How many lithospheric plates are there. List of lithospheric plates. Cause of plate movement
The word "tectonics" literally translated from ancient Greek means building art, structure. In the geosciences, this term is usually understood as geological structure and the patterns of development of the earth's crust, and under the lithosphere - the stone (i.e., solid and durable) shell of the Earth. In the modern sense, the term "lithosphere" includes not only the earth's crust, but also a part of the upper mantle, in which the mantle substance has cooled so much that it completely crystallized and turned into rock. The word "plates" in the title of the new theory shows that the Earth's lithospheric shell is divided into separate blocks, the vertical dimensions of which are usually much smaller than the horizontal ones.
Figure 3: Plate tectonics on a world map. It is at the boundaries between the plates that the most intense geological activity of the planet is found - active volcanoes, frequent seismic failures and earthquakes, the uplift of mountain ranges, the formation and destruction of plates and crust. There are three different types of boundaries between lithospheric plates. Transformative or conservative boundaries. divergent borders; Border convergence.
Figure 4: Limits between plates. Bounce conversion limits. Within the boundaries where the plates slide relative to each other, the lithosphere is neither created nor destroyed. These limits represent failure transformations: cracks along which relative displacement occurs when horizontal slip occurs between adjacent blocks. Transformer failure limits are commonly found along the Meso-oceanic ridges, where the diverging boundary has its own continuity and moves in a pattern similar to a staggering one.
So under tectonics lithospheric plates we will understand the geological theory, which considers the structure, formation and mutual movements of lithospheric plates, accompanied by their deformations, magmatic manifestations and other processes leading to the formation of the earth's crust and associated minerals. This definition does not say anything about the causes of the movement of lithospheric plates, since this problem is solved by a related discipline - geodynamics, which was discussed in previous chapters.
The Santo André Defect in California, where the Pacific Plate is slipping relative to the N American Plate, is a great example of the failure of a transformational continent, as shown in the figure. Since the plates are shifted relative to each other for millions of years, the adjacent rocks on either side of the fault are of different types and ages.
Figure 5: View looking northwest along the Andreevsky Fault in the Carrizo Plain of Central California. There are two types of divergent boundaries: plate separation in continents and plate separation in oceans. Separation of plates on the continents. This divergent boundary is characterized by valleys in rift, volcanic activity, and earthquakes distributed over a wider area than centers of oceanic expansion as separation occurs with a continental plate and another continental plate.
A feature of lithospheric plates is their long-term rigidity and the ability, in the absence of external influences, to maintain their shape and structure unchanged for a long time. In order to destroy or deform a lithospheric plate, it is necessary to apply additional mechanical stresses to it, exceeding the ultimate strength of its constituent rocks, approximately equal to 1 t/cm 2 .
This applies to the Red Sea and the Gulf of California, rifts that are in a more advanced stage of expansion. In these cases, the continents have already separated enough that a new ocean floor could be formed along the expansion axis and the rift valleys would be flooded by the ocean. Occasionally, a continental gap may slow down or stop before a continental gap opens up and a new ocean basin opens up. Another example is the Gulf of California, an ocean in the process of opening caused by plate movement, indicating a widening split between California and Mexico.
In the process of studying the upper, rigid shell of the Earth - the lithosphere - it was found that it consists of the earth's crust and the underlying subcrustal part of the lithosphere. As noted in section 2.3, Earth's crust continents is mainly composed of granitoids and rocks of intermediate composition, from above the continental crust is usually covered by sediments. The total thickness of the crust varies from 30 to 80 km (close to 40 km on average). Consolidated oceanic crust is thinner - usually reaches 6.5-7 km - and is composed (from top to bottom) of basalts, gabbro and serpentinites. The thickness of sedimentary deposits on the oceanic crust is not constant: it increases in the coastal zones of the oceans and wedges out on the crests of the mid-ocean ridges. On average, the thickness of oceanic sediments reaches 500 m. From below, the oceanic crust is underlain by massive ultrabasic rocks - peridotites and lherzolites. The total thickness of oceanic lithospheric plates varies from 2-3 km in the region of the rift zones of the oceans to 80-90 km near the ocean coasts. The thickness of the ancient continental plates reaches 200-250 km.
