The lithosphere and the earth's crust. The names of the largest plates of the Earth. Versions of the formation of the planet Lithospheric plates and what is on them

According to modern lithospheric plate theory The entire lithosphere is divided by narrow and active zones - deep faults - into separate blocks that move in the plastic layer of the upper mantle relative to each other at a rate 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 keep 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 diverge, approach or slide relative to each other. In the first case, tension zones with cracks appear between the plates along the boundaries of the plates, in the second - zones of compression, accompanied by the thrust of one plate onto another (thrust - obduction; thrust - subduction), in the third - shear zones - faults along which the neighboring plates slide. ...

At the points of convergence of the continental plates, they collide, and mountain belts are formed. This is how, for example, the Himalayan mountain system appeared on the border of the Eurasian and Indo-Australian plates (Fig. 1).

Rice. 1. Collision of continental lithospheric plates

With the interaction of the continental and oceanic plates, 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 an oceanic type of crust is shown in Fig. 3.

The axial zones of the mid-oceanic ridges are characterized by rifts(from the English. rift - crevice, crack, fault) - a large linear tectonic structure of the earth's crust with a length of hundreds, thousands, 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 movements and friction of adjacent plates occur. These zones were named 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 the formation of the earth's crust in these zones is currently going on very intensively.

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

Rice. 4. Rift formation diagram

Most of the fractures of 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 the east of Africa. It stretches for 4000 km. The width of this fault is 80-120 km.

At present, seven of the largest slabs can be distinguished (Fig. 5). Of these, the largest in area is the Pacific Ocean, which consists entirely of the 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. At the same time, scientists suggest that in fact the Nazca plate is much larger than we see it on the map (see Fig. 5), since a significant part of it went under the neighboring plates. This plate also consists only of the oceanic lithosphere.

Rice. 5. Lithospheric plates of the Earth

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

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

Rice. 6. The outlines of the continents do seem to be 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 the map, one can notice that the shores of Africa and South America, when approaching, seem to be compatible (Fig. 6).

The emergence of the hypothesis of the movement of 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 continents first occurred to me ... when I was struck by the similarity of coastlines 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 was the first seafood - 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 Pangea (white - land, points - shallow sea)

Approximately 180 million years ago, the Pangea continent again began to separate into its component parts, which were mixed on the surface of our planet. The division took place 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; old - Quiet. The Arctic Ocean has become isolated with an 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 many confirmations of the existence of a single continent of the Earth. The existence in Africa and South America of the remains of ancient animals - the listosaurs - seemed to him especially convincing. They were reptiles, similar to small hippos, that lived only in freshwater bodies of water. This means that they could not swim great distances in salty sea water. He found similar evidence in the plant kingdom.

Interest in the hypothesis of the movement of continents in the 30s of the XX century. slightly decreased, 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 "diving" of some parts of the crust under others (subduction).

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

For the first time, the hypothesis of the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s within the framework of the hypothesis of "continental drift", but this hypothesis did not receive support at that time.

Only in the 1960s, studies of the ocean floor provided conclusive evidence of horizontal plate movements 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 took place in the framework of the "mobilistic" direction, 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 the earlier (1961-62) ideas of American scientists G. Hess and R. Digz on the expansion (spreading) of the ocean floor.

It is argued that scientists are not entirely sure what is causing 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 continents first occurred to me ... when I was struck by the similarity of coastlines 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 was the first seafood - 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 Pangea continent again began to separate into its component parts, which were mixed on the surface of our planet. The division took place 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; old - Quiet. The Arctic Ocean has become isolated with an increase in land mass in the Northern Hemisphere.

A. Wegener found many confirmations of the existence of a single continent of the Earth. The existence in Africa and South America of the remains of ancient animals - the listosaurs - seemed to him especially convincing. They were reptiles, similar to small hippos, that lived only in freshwater bodies of water. This means that they could not swim great distances in salty sea water. He found similar evidence in the plant kingdom.

Interest in the hypothesis of the movement of continents in the 30s of the XX century. slightly decreased, 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 "diving" of some parts of the crust under others (subduction).

