Divergence of lithospheric plates. Tectonic plates and their movement

Then surely you would like to know what are lithospheric plates.

So, lithospheric plates are huge blocks into which the solid surface layer of the earth is divided. Given the fact that the rock beneath them is molten, the plates move slowly, at a speed of 1 to 10 centimeters per year.

Today there are 13 largest lithospheric plates, which cover 90% earth's surface.

Largest lithospheric plates:

• Australian plate- 47,000,000 km²
• Antarctic plate- 60,900,000 km²
• Arabian subcontinent- 5,000,000 km²
• African plate- 61,300,000 km²
• Eurasian plate- 67,800,000 km²
• Hindustan plate- 11,900,000 km²
• Coconut Plate - 2,900,000 km²
• Nazca Plate - 15,600,000 km²
• Pacific Plate- 103,300,000 km²
• North American Plate- 75,900,000 km²
• Somali plate- 16,700,000 km²
• South American Plate- 43,600,000 km²
• Philippine plate- 5,500,000 km²

Here it must be said that there is a continental and oceanic crust. Some plates consist exclusively of one type of crust (for example, the Pacific plate), and some mixed types, when the plate begins in the ocean and smoothly passes to the continent. The thickness of these layers is 70-100 kilometers.

Lithospheric plates float on the surface of a partially molten layer of the earth - the mantle. When the plates move apart, liquid rock called magma fills the cracks between them. When magma solidifies, it forms new crystalline rocks. We’ll talk more about magma in the article on volcanoes.

Map of lithospheric plates

At the beginning of the 20th century, American F.B. Taylor and the German Alfred Wegener simultaneously came to the conclusion that the location of the continents was slowly changing. By the way, this is, to a large extent, what it is. But scientists were unable to explain how this happens until the 60s of the twentieth century, until the doctrine of geological processes on the seabed.

It was fossils that played the main role here. Fossilized remains of animals that clearly could not swim across the ocean were found on different continents. This led to the assumption that once all the continents were connected and animals calmly moved between them.

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The lithosphere is the rocky shell of the Earth. From the Greek “lithos” - stone and “sphere” - ball

The lithosphere is the outer solid shell of the Earth, which includes the entire Earth's crust with part of the Earth's upper mantle and consists of sedimentary, igneous and metamorphic rocks. The lower boundary of the lithosphere is unclear and is determined by a sharp decrease in the viscosity of rocks, a change in the speed of propagation of seismic waves and an increase in the electrical conductivity of rocks. The thickness of the lithosphere on continents and under the oceans varies and averages 25 - 200 and 5 - 100 km, respectively.

Let's consider in general view geological structure of the Earth. The third planet beyond the distance from the Sun, Earth, has a radius of 6370 km, an average density of 5.5 g/cm3 and consists of three shells - bark, mantle and and. The mantle and core are divided into internal and external parts.

The Earth's crust is the thin upper shell of the Earth, which is 40-80 km thick on the continents, 5-10 km under the oceans and makes up only about 1% of the Earth's mass. Eight elements - oxygen, silicon, hydrogen, aluminum, iron, magnesium, calcium, sodium - form 99.5% of the earth's crust.

According to scientific research, scientists were able to establish that the lithosphere consists of:

• Oxygen – 49%;
• Silicon – 26%;
• Aluminum – 7%;
• Iron – 5%;
• Calcium – 4%
• The lithosphere contains many minerals, the most common being spar and quartz.

On continents, the crust has three layers: sedimentary rocks cover granite rocks, and granite rocks overlie basaltic rocks. Under the oceans the crust is “oceanic”, of a two-layer type; sedimentary rocks simply lie on basalts, there is no granite layer. There is also a transitional type of the earth's crust (island-arc zones on the margins of the oceans and some areas on continents, for example the Black Sea).

Greatest thickness the earth's crust is in mountainous regions(under the Himalayas - over 75 km), the average - in the areas of the platforms (under the West Siberian Lowland - 35-40, within the borders of the Russian Platform - 30-35), and the smallest - in central regions oceans (5-7 km). The predominant part of the earth's surface is the plains of the continents and ocean floor.

The continents are surrounded by a shelf - a shallow strip with a depth of up to 200 g and an average width of about 80 km, which, after a sharp steep bend of the bottom, turns into a continental slope (the slope varies from 15-17 to 20-30°). The slopes gradually level out and turn into abyssal plains (depths 3.7-6.0 km). Greatest depths(9-11 km) have oceanic trenches, the vast majority of which are located on the northern and western edges of the Pacific Ocean.

The main part of the lithosphere consists of igneous igneous rocks (95%), among which granites and granitoids predominate on the continents, and basalts in the oceans.

Blocks of the lithosphere - lithospheric plates - move along a relatively plastic asthenosphere. The section of geology on plate tectonics is devoted to the study and description of these movements.

To designate the outer shell of the lithosphere, the now obsolete term sial, derived from the name of the main elements, was used rocks Si (lat. Silicium - silicon) and Al (lat. Aluminum - aluminum).

Lithospheric plates

It is worth noting that the largest tectonic plates are very clearly visible on the map and they are:

• Pacific– the most large stove a planet along the boundaries of which constant collisions of tectonic plates occur and faults form - this is the reason for its constant decrease;
• Eurasian– covers almost the entire territory of Eurasia (except for Hindustan and the Arabian Peninsula) and contains the largest part of the continental crust;
• Indo-Australian– it includes the Australian continent and the Indian subcontinent. Due to constant collisions with the Eurasian plate, it is in the process of breaking;
• South American– consists of the South American continent and part of the Atlantic Ocean;
• North American– consists of the North American continent, part of northeastern Siberia, the northwestern part of the Atlantic and half of the North Arctic Oceans;
• African– consists of the African continent and the oceanic crust of the Atlantic and Indian oceans. Interestingly, the plates adjacent to it move in the opposite direction from it, so the largest fault on our planet is located here;
• Antarctic plate– consists of the continent of Antarctica and the nearby oceanic crust. Due to the fact that the plate is surrounded by mid-ocean ridges, the remaining continents are constantly moving away from it.

Movement of tectonic plates in the lithosphere

Lithospheric plates, connecting and separating, constantly change their outlines. This allows scientists to put forward the theory that about 200 million years ago the lithosphere had only Pangea - a single continent, which subsequently split into parts, which began to gradually move away from each other at a very low speed (on average about seven centimeters per year ).

This is interesting! There is an assumption that, thanks to the movement of the lithosphere, in 250 million years a new continent due to the unification of moving continents.

When the oceanic and continental plates collide, the edge of the oceanic crust is subducted under the continental crust, while on the other side of the oceanic plate its boundary diverges from the adjacent plate. The boundary along which the movement of lithospheres occurs is called the subduction zone, where the upper and subducting edges of the plate are distinguished. It is interesting that the plate, plunging into the mantle, begins to melt when the upper part of the earth’s crust is compressed, as a result of which mountains are formed, and if magma also erupts, then volcanoes.

