Why does the earth need a core? Why doesn't the earth's core cool down?

History of the study

One of the first to suggest the existence of a region of increased density inside the Earth was Henry Cavendish, who calculated the mass and average density Earth and found that it is much greater than the density characteristic of rocks exposed on the earth's surface.
The existence of the core was proven in 1897 by the German seismologist E. Wichert for the presence of the so-called “seismic shadow” effect. In 1910, due to a sharp jump in the velocities of longitudinal seismic waves, the American geophysicist B. Gutenberg determined the depth of its surface - 2900 km.

Founder of geochemistry V. M. Goldschmidt (German) Victor Moritz Goldschmidt(1888-1947) in 1922 proposed that the core was formed by gravitational differentiation of the primordial Earth during its growth or later periods. An alternative hypothesis that the iron core arose in the protoplanetary cloud was developed by the German scientist A. Eiken (1944), the American scientist E. Orovan and the Soviet scientist A.P. Vinogradov (60-70s).

In 1941, Kuhn and Ritman, based on the hypothesis of the identity of the chemical composition of the Sun and the Earth and on calculations of the phase transition in hydrogen, suggested that the earth's core consists of metallic hydrogen. This hypothesis has not been tested experimentally. Shock compression experiments have shown that the density of metallic hydrogen is approximately an order of magnitude less than the density of the nucleus. However, this hypothesis was later adapted to explain the structure of the giant planets - Jupiter, Saturn, etc. Modern science believes that the magnetic field arises precisely in the metallic hydrogen core.

In addition, V.N. Lodochnikov and U. Ramsay suggested that the lower mantle and the core have the same chemical composition - at the core-mantle boundary at a pressure of 1.36 MBar, mantle silicates pass into the liquid metal phase (metallized silicate core).

Kernel composition

The composition of the kernel can only be estimated from a few sources.

The samples of iron meteorites, which are fragments of the nuclei of asteroids and protoplanets, are considered to be closest to the core material. However, iron meteorites are not equivalent to the material of the earth's core, since they formed in much smaller bodies, i.e. with other physicochemical parameters.

The core density is known from gravimetric data, which further limits component composition. Since the core density is approximately 10% less than the density of iron-nickel alloys, the Earth's core accordingly contains more light elements than iron meteorites.

Based on geochemical considerations, calculating the primary composition of the Earth and calculating the proportion of elements found in other geospheres, it is possible to construct an approximate estimate of the composition of the core. Help in such calculations is provided by high-temperature and high-pressure experiments on the distribution of elements between molten iron and silicate phases.

Formation of the earth's core

Formation time

Nucleation – key moment history of the Earth. The following considerations were used to determine the age of this event:

The substance from which the Earth was formed contained the isotope 182 Hf, which has a half-life of 9 million years and turns into the isotope 182 W. Hafnium is a lithophile element, i.e. When the primary substance of the Earth was divided into silicate and metallic phases, it was predominantly concentrated in the silicate phase, and tungsten, a siderophile element, was concentrated in the metallic phase. In the metallic core of the Earth, the Hf/W ratio is close to zero, while in the silicate shell this ratio is close to 15.

From the analysis of unfractionated chondrites and iron meteorites, the primary ratio of hafnium and tungsten isotopes is known.
If the core formed after a time much longer than the half-life of 182 Hf, then it would have time to almost completely transform into 182 W, and the isotopic composition of tungsten in the silicate part of the Earth and its core would be the same, the same as in chondrites.
If the core was formed while 182 Hf had not yet decayed, then the silicate shell of the Earth should contain some excess of 182 W compared to chondrites, which is actually observed.

Based on this model of the separation of the metallic and silicate parts of the Earth, calculations showed that the core formed in less than 30 million years, since the first formation of solid particles in the solar system. Similar calculations can be made for metal meteorites, which are fragments of the nuclei of small planetary bodies. In them, the formation of the core occurred much faster - over several million years. The age of the inner solid core is estimated at 2-4 billion years.

Sorokhtin–Ushakov theory

According to the Sorokhtin-Ushakov model, the process of formation of the earth's core lasted approximately 1.6 billion years (from 4 to 2.6 billion years ago). According to the authors, the formation of the earth's core occurred in two stages. At first the planet was cold, and no movements occurred in its depths. Then she warmed up with energy radioactive decay before the smelting of metallic iron began, which began to penetrate to the center of the Earth. At the same time, due to gravitational differentiation, a large number of heat, and the process of core separation only accelerated. This process only went to a depth below which the substance, under ultra-high pressure, became so viscous that the iron could no longer sink deeper. As a result, a dense annular layer of molten iron and its oxide was formed. It was located above the lighter substance of the original “core” of the Earth. Later, the silicate substance was squeezed out from the center of the Earth at the equator, which led to the asymmetry of the planet.

