Our planet Earth has a layered structure and consists of three main parts: the earth's crust, mantle and core. What is the center of the earth? Core. The depth of the core is 2900 km, and the diameter is approximately 3.5 thousand km. Inside - a monstrous pressure of 3 million atmospheres and an incredibly high temperature - 5000 ° C. In order to find out what is in the center of the Earth, it took scientists several centuries. Even modern technology could not penetrate deeper than twelve thousand kilometers. The deepest borehole, located on the Kola Peninsula, has a depth of 12,262 meters. Far from the center of the earth.
The history of the discovery of the earth's core
One of the first to guess about the presence of a nucleus in the center of the planet was the English physicist and chemist Henry Cavendish at the end of the 18th century. With the help of physical experiments, he calculated the mass of the Earth and, based on its size, determined the average density of the substance of our planet - 5.5 g / cm3. The density of known rocks and minerals in the earth's crust turned out to be approximately two times less. From this followed a logical assumption that in the center of the Earth there is an area of denser matter - the core.
In 1897, the German seismologist E. Wiechert, studying the passage of seismological waves through the inner parts of the Earth, was able to confirm the assumption of the presence of a core. And in 1910, the American geophysicist B. Gutenberg determined the depth of its location. Subsequently, hypotheses about the process of formation of the nucleus were also born. It is assumed that it was formed as a result of the settling of heavier elements to the center, and initially the substance of the planet was homogeneous (gaseous).
What is the core made of?
It is quite difficult to study a substance whose sample cannot be obtained in order to study its physical and chemical parameters. Scientists have only to assume the presence of certain properties, as well as the structure and composition of the nucleus by indirect signs. Especially helpful in the study of the internal structure of the Earth was the study of the propagation of seismic waves. Seismographs, located at many points on the surface of the planet, record the speed and types of passing seismic waves arising from tremors of the earth's crust. All these data make it possible to judge the internal structure of the Earth, including the core.
To date, scientists suggest that the central part of the planet is heterogeneous. What is at the center of the earth? The part adjacent to the mantle is a liquid core, consisting of molten matter. Apparently, it contains a mixture of iron and nickel. This idea led scientists to the study of iron meteorites, which are pieces of asteroid nuclei. On the other hand, the obtained iron-nickel alloys have a higher density than the expected density of the core. Therefore, many scientists tend to assume that in the center of the Earth, the core, there are also lighter chemical elements.
Geophysicists also explain the existence of a magnetic field by the presence of a liquid core and the rotation of the planet around its own axis. It is known that an electromagnetic field around a conductor arises when current flows. The molten layer adjacent to the mantle serves as such a giant current-carrying conductor.
The inner part of the nucleus, despite the temperature of several thousand degrees, is a solid. This is due to the fact that the pressure in the center of the planet is so high that hot metals become solid. Some scientists suggest that the solid core consists of hydrogen, which, under the influence of incredible pressure and enormous temperature, becomes like a metal. Thus, what is the center of the Earth, even geophysicists are still not known for certain. But if we consider the issue from a mathematical point of view, we can say that the center of the Earth is located approximately 6378 km. from the surface of the planet.
By tightly compressing both substances with diamonds, the scientists were able to push the molten iron through the silicate. “This pressure significantly changes the interaction properties of iron with silicates,” says Mao. - At high pressure, a "melting network" is formed.
This may suggest that iron gradually slipped through the Earth's rocks over millions of years until it reached the core.
At this point, you may be asking: how do we actually know the size of the kernel? Why do scientists believe that it starts 3,000 kilometers away? There is only one answer: seismology.
When an earthquake occurs, it sends shockwaves all over the planet. Seismologists record these vibrations. As if we are hitting one side of the planet with a giant hammer and listening to the noise on the other side.
“There was an earthquake in Chile in the 1960s that gave us a huge amount of data,” says Redfern. “All seismic stations around the Earth recorded the shocks of this earthquake.”
Depending on the route of these oscillations, they pass through different parts of the Earth, and this affects what kind of "sound" they make at the other end.
Early in the history of seismology, it became apparent that some vibrations were missing. These "S-waves" were expected to be seen at the other end of the Earth after originating at one, but they were not. The reason for this is simple. S-waves reverberate through solid material and cannot travel through liquid.
