Oceanic and continental crust. The earth's crust and its structure What types of earth's crust are distinguished

The earth's crust is the upper part of the lithosphere. On the scale of the entire globe, it can be compared to the thinnest film - its thickness is so insignificant. But we don’t know even this uppermost shell of the planet very well. How can one learn about the structure of the earth’s crust if even the deepest wells drilled in the crust do not go beyond the first ten kilometers? Seismic location comes to the aid of scientists. By deciphering the speed of seismic waves passing through different media, it is possible to obtain data on the density of the earth's layers and draw conclusions about their composition. Under continents and ocean basins, the structure of the earth's crust is different.

OCEANIC CRUST

The oceanic crust is thinner (5-7 km) than the continental crust, and consists of two layers - lower basalt and upper sedimentary. Below the basalt layer is the Moho surface and the upper mantle. The topography of the ocean floor is very complex. Among the various landforms, the huge mid-ocean ridges stand out. In these places, the birth of young basaltic oceanic crust from mantle material occurs. Through a deep fault running along the peaks in the center of the ridge - a rift - magma comes to the surface, spreading in different directions in the form of underwater lava flows, constantly pushing the walls of the rift gorge in different directions. This process is called spreading.

Mid-ocean ridges rise several kilometers above the ocean floor, and their length reaches 80 thousand km. The ridges are cut by parallel transverse faults. They are called transformative. Rift zones are the most turbulent seismic zones on Earth. The basalt layer is overlain by layers of marine sedimentary deposits - silts and clays of various compositions.

CONTINENTAL CRUST

The continental crust occupies a smaller area (about 40% of the Earth's surface - note from geoglobus.ru), but has a more complex structure and much greater thickness. Under high mountains its thickness is measured 60-70 kilometers. The structure of the continental crust is three-membered - basalt, granite and sedimentary layers. The granite layer comes to the surface in areas called shields. For example, the Baltic Shield, part of which is occupied by the Kola Peninsula, is composed of granite rocks. It was here that deep drilling was carried out, and the Kola superdeep well reached 12 km. But attempts to drill through the entire granite layer were unsuccessful.

The shelf - the underwater margin of the continent - also has continental crust. The same applies to the large islands - New Zealand, the islands of Kalimantan, Sulawesi, New Guinea, Greenland, Sakhalin, Madagascar and others. Marginal seas and internal seas, such as the Mediterranean, Black, and Azov, are located on continental-type crust.

It is possible to talk about basalt and granite layers of the continental crust only conditionally. This means that the speed of passage of seismic waves in these layers is similar to the speed of their passage in rocks of basalt and granite composition. The boundary between the granite and basalt layers is not very clearly defined and varies in depth. The basalt layer borders the Moho surface. The upper sedimentary layer changes its thickness depending on the surface topography. So, in mountainous areas it is thin or absent altogether, since the external forces of the Earth move loose material down the slopes - approx. from geoglobus.ru. But in the foothills, plains, basins and depressions it reaches significant power. For example, in the Caspian lowland, which is undergoing subsidence, the sedimentary layer reaches 22 km.

FROM THE HISTORY OF THE KOLA SUPERDEEP WELL

Since the start of drilling this well in 1970, scientists have set a purely scientific goal for this experiment: to determine the boundary between the granite and basalt layers. The location was chosen taking into account the fact that it is in the areas of the shields that the granite layer, not covered by the sedimentary one, can be passed “through and through”, which would allow one to touch the rocks of the basalt layer and see the difference. It was previously assumed that such a boundary on the Baltic Shield, where ancient igneous rocks come to the surface, should be located at a depth of approximately 7 km.

Over several years of drilling, the well repeatedly deviated from the specified vertical direction, intersecting layers with different strengths. Sometimes the drills broke, and then we had to start drilling again, using bypass shafts. The material that was delivered to the surface was studied by various scientists and constantly brought amazing discoveries. Thus, at a depth of about 2 km, copper-nickel ores were found, and from a depth of 7 km, a core was delivered (this is the name of a rock sample from a drill in the form of a long cylinder - note from geoglobus.ru), in which the fossilized remains of ancient organisms were discovered .

But, having traveled more than 12 km by 1990, the well never went beyond the granite layer. In 1994, drilling was stopped. The Kola superdeep well is not the only well in the world that was laid for deep drilling. Similar experiments were carried out in different places by several countries. But only Kola reached such marks, for which it was included in the Guinness Book of Records.

The most significant features of the earth's crust in the seas and oceans are its small thickness and the absence of a granite layer in its structure.

Based on the relationship between the deep structure of the crust and the major morphological features of the ocean floor, the following types of structure of the oceanic crust can be distinguished.

Marginal-continental type The crust is distributed in areas of the continental shallows (shelf), representing a direct continuation of continental structures within the shelf.

Its thickness is from 25 to 35 km. The structure of the crust here includes sedimentary, granite and basalt layers. In some cases, it differs from continental platforms in its thicker sedimentary cover.

Marine geosynclinal type crust is inherent in marine geosynclinal depressions of various geosynclinal seas (inland, intercontinental, marginal-continental). This type of crust underlies the Mediterranean, Caribbean, Black, Caspian, Japanese, Okhotsk, and Bering seas.

It is characterized by a large thickness of sedimentary cover and surface loose sediments, which together make up a sedimentary thickness of up to 20 km or more. This sequence lies directly on the basalt layer. This structure is characteristic of the central parts of deep sea depressions. On the slopes of these depressions, rocks belonging to the granite layer gradually wedge out, which is accompanied by a steep drop in the layers of sedimentary rocks (Mesozoic and Cenozoic in age) that make up the adjacent spaces.

Suboceanic type crust is distributed within the continental slope.

The thickness of loose marine sediments increases sharply with increasing depth, reaching 2-3 km near the base of the continental slope. In other parts of the continental slope, where the basement is sharply dissected, its structurally determined irregularities are gradually leveled out by the thickness of sediments.

As the depth on the continental slope increases, the thickness of the granite layer gradually decreases and the angle of dip of the sediments on it, which often have a transgressive nature of occurrence, increases. With a decrease in the granite layer and the sediments covering it, the thickness of the crust in the lower part of the slope decreases to 10 km. The nature of the occurrence of the foundation and the sedimentary rocks covering it most closely corresponds to the structure of the continental flexure. In this case, the most depressed part of the continental slope (at its base), filled with thick loose sediments, represents a growing geosynclinal trough.

In most cases, it is compensated by the accumulation of loose sediments carried down from the slope. In other cases, deep fault lines extend along the continental slope, expressed in the relief of the continental slope. They can determine the further development of the geosynclinal trough between the edge of the continent and the ocean floor.

Type of abyssal oceanic plains the structure of the earth's crust is distributed over the predominant part of the bottom of ocean basins with depths of more than 4500-5000 m.

This type of crust is characterized by the absence of a granite layer and its smallest total thickness (from 2-3 to 10-12 km). Loose oceanic sediments, often containing layers of volcanic rocks, directly overlie the basalt layer. Among the abyssal plains, based on the thickness of the upper layer of sediments, one can distinguish between abyssal volcanic plains and abyssal accumulative plains. The former are characterized by a relatively small thickness of sedimentary deposits (no more than 400-500 m) and, what is especially important, individual layers of volcanic rocks.

Abyssal accumulative plains are distinguished by a large thickness of loose surface cover, reaching 2.5-3 km (usually more than 1 km). It is considered most likely that the greater thickness of loose sediments in this type of crust is associated with turbidity currents. At the same time, it is obvious that such significant sediments could be deposited in this way only under conditions of stable subsidence. Thus, different conditions for the accumulation of sedimentary deposits on the ocean floor reflect their neotectonic development.

