Boundaries of lithospheric plates on the world map. Lithosphere as an element of the geographical shell

Together with part of the upper mantle, it consists of several very large blocks called lithospheric plates. Their thickness varies - 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 a 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 by comparing images taken from artificial Earth satellites. They suggest that the configuration in the future may be completely different from the present one, since it is known that the American lithospheric plate is moving towards the Pacific, and the Eurasian plate is moving closer to the African, Indo-Australian, and also the Pacific. The American and African lithospheric plates are slowly moving apart.

Forces that cause divergence lithospheric plates, arise when the mantle material moves. Powerful upward flows of this substance push the plates apart, tearing apart the earth's crust, forming deep faults in it. Due to underwater outpourings of lavas, strata are formed along faults. By freezing, they seem to heal wounds - cracks. However, the stretching increases again, and ruptures occur again. So, gradually increasing, lithospheric plates diverge in different directions.

There are fault zones on land, but most of them are in the oceanic ridges, where Earth's crust thinner. The largest fault on land is located in the east. It stretches for 4000 km. The width of this fault is 80-120 km. Its outskirts are dotted with extinct and active ones.

Along other plate boundaries, plate collisions are observed. It happens in different ways. If plates, one of which has oceanic crust and the other continental, come closer together, then the lithospheric plate, covered by the sea, sinks under the continental one. In this case, arcs () or mountain ranges () appear. If two plates that have continental crust collide, the edges of these plates are crushed into folds of rock, and mountainous regions are formed. This is how they arose, for example, on the border of the Eurasian and Indo-Australian plates. The presence of mountainous areas in the internal parts of the lithospheric plate suggests that once there was a boundary of two plates that were firmly fused with 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, zones, mountain areas, mid-ocean ridges, deep-sea depressions and trenches are confined. It is at the border of lithospheric plates that they are formed, the origin of which is associated with magmatism.

Divergent boundaries are 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).

Structure of the continental rift



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: their typical elements are 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).

Model of the subduction 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 ocean plate is separated, and only its upper part - the crust and several kilometers of the upper mantle - moves forward.

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 the collision, the edges of colliding continental plates are crushed, crushed, and systems of large thrusts are formed, which leads to the growth of mountain structures with a complex fold-thrust structure. A classic example of such a process is the collision of the Hindustan plate with the Eurasian plate, accompanied by the growth of the grandiose mountain systems of the Himalayas and Tibet.

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.


Figure - 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 metal 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.

Rocks heated in the central zones of the Earth 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 in a horizontal plane, and it is this part of the flow that determines the horizontal movement of the matter 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.

Attached to the base of the intraplate parts of the lithosphere are the main driving forces plate tectonics - 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 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.

Figure - Schematic diagram of mantle convection.


Figure - Alternative schemes of mantle convection

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 material are located in northeastern Africa (approximately under the junction zone of the African, Somali and Arabian plates) and in the Easter Island region (under the middle ridge of the Pacific Ocean - the 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 change to a single-cell regime (according to the model of through-mantle convection) or (according to an alternative model) convection will become through-mantle due to the collapse of slabs across a 670 km divide. 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 of 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.

The word “tectonics” literally translated from ancient Greek means construction art, structure. In geosciences, this term usually means geological structure and patterns of development of the earth's crust, and under the lithosphere - the stone (i.e., hard and durable) shell of the Earth. In the modern sense, the term “lithosphere” includes not only the earth’s crust, but also part of the upper mantle, in which the mantle material has cooled so much that it has completely crystallized and turned into rock. The word “plates” in the name of the new theory shows that the lithospheric shell of the Earth is divided into separate blocks, the vertical dimensions of which are usually much smaller than the horizontal ones.

Thus, by lithospheric plate tectonics we will understand a geological theory that 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 says nothing about the reasons for the movement of lithospheric plates, since this problem is solved by a related discipline - geodynamics, which was discussed in previous chapters.

A feature of lithospheric plates is their long-term rigidity and 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 tensile strength of the rocks composing it, approximately equal to 1 t/cm2.