They must have hit something molten at the center of the earth. Having mapped the paths of S-waves, scientists came to the conclusion that at a depth of about 3,000 kilometers, the rocks become liquid. This also suggests that the entire core is molten. But seismologists had another surprise in this story. 
In the 1930s, Danish seismologist Inge Lehman discovered that another type of wave, P-waves, had unexpectedly traveled through the core and been found on the other side of the planet. The assumption immediately followed that the core was divided into two layers. The "inner" core, which starts 5,000 kilometers below, was solid. Only the "outer" core is melted.
Lehman's idea was confirmed in 1970 when more sensitive seismographs showed that P-waves do indeed pass through the core and, in some cases, bounce off it at certain angles. No wonder they end up on the other side of the planet.
Shock waves send more than just earthquakes across the Earth. In fact, seismologists owe a lot to the development of nuclear weapons.
A nuclear explosion also creates waves on the ground, so states turn to seismologists for help during nuclear weapons testing. During the Cold War, this was extremely important, so seismologists like Lehman received a lot of support.
Competing countries were learning about each other's nuclear capabilities, and in parallel, we were learning more and more about the Earth's core. Seismology is still used to detect nuclear explosions today. 
Now we can draw a rough picture of the structure of the Earth. There is a molten outer core that starts about halfway to the center of the planet, and inside it is a solid inner core with a diameter of about 1220 kilometers.
There are no fewer questions from this, especially on the topic of the inner core. For example, how hot is it? Figuring it out hasn't been easy, and scientists have been racking their brains for a long time, says Lidunka Vokadlo of University College London in the UK. We can't stick a thermometer in there, so the only option is to create the right pressure in the lab. 
Under normal conditions, iron melts at a temperature of 1538 degrees
In 2013, a group of French scientists produced the best estimate to date. They subjected pure iron to a pressure of half what is in the core, and started from this. The melting point of pure iron in the core is approximately 6230 degrees. The presence of other materials may lower the melting point slightly, up to 6000 degrees. But it's still hotter than on the surface of the Sun.
Being a kind of fried potato in their skins, the core of the Earth remains hot, thanks to the heat left over from the formation of the planet. It also extracts heat from the friction generated by the movement of dense materials, as well as the decay of radioactive elements. It cools down by about 100 degrees Celsius every billion years.
It is useful to know this temperature because it affects the rate at which vibrations travel through the nucleus. And this is convenient, because there is something strange in these vibrations. P-waves travel surprisingly slowly through the inner core - slower than if it were made of pure iron.
“The wave velocities that seismologists have measured in earthquakes are much lower than experimental or computer simulations indicate,” Vocadlo says. “No one knows yet why that is.”
Obviously, another material is mixed with iron. Possibly nickel. But the scientists calculated how the seismic waves should travel through the iron-nickel alloy, and were unable to fit the calculations to the observations.
Vocadlo and her colleagues are currently considering the presence of other elements in the core, such as sulfur and silicon. So far, no one has been able to come up with a theory of the composition of the inner core that would satisfy everyone. Cinderella's Problem: The shoe doesn't fit anyone. Vocadlo tries to experiment with the materials of the inner core on the computer. She hopes to find a combination of materials, temperatures and pressures that will slow down seismic waves by just the right amount. 
She says the secret may lie in the fact that the inner core is almost at its melting point. As a result, the exact properties of the material may differ from those of a perfectly solid substance. It could also explain why seismic waves travel slower than expected.
“If this effect is real, we could reconcile the results of mineral physics with the results of seismology,” Vocadlo says. "People can't do it yet."
There are still many mysteries related to the Earth's core that have yet to be solved. But unable to dive to these unimaginable depths, scientists accomplish the feat of finding out what is thousands of kilometers below us. The hidden processes of the Earth's interior are extremely important to study. The Earth has a powerful magnetic field, which is generated due to the partially molten core. The constant movement of the molten core generates an electric current inside the planet, and it, in turn, generates a magnetic field that reaches far into space.
This magnetic field protects us from harmful solar radiation. If the core of the Earth were not the way it is, there would be no magnetic field, and we would seriously suffer from this. It is unlikely that any of us will be able to see the core with our own eyes, but it is good just to know that it is there. 
