A characteristic feature of the evolution of the Earth is the differentiation of matter, the expression of which is the shell structure of our planet. The lithosphere, hydrosphere, atmosphere, biosphere form the main shells of the Earth, differing in chemical composition, power and state of matter.

The internal structure of the Earth

The chemical composition of the Earth(Fig. 1) is similar to the composition of other terrestrial planets, such as Venus or Mars.

In general, elements such as iron, oxygen, silicon, magnesium, and nickel predominate. The content of light elements is low. The average density of the Earth's matter is 5.5 g/cm 3 .

There is very little reliable data on the internal structure of the Earth. Consider Fig. 2. It depicts the internal structure of the Earth. The earth consists of the earth's crust, mantle and core.

Rice. 1. The chemical composition of the Earth

Rice. 2. The internal structure of the Earth

Core

Core(Fig. 3) is located in the center of the Earth, its radius is about 3.5 thousand km. The core temperature reaches 10,000 K, i.e., it is higher than the temperature of the outer layers of the Sun, and its density is 13 g / cm 3 (compare: water - 1 g / cm 3). The core presumably consists of alloys of iron and nickel.

The outer core of the Earth has a greater power than the inner core (radius 2200 km) and is in a liquid (molten) state. The inner core is under enormous pressure. The substances that compose it are in a solid state.

Mantle

Mantle- the geosphere of the Earth, which surrounds the core and makes up 83% of the volume of our planet (see Fig. 3). Its lower boundary is located at a depth of 2900 km. The mantle is divided into a less dense and plastic upper part (800-900 km), from which magma(translated from Greek means "thick ointment"; this is the molten substance of the earth's interior - a mixture of chemical compounds and elements, including gases, in a special semi-liquid state); and a crystalline lower one, about 2000 km thick.

Rice. 3. Structure of the Earth: core, mantle and earth's crust

Earth's crust

Earth's crust - the outer shell of the lithosphere (see Fig. 3). Its density is approximately two times less than the average density of the Earth - 3 g/cm 3 .

Separates the earth's crust from the mantle Mohorovicic border(it is often called the Moho boundary), characterized by a sharp increase in seismic wave velocities. It was installed in 1909 by a Croatian scientist Andrey Mohorovichich (1857- 1936).

Since the processes occurring in the uppermost part of the mantle affect the movement of matter in the earth's crust, they are combined under the general name lithosphere(stone shell). The thickness of the lithosphere varies from 50 to 200 km.

Below the lithosphere is asthenosphere- less hard and less viscous, but more plastic shell with a temperature of 1200 °C. It can cross the Moho boundary, penetrating into the earth's crust. The asthenosphere is the source of volcanism. It contains pockets of molten magma, which is introduced into the earth's crust or poured onto the earth's surface.

The composition and structure of the earth's crust

Compared to the mantle and core, the earth's crust is a very thin, hard, and brittle layer. It is composed of a lighter substance, which currently contains about 90 natural chemical elements. These elements are not equally represented in the earth's crust. Seven elements—oxygen, aluminum, iron, calcium, sodium, potassium, and magnesium—account for 98% of the mass of the earth's crust (see Figure 5).

Peculiar combinations of chemical elements form various rocks and minerals. The oldest of them are at least 4.5 billion years old.

Rice. 4. The structure of the earth's crust

Rice. 5. The composition of the earth's crust

Mineral is a relatively homogeneous in its composition and properties of a natural body, formed both in the depths and on the surface of the lithosphere. Examples of minerals are diamond, quartz, gypsum, talc, etc. (You will find a description of the physical properties of various minerals in Appendix 2.) The composition of the Earth's minerals is shown in fig. 6.

Rice. 6. General mineral composition of the Earth

Rocks are made up of minerals. They can be composed of one or more minerals.

Sedimentary rocks - clay, limestone, chalk, sandstone, etc. - formed by the precipitation of substances in the aquatic environment and on land. They lie in layers. Geologists call them pages of the history of the Earth, because they can learn about the natural conditions that existed on our planet in ancient times.

