Map of the plates of the earth's crust. Plate tectonics: Definition, movement, types. Version as an axiom

December 10th, 2015

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According to modern theories of lithospheric plates the entire lithosphere is divided by narrow and active zones - deep faults - into separate blocks 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.

Alfred Wegener first suggested horizontal movement of crustal blocks in the 1920s as part of the “continental drift” hypothesis, but this hypothesis did not receive support at that time.

Only in the 1960s, studies of the ocean floor provided indisputable evidence of the horizontal movement of plates and the processes of expansion of the oceans due to the formation (spreading) of the oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the "mobilistic" direction, the development of which led to the development of the modern theory of plate tectonics. The main provisions of plate tectonics were formulated in 1967-68 by a group of American geophysicists - W. J. Morgan, C. Le Pichon, J. Oliver, J. Isaacs, L. Sykes in the development of earlier (1961-62) ideas of American scientists G. Hess and R. Digts on the expansion (spreading) of the ocean floor.

It is argued that scientists are not entirely sure what causes these very shifts and how the boundaries of tectonic plates were designated. There are countless different theories, but none of them fully 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 the continents first occurred to me ... when I was struck by the similarity of the outlines of the coasts on both sides Atlantic Ocean". He suggested that in the early Paleozoic there were two large continents on Earth - Laurasia and Gondwana.

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

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

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

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

A. Wegener found a lot of evidence for the existence of a single continent of the Earth. The existence in Africa and South America of the remains of ancient animals - leafosaurs seemed especially convincing to him. These were reptiles, similar to small hippos, that lived only in freshwater reservoirs. So, to swim huge distances on the salty sea ​​water they couldn't. He found similar evidence in the plant world.

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

The structure of the continental rift

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

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; inner regions plates are weakly seismic and are 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.

Diagram of rift formation

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

Divergent boundaries are boundaries along which plates move apart. The geodynamic setting in which the process of horizontal stretching occurs earth's crust, accompanied by the appearance of extended linearly elongated slot or rov-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 be laid both on the continental and oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to a break in the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of break of the continental crust, it is filled with sediments, turning into an aulacogen).

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

The structure of the mid-ocean ridge. 1 - asthenosphere, 2 - ultrabasic rocks, 3 - basic rocks (gabbroids), 4 - complex of parallel dikes, 5 - basalts of the ocean floor, 6 - segments of the oceanic crust that formed in different time(I-V as it gets older), 7 - near-surface magma chamber (with ultramafic magma in the lower part and basic in the upper part), 8 - sediments of the ocean floor (1-3 as they accumulate)

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

Collision of continental and oceanic lithospheric plates

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

When continental and oceanic plates collide, a natural phenomenon is the subduction of the oceanic (heavier) plate under the edge of the continental one; when two oceanic ones collide, the older one (that is, the cooler and denser) of them sinks.

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

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

The subduction of the subducting plate into the mantle is traced by earthquake foci that occur at the contact of the plates and inside the subducting plate (which is colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone is called the Benioff-Zavaritsky zone. In subduction zones, the process of formation of a new continental crust begins. A much rarer process of interaction between the continental and oceanic plates is the process of obduction - thrusting of a part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that in the course of this process, the oceanic plate is stratified, and only its upper part is advancing - the crust and several kilometers of the upper mantle.

Collision of continental lithospheric plates

When continental plates collide, the crust of which is lighter than the substance of the mantle and, as a result, is not able to sink into it, a collision process occurs. In the course of 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 one, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet. The collision process replaces the subduction process, completing the closure of the ocean basin. At the same time, at the beginning of the collision process, when the edges of the continents have already approached, the collision is combined with the subduction process (the remains of the oceanic crust continue to sink under the edge of the continent). Collision processes are characterized by large-scale regional metamorphism and intrusive granitoid magmatism. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).

The main cause of plate movement is mantle convection, caused by mantle heat and gravity currents.