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 already noted in Section 2.3, the crust of the continents is mainly composed of granitoids and rocks of intermediate composition; the continental crust is usually covered by sediments on top. The total thickness of the crust varies from 30 to 80 km (on average close to 40 km). The consolidated oceanic crust is thinner - usually reaching 6.5-7 km - and is composed (from top to bottom) of basalts, gabbros and serpentinites. The thickness of sedimentary deposits on the oceanic crust is not constant: it increases in the coastal zones of the oceans and pinches out on the crests of mid-ocean ridges. On average, the thickness of oceanic sediments reaches 500 m. 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 area of ​​rift zones of the oceans to 80-90 km near the ocean coasts. The thickness of ancient continental plates reaches 200-250 km.

As is known, the temperature in the Earth gradually increases with depth. Beneath the oceanic plates, the temperature of the mantle reaches the melting temperature of mantle rocks (see Fig. 19).

Figure 19.
The numbers on geotherms indicate the age of lithospheric plates in millions of years. T oc—geotherms of oceanic plates; Tm—adiabatic temperature of the upper mantle; T s is the solidus temperature of the mantle material; T cl—geotherm of ancient (Archean) continental lithospheric plates; CC—base of continental crust; I – boundary of the phase transition of basalts to eclogites; II - endothermic transition from the rigid lithosphere under the continents to a plastic state; III—the base of the Archean sections of the continental lithosphere.

Therefore, the surface of the beginning of melting of mantle material with the temperature of its solidus is taken as the base of the lithosphere under the oceans. Below the oceanic lithosphere, the mantle material appears to be partially molten and plastic with 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 oceanic plates (under the oceans it was first discovered as a layer forming a seismic waveguide). Under thick continental plates there is virtually no asthenosphere, although they are also underlain by plastic material of the upper mantle (see 19).

The asthenosphere plays a decisive role in the formation of basaltic magmatism in oceanic plates and in the interaction of convective mass transfer of the mantle with the lithospheric shell. Basaltic magmatism of continental plates can only manifest itself in the case when hot mantle material, due to the splitting of the plate, can rise to the level at which this substance begins to melt (approximately at depths of about 80-100 km).

Unlike the lithosphere, the asthenosphere does not have a tensile strength, and its substance can deform (flow) under the influence of even very small excess pressures, although this process, due to the high viscosity of the asthenosphere substance - about 10 18 -10 20 P, develops extremely slowly (for comparison note that the viscosity of water is 10 -2 P, liquid basaltic lava - 10 4 -10 6, ice - about 10 13 and rock salt - about 10 18 P). Under the influence of the high hydrostatic pressures prevailing in the interior of the Earth, the melting temperature of silicates with depth increases 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.

There are seven large plates on Earth: the 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 eastern Pacific Ocean, the Philippine, Arabian, Somali, Caribbean and Scotia plates, located between South America and Antarctica. Sometimes, within large continental plates, medium plates are distinguished as independent ones, 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).


Figure 6.
Barazangi, Dorman, 1968

Movements of lithospheric plates along the surface of the asthenosphere occur 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 rift cracks along the plate boundaries appear between the plates, in the second - compression zones, accompanied by the thrust of one of the plates onto the other, in the third - shear zones, transform faults, along which the neighboring plates are displaced.

In accordance with the different nature of 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—isotherm 500 °C (beginning of serpentinization).

In the oceans, these boundaries correspond to the crests of mid-ocean ridges: in the Arctic Ocean - ridge. Gakkel, Knipovich, Mohn and Kolbensei; in the Atlantic - ridge. Reykjanes, North Atlantic, South Atlantic and African-Antarctic; in the Indian Ocean - ridge. West Indian, Arabian-Indian, Central Indian and Australasian-Antarctic rise; V Pacific Ocean- South Pacific and East Pacific rises. On continents, boundaries of this type include the East African Rift Zone and the Baikal Rift in Asia. An example of rift zones that only relatively recently turned from continental to oceanic are the rifts of the Red Sea and the Gulf of Aden Indian Ocean.