Type of oceanic ridges and rises.

Structures of this type have an enormous extent and a complexly dissected topography with a large participation in its formation of faults and movements along them (rift valleys).

This type includes mid-ocean ridges and oceanic mountainous countries (for example, in the Pacific Ocean), as well as individual significant mountains and hills on the ocean floor, which often serve as the foundation of oceanic islands.

This type of oceanic crust structure is characterized by a significant total thickness, reaching 20-30 km. In the structure of such a crust, the surface part of the section is composed of sedimentary-volcanic rocks; at depth they are replaced by rocks of the basalt layer, which, in comparison with other parts of the structure of the crust of the ocean floor, have significantly different properties.

At the base of oceanic mountain ranges and mountains, these rocks are more dense, which is explained by the mixing of basalts with mantle rocks. The interface surface M beneath the oceanic ridges decreases significantly. The underwater ridges of marine geosynclinal depressions also have a similar nature of deep structure.

They differ only in the great similarity of the rocks of the surface part of the section with the rocks of the adjacent continental structures.

Type of abyssal oceanic trenches. Crustal structures of this type are characterized by a very small thickness of the crust with a sharp subsidence of the M interface.

The association of abyssal trenches with deep fault lines, their modern seismicity, volcanism, and sedimentation conditions - all this indicates their belonging to modern significant geosynclinal troughs, the development of which continues.

In some trenches, thick sedimentary rocks are known, for example in the Puerto Rico Trench (8 km). In other trenches (Japanese, Tonga) rocks related to the granite shell of the crust are known. The sedimentary sequence rests on a thin basalt layer. The most reasonable idea in this case is the stretching of the earth's crust under the oceanic trenches, due to which the thickness of the basalt layer decreases. Negative gravity anomalies here are associated with deposits of loose sediments of high thickness.

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(Vp) less than 5 km/s.

2) The second - traditionally called “granite” layer is 50% composed of granites, 40% - gneisses and other metamorphosed rocks to varying degrees.

Based on these data, it is often called granite-gneiss. Its average thickness is 15-20 km (sometimes in mountain structures up to 20-25 km). Seismic wave speed (Vp) — 5.5-6.0 (6.4) km/s.

3) The third, bottom layer is called “basalt”.

In terms of average chemical composition and seismic wave speed, this layer is close to basalts. It would be more correct to call this layer granulite-mafic (Vp) 6.5-6.7 (7.4) km/s.

Conrad's section.

7 Continental and subcontinental crust.

Continental type of earth's crust.

The thickness of the continental crust varies from 35-40 (45) km within platforms to 55-70 (75) km in young mountain structures.

The continental crust consists of three layers.

1) The first - the uppermost layer is represented by sedimentary rocks, with a thickness of 0 to 5 (10) km within platforms, up to 15-20 km in tectonic troughs of mountain structures.

Velocity of longitudinal seismic waves (Vp) less than 5 km/s.

2) The second - traditionally called “granite” layer is 50% composed of granites, 40% - gneisses and other metamorphosed rocks to varying degrees. Based on these data, it is often called granite-gneiss.

Its average thickness is 15-20 km (sometimes in mountain structures up to 20-25 km). Seismic wave speed (Vp) — 5.5-6.0 (6.4) km/s.

3) The third, bottom layer is called “basalt”. In terms of average chemical composition and seismic wave speed, this layer is close to basalts. It would be more correct to call this layer granulite-mafic. Its thickness varies from 15-20 to 35 km. Wave speed (Vp) 6.5-6.7 (7.4) km/s.

The boundary between granite-gneiss and granulite-mafic layers is called seismic Conrad's section.

The subcontinental type of the earth's crust is similar in structure to the continental one, but began to stand out due to the vaguely defined Conrad boundary.

8 Oceanic and suboceanic types of the earth's crust

Oceanic crust has a three-layer structure with a thickness of 5 to 9 (12) km, more often 6-7 km.

Some increase in power is observed under the ocean islands.

1. The upper, first layer of the ocean crust is sedimentary, consisting mainly of various sediments that are in a loose state. Its thickness ranges from several hundred meters to 1 km. The speed of propagation of seismic waves (Vp) in it is 2.0-2.5 km/s.

The second oceanic layer, located below, according to drilling data, is composed mainly of basalts with interlayers of carbonate and siliceous rocks. Its thickness is from 1.0-1.5 to 2.5-3.0 km. The speed of propagation of seismic waves (Vp) is 3.5-4.5 (5) km/s.

3. The third, lower high-velocity oceanic layer has not yet been opened by drilling - it is composed of basic igneous rocks such as gabbro with subordinate ultrabasic rocks (serpentinites, pyroxenites).

Its thickness according to seismic data is from 3.5 to 5.0 km. The speed of seismic waves (Vp) is from 6.3-6.5 km/s, and in some places increases to 7.0 (7.4) km/s

The suboceanic type of the earth's crust is confined to the basin parts (with a depth of more than 2 km) of the marginal and inland seas (Okhotsk, Japan, Mediterranean, Black, etc.).

In structure, this type is close to the oceanic one, but differs from it in the increased thickness (4-10 or more km) of the sedimentary layer, located on the third oceanic layer with a thickness of 5-10 km.

9 Relative and absolute geochronology. Characteristics of geochronological and stratigraphic scales.

RELATIVE GEOCHRONOLOGY

stratigraphy- one of the branches of geological science, the task of which includes the division of sedimentary and volcanogenic rocks into separate layers and their units; description of the remains of fauna and flora contained in them; establishing the age of layers; comparison of the selected layers of a given area with others; compiling a consolidated section of the region's sediments and developing a stratigraphic scale not only for individual regions - regional stratigraphic scales, but also a unified or international stratigraphic scale for the entire Earth.

1) lithological method– any section of sediments must be divided into separate layers or their units.

2) paleontological - is based on the identification of layers containing various complexes of organic residues.

3) micropaleontological method, the object of which is the remains of calcareous and siliceous skeletons of simple organisms.

4) spore-pollen method, based on the study of the remains of spores and pollen grains, which are extremely stable and do not collapse, being carried by the wind over long distances in huge quantities.

The paleontological methods discussed are applicable only to layered sedimentary deposits.

However, large areas of the globe are composed of igneous and metamorphic rocks, devoid of organic remains. This method is not applicable to them.

5) paleomagnetic method, based on the ability of rocks to retain the magnetization character of the era in which they were formed. It should be noted that the paleomagnetic method is extremely widely used to determine the movements of lithospheric plates in the geological past.

Absolute geochronology

1) radiometric methods

table).

2) Luminescent methods

It is also based on changes that gradually accumulate in the crystal under the influence of radiation. Only in this case we are not talking about the number of “excited” electrons capable of “calming down” with the emission of light, but about the number of electrons with a changed spin.

4) amino acid method

Or dating by tree rings, which is highly favored by archaeologists. This method allows you to date only the youngest sediments (up to 5–8 thousand years old), but with very high accuracy, up to one year! It is only necessary that a sufficient amount of wood be found in the excavation.

In the trunks of most trees, annual rings are formed, the width of which varies depending on the weather conditions of the corresponding year.

10 Characteristics of absolute geochronology methods

Absolute geochronology

1) radiometric methods, based on the constancy of the decay rate of radioactive isotopes (see.

table).

While the substance is in a liquid state (liquid magma, for example), its chemical composition is changeable: mixing, diffusion occurs, many components can evaporate, etc.

d. But when the mineral hardens, it begins to behave as a relatively closed system. This means that the radioactive isotopes present in it are not washed out or evaporated from it, and their quantity decreases only due to decay, which occurs at a known constant rate.

