Movement of lithospheric plates. Theories of continental drift and lithospheric plates

Basic principles 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, the basis of global tectonic processes is the horizontal movement of relatively integral blocks of the lithosphere - lithospheric plates. Thus, plate tectonics examines the movements and interactions of lithospheric plates. The horizontal movement of crustal blocks was first proposed 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 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.

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

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.
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.

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 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).

Structure of the continental rift

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).

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.

* Convergent boundaries are boundaries along which plates collide. 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 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 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. 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).

* Transform boundaries are boundaries along which plate displacements occur.

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.
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.

6 ). 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.

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.

The peculiarity 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. Individual lithospheric plates can move apart, move closer together, or slide relative to each other. In the first case, tension zones with cracks along the boundaries of the plates appear between the plates, in the second - compression zones, accompanied by the pushing of one plate onto another (thrusting - obduction; thrusting - subduction), in the third - shear zones - faults along which sliding of neighboring plates occurs .

Where continental plates converge, they collide and mountain belts are formed. This is how, for example, the Himalaya mountain system arose 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 resulting formation of the oceanic crust 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 hundreds, thousands in length, tens and sometimes hundreds of kilometers wide, 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 movements and friction of adjacent plates occur. These zones are called seismic belts. Reefs, mid-ocean ridges and deep-sea trenches are mobile regions 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 occurring very intensively.

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

Rice. 4. Rift formation scheme

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

Currently, seven of the 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, which is several times smaller in size than each of the seven largest ones, is also classified as large. At the same time, scientists suggest that in fact the Nazca plate is much larger than we see on the map (see Fig. 5), since a significant part of it went under 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 continental lithosphere.

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

Rice. 6. The shapes 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 have noted that when looking at a map, one can notice that the coasts of Africa and South America seem compatible when approaching (Fig. 6).

The emergence of the hypothesis of continental movement 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 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) (Fig. 8).

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

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.

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

A. Wegener found many confirmations of the existence of a single continent of the Earth. He found the existence of remains of ancient animals—listosaurus—in Africa and South America especially convincing. 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).

December 10th, 2015

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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 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.

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)

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.

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 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 that 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)

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.

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.

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.

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 -

The theory of plate tectonics is a modern science about the origin and development of the Earth's lithosphere. The basic ideas of the theory of plate tectonics are as follows. Lithospheric plates are located above a plastic and viscous shell, asthenosphere. The asthenosphere is a layer of reduced hardness and viscosity in the upper part of the Earth's mantle. The plates float and move slowly horizontally through the asthenosphere.

As the plates move apart, cracks appear on the opposite side of the oceanic reefs in the middle of the valley, which are filled with young basalts rising from the Earth's mantle. Oceanic plates sometimes end up underneath continental plates, or slide relative to each other along vertical fault planes. The spreading and creeping of plates is compensated by the birth of new oceanic crust at the crack sites.

Modern science explains the reasons for the movement of lithospheric plates by the fact that heat accumulates in the bowels of the Earth, which causes convection currents mantle substances. Mantle plumes occur even at the core-mantle boundary. And cooled oceanic plates gradually sink into the mantle. This gives impetus to hydrodynamic processes. Falling plates linger for about 400 million years at a 700 km boundary, and after accumulating sufficient weight "fail"through the boundaries, into the lower mantle, reaching the surface of the core. This causes mantle plumes to rise to the surface. At the 700 km boundary, these jets split and penetrate into the upper mantle, generating an upward flow in it. A line of plate separation is formed above these currents. Under the influence of mantle flows, plate tectonics occurs.

In 1912, the German geophysicist and meteorologist Alfred Wegener, based on the similarity of the Atlantic coasts of North and South America with Europe and Africa, as well as on the basis of paleontological and geological data, proved “ continental drift" He published these data in 1915 in Germany.

According to this theory, the continents “float” on the lower basalt “lake” like icebergs. According to Wegener's hypothesis, a supercontinent existed 250 million years ago Pangea(gr. pan - everything, and gaya - Earth, i.e. The whole Earth). About 200 million years ago, Pangea split into Laurasia in the north and Gondwana on South. Between them was the Tethys Sea.

The existence of the supercontinent Gondwana at the beginning of the Mesozoic era is confirmed by the similarity of the topography of South America, Africa, Australia and the Hindustan Peninsula. Coal deposits have been found in Antarctica, indicating that in the distant past these places had a hot climate and abundant vegetation.

Paleontologists have proven that the flora and fauna of the continents that formed after the collapse of Gondwana are the same and form one family. The similarity of the coal seams of Europe and North America and the similarity of dinosaur remains indicate that these continents separated after Triassic period.