Mechanism of formation of the earth's core

Very little is known about the mechanism of nucleation. According to various estimates, the formation took place at a pressure and temperature close to that which now prevails in the upper and middle mantle, and not in planetesimals and asteroids. This means that during the accretion of the Earth, its new homogenization occurred.

Mechanism for constantly updating the internal core

A number of studies recent years showed anomalous properties of the earth's core - it was found that seismic waves cross the eastern part of the core faster than the western. Classical models suggest that the inner core of our planet is a symmetrical, homogeneous and practically stable formation, slowly growing due to the solidification of the material of the outer core. However, the inner core is a rather dynamic structure.
A group of researchers from Joseph Fourier universities University Joseph Fourier and Lyon (fr. Universite de Lyon) put forward the assumption that the inner core of the Earth is constantly crystallizing in the west and melting in the east. The geometric center of the inner core is shifted relative to the center of the Earth. Parts of the core in the west and east have different temperatures, which leads to one-sided melting and crystallization. Sets in motion the entire mass of the inner core, slowly shifts from the western side to the eastern side, where it collapses solid replenishes the composition of the liquid shell at a rate of 1.5 cm/year. Those. complete remelting in 100 million years. The difference in the ratio of light and heavy elements in the west and east of the core naturally leads to a difference in the velocities of seismic waves.

Such powerful processes of solidification and melting cannot but affect convective flows in the outer core. They affect the planetary dynamo, the Earth's magnetic field, the behavior of the mantle and the movement of continents. The hypothesis explains the discrepancy between the rotation speed of the core and the rest of the planet, the accelerated shift of the magnetic poles.

With a thickness of about 2200 km, between which a transition zone is sometimes distinguished. Core mass - 1.932 10 24 kg.

Very little is known about the core - all information is obtained by indirect geophysical or geochemical methods, and images of the core material are not available, and are unlikely to be obtained in the foreseeable future. However, science fiction writers have already described in detail several times travel to the core of the Earth and the untold riches hidden there. The hope for the treasures of the core has some basis, since according to modern geochemical models, the core contains a relatively high content noble metals and other valuable elements.

History of the study

Probably one of the first to suggest the existence of a region of increased density inside the Earth was Henry Cavendish, who calculated the mass and average density of the Earth and found that it was significantly greater than the density characteristic of rocks exposed to the Earth’s surface.

The existence was proven in 1897 by the German seismologist E. Wichert, and the depth of occurrence (2900 km) was determined in 1910 by the American geophysicist B. Gutenberg.

Similar calculations can be made for metal meteorites, which are fragments of the nuclei of small planetary bodies. It turned out that in them the formation of the nucleus occurred much faster, over a period of about several million years.

Theory of Sorokhtin and Ushakov

The described model is not the only one. So, according to the model of Sorokhtin and Ushakov, set out in the book “Development of the Earth,” the process of formation of the earth’s core lasted approximately 1.6 billion years (from 4 to 2.6 billion years ago). According to the authors, the formation of the nucleus occurred in two stages. At first the planet was cold, and no movements occurred in its depths. It was then heated enough by radioactive decay to begin to melt. metallic iron. It began to flock to the center of the earth, while due to gravitational differentiation a large amount of heat was released, and the process of separating the core only accelerated. This process went only to a certain depth, below which the substance was so viscous that the iron could no longer sink. As a result, a dense (heavy) annular layer of molten iron and its oxide was formed. It was located above the lighter substance of the primordial “core” of the Earth.

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 there is a monstrous pressure of 3 million atmospheres and incredible high temperature- 5000°C. It took scientists several centuries to find out what was in the center of the Earth. 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. It's a long way from the center of the Earth.

History of the discovery of the earth's core

One of the first to guess about the presence of a core in the center of the planet was the English physicist and chemist Henry Cavendish at the end of the 18th century. By using 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. Density of known rocks and there were approximately two times less minerals in the earth’s crust. This led to the logical assumption that in the center of the Earth there is a region of denser matter - the core.

In 1897, the German seismologist E. Wichert, studying the passage of seismological waves through the interior 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 nucleus formation were born. It is assumed that it was formed due to the settling of heavier elements towards the center, and initially the substance of the planet was homogeneous (gaseous).

What does the core consist of?

It is quite difficult to study a substance of which a sample cannot be obtained in order to study its physical and chemical parameters. Scientists only have to assume the presence of certain properties, as well as the structure and composition of the nucleus based on indirect evidence. Particularly helpful in research internal structure Earth study of the propagation of seismic waves. Seismographs located at many points on the planet's surface record the speed and types of passing seismic waves resulting from shaking of the earth's crust. All these data make it possible to judge the internal structure of the Earth, including its core.

