There are two types of lithosphere. The oceanic lithosphere has an oceanic crust about 6 km thick. It is mostly covered by the sea. The continental lithosphere is covered by a continental crust with a thickness of 35 to 70 km. For the most part, this bark protrudes above, forming land.
Plates
Rocks and minerals
moving plates
The plates of the earth's crust are constantly moving in different directions, albeit very slowly. The average speed of their movement is 5 cm per year. Your nails grow at about the same rate. Since all the plates are tightly adjacent to each other, the movement of any of them acts on the surrounding plates, causing them to gradually move. The plates can move in different ways, which can be seen at their boundaries, but the reasons that cause the movement of the plates are still unknown to scientists. Apparently, this process may have neither beginning nor end. Nevertheless, some theories argue that one type of plate movement can be, so to speak, "primary", and from it all other plates are already set in motion.
One type of plate movement is the "diving" of one plate under another. Some scholars believe that it is this type of movement that causes all other plate movements. At some boundaries, molten rock, breaking through to the surface between two plates, hardens along their edges, pushing these plates apart. This process can also cause the movement of all other plates. It is also believed that, in addition to the primary push, the movement of the plates is stimulated by giant heat flows circulating in the mantle (see the article "").
drifting continents
Scientists believe that since the formation of the primary earth's crust, the movement of plates has changed the position, shape and size of the continents and oceans. This process has been called tectonics slabs. Various proofs of this theory are given. For example, the outlines of continents such as South America and Africa look as if they once formed a single whole. Undoubted similarities were also found in the structure and age of the rocks that make up the ancient mountain ranges on both continents.
1. According to scientists, the land masses that now form South America and Africa were connected to each other more than 200 million years ago.
2. Apparently, the bottom of the Atlantic Ocean gradually expanded when new rock was formed at the boundaries of the plates.
3. Now South America and Africa are moving away from each other at a rate of about 3.5 cm per year due to plate movement.
tectonic fault lithospheric geomagnetic
Beginning with the Early Proterozoic, the rate of movement of lithospheric plates consistently decreased from 50 cm/yr to its current value of about 5 cm/yr.
The decrease in the average speed of plate movement will continue, up to the moment when, due to an increase in the power of oceanic plates and their friction against each other, it will not stop at all. But this will happen, apparently, only after 1-1.5 billion years.
To determine the velocities of the movement of lithospheric plates, data on the location of banded magnetic anomalies on the ocean floor are usually used. These anomalies, as has now been established, appear in the rift zones of the oceans due to the magnetization of the basalt erupted on them by the magnetic field that existed on Earth at the time of the basalt outpouring.
But, as you know, the geomagnetic field from time to time changed direction to the exact opposite. This led to the fact that basalts that erupted during different periods of geomagnetic field reversals turned out to be magnetized in opposite directions.
But due to the expansion of the ocean floor in the rift zones of the mid-ocean ridges, the older basalts always turn out to be moved to greater distances from these zones, and together with the ocean floor, the ancient magnetic field of the Earth “frozen” into the basalts also moves away from them.
Rice.
The expansion of the oceanic crust together with differently magnetized basalts usually develops strictly symmetrically on both sides of the rift fault. Therefore, the associated magnetic anomalies are also located symmetrically along both slopes of the mid-ocean ridges and the surrounding abyssal basins. Such anomalies can now be used to determine the age of the ocean floor and its expansion rate in rift zones. However, for this it is necessary to know the age of individual reversals of the Earth's magnetic field and compare these reversals with the magnetic anomalies observed on the ocean floor.
The age of magnetic reversals was determined from detailed paleomagnetic studies of well-dated sequences of basaltic sheets and sedimentary rocks of the continents and ocean floor basalts. As a result of comparing the geomagnetic time scale obtained in this way with magnetic anomalies on the ocean floor, it was possible to determine the age of the oceanic crust in most of the waters of the World Ocean. All oceanic plates that formed earlier than the Late Jurassic have already subsided into the mantle under modern or ancient zones of plate underthrust, and, consequently, no magnetic anomalies older than 150 million years have been preserved on the ocean floor.