In the 20th century, it became clear that in the middle of the oceans there are seamounts about 2 km high, 200 to 500 km wide and up to several thousand km long. They were called mid-ocean ridges (CR). These ridges covered the entire planet in a ring. It has been established that the most seismically active places on the earth's surface are SKh. The main material of these mountains is basalt.

Scientists have discovered deep (about 10 km) oceanic trenches under the oceans, which are mainly located on the shores of continents or islands. They were discovered in the Pacific and Indian oceans. But there are none in the Atlantic Ocean. The deepest gutter is Mariana Trench, 11022 m deep, located in the Pacific Ocean. IN deep gutters There is great seismic activity, and the earth's crust in such places falls into the mantle.

The American scientist G. Hess suggested that the mantle material through rift (English rift - removal, expansion) cracks rises up to the central parts of the SC, and, filling the cracks, crystallizes, oriented in the direction of the Earth's magnetic field. After some time, while moving away from each other, a new crack appears again, and the process repeats. Scientists, taking into account the direction of the magnetic field of crystals of volcanic origin and the Earth, through correlation, established the location and direction of movement of continents in different geological times. Extrapolating in the opposite direction the movement of the continents, they received the supercontinents Gondwana and Pangea.

The most active place of mountain ranges is the line passing in the middle of the ridges, where faults appear that reach the mantle. The length of the faults ranges from 10 km to 100 km. Rifts divide the SH into two parts. Rifts located between the peninsula Arabia and Africa have a length of about 6500 km. In total, the length of oceanic rifts is about 90 thousand km.

Sedimentary rocks have accumulated since Jurassic period. There are no sedimentary rocks near the SKh, and the direction of the magnetic field of the crystals coincides with the direction of the Earth's magnetic field. Based on these data, in 1962, American geologists G. Hess and R. Dietz explained the reasons for the occurrence of the SH by the fact that the earth's crust under the oceans slides in the opposite direction. And for this reason, rift cracks appear and SH. The causes of continental drift are associated with the emergence of continental continents, which, expanding, push away lithospheric plates, and thereby set them in motion.

Underwater the slabs are heavy, when they meet continental plates, they fall into the Earth's mantle. Near Venezuela, the Caribbean Plate is moving under the South American Plate. In recent years, with the help of spacecraft, it has been established that the speeds of plate movement are different. For example, the speed of movement of the peninsula Hindustan to the north is about 6 cm/year, North America towards the west - 5 cm/year and Australia to the northeast - 14 cm/year.

The rate of formation of new earth's crust is 2.8 km 2 /year. The area of the SKh is 310 million km 2, therefore, they were formed over 110 million years. The age of the crustal rocks of the western Pacific Ocean is 180 million years. Over the past 2 billion years, new oceans have appeared and old oceans have disappeared about 20 times.

South America separated from Africa 135 million years ago. North America separated from Europe 85 million years ago. Hindustan plate 40 million years ago collided with Eurasian, as a result of which mountains appeared Tibet and Himalayas. Science has established that after the formation of the earth’s crust (4.2 billion years ago) as a result of tectonic processes disintegrated four times and the formation of Pangea with a period of about one billion years.

Volcanic activity is concentrated at plate junctions. Along the junction line of the plates there are volcano chains, for example, in the Hawaiian Islands and Greenland. The length of the volcanic chains is currently about 37 thousand km. Scientists believe that in a few hundred million years, Asia will unite with North and South America. The Pacific Ocean will close and the Atlantic Ocean will expand.

Questions for self-control

1. What is the name of the theory about the origin and development of the Earth's lithosphere?

2. What is the name of the layer of reduced hardness and viscosity in the upper part of the Earth’s mantle?

3. Where do the oceanic plates move apart on the opposite side?

4. How does modern science explain the reasons for the movement of lithospheric plates?

5. What plates are plunging into the Earth's mantle?

6. What causes mantle plumes to rise to the surface?

7. Who and when, based on the similarity of the Atlantic coasts of North and South America with Europe and Africa, proved “ continental drift».

8. How many millions of years ago did the supercontinent exist? Pangea?

9. How many million years ago did Pangea split into Laurasia in the north and Gondwana on South?

10. Where was the Tethys Sea?

11. Where were coal deposits found, indicating that in the distant past these places had a hot climate and abundant vegetation?

12. The flora and fauna of which continents are the same and form one family?

13. What does the similarity of coal seams in Europe and North America indicate?

14. When they found out that in the middle of the oceans there are mid-ocean ridges?

15.Mid-ocean ridges do they cover the entire planet in a ring or not?

