The movement of the tectonic plates of the earth. tectonic plates

The main provisions of the theory of lithospheric plate tectonics :

Plate tectonics(plate tectonics) - a modern geological theory about the movement of the lithosphere. According to this theory, global tectonic processes are based on horizontal movement of relatively integral blocks of the lithosphere - lithospheric plates. Thus, plate tectonics considers the movements and interactions of lithospheric plates. For the first time, the assumption of the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s as part of the “continental drift” hypothesis, but this hypothesis did not receive support at that time. Only in the 1960s, studies of the ocean floor provided indisputable evidence of the horizontal movement of plates and the processes of expansion of the oceans due to the formation (spreading) of the oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the "mobilistic" direction, the development of which led to the development of the modern theory of plate tectonics. The main provisions 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. Digts on the expansion (spreading) of the ocean floor.

The basic principles of plate tectonics can be traced back to a few fundamental ones:

1). The upper stone part of the planet is divided into two shells, which differ significantly 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 mantle material at lithostatic pressure existing at depths of a few hundreds of kilometers. The rocks lying in the Earth above this isotherm are quite cold and behave like a rigid material, while the underlying rocks of the same composition are quite heated and deform relatively easily.

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 inner areas of the plates are weakly seismic and are characterized by a 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 Cocos 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 plate movements: divergence (divergence), convergence (convergence) and shear movements.

Accordingly, three types of main plate boundaries are distinguished.

* Divergent boundaries are boundaries along which plates move apart. The geodynamic setting in which the process of horizontal stretching of the earth's crust occurs, accompanied by the appearance of extended linearly elongated fissured or ravine-shaped 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 be laid both on the 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 break of the continental crust, it is filled with sediments, turning into an aulacogen).

The structure of the continental rift

The process of plate expansion in the zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of a new oceanic crust due to magmatic basalt melts coming from the asthenosphere. Such a process of formation of a new oceanic crust due to the influx of mantle matter 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 - ocean floor basalts, 6 - oceanic crust segments that formed at different times (I-V as they age), 7 - near-surface igneous chamber (with ultrabasic magma in the lower part and basic in the upper part), 8 – sediments of the ocean floor (1-3 as they accumulate)

In the course of spreading, each stretching pulse is accompanied by the inflow of a new portion of mantle melts, which, while solidifying, build up the edges of the plates diverging from the MOR axis. It is in these zones that the formation of young oceanic crust occurs.

* Convergent boundaries are boundaries along which plates collide. There can be three main variants of interaction in a collision: "oceanic - oceanic", "oceanic - continental" and "continental - continental" lithosphere. Depending on the nature of the colliding plates, several different processes can take place.
Subduction is the process of subduction of an oceanic plate under a continental or other oceanic one. The subduction zones are confined to the axial parts of deep-sea trenches conjugated with island arcs (which are elements of active margins). Subduction boundaries account for about 80% of the length of all convergent boundaries.
When continental and oceanic plates collide, a natural phenomenon is the subduction of the oceanic (heavier) plate under the edge of the continental one; when two oceanic ones collide, the older one (that is, the cooler and denser) of them sinks.
The subduction zones have a characteristic structure: their typical elements are a deep-water trough - a volcanic island arc - a back-arc basin. A deep-water trench is formed in the zone of bending and underthrusting of the subducting plate. As this plate sinks, it begins to lose water (which is 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 island arc volcanoes. In the rear of the volcanic arc, some extension usually occurs, which determines the formation of a back-arc basin. In the zone of the back-arc basin, the extension can be so significant that it leads to the rupture of the plate crust and the opening of the basin with oceanic crust (the so-called back-arc spreading process).

