According to modern theories of lithospheric plates the entire lithosphere is divided into separate blocks by narrow and active zones - deep faults - moving in the plastic layer of the upper mantle relative to each other at a speed of 2-3 cm per year. These blocks are called lithospheric plates.
A feature of lithospheric plates is their rigidity and ability, in the absence of external influences, to maintain their shape and structure unchanged for a long time.
Lithospheric plates are mobile. Their movement along the surface of the asthenosphere occurs under the influence of convective currents in the mantle. Separate lithospheric plates can diverge, approach or slide relative to each other. In the first case, tension zones with cracks along the plate boundaries appear between the plates, in the second case, compression zones accompanied by thrusting of one plate onto another (thrust - obduction; underthrust - subduction), in the third case - shear zones - faults along which sliding of neighboring plates occurs. .
At the convergence of continental plates, they collide, forming mountain belts. This is how the Himalaya mountain system arose, for example, on the border of the Eurasian and Indo-Australian plates (Fig. 1).
Rice. 1. Collision of continental lithospheric plates
When the continental and oceanic plates interact, the plate with the oceanic crust moves under the plate with the continental crust (Fig. 2).
Rice. 2. Collision of continental and oceanic lithospheric plates
As a result of the collision of continental and oceanic lithospheric plates, deep-sea trenches and island arcs are formed.
The divergence of lithospheric plates and the formation of an oceanic type of earth's crust as a result of this is shown in Fig. 3.
The axial zones of mid-ocean ridges are characterized by rifts(from English. rift- fissure, crack, fault) - a large linear tectonic structure of the earth's crust with a length of hundreds, thousands, a width of tens, and sometimes hundreds of kilometers, formed mainly during horizontal stretching of the crust (Fig. 4). Very large rifts are called rift belts, zones or systems.
Since the lithospheric plate is a single plate, each of its faults is a source of seismic activity and volcanism. These sources are concentrated within relatively narrow zones, along which mutual displacements and frictions of adjacent plates occur. These zones are called seismic belts. Reefs, mid-ocean ridges and deep-sea trenches are mobile areas of the Earth and are located at the boundaries of lithospheric plates. This indicates that the process of formation of the earth's crust in these zones is currently very intensive.
Rice. 3. Divergence of lithospheric plates in the zone among the nano-oceanic ridge
Rice. 4. Scheme of rift formation
Most of the faults of the lithospheric plates are at the bottom of the oceans, where the earth's crust is thinner, but they are also found on land. The largest fault on land is located in eastern Africa. It stretched for 4000 km. The width of this fault is 80-120 km.
At present, seven largest plates can be distinguished (Fig. 5). Of these, the largest in area is the Pacific, which consists entirely of oceanic lithosphere. As a rule, the Nazca plate is also referred to as large, which is several times smaller in size than each of the seven largest ones. At the same time, scientists suggest that in fact the Nazca plate is much larger than we see it on the map (see Fig. 5), since a significant part of it went under the neighboring plates. This plate also consists only of oceanic lithosphere.
Rice. 5. Earth's lithospheric plates
An example of a plate that includes both continental and oceanic lithosphere is, for example, the Indo-Australian lithospheric plate. The Arabian Plate consists almost entirely of the continental lithosphere.
The theory of lithospheric plates is important. First of all, it can explain why mountains are located in some places on the Earth, and plains in others. With the help of the theory of lithospheric plates, it is possible to explain and predict catastrophic phenomena occurring at the boundaries of plates.
Rice. 6. The outlines of the continents really seem compatible
Continental drift theory
The theory of lithospheric plates originates from the theory of continental drift. Back in the 19th century many geographers noted that when looking at a map, one can notice that the coasts of Africa and South America seem compatible when approaching (Fig. 6).
The emergence of the hypothesis of the movement of the continents is associated with the name of the German scientist Alfred Wegener(1880-1930) (Fig. 7), who most fully developed this idea.
Wegener wrote: "In 1910, the idea of moving the continents first came to my mind ... when I was struck by the similarity of the outlines of the coasts on both sides of the Atlantic Ocean." He suggested that in the early Paleozoic there were two large continents on Earth - Laurasia and Gondwana.
Laurasia was the northern mainland, which included the territories of modern Europe, Asia without India and North America. The southern mainland - Gondwana united the modern territories of South America, Africa, Antarctica, Australia and Hindustan.