Among sedimentary rocks, organogenic and inorganic (detrital and chemogenic) are distinguished.

Organogenic rocks are formed as a result of the accumulation of the remains of animals and plants.

Clastic rocks are formed as a result of weathering, the formation of destruction products of previously formed rocks with the help of water, ice or wind (Table 1).

Table 1. Clastic rocks depending on the size of the fragments

Breed name	Size of bummer con (particles)
	Over 50 cm
	5 mm - 1 cm
	1 mm - 5 mm
Sand and sandstones	0.005 mm - 1 mm
	Less than 0.005mm

Chemogenic rocks are formed as a result of sedimentation from the waters of the seas and lakes of substances dissolved in them.

In the thickness of the earth's crust, magma forms igneous rocks(Fig. 7), such as granite and basalt.

Sedimentary and igneous rocks, when immersed to great depths under the influence of pressure and high temperatures, undergo significant changes, turning into metamorphic rocks. So, for example, limestone turns into marble, quartz sandstone into quartzite.

Three layers are distinguished in the structure of the earth's crust: sedimentary, "granite", "basalt".

Sedimentary layer(see Fig. 8) is formed mainly by sedimentary rocks. Clays and shales predominate here, sandy, carbonate and volcanic rocks are widely represented. In the sedimentary layer there are deposits of such mineral, like coal, gas, oil. All of them are of organic origin. For example, coal is a product of the transformation of plants of ancient times. The thickness of the sedimentary layer varies widely - from complete absence in some areas of land to 20-25 km in deep depressions.

Rice. 7. Classification of rocks by origin

"Granite" layer consists of metamorphic and igneous rocks similar in their properties to granite. The most common here are gneisses, granites, crystalline schists, etc. The granite layer is not found everywhere, but on the continents, where it is well expressed, its maximum thickness can reach several tens of kilometers.

"Basalt" layer formed by rocks close to basalts. These are metamorphosed igneous rocks, denser than the rocks of the "granite" layer.

The thickness and vertical structure of the earth's crust are different. There are several types of the earth's crust (Fig. 8). According to the simplest classification, oceanic and continental crust are distinguished.

Continental and oceanic crust are different in thickness. Thus, the maximum thickness of the earth's crust is observed under mountain systems. It is about 70 km. Under the plains, the thickness of the earth's crust is 30-40 km, and under the oceans it is the thinnest - only 5-10 km.

Rice. 8. Types of the earth's crust: 1 - water; 2 - sedimentary layer; 3 - interbedding of sedimentary rocks and basalts; 4, basalts and crystalline ultramafic rocks; 5, granite-metamorphic layer; 6 - granulite-mafic layer; 7 - normal mantle; 8 - decompressed mantle

The difference between the continental and oceanic crust in terms of rock composition is manifested in the absence of a granite layer in the oceanic crust. Yes, and the basalt layer of the oceanic crust is very peculiar. In terms of rock composition, it differs from the analogous layer of the continental crust.

The boundary of land and ocean (zero mark) does not fix the transition of the continental crust into the oceanic one. The replacement of the continental crust by oceanic occurs in the ocean approximately at a depth of 2450 m.

Rice. 9. The structure of the continental and oceanic crust

There are also transitional types of the earth's crust - suboceanic and subcontinental.

Suboceanic crust located along the continental slopes and foothills, can be found in the marginal and Mediterranean seas. It is a continental crust up to 15-20 km thick.

subcontinental crust located, for example, on volcanic island arcs.

Based on materials seismic sounding - seismic wave velocity - we get data on the deep structure of the earth's crust. Thus, the Kola superdeep well, which for the first time made it possible to see rock samples from a depth of more than 12 km, brought a lot of unexpected things. It was assumed that at a depth of 7 km, a “basalt” layer should begin. In reality, however, it was not discovered, and gneisses predominated among the rocks.

