Lithospheric plates: theory of tectonics and its main provisions. Plate theory

Together with part of the upper mantle, it consists of several very large blocks called lithospheric plates. Their thickness varies - from 60 to 100 km. Most plates include both continental and oceanic crust. There are 13 main plates, of which 7 are the largest: American, African, Indo-, Amur.

The plates lie on a plastic layer of the upper mantle (asthenosphere) and slowly move relative to each other at a speed of 1-6 cm per year. This fact was established by comparing images taken from artificial Earth satellites. They suggest that the configuration in the future may be completely different from the present one, since it is known that the American lithospheric plate is moving towards the Pacific, and the Eurasian plate is moving closer to the African, Indo-Australian, and also the Pacific. The American and African lithospheric plates are slowly moving apart.

The forces that cause the divergence of lithospheric plates arise when the material of the mantle moves. Powerful upward flows of this substance push the plates apart, tearing apart the earth's crust, forming deep faults in it. Due to underwater outpourings of lavas, strata are formed along faults. By freezing, they seem to heal wounds - cracks. However, the stretching increases again, and ruptures occur again. So, gradually increasing, lithospheric plates diverge in different directions.

There are fault zones on land, but most of them are in the ocean ridges, where the earth's crust is thinner. The largest fault on land is located in the east. It stretches for 4000 km. The width of this fault is 80-120 km. Its outskirts are dotted with extinct and active ones.

Along other plate boundaries, plate collisions are observed. It happens in different ways. If plates, one of which has oceanic crust and the other continental, come closer together, then the lithospheric plate, covered by the sea, sinks under the continental one. In this case, arcs () or mountain ranges () appear. If two plates that have continental crust collide, the edges of these plates are crushed into folds of rock, and mountainous regions are formed. This is how they arose, for example, on the border of the Eurasian and Indo-Australian plates. The presence of mountainous areas in the internal parts of the lithospheric plate suggests that once there was a boundary of two plates that were firmly fused with each other and turned into a single, larger lithospheric plate. Thus, we can draw a general conclusion: the boundaries of lithospheric plates are mobile areas to which volcanoes, zones, mountain areas, mid-ocean ridges, deep-sea depressions and trenches are confined. It is at the border of lithospheric plates that they are formed, the origin of which is associated with magmatism.

Lithospheric plates have high rigidity and are capable of maintaining their structure and shape without changes for a long time in the absence of external influences.

Plate movement

Lithospheric plates are in constant motion. This movement, occurring in the upper layers, is due to the presence of convective currents present in the mantle. Individual lithospheric plates approach, diverge, and slide relative to each other. When the plates come together, compression zones arise and subsequent thrusting (obduction) of one of the plates onto the neighboring one, or pushing (subduction) of adjacent formations. When divergence occurs, tension zones appear with characteristic cracks appearing along the boundaries. When sliding, faults are formed, in the plane of which nearby plates are observed.

Movement results

In areas of convergence of huge continental plates, when they collide, mountain ranges arise. Similarly, at one time the Himalaya mountain system arose, formed on the border of the Indo-Australian and Eurasian plates. The result of the collision of oceanic lithospheric plates with continental formations is island arcs and deep-sea trenches.

In the axial zones of mid-ocean ridges, rifts (from the English Rift - fault, crack, crevice) of a characteristic structure arise. Similar formations of the linear tectonic structure of the earth's crust, with a length of hundreds and thousands of kilometers, with a width of tens or hundreds of kilometers, arise as a result of horizontal stretching of the earth's crust. Very large rifts are usually called rift systems, belts or zones.

Due to the fact that each lithospheric plate is a single plate, increased seismic activity and volcanism are observed in its faults. These sources are located within fairly narrow zones, in the plane of which friction and mutual movements of neighboring plates occur. These zones are called seismic belts. Deep-sea trenches, mid-ocean ridges and reefs are mobile areas of the earth's crust, they are located at the boundaries of individual lithospheric plates. This once again confirms that the process of formation of the earth’s crust in these places continues quite intensively at the present time.