The subduction of the subducting plate into the mantle is traced by earthquake foci that occur at the contact of the plates and inside the subducting plate (which is colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone is called the Benioff-Zavaritsky zone. In subduction zones, the process of formation of a new continental crust begins. A much rarer process of interaction between the continental and oceanic plates is the process of obduction - thrusting of a part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that in the course of this process, the oceanic plate is stratified, and only its upper part is advancing - the crust and several kilometers of the upper mantle. When continental plates collide, the crust of which is lighter than the substance of the mantle and, as a result, is not able to sink into it, a collision process occurs. During the collision, the edges of the 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 one, 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. At the same time, at the beginning of the collision process, when the edges of the continents have already approached, the collision is combined with the subduction process (the remains of the oceanic crust continue to sink under the edge of the continent). Collision processes are characterized by large-scale regional metamorphism and intrusive granitoid magmatism. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).

* Transform boundaries are boundaries along which shear displacements of plates occur.

4 ). The volume of oceanic crust absorbed in subduction zones is equal to the volume of crust formed in spreading zones. This provision emphasizes the opinion about the constancy of the volume of the Earth. But such an opinion is not the only and definitively proven. It is possible that the volume of the plans changes pulsatingly, or there is a decrease in its decrease due to cooling.

5 ). The main cause of plate movement is mantle convection, caused by mantle heat and gravity currents.
The source of energy for these currents is the temperature difference between the central regions of the Earth and the temperature of its near-surface parts. At the same time, the main part of the endogenous heat is released at the boundary of the core and mantle during the process of deep differentiation, which determines the decay of the primary chondrite substance, during which the metal part rushes to the center, increasing the core of the planet, and the silicate part is concentrated in the mantle, where it further undergoes differentiation.
The rocks heated in the central zones of the Earth expand, their density decreases, and they float, giving way to descending colder and therefore heavier masses, which have already given up part of the heat in near-surface zones. This process of heat transfer goes on continuously, resulting in the formation of ordered closed convective cells. At the same time, in the upper part of the cell, the flow of matter occurs in an almost 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 "drag" by convective currents. In addition, a number of other factors act on the plates. In particular, the surface of the asthenosphere turns out to be somewhat elevated above the zones of ascending branches and more lowered in the zones of subsidence, which determines the gravitational "slip" of the lithospheric plate located on an inclined plastic surface. Additionally, there are processes of pulling the heavy cold oceanic lithosphere in the subduction zones into the hot, and as a result 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 bottom 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 velocity of the asthenospheric current, 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 higher than under the oceans, the magnitude of the FDC force is almost an order of magnitude inferior to that of the FDO. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost wedges out, so the continents seem to be “sitting aground”. Since most of the lithospheric plates of the modern Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the composition of the plate in the general case should “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving are the almost purely oceanic plates Pacific, Cocos and Nasca; the slowest are the Eurasian, North American, South American, Antarctic and African, 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 (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 episodically and only in certain geodynamic settings, for example, in the cases of the collapse of slabs through the 670 km section described above.
Thus, the mechanisms that set the lithospheric plates in motion can be conventionally assigned to the following two groups: 1) associated with the forces of the mantle “dragging” (mantle drag mechanism) applied to any points of the bottom of the plates, in the figure - the forces of FDO and FDC; 2) associated with the forces applied to the edges of the plates (edge-force mechanism), in the figure - the forces FRP and FNB. The role of this or that driving mechanism, as well as these or those forces, is evaluated individually for each lithospheric plate.

The totality of these processes reflects the general geodynamic process, covering areas from the surface to deep zones of the Earth. At present, a two-cell closed-cell mantle convection is developing in the Earth's mantle (according to the through-mantle convection model) 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 the mantle matter are located in northeast Africa (approximately under the junction zone of the African, Somali and Arabian plates) and in the area of ​​Easter Island (under the middle ridge of the Pacific Ocean - the East Pacific Rise). The mantle subsidence equator runs along an approximately continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian Oceans. 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 the collision of the continents and the formation of a new supercontinent, the fifth in the history of the Earth.

