Movement of continental plates. Tectonic plates and their movement

Earth's lithospheric plates are huge blocks. Their foundation is formed by highly folded granite metamorphosed igneous rocks. The names of the lithospheric plates will be given in the article below. From above they are covered with a three-four-kilometer "cover". It is formed from sedimentary rocks. The platform has a relief consisting of individual mountain ranges and vast plains. Next, the theory of the movement of lithospheric plates will be considered.

The emergence of the hypothesis

The theory of the movement of lithospheric plates appeared at the beginning of the twentieth century. Subsequently, she was destined to play a major role in the exploration of the planet. The scientist Taylor, and after him Wegener, put forward the hypothesis that over time there is a drift of lithospheric plates in a horizontal direction. However, in the thirties of the 20th century, a different opinion was established. According to him, the movement of lithospheric plates was carried out vertically. This phenomenon was based on the process of differentiation of the planet's mantle matter. It became known as fixism. Such a name was due to the fact that the permanently fixed position of sections of the crust relative to the mantle was recognized. But in 1960, after the discovery of a global system of mid-ocean ridges that encircle the entire planet and come out on land in some areas, there was a return to the hypothesis of the early 20th century. However, the theory has taken on a new form. Block tectonics has become the leading hypothesis in the sciences that study the structure of the planet.

Basic provisions

It was determined that there are large lithospheric plates. Their number is limited. There are also smaller lithospheric plates of the Earth. The boundaries between them are drawn according to the concentration in the sources of earthquakes.

The names of the lithospheric plates correspond to the continental and oceanic regions located above them. There are only seven blocks with a huge area. The largest lithospheric plates are the South and North American, Euro-Asian, African, Antarctic, Pacific and Indo-Australian.

Blocks floating in the asthenosphere are characterized by solidity and rigidity. The above areas are the main lithospheric plates. In accordance with the initial ideas, it was believed that the continents make their way through the ocean floor. At the same time, the movement of lithospheric plates was carried out under the influence of an invisible force. As a result of the research, it was revealed that the blocks float passively over the material of the mantle. It is worth noting that their direction is vertical at first. The mantle material rises under the crest of the ridge. Then there is a spread in both directions. Accordingly, there is a divergence of lithospheric plates. This model represents the ocean floor as a giant. It comes to the surface in the rift areas of the mid-ocean ridges. Then hides in deep-sea trenches.

The divergence of lithospheric plates provokes the expansion of oceanic beds. However, the volume of the planet, despite this, remains constant. The fact is that the birth of a new crust is compensated by its absorption in subduction (underthrust) areas in deep-sea trenches.

Why does lithospheric plates move?

The reason is the thermal convection of the planet's mantle material. The lithosphere is stretched and uplifted, which occurs over ascending branches from convective currents. This provokes the movement of lithospheric plates to the sides. As the platform moves away from the mid-ocean rifts, the platform becomes compacted. It becomes heavier, its surface sinks down. This explains the increase in ocean depth. As a result, the platform plunges into deep-sea trenches. When attenuating from the heated mantle, it cools and sinks with the formation of basins, which are filled with sediments.

Plate collision zones are areas where the crust and platform experience compression. In this regard, the power of the first increases. As a result, the upward movement of lithospheric plates begins. It leads to the formation of mountains.

Research

The study today is carried out using geodetic methods. They allow us to conclude that the processes are continuous and ubiquitous. Collision zones of lithospheric plates are also revealed. The lifting speed can be up to tens of millimeters.

Horizontally large lithospheric plates float somewhat faster. In this case, the speed can be up to ten centimeters during the year. So, for example, St. Petersburg has already risen by a meter over the entire period of its existence. Scandinavian peninsula - by 250 m in 25,000 years. The mantle material moves relatively slowly. However, earthquakes and other phenomena occur as a result. This allows us to draw a conclusion about the high power of moving the material.

Using the tectonic position of the plates, researchers explain many geological phenomena. At the same time, during the study, it turned out that the complexity of the processes occurring with the platform is much greater than it seemed at the very beginning of the appearance of the hypothesis.

Plate tectonics could not explain changes in the intensity of deformations and movement, the presence of a global stable network of deep faults, and some other phenomena. The question of the historical beginning of the action also remains open. Direct signs indicating plate-tectonic processes have been known since the late Proterozoic. However, a number of researchers recognize their manifestation from the Archean or early Proterozoic.

Expanding Research Opportunities

The advent of seismic tomography led to the transition of this science to a qualitatively new level. In the mid-eighties of the last century, deep geodynamics became the most promising and young direction of all the existing geosciences. However, the solution of new problems was carried out using not only seismic tomography. Other sciences also came to the rescue. These include, in particular, experimental mineralogy.

Thanks to the availability of new equipment, it became possible to study the behavior of substances at temperatures and pressures corresponding to the maximum at the depths of the mantle. The methods of isotope geochemistry were also used in the studies. This science studies, in particular, the isotopic balance of rare elements, as well as noble gases in various earthly shells. In this case, the indicators are compared with meteorite data. Methods of geomagnetism are used, with the help of which scientists are trying to uncover the causes and mechanism of reversals in a magnetic field.

Modern painting

The platform tectonics hypothesis continues to satisfactorily explain the process of crustal development during at least the last three billion years. At the same time, there are satellite measurements, according to which the fact that the main lithospheric plates of the Earth do not stand still is confirmed. As a result, a certain picture emerges.

There are three most active layers in the cross section of the planet. The thickness of each of them is several hundred kilometers. It is assumed that the main role in global geodynamics is assigned to them. In 1972, Morgan substantiated the hypothesis put forward in 1963 by Wilson about ascending mantle jets. This theory explained the phenomenon of intraplate magnetism. The resulting plume tectonics has become increasingly popular over time.

Geodynamics

With its help, the interaction of rather complex processes that occur in the mantle and the crust is considered. In accordance with the concept set forth by Artyushkov in his work "Geodynamics", the main source of energy is the gravitational differentiation of matter. This process is noted in the lower mantle.

