Plate tectonics (plate tectonics) is a modern geodynamic concept based on the concept of large-scale horizontal movements of relatively integral fragments of the lithosphere (lithospheric plates). Thus, plate tectonics deals with the movements and interactions of lithospheric plates.
The first suggestion about the horizontal movement of crustal blocks was made by Alfred Wegener in the 1920s within the framework of the “continental drift” hypothesis, but this hypothesis did not receive support at that time. Only in the 1960s did studies of the ocean floor provide conclusive evidence of horizontal plate movements and ocean expansion processes due to the formation (spreading) of oceanic crust. The revival of ideas about the predominant role of horizontal movements occurred within the framework of the “mobilist” trend, the development of which led to the development modern theory plate tectonics. The main principles of plate tectonics were formulated in 1967-68 by a group of American geophysicists - W. J. Morgan, C. Le Pichon, J. Oliver, J. Isaacs, L. Sykes in the development of earlier (1961-62) ideas of American scientists G. Hess and R. Digtsa on the expansion (spreading) of the ocean floor
Fundamentals of Plate Tectonics
The basic principles of plate tectonics can be summarized in several fundamental
1. The upper rocky part of the planet is divided into two shells, significantly different in rheological properties: a rigid and brittle lithosphere and an underlying plastic and mobile asthenosphere.
2. The lithosphere is divided into plates, constantly moving along the surface of the plastic asthenosphere. The lithosphere is divided into 8 large plates, dozens of medium plates and many small ones. Between the large and medium slabs there are belts composed of a mosaic of small crustal slabs.
Plate boundaries are areas of seismic, tectonic, and magmatic activity; the internal regions of the plates are weakly seismic and characterized by weak manifestation of endogenous processes.
More than 90% of the Earth's surface falls on 8 large lithospheric plates:
Australian plate,
Antarctic Plate,
African plate,
Eurasian Plate,
Hindustan plate,
Pacific Plate,
North American Plate,
South American Plate.
Middle plates: Arabian (subcontinent), Caribbean, Philippine, Nazca and Coco and Juan de Fuca, etc.
Some lithospheric plates are composed exclusively of oceanic crust (for example, the Pacific Plate), others include fragments of both oceanic and continental crust.
3. There are three types of relative movements of plates: divergence (divergence), convergence (convergence) and shear movements.
Accordingly, three types of main plate boundaries are distinguished.
Divergent boundaries– boundaries along which plates move apart.
The processes of horizontal stretching of the lithosphere are called rifting. These boundaries are confined to continental rifts and mid-ocean ridges in ocean basins.
The term "rift" (from the English rift - gap, crack, gap) is applied to large linear structures of deep origin, formed during the stretching of the earth's crust. In terms of structure, they are graben-like structures.
Rifts can form on both continental and oceanic crust, forming a single global system oriented relative to the geoid axis. In this case, the evolution of continental rifts can lead to a break in the continuity of the continental crust and the transformation of this rift into an oceanic rift (if the expansion of the rift stops before the stage of rupture of the continental crust, it is filled with sediments, turning into an aulacogen).
The process of plate separation in zones of oceanic rifts (mid-ocean ridges) is accompanied by the formation of new oceanic crust due to magmatic basaltic melt coming from the asthenosphere. This process of formation of new oceanic crust due to the influx of mantle material is called spreading(from the English spread - spread out, unfold).
Structure of the mid-ocean ridge
During spreading, each extension pulse is accompanied by the arrival of a new portion of mantle melts, which, when solidified, build up the edges of plates diverging from the MOR axis.
It is in these zones that the formation of young oceanic crust occurs.
Convergent boundaries– boundaries along which plate collisions occur. There can be three main options for interaction during a collision: “oceanic - oceanic”, “oceanic - continental” and “continental - continental” lithosphere. Depending on the nature of the colliding plates, several different processes can occur.
Subduction- the process of subduction of an oceanic plate under a continental or other oceanic one. Subduction zones are confined to the axial parts of deep-sea trenches associated with island arcs (which are elements of active margins). Subduction boundaries account for about 80% of the length of all convergent boundaries.
When the continental and oceanic plates collide, a natural phenomenon is the displacement of the oceanic (heavier) plate under the edge of the continental one; When two oceans collide, the more ancient (that is, cooler and denser) of them sinks.