The structure of the continental rift

The upper stony part of the planet is divided into two shells, significantly different in rheological properties: the rigid and fragile lithosphere and the underlying plastic and mobile asthenosphere.
The bottom of the lithosphere is an isotherm of approximately 1300 ° C, which corresponds to the melting point (solidus) of the mantle material at lithostatic pressure existing at depths of the first hundreds of kilometers. The rocks lying in the Earth above this isotherm are cold enough and behave like a hard material, while the underlying rocks of the same composition are sufficiently heated and relatively easily deformed.

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 mosaics of small crustal slabs.

Plate boundaries are areas of seismic, tectonic, and magmatic activity; the inner regions 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:

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 diagram

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 of the earth's crust occurs, accompanied by the emergence of extended linearly elongated slotted or ditch-like depressions, is called rifting. These boundaries are confined to continental rifts and mid-ocean ridges in oceanic basins. The term "rift" (from the English rift - rupture, 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 on the oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to the rupture of the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of rupture of the continental crust, it is filled with sediments, turning into an aulacogen).

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

The structure of the mid-ocean ridge. 1 - asthenosphere, 2 - ultrabasic rocks, 3 - basic rocks (gabbroids), 4 - a complex of parallel dikes, 5 - basalts of the oceanic bottom, 6 - segments of the oceanic crust that formed at different times (IV with aging), 7 - shallow magmatic chamber (with ultrabasic magma in the lower part and main in the upper part), 8 - sediments of the oceanic bottom (1-3 as they accumulate)

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

Collision of continental and oceanic lithospheric plates

Subduction is the process of shifting an oceanic plate under a continental or other oceanic plate. 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 the continental and oceanic plates collide, a natural phenomenon is the underdling of the oceanic (heavier) plate under the edge of the continental; when two oceanic ones collide, the older (that is, the 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 bend and sub-motor subduction plate. As it sinks, this plate begins to lose water (which is abundant in the composition of 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 the volcanoes of the island arcs. In the rear of a volcanic arc, some stretching usually occurs, which determines the formation of a back-arc basin. In the zone of the back-arc basin, the tension can be so significant that it leads to rupture of the plate crust and the opening of the basin with the oceanic crust (the so-called back-arc spreading process).

The volume of the oceanic crust absorbed in the subduction zones is equal to the volume of the crust arising in the spreading zones. This position emphasizes the opinion about the constancy of the volume of the Earth. But this 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 subsidence of the subducting plate into the mantle is traced by earthquake foci arising at the contact of the plates and inside the subducting plate (colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone was named the Benioff-Zavaritsky zone. In the subduction zones, the process of the formation of a new continental crust begins. A much rarer process of interaction between the continental and oceanic plates is the process of obduction - the thrust 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 separation of the oceanic plate occurs, and only its upper part - the crust and several kilometers of the upper mantle - is advancing.

Collision of continental lithospheric plates

When the continental plates collide, the crust of which is lighter than the material of the mantle, and as a result, is not able to submerge in it, the collision process takes place. In the course of the collision, the edges of the colliding continental plates are crushed, crumpled, 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 immense mountain systems of the Himalayas and Tibet. The collision process replaces the subduction process, completing the closure of the oceanic 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 process of subduction (the subsidence of the oceanic crust continues under the edge of the continent). Large-scale regional metamorphism and intrusive granitoid magmatism are typical for collisional processes. 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-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. In this case, the main part of endogenous heat is released at the boundary of the core and mantle during the process of deep differentiation, which determines the decay of the primary chondrite material, 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 rise, giving way to sinking colder and therefore heavier masses, which have already given off part of the heat in the near-surface zones. This process of heat transfer goes on 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 the 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 the convective cells are located under the zones of divergent boundaries (MOR and continental rifts), the descending branches - 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 plates. In particular, the surface of the asthenosphere turns out to be somewhat raised above the zones of ascending branches and more lowered in the zones of immersion, 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 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 bottom of the intraplate parts of the lithosphere - the forces of mantle drag FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the asthenospheric current velocity, 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 force FDC is almost an order of magnitude inferior to the magnitude of FDO. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost wedges out, so the continents seem to be “stranded”. 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 plate should generally “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving are the almost purely oceanic plates of the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African, a significant part of which is occupied by continents). Finally, at convergent plate boundaries, where the heavy and cold edges of the lithospheric plates (slabs) sink into the mantle, their negative buoyancy creates a force FNB (the 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 the slab collapse described above through the 670 km section.

