Lithospheric plates have high rigidity and are capable of retaining their structure and shape unchanged for a long time in the absence of external influences.
Plate movement
Lithospheric plates are in constant motion. This movement, which takes place in the upper layers, is due to the presence of convective currents present in the mantle. Separately taken lithospheric plates approach, diverge and slide relative to each other. When the plates approach each other, compression zones arise and the subsequent thrust (obduction) of one of the plates onto the adjacent one, or the thrust (subduction) of the adjacent formations. When the divergence occurs, tension zones with characteristic cracks appear along the boundaries. When sliding, faults are formed, in the plane of which adjacent plates are observed.
Motion results
In the areas of convergence of huge continental plates, when they collide, mountain ranges arise. Similarly, at one time the Himalayan mountain system arose, formed on the border of the Indo-Australian and Eurasian plates. Island arcs and deep-sea depressions result from the collision of oceanic lithospheric plates with continental formations.
In the axial zones of the mid-oceanic ridges, rifts (from the English Rift - fault, crack, crevice) of a characteristic structure appear. Such formations of the linear tectonic structure of the earth's crust, which are hundreds and thousands of kilometers long, tens or hundreds of kilometers wide, arise as a result of horizontal stretching of the earth's crust. Rifts of very large sizes are usually called rift systems, belts or zones.
Since each lithospheric plate is a single plate, increased seismic activity and volcanism are observed in its faults. These sources are located within rather narrow zones, in the plane of which friction and mutual displacements of adjacent plates arise. These zones are called seismic belts. Deep-sea trenches, mid-ocean ridges and reefs are mobile regions of the earth's crust located at the boundaries of individual lithospheric plates. This once again confirms that the course of the formation of the earth's crust in these places is still going on quite intensively.
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 areas of the Earth, in others -. The theory of lithospheric plates makes it possible to explain and foresee the occurrence of catastrophic phenomena that can occur in the area of their boundaries.
Lithospheric plates of the Earth are huge blocks. Their foundation is formed by granite metamorphosed igneous rocks strongly crumpled into folds. The names of the lithospheric plates will be given in the article below. From above they are covered with a three-four-kilometer "cover". It is formed from sedimentary rocks... The platform has a relief consisting of individual mountain ranges and vast plains. Further, the theory of the movement of lithospheric plates will be considered.
The emergence of a hypothesis
The theory of the movement of lithospheric plates appeared at the beginning of the twentieth century. Subsequently, she was destined to play a major role in planetary exploration. Scientist Taylor, and after him Wegener, put forward a hypothesis that over time there is a drift of lithospheric plates in the horizontal direction. However, in the thirties of the 20th century, a different opinion was established. According to him, the movement of lithospheric plates was carried out vertically. This phenomenon was based on the process of differentiation of the planet's mantle matter. It came to be called fixism. This name was due to the fact that the permanently fixed position of the crustal areas relative to the mantle was recognized. But in 1960, after the discovery of the global system of mid-ocean ridges that encircle the entire planet and come out on land in some areas, there was a return to the hypothesis of the early 20th century. However, the theory took on a new form. Block tectonics has become a leading hypothesis in the sciences that study the structure of the planet.
Basic Provisions
It was determined that there are large lithospheric plates. Their number is limited. There are also smaller lithospheric plates of the Earth. The boundaries between them are drawn along the thickening in the foci of earthquakes.
The names of the lithospheric plates correspond to the continental and oceanic regions located above them. There are only seven boulders with a huge area. The largest lithospheric plates are South and North American, Euro-Asian, African, Antarctic, Pacific and Indo-Australian.
Lumps floating in the asthenosphere are solid and rigid. The above areas are the main lithospheric plates. In accordance with the initial ideas, it was believed that the continents make their way through the ocean floor. In this case, the movement of lithospheric plates was carried out under the influence of an invisible force. As a result of the studies carried out, it was revealed that the blocks float passively over the mantle material. It is worth noting that their direction is vertical at first. The mantle material rises upward under the ridge crest. Then there is a spread in both directions. Accordingly, there is a divergence of the lithospheric plates. This model presents the ocean floor as a giant one. It comes to the surface in the rift regions of the mid-ocean ridges. Then it hides in deep-sea trenches.
The divergence of lithospheric plates provokes the expansion of oceanic beds. However, the volume of the planet, despite this, remains constant. The fact is that the birth of a new crust is compensated by its absorption in the areas of subduction (underthrust) in deep-sea trenches.
Why does the movement of lithospheric plates occur?
The reason lies in the thermal convection of the planet's mantle material. The lithosphere is stretched and lifted, which occurs above the ascending branches from convective currents. This provokes the movement of the lithospheric plates to the sides. With increasing distance from the mid-oceanic rifts, the platform compaction occurs. It becomes heavier, its surface sinks down. This explains the increase in ocean depth. As a result, the platform sinks into deep-sea trenches. When decaying from the heated mantle, it cools and sinks with the formation of basins that are filled with sediments.
Lithospheric plate collision zones are areas where crust and plate are compressed. In this regard, the power of the former is increased. As a result, the upward movement of lithospheric plates begins. It leads to the formation of mountains.
Research
The study today is carried out using geodetic methods. They allow us to draw a conclusion about the continuity and ubiquity of processes. The zones of collision of lithospheric plates are also revealed. The lifting speed can be up to ten millimeters.
Horizontally large lithospheric plates float somewhat faster. In this case, the speed can be up to ten centimeters during the year. So, for example, St. Petersburg has already risen by a meter over the entire period of its existence. The Scandinavian Peninsula - 250 m in 25,000 years. The mantle material moves relatively slowly. However, as a result, earthquakes and other phenomena occur. This allows us to conclude about the high power of material movement.
Using the tectonic position of the plates, researchers explain a variety of geological phenomena. At the same time, in the course of the study, it became clear that the complexity of the processes taking place with the platform is much greater than it seemed at the very beginning of the hypothesis.
Plate tectonics could not explain the changes in the intensity of deformations and movement, the presence of a global stable network of deep faults, and some other phenomena. Remains also open question about the historical beginning of the action. Direct signs indicating plate tectonic processes have been known since the Late Proterozoic. However, a number of researchers recognize their manifestation from the Archean or Early Proterozoic.
Expanding Research Opportunities
The advent of seismic tomography led to the transition of this science to a qualitatively new level. In the mid-eighties of the last century, deep geodynamics became the most promising and young direction of all the existing earth sciences. However, the solution of new problems was carried out using not only seismotomography. Other sciences also came to the rescue. These include, in particular, experimental mineralogy.
Thanks to the availability of new equipment, it became possible to study the behavior of substances at temperatures and pressures corresponding to the maximum at the depths of the mantle. Also, the research used the methods of isotope geochemistry. This science studies, in particular, the isotopic balance of rare elements, as well as noble gases in various earthly shells... In this case, the indicators are compared with meteorite data. The methods of geomagnetism are used, with the help of which scientists are trying to reveal the causes and mechanism of reversals in the magnetic field.