Subduction zones have a characteristic structure: they typical elements serve as a deep-sea trench - a volcanic island arc - a back-arc basin. A deep-sea trench is formed in the zone of bending and underthrusting of the subducting plate. As this plate sinks, it begins to lose water (found in abundance in sediments and minerals), the latter, as is known, significantly reduces the melting temperature of rocks, which leads to the formation of melting centers that feed volcanoes of island arcs. In the rear of a volcanic arc, some stretching usually occurs, which determines the formation of a back-arc basin. In the back-arc basin zone, stretching can be so significant that it leads to rupture of the plate crust and the opening of a basin with oceanic crust (the so-called back-arc spreading process).
The immersion of the subducting plate into the mantle is traced by the foci of earthquakes that occur at the contact of the plates and inside the subducting plate (colder and, therefore, more fragile than the surrounding mantle rocks). This seismic focal zone is called Benioff-Zavaritsky zone.
In subduction zones, the process of formation of new continental crust begins.
A much rarer process of interaction between continental and oceanic plates is the process obduction– thrusting of part of the oceanic lithosphere onto the edge of the continental plate. It should be emphasized that during this process, the oceanic plate is separated, and only its upper part– crust and several kilometers of upper mantle.
When continental plates collide, the crust of which is lighter than the mantle material, and as a result is not capable of plunging into it, a process occurs collisions. During a collision, the edges of colliding continental plates are crushed, crushed, and systems of large thrusts are formed, which leads to growth mining structures with a complex fold-thrust structure. Classic example The collision of the Hindustan plate with the Eurasian plate, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet, serves as such a process.
Collision Process Model
The collision process replaces the subduction process, completing the closure of the ocean basin. Moreover, at the beginning of the collision process, when the edges of the continents have already moved closer together, the collision is combined with the process of subduction (the remnants of the oceanic crust continue to sink under the edge of the continent).
Large-scale regional metamorphism and intrusive granitoid magmatism are typical for collision processes. These processes lead to the creation of a new continental crust (with its typical granite-gneiss layer).
Transform boundaries– boundaries along which shear displacements of plates occur.
Boundaries of the Earth's lithospheric plates
1 – divergent boundaries ( A - mid ocean ridges, b – continental rifts); 2 – transform boundaries; 3 – convergent boundaries ( A - island-arc, b – active continental margins, V - conflict); 4 – direction and speed (cm/year) of plate movement.
4. The volume of oceanic crust absorbed in subduction zones is equal to the volume of crust emerging in spreading zones. This position emphasizes the idea that the volume of the Earth is constant. But this opinion is not the only and definitively proven one. It is possible that the volume of the plane changes pulsatingly, or that it decreases due to cooling.
5. The main reason for plate movement is mantle convection , caused by mantle thermogravitational currents.
The source of energy for these currents is the difference in temperature between the central regions of the Earth and the temperature of its near-surface parts. In this case, the main part of the endogenous heat is released at the boundary of the core and the mantle during the process of deep differentiation, which determines the disintegration of the primary chondritic substance, during which the metallic part rushes to the center, building up the core of the planet, and the silicate part is concentrated in the mantle, where it further undergoes differentiation.
Heated in central zones The rocks expand, their density decreases, and they float up, giving way to sinking colder and therefore heavier masses that have already given up some of the heat in the near-surface zones. This process of heat transfer occurs continuously, resulting in the formation of ordered closed convective cells. In this case, in the upper part of the cell the flow of matter occurs almost at horizontal plane, and it is this part of the flow that determines the horizontal movement of the substance of the asthenosphere and the plates located on it. In general, the ascending branches of convective cells are located under the zones of divergent boundaries (MOR and continental rifts), while the descending branches are located under the zones of convergent boundaries.
Thus, the main reason for the movement of lithospheric plates is “dragging” by convective currents.
In addition, a number of other factors act on the slabs. In particular, the surface of the asthenosphere turns out to be somewhat elevated above the zones of ascending branches and more depressed in the zones of subsidence, which determines the gravitational “sliding” of the lithospheric plate located on an inclined plastic surface. Additionally, there are processes of drawing heavy cold oceanic lithosphere in subduction zones into the hot, and as a consequence less dense, asthenosphere, as well as hydraulic wedging by basalts in the MOR zones.