The importance of the theory of lithospheric plates cannot be denied. Since it is she who is able to explain the presence of mountains in some regions of the Earth, and in others. The theory of lithospheric plates makes it possible to explain and predict the occurrence of catastrophic phenomena that can occur in the area of their boundaries.

December 10th, 2015

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According to modern plate theories 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.

Only in the 1960s did studies of the ocean floor provide conclusive evidence of horizontal plate movements and ocean expansion processes due to the formation (spreading) of oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the “mobilistic” trend, the development of which led to the development of the modern theory of plate tectonics. The main principles of plate tectonics were formulated in 1967-68 by a group of American geophysicists - W. J. Morgan, C. Le Pichon, J. Oliver, J. Isaacs, L. Sykes in the development of earlier (1961-62) ideas of American scientists G. Hess and R. Digtsa about the expansion (spreading) of the ocean floor.

It is argued that scientists are not entirely sure what causes these shifts and how the boundaries of tectonic plates are defined. There are countless different theories, but none completely explains all aspects of tectonic activity.

Let's at least find out how they imagine it now.

Wegener wrote: “In 1910, the idea of moving continents first occurred to me ... 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 continent, which included the territories of modern Europe, Asia without India and North America. The southern continent - Gondwana united the modern territories of South America, Africa, Antarctica, Australia and Hindustan.

Between Gondwana and Laurasia there 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)

About 180 million years ago, the continent of Pangea again began to separate into its component parts, which mixed on the surface of our planet. The division occurred as follows: first Laurasia and Gondwana reappeared, then Laurasia split, and then Gondwana split. Due to the split and divergence of parts of Pangea, oceans were formed. The Atlantic and Indian oceans can be considered young oceans; old - Quiet. The Arctic Ocean became isolated as landmass increased in the Northern Hemisphere.

A. Wegener found many confirmations of the existence of a single continent of the Earth. What seemed especially convincing to him was the existence in Africa and South America of the remains of ancient animals - listosaurs. These were reptiles, similar to small hippopotamuses, that lived only in freshwater bodies of water. 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 continental movement in the 30s of the 20th century. decreased somewhat, but was revived again in the 60s, 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).

Structure of the continental rift

The upper rocky part of the planet is divided into two shells, significantly different in rheological properties: a rigid and brittle lithosphere and an underlying plastic and mobile asthenosphere.
The base of the lithosphere is an isotherm approximately equal to 1300°C, which corresponds to the melting temperature (solidus) of the mantle material at lithostatic pressure existing at depths of the first hundreds of kilometers. Rocks in the Earth above this isotherm are quite cold and behave like rigid materials, while underlying rocks of the same composition are quite heated and deform relatively easily.

The lithosphere is divided into plates, constantly moving along the surface of the plastic asthenosphere. The lithosphere is divided into 8 large plates, dozens of medium plates and many small ones. Between the large and medium slabs there are belts composed of a mosaic of small crustal slabs.

Plate boundaries are areas of seismic, tectonic, and magmatic activity; the internal regions of the plates are weakly seismic and characterized by weak manifestation of endogenous processes.
More than 90% of the Earth's surface falls on 8 large lithospheric plates:

Some lithospheric plates are composed exclusively of oceanic crust (for example, the Pacific Plate), others include fragments of both oceanic and continental crust.

Rift formation scheme

There are three types of relative movements of plates: divergence (divergence), convergence (convergence) and shear movements.

Divergent boundaries are boundaries along which plates move apart. The geodynamic situation in which the process of horizontal stretching of the earth's crust occurs, accompanied by the appearance of extended linearly elongated slot or ditch-like depressions, is called rifting. These boundaries are confined to continental rifts and mid-ocean ridges in ocean basins. The term "rift" (from the English rift - gap, crack, gap) is applied to large linear structures of deep origin, formed during the stretching of the earth's crust. In terms of structure, they are graben-like structures. Rifts can form on both continental and oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to a break in the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of rupture of the continental crust, it is filled with sediments, turning into an aulacogen).