Separation of plaques in the oceans. At the bottom of the sea, the boundary between the separating plates is marked by a meso-oceanic ridge that exhibits active volcanism, earthquakes and rivets caused by tensile forces pulling on the two separate plates. Figure 8 shows that the expansion of the ocean floor, which occurs as the North American and Eurasian plates, is separating, and a new floor of the Atlantic Ocean is being created by mantle ascension.
Figure 9: Rupture and expansion of the ocean floor in the Meso-Atlantic Ridge creates a chain of volcanic mountains where faults, earthquakes and volcanism are concentrated along a narrow meso-ocean expansion center. Another example is the island of Iceland, which is a segment of the Meso-Atlantic Ridge. These centers of expansion gave rise to the millions of square kilometers of oceanic crust that now make up half of all oceans. Figure 10: The Meso-Atlantic Ridge, a diverging plate boundary, rises above sea level in Iceland.
With depth, as you know, the temperature in the Earth gradually increases. Beneath the oceanic plates, the mantle temperature reaches the melting temperature of mantle rocks (see Fig. 19).
Figure 19.
The numbers on the geotherms indicate the age of the lithospheric plates in millions of years. T oc are oceanic plate geotherms; Tm is the adiabatic temperature of the upper mantle; T s is the temperature of the solidus of the mantle matter; Tcl, geotherm of ancient (Archean) continental lithospheric plates; KK, base of the continental crust; I, boundary of phase transition from basalts to eclogites; II - endothermic transition from a rigid lithosphere under the continents to a plastic state; III - the sole of the Archean sections of the continental lithosphere.
A fractured rift valley filled with new volcanic rock indicates that the plates are being removed. They occur where lithospheric plates collide frontally, with consequences depending on differences in density between the plates. As a rule, a plate with a higher density sinks under another, partially melts in depth and generates a large volume of magma and lava, for example, on the Pacific margin of South America, between Nazca and the South American plates. When plates with the same density collide, the process is complex, with intense compression deformations and related phenomena, with folding, reverse failure, cavalierization of flakes of one plate, on the other, with this, enhanced thickening of the crust.
Therefore, the surface of the beginning of the melting of mantle matter with the temperature of its solidus is taken as the sole of the lithosphere under the oceans. Below the oceanic lithosphere, the mantle matter is partially molten and plastic with a reduced viscosity. The plastic layer of the mantle under such a lithosphere is usually distinguished as an independent shell - the asthenosphere. The latter is clearly expressed only under the oceanic plates (under the oceans, it was first discovered as a layer that forms a seismic waveguide). There is virtually no asthenosphere under the thick continental plates, although they are also underlain by the plastic material of the upper mantle (see Ref. 19).
An example of this process is the formation of the Himalayas. There are three types of convergent boundaries: ocean-ocean convergence, ocean-continent convergence, and continent-continent convergence. If two plates are involved in the ocean, one sinks below the other in a process known as subduction. The oceanic lithosphere of the subductive plate sinks into the asthenosphere and is eventually recirculated by the mantle convection system. This downward bend creates a long, narrow deep water trench.
When a cold lithospheric plate falls, the pressure increases; water trapped in the rocks of the underwater oceanic crust "compresses" and rises to the asthenosphere above the plate. This fluid melts the mantle, creating a chain of volcanoes called an arch of islands on the ocean floor behind the pit. An example of this ocean-to-ocean convergence is the island arches in Japan.
The asthenosphere plays a decisive role in the formation of basalt magmatism of oceanic plates and in the interaction of the convective mass transfer of the mantle with the lithospheric shell. The basaltic magmatism of continental plates can manifest itself only when the hot mantle material, due to the split of the plate, can rise to the level of the beginning of melting of this material (approximately at depths of about 80-100 km).