Modern painting
The platform tectonics hypothesis continues to provide a satisfactory explanation of the crustal evolutionary process over at least the last three billion years. At the same time, there are satellite measurements, according to which the fact is confirmed that the main lithospheric plates of the Earth do not stand still. As a result, a certain picture emerges.
There are three most active layers in the cross section of the planet. The capacity of each of them is several hundred kilometers. It is assumed that the main role in global geodynamics is assigned to them. In 1972, Morgan substantiated the hypothesis of ascending mantle jets put forward in 1963 by Wilson. This theory explained the phenomenon of intraplate magnetism. The resulting plume tectonics has become increasingly popular over time.
Geodynamics
With its help, the interaction of rather complex processes that occur in the mantle and crust is considered. In accordance with the concept outlined by Artyushkov in his work "Geodynamics", gravitational differentiation of matter acts as the main source of energy. This process is noted in the lower mantle.
After the heavy components (iron, etc.) are separated from the rock, a lighter mass of solids remains. She sinks into the core. The location of the lighter layer under the heavy is unstable. In this regard, the accumulating material collects periodically into large enough blocks that float to the upper layers. The size of such formations is about one hundred kilometers. This material was the basis for the formation of the upper
The lower layer is probably an undifferentiated primary substance. During the evolution of the planet, due to the lower mantle, the upper mantle grows and the core increases. It is more likely that blocks of light material rise in the lower mantle along the channels. The temperature of the mass in them is quite high. At the same time, the viscosity is significantly reduced. An increase in temperature is facilitated by the release of a large volume of potential energy in the process of ascent of matter into the region of gravity over a distance of about 2000 km. In the course of movement along such a channel, a strong heating of light masses occurs. In this regard, matter enters the mantle, having a sufficiently high temperature and significantly less weight in comparison with the surrounding elements.
Due to the lowered density, light material floats into the upper layers to a depth of 100-200 kilometers or less. With decreasing pressure, the melting point of the components of the substance decreases. After primary differentiation at the core-mantle level, a secondary one occurs. At shallow depths, light matter undergoes partial melting. During differentiation, denser substances are released. They sink into the lower layers of the upper mantle. The lighter components that stand out, respectively, rise up.
The complex of movements of substances in the mantle associated with the redistribution of masses with different densities as a result of differentiation is called chemical convection. The rise of light masses occurs at a frequency of about 200 million years. At the same time, intrusion into the upper mantle is not observed everywhere. In the lower layer, the channels are located at a fairly large distance from each other (up to several thousand kilometers).
Lump lifting
As mentioned above, in those zones where large masses of light heated material are introduced into the asthenosphere, it partially melts and differentiates. In the latter case, the selection of components and their subsequent emergence are noted. They quickly pass through the asthenosphere. Upon reaching the lithosphere, their speed decreases. In some areas, matter forms clusters of anomalous mantle. They usually occur in the upper layers of the planet.
Abnormal mantle
Its composition roughly corresponds to normal mantle material. The difference between the anomalous accumulation is a higher temperature (up to 1300-1500 degrees) and a reduced speed of elastic longitudinal waves.
The influx of matter under the lithosphere provokes isostatic uplift. Due to the increased temperature, the anomalous cluster has a lower density than the normal mantle. In addition, there is a low viscosity of the composition.
In the process of entering the lithosphere, the anomalous mantle is rather quickly distributed along the base. At the same time, it displaces the denser and less heated matter of the asthenosphere. In the course of movement, the anomalous accumulation fills those areas where the base of the platform is in a raised state (traps), and it flows around deeply submerged areas. As a result, in the first case, isostatic uplift is noted. Above the submerged areas, the crust remains stable.
Traps
The process of cooling the mantle upper layer and crust to a depth of about one hundred kilometers is slow. In general, it takes several hundred million years. In this regard, heterogeneities in the thickness of the lithosphere, explained by horizontal temperature differences, have a fairly large inertia. In the event that the trap is located near the upward flow of the anomalous cluster from the depths, a large number of the substance is captured when it is very hot. As a result, a rather large rock element is formed. In accordance with this scheme, high uplifts occur at the site of epiplatform orogenesis in
Description of processes
In the trap, the anomalous layer is compressed by 1–2 kilometers during cooling. The bark located on top sinks. In the formed trough, sediments begin to accumulate. Their severity contributes to an even greater sinking of the lithosphere. As a result, the depth of the basin can be from 5 to 8 km. At the same time, during compaction of the mantle in the lower part of the basalt layer in the crust, a phase transformation of the rock into eclogite and garnet granulite can be noted. Due to the heat flow escaping from the anomalous substance, the overlying mantle heats up and its viscosity decreases. In this regard, a gradual displacement of the normal accumulation is observed.
Horizontal displacements
With the formation of uplifts in the process of anomalous mantle inflow to the crust on the continents and oceans, there is an increase in the potential energy stored in the upper layers of the planet. To dump excess substances, they tend to disperse to the sides. As a result, additional stresses are formed. Various types of movement of plates and crust are associated with them.
The expansion of the ocean floor and the floating of continents are a consequence of the simultaneous expansion of the ridges and the immersion of the platform into the mantle. Under the first are large masses of highly heated anomalous matter. In the axial part of these ridges, the latter is located directly under the crust. The lithosphere is much less powerful here. At the same time, the abnormal mantle spreads out in the area of increased pressure - in both directions from under the ridge. At the same time, it tears apart the ocean crust quite easily. The crevice is filled with basalt magma. She, in turn, is smelted from the anomalous mantle. In the process of solidification of magma, a new one is formed. This is how the bottom grows.
Process features
Below the middle ridges, the anomalous mantle has a reduced viscosity due to the increased temperature. The substance is capable of spreading quickly enough. In this regard, the growth of the bottom occurs at an increased rate. The oceanic asthenosphere also has a relatively low viscosity.
The main lithospheric plates of the Earth float from ridges to diving sites. If these areas are in the same ocean, then the process occurs at a relatively high speed. This situation is typical today for The Pacific... If the expansion of the bottom and subsidence occurs in different areas, then the continent located between them drifts in the direction where the deepening occurs. Under the continents, the viscosity of the asthenosphere is higher than under the oceans. Due to the friction that occurs, significant resistance to movement appears. As a result, the rate at which the bottom expands is reduced if there is no compensation for the subsidence of the mantle in the same area. Thus, proliferation in the Pacific Ocean is faster than in the Atlantic.