The process of plate separation in zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of new oceanic crust due to magmatic basaltic melt coming from the asthenosphere. This process of formation of new oceanic crust due to the influx of mantle material is called spreading (from the English spread - to spread, unfold).

The structure of the mid-ocean ridge. 1 – asthenosphere, 2 – ultrabasic rocks, 3 – basic rocks (gabbroids), 4 – complex of parallel dikes, 5 – basalts of the ocean floor, 6 – segments of the oceanic crust formed at different times (I-V as they become more ancient), 7 – near-surface igneous chamber (with ultrabasic magma in the lower part and basic magma in the upper), 8 – sediments of the ocean floor (1-3 as they accumulate)

During spreading, each extension pulse is accompanied by the arrival of a new portion of mantle melts, which, when solidified, build up the edges of plates diverging from the MOR axis. It is in these zones that the formation of young oceanic crust occurs.

Collision of continental and oceanic lithospheric plates

Subduction is the process of pushing an oceanic plate under a continental or other oceanic one. Subduction zones are confined to the axial parts of deep-sea trenches associated with island arcs (which are elements of active margins). Subduction boundaries account for about 80% of the length of all convergent boundaries.

When the continental and oceanic plates collide, a natural phenomenon is the displacement of the oceanic (heavier) plate under the edge of the continental one; When two oceans collide, the more ancient (that is, cooler and denser) of them sinks.

Subduction zones have a characteristic structure: their typical elements are a deep-sea trench - a volcanic island arc - a back-arc basin. A deep-sea trench is formed in the zone of bending and underthrusting of the subducting plate. As this plate sinks, it begins to lose water (found in abundance in sediments and minerals), the latter, as is known, significantly reduces the melting temperature of rocks, which leads to the formation of melting centers that feed volcanoes of island arcs. In the rear of a volcanic arc, some stretching usually occurs, which determines the formation of a back-arc basin. In the back-arc basin zone, stretching can be so significant that it leads to rupture of the plate crust and the opening of a basin with oceanic crust (the so-called back-arc spreading process).

The volume of oceanic crust absorbed in subduction zones is equal to the volume of crust emerging in spreading zones. This position emphasizes the idea that the volume of the Earth is constant. But this opinion is not the only and definitively proven one. It is possible that the volume of the plane changes pulsatingly, or that it decreases due to cooling.

The immersion of the subducting plate into the mantle is traced by the foci of earthquakes that occur at the contact of the plates and inside the subducting plate (colder and, therefore, more fragile than the surrounding mantle rocks). This seismofocal zone is called the Benioff-Zavaritsky zone. In subduction zones, the process of formation of new continental crust begins. A much rarer process of interaction between the continental and oceanic plates is the process of obduction - the pushing of part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that during this process, the ocean plate is separated, and only its upper part - the crust and several kilometers of the upper mantle - moves forward.

Collision of continental plates

When continental plates collide, the crust of which is lighter than the mantle material and, as a result, is not able to sink into it, a collision process occurs. During the collision, the edges of colliding continental plates are crushed, crushed, and systems of large thrusts are formed, which leads to the growth of mountain structures with a complex fold-thrust structure. A classic example of such a process is the collision of the Hindustan plate with the Eurasian plate, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet. The collision process replaces the subduction process, completing the closure of the ocean basin. Moreover, at the beginning of the collision process, when the edges of the continents have already moved closer together, the collision is combined with the process of subduction (the remnants of the oceanic crust continue to sink under the edge of the continent). Large-scale regional metamorphism and intrusive granitoid magmatism are typical for collision processes. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).

The main reason for plate movement is mantle convection, caused by mantle thermogravitational currents.