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

The combination of these processes reflects the general geodynamic process, covering areas from the surface to deep zones of the Earth. Currently, a two-cell mantle convection with closed cells (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with accumulation of slabs under subduction zones (according to a two-tiered model) is developing in the Earth's mantle. The probable poles of the uplift of mantle matter are located in northeastern 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 Uplift). The equator of the subsidence of mantle material runs along an approximately continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern parts of the Indian Oceans. convection) or (according to an alternative model) convection will become through the mantle due to the collapse of slabs through the 670 km section. This, possibly, will lead to the collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.

Displacements of plates obey the laws of spherical geometry and can be described based on Euler's theorem. Euler's Rotation Theorem states that any rotation in 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. Analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which undergoes further disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, the modern continents were formed.

Plate tectonics is the first general geological concept that could be tested. Such a check was carried out. In the 70s. a deep water drilling program was organized. Within the framework of this program, the drilling vessel "Glomar Challenger" drilled several hundred wells, which showed good convergence of ages estimated from magnetic anomalies with ages determined from basalts or sedimentary horizons. The distribution scheme of the different-aged sections of the oceanic crust is shown in Fig.:

The age of the oceanic crust based on magnetic anomalies (Kenneth, 1987): 1 - areas of lack of data and 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)

In the late 80s. another experiment to check the movement of lithospheric plates was completed. It was based on measuring baselines in relation to distant quasars. On two plates, points were selected 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 the first centimeters. Several years later, the measurements were repeated. Very good convergence of the results calculated from the magnetic anomalies with the data determined from the baselines was obtained.

Diagram illustrating the results of measurements of the mutual displacement of lithospheric plates, obtained by the method of interferometry with a very 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 that have been measured by the ISDB method with 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, 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 modern movement of the lithospheric plates, the speed of the 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.

This forecast was made by the 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 will be gradually eliminated. The Strait of Gibraltar will disappear, and the "turned" Spain will close the Bay of Biscay. Africa will be split by the great African rifts and its eastern part will be displaced 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 that the Himalayan mountains will grow. California along the San Andreas Fault will separate from North America, and a new oceanic basin will begin to form at this point. 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.

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

And let me remind you, but interesting and like this. Look at and The original article is on the site InfoGlaz.rf The link to the article this copy was made from is

The lithosphere of planet Earth is a hard shell of the earth, which includes multilayer blocks called lithospheric plates. As Wikipedia points out, translated from Greek it is "stone ball". It has a heterogeneous structure depending on the landscape and the plasticity of the rocks in the upper soil layers.

The boundaries of the lithosphere and the location of its plates are not fully understood. Modern geology has only a limited amount of data on the internal structure of the globe. It is known that lithospheric blocks have boundaries with the hydrosphere and atmospheric space of the planet. They are closely related to each other and touch each other. The structure itself consists of the following elements:

1. Asthenosphere. A layer of reduced hardness, which is located in the upper part of the planet in relation to the atmosphere. In some places it has very low strength, is prone to fractures and toughness, especially if groundwater flows inside the asthenosphere.
2. Mantle. It is a part of the Earth called the geosphere, located between the asthenosphere and the inner core of the planet. It has a semi-fluid structure, and its boundaries begin at a depth of 70–90 km. It is characterized by high seismic velocities, and its movement directly affects the thickness of the lithosphere and the activity of its plates.
3. Core. The center of the globe, which has a liquid etiology, and the movement of its mineral components and the molecular structure of molten metals depends on the preservation of the planet's magnetic polarity and its rotation around its axis. The main constituent of the earth's core is an alloy of iron and nickel.

What is the lithosphere? In fact, it is the solid shell of the Earth, which acts as an intermediate layer between fertile soil, mineral deposits, ores and the mantle. On the plain, the thickness of the lithosphere is 35–40 km.