After the heavy components (iron, etc.) are separated from the rock, a lighter mass of solids remains. She descends into the core. The location of the lighter layer under the heavy one is unstable. In this regard, the accumulating material is collected periodically into fairly large blocks that float into the upper layers. The size of such formations is about a hundred kilometers. This material was the basis for the formation of the upper

The lower layer is probably an undifferentiated primary substance. During the evolution of the planet, due to the lower mantle, the upper mantle grows and the core increases. It is more likely that blocks of light material are uplifted in the lower mantle along the channels. In them, the temperature of the mass is quite high. At the same time, the viscosity is significantly reduced. The increase in temperature is facilitated by the release of a large amount of potential energy in the process of lifting matter into the region of gravity at a distance of about 2000 km. In the course of movement along such a channel, a strong heating of light masses occurs. In this regard, the substance enters the mantle, having a sufficiently high temperature and significantly less weight in comparison with the surrounding elements.

Due to the reduced density, light material floats into the upper layers to a depth of 100-200 kilometers or less. With decreasing pressure, the melting point of the components of the substance decreases. After the primary differentiation at the "core-mantle" level, the secondary one occurs. At shallow depths, light matter is partially subjected to melting. During differentiation, denser substances are released. They sink into the lower layers of the upper mantle. The released lighter components rise accordingly.

The complex of motions of substances in the mantle, associated with the redistribution of masses with different densities as a result of differentiation, is called chemical convection. The rise of light masses occurs at intervals of about 200 million years. At the same time, intrusion into the upper mantle is not observed everywhere. In the lower layer, the channels are located at a sufficiently large distance from each other (up to several thousand kilometers).

Boulder lifting

As mentioned above, in those zones where large masses of light heated material are introduced into the asthenosphere, its partial melting and differentiation occur. In the latter case, the separation of components and their subsequent ascent are noted. They quickly pass through the asthenosphere. When they reach the lithosphere, their speed decreases. In some areas, matter forms accumulations of anomalous mantle. They lie, as a rule, in the upper layers of the planet.

anomalous mantle

Its composition approximately corresponds to normal mantle matter. The difference between the anomalous accumulation is a higher temperature (up to 1300-1500 degrees) and a reduced speed of elastic longitudinal waves.

The influx of matter under the lithosphere provokes isostatic uplift. Due to the elevated temperature, the anomalous cluster has a lower density than the normal mantle. In addition, there is a small viscosity of the composition.

In the process of entering the lithosphere, the anomalous mantle is rather quickly distributed along the sole. At the same time, it displaces the denser and less heated matter of the asthenosphere. In the course of movement, the anomalous accumulation fills those areas where the sole of the platform is in an elevated state (traps), and it flows around deeply submerged areas. As a result, in the first case, an isostatic uplift is noted. Above submerged areas, the crust remains stable.

Traps

The process of cooling the upper mantle layer and the crust to a depth of about a hundred kilometers is slow. In general, it takes several hundred million years. In this regard, inhomogeneities in the thickness of the lithosphere, explained by horizontal temperature differences, have a rather large inertia. In the event that the trap is located not far from the upward flow of the anomalous accumulation from the depth, a large amount of the substance is captured very heated. As a result, a rather large mountain element is formed. In accordance with this scheme, high uplifts occur in the area of epiplatform orogeny in

Description of processes

In the trap, the anomalous layer undergoes compression by 1–2 kilometers during cooling. The bark located on top is immersed. Precipitation begins to accumulate in the formed trough. Their heaviness contributes to even greater subsidence of the lithosphere. As a result, the depth of the basin can be from 5 to 8 km. At the same time, during the compaction of the mantle in the lower part of the basalt layer, a phase transformation of the rock into eclogite and garnet granulite can be observed in the crust. Due to the heat flow leaving the anomalous substance, the overlying mantle is heated and its viscosity decreases. In this regard, a gradual displacement of the normal cluster is observed.

Horizontal offsets

During the formation of uplifts in the process of the anomalous mantle reaching the crust on the continents and oceans, there is an increase in the potential energy stored in the upper layers of the planet. To dump excess substances, they tend to disperse to the sides. As a result, additional stresses are formed. They are associated with different types of movement of plates and crust.

The expansion of the ocean floor and the floating of the continents are the result of the simultaneous expansion of the ridges and the sinking of the platform into the mantle. Under the first are large masses of highly heated anomalous matter. In the axial part of these ridges, the latter is directly under the crust. The lithosphere here has a much smaller thickness. At the same time, the anomalous mantle spreads in the area of high pressure - in both directions from under the ridge. At the same time, it quite easily breaks the ocean's crust. The crevice is filled with basaltic magma. It, in turn, is melted out of the anomalous mantle. In the process of solidification of magma, a new one is formed. This is how the bottom grows.

Process Features

Beneath the mid-ridges, the anomalous mantle has reduced viscosity due to elevated temperatures. The substance is able to spread quite quickly. As a result, the growth of the bottom occurs at an increased rate. The oceanic asthenosphere also has a relatively low viscosity.

The main lithospheric plates of the Earth float from the ridges to the places of immersion. If these areas are in the same ocean, then the process occurs at a relatively high speed. This situation is typical today for the Pacific Ocean. If the expansion of the bottom and the subsidence occurs in different areas, then the continent located between them drifts in the direction where the deepening occurs. Under the continents, the viscosity of the asthenosphere is higher than under the oceans. Due to the resulting friction, there is a significant resistance to movement. As a result, the rate at which the bottom expands is reduced if there is no compensation for the mantle subsidence in the same area. Thus, the expansion in the Pacific is faster than in the Atlantic.

Characteristic geological structure with a certain ratio of plates. In the same geodynamic setting, the same type of tectonic, magmatic, seismic, and geochemical processes occur.