Between Gondwana and Laurasia was the first sea - Tethys, like a huge bay. The rest of the Earth's space was occupied by the Panthalassa ocean.
About 200 million years ago, Gondwana and Laurasia were united into a single continent - Pangea (Pan - universal, Ge - earth) (Fig. 8).
Rice. 8. The existence of a single mainland Pangea (white - land, dots - shallow sea)
Approximately 180 million years ago, the mainland of Pangea again began to be divided into constituent parts, which mixed up on the surface of our planet. The division took place as follows: first, Laurasia and Gondwana reappeared, then Laurasia divided, and then Gondwana also split. Due to the split and divergence of parts of Pangea, oceans were formed. The young oceans can be considered the Atlantic and Indian; old - Quiet. The Arctic Ocean became isolated with the increase in land mass in the Northern Hemisphere.
Rice. 9. Location and directions of continental drift in the Cretaceous period 180 million years ago
A. Wegener found a lot of evidence for the existence of a single continent of the Earth. Particularly convincing seemed to him the existence in Africa and South America of the remains of ancient animals - leafosaurs. These were reptiles, similar to small hippos, that lived only in freshwater reservoirs. This means that they could not swim huge distances in salty sea water. He found similar evidence in the plant world.
Interest in the hypothesis of the movement of the continents in the 30s of the XX century. decreased slightly, but in the 60s it revived again, when, as a result of studies of the relief and geology of the ocean floor, data were obtained indicating the processes of expansion (spreading) of the oceanic crust and the “diving” of some parts of the crust under others (subduction).
Lithospheric plates- large rigid blocks of the Earth's lithosphere, limited by seismically and tectonically active fault zones.
The plates, as a rule, are separated by deep faults and move along the viscous layer of the mantle relative to each other at a rate of 2-3 cm per year. Where continental plates collide, they form mountain belts . When the continental and oceanic plates interact, the plate with the oceanic crust moves under the plate with the continental crust, resulting in the formation of deep-sea trenches and island arcs.
The movement of lithospheric plates is associated with the movement of matter in the mantle. In separate parts of the mantle, there are powerful flows of heat and matter rising from its depths to the surface of the planet.
Over 90% of the Earth's surface is covered 13 the largest lithospheric plates.
Rift– a huge fracture in the earth's crust, formed during its horizontal stretching (i.e., where the flows of heat and matter diverge). In the rifts there is an outpouring of magma, new faults, horsts, grabens appear. Mid-ocean ridges are forming.
First continental drift hypothesis (i.e. the horizontal movement of the earth's crust) put forward at the beginning of the twentieth century A. Wegener. On its basis, created theory of lithospheric plates m. According to this theory, the lithosphere is not a monolith, but consists of large and small plates, "floating" on the asthenosphere. The boundary regions between lithospheric plates are called seismic belts - these are the most "restless" areas of the planet.
The earth's crust is divided into stable (platforms) and mobile sections (folded areas - geosynclines).
- powerful underwater mountain structures within the ocean floor, most often occupying a middle position. Near mid-ocean ridges, lithospheric plates move apart and young basalt oceanic crust appears. The process is accompanied by intense volcanism and high seismicity.
Continental rift zones are, for example, the East African rift system, the Baikal rift system. Rifts, like mid-ocean ridges, are characterized by seismic activity and volcanism.
Plate tectonics- a hypothesis suggesting that the lithosphere is divided into large plates that move along the mantle in a horizontal direction. Near mid-ocean ridges, lithospheric plates move apart and build up due to matter rising from the bowels of the Earth; in deep-sea trenches, one plate moves under another and is absorbed by the mantle. In places where plates collide, folded structures are formed.
Plate tectonics
Definition 1
A tectonic plate is a moving part of the lithosphere that moves on the asthenosphere as a relatively rigid block.
Remark 1
Plate tectonics is the science that studies the structure and dynamics of the earth's surface. It has been established that the upper dynamic zone of the Earth is fragmented into plates moving along the asthenosphere. Plate tectonics describes the direction in which lithospheric plates move, as well as the features of their interaction.
The entire lithosphere is divided into larger and smaller plates. Tectonic, volcanic and seismic activity is manifested along the edges of the plates, which leads to the formation of large mountain basins. Tectonic movements can change the relief of the planet. At the place of their connection, mountains and hills are formed, at the places of divergence, depressions and cracks in the ground are formed.
Currently, the movement of tectonic plates continues.
