The continental crust, both in composition and structure, differs sharply from the oceanic one. Its thickness varies from 20-25 km under island arcs and areas with a transitional type of crust to 80 km under the young folded belts of the Earth, for example, under the Andes or the Alpine-Himalayan belt. On average, the thickness of the continental crust under the ancient platforms is approximately 40 km, and its mass, including the subcontinental crust, reaches 2.2510 × 25 g. The relief of the continental crust is very complex. However, it distinguishes vast sediment-filled plains, usually located above the Proterozoic platforms, protrusions of the most ancient (Archaean) shields, and younger mountain systems. The relief of the continental crust is also characterized by maximum height differences, reaching 16-17 km from the foot of the continental slopes in deep-water trenches to the highest mountain peaks.

The structure of the continental crust is very heterogeneous, however, as in the oceanic crust, in its thickness, especially in ancient platforms, three layers are sometimes distinguished: the upper sedimentary and two lower layers composed of crystalline rocks. Under the young mobile belts, the structure of the crust is more complex, although its general dissection approaches two-layer.

The sedimentary layer on the continents has been studied quite fully both with the help of geophysical exploration methods and direct drilling. The structure of the surface of the consolidated crust in places where it was exposed on ancient shields was studied both by direct geological and geophysical methods, and on continental platforms covered by sediments, mainly by geophysical research methods. Thus, it was found that the velocities of seismic waves in the layers of the earth's crust increase from top to bottom from 2-3 to 4.5-5.5 km / s in the lower sedimentary strata; up to 6-6.5 km/s in the upper layer of crystalline rocks and up to 6.6-7.0 km/s in the lower layer of the crust. Almost everywhere, the continental crust, like the oceanic one, is underlain by high-velocity rocks of the Mokhorovichich boundary with seismic wave velocities from 8.0 to 8.2 km/s, but these are already properties of the subcrustal lithosphere composed of mantle rocks.

The thickness of the upper sedimentary layer of the continental crust varies over a wide range - from zero on ancient shields to 10-12 and even 15 km on the passive margins of the continents and in the marginal troughs of the platforms. The average thickness of sediments on stable Proterozoic platforms is usually close to 2-3 km. The sediments on such platforms are dominated by clayey sediments and carbonates from shallow marine basins. In marginal troughs and on the passive margins of Atlantic-type continents, sedimentary sections usually begin with coarse clastic facies, which are replaced upstream by sandy-argillaceous deposits and carbonates of coastal facies. Both at the base and in the uppermost parts of the sections of the sedimentary strata of the marginal troughs, chemogenic sediments are sometimes found - evaporites, which mark the conditions of sedimentation in narrow semi-enclosed marine basins with an arid climate. Typically, such basins arise only at the initial or final stage of the development of marine basins and oceans, if, of course, these oceans and basins at the time of their formation or closure were located in arid climate zones. Examples of the deposition of such formations at the early stages of the formation of oceanic basins are evaporites at the base of sedimentary sections of the African shelf zones in the Atlantic Ocean and salt-bearing deposits of the Red Sea. Examples of the deposition of salt-bearing formations confined to closing basins are the evaporites of the Reno-Hercynian zone in Germany and the Permian salt-gypsum-bearing sequences in the Cis-Ural marginal foredeep in the east of the Russian Platform.

The upper part of the section of the consolidated continental crust is usually represented by ancient, mainly Precambrian rocks of granite-gneiss composition or alternation of granitoids with belts of greenstone rocks of basic composition. Sometimes this part of the section of the hard crust is called the "granite" layer, thereby emphasizing the predominance of rocks of the granitoid series in it and the subordination of basaltoids. The rocks of the "granite" layer are usually transformed by processes of regional metamorphism up to and including the amphibolite facies. The upper part of this layer is always a denudation surface, along which the erosion of tectonic structures and igneous formations of the ancient folded (mountainous) belts of the Earth once occurred. Therefore, the overlying sediments on the bedrocks of the continental crust always occur with a structural unconformity and usually with a large time shift in age.