Change in the temperature of the earth's crust with depth. The surface layer of the earth's crust has a temperature determined by solar heat. This is heliometric layer(from the Greek Helio - the Sun), experiencing seasonal temperature fluctuations. Its average thickness is about 30 m.

Below is an even thinner layer, the characteristic feature of which is a constant temperature corresponding to the average annual temperature of the observation site. The depth of this layer increases in the continental climate.

Even deeper in the earth's crust, a geothermal layer is distinguished, the temperature of which is determined by the internal heat of the Earth and increases with depth.

The increase in temperature occurs mainly due to the decay of radioactive elements that make up the rocks, primarily radium and uranium.

The magnitude of the increase in temperature of rocks with depth is called geothermal gradient. It varies over a fairly wide range - from 0.1 to 0.01 ° C / m - and depends on the composition of the rocks, the conditions of their occurrence and a number of other factors. Under the oceans, the temperature rises faster with depth than on the continents. On average, with every 100 m of depth it becomes warmer by 3 °C.

The reciprocal of the geothermal gradient is called geothermal step. It is measured in m/°C.

The heat of the earth's crust is an important energy source.

The part of the earth's crust extending to the depths available for geological study forms bowels of the earth. The bowels of the Earth require special protection and reasonable use.

The upper layer of the Earth, which gives life to the inhabitants of the planet, is just a thin shell covering many kilometers of inner layers. Little more is known about the hidden structure of the planet than about outer space. The deepest Kola well, drilled into the earth's crust to study its layers, has a depth of 11 thousand meters, but this is only four hundredth of the distance to the center of the globe. Only seismic analysis can get an idea of ​​the processes taking place inside and create a model of the Earth's structure.

Inner and outer layers of the Earth

The structure of the planet Earth is heterogeneous layers of inner and outer shells, which differ in composition and role, but are closely related to each other. The following concentric zones are located inside the globe:

• The core - with a radius of 3500 km.
• Mantle - approximately 2900 km.
• The earth's crust is an average of 50 km.

The outer layers of the earth make up a gaseous shell, which is called the atmosphere.

Center of the planet

The central geosphere of the Earth is its core. If we raise the question of which layer of the Earth is practically the least studied, then the answer will be - the core. It is not possible to obtain exact data on its composition, structure and temperature. All information that is published in scientific papers has been obtained by geophysical, geochemical methods and mathematical calculations and is presented to the general public with the proviso "presumably". As the results of the analysis of seismic waves show, the earth's core consists of two parts: internal and external. The inner core is the most unexplored part of the Earth, since seismic waves do not reach its limits. The outer core is a mass of hot iron and nickel, with a temperature of about 5 thousand degrees, which is constantly in motion and is a conductor of electricity. It is with these properties that the origin of the Earth's magnetic field is associated. The composition of the inner core, according to scientists, is more diverse and is supplemented by even lighter elements - sulfur, silicon, and possibly oxygen.

Mantle

The geosphere of the planet, which connects the central and upper layers of the Earth, is called the mantle. It is this layer that makes up about 70% of the mass of the globe. The lower part of the magma is the shell of the core, its outer boundary. Seismic analysis shows here a sharp jump in the density and velocity of compressional waves, which indicates a material change in the composition of the rock. The composition of the magma is a mixture of heavy metals, dominated by magnesium and iron. The upper part of the layer, or asthenosphere, is a mobile, plastic, soft mass with a high temperature. It is this substance that breaks through the earth's crust and splashes to the surface in the process of volcanic eruptions.