At the moment, scientists assume that the central part of the planet is heterogeneous. What is at the center of the Earth? The part adjacent to the mantle is liquid core consisting of molten matter. Apparently it contains a mixture of iron and nickel. Scientists were led to this idea by a study of iron meteorites, which are pieces of asteroid cores. On the other hand, the resulting iron-nickel alloys have more high density than the estimated core density. Therefore, many scientists are inclined to assume that in the center of the Earth, the core, there are lighter chemical elements.

Geophysicists explain the existence of the planet by the presence of a liquid core and the rotation of the planet around its own axis. magnetic field. 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.

Interior The core, despite the temperature of several thousand degrees, is a solid substance. This is because the pressure at 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 metal. Thus, even geophysicists still do not know for certain what the center of the Earth is. But if we consider the issue from a mathematical point of view, we can say that the center of the Earth is approximately 6378 km away. from the surface of the planet.

When you drop your keys into a stream of molten lava, say goodbye to them because, well, dude, they're everything.
- Jack Handy

We all know why this is so: because the Earth's oceans float above the rocks and dirt that make up the land. The concept of buoyancy, in which less dense objects float above denser ones that sink below, explains much more than just the oceans.

The same principle that explains why ice floats in water, a helium balloon rises in the atmosphere, and rocks sink in a lake explains why the layers of planet Earth are arranged the way they are.

The least dense part of the Earth, the atmosphere, floats above the water oceans that float above earth's crust, which lies above the denser mantle, which does not sink into the densest part of the Earth: the core.

Ideally, the most stable state of the Earth would be one that would be ideally distributed into layers, like an onion, with the densest elements in the center, and as you move outward, each subsequent layer would be composed of less dense elements. And every earthquake, in fact, moves the planet towards this state.

And this explains the structure of not only the Earth, but also all the planets, if you remember where these elements came from.

When the Universe was young—just a few minutes old—only hydrogen and helium existed. Increasingly heavier elements were created in stars, and only when these stars died did the heavier elements escape into the Universe, allowing new generations of stars to form.

But this time, a mixture of all these elements - not only hydrogen and helium, but also carbon, nitrogen, oxygen, silicon, magnesium, sulfur, iron and others - forms not only a star, but also a protoplanetary disk around this star.

Pressure from the inside out in a forming star pushes out lighter elements, and gravity causes irregularities in the disk to collapse and form planets.

When solar system four inner world are the densest of all the planets in the system. Mercury consists of the densest elements, which could not hold large amounts of hydrogen and helium.

Other planets, more massive and farther from the Sun (and therefore receiving less of its radiation), were able to retain more of these ultra-light elements - this is how gas giants formed.

On all worlds, as on Earth, on average, the densest elements are concentrated in the core, and the light ones form increasingly less dense layers around it.

It is not surprising that iron, the most stable element, and the heaviest element created in large quantities at the supernova boundary, and is the most common element of the earth's core. But what may be surprising is that between hard core and the solid mantle is a liquid layer more than 2000 km thick: the outer core of the Earth.

The Earth has a thick liquid layer containing 30% of the planet's mass! And we learned about its existence using a rather ingenious method - thanks to seismic waves originating from earthquakes!

In earthquakes, seismic waves of two types are born: the main compression wave, known as P-wave, which travels along a longitudinal path

And a second shear wave, known as an S-wave, similar to waves on the surface of the sea.

Seismic stations around the world are able to pick up P- and S-waves, but S-waves do not travel through liquid, and P-waves not only travel through liquid, but are refracted!

As a result, we can understand that the Earth has a liquid outer core, outside of which there is a solid mantle, and inside - a solid inner core! This is why the Earth's core contains the heaviest and densest elements, and this is how we know that the outer core is a liquid layer.

But why is the outer core liquid? Like all elements, the state of iron, whether solid, liquid, gas, or other, depends on the pressure and temperature of the iron.

Iron is a more complex element than many you are used to. Of course, it may have different crystalline solid phases, as indicated in the graph, but we are not interested in ordinary pressures. We are descending into the earth's core, where pressures are a million times higher than sea level. What does the phase diagram look like for such high pressures?

The beauty of science is that even if you don't have the answer to a question right away, chances are someone has already done the right research that may reveal the answer! In this case, Ahrens, Collins and Chen in 2001 found the answer to our question.

And although the diagram shows gigantic pressures of up to 120 GPa, it is important to remember that the atmospheric pressure is only 0.0001 GPa, while in the inner core pressures reach 330-360 GPa. The upper solid line shows the boundary between melting iron (top) and solid iron (bottom). Did you notice how the solid line at the very end makes a sharp upward turn?

In order for iron to melt at a pressure of 330 GPa, an enormous temperature is required, comparable to that prevailing on the surface of the Sun. The same temperatures at lower pressures will easily maintain iron in liquid state, and at higher levels - in the solid. What does this mean in terms of the Earth's core?