In places where tectonic plates come into contact with each other, zones of maximum volcanic and seismic activity are located: during the movement and collision of the lithosphere, the earth's crust is destroyed, and when they diverge, faults and depressions are formed (the lithosphere and the Earth's topography are connected to each other). This is the reason that the Earth's largest landforms are located along the edges of tectonic plates - mountain ranges With active volcanoes and deep-sea trenches.

Lithosphere problems

The intensive development of industry has led to the fact that man and the lithosphere in lately began to get along extremely poorly with each other: the pollution of the lithosphere is acquiring catastrophic proportions. This happened due to the increase in industrial waste combined with household waste and used in agriculture fertilizers and pesticides, which negatively affects chemical composition soil and living organisms. Scientists have calculated that about one ton of garbage is generated per person per year, including 50 kg of hard-to-degrade waste.

Today, lithosphere pollution has become actual problem, since nature is not able to cope with it on its own: the self-cleaning of the earth’s crust occurs very slowly, and therefore harmful substances gradually accumulate and over time have a negative impact on the main culprit of the problem - the person.

Long time in geological science The prevailing hypothesis was that the position of the continents and oceans remained unchanged. It was generally accepted that both of them arose hundreds of millions of years ago and never changed their position. Only occasionally, when the height of the continents decreased significantly and the level of the World Ocean rose, did the sea advance on the lowlands and flood them.

Among geologists, the opinion has become established that the earth's crust experiences only slow vertical movement and thanks to this, land and underwater relief is created.

The vast majority of geologists agreed long ago with the idea that the “firmament of the earth” is in constant vertical movement, due to which the relief of the Earth is formed. Often these movements have large amplitude and speed and lead to major disasters, such as earthquakes. However, there are also very slow vertical movements with a variable sign that are not noticeable even by the most sensitive instruments. These are the so-called oscillatory movements. Only over a very long period of time it is discovered that mountain peaks grew by several centimeters, and the river valleys deepened.

At the end of the 19th - beginning of the 20th century. Some naturalists doubted the validity of these assumptions and began to cautiously express ideas about the unity of continents in the geological past, currently separated by vast oceans. These scientists, like many progressives, found themselves in a difficult position because their assumption was unproven. Indeed, if vertical oscillation the earth's crust could be explained by some internal forces(for example, under the influence of the Earth’s heat), the movement of huge continents across the earth’s surface was difficult to imagine.

WEGENER'S HYPOTHESIS

At the beginning of the 20th century. The idea of ​​moving continents gained great popularity among naturalists, thanks to the works of the German geophysicist A. Wegener. He spent many years on expeditions and in November 1930 ( exact date unknown) died on the glaciers of Greenland. Scientific world was shocked by the news of the death of A. Wegener, who was in the prime of his creative powers. By this time, the popularity of his idea of ​​continental drift had reached its zenith. Many geologists and geophysicists, paleogeographers and biogeographers perceived them with interest, and talented works began to appear in which these ideas were developed.

A. Wegener came up with the idea of ​​​​the possible movement of continents when he carefully examined the geographical map of the world. He was struck by the amazing similarity between the outlines of the coasts of South America and Africa. Later, A. Wegener became acquainted with paleontological materials indicating the existence of once land connections between Brazil and Africa. In turn, this served as an impetus for more detailed analysis available geological and paleontological data and led to a firm conviction about the correctness of his assumption.

It was difficult at first to overcome the dominance of a well-developed concept of the immutability of the position of the continents, or the hypothesis of fixism, with the ingenious, purely speculative assumption of the mobilists, based so far only on the similarity of the configurations of the opposite shores of the Atlantic Ocean. A. Wegener believed that he would be able to convince all his opponents of the validity of continental drift only when strong evidence based on extensive geological and paleontological materials was collected.

To confirm continental drift, A. Wegener and his supporters cited four groups of independent evidence: geomorphological, geological, paleontological and paleoclimatic. So it all started with a certain similarity coastlines continents located on both sides of the Atlantic Ocean, the outlines of the coastlines of the continents surrounding the Indian Ocean have a less clear coincidence. A. Wegener suggested that about 250 million years ago all the continents were grouped into a single giant supercontinent - Pangea. This supercontinent consisted of two parts. In the north was Laurasia, which united Eurasia (without India) and North America, and in the south was Gondwana, represented by South America, Africa, Hindustan, Australia and Antarctica.

The reconstruction of Pangea was based primarily on geomorphological data. They are fully confirmed by the similarity of the geological sections of individual continents and the areas of development of certain types of animal and plant kingdoms. All the ancient flora and fauna of the southern Gondwanan continents form a single community. Many terrestrial and freshwater vertebrates, as well as shallow-water invertebrate forms, unable to actively move over long distances and seemingly living on different continents, turned out to be surprisingly close and similar to each other. It is difficult to imagine how the ancient flora could have settled if the continents had been separated from each other by the same enormous distance as they are now.

Convincing evidence in favor of the existence of Pangea, Gondwana and Laurasia was obtained by A. Wegener after summarizing paleoclimatic data. At that time, it was already well known that traces of the largest sheet glaciation, which occurred about 280 million years ago, were found on almost all southern continents. Glacial formations in the form of fragments of ancient moraines (they are called tillites), remains of forms glacial relief and traces of glacier movement are known in South America (Brazil, Argentina), South Africa, India, Australia and Antarctica. It is difficult to imagine how, given the current position of the continents, glaciation could occur almost simultaneously in areas so distant from each other. In addition, most of the listed glaciation areas are currently located in equatorial latitudes.

Opponents of the continental drift hypothesis put forward the following arguments. In their opinion, although all these continents in the past were located in equatorial and tropical latitudes, they were at a much higher hypsometric position than at present, which caused the appearance of ice and snow within their borders. After all, now there is long-term snow and ice on Mount Kilimanjaro. However, it is unlikely that the total height of the continents at that distant time was 3500-4000 m. There is no basis for this assumption, since in this case the continents would have been subjected to intense erosion and on their frames thicknesses of coarse material would have accumulated, similar to accumulations in terminal basins flow of mountain rivers. In reality, only fine-grained and chemogenic sediments were deposited on continental shelves.

Therefore, the most acceptable explanation for this is unique phenomenon, i.e., the presence of ancient moraines in the modern equatorial and tropical regions of the Earth, is that 260 - 280 million years ago the continent of Gondwana, consisting of South America, India, Africa, Australia and Antarctica collected together, was located in high latitudes , near the South Geographic Pole.

Opponents of the drift hypothesis could not imagine how continents moved over such long distances. A. Wegener explained this using the example of the movement of icebergs, which was carried out under the influence of centrifugal forces caused by the rotation of the planet.