In deeper parts of the crust (approximately at depths of about 15-20 km), a scattered and unstable boundary is often traced, along which the propagation velocity of longitudinal waves increases by about 0.5 km/s. This is the so-called Konrad boundary, outlining from above the lower layer of the continental crust, sometimes conditionally called "basalt", although we still have very little definite data on its composition. Most likely, the lower parts of the continental crust are composed of rocks of intermediate and basic composition, metamorphosed to amphibolite or even to granulite facies (at temperatures above 600 °C and pressure above 3–4 kbar). It is possible that at the base of those blocks of continental crust that were once formed due to collisions of island arcs, there may be fragments of ancient oceanic crust, including not only basic, but also serpentinized ultrabasic rocks.

The heterogeneity of the continental crust is especially clearly visible even with a simple glance at the geological map of the continents. Usually, separate and closely intertwined blocks of the crust, heterogeneous in composition and structure, are geological structures of different ages - the remains of ancient folded belts of the Earth, successively adjoining each other during the growth of continental masses. Sometimes such structures, on the contrary, are traces of former splits of ancient continents (for example, aulacogenes). Such blocks are usually in contact with each other along suture zones, often called, not very successfully, deep faults.

The studies of the deep structure of the continental crust carried out in the last decade by the seismic method of reflected waves with signal accumulation (COCORT project) have shown that the suture zones separating folded belts of different ages are, as a rule, giant thrust faults. The thrust surfaces, which are steep in the upper parts of the crust, rapidly flatten with depth. Horizontally, such thrust structures are often traced for many tens and up to hundreds of kilometers, while in depth they sometimes approach the very base of the continental crust, marking ancient and now dead zones of lithospheric plate underthrust or associated secondary thrusts.

At one time I read many books by Wells, Doyle, Verne, and each of these authors has a work describing underwater life. As a rule, it mentions the features of life on the ocean floor or penetrating through the earth's crust. Therefore, I wanted to figure out how the land differs from the bottom of the sea.

Continental crust is different from oceanic

Of course, the main difference between them will be their location: the first carries all the land and continents, and the second - the seas, oceans, and indeed all water bodies. But they also differ in other ways:

• the first consists of granulites, the second - of basalt;
• the continental crust is thicker than the oceanic;
• the land crust is inferior to the oceanic in area, but wins in total volume;
• the oceanic crust is more mobile and is able to layer on the continental one.

The process described in the last paragraph is called obduction and means the layering of tectonic plates one on top of the other.

Main characteristics of the continental crust

Such a crust is also called continental, and it consists of 3 layers.

1. Upper sedimentary - consists of rocks of the same name, different in origin, age, location. Usually its thickness reaches 25 km.
2. Medium granite-metaphorical - formed from acidic rocks, similar in composition to granite. The thickness of the layer varies from 15 to 30 km (its greatest thickness is recorded under the highest mountains).
3. Lower basaltic - formed by metamorphosed rocks. Its thickness reaches 10–30 km.

It is noteworthy that the third layer is called "basalt" conditionally: seismic waves pass through it at the same speed as they would pass through basalt.

Oceanic crust parameters

Some scientists distinguish only 2 main ones, but, in my opinion, it is better to take a three-level interpretation of the structure of this cortex.

1. The upper layer is represented by sedimentary rocks, which can reach a thickness of 15 km.
2. The middle layer is composed of pillow lavas; its thickness does not exceed 20 km.
3. The third layer consists of basic igneous rocks, its thickness is 4–7 km.

The last layer is also called "gabbro" due to the crystalline structure of the rock.

The earth's crust is a multi-layered formation. Its upper part - the sedimentary cover, or the first layer - is formed by sedimentary rocks and sediments that are not compacted to the state of rocks. Below, both on the continents and in the oceans, lies a crystalline foundation. In its structure lies the main differences between the continental and oceanic types of the earth's crust. On the continents, two thick layers are distinguished in the composition of the basement - "granite" and basalt. There is no "granite" layer under the abyssal bed of the oceans. However, the basalt basement of the ocean is by no means homogeneous in section; it is divided into the second and third layers.

Before ultra-deep and deep-water drilling, the structure of the earth's crust was judged mainly from geophysical data, namely, from the velocities of longitudinal and transverse seismic waves. Depending on the composition and density of the rocks that make up certain layers of the earth's crust, the velocities of the passage of seismic waves change significantly. In the upper horizons, where weakly compacted sedimentary formations predominate, they are relatively small, while in crystalline rocks they increase sharply as their density increases.