This means that as the Earth cools, its internal temperature drops, but the pressure remains unchanged. That is, during the formation of the Earth, most likely, the entire core was liquid, and as it cools, the inner core grows! And in the process, since solid iron has a higher density than liquid iron, the Earth slowly contracts, which leads to earthquakes!

So, the Earth's core is liquid because it is hot enough to melt iron, but only in regions with low enough pressure. As the Earth ages and cools, more and more of the core becomes solid, and so the Earth shrinks a little!

If we want to look far into the future, we can expect the same properties to appear as those observed in Mercury.

Mercury, due to its small size, has already cooled and contracted significantly, and has fractures hundreds of kilometers long that have appeared due to the need for compression due to cooling.

So why does the Earth have a liquid core? Because it hasn't cooled down yet. And each earthquake is a small approach of the Earth to its final, cooled and completely solid state. But don't worry, long before that moment the Sun will explode and everyone you know will be dead for a very long time.

The Earth, along with other bodies of the Solar System, was formed from a cold gas and dust cloud through the accretion of its constituent particles. After the emergence of the planet, it began completely new stage its development, which in science is usually called pre-geological.
The name of the period is due to the fact that the earliest evidence of past processes - igneous or volcanic rocks - is no older than 4 billion years. Only scientists can study them today.
The pre-geological stage of the Earth’s development is still fraught with many mysteries. It covers a period of 0.9 billion years and is characterized by widespread volcanism on the planet with the release of gases and water vapor. It was at this time that the process of separation of the Earth into its main shells began - the core, mantle, crust and atmosphere. It is assumed that this process was provoked by intense meteorite bombardment of our planet and the melting of its individual parts.
One of key events in the history of the Earth was the formation of its inner core. This probably happened during the pre-geological 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 Earth's core form? Scientists offer two main hypotheses to answer this question.
According to the first version, the matter immediately after the emergence of the Earth was homogeneous.
It consisted entirely of microparticles that can be observed today in meteorites. But after a certain period of time, this primary homogeneous mass was divided into a heavy core, into which all the iron had flowed, and a lighter silicate mantle. In other words, drops of molten iron and accompanying heavy chemical compounds settled to the center of our planet and formed a core there, which remains largely molten to this day. As heavy elements tended to the center of the Earth, light slags, on the contrary, floated upward - to the outer layers of the planet. Today, these light elements make up the upper mantle and crust.
Why did such differentiation of matter occur? It is believed that immediately after the completion of the process of its formation, the Earth began to warm up intensively, primarily due to the energy released during the gravitational accumulation of particles, as well as due to the energy of the radioactive decay of individual chemical elements.
Additional heating of the planet and the formation of an iron-nickel alloy, which, due to its significant specific gravity gradually sank towards the center of the Earth, facilitated by the supposed 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, was able to descend more than a thousand kilometers and reach the region of the planet’s core.
In accordance with the second hypothesis, the Earth's core 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 must exist separately. Modern research shows that iron meteorites could only have arisen in the depths of a planet that disintegrated under significant pressure, that is, after the formation of our Solar System and all the planets.
The first version seems more logical, since it provides for a dynamic boundary between the Earth's core and the mantle. This means that the process of division of matter between them could continue on the planet for a very long time. for a 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, the process of differentiation of matter lasted approximately 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 material of the primordial core of our planet. Next, a light silicate substance was squeezed out from the center of the Earth. Moreover, it was displaced at the equator, which may have 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 now decreased. The light elements and their compounds that “floated” to the surface formed a thin primary crust, which, like all planets, consisted terrestrial group, from volcanic basalts, overlain by a thick 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 planet’s evolution, under the influence of high temperatures and pressures, primary basalts metamorphosed, melted and transformed into the granite-gneiss rocks known to us.
What is the core of our planet, which was probably formed 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 the liquid.
The outer core is a stream of molten iron and nickel that conducts electricity well. It is with this core that scientists associate the origin of the earth's magnetic field. The remaining 1,270 km gap 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 hot. If the outer is directly connected to the mantle, then the inner core of the Earth exists on its own. Its hardness, despite high temperatures, is ensured by gigantic pressure in the center of the planet, which can reach 3 million atmospheres.
As a result, many chemical elements transform into a metallic state. Therefore, it was even 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 geosphere layers of the Earth from scattering.
Probably, such a field was characteristic of the core from the moment the planet was formed, no matter what it was then chemical composition and structure. It contributed to the contraction of the formed particles towards the center.
Yet the origin of the core and the study of the internal structure of the Earth are the most current problem for scientists closely involved in the study of the geological history of our planet. There is still a long way to go before a final solution to this issue is achieved. To avoid various contradictions, in modern science the hypothesis has been accepted that the process of core formation began to occur simultaneously with the formation of the Earth.