Figure - Forces acting on lithospheric plates.
The main driving forces of plate tectonics are applied to the base of the intraplate parts of the lithosphere - the mantle drag forces FDO under the oceans and FDC under the continents, the magnitude of which depends primarily on the speed of the asthenospheric flow, and the latter is determined by the viscosity and thickness of the asthenospheric layer. Since under the continents the thickness of the asthenosphere is much less, and the viscosity is much greater than under the oceans, the magnitude of the force FDC almost an order of magnitude smaller than FDO. Under the continents, especially their ancient parts (continental shields), the asthenosphere almost pinches out, so the continents seem to be “stranded.” Since most lithospheric plates of the modern Earth include both oceanic and continental parts, it should be expected that the presence of a continent in the plate should, in general, “slow down” the movement of the entire plate. This is how it actually happens (the fastest moving almost purely oceanic plates are the Pacific, Cocos and Nazca; the slowest are the Eurasian, North American, South American, Antarctic and African plates, a significant part of whose area is occupied by continents). Finally, at convergent plate boundaries, where the heavy and cold edges of the lithospheric plates (slabs) sink into the mantle, their negative buoyancy creates a force FNB(index in the designation of strength - from English negative buoyance). The action of the latter leads to the fact that the subducting part of the plate sinks in the asthenosphere and pulls the entire plate along with it, thereby increasing the speed of its movement. Obviously strength FNB acts episodically and only in certain geodynamic situations, for example in cases of the collapse of slabs described above through the 670 km section.
Thus, the mechanisms that set lithospheric plates in motion can be conditionally classified into the following two groups: 1) associated with the forces of mantle “drag” ( mantle drag mechanism), applied to any points of the base of the slabs, in Fig. 2.5.5 – forces FDO And FDC; 2) associated with forces applied to the edges of the plates ( edge-force mechanism), in the figure - forces FRP And FNB. The role of one or another driving mechanism, as well as certain forces, is assessed individually for each lithospheric plate.
The combination of these processes reflects the general geodynamic process, covering areas from the surface to the deep zones of the Earth.
Mantle convection and geodynamic processes
Currently, two-cell mantle convection with closed cells is developing in the Earth's mantle (according to the model of through-mantle convection) or separate convection in the upper and lower mantle with the accumulation of slabs under subduction zones (according to the two-tier model). The probable poles of the rise of mantle matter are located in northeast africa(approximately under the junction zone of the African, Somali and Arabian plates) and in the area of Easter Island (under middle ridge Pacific Ocean– East Pacific Rise).
The equator of mantle subsidence follows a roughly continuous chain of convergent plate boundaries along the periphery of the Pacific and eastern Indian Oceans.
The modern regime of mantle convection, which began approximately 200 million years ago with the collapse of Pangea and gave rise to modern oceans, will in the future be replaced by a single-cell regime (according to the model of through-mantle convection) or (according to alternative model) convection will become through the mantle due to the collapse of slabs through the 670 km section. This may lead to a collision of continents and the formation of a new supercontinent, the fifth in the history of the Earth.
6. The movements of plates obey the laws of spherical geometry and can be described based on Euler’s theorem. Euler's rotation theorem states that any rotation three-dimensional space has an axis. Thus, rotation can be described by three parameters: the coordinates of the rotation axis (for example, its latitude and longitude) and the rotation angle. Based on this position, the position of the continents in past geological eras can be reconstructed. An analysis of the movements of the continents led to the conclusion that every 400-600 million years they unite into a single supercontinent, which subsequently undergoes disintegration. As a result of the split of such a supercontinent Pangea, which occurred 200-150 million years ago, modern continents were formed.
Some evidence of the reality of the mechanism of lithospheric plate tectonics
Older age of oceanic crust with distance from spreading axes(see picture). In the same direction, an increase in the thickness and stratigraphic completeness of the sedimentary layer is noted.
Drawing - Rock age map ocean floor North Atlantic (according to W. Pitman and M. Talvani, 1972). Sections of the ocean floor of different age intervals are highlighted in different colors; The numbers indicate the age in millions of years.
Geophysical data.