Figure 10: Connecting an oceanic plate with another oceanic plate forming a deep pit and arch of a volcanic island. Convergence of ocean and continent. This type of convergence occurs between an oceanic plate and a continental plate. The continental plate rides on the oceanic plate because it is lighter and more complex than the oceanic crust. The continental boundary is rugose and rises on a belt of mountains, roughly parallel to the deep sea trench. Huge collisions and subduction forces create large earthquakes along the subduction interface.
Unlike the lithosphere, the asthenosphere does not have a tensile strength, and its substance can deform (flow) under the action of even very small excess pressures, although this process develops extremely slowly due to the high viscosity of the asthenosphere substance - about 10 18 -10 20 P (for comparison note that the viscosity of water is 10 -2 P, liquid basalt lava - 10 4 -10 6, ice - about 10 13 and rock salt - about 10 18 P). Under the influence of high hydrostatic pressures prevailing in the bowels of the Earth, the melting point of silicates increases with depth faster than the temperature of the mantle itself. Consequently, deeper than the asthenosphere, partial melting of the mantle substance should no longer occur, although its properties remain plastic, resembling a superviscous liquid with a viscosity of about 10 22 -10 23 P.
For example, the western coast of South America, where the South American plate collides with the Nazca plate, of an oceanic nature, is a subduction zone of this type. There is a big chain near the coast high mountains, Andes, rising on the continental side of the border, and a deep-water trench. Here the volcanoes are active and deadly. One of them, Nevado del Ruiz, in Colombia, killed 25,000 people during the eruption. Some of the largest earthquakes in the world have also been recorded along this limit.
Figure 11: Summing up of an oceanic plate at the continental margin, forming a volcanic mountain belt at the deformed edge of the continent rather than an island arc. Convergence of the continental continent. Plate convergence involving two continents. One example is the collision of Indiana and the Eurasian plates, both with continents at the forefront. Eurasian plate rides on the Indian plate, but India and Asia remain buoyant, creating twice the thickness of the earth's crust and forming the world's highest mountain range, the Himalayas, and the vast and lofty plateau of Tibet.
On Earth, there are seven large slabs: Pacific, Eurasian, Indo-Australian, Antarctic, African, North American and South American, and the same number of medium-sized plates: the Nazca and Cocos plates in the east Pacific Ocean, Philippine, Arabian, Somali, Caribbean and Scotia plate, located between South America and Antarctica. Sometimes, within large continental plates, they are distinguished as independent middle plates, for example, the Amur, South China, Indonesian, and many small ones: Panonian, Anatolian, Tarim, etc. All plates move relative to each other, so their boundaries are usually clearly marked by zones of increased seismicity ( see Fig. 6).
In this and other areas of collision of continents and continents in the earth's crust, which are subject to wrinkles, strong earthquakes occur. They form the outer and continuous layer of the Earth, called the lithosphere, where the continents and oceans are located. This layer is made up of seven basic hard rock slabs that change position and fit like a jigsaw puzzle.
The movement of the plates is associated with the formation of mountain ranges, volcanoes, earthquakes, tsunamis, landscapes and maps of the planet. Nameplate tectonics is a concept that deals with the geological history of the Earth. There are also smaller plates labeled: Adriatic Plate, Anatolian Plate, Arabica Plate, Caribbean Plate, Caroline Plate, Coco Plate, East American Plate, Gorda Plate, Greek Plate, Indian Plate, Indo-Australian Plate, Iranian Plate, Juan de Fuca Plate , Nazca plate, Philippine tile, Somali plate, Scotia plate, Sunda plate and Tonga tile.
Figure 6
Barazangi and Dorman, 1968
Movements of lithospheric plates on the surface of the asthenosphere occur under the influence of convective currents in the mantle. Separate lithospheric plates can diverge, approach or slide relative to each other. In the first case, tension zones with rift cracks along the plate boundaries appear between the plates, in the second case, compression zones accompanied by thrusting of one of the plates onto the other, in the third case, shear zones, transform faults, along which the displacement of neighboring plates occurs.