The source of energy for these currents is the difference in temperature between the central regions of the Earth and the temperature of its near-surface parts. In this case, the main part of the endogenous heat is released at the boundary of the core and the mantle during the process of deep differentiation, which determines the disintegration of the primary chondritic substance, during which the metal part rushes to the center, building up the core of the planet, and the silicate part is concentrated in the mantle, where it further undergoes differentiation.

Rocks heated in the central zones of the Earth expand, their density decreases, and they float up, giving way to sinking colder and therefore heavier masses that have already given up some of the heat in the near-surface zones. This process of heat transfer occurs continuously, resulting in the formation of ordered closed convective cells. In this case, in the upper part of the cell, the flow of matter occurs almost in a horizontal plane, and it is this part of the flow that determines the horizontal movement of the matter of the asthenosphere and the plates located on it. In general, the ascending branches of convective cells are located under the zones of divergent boundaries (MOR and continental rifts), while the descending branches are located under the zones of convergent boundaries. Thus, the main reason for the movement of lithospheric plates is “dragging” by convective currents. In addition, a number of other factors act on the slabs. In particular, the surface of the asthenosphere turns out to be somewhat elevated above the zones of ascending branches and more depressed in the zones of subsidence, which determines the gravitational “sliding” of the lithospheric plate located on an inclined plastic surface. Additionally, there are processes of drawing heavy cold oceanic lithosphere in subduction zones into the hot, and as a consequence less dense, asthenosphere, as well as hydraulic wedging by basalts in the MOR zones.

The main driving forces of plate tectonics are applied to the base of the intraplate parts of the lithosphere - the mantle drag forces FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the speed of the asthenospheric flow, and the latter is determined by the viscosity and thickness of the asthenospheric layer. Since the thickness of the asthenosphere under the continents is much less, and the viscosity is much greater than under the oceans, the magnitude of the FDC force is almost an order of magnitude lower than the FDO value. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost pinches out, so the continents seem to be “stranded.” Since most lithospheric plates of the modern Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the plate should, in general, “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving almost purely oceanic plates are the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African plates, a significant part of whose area is occupied by continents). Finally, at convergent plate boundaries, where the heavy and cold edges of lithospheric plates (slabs) sink into the mantle, their negative buoyancy creates the FNB force (an index in the designation of force - from the English negative buoyance). The action of the latter leads to the fact that the subducting part of the plate sinks in the asthenosphere and pulls the entire plate along with it, thereby increasing the speed of its movement. Obviously, the FNB force acts sporadically and only in certain geodynamic settings, for example in the cases of slab failure across the 670 km divide described above.

Thus, the mechanisms that set lithospheric plates in motion can be conditionally classified into the following two groups: 1) associated with the forces of mantle drag mechanism applied to any points of the base of the plates, in the figure - forces FDO and FDC; 2) associated with forces applied to the edges of the slabs (edge-force mechanism), in the figure - FRP and FNB forces. The role of one or another driving mechanism, as well as certain forces, is assessed individually for each lithospheric plate.

The combination of these processes reflects the general geodynamic process, covering areas from the surface to the deep zones of the Earth. Currently, two-cell mantle convection with closed cells is developing in the Earth's mantle (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with the accumulation of slabs under subduction zones (according to the two-tier model). The probable poles of the rise of mantle material are located in northeastern Africa (approximately under the junction zone of the African, Somali and Arabian plates) and in the Easter Island region (under the middle ridge of the Pacific Ocean - the East Pacific Rise). The equator of subsidence of mantle matter passes approximately along a continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian Oceans. The modern regime of mantle convection, which began approximately 200 million years ago with the collapse of Pangea and gave rise to modern oceans, will in the future be replaced by a single-cell regime (according to the model of through-mantle convection convection) or (according to an alternative model) convection will become through the mantle due to the collapse of slabs through the 670 km section. This may lead to a collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.