Thanks to the simplicity and clarity and, most importantly, the convincingness of the facts cited in defense of the hypothesis of continental drift, it quickly became popular. However, after success, a crisis came quite soon. The critical attitude towards the hypothesis began with geophysicists. They received a large number of facts and physical contradictions in the chain of logical evidence of the movement of continents. This allowed them to prove the inconclusiveness of the method and causes of continental drift, and by the beginning of the 40s this hypothesis had lost almost all of its supporters. By the 50s of the XX century. it seemed to most geologists that the hypothesis of continental drift should be finally abandoned and could be considered only as one of the historical paradoxes of science that had not received confirmation and did not stand the test of time.

PALEOMAGNETISM AND NEOMOBILISM

From the middle of the 20th century. Scientists began an intensive study of the topography and geology of the ocean floor in its deep interior, as well as the physics, chemistry and biology of ocean waters. They began to probe the seabed with numerous instruments. By deciphering the records of seismographs and magnetometers, geophysicists obtained new facts. It was found that many rocks during their formation acquired magnetization in the direction of the existing geomagnetic pole. In most cases, this remanent magnetization remains unchanged for many millions of years.

Currently, methods for selecting samples and determining their magnetization using special devices - magnetometers - have already been well developed. By determining the direction of magnetization of rocks of different ages, you can find out how the direction changed in each specific area geomagnetic field for a given period of time.

The study of remanent magnetization in rocks led to two fundamental discoveries. Firstly, it has been established that during long history The Earth's magnetization has changed many times - from normal, i.e., corresponding to modern, to reverse. This discovery was confirmed in the early 60s of our century. It turned out that the orientation of magnetization clearly depends on time, and on this basis scales of magnetic field reversals were constructed.

Secondly, when studying lava columns lying on both sides of mid-ocean ridges, a certain symmetry was discovered. This phenomenon is called a stripe magnetic anomaly. Such anomalies are located symmetrically on both sides of the mid-ocean ridge, and each symmetrical pair of them is of the same age. Moreover, the latter naturally increases with distance from the axis of the mid-ocean ridge towards the continents. Strip magnetic anomalies are like a record of inversions, i.e. changes in the direction of the magnetic field on a giant “magnetic tape”.

The American scientist G. Hess suggested, which was subsequently confirmed many times, that partially molten mantle material rises to the surface along cracks and through rift valleys, located in the axial part of the mid-ocean ridge. It spreads in different directions from the axis of the ridge and at the same time, as it were, pulls apart and reveals the ocean floor. The mantle material gradually fills the rift crack, solidifies in it, is magnetized based on the existing magnetic polarity, and then, breaking approximately in the middle, is pushed away by a new portion of the melt. Based on the inversion time and the order of alternation of direct and reverse magnetization, the age of the oceans is determined and the history of their development is deciphered.

Strip magnetic anomalies of the ocean floor turned out to be the most convenient information for reconstructing geomagnetic field polarity epochs in the geological past. But there is still a very important direction in the study of igneous rocks. Based on the remanent magnetization of ancient rocks, it is possible to determine the direction of the paleomeridians, and therefore the coordinates of the North and South Poles in a particular geological era.

The first determinations of the position of the ancient poles showed that the older the era under study, the more different the location of the magnetic pole is from the modern one. However, the main thing is that the coordinates of the poles, determined from rocks of the same age, are the same for each individual continent, but for different continents they have a discrepancy, which increases as we go deeper into the distant past.

One of the phenomena of paleomagnetic research was the incompatibility of the positions of the ancient and modern magnetic poles. When trying to combine them, it was necessary to move the continents each time. It is noteworthy that when combining the Late Paleozoic and Early Mesozoic magnetic poles With modern times, the continents shifted into a single huge continent, very similar to Pangea.

Such stunning results of paleomagnetic research contributed to a return to the hypothesis of continental drift by the wider scientific community. The English geophysicist E. Bullard and his colleagues decided to test the initial premise of continental drift - the similarity of the contours of continental blocks currently separated by the Atlantic Ocean. The alignment was carried out using electronic computers, but not along the contour of the coastlines, as A. Wegener did, but along an isobath of 1800 m, which runs approximately in the middle of the continental slope. The contours of the continents located on both sides of the Atlantic coincided over a considerable distance.

TECTONICS OF LITHOSPHERIC PLATES

The discoveries of primary magnetization, poles of magnetic anomalies with alternating signs, symmetrical to the axes of mid-ocean ridges, changes in the position of magnetic poles over time, and a number of other discoveries led to the revival of the continental drift hypothesis.

The idea of ​​the expansion of the ocean floor from the axes of mid-ocean ridges to the periphery has received repeated confirmation, especially after deep-sea drilling. Seismologists made a great contribution to the development of the ideas of mobilism (continental drift). Their research made it possible to clarify the picture of the distribution of seismic activity zones on the earth's surface. It turned out that these zones are quite narrow, but extensive. They are confined to continental margins, island arcs, and mid-ocean ridges.

The revived hypothesis of continental drift is called plate tectonics. These plates move slowly across the surface of our planet. Their thickness sometimes reaches 100-120 km, but more often it is 80-90 km. There are few lithospheric plates on Earth (Fig. 1) - eight large and about one and a half dozen small ones. The latter are often called microplates. Two large plates are located within the Pacific Ocean and are represented by thin and easily permeable oceanic crust. The Antarctic, Indo-Australian, African, North American, South American and Eurasian lithospheric plates have continental-type crust. They have different edges (borders). When plates move apart, their edges are called divergent. As they diverge, mantle material enters the resulting crack (rift zone). It hardens on the surface of the bottom and builds up the oceanic crust. New portions of mantle material expand the rift zone, which causes lithospheric plates to move. At the place where they move apart, an ocean is formed, the size of which is constantly increasing. This type of boundary is recorded by modern oceanic rift fractures along the axes of mid-ocean ridges.

Rice. 1. Modern lithospheric plates of the Earth and the direction of their movement.

1 - expansion axes and faults; 2 - planetary compression belts; 3 - convergent plate boundaries; 4 - modern continents

When lithospheric plates converge, their boundaries are called convergent. Complex processes occur in the convergence zone. There are two main ones. When an oceanic plate collides with another oceanic or continental plate, it sinks into the mantle. This process is accompanied by warping and breaking. Deep-focus earthquakes occur in the immersion zone. It is in these places that the Zavaritsky-Benioff zones are located.

The oceanic plate enters the mantle and is partially melted there. At the same time, its lightest components, melting, rise to the surface again in the form of volcanic eruptions. This is precisely the nature of the Pacific fire ring. Heavy components slowly sink into the mantle and can descend all the way to the boundaries of the core.

When two continental lithospheric plates collide, a hummocking-type effect occurs.