Movement of tectonic plates
Lithospheric plates move relative to each other at an average rate of 2.5 cm per year. When moving, the plates interact with each other, especially along the boundaries, causing significant deformations in the earth's crust.
As a result of the interaction of tectonic plates, massive mountain ranges and associated fault systems were formed (for example, the Himalayas, the Pyrenees, the Alps, the Urals, the Atlas, the Appalachians, the Apennines, the Andes, the San Andreas fault system, etc.).
The friction between the plates causes most of the planet's earthquakes, volcanic activity, and the formation of oceanic pits.
The composition of tectonic plates includes two types of lithosphere: continental crust and oceanic crust.
The tectonic plate can be of three types:
- continental Plate,
- ocean Plate,
- mixed board.
Theories of tectonic plate movement
In the study of the movement of tectonic plates, special merit belongs to A. Wegener, who suggested that Africa and the eastern part of South America were previously a single continent. However, after the break that happened many million years ago, parts of the earth's crust began to shift.
According to Wegener's hypothesis, tectonic platforms with different masses and rigid structures were located on the plastic asthenosphere. They were in an unstable state and moved all the time, as a result of which they collided, entered each other, and zones of plate separation and joints were formed. At the collision sites, areas with increased tectonic activity formed, mountains formed, volcanoes erupted and earthquakes occurred. The displacement occurred at a rate of up to 18 cm per year. Magma penetrated the faults from the deep layers of the lithosphere.
Some researchers believe that the magma that came to the surface gradually cooled down and formed a new bottom structure. The unused earth's crust, under the influence of plate drift, sank into the bowels and again turned into magma.
Wegener's research touched upon the processes of volcanism, the study of the stretching of the surface of the ocean floor, as well as the viscous-liquid internal structure of the earth. The works of A. Wegener became the foundation for the development of the theory of lithospheric plate tectonics.
Schmelling's research proved the existence of convective movement inside the mantle and leading to the movement of lithospheric plates. The scientist believed that the main reason for the movement of tectonic plates is thermal convection in the planet's mantle, in which the lower layers of the earth's crust heat up and rise, and the upper layers cool down and gradually descend.
The main position in the theory of plate tectonics is occupied by the concept of a geodynamic setting, a characteristic structure with a certain ratio of tectonic plates. In the same geodynamic setting, the same type of magmatic, tectonic, geochemical and seismic processes are observed.
The theory of plate tectonics does not fully explain the relationship between plate movements and processes occurring in the depths of the planet. A theory is needed that could describe the internal structure of the earth itself, the processes taking place in its depths.
Provisions of modern plate tectonics:
- the upper part of the earth's crust includes the lithosphere, which has a fragile structure, and the asthenosphere, which has a plastic structure;
- the main cause of plate movement is convection in the asthenosphere;
- the modern lithosphere consists of eight large tectonic plates, about ten medium plates and many small ones;
- small tectonic plates are located between large ones;
- magmatic, tectonic and seismic activity are concentrated at plate boundaries;
- the movement of tectonic plates obeys Euler's rotation theorem.
Types of tectonic plate movements
There are different types of tectonic plate movements:
- divergent movement - two plates diverge, and an underwater mountain range or an abyss in the ground forms between them;
- convergent movement - two plates converge and a thinner plate moves under a larger plate, resulting in the formation of mountain ranges;
- sliding motion - plates move in opposite directions.
Depending on the type of movement, divergent, convergent and sliding tectonic plates are distinguished.
Convergence leads to subduction (one plate is on top of another) or to collision (two plates are crushed and mountain ranges are formed).
Divergence leads to spreading (divergence of plates and formation of oceanic ridges) and rifting (formation of a break in the continental crust).
The transform type of movement of tectonic plates implies their movement along the fault.
Figure 1. Types of tectonic plate movements. Author24 - online exchange of student papers
Plate tectonics
Definition 1
A tectonic plate is a moving part of the lithosphere that moves on the asthenosphere as a relatively rigid block.
Remark 1
Plate tectonics is the science that studies the structure and dynamics of the earth's surface. It has been established that the upper dynamic zone of the Earth is fragmented into plates moving along the asthenosphere. Plate tectonics describes the direction in which lithospheric plates move, as well as the features of their interaction.
The entire lithosphere is divided into larger and smaller plates. Tectonic, volcanic and seismic activity is manifested along the edges of the plates, which leads to the formation of large mountain basins. Tectonic movements can change the relief of the planet. At the place of their connection, mountains and hills are formed, at the places of divergence, depressions and cracks in the ground are formed.