Important! In mountainous areas, this figure can reach 70 km. In the area of ​​such geological heights as the Himalayan or the Caucasus Mountains, the depth of this layer reaches 90 km.

Structure of the earth

Layers of the lithosphere

If we consider the structure of lithospheric plates in more detail, then they are classified into several layers, which form the geological features of a particular region of the Earth. They form the basic properties of the lithosphere. Based on this, the following layers of the hard shell of the globe are distinguished:

1. Sedimentary. Covers most of the top layer of all earth blocks. It mainly consists of volcanic rocks, as well as the remains of organic matter, which decomposed into humus over many millennia. Fertile soils are also part of the sedimentary layer.
2. Granite. These are lithospheric plates in constant motion. They are predominantly composed of extra-strong granite and gneiss. The last component is a metamorphic rock, the overwhelming part of which is filled with minerals such as potassium spar, quartz and plagioclase. The seismic activity of this layer of the hard shell is at the level of 6.4 km / sec.
3. Basaltic. Mostly composed of basaltic deposits. This part of the hard shell of the Earth was formed under the influence of volcanic activity in ancient times, when the formation of the planet took place and the first conditions for the development of life were born.

What is the lithosphere and its multilayer structure? Based on the foregoing, we can conclude that this is a solid part of the globe, which has a heterogeneous composition. Its formation took place over several millennia, and its qualitative composition depends on what metaphysical and geological processes took place in a particular region of the planet. The influence of these factors is reflected in the thickness of the lithospheric plates, their seismic activity in relation to the structure of the Earth.

Layers of the lithosphere

Oceanic lithosphere

This type of the earth's shell differs significantly from its mainland. This is due to the fact that the boundaries of the lithospheric blocks and the hydrosphere are closely intertwined, and in some parts of it the water space extends beyond the surface layer of the lithospheric plates. This applies to bottom faults, depressions, cavernous formations of various etiologies.

Ocean crust

That is why oceanic-type plates have their own structure and consist of the following layers:

• marine sediments, which have a total thickness of at least 1 km (in deep ocean areas they may be absent altogether);
• the secondary layer (responsible for the propagation of medium and longitudinal waves moving at a speed of up to 6 km / s, takes an active part in the movement of plates, which provokes earthquakes of various strengths);
• the lower layer of the earth's hard shell in the area of ​​the ocean floor, which is mainly composed of gabbro and borders on the mantle (the average activity of seismic waves is from 6 to 7 km / sec.).

A transitional type of lithosphere is also distinguished, located in the area of ​​oceanic soil. It is typical for island zones formed in an arcuate manner. In most cases, their appearance is associated with the geological process of movement of lithospheric plates, which layered on top of each other, forming such irregularities.

Important! A similar structure of the lithosphere can be found on the outskirts of the Pacific Ocean, as well as in some parts of the Black Sea.

Useful video: lithospheric plates and modern relief

Chemical composition

In terms of filling with organic and mineral compounds, the lithosphere does not differ in diversity and is mainly presented in the form of 8 elements.

Most of these are rocks that were formed during the period of active eruption of volcanic magma and plate movement. The chemical composition of the lithosphere is as follows:

1. Oxygen. It occupies at least 50% of the entire structure of the hard shell, filling its faults, depressions and cavities formed during the movement of plates. Plays a key role in the balance of compression pressure during geological processes.
2. Magnesium. This is 2.35% of the Earth's solid shell. Its appearance in the lithosphere is associated with magmatic activity in the early periods of the planet's formation. It is found throughout the continental, marine and oceanic parts of the planet.
3. Iron. Rock, which is the main mineral of the lithospheric plates (4.20%). Its main concentration is in the mountainous regions of the globe. It is in this part of the planet that the highest density of this chemical element. It is not presented in pure form, but is in the composition of lithospheric plates in a mixed form together with other mineral deposits.

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

For the first time, the hypothesis of the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s within the framework of the hypothesis of "continental drift", but this hypothesis did not receive support at that time. Only in the 1960s, studies of the ocean floor provided conclusive evidence of horizontal plate movements 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 took place in 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 the earlier (1961-62) ideas of American scientists G. Hess and R. Digz on the expansion (spreading) of the ocean floor

Basics of plate tectonics

The fundamentals of plate tectonics can be summarized in several fundamental

1. The upper rocky part of the planet is divided into two shells, significantly different in rheological properties: the rigid and fragile lithosphere and the 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 mosaics of small crustal slabs.