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

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

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

Thus, the mechanisms that set the lithospheric plates in motion can be conventionally assigned to the following two groups: 1) associated with the forces of the mantle “dragging” (mantle drag mechanism) applied to any points of the bottom of the plates, in the figure - the forces of FDO and FDC; 2) associated with the forces applied to the edges of the plates (edge-force mechanism), in the figure - the forces FRP and FNB. The role of this or that driving mechanism, as well as these or those forces, is evaluated individually for each lithospheric plate.

The totality of these processes reflects the general geodynamic process, covering areas from the surface to deep zones of the Earth. At present, two-cell closed-cell mantle convection is developing in the Earth's mantle (according to the through-mantle convection model) or separate convection in the upper and lower mantle with the accumulation of slabs under subduction zones (according to the two-tier model). The probable poles of the rise of the mantle matter are located in northeast Africa (approximately under the junction zone of the African, Somali and Arabian plates) and in the area of ​​Easter Island (under the median ridge Pacific Ocean– East Pacific Rise). The equator of mantle subsidence runs along an approximately continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian oceans. The current regime of mantle convection, which began about 200 million years modern oceans, in the future will change to a single-cell mode (according to the model of through-mantle convection) or (according to an alternative model) convection will become through-mantle due to the collapse of slabs through the 670 km section. This may lead to the collision of the continents and the formation of a new supercontinent, the fifth in the history of the Earth.

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

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

The age of the oceanic crust according to magnetic anomalies (Kenneth, 1987): 1 - areas of lack of data and dry land; 2–8 - age: 2 - Holocene, Pleistocene, Pliocene (0–5 Ma); 3 - Miocene (5–23 Ma); 4 - Oligocene (23–38 Ma); 5 - Eocene (38–53 Ma); 6 - Paleocene (53–65 Ma) 7 - Cretaceous (65–135 Ma) 8 - Jurassic (135–190 Ma)

At the end of the 80s. completed another experiment to test the movement of lithospheric plates. It was based on baseline measurements relative to distant quasars. Points were selected on two plates, at which, using modern radio telescopes, the distance to quasars and their declination angle were determined, and, accordingly, the distances between points on two plates were calculated, i.e., the baseline was determined. The accuracy of the determination was a few centimeters. Several years later, the measurements were repeated. Very good convergence of results calculated from magnetic anomalies with data determined from baselines was obtained.

Scheme illustrating the results of measurements of the mutual displacement of lithospheric plates, obtained by the method of interferometry with an extra long baseline - 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 the baselines from which the ISDB measured enough data 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, lithospheric 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 the current movement of lithospheric plates, the speed of expansion 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.

Such a forecast was made by American geologists R. Dietz and J. Holden. After 50 million years, according to their assumptions, the Atlantic and Indian oceans will grow at the expense of the Pacific, Africa will shift to the north and due to this, the Mediterranean Sea will gradually be liquidated. The Strait of Gibraltar will disappear, and the “turned” Spain will close the Bay of Biscay. Africa will be split by the great African faults and the eastern part of it 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 Persian Gulf. India will increasingly move towards Asia, which means that 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 refinement. Much here is still debatable and unclear.

The earthquake that caused record destruction and the subsequent tsunami that hit Japan early Friday morning is a stark reminder of the devastating natural disasters that can hit populated cities - especially those in the high risk, for example, along the lines of the main faults of the earth's crust.

Take a look at the five cities that are most at risk from such disasters due to their location.

1. Tokyo, Japan


Built exactly on the triple intersection of three major tectonic plates - the North American Plate, the Philippine Plate and the Pacific Plate - Tokyo is constantly on the move. A long history and familiarity with earthquakes have pushed the city to create maximum levels tectonic protection.


Tokyo is by far the most earthquake-prepared city, which means we're probably underestimating the potential damage nature can cause.


Faced with an 8.9 magnitude earthquake, the strongest earthquake in Japanese history, Tokyo, 370 km from the epicenter, went into an automated stop mode: elevators stopped working, the subway stopped, people had to walk many kilometers on a cold night to get to their houses outside the city, where the greatest destruction occurred.