Plate movements obey the laws of spherical geometry and can be described based on Euler's theorem. Euler's rotation theorem states that any rotation of three-dimensional space has an axis. Thus, rotation can be described by three parameters: the coordinates of the rotation axis (for example, its latitude and longitude) and the rotation angle. Based on this position, the position of the continents in past geological eras can be reconstructed. An analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which subsequently undergoes disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, modern continents were formed.

Plate tectonics was the first general geological concept that could be tested. Such a check was carried out. In the 70s a deep-sea drilling program was organized. As part of this program, several hundred wells were drilled by the Glomar Challenger drilling vessel, which showed good agreement between ages estimated from magnetic anomalies and ages determined from basalts or sedimentary horizons. The distribution diagram of sections of the oceanic crust of different ages is shown in Fig.:

Age of the ocean crust based on magnetic anomalies (Kennet, 1987): 1 - areas of missing data and land; 2–8 - age: 2 - Holocene, Pleistocene, Pliocene (0–5 million years); 3 - Miocene (5–23 million years); 4 - Oligocene (23–38 million years); 5 - Eocene (38–53 million years); 6 - Paleocene (53–65 million years) 7 - Cretaceous (65–135 million years) 8 - Jurassic (135–190 million years)

At the end of the 80s. Another experiment to test the movement of lithospheric plates was completed. It was based on measuring baselines relative to distant quasars. Points were selected on two plates at which, using modern radio telescopes, the distance to the quasars and their declination angle were determined, and, accordingly, the distances between the points on the two plates were calculated, i.e., the base line was determined. The accuracy of the determination was a few centimeters. After several years, the measurements were repeated. A very good agreement was obtained between the results calculated from magnetic anomalies and the data determined from the baselines

Diagram illustrating the results of measurements of the mutual movement of lithospheric plates obtained by the very long baseline interferometry method - ISDB (Carter, Robertson, 1987). The movement of the plates changes the length of the baseline between radio telescopes located on different plates. The map of the Northern Hemisphere shows baselines from which sufficient data have been obtained using the ISDB method to make a reliable estimate of the rate of change in their length (in centimeters per year). The numbers in parentheses indicate the amount of plate displacement calculated from the theoretical model. In almost all cases the calculated and measured values are very close

Thus, plate tectonics has been tested over the years by a number of independent methods. It is recognized by the world scientific community as the paradigm of geology at the present time.

Knowing the position of the poles and the speed of modern movement of lithospheric plates, the speed of spreading and absorption of the ocean floor, it is possible to outline the path of movement of the continents in the future and imagine their position for a certain period of time.

This forecast was made by American geologists R. Dietz and J. Holden. In 50 million years, according to their assumptions, the Atlantic and Indian oceans will expand at the expense of the Pacific, Africa will shift to the north and thanks to this the Mediterranean Sea will gradually be eliminated. The Strait of Gibraltar will disappear, and a “turned” Spain will close the Bay of Biscay. Africa will be split by the great African faults and its eastern part will shift to the northeast. The Red Sea will expand so much that it will separate the Sinai Peninsula from Africa, Arabia will move to the northeast and close the Persian Gulf. India will increasingly move towards Asia, which means the Himalayan mountains will grow. California will separate from North America along the San Andreas Fault, and a new ocean basin will begin to form in this place. Significant changes will occur in the southern hemisphere. Australia will cross the equator and come into contact with Eurasia. This forecast requires significant clarification. Much here still remains debatable and unclear.

sources

http://www.pegmatite.ru/My_Collection/mineralogy/6tr.htm

http://www.grandars.ru/shkola/geografiya/dvizhenie-litosfernyh-plit.html

http://kafgeo.igpu.ru/web-text-books/geology/platehistory.htm

http://stepnoy-sledopyt.narod.ru/geologia/dvizh/dvizh.htm

Let me remind you, but here are the interesting ones and this one. Look at and The original article is on the website InfoGlaz.rf Link to the article from which this copy was made -

Plate tectonics (plate tectonics) is a modern geodynamic concept based on the concept of large-scale horizontal movements of relatively integral fragments of the lithosphere (lithospheric plates). Thus, plate tectonics deals with the movements and interactions of lithospheric plates.