The main structural units at the level of the lithosphere are lithospheric plates, reflecting its lateral heterogeneities. Their boundaries cross the earth's crust and the suprastenospheric mantle, and often, according to seismic data, can be traced to significant depths in the lower mantle. Among the structures of the second order within the lithospheric plates, their continental and oceanic segments (continents and oceans) are distinguished, which are most sharply distinguished by the structure of the earth's crust. The development of the main structural units of the lithosphere is described by the tectonics of the lithospheric plates.
In the main provisions of tectonics of lithospheric plates there are six postulates.
1) In the upper shells of the solid Earth, a fragile shell is distinguished by rheological properties - lithosphere and the underlying plastic shell - asthenosphere.
2) The lithosphere is divided into a limited number of large and small plates. Large lithospheric plates are - Eurasian, African, North American, South American, Pacific, Australian, Nazca... Among small slabs and microplates stand out: Juan de Fuca, Coconut, Caribbean, Arabian, Chinese, Indochinese, Okhotsk, Philippine.
3) There are three types of lithospheric plate boundaries: divergent boundaries along which the slabs move apart; convergent borders, along which the plates approach and submerge one under the other or collide with each other, transform boundaries where the plates slide against each other.
4) The horizontal movement of the plates can be described by the laws of Euler's spherical geometry, according to which any movement of two conjugate points on a sphere occurs along a circle drawn relative to an axis passing through the center of the Earth. The exit of this axis to the earth's surface is called the pole of rotation or opening.
5) The area absorbed at the convergent boundaries of the oceanic crust is equal to the area of the crust formed at the divergent boundaries.
6) The main reason for the movement of lithospheric plates is convection in the mantle.
An important addition to the "classical" plate tectonics is plume tectonics , ideas of which began to form simultaneously with plate tectonics, which used "hot Spots" oceans to trace the movement of lithospheric plates. At present, according to seismic tomography data, flows of decompressed heated matter (plumes) are distinguished, emanating from different deep shells of the Earth.
Divergent boundaries of lithospheric plates caused by the processes of rifting and reflect the geodynamic conditions of lateral extension, oriented mainly across the strike of divergent boundaries. Morphologically rift structures are expressed by complex systems of grabens, limited by faults. Most of the rift structures form a single global system that crosses continents and oceans. Most of the system (about 60 thousand km) is located in the oceans and is expressed by mid-oceanic ridges. On continents ocean rifts often continue continental rifts . When crossing active continental margins, mid-ocean ridges can be absorbed in subduction zones. The extinction of rift zones along strike is gradual or interrupted by transform faults. Rift zones form an almost complete ring around South Pole at latitudes 40-60 °. Three branches decaying to the north extend from this ring in the meridional direction: East Pacific, Atlantic and Indian Ocean... Only a few of the major rift zones are outside the global system.
The mechanisms of rifting include deformation rifting and the mechanism of hydraulic wedging. During deformation rifting stretching is realized by discontinuous and viscous deformations in a relatively narrow strip with a decrease in the thickness of this strip and the formation of a "neck". Several models of deformation rifting have been proposed. R. Smith et al. Model with sub-horizontal disruption between the brittle and plastic deformations; model by W. Hamilton et al. with a lenticular nature of deformations; B. Wernicke's model considering asymmetric deformation based on shallow fault.
Hydraulic wedging mechanism provides as an active force basaltic magma, which pushes the rocks, penetrating from below into vertical cracks between them and forming swarms of parallel dikes. Fractures result from hydraulic fracturing under the influence of the same magma.
Opening of spreading zones can be done in two ways. The first one active rifting proceeds from the primacy of the ascending flow of asthenospheric matter. The stream lifts and expands the lithosphere, which ultimately leads to its thinning and rupture. Passive rifting due to tensile forces that are applied directly to the deformable layer.
Transform boundaries of lithospheric plates combine and complement divergent boundaries. They are most pronounced within the mid-oceanic ridges, where they are divided into fragments of different ages and displaced across the strike.
The most important property divergent and transform boundaries is that within them, in the course of spreading, a new oceanic crust.
Convergent lithospheric plate boundaries characterized by the convergence of plates in geodynamic conditions of prevailing lateral compression. They are expressed subduction zones, in which the oceanic crust plunges under the continental, or the oceanic crust plunges under the oceanic, but younger. When approaching with the subsequent collision of continental segments of lithospheric plates, the convergent boundaries are expressed collision. Under certain conditions, subduction and collision can be accompanied by obduction- thrusting of the oceanic crust onto the continental one. Most of the subduction zones are located on the periphery of the Pacific Ocean. Another system departs from the Pacific to the west and, alternating with collisional areas, follows from the Sunda zone to the Calabrian zone in the Mediterranean and Gibraltar. Modern collision zones are mainly associated with the Mediterranean-Himalayan fold belt. Within them there is tectonic crowding leading to intense fold-thrust deformations and the formation of mountain structures - orogens.
Just as at divergent and transform boundaries, within the convergent boundaries, a new crust is formed, but crust of the continental type.
Intraplate tectonic processes and the structures they generate are currently the subject of intensive study. Among the main types of intraplate dislocations, planetary fracturing and lineaments closely related to it, zones of folded dislocations, and ring structures are distinguished.
Planetary fracturing seems to be the most universal and widespread type of intraplate dislocations. It is best studied on the continental segments of the lithospheric plates, where it is best manifested in an undeformed form in the deposits of the platform cover. Its most important feature is the predominance of two generations of cracks: layer-by-layer (sub-horizontal) and normal (perpendicular to the boundaries of the layer). Distances between normal fractures are a function of the thickness of the layer and the composition of its constituent rocks. In general, the greater the thickness of the layer broken by cracks, the greater the distance (step) between them. In addition, normal cracks are divided into systems - sets of cracks with close occurrence elements. Among the systems, submeridional, sublatitudinal and two diagonal (northwestern and northeastern) are most often distinguished. The peculiarities of planetary fracturing are associated with rotational factors - nonstationarity of the planet's rotation speed around its axis.
Term lineament was first proposed by the American geologist W. Hobbs in 1911 to designate global relief and structure elements elongated in one direction. It received its new meaning in the process of widespread use in the geology of aerial and space images, as a reflection on earth surface ruptured faults of various ranks (including planetary fracturing).
Intraplate zones of folded dislocations are found on all continents, and are currently beginning to stand out within the ocean floor. Their length reaches hundreds of kilometers and a width of many tens of kilometers. Some of them are formed over ancient rifts as a result of inversion of movements, others are formed parallel to the nearest fold belts and synchronously with them. By origin, epiplatform orogens are closely related to them. Gentle linear uplifts and troughs, considered as lithospheric folds, are widespread.