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 continents, divergent plate boundaries are marked by eruptions of trap basalts and contrasting bimodal basaltic-sialic and alkaline magmatism and somewhat deeper-focus earthquakes (up to 200 km).

The boundaries of the second, or convergent, type include zones of plate thrust (subduction zones), in which oceanic lithospheric plates are pushed under island arcs or under Andean-type continental margins. These boundaries usually correspond to characteristic relief forms: conjugate 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 the thrust occurs under the continents. Examples of such boundaries in the oceans include deep-sea trenches in front of the Aleutian, Kuril-Kamchatka, Japanese, Mariana, Philippine island arcs, deep-sea trenches at the foot of New Britain, the Solomon Islands, the New Hebrides, Tonga-Kermadec islands, as well as at the foot of west coasts Central and South America in the Pacific Ocean. In the Indian Ocean, these are the trenches of the Andoman, Greater and Lesser Sunda Islands. IN 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 subduction of lithospheric plates are always inclined (“fall”) under island arcs or continental margins and are usually clearly distinguished by chains of earthquake sources. Plates sinking into the mantle are also characterized by increased values ​​of the seismic quality factor Q, since in a sinking cold lithospheric plate the attenuation of seismic waves is always less than in the hot and partially molten mantle surrounding this plate. The 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 strata; 6—ocean 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 subduction of oceanic plates under the continents, if it is not compensated by their moving apart at mid-ocean ridges, usually leads to a gradual closure of the ocean, accompanied by the collision of the continents that frame it, and the emergence of a collisional fold belt along the plate subduction zone. In this way, for example, the Alpine-Himalayan mountain belt arose on the site of the ancient Tethys Ocean. The process of plate underthrusting continues here at the present time, as evidenced by the increased seismicity of this region, therefore the Alpine-Himalayan belt can also be considered as a convergent or collisional plate boundary.

Detailed studies of mid-ocean ridges have established that their crests and rift valleys They 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 plate boundaries of the third type, or transform faults. As a rule, they are always located perpendicular to the strike of rift cracks. In this case, active sections of faults are only their segments that connect two adjacent rift zones (transforming one of them into the other). Outside these active areas, no plate displacements along transform faults occur. The amplitude of displacement along most of these faults does not exceed ten or several tens of kilometers, but sometimes it reaches hundreds of kilometers.

Transform faults sometimes cross plate thrust 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 Romanche faults in the Atlantic Ocean; the Owen and Amsterdam faults in the Indian Ocean; Eltanin and Challenger faults in the Pacific Ocean. In addition, in the northern half of the Pacific Ocean there are traces of now extinct, but once gigantic faults, displacements along which occurred by many hundreds and even 1,200 km. These are the so-called great faults of the Pacific Ocean floor: Mendocino, Pioneer, Murray, Molokai, Clarion and Clipperton. An example of the third type of boundary on continents is the San Andreas Fault in California. In the relief, oceanic transform faults are clearly identified by conjugate parallel structures of narrow ridges and troughs with a steep common wall (Fig. 81). Moreover, due to the “soldering” of lithospheric plates to 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 on the side of younger plates and, conversely, troughs arise only on 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) basaltic volcanoes with an alkaline orientation may arise on their flanks.

Figure 81.

Movements of lithospheric plates are accompanied by their friction against each other and the occurrence of earthquakes along the plate boundaries. Therefore, the boundaries of lithospheric plates can be distinguished not only by geomorphological characteristics, but also by zones of increased seismicity. At the same time, different earthquake mechanisms correspond to different plate boundaries. Thus, in ocean rift zones, all earthquakes located under the crests of mid-ocean ridges are shallow-focus with a focal 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 shear. Seismically, the zones of plate underthrust are the most active. In these zones there are both shallow-focus earthquakes with a focal depth of up to 30 km, intermediate earthquakes at depths from 30 to 150-200 km, and deep-focus earthquakes with a focal depth of up to 600-700 km. The main seismofocal surface of plate underthrust zones usually descends at an angle of about 30-50° from the axis of the deep-sea trench under an island arc or continental margin, outlining the body of the oceanic plate being subducted into the mantle. In zones of plate subduction, earthquakes of various types occur, but among shallow earthquakes, shear and reverse-thrust mechanisms predominate, and at medium and great depths, shear and compression mechanisms predominate.