There are three types of tectonic plate boundaries: divergent, convergent, and conserved. This happens when the plates track away from each other, causing the "birth" of new oceanic crust. The movement is carried out in a horizontal direction. This limit is determined in three stages: the first is the opening of the ocean, which occurs with the destruction of the crust, the intrusion into the water and the formation of salt lakes. At this stage, intense volcanic activity occurs.
In the second stage, fragmentation is general and there are two continents effectively separated by an ocean. Volcanic activity persists as magma rises. It is the constancy of magma activity that determines the entry into the third stage, the denominated formation of the ocean. The main example of the Divergent Frontier in its three phases lies in Atlantic Ocean, which separates Europe, Africa and America.
In accordance with the different nature of the deformations that occur along the periphery of the plates, three types of boundaries are distinguished. The first, or divergent, includes plate boundaries, along which lithospheric plates move apart (spreading) with the formation of rift zones (see Fig. 5).
Figure 5
1 - ocean level; 2 - precipitation; 3, pillow basaltic lavas (layer 2a); 4, dike complex, dolerites (layer 2b); 5 - gabbro; 6 - layered complex; 7, serpentinites; 8, lherzolites of lithospheric plates; 9 - asthenosphere; 10 - 500 °C isotherm (beginning of serpentinization).
The separation of the continents began 180 million years ago at an average rate of 1 centimeter per year. This is the definition of the movement of collisions of one plate over another. There are three types of convergence between tectonic plates, namely ocean-continental, continental-continental, and ocean-oceanic.
Continental ocean convergence occurs when there are deep sections in the oceans and plates meet and volcanic formation is possible. The friction between them has a large stress that can cause earthquakes. An example of this error is San Andreas, which cuts off the coast of California and the west coast of Mexico. A denser plate sinks from a less dense one. This is a case of a collision between an oceanic and a continental plate. When these plates are compressed, they eventually give rise to mountain ranges.
In the oceans, these boundaries correspond to the ridges of the mid-ocean ridges: in the Arctic Ocean - Mt. Gakkel, Knipovich, Mona and Kolbensey; in the Atlantic - Mt. Reykjanes, North Atlantic, South Atlantic and African-Antarctic; in the Indian Ocean - Mt. West Indian, Arabian-Indian, Central Indian and Australo-Antarctic uplift; in the Pacific Ocean - the South Pacific and East Pacific Rise. On the continents, this type of boundary includes the East African Rift Zone and the Baikal Rift in Asia. An example of rift zones that have only relatively recently changed from continental to oceanic are the Red Sea rifts and Gulf of Aden indian ocean.
For example, the Andes were born because of a collision between these two plates, the oceanic Nazca and the continental South American. Areas where this type of shock occurs are prone to earthquakes. Due to the open fault in the earth's crust, magma can be avoided, which leads to the formation of volcanic islands.
The Atlantic Ocean is a turtle due to this type, which leaves South America from Africa. The Himalayas, for example, are the result of a collision of Euro-Asian and Indian saucers. The cases are getting more and more interesting, even if from the cadastral point of view he was so critical of his accuracy for our selfish goals.
Divergent plate boundaries in the oceans correspond to powerful basaltic volcanism, which forms the oceanic crust in the rift zones of mid-ocean ridges, and shallow-focus seismicity. On the continents, divergent plate boundaries are marked by trap basalt effusions and contrasting bimodal basalt-sialic and alkaline magmatism and somewhat deeper earthquakes (up to 200 km).