2) Luminescent methods Absolute dating is based on the ability of some common minerals (for example, quartz and feldspar) to accumulate the energy of ionizing radiation and then, under certain conditions, quickly release it in the form of light.

Ionizing radiation not only comes to us from space, but is also generated by rocks during the decay of radioactive elements.

3) Electron-paramagnetic or electron-spin resonance method is also based on changes that gradually accumulate in the crystal under the influence of radiation.

Only in this case we are not talking about the number of “excited” electrons capable of “calming down” with the emission of light, but about the number of electrons with a changed spin.

4) amino acid method, based on the fact that “left-handed” amino acids, from which the proteins of all living organisms are built, gradually racemize after death, that is, they turn into a mixture of “right-handed” and “left-handed” forms.

The method is applicable only to very well preserved specimens in which a sufficient amount of primary organic matter has been preserved.

5) Dendrochronological method, or tree-ring dating, is highly favored by archaeologists.

Continental type of earth's crust.

This method allows you to date only the youngest sediments (up to 5–8 thousand years old), but with very high accuracy, up to one year! It is only necessary that a sufficient amount of wood be found in the excavation. In the trunks of most trees, annual rings are formed, the width of which varies depending on the weather conditions of the corresponding year.

11 Tectonic movements of the earth's crust.

Oscillatory movements.

Oscillatory movements are an important link in a complex chain of various geological processes. They are closely related to fold-forming and rupture-forming movements; they largely determine the course of transgression and regression of the sea, changes in the outlines of continents, the nature and intensity of the processes of sedimentation and denudation, etc.

In other words, oscillatory movements are the key to paleogeographical constructions; they make it possible to understand the physical and geographical situation of past times and genetically link a number of geological events.

Some general properties of oscillatory movements:

1) Multiple periods of oscillatory movements.

2) Wide area distribution of oscillatory movements. Oscillatory movements are common everywhere.

3) Reversibility of oscillatory movements.

This is the phenomenon of changing the sign of movement: a rise in the same place over time is replaced by a fall, etc. But each cycle is not a repetition of the previous one, it changes and becomes more complex.

4) Oscillatory movements are not accompanied by the development of linear folding and ruptures.

5) Oscillatory movements and thickness of sedimentary strata. When studying oscillatory movements, the analysis of the thickness of sedimentary strata is of utmost importance. The thickness of a given series of sediments in general terms corresponds in total to the depth of subsidence of the section of the crust within which the given sequence accumulated.

6) Oscillatory movements and paleogeographic reconstructions.

Tectonic movements are movements of the earth's crust caused by processes taking place in its depths.

The main cause of tectonic movements is considered to be convective currents in the mantle, excited by the heat of decay of radioactive elements and gravitational differentiation of its substance in combination with the action of gravity and the tendency of the lithosphere to gravitational equilibrium in relation to the surface of the asteposphere.

1.Vertical tectonic movements.

Any section of the earth's surface has repeatedly experienced ascending and descending tectonic movements over time.

Uplifts.

Marine sediments can often be found high in the mountains. They accumulated initially below sea level, but were later raised to higher altitudes. The amplitude of the rise in some cases can reach 10 km.

2. Horizontal tectonic movements.

They appear in two forms: compression and tension.

Compression. Sedimentary layers collected in folds indicate a decrease in horizontal distances between individual points, which occurred perpendicular to the axes of the folds.

The explanation for the compression was based on the observed loss of heat by the Earth and its possible cooling, which should cause a reduction in its volume.

Stretching.

When stretched, cracks appear through which a huge amount of basaltic magma enters the surface, forming dikes and flows.

13 Main types of faults

The main types of faults are normal faults, thrust faults and shear faults.

Reset - the recumbent wing is raised, the trailing wing is lowered. The displacement falls towards the lowered wing. The angle of incidence is most often 40-60¦, but can be anything. Reset is a tensile deformation.

Large faults outline the depressions of Lake Baikal, Lake Teletskoye, the Red Sea, etc.

Thrust - the recumbent wing is lowered, the hanging wing is raised. The displacement falls towards the raised wing. The angle of incidence is most often 40-60¦. Thrust is a shearing deformation under compression conditions. Hadwigs with a very steep displacement, more than 60¦, are called reverse faults.

A strike-slip fault is a tectonic rupture with movement of the wings mainly in the horizontal direction along the strike of the fault plane.

It is oriented, as a rule, at an angle to the direction of tectonic forces and has a steep or vertical displacement.

In nature, combinations of various types of these faults are possible (fault-slip faults, strike-slip faults, etc.). Based on the nature of the relationship between the fault plane and the strike of layers in a folded structure, longitudinal, transverse, oblique, conformable and unconformable faults are distinguished.

14 Magmatism and igneous rocks

Magma is the Earth's substance in a molten liquid state.

It is formed in the Earth's crust and upper mantle at depths of 30-400 km.

Characteristics of igneous rocks.

1. Mineral composition - minerals are divided into rock-forming (major and secondary) and accessory.

Rock-forming minerals - make up >90% of the rock volume and are represented mainly by silicates:

feldspars, quartz, nepheline - light-colored,

pyroxene, olivine, amphiboles, micas are dark-colored.

In rocks with different chemical compositions, the same mineral can be major or minor.

Accessory minerals constitute, on average, ~1% of the rock volume, and are: apatite, magnetite, zircon, rutile, chromite, gold, platinum, etc.

Classification of igneous rocks

The classification is based on characteristics - chemical composition and genesis.

According to the chemical composition and in particular the content of silica SiO 2, all rocks are divided into:

ultrabasic SiO2 >45%

basic SiO2 up to 45-52%

average SiO2 up to 52-65%

acidic SiO2 up to 65-75%

In turn, among these groups, each is divided according to its genesis into intrusive and effusive.

15 INTRUSIVE MAGMATISM

I. Intrusive magmatism is the process of intrusion of magma into overlying strata and its crystallization in the earth’s crust without reaching the surface at different depths.

This process is characterized by a slow decrease in temperature and pressure, crystallization in a confined space. Igneous rocks consist of completely crystallized granular aggregates of rock-forming minerals.

Such igneous rocks are called intrusive.

Depending on the depth of formation, intrusive massifs are divided into near-surface or subvolcanic (the latter word means that the magma almost approached the surface, but still did not reach it, i.e.

an “almost volcano” or subvolcano has formed) - up to the first hundred meters; medium-depth, or hypabyssal, up to 1-1.5 km and deep, or abyssal, deeper than 1-1.5 km.

Deep veins include secant and stratal veins. A) secant veins Dikes that cross a layer of rock at different angles are called dikes. They are formed as a result of stretching of rocks and filling of space with magma.

Rocks: porphyrites, granite - porphyries, diabases, negmatites. b) strata veins– sills – lie in conformity with the host rocks and are formed as a result of the pushing apart of these rocks by magma.

Deep ones also include:

lopolit(bowl) S = 300 km2, m – 15 km.

in diameter, characteristic of platforms.

phacolite(lentils) – formed simultaneously with folds; S ~ 300 km2, m ~ 10 km.

laccolith– mushroom-shaped, the upper layers are raised; S – 300 km2, m – 10 – 15 km.

There are deep forms such as:

batholiths– large granite intrusions, S – hundreds and thousands of km2, depth – uncertain.

rods– columnar bodies, isometric, S< 100 – 150 км2.

Types of structure of the earth's crust

When studying the earth's crust, its structure was discovered to be different in different areas.