The above conclusions of the theory make it possible to quantitatively calculate the motion parameters at the beginning of two adjacent plates, and then for the third, taken in tandem with one of the previous ones. In this way, one can gradually involve the main of the identified lithospheric plates in the calculation and determine the mutual displacements of all plates on the Earth's surface. Abroad, such calculations were performed by J. Minster and his colleagues, and in Russia by S.A. Ushakov and Yu.I. Galushkin. It turned out that the ocean floor is moving apart with maximum speed in the southeastern part of the Pacific Ocean (near Easter Island). In this place, up to 18 cm of new oceanic crust grows annually. In terms of geological scale, this is a lot, since only in 1 million years a strip of a young bottom up to 180 km wide is formed in this way, while approximately 360 km3 of basalt lavas are poured out at each kilometer of the rift zone in the same time! According to the same calculations, Australia is moving away from Antarctica at a rate of about 7 cm/year, and South America is moving away from Africa at a rate of about 4 cm/year. The pushing away of North America from Europe is slower - 2-2.3 cm/year. The Red Sea expands even more slowly - by 1.5 cm/year (accordingly, there is less basalt outflow here - only 30 km3 per linear kilometer of the Red Sea Rift in 1 million years). On the other hand, the rate of "collision" between India and Asia reaches 5 cm/year, which explains the intense neotectonic deformations that are developing before our eyes and the growth of the mountain systems of the Hindu Kush, the Pamirs and the Himalayas. These deformations create a high level of seismic activity in the entire region (the tectonic impact of the collision of India with Asia affects far beyond the plate collision zone itself, extending all the way to Lake Baikal and the regions of the Baikal-Amur Mainline). The deformations of the Greater and Lesser Caucasus are caused by the pressure of the Arabian Plate on this region of Eurasia, however, the rate of convergence of the plates here is much less - only 1.5-2 cm / year. Therefore, the seismic activity of the region is also less here.
Modern geodetic methods, including space geodesy, high-precision laser measurements and other methods, have established the speed of movement of lithospheric plates and it has been proved that oceanic plates move faster than those in the structure of which the continent is included, and the thicker the continental lithosphere, the lower the speed of the plate movement.
Last week, the public was stirred 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 territorially located? What makes them move and where? Which territories still want to "join" Russia, and which ones threaten to "escape" to the USA?
"And we're going somewhere"
Yes, we are all going somewhere. While you are reading these lines, you are slowly moving: if you are in Eurasia, then to the 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 takes you to the northwest by 10 centimeters a year.
If you sit back in your chair and wait about 250 million years, you will find yourself on a new supercontinent that will unite all the earth's land - on the mainland 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, together with Russia, Ukraine, Siberia and the European Union, is part of the Eurasian lithospheric plate, and all of them have 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) - 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 completely different smaller "tiles". If very conditionally, then the earth's crust is composed of continental lithospheric plates. They themselves consist of ancient and very stable platforms.and mountain building zones (ancient and modern). And already the platforms themselves are divided into slabs - smaller sections of the crust, consisting of two "layers" - the foundation and the cover, and shields - "single-layer" outcrops.
The cover of these non-lithospheric 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 Crimea) or igneous rocks (thrown from volcanoes and solidified lava masses). 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 of high temperatures and enormous pressure, various changes occur with 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 (IPA RAS), Doctor of Physical and Mathematical Sciences Alexander Ipatov, said 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 plates of the earth's crust. 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 Peter 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 around for several decades, and it itself began back in the Cenozoic era.
Wegener's theory was received with skepticism - mainly because he could not offer a satisfactory mechanism to explain the movement of the continents. He believed that the continents move, breaking through the earth's crust, like icebreakers through ice, due to the centrifugal force from the rotation of the Earth and tidal forces. His opponents said that the continents-"icebreakers" in the process of movement would change their appearance beyond recognition, and centrifugal and tidal forces are 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 fast (Wegener estimated their speed at 250 centimeters per year), it would stop the rotation of the Earth 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 new crust was continuously forming in the zone of these ridges, due to which the continents were "moving apart" . Geophysicists have studied the magnetization of rocks along the mid-ocean ridges and found "bands" with multidirectional magnetization.
