A distinctive feature of the earth's lithosphere, associated with the phenomenon of the global tectonics of our planet, is the presence of two types of crust: continental, which makes up continental masses, and oceanic. They differ in composition, structure, thickness and nature of the prevailing tectonic processes. An important role in the functioning of a single dynamic system, which is the Earth, belongs to the oceanic crust. To clarify this role, it is first necessary to turn to the consideration of its inherent features.

general characteristics

The oceanic type of crust forms the largest geological structure of the planet - the ocean bed. This crust has a small thickness - from 5 to 10 km (for comparison, the thickness of the continental-type crust is on average 35-45 km and can reach 70 km). It occupies about 70% of the total surface area of ​​the Earth, but in terms of mass it is almost four times inferior to the continental crust. The average density of rocks is close to 2.9 g/cm 3 , that is, higher than that of the continents (2.6-2.7 g/cm 3 ).

Unlike isolated blocks of the continental crust, the oceanic one is a single planetary structure, which, however, is not monolithic. The Earth's lithosphere is divided into a number of mobile plates formed by sections of the crust and the underlying upper mantle. The oceanic type of crust is present on all lithospheric plates; there are plates (for example, the Pacific or Nazca) that do not have continental masses.

Plate tectonics and crustal age

In the oceanic plate, such large structural elements as stable platforms - thalassocratons - and active mid-ocean ridges and deep-sea trenches are distinguished. Ridges are areas of spreading, or moving apart of plates and the formation of new crust, and trenches are subduction zones, or subduction of one plate under the edge of another, where the crust is destroyed. Thus, its continuous renewal takes place, as a result of which the age of the most ancient crust of this type does not exceed 160-170 million years, that is, it was formed in the Jurassic period.

On the other hand, it should be borne in mind that the oceanic type appeared on Earth earlier than the continental type (probably at the turn of the Catarcheans - Archeans, about 4 billion years ago), and is characterized by a much more primitive structure and composition.

What and how is the earth's crust under the oceans

Currently, there are usually three main layers of oceanic crust:

1. Sedimentary. It is formed mainly by carbonate rocks, partly by deep-water clays. Near the slopes of the continents, especially near the deltas of large rivers, there are also terrigenous sediments entering the ocean from land. In these areas, the thickness of precipitation can be several kilometers, but on average it is small - about 0.5 km. Precipitation is practically absent near mid-ocean ridges.
2. Basaltic. These are pillow-type lavas erupted, as a rule, under water. In addition, this layer includes a complex complex of dikes located below - special intrusions - of dolerite (that is, also basalt) composition. Its average thickness is 2-2.5 km.
3. Gabbro-serpentinite. It is composed of an intrusive analogue of basalt - gabbro, and in the lower part - serpentinites (metamorphosed ultrabasic rocks). The thickness of this layer, according to seismic data, reaches 5 km, and sometimes more. Its sole is separated from the upper mantle underlying the crust by a special interface - the Mohorovichic boundary.

The structure of the oceanic crust indicates that, in fact, this formation can, in a sense, be considered as a differentiated upper layer of the earth's mantle, consisting of its crystallized rocks, which is overlain from above by a thin layer of marine sediments.

"Conveyor" of the ocean floor

It is clear why there are few sedimentary rocks in this crust: they simply do not have time to accumulate in significant quantities. Growing from spreading zones in the areas of mid-ocean ridges due to the influx of hot mantle matter during the convection process, lithospheric plates, as it were, carry the oceanic crust further and further away from the place of formation. They are carried away by the horizontal section of the same slow but powerful convective current. In the subduction zone, the plate (and the crust in its composition) plunges back into the mantle as a cold part of this flow. At the same time, a significant part of the sediments is torn off, crushed, and ultimately goes to increase the crust of the continental type, that is, to reduce the area of ​​the oceans.

The oceanic type of crust is characterized by such an interesting property as strip magnetic anomalies. These alternating areas of direct and reverse magnetization of basalt are parallel to the spreading zone and are located symmetrically on both sides of it. They arise during the crystallization of basaltic lava, when it acquires remanent magnetization in accordance with the direction of the geomagnetic field in a particular epoch. Since it repeatedly experienced inversions, the direction of magnetization periodically changed to the opposite. This phenomenon is used in paleomagnetic geochronological dating, and half a century ago it served as one of the strongest arguments in favor of the correctness of the theory of plate tectonics.