It has a special composition, differing from the composition of the earth's crust covering it. Data on the chemical composition of the mantle were obtained from analyzes of the deepest igneous rocks that entered the upper horizons of the Earth as a result of powerful tectonic uplifts with the removal of mantle material. These rocks include ultrabasic rocks - dunites, peridotites occurring in mountain systems. The rocks of the St. Paul Islands in the middle part of the Atlantic Ocean, according to all geological data, belong to the mantle material. The mantle material also includes rock fragments collected by Soviet oceanographic expeditions from the bottom of the Indian Ocean in the area of the Indian Ocean Ridge. As regards the mineralogical composition of the mantle, significant changes can be expected here, starting from the upper horizons and ending with the base of the mantle, due to an increase in pressure. The upper mantle is composed mainly of silicates (olivines, pyroxenes, garnets), which are stable and within relatively low pressures. The lower mantle is composed of high-density minerals.
The most common component of the mantle is silicon oxide in the composition of silicates. But at high pressures, silica can go into a denser polymorphic modification - stishovite. This mineral was obtained by the Soviet researcher Stishov and named after him. If ordinary quartz has a density of 2.533 r/cm 3 , then stishovite, formed from quartz at a pressure of 150,000 bar, has a density of 4.25 g/cm 3 .
In addition, denser mineral modifications of other compounds are also probable in the lower mantle. Based on the foregoing, it can be reasonably assumed that with increasing pressure, the usual iron-magnesian silicates of olivines and pyroxenes decompose into oxides, which individually have a higher density than silicates, which turn out to be stable in the upper mantle.
The upper mantle consists mainly of ferruginous-magnesian silicates (olivines, pyroxenes). Some aluminosilicates can transform here into denser minerals such as garnets. Beneath the continents and oceans, the upper mantle has different properties and probably a different composition. One can only assume that in the area of continents the mantle is more differentiated and has less SiO 2 due to the concentration of this component in the aluminosilicate crust. Beneath the oceans, the mantle is less differentiated. In the upper mantle, denser polymorphic modifications of olivine with a spinel structure, etc., can occur.
The transitional layer of the mantle is characterized by a constant increase in seismic wave velocities with depth, which indicates the appearance of denser polymorphic modifications of matter. Here, obviously, FeO, MgO, GaO, SiO 2 oxides appear in the form of wustite, periclase, lime, and stishovite. Their number increases with depth, while the amount of ordinary silicates decreases, and below 1000 km they make up an insignificant fraction.
The lower mantle within the depths of 1000-2900 km almost completely consists of dense varieties of minerals - oxides, as evidenced by its high density in the range of 4.08-5.7 g/cm 3 . Under the influence of increased pressure, dense oxides are compressed, further increasing their density. The content of iron also probably increases in the lower mantle.
Earth's core. The question of the composition and physical nature of the core of our planet is one of the most exciting and mysterious problems of geophysics and geochemistry. Only recently there has been a little enlightenment in solving this problem.
The vast central core of the Earth, which occupies the inner region deeper than 2900 km, consists of a large outer core and a small inner one. According to seismic data, the outer core has the properties of a liquid. It does not transmit transverse seismic waves. The absence of cohesive forces between the core and the lower mantle, the nature of the tides in the mantle and crust, the peculiarities of the movement of the Earth's rotation axis in space, the nature of the passage of seismic waves deeper than 2900 km indicate that the outer core of the Earth is liquid.
Some authors assumed that the composition of the core for a chemically homogeneous model of the Earth was silicate, and under the influence of high pressure, the silicates passed into a “metallized” state, acquiring an atomic structure in which the outer electrons are common. However, the geophysical data listed above contradict the assumption of a "metallized" state of the silicate material in the Earth's core. In particular, the absence of cohesion between the core and the mantle cannot be compatible with a "metallized" solid core, which was assumed in the Lodochnikov-Ramsay hypothesis. Very important indirect data on the core of the Earth were obtained during experiments with silicates under high pressure. In this case, the pressure reached 5 million atm. Meanwhile, in the center of the Earth, the pressure is 3 million atm., and at the boundary of the core - approximately 1 million atm. Thus, experimentally, it was possible to block the pressures that exist in the very depths of the Earth. In this case, for silicates, only linear compression was observed without a jump and transition to a “metallized” state. In addition, at high pressures and within the depths of 2900-6370 km, silicates cannot be in a liquid state, like oxides. Their melting point increases with increasing pressure.