It turned out that the new oceanic crust "records" the state of the Earth's magnetic field at the time of formation, and scientists have received an excellent "ruler" to measure the speed of this conveyor. So, in the 1960s, the theory of continental drift returned for the second time, for good. And this time, scientists were able to understand what moves the continents.
Ice floes in the 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 also frozen into some ice floes. Ice is lithospheric plates, rafts are continents, and they float in the substance of the mantle," explains Corresponding Member of the Russian Academy of Sciences Valery Trubitsyn, chief researcher at the Institute of Physics of the Earth named after O.Yu. Schmidt.
Back in the 1960s, he put forward the 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 in the center of the Earth - the mantle - consists of silicate rocks. The temperature in it varies from 500 degrees Celsius in the upper part to 4000 degrees Celsius at the border of the core. Therefore, from a depth of 100 kilometers, where the temperature is already more than 1300 degrees, the mantle substance behaves like a very thick resin and flows at a speed of 5-10 centimeters per year, says Trubitsyn.
As a result, in the mantle, as in a pot of boiling water, convective cells appear - areas where hot matter rises from one edge, and cooled down from the other.
"There are about eight of these large cells in the mantle and many more small ones," the scientist says. Mid-ocean ridges (for example, in the center of the Atlantic) are the place where the material of the mantle rises to the surface and where new crust is born. In addition, there are subduction zones, places where a plate begins to "creep" under the neighboring one and sinks down into the mantle. Subduction zones are, for example, the western coast of South America. This is where the most powerful earthquakes occur.
“In this way, the plates take part in the convective circulation of the mantle substance, which temporarily becomes solid while on the surface. Plunging into the mantle, the plate substance heats up and softens again,” explains the geophysicist.
In addition, individual jets of matter rise to the surface from the mantle - plumes, and these jets have every chance to destroy humanity. After all, it is mantle plumes that are the cause of 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 exits, a giant volcano arises. There are many such volcanoes, they are in Hawaii, in Iceland, a similar example is the Yellowstone caldera. Supervolcanoes can generate eruptions thousands of times more powerful than most ordinary volcanoes like Vesuvius or Etna.
"250 million years ago, such a volcano on the territory of modern Siberia killed almost all life, only the ancestors of dinosaurs survived," says Trubitsyn.
Agreed - dispersed
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 sink 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 with the Eurasian plate, the Himalayas, the highest mountains on Earth, grew at the point of collision.
The movement of the plates will sooner or later bring all the continents into one, as leaves converge into one island in a whirlpool. In the history of the Earth, the continents have united and broken up approximately four to six times. The last supercontinent Pangea existed 250 million years ago, before it was the supercontinent Rodinia, 900 million years ago, before it - two more. "And already, it seems, the unification of the new continent will soon begin," the scientist clarifies.
He explains that the continents act as a thermal insulator, the mantle beneath them begins to heat up, updrafts occur, and therefore the supercontinents break apart again after a while.
America will "take away" Chukotka
Large lithospheric plates are drawn in textbooks, anyone can name them: Antarctic plate, Eurasian, North American, South American, Indian, Australian, Pacific. But at the boundaries between the plates there is a real chaos of 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 to the west, along the Chersky Ridge. Chukotka thus turns out to be part of the North American Plate. At the same time, 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 rests on 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", and in this case, a giant circular seam may appear on Earth, which will pass through the Atlantic, the 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 has been resurrected, not least because scientists have the ability to accurately measure the displacement of continents. Now satellite navigation systems are used for this, but there are other methods. All of them are needed to build a single international coordinate system - the International Terrestrial Reference Frame (ITRF).
One of these methods is very long baseline radio interferometry (VLBI). Its essence lies in simultaneous observations with the help of several radio telescopes in different parts of the Earth. The difference in signal acquisition time makes it possible to determine offsets with high accuracy. Two other ways to measure speed are laser ranging observations using satellites and Doppler measurements. All these observations, including with the help of GPS, are carried out at hundreds of stations, all these data are brought together, and as a result, we get a picture of continental drift.