The first suggestion about the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s within the framework of the “continental drift” hypothesis, but this hypothesis did not receive support at that time. Only in the 1960s did studies of the ocean floor provide conclusive evidence of horizontal plate movements and ocean expansion processes due to the formation (spreading) of oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the “mobilistic” trend, the development of which led to the development of the modern theory of plate tectonics. The main principles of plate tectonics were formulated in 1967-68 by a group of American geophysicists - W. J. Morgan, C. Le Pichon, J. Oliver, J. Isaacs, L. Sykes in the development of earlier (1961-62) ideas of American scientists G. Hess and R. Digtsa on the expansion (spreading) of the ocean floor

Fundamentals of Plate Tectonics

The basic principles of plate tectonics can be summarized in several fundamental

1. The upper rocky part of the planet is divided into two shells, significantly different in rheological properties: a rigid and brittle lithosphere and an underlying plastic and mobile asthenosphere.

2. The lithosphere is divided into plates, constantly moving along the surface of the plastic asthenosphere. The lithosphere is divided into 8 large plates, dozens of medium plates and many small ones. Between the large and medium slabs there are belts composed of a mosaic of small crustal slabs.

Plate boundaries are areas of seismic, tectonic, and magmatic activity; the internal regions of the plates are weakly seismic and characterized by weak manifestation of endogenous processes.

More than 90% of the Earth's surface falls on 8 large lithospheric plates:

Australian plate,
Antarctic Plate,
African plate,
Eurasian Plate,
Hindustan plate,
Pacific Plate,
North American Plate,
South American Plate.

Middle plates: Arabian (subcontinent), Caribbean, Philippine, Nazca and Coco and Juan de Fuca, etc.

Some lithospheric plates are composed exclusively of oceanic crust (for example, the Pacific Plate), others include fragments of both oceanic and continental crust.

3. There are three types of relative movements of plates: divergence (divergence), convergence (convergence) and shear movements.

Accordingly, three types of main plate boundaries are distinguished.

Divergent boundaries– boundaries along which plates move apart.

The processes of horizontal stretching of the lithosphere are called rifting. These boundaries are confined to continental rifts and mid-ocean ridges in ocean basins.

The term "rift" (from the English rift - gap, crack, gap) is applied to large linear structures of deep origin, formed during the stretching of the earth's crust. In terms of structure, they are graben-like structures.

Rifts can form on both continental and oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to a break in the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of rupture of the continental crust, it is filled with sediments, turning into an aulacogen).

The process of plate separation in zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of new oceanic crust due to magmatic basaltic melt coming from the asthenosphere. This process of formation of new oceanic crust due to the influx of mantle material is called spreading(from the English spread - spread out, unfold).

Structure of the mid-ocean ridge

During spreading, each extension pulse is accompanied by the arrival of a new portion of mantle melts, which, when solidified, build up the edges of plates diverging from the MOR axis.

It is in these zones that the formation of young oceanic crust occurs.

Convergent boundaries– boundaries along which plate collisions occur. There can be three main options for interaction during a collision: “oceanic - oceanic”, “oceanic - continental” and “continental - continental” lithosphere. Depending on the nature of the colliding plates, several different processes can occur.

Subduction- the process of subduction of an oceanic plate under a continental or other oceanic one. Subduction zones are confined to the axial parts of deep-sea trenches associated with island arcs (which are elements of active margins). Subduction boundaries account for about 80% of the length of all convergent boundaries.

In subduction zones, the process of formation of new continental crust begins.

A much rarer process of interaction between continental and oceanic plates is the process obduction– thrusting of part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that during this process, the ocean plate is separated, and only its upper part - the crust and several kilometers of the upper mantle - moves forward.