The boundaries of the second, or convergent, type include plate underthrust zones (subduction zones), in which oceanic lithospheric plates are subducted under island arcs or under Andean-type continental margins. These boundaries usually correspond to characteristic landforms: conjugated structures of deep-sea trenches (bottom depths in which sometimes exceed 10 km) with a chain of volcanic island arcs or the highest mountain structures (reaching 7-8 km in height), if underthrust occurs under the continents. Examples of such boundaries in the oceans are deep-sea trenches in front of the Aleutian, Kurile-Kamchatka, Japanese, Mariana, Philippine island arcs, deep-sea trenches at the foot of New Britain, the Solomon Islands, the New Hebrides, Tonga-Kermadec, and also at the foot of western coasts Central and South America in the Pacific. In the Indian Ocean, these are the trenches of the Andoman, Greater and Lesser Sunda Islands. In the Atlantic Ocean, these are the Cayman and Puerto Rico trenches in front of the Greater and Lesser Antilles in the Caribbean Sea and the South Sandwich Trench in front of the islands of the same name in the South Atlantic. Zones of lithospheric plate movement are always inclined (“fall”) under island arcs or continental margins and are usually well distinguished by chains of earthquake sources. Plates sinking into the mantle are also characterized by increased values of the seismic figure of merit Q, since the attenuation of seismic waves in a descending cold lithospheric plate is always less than in the hot and partially molten mantle surrounding this plate. Plate underthrust zones are characterized by calc-alkaline magmatism of andesitic composition. Andesitic volcanoes are usually located in the rear parts of island-arc structures (see Fig. 7).
Figure 7
1 - asthenosphere; 2 - lithosphere; 3, oceanic crust; 4-5 - sedimentary-volcanogenic sequence; 6 - oceanic sediments; isolines show seismic activity in A 10 units (Fedotov et al., 1969); β is the angle of incidence of the Wadati-Benief zone; α is the angle of incidence of the plastic deformation zone.
The pushing of oceanic plates under the continents, if it is not compensated by their spreading in the mid-ocean ridges, usually leads to a gradual closing of the ocean, accompanied by a collision of the continents surrounding it, and the appearance of a collisional folded belt along the zone of plate underthrust. In this way, for example, the Alpine-Himalayan mountain belt arose on the site of the ancient Tethys ocean. The process of plate underthrust here continues at the present time, as evidenced by the increased seismicity of this region, so the Alpine-Himalayan belt can also be considered as a convergent or collisional plate boundary.
Detailed studies of the mid-ocean ridges have established that their crests and rift valleys do not stretch along the ridges continuously, but are, as it were, torn into separate sections by transform faults, along which only shear displacements of plates usually occur. These are the boundaries of plates of the third type, or transform faults. As a rule, they are always located perpendicular to the strike of rift fractures. At the same time, only their segments connecting two adjacent rift zones (transforming one of them into another) are active sections of faults. Outside these active areas, no plate displacements along transform faults occur. The displacement amplitude along most of these faults does not exceed ten or several tens of kilometers, but sometimes it reaches hundreds of kilometers.
Transform faults sometimes intersect plate underthrust zones or extend from them to rift zones, but most of them cut only mid-ocean ridges. The largest of them are the Gibbs, Atlantis, Vima and Romansh faults in the Atlantic Ocean; the Owen and Amsterdam faults in the Indian Ocean; the Eltanin and Challenger faults in the Pacific Ocean. In addition, in the northern half of the Pacific Ocean there are traces of now dead, but once gigantic faults, displacements along which occurred for many hundreds and even 1,200 km. These are the so-called great faults of the Pacific floor: Mendocino, Pioneer, Murray, Molokai, Clarion and Clipperton. An example of the boundaries of the third type on the continents is the San Andreas Fault in California. In the relief, oceanic transform faults are clearly identified by conjugated parallel structures of narrow ridges and troughs with a common steep wall (Fig. 81). At the same time, due to the “soldering” of lithospheric plates with each other on the passive flanks of transform faults and the faster subsidence of young plates, transform faults are always framed by narrow ridges only from the side of younger plates and, conversely, hollows appear only from the side of older plates. As a rule, transform faults are amagmatic, although in some cases (in the presence of a sliding component in the movement of plates) basalt volcanoes with alkaline orientation may appear on their flanks.
Figure 81.