For example, the Crimean Simeiz, where a laser sounding station is located, as well as a satellite station for determining coordinates, "travels" to the northeast (in azimuth about 65 degrees) at a speed of about 26.8 millimeters per year. Zvenigorod, near Moscow, is moving about a millimeter a year faster (27.8 millimeters a year) and keeps its course to the 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 moving independently 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 that this independence is partly explained by the "personal history" of different parts of the continents: the main parts of the continents, the platforms, may be fragments of ancient lithospheric plates that "merged" with their neighbors. For example, the Ural Range is one of the seams. Platforms are relatively rigid, but parts around them can deform and move at will.
Solid planets in their development go through a period of heating, the main energy for which is provided by fragments of cosmic bodies falling on the surface of the planet ( cm. Hypothesis of a gas and dust cloud). When these objects collide with the planet, almost all the kinetic energy of the falling object is instantly converted into thermal energy, since its speed of movement, which is several tens of kilometers per second, drops sharply to zero at the moment of impact. To all the inner planets of the solar system - Mercury, Venus, Earth, Mars - this heat was enough, if not to completely or partially melt, then at least to soften and become plastic and fluid. During this period, substances with the highest density moved to the center of the planets, forming core, and the least dense, on the contrary, rose to the surface, forming the earth's crust. About the same way, salad dressing is stratified if it is left on the table for a long time. This process, called magma differentiation explains the internal structure of the earth.
For the smallest inner planets, Mercury and Mars (as well as the Moon), this heat eventually escaped to the surface and was dissipated into space. The planets then solidified and (as in the case of Mercury) exhibited low geological activity over the next few billion years. The history of the Earth was very different. Since the Earth is the largest of the inner planets, it also has the largest storage of heat. And the larger the planet, the smaller its ratio of surface area to volume and the less heat it loses. Consequently, the Earth cooled more slowly than the other inner planets. (The same can be said for Venus, which is slightly smaller than Earth.)
In addition, from the beginning of the formation of the Earth, the decay of radioactive elements occurred in it, which increased the supply of heat in its depths. Therefore, the Earth can be considered as a spherical furnace. Inside it, heat is continuously generated, transferred to the surface and radiated into space. Heat transfer causes a reciprocal movement robes - shell of the Earth, located between the core and the earth's crust at a depth of several tens to 2900 km ( cm. Heat exchange). Hot matter rises from the depths of the mantle, cools, and then sinks again, being replaced by new hot matter. This is a classic example of a convective cell.
We can say that the rock of the mantle seethes in the same way as water in a kettle: in both cases, heat is transferred in the process of convection. Some geologists believe that it takes hundreds of millions of years for mantle rocks to complete a complete convective cycle, a very long time by human standards. It is known that many substances slowly deform over time, although during the course of a human life they look absolutely solid and motionless. For example, in medieval cathedrals, antique window panes are thicker at the bottom than at the top because glass has flowed down through the ages under the force of gravity. If in a few centuries this happens with solid glass, then it is not difficult to imagine that the same thing can happen with solid rocks in hundreds million years.
On top of the convective cells of the earth's mantle, the rocks that make up the solid surface of the Earth float - the so-called tectonic plates. These slabs are composed of basalt, the most common erupted igneous rock. The thickness of these plates is approximately 10-120 km, and they move along the surface of the partially molten mantle. Continents, composed of relatively light rocks such as granite, form the uppermost layer of the plates. In most cases, the thickness of the plates under the continents is greater than under the oceans. Over time, processes occurring inside the Earth move the plates, causing them to collide and crack, up to the formation of new plates or the disappearance of old ones. It is thanks to this slow but continuous movement of plates that the surface of our planet is constantly in dynamics, constantly changing.
It is important to understand that the concepts of "slab" and "mainland" are not the same thing. For example, the North American tectonic plate extends from the mid-Atlantic Ocean to the western coast of the North American continent. Part of the plate is covered with water, part - with land. The Anatolian Plate, on which Turkey and the Middle East are located, is completely covered by land, while the Pacific Plate is located completely under the Pacific Ocean. That is, the boundaries of the plates and the coastlines of the continents do not necessarily coincide. By the way, the word "tectonics" comes from the Greek word tekton("builder") - the same root is in the word "architect" - and refers to the process of building or assembling.
Plate tectonics is most noticeable where plates touch each other. It is customary to distinguish three types of boundaries between plates.