The 10-meter-high tsunami that followed the earthquake washed away hundreds of bodies on the northeast coast, thousands of people are considered missing.

2. Istanbul, Türkiye


The Eastern San Andreas, North Anatolian strike-slip fault is the world's longest fractured fault, which has been tearing westward along the fault line since 1939.


The city is a mixture of rich and poor infrastructure, putting a large proportion of its 13 million inhabitants at risk. In 1999 A 7.4 magnitude earthquake hit the city of Izmit, just 97km from Istanbul.


While older buildings such as mosques have persevered, newer buildings from the 20th century, often built from concrete mixed with salty groundwater and disregarding local building codes, have crumbled to dust. About 18,000 people died in the region.


In 1997 seismologists predicted that with a 12% chance, the same earthquake could happen again in the region before 2026. Last year, seismologists in the journal Nature Geoscience published data that the next earthquake is likely to occur in the west of Izmit along the fault - a dangerous 19km south of Istanbul.

3. Seattle, Washington


When residents of a Pacific Northwest city think of disasters, two scenarios come to mind: a mega-quake and the eruption of Mount Rainier.


In 2001 An earthquake in Nisqually Indian Territory prompted the city to improve its earthquake preparedness plan, and several new building code improvements were made. Be that as it may, many older buildings, bridges and roads are still not modernized in accordance with the new norms.


The city lies on an active tectonic boundary along the North American Plate, the Pacific Plate, and the Juan de Fuca Plate. Ancient history both earthquakes and tsunamis are recorded in the ground turned to stone in the water forests, as well as in the oral histories passed down from generation to generation of Pacific Northwest Native Americans.


Faintly looming in the distance, and when the cloud cover is high enough, the impressive view of Mount Rainier reveals that this is a dormant volcano and at any time it can push Mount St. Helena as well.
While seismologists are extremely good at monitoring volcanic shocks and alerting authorities to an impending eruption, Iceland's Eyjafjallajökull volcano eruption last year showed that the extent and duration of the eruption is just someone's guess. Most of the devastation will affect the east of the volcano.


But if an uncharacteristic northwest wind blows, the Seattle airport and the city itself will face more hot ash.

4. Los Angeles, California


Disasters aren't new to the Los Angeles area - and they don't talk about everyone on TV.


Over the past 700 years, powerful earthquakes have occurred in the region every 45-144 years. The last strong 7.9 magnitude earthquake occurred 153 years ago. In other words, Los Angeles should be subject to the next major earthquake.


Los Angeles, with a population of about 4 million, could be hit hard by the next major quake. According to some estimates, taking into account the whole of Southern California with a population of about 37 million people, a natural disaster could kill 2,000 to 50,000 people and cause billions of dollars in damage.

5. San Francisco, California


San Francisco, with a population of over 800,000, is another Big city on the west coast of the United States that could be devastated by a major earthquake and/or tsunami.
San Francisco is located nearby, although not exactly on the northern part of the San Andreas Fault. There are also several related faults running in parallel across the San Francisco region, raising the possibility of an extremely destructive quake.


There has already been one such catastrophe in the history of the city. April 18, 1906 San Francisco was hit by an earthquake measuring between 7.7 and 8.3. The disaster caused the death of 3,000 people, caused losses of half a billion dollars and leveled most of the city to the ground.


In 2005 Earthquake expert David Schwartz, a San Francisco resident, has estimated that the region has a 62% chance of being hit by a major earthquake within the next 30 years. While some buildings in the city are built or reinforced to withstand an earthquake, according to Schwartz, many are still at risk. Residents are also advised to keep emergency kits ready at all times.

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

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

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

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

Rice. 1. Collision of continental lithospheric plates

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

Rice. 2. Collision of continental and oceanic lithospheric plates

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

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

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

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

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

Rice. 4. Scheme of rift formation

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

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

Rice. 5. Earth's lithospheric plates

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

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

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

Continental drift theory

The theory of lithospheric plates originates from the theory of continental drift. Back in the 19th century many geographers noted that when looking at 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 the movement of the continents is associated with the name of the German scientist Alfred Wegener(1880-1930) (Fig. 7), who most fully developed this idea.