Movements of lithospheric plates are accompanied by their friction against each other and the occurrence of earthquakes along the boundaries of plates. Therefore, the boundaries of lithospheric plates can be distinguished not only by geomorphological features, but also by zones of increased seismicity. At the same time, different mechanisms of earthquakes correspond to different plate boundaries. Thus, in oceanic rift zones, all earthquakes located under the crests of mid-ocean ridges are shallow-focus with a source depth of up to 5–10 km and are characterized by extension mechanisms. The depth of earthquakes in transform faults reaches 30-40 km, and their mechanisms are purely strike-slip. The most seismically active are the plate underthrust zones. In these zones, there are both shallow-focus earthquakes with a focus depth of up to 30 km, intermediate earthquakes at depths from 30 to 150-200 km, and deep-focus earthquakes with a focus depth of up to 600-700 km. The main seismic focal surface of plate underthrust zones usually descends at an angle of about 30-50° from the axis of the deep-water trench under the island arc or continental margin, outlining the body of the oceanic plate being subducted into the mantle. Earthquakes of various types occur in plate underthrust zones, but shear and reverse-thrust mechanisms prevail among shallow-focus earthquakes, and shear and compression mechanisms prevail at medium and large depths.
As a rule, the limiting depth of deep-focus earthquakes corresponds to the position of the endothermic phase boundary at a depth of about 670 km (see Fig. 58). Deeper than this boundary, the crystalline bonds in the mantle matter are broken, and it apparently acquires the properties of an amorphous substance. Nevertheless, judging by the data of seismic tomography, traces of sinking oceanic plates can be traced even deeper in the lower mantle, all the way to the Earth's core. This can also be seen from the relief of its surface: everywhere under the zones of plate underthrust, framing, for example, the Pacific and Indian oceans, depressions are traced on the surface of the core with an amplitude of up to 4 km, and under ascending flows in the centers of these oceans, as well as under the North Atlantic, on the contrary , there are rises in its relief with an amplitude of up to 6 km (see Fig. 12).
Figure 58.
T S is the temperature of the mantle material solidus (using data from Green and Ringwood, 1967 and Takahashi, 1986); T M is the adiabatic temperature of the convecting mantle (Sorokhtin, 2001); T Cont, continental geotherm under the Archean cratons (Sorokhtin et al., 1996); dots show the region of existence of juvenile melts in the mantle. Exothermic phase transitions: I — transition from plagioclase to pyroxene lherzolites (Lpx); II, transition from pyroxene to garnet lherzolites (Lgr); IV, transition of olivines (a) to spinel structures (γ and β); V is the transition of silica into the structure of stishovite (St) and pyroxenes into the structure of ilmenite (Ilm). Endothermic transitions: III - the proposed transition from a rigid polycrystalline substance to its plastic state; VI - transition of pyroxenes to the structure of perovskite (Pv) and magnesiowustite (Mw). Phase transitions I and II are constructed according to the data of Green and Ringwood (1967), generalized transitions IV, V and VI are constructed according to the data of Kuskov and Fabrichnaya (1990).
Figure 12.
contour lines are drawn after 2 km, according to Morelli, Dziewonski, 1987
It is noteworthy that many plates include both continental masses and segments of the oceanic lithosphere soldered to them. For example, the African plate includes the continent of Africa itself and the eastern halves of the Central and South Atlantic adjacent to it, the western parts of the bottom of the Indian Ocean, as well as the sections of the bottom of the Mediterranean and Red Seas adjacent to the continent. In addition to plates of mixed continental-ocean structure, there are plates consisting only of oceanic lithosphere with oceanic crust on the surface. This type includes the Pacific, Nazca, Cocos and Philippine plates.
In the first approximation, lithospheric plates can be considered as fragments of a rigid spherical shell moving along the Earth's surface. In this case, for a quantitative description of the movements of lithospheric plates on the spherical surface of the Earth, Euler's theorem, formulated by him back in 1777, is usually used. With regard to the problem of determining the parameters of the movement of rigid spherical shells - lithospheric plates on the surface of the globe, this theorem states that at any given moment time, any such movement can be represented by a rotation of a plate with a certain angular velocity about an axis passing through the center of the Earth and a certain point on its surface, called the pole of rotation of this plate.