We observe it many times during ice drift, when the ice floes collide and are crushed, moving towards each other. The crust of the continents is much lighter than the mantle, so the plates do not sink into the mantle. When they collide, they compress and large mountain structures appear at their edges.

Numerous and long-term observations have allowed geophysicists to establish the average speeds of movement of lithospheric plates. Within the Alpine-Himalayan compression belt, which was formed as a result of the collision of the African and Hindustan plates with the Eurasian plates, convergence rates range from 0.5 cm/year in the Gibraltar region to 6 cm/year in the Pamir and Himalaya regions.

Europe is currently “sailing away” from North America at a rate of up to 5 cm/year. However, Australia is “moving away” from Antarctica with maximum speed- on average 14 cm/year.

Oceanic lithospheric plates have the highest movement speeds - their speed is 3-7 times higher than the speed of continental lithospheric plates. The “fastest” is the Pacific plate, and the “slowest” is the Eurasian plate.

MECHANISM OF LITHOSPHERIC PLATE MOVEMENT

It is difficult to imagine that vast and massive continents can move slowly. Even more difficult to answer is the question of why they move? The earth's crust is a cooled and completely crystallized mass. From below it is underlain by partially molten asthenosphere. It is easy to assume that lithospheric plates arose during the cooling of the partially molten substance of the asthenosphere, similar to the process of ice formation in reservoirs in winter. However, the difference is that ice is lighter than water, and crystallized silicates of the lithosphere are heavier than their melt.

How are oceanic lithospheric plates formed?

The hot and partially molten substance of the asthenosphere rises into the space between them, which, falling on the surface of the ocean floor, cools and, crystallizing, turns into rocks of the lithosphere (Fig. 2). The previously formed sections of the lithosphere seem to “freeze” even more strongly and split into cracks. A new portion of hot substance enters these cracks and, solidifying, increasing in volume, pushes them apart. The process is repeated many times.

Rice. 2. Scheme of the movement of rigid lithospheric plates (according to B. Isaacs and others)

The rocks of the lithosphere are heavier than the underlying hot substance of the asthenosphere and, therefore, the thicker it is, the deeper it sinks, or sinks, into the mantle. Why do lithospheric plates, if they are heavier than the substance of the molten mantle, not sink in it? The answer is quite simple. They do not sink because the light earth’s crust is “soldered” to the heavy mantle part of the continental plates on top, acting as a float. Therefore, the average density of continental plate rocks is always less than the average density of hot mantle matter.

Oceanic plates are heavier than the mantle, and therefore they sooner or later sink into the mantle and sink under the lighter continental plates.

Enough long time The oceanic lithosphere, like giant “flattened saucers,” is held on the surface. In accordance with Archimedes' law, the mass of the asthenosphere displaced from under them is equal to the mass of the plates themselves and the water filling the lithospheric depressions. Buoyancy that exists for a long time appears. However, this cannot continue for long. The integrity of the “saucer” is sometimes disrupted in places where excess stresses arise, and they are stronger the deeper the plates sink into the mantle, and therefore, the older they are. Probably, in lithospheric plates that were older than 150 million years, stresses arose that far exceeded the tensile strength of the lithosphere itself; they split and sank into the hot mantle.

GLOBAL RECONSTRUCTIONS

Based on the study of the remanent magnetization of continental rocks and the ocean floor, the position of the poles and latitudinal zonation in the geological past are established. Paleolatitudes, as a rule, do not coincide with modern ones geographical latitudes, and this difference increases more and more as we move away from the present time.

The combined use of geophysical (palaeomagnetic and seismic), geological, paleogeographic and paleoclimatic data makes it possible to reconstruct the position of continents and oceans for various periods of time in the geological past. Many specialists take part in these studies: geologists, paleontologists, paleoclimatologists, geophysicists, as well as specialists in computer technology, since not the calculations of the residual magnetization vectors themselves, but their interpretation is unthinkable without the use of a computer. Reconstructions were carried out independently of each other by Soviet, Canadian and American scientists.

Throughout almost the entire Paleozoic, the southern continents were united into a single huge continent, Gondwana. There is no reliable evidence of existence in the Paleozoic South Atlantic and the Indian Ocean.

At the beginning of the Cambrian period, approximately 550 - 540 million years ago, the largest continent was Gondwana. It was opposed in the northern hemisphere by separated continents (North American, East European and Siberian), as well as a small number of microcontinents. Between the Siberian and East European continents, on the one hand, and Gondwana, on the other, was the Paleo-Asian Ocean, and between the North American continent and Gondwana was the Paleo-Atlantic Ocean. In addition to them, at that distant time there was a vast oceanic space - an analogue of the modern Pacific Ocean. The end of the Ordovician, about 450 - 480 million years ago, was characterized by the convergence of continents in the northern hemisphere. Their collisions with island arcs led to the buildup of the marginal parts of the Siberian and North American land masses. The Paleo-Asian and Paleo-Atlantic oceans are beginning to shrink in size. After some time, a new ocean appears in this place - Paleotethys. It occupied the territory of modern Southern Mongolia, Tien Shan, Caucasus, Turkey, and the Balkans. A new water basin also arose on the site of the modern Ural ridge. The width of the Ural Ocean exceeded 1500 km. According to paleomagnetic definitions, South Pole at this time was in the northwestern part of Africa.

In the first half of the Devonian period, 370 - 390 million years ago, the continents began to unite: North American with Western Europe, as a result of which a new continent, Euramerica, emerged, although not for long. Modern mountain structures of Appalachia and Scandinavia were formed due to the collision of these continents. Paleotethys shrunk somewhat in size. In place of the Ural and Paleo-Asian oceans, small relict basins remained. The South Pole was located in what is now Argentina.

Much of North America was located in southern hemisphere. In tropical and equatorial latitudes there were the Siberian, Chinese, Australian continents and the eastern part of Euramerica.

The Early Carboniferous, approximately 320-340 million years ago, was characterized by the continued convergence of the continents (Fig. 3). In the places where they collided, folded regions and mountain structures arose - the Urals, Tien Shan, the mountain ranges of Southern Mongolia and Western China, Salair, etc. A new ocean, Paleotethys II (Paleotethys of the second generation), appears. It separated the Chinese continent from Siberia and Kazakhstan.

Fig.3. Position of continents in the Early Carboniferous (340 million years ago)

In the middle of the Carboniferous period, large parts of Gondwana found themselves in the polar region of the southern hemisphere, which led to one of the greatest glaciations in the history of the Earth.

The Late Carboniferous - the beginning of the Permian period 290 - 270 million years ago, was marked by the unification of continents into a giant continental block - the supercontinent Pangea (Fig. 4). It consisted of Gondwana in the south and Laurasia in the north. Only the Chinese continent was separated from Pangea by the Paleotethys II ocean.