Figure - Tomographic profile through the Hellenic Trench, Crete and the Aegean Sea. Gray circles are earthquake hypocenters. The plate of the subducting cold mantle is shown in blue, the hot mantle is shown in red (according to V. Spackman, 1989)
The remains of the huge Faralon plate, which disappeared in the subduction zone under North and South America, are recorded in the form of slabs of the “cold” mantle (section across North America, along S-waves). According to Grand, Van der Hilst, Widiyantoro, 1997, GSA Today, v. 7, No. 4, 1-7
Linear magnetic anomalies in the oceans were discovered in the 50s during geophysical studies of the Pacific Ocean. This discovery allowed Hess and Dietz to formulate the theory of ocean floor spreading in 1968, which grew into the theory of plate tectonics. They became one of the most compelling evidence of the correctness of the theory.
Figure - Formation of stripes magnetic anomalies during spreading.
The reason for the origin of strip magnetic anomalies is the process of birth of oceanic crust in the spreading zones of mid-ocean ridges; erupted basalts, when cooling below the Curie point in the Earth's magnetic field, acquire remanent magnetization. The direction of magnetization coincides with the direction magnetic field Earth, however, due to periodic reversals of the Earth’s magnetic field, erupted basalts form strips with in different directions magnetization: direct (coincides with the modern direction of the magnetic field) and reverse.
Figure - Scheme of the formation of the strip structure of the magnetically active layer and magnetic anomalies of the ocean (Vine – Matthews model).
The Earth's crust is divided by faults into lithospheric plates, which are huge solid blocks reaching upper layers mantle. They are large, stable parts of the earth's crust and are in continuous motion, sliding along the surface of the Earth. Lithospheric plates consist of either continental or oceanic crust, and some combine a continental mass with an oceanic one. There are 7 largest lithospheric plates, which occupy 90% of the surface of our planet: Antarctic, Eurasian, African, Pacific, Indo-Australian, South American, North American. In addition to them, there are dozens of medium-sized slabs and many small ones. Between the medium and large slabs there are belts in the form of mosaics of small slabs of bark.
Theory of plate tectonics
The theory of lithospheric plates studies their movement and the processes associated with this movement. This theory states that the cause of global tectonic changes is the horizontal movement of lithosphere blocks - plates. Plate tectonics examines the interaction and movement of blocks of the earth's crust.
Wagner's theory
The idea that lithospheric plates move horizontally was first suggested in the 1920s by Alfred Wagner. He put forward a hypothesis about “continental drift”, but it was not recognized as reliable at that time. Later, in the 1960s, studies of the ocean floor were carried out, as a result of which Wagner’s guesses about the horizontal movement of plates were confirmed, and the presence of processes of ocean expansion, caused by the formation of oceanic crust (spreading), was revealed. The main provisions of the theory were formulated in 1967-68 by American geophysicists J. Isaacs, C. Le Pichon, L. Sykes, J. Oliver, W. J. Morgan. According to this theory, plate boundaries are located in tectonic, seismic and volcanic activity. Boundaries are divergent, transformative and convergent.
Movement of lithospheric plates
Lithospheric plates begin to move due to the movement of matter located in the upper mantle. In rift zones, this substance breaks through the crust, pushing plates apart. Most rifts are located on the ocean floor, since the earth's crust there is much thinner. The largest rifts that exist on land are located near Lake Baikal and the African Great Lakes. The movement of lithospheric plates occurs at a speed of 1-6 cm per year. When they collide with each other, mountain systems arise on their borders if there is continental crust, and in the case when one of the plates has a crust of oceanic origin, deep-sea trenches are formed.
The basic principles of plate tectonics come down to several points.
- In the upper rocky part of the Earth, there are two shells that differ significantly in geological characteristics. These shells are the hard and brittle lithosphere and the mobile asthenosphere underneath. The base of the lithosphere is a hot isotherm with a temperature of 1300°C.
- The lithosphere consists of plates of the earth's crust continuously moving along the surface of the asthenosphere.
According to modern plate theories the entire lithosphere in narrow and active zones - deep faults- divided into separate blocks, moving in the plastic layer of the upper mantle relative to each other at a speed of 2-3 cm per year. These blocks are called lithospheric plates.
The peculiarity of lithospheric plates is their rigidity and ability, in the absence of external influences, to maintain their shape and structure unchanged for a long time.