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

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

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

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

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

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

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

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

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

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

Fundamentals of plate tectonics

The fundamentals of plate tectonics can be traced back to a few fundamental

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

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

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

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

australian plate,
Antarctic Plate,
african plate,
Eurasian Plate,
Hindustan Plate,
Pacific Plate,
North American Plate,
South American plate.

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

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

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

Accordingly, three types of main plate boundaries are distinguished.

Divergent boundaries are the boundaries along which the plates move apart.

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

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

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

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

The structure of the mid-ocean ridge

In the course of spreading, each stretching pulse is accompanied by the inflow of a new portion of mantle melts, which, while solidifying, build up the edges of the plates diverging from the MOR axis.

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

convergent borders are the boundaries along which plates collide. There can be three main variants of interaction in a collision: "oceanic - oceanic", "oceanic - continental" and "continental - continental" lithosphere. Depending on the nature of the colliding plates, several different processes can take place.

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

When continental and oceanic plates collide, a natural phenomenon is the subduction of the oceanic (heavier) plate under the edge of the continental one; when two oceanic ones collide, the older one (that is, the cooler and denser) of them sinks.

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

The subduction of the subducting plate into the mantle is traced by earthquake foci that occur at the contact of the plates and inside the subducting plate (which is colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone is called Benioff-Zavaritsky zone.

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

A much rarer process of interaction between continental and oceanic plates is the process obduction– thrusting of a part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that in the course of this process, the oceanic plate is stratified, and only its upper part is advancing - the crust and several kilometers of the upper mantle.

In the collision of continental plates, the crust of which is lighter than the substance of the mantle, and therefore is not able to sink into it, the process collisions. In the course of 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 one, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet.

Collision process model

The collision process replaces the subduction process, completing the closure of the ocean basin. At the same time, at the beginning of the collision process, when the edges of the continents have already approached, the collision is combined with the subduction process (the remains of the oceanic crust continue to sink under the edge of the continent).

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

Transform borders are the boundaries along which shear displacements of plates occur.

The boundaries of the lithospheric plates of the Earth

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

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

5. The main cause of plate movement is mantle convection. , caused by mantle thermogravitational currents.

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

The rocks heated in the central zones of the Earth expand, their density decreases, and they float, giving way to descending colder and therefore heavier masses, which have already given up part of the heat in near-surface zones. This process of heat transfer goes on continuously, resulting in the formation of ordered closed convective cells. At the same time, in the upper part of the cell, the flow of matter occurs in an almost horizontal plane, and it is this part of the flow that determines the horizontal movement of the matter of the asthenosphere and the plates located on it. In general, the ascending branches of convective cells are located under the zones of divergent boundaries (MOR and continental rifts), while the descending branches are located under the zones of convergent boundaries.

Thus, the main reason for the movement of lithospheric plates is "drag" by convective currents.

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

Figure - Forces acting on lithospheric plates.

The main driving forces of plate tectonics are applied to the bottom of the intraplate parts of the lithosphere: the forces of mantle “drag” (English drag) FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the speed of the asthenospheric current, and the latter is determined by the viscosity and thickness of the asthenospheric layer. Since under the continents the thickness of the asthenosphere is much less, and the viscosity is much greater than under the oceans, the magnitude of the force FDC almost an order of magnitude smaller than FDO. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost wedges out, so the continents seem to be “sitting aground”. Since most of the lithospheric plates of the modern Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the composition of the plate in the general case should “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving are the almost purely oceanic plates Pacific, Cocos and Nasca; the slowest are the Eurasian, North American, South American, Antarctic and African, a significant part of the area of ​​​​which 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 a force FNB(index in the designation of strength - from English negative feedback). 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 strength FNB operates episodically and only in certain geodynamic settings, for example, in cases of the collapse of slabs described above through a section of 670 km.