Currently, the movement of tectonic plates continues.
Movement of tectonic plates
Lithospheric plates move relative to each other at an average rate of 2.5 cm per year. When moving, the plates interact with each other, especially along the boundaries, causing significant deformations in the earth's crust.
As a result of the interaction of tectonic plates, massive mountain ranges and associated fault systems were formed (for example, the Himalayas, the Pyrenees, the Alps, the Urals, the Atlas, the Appalachians, the Apennines, the Andes, the San Andreas fault system, etc.).
The friction between the plates causes most of the planet's earthquakes, volcanic activity, and the formation of oceanic pits.
The composition of tectonic plates includes two types of lithosphere: continental crust and oceanic crust.
The tectonic plate can be of three types:
- continental Plate,
- ocean Plate,
- mixed board.
Theories of tectonic plate movement
In the study of the movement of tectonic plates, special merit belongs to A. Wegener, who suggested that Africa and the eastern part of South America were previously a single continent. However, after the break that happened many million years ago, parts of the earth's crust began to shift.
According to Wegener's hypothesis, tectonic platforms with different masses and rigid structures were located on the plastic asthenosphere. They were in an unstable state and moved all the time, as a result of which they collided, entered each other, and zones of plate separation and joints were formed. At the collision sites, areas with increased tectonic activity formed, mountains formed, volcanoes erupted and earthquakes occurred. The displacement occurred at a rate of up to 18 cm per year. Magma penetrated the faults from the deep layers of the lithosphere.
Some researchers believe that the magma that came to the surface gradually cooled down and formed a new bottom structure. The unused earth's crust, under the influence of plate drift, sank into the bowels and again turned into magma.
Wegener's research touched upon the processes of volcanism, the study of the stretching of the surface of the ocean floor, as well as the viscous-liquid internal structure of the earth. The works of A. Wegener became the foundation for the development of the theory of lithospheric plate tectonics.
Schmelling's research proved the existence of convective movement inside the mantle and leading to the movement of lithospheric plates. The scientist believed that the main reason for the movement of tectonic plates is thermal convection in the planet's mantle, in which the lower layers of the earth's crust heat up and rise, and the upper layers cool down and gradually descend.
The main position in the theory of plate tectonics is occupied by the concept of a geodynamic setting, a characteristic structure with a certain ratio of tectonic plates. In the same geodynamic setting, the same type of magmatic, tectonic, geochemical and seismic processes are observed.
The theory of plate tectonics does not fully explain the relationship between plate movements and processes occurring in the depths of the planet. A theory is needed that could describe the internal structure of the earth itself, the processes taking place in its depths.
Provisions of modern plate tectonics:
- the upper part of the earth's crust includes the lithosphere, which has a fragile structure, and the asthenosphere, which has a plastic structure;
- the main cause of plate movement is convection in the asthenosphere;
- the modern lithosphere consists of eight large tectonic plates, about ten medium plates and many small ones;
- small tectonic plates are located between large ones;
- magmatic, tectonic and seismic activity are concentrated at plate boundaries;
- the movement of tectonic plates obeys Euler's rotation theorem.
Types of tectonic plate movements
There are different types of tectonic plate movements:
- divergent movement - two plates diverge, and an underwater mountain range or an abyss in the ground forms between them;
- convergent movement - two plates converge and a thinner plate moves under a larger plate, resulting in the formation of mountain ranges;
- sliding motion - plates move in opposite directions.
Depending on the type of movement, divergent, convergent and sliding tectonic plates are distinguished.
Convergence leads to subduction (one plate is on top of another) or to collision (two plates are crushed and mountain ranges are formed).
Divergence leads to spreading (divergence of plates and formation of oceanic ridges) and rifting (formation of a break in the continental crust).
The transform type of movement of tectonic plates implies their movement along the fault.
Figure 1. Types of tectonic plate movements. Author24 - online exchange of student papers
In the process of formation and then development of geology as a science, many hypotheses were proposed, each of which, from one position or another, considered and explained either individual problems or a complex of problems related to the development of the earth's crust or the Earth as a whole. These hypotheses are called geotectonic. Some of them, due to insufficient persuasiveness, quickly lost their significance in science, while others turned out to be more durable, again until new facts and ideas accumulated, which formed the basis of new hypotheses that are more appropriate for a given stage in the development of science. Despite the great successes achieved in the study of the structure and development of the earth's crust, none of the modern hypotheses and theories (even recognized ones) is able to adequately and fully explain all the conditions for the formation of the earth's crust.