A generalization of a large amount of factual material has made it possible to distinguish two types of structure of the earth's crust - continental and oceanic.

Continental type

The continental type is characterized by a very significant thickness of the crust and the presence of a granite layer.

The boundary of the upper mantle here is located at a depth of 40-50 km or more. The thickness of the sedimentary rock strata in some places reaches 10-15 km, in others the thickness may be completely absent. The average thickness of sedimentary rocks of the continental crust is 5.0 km, the granite layer is about 17 km (from 10-40 km), the basalt layer is about 22 km (up to 30 km).

As mentioned above, the petrographic composition of the basaltic layer of the continental crust is variegated and most likely it is dominated not by basalts, but by metamorphic rocks of basic composition (granulites, eclogites, etc.).

For this reason, some researchers proposed calling this layer granulite.

The thickness of the continental crust increases over the area of ​​folded mountain structures. For example, on the East European Plain the thickness of the crust is about 40 km (15 km - granite layer and more than 20 km - basalt), and in the Pamirs - one and a half times more (about 30 km in total are the thickness of sedimentary rocks and granite layer and the same amount basalt layer).

The continental crust reaches especially great thickness in mountainous areas located along the edges of continents. For example, in the Rocky Mountains (North America) the thickness of the crust significantly exceeds 50 km. The earth's crust, which forms the bottom of the oceans, has a completely different structure. Here the thickness of the crust sharply decreases and the mantle material comes close to the surface.

There is no granite layer, and the thickness of the sedimentary strata is relatively small.

There is an upper layer of unconsolidated sediments with a density of 1.5-2 g/cm3 and a thickness of about 0.5 km, a volcanic-sedimentary layer (interlayering of loose sediments with basalts) with a thickness of 1-2 km, and a basalt layer, the average thickness of which is estimated at 5- 6 km.

At the bottom of the Pacific Ocean, the earth's crust has a total thickness of 5-6 km; At the bottom of the Atlantic Ocean, under a sedimentary layer of 0.5-1.0 km, there is a basalt layer 3-4 km thick. Note that with increasing ocean depth, the thickness of the crust does not decrease.

Currently, transitional subcontinental and suboceanic types of crust are also distinguished, corresponding to the underwater margin of continents.

Within the crust of the subcontinental type, the granite layer is greatly reduced, which is replaced by a thickness of sediments, and then towards the ocean floor the thickness of the basalt layer begins to decrease. The thickness of this transition zone of the earth's crust is usually 15-20 km. The boundary between the oceanic and subcontinental crust passes within the continental slope in the depth range of 1 -3.5 km.

Ocean type

Although oceanic crust occupies a larger area than continental and subcontinental crust, due to its small thickness, only 21% of the volume of the earth's crust is concentrated in it.

Information about the volume and mass of different types of the earth's crust is shown in Fig. 1.

Fig.1. Volume, thickness and mass of horizons of different types of the earth's crust

The earth's crust lies on the subcrustal mantle substrate and makes up only 0.7% of the mass of the mantle. In the case of low crustal thickness (for example, on the ocean floor), the uppermost part of the mantle will also be in a solid state, usual for rocks of the earth's crust.

Therefore, as noted above, along with the concept of the earth's crust as a shell with certain indicators of density and elastic properties, there is the concept of the lithosphere - a stone shell, thicker than solid matter covering the surface of the Earth.

Structures of crustal types

The types of earth's crust also differ in their structures.

The oceanic crust is characterized by a variety of structures. Powerful mountain systems - mid-ocean ridges - stretch along the central part of the ocean floor. In the axial part, these ridges are dissected by deep and narrow rift valleys with steep sides. These formations represent zones of active tectonic activity. Deep-sea trenches are located along island arcs and mountain structures on the edges of continents. Along with these formations, there are deep-sea plains that occupy vast areas.

The continental crust is just as heterogeneous.

Within its boundaries, one can distinguish young mountain-fold structures, where the thickness of the crust as a whole and each of its horizons increases greatly. Areas are also identified where the crystalline rocks of the granite layer represent ancient folded areas, leveled over a long geological time. Here the thickness of the crust is much less. These large areas of continental crust are called platforms. Inside the platforms, a distinction is made between shields - areas where the crystalline foundation comes directly to the surface, and slabs, the crystalline base of which is covered with a thickness of horizontally occurring sediments.

An example of a shield is the territory of Finland and Karelia (Baltic Shield), while on the East European Plain the folded basement is deeply depressed and covered by sedimentary deposits. The average thickness of precipitation on the platforms is about 1.5 km. Mountain-fold structures are characterized by a significantly greater thickness of sedimentary rocks, the average value of which is estimated at 10 km. The accumulation of such thick deposits is achieved by long-term gradual subsidence, subsidence of individual sections of the continental crust, followed by their uplift and folding.

Such areas are called geosynclines. These are the most active zones of the continental crust. About 72% of the total mass of sedimentary rocks is confined to them, while about 28% is concentrated on the platforms.

Manifestations of magmatism on platforms and geosynclines vary sharply. During periods of subsidence of geosynclines, magma of basic and ultrabasic composition enters along deep faults.

In the process of transforming a geosyncline into a folded region, the formation and intrusion of huge masses of granitic magma occurs. The later stages are characterized by volcanic outpourings of lavas of intermediate and acidic composition.

On platforms, magmatic processes are much less pronounced and are represented mainly by outpourings of basalts or lavas of alkaline-basic composition. Among the sedimentary rocks of the continents, clays and shales predominate.

At the bottom of the oceans, the content of calcareous sediments increases. So, the earth's crust consists of three layers. Its upper layer is composed of sedimentary rocks and weathering products. The volume of this layer is about 10% of the total volume of the earth's crust. Most of the matter is located on the continents and transition zone; within the oceanic crust, no more than 22% of the layer volume.

In the so-called granite layer, the most common rocks are granitoids, gneisses and schists.

More basic rocks account for about 10% of this horizon. This circumstance is well reflected in the average chemical composition of the granite layer. When comparing the average composition values, attention is drawn to the clear difference between this layer and the sedimentary sequence (Fig.

Fig.2. Chemical composition of the earth's crust (in weight percent)

The composition of the basalt layer in the two main types of earth's crust is different. On continents, this sequence is characterized by a variety of rocks. There are deeply metamorphosed and igneous rocks of basic and even acidic composition.

Basic rocks make up about 70% of the total volume of this layer. The basalt layer of the oceanic crust is much more homogeneous. The predominant type of rocks are the so-called tholeiitic basalts, which differ from continental basalts in their low potassium, rubidium, strontium, barium, uranium, thorium, zirconium content and high Na/K ratio.

This is due to the lower intensity of differentiation processes during their melting from the mantle. Ultrabasic rocks of the upper mantle emerge in deep reef fractures. The prevalence of rocks in the earth's crust, grouped to determine the ratio of their volume and mass, is shown in Fig. 3.

Fig.3.

Occurrence of rocks in the earth's crust

Formation of the earth's crust

The continental crust consists of crystalline rocks of basalt and granite geophysical layers (59.2% and 29.8%, respectively, of the total volume of the earth’s crust), covered by a sedimentary shell (stratisphere). The area of ​​continents and islands is 149 million.

Types of structure of the earth's crust

km2. The sedimentary shell covers 119 million km2, i.e. 80% of the total land area, wedging out towards the ancient platform shields. It is composed predominantly of Late Proterozoic and Phanerozoic sedimentary and volcanogenic rocks, although it also contains in small quantities older Middle and Early Proterozoic weakly metamorphosed sediments of protoplatforms.