Ring structures (morphostructures of the central type ) actively began to be studied in close connection with the development of space geology. Among them, structures of magmatogenic origin (volcanic, volcanic-plutonic, plutonic) are distinguished; metamorphogenic (granite-gneiss domes); diapir structures of salt and clay strata, arched uplifts and subsidence; as well as thermokarst and karst forms associated with exogenous processes. Structures of impact (meteorite) origin form a special group. A significant part of the ring objects identified during decryption is referred to the category of cryptostructures (structures of unknown origin).
Impact (meteorite, cosmogenic) structures formed when celestial bodies fall to Earth of various types and size. Meteorite craters include basins on the Earth's surface that retain the morphological features of impact origin. Structures that have lost these features due to denudation are usually called astroblemes(star scars).
The speeds of approach of space bodies to the Earth vary from 11 to 76 km / s. Small bodies, when entering the atmosphere, lose speed due to deceleration. They can completely "burn" in the atmosphere. But already bodies 10-20 m in size, colliding with the Earth at a speed of the first kilometers per second, are able to form craters and leave their debris in them. If the velocity of such bodies upon impact is 30 km / s or more, a pressure of 1500 GPa develops, which is about 50 times greater than at the center of the Earth. In this case, the temperature is tens of thousands of degrees. In such conditions, almost complete evaporation of meteoric matter occurs. The craters are filled with impact breccias overlying fractured bedrocks. In the central part of the craters, there is often a central uplift composed of chaotic breccias. Rocks filling crater (impactites), are formed under enormous pressure and high temperature. Among them, the following varieties stand out.
Authigenic breccia Are crushed bedrocks that have not experienced significant movement. They lie at the base of the section.
Allogeneic breccia formed by debris of various sizes that fell back into the crater, cemented by loose debris ( coptoclast). The breccia thickness can reach 100 meters or more.
Suvites, which is a sintered mass of glass fragments and rocks, together with other rocks fill the inner parts of the craters. In addition, they are spread in separate languages outside the craters.
Tagamites lie inside the craters. They form irregular sheet-like and lenticular bodies on the surface of authigenic breccias or above allogeneic breccias and suvites, and also form dikes and vents in authigenic breccias and pseudo-veils. Tagamites are represented by uniform spotted rocks with a porous, sometimes pumice-like structure, consisting of fragments of dark gray or colored glass.
Pseudotachylitis- remelted glassy or crystallized rocks that form veins in authigenic breccias. They are formed as a result of frictional melting at the boundaries of blocks rubbing against each other.
Oceans
The most important morphostructural elements of the oceans are mid-ocean ridges, transform faults, and abyssal plains.
Mid-ocean ridges and transform faults, being part of the global rift system, appear in all oceans as zones spreading- the expansion of the ocean floor due to the new crust formed in their axial parts. The ridges are grandiose mountain structures, the average width of which varies from several hundred kilometers to 2000-4000 km, the relative elevation over the ocean bed is 1-3 km. The tops of the ridges are at an average depth of 2.5 km. The relief of the ridges is highly dissected. At the same time, with distance from the axis, the mountain spiers are replaced by a hilly relief, which gradually smoothes out at the transition to the abyssal plains. The ridges are thus subdivided into two geomorphological zones: ridge area and area of slopes (flanks)... Ridge zones consist of mountain systems and valley-like depressions separating them, elongated in accordance with the general strike. In the central axial zone of the mid-oceanic ridges, the height of the mountains is maximum. Here they are associated with a narrow (10-40 km) and deep (1-4 km) rift valley with steep (about 40 °) sides, which are divided into several ledges. Pillow lavas are exposed in the ledges ( pillow lava). The rift valley is characterized by block-ridge dissection. Its central part consists of solidified basalt domes and sleeve-like flows, dissected jyaram- gaping tensile cracks without vertical displacement 0.5 - 3 m wide (sometimes up to 20 m) and tens of meters long. The mid-oceanic ridges of the Pacific Ocean, in comparison with the ridges of the Atlantic, Indian and Arctic (Arctic) oceans, are characterized by less contrasting relief forms, the rift valley in them is not clearly expressed, and volcanic forms are widely developed.
Mid-ocean ridges intersect transform faults(J. T. Wilson, 1965), which displace fragments of ridges in directions transverse to the strike of the ridges. The amplitude of the displacement is hundreds of kilometers (up to 750 km in the equatorial regions of the Atlantic). In the topography of the ocean floor, transform faults are expressed by narrow troughs with steep slopes. Their depth reaches 7-8 km (Eltanin and Romansh faults). Transform faults are a special type of shear displacement faults that transfer (transform) the horizontal movement of the lithosphere from one active boundary to another. Rift transform faults correspond to the ridge-ridge type (relieve stress between two segments of the rift zone). The reasons for the accumulation of stresses between the ridge segments are associated with the uneven spreading. Active and passive parts are distinguished in the structure of transform faults. Within the active part, a new oceanic crust is being formed. In terms of length, among the transform faults, main (according to V.E. Khain), or demarcation (according to Yu.M. Pushcharovsky) Their length is tens of thousands of kilometers, and the distance separating them is about a thousand kilometers. They cross oceans and can extend out onto continents. Such transform faults divide the oceans into segments that have opened at different times. Less extended transform faults cross mid-ocean ridges every 100-200 km and continue for some distances within the abyssal plains. The faults of the next category do not go beyond the ridges and are separated from each other by tens of kilometers. Finally, smaller faults intersect only crestal zones and rift valleys.
In geophysical fields the mid-oceanic ridges are very distinct. The ridge zone is characterized by increased seismicity. Moreover, the depth of earthquake hypocenters usually does not exceed the first kilometers. In the gravitational field along the axis of the ridge, negative anomalies are distinguished. In combination with the increased heat flow of the ridge zone, they record magma chambers in which magmas are concentrated, which are the result of the melting of the basalt component from the asthenosphere lying near the surface. The magnetic field of the mid-oceanic ridges is characterized by strip magnetic anomalies. They run parallel and symmetrically to the axis of the ridge and represent an alternation of forward and reverse polarity. Anomalies are assigned numbers, the counting of which starts symmetrically on both sides of the center zone. The distance between the anomalies of the same name in different rift zones can be different. It does not remain constant along the same anomaly. Sometimes the symmetry of anomalies relative to the rift axis is different on different sides: on one side the anomalies are located compressed, and on the other - sparsely. All these features are explained by the fact that, during the crystallization of magma in the zone of expansion, the remanent magnetization fixes in rocks geomagnetic characteristics(model F. Vine - D. Matthews from the University of Cambridge, USA, 1963). As it forms, the newly formed oceanic crust moves away from the spreading axis and, like a magnetic tape, records variations in the geomagnetic field, including polarity reversals. Since the crust grows on both sides of the spreading axis, two overlapping magnetic records are formed. The distance between the anomalies of the same name, provided that their age is dated, makes it possible to determine the spreading rate. The velocities obtained by this method vary from fractions of a centimeter to 15-18 cm / year. Since spreading usually develops symmetrically, the total spreading rate of the lithospheric plates is twice the spreading rate. The global anomaly scale is currently developed in sufficient detail. In particular, anomaly 34, which has normal polarity, occupies a wide strip of the bottom and is interpreted as a “Cretaceous zone of a calm magnetic field (120-84 million years). More ancient anomalies with dates up to 167.5 million years (Jurassic) are also distinguished. Thus, the use of data on strip anomalies made it possible to reconstruct the history of the oceans, as well as the entire global system of relative displacement of lithospheric plates from the middle of the Mesozoic to the present.