In the process of a detailed study of the tectonic structure of the ocean floor, one remarkable rule was revealed. It turned out that almost all rift faults are always oriented to the corresponding poles of plate expansion, and the transform faults associated with them are always perpendicular to these directions. Consequently, the network of rift and transform faults that arise between two moving apart plates is always oriented along the meridians and latitudinal circles drawn from the pole of mutual rotation of the plates. It follows from Euler's theory that the rate of mutual displacement of two lithospheric plates will change with distance from the pole of rotation according to the law of the sine of the polar angle of a given point, measured from the same pole of rotation of the plates. As a result of taking into account the features of plate movements, Euler's theorem made it possible to quantitatively calculate the movements of the entire ensemble of lithospheric plates on the Earth's surface from paleomagnetic anomalies on the ocean floor and build paleogeodynamic reconstructions of the positions of ancient oceans and continents in past geological epochs.
Figure 8
(Heirtzler et al., 1966). Positive anomalies are marked in black; AA is the zero anomaly of the rift zone.
To determine the velocities of lithospheric plates, data on the location of banded magnetic anomalies on the ocean floor are usually used (see Fig. 8). Let us recall once again that these anomalies, as has now been established, appear in the rift zones of the oceans due to the magnetization of the basalts erupted on them by the magnetic field that existed on Earth at the time of the basalt outpouring. But, as you know, the geomagnetic field from time to time changed direction to the exact opposite. This led to the fact that basalts that erupted during different periods of geomagnetic field reversals turned out to be magnetized in opposite directions. But due to the expansion of the ocean floor in the rift zones of the mid-ocean ridges, the older basalts always turn out to be moved a long distance from these zones, and together with the ocean floor, the ancient magnetic field of the Earth “frozen” into the basalts also moves away from them.
The expansion of the oceanic crust together with differently magnetized basalts usually develops strictly symmetrically on both sides of the rift fault. Therefore, the magnetic anomalies associated with them are also located symmetrically along both slopes of the mid-ocean ridges and the abyssal basins surrounding them (see Fig. 8). Such anomalies can now be used to determine the age of the ocean floor and its expansion rate in rift zones. However, for this it is necessary to know the age of individual reversals of the Earth's magnetic field and compare these reversals with the magnetic anomalies observed on the ocean floor.
The age of magnetic reversals was determined from detailed paleomagnetic studies of well-dated strata of basalt covers and sedimentary rocks of the continents and ocean floor basalts (Fig. 82). 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 water areas of the World Ocean (see Fig. 9).
Figure 82.
(time given in millions of years)
Figure 9
after Larson, Pitman et al., 1985
The above conclusions of the theory make it possible to quantitatively calculate the motion parameters at the beginning of two adjacent plates, and then for the third, taken in tandem with one of the previous ones. In this way, one can gradually involve the main of the identified lithospheric plates in the calculation and determine the mutual displacements of all plates on the Earth's surface.
Abroad, such calculations were performed by J. Minster and his colleagues, and in Russia by S. A. Ushakov and Yu. I. Galushkin (Fig. 83). It turned out that the ocean floor is moving apart with maximum speed in the southeastern part of the Pacific Ocean (near Easter Island). In this place, up to 18 cm of new oceanic crust grows annually. In terms of geological scale, this is a lot, since only in 1 million years a strip of a young bottom up to 180 km wide is formed in this way, while approximately 360 km 3 of basaltic lavas are poured out at each kilometer of the rift zone in the same time! According to the same calculations, Australia is moving away from Antarctica at a rate of about 7 cm/year, and South America from Africa - at a rate of about 4 cm / year. The pushing away of North America from Europe is slower - 2-2.3 cm/year. The Red Sea expands even more slowly - by 1.5 cm / year (correspondingly, less basalts are poured out here - only 30 km3 per linear kilometer of the Red Sea rift for 1 million years). On the other hand, the rate of "collision" between India and Asia reaches 5 cm/year, which explains the intense neotectonic deformations developing before our eyes and the growth of the mountain systems of the Hindu Kush, the Pamirs and the Himalayas. These deformations create a high level of seismic activity in the entire region (the tectonic impact of the collision of India with Asia affects far beyond the plate collision zone itself, extending all the way to Lake Baikal and the regions of the Baikal-Amur Mainline). The deformations of the Greater and Lesser Caucasus are caused by the pressure of the Arabian Plate on this region of Eurasia, however, the rate of convergence of the plates here is much less - only 1.5-2 cm / year. Therefore, the seismic activity of the region is also less here.