The thickness of the magma layer in the mantle is from 200 to 250 kilometers, the temperature is about 2000 ° C. The mantle is separated from the lower globe of the earth's crust by the Moho layer, or the Mohorovichic boundary, by a Serbian scientist who determined a sharp change in the speed of seismic waves in this part of the mantle.

hard shell

What is the name of the layer of the Earth that is the hardest? This is the lithosphere, a shell that connects the mantle and the earth's crust, it is located above the asthenosphere, and cleans the surface layer from its hot influence. The main part of the lithosphere is part of the mantle: out of the entire thickness from 79 to 250 km, the earth's crust accounts for 5-70 km, depending on the location. The lithosphere is heterogeneous, it is divided into lithospheric plates, which are in constant slow motion, sometimes diverging, sometimes approaching each other. Such fluctuations of the lithospheric plates are called tectonic movement, it is their fast tremors that cause earthquakes, cracks in the earth's crust, and magma splashing to the surface. The movement of lithospheric plates leads to the formation of troughs or hills, the frozen magma forms mountain ranges. Plates do not have permanent boundaries, they join and separate. Territories of the Earth's surface, above the faults of tectonic plates, are places of increased seismic activity, where earthquakes, volcanic eruptions occur more often than in others, and minerals are formed. At this time, 13 lithospheric plates have been recorded, the largest of them: American, African, Antarctic, Pacific, Indo-Australian and Eurasian.

Earth's crust

Compared to other layers, the earth's crust is the thinnest and most fragile layer of the entire earth's surface. The layer in which organisms live, which is the most saturated with chemicals and microelements, is only 5% of the total mass of the planet. The earth's crust on planet Earth has two varieties: continental or mainland and oceanic. The continental crust is harder, consists of three layers: basalt, granite and sedimentary. The ocean floor is made up of basalt (basic) and sedimentary layers.

• Basalt rocks- These are igneous fossils, the densest of the layers of the earth's surface.
• granite layer- absent under the oceans, on land it can approach a thickness of several tens of kilometers of granite, crystalline and other similar rocks.
• Sedimentary layer formed during the destruction of rocks. In some places it contains deposits of minerals of organic origin: coal, table salt, gas, oil, limestone, chalk, potassium salts and others.

Hydrosphere

Characterizing the layers of the Earth's surface, one cannot fail to mention the vital water shell of the planet, or the hydrosphere. The water balance on the planet is maintained by ocean waters (the main water mass), groundwater, glaciers, inland waters of rivers, lakes and other bodies of water. 97% of the entire hydrosphere falls on the salt water of the seas and oceans, and only 3% is fresh drinking water, of which the bulk is in glaciers. Scientists suggest that the amount of water on the surface will increase over time due to deep balls. Hydrospheric masses are in constant circulation, they pass from one state to another and closely interact with the lithosphere and atmosphere. The hydrosphere has a great influence on all earthly processes, the development and life of the biosphere. It was the water shell that became the environment for the origin of life on the planet.

The soil

The thinnest fertile layer of the Earth called soil, or soil, together with the water shell, is of the greatest importance for the existence of plants, animals and humans. This ball arose on the surface as a result of erosion of rocks, under the influence of organic decomposition processes. Processing the remnants of life, millions of microorganisms have created a layer of humus - the most favorable for crops of all kinds of land plants. One of the important indicators of high soil quality is fertility. The most fertile soils are those with an equal content of sand, clay and humus, or loam. Clay, rocky and sandy soils are among the least suitable for agriculture.

Troposphere

The air shell of the Earth rotates together with the planet and is inextricably linked with all processes occurring in the earth's layers. The lower part of the atmosphere through the pores penetrates deep into the body of the earth's crust, the upper part gradually connects with space.

The layers of the Earth's atmosphere are heterogeneous in composition, density and temperature.

At a distance of 10 - 18 km from the earth's crust extends the troposphere. This part of the atmosphere is heated by the earth's crust and water, so it gets colder with height. The decrease in temperature in the troposphere occurs by about half a degree every 100 meters, and at the highest points it reaches from -55 to -70 degrees. This part of the airspace occupies the largest share - up to 80%. It is here that the weather is formed, storms, clouds gather, precipitation and winds form.

high layers

• Stratosphere- the ozone layer of the planet, which absorbs the ultraviolet radiation of the sun, preventing it from destroying all life. The air in the stratosphere is rarefied. Ozone maintains a stable temperature in this part of the atmosphere from -50 to 55 ° C. In the stratosphere, an insignificant part of moisture, therefore, clouds and precipitation are not characteristic of it, in contrast to air currents that are significant in speed.
• Mesosphere, thermosphere, ionosphere- the air layers of the Earth above the stratosphere, in which a decrease in the density and temperature of the atmosphere is observed. The layer of the ionosphere is the place where the glow of charged gas particles occurs, which is called the aurora.
• Exosphere- a sphere of dispersion of gas particles, a blurred border with space.