History of the theory

The basis of theoretical geology at the beginning of the 20th century was the contraction hypothesis. The earth cools like a baked apple, and wrinkles appear on it in the form of mountain ranges. These ideas were developed by the theory of geosynclines, created on the basis of the study of folded formations. This theory was formulated by James Dana, who added the principle of isostasy to the contraction hypothesis. According to this concept, the Earth consists of granites (continents) and basalts (oceans). When the Earth is compressed in the oceans-troughs, tangential forces arise that put pressure on the continents. The latter rise up into the mountain ranges and then collapse. The material that is obtained as a result of destruction is deposited in the depressions.

In addition, Wegener began to look for geophysical and geodetic evidence. However, at that time the level of these sciences was clearly not sufficient to fix the current movement of the continents. In 1930, Wegener died during an expedition to Greenland, but before his death he already knew that the scientific community did not accept his theory.

Initially continental drift theory was accepted favorably by the scientific community, but in 1922 it was severely criticized by several well-known experts at once. The main argument against the theory was the question of the force that moves the plates. Wegener believed that the continents move along the basalts of the ocean floor, but this required a huge effort, and no one could name the source of this force. The Coriolis force, tidal phenomena and some others were proposed as a source of plate movement, however, the simplest calculations showed that all of them are absolutely not enough to move huge continental blocks.

Critics of Wegener's theory put the question of the force that moves the continents at the forefront, and ignored all the many facts that unconditionally confirmed the theory. In fact, they found the only issue in which the new concept was powerless, and without constructive criticism, they rejected the main evidence. After the death of Alfred Wegener, the theory of continental drift was abandoned, given the status of a fringe science, and the vast majority of research continued to be carried out within the theory of geosynclines. True, she also had to look for explanations for the history of the settlement of animals on the continents. For this, land bridges were invented that connected the continents, but plunged into the depths of the sea. This was another birth of the legend of Atlantis. It is worth noting that some scientists did not recognize the verdict of world authorities and continued to search for evidence of the movement of the continents. So du Toit Alexander du Toit) explained the formation of the Himalayan mountains by the collision of Hindustan and the Eurasian plate.

The sluggish struggle between the fixists, as the supporters of the absence of significant horizontal movements were called, and the mobilists, who argued that the continents did move, flared up with renewed vigor in the 1960s, when, as a result of studying the bottom of the oceans, the keys to understanding the “machine” called Earth.

By the early 1960s, a topography map of the bottom of the World Ocean was compiled, which showed that mid-ocean ridges are located in the center of the oceans, which rise 1.5-2 km above the abyssal plains covered with sediments. These data allowed R. Dietz (English)Russian and G. Hess (English)Russian in -1963 put forward the spreading hypothesis. According to this hypothesis, convection occurs in the mantle at a rate of about 1 cm/year. Ascending branches of convection cells carry mantle material under the mid-ocean ridges, which renews the ocean floor in the axial part of the ridge every 300-400 years. Continents do not float on the oceanic crust, but move along the mantle, being passively "soldered" into the lithospheric plates. According to the concept of spreading, ocean basins are unstable structures, while continents are stable.

The age of the ocean floor (red color corresponds to young crust)

The same driving force (height difference) determines the degree of elastic horizontal compression of the crust by the force of viscous friction of the flow against the earth's crust. The magnitude of this compression is small in the region of the mantle flow ascending and increases as it approaches the place of flow descending (due to the transfer of compression stress through the immovable solid crust in the direction from the place of rise to the place of flow descent). Above the descending flow, the compression force in the crust is so great that from time to time the strength of the crust is exceeded (in the area of the lowest strength and highest stress), an inelastic (plastic, brittle) deformation of the crust occurs - an earthquake. At the same time, entire mountain ranges, for example, the Himalayas, are squeezed out of the place of deformation of the crust (in several stages).

With plastic (brittle) deformation, the stress in it decreases very quickly (at the rate of displacement of the crust during an earthquake) - the compressive force in the earthquake source and its environs. But immediately after the end of inelastic deformation, a very slow increase in stress (elastic deformation) interrupted by the earthquake continues due to the very slow movement of the viscous mantle flow, starting the cycle of preparation for the next earthquake.

Thus, the movement of the plates is a consequence of the transfer of heat from the central zones of the Earth by very viscous magma. In this case, part of the thermal energy is converted into mechanical work to overcome friction forces, and part, having passed through the earth's crust, is radiated into the surrounding space. So our planet is, in a sense, a heat engine.

There are several hypotheses regarding the cause of the high temperature of the Earth's interior. At the beginning of the 20th century, the hypothesis of the radioactive nature of this energy was popular. It seemed to be confirmed by estimates of the composition of the upper crust, which showed very significant concentrations of uranium, potassium and other radioactive elements, but it later turned out that the content of radioactive elements in the rocks of the earth's crust is completely insufficient to ensure the observed flow of deep heat. And the content of radioactive elements in the subcrustal matter (in composition close to the basalts of the ocean floor), one might say, is negligible. However, this does not exclude a sufficiently high content of heavy radioactive elements that generate heat in the central zones of the planet.

Another model explains the heating by chemical differentiation of the Earth. Initially, the planet was a mixture of silicate and metallic substances. But simultaneously with the formation of the planet, its differentiation into separate shells began. The denser metal part rushed to the center of the planet, and the silicates were concentrated in the upper shells. In this case, the potential energy of the system decreased and turned into thermal energy.

Other researchers believe that the heating of the planet occurred as a result of accretion during impacts of meteorites on the surface of a nascent celestial body. This explanation is doubtful - during accretion, heat was released practically on the surface, from where it easily escaped into space, and not into the central regions of the Earth.