Tectonomagmatic processes in spreading zones form the oceanic crust from material separated from the mantle. In terms of the volume of products of modern volcanism, oceanic spreading zones are three times larger than all other types of volcanism combined and amount to about 4 km³ / year. The main varieties of igneous rocks of the mid-oceanic ridges are formed by basaltoids, gabbroids, and also peridotites - a refractory remnant of mantle material. The ridges are characterized by a special geochemical type of basaltoids, usually denoted by the abbreviation MORB (Mid-Oceanic Ridge Basalts) or MOR (Mid-Oceanic Ridges), or tholeiitic basalts... For oceanic tholeiites of normal type(N-MORB) there is a low content of mobile ( incoherent) elements, by which we mean elements with ionic radii and charges that do not allow easy entry into rock-forming minerals. Therefore, they have very low crystal-liquid distribution coefficients and accumulate in the system as they crystallize. These include potassium, zirconium, barium, most TR, etc. Such basalts are considered the result of partial melting of geochemically depleted ( depleted) mantle at relatively shallow depths. At the same time, the degree of melting of the initial rocks was high, which was reflected in the enrichment of the melt with elements of the iron group.
The volcanic zones of the mid-oceanic ridges are confined to hydrothermal outlets... They are associated with metal-bearing sediments and specific deposits of "black and white smokers".
Metalliferous sediments Are loose polygenic formations enriched mainly in iron and manganese of hydrothermal origin. Modern sediments are confined to the axial parts and flanks of spreading ridges, to the vicinity of hydrothermal fields. As spreading develops, metalliferous sediments pass into a buried state and lie at the base of the section of the oceanic sedimentary cover, where their thickness can reach several tens of meters. These formations stand out as an independent metal-bearing basal formation.
"Black smokers"- pipe-like cones of sulphide buildings, through which hydrothermal solutions with a temperature of 350-400 ° C, saturated with a suspension of mineral particles dispersed in the aquatic environment like smoke, enter. They are accompanied by a unique biota complex completely independent of exogenous food sources. Hills and conical structures form massive sulfide ore deposits weighing several thousand tons. There are also mantle-like covers of massive sulfide ores, up to 10 m thick. The mass of some of these formations can reach 2 million tons. Sulfide ores are localized mainly in the axial zones of the mid-ocean ridges.
"White smokers"- a type of relatively low-temperature hydrothermal springs with a temperature of less than 300 ° C, functioning in paragenesis with "black smokers". However, if the smoke of "black smokers" consists of sulfides of iron, zinc, copper with an admixture of amorphous silica, then the smoke of "white smokers" is formed by sulfates (anhydrite, barite) and amorphous silica.
Relatively recently, another previously unknown type of fluid was discovered on the summit of the Atlantis Seamount within the Mid-Atlantic Ridge, 15 km west of its axis at a depth of 2600 feet. In the bottom relief, these hydrothermal waters are represented by huge dazzling white towers up to 60 m high and about 100 m wide at the base, based on peridotites. They got the name Lost City... The towers are composed of carbonates - calcite, aragonite, brookite. They are devoid of smoke, instead of which streams of water with a temperature of 50-80 ° C pour out of the cracks. The heat source is the cooling process of ultrabasic rocks. Additionally, it is produced by a chemical reaction in which olivine (the main mineral of peridotite) interacts with seawater, salts dissolved in it and transforms into serpentinite and carbonates that make up the described hydrothermal structures. The Lost City is heavily populated with bacteria that form vast mats. They feed on methane and hydrogen, which are released during the reaction.
Depending on the spreading speed zones with fast spreading (speed more than 7 cm / year), medium spreading (speed 3-7 cm / year), slow spreading (speed 1-3 cm / year) and ultra-slow spreading (speed up to 1 cm / year) are distinguished. The spreading rate is closely related to the topography of the oceanic spreading zones. An example of high-speed spreading is the East Pacific Rise, which is characterized by a large width, weakly expressed rift depression (up to its complete absence and replacement with a horst-like protrusion). The Mid-Atlantic Ridge has low and medium spreading rates in different areas. Its relief is the relief of the "classic" mid-ocean ridge. Ultraslow spreading rift zones include the Gakkel Ridge in the Arctic Ocean. In the bottom topography, it is represented by almost one narrow rift valley. Changes in the spreading rate in mid-oceanic ridges are cyclical, which is expressed in tectonoeustatic transgressions and regressions. With rapid spreading, a new crust is formed in large volumes, the crestal part of the ridges does not have time to cool down, and the ridges become wider, "squeezing" the water of the oceans onto land, which causes a global transgression. With slow spreading, the newly formed oceanic crust forms in smaller volumes and has time to cool down. The depth of the oceanic trenches increases, as does their volume. Water from the continents is "drawn" into the ocean, a global regression takes place.
The separation of basaltic magma also depends on the rate of divergence. With an increase in the spreading rate, the magma chamber of the ridges is located closer and closer to the surface. Magma has more high fever and low viscosity, therefore, when erupting, it forms extensive covers, similar to the plateau-basalts of the continents. Pillow lavas are formed during slow spreading. Low spreading rates make it difficult for the melt to emerge to the surface, the degree of magma differentiation increases, and porphyry basalt varieties appear. With an increase in the spreading rate in the rocks, the titanium content increases, and the ratio of the amount of iron to the amount of magnesium increases. In spreading zones with a high spreading rate, hydraulic wedging mechanism... It is expressed in the fact that with the rapid ascent of basalt magma, a wedging effect is provided, which magma has on the rocks of the earth's crust. Solidified magma wedges are expressed by systems of parallel dikes at the base of the oceanic crust. Under conditions of slow spreading, an important role can be played by deformation mechanism of rifting, in which tension is realized by discontinuous and viscous deformations of the earth's crust in a relatively narrow strip with a decrease in its thickness.
Dying off of zones of oceanic rifting can occur when the external geodynamic conditions change. As a result, paleospreading ridges... One of the options for such a withering away is a sharp shift, jumping axis spreading. After the spreading rate decreases to minimum values, tensile stresses cease and a long passive phase sets in, when the lithosphere under the ridge cools and increases its thickness from below due to crystallization of the asthenospheric material. This is accompanied by isostatic subsidence, the relief of the ridge is smoothed out, and it is more and more covered by a sedimentary cover.