The study of the internal structure of the planets, including our Earth, is an extremely difficult task. We cannot physically "drill" the earth's crust down to the core of the planet, so all the knowledge we have received at the moment is knowledge obtained "by touch", and in the most literal way.

How seismic exploration works on the example of oil exploration. We “call” the ground and “listen” to what the reflected signal will bring us

The fact is that the simplest and most reliable way to find out what is under the surface of the planet and is part of its crust is to study the propagation velocity seismic waves in the depths of the planet.

It is known that the velocity of longitudinal seismic waves increases in denser media and, on the contrary, decreases in loose soils. Accordingly, knowing the parameters of different types of rock and having calculated data on pressure, etc., “listening” to the received answer, one can understand through which layers of the earth’s crust the seismic signal passed and how deep they are under the surface.

Studying the structure of the earth's crust using seismic waves

Seismic vibrations can be caused by two types of sources: natural and artificial. Earthquakes are natural sources of vibrations, the waves of which carry the necessary information about the density of the rocks through which they penetrate.

The arsenal of artificial vibration sources is more extensive, but first of all, artificial vibrations are caused by an ordinary explosion, but there are also more “subtle” ways of working - generators of directed impulses, seismic vibrators, etc.

Conducting blasting and studying the velocities of seismic waves is engaged in seismic exploration- one of the most important branches of modern geophysics.

What did the study of seismic waves inside the Earth give? An analysis of their propagation revealed several jumps in the change in speed when passing through the bowels of the planet.

Earth's crust

The first jump, at which speeds increase from 6.7 to 8.1 km / s, according to geologists, registers bottom of the earth's crust. This surface is located in different places on the planet at different levels, from 5 to 75 km. The boundary of the earth's crust and the underlying shell - the mantle, is called "Mohorovicic surfaces", named after the Yugoslav scientist A. Mohorovichich, who first established it.

Mantle

Mantle lies at depths up to 2,900 km and is divided into two parts: upper and lower. The boundary between the upper and lower mantle is also fixed by the jump in the propagation velocity of longitudinal seismic waves (11.5 km/s) and is located at depths from 400 to 900 km.

The upper mantle has a complex structure. In its upper part there is a layer located at depths of 100-200 km, where transverse seismic waves attenuate by 0.2-0.3 km / s, and the velocities of longitudinal waves, in essence, do not change. This layer is called waveguide. Its thickness is usually 200-300 km.

The part of the upper mantle and the crust overlying the waveguide is called lithosphere, and the layer of low velocities itself - asthenosphere.

Thus, the lithosphere is a rigid hard shell underlain by a plastic asthenosphere. It is assumed that processes arise in the asthenosphere that cause the movement of the lithosphere.

The internal structure of our planet

Earth's core

At the base of the mantle, there is a sharp decrease in the propagation velocity of longitudinal waves from 13.9 to 7.6 km/s. At this level lies the boundary between the mantle and the core of the earth, deeper than which transverse seismic waves no longer propagate.

The radius of the core reaches 3500 km, its volume: 16% of the planet's volume, and mass: 31% of the mass of the Earth.

Many scientists believe that the core is in a molten state. Its outer part is characterized by sharply reduced P-wave velocities, while in the inner part (with a radius of 1200 km), seismic wave velocities increase again to 11 km/s. The density of the core rocks is 11 g/cm 3 , and it is determined by the presence of heavy elements. Such a heavy element can be iron. Most likely, iron is an integral part of the core, since the core of a purely iron or iron-nickel composition should have a density that is 8-15% higher than the existing density of the core. Therefore, oxygen, sulfur, carbon and hydrogen appear to be attached to the iron in the core.