The areas of outcrops of sedimentary rocks decrease with increasing age, while those of crystalline rocks increase.

The sedimentary shell of the earth's crust of the oceans, occupying 58% of the total area of ​​the Earth, rests on a basalt layer. The age of its deposits, according to deep-sea drilling data, covers the time interval from the Upper Jurassic to the Quaternary period inclusive. The average thickness of the Earth's sedimentary shell is estimated at 2.2 km, which corresponds to 1/3000 of the radius of the planet. The total volume of its constituent formations is approximately 1100 million.

km3, which is 10.9% of the total volume of the earth's crust and 0.1% of the total volume of the Earth. The total volume of ocean sediments is estimated at 280 million km3. The average thickness of the earth's crust is estimated at 37.9 km, which is 0.94% of the total volume of the Earth. Volcanic rocks account for 4.4% on platforms and 19.4% in folded areas of the total volume of the sedimentary shell.

In platform areas and especially in the oceans, basalt covers are widespread, occupying more than two-thirds of the Earth's surface.

The Earth's crust, atmosphere and hydrosphere of the Earth were formed as a result of the geochemical differentiation of our planet, accompanied by the melting and degassing of deep matter. The formation of the earth's crust is caused by the interaction of endogenous (magmatic, fluid-energy) and exogenous (physical and chemical weathering, destruction, decomposition of rocks, intensive terrigenous sedimentation) factors.

The isotopic systematics of igneous rocks is of great importance, since it is magmatism that carries information about geological time and the material specificity of surface tectonic and deep mantle processes responsible for the formation of oceans and continents and reflects the most important features of the processes of transformation of the Earth's deep substance into the earth's crust. The most reasonable is considered to be the sequential formation of the oceanic crust due to the depleted mantle, which in zones of convergent interaction of plates forms the transitional crust of island arcs, and the latter, after a series of structural and material transformations, turns into the continental crust.

Structure and types of the earth's crust

Earth's crust, which makes up the upper shell of the Earth, is heterogeneous vertically and horizontally.

The upper boundary of the earth's crust is the upper solid surface of the planet, the lower - the surface of the mantle. In terms of its state of aggregation, the upper part of the mantle is closer to the earth's crust, so they are combined into a single rock shell - the lithosphere.

The upper boundary of the lithosphere and the earth's crust coincide, the lower boundary runs along the surface of the asthenosphere. Under the continents, both the earth's crust and the lithosphere have greater thickness than under the oceans, while the thickness of both the earth's crust and the suprasthenospheric layer of the mantle increases or decreases synchronously.

The most consistent structure is found in ancient blocks of the earth's crust, or continental cores, which are more than 2 billion years old. Three layers (shells) are distinguished in them: the upper one is a sedimentary layer, then granite and even lower basalt.

These names are given based on the physical properties of the layers, and not on the composition, and therefore are arbitrary.

Sedimentary layer composed of sedimentary and volcanogenic-sedimentary rocks. Soils and modern, including technogenic, sediments are not included in it. The bulk of the rocks are clayey and sandy (almost 70%): loose (clay, sand) and cemented (shales, sandstones).

Carbonate rocks (limestones, marls, etc.) are cemented. Rocks that have undergone thermodynamic transformations (decrystallization) are absent or are rare and local. Such layers occur horizontally and subhorizontally.

Occasionally, this layer is broken through by silicate melts similar in composition to basalts. Sedimentary rocks often contain layers of coal and layers saturated with gases and oil. The average density of rocks is 2.45 g/cm3.

The thickness of the layer varies from 0 to 20 km, averaging about 3.5 km. It is underlain by granite or basalt layers.

granite layer consists of gneisses, similar in composition to granites, and granites, together accounting for almost 80%.

Therefore, this layer is often called granite-gneiss. The rocks that make up this layer form bodies in the form of layers, lenses, veins, often break through layered strata and are introduced along faults in the form of intrusions. All these bodies are deformed, crushed, crushed into folds, broken into blocks, i.e.

e. experience thermodynamic and tectonic influences and recrystallization. The thickness of the layer varies from 0 to 25 km. It is covered by a sedimentary layer.

Below the granite layer lies a basalt layer. The boundary between them is called surface (section) of Conrad and is usually not clearly expressed. The average density of the layer is 2.7 g/cm3.

Basalt layer consists mainly of gneisses, similar in composition to mafic rocks, gabbroids and granulites, and therefore is often called mafic-gneiss or granulite-gneiss.

Below the basalt layer of the earth's crust lies suprasthenospheric layer the mantle, which, as already mentioned, enters the lithosphere along with the earth's crust.

This layer is close in composition to peridotites and is called ultramafic. The average density is 3.3 g/cm3, significantly higher than that of the rocks of the lower crust. Under the continents, this layer is depleted in silicon, potassium, aluminum and volatile components (sialic). Such a mantle is called “depleted,” that is, it has given up a significant part of its light elements for the formation of the earth’s crust. The mafic-gneiss layer of the continents is also different from the basalt layer of the oceanic crust.

In the crust of the oceans there are two “basalt” layers: continental and oceanic types. This pattern is characteristic of ancient oceanic crust near continental margins.

Based on the basic elements of the earth's crust, composition and thickness, there are two main types of the earth's crust: continental and oceanic.

Continental crust - the crust of the continents (and the adjacent shallow shelf) is characterized by great thickness, reaching 75-80 km in young mountain structures and 35-45 km within platforms.

It is composed of igneous, sedimentary and metamorphic rocks, forming three layers (Fig. 5.1). The uppermost sedimentary layer, represented by sedimentary rocks, has a thickness of 0 to 5 (10) km and is characterized by discontinuous distribution. It is absent from the most elevated areas of ancient cratons - ledges and shields.

In some of the most depressed structures of the earth's crust - depressions and syneclises - the thickness of the sedimentary layer reaches 15-20 km. The rock density values ​​here are small, and the propagation speed of longitudinal seismic waves is (V) 2-5 km/s.

Below lies granite(now called granite-gneiss) layer composed mainly of granites, gneisses and other metamorphic rocks of different metamorphic facies.

The most complete sections of this layer are presented on crystalline shields of ancient cratons. The rock density values ​​here are measured in the range of 2.5-2.7 g/cm3, and the speed of propagation of longitudinal seismic waves (K) is up to 5-6.5 km/s. Its average thickness is 15-20 km, and sometimes reaches 25 km.

The third, bottom layer is called basalt.

In terms of the average chemical composition and the speed of propagation of seismic waves, this layer is close to basalts. True, there is an assumption that the layer is composed of basic rocks such as gabbro and metamorphic varieties of rocks of amphibolite and granulite facies.

The presence of ultramafic rocks of garnet-pyroxene composition - eclogites - cannot be ruled out. Therefore, it would be more correct to call it granulite-mafic. The thickness of the layer varies within 15-20-35 km, the speed of propagation of longitudinal seismic waves increases (K) to 6.5-6.7-7.4 km/s.

The boundary between the granite-gneiss and granulite-mafic layers is called the Conrad seismic section, which is distinguished by the jump in V waves from 6.5 to 7.4 km/s at the base of the third layer.

In recent years, deep seismic data have shown that the Conrad boundary does not exist everywhere.

V.V. Belousov and N.I. Pavlenkova proposed a new four-layer model of the earth’s crust (Fig. 5.2). This model identifies the upper sedimentary layer with a clear velocity boundary - K0.

