Earth's lithospheric plates are huge boulders. Their foundation is formed by granite metamorphosed igneous rocks strongly crushed into folds. The names of lithospheric plates will be given in the article below. 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. Next, the theory of motion of lithospheric plates will be considered.
The emergence of a hypothesis
The theory of motion of lithospheric plates appeared at the beginning of the twentieth century. Subsequently, she was destined to play a major role in the exploration of the planet. Scientist Taylor, and after him Wegener, put forward the hypothesis that over time, lithospheric plates drift 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. The basis of this phenomenon was the process of differentiation of the mantle matter of the planet. It began to be called fixism. This name was due to the fact that a permanently fixed position of the crustal sections relative to the mantle was recognized. But in 1960, after the discovery of the global system of mid-ocean ridges, which encircled the entire planet and reached land in some areas, there was a return to the hypothesis of the beginning of the 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.
Key Points
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 by thickening in the centers of earthquakes.
The names of lithospheric plates correspond to the continental and oceanic regions located above them. There are only seven blocks with a huge area. The largest lithospheric plates are South and North American, Euro-Asian, African, Antarctic, Pacific and Indo-Australian.
The blocks floating in the asthenosphere are monolithic and stiff. The above sections 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, it was found that the blocks swim passively along the mantle material. It is worth noting that their direction is initially vertical. Mantle material rises under the crest of the ridge up. Then there is a spread in both directions. Accordingly, there is a discrepancy in lithospheric plates. This model represents the ocean floor as a giant conveyor belt. It comes to the surface in the rift areas 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 (sub-feat) in deep-sea trenches.
Why is the movement of lithospheric plates?
The reason is the thermal convection of the mantle material of the planet. The lithosphere undergoes stretching and experiences a rise, which occurs over ascending branches from convective currents. This provokes the movement of lithospheric plates to the sides. As you move away from the mid-ocean rifts, the platform becomes denser. It is heavier, its surface drops down. This explains the increase in oceanic depth. As a result, the platform is immersed in deep-sea trenches. With the attenuation of the ascending flows from the heated mantle, it cools and lowers with the formation of basins, which are filled with sediments.
Collision zones of lithospheric plates are areas where the crust and platform experience compression. In this regard, the power of the first increases. As a result, the upward movement of lithospheric plates begins. It leads to the formation of mountains.
Research
The study today is carried out using geodetic methods. They allow us to conclude that the processes are continuous and ubiquitous. Collision zones of lithospheric plates are also identified. 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 a dozen centimeters during the year. So, for example, St. Petersburg has already risen by a meter for the entire period of its existence. The Scandinavian Peninsula - at 250 m in 25,000 years. Mantle material moves relatively slowly. However, earthquakes, volcanic eruptions, and other phenomena occur as a result. This allows us to conclude about the high power of material movement.
Using the tectonic position of the plates, the researchers explain many geological phenomena. At the same time, during the study, it turned out to be much greater than it seemed at the very beginning of the appearance of the hypothesis, the complexity of the processes occurring with the platform.
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. The question of the historical beginning of the action also remains open. Direct signs indicating plate-tectonic processes have been known since the late Proterozoic. However, a number of researchers recognize their manifestation from the Archean or the early Proterozoic.
Expanding Research Opportunities
The advent of seismotomography 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 youngest direction of all the existing Earth sciences. However, the solution of new problems was carried out using not only seismotomography. Other sciences 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, isotope geochemistry methods were used in the studies. This science studies, in particular, the isotopic balance of rare elements, as well as noble gases in various earth 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 uncover the causes and mechanism of inversions in a magnetic field.
Contemporary painting
The platform tectonics hypothesis continues to satisfactorily explain the process of development of the crust of the oceans and continents for 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 definite picture emerges.
In the cross section of the planet there are three most active layers. The power 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 advanced in 1963 by Wilson. This theory explained the phenomenon of intraplate magnetism. The resulting plume tectonics is becoming increasingly popular over time.
Geodynamics
With its help, the interaction of fairly complex processes that occur in the mantle and crust is considered. In accordance with the concept set forth by Artyushkov in his work “Geodynamics”, the 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. It sinks into the core. The location of the lighter layer under the heavy is unstable. In this regard, the accumulating material is collected periodically in large enough blocks that float into the upper layers. The size of such formations is about one hundred kilometers. This material was the basis for the formation of the upper mantle of the Earth.