Plate boundaries are areas of seismic, tectonic, and magmatic activity; the inner regions 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 displacements of plates: divergence (divergence), convergence (convergence) and shear displacements.

Accordingly, three types of main plate boundaries are distinguished.

Divergent boundaries- boundaries along which the slabs 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 oceanic basins.

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




The process of sliding plates in zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of a new oceanic crust due to magmatic basaltic melt coming from the asthenosphere. This 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



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

It is in these zones that the formation of a young oceanic crust takes place.

Convergent boundaries- boundaries along which the collision of plates occurs. 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 shifting the oceanic plate under the continental or other oceanic. 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 the continental and oceanic plates collide, a natural phenomenon is the underdling of the oceanic (heavier) plate under the edge of the continental; when two oceanic ones collide, the older (that is, the 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 bend and underthrust zone of the subducting plate. As it sinks, this plate begins to lose water (which is abundant in the composition of 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 the volcanoes of the island arcs. In the rear of a volcanic arc, some stretching usually occurs, which determines the formation of a back-arc basin. In the zone of the back-arc basin, the tension can be so significant that it leads to rupture of the plate crust and the opening of the basin with the oceanic crust (the so-called back-arc spreading process).



The subsidence of the subducting plate into the mantle is traced by earthquake foci arising at the contact of the plates and inside the subducting plate (colder and therefore more fragile than the surrounding mantle rocks). This seismic focal zone was named Benioff-Zavaritsky zone.

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

A much rarer process of interaction of the 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 separation of the oceanic plate occurs, and only its upper part - the crust and several kilometers of the upper mantle - is advancing.

In the collision of continental plates, the crust of which is lighter than the material of the mantle, and as a result, is not able to submerge in it, the process takes place collisions... In the course of the collision, the edges of the colliding continental plates are crushed, crumpled, 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 immense mountain systems of the Himalayas and Tibet.

Collision process model



The collision process replaces the subduction process, completing the closure of the oceanic 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 process of subduction (the subsidence of the oceanic crust continues under the edge of the continent).

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

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

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 - collisional); 4 – direction and speed (cm / year) of plate movement.

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

5. The main cause of plate movement is mantle convection. caused by mantle heat-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. In this case, the main part of endogenous heat is released at the boundary of the core and mantle during the process of deep differentiation, which determines the decay of the primary chondrite material, 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 rise, giving way to sinking colder and therefore heavier masses, which have already given off part of the heat in the near-surface zones. This process of heat transfer goes on 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 the 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 the convective cells are located under the zones of divergent boundaries (MOR and continental rifts), the descending branches - 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 plates. In particular, the surface of the asthenosphere turns out to be somewhat raised above the zones of ascending branches and more lowered in the zones of immersion, 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 consequence, less dense asthenosphere, as well as hydraulic wedging by basalts in the MOR zones.



Figure - Forces acting on lithospheric plates.

The main driving forces of plate tectonics are applied to the bottom of the intraplate parts of the lithosphere - the forces of mantle drag FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the asthenospheric current velocity, 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 higher than under the oceans, the magnitude of the force FDC almost an order of magnitude inferior to FDO... Under the continents, especially their ancient parts (continental shields), the asthenosphere almost wedges out, so the continents seem to be “stranded”. 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 plate should generally “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving are the almost purely oceanic plates of the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African, a significant part 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(the index in the designation of strength - from English negative buoyance). The action of the latter leads to the fact that the subducting part of the plate sinks in the asthenosphere and pulls the entire plate along with it, thereby increasing the speed of its movement. Obviously the strength FNB acts sporadically and only in certain geodynamic settings, for example, in cases of the above-described slab collapse through the 670 km section.

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 base of the slabs, in Fig. 2.5.5 - forces FDO and FDC; 2) associated with 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 those or other forces, is assessed individually for each lithospheric plate.