Divergent boundaries
In the middle of the Atlantic Ocean, hot magma rises to the surface, formed in the depths of the mantle. It breaks through the surface and spreads, gradually filling the crack between the sliding plates. Because of this, the seabed is expanding and Europe and North America are moving apart at a rate of several centimeters per year. (This movement could be measured using radio telescopes located on two continents, comparing the time of arrival of the radio signal from distant quasars.)
If the divergent boundary is located under the ocean, a mid-ocean ridge, a mountain range formed by the accumulation of matter at the point where it comes to the surface, arises as a result of the divergence of plates. The Mid-Atlantic Ridge, stretching from Iceland to the Falklands, is the longest mountain range on earth. If the divergent boundary is located under the mainland, it literally tears it apart. An example of such a process taking place today is the Great Rift Valley, which extends from Jordan south into East Africa.
convergent borders
If a new crust forms at the divergent boundaries, then somewhere else the crust must be destroyed, otherwise the Earth would increase in size. When two plates collide, one of them moves under the other (this phenomenon is called subduction, or pushing). In this case, the plate, which is below, is immersed in the mantle. What happens on the surface above the subduction zone depends on the location of the plate boundaries: under the mainland, at the edge of the mainland, or under the ocean.
If the subduction zone is located under the oceanic crust, then as a result of subduction, a deep mid-ocean trench (trough) is formed. An example of this is the deepest place in the oceans - the Mariana Trench near the Philippines. The material of the lower plate gets deep into the magma and melts there, and then can rise again to the surface, forming a ridge of volcanoes - such as, for example, a chain of volcanoes in the eastern Caribbean Sea and on the western coast of the United States.
If both plates at the convergent boundary are under the continents, the result will be very different. The continental crust is composed of light materials, and both plates actually float above the subduction zone. As one plate slides under the other, the two continents collide and their boundaries crumple, forming a continental mountain range. This is how the Himalayas were formed when the Indian plate collided with the Eurasian one about 50 million years ago. As a result of the same process, the Alps were formed when Italy joined Europe. And the Ural Mountains, an old mountain range, can be called a "weld" formed when the European and Asian massifs united.
If the continent rests on only one of the plates, it will develop folds and folds as it creeps into the subduction zone. An example of this is the Andes on the West Coast of South America. They were formed after the South American Plate floated on the Nazca Plate submerged under it in the Pacific Ocean.
Transform borders
Sometimes it happens that two plates do not diverge and do not move under each other, but simply rub along the edges. The most famous example of such a boundary is the San Andreas Fault in California, where the Pacific and North American plates move side by side. In the case of a transform boundary, the plates collide for a while and then move apart, releasing a lot of energy and causing large earthquakes.
In conclusion, I would like to emphasize that although plate tectonics includes the concept of continental movement, it is not the same as the continental drift hypothesis proposed at the beginning of the 20th century. This hypothesis was rejected (rightly, according to the author) by geologists due to some experimental and theoretical inconsistencies. And the fact that our modern theory includes one aspect of the continental drift hypothesis - the movement of the continents - does not mean that scientists rejected plate tectonics at the beginning of the last century only to accept it later. The theory that is now accepted is fundamentally different from the previous one.
In the process of formation and then development of geology as a science, many hypotheses were proposed, each of which, from one position or another, considered and explained either individual problems or a complex of problems related to the development of the earth's crust or the Earth as a whole. These hypotheses are called geotectonic. Some of them, due to insufficient persuasiveness, quickly lost their significance in science, while others turned out to be more durable, again until new facts and ideas accumulated, which formed the basis of new hypotheses that are more appropriate for a given stage in the development of science. Despite the great successes achieved in the study of the structure and development of the earth's crust, none of the modern hypotheses and theories (even recognized ones) is able to adequately and fully explain all the conditions for the formation of the earth's crust.
The first scientific hypothesis, the uplift hypothesis, was formulated in the first half of the 19th century. based on the ideas of the Plutonists about the role of the internal forces of the Earth, which played a positive role in the fight against the erroneous ideas of the Neptunists. In the 50s. 19th century it was replaced by the more substantiated at that time contraction hypothesis (compressed), set forth by the French scientist Elie de Beaumont. The contraction hypothesis was based on Laplace's cosmogonic hypothesis, which recognized, as is known, the primary hot state of the Earth and its subsequent gradual cooling.