Oceanic type of crust in the cycle of matter and in the heat balance of the Earth

Participating in the processes of lithospheric plate tectonics, the oceanic crust is an important element of long-term geological cycles. Such, for example, is the slow mantle-oceanic water cycle. The mantle contains a lot of water, and a considerable amount of it enters the ocean during the formation of the basalt layer of the young crust. But during its existence, the crust, in turn, is enriched due to the formation of the sedimentary layer with ocean water, a significant proportion of which, partly in a bound form, goes into the mantle during subduction. Similar cycles operate for other substances, for example, for carbon.

Plate tectonics play a key role in the Earth's energy balance, allowing heat to move slowly away from hot interiors and away from the surface. Moreover, it is known that in the entire geological history of the planet gave up to 90% of the heat through the thin crust under the oceans. If this mechanism did not work, the Earth would get rid of excess heat in a different way - perhaps, like Venus, where, as many scientists suggest, there was a global destruction of the crust when the superheated mantle substance broke through to the surface. Thus, the importance of the oceanic crust for the functioning of our planet in a mode suitable for the existence of life is also exceptionally great.

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 most ancient (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 caused the penetration of the upper mantle to the basalt chambers. Basaltic volcanism is superimposed on the earlier 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 the earth's, 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 between the lithosphere and the asthenosphere, located approximately at a depth of 100 km, the temperature should be close to 1500°C. It has been established that basalt melts at this temperature. 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.

Earth's crust – outer solid shell of the Earth, the upper part of the lithosphere. The Earth's crust is separated from the Earth's mantle by the Mohorovichic surface.

It is customary to distinguish continental and oceanic crust, which differ in their composition, power, structure and age. continental crust located under the continents and their underwater margins (shelf). The earth's crust of the continental type with a thickness of 35-45 km is located under the plains up to 70 km in the area of ​​young mountains. The most ancient sections of the continental crust have a geological age exceeding 3 billion years. It consists of such shells: weathering crust, sedimentary, metamorphic, granite, basalt.

oceanic crust much younger, its age does not exceed 150-170 million years. It has less power – 5-10 km. There is no boundary layer within the oceanic crust. In the structure of the earth's crust of the oceanic type, the following layers are distinguished: unconsolidated sedimentary rocks (up to 1 km), volcanic oceanic, which consists of compacted sediments (1-2 km), basalt (4-8 km).

The stone shell of the Earth is not a single whole. It is made up of individual blocks. – lithospheric plates. In total, there are 7 large and several smaller plates on the globe. The large ones include the Eurasian, North American, South American, African, Indo-Australian (Indian), Antarctic and Pacific plates. Within all large plates, with the exception of the last, there are continents. The boundaries of lithospheric plates usually run along mid-ocean ridges and deep-sea trenches.

Lithospheric plates are constantly changing: two plates can be soldered into a single one as a result of a collision; As a result of rifting, the slab can split into several parts. Lithospheric plates can sink into the mantle of the earth, while reaching the earth's core. Therefore, the division of the earth's crust into plates is not unambiguous: with the accumulation of new knowledge, some plate boundaries are recognized as non-existent, and new plates are distinguished.

Within the lithospheric plates are areas with different types of the earth's crust. So, the eastern part of the Indo-Australian (Indian) plate is the mainland, and the western part is located at the base of the Indian Ocean. At the African Plate, the continental crust is surrounded on three sides by the oceanic crust. The mobility of the atmospheric plate is determined by the ratio of the continental and oceanic crust within it.

When lithospheric plates collide, folding of rock layers. Pleated belts – mobile, highly dissected parts of the earth's surface. There are two stages in their development. At the initial stage, the earth's crust experiences predominantly subsidence; sedimentary rocks accumulate and metamorphize. At the final stage, the lowering is replaced by an uplift, the rocks are crushed into folds. During the last billion years, there have been several epochs of intense mountain building on Earth: Baikal, Caledonian, Hercynian, Mesozoic and Cenozoic. In accordance with this, different areas of folding are distinguished.

Subsequently, the rocks that make up the folded area lose their mobility and begin to collapse. Sedimentary rocks accumulate on the surface. Stable areas of the earth's crust are formed – platforms. They usually consist of a folded basement (remains of ancient mountains) overlain from above by layers of horizontally deposited sedimentary rocks that form a cover. In accordance with the age of the foundation, ancient and young platforms are distinguished. Rock areas where the foundation is submerged to a depth and covered by sedimentary rocks are called slabs. The places where the foundation comes to the surface are called shields. They are more characteristic of ancient platforms. At the base of all continents there are ancient platforms, the edges of which are folded areas of different ages.

The spread of platform and fold areas can be seen on a tectonic geographical map, or on a map of the structure of the earth's crust.