After the velocities of seismic wave propagation in the rocks of the ocean floor were measured for the first time in 1949, it became clear that the velocity sections of the crust of the continents and oceans are very different. At a shallow depth from the bottom, in the basement under the abyssal basin, these velocities reached values ​​that were recorded on the continents in the deepest layers of the earth's crust. The reason for this discrepancy soon became clear. The fact is that the crust of the oceans turned out to be amazingly thin. If on the continents the thickness of the earth's crust is on average 35 km, and under mountain-fold systems even 60 and 70 km, then in the ocean it does not exceed 5-10, rarely 15 km, and in some areas the mantle is located almost at the very bottom.

The standard velocity section of the continental crust includes the upper, sedimentary layer with a longitudinal wave velocity of 1–4 km/s, an intermediate, “granitic” layer, 5.5–6.2 km/s, and a lower, basaltic layer, 6.1–7.4 km /with. Below, it is believed, lies the so-called peridotite layer, which is already part of the asthenosphere, with velocities of 7.8–8.2 km/s. The names of the layers are conditional, since no one has yet seen real continuous sections of the continental crust, although the Kola superdeep well has already penetrated 12 km deep into the Baltic Shield.

In the abyssal basins of the ocean, under a thin sedimentary mantle (0.5–1.5 km), where seismic wave velocities do not exceed 2.5 km/s, there is a second layer of oceanic crust. According to the American geophysicist J. Worzel and other scientists, it has surprisingly similar speeds - 4.93–5.23 km / s, an average of 5.12 km / s, and the average thickness under the ocean floor is 1.68 km ( in the Atlantic - 2.28, in the Pacific - 1.26 km). However, in the peripheral parts of the abyssal, closer to the continental margins, the thicknesses of the second layer increase quite sharply. Under this layer, a third layer of the crust stands out with no less uniform velocities of propagation of longitudinal seismic waves, equal to 6.7 km/s. Its thickness ranges from 4.5 to 5.5 km.

In recent years, it has become clear that the velocity sections of the oceanic crust are characterized by a greater scatter of values ​​than previously thought, which is apparently associated with deep heterogeneities that exist in it (Pushcharovsky, 1987).

As we can see, the propagation velocities of longitudinal seismic waves in the upper (first and second) layers of the continental and oceanic crust are significantly different.

As for the sedimentary cover, this is due to the predominance of ancient Mesozoic, Paleozoic and Precambrian formations in its composition on the continents, which have undergone rather complex transformations in the bowels. The ocean floor, as mentioned above, is relatively young, and the sediments overlying the basement basalts are weakly compacted. This is due to the action of a number of factors that determine the effect of underconsolidation, which is known as the paradox of deep sea diagenesis.

It is more difficult to explain the difference in the velocities of seismic waves during their propagation through the second ("granite") layer of the continental and the second (basalt) layer of the oceanic crust. Oddly enough, in the basalt layer of the ocean these velocities turned out to be lower (4.82–5.23 km/s) than in the “granite” layer (5.5–6.2 km/s). The point here is that the velocities of longitudinal seismic waves in crystalline rocks with a density of 2.9 g/cm 3 approach 5.5 km/s. It follows from this that if the "granite" layer on the continents is indeed composed of crystalline rocks, among which metamorphic formations of the lower stages of transformation predominate (according to the data of ultra-deep drilling on the Kola Peninsula), then the composition of the second layer of the oceanic crust, in addition to basalts, should include formations with a density less than that of crystalline rocks (2–2.55 g / cm 3).

Indeed, on the 37th voyage of the drilling vessel "Glomar Challenger" the rocks of the oceanic basement were uncovered. The drill penetrated several basalt sheets, between which there were horizons of carbonate pelagic sediments. In one of the wells, an 80-meter stratum of basalts with limestone interbeds was drilled, in the other, a 300-meter series of rocks of volcanogenic-sedimentary origin. Drilling of the first of these wells was stopped in ultramafic rocks - gabbro and ultramafic rocks, which probably already belong to the third layer of the oceanic crust.