Secondary forces

The force of viscous friction arising from thermal convection plays a decisive role in the movements of the plates, but besides it, other, smaller, but also important forces act on the plates. These are the forces of Archimedes, which ensure that the lighter crust floats on the surface of the heavier mantle. Tidal forces, due to the gravitational influence of the Moon and the Sun (the difference in their gravitational influence on points of the Earth at different distances from them). Now the tidal “hump” on Earth, caused by the attraction of the Moon, is on average about 36 cm. Previously, the Moon was closer, and this was on a large scale, the deformation of the mantle leads to its heating. For example, the volcanism observed on Io (a satellite of Jupiter) is caused precisely by these forces - the tide on Io is about 120 m. As well as the forces arising from changes in atmospheric pressure on various parts of the earth's surface - atmospheric pressure forces quite often change by 3%, which equivalent to a continuous layer of water 0.3 m thick (or granite at least 10 cm thick). Moreover, this change can occur in a zone hundreds of kilometers wide, while the change in tidal forces occurs more smoothly - at distances of thousands of kilometers.

Divergent or plate separation boundaries

These are the boundaries between plates moving in opposite directions. In the Earth's relief, these boundaries are expressed by rifts, tensile deformations prevail in them, the thickness of the crust is reduced, the heat flow is maximum, and active volcanism occurs. If such a boundary is formed on the continent, then a continental rift is formed, which can later turn into an oceanic basin with an oceanic rift in the center. In oceanic rifts, spreading results in the formation of new oceanic crust.

ocean rifts

Diagram of the structure of the mid-ocean ridge

On the oceanic crust, rifts are confined to the central parts of the mid-ocean ridges. They form a new oceanic crust. Their total length is more than 60 thousand kilometers. A lot of them are confined to them, which carry a significant part of the deep heat and dissolved elements into the ocean. High-temperature sources are called black smokers, significant reserves of non-ferrous metals are associated with them.

continental rifts

The splitting of the continent into parts begins with the formation of a rift. The crust thins and moves apart, magmatism begins. An extended linear depression with a depth of about hundreds of meters is formed, which is limited by a series of normal faults. After that, two scenarios are possible: either the expansion of the rift stops and it is filled with sedimentary rocks, turning into aulacogen, or the continents continue to move apart and between them, already in typically oceanic rifts, the oceanic crust begins to form.

convergent borders

Convergent boundaries are boundaries where plates collide. Three options are possible (Convergent plate boundary):

Continental plate with oceanic. Oceanic crust is denser than continental crust and subducts under the continent in a subduction zone.
Oceanic plate with oceanic. In this case, one of the plates crawls under the other and a subduction zone is also formed, above which an island arc is formed.
Continental plate with continental. A collision occurs, a powerful folded area appears. The classic example is the Himalayas.

In rare cases, the thrusting of the oceanic crust on the continental occurs - obduction. Through this process, the ophiolites of Cyprus, New Caledonia, Oman and others have come into being.

In subduction zones, oceanic crust is absorbed, and thereby its appearance in mid-ocean ridges is compensated. Exceptionally complex processes of interaction between the crust and the mantle take place in them. Thus, oceanic crust can pull blocks of continental crust into the mantle, which, due to their low density, are exhumed back into the crust. This is how metamorphic complexes of ultrahigh pressures arise, one of the most popular objects of modern geological research.

Most modern subduction zones are located along the periphery of the Pacific Ocean, forming the Pacific ring of fire. The processes taking place in the plate convergence zone are considered to be among the most complex in geology. It mixes blocks of different origin, forming a new continental crust.

Active continental margins

Active continental margin

An active continental margin occurs where oceanic crust sinks under a continent. The western coast of South America is considered the standard for this geodynamic setting, it is often called Andean type of continental margin. The active continental margin is characterized by numerous volcanoes and powerful magmatism in general. The melts have three components: the oceanic crust, the mantle above it, and the lower parts of the continental crust.

Under the active continental margin, there is an active mechanical interaction between the oceanic and continental plates. Depending on the speed, age, and thickness of the oceanic crust, several equilibrium scenarios are possible. If the plate moves slowly and has a relatively low thickness, then the continent scrapes off the sedimentary cover from it. Sedimentary rocks are crushed into intense folds, metamorphosed and become part of the continental crust. The resulting structure is called accretionary wedge. If the speed of the subducting plate is high and the sedimentary cover is thin, then the oceanic crust erases the bottom of the continent and draws it into the mantle.

island arcs

island arc

Island arcs are chains of volcanic islands above a subduction zone, occurring where an oceanic plate subducts under another oceanic plate. The Aleutian, Kuril, Mariana Islands, and many other archipelagos can be named as typical modern island arcs. The Japanese islands are also often referred to as an island arc, but their foundation is very ancient and in fact they are formed by several island arc complexes of different times, so that the Japanese islands are a microcontinent.

Island arcs are formed when two oceanic plates collide. In this case, one of the plates is at the bottom and is absorbed into the mantle. Island arc volcanoes form on the upper plate. The curved side of the island arc is directed towards the absorbed slab. On this side are a deep-water trench and a fore-arc trough.

Behind the island arc there is a back-arc basin (typical examples: the Sea of Okhotsk, the South China Sea, etc.), in which spreading can also occur.

Collision of continents

The collision of continental plates 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 by 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, it is intensively destroyed by surface and tectonic erosion. Granites are smelted from metamorphosed sedimentary and igneous rocks in the crust with a sharply increased thickness. This is how the largest batholiths were formed, for example, Angara-Vitimsky and Zerenda.

Transform borders

Where plates move in a parallel course, but at different speeds, transform faults occur - grandiose shear faults that are widespread in the oceans and rare on the continents.

Transform Rifts

In the oceans, transform faults run perpendicular to mid-ocean ridges (MORs) and break them into segments averaging 400 km wide. Between the segments of the ridge there is an active part of the transform fault. Earthquakes and mountain building constantly occur in this area, numerous feathering structures are formed around the fault - thrusts, folds and grabens. As a result, mantle rocks are often exposed in the fault zone.

On both sides of the MOR segments are inactive parts of transform faults. Active movements do not occur in them, but they are clearly expressed in the topography of the ocean floor as linear uplifts with a central depression.