6 ). Plate movements obey the laws of spherical geometry and can be described on the basis of 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 angle of rotation. Based on this position, the position of the continents in past geological epochs 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 is further disintegrated. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, modern continents were formed.

Lithospheric plates have high rigidity and are able to maintain their structure and shape unchanged for a long time in the absence of outside influences.

plate movement

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

Movement Results

In the areas of convergence of huge continental plates, when they collide, mountain ranges arise. In a similar way, the Himalayas mountain system arose at one time, formed on the border of the Indo-Australian and Eurasian plates. The result of the collision of oceanic lithospheric plates with continental formations are island arcs and deep-water depressions.

In the axial zones of the mid-ocean ridges, rifts (from the English. Rift - a fault, a crack, a crevice) of a characteristic structure arise. Similar formations of the linear tectonic structure of the earth's crust, having 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.

In view of 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 displacements 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 course of the process of formation of the earth's crust in these places and is currently continuing quite intensively.

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 areas of the Earth, in others -. The theory of lithospheric plates makes it possible to explain and foresee the occurrence of catastrophic phenomena that can occur in the region of their boundaries.

Last week, the public was stirred by the news that the Crimean peninsula is moving towards Russia, not only thanks to the political will of the population, but also according to the laws of nature. What are lithospheric plates and on which of them is Russia territorially located? What makes them move and where? Which territories still want to "join" Russia, and which ones threaten to "escape" to the USA?

"And we're going somewhere"

Yes, we are all going somewhere. While you are reading these lines, you are moving slowly: if you are in Eurasia, then east at a speed of about 2-3 centimeters per year, if in North America, then at the same speed west, and if somewhere at the bottom of the Pacific Ocean (how did you get there?), then it takes you to the northwest by 10 centimeters a year.

If you sit back in your chair and wait about 250 million years, you will find yourself on a new supercontinent that will unite all the earth's land - on the mainland Pangea Ultima, named so in memory of the ancient supercontinent Pangea, which existed just 250 million years ago.

Therefore, the news that "Crimea is moving" can hardly be called news. Firstly, because Crimea, together with Russia, Ukraine, Siberia and the European Union, is part of the Eurasian lithospheric plate, and all of them have been moving together in one direction for the last hundred million years. However, Crimea is also part of the so-called Mediterranean mobile belt, it is located on the Scythian plate, and most of the European part of Russia (including the city of St. Petersburg) - on the East European platform.

And this is where confusion often arises. The fact is that in addition to huge sections of the lithosphere, such as the Eurasian or North American plates, there are completely different smaller "tiles". If very conditionally, then the earth's crust is composed of continental lithospheric plates. They themselves consist of ancient and very stable platforms.and mountain building zones (ancient and modern). And already the platforms themselves are divided into slabs - smaller sections of the crust, consisting of two "layers" - the foundation and the cover, and shields - "single-layer" outcrops.

The cover of these non-lithospheric plates consists of sedimentary rocks (for example, limestone, composed of many shells of marine animals that lived in the prehistoric ocean above the surface of Crimea) or igneous rocks (thrown from volcanoes and solidified lava masses). A fslab foundations and shields most often consist of very old rocks, mainly of metamorphic origin. This is the name given to igneous and sedimentary rocks that have sunk into the depths of the earth's crust, where, under the influence of high temperatures and enormous pressure, various changes occur with them.

In other words, most of Russia (with the exception of Chukotka and Transbaikalia) is located on the Eurasian lithospheric plate. However, its territory is "divided" between the West Siberian plate, the Aldan shield, the Siberian and East European platforms and the Scythian plate.

Probably, the director of the Institute of Applied Astronomy (IPA RAS), Doctor of Physical and Mathematical Sciences Alexander Ipatov, said about the movement of the last two plates. And later, in an interview with Indicator, he clarified: "We are engaged in observations that allow us to determine the direction of movement of the plates of the earth's crust. The plate on which the Simeiz station is located moves at a speed of 29 millimeters per year to the northeast, that is, to where Russia And the plate where Peter is located is moving, one might say, towards Iran, to the south-southwest."However, this is not such a discovery, because this movement has been around for several decades, and it itself began back in the Cenozoic era.