The first scientific hypothesis, the uplift hypothesis, was formulated in the first half of the 19th century. based on the ideas of the Plutonists about the role of the internal forces of the Earth, which played a positive role in the fight against the erroneous ideas of the Neptunists. In the 50s. 19th century it was replaced by the more substantiated at that time contraction hypothesis (compressed), set forth by the French scientist Elie de Beaumont. The contraction hypothesis was based on Laplace's cosmogonic hypothesis, which recognized, as is known, the primary hot state of the Earth and its subsequent gradual cooling.
The essence of the contraction hypothesis is that the cooling of the Earth causes its compression, followed by a decrease in its volume. As a result, the earth's crust, which had hardened before the inner zones of the planet, is forced to wrinkle, which is why folded mountains are formed.
In the second half of the XIX century. American scientists J. Hall and J. Dan formulated the doctrine of geosynclines - special mobile zones of the earth's crust over time turning into folded mountain structures. This teaching significantly strengthened the position of the contraction hypothesis. However, by the beginning of the 20th century. in connection with the receipt of new data on the Earth, this hypothesis began to lose its significance, since it turned out to be unable to explain the periodicity of mountain-building movements and magmatism processes, it ignored stretching processes, etc. In addition, ideas arose in science about the formation of a planet from cold particles , which deprived the hypothesis of its main support.
At the same time, the doctrine of geosynclines continued to be supplemented and developed. In this regard, a great contribution was also made by the Soviet scientists A. D. Arkhangelsky, N. S. Shatsky, M. V. Muratov and others. and especially since the beginning of the 20th century. the doctrine of relatively stable continental areas - platforms began to develop; of the domestic scientists who developed this doctrine, we must first of all name A. P. Karpinsky, A. D. Arkhangelsky, N. S. Shatsky, A. A. Bogdanov, A. L. Yanshin.
The doctrine of geosynclines and platforms has firmly entered the geological science and retains its significance to the present day. However, it still lacks a solid theoretical basis.
The desire to supplement and eliminate shortcomings in the contraction hypothesis or, conversely, to completely replace it, led to the appearance during the first half of the 20th century. a number of new geotectonic hypotheses. Let's note some of them.
pulsation hypothesis. It is based on the idea of the alternation of the processes of compression and expansion of the Earth - processes that are very characteristic of the Universe as a whole. M. A. Usov and V. A. Obruchev, who developed this hypothesis, associated folding, overthrusts, and the intrusion of acid intrusions with the compression phases, and the appearance of cracks in the earth's crust and the outpouring of mainly basic lavas along them with expansion phases.
Hypothesis of differentiation of the subcrustal substance and migration of radioelements. Under the influence of gravitational differentiation and radiogenic heating, periodic melting of liquid components from the atmosphere occurs, which entails ruptures of the earth's crust, volcanism, mountain building and other phenomena. One of the authors of this hypothesis is the famous Soviet scientist VV Belousov.
Continental drift hypothesis. It was presented in 1912 by the German scientist A. Wegener and is fundamentally different from all other hypotheses. Based on the principles of mobilism - recognition of significant horizontal movements of vast continental masses. Most of the hypotheses proceeded from the principles of fixism - the recognition of a stable, fixed position of individual parts of the earth's crust relative to the underlying mantle (such are the hypotheses of contraction, differentiation of subcrustal matter and migration of radioelements, etc.).
According to the ideas of A. Wegener, the granitic layer of the earth's crust "floats" on the basalt layer. Under the influence of the rotation of the Earth, it turned out to be collected in a single continent of Pangea. At the end of the Paleozoic era (about 200-300 million years ago), Pangea was divided into separate blocks and their drift began until they occupied their present position. Under the influence of the drift of the blocks of North and South America to the west, the Atlantic Ocean arose, and the resistance experienced by these continents as they moved along the basalt layer contributed to the emergence of such mountains as the Andes and the Cordillera. For the same reasons, Australia and Antarctica moved apart and moved south, etc.
A. Wegener saw confirmation of his hypothesis in the similarity of the contours and geological structure of the coasts on both sides of the Atlantic Ocean, in the similarity of fossil organisms of continents far apart from each other, in the different structure of the earth's crust within the oceans and continents.