Geochemical method for studying the structure of planets

There is another way to study the deep structure of planets - geochemical method. The identification of various shells of the Earth and other terrestrial planets by physical parameters finds a fairly clear geochemical confirmation based on the theory of heterogeneous accretion, according to which the composition of the cores of the planets and their outer shells in its main part is initially different and depends on the earliest stage of their development.

As a result of this process, the heaviest ( iron-nickel) components, and in the outer shells - lighter silicate ( chondrite), enriched in the upper mantle with volatiles and water.

The most important feature of the terrestrial planets ( , Earth, ) is that their outer shell, the so-called bark, consists of two types of matter: mainland" - feldspar and " oceanic» - basalt.

Continental (continental) crust of the Earth

The continental (continental) crust of the Earth is composed of granites or rocks similar in composition to them, that is, rocks with a large amount of feldspars. The formation of the "granite" layer of the Earth is due to the transformation of older sediments in the process of granitization.

The granite layer should be considered as specific the shell of the Earth's crust - the only planet on which the processes of differentiation of matter with the participation of water and having a hydrosphere, an oxygen atmosphere and a biosphere have been widely developed. On the Moon and, probably, on the terrestrial planets, the continental crust is composed of gabbro-anorthosites - rocks consisting of a large amount of feldspar, however, of a slightly different composition than in granites.

These rocks form the oldest (4.0-4.5 billion years) surfaces of the planets.

Oceanic (basalt) crust of the Earth

Oceanic (basalt) crust The earth was formed as a result of stretching and is associated with zones of deep faults, which led to the penetration of the upper mantle to the basalt chambers. Basalt volcanism is superimposed on previously formed continental crust and is a relatively younger geological formation.

Manifestations of basalt volcanism on all terrestrial planets are apparently similar. The wide development of basalt "seas" on the Moon, Mars, and Mercury is obviously associated with stretching and the formation of permeability zones as a result of this process, along which basalt melts of the mantle rushed to the surface. This mechanism of manifestation of basaltic volcanism is more or less similar for all planets of the terrestrial group.

The satellite of the Earth - the Moon also has a shell structure, which on the whole repeats that of the earth, although it has a striking difference in composition.

Heat flow of the Earth. It is hottest in the region of faults in the earth's crust, and colder in the regions of ancient continental plates

Method for measuring heat flow for studying the structure of planets

Another way to study the deep structure of the Earth is to study its heat flow. It is known that the Earth, hot from the inside, gives off its heat. The heating of deep horizons is evidenced by volcanic eruptions, geysers, and hot springs. Heat is the main energy source of the Earth.

The increase in temperature with deepening from the Earth's surface averages about 15 ° C per 1 km. This means that at the boundary of the lithosphere and asthenosphere, located approximately at a depth of 100 km, the temperature should be close to 1500 ° C. It has been established that at this temperature basalt melts. This means that the asthenospheric shell can serve as a source of basaltic magma.

With depth, the change in temperature occurs according to a more complex law and depends on the change in pressure. According to the calculated data, at a depth of 400 km the temperature does not exceed 1600°C, and at the core-mantle boundary it is estimated at 2500-5000°C.

It is established that the release of heat occurs constantly over the entire surface of the planet. Heat is the most important physical parameter. Some of their properties depend on the degree of heating of rocks: viscosity, electrical conductivity, magneticness, phase state. Therefore, according to the thermal state, one can judge the deep structure of the Earth.

Measuring the temperature of our planet at great depths is a technically difficult task, since only the first kilometers of the earth's crust are available for measurements. However, the internal temperature of the Earth can be studied indirectly by measuring the heat flux.

Despite the fact that the main source of heat on Earth is the Sun, the total power of the heat flow of our planet exceeds the power of all power plants on Earth by 30 times.