Very interesting results have been obtained in recent years on the effect of very high pressures on the melting point of metals. It turned out that a number of metals at high pressures (300,000 atm. and above) go into a liquid state at relatively low temperatures. According to some calculations, an alloy of iron with an admixture of nickel and silicon (76% Fe, 10% Ni, 14% Si) at a depth of 2900 km under the influence of high pressure should be in a liquid state already at a temperature of 1000 ° C. But the temperature at these depths, according to the most conservative estimates of geophysicists, it should be much higher.
Therefore, in the light of modern geophysics and high-pressure physics data, as well as cosmochemistry data indicating the leading role of iron as the most abundant metal in space, it should be assumed that the Earth's core is mainly composed of liquid iron with an admixture of nickel. However, the calculations of the American geophysicist F. Birch showed that the density of the earth's core is 10% lower than that of an iron-nickel alloy at temperatures and pressures prevailing in the core. It follows that the metallic core of the Earth must contain a significant amount (10-20%) of some kind of lung. Of all the lightest and most common elements, silicon (Si) and sulfur (S) are the most probable | The presence of one or the other can explain the observed physical properties of the earth's core. Therefore, the question of what is an admixture of the earth's core - silicon or sulfur, turns out to be debatable and is connected with the way our planet is formed in practice.
A. Ridgwood in 1958 assumed that the earth's core contains silicon as a light element, arguing this assumption by the fact that elemental silicon in an amount of several weight percent is found in the metal phase of some reduced chondrite meteorites (enstatite). However, there are no other arguments in favor of the presence of silicon in the earth's core.
The assumption that there is sulfur in the Earth's core follows from a comparison of its distribution in the chondrite material of meteorites and the Earth's mantle. Thus, a comparison of the elementary atomic ratios of some volatile elements in a mixture of the crust and mantle and in chondrites shows a sharp lack of sulfur. In the material of the mantle and crust, the concentration of sulfur is three orders of magnitude lower than in the average material of the solar system, which is taken as chondrites.
The possibility of loss of sulfur at the high temperatures of the primitive Earth is eliminated, since other more volatile elements than sulfur (for example, H2 in the form of H2O), found to be much less deficient, would be lost to a much greater extent. In addition, when solar gas cools, sulfur chemically bonds with iron and ceases to be a volatile element.
In this regard, it is quite possible that large amounts of sulfur enter the earth's core. It should be noted that, other things being equal, the melting point of the Fe-FeS system is much lower than the melting point of iron or mantle silicate. So, at a pressure of 60 kbar, the melting point of the system (eutectic) Fe-FeS will be 990 ° C, while pure iron - 1610 °, and mantle pyrolite - 1310. Therefore, with an increase in temperature in the bowels of the initially homogeneous Earth, an iron melt enriched with sulfur , will form first and, due to its low viscosity and high density, will easily drain into the central parts of the planet, forming a ferruginous-sulphurous core. Thus, the presence of sulfur in the nickel-iron environment acts as a flux, lowering its melting point as a whole. The hypothesis of the presence of significant amounts of sulfur in the earth's core is very attractive and does not contradict all the known data of geochemistry and cosmochemistry.
Thus, modern ideas about the nature of the interior of our planet correspond to a chemically differentiated globe, which turned out to be divided into two different parts: a powerful solid silicate-oxide mantle and a liquid, mostly metallic core. The earth's crust is the lightest upper solid shell, consisting of aluminosilicates and having the most complex structure.
Summarizing the above, we can draw the following conclusions.
- The earth has a layered zonal structure. It consists of two-thirds of a solid silicate-oxide shell - the mantle and one-third of a metallic liquid core.
- The main properties of the Earth indicate that the core is in a liquid state and only iron from the most common metals with an admixture of some light elements (most likely sulfur) is able to provide these properties.
- In its upper horizons, the Earth has an asymmetric structure, covering the crust and upper mantle. The oceanic hemisphere within the upper mantle is less differentiated than the opposite continental hemisphere.
The task of any cosmogonic theory of the origin of the Earth is to explain these basic features of its internal nature and composition.