The appearance of A. Wegener's hypothesis aroused great interest, but it died out relatively quickly, since it was not able to explain many phenomena, and most importantly, the possibility of the movement of continents along the basalt layer. Nevertheless, as we will see below, mobilist views, but on a completely new basis, were revived and received wide recognition in the second half of the 20th century.
rotational hypothesis. It occupies a separate place among geotectonic hypotheses, as it sees the manifestation of tectonic processes on the Earth under the influence of extraterrestrial causes, namely the attraction of the Moon and the Sun, causing solid tides in the earth's crust and mantle, slowing down the rotation of the Earth and changing its shape. The consequence of this is not only vertical, but also horizontal displacements of individual blocks of the earth's crust. The hypothesis is not widely accepted, since the vast majority of scientists believe that tectogenesis is the result of the manifestation of the internal forces of the Earth. At the same time, the influence of extraterrestrial causes on the formation of the earth's crust, obviously, must also be taken into account.
The theory of new global tectonics, or lithospheric plate tectonics. Since the beginning of the second half of the XX century. extensive geological and geophysical studies of the bottom of the oceans were launched. They resulted in the emergence of completely new ideas about the development of the oceans, such as, for example, the separation of lithospheric plates and the formation of a young oceanic crust in rift valleys, the formation of continental crust in zones of subduction of lithospheric plates, etc. These ideas led to the revival in geological science of mobilist ideas and to the emergence of the theory of a new global tectonics, or lithospheric plate tectonics.
The new theory is based on the idea that the entire lithosphere (i.e., the earth's crust together with the upper mantle layer) is divided by narrow tectonically active zones into separate rigid plates moving along the asthenosphere (plastic layer in the upper mantle). Active tectonic zones characterized by high seismicity and volcanism are rift zones of mid-ocean ridges, systems of island arcs and deep ocean trenches, and rift valleys on the continents. In the rift zones of the mid-ocean ridges, plates move apart and a new oceanic crust is formed, and in deep-sea trenches, some plates are pushed under others and the continental crust is formed. A collision of plates is also possible - the formation of the Himalayan fold zone is considered to be the result of such a phenomenon.
There are seven large lithospheric plates and a slightly larger number of small ones. These plates have received the following names: 1) Pacific, 2) North American, 3) South American, 4) Eurasian, 5) African, 6) Indo-Australian and 7) Antarctic. Each of them includes one or more continents or parts of them and oceanic crust, with the exception of the Pacific Plate, which is almost entirely composed of oceanic crust. Simultaneously with the horizontal displacements of the plates, their rotations also occurred.
The movement of lithospheric plates, according to this theory, is caused by convective currents of matter in the mantle, generated by heat released during the radioactive decay of elements and gravitational differentiation of matter in the bowels of the Earth. However, the argumentation of thermal convection in the mantle, according to many scientists, is insufficient. This also applies to the possibility of submersion of oceanic plates into the mantle to a great depth and a number of other provisions. The surface expression of the convective motion is the rift zones of the mid-ocean ridges, where the relatively warmer mantle rises to the surface and undergoes melting. It pours out in the form of basaltic lavas and freezes. Further, basaltic magma again intrudes into these frozen rocks and pushes older basalts in both directions. This happens many times. At the same time, the ocean floor is growing, growing. Such a process is called spreading. The growth rate of the ocean floor ranges from a few mm to 18 cm per year.
Other boundaries between lithospheric plates are convergent, that is, the earth's crust in these areas is absorbed. Such zones were called subduction zones. They are located along the edges of the Pacific Ocean and in the east of the Indian. The heavy and cold oceanic lithosphere, approaching the thicker and lighter continental one, goes under it, as if diving. If two oceanic plates come into contact, then the older one sinks, since it is heavier and colder than the young plate.
The zones where subduction occurs are morphologically expressed by deep-water trenches, and the sinking oceanic cold and elastic lithosphere itself is well established from seismic tomography data. The angle of subsidence of oceanic plates is different, up to vertical, and the plates can be traced to the boundary of the upper and lower mantles at a depth of approximately 670 km.
When the oceanic plate begins to sharply bend when approaching the continental one, stresses arise in it, which, when discharged, provoke earthquakes. Earthquake hypocenters or foci clearly mark the friction boundary between the two plates and form an inclined seismic focal zone that plunges under the continental lithosphere to a depth of 700 km. These zones are called Benioff zones, in honor of the American seismologist who studied them.