When continental plates collide, the crust of which is lighter than the mantle material, and as a result is not capable of plunging into it, a process occurs collisions. During the collision, the edges of colliding continental plates are crushed, crushed, and systems of large thrusts are formed, which leads to the growth of mountain structures with a complex fold-thrust structure. A classic example of such a process is the collision of the Hindustan plate with the Eurasian plate, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet.

Collision Process Model

The collision process replaces the subduction process, completing the closure of the ocean basin. Moreover, at the beginning of the collision process, when the edges of the continents have already moved closer together, the collision is combined with the process of subduction (the remnants of the oceanic crust continue to sink under the edge of the continent).

Large-scale regional metamorphism and intrusive granitoid magmatism are typical for collision processes. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).

Transform boundaries– boundaries along which shear displacements of plates occur.

Boundaries of the Earth's lithospheric plates

1 – divergent boundaries ( A - mid ocean ridges, b – continental rifts); 2 – transform boundaries; 3 – convergent boundaries ( A - island-arc, b – active continental margins, V - conflict); 4 – direction and speed (cm/year) of plate movement.

4. The volume of oceanic crust absorbed in subduction zones is equal to the volume of crust emerging in spreading zones. This position emphasizes the idea that the volume of the Earth is constant. But this opinion is not the only and definitively proven one. It is possible that the volume of the plane changes pulsatingly, or that it decreases due to cooling.

5. The main reason for plate movement is mantle convection , caused by mantle thermogravitational currents.

Thus, the main reason for the movement of lithospheric plates is “dragging” by convective currents.

In addition, a number of other factors act on the slabs. In particular, the surface of the asthenosphere turns out to be somewhat elevated above the zones of ascending branches and more depressed in the zones of subsidence, which determines the gravitational “sliding” of the lithospheric plate located on an inclined plastic surface. Additionally, there are processes of drawing heavy cold oceanic lithosphere in subduction zones into the hot, and as a consequence less dense, asthenosphere, as well as hydraulic wedging by basalts in the MOR zones.

Figure - Forces acting on lithospheric plates.

The main driving forces of plate tectonics are applied to the base of the intraplate parts of the lithosphere - the mantle drag forces FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the speed of the asthenospheric flow, and the latter is determined by the viscosity and thickness of the asthenospheric layer. Since under the continents the thickness of the asthenosphere is much less, and the viscosity is much greater than under the oceans, the magnitude of the force FDC almost an order of magnitude smaller than FDO. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost pinches out, so the continents seem to be “stranded.” Since most lithospheric plates of the modern Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the plate should, in general, “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving almost purely oceanic plates are the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African plates, a significant part of whose area is occupied by continents). Finally, at convergent plate boundaries, where the heavy and cold edges of the lithospheric plates (slabs) sink into the mantle, their negative buoyancy creates a force FNB(index in the designation of strength - from English negative buoyance). The action of the latter leads to the fact that the subducting part of the plate sinks in the asthenosphere and pulls the entire plate along with it, thereby increasing the speed of its movement. Obviously strength FNB acts episodically and only in certain geodynamic situations, for example in cases of the collapse of slabs described above through the 670 km section.

Thus, the mechanisms that set lithospheric plates in motion can be conditionally classified into the following two groups: 1) associated with the forces of mantle “drag” ( mantle drag mechanism), applied to any points of the base of the slabs, in Fig. 2.5.5 – forces FDO And FDC; 2) associated with forces applied to the edges of the plates ( edge-force mechanism), in the figure - forces FRP And FNB. The role of one or another driving mechanism, as well as certain forces, is assessed individually for each lithospheric plate.

The combination of these processes reflects the general geodynamic process, covering areas from the surface to the deep zones of the Earth.

Mantle convection and geodynamic processes

Currently, two-cell mantle convection with closed cells is developing in the Earth's mantle (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with the accumulation of slabs under subduction zones (according to the two-tier model). The probable poles of the rise of mantle material are located in northeastern Africa (approximately under the junction zone of the African, Somali and Arabian plates) and in the Easter Island region (under the middle ridge of the Pacific Ocean - the East Pacific Rise).