As a rule, the maximum 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 substance are disrupted, and it apparently acquires the properties of an amorphous substance. However, judging by seismic tomography data, traces of sinking oceanic plates can be traced deeper into the lower mantle, right down 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 can be traced on the surface of the core with an amplitude of up to 4 km, and under the ascending flows in the centers of these same oceans, as well as under North Atlantic On the contrary, rises in its relief with an amplitude of up to 6 km are observed (see Fig. 12).


Figure 58.
T S is the solidus temperature of the mantle material (using data from Green, Ringwood, 1967 and Takahashi, 1986); T M is the adiabatic temperature of the convecting mantle (Sorokhtin, 2001); T Cont—continental geotherm beneath Archean cratons (Sorokhtin et al., 1996); The dotted line shows 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—transition of silica into the stishovite structure (St) and pyroxenes into the ilmenite structure (Ilm). Endothermic transitions: III - the supposed transition from a rigid polycrystalline substance to its plastic state; VI—transition of pyroxenes into the structure of perovskite (Pv) and magnesiowüstite (Mw). Phase transitions I and II are constructed according to the data of Green and Ringwood (1967), generalized transitions IV, V and VI - according to the data of Kuskov and Fabrichnaya (1990).


Figure 12.
isolines are drawn every 2 km, according to Morelli, Dziewonski, 1987

It is noteworthy that many plates include both continental massifs and sections of oceanic lithosphere welded to them. For example, the African Plate includes the continent of Africa itself and the adjacent eastern halves of the Central and South Atlantic, the western parts of the floor of the Indian Ocean, as well as the areas 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, Coco and Philippine plates.

To a first approximation, lithospheric plates can be considered as fragments of a rigid spherical shell moving along the surface of the Earth. 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. In relation 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 the rotation of the plate with a certain angular velocity relative to an axis passing through the center of the Earth and a certain point on its surface, called the rotation pole of this plate.

In the process of detailed study of the tectonic structure of the ocean floor, one remarkable rule emerged. It turned out that almost all rift faults are always oriented towards the corresponding poles of plate movement, 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 meridians and latitudinal circles drawn from the pole of mutual rotation of the plates. From Euler's theory it follows that the speed 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 peculiarities of plate movements, Euler’s theorem made it possible, using paleomagnetic anomalies on the ocean floor, to quantitatively calculate the movements of the entire ensemble of lithospheric plates on the Earth’s surface and to construct paleogeodynamic reconstructions of the positions of ancient oceans and continents in past geological eras.


Figure 8.
(Heirtzler et al., 1966). Positive anomalies are indicated in black; AA—zero anomaly of the rift zone.

To determine the speed of movement 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 that poured out onto them by the magnetic field that existed on Earth at the time of the basalts’ outpouring. But, as is known, the geomagnetic field from time to time changed direction to the exact opposite. This led to the fact that basalts that erupted during different periods of geomagnetic field reversals turned out to be magnetized in opposite directions. But thanks to the spreading of the ocean floor in the rift zones of mid-ocean ridges, more ancient basalts are always moved to great distances from these zones, and along with the ocean floor, the ancient magnetic field of the Earth “frozen” into the basalts moves away from them.

The expansion of the oceanic crust, together with differently magnetized basalts, usually develops strictly symmetrically on both sides of the rift fault. Therefore, the associated magnetic anomalies are also located symmetrically on both slopes of 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 the rate of its expansion in rift zones. However, for this it is necessary to know the age of individual reversals of the Earth's magnetic field and compare these reversals with magnetic anomalies observed on the ocean floor.

The age of magnetic reversals was determined from detailed paleomagnetic studies of well-dated strata of basalt covers and sedimentary rocks of 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 waters of the World Ocean (see Fig. 9).