Below are three layers of consolidated crust: upper, intermediate and lower, separated by the boundaries K1 and K2. The K1 boundary is established at a depth of 10-15 km, above it there are rocks with velocities V = 5.9-6.3 km/s. The K2 boundary passes at a depth of about 30 km and the rocks between K1 and K2 are characterized by Vр = 6.4-6.5 km/s. In the lower layer, V reaches 6.8-7.0 km/s.

The material composition of the lower layer is represented by rocks of granulite facies metamorphism and basic and ultrabasic igneous rocks.

The middle and upper layers are considered to be composed of igneous and metamorphic rocks of felsic composition.

Thus, the proposed three-layer model of the consolidated part of the continental crust is based only on seismic data, and the petrographic composition actually corresponds to a two-layer model: granulite-gneiss and granulite-mafic layers.

Oceanic crust. It was previously believed that the oceanic crust consists of two layers: upper sedimentary and lower basaltic.

Long-term studies of the ocean floor through drilling, dredging and seismic work have established that the oceanic crust has a three-layer structure with an average thickness of 5-7 km.

1. Sedimentary The upper layer consists of loose sediments of different composition and thickness, varying over a very wide range, from several hundred meters to 6-7 km.

The sedimentary layer reaches its maximum thickness in oceanic trenches (6.5 km in southwestern Japan) or in submarine alluvial fans (for example, the Bengal cone along the continuation of the Ganges and Brahmaputra rivers, the Amazonian, Mississippian, where the thickness of sediments reaches 3-5 km).

Propagation speed Vр = 1.0-2.5 km/s.

2. The second layer, located below, is composed mainly of basaltic lavas of the pillow and cover types. The relationship between different types of lavas at the bottom of the caldera of Mount Axial (Juan de Fuca Ridge) was mapped in detail during one of the expeditions of the R/V Mstislav Keldysh in 1985 (Fig. 5.3).

3. The third, lower layer, according to dredging and deep-sea drilling data, is composed of basic igneous rocks such as gabbro and ultrabasic (peridotites, pyroxenites).

The section of oceanic crust exposed in the Hess Basin in the Galapagos Rift of the Pacific Ocean was sampled by dredging and examined from the French Nautilus lander (Fig. 5.4).

Structure of the continental crust

At the base of the section there are gabbros with K = 6.8 km/s, which above are replaced by dolerites with a thickness of up to 1 km and F = 5.5 km/s, and the section ends with pillow and cover lavas of tholeiitic basalts with a thickness of about 1 km.

At the base of the section there are peridotites. The layered structure of the oceanic crust can be traced over long distances, which is confirmed by multichannel seismic profiling data.


The results of geophysical research in recent decades have resulted in the identification of two more intermediate (transitional) types of the earth's crust: subcontinental and suboceanic.

Subcontinental type of earth's crust its structure is close to the continental crust, has a smaller thickness of 20-30 km and a vaguely defined Conrad boundary.

Characteristic of island arcs and continental margins.

Suboceanic type of the earth's crust is isolated in the deep-sea basins of the marginal and inland seas (Okhotsk, Japan, Mediterranean, Black, etc.). This type differs from the oceanic crust in the increased thickness of the sedimentary layer (4-10 km or more), and its total thickness is 10-20, in some places 25-30 km.

The Earth's crust (lithosphere) is the upper shell of the Earth. There are two types of earth's crust: oceanic And continental (mainland). The coincidence of their boundaries with the coastline of the world's oceans is observed over most of the latter's length, but there are also significant areas where they diverge. At the same time, the areas of continents located below sea level significantly predominate.

It is customary to distinguish three layers in the composition of the bark - the upper sedimentary, average granite and lower basaltic(Fig. 1.9).

Rice. 1.9.

The identification of layers is based on geophysics data on the speed of seismic waves. Sedimentary and granite layers are not widespread; basalt layers are present everywhere. The names of the two lower layers should not be taken literally. There are rocks there with seismic wave velocities corresponding to granites and basalts. In reality, there may be other breeds, similar or not similar to them.

The separation of granite and basalt layers during well drilling has not been confirmed in many cases. Wells buried in granites, instead of the granite-basalt boundary, revealed granites, gneisses or some other rocks. Basalts were repeatedly exposed only where the granite layer was completely absent. As a result, the question arose about the legality of identifying a granite layer, and this question remains open, but geologists do not abandon the three-layer structure of the earth’s crust.

Two types of earth's crust - oceanic crust and continental crust are distinguished on the basis of geophysical data. The oceanic crust is thinner and is 5-15 km (average 10 km), and lacks a granite layer. The continental crust is thicker - 30-40 km (occasionally up to 80 km). The connection between the two types of crust and the presence of land and oceans is clear in some places, but not in others. The thicker continental crust is more submerged in the mantle and is more uplifted, protruding above sea level.

The continental crust is less dense and seems to float on the surface of the mantle, preserving for billions of years. The oceanic crust is denser, its sections are drawn into the convective movement of mantle matter, i.e. in some places they sink into the mantle and melt there. In other places, mantle material rises to the surface, solidifies, and new oceanic crust grows (Fig. 1.10).

Therefore, in the oceans (on the oceanic crust) sediments older than 250 million years are not found.

Rice. 1.10.

The figure shows that at the site of ascent the thickness of the oceanic crust is minimal, and at the site of descent it is maximum. The continental crust does not participate in convection.

The parts of the continents that fall below ocean level are called shelf. The depth of the sea within the shelf usually does not exceed 200 m. Currently, for example, the shelf includes the North Atlantic and a significant part of the Arctic Ocean (the bottom of the North, Baltic, White, Kara, East Siberian Seas, Laptev Sea, East China Sea ), a strip of the Atlantic Ocean near the southern coast of Argentina, the space between Australia and Indochina, vast areas around New Zealand and Antarctica.

In the geological past, shelf marine conditions regularly arose on continents in one place or another. This is indicated by the presence of a sedimentary layer - a cover of marine rocks that are widespread on the continents. For example, in Moscow the thickness of the cover is about 1.5 km.

It is believed that in the geological past, land and sea regularly replaced each other here, and the land existed approximately

2/3, and the sea 1/3 of the time, the continental type of crust was preserved (Fig. 1.11).

Rice. 1.11.

There are few areas of oceanic crust that rise above sea level and form land - the island of Iceland and a few small islands in the Pacific Ocean. According to modern ideas, the main structures of the earth's crust are the so-called lithospheric plates - areas of the earth's crust that undergo independent horizontal movements. The current location of lithospheric plates is shown in Fig. 1.12.

Rice. 1.12.

7 - Eurasian (/, A- Chinese; 1,6 - Iranian; 1, in- Turkish; 1,g- Hellenic; 1, d- Adriatic); 2 - African (2, A- Arabian); 3 - Indo-Australian (3, A- Fiji; 3,6 - Solomonova); 4 - Pacific ( 4, a- Nazca; 4,6 - Coconut; 4, in- Caribbean; 4, g- Proud; 4, d- Philippine; 4, e- Bismarck); 5 - American (5, A- North American; 5 B- South American);

b - Antarctic

The speed of movement of lithospheric plates is up to several centimeters per year, the total movements in geological time are many thousands of kilometers horizontally. A lithospheric plate can consist either of only a piece of continental or oceanic crust, or of a combined section of both crusts. In many places where lithospheric plates contact, increased tectonic, volcanic and other activity is observed.

Test questions and assignments

• 1. Tell us about the origin of the Universe and the Earth.
• 2. Describe the structure of the solar system.
• 3. Based on what methods are ideas about the structure of the Earth formed?
• 4. What are geophysical methods for studying the deep structure of the Earth?
• 5. What are the shape, size, density, chemical composition of the Earth?
• 6. What is the structure of the Earth according to geophysical data?
• 7. Name the main types of the earth's crust. What is a shelf?
• 8. What are sedimentary, granite and basalt layers?