The measurements showed that the average heat flow on the continents and in the oceans is the same. This result is explained by the fact that in the oceans, most of the heat (up to 90%) comes from the mantle, where the process of transfer of matter by moving streams occurs more intensively - convection.

Convection is a process in which a heated liquid expands, becomes lighter, and rises, while colder layers sink. Since the mantle substance is closer in its state to a solid body, convection in it proceeds under special conditions, at low material flow rates.

What is the thermal history of our planet? Its initial heating is probably associated with the heat generated by the collision of particles and their compaction in their own gravity field. Then the heat was the result of radioactive decay. Under the influence of heat, a layered structure of the Earth and the terrestrial planets arose.

Radioactive heat in the Earth is released even now. There is a hypothesis according to which, at the boundary of the molten core of the Earth, the processes of splitting of matter continue to this day with the release of a huge amount of thermal energy that heats up the mantle.

- limited to the surface of the land or the bottom of the oceans. It also has a geophysical boundary, which is the section Moho. The boundary is characterized by the fact that seismic wave velocities sharply increase here. It was installed in $1909 by a Croatian scientist A. Mohorovic ($1857$-$1936$).

The earth's crust is made up sedimentary, igneous and metamorphic rocks, and in terms of composition it stands out three layers. Rocks of sedimentary origin, the destroyed material of which was redeposited in the lower layers and formed sedimentary layer the earth's crust, covers the entire surface of the planet. In some places it is very thin and may be interrupted. In other places, it reaches a thickness of several kilometers. Sedimentary are clay, limestone, chalk, sandstone, etc. They are formed by sedimentation of substances in water and on land, they usually lie in layers. From sedimentary rocks, you can learn about the natural conditions that existed on the planet, so geologists call them pages of the history of the Earth. Sedimentary rocks are subdivided into organogenic, which are formed by the accumulation of the remains of animals and plants and non-organogenic, which are further subdivided into clastic and chemogenic.

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clastic rocks are the product of weathering, and chemogenic- the result of the precipitation of substances dissolved in the water of the seas and lakes.

Igneous rocks make up granite layer of the earth's crust. These rocks were formed as a result of solidification of molten magma. On the continents, the thickness of this layer is $15$-$20$ km, it is completely absent or very much reduced under the oceans.

Igneous matter, but poor in silica composes basaltic layer with a high specific gravity. This layer is well developed at the base of the earth's crust of all regions of the planet.

The vertical structure and thickness of the earth's crust are different, therefore, several types of it are distinguished. According to a simple classification, there is oceanic and continental Earth's crust.

continental crust

Continental or continental crust is different from oceanic crust thickness and device. The continental crust is located under the continents, but its edge does not coincide with the coastline. From the point of view of geology, the real continent is the entire area of ​​the continuous continental crust. Then it turns out that the geological continents are larger than the geographical continents. Coastal areas of the continents, called shelf- these are parts of the continents temporarily flooded by the sea. Such seas as the White, East Siberian, Azov Seas are located on the continental shelf.

There are three layers in the continental crust:

• The upper layer is sedimentary;
• The middle layer is granite;
• The bottom layer is basalt.

Under young mountains this type of crust has a thickness of $75$ km, under plains up to $45$ km, and under island arcs up to $25$ km. The upper sedimentary layer of the continental crust is formed by clay deposits and carbonates of shallow marine basins and coarse clastic facies in foredeeps, as well as on the passive margins of Atlantic-type continents.

Magma invading the cracks in the earth's crust formed granite layer which contains silica, aluminum and other minerals. The thickness of the granite layer can be up to $25$ km. This layer is very ancient and has a solid age of $3 billion years. Between the granite and basalt layers, at a depth of up to $20$ km, there is a boundary Conrad. It is characterized by the fact that the propagation velocity of longitudinal seismic waves here increases by $0.5$ km/sec.