Thus, the mechanisms that set the lithospheric plates in motion can be conditionally assigned to the following two groups: 1) associated with the forces of mantle “dragging” ( mantle drag mechanism) applied to any points of the soles of the plates, in Fig. 2.5.5 - forces FDO And FDC; 2) related to the forces applied to the edges of the plates ( edge force mechanism), in the figure - forces FRP And FNB. The role of this or that driving mechanism, as well as these or those forces, is evaluated individually for each lithospheric plate.

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

Mantle convection and geodynamic processes

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

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

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

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

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

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

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

geophysical data.

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

Remains of the huge Faralon Plate, which disappeared in the subduction zone under North and South America, fixed in the form of “cold” mantle slabs (section across North America, along S-waves). After Grand, Van der Hilst, Widiyantoro, 1997, GSA Today, v. 7, no. 4, 1-7

​Linear magnetic anomalies in the oceans were discovered in the 1950s during geophysical studies of the Pacific Ocean. This discovery allowed Hess and Dietz to formulate the theory of ocean floor spreading in 1968, which grew into the theory of plate tectonics. They became one of the strongest proofs of the correctness of the theory.

Figure - Formation of strip magnetic anomalies during spreading.

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

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

What do we know about the lithosphere?

Tectonic plates are large stable areas of the Earth's crust that are the constituent parts of the lithosphere. If we turn to tectonics, the science that studies lithospheric platforms, we learn that large areas of the earth's crust are limited on all sides by specific zones: volcanic, tectonic and seismic activities. It is at the junctions of neighboring plates that phenomena occur, which, as a rule, have catastrophic consequences. These include both volcanic eruptions and strong earthquakes on the scale of seismic activity. In the process of studying the planet, platform tectonics played a very important role. Its significance can be compared to the discovery of DNA or the heliocentric concept in astronomy.

If we recall the geometry, then we can imagine that one point can be the point of contact of the boundaries of three or more plates. The study of the tectonic structure of the earth's crust shows that the most dangerous and rapidly collapsing are the junctions of four or more platforms. This formation is the most unstable.

The lithosphere is divided into two types of plates, different in their characteristics: continental and oceanic. It is worth highlighting the Pacific platform, composed of oceanic crust. Most of the others consist of the so-called block, when the continental plate is soldered into the oceanic one.

The location of the platforms shows that about 90% of the surface of our planet consists of 13 large, stable areas of the earth's crust. The remaining 10% fall on small formations.

Scientists have compiled a map of the largest tectonic plates:

• Australian;
• Arabian subcontinent;
• Antarctic;
• African;
• Hindustan;
• Eurasian;
• Nazca plate;
• Cooker Coconut;
• Pacific;
• North and South American platforms;
• Scotia plate;
• Philippine plate.

From theory, we know that the solid shell of the earth (lithosphere) consists not only of the plates that form the relief of the surface of the planet, but also of the deep part - the mantle. Continental platforms have a thickness of 35 km (in the flat areas) to 70 km (in the zone of mountain ranges). Scientists have proven that the plate in the Himalayas has the greatest thickness. Here the thickness of the platform reaches 90 km. The thinnest lithosphere is found in the ocean zone. Its thickness does not exceed 10 km, and in some areas this figure is 5 km. Based on the information about the depth at which the epicenter of the earthquake is located and what is the speed of propagation of seismic waves, calculations are made of the thickness of the sections of the earth's crust.

The process of formation of lithospheric plates

The lithosphere consists mainly of crystalline substances formed as a result of cooling of magma upon reaching the surface. The description of the structure of the platforms speaks of their heterogeneity. The process of formation of the earth's crust took place over a long period, and continues to this day. Through microcracks in the rock, molten liquid magma came to the surface, creating new bizarre forms. Its properties changed depending on the change in temperature, and new substances were formed. For this reason, minerals that are at different depths differ in their characteristics.