Lithospheric plates are mobile. Their movement along the surface of the asthenosphere occurs under the influence of convective currents in the mantle. Individual lithospheric plates can move apart, move closer together, or slide relative to each other. In the first case, tension zones with cracks along the boundaries of the plates appear between the plates, in the second - compression zones, accompanied by the pushing of one plate onto another (thrusting - obduction; thrusting - subduction), in the third - shear zones - faults along which sliding of neighboring plates occurs .
Where continental plates converge, they collide and mountain belts are formed. This is how, for example, the Himalaya mountain system arose on the border of the Eurasian and Indo-Australian plates (Fig. 1).
Rice. 1. Collision of continental lithospheric plates
When the continental and oceanic plates interact, the plate with the oceanic crust moves under the plate with the continental crust (Fig. 2).
Rice. 2. Collision of continental and oceanic lithospheric plates
As a result of the collision of continental and oceanic lithospheric plates, deep-sea trenches and island arcs are formed.
The divergence of lithospheric plates and the resulting formation of the oceanic crust is shown in Fig. 3.
The axial zones of mid-ocean ridges are characterized by rifts(from English rift - crevice, crack, fault) - large linear tectonic structure the earth's crust with a length of hundreds, thousands, a width of tens and sometimes hundreds of kilometers, formed mainly during horizontal stretching of the crust (Fig. 4). Very large rifts are called rift belts, zones or systems.
Since the lithospheric plate is a single plate, each of its faults is a source of seismic activity and volcanism. These sources are concentrated within relatively narrow zones along which mutual movements and friction of adjacent plates occur. These zones are called seismic belts. Reefs, mid-ocean ridges and deep-sea trenches are mobile regions of the Earth and are located at the boundaries of lithospheric plates. This indicates that the process of formation of the earth's crust in these zones is currently occurring very intensively.
Rice. 3. Divergence of lithospheric plates in the zone among the oceanic ridge
Rice. 4. Rift formation scheme
Most of the faults in lithospheric plates occur at the bottom of the oceans, where the earth’s crust is thinner, but they also occur on land. The largest fault on land is located in eastern Africa. It stretches for 4000 km. The width of this fault is 80-120 km.
Currently, seven of the largest plates can be distinguished (Fig. 5). Of these, the largest in area is the Pacific, which consists entirely of oceanic lithosphere. As a rule, the Nazca plate, which is several times smaller in size than each of the seven largest ones, is also classified as large. At the same time, scientists suggest that in fact the Nazca plate is much more larger size, than we see it on the map (see Fig. 5), since a significant part of it went under the neighboring plates. This plate also consists only of oceanic lithosphere.
Rice. 5. Earth's lithospheric plates
An example of a plate that includes both continental and oceanic lithosphere is, for example, the Indo-Australian lithospheric plate. The Arabian plate consists almost entirely of continental lithosphere.
The theory of lithospheric plates is important. First of all, it can explain why there are mountains in some places on Earth and plains in others. Using the theory of lithospheric plates, it is possible to explain and predict catastrophic phenomena that occur at plate boundaries.
Rice. 6. The shapes of the continents really seem compatible.
Continental drift theory
The theory of lithospheric plates originates from the theory of continental drift. Back in the 19th century. many geographers have noted that when looking at a map one can notice that the coasts of Africa and South America when approaching, they appear compatible (Fig. 6).
The emergence of the hypothesis of continental movement is associated with the name of the German scientist Alfred Wegener(1880-1930) (Fig. 7), who most fully developed this idea.
Wegener wrote: “In 1910, the idea of moving continents first occurred to me... when I was struck by the similarity of the outlines of the coasts on both sides of the Atlantic Ocean.” He suggested that in the early Paleozoic there were two large continents on Earth - Laurasia and Gondwana.
Laurasia - it was northern continent, which included the territories of modern Europe, Asia without India and North America. Southern continent - Gondwana united modern territories South America, Africa, Antarctica, Australia and Hindustan.
Between Gondwana and Laurasia there was the first sea - Tethys, like a huge bay. The rest of the Earth's space was occupied by the Panthalassa Ocean.
About 200 million years ago, Gondwana and Laurasia were united into a single continent - Pangea (Pan - universal, Ge - earth) (Fig. 8).