Formation basalt layer occurred as a result of outpouring of basalt lavas onto the land surface in zones of intraplate magmatism. Basalts contain more iron, magnesium and calcium, so they are heavier than granite. Within this layer, the propagation velocity of longitudinal seismic waves is from $6.5$-$7.3$ km/sec. Where the boundary becomes blurred, the velocity of longitudinal seismic waves increases gradually.

Remark 2

The total mass of the earth's crust of the mass of the entire planet is only $0.473$%.

One of the first tasks associated with determining the composition upper continental bark, young science undertook to solve geochemistry. Since the bark is made up of a wide variety of rocks, this task was very difficult. Even in one geological body, the composition of rocks can vary greatly, and different types of rocks can be common in different areas. Based on this, the task was to determine the general, average composition that part of the earth's crust that comes to the surface on the continents. This first estimate of the composition of the upper crust was made by Clark. He worked as an employee of the US Geological Survey and was engaged in the chemical analysis of rocks. In the course of many years of analytical work, he managed to summarize the results and calculate the average composition of the rocks, which was close to to granite. Work Clark was subjected to harsh criticism and had opponents.

The second attempt to determine the average composition of the earth's crust was made by W. Goldschmidt. He suggested that moving along the continental crust glacier, can scrape and mix exposed rocks that would be deposited during glacial erosion. They will then reflect the composition of the middle continental crust. Having analyzed the composition of banded clays, which were deposited during the last glaciation in Baltic Sea, he got a result close to the result Clark. Different methods gave the same scores. Geochemical methods were confirmed. These issues have been addressed, and the assessments received wide recognition. Vinogradov, Yaroshevsky, Ronov and others.

oceanic crust

oceanic crust located where the depth of the sea is more than $ 4 $ km, which means that it does not occupy the entire space of the oceans. The rest of the area is covered with bark intermediate type. The oceanic-type crust is not organized in the same way as the continental crust, although it is also divided into layers. It has almost no granite layer, while the sedimentary one is very thin and has a thickness of less than $1$ km. The second layer is still unknown, so it is simply called second layer. Bottom third layer basaltic. The basalt layers of the continental and oceanic crust are similar in seismic wave velocities. The basalt layer in the oceanic crust prevails. According to the theory of plate tectonics, the oceanic crust is constantly formed in the mid-ocean ridges, then it moves away from them and in areas subduction absorbed into the mantle. This indicates that the oceanic crust is relatively young. The largest number of subduction zones is typical for Pacific Ocean where powerful seaquakes are associated with them.

Definition 1

Subduction- this is the lowering of rock from the edge of one tectonic plate into a semi-molten asthenosphere

In the case when the upper plate is a continental plate, and the lower one is an oceanic one, ocean trenches.
Its thickness in different geographical areas varies from $5$-$7$ km. Over time, the thickness of the oceanic crust practically does not change. This is due to the amount of melt released from the mantle in the mid-ocean ridges and the thickness of the sedimentary layer at the bottom of the oceans and seas.

Sedimentary layer oceanic crust is small and rarely exceeds a thickness of $0.5$ km. It consists of sand, deposits of animal remains and precipitated minerals. Carbonate rocks of the lower part are not found at great depths, and at a depth of more than $4.5$ km, carbonate rocks are replaced by red deep-water clays and siliceous silts.

Basalt lavas of tholeiite composition formed in the upper part basalt layer, and below lies dike complex.

Definition 2

dikes- these are channels through which basalt lava flows to the surface

Basalt layer in zones subduction turns into ecgoliths, which submerge in depth because they have a high density of surrounding mantle rocks. Their mass is about $7$% of the mass of the entire Earth's mantle. Within the basalt layer, the velocity of longitudinal seismic waves is $6.5$-$7$ km/sec.

The average age of the oceanic crust is $100$ million years, while its oldest sections are $156$ million years old and are located in the basin Pijafeta in the Pacific Ocean. The oceanic crust is concentrated not only within the World Ocean floor, it can also be in closed basins, for example, the northern basin of the Caspian Sea. Oceanic the earth's crust has a total area of ​​$306$ million sq. km.