Abyssal plains in terms of area, they are the predominant element in the structure of the oceanic bed. They are located between the mid-ocean ridges and the foothills of the continents and have a depth of 4 to 6 km. The crust within the abyssal plains is consistent in thickness, except that the sedimentary layer in the direction of the continental margins increases in thickness due to the appearance of more and more ancient horizons, up to the upper Middle Jurassic.
Some plains (especially in the Atlantic and Indian Oceans) have a perfectly flat bottom surface, others, mainly in the Pacific Ocean, are characterized by hilly relief. Underwater volcanic mountains rise among the plains. There are especially many of them within the Pacific Ocean. A special kind of seamounts form guyots - flat-topped uplands of volcanic origin, found at a depth of about 2 km. Their tops were previously cut off by marine abrasion, then covered with shallow sediments, sometimes reefs, and then sank as a result of cooling of the crust below sea level.
The abyssal plains are divided into separate basins by large underwater ridges and hills. Among the underwater uplifts, isometric elevations of an oval-rounded shape (Bermuda in the Atlantic) stand out, flat elevations due to the sedimentary cover - ocean plateaus(Ontong Java in the Pacific Ocean). Others are linear, stretching for thousands of kilometers with a width of hundreds of kilometers (Maldives and East Indian ridges in the Indian Ocean). All these ridges and hills rise above the adjacent basins by 2-3 km. In some places, their tops protrude above sea level in the form of islands (Bermuda). Most of the uplifts are obviously of volcanic origin. For the Imperial-Hawaiian Ridge, it is proved by modern volcanism on the island. Hawaii, the volcanic nature of the remaining islands of the Hawaiian chain. For these and other islands, in addition to effusive rocks, intrusions of rocks - differentiates of alkaline-basaltic magma are known. A thickening of the crust, which can exceed 30 km, is noted under almost all underwater uplifts. Initially, a significant part of the internal uplifts of the ocean with a thickened crust belonged to microcontinent... However, subsequent studies have shown that the number of modern representatives of this category of structures is very limited. In the Atlantic, they include the Roccol Plateau, and in the Indian Ocean, Madagascar. In the Pacific Ocean, New Zealand with the New Zealand Submarine Plateau. In the Arctic Ocean - ridge. Lomonosov. Microcontinents have a flat surface lying at a depth of about 2 km, but some of them can protrude above the water in the form of islands. Compared to the abyssal plains, the sedimentary cover of the microcontinent has an increased thickness. It may contain sediments prior to the opening of this ocean. The basement age can vary from Paleozoic to Archean. Microcontinents split off from continents in the early stages of ocean opening. Then the spreading axis jumped to the central part of the modern ocean.
Modern World Ocean consists of several oceans. Of them Pacific Ocean- the largest ocean on our planet. It takes up about a third of the surface the globe and almost half of the area of the World Ocean - 178.6 million km². This is the most deep ocean, its average depth is more than 4 km, and the maximum - 11022 m is noted in the Mariana Trench. The ocean floor occupies 63% of its area. It is divided by a system of uplifts into a series of basins, the largest of which are located along the central axis of the bed. In the west, the basins are characterized by a hilly surface, in the eastern part of the ocean (northeastern, southern basins, etc.), a ridge-hilly relief is noted. The bed is complicated by volcanic ridges (Imperial, Hawaiian ridges, etc.). Numerous (about 7 thousand) guyots are also characteristic. They are mainly located on arched uplifts, swells, as well as along faults. In the eastern part, the Pacific midline ridge is located, shifted to the east relative to the midline. Its area is 13% total area ocean. A significant part of the ridge in the northern hemisphere goes under North America. A distinctive feature is its comparative low height(from 1 to 2.5 km), considerable width (up to 3 thousand km), the absence of a clearly defined rift valley. The axial block is often represented by a ridge several hundred meters high and several tens of kilometers wide. The Pacific Ridge is divided into several links. Among them are the South and East Pacific uplifts, the Gordn and Juan de Fuca ridges. There are also two large branches - Galapagos and Chilean. Among the largest transform faults that cut the ridge into segments displaced relative to each other in the latitudinal direction, there are: Eltanin, Galapagos, Mendocino, Clarion, Clipperon. The specific morphostructure of the Pacific Ocean is the New Zealand Plateau - a block of continental crust that is not connected with the surrounding continents.
Atlantic Ocean makes up about a quarter of the World Ocean (area 90.5 million km²). Its average depth is 3844 m. The ocean floor (about 35% of its total area) is characterized by a combination of deep-water basins (North American, Canary, Western European, Brazilian, Angola, Cape) and submarine uplifts. The hollows are characterized by an abyssal hilly relief.
The Mid-Atlantic Ridge covers almost half of the ocean area. Its width is about 1400 km with an excess of up to 4 km above the bottom, its slopes are steep. The rift zone is distinctly expressed along its entire length. The ridge is divided into several fragments by transform faults: the northern one (Knipovich and Mona ridges) reaches about. Jan Mayen; further followed by the Kolbeinst ridge and the Great Icelandic Graben (Iceland Island). To the south, it continues with the Reykjanes ridge and has a strictly meridional strike to the Azores. In the equatorial region, the Romanche, Vima, Sao Paulo, Chain, and others transform faults displace it by several hundred kilometers. The South Atlantic Ridge maintains a submeridional position.
Mediterranean Basin in oceanological terms, it belongs to the Atlantic Ocean basin, and in the tectonic sense it is distinguished by a complex structure reflecting its long-term development, largely inherited from the polycyclic Ocean Tethys... Mediterranean Sea via Dardanelles - Sea of Marmara - Bosphorus connects to the deep Black Sea. Within the Mediterranean there are deep-sea basins, in many respects similar to oceanic ones, extensive shallow-water plateaus, deep-sea trenches and rift zones, underwater ridges and individual volcanoes.
The eastern Mediterranean Sea is the same age as the main Tethys Ocean. It represents the southern deep-sea basins of this ocean.
Western part Mediterranean Sea (Western Mediterranean Basin) emerged at the neotectonic stage (in the Oligocene) as a small oceanic basin after the closure of the Tethys Ocean.
Indian Ocean has an area of 76.8 million km² (about 20% of the area of the World Ocean). Its average depth is 3963 m. The ocean floor consists of 24 deep-water basins of which the largest: Central, Western Australian, Madagascar, Somali. The bed is complicated by meridional faults. About a thousand guyots have been identified within the basins. The basins are separated by underwater uplifts (ridges): Maldives, East Indian, Madagascar, Mozambique, Mascarensky, Amiranta, etc.