The lower layer is probably an undifferentiated primary substance. During the evolution of the planet due to the lower mantle, the upper one grows and the core grows. It is more likely that blocks of light material rise in the lower mantle along the channels. In them, the temperature of the mass is quite high. Viscosity is significantly reduced. The increase in temperature is facilitated by the release of a large amount of potential energy during the lifting of a substance into the region of gravity by a distance of about 2000 km. In the direction of travel along such a channel, strong heating of light masses occurs. In this regard, the substance enters the mantle, having a sufficiently high temperature and significantly lower weight in comparison with the surrounding elements.
Due to the reduced density, light material floats into the upper layers to a depth of 100-200 or less kilometers. With decreasing pressure, the melting temperature of the components of the substance decreases. After primary differentiation at the core-mantle level, secondary occurs. At shallow depths, light material is partially melted. With differentiation, more dense substances are released. They plunge into the lower layers of the upper mantle. Standing out lighter components, respectively, rise up.
The complex of motions 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 intervals of approximately 200 million years. At the same time, penetration into the upper mantle is not observed everywhere. In the lower layer, the channels are located at a sufficiently large distance from each other (up to several thousand kilometers).
Lifting blocks
As mentioned above, in those zones where large masses of light heated material are introduced into the asthenosphere, its partial melting and differentiation takes place. In the latter case, the separation of components and their subsequent ascent is noted. They quickly pass through the asthenosphere. Upon reaching the lithosphere, their speed decreases. In some areas, matter forms clusters of anomalous mantle. They occur, as a rule, in the upper layers of the planet.
Abnormal Mantle
Its composition approximately corresponds to the normal mantle substance. The difference between the abnormal accumulation is a higher temperature (up to 1300-1500 degrees) and a reduced speed of elastic longitudinal waves.
The entry of matter under the lithosphere provokes an isostatic uplift. Due to the elevated temperature, the abnormal cluster has a lower density than the normal mantle. In addition, there is a small viscosity of the composition.
In the process of entering the lithosphere, the anomalous mantle is fairly quickly distributed along the sole. At the same time, it displaces the denser and less heated substance of the asthenosphere. In the direction of travel, an abnormal accumulation fills those areas where the sole of the platform is in an elevated state (traps), and it flows around deeply submerged areas. As a result, in the first case, an isostatic uplift is noted. Over 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 total, it takes several hundred million years. In this regard, heterogeneities in the thickness of the lithosphere, explained by horizontal temperature differences, have a sufficiently large inertia. In the event that the trap is located near the upward flow of an abnormal accumulation from the depths, a large amount of matter is trapped very hot. As a result, a fairly large mountain element is formed. In accordance with this scheme, high uplifts occur at the site of epiplatform orogenesis in the folded belts.
Process description
In the trap, the anomalous layer is compressed for 1-2 kilometers during cooling. The bark, located on top, sinks. In the formed trough, precipitation begins to accumulate. Their severity contributes to even greater immersion of the lithosphere. As a result, the depth of the pool can be from 5 to 8 km. At the same time, during compaction of the mantle in the lower part of the basaltic layer in the crust, phase transformation of the rock into eclogite and garnet granulite can be noted. Due to the heat flow emerging from the anomalous substance, the overlying mantle is heated and its viscosity decreases. In this regard, a gradual crowding out of the normal cluster is observed.
Horizontal displacements
With the formation of uplifts during the arrival of the anomalous mantle to the crust on continents and oceans, an increase in the potential energy stored in the upper layers of the planet occurs. To discharge excess substances tend to diverge. As a result, additional stresses are formed. Various types of plate and crust movement are associated with them.
The expansion of the ocean floor and the swimming of the continents are the result of the simultaneous expansion of the ridges and the immersion of the platform in the mantle. Under the first are large masses of highly heated anomalous substances. In the axial part of these ridges, the latter is located directly below the crust. The lithosphere here has much less power. The abnormal mantle at the same time spreads in the area of high pressure - on both sides from under the ridge. Along with this, it tears the ocean crust quite easily. The cleft is filled with basaltic magma. She, in turn, is smelted from the anomalous mantle. In the process of solidification of magma, a new oceanic crust is formed. This is how the bottom grows.
Process features
Under the middle ridges, the anomalous mantle has a reduced viscosity due to elevated temperature. The substance is able to spread 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 the ridges to the places of immersion. If these sites are in the same ocean, then the process occurs at a relatively high speed. This situation is typical for the Pacific today. If the bottom grows and sinks in different areas, then the continent located between them drifts in the direction where the deepening takes place. Under the continents, the viscosity of the asthenosphere is higher than under the oceans. In connection with the friction that arises, considerable resistance to motion appears. As a result, the rate at which the bottom expands decreases if there is no compensation for the mantle sinking in the same region. Thus, sprawl in the Pacific Ocean is faster than in the Atlantic.