The subsidence of the oceanic lithosphere leads to one more important consequence. When the lithosphere reaches a depth of 100 - 200 km in the area of high temperatures and pressures, fluids are released from it - special superheated mineral solutions that cause the melting of rocks of the continental lithosphere and the formation of magma chambers that feed the chains of volcanoes developed parallel to deep-sea trenches on active continental margins.
Thus, due to subduction, a strongly dissected topography, high seismicity, and vigorous volcanic activity are observed on the active continental margin.
In addition to the phenomenon of subduction, there is the so-called obduction, that is, the thrusting of the oceanic lithosphere onto the continental one, an example of which is the huge tectonic cover on the eastern margin of the Arabian Peninsula, composed of typical oceanic crust.
It should also mention the collision, or collisions, two continental plates, which, due to the relative lightness of the material that composes them, cannot sink under each other, but collide, forming a mountain-fold belt with a very complex internal structure.
The main provisions of lithospheric plate tectonics are as follows:
1.The first premise Plate tectonics is the division of the upper part of the solid Earth into two shells that differ significantly in rheological properties (viscosity) - a rigid and brittle lithosphere and a more plastic and mobile asthenosphere. As already mentioned, these two shells are distinguished from seismological or magnetotelluric data.
2.Second position Plate tectonics, to which it owes its name, lies in the fact that the lithosphere is naturally subdivided into a limited number of plates - currently seven large and the same number of small ones. The basis for their selection and drawing boundaries between them is the location of earthquake sources.
3.Third position Plate tectonics concerns the nature of their mutual movement. There are three types of such displacements and, accordingly, the boundaries between the plates: 1) divergent borders, along which the plates move apart - spreading; 2) convergent borders, on which there is a convergence of plates, usually expressed by the subduction of one plate under another; when an oceanic plate moves under a continental one, this process is called subduction, if the oceanic plate is moving towards the continental - obduction; if two continental plates collide, also usually with subduction of one under the other, - collision; 3)transform borders, along which there is a horizontal sliding of one plate relative to the other along the plane of the vertical transform fault.
In nature, the boundaries of the first two types predominate.
At divergent boundaries, in spreading zones, there is a continuous birth of new oceanic crust; Therefore, these boundaries are also called constructive. This crust is moved by the asthenospheric current towards subduction zones, where it is absorbed at depth; this gives grounds to call such boundaries destructive.
Fourth position plate tectonics lies in the fact that during their movements, the plates obey the laws of spherical geometry, or rather Euler's theorem, according to which any movement of two conjugate points on a sphere is performed along a circle drawn relative to an axis passing through the center of the Earth.
5.Fifth provision Plate tectonics states that the volume of oceanic crust absorbed in subduction zones is equal to the volume of crust originating in spreading zones.
6.sixth position plate tectonics sees the main cause of plate movement in the mantle convection. This convection in the classical 1968 model is purely thermal and general mantle, and the way it affects lithospheric plates is that these plates, which are in viscous adhesion to the asthenosphere, are carried along by the latter and move in the manner of a conveyor belt from spreading axes to subduction zones. In general, the scheme of mantle convection, leading to a plate tectonic model of lithosphere movements, consists in the fact that ascending branches of convective cells are located under the mid-ocean ridges, descending branches are located under subduction zones, and horizontal segments of these cells.
The theory of new global tectonics, or lithospheric plate tectonics, is especially popular abroad: it is also recognized by many Soviet scientists, who do not confine themselves to general recognition, but work hard to clarify its main provisions, supplementing, deepening and developing them. The Soviet mobilist scientist A.V. Peivs, developing this theory, however, came to the conclusion that giant rigid lithospheric plates do not exist at all, and the lithosphere, due to the fact that it is penetrated by horizontal, inclined and vertical mobile zones, consists of separate plates (“lithoplasts”) moving differentially. This is an essentially new look at one of the main, but controversial provisions of this theory.
It should be noted that a certain part of mobilist scientists (both abroad and domestic) in their views show an extremely negative attitude towards the classical doctrine of geosynclines in fact, they completely reject it, ignoring the fact that many of the provisions of this doctrine are based on reliable facts and observations established and carried out during geological studies of the continents.
It is obvious that the most correct way to create a truly global theory of the Earth is not to contrast, but to reveal the unity and relationship between everything positive, reflected in the classical theory of geosynclines, and everything new that is revealed in the theory of new global tectonics.