The mid-ocean ridges of the Indian Ocean are a complex system underwater mountain ranges, which include: the West Indian ridge, which continues the system of the Mid-Atlantic ridges; The Australian-Antarctic Ridge, connecting with the Pacific ridges; Central Indian Ridge, which arose at the confluence of the first two ranges .; Arabian-Indian; ridge (Carlsberg). Mid-Oceanic ridges are complicated by transform faults.
Northern Arctic Ocean - the smallest ocean. Its area is 15.2 million km² (4.2% of the area of the World Ocean). The average depth is 1300 m. The ocean floor makes up 40% of its area and is formed by small deep-water basins: Amundsen, Nansen, Makarov, Tolya, Beaufort. They are separated by underwater uplifts - submerged blocks of the continental crust, expressed by ridges: Lomonosov, Mendeleev, Alpha.
The Mid-Ocean Ridge continues the Mid-Atlantic Ridge. It begins with the Gakkel ridge, which has an insignificant width, with reduced flanks. In essence, it is formed by a single rift valley. It is supposed to continue on land in the Lena delta in the Momsky rift system.
Age of the oceans, limited by passive margins, is determined by the age of their most ancient crust, corresponding to the beginning of the opening of the oceans. For the Atlantic Ocean, this is 170 million years (Bathonian-Callovian ages of the Middle Jurassic). For the Indian Ocean - 158 million years (Oxfordian Late Jurassic). For the Arctic Ocean - 120 million years (Early Cretaceous). For the Pacific Ocean, surrounded by active margins, on the basis of paleogeographic reconstructions, fragments of former passive margins with an age related to the Late Riphean (in the North American Cordilleras), Late Riphean - Early Cambrian (Adelaide fold system in Australia) were identified. Thus, the modern young crust of the Pacific Ocean is only renewed, and the very beginning of the existence of this ocean belongs to the late Proterozoic, although since that time its area and configuration have undergone significant changes.
The given datings of the age of modern oceans belong to the most ancient parts of them. However, the opening of the oceans did not occur all at once, but along separate segments, separated by main transform faults. At the end of the Middle Jurassic and throughout the Late Jurassic, the central segment of the Atlantic opened up between the Azoro-Gibraltar Fault in the north and the Equatorial Fault Zone in the south. During the Early Cretaceous, the process spread northward to the main Charlie - Gibbs transform fault. At the end of the Cretaceous, the spreading reached the Greenland-Faroe Rapids passing through Iceland. At this stage, a subsidiary - the Labrador branch of spreading was formed, which separated Greenland from North America by the end of the Eocene. At the end of the Paleocene - beginning of the Eocene, spreading spread from the North Atlantic to the Norwegian-Greenland basin of the Arctic, then, having overcome the Svalbard fault, penetrated into the Eurasian basin of the Arctic Ocean, forming the Gakkel ridge.
In the South Atlantic, spreading propagation also proceeded from south to north. In the Late Jurassic, Africa separated from South America and Antarctica, and by the beginning of the Cretaceous, the opening reached the Falklands-Agulhas fault. In the Neocom, it moved northward to the Rio Grande Fault. At the end of the Aptian - Albe, the Angolo-Brazilian segment opened, and at the end of the Cenomanian, the South and Central Atlantic were unified.
In the Indian Ocean in the Late Jurassic, spreading spread to the southwest, separating Africa from India, Madagascar and Antarctica., And then from north to south and southeast, separating India from Australia at the end of the Jurassic - early Cretaceous and Australia at the beginning of the Cenomanian from Antarctica. In the Late Miocene, spreading developed from the Owen Fault into the Gulf of Aden and the Red Sea.
The development of the Pacific Ocean was more complicated, where the plan for the location of the spreading axes was rearranged. Their modern outlines began to form at the end of the Cretaceous.
What do we know about the lithosphere?
Tectonic plates are large, stable areas of the Earth's crust that are part of the lithosphere. If we turn to tectonics, a science that studies lithospheric platforms, then we learn that large areas of the earth's crust are limited on all sides by specific zones: volcanic, tectonic and seismic activities. It is at the joints of adjacent plates that phenomena occur, which, as a rule, have catastrophic consequences. These include both volcanic eruptions and earthquakes that are strong on the scale of seismic activity. In the process of studying the planet, plate tectonics played a very important role. Its significance can be compared to the discovery of DNA or the heliocentric concept in astronomy.
If we recall the geometry, then we can imagine that one point can be the point of contact of the boundaries of three or more plates. The study of the tectonic structure of the earth's crust shows that the most dangerous and rapidly decaying are the joints of four or more platforms. This formation the most unstable.
The lithosphere is divided into two types of plates, different in their characteristics: continental and oceanic. It is worth highlighting the Pacific platform, made up of oceanic crust. Most others are made up of what is called a block, where a continental plate is soldered into an oceanic plate.
The location of the platforms shows that about 90% of the surface of our planet consists of 13 large, stable sections of the earth's crust. The remaining 10% falls on small formations.
Scientists have mapped the largest tectonic plates:
- Australian;
- The Arabian subcontinent;
- Antarctic;
- African;
- Hindustan;
- Eurasian;
- Nazca plate;
- Coconut plate;
- Pacific;
- North and South American platforms;
- Scotia plate;
- Filipino plate.
From the theory we know that the solid shell of the earth (lithosphere) consists not only of plates that form the relief of the planet's surface, but also of the deepest part - the mantle. Continental platforms are 35 km thick (in flat areas) to 70 km (in mountainous areas). Scientists have proven that greatest thickness has a slab in the Himalayan zone. Here the thickness of the platform reaches 90 km. The thinnest lithosphere is in the oceans. Its thickness does not exceed 10 km, and in some areas this figure is 5 km. Based on information about the depth at which the epicenter of the earthquake is and what is the speed of propagation of seismic waves, calculations of the thickness of the sections of the earth's crust are made.
The process of formation of lithospheric plates
The lithosphere consists mainly of crystalline substances formed as a result of the cooling of magma upon reaching the surface. The description of the structure of the platforms indicates their heterogeneity. The process of the formation of the earth's crust took place for a long period, and continues to this day. Through microcracks in the rock, molten liquid magma was released to the surface, creating new bizarre shapes. Its properties changed depending on the change in temperature, and new substances were formed. For this reason, minerals that are at different depths differ in their characteristics.
The surface of the earth's crust depends on the influence of the hydrosphere and atmosphere. Weathering constantly occurs. Under the influence of this process, forms change, and minerals are crushed, changing their characteristics with a constant chemical composition. As a result of weathering, the surface became looser, cracks and microdepressions appeared. In these places, sediments appeared, which we know as soil.