Figure 82.
(time is given in millions of years)


Figure 9.
from Larson, Pitman et al., 1985

The presented conclusions of the theory make it possible to quantitatively calculate the parameters of motion at the beginning of two adjacent plates, and then for the third, taken in tandem with one of the previous ones. In this way, it is gradually possible to involve the main of the identified lithospheric plates in the calculation and determine the mutual movements of all plates on the Earth's surface.

Abroad, such calculations were performed by J. Minster and his colleagues, and in Russia by S. A. Ushakov and Yu. I. Galushkin (Fig. 83). It turned out that the ocean floor is moving apart at maximum speed in the southeastern part of the Pacific Ocean (near Easter Island). In this place, up to 18 cm of new oceanic crust grows annually. On a geological scale, this is a lot, since in just 1 million years a strip of young bottom up to 180 km wide is formed in this way, while for each kilometer of the rift zone approximately 360 km 3 of basalt lavas flow out during the same time! According to the same calculations, Australia is moving away from Antarctica at a speed of about 7 cm/year, and South America from Africa - at a rate of about 4 cm/year. Moving aside North America from Europe it occurs more slowly - 2-2.3 cm/year. The Red Sea is expanding even more slowly - by 1.5 cm/year (accordingly, less basalts are poured out here - only 30 km3 for each linear kilometer of the Red Sea rift over 1 million years). But the speed of the “collision” of India with Asia reaches 5 cm/year, which explains the intense neotectonic deformations developing before our eyes and the growth of the mountain systems of the Hindu Kush, Pamir and Himalayas. These deformations create a high level of seismic activity in the entire region (the tectonic influence of the collision of India with Asia affects far beyond the plate collision zone itself, spreading all the way to Lake Baikal and areas of the Baikal-Amur Mainline). Deformations of the Greater and Lesser Caucasus are caused by the pressure of the Arabian Plate on this region of Eurasia, but the rate of convergence of the plates here is significantly less - only 1.5-2 cm/year. Therefore, the seismic activity of the region is also less here.


Figure 83.
1 - oceanic rift zones and transform faults; 2—continental rift zones; 3—zones of subduction of oceanic lithospheric plates under island arcs; 4 - the same, under the active margins of Andean-type continents; 5 - zones of “collision” (collision) of continental plates; 6—transform (shear) plate boundaries; 7 - lithospheric plates; 8 - directions and speeds (cm/year) of the relative movement of the plates.

The importance of the above calculations is obvious, since they make it possible to quantify the modern tectonic activity of the Earth and the volume of magmatic outpourings in modern rift zones. But it turns out that using a similar technique and consistently combining coeval magnetic anomalies with each other, it is possible for past geological times to construct accurate reconstructions of the position of continents and oceans (with mid-ocean ridges in them) and determine the speed of expansion or subtraction of the ocean floor under 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 had already sunk into the mantle under modern or ancient zones of plate thrust, and, therefore, no magnetic anomalies with an age exceeding 150 million years were preserved on the ocean floor. Therefore, for the more ancient geological epochs Only approximate paleogeographic reconstructions can be constructed using paleomagnetic data on continents. Such reconstructions, constructed 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).

Lithospheric plates have high rigidity and are capable of maintaining their structure and shape without changes for a long time in the absence of external influences.

Plate movement

Lithospheric plates are in constant motion. This movement, occurring in the upper layers of the asthenosphere, is due to the presence of convective currents present in the mantle. Individual lithospheric plates approach, diverge, and slide relative to each other. When the plates come together, compression zones arise and subsequent thrusting (obduction) of one of the plates onto the neighboring one, or pushing (subduction) of adjacent formations. When divergence occurs, tension zones appear with characteristic cracks appearing along the boundaries. When sliding, faults are formed, in the plane of which sliding of nearby plates is observed.

Movement results

In areas of convergence of huge continental plates, when they collide, mountain ranges. Similarly, at one time the Himalaya mountain system arose, formed on the border of the Indo-Australian and Eurasian plates. The result of the collision of oceanic lithospheric plates with continental formations is island arcs and deep-sea trenches.