Deep-sea drilling and the study of rift zones from manned underwater vehicles (UAVs) made it possible to elucidate in general terms the structure of the oceanic crust. True, it is impossible to assert with certainty that we know its complete and continuous section, not distorted by subsequent superimposed processes. At present, the upper, sedimentary layer, partially or completely exposed at almost 1000 points of the bottom, has been studied in the most detail by the Glomar Challenger and Joydes Resolution drills. Much less explored is the second layer of the oceanic crust, which has been penetrated to a certain depth by a much smaller number of boreholes (a few dozen). However, it is now obvious that this layer was formed mainly by lava covers of basalts, between which various sedimentary formations of small thickness are enclosed. Basalts belong to tholeiite varieties that arose in underwater conditions. These are pillow lavas, often composed of hollow lava tubes and pillows. The sediments located between the basalts in the central parts of the ocean consist of the remains of the smallest planktonic organisms with a carbonate or siliceous function.

Finally, the third layer of the oceanic crust is identified with the so-called dike belt - a series of small igneous bodies (intrusions), closely fitted one to the other. The composition of these intrusions is basic to ultrabasic. These are gabbro and hyperbasite, which were formed not during the outpouring of magmas on the bottom surface, like basalts of the second layer, but in the depths of the crust itself. In other words, we are talking about magmatic melts that solidified near the magma chamber without reaching the bottom surface. Their "heavier" ultramafic composition indicates the residual nature of these magmatic melts. If we recall that the thickness of the third layer is usually 3 times the thickness of the second layer of the oceanic crust, then its definition as basaltic may seem like a great exaggeration.

Similarly, the “granite” layer of the continental crust, as it turned out during drilling of the Kola superdeep well, turned out to be not granite at all, at least in its upper half. As mentioned above, the section passed here was dominated by metamorphic rocks of the lower and middle stages of transformation. For the most part, they are ancient sedimentary rocks modified at high temperatures and pressures that exist in the bowels of the Earth. In this regard, a paradoxical situation has arisen, which consists in the fact that we now know more about the oceanic crust than about the continental one. And this despite the fact that the first one has been studied intensively for two decades, while the second one has been the object of research for at least a century and a half.

Both varieties of the earth's crust are not antagonists. In the marginal parts of the young oceans, the Atlantic and Indian, the boundary between the continental and oceanic crust is somewhat "blurred" due to the gradual thinning of the first of them in the transition region from the continent to the ocean. On the whole, this boundary is tectonically calm, i.e., it does not manifest itself either as powerful seismic shocks, which occur here extremely rarely, or as volcanic eruptions.

However, this situation does not hold everywhere. In the Pacific, the boundary between continental and oceanic crust is perhaps one of the most dramatic dividing lines on our planet. So what, after all, are these two varieties of the earth's crust antipodes or not? It seems that we can justifiably consider them as such. Indeed, despite the existence of a number of hypotheses suggesting the oceanization of the continental crust or, on the contrary, the transformation of the oceanic substrate into a continental one due to a number of mineral transformations of basalts, in fact there is no evidence of a direct transition of one type of crust to another. As will be shown below, the continental crust is formed in specific tectonic settings in active transition zones between the mainland and the ocean, and mainly as a result of the transformation of another type of earth's crust, called suboceanic. The oceanic substrate disappears in the Benioff zones, or is squeezed out like paste from a tube to the edge of the continent, or turns into tectonic melange (crushed ground rocks) in the areas of "collapsing" oceans. However, more on that later.

Hypotheses explaining the origin and development of the earth's crust

The concept of the earth's crust.

Earth's crust is a complex of surface layers of the solid body of the Earth. In the scientific geographical literature there is no single idea of ​​the origin and development of the earth's crust.

There are several concepts (hypotheses) that reveal the mechanisms of formation and development of the earth's crust, the most justified of which are the following:

1. The theory of fixism (from lat. fixus - motionless, unchanging) claims that the continents have always remained in the places they currently occupy. This theory denies any movement of continents and large parts of the lithosphere.

2. The theory of mobilism (from Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has been especially established in recent years in connection with the receipt of new scientific data in the study of the bottom of the World Ocean.

3. The concept of the growth of continents at the expense of the ocean floor assumes that the original continents were formed in the form of relatively small massifs, which now make up the ancient continental platforms. Subsequently, these massifs grew due to the formation of mountains on the ocean floor adjacent to the edges of the original land cores. The study of the ocean floor, especially in the zone of mid-ocean ridges, gave reason to doubt the correctness of the concept of the growth of continents due to the ocean floor.

4. The theory of geosynclines states that the increase in the size of land occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the earth's crust of the continents, is the basis for many modern scientific explanations of the process of origin and development of the earth's crust.