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The earth's crust in the scientific sense is the uppermost and hardest geological part of the shell of our planet.

Scientific research allows you to study it thoroughly. This is facilitated by repeated drilling of wells both on the continents and on the ocean floor. The structure of the earth and the earth's crust in different parts of the planet differ both in composition and in characteristics. The upper boundary of the earth's crust is the visible relief, and the lower boundary is the zone of separation of the two media, which is also known as the Mohorovichic surface. It is often referred to simply as the "M boundary". She received this name thanks to the Croatian seismologist Mohorovichich A. For many years he observed the speed of seismic movements depending on the depth level. In 1909, he established the existence of a difference between the earth's crust and the red-hot mantle of the Earth. The M boundary lies at the level where the seismic wave velocity increases from 7.4 to 8.0 km/s.

The chemical composition of the Earth

Studying the shells of our planet, scientists made interesting and even amazing conclusions. The structural features of the earth's crust make it similar to the same areas on Mars and Venus. More than 90% of its constituent elements are represented by oxygen, silicon, iron, aluminum, calcium, potassium, magnesium, sodium. Combining with each other in various combinations, they form homogeneous physical bodies - minerals. They can enter the composition of rocks in different concentrations. The structure of the earth's crust is very heterogeneous. So, rocks in a generalized form are aggregates of a more or less constant chemical composition. These are independent geological bodies. They are understood as a clearly defined area of ​​the earth's crust, which has the same origin and age within its boundaries.

Rocks by groups

1. Magmatic. The name speaks for itself. They arise from cooled magma flowing from the vents of ancient volcanoes. The structure of these rocks directly depends on the rate of lava solidification. The larger it is, the smaller the crystals of the substance. Granite, for example, was formed in the thickness of the earth's crust, and basalt appeared as a result of a gradual outpouring of magma on its surface. The variety of such breeds is quite large. Considering the structure of the earth's crust, we see that it consists of magmatic minerals by 60%.

2. Sedimentary. These are rocks that were the result of the gradual deposition on land and the ocean floor of fragments of various minerals. These can be loose components (sand, pebbles), cemented (sandstone), microorganism residues (coal, limestone), chemical reaction products (potassium salt). They make up to 75% of the entire earth's crust on the continents.
According to the physiological method of formation, sedimentary rocks are divided into:

• Clastic. These are the remains of various rocks. They were destroyed under the influence of natural factors (earthquake, typhoon, tsunami). These include sand, pebbles, gravel, crushed stone, clay.
• Chemical. They are gradually formed from aqueous solutions of various mineral substances (salts).
• organic or biogenic. Consist of the remains of animals or plants. These are oil shale, gas, oil, coal, limestone, phosphorites, chalk.

3. Metamorphic rocks. Other components can turn into them. This happens under the influence of changing temperature, high pressure, solutions or gases. For example, marble can be obtained from limestone, gneiss from granite, and quartzite from sand.

Minerals and rocks that humanity actively uses in its life are called minerals. What are they?

These are natural mineral formations that affect the structure of the earth and the earth's crust. They can be used in agriculture and industry both in their natural form and being processed.

Types of useful minerals. Their classification

Depending on the physical state and aggregation, minerals can be divided into categories:

1. Solid (ore, marble, coal).
2. Liquid (mineral water, oil).
3. Gaseous (methane).

Characteristics of individual types of minerals

According to the composition and features of the application, there are:

1. Combustible (coal, oil, gas).
2. Ore. They include radioactive (radium, uranium) and noble metals (silver, gold, platinum). There are ores of ferrous (iron, manganese, chromium) and non-ferrous metals (copper, tin, zinc, aluminum).
3. Non-metallic minerals play a significant role in such a concept as the structure of the earth's crust. Their geography is extensive. These are non-metallic and non-combustible rocks. These are building materials (sand, gravel, clay) and chemicals (sulfur, phosphates, potassium salts). A separate section is devoted to precious and ornamental stones.

The distribution of minerals on our planet directly depends on external factors and geological patterns.

Thus, fuel minerals are primarily mined in oil and gas bearing and coal basins. They are of sedimentary origin and form on the sedimentary covers of platforms. Oil and coal rarely occur together.

Ore minerals most often correspond to the basement, ledges and folded areas of platform plates. In such places they can create huge belts.

Core

The earth's shell, as you know, is multi-layered. The core is located in the very center, and its radius is approximately 3,500 km. Its temperature is much higher than that of the Sun and is about 10,000 K. Accurate data on the chemical composition of the core have not been obtained, but presumably it consists of nickel and iron.