The Earth's core includes two layers with a boundary zone between them: the outer liquid shell of the core reaches a thickness of 2266 kilometers, under it there is a massive dense core, the diameter of which, according to estimates, reaches 1300 km. The transition zone has a non-uniform thickness and gradually hardens, passing into the inner core. On the surface of the upper layer, the temperature is in the region of 5960 degrees Celsius, although these data are considered approximate.
Approximate composition of the outer core and methods for its determination
Very little is known about the composition of even the outer layer of the earth's core, since it is not possible to obtain samples for study. The main elements of which the outer core of our planet can consist are iron and nickel. Scientists came to this hypothesis as a result of analyzing the composition of meteorites, since wanderers from outer space are fragments of the nuclei of asteroids and other planets.
Nevertheless, meteorites cannot be considered absolutely identical in chemical composition, since the original cosmic bodies were much smaller than the Earth in size. After much research, scientists came to the conclusion that the liquid part of the nuclear substance is highly diluted with other elements, including sulfur. This explains its lower density than iron-nickel alloys.
What happens in the outer part of the planet's core?
The outer surface of the core at the boundary with the mantle is inhomogeneous. Scientists suggest that it has a different thickness, forming a kind of internal relief. This is due to the constant mixing of heterogeneous deep substances. They are different in chemical composition and also have different densities, so the thickness of the boundary between the core and the mantle can vary from 150 to 350 km.
Fantasists of the past years in their works described a journey to the center of the Earth through deep caves and underground passages. Is it really possible? Alas, the pressure on the surface of the core exceeds 113 million atmospheres. This means that any cave would tightly “slam” even at the stage of approaching the mantle. This explains why there are no caves deeper than even 1 km on our planet.
How is the outer layer of the nucleus studied?
Scientists can judge what the core looks like and what it consists of by monitoring seismic activity. So, for example, it was found that the outer and inner layers rotate in different directions under the influence of a magnetic field. The core of the Earth still holds dozens of unsolved mysteries and is waiting for new fundamental discoveries.
Why does the Earth's core not cool down and remains heated to a temperature of approximately 6000°C for 4.5 billion years? The question is extremely complex, to which, moreover, science cannot give a 100% accurate intelligible answer. However, there are objective reasons for this.
Too much mystery
Excessive, so to speak, the mystery of the earth's core is associated with two factors. Firstly, no one knows for sure how, when and under what circumstances it was formed - it happened during the formation of the proto-Earth or already in the early stages of the existence of the formed planet - all this is a big mystery. Secondly, it is absolutely impossible to get samples from the earth's core - for sure no one knows what it consists of. Moreover, all the data that we know about the nucleus is collected by indirect methods and models.
Why does the Earth's core stay hot?
To try to understand why the earth's core does not cool down for such a long time, you first need to figure out what caused it to warm up in the first place. The bowels of ours, like any other planet, are heterogeneous, they are relatively clearly demarcated layers of different densities. But this was not always the case: the heavy elements slowly descended, forming the inner and outer core, while the light ones were forced out to the top, forming the mantle and the earth's crust. This process proceeds extremely slowly and is accompanied by the release of heat. However, this was not the main reason for the heating. The entire mass of the Earth with great force presses on its center, producing a phenomenal pressure of approximately 360 GPa (3.7 million atmospheres), as a result of which the decay of radioactive long-lived elements contained in the iron-silicon-nickel core began to occur, which was accompanied by colossal heat emissions .
An additional source of heating is the kinetic energy generated as a result of friction between different layers (each layer rotates independently of the other): the inner core with the outer and the outer with the mantle.
The bowels of the planet (the proportions are not met). Friction between the three inner layers serves as an additional source of heating.
Based on the above, we can conclude that the Earth and, in particular, its bowels are a self-sufficient machine that heats itself. But it cannot continue so naturally forever: the stocks of radioactive elements inside the core are slowly disappearing and there will be nothing else to maintain the temperature.
It's getting cold!
In fact, the cooling process has already begun a very long time ago, but it proceeds extremely slowly - by a fraction of a degree per century. According to rough estimates, it will take at least 1 billion years for the core to cool completely and stop chemical and other reactions in it.
Short answer: The earth, and in particular the earth's core, is a self-sufficient machine that heats itself. The entire mass of the planet presses on its center, producing phenomenal pressure and thereby starting the process of decay of radioactive elements, as a result of which heat is released.