The surface of the earth's crust depends on the influence of the hydrosphere and atmosphere. There is constant weathering. Under the influence of this process, the forms change, and the minerals are crushed, changing their characteristics with the same chemical composition. As a result of weathering, the surface became looser, cracks and microdepressions appeared. In these places deposits appeared, which we know as soil.

Map of tectonic plates

At first glance it seems that the lithosphere is stable. Its upper part is such, but the lower part, which is distinguished by viscosity and fluidity, is mobile. The lithosphere is divided into a certain number of parts, the so-called tectonic plates. Scientists cannot say how many parts the earth's crust consists of, since in addition to large platforms, there are also smaller formations. The names of the largest plates were given above. The process of formation of the earth's crust is ongoing. We do not notice this, since these actions occur very slowly, but by comparing the results of observations for different periods, you can see how many centimeters per year the boundaries of formations are shifting. For this reason tectonic map the world is constantly updated.

Tectonic Plate Cocos

The Cocos platform is a typical representative of the oceanic parts of the earth's crust. It is located in the Pacific region. In the west, its boundary runs along the ridge of the East Pacific Rise, and in the east its boundary can be defined by a conventional line along the coast of North America from California to the Isthmus of Panama. This plate is subducting under the neighboring Caribbean plate. This zone is characterized by high seismic activity.

Mexico suffers the most from earthquakes in this region. Among all the countries of America, it is on its territory that the most extinct and active volcanoes are located. The country moved a large number of earthquakes with a magnitude greater than 8. The region is quite densely populated, therefore, in addition to destruction, seismic activity also leads to a large number of victims. Unlike Cocos, located in another part of the planet, the Australian and West Siberian platforms are stable.

Movement of tectonic plates

For a long time, scientists have been trying to find out why one region of the planet has mountainous terrain, while another is flat, and why earthquakes and volcanic eruptions occur. Various hypotheses were built mainly on the knowledge that was available. Only after the 50s of the twentieth century was it possible to study the earth's crust in more detail. Mountains formed at the sites of plate faults, the chemical composition of these plates were studied, and maps of regions with tectonic activity were also created.

In the study of tectonics, a special place was occupied by the hypothesis of the displacement of lithospheric plates. Back in the early twentieth century, the German geophysicist A. Wegener put forward a bold theory about why they move. He carefully studied the outlines of the western coast of Africa and the eastern coast of South America. The starting point in his research was precisely the similarity of the outlines of these continents. He suggested that, perhaps, these continents used to be a single whole, and then a break occurred and the shift of parts of the Earth's crust began.

His research touched upon the processes of volcanism, the stretching of the surface of the ocean floor, the viscous-liquid structure the globe. It was the works of A. Wegener that formed the basis of the research conducted in the 60s of the last century. They became the foundation for the emergence of the theory of "lithospheric plate tectonics".

This hypothesis described the model of the Earth as follows: tectonic platforms with a rigid structure and different masses were placed on the plastic substance of the asthenosphere. They were in a very unstable state and were constantly moving. For a simpler understanding, we can draw an analogy with icebergs that are constantly drifting in ocean waters. Similarly, tectonic structures, being on a plastic substance, are constantly moving. During displacements, the plates constantly collided, came one on top of the other, joints and zones of separation of the plates arose. This process was due to the difference in mass. Areas of increased tectonic activity were formed at the collision sites, mountains arose, earthquakes and volcanic eruptions occurred.

The displacement rate was no more than 18 cm per year. Faults formed, into which magma entered from the deep layers of the lithosphere. For this reason, the rocks that make up ocean platforms have different age. But scientists have put forward even more incredible theory. According to some representatives of the scientific world, magma came to the surface and gradually cooled, creating a new bottom structure, while the "excess" of the earth's crust, under the influence of plate drift, sank into the earth's interior and again turned into liquid magma. Be that as it may, the movements of the continents occur in our time, and for this reason new maps are being created to further study the process of drifting tectonic structures.