In the second half of the Triassic period, 200 - 220 million years ago, although the location of the continents was approximately the same as at the end of the Paleozoic, changes nevertheless occurred in the outlines of the continents and oceans (Fig. 5). The Chinese continent united with Eurasia, Paleotethys II ceased to exist.

However, almost simultaneously, a new oceanic basin, the Tethys, arose and began to rapidly expand. He separated Gondwana from Eurasia. Inside it, isolated microcontinents have been preserved - Indochina, Iran, Rhodope, Transcaucasia, etc.

The emergence of a new ocean was due to further development lithosphere - the collapse of Pangea and the separation of all currently known continents. At the beginning, Laurasia split - in the area of ​​​​the modern Atlantic and Arctic oceans. Then its individual parts began to move away from each other, thereby making room for the North Atlantic.

The Late Jurassic era, about 140 - 160 million years ago, is the time of the fragmentation of Gondwana (Fig. 6). At the site of the split, the Atlantic Ocean Basin and mid-ocean ridges arose. The Tethys Ocean continued to develop, in the north of which there was a system of island arcs. They were located on the site of the modern Lesser Caucasus, Elburz and the mountains of Afghanistan and separated from the ocean marginal seas.

During the Late Jurassic and Cretaceous times, continents moved in a latitudinal direction. The Labrador Sea and the Bay of Biscay arose, Hindustan and Madagascar separated from Africa. A strait appeared between Africa and Madagascar. The long journey of the Hindustan Plate ended at the end of the Paleogene with a collision with Asia. This is where giant mountain structures - the Himalayas - were formed.

The Tethys Ocean began to gradually shrink and become closed, mainly due to the rapprochement of Africa and Eurasia. A chain of volcanic island arcs arose on its northern edge. A similar volcanic belt formed on the eastern edge of Asia. At the end of the Cretaceous period, North America and Eurasia united in the region of Chukotka and Alaska.

During the Cenozoic, the Tethys Ocean became completely closed, a relic of which is now the Mediterranean Sea. The collision of Africa with Europe led to the formation of the Alpine-Caucasian mountain system. The continents began to gradually converge in the northern hemisphere and move apart in the southern, breaking up into separate isolated blocks and massifs.

Comparing the positions of the continents into separate geological periods, we come to the conclusion that in the development of the Earth there were large cycles, during which the continents either came together or diverged in different directions. The duration of each such cycle is at least 600 million years. There is reason to believe that the formation of Pangea and its collapse were not isolated moments in the history of our planet. A similar supergiant continent arose in ancient times, approximately 1 billion years ago.

GEOSYNCLINALS - FOLDED MOUNTAIN SYSTEMS

In the mountains we admire the colorful panorama that opens, we are amazed at the boundless creative and destructive forces nature. Gray mountain peaks stand majestically, huge glaciers descend like tongues into the valleys, mountain rivers bubble in deep canyons. We are surprised not only by the wild beauty of the mountainous regions, but also by the facts that we hear about from geologists, and they claim that in place of vast mining structures in the distant past there were vast expanses of sea.

When Leonardo da Vinci discovered the remains of sea mollusk shells high in the mountains, he made the correct conclusion about the existence of a sea there in ancient times, but few people believed him then. How could there be a sea in the mountains at an altitude of 2-3 thousand meters? More than one generation of natural scientists has made great efforts to prove the likelihood of such a seemingly unprecedented case.

The great Italian was right. The surface of our planet is constantly in motion - horizontal or vertical. During its descent, grandiose transgressions repeatedly occurred, when over 40% of the modern land surface was covered by the sea. With the upward movement of the earth's crust, the height of the continents increased and the sea retreated. The so-called regression of the sea took place. But how did the grandiose mountain structures and vast mountain ranges form?

For a long time, geology was dominated by the idea of ​​the predominance of vertical movements. In this regard, there was an opinion that thanks to such movements, mountains were formed. Most mountain structures globe concentrated in certain belts with a length of thousands of kilometers and a width of several tens or even a few hundred kilometers. They are characterized by intense folding, manifestations of various faults, intrusions of igneous rocks, dikes cutting through strata of sedimentary and metamorphic rocks. Continuous slow uplift, accompanied by erosion processes, shapes the relief of mountain structures.

The mountainous regions of the Appalachians, Cordillera, Urals, Altai, Tien Shan, Hindu Kush, Pamir, Himalayas, Alps, Caucasus are folded systems that formed during different periods of the geological past during eras of tectonic and magmatic activity. These mountain systems are characterized by the enormous thickness of accumulated sedimentary formations, often exceeding 10 km, which is tens of times greater than the thickness of similar rocks within the flat, platform part.

The discovery of unusually thick strata of sedimentary rocks, crumpled into folds, penetrated by intrusions and dikes of igneous rocks, moreover, having a large extent with a relatively small width, led to the creation in the middle of the 19th century. geosynclinal theory of mountain formation. An extended area of ​​thick sedimentary strata, which over time turns into a mountain system, is called a geosyncline. In contrast, stable areas of the earth's crust with a large thickness of sedimentary rocks are called platforms.

Almost all the mountain systems of the globe, characterized by folding, discontinuities and magmatism, are ancient geosynclines located on the edges of continents. Despite the enormous thickness, the vast majority of sediments are of shallow water origin. Often on the bedding surfaces there are imprints of ripple marks, remains of shallow-water bottom animals, and even desiccation cracks. The large thickness of the sediments indicates a significant and at the same time fairly rapid subsidence of the earth's crust. Along with typically shallow-water sediments, deep-water ones are also found (for example, radiolarites and fine-grained sediments with peculiar layering and textures).

Geosynclinal systems have been studied for a whole century, and thanks to the work of many generations of scientists, a seemingly harmonious system of the sequence of their occurrence and evolution has been developed. The only inexplicable fact still remains the absence of a modern analogue of the geosyncline. What can be considered a modern geosyncline? The marginal sea or the entire ocean?

However, with the development of the concept of lithospheric plate tectonics, the geosynclinal theory underwent some changes and the place of geosynclinal systems during periods of stretching, movement and collision of lithospheric plates was found.

How did the development of folded systems occur? On the tectonically active margins of the continents there were extended areas experiencing slow subsidence. In the marginal seas, sediments with a thickness of 6 to 20 km accumulated. At the same time, volcanic formations were formed here in the form of magmatic intrusions, dikes and lava covers. Sedimentation lasted tens and sometimes even hundreds of millions of years.

Then, during the orogenic stage, slow deformation and transformation of the geosynclinal system occurred. Its area has decreased, it seems to have been flattened. Folds and breaks appeared, as well as intrusions of molten igneous rocks. During the deformation process, deep-sea and shallow-sea sediments shifted, and at high pressures and temperatures they underwent metamorphism.