There are 2 main types of the earth's crust: continental and oceanic, and 2 transitional types - subcontinental and suboceanic (see figure).

1- sedimentary rocks;

2- volcanic rocks;

3- granite layer;

4- basalt layer;

5- Mohorovicic border;

6- upper mantle.

The continental type of the earth's crust has a thickness of 35 to 75 km, in the shelf area - 20 - 25 km, and pinches out on the continental slope. There are 3 layers of continental crust:

1st – upper, composed of sedimentary rocks with a thickness of 0 to 10 km. on platforms and 15 – 20 km. in tectonic deflections of mountain structures.

2nd – medium “granite-gneiss” or “granite” - 50% granites and 40% gneisses and other metamorphosed rocks. Its average thickness is 15–20 km. (in mountain structures up to 20 - 25 km.).

3rd – lower, “basalt” or “granite-basalt”, compositionally close to basalt. Power from 15 – 20 to 35 km. The boundary between the “granite” and “basalt” layers is the Conrad section.

According to modern data, the oceanic type of the earth’s crust also has a three-layer structure with a thickness of 5 to 9 (12) km, more often 6–7 km.

1st layer – upper, sedimentary, consists of loose sediments. Its thickness ranges from several hundred meters to 1 km.

2nd layer – basalts with interlayers of carbonate and silicon rocks. Thickness from 1 – 1.5 to 2.5 – 3 km.

The 3rd layer is the bottom one, not opened by drilling. It is composed of basic igneous rocks of the gabbro type with subordinate, ultrabasic rocks (serpentinites, pyroxenites).

The subcontinental type of earth's surface is similar in structure to the continental one, but does not have a clearly defined Conrad section. This type of crust is usually associated with island arcs - the Kuril, Aleutian and continental margins.

1st layer – upper, sedimentary – volcanic, thickness – 0.5 – 5 km. (on average 2 – 3 km.).

2nd layer – island arc, “granite”, thickness 5 – 10 km.

The 3rd layer is “basalt”, at depths of 8 – 15 km, with a thickness from 14 – 18 to 20 – 40 km.

The suboceanic type of the earth's crust is confined to the basin parts of the marginal and inland seas (Okhotsk, Japan, Mediterranean, Black, etc.). It is close in structure to the oceanic one, but is distinguished by an increased thickness of the sedimentary layer.

1st upper – 4 – 10 or more km, located directly on the third oceanic layer with a thickness of 5 – 10 km.

The total thickness of the earth's crust is 10–20 km, in some places up to 25–30 km. due to an increase in the sedimentary layer.

A peculiar structure of the earth's crust is observed in the central rift zones of the mid-ocean ridges (Mid-Atlantic). Here, under the second oceanic layer, there is a lens (or protrusion) of low-speed material (V = 7.4 - 7.8 km / s). It is believed that this is either a protrusion of an abnormally heated mantle, or a mixture of crustal and mantle matter.

Structure of the earth's crust

On the surface of the Earth, on continents, rocks of different ages are found in different places.

Some areas of the continents are composed on the surface of the most ancient rocks of Archean (AR) and Proterozoic (PT) age. They are highly metamorphosed: clays turned into metamorphic shales, sandstones into crystalline quartzites, limestones into marbles. There are many granites among them. The areas on the surface of which these most ancient rocks emerge are called crystalline massifs or shields (Baltic, Canadian, African, Brazilian, etc.).

Other areas on the continents are occupied by rocks of predominantly younger age - Paleozoic, Mesozoic, Cenozoic (Pz, Mz, Kz). These are mainly sedimentary rocks, although among them there are also rocks of igneous origin, erupted on the surface in the form of volcanic lava or embedded and frozen at some depth. There are two categories of land areas: 1) platforms - plains: layers of sedimentary rocks lie calmly, almost horizontally, with rare and small folds observed in them. There is very little igneous, especially intrusive, rock in such rocks; 2) folded zones (geosynclines) - mountains: sedimentary rocks are strongly folded, penetrated by deep cracks; Intruded or erupted igneous rocks are often encountered. The differences between platforms or folded zones lie in the age of the resting or folded rocks. Therefore, there are ancient and young platforms. By saying that the platforms could have formed at different times, we thereby indicate different ages of the folded zones.

Maps depicting the location of platforms and folded zones of different ages and some other features of the structure of the earth's crust are called tectonic. They serve as a complement to geological maps, which represent the most objective geological documents illuminating the structure of the earth's crust.

Types of the earth's crust

The thickness of the earth's crust is not the same under continents and oceans. It is larger under mountains and plains, thinner under oceanic islands and oceans. Therefore, there are two main types of the earth's crust - continental and oceanic.

The average thickness of the continental crust is 42 km. But in the mountains it increases to 50-60 and even 70 km. Then they talk about “the roots of the mountains.” The average thickness of the oceanic crust is about 11 km.

Thus, the continents represent, as it were, unnecessary accumulations of masses. But these masses should create a stronger attraction, and in the oceans, where the attracting body is lighter water, the force of gravity should weaken. But in reality there are no such differences. The force of gravity is approximately the same everywhere on the continents and oceans. This leads to the conclusion: continental and oceanic masses are balanced. They obey the law of isostasy (equilibrium), which reads like this: additional masses on the surface of the continents correspond to a lack of masses at depth, and vice versa - the lack of masses on the surface of the oceans must correspond to some heavy masses at depth.

All described types of rocks participate in the structure of the earth's crust - igneous, sedimentary and metamorphic, occurring above the Moho boundary. Both within the continents and within the oceans, mobile belts and relatively stable areas of the earth's crust are distinguished. On continents, stable areas include vast flat spaces - platforms (East European, Siberian), within which the most stable areas are located - shields (Baltic, Ukrainian), which are outcrops of ancient crystalline rocks. Mobile belts include young mountain structures, such as the Alps, Caucasus, Himalayas, Andes and others (Figure 3.1).

Figure 3.1. Generalized profile of the ocean floor (according to O. K. Leontiev)

Continental structures are not limited only to continents; in some cases they extend into the ocean, forming the so-called underwater margin of continents, consisting of a shelf, up to 200 m deep, a continental slope with a foot to depths of 2500 -3000 m. Stable areas are also distinguished within the oceans - ocean platforms - significant areas of the ocean floor - vast abyssal (Greek "abyssos" - abyss) plains 4-6 km deep, and mobile belts, which include mid-ocean ridges and active margins of the Pacific Ocean with developed marginal seas (Okhotsk, Japanese etc.), island arcs (Kuril, Japanese, etc.) and deep-sea trenches (8-10 km or more deep).

At the first stages of geophysical research, two main types of the earth's crust were distinguished: 1) continental and 2) oceanic, sharply different from each other in the structure and thickness of the constituent rocks. Subsequently, two transitional types began to be distinguished: 1) subcontinental and 2) suboceanic (Figure 3.2).

Legend:

1 - water; 2 - sedimentary layer; 3 - granite layer; 4 - basalt layer of continental crust; 5 - basalt layer of oceanic crust; 6 - magmatic layer of oceanic crust; 7 - volcanic islands; 8.9 - mantle (ultrabasic igneous rocks).