16. Where are ocean trenches located?

17. Which oceanic trench is the deepest and where is it located?

18. How many parts are divided by rifts (cracks) of the mid-ocean ridges?

19. How many thousand km in total is the length of oceanic rifts?

20. Who and when connected the causes of continental drift with the emergence of the mid-ocean ridges?

21. Why do underwater plates, when they meet continental plates, fall into the Earth’s mantle?

22. How many cm/year is the speed of movement? North America towards the west?

23. How many cm/year is the speed of movement? Australia to the northeast?

24. How many km 2 /year is the rate of formation of the new earth’s crust?

25. How many million km 2 area mid-ocean ridges?

26. How many millions of years did they form? mid-ocean ridges?

27. For what reason do they arise? chains of volcanoes?

28. On which islands is there a chain of volcanoes?

29. How many thousands of kilometers are the length of the volcanic chains at present?

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Topic 21. Environment and health

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

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

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

By the early 60s, a relief map of the ocean floor 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 sediment. These data allowed R. Dietz and G. Hess to put forward the spreading hypothesis in 1962–1963. According to this hypothesis, convection occurs in the mantle at a speed of about 1 cm/year. The ascending branches of convection cells carry out mantle material under 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 lithospheric plates. According to the concept of spreading, ocean basins have a variable and unstable structure, while continents are stable.

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

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

Current state of plate tectonics

Over the past decades, plate tectonics has significantly changed its basic principles. Nowadays they can be formulated as follows:

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

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

To a first approximation, lithospheric plates are described as rigid bodies, and their motion obeys Euler's rotation theorem.

There are three main types of relative plate movements

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

Spreading in the oceans is compensated by subduction and collision along their periphery, and the radius and volume of the Earth are constant (this statement is constantly discussed, but it has never been refuted)

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

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

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

Medium-sized plates include the Arabian subcontinent and the Cocos and Juan de Fuca plates, remnants of the enormous Faralon plate that formed much of the Pacific Ocean floor but has now disappeared into the subduction zone beneath the Americas.

The force that moves the plates

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

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

There are several hypotheses regarding the cause of the high temperature of the Earth's interior. At the beginning of the 20th century, the hypothesis of the radioactive nature of this energy was popular. It seemed to be confirmed by estimates of the composition of the upper crust, which showed very significant concentrations of uranium, potassium and other radioactive elements, but it later turned out that the content of radioactive elements sharply decreases with depth. Another model explains the heating by chemical differentiation of the Earth. The planet was originally 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 silicates concentrated in the upper shells. At the same time, the potential energy of the system decreased and was converted into thermal energy. Other researchers believe that the heating of the planet occurred as a result of accretion during meteorite impacts on the surface of the nascent celestial body.

Secondary forces

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

As oceanic crust sinks into the mantle, the basalts of which it is composed transform into eclogites, rocks denser than ordinary mantle rocks - peridotites. Therefore, this part of the oceanic plate sinks into the mantle, and pulls with it the part that has not yet been eclogitized.

Divergent boundaries or plate boundaries

These are boundaries between plates moving in opposite directions. In the Earth's topography, these boundaries are expressed as rifts, where tensile deformations predominate, the thickness of the crust is reduced, the heat flow is maximum, and active volcanism occurs. If such a boundary forms on a 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, new oceanic crust is formed as a result of spreading.

Ocean rifts

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

Continental rifts

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

Convergent boundaries

Read more in the article Subduction Zone

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

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

In rare cases, oceanic crust is pushed onto continental crust - obduction. Thanks to this process, ophiolites of Cyprus, New Caledonia, Oman and others arose.

In subduction zones, oceanic crust is absorbed, thereby compensating for its appearance in the MOR. Extremely complex processes and interactions between the crust and mantle take place in them. Thus, the 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 ultra-high 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 occurring in the plate convection zone are rightfully considered to be among the most complex in geology. It mixes blocks of different origins, forming a new continental crust.

Active continental margins

Read more in the article Active continental margin

An active continental margin occurs where oceanic crust subducts beneath a continent. The standard of this geodynamic situation is considered to be the western coast of South America; it is often called Andean type of continental margin. The active continental margin is characterized by numerous volcanoes and generally powerful magmatism. Melts have three components: the oceanic crust, the mantle above it, and the lower continental crust.

Beneath the active continental margin, there is 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 structure that forms 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

Read more in the article Island Arc

Island arcs are chains of volcanic islands above a subduction zone, occurring where an oceanic plate subducts beneath an oceanic plate. Typical modern island arcs include the Aleutian, Kuril, Mariana Islands, and many other archipelagos. The Japanese Islands are also often called an island arc, but their foundation is very ancient and in fact they were formed by several island arc complexes at different times, so the Japanese Islands are a microcontinent.