At this time, uplift occurred, the sea completely left the territory and mountain ranges formed. Subsequent processes of erosion of rocks, transportation and accumulation of clastic sediments eventually led to the fact that these mountains were gradually destroyed down to elevations close to sea level. The slow subsidence of folded systems located at the edges of the continental plate also led to the same result.

In the process of formation of geosynclinal systems, not only horizontal movements take part, but also vertical ones, carried out mainly as a result of the slow movement of lithospheric plates. In the case when one plate was subducted under another, thick sediments of geosynclines within marginal seas, island arcs and deep-sea trenches were actively exposed to high temperatures and pressure. The areas where plates subduct are called subduction zones. Here rocks descend into the mantle, melt and recycle. This zone is characterized by strong earthquakes and volcanism.

Where the pressure and temperature were not so high, the rocks were crushed into a system of folds, and in places where the rocks were the most hard, their continuity was disrupted by ruptures and movements of individual blocks.

In areas of convergence and then collision of continental lithospheric plates, the width of the geosynclinal system greatly decreased. Some parts of it sank deep into the mantle, while others, on the contrary, advanced onto the nearest plate. Squeezed out of the depths and crushed into folds, sedimentary and metamorphic formations were repeatedly layered on top of each other in the form of giant scales, and eventually mountain ranges arose. For example, the Himalayas were formed as a result of the collision of two large lithospheric plates - the Hindustan and the Eurasian. Mountain systems of southern Europe and North Africa, Crimea, the Caucasus, the mountainous regions of Turkey, Iran, Afghanistan were mainly formed as a result of the collision of the African and Eurasian plates. In a similar way, but in more ancient times, the Ural Mountains, the Cordillera, the Appalachians and other mountainous regions arose.

HISTORY OF THE MEDITERRANEAN SEA

The seas and oceans were formed over a long period of time until they acquired modern look. From the history of the development of sea basins special interest represents the evolution of the Mediterranean Sea. The first civilized states arose around it, and the history of the peoples who inhabited its coast is well known. But we will have to begin our description many millions of years before the first man appeared here.

In ancient times, almost 200 million years ago, on the site of the modern Mediterranean Sea there was a wide and deep Tethys Ocean; Africa at that time was several thousand kilometers away from Europe. There were large and small archipelagos of islands in the ocean. These well-known areas, currently located in Southern Europe, the Near and Middle East - Iran, Turkey, the Sinai Peninsula, the Rhodope, Apulian, Tatra massifs, Southern Spain, Calabria, Meseta, the Canary Islands, Corsica, Sardinia, were far away south of their modern location.

In the Mesozoic, a fault arose between Africa and North America. He separated the Rhodope-Turkish massif and Iran from Africa, and basaltic magma penetrated along it, oceanic lithosphere was formed and the earth's crust moved apart, or spreading. The Tethys Ocean was located in the tropical region of the Earth and extended from the modern Atlantic Ocean through the Indian Ocean (the latter was part of it) to the Pacific. Tethys reached its maximum latitude approximately 100-120 million years ago, and then its gradual reduction began. Slowly, the African lithospheric plate moved closer to the Eurasian plate. About 50 - 60 million years ago, India separated from Africa and began its unprecedented drift to the north until it collided with Eurasia. The size of the Tethys Ocean gradually decreased. Just 20 million years ago, in place of a vast ocean, marginal seas remained - the Mediterranean, Black and Caspian, the sizes of which, however, were much larger than today. No less large-scale events took place in subsequent times.

In the early 70s of this century, evaporites - various rock salts, gypsum and anhydrites - were discovered in the Mediterranean Sea under a layer of loose sediments several hundred meters thick. They were formed by increased evaporation of water about 6 million years ago. But could the Mediterranean Sea really dry up? This is precisely the hypothesis that has been expressed and supported by many geologists. It is assumed that 6 million years ago the Strait of Gibraltar closed and after about a thousand years the Mediterranean Sea turned into a huge basin 2 - 3 km deep with small drying up salt lakes. The bottom of the sea was covered with a layer of hardened dolomite silt, gypsum and rock salt.

Geologists have established that the Strait of Gibraltar periodically opened and water through it from the Atlantic Ocean fell to the bottom of the Mediterranean Sea. At the opening of Gibraltar Atlantic waters fell in the form of a waterfall, which was at least 15 - 20 times higher than the flow of the largest Victoria Falls on the river. Zambezi in Africa (200 km 3 / year). The closing and opening of Gibraltar occurred at least 11 times, and this ensured the accumulation of a sequence of evaporites about 2 km thick.

During periods of drying of the Mediterranean Sea, on the steep slopes of its deep basin, rivers flowing from the land cut long and deep canyons. One of these canyons was discovered and traced at a distance of about 250 km from the modern river delta. Rhone along the continental slope. It is filled with very young, Pliocene sediments. Another example of such a canyon is the underwater continuation of the river. Nile in the form of a sediment-filled canyon, traced at a distance of 1200 km from the delta.

During the loss of communication between the Mediterranean Sea and the open ocean, a unique highly desalinated basin was located in its place, the remnants of which are currently the Black and Caspian Sea, this freshwater and at times saline basin extended from Central Europe to the Urals and Aral Sea and named Paratethys.

Knowing the position of the poles and the speed of modern movement of lithospheric plates, the speed of spreading and absorption of the ocean floor, it is possible to outline the path of movement of the continents in the future and imagine their position for a certain period of time.

This forecast was made by American geologists R. Dietz and J. Holden. After 50 million years, according to their assumptions, the Atlantic and Indian Oceans will grow at the expense of the Pacific, Africa will shift to the north and thanks to this the Mediterranean Sea will gradually be eliminated. The Strait of Gibraltar will disappear, and a “turned” Spain will close the Bay of Biscay. Africa will be split by the great African faults and its eastern part will shift to the northeast. The Red Sea will expand so much that it will separate the Sinai Peninsula from Africa, Arabia will move to the northeast and close the Persian Gulf. India will increasingly move towards Asia, which means the Himalayan mountains will grow. California will separate from North America along the San Andreas Fault, and a new ocean basin will begin to form in this place. Significant changes will occur in the southern hemisphere. Australia will cross the equator and come into contact with Eurasia. This forecast requires significant clarification. Much here still remains debatable and unclear.

From the book " Modern geology" N.A. Yasamanov. M. Nedra. 1987

The Earth's lithospheric plates are huge blocks. Their foundation is formed by strongly folded granite metamorphosed igneous rocks. The names of lithospheric plates will be given in the article below. From above they are covered with a three- to four-kilometer “cover.” It is formed from sedimentary rocks. The platform has a topography consisting of isolated mountain ranges and vast plains. Next, the theory of the movement of lithospheric plates will be considered.

