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

When moving the slabs apart opposite side In the oceanic reefs located in the middle of the valley, cracks appear that are filled with young basalts rising from the Earth's mantle. Oceanic plates sometimes end up under continental plates, or slide relative to each other along vertical plane faults. The spreading and creeping of the slabs is compensated by the birth of a new oceanic crust cracks in places.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Questions for self-control

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

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

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

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

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

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

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

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

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

10. Where was the Tethys Sea?

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

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

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

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

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

16. Where are ocean trenches located?

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

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

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

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

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

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

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

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

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

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

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

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

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

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Topic 21. Environment and health

Plate tectonics

Definition 1

A tectonic plate is a moving part of the lithosphere that moves on the asthenosphere as a relatively rigid block.

Note 1

Plate tectonics is the science that studies the structure and dynamics of the earth's surface. It has been established that the upper dynamic zone of the Earth is fragmented into plates moving along the asthenosphere. Plate tectonics describes the direction in which lithospheric plates move and how they interact.

The entire lithosphere is divided into larger and smaller plates. Tectonic, volcanic and seismic activity occurs along the edges of plates, leading to the formation of large mountain basins. Tectonic movements can change the topography of the planet. At the point of their connection, mountains and hills are formed, at the points of divergence, depressions and cracks in the ground are formed.

Currently, the movement of tectonic plates continues.

Movement of tectonic plates

Lithospheric plates move relative to each other at an average speed of 2.5 cm per year. As plates move, they interact with each other, especially along their boundaries, causing significant deformations in the earth's crust.

As a result of the interaction of tectonic plates with each other, massive mountain ranges and associated fault systems were formed (for example, the Himalayas, Pyrenees, Alps, Urals, Atlas, Appalachians, Apennines, Andes, San Andreas fault system, etc.).

Friction between plates causes most of the planet's earthquakes, volcanic activity and the formation of ocean pits.

Tectonic plates contain two types of lithosphere: continental crust and oceanic crust.

A tectonic plate can be of three types:

• continental plate,
• oceanic plate,
• mixed slab.

Theories of tectonic plate movement

In the study of the movement of tectonic plates, special merit belongs to A. Wegener, who suggested that Africa and the eastern part of South America were previously a single continent. However, after a fault that occurred many millions of years ago, parts of the earth’s crust began to shift.

According to Wegener's hypothesis, tectonic platforms with different masses and a rigid structure were located on a plastic asthenosphere. They were in an unstable state and moved all the time, as a result of which they collided, overlapped each other, and zones of moving apart plates and joints were formed. In places of collisions, areas with increased tectonic activity were formed, mountains were formed, volcanoes erupted and earthquakes occurred. The displacement occurred at a rate of up to 18 cm per year. Magma penetrated into the faults from the deep layers of the lithosphere.

Some researchers believe that the magma that came to the surface gradually cooled and formed a new bottom structure. The unused earth's crust, under the influence of plate drift, sank into the depths and again turned into magma.

Wegener's research affected the processes of volcanism, the study of stretching of the surface of the ocean floor, as well as the viscous-liquid internal structure of the earth. The works of A. Wegener became the foundation for the development of the theory of lithospheric plate tectonics.

Schmelling's research proved the existence of convective movement within the mantle leading to the movement of lithospheric plates. The scientist believed that the main reason for the movement of tectonic plates is thermal convection in the planet’s mantle, during which the lower layers of the earth’s crust heat up and rise, and the upper layers cool and gradually sink.

The main position in the theory of plate tectonics is occupied by the concept of geodynamic setting, a characteristic structure with a certain relationship of tectonic plates. In the same geodynamic setting, the same type of magmatic, tectonic, geochemical and seismic processes are observed.

The theory of plate tectonics does not fully explain the relationship between plate movements and processes occurring deep within the planet. A theory is needed that could describe the internal structure of the earth itself, the processes occurring in its depths.

Positions of modern plate tectonics:

• the upper part of the earth's crust includes the lithosphere, which has a fragile structure, and the asthenosphere, which has a plastic structure;
• the main reason for plate movement is convection in the asthenosphere;
• the modern lithosphere consists of eight large tectonic plates, about ten medium plates and many small ones;
• small tectonic plates are located between large ones;
• igneous, tectonic and seismic activity is concentrated at plate boundaries;
• The movement of tectonic plates obeys Euler's rotation theorem.

Types of tectonic plate movements

Highlight Various types movements of tectonic plates:

• divergent movement - two plates diverge, and an underwater mountain range or chasm in the ground forms between them;
• convergent movement - two plates converge and a thinner plate moves under a larger plate, resulting in the formation of mountain ranges;
• sliding movement - plates move in opposite directions.