Figure 83
1, oceanic rift zones and transform faults; 2, continental rift zones; 3, zones of underthrusting of oceanic lithospheric plates under island arcs; 4 - the same, under the active margins of the Andean-type continents; 5 - zones of "collision" (collisions) of continental plates; 6, transform (shear) plate boundaries; 7, lithospheric plates; 8—directions and velocities (cm/yr) of the relative motion of the plates.
The importance of the above calculations is obvious, since they allow us to quantify the modern tectonic activity of the Earth and the volume of magmatic eruptions in modern rift zones. But it turns out that using a similar technique and successively combining magnetic anomalies of the same age with each other, it is also possible to build accurate reconstructions of the position of continents and oceans (with mid-ocean ridges in them) for past geological times and determine the speed of expansion or subduction of the ocean floor under the island arcs. . Behind last years a large number of such paleogeodynamic reconstructions were constructed by L.P. Zonenshain and his colleagues (1976, 1977) for the entire time interval of the existence of magnetic anomalies on the modern ocean floor, i.e., from the late Mesozoic to the present day.
All oceanic plates that formed earlier than the Late Jurassic have already subsided into the mantle under modern or ancient zones of plate underthrust, and, consequently, no magnetic anomalies older than 150 million years have been preserved on the ocean floor. Therefore, for older geological epochs only approximate paleogeographic reconstructions can be built using paleomagnetic data for continents. Such reconstructions, built by A. Smith and J. Bryden (1977), cover the time interval up to and including the Early Triassic (220 Ma). In Russia, similar reconstructions were built by A. M. Gorodnitsky and L. P. Zonenshain for the entire Phanerozoic (1977).
Together with part of the upper mantle, it consists of several very large blocks, which are called lithospheric plates. Their thickness is different - from 60 to 100 km. Most plates include both continental and oceanic crust. There are 13 main plates, of which 7 are the largest: American, African, Indo-, Amur.
The plates lie on the plastic layer of the upper mantle (asthenosphere) and slowly move relative to each other at a speed of 1-6 cm per year. This fact was established as a result of a comparison of images taken from artificial earth satellites. They suggest that the configuration in the future may be completely different from the current one, since it is known that the American lithospheric plate is moving towards the Pacific, and the Eurasian one is approaching the African, Indo-Australian, and also the Pacific. The American and African lithospheric plates are slowly moving apart.
The forces that cause the separation of lithospheric plates arise when the mantle substance moves. Powerful ascending flows of this substance push apart the plates, break the earth's crust, forming deep faults in it. Due to underwater outpourings of lavas, strata are formed along the faults. Freezing, they seem to heal wounds - cracks. However, the stretch increases again, and breaks occur again. So, gradually increasing lithospheric plates diverge in different directions.
There are fault zones on land, but most of them are in ocean ridges on where the earth's crust is thinner. The largest fault on land is located in the east. It stretched for 4000 km. The width of this fault is 80-120 km. Its outskirts are dotted with extinct and active ones.
Collision is observed along other plate boundaries. It happens in different ways. If the plates, one of which has an oceanic crust and the other a continental one, approach each other, then the lithospheric plate, covered by the sea, sinks under the continental one. In this case, arcs () or mountain ranges () arise. If two plates with a continental crust collide, then the edges of these plates are crushed into folds of rocks, and mountainous regions are formed. So they arose, for example, on the border of the Eurasian and Indo-Australian plates. The presence of mountainous areas in the inner parts of the lithospheric plate suggests that once there was a boundary between two plates, firmly soldered to each other and turned into a single, larger lithospheric plate. Thus, we can draw a general conclusion: the boundaries of lithospheric plates are mobile areas to which volcanoes are confined, zones, mountainous areas, mid-ocean ridges, deep-water depressions and trenches. It is at the boundary of lithospheric plates that are formed, the origin of which is associated with magmatism.