The emergence of a hypothesis

The theory of the movement of lithospheric plates appeared at the beginning of the twentieth century. Subsequently, she was destined to play a major role in planetary exploration. The scientist Taylor, and after him Wegener, put forward the hypothesis that over time, lithospheric plates drift in a horizontal direction. However, in the thirties of the 20th century, a different opinion took hold. According to him, the movement of lithospheric plates was carried out vertically. This phenomenon was based on the process of differentiation of the planet's mantle matter. It came to be called fixism. This name was due to the fact that the permanently fixed position of sections of the crust relative to the mantle was recognized. But in 1960, after the discovery of a global system of mid-ocean ridges that encircle the entire planet and reach land in some areas, there was a return to the hypothesis of the early 20th century. However, the theory took on a new form. Block tectonics has become a leading hypothesis in sciences studying the structure of the planet.

Basic provisions

It was determined that large lithospheric plates exist. Their number is limited. There are also smaller lithospheric plates of the Earth. The boundaries between them are drawn according to the concentration in the earthquake foci.

The names of lithospheric plates correspond to the continental and oceanic regions located above them. There are only seven blocks with a huge area. The largest lithospheric plates are the South and North American, Euro-Asian, African, Antarctic, Pacific and Indo-Australian.

The blocks floating on the asthenosphere are distinguished by their solidity and rigidity. The above areas are the main lithospheric plates. In accordance with the initial ideas, it was believed that continents make their way through the ocean floor. In this case, the movement of lithospheric plates was carried out under the influence of an invisible force. As a result of the studies, it was revealed that the blocks float passively along the mantle material. It is worth noting that their direction is initially vertical. Mantle material rises upward under the crest of the ridge. Then propagation occurs in both directions. Accordingly, the divergence of lithospheric plates is observed. This model represents the ocean floor as a giant one. It comes to the surface in the rift regions of mid-ocean ridges. Then it hides in deep-sea trenches.

The divergence of lithospheric plates provokes the expansion of ocean floors. However, the volume of the planet, despite this, remains constant. The fact is that the birth of new crust is compensated by its absorption in areas of subduction (underthrust) in deep-sea trenches.

Why do lithospheric plates move?

The reason is thermal convection of the planet's mantle material. The lithosphere is stretched and rises, which occurs above the ascending branches of convective currents. This provokes the movement of lithospheric plates to the sides. As the platform moves away from the mid-ocean rifts, the platform becomes denser. It becomes heavier, its surface sinks down. This explains the increase in ocean depth. As a result, the platform sinks into deep-sea trenches. As the heated mantle decays, it cools and sinks, forming basins that are filled with sediment.

Plate collision zones are areas where the crust and platform experience compression. In this regard, the power of the first increases. As a result, the upward movement of lithospheric plates begins. It leads to the formation of mountains.

Research

The study today is carried out using geodetic methods. They allow us to draw a conclusion about the continuity and ubiquity of processes. Collision zones of lithospheric plates are also identified. The lifting speed can be up to tens of millimeters.

Horizontally large lithospheric plates float somewhat faster. In this case, the speed can be up to ten centimeters during the year. So, for example, St. Petersburg has already risen by a meter over the entire period of its existence. Scandinavian Peninsula - by 250 m in 25,000 years. Mantle material moves relatively slowly. However, as a result, earthquakes and other phenomena occur. This allows us to conclude about the high power of material movement.

Using the tectonic position of plates, researchers explain many geological phenomena. At the same time, during the study it became clear that the complexity of the processes occurring with the platform was much greater than it seemed at the very beginning of the hypothesis.

Plate tectonics could not explain changes in the intensity of deformation and movement, the presence of a global stable network of deep faults and some other phenomena. The question of the historical beginning of the action also remains open. Direct signs indicating plate tectonic processes have been known since the late Proterozoic period. However, a number of researchers recognize their manifestation from the Archean or Early Proterozoic.

Expanding Research Opportunities

The advent of seismic tomography led to the transition of this science to qualitatively new level. In the mid-eighties of the last century, deep geodynamics became the most promising and youngest direction of all existing geosciences. However, new problems were solved using not only seismic tomography. Other sciences also came to the rescue. These include, in particular, experimental mineralogy.

Thanks to the availability of new equipment, it became possible to study the behavior of substances at temperatures and pressures corresponding to the maximum at the depths of the mantle. The research also used isotope geochemistry methods. This science studies, in particular, isotope balance rare elements, as well as noble gases in various earth's shells. In this case, the indicators are compared with meteorite data. Geomagnetism methods are used, with the help of which scientists try to uncover the causes and mechanism of reversals in the magnetic field.

Modern painting

The platform tectonics hypothesis continues to satisfactorily explain the process of crustal development over at least the last three billion years. At the same time, there are satellite measurements, according to which the fact is confirmed that the main lithospheric plates of the Earth do not stand still. As a result, a certain picture emerges.

In the cross section of the planet there are three most active layers. The thickness of each of them is several hundred kilometers. It is assumed that the execution leading role in global geodynamics is entrusted to them. In 1972, Morgan substantiated the hypothesis of ascending mantle jets put forward in 1963 by Wilson. This theory explained the phenomenon of intraplate magnetism. The resulting plume tectonics has become increasingly popular over time.

Geodynamics

With its help, the interaction of rather complex processes that occur in the mantle and crust is examined. In accordance with the concept outlined by Artyushkov in his work “Geodynamics”, gravitational differentiation of matter acts as the main source of energy. This process is observed in the lower mantle.

After heavy components (iron, etc.) are separated from the rock, a lighter mass remains solids. It descends into the core. The placement of a lighter layer under a heavier one is unstable. In this regard, the accumulating material is periodically collected into fairly large blocks that float to the upper layers. The size of such formations is about one hundred kilometers. This material was the basis for the formation of the upper

The lower layer probably represents undifferentiated primary substance. During the evolution of the planet, due to the lower mantle, the upper mantle grows and the core increases. It is more likely that blocks of light material rise in the lower mantle along the channels. The mass temperature in them is quite high. The viscosity is significantly reduced. The increase in temperature is facilitated by the release of a large amount of potential energy during the rise of matter into the gravity region at a distance of approximately 2000 km. As it moves through such a channel, strong heating of light masses occurs. In this regard, the substance enters the mantle with a fairly high temperature and significantly less weight in comparison with the surrounding elements.

Due to the reduced density lightweight material floats to the upper layers to a depth of 100-200 kilometers or less. As the pressure decreases, the melting point of the components of the substance decreases. After primary differentiation at the core-mantle level, secondary differentiation occurs. At shallow depths, the light substance partially undergoes melting. During differentiation, denser substances are released. They sink into the lower layers of the upper mantle. The released lighter components, accordingly, rise upward.

The complex of movements of substances in the mantle associated with the redistribution of masses having different densities as a result of differentiation is called chemical convection. The rise of light masses occurs with a periodicity of approximately 200 million years. However, penetration into the upper mantle is not observed everywhere. In the lower layer, the channels are located quite long distance from each other (up to several thousand kilometers).