Depending on the type of movement, divergent, convergent and sliding tectonic plates are distinguished.

Convergence leads to subduction (one plate sits on top of another) or collision (two plates crush to form mountain ranges).

Divergence leads to spreading (the separation of plates and the formation of ocean ridges) and rifting (the formation of a break in the continental crust).

The transform type of movement of tectonic plates involves their movement along a fault.

Figure 1. Types of tectonic plate movements. Author24 - online exchange of student works

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

"We're going somewhere"

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

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

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

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

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

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

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

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

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

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

"Ice floes" in a boiling ocean

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

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

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

As a result, convective cells appear in the mantle, like in a pan of boiling water - areas where hot substance rises upward at one end, and cooled substance sinks down at the other.

“There are about eight of these large cells in the mantle and many more small ones,” says the scientist. Mid-ocean ridges (such as those in the mid-Atlantic) are where mantle material rises to the surface and where new crust is born. In addition, there are subduction zones, places where a plate begins to “crawl” under the neighboring one and sinks down into the mantle. Subduction zones are, for example, the west coast of South America. The most powerful earthquakes occur here.

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

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

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

We agreed - we separated

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

For example, about 60 million years ago, the Indian plate broke away from what later became Africa and went north, and about 45 million years ago it met the Eurasian plate, and the Himalayas grew at the site of the collision - the most high mountains on the ground.

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

He explains that continents act as a thermal insulator, the mantle underneath them begins to heat up, updrafts arise and therefore supercontinents break up again after some time.

America will “take away” Chukotka

Large lithospheric plates are depicted in textbooks; anyone can name them: Antarctic plate, Eurasian, North American, South American, Indian, Australian, Pacific. But at the boundaries between plates, real chaos arises from many microplates.

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

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

Speedometer for the lithosphere

Wegener's theory was revived not least because scientists had the opportunity to high accuracy measure the displacement of continents. Nowadays satellite navigation systems are used for this, but there are other methods. All of them are needed to build a single international system coordinates - International Terrestrial Reference Frame (ITRF).

One of these methods is very long baseline radio interferometry (VLBI). Its essence lies in simultaneous observations using several radio telescopes at different points on the Earth. The difference in the time at which signals are received allows displacements to be determined with high accuracy. Two other ways to measure speed are laser ranging observations from satellites and Doppler measurements. All these observations, including using GPS, are carried out at hundreds of stations, all this data is brought together, and as a result we get a picture of continental drift.

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

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

Trubitsyn says this independence is partly explained by “personal history” different parts continents: the main parts of continents, platforms, may be fragments of ancient lithospheric plates that have “fused” with their neighbors. For example, the Ural ridge is one of the seams. The platforms are relatively rigid, but the parts around them can warp and move of their own accord.

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

The idea of ​​the movement of crustal blocks was first proposed in the theory of continental drift, proposed by Alfred Wegener in the 1920s. This theory was initially rejected. The revival of the idea of ​​​​movements in the solid shell of the Earth (“mobilism”) occurred in the 1960s, when, as a result of studies of the relief and geology of the ocean floor, data were obtained indicating the processes of expansion (spreading) of the oceanic crust and the subduction of some parts of the crust under others ( subduction). Combining these ideas with the old theory of continental drift gave rise to the modern theory of plate tectonics, which soon became a generally accepted concept in the earth sciences.

In the theory of plate tectonics, a key position is occupied by the concept of geodynamic setting - a characteristic geological structure with a certain ratio of plates. In the same geodynamic setting, the same type of tectonic, magmatic, seismic and geochemical processes occur.

Current state of plate tectonics

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

Top part solid earth It is divided into a brittle lithosphere and a plastic asthenosphere. Convection in the asthenosphere is the main cause of plate movement.

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

Lithospheric plates, to a first approximation, are described as solids, and their motion obeys Euler's rotation theorem.

There are three main types of relative plate movements

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

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

3) shear movements along transform geological faults.

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

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

There are two fundamentally different types of earth's crust - continental crust (more ancient) and oceanic crust (no older than 200 million years). Some lithospheric plates are composed exclusively of oceanic crust (an example is the largest Pacific plate), others consist of a block of continental crust welded into the oceanic crust.

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

1. Australian stove.

2. Antarctic plate.

3. African plate.

4. Eurasian plate.

5. Hindustan plate.

6. Pacific plate.

7. North American Plate.

8. South American Plate.

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