The equator of mantle subsidence follows a roughly continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian Oceans.

The modern regime of mantle convection, which began approximately 200 million years ago with the collapse of Pangea and gave rise to modern oceans, will in the future change to a single-cell regime (according to the model of through-mantle convection) or (according to an alternative model) convection will become through-mantle due to the collapse of slabs across a 670 km divide. This may lead to a collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.

6. The movements of plates obey the laws of spherical geometry and can be described based on Euler’s theorem. Euler's rotation theorem states that any rotation of three-dimensional space has an axis. Thus, rotation can be described by three parameters: the coordinates of the rotation axis (for example, its latitude and longitude) and the rotation angle. Based on this position, the position of the continents in past geological eras can be reconstructed. An analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which subsequently undergoes disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, modern continents were formed.

Some evidence of the reality of the mechanism of lithospheric plate tectonics

Older age of oceanic crust with distance from spreading axes(see picture). In the same direction, an increase in the thickness and stratigraphic completeness of the sedimentary layer is noted.

Figure - Map of the age of rocks of the ocean floor of the North Atlantic (according to W. Pitman and M. Talvani, 1972). Sections of the ocean floor of different age intervals are highlighted in different colors; The numbers indicate the age in millions of years.

Geophysical data.

Figure - Tomographic profile through the Hellenic Trench, Crete and the Aegean Sea. Gray circles are earthquake hypocenters. The plate of the subducting cold mantle is shown in blue, the hot mantle is shown in red (according to V. Spackman, 1989)

The remains of the huge Faralon plate, which disappeared in the subduction zone under North and South America, are recorded in the form of slabs of the “cold” mantle (section across North America, along S-waves). According to Grand, Van der Hilst, Widiyantoro, 1997, GSA Today, v. 7, No. 4, 1-7

Linear magnetic anomalies in the oceans were discovered in the 50s during geophysical studies of the Pacific Ocean. This discovery allowed Hess and Dietz to formulate the theory of ocean floor spreading in 1968, which grew into the theory of plate tectonics. They became one of the most compelling evidence of the correctness of the theory.

Figure - Formation of strip magnetic anomalies during spreading.

The reason for the origin of stripe magnetic anomalies is the process of birth of oceanic crust in the spreading zones of mid-ocean ridges; erupted basalts, when cooling below the Curie point in the Earth's magnetic field, acquire remanent magnetization. The direction of magnetization coincides with the direction of the Earth's magnetic field, however, due to periodic inversions of the Earth's magnetic field, erupted basalts form strips with different directions of magnetization: direct (coinciding with the modern direction of the magnetic field) and reverse.

Figure - Scheme of the formation of the strip structure of the magnetically active layer and magnetic anomalies of the ocean (Vine – Matthews model).

Lithospheric plates– large rigid blocks of the Earth’s lithosphere, bounded by seismically and tectonically active fault zones.

The plates, as a rule, are separated by deep faults and move through the viscous layer of the mantle relative to each other at a speed of 2-3 cm per year. Where continental plates converge, they collide and form mountain belts . When the continental and oceanic plates interact, the plate with the oceanic crust is pushed 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 certain parts of the mantle there are powerful flows of heat and matter rising from its depths to the surface of the planet.

More than 90% of the Earth's surface is covered 13 -th 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 rifts, magma outflows, new faults, horsts, and grabens arise. Mid-ocean ridges form.

First continental drift hypothesis (i.e. horizontal movement of the earth's crust) put forward at the beginning of the twentieth century A. Wegener. Created on its basis lithospheric theory t. According to this theory, the lithosphere is not a monolith, but consists of large and small plates “floating” on the asthenosphere. The boundary areas 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 areas (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 basaltic 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 horizontally through the mantle. Near mid-ocean ridges, lithospheric plates move apart and grow due to material rising from the bowels of the Earth; in deep-sea trenches, one plate moves under another and is absorbed by the mantle. Fold structures are formed where plates collide.