5. The rotational theory bases its explanation on the proposition that since the figure of the Earth does not coincide with the surface of a mathematical spheroid and is rebuilt due to uneven rotation, zonal bands and meridional sectors on a rotating planet are inevitably tectonically unequal. They react with varying degrees of activity to tectonic stresses caused by intraterrestrial processes.

There are two main types of earth's crust: oceanic and continental. There is also a transitional type of the earth's crust.

Oceanic crust. The thickness of the oceanic crust in the modern geological epoch ranges from 5 to 10 km. It consists of the following three layers:

1) the upper thin layer of marine sediments (thickness is not more than 1 km);

2) middle basalt layer (thickness from 1.0 to 2.5 km);

3) the lower gabbro layer (about 5 km thick).

Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic crust. Its average thickness is 35-45 km, and in mountainous countries it increases to 70 km. It also consists of three layers, but differs significantly from the ocean:

1) the lower layer composed of basalts (about 20 km thick);

2) the middle layer occupies the main thickness of the continental crust and is conditionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;

3) the upper layer is sedimentary. Its average thickness is about 3 km. In some areas, the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some regions of the Earth, the sedimentary layer is absent altogether and a granite layer comes to the surface. Such areas are called shields (eg Ukrainian Shield, Baltic Shield).

On the continents, as a result of weathering of rocks, a geological formation is formed, called weathering crusts.

The granite layer is separated from the basalt Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km/sec.

The boundary between the earth's crust and mantle (both on the continents and on the oceans) runs along Mohorovichic surface (Moho line). The speed of seismic waves on it jumps up to 8 km/h.

In addition to the two main types - oceanic and continental - there are also areas of a mixed (transitional) type.

On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out in it. In East Asia, in the area of ​​island arcs (the Kuril Islands, the Aleutian Islands, the Japanese Islands, and others), the earth's crust is of a transitional type. Finally, the earth's crust of the mid-ocean ridges is very complex and still little studied. There is no Moho boundary here, and the material of the mantle rises along faults into the crust and even to its surface.

The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than "the earth's crust". In the lithosphere, modern science includes not only the earth's crust, but also the uppermost mantle to the asthenosphere, that is, to a depth of about 100 km.

The concept of isostasy . The study of the distribution of gravity has shown that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density. Heavy oceanic crust is thinner than lighter continental crust.

Isostasy is, in essence, not even an equilibrium, but a striving for equilibrium, continuously disturbed and restored again. So, for example, the Baltic Shield, after the melting of continental ice of the Pleistocene glaciation, rises by about 1 meter per century. The area of ​​Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero line of balance passes at the present time somewhat to the south of 60 0 N. latitude. Modern St. Petersburg is about 1.5 m higher than St. Petersburg during the time of Peter the Great. As the data of modern scientific research show, even the heaviness of large cities is sufficient for the isostatic fluctuation of the territory under them. Consequently, the earth's crust in the areas of large cities is very mobile. On the whole, the relief of the earth's crust is a mirror image of the Moho surface, the sole of the earth's crust: elevated areas correspond to depressions in the mantle, and lower areas correspond to a higher level of its upper boundary. So, under the Pamirs, the depth of the Moho surface is 65 km, and in the Caspian lowland - about 30 km.

Thermal properties of the earth's crust . Daily fluctuations in soil temperature extend to a depth of 1.0–1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate to a depth of 20–30 m. a layer of constant soil temperature. It is called isothermal layer . Below the isothermal layer deep into the Earth, the temperature rises, and this is already caused by the internal heat of the earth's interior. Internal heat does not participate in the formation of climates, but it serves as the energy basis for all tectonic processes.

The number of degrees by which the temperature increases for every 100 m of depth is called geothermal gradient . The distance in meters, when lowered by which the temperature rises by 1 0 C, is called geothermal stage . The value of the geothermal step depends on the relief, the thermal conductivity of rocks, the proximity of volcanic foci, the circulation of groundwater, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be only about 5 m, and in geologically calm areas (for example, on platforms) it can reach 100 m.

