Earth's crust. The layers of the earth and its structure What gases does the earth's crust consist of

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.

Nucleus

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.

Such a question as the structure of the Earth is of interest to many scientists, researchers and even believers. With the rapid development of science and technology since the beginning of the 18th century, many worthy workers of science have spent a lot of effort in order to understand our planet. The daredevils descended to the bottom of the ocean, flew to the highest layers of the atmosphere, drilled deep wells to explore the soil.

Today there is a fairly complete picture of what the Earth consists of. True, the structure of the planet and all its regions is still not 100% known, but scientists are gradually expanding the boundaries of knowledge and getting more and more objective information about this.

The shape and size of the planet Earth

The shape and geometric dimensions of the Earth are the basic concepts by which it is described as a celestial body. In the Middle Ages, it was believed that the planet has a flat shape, is located in the center of the universe, and the Sun and other planets revolve around it.

But such bold naturalists as Giordano Bruno, Nicolaus Copernicus, Isaac Newton refuted such judgments and mathematically proved that the Earth has the shape of a ball with flattened poles and revolves around the Sun, and not vice versa.

The structure of the planet is very diverse, despite the fact that its dimensions are quite small by the standards of even the solar system - the length of the equatorial radius is 6378 kilometers, the polar radius is 6356 km.

The length of one of the meridians is 40,008 km, and the equator extends for 40,007 km. This also shows that the planet is somewhat "flattened" between the poles, its weight is 5.9742 × 10 24 kg.

Earth shells

The earth consists of many shells that form peculiar layers. Each layer is centrally symmetrical with respect to the base center point. If you visually cut the soil along its entire depth, then layers with different composition, state of aggregation, density, etc. will open.

All shells are divided into two large groups:

1. The internal structure is described, respectively, by internal shells. They are the earth's crust and mantle.
2. The outer shells, which include the hydrosphere and atmosphere.

The structure of each shell is the subject of study of individual sciences. Scientists still, in the age of rapid technological progress, not all questions have been clarified to the end.

The earth's crust and its types

The Earth's crust is one of the shells of the planet, occupying only about 0.473% of its mass. The depth of the crust is 5 - 12 kilometers.

It is interesting to note that scientists practically did not penetrate deeper, and if we draw an analogy, then the bark is like a peel on an apple in relation to its entire volume. Further and more precise study requires a completely different level of development of technology.

If you look at the planet in a section, then according to the different depths of penetration into its structure, the following types of the earth's crust can be distinguished in order:

1. oceanic crust- consists mainly of basalts, is located at the bottom of the oceans under huge layers of water.
2. Continental or mainland crust- covers the land, consists of a very rich chemical composition, including 25% silicon, 50% oxygen, and 18% other main elements of the periodic table. For the purpose of convenient study of this bark, it is also divided into lower and upper. The most ancient belong to the lower part.

The temperature of the crust increases as it deepens.

Mantle

The main volume of our planet is the mantle. It occupies the entire space between the crust and the nucleus discussed above and consists of many layers. The smallest thickness to the mantle is about 5-7 km.

The current level of development of science and technology does not allow direct study of this part of the Earth, therefore, indirect methods are used to obtain information about it.

Very often, the birth of a new earth's crust is accompanied by its contact with the mantle, which is especially active in places under ocean waters.

Today, it is believed that there is an upper and lower mantle that are separated by the Mohorovicic boundary. The percentages of this distribution are calculated quite accurately, but require clarification in the future.

outer core

The core of the planet is also not homogeneous. Huge temperatures and pressure make many chemical processes take place here, the distribution of masses and substances is carried out. The nucleus is divided into inner and outer.

The outer core is about 3,000 kilometers thick. The chemical composition of this layer is iron and nickel, which are in the liquid phase. The temperature of the environment here ranges from 4400 to 6100 degrees Celsius as you approach the center.

inner core

The central part of the Earth, the radius of which is approximately 1200 kilometers. The lowest layer, which also consists of iron and nickel, as well as some impurities of light elements. The aggregate state of this nucleus is similar to the amorphous one. The pressure here reaches an incredible 3.8 million bar.

Do you know how many kilometers to the core of the earth? The distance is approximately 6371 km, which is easily calculated if you know the diameter and other parameters of the ball.

Comparison of the thickness of the inner layers of the Earth

The geological structure is sometimes estimated by such a parameter as the thickness of the inner layers. It is believed that the mantle is the most powerful, since it has the largest thickness.

Outer spheres of the globe

Planet Earth differs from any other space object known to scientists in that it also has outer spheres, to which they belong:

• hydrosphere;
• atmosphere;
• biosphere.

The research methods of these spheres are significantly different, because they all differ greatly in their composition and object of study.

Hydrosphere

The hydrosphere is understood as the entire water shell of the Earth, including both the huge oceans, which occupy approximately 74% of the surface, and the seas, rivers, lakes, and even small streams and reservoirs.

The greatest thickness of the hydrosphere is about 11 km and is observed in the area of ​​the Mariana Trench. It is water that is considered the source of life and what distinguishes our ball from all the others in the Universe.

The hydrosphere occupies approximately 1.4 billion km 3 of volume. Life is in full swing here, and the conditions for the functioning of the atmosphere are provided.

Atmosphere

The gaseous shell of our planet, reliably closing its bowels from space objects (meteorites), cosmic cold and other phenomena incompatible with life.

The thickness of the atmosphere is, according to various estimates, about 1000 km. Near the ground surface, the density of the atmosphere is 1.225 kg/m 3 .

78% of the gas envelope consists of nitrogen, 21% of oxygen, the rest is accounted for by elements such as argon, carbon dioxide, helium, methane and others.

Biosphere

Regardless of how scientists study the issue under consideration, the biosphere is the most important part of the structure of the Earth - this is the shell that is inhabited by living beings, including people themselves.

The biosphere is not only inhabited by living beings, but also constantly changing under their influence, in particular, under the influence of man and his activities. A holistic doctrine of this area was developed by the great scientist V. I. Vernadsky. This very definition was introduced by the Austrian geologist Suess.

Conclusion

The surface of the Earth, as well as all the shells of its external and internal structure, are a very interesting subject of study for entire generations of scientists.

Although at first glance it seems that the considered spheres are rather disparate, in fact they are connected by indestructible ties. For example, life and the entire biosphere are simply impossible without the hydrosphere and atmosphere, the same, in turn, originate from the depths.

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

Nucleus

Nucleus(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. it 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.

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, partially 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 regime suitable for the existence of life is also exceptionally great.