Figure 3.2 - Scheme of the structure of various types of the earth’s crust

Continental type of earth's crust

Continental type of earth's crust. The thickness of the continental crust varies from 35-40 (45) km within platforms to 55-70 (75) km in young mountain structures. The continental crust continues into the submarine margins of the continents. In the shelf area, its thickness decreases to 20-25 km, and on the continental slope (at a depth of about 2.0-2.5 km) it pinches out. The continental crust consists of three layers. The first - the uppermost layer is represented by sedimentary rocks, with a thickness of 0 to 5 (10) km within platforms, up to 15-20 km in tectonic troughs of mountain structures. The velocity of longitudinal seismic waves (Vp) is less than 5 km/s. The second - traditionally called "granite" layer is 50% composed of granites, 40% - gneisses and other metamorphosed rocks to varying degrees. Based on these data, it is often called granite-gneiss or granite-metamorphic. Its average thickness is 15-20 km (sometimes in mountain structures up to 20-25 km). Seismic wave speed (Vp) - 5.5-6.0 (6.4) km/s. The third, lower layer is called "basalt". In terms of average chemical composition and seismic wave speed, this layer is close to basalts.

However, it is suggested that it is composed of basic intrusive rocks such as gabbro, as well as metamorphic rocks of the amphibolite and granulite facies of metamorphism; the presence of ultrabasic rocks is not excluded. It is more correct to call this layer granulite-mafic (mafic is the main rock). Its thickness varies from 15-20 to 35 km. Wave propagation speed (Vp) 6.5-6.7 (7.4) km/s. The boundary between granite-metamorphic and granulite-mafic layers is called the Conrad seismic section. For a long time, the prevailing idea was that the Conrad boundary exists everywhere in the continental crust. However, subsequent deep seismic sounding data showed that the Conrad surface is not expressed everywhere, but is recorded only in certain places. Naturally, new interpretations of the structure of the continental crust arise. Thus, N.I. Pavlenkova and others proposed a four-layer model (Fig. 3.3). This model identifies an upper sedimentary layer with a clear velocity boundary, designated Ko. The lower parts of the earth's crust are combined into the concept of a crystalline foundation, or consolidated crust, within which three layers are distinguished: upper, intermediate and lower, separated by the boundaries K1 and K2. There is sufficient stability of the K2 boundary - between the intermediate and lower floors. The upper floor is characterized by a vertically layered structure and differentiation of individual blocks in composition and physical parameters. For the intermediate floor, thin horizontal layering and the presence of individual plates with a reduced seismic wave velocity (Vp) - 6 km/s (with a total velocity in the layer of 6.4-6.7 km/s) and an anomalous density are noted.

Based on this, it is concluded that the intermediate layer can be classified as a weakened layer, along which horizontal movements of the substance are possible. Currently, other researchers are paying attention to the presence of individual lenses in the continental crust with relatively (0.1-0.2 km/s) reduced seismic wave velocities at depths of 10-20 km, with a lens power of 5-10 km. It is assumed that these zones (or lenses) are associated with strong fracturing and water content in the rocks.

S. R. Taylor's data also indicate that within the continental crust there is no single layer with a reduced velocity, but discontinuous layering is noted. All of the above indicates the great complexity of the continental crust and the ambiguity of its interpretation. Quite convincing evidence of this is the data obtained during drilling of the ultra-deep Kola well, which has already reached a depth of over 12 km. According to preliminary seismic data, in the area where the well was laid, the boundary between the “granite” and “basalt” layers should be encountered at a depth of about 7 km. In reality, there was no geophysical “basalt” layer. At this depth, under a thick metamorphosed volcanogenic-sedimentary strata of Proterozoic age, plagioclase gneisses, granite-gneisses, and amphibolites were discovered - rocks of the medium-temperature stage of metamorphism, the percentage of which increases with depth. What caused the change in the speed of seismic waves (from 6.1 to 6.5-6.6 km/s) at a depth of about 7 km, where the presence of a geophysical “basalt” layer was assumed? It is possible that this is due to amphibolites and their role in changing the elastic properties of rocks. It is also possible that the boundary indicated earlier (before drilling the well) is not associated with a change in the composition of the rocks, but with an increase in the stress field caused by intense deformations and repeated manifestations of metamorphism.

– 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 the velocities of seismic waves sharply increase here. It was installed in $1909 by a Croatian scientist A. Mohorovicic ($1857$-$1936$).

The earth's crust is composed sedimentary, igneous and metamorphic rocks, and according to its composition it stands out three layers. Rocks of sedimentary origin, the destroyed material of which was redeposited into the lower layers and formed sedimentary layer The earth's crust covers the entire surface of the planet. It is very thin in some places and may be interrupted. In other places it reaches a thickness of several kilometers. Sedimentary rocks are clay, limestone, chalk, sandstone, etc. They are formed by sedimentation of substances in water and on land, and usually lie in layers. From sedimentary rocks one can learn about the natural conditions that existed on the planet, which is why geologists call them pages of Earth's history. Sedimentary rocks are divided into organogenic, which are formed by the accumulation of animal and plant remains and inorganogenic, which in turn are divided into clastic and chemogenic.

Clastic rocks are a product of weathering, and chemogenic- the result of sedimentation of substances dissolved in the water of seas and lakes.

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

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

The vertical structure and thickness of the earth's crust are different, so there are several types. 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. Continental crust is located under the continents, but its edge does not coincide with the coastline. From a geological point of view, a real continent is the entire area of ​​continuous continental crust. Then it turns out that geological continents are larger than geographical continents. Coastal zones of continents, called shelf- these are parts of continents temporarily flooded by the sea. Seas such as the White, East Siberian, and Azov seas are located on the continental shelf.

There are three layers in the continental crust:

• The top 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 marginal troughs, as well as on the passive margins of Atlantic-type continents.

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

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

Note 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 crust, young science began to solve geochemistry. Since the bark consists of many different rocks, this task was quite difficult. Even within the same geological body, the composition of rocks can vary greatly, and different types of rocks can be distributed 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 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 was able to summarize the results and calculate the average composition of rocks, which was close to granite. Job 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 V. Goldshmidt. He suggested that moving along the continental crust glacier, can scrape and mix exposed rocks that will be deposited during glacial erosion. They will then reflect the composition of the middle continental crust. Having analyzed the composition of ribbon clays that were deposited in the last glaciation Baltic Sea, he got a result close to the result Clark. Different methods gave similar estimates. Geochemical methods were confirmed. These issues have been addressed and the assessments Vinogradov, Yaroshevsky, Ronov, etc..

Oceanic crust

Oceanic crust is located where the sea depth 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 crust is structured differently from the continental crust, although it is also divided into layers. It is almost completely absent granite layer, and 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 have similar seismic wave velocities. The basalt layer predominates in the oceanic crust. According to the theory of plate tectonics, oceanic crust is constantly formed at mid-ocean ridges, then it moves away from them and into areas subduction absorbed into the mantle. This indicates that the oceanic crust is relatively young. The largest number of subduction zones is characteristic of Pacific Ocean, where powerful seaquakes are associated with them.

Definition 1

Subduction is the descent of rock from the edge of one tectonic plate into the 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 zones varies from $5$-$7$ km. Over time, the thickness of the oceanic crust remains virtually unchanged. This is due to the amount of melt released from the mantle at mid-ocean ridges and the thickness of the sedimentary layer at the bottom of the oceans and seas.

Sedimentary layer The 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 depths greater than $4.5 km, carbonate rocks are replaced by red deep-sea clays and siliceous silts.

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

Definition 2

Dykes- these are channels through which basaltic lava flows to the surface

Basalt layer in zones subduction turns into ecgoliths, which plunge into 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 the oldest sections of it are $156$ million years old and are located in the depression Jacket in the Pacific Ocean. The oceanic crust is concentrated not only within the bed of the World Ocean, 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 km sq.