Continental crust or continental crust - the earth's crust of the continents, which consists of sedimentary, granite and basalt layers. The average thickness is 35-45 km, the maximum thickness is up to 75 km (under mountain ranges). It is opposed to the oceanic crust, which is different in structure and composition. The continental crust has a three-layer structure. The upper layer is represented by a discontinuous cover of sedimentary rocks, which is widely developed, but rarely has a large thickness. Most of the crust is composed of the upper crust, a layer composed mainly of granites and gneisses of low density and ancient history. Studies show that most of these rocks were formed very long ago, about 3 billion years ago. Below is the lower crust, consisting of metamorphic rocks - granulites and the like.

5. Types of ocean structures. The land surface of the continents makes up only one third of the Earth's surface. The surface area occupied by the World Ocean is 361.1 ml sq. km. The underwater margins of the continents (shelf plateaus and continental slope) account for about 1/5 of its surface area, the so-called. “transitional” zones (deep trenches, island arcs, marginal seas) – about 1/10 of the area. The rest of the surface (about 250 ml sq. km.) Is occupied by oceanic deep-water plains, depressions and interoceanic uplifts separating them. The ocean floor differs sharply in the nature of seismicity. It is possible to distinguish areas with high seismic activity and aseismic areas. The first are extended zones occupied by systems of mid-ocean ridges, stretching across all oceans. These areas are sometimes called oceanic mobile belts. Mobile belts are characterized by intense volcanism (tholeiitic basalts), increased heat flow, sharply dissected relief with systems of longitudinal and transverse ridges, trenches, ledges, and shallow mantle surface. Seismically inactive areas are expressed in the relief by large oceanic basins, plains, plateaus, as well as underwater ridges limited by fault-type ledges and intra-oceanic swell-like uplifts topped by cones of active and extinct volcanoes. Within the regions of the second type, there are underwater plateaus and uplifts with continental-type crust (microcontinents). Unlike mobile oceanic belts, these regions, by analogy with the structures of continents, are sometimes called thalassocratons.

6. The structure of the oceanic crust in structures of various types. Oceanic depressions, as the largest negative structures on the surface of the earth's crust, have a number of structural features that allow them to be opposed to positive structures (continents) and compared with each other.

The main thing that unites and distinguishes all oceanic depressions is the low position of the surface of the earth's crust within them and the absence of a geophysical granite-metamorphic layer characteristic of continents. Mobile belts stretch through all oceanic depressions - mountain systems of mid-ocean ridges with a high heat flow, an elevated position of the mantle layer, which is not typical for continents. The system of mid-ocean ridges, the longest on the surface of the Earth, penetrates and thus connects all oceanic depressions, occupying a central or marginal position in them. It is also characteristic that the tectonic structures of the ocean floor are often closely related to the structures of the continents. First of all, these connections are expressed in the presence of common faults, in the transitions of rift valleys of mid-ocean ridges into continental rifts (Gulfs of California and Aden), in the presence of large submerged blocks of continental crust in the oceans, as well as depressions with graniteless crust on continents, in transitions trap fields of the continents to the shelf and ocean floor. The internal structure of oceanic depressions is also different. According to the position of the zone of modern spreading, it is possible to oppose the depression of the Atlantic Ocean with the median position of the Mid-Atlantic Ridge to all other oceans, in which the so-called. the median ridge is displaced to one of the edges. The internal structure of the Indian Ocean depression is complex. In the western part it resembles the structure of the Atlantic Ocean, in the eastern part it is closer to the western region of the Pacific Ocean. Comparing the structure of the western region of the Pacific Ocean with the eastern part of the Indian Ocean, one draws attention to their certain similarities: the depths of the bottom, the age of the crust (the Cocos and Western Australian Basins of the Indian Ocean, the Western Basin of the Pacific Ocean). In both oceans, these parts are separated from the continent and the basins of the marginal seas by systems of deep-sea trenches and island arcs. The connection between active ocean margins and young folded structures of the continents is observed in Central America, where the Atlantic Ocean is separated from the Caribbean Sea by a deep-sea trench and island arc. The close relationship between the deep-water trenches separating the ocean basins from the continental massifs with the structures of the continental crust can be traced in the example of the northern extension of the Sunda deep-sea trench, which passes into the Pre-Arakan foredeep.