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





Mantle convection and geodynamic processes

Currently, a two-cell mantle convection with closed cells (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with accumulation of slabs under subduction zones (according to a two-tiered model) is developing in the Earth's mantle. The probable poles of the uplift of mantle matter are located in northeastern 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 Uplift).

The equator of the subsidence of mantle material 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 ago with the disintegration 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) or (according to an alternative model) convection will become through the mantle due to the collapse of slabs through the 670 km section. This, possibly, will lead to the collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.

6. Displacements 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 in 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. Analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which undergoes further disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, the modern continents were formed.

Some evidence of the reality of the mechanism of plate tectonics

Aging of the oceanic crust age with distance from the spreading axes(see figure). An increase in the thickness and stratigraphic completeness of the sedimentary layer is noted in the same direction.



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

Geophysical data.



Figure - Tomographic profile through the Hellenic Trench, Crete and the Aegean Sea. Gray circles are earthquake hypocenters. Blue color shows a plate of a plunging cold mantle, red - a hot mantle (according to V. Spekman, 1989)



Remains of the huge Faralon plate, which disappeared in the subduction zone under the North and South America, recorded as slabs of the "cold" mantle (section across North America, along S-waves). By 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 the geophysical study of the Pacific Ocean. This discovery allowed Hess and Diez in 1968 to formulate the theory of ocean floor spreading, which grew into the theory of plate tectonics. They have become one of the strongest proofs of the theory's correctness.



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 mid-ocean ridges, the erupted basalts, when they cool below the Curie point in the Earth's magnetic field, acquire remanent magnetization. The direction of magnetization coincides with the direction of the Earth's magnetic field, however, due to periodic inversions of the Earth's magnetic field, the erupted basalts form stripes with different directions of magnetization: direct (coincides with the modern direction of the magnetic field) and reverse.



Figure - Diagram of the formation of the strip structure of the magnetoactive layer and magnetic anomalies of the ocean (Vine - Matthews model).

The theory of lithospheric plates is the most interesting direction in geography. As suggested by modern scientists, the entire lithosphere is divided into blocks that drift in the upper layer. Their speed is 2-3 cm per year. They are called lithospheric plates.

Founder of the theory of lithospheric plates

Who founded the theory of lithospheric plates? A. Wegener was one of the first in 1920 to make the assumption that the plates move horizontally, but he was not supported. And only in the 60s, the survey of the ocean floor confirmed his assumption.

The resurrection of these ideas led to the creation of the modern theory of tectonics. Its most important provisions were determined by a team of geophysicists from America D. Morgan, J. Oliver, L. Sykes and others in 1967-68.

Scientists cannot say in the affirmative what causes such displacements and how boundaries are formed. Back in 1910, Wegener believed that at the very beginning of the Paleozoic period, the Earth consisted of two continents.

Laurasia covered the area of ​​present-day Europe, Asia (India was not included), North America. She was the northern mainland. Gondwana included South America, Africa, Australia.

Somewhere two hundred million years ago, these two continents merged into one - Pangea. And 180 million years ago, it is again divisible by two. Subsequently, Laurasia and Gondwana were also separated. Through this split, the oceans were formed. Moreover, Wegener found evidence that confirmed his hypothesis about a single continent.

Map of lithospheric plates of the world

Over the billions of years during which the movement of the plates was carried out, they repeatedly merged and separated. The strength and vigor of the movement of continents is greatly influenced by the internal temperature of the Earth. With its increase, the speed of movement of the plates increases.



How many plates and how are lithospheric plates located on the world map today? Their boundaries are very conditional. Now there are 8 of the most important slabs. They cover 90% of the entire planet:

• Australian;
• Antarctic;
• African;
• Eurasian;
• Hindustan;
• Pacific;
• North American;
• South American.

Scientists constantly inspect and analyze the ocean floor, and investigate faults. They open new slabs and correct the lines of the old ones.

The largest lithospheric plate

What is the largest lithospheric plate? The most impressive is the Pacific plate, the crust of which is of the oceanic type. Its area is 10,300,000 km ². The size of this plate, like the size of the Pacific Ocean, is gradually decreasing.

In the south, it borders the Antarctic Plate. From the north side it creates the Aleutian Trench, and from the west - the Mariana Trench.