Lifting blocks

As mentioned above, in those zones where large masses of light heated material are introduced into the asthenosphere, partial melting and differentiation occurs. IN the latter case the release of components and their subsequent ascent are noted. They pass through the asthenosphere quite quickly. When reaching the lithosphere, their speed decreases. In some areas, the substance forms accumulations of anomalous mantle. They usually lie in upper layers planets.

Anomalous mantle

Its composition approximately corresponds to normal mantle matter. The difference between the anomalous cluster is that it is more high temperature(up to 1300-1500 degrees) and reduced speed of elastic longitudinal waves.

The entry of matter under the lithosphere provokes isostatic uplift. Due to the increased temperature, the anomalous cluster has a lower density than the normal mantle. In addition, there is a slight viscosity of the composition.

In the process of reaching the lithosphere, the anomalous mantle is quite quickly distributed along the base. At the same time, it displaces the denser and less heated substance of the asthenosphere. As the movement progresses, the anomalous accumulation fills those areas where the base of the platform is in an elevated state (traps), and it flows around deeply submerged areas. As a result, in the first case there is an isostatic rise. Above submerged areas, the crust remains stable.

Traps

The cooling process of the upper mantle layer and crust to a depth of about one hundred kilometers occurs slowly. Overall, it takes several hundred million years. In this regard, heterogeneities in the thickness of the lithosphere, explained by horizontal temperature differences, have a fairly large inertia. In the event that the trap is located near the upward flow of an anomalous accumulation from the depths, large number substances are captured by highly heated ones. As a result, a fairly large mountain element is formed. In accordance with this scheme, high uplifts occur in the area of ​​epiplatform orogenesis in

Description of processes

In the trap, the anomalous layer is compressed by 1-2 kilometers during cooling. The crust located on top sinks. Sediment begins to accumulate in the formed trough. Their severity contributes to even greater subsidence of the lithosphere. As a result, the depth of the basin can be from 5 to 8 km. At the same time, when the mantle compacts in the lower part of the basalt layer in the crust, a phase transformation of the rock into eclogite and garnet granulite can be observed. Due to the heat flow escaping from the anomalous substance, the overlying mantle is heated and its viscosity decreases. In this regard, there is a gradual displacement of the normal accumulation.

Horizontal offsets

When uplifts form as anomalous mantle enters the crust on the continents and oceans, the potential energy stored in the upper layers of the planet increases. To discharge excess substances they tend to move apart. As a result, additional stresses are formed. Associated with them different types movements of plates and crust.

The expansion of the ocean floor and the floating of continents are a consequence of the simultaneous expansion of the ridges and the subsidence of the platform into the mantle. Underneath the former are large masses of highly heated anomalous matter. In the axial part of these ridges the latter is located directly under the crust. The lithosphere here has significantly less thickness. At the same time, the anomalous mantle spreads in an area of ​​​​high pressure - in both directions from under the ridge. At the same time, it quite easily tears the ocean crust. The crevice is filled with basaltic magma. It, in turn, is melted out of the anomalous mantle. In the process of solidification of magma, a new one is formed. This is how the bottom grows.

Process Features

Beneath the median ridges, the anomalous mantle has reduced viscosity due to increased temperature. The substance can spread quite quickly. In this regard, the growth of the bottom occurs at an increased rate. The oceanic asthenosphere also has relatively low viscosity.

The main lithospheric plates of the Earth float from ridges to subsidence sites. If these areas are located in the same ocean, then the process occurs at a relatively high speed. This situation is typical for the Pacific Ocean today. If the bottom expands and subsidence occurs in different areas, then the continent located between them drifts in the direction where the deepening occurs. Under continents, the viscosity of the asthenosphere is higher than under the oceans. Due to the resulting friction, significant resistance to movement appears. The result is a reduction in the rate at which seafloor expansion occurs unless there is compensation for mantle subsidence in the same area. Thus, expansion in the Pacific Ocean is faster than in the Atlantic.

tectonic fault lithospheric geomagnetic

Starting from the Early Proterozoic, the speed of movement of lithospheric plates consistently decreased from 50 cm/year to its modern value of about 5 cm/year.

The decrease in the average speed of plate movement will continue to occur, until the moment when, due to the increase in the power of the oceanic plates and their friction against each other, it will not stop at all. But this will happen, apparently, only in 1-1.5 billion years.

To determine the speed of 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 basalts poured onto them by the magnetic field that existed on Earth at the time of the basalts eruption.

But, as is known, 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 thanks to the spreading of the ocean floor in the rift zones of mid-ocean ridges, more ancient basalts are always moved to greater distances from these zones, and along with the ocean floor, the ancient magnetic field of the Earth “frozen” into the basalts 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 on both slopes of mid-ocean ridges and the abyssal basins surrounding them. Such anomalies can now be used to determine the age of the ocean floor and the rate of its expansion 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 magnetic anomalies observed on the ocean floor.

The age of magnetic reversals was determined from detailed paleomagnetic studies of well-dated basaltic strata and sedimentary rocks of 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 had already sunk into the mantle under modern or ancient plate thrust zones, and, therefore, no magnetic anomalies older than 150 million years were preserved on the ocean floor.

The presented conclusions of the theory make it possible to quantitatively calculate the parameters of motion at the beginning of two adjacent plates, and then for the third, taken in tandem with one of the previous ones. In this way, it is gradually possible to involve the main of the identified lithospheric plates into the calculation and determine the mutual movements 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 at 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. On a geological scale, this is a lot, since in just 1 million years a strip of young bottom up to 180 km wide is formed in this way, while approximately 360 km3 of basaltic lavas flow out on each kilometer of the rift zone during the same time! According to the same calculations, Australia is moving away from Antarctica at a speed of about 7 cm/year, and South America from Africa at a speed of about 4 cm/year. The movement of North America from Europe occurs more slowly - 2-2.3 cm/year. The Red Sea is expanding even more slowly - by 1.5 cm/year (accordingly, less basalts are poured out here - only 30 km3 for each linear kilometer of the Red Sea rift over 1 million years). But the speed of the “collision” between India and Asia reaches 5 cm/year, which explains the intense neotectonic deformations developing before our eyes and the growth of the mountain systems of the Hindu Kush, Pamir and Himalayas. These deformations create high level seismic activity of the entire region (the tectonic influence of the collision of India with Asia affects far beyond the plate collision zone itself, spreading all the way to Lake Baikal and areas of the Baikal-Amur Mainline). Deformations of the Greater and Lesser Caucasus are caused by the pressure of the Arabian Plate on this region of Eurasia, but the rate of convergence of the plates here is significantly 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 in other ways the speeds of movement of lithospheric plates have been established and it has been proven that oceanic plates move faster than those that include a continent in their structure, and the thicker the continental lithosphere, the lower the speed of plate movement.