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.
In what time immemorial did this happen? All these questions have long troubled mankind. And many scientists wanted to quickly find out what is there, in the depths? But it turned out that to study all this is not so easy. After all, even today, having all the modern devices for conducting all kinds of research, humanity is able to drill wells into the bowels of only some fifteen kilometers - no more. And for full-fledged and comprehensive experiments, the required depth should be an order of magnitude greater. Therefore, scientists have to calculate how the Earth's core was formed using a variety of high-precision instruments.
Exploring the Earth
Since ancient times, people have studied naturally exposed rocks. Cliffs and slopes of mountains, steep banks of rivers and seas... Here you can see with your own eyes what existed probably millions of years ago. Wells are being drilled in some suitable places. One of these - at its depth - fifteen thousand meters. The mines that people break through for also help to study the inner Core, of course, they cannot “get it”. But on the other hand, from these mines and wells, scientists can extract rock samples, learning in this way about their change and origin, structure and composition. The disadvantage of these methods is that they are able to explore only the land and only the upper part of the Earth's crust.
Recreating Conditions at the Earth's Core
But geophysics and seismology, the sciences of earthquakes and the geological composition of the planet, help scientists to penetrate deeper and deeper without contact. By studying seismic waves and their propagation, it turns out what both the mantle and the core consist of (it is determined similarly, for example, with the composition of fallen meteorites). Such knowledge is based on the obtained data - indirect - about the physical properties of substances. Also today, the study is facilitated by modern data obtained from artificial satellites in orbit.
The structure of the planet
Scientists managed to understand, summarizing the data obtained, that the structure of the Earth is complex. It consists of at least three unequal parts. In the center is a small core, which is surrounded by a huge mantle. The mantle occupies about five-sixths of the entire volume of the globe. And from above everything is covered by a rather thin outer crust of the Earth.
The structure of the nucleus
The core is the central, middle part. It is divided into several layers: internal and external. According to most modern scientists, the inner core is solid, and the outer one is liquid (it is in a molten state). And the core is very heavy: it weighs more than a third of the mass of the entire planet with a volume of just over 15. In the core, the temperature is quite high, it ranges from 2000 to 6000 degrees Celsius. According to the assumptions of science, the center of the Earth consists mainly of iron and nickel. The radius of this heavy segment is 3470 kilometers. And its surface area is about 150 million square kilometers, which is approximately equal to the area of all the continents on the surface of the Earth.
How was the Earth's core formed?
There is very little information about the core of our planet, and it can only be obtained indirectly (there are no core rock samples). Therefore, theories can be expressed only hypothetically about how the core of the Earth was formed. The history of the Earth is billions of years old. Most scientists adhere to the theory that in the beginning the planet formed as a fairly homogeneous one. The process of isolating the nucleus began later. And its composition is nickel and iron. How was the Earth's core formed? The melt of these metals gradually descended to the center of the planet, forming the core. This was due to the higher specific gravity of the melt.
Alternative theories
There are also opponents of this theory, who bring their own, quite reasonable arguments. First, these scientists question the passage of an alloy of iron and nickel to the center of the nucleus (and this is more than 100 kilometers). Secondly, if we assume the release of nickel and iron from silicates similar to meteoric ones, then the corresponding reduction reaction should have occurred. It, in turn, had to be accompanied by the release of a huge amount of oxygen, forming an atmospheric pressure of several hundred thousand atmospheres. And there is no evidence of the existence of such an atmosphere in the past of the Earth. Therefore, theories were put forward about the initial formation of the core during the formation of the entire planet.
In 2015, Oxford scientists even proposed a theory according to which the core of the planet Earth consists of uranium and has radioactivity. This indirectly proves both such a long existence of the magnetic field near the Earth, and the fact that at the present time our planet radiates much more heat than was assumed by previous scientific hypotheses.
The Earth, along with other bodies of the solar system, was formed from a cold gas and dust cloud by accretion of the particles that made it up. After the appearance of the planet, a completely new stage of its development began, which in science is usually called pregeological.
The name of the period is due to the fact that the earliest evidence of past processes - igneous or volcanic rocks - is not older than 4 billion years. Only scientists today can study them.
The pre-geological stage of the development of the Earth is still fraught with many mysteries. It covers a period of 0.9 billion years and is characterized by a wide manifestation of volcanism on the planet with the release of gases and water vapor. It was at this time that the process of stratification of the Earth into the main shells began - the core, mantle, crust and atmosphere. It is assumed that this process was provoked by an intense meteorite bombardment of our planet and the melting of its individual parts.
One of the key events in the history of the Earth was the formation of its inner core. This probably happened at the pregeological stage of the planet's development, when all matter was divided into two main geospheres - the core and the mantle.