Transform faults form a regular grid and, obviously, do not arise by chance, but due to objective physical reasons. The combination of numerical modeling data, thermophysical experiments and geophysical observations made it possible to find out that mantle convection has a three-dimensional structure. In addition to the main flow from the MOR, longitudinal flows arise in the convective cell due to the cooling of the upper part of the flow. This cooled matter rushes down along the main direction of the mantle flow. It is in the zones of this secondary descending flow that the transform faults are located. This model is in good agreement with the data on the heat flow: a decrease is observed over the transform faults.

Shifts across the continents

Shear plate boundaries on continents are relatively rare. Perhaps the only currently active example of this type of boundary is the San Andreas Fault, which separates the North American Plate from the Pacific. The 800-mile San Andreas Fault is one of the most seismically active regions on the planet: the plates shift relative to each other by 0.6 cm per year, earthquakes with a magnitude of more than 6 units occur on average once every 22 years. The city of San Francisco and much of the San Francisco Bay Area are built in close proximity to this fault.

Intraplate processes

The first formulations of plate tectonics claimed that volcanism and seismic phenomena were concentrated along the boundaries of the plates, but it soon became clear that specific tectonic and magmatic processes were taking place inside the plates, which were also interpreted within the framework of this theory. Among intraplate processes, a special place was occupied by the phenomena of long-term basaltic magmatism in some areas, the so-called hot spots.

Hot Spots

Numerous volcanic islands are located at the bottom of the oceans. Some of them are located in chains with successively changing age. A classic example of such an underwater ridge is the Hawaiian submarine ridge. It rises above the ocean surface in the form of the Hawaiian Islands, from which a chain of seamounts with continuously increasing age extends to the northwest, some of which, for example, Midway Atoll, come to the surface. At a distance of about 3000 km from Hawaii, the chain turns slightly to the north and is already called the Imperial Range. It is interrupted in a deep-water trough in front of the Aleutian island arc.

To explain this amazing structure, it was suggested that there is a hot spot under the Hawaiian Islands - a place where a hot mantle flow rises to the surface, which melts the oceanic crust moving above it. There are many such points on Earth now. The mantle flow that causes them has been called a plume. In some cases, an exceptionally deep origin of plume matter is assumed, up to the core-mantle boundary.

The hot spot hypothesis also raises objections. So, in their monograph, Sorokhtin and Ushakov consider it incompatible with the model of general convection in the mantle, and also point out that the erupting magmas in Hawaiian volcanoes are relatively cold, and do not indicate an increased temperature in the asthenosphere under the fault. “In this regard, the hypothesis of D. Tarkot and E. Oxburg (1978) is fruitful, according to which lithospheric plates, moving along the surface of the hot mantle, are forced to adapt to the variable curvature of the Earth's rotation ellipsoid. And although the radii of curvature of the lithospheric plates change insignificantly (only by fractions of a percent), their deformation causes the appearance of excess tensile or shear stresses of the order of hundreds of bars in the body of large plates.

Traps and oceanic plateaus

In addition to long-term hotspots, sometimes grandiose outpourings of melts occur inside the plates, which form traps on the continents, and oceanic plateaus in the oceans. The peculiarity of this type of magmatism is that it occurs in a geologically short time - on the order of several million years, but covers vast areas (tens of thousands of km²); at the same time, a colossal volume of basalts is poured out, comparable to their number, crystallizing in the mid-ocean ridges.

Siberian traps are known on the East Siberian platform, traps of the Deccan plateau on the Hindustan continent, and many others. Traps are also thought to be caused by hot mantle flows, but unlike hotspots, they are short-lived and the difference between them is not entirely clear.

Hot spots and traps gave rise to the creation of the so-called plume geotectonics, which states that not only regular convection, but also plumes play a significant role in geodynamic processes. Plume tectonics does not contradict plate tectonics, but complements it.

Plate tectonics as a system of sciences

Tectonics can no longer be viewed as a purely geological concept. It plays a key role in all geosciences; it has several methodological approaches with different basic concepts and principles.

From point of view kinematic approach, the movements of the plates can be described by the geometric laws of the movement of figures on the sphere. The Earth is viewed as a mosaic of plates of different sizes moving relative to each other and the planet itself. Paleomagnetic data make it possible to reconstruct the position of the magnetic pole relative to each plate at different times. Generalization of data on different plates led to the reconstruction of the entire sequence of relative displacements of plates. Combining this data with information from static hotspots made it possible to determine the absolute movements of the plates and the history of the movement of the Earth's magnetic poles.

Thermophysical approach considers the Earth as a heat engine, in which thermal energy is partially converted into mechanical energy. Within the framework of this approach, the movement of matter in the inner layers of the Earth is modeled as a flow of a viscous fluid, described by the Navier-Stokes equations. Mantle convection is accompanied by phase transitions and chemical reactions, which play a decisive role in the structure of mantle flows. Based on geophysical sounding data, the results of thermophysical experiments, and analytical and numerical calculations, scientists are trying to detail the structure of mantle convection, find flow rates and other important characteristics of deep processes. These data are especially important for understanding the structure of the deepest parts of the Earth - the lower mantle and the core, which are inaccessible for direct study, but undoubtedly have a huge impact on the processes taking place on the surface of the planet.

Geochemical approach. For geochemistry, plate tectonics is important as a mechanism for the continuous exchange of matter and energy between the various shells of the Earth. Each geodynamic setting is characterized by specific associations of rocks. In turn, these characteristic features can be used to determine the geodynamic setting in which the rock was formed.

Historical approach. In the sense of the history of planet Earth, plate tectonics is the history of connecting and splitting continents, the birth and extinction of volcanic chains, the appearance and closing of oceans and seas. Now, for large blocks of the crust, the history of movements has been established with great detail and over a considerable period of time, but for small plates, the methodological difficulties are much greater. The most complex geodynamic processes occur in plate collision zones, where mountain ranges are formed, composed of many small heterogeneous blocks - terranes. When studying the Rocky Mountains, a special direction of geological research was born - terrane analysis, which incorporated a set of methods for identifying terranes and reconstructing their history.