7. Structures of the margins of continents (oceans) and types of crust.

8. Types of boundaries of continental blocks and oceanic depressions. Continental massifs and oceanic depressions can have two types of boundaries - passive (Atlantic) and active (Pacific). The first type is distributed along the framing of most of the Atlantic, Indian, and Arctic oceans. This type is characterized by the fact that through a continental slope of one or another steepness with a system of stepped normal faults, ledges and a relatively gentle continental foot, continental massifs merge with the area of ​​abyssal plains of the ocean floor. In the zone of the continental foot, systems of deep troughs are known, but they are smoothed out by thick layers of loose sediments. The second type of margins is expressed along the framing of the Pacific Ocean, along the northeastern margin of the Indian Ocean and on the margin of the Atlantic Ocean adjacent to Central America. In these areas, between the continental massifs and the abyssal plains of the ocean floor, there is a zone of varying width with deep-sea trenches, island arcs, and basins of marginal seas.

9. Lithospheric plates and types of their boundaries. Studying the lithosphere, which includes the earth's crust and upper mantle, geophysicists came to the conclusion that it contains its own heterogeneities. First of all, these inhomogeneities of the lithosphere are expressed by the presence of strip zones crossing it for the entire thickness with a high heat flow, high seismicity, and active modern volcanism. The areas located between such strip zones are called lithospheric plates, and the zones themselves are considered as the boundaries of lithospheric plates. At the same time, one type of boundaries is characterized by tensile stresses (borders of divergence of plates), another type is characterized by compressive stresses (borders of convergence of plates), and the third type is characterized by tensions and compressions that occur during shears. The first type of boundaries are divergent (constructive) boundaries, which on the surface correspond to rift zones. The second type of boundaries are subduction (when oceanic blocks are pushed under continental ones), obductive (when oceanic blocks are thrust onto continental ones), and collisional (when continental blocks are shifted). On the surface, they are expressed by deep-water trenches, foredeeps, and zones of large thrusts, often with ophiolites (sutures). The third type of boundaries (shear) is called transform boundaries. It is also often accompanied by discontinuous chains of rift depressions. There are several large and small lithospheric plates. Large plates include the Eurasian, African, Indo-Australian, South American, North American, Pacific, and Antarctic. Small plates include the Caribbean, Scotia, Philippine, Cocos, Nazca, Arabian, etc.

10. Rifting, spreading, subduction, obduction, collision. Rifting is the process of the emergence and development in the earth's crust of continents and oceans of band-like zones of horizontal stretching on a global scale. In its upper brittle part, it manifests itself in the formation of rifts expressed in the form of large linear grabens, sliding cavities and related structural forms, and their filling with sediments and (or) products of volcanic eruptions, usually accompanying rifting. In the lower, more heated part of the crust, brittle deformations during rifting are replaced by plastic tension, leading to its thinning (formation of a “neck”), and, with especially intense and prolonged stretching, to a complete break in the continuity of the pre-existing crust (continental or oceanic) and the formation of "gaps" of the new crust of the oceanic type. The last process, called spreading, proceeded powerfully in the late Mesozoic and Cenozoic within the modern oceans, and on a smaller (?) scale periodically manifested itself in some zones of older mobile belts.

Subduction - subduction of lithospheric plates of the oceanic crust and mantle rocks under the edges of other plates (according to the concepts of plate tectonics). Accompanied by the emergence of zones of deep-focus earthquakes and the formation of active volcanic island arcs.

Obduction - thrusting of tectonic plates, composed of fragments of the oceanic lithosphere, onto the continental margin. As a result, an ophiolite complex is formed. Obduction occurs when any factors disrupt the normal absorption of the oceanic crust into the mantle. One of the mechanisms of obduction is the lifting of the oceanic crust to the continental margin when it enters the subduction zone of the mid-ocean ridge. Obduction is a relatively rare phenomenon and has occurred only periodically in the Earth's history. Some researchers believe that in our time this process is taking place on the southwestern coast of South America.

A continental collision is a collision of continental plates, which always leads to the collapse of the crust and the formation of mountain ranges. An example of a collision is the Alpine-Himalayan mountain belt, formed as a result of the closure of the Tethys Ocean and a collision with the Eurasian plate of Hindustan and Africa. As a result, the thickness of the crust increases significantly, under the Himalayas it is 70 km. This is an unstable structure, its sides are intensively destroyed by surface and tectonic erosion. In the crust with a sharply increased thickness, granites are smelted from metamorphosed sedimentary and igneous rocks.