Tectonic plate map
At first glance, it seems that the lithosphere is stable. Its upper part is such, but the lower one, which differs in viscosity and fluidity, is mobile. The lithosphere is divided into a certain number of parts, the so-called tectonic plates. Scientists cannot say how many parts the earth's crust consists of, since in addition to large platforms, there are also smaller formations. The names of the most large slabs were given above. The formation of the earth's crust is ongoing. We do not notice this, since these actions occur very slowly, but comparing the results of observations for different periods, you can see how many centimeters per year the boundaries of the formations are shifted. For this reason, the tectonic map of the world is constantly being updated.
Coconut tectonic plate
The Cocos Platform is a typical representative of the oceanic parts of the earth's crust. It is located in the Pacific region. In the west, its border runs along the ridge of the East Pacific Rise, and in the east, its border can be determined by a conditional line along the coast of North America from California to the Isthmus of Panama. This plate is sliding under the neighboring Caribbean Plate. This zone is characterized by high seismic activity.
Mexico is hit hardest by earthquakes in this region. Among all the countries of America, it is on its territory that the most extinct and active volcanoes... The country has suffered a large number of earthquakes with a magnitude above 8 points. The region is quite densely populated, therefore, in addition to destruction, seismic activity leads to a large number of victims. Unlike Cocos, located in another part of the planet, the Australian and West Siberian platforms are stable.
Tectonic plate movement
For a long time, scientists have tried to find out why in one region of the planet there is mountainous terrain, and in another it is flat, and why earthquakes and volcanic eruptions occur. Various hypotheses were based mainly on the knowledge that was available. Only after the 50s of the twentieth century was it possible to study the earth's crust in more detail. The mountains formed at the sites of fracture of plates were studied, chemical composition these plates, as well as maps of regions with tectonic activity were created.
In the study of tectonics special place took the hypothesis of the displacement of lithospheric plates. Back in the early twentieth century, the German geophysicist A. Wegener put forward a bold theory about why they move. He carefully examined the outline of the west coast of Africa and the east coast of South America. The starting point in his research was precisely the similarity of the outlines of these continents. He suggested that, perhaps, these continents were previously a single whole, and then a break occurred and parts of the Earth's crust began to shift.
His research touched upon the processes of volcanism, the stretching of the surface of the ocean floor, the viscous-liquid structure of the globe. It was the works of A. Wegener that formed the basis for the research carried out in the 60s of the last century. They became the foundation for the emergence of the theory of "plate tectonics".
This hypothesis described the model of the Earth as follows: tectonic platforms with a rigid structure and different masses were located on the plastic matter of the asthenosphere. They were in a very unstable state and constantly moved. For a simpler understanding, you can draw an analogy with icebergs that constantly drift in ocean waters. Likewise, tectonic structures, being on plastic matter, are constantly moving. During displacements, the slabs constantly collided, went one on top of the other, joints and zones of sliding of the slabs arose. This process was due to the difference in mass. In places of collisions, areas with increased tectonic activity were formed, mountains appeared, earthquakes and volcanic eruptions occurred.
The displacement rate was no more than 18 cm per year. Faults were formed, into which magma entered from the deep layers of the lithosphere. For this reason, the rocks that make up the oceanic platforms have different age... But scientists have put forward an even more incredible theory. According to some representatives of the scientific world, magma came to the surface and gradually cooled, creating a new structure of the bottom, while the "excess" of the earth's crust under the influence of plate drift, sank into the earth's interior and again turned into liquid magma. Be that as it may, but the movements of the continents occur in our time, and for this reason new maps are created for further study of the drift of tectonic structures.
According to modern lithospheric plate theory The entire lithosphere is divided by narrow and active zones - deep faults - into separate blocks that move in the plastic layer of the upper mantle relative to each other at a rate 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 keep 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 diverge, approach or slide relative to each other. In the first case, tension zones with cracks appear between the plates along the boundaries of the plates, in the second - zones of compression, accompanied by the thrust of one plate onto another (thrust - obduction; thrust - subduction), in the third - shear zones - faults along which the neighboring plates slide. ...
At the points of convergence of the continental plates, they collide, and mountain belts are formed. This is how, for example, the Himalayan mountain system appeared on the border of the Eurasian and Indo-Australian plates (Fig. 1).
Rice. 1. Collision of continental lithospheric plates
With the interaction of the continental and oceanic plates, 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 an oceanic type of crust is shown in Fig. 3.
The axial zones of the mid-oceanic ridges are characterized by rifts(from the English. rift - crevice, crack, fault) - a large linear tectonic structure of the earth's crust with a length of hundreds, thousands, 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 were named 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 the formation of the earth's crust in these zones is currently going on very intensively.
Rice. 3. Divergence of lithospheric plates in the zone among the nno-oceanic ridge
Rice. 4. Rift formation diagram
Most of the fractures of lithospheric plates are at the bottom of the oceans, where the earth's crust is thinner, but they are also found on land. The largest fault on land is located in the east of Africa. It stretches for 4000 km. The width of this fault is 80-120 km.
At present, seven of the largest slabs can be distinguished (Fig. 5). Of these, the largest in area is the Pacific Ocean, which consists entirely of the oceanic lithosphere. As a rule, the Nazca plate is also referred to as large, which is several times smaller in size than each of the seven largest. At the same time, scientists suggest that in fact the Nazca plate is much larger 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 the oceanic lithosphere.
Rice. 5. Lithospheric plates of the Earth
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 the continental lithosphere.
The theory of lithospheric plates is important. First of all, it can explain why there are mountains in some parts of the Earth, and plains in others. With the help of the theory of lithospheric plates, it is possible to explain and predict the catastrophic phenomena occurring at the boundaries of the plates.
Rice. 6. The outlines of the continents do seem to be 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 the map, one can notice that the shores of Africa and South America, when approaching, seem to be compatible (Fig. 6).
The emergence of the hypothesis of the movement of continents 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 coastlines 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 was the northern continent, which included the territories of modern Europe, Asia without India and North America. Southern mainland- Gondwana united the modern territories of South America, Africa, Antarctica, Australia and Hindustan.
Between Gondwana and Laurasia was the first seafood - 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 Pangea (white - land, points - shallow sea)
Approximately 180 million years ago, the Pangea continent again began to separate into its component parts, which were mixed on the surface of our planet. The division took place 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; old - Quiet. The Arctic Ocean has become isolated with an increase in land mass in the Northern Hemisphere.
Rice. 9. Location and directions of continental drift in the Cretaceous period 180 million years ago
A. Wegener found many confirmations of the existence of a single continent of the Earth. The existence in Africa and South America of the remains of ancient animals - the listosaurs - seemed to him especially convincing. They were reptiles, similar to small hippos, that lived only in freshwater bodies of water. So, to swim huge distances on the salty sea water they couldn't. He found similar evidence in the plant kingdom.
Interest in the hypothesis of the movement of continents in the 30s of the XX century. slightly decreased, but in the 60s it revived again, 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 "diving" of some parts of the crust under others (subduction).