Then surely you would like to know what are lithospheric plates.

So, lithospheric plates are huge blocks into which the solid surface layer of the earth is divided. Given the fact that the rocks beneath them are melted, the plates move slowly, at a speed of 1 to 10 centimeters per year.

To date, there are 13 largest lithospheric plates that cover 90% of the earth's surface.

The largest lithospheric plates:

australian plate- 47,000,000 km²
Antarctic Plate- 60,900,000 km²
Arabian subcontinent- 5,000,000 km²
African plate- 61,300,000 km²
Eurasian plate- 67,800,000 km²
Hindustan plate- 11,900,000 km²
Coconut Plate - 2,900,000 km²
Nazca Plate - 15,600,000 km²
Pacific Plate- 103,300,000 km²
North American Plate- 75,900,000 km²
Somali plate- 16,700,000 km²
South American Plate- 43,600,000 km²
Philippine plate- 5,500,000 km²

Here it must be said that there is a continental and oceanic crust. Some plates are composed entirely of one type of crust (such as the Pacific Plate), and some are of mixed types, where the plate begins in the ocean and smoothly transitions to the continent. The thickness of these layers is 70-100 kilometers.

Lithospheric plates float on the surface of a partially molten layer of the earth - the mantle. When the plates move apart, liquid rock called magma fills the cracks between them. When magma solidifies, it forms new crystalline rocks. We will talk about magma in more detail in the article on volcanoes.

Map of lithospheric plates

The largest lithospheric plates (13 pcs.)

At the beginning of the 20th century, the American F.B. Taylor and the German Alfred Wegener simultaneously came to the conclusion that the location of the continents is slowly changing. By the way, this is exactly what, to a large extent, is. But scientists could not explain how this happens until the 60s of the twentieth century, when the doctrine of geological processes on the seabed was developed.

Map of the location of lithospheric plates

It was the fossils that played the main role here. On different continents, fossilized remains of animals were found that clearly could not swim across the ocean. This led to the assumption that once all the continents were connected and animals calmly passed between them.

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Read more in the article History of the theory of plate tectonics

The basis of theoretical geology at the beginning of the 20th century was the contraction hypothesis. The earth cools like a baked apple, and wrinkles appear on it in the form of mountain ranges. These ideas were developed by the theory of geosynclines, created on the basis of the study of folded structures. This theory was formulated by J. Dan, who added the principle of isostasy to the contraction hypothesis. According to this concept, the Earth consists of granites (continents) and basalts (oceans). When the Earth is compressed in the oceans-troughs, tangential forces arise that put pressure on the continents. The latter rise up into the mountain ranges and then collapse. The material that is obtained as a result of destruction is deposited in the depressions.

The sluggish struggle between the fixists, as the supporters of the absence of significant horizontal movements were called, and the mobilists, who claimed that they still move, flared up with renewed vigor in the 1960s, when, as a result of studying the bottom of the oceans, the keys to understanding the “machine” called the Earth were found. .

By the beginning of the 60s, a relief map of the bottom of the World Ocean was compiled, which showed that mid-ocean ridges are located in the center of the oceans, which rise 1.5–2 km above the abyssal plains covered with sediments. These data allowed R. Dietz and G. Hess to put forward the spreading hypothesis in 1962–1963. According to this hypothesis, convection occurs in the mantle at a rate of about 1 cm/year. Ascending branches of convection cells carry mantle material under the mid-ocean ridges, which renews the ocean floor in the axial part of the ridge every 300–400 years. Continents do not float on the oceanic crust, but move along the mantle, being passively "soldered" into the lithospheric plates. According to the concept of spreading, the oceanic basins of the structure are unstable, unstable, while the continents are stable.

In 1963, the spreading hypothesis received strong support in connection with the discovery of strip magnetic anomalies on the ocean floor. They have been interpreted as a record of reversals of the Earth's magnetic field, recorded in the magnetization of the ocean floor basalts. After that, plate tectonics began its triumphal march in the earth sciences. More and more scientists understood that, rather than wasting time defending the concept of fixism, it was better to look at the planet from the point of view of a new theory and, finally, begin to give real explanations for the most complex earthly processes.

Plate tectonics has now been confirmed by direct measurements of plate velocities using radiation interferometry from distant quasars and GPS measurements. The results of many years of research have fully confirmed the main provisions of the theory of plate tectonics.

Current state of plate tectonics

Over the past decades, plate tectonics has changed its fundamentals significantly. Now they can be formulated as follows:

The upper part of the solid Earth is divided into a fragile lithosphere and a plastic asthenosphere. Convection in the asthenosphere is the main cause of plate movement.

The lithosphere is divided into 8 large plates, dozens of medium plates and many small ones. Small slabs are located in belts between large slabs. Seismic, tectonic and magmatic activity is concentrated at plate boundaries.

Lithospheric plates in the first approximation are described as solid bodies, and their movement obeys the Euler rotation theorem.

There are three main types of relative plate movements

divergence (divergence) expressed by rifting and spreading;
convergence (convergence) expressed by subduction and collision;
strike-slip movements along transform faults.

Spreading in the oceans is compensated by subduction and collision along their periphery, and the radius and volume of the Earth are constant (this statement is constantly discussed, but it is so reliable and not refuted)

The movement of lithospheric plates is caused by their entrainment by convective currents in the asthenosphere.

There are two fundamentally different types of earth's crust - continental crust and oceanic crust. Some lithospheric plates are composed exclusively of oceanic crust (an example is the largest Pacific plate), others consist of a block of continental crust soldered into the oceanic crust.