Wegener's theory was received with skepticism - mainly because he could not offer a satisfactory mechanism to explain the movement of the continents. He believed that the continents move, breaking through the earth's crust, like icebreakers through ice, due to the centrifugal force from the rotation of the Earth and tidal forces. His opponents said that the continents-"icebreakers" in the process of movement would change their appearance beyond recognition, and centrifugal and tidal forces are too weak to serve as a "motor" for them. One critic calculated that if the tidal force were strong enough to move the continents so fast (Wegener estimated their speed at 250 centimeters per year), it would stop the rotation of the Earth in less than a year.

By the end of the 1930s, the theory of continental drift was rejected as unscientific, but by the middle of the 20th century it had to be returned to: mid-ocean ridges were discovered and it turned out that new crust was continuously forming in the zone of these ridges, due to which the continents were "moving apart" . Geophysicists have studied the magnetization of rocks along the mid-ocean ridges and found "bands" with multidirectional magnetization.

It turned out that the new oceanic crust "records" the state of the Earth's magnetic field at the time of formation, and scientists have received an excellent "ruler" to measure the speed of this conveyor. So, in the 1960s, the theory of continental drift returned for the second time, for good. And this time, scientists were able to understand what moves the continents.

Ice floes in the boiling ocean

"Imagine an ocean where ice floes float, that is, there is water in it, there is ice, and, let's say, wooden rafts are also frozen into some ice floes. Ice is lithospheric plates, rafts are continents, and they float in the substance of the mantle," explains Corresponding Member of the Russian Academy of Sciences Valery Trubitsyn, chief researcher at the Institute of Physics of the Earth named after O.Yu. Schmidt.

Back in the 1960s, he put forward the theory of the structure of giant planets, and at the end of the 20th century he began to create a mathematically based theory of continental tectonics.

The intermediate layer between the lithosphere and the hot iron core in the center of the Earth - the mantle - consists of silicate rocks. The temperature in it varies from 500 degrees Celsius in the upper part to 4000 degrees Celsius at the border of the core. Therefore, from a depth of 100 kilometers, where the temperature is already over 1300 degrees, the mantle substance behaves like a very thick resin and flows at a speed of 5-10 centimeters per year, says Trubitsyn.

As a result, in the mantle, as in a pot of boiling water, convective cells appear - areas where hot matter rises from one edge, and cooled down from the other.

"There are about eight of these large cells in the mantle and many more small ones," the scientist says. Mid-ocean ridges (for example, in the center of the Atlantic) are the place where the material of the mantle rises to the surface and where new crust is born. In addition, there are subduction zones, places where a plate begins to "creep" under the neighboring one and sinks down into the mantle. Subduction zones are, for example, the western coast of South America. This is where the most powerful earthquakes occur.

“In this way, the plates take part in the convective circulation of the mantle substance, which temporarily becomes solid while on the surface. Plunging into the mantle, the plate substance heats up and softens again,” explains the geophysicist.

In addition, separate jets of matter rise to the surface from the mantle - plumes, and these jets have every chance to destroy humanity. After all, it is the mantle plumes that are the cause of the appearance of supervolcanoes (see). Such points are in no way connected with lithospheric plates and can remain in place even when the plates move. When the plume exits, a giant volcano arises. There are many such volcanoes, they are in Hawaii, in Iceland, a similar example is the Yellowstone caldera. Supervolcanoes can generate eruptions thousands of times more powerful than most ordinary volcanoes like Vesuvius or Etna.

"250 million years ago, such a volcano on the territory of modern Siberia killed almost all life, only the ancestors of dinosaurs survived," says Trubitsyn.

