, and between them lies a layer of terrestrial rocks - the mantle.


It is noticeably thicker than the crust and occupies more than 80% of the entire volume of the globe. The mantle begins at a depth of about 30-50 km (under the oceans) and much lower - under the continents. At a depth of about 30,000 km, it borders on the core.

How do they study the structure of the Earth at such great depths?

Of course, the subsoil is not the abyss of the ocean or space. No expeditions or robots can be sent inside the planet. However, methods have been developed that allow you to "look" there. There are several ways to do this.

1. Geophysical research. For example, register the propagation of waves from earthquakes. While these waves get, for example, from Japan to Germany, they change their direction and speed more than once. According to the layers in which they move more slowly, in which - faster, one can judge the structure of these layers, their composition.

2. Geological collections. Experts are often able to distinguish between "pebbles" by the place of their birth. So, recently it was possible to decipher the biography of six diamonds by impurities. Once upon a time, tiny pieces of carbon descended from the crust into the mantle and “drowned” in it. Monstrous pressure turned them into, and the updraft carried them into the crust. They ended up in volcanic rock, which, after 200 million years, people lifted from a Brazilian mine.

3. Experiments. Roughly imagining the conditions in the bowels of the Earth, you can reproduce them in laboratories and look at the results.

4. Drilling of superdeep wells. True, so far the deepest of them, on the Kola Peninsula, has reached only 12,262 meters. It is possible to get to the mantle by drilling the ocean floor - here the crust is much thinner. This may be within the power of drilling ships already created specifically for such work.

What is the mantle made of? What are the processes in it?

The mantle can be judged by its fragments, which were brought to the surface of the land or valleys of the ocean floor billions of years ago. It is assumed that the mantle is greenish-black and consists of rocks containing silicon, magnesium, calcium, iron, oxygen. In composition, it is similar to. Once upon a time, before the formation of the crust, this was the entire surface of the Earth.

Now the decay of radioactive substances heats the core, and it transfers its heat to the mantle. The temperature of its lowest layer is measured in thousands of degrees. Therefore, its rocks are softened, colossal pressure makes them fluid. Outside, the temperature of the mantle gradually drops. The cooled outer masses descend, the warmed inner masses rise. Due to the high viscosity, the speed of movement is low - up to several tens of centimeters per year. But this cycle never stops. From time to time, flows of mantle matter penetrate the crust; volcanoes help these movements.

Why is it important to study the Earth's mantle?

The mantle is far from us (more precisely, deep), but, of course, affects the lives of people and all the nature around us. Movements in the mantle cause the huge slabs of crust standing on it, which carry the continents, to move. The result is known - earthquakes, volcanic eruptions and mass extinctions of organisms, the birth and death of islands, the movement of continents. By understanding the processes in the mantle, we will get a chance to foresee global catastrophes.

Thermal movements in the mantle affect the appearance of underground heat zones. Imagining its "behavior", it will be easier to find such areas for the construction of geothermal power plants, hot groundwater, metal ores. Yes, and other minerals too.


For example, it was believed that the combustible gas methane is formed from decaying organic matter due to bacteria. But not so long ago, a group of physicists proved that it is otherwise. The scientists mixed water, iron oxide and the mineral calcite. The mixture was heated to 1000 ° C under a pressure of 110 thousand atmospheres and methane was obtained! These meant that he could also appear in the depths of the mantle. It is possible that from there it rises into the thickness of the crust. So here you need to look for its accumulations and extract it.