More than 90% of the Earth's surface is covered by 8 major lithospheric plates:

Medium-sized plates include the Arabian subcontinent, and the Cocos and Juan de Fuca plates, remnants of the huge Faralon Plate that formed a large part of the Pacific Ocean floor, but has now disappeared in the subduction zone under the Americas.

The force that moves the plates

Now there is no doubt that the movement of plates occurs due to mantle heat-gravity currents - convection. The source of energy for these currents is the transfer of heat from the central parts of the Earth, which have a very high temperature (according to estimates, the temperature of the core is about 5000 ° C). Heated rocks expand (see thermal expansion), their density decreases, and they float up, giving way to colder rocks. These currents can close and form stable convective cells. At the same time, in the upper part of the cell, the flow of matter occurs in a horizontal plane, and it is this part of it that transfers the plates.

Thus, the movement of the plates is a consequence of the cooling of the Earth, in which part of the thermal energy is converted into mechanical work, and our planet in a sense is a heat engine.

There are several hypotheses regarding the cause of the high temperature of the Earth's interior. At the beginning of the 20th century, the hypothesis of the radioactive nature of this energy was popular. It seemed to be confirmed by estimates of the composition of the upper crust, which showed very significant concentrations of uranium, potassium and other radioactive elements, but later it turned out that the content of radioactive elements drops sharply with depth. Another model explains the heating by the Earth's chemical differentiation. Initially, the planet was a mixture of silicate and metallic substances. But simultaneously with the formation of the planet, its differentiation into separate shells began. The denser metal part rushed to the center of the planet, and the silicates were concentrated in the upper shells. In this case, the potential energy of the system decreased and turned into thermal energy. Other researchers believe that the heating of the planet occurred as a result of accretion during impacts of meteorites on the surface of a nascent celestial body.

Secondary forces

Thermal convection plays a decisive role in the movements of plates, but in addition to it, smaller, but no less important forces act on the plates.

When the oceanic crust sinks into the mantle, the basalts of which it consists turn into eclogites, rocks that are denser than ordinary mantle rocks - peridotites. Therefore, this part of the oceanic plate sinks into the mantle, and pulls the not yet eclogitized part along with it.

Divergent or plate separation boundaries

ocean rifts

continental rifts

convergent borders

Read more in the article Subduction zone

Convergent boundaries are boundaries where plates collide. Three options are possible:

Continental plate with oceanic. Oceanic crust is denser than continental crust and subducts under the continent in a subduction zone.
Oceanic plate with oceanic. In this case, one of the plates crawls under the other and a subduction zone is also formed, above which an island arc is formed.
Continental plate with continental. A collision occurs, a powerful folded area appears. The classic example is the Himalayas.

In rare cases, the thrusting of the oceanic crust on the continental occurs - obduction. Through this process, the ophiolites of Cyprus, New Caledonia, Oman and others have come into being.

In subduction zones, the oceanic crust is absorbed, and thus its appearance in MORs is compensated. Exceptionally complex processes, interactions between the crust and the mantle take place in them. Thus, oceanic crust can pull blocks of continental crust into the mantle, which, due to their low density, are exhumed back into the crust. This is how metamorphic complexes of ultrahigh pressures arise, one of the most popular objects of modern geological research.

Most of today's subduction zones are located along the periphery of the Pacific Ocean, forming the Pacific Ring of Fire. The processes taking place in the zone of plate convergence are considered to be among the most complex in geology. It mixes blocks of different origin, forming a new continental crust.

Active continental margins

Read more in the article Active continental margin

island arcs

island arc

Read more in the article Island arc

Island arcs are chains of volcanic islands above a subduction zone, occurring where an oceanic plate subducts under an oceanic one. The Aleutian, Kuril, Mariana Islands, and many other archipelagos can be named as typical modern island arcs. The Japanese islands are also often referred to as an island arc, but their foundation is very ancient and in fact they are formed by several island arc complexes of different times, so that the Japanese islands are a microcontinent.

Behind the island arc there is a back-arc basin (typical examples: the Sea of Okhotsk, the South China Sea, etc.) in which spreading can also occur.

Collision of continents

Read more in the article Collision of continents

Transform borders

Where plates move in a parallel course, but at different speeds, transform faults occur - grandiose shear faults that are widespread in the oceans and rare on the continents.

Transform Rifts

Read more in the article Transform Fault

Shifts across the continents

Read more in the article Shift

Intraplate processes

Hot Spots

Numerous volcanic islands are located at the bottom of the oceans. Some of them are located in chains with successively changing age. A classic example of such an underwater ridge is the Hawaiian submarine ridge. It rises above the ocean surface in the form of the Hawaiian Islands, from which a chain of seamounts with continuously increasing age extends to the northwest, some of which, for example, Midway Atoll, come to the surface. At a distance of about 3000 km from Hawaii, the chain turns slightly to the north, and is already called the Imperial Range. It is interrupted in a deep-water trench in front of the Aleutian island arc.

Traps and oceanic plateaus

In addition to long-term hotspots, sometimes grandiose outpourings of melts occur inside the plates, which form traps on the continents, and oceanic plateaus in the oceans. The peculiarity of this type of magmatism is that it occurs in a geologically short time of the order of several million years, but captures vast areas (tens of thousands of km²) and pours out a colossal volume of basalts, comparable to their number, crystallizing in the mid-ocean ridges.

Hot spots and traps gave rise to the creation of the so-called plume geotectonics, which states that not only regular convection, but also plumes play a significant role in geodynamic processes. Plume tectonics does not contradict plate tectonics, but complements it.

Plate tectonics as a system of sciences

Map of tectonic plates

Tectonics can no longer be viewed as a purely geological concept. It plays a key role in all geosciences; it has several methodological approaches with different basic concepts and principles.

From point of view kinematic approach, the movements of the plates can be described by the geometric laws of the movement of figures on the sphere. The Earth is viewed as a mosaic of plates of different sizes moving relative to each other and the planet itself. Paleomagnetic data make it possible to reconstruct the position of the magnetic pole relative to each plate at different times. Generalization of data on different plates led to the reconstruction of the entire sequence of relative displacements of plates. Combining this data with information from static hotspots made it possible to determine the absolute movements of the plates and the history of the movement of the Earth's magnetic poles.