Agreed - dispersed

Lithospheric plates consist of relatively heavy and thin basaltic oceanic crust and lighter, but much thicker continents. A plate with a continent and oceanic crust "frozen" around it can move forward, while the heavy oceanic crust sinks under its neighbor. But when continents collide, they can no longer sink under each other.

For example, about 60 million years ago, the Indian plate broke away from what later became Africa and went north, and about 45 million years ago it met with the Eurasian plate, the Himalayas, the highest mountains on Earth, grew at the point of collision.

The movement of the plates will sooner or later bring all the continents into one, as leaves converge into one island in a whirlpool. In the history of the Earth, the continents have united and broken up approximately four to six times. The last supercontinent Pangea existed 250 million years ago, before it was the supercontinent Rodinia, 900 million years ago, before it - two more. "And already, it seems, the unification of the new continent will soon begin," the scientist clarifies.

He explains that the continents act as a thermal insulator, the mantle beneath them begins to heat up, updrafts occur, and therefore the supercontinents break apart again after a while.

America will "take away" Chukotka

Large lithospheric plates are drawn in textbooks, anyone can name them: Antarctic plate, Eurasian, North American, South American, Indian, Australian, Pacific. But at the boundaries between the plates there is a real chaos of many microplates.

For example, the boundary between the North American Plate and the Eurasian Plate does not run along the Bering Strait at all, but much to the west, along the Chersky Ridge. Chukotka thus turns out to be part of the North American Plate. At the same time, Kamchatka is partly located in the zone of the Okhotsk microplate, and partly in the zone of the Bering Sea microplate. And Primorye is located on the hypothetical Amur Plate, the western edge of which rests on Baikal.

Now the eastern edge of the Eurasian plate and the western edge of the North American plate are "spinning" like gears: America is turning counterclockwise, and Eurasia is turning clockwise. As a result, Chukotka may finally come off "along the seam", and in this case, a giant circular seam may appear on Earth, which will pass through the Atlantic, the Indian, Pacific and Arctic Oceans (where it is still closed). And Chukotka itself will continue to move "in the orbit" of North America.

Speedometer for the lithosphere

Wegener's theory has been resurrected, not least because scientists have the ability to accurately measure the displacement of continents. Now satellite navigation systems are used for this, but there are other methods. All of them are needed to build a single international coordinate system - the International Terrestrial Reference Frame (ITRF).

One of these methods is very long baseline radio interferometry (VLBI). Its essence lies in simultaneous observations with the help of several radio telescopes in different parts of the Earth. The difference in signal acquisition time makes it possible to determine offsets with high accuracy. Two other ways to measure speed are laser ranging observations using satellites and Doppler measurements. All these observations, including with the help of GPS, are carried out at hundreds of stations, all these data are brought together, and as a result, we get a picture of continental drift.

For example, Crimean Simeiz, where a laser sounding station is located, as well as a satellite station for determining coordinates, "moves" to the northeast (in azimuth about 65 degrees) at a speed of about 26.8 millimeters per year. Zvenigorod, near Moscow, is moving about a millimeter a year faster (27.8 millimeters a year) and keeps its course to the east - about 77 degrees. And, say, the Hawaiian volcano Mauna Loa is moving northwest twice as fast - 72.3 millimeters per year.

Lithospheric plates can also be deformed, and their parts can "live their own lives", especially at the boundaries. Although the scale of their independence is much more modest. For example, Crimea is still moving independently to the northeast at a speed of 0.9 millimeters per year (and at the same time growing by 1.8 millimeters), and Zvenigorod is moving somewhere to the southeast at the same speed (and down - by 0 .2 millimeters per year).

Trubitsyn says that this independence is partly explained by the "personal history" of different parts of the continents: the main parts of the continents, the platforms, may be fragments of ancient lithospheric plates that "merged" with their neighbors. For example, the Ural Range is one of the seams. Platforms are relatively rigid, but parts around them can deform and move at will.