The essence of the contraction hypothesis is that the cooling of the Earth causes its compression, followed by a decrease in its volume. As a result, the earth's crust, which had hardened before the inner zones of the planet, is forced to wrinkle, which is why folded mountains are formed.
In the second half of the XIX century. American scientists J. Hall and J. Dan formulated the doctrine of geosynclines - special mobile zones of the earth's crust over time turning into folded mountain structures. This teaching significantly strengthened the position of the contraction hypothesis. However, by the beginning of the 20th century. in connection with the receipt of new data on the Earth, this hypothesis began to lose its significance, since it turned out to be unable to explain the periodicity of mountain-building movements and magmatism processes, it ignored stretching processes, etc. In addition, ideas arose in science about the formation of a planet from cold particles , which deprived the hypothesis of its main support.
At the same time, the doctrine of geosynclines continued to be supplemented and developed. In this regard, a great contribution was also made by the Soviet scientists A. D. Arkhangelsky, N. S. Shatsky, M. V. Muratov and others. and especially since the beginning of the 20th century. the doctrine of relatively stable continental areas - platforms began to develop; of the domestic scientists who developed this doctrine, we must first of all name A. P. Karpinsky, A. D. Arkhangelsky, N. S. Shatsky, A. A. Bogdanov, A. L. Yanshin.
The doctrine of geosynclines and platforms has become firmly established in geological science and retains its significance to the present day. However, it still lacks a solid theoretical basis.
The desire to supplement and eliminate shortcomings in the contraction hypothesis or, conversely, to completely replace it, led to the appearance during the first half of the 20th century. a number of new geotectonic hypotheses. Let's note some of them.
pulsation hypothesis. It is based on the idea of the alternation of the processes of compression and expansion of the Earth - processes that are very characteristic of the Universe as a whole. M. A. Usov and V. A. Obruchev, who developed this hypothesis, associated folding, overthrusts, and the intrusion of acid intrusions with the compression phases, and the appearance of cracks in the earth's crust and the outpouring of mainly basic lavas along them with expansion phases.
Hypothesis of differentiation of the subcrustal substance and migration of radioelements. Under the influence of gravitational differentiation and radiogenic heating, periodic melting of liquid components from the atmosphere occurs, which entails ruptures of the earth's crust, volcanism, mountain building and other phenomena. One of the authors of this hypothesis is the famous Soviet scientist VV Belousov.
Continental drift hypothesis. It was presented in 1912 by the German scientist A. Wegener and is fundamentally different from all other hypotheses. Based on the principles of mobilism - recognition of significant horizontal movements of vast continental masses. Most of the hypotheses proceeded from the principles of fixism - the recognition of a stable, fixed position of individual parts of the earth's crust relative to the underlying mantle (such are the hypotheses of contraction, differentiation of subcrustal matter and migration of radioelements, etc.).
According to the ideas of A. Wegener, the granitic layer of the earth's crust "floats" on the basalt layer. Under the influence of the rotation of the Earth, it turned out to be collected in a single continent of Pangea. At the end of the Paleozoic era (about 200-300 million years ago), Pangea was divided into separate blocks and their drift began until they occupied their present position. Under the influence of the drift of the blocks of North and South America to the west, the Atlantic Ocean arose, and the resistance experienced by these continents as they moved along the basalt layer contributed to the emergence of such mountains as the Andes and the Cordillera. For the same reasons, Australia and Antarctica moved apart and moved south, etc.
A. Wegener saw confirmation of his hypothesis in the similarity of the contours and geological structure of the coasts on both sides of the Atlantic Ocean, in the similarity of fossil organisms of continents far apart from each other, in the different structure of the earth's crust within the oceans and continents.