Rice. 8. The existence of a single continent of Pangea (white - land, dots - shallow sea)
About 180 million years ago, the continent of Pangea again began to separate into its component parts, which mixed on the surface of our planet. The division occurred as follows: first Laurasia and Gondwana reappeared, then Laurasia split, and then Gondwana split. Due to the split and divergence of parts of Pangea, oceans were formed. The Atlantic and Indian oceans can be considered young oceans; old - Quiet. The Arctic Ocean became isolated as landmass increased in the Northern Hemisphere.
Rice. 9. Location and directions of continental drift during the Cretaceous period 180 million years ago
A. Wegener found many confirmations of the existence of a single continent of the Earth. He found the existence of remains of ancient animals—listosaurus—in Africa and South America especially convincing. These were reptiles, similar to small hippopotamuses, that lived only in freshwater bodies of water. This means swimming huge distances on the salty sea water they couldn't. He found similar evidence in the plant world.
Interest in the hypothesis of continental movement in the 30s of the 20th century. decreased somewhat, but was revived again in the 60s, when, as a result of studies of the relief and geology of the ocean floor, data were obtained indicating the processes of expansion (spreading) of the oceanic crust and the “diving” of some parts of the crust under others (subduction).
For a long time, geological science was dominated by the hypothesis of the unchanged position of the continents and oceans. 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 is it discovered that the mountain peaks have grown by several centimeters, and the river valleys have deepened.
At the end of the 19th - beginning of the 20th centuries. 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. Thanks to the works of the German geophysicist A. Wegener, the idea of moving continents gained great popularity among naturalists. He spent many years on expeditions and in November 1930 (the exact date is unknown) died on the glaciers of Greenland. The 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 possible relocation continents, when he carefully examined geographical map peace. 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 in the coastlines of the 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 mainly 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, not capable of actively moving over long distances and living as if 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), remnants of glacial relief forms 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 so far 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 completely 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 relief and geology of the ocean floor in its deep interior, as well as physics, chemistry and biology 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 the process of 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 of the geomagnetic field changed in each specific area over 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 into different sides 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 slabs 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 bottom surface and builds up 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. In the convergence zone there are complex processes. 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.
Oceanic plate enters the mantle and is partially melted there. In this case, its lightest components, melting, rise to the surface again in the form volcanic eruptions. This is precisely the nature of the Pacific Ring of Fire. 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.
Currently, Europe is “sailing away” from North America at a speed of up to 5 cm/year. However, Australia is “moving away” from Antarctica at the maximum speed - an average of 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.
For quite a long time, the oceanic lithosphere, like giant “flattened saucers,” remains 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 residual magnetization of rocks of the continents and the ocean floor, the positions of the poles and latitudinal zonation in the geological past. Paleolatitudes, as a rule, do not coincide with modern geographic latitudes, and this difference increases more and more with distance from the present.
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 southern continents were united into a single huge continent, Gondwana. There is no reliable evidence of the existence of the South Atlantic and Indian Ocean in the Paleozoic.
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 determinations, the South Pole was located in northwestern Africa at that time.
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 the 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 places where they collided, folded areas and mountain structures arose - the Urals, Tien Shan, 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 the further development of the 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 Northern 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 the marginal seas from the ocean.
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 in individual 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 the distant past, on the site of vast mountain structures, 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 layers 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, and Caucasus are folded systems that formed during various 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 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 one an inexplicable fact There is still a lack 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 affected 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, Caucasus, mountainous areas Turkey, Iran, Afghanistan were mainly formed as a result of the collision of the African and Eurasian plates. In a similar way, but more ancient times arose Ural Mountains, Cordilleras, Appalachians and other mountainous regions.
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 development sea basins special interest represents evolution 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, in 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 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 dimensions of which, however, were much larger than today. No less large-scale events took place in subsequent times.
In the early 70s of our century, evaporites were discovered in the Mediterranean Sea under a layer of loose sediments several hundred meters thick - a variety of rock salts, gypsum and anhydrite. 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 about a thousand years later 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. When Gibraltar was discovered, 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 open ocean in its place there was a kind of highly desalinated pool, the remains of which are currently Chernoe and Caspian Sea, this freshwater and at times saline basin extended from Central Europe to the Urals and the Aral Sea and was 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. In 50 million years, according to their assumptions, the Atlantic and Indian oceans will expand at the expense of the Pacific, Africa will shift to the north and thanks to this the Mediterranean 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