• 1)_The first hypothesis arose in the second half of the 18th century and was called the uplift hypothesis. It was proposed by M. V. Lomonosov, German scientists A. von Humboldt and L. von Buch, Scot J. Hutton. The essence of the hypothesis is as follows - mountain uplifts are caused by the rise of molten magma from the depths of the Earth, which on its way had a pushing effect on the surrounding layers, leading to the formation of folds, abysses of various sizes. Lomonosov was the first to distinguish two types of tectonic movements - slow and fast, causing earthquakes.
• 2) In the middle of the 19th century, this hypothesis was replaced by the contraction hypothesis of the French scientist Elie de Beaumont. It was based on the cosmogonic hypothesis of Kant and Laplace about the origin of the Earth as an initially hot body with subsequent gradual cooling. This process led to a decrease in the volume of the Earth, and as a result, the Earth's crust was compressed, and folded mountain structures similar to giant "wrinkles" arose.
• 3) In the middle of the 19th century, the Englishman D. Airy and the priest from Calcutta D. Pratt discovered a pattern in the positions of gravity anomalies - high in the mountains, the anomalies turned out to be negative, i.e., a mass deficit was detected, and in the oceans the anomalies were positive. To explain this phenomenon, a hypothesis was proposed, according to which the earth's crust floats on a heavier and more viscous substrate and is in isostatic equilibrium, which is disturbed by the action of external radial forces.
• 4) The cosmogonic hypothesis of Kant-Laplace was replaced by the hypothesis of O. Yu. Schmidt about the initial solid, cold and homogeneous state of the Earth. There was a need for a different approach in explaining the formation of the earth's crust. Such a hypothesis was proposed by V. V. Belousov. It's called radio migration. The essence of this hypothesis:
• 1. The main energy factor is radioactivity. The heating of the Earth with subsequent compaction of matter occurred due to the heat of radioactive decay. Radioactive elements at the initial stages of the Earth's development were distributed evenly, and therefore the heating was strong and ubiquitous.
• 2. Heating of the primary substance and its compaction led to the separation of magma or its differentiation into basalt and granite. The latter concentrated radioactive elements. As a lighter granitic magma “floated up” to the upper part of the Earth, while the basalt magma sank down. At the same time, there was also a temperature difference.

Modern geotectonic hypotheses are developed using the ideas of mobilism. This idea is based on the concept of the predominance of horizontal movements in the tectonic movements of the earth's crust.

• 5) For the first time, to explain the mechanism and sequence of geotectonic processes, the German scientist A. Wegener proposed the hypothesis of horizontal continental drift.
• 1. The similarity of the outlines of the coasts of the Atlantic Ocean, especially in the southern hemisphere (near South America and Africa).
• 2. Similarity of the geological structure of the continents (coincidence of some regional tectonic strikes, similarity in composition and age of rocks, etc.).

hypothesis of lithospheric plate tectonics or new global tectonics. The main points of this hypothesis are:

• 1. The earth's crust with the upper part of the mantle forms the lithosphere, which is underlain by the plastic asthenosphere. The lithosphere is divided into large blocks (plates). The boundaries of the plates are rift zones, deep-water trenches, which are adjacent to faults that penetrate deep into the mantle - these are the Benioff-Zavaritsky zones, as well as zones of modern seismic activity.
• 2. Lithospheric plates move horizontally. This movement is determined by two main processes - pushing apart plates or spreading, submersion of one plate under another - subduction or thrusting of one plate onto another - obduction.
• 3. Basalts from the mantle periodically enter the pull apart zone. Evidence of the separation is provided by strip magnetic anomalies in basalts.
• 4. In the regions of island arcs, zones of accumulation of sources of deep-focus earthquakes are distinguished, which reflect zones of subsidence of a plate with basaltic oceanic crust under the continental crust, i.e., these zones reflect subduction zones. In these zones, due to crushing and melting, part of the material sinks, while the other part penetrates into the continent in the form of volcanoes and intrusions, and thereby the thickness of the continental crust increases.