Thermophysical approach considers the Earth as a heat engine, in which thermal energy is partially converted into mechanical energy. Within the framework of this approach, the movement of matter in the inner layers of the Earth is modeled as a flow of a viscous fluid, described by the Navier-Stokes equations. Mantle convection is accompanied by phase transitions and chemical reactions, which play a decisive role in the structure of mantle flows. Based on geophysical sounding data, the results of thermophysical experiments, and analytical and numerical calculations, scientists are trying to detail the structure of mantle convection, find flow rates and other important characteristics of deep processes. These data are especially important for understanding the structure of the deepest parts of the Earth - the lower mantle and the core, which are inaccessible for direct study, but undoubtedly have a huge impact on the processes taking place on the surface of the planet.

Geochemical approach. For geochemistry, plate tectonics is important as a mechanism for the continuous exchange of matter and energy between the various shells of the Earth. Each geodynamic setting is characterized by specific associations of rocks. In turn, these characteristic features can be used to determine the geodynamic setting in which the rock was formed.

Historical approach. In the sense of the history of the planet Earth, plate tectonics is the history of connecting and splitting continents, the birth and extinction of volcanic chains, the appearance and closing of oceans and seas. Now, for large blocks of the crust, the history of displacements has been established with great detail and over a considerable period of time, but for small slabs, the methodological difficulties are much greater. The most complex geodynamic processes occur in the zones of collision of plates, where mountain ranges are formed, composed of many small heterogeneous blocks - terranes, carried out in 1999 by the Proterozoic space station. Prior to this, the mantle may have had a different structure of mass transfer, in which a large role was played not by steady convective flows, but by turbulent convection and plumes.

Past plate movements

Read more in the article History of moving plates

Reconstruction of past plate movements is one of the main subjects of geological research. With varying degrees of detail, the positions of the continents and the blocks from which they formed have been reconstructed up to the Archean.

It moves to the north and crushes the Eurasian plate, but, apparently, the resource of this movement is already almost exhausted, and in the near future a new subduction zone will appear in the Indian Ocean, in which the oceanic crust of the Indian Ocean will be absorbed under the Indian continent.

Effect of plate movements on climate

The location of large continental masses in the polar regions contributes to a general decrease in the temperature of the planet, since ice sheets can form on the continents. The more developed glaciation, the greater the albedo of the planet and the lower the average annual temperature.

In addition, the relative position of the continents determines oceanic and atmospheric circulation.

However, a simple and logical scheme: continents in the polar regions - glaciation, continents in the equatorial regions - temperature increase, turns out to be incorrect when compared with geological data about the Earth's past. The Quaternary glaciation really happened when Antarctica appeared in the region of the South Pole, and in the northern hemisphere, Eurasia and North America approached the North Pole. On the other hand, the strongest Proterozoic glaciation, during which the Earth was almost completely covered with ice, occurred when most of the continental masses were in the equatorial region.

In addition, significant changes in the position of the continents occur over a period of about tens of millions of years, while the total duration of ice ages is about several million years, and during one ice age there are cyclic changes of glaciations and interglacial periods. All of these climatic changes occur quickly compared to the speeds at which the continents move, and therefore plate movement cannot be the cause.

It follows from the above that plate movements do not play a decisive role in climate change, but can be an important additional factor “pushing” them.

Significance of plate tectonics

Plate tectonics has played a role in the Earth sciences comparable to the heliocentric concept in astronomy, or the discovery of DNA in genetics. Before the adoption of the theory of plate tectonics, the earth sciences were descriptive. They achieved a high level of perfection in describing natural objects, but were rarely able to explain the causes of processes. Opposite concepts could dominate in different branches of geology. Plate tectonics connected the various sciences of the Earth, gave them predictive power.

V. E. Khain. over smaller regions and smaller time scales.

. - The main lithospheric plates. - - - Lithospheric plates of Russia.

What is the composition of the lithosphere.

At this time, on the boundary opposite from the fault, collision of lithospheric plates. This collision can proceed in different ways depending on the types of colliding plates.

If the oceanic and continental plates collide, the first sinks under the second. In this case, deep-sea trenches, island arcs (Japanese islands) or mountain ranges (Andes) arise.
If two continental lithospheric plates collide, then at this point the edges of the plates are crumpled into folds, which leads to the formation of volcanoes and mountain ranges. Thus, the Himalayas arose on the border of the Eurasian and Indo-Australian plates. In general, if there are mountains in the center of the mainland, this means that once it was a place of collision of two lithospheric plates welded into one.

Thus, the earth's crust is in constant motion. In its irreversible development, mobile areas - geosynclines- are transformed through long-term transformations into relatively calm areas - platforms.

Lithospheric plates of Russia.

Russia is located on four lithospheric plates.

Eurasian plate- most of the western and northern parts of the country,
North American Plate- northeastern part of Russia,
Amur lithospheric plate- south of Siberia,
Sea of Okhotsk plate The Sea of Okhotsk and its coast.

Fig 2. Map of the lithospheric plates of Russia.

In the structure of lithospheric plates, relatively even ancient platforms and mobile folded belts stand out. Plains are located on stable areas of the platforms, and mountain ranges are located in the region of folded belts.

Fig 3. Tectonic structure of Russia.

Russia is located on two ancient platforms (East European and Siberian). Within the platforms stand out plates and shields. A plate is a section of the earth's crust, the folded base of which is covered with a layer of sedimentary rocks. Shields, in contrast to slabs, have very little sedimentary deposits and only a thin layer of soil.

In Russia, the Baltic Shield is distinguished on the East European Platform and the Aldan and Anabar Shields on the Siberian Platform.

Figure 4. Platforms, slabs and shields in Russia.