What causes the tectonic plates of the earth's crust to move. Tectonic plates are moving

This is a modern geological theory about the movement of the lithosphere, according to which the earth's crust consists of relatively integral blocks - lithospheric plates that are in constant motion relative to each other. At the same time, in expansion zones (mid-ocean ridges and continental rifts), as a result of spreading (English seafloor spreading - spreading of the seabed), a new oceanic crust is formed, and the old one is absorbed in subduction zones. The theory of plate tectonics explains the occurrence of earthquakes, volcanic activity and mountain building processes, for the most part confined to plate boundaries.

The idea of the movement of crustal blocks was first expressed in the theory of continental drift proposed by Alfred Wegener in the 1920s. This theory was initially rejected. The revival of the idea of movements in the solid shell of the Earth (“mobilism”) occurred in the 1960s, 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 subduction of some parts of the crust under others ( subduction). The combination of these ideas with the old theory of continental drift gave rise to the modern theory of plate tectonics, which soon became an accepted concept in the earth sciences.

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.

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 modern 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 motion obeys the Euler rotation theorem.

There are three main types of relative plate movements

1) divergence (divergence), expressed by rifting and spreading;

2) convergence (convergence) expressed by subduction and collision;

3) shear movements along transform geological faults.

Spreading in the oceans is compensated by subduction and collision along their periphery, and the radius and volume of the Earth are constant up to the thermal compression of the planet (in any case, the average temperature of the Earth's interior slowly, over billions of years, decreases).

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

There are two fundamentally different types earth's crust- continental crust (more ancient) and oceanic crust (not older than 200 million years). 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 in the modern era is covered by 8 largest lithospheric plates:

1. Australian plate.

2. Antarctic plate.

3. African plate.

4. Eurasian plate.

5. Hindustan plate.

6. Pacific plate.

7. North American Plate.

8. South American plate.

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

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 modern movement 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)

This 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 region of the lowest strength and highest voltage), 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.

Regarding the reason high temperature the interior of the Earth, there are several hypotheses. 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 enough high content 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 moon of Jupiter) is caused precisely by these forces - the tide on Io is about 120 m. And also the forces arising from the change atmospheric pressure on various parts of the earth's surface - atmospheric pressure forces often change by 3%, which is 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. 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 the plates move in a parallel course, but with different speed, transform faults arise - grandiose shear disturbances, 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 heat flow: its decrease is observed above 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: 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. Oksburg (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 captures 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; several methodological approaches with different basic concepts and principles have been identified in it.

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 seen as a mosaic of plates different size 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 the plates. Combining this data with information obtained from stationary hotspots made it possible to determine the absolute movements of the plates and the history of the movement. magnetic poles Earth.

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 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 rocks. In turn, according to these characteristic features it is possible 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 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 absorbed a set of methods for identifying terranes and reconstructing their history.

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 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.

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 rejected, given the status of a marginal science, and the vast majority of research continued to be carried out within the framework of 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.

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 and Harry Hess to put forward the spreading hypothesis in 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.

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 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).

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.