Plate tectonics is a modern geological theory about the movement of the lithosphere. According to this theory, global tectonic processes are based on horizontal movement of relatively integral blocks of the lithosphere - lithospheric plates. Thus, plate tectonics considers the movements and interactions of lithospheric plates. Alfred Wegener first suggested horizontal movement of crustal blocks in the 1920s as part of the “continental drift” hypothesis, but this hypothesis did not receive support at that time. Only in the 1960s, studies of the ocean floor provided indisputable evidence of the horizontal movement of plates and the processes of expansion of the oceans due to the formation (spreading) of the oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the "mobilistic" direction, the development of which led to the development of the modern theory of plate tectonics. The main provisions 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. Digts on the expansion (spreading) of the ocean floor. one). The upper stone part of the planet is divided into two shells, which differ significantly 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. 3). There are three types of relative plate movements: divergence (divergence), convergence (convergence) and shear movements. 4). The volume of oceanic crust absorbed in subduction zones is equal to the volume of crust formed in spreading zones. This provision emphasizes the opinion about the constancy of the volume of the Earth. 5). The main cause of plate movement is mantle convection, caused by mantle heat and gravity currents.

The source of energy for these currents is the temperature difference between the central regions of the Earth and the temperature of its near-surface parts. At the same time, the main part of the endogenous heat is released at the boundary of the core and mantle during the process of deep differentiation, which determines the decay of the primary chondrite substance, during which the metal part rushes to the center, increasing the core of the planet, and the silicate part is concentrated in the mantle, where it further undergoes differentiation. 6). Plate movements obey the laws of spherical geometry and can be described on the basis of 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 angle of rotation.

Geographical consequences of the movement of Lith plates (Seismic activity increases, faults form, ridges appear, and so on). In the theory of plate tectonics, the key position is occupied by the concept of the geodynamic setting - a 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.

According to modern theories of lithospheric plates the entire lithosphere is divided into separate blocks by narrow and active zones - deep faults - moving in the plastic layer of the upper mantle relative to each other at a speed of 2-3 cm per year. These blocks are called lithospheric plates.

A feature of lithospheric plates is their rigidity and ability, in the absence of external influences, to maintain their shape and structure unchanged for a long time.

Lithospheric plates are mobile. Their movement along the surface of the asthenosphere occurs under the influence of convective currents in the mantle. Separate lithospheric plates can diverge, approach or slide relative to each other. In the first case, tension zones with cracks along the plate boundaries appear between the plates, in the second case, compression zones accompanied by thrusting of one plate onto another (thrust - obduction; underthrust - subduction), in the third case - shear zones - faults along which sliding of neighboring plates occurs. .

At the convergence of continental plates, they collide, forming mountain belts. This is how the Himalaya mountain system arose, for example, on the border of the Eurasian and Indo-Australian plates (Fig. 1).

Rice. 1. Collision of continental lithospheric plates

When the continental and oceanic plates interact, the plate with the oceanic crust moves under the plate with the continental crust (Fig. 2).

Rice. 2. Collision of continental and oceanic lithospheric plates

As a result of the collision of continental and oceanic lithospheric plates, deep-sea trenches and island arcs are formed.

The divergence of lithospheric plates and the formation of an oceanic type of earth's crust as a result of this is shown in Fig. 3.

The axial zones of mid-ocean ridges are characterized by rifts(from English. rift- crevice, crack, fault) - a large linear tectonic structure of the earth's crust with a length of hundreds, thousands, a width of tens, and sometimes hundreds of kilometers, formed mainly during horizontal stretching of the crust (Fig. 4). Very large rifts are called rift belts, zones or systems.

Since the lithospheric plate is a single plate, each of its faults is a source of seismic activity and volcanism. These sources are concentrated within relatively narrow zones, along which mutual displacements and frictions of adjacent plates occur. These zones are called seismic belts. Reefs, mid-ocean ridges and deep-sea trenches are mobile areas of the Earth and are located at the boundaries of lithospheric plates. This indicates that the process of formation of the earth's crust in these zones is currently very intensive.