In the axial zones of mid-ocean ridges, rifts (from the English Rift - fault, crack, crevice) of a characteristic structure arise. Similar formations of the linear tectonic structure of the earth's crust, with a length of hundreds and thousands of kilometers, with a width of tens or hundreds of kilometers, arise as a result of horizontal stretching of the earth's crust. Very large rifts are usually called rift systems, belts or zones.

Due to the fact that each lithospheric plate is a single plate, increased seismic activity and volcanism are observed in its faults. These sources are located within fairly narrow zones, in the plane of which friction and mutual movements of neighboring plates occur. These zones are called seismic belts. Deep-sea trenches, mid-ocean ridges and reefs are mobile areas of the earth's crust, they are located at the boundaries of individual lithospheric plates. This circumstance once again confirms that the process of formation of the earth’s crust in these places continues quite intensively at the present time.

The importance of the theory of lithospheric plates cannot be denied. Since it is she who is able to explain the presence of mountains in some regions of the Earth, and plains in others. The theory of lithospheric plates makes it possible to explain and predict the occurrence of catastrophic phenomena that can occur in the area of ​​their boundaries.

Consists of many layers piled on top of each other. However, what we know best is the earth's crust and lithosphere. This is not surprising - after all, we not only live on them, but also draw from the depths most of the resources available to us. natural resources. But the upper shells of the Earth still preserve millions of years of history of our planet and the entire solar system.

These two concepts appear so often in the press and literature that they have entered the everyday vocabulary of modern man. Both words are used to refer to the surface of the Earth or another planet - however, there is a difference between the concepts, based on two fundamental approaches: chemical and mechanical.

Chemical aspect - earth's crust

If we divide the Earth into layers, guided by differences in chemical composition, top layer the planet will have the earth's crust. This is a relatively thin shell, ending at a depth of 5 to 130 kilometers below sea level - the oceanic crust is thinner, and the continental crust, in mountainous areas, is thickest. Although 75% of the crust's mass is composed only of silicon and oxygen (not pure, bound in different substances), it has the greatest chemical diversity of all layers of the Earth.

The wealth of minerals also plays a role - various substances and mixtures created over billions of years of the planet’s history. The Earth's crust contains not only "native" minerals that were created by geological processes, but also massive organic heritage, such as oil and coal, as well as alien inclusions.

Physical aspect - lithosphere

Based on the physical characteristics of the Earth, such as hardness or elasticity, we will get a slightly different picture - the interior of the planet will be enveloped by the lithosphere (from the Greek lithos, “rocky, hard” and “sphaira” sphere). It is much thicker than the earth's crust: the lithosphere extends up to 280 kilometers deep and even covers the upper solid part of the mantle!

The characteristics of this shell fully correspond to the name - it is the only solid layer of the Earth, besides the inner core. Strength, however, is relative - the Earth’s lithosphere is one of the most mobile in solar system, which is why the planet has changed its appearance more than once. But significant compression, curvature and other elastic changes require thousands of years, if not more.


  • An interesting fact is that the planet may not have a surface crust. So, the surface is its hardened mantle; The planet closest to the Sun lost its crust a long time ago as a result of numerous collisions.

To summarize, the Earth's crust is the upper, chemically diverse part of the lithosphere, the hard shell of the Earth. Initially they had almost the same composition. But when only the underlying asthenosphere and high temperatures affected the depths, the hydrosphere, atmosphere, meteorite remains and living organisms actively participated in the formation of minerals on the surface.

Lithospheric plates

Another feature that distinguishes the Earth from other planets is the diversity of different types of landscapes on it. Of course, water also played an incredibly important role, which we will talk about a little later. But even the basic forms of the planetary landscape of our planet differ from the same Moon. The seas and mountains of our satellite are pits from bombardment by meteorites. And on Earth they were formed as a result of hundreds and thousands of millions of years of movement of lithospheric plates.