The outer core is in a molten state and has even more power than the inner one. The latter is under enormous pressure. The substances of which it is composed are in a permanent solid state.

Mantle

The geosphere of the Earth surrounds the core and makes up about 83 percent of the entire shell of our planet. The lower boundary of the mantle is located at a great depth of almost 3000 km. This shell is conventionally divided into a less plastic and dense upper part (it is from it that magma is formed) and a lower crystalline one, the width of which is 2000 kilometers.

The composition and structure of the earth's crust

In order to talk about what elements make up the lithosphere, it is necessary to give some concepts.

The earth's crust is the outermost shell of the lithosphere. Its density is less than two times compared to the average density of the planet.

The earth's crust is separated from the mantle by the boundary M, which has already been mentioned above. Since the processes occurring in both areas mutually influence each other, their symbiosis is usually called the lithosphere. It means "stone shell". Its power ranges from 50-200 kilometers.

Below the lithosphere is the asthenosphere, which has a less dense and viscous consistency. Its temperature is about 1200 degrees. A unique feature of the asthenosphere is the ability to violate its boundaries and penetrate into the lithosphere. It is the source of volcanism. Here are molten pockets of magma, which is introduced into the earth's crust and pours out to the surface. By studying these processes, scientists have been able to make many amazing discoveries. This is how the structure of the earth's crust was studied. The lithosphere was formed many thousands of years ago, but even now active processes are taking place in it.

Structural elements of the earth's crust

Compared to the mantle and core, the lithosphere is a hard, thin, and very fragile layer. It is composed of a combination of substances, in which more than 90 chemical elements have been found to date. They are distributed unevenly. 98 percent of the mass of the earth's crust is accounted for by seven components. These are oxygen, iron, calcium, aluminum, potassium, sodium and magnesium. The oldest rocks and minerals are over 4.5 billion years old.

By studying the internal structure of the earth's crust, various minerals can be distinguished.
A mineral is a relatively homogeneous substance that can be located both inside and on the surface of the lithosphere. These are quartz, gypsum, talc, etc. Rocks are made up of one or more minerals.

Processes that form the earth's crust

The structure of the oceanic crust

This part of the lithosphere mainly consists of basalt rocks. The structure of the oceanic crust has not been studied as thoroughly as the continental one. The plate tectonic theory explains that the oceanic crust is relatively young, and its most recent sections can be dated to the Late Jurassic.
Its thickness practically does not change with time, since it is determined by the amount of melts released from the mantle in the zone of mid-ocean ridges. It is significantly affected by the depth of sedimentary layers on the ocean floor. In the most voluminous sections, it ranges from 5 to 10 kilometers. This type of earth shell belongs to the oceanic lithosphere.

continental crust

The lithosphere interacts with the atmosphere, hydrosphere and biosphere. In the process of synthesis, they form the most complex and reactive shell of the Earth. It is in the tectonosphere that processes occur that change the composition and structure of these shells.
The lithosphere on the earth's surface is not homogeneous. It has several layers.

1. Sedimentary. It is mainly formed by rocks. Clays and shales predominate here, as well as carbonate, volcanic and sandy rocks. In the sedimentary layers one can find such minerals as gas, oil and coal. All of them are of organic origin.
2. granite layer. It consists of igneous and metamorphic rocks, which are closest in nature to granite. This layer is not found everywhere, it is most pronounced on the continents. Here, its depth can be tens of kilometers.
3. The basalt layer is formed by rocks close to the mineral of the same name. It is denser than granite.

Depth and change in the temperature of the earth's crust

The surface layer is heated by solar heat. This is a heliometric shell. It experiences seasonal fluctuations in temperature. The average layer thickness is about 30 m.

Below is a layer that is even thinner and more fragile. Its temperature is constant and approximately equal to the average annual temperature characteristic of this region of the planet. Depending on the continental climate, the depth of this layer increases.
Even deeper in the earth's crust is another level. This is the geothermal layer. The structure of the earth's crust provides for its presence, and its temperature is determined by the internal heat of the Earth and increases with depth.

The increase in temperature occurs due to the decay of radioactive substances that are part of the rocks. First of all, it is radium and uranium.

Geometric gradient - the magnitude of the increase in temperature depending on the degree of increase in the depth of the layers. This setting depends on various factors. The structure and types of the earth's crust affect it, as well as the composition of rocks, the level and conditions of their occurrence.

The heat of the earth's crust is an important energy source. His study is very relevant today.

- 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.