Rice. 3. Divergence of lithospheric plates in the zone among the nano-oceanic ridge

Rice. 4. Scheme of rift formation

Most of the faults of the lithospheric plates are at the bottom of the oceans, where the earth's crust is thinner, but they are also found on land. The largest fault on land is located in eastern Africa. It stretched for 4000 km. The width of this fault is 80-120 km.

At present, seven largest plates can be distinguished (Fig. 5). Of these, the largest in area is the Pacific, which consists entirely of oceanic lithosphere. As a rule, the Nazca plate is also referred to as large, which is several times smaller in size than each of the seven largest ones. At the same time, scientists suggest that in fact the Nazca plate is much larger than we see it on the map (see Fig. 5), since a significant part of it went under the neighboring plates. This plate also consists only of oceanic lithosphere.

Rice. 5. Earth's lithospheric plates

An example of a plate that includes both continental and oceanic lithosphere is, for example, the Indo-Australian lithospheric plate. The Arabian Plate consists almost entirely of the continental lithosphere.

The theory of lithospheric plates is important. First of all, it can explain why there are mountains in some places on the Earth, and plains in others. With the help of the theory of lithospheric plates, it is possible to explain and predict catastrophic phenomena occurring at the boundaries of plates.

Rice. 6. The outlines of the continents really seem compatible

Continental drift theory

The theory of lithospheric plates originates from the theory of continental drift. Back in the 19th century many geographers noted that when looking at the map, one can notice that the coasts of Africa and South America seem compatible when approaching (Fig. 6).

The emergence of the hypothesis of the movement of the continents is associated with the name of the German scientist Alfred Wegener(1880-1930) (Fig. 7), who most fully developed this idea.

Wegener wrote: "In 1910, the idea of ​​moving the continents first came to my mind ... when I was struck by the similarity of the outlines of the coasts on both sides of the Atlantic Ocean." He suggested that in the early Paleozoic there were two large continents on Earth - Laurasia and Gondwana.

Laurasia was the northern continent, which included the territories of modern Europe, Asia without India and North America. The southern mainland - Gondwana united the modern territories of South America, Africa, Antarctica, Australia and Hindustan.

Between Gondwana and Laurasia was the first sea - Tethys, like a huge bay. The rest of the Earth's space was occupied by the Panthalassa ocean.

About 200 million years ago, Gondwana and Laurasia were united into a single continent - Pangea (Pan - universal, Ge - earth) (Fig. 8).

Rice. 8. The existence of a single mainland Pangea (white - land, dots - shallow sea)

Approximately 180 million years ago, the mainland of Pangea again began to be divided into constituent parts, which mixed up on the surface of our planet. The division took place as follows: first, Laurasia and Gondwana reappeared, then Laurasia divided, and then Gondwana also split. Due to the split and divergence of parts of Pangea, oceans were formed. The young oceans can be considered the Atlantic and Indian; old - Quiet. The Arctic Ocean became isolated with the increase in land mass in the Northern Hemisphere.

Rice. 9. Location and directions of continental drift in the Cretaceous period 180 million years ago

A. Wegener found a lot of evidence for the existence of a single continent of the Earth. Particularly convincing seemed to him the existence in Africa and South America of the remains of ancient animals - leafosaurs. These were reptiles, similar to small hippos, that lived only in freshwater reservoirs. This means that they could not swim huge distances in salty sea water. He found similar evidence in the plant world.

Interest in the hypothesis of the movement of the continents in the 30s of the XX century. decreased slightly, but in the 60s it revived again, when, as a result of studies of the relief and geology of the ocean floor, data were obtained indicating the processes of expansion (spreading) of the oceanic crust and the “diving” of some parts of the crust under others (subduction).