You've probably already heard about plates - these are huge stable fragments of the lithosphere that drift along the fluid asthenosphere, like broken ice on a river. However, there are two main differences between the lithosphere and ice:

  • The gaps between the plates are small and are quickly closed due to the molten substance erupting from them, and the plates themselves are not destroyed by collisions.
  • Unlike water, there is no constant flow in the mantle, which could set a constant direction for the movement of the continents.

So, driving force The drift of lithospheric plates is the convection of the asthenosphere, the main part of the mantle - hotter flows from the earth's core rise to the surface when cold ones fall back down. Considering that the continents differ in size, and the topography of their lower side mirrors the irregularities of the upper side, they also move unevenly and inconsistently.


Main plates

Over billions of years of movement of lithospheric plates, they repeatedly merged into supercontinents, after which they separated again. In the near future, in 200–300 million years, the formation of a supercontinent called Pangea Ultima is also expected. We recommend watching the video at the end of the article - it clearly shows how lithospheric plates have migrated over the past several hundred million years. In addition, the strength and activity of continental movement is determined by the internal heating of the Earth - the higher it is, the more the planet expands, and the faster and freer the lithospheric plates move. However, since the beginning of the Earth's history, its temperature and radius have been gradually decreasing.

  • An interesting fact is that plate drift and geological activity do not necessarily have to be powered by the internal self-heating of the planet. For example, the satellite of Jupiter has many active volcanoes. But the energy for this is not provided by the satellite’s core, but by gravitational friction c, due to which Io’s interior heats up.

The boundaries of lithospheric plates are very arbitrary - some parts of the lithosphere sink under others, and some, like the Pacific plate, are completely hidden under water. Geologists today count 8 main plates that cover 90 percent of the entire Earth's area:

  • Australian
  • Antarctic
  • African
  • Eurasian
  • Hindustan
  • Pacific
  • North American
  • South American


This division appeared recently - so, Eurasian plate 350 million years ago it consisted of separate parts, during the merger of which the Ural Mountains were formed, one of the oldest on Earth. Scientists to this day continue to study faults and the ocean floor, discovering new plates and clarifying the boundaries of old ones.

Geological activity

Lithospheric plates move very slowly - they creep over each other at a speed of 1–6 cm/year, and move away by a maximum of 10–18 cm/year. But it is the interaction between the continents that creates the geological activity of the Earth, noticeable on the surface - volcanic eruptions, earthquakes and the formation of mountains always occur in the contact zones of lithospheric plates.

However, there are exceptions - so-called hot spots, which can also exist deep in lithospheric plates. In them, molten flows of asthenosphere matter break upward, melting the lithosphere, which leads to increased volcanic activity and regular earthquakes. Most often, this happens near those places where one lithospheric plate creeps onto another - the lower, depressed part of the plate sinks into the Earth's mantle, thereby increasing the pressure of magma on the upper plate. However, now scientists are inclined to believe that the “drowned” parts of the lithosphere are melting, increasing pressure in the depths of the mantle and thereby creating upward flows. This can explain the anomalous distance of some hot spots from tectonic faults.


  • Interesting fact - hot spots often form shield volcanoes, characterized by their flat shape. They erupt many times, growing due to flowing lava. This is also a typical alien volcano format. The most famous of them on Mars, the most high point planet - its height reaches 27 kilometers!

Oceanic and continental crust of the Earth

The interaction of plates also leads to the formation of two various types earth's crust - oceanic and continental. Since the oceans, as a rule, are the junctions of different lithospheric plates, their crust is constantly changing - being broken or absorbed by other plates. At the site of faults, direct contact occurs with the mantle, from where hot magma rises. As it cools under the influence of water, it creates a thin layer of basalts, the main volcanic rock. Thus, the oceanic crust is completely renewed every 100 million years - the oldest sections, which are located in the Pacific Ocean, reach maximum age at 156–160 million years.

Important! The oceanic crust is not all of the earth’s crust that is under water, but only its young sections at the junction of continents. Part of the continental crust is under water, in the zone of stable lithospheric plates.