Unfortunately, a reliable theory about the formation of the earth's core, which would be confirmed by serious scientific information and evidence, does not yet exist. How did the core of the Earth form? To this question, scientists offer two main hypotheses.
According to the first version, the substance immediately after the formation of the Earth was homogeneous.
It consisted entirely of microparticles, which can be observed today in meteorites. But after a certain period of time, this initially homogeneous mass was divided into a heavy core, where all the iron glassed, and a lighter silicate mantle. In other words, drops of molten iron and the heavy chemical compounds that accompanied it settled to the center of our planet and formed a core there, which remains largely molten to this day. As heavy elements aspired to the center of the Earth, light slags, on the contrary, floated up - to the outer layers of the planet. Today, these light elements make up the upper mantle and the earth's crust.
Why did such a differentiation of matter occur? It is believed that immediately after the completion of the process of its formation, the Earth began to heat up intensively, primarily due to the energy released in the process of gravitational accumulation of particles, as well as due to the energy of the radioactive decay of individual chemical elements.
An additional heating of the planet and the formation of an iron-nickel alloy, which, due to its significant specific gravity, gradually descended to the center of the Earth, was facilitated by the alleged meteorite bombardment.
However, this hypothesis faces some difficulties. For example, it is not entirely clear how an iron-nickel alloy, even in a liquid state, could sink more than a thousand kilometers and reach the region of the planet's core.
In accordance with the second hypothesis, the core of the Earth was formed from iron meteorites that collided with the surface of the planet, and later it was overgrown with a silicate shell of stone meteorites and formed the mantle.
There is a serious flaw in this hypothesis. In this situation, in outer space, iron and stone meteorites should exist separately. Modern studies show that iron meteorites could have arisen only in the bowels of a planet that broke up under significant pressure, that is, after the formation of our solar system and all planets.
The first version looks more logical, since it provides for a dynamic boundary between the Earth's core and the mantle. This means that the process of separation of matter between them could continue on the planet for a very long time, thereby exerting a great influence on the further evolution of the Earth.
Thus, if we take the first hypothesis of the formation of the planet's core as a basis, then the process of differentiation of matter stretched for about 1.6 billion years. Due to gravitational differentiation and radioactive decay, the separation of matter was ensured.
Heavy elements sank only to a depth below which the substance was so viscous that iron could no longer sink. As a result of this process, a very dense and heavy annular layer of molten iron and its oxide was formed. It was located above the lighter substance of the primordial core of our planet. Further, a light silicate substance was squeezed out from the center of the Earth. Moreover, it was forced out at the equator, which, perhaps, marked the beginning of the asymmetry of the planet. 
It is assumed that during the formation of the iron core of the Earth, a significant decrease in the volume of the planet occurred, as a result of which its surface has decreased by now. The light elements and their compounds that “surfaced” to the surface formed a thin primary crust, which, like all planets of the terrestrial group, consisted of volcanic basalts overlain from above by a layer of sediments.
However, it is not possible to find living geological evidence of past processes associated with the formation of the earth's core and mantle. As already noted, the oldest rocks on planet Earth are about 4 billion years old. Most likely, at the beginning of the evolution of the planet, under the influence of high temperatures and pressures, primary basalts were metamorphosed, melted down and transformed into granite-gneiss rocks known to us.
What is the core of our planet, which was formed, probably, at the earliest stages of the Earth's development? It consists of outer and inner shells. According to scientific assumptions, at a depth of 2900-5100 km there is an outer core, which in its physical properties approaches liquid.
The outer core is a stream of molten iron and nickel, a good conductor of electricity. It is with this core that scientists associate the origin of the earth's magnetic field. The 1270 km gap remaining to the center of the Earth is occupied by the inner core, which is 80% iron and 20% silicon dioxide.
The inner core is hard and high temperature. If the outer is directly connected with the mantle, then the inner core of the Earth exists by itself. Its hardness, despite the high temperatures, is ensured by the gigantic pressure in the center of the planet, which can reach 3 million atmospheres. 
Many chemical elements as a result pass into a metallic state. Therefore, it has even been suggested that the inner core of the Earth consists of metallic hydrogen.
The dense inner core has a serious impact on the life of our planet. The planetary gravitational field is concentrated in it, which keeps light gas shells, the hydrosphere and geospheric layers of the Earth from scattering.
Probably, such a field has been characteristic of the core since the formation of the planet, whatever it was then in terms of its chemical composition and structure. It contributed to the contraction of the formed particles to the center.
Nevertheless, the origin of the core and the study of the internal structure of the Earth is the most urgent problem for scientists who are closely involved in the study of the geological history of our planet. The final solution of this issue is still very far away. To avoid various contradictions, modern science has adopted the hypothesis that the process of core formation began to occur simultaneously with the formation of the Earth.
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, 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 left 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.
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