The appearance of A. Wegener's hypothesis aroused great interest, but it died out relatively quickly, since it was not able to explain many phenomena, and most importantly, the possibility of the movement of continents along the basalt layer. Nevertheless, as we will see below, mobilist views, but on a completely new basis, were revived and received wide recognition in the second half of the 20th century.
rotational hypothesis. It occupies a separate place among geotectonic hypotheses, as it sees the manifestation of tectonic processes on the Earth under the influence of extraterrestrial causes, namely the attraction of the Moon and the Sun, causing solid tides in the earth's crust and mantle, slowing down the rotation of the Earth and changing its shape. The consequence of this is not only vertical, but also horizontal displacements of individual blocks of the earth's crust. The hypothesis is not widely accepted, since the vast majority of scientists believe that tectogenesis is the result of the manifestation of the internal forces of the Earth. At the same time, the influence of extraterrestrial causes on the formation of the earth's crust, obviously, must also be taken into account.
The theory of new global tectonics, or lithospheric plate tectonics. Since the beginning of the second half of the XX century. extensive geological and geophysical studies of the bottom of the oceans were launched. They resulted in the emergence of completely new ideas about the development of the oceans, such as, for example, the separation of lithospheric plates and the formation of a young oceanic crust in rift valleys, the formation of continental crust in zones of subduction of lithospheric plates, etc. These ideas led to the revival in geological science of mobilist ideas and to the emergence of the theory of a new global tectonics, or lithospheric plate tectonics.
The new theory is based on the idea that the entire lithosphere (i.e., the earth's crust together with the upper mantle layer) is divided by narrow tectonically active zones into separate rigid plates moving along the asthenosphere (plastic layer in the upper mantle). Active tectonic zones characterized by high seismicity and volcanism are rift zones of mid-ocean ridges, systems of island arcs and deep ocean trenches, and rift valleys on the continents. In the rift zones of the mid-ocean ridges, plates move apart and a new oceanic crust is formed, and in deep-sea trenches, some plates are pushed under others and the continental crust is formed. A collision of plates is also possible - the formation of the Himalayan fold zone is considered to be the result of such a phenomenon.
There are seven large lithospheric plates and a slightly larger number of small ones. These plates have received the following names: 1) Pacific, 2) North American, 3) South American, 4) Eurasian, 5) African, 6) Indo-Australian and 7) Antarctic. Each of them includes one or more continents or parts of them and oceanic crust, with the exception of the Pacific Plate, which consists almost entirely of oceanic crust. Simultaneously with the horizontal displacements of the plates, their rotations also occurred.
The movement of lithospheric plates, according to this theory, is caused by convective currents of matter in the mantle, generated by heat released during the radioactive decay of elements and gravitational differentiation of matter in the bowels of the Earth. However, the argumentation of thermal convection in the mantle, according to many scientists, is insufficient. This also applies to the possibility of submersion of oceanic plates into the mantle to a great depth and a number of other provisions. The surface expression of the convective motion is the rift zones of the mid-ocean ridges, where the relatively warmer mantle rises to the surface and undergoes melting. It pours out in the form of basaltic lavas and freezes. Further, basaltic magma again intrudes into these frozen rocks and pushes older basalts in both directions. This happens many times. At the same time, the ocean floor is growing, growing. Such a process is called spreading. The growth rate of the ocean floor ranges from a few mm to 18 cm per year.
Other boundaries between lithospheric plates are convergent, that is, the earth's crust in these areas is absorbed. Such zones were called subduction zones. They are located along the edges of the Pacific Ocean and in the east of the Indian. The heavy and cold oceanic lithosphere, approaching the thicker and lighter continental one, goes under it, as if diving. If two oceanic plates come into contact, then the older one sinks, since it is heavier and colder than the young plate.
The zones where subduction occurs are morphologically expressed by deep-water trenches, and the sinking oceanic cold and elastic lithosphere itself is well established from seismic tomography data. The angle of subsidence of oceanic plates is different, up to vertical, and the plates can be traced to the boundary of the upper and lower mantles at a depth of approximately 670 km.
When the oceanic plate begins to sharply bend when approaching the continental one, stresses arise in it, which, when discharged, provoke earthquakes. Earthquake hypocenters or foci clearly mark the friction boundary between two plates and form an inclined seismic focal zone that plunges under the continental lithosphere to depths of 700 km. These zones are called Benioff zones, in honor of the American seismologist who studied them.
The subsidence of the oceanic lithosphere leads to one more important consequence. When the lithosphere reaches a depth of 100 - 200 km in the area of high temperatures and pressures, fluids are released from it - special superheated mineral solutions that cause the melting of rocks of the continental lithosphere and the formation of magma chambers that feed the chains of volcanoes developed parallel to deep-sea trenches on active continental margins.