Island arcs are formed when two oceanic plates collide. In this case, one of the plates ends up 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 plate. On this side there is a deep-sea trench and a forearc trough.

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

Continental collision

Collision of continents

Read more in the article Continental Collision

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 as a result of the closure of the Tethys Ocean and the collision with the Eurasian Plate of Hindustan and Africa. As a result, the thickness of the crust increases significantly; under the Himalayas it reaches 70 km. This is an unstable structure; it is intensively destroyed by surface and tectonic erosion. In the crust with a sharply increased thickness, granites are smelted from metamorphosed sedimentary and igneous rocks. This is how the largest batholiths were formed, for example, Angara-Vitimsky and Zerendinsky.

Transform boundaries

Where plates move in parallel courses, but at different speeds, transform faults arise - enormous shear faults, widespread in the oceans and rare on continents.

Transform faults

More details in the article Transform fault

In the oceans, transform faults run perpendicular to mid-ocean ridges (MORs) and break them into segments averaging 400 km wide. Between the ridge segments 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 there are inactive parts of transform faults. There are no active movements in them, but they are clearly expressed in the topography of the ocean floor by linear uplifts with a central depression. .

Transform faults form a regular network and, obviously, do not arise by chance, but due to objective physical reasons. A 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 currents arise in the convective cell due to the cooling of the upper part of the flow. This cooled substance rushes down along the main direction of the mantle flow. Transform faults are located in the zones of this secondary descending flow. This model agrees well with the data on heat flow: a decrease in heat flow is observed above transform faults.

Continental shifts

More details in the article Shift

Strike-slip plate boundaries on continents are relatively rare. Perhaps the only currently active example of a boundary of this type is the San Andreas Fault, separating the North American Plate from the Pacific Plate. The 800-mile San Andreas Fault is one of the most seismically active areas on the planet: plates move 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.

Within-plate processes

The first formulations of plate tectonics argued that volcanism and seismic phenomena are concentrated along plate boundaries, but it soon became clear that specific tectonic and magmatic processes also occur within 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

There are numerous volcanic islands at the bottom of the oceans. Some of them are located in chains with successively changing ages. A classic example of such an underwater ridge is the Hawaiian Underwater Ridge. It rises above the surface of the ocean 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 Ridge. It is interrupted in a deep-sea trench in front of the Aleutian island arc.

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

Traps and oceanic plateaus

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

The Siberian traps on the East Siberian Platform, the Deccan Plateau traps on the Hindustan continent and many others are known. Hot mantle flows are also considered to be the cause of the formation of traps, but unlike hot spots, they act for a short time, 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

Tectonic plate map

Now tectonics can no longer be considered as a purely geological concept. It plays a key role in all geosciences; several methodological approaches with different basic concepts and principles have emerged in it.

From point of view kinematic approach, the movements of the plates can be described by the geometric laws of movement of figures on a sphere. The Earth is seen as a mosaic of plates of different sizes moving relative to each other and the planet itself. Paleomagnetic data allows us to reconstruct the position of the magnetic pole relative to each plate at different points in time. Generalization of data for different plates led to the reconstruction of the entire sequence of relative movements of the plates. Combining this data with information obtained from fixed hot spots made it possible to determine the absolute movements of the plates and the history of the movement of the Earth's magnetic poles.

Thermophysical approach considers the Earth as a heat engine, in which thermal energy is partially converted into mechanical energy. Within 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 velocities 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 occurring 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 different layers of the Earth. Each geodynamic setting is characterized by specific rock associations. In turn, these characteristic features can be used to determine the geodynamic environment in which the rock was formed.

Historical approach. In terms of the history of planet Earth, plate tectonics is the history of continents joining and breaking apart, the birth and decay of volcanic chains, and the appearance and closure of oceans and seas. Now for large blocks of the crust the history of movements has been established in great detail and over a significant 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, carried out in 1999 by the Proterozoic space station. Before this, the mantle may have had a different mass transfer structure, in which turbulent convection and plumes played a major role rather than steady convective flows.

Past plate movements

Read more in the article History of plate movement

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

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

The influence of plate movements on climate

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

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

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

In addition, significant changes in the position of the continents occur over a period of about tens of millions of years, while the total duration of ice ages is about several million years, and during one ice age cyclical changes of glaciations and interglacial periods occur. All of these climate changes occur quickly compared to the speed of continental movement, and therefore plate movement cannot be the cause.

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

The meaning of plate tectonics

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

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