Thus, due to subduction, a strongly dissected topography, high seismicity, and vigorous volcanic activity are observed on the active continental margin.
In addition to the phenomenon of subduction, there is the so-called obduction, that is, the thrusting of the oceanic lithosphere onto the continental one, an example of which is the huge tectonic cover on the eastern margin of the Arabian Peninsula, composed of typical oceanic crust.
It should also mention the collision, or collisions, two continental plates, which, due to the relative lightness of the material that composes them, cannot sink under each other, but collide, forming a mountain-fold belt with a very complex internal structure.
The main provisions of lithospheric plate tectonics are as follows:
1.The first premise Plate tectonics is the division of the upper part of the solid Earth into two shells that differ significantly in rheological properties (viscosity) - a rigid and brittle lithosphere and a more plastic and mobile asthenosphere. As already mentioned, these two shells are distinguished from seismological or magnetotelluric data.
2.Second position Plate tectonics, to which it owes its name, lies in the fact that the lithosphere is naturally subdivided into a limited number of plates - currently seven large and the same number of small ones. The basis for their selection and drawing boundaries between them is the location of earthquake sources.
3.Third position Plate tectonics concerns the nature of their mutual movement. There are three types of such displacements and, accordingly, the boundaries between the plates: 1) divergent borders, along which the plates move apart - spreading; 2) convergent borders, on which there is a convergence of plates, usually expressed by the subduction of one plate under another; when an oceanic plate moves under a continental one, this process is called subduction, if the oceanic plate is moving towards the continental - obduction; if two continental plates collide, also usually with subduction of one under the other, - collision; 3)transform borders, along which there is a horizontal sliding of one plate relative to the other along the plane of the vertical transform fault.
In nature, the boundaries of the first two types predominate.
At divergent boundaries, in spreading zones, there is a continuous birth of new oceanic crust; Therefore, these boundaries are also called constructive. This crust is moved by the asthenospheric current towards subduction zones, where it is absorbed at depth; this gives grounds to call such boundaries destructive.
Fourth position plate tectonics lies in the fact that during their movements, the plates obey the laws of spherical geometry, or rather Euler's theorem, according to which any movement of two conjugate points on a sphere is performed along a circle drawn relative to an axis passing through the center of the Earth.
5.Fifth provision Plate tectonics states that the volume of oceanic crust absorbed in subduction zones is equal to the volume of crust originating in spreading zones.
6.sixth position plate tectonics sees the main cause of plate movement in the mantle convection. This convection in the classical 1968 model is purely thermal and general mantle, and the way it affects lithospheric plates is that these plates, which are in viscous adhesion to the asthenosphere, are carried along by the latter and move in the manner of a conveyor belt from spreading axes to subduction zones. In general, the scheme of mantle convection, leading to a plate tectonic model of lithosphere movements, consists in the fact that ascending branches of convective cells are located under the mid-ocean ridges, descending branches are located under subduction zones, and horizontal segments of these cells.
The theory of new global tectonics, or lithospheric plate tectonics, is especially popular abroad: it is also recognized by many Soviet scientists, who do not confine themselves to general recognition, but work hard to clarify its main provisions, supplementing, deepening and developing them. The Soviet mobilist scientist A. V. Peivs, developing this theory, however, came to the conclusion that giant rigid lithospheric plates do not exist at all, and the lithosphere, due to the fact that it is penetrated by horizontal, inclined and vertical mobile zones, consists of separate plates (“lithoplasts”) moving differentially. This is an essentially new look at one of the main, but controversial provisions of this theory.
It should be noted that a certain part of mobilist scientists (both abroad and domestic) in their views show an extremely negative attitude towards the classical doctrine of geosynclines in fact, they completely reject it, ignoring the fact that many of the provisions of this doctrine are based on reliable facts and observations established and carried out during geological studies of the continents.
It is obvious that the most correct way to create a truly global theory of the Earth is not to contrast, but to reveal the unity and relationship between everything positive, reflected in the classical theory of geosynclines, and everything new that is revealed in the theory of new global tectonics.
