The lithospheric plates of the Earth are huge boulders. Their basement is formed by highly folded granite metamorphosed igneous rocks. 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. Next, 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 the exploration of the planet. The scientist Taylor, and after him Wegener, put forward the hypothesis that over time there is a drift of lithospheric plates in a 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 became known as fixism. This name was due to the fact that a permanently fixedposition of crustal regions relative to the mantle. But in 1960, after the discovery of a 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 has taken on a new form. Block tectonics has become the leading hypothesis in the sciences that study the structure of the planet.
Basics
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 according to the concentration in the sources of earthquakes.
The names of the lithospheric plates correspond to the continental and oceanic areas located above them. There are only seven blocks with a huge area. The largest lithospheric plates are the South and North American, Euro-Asian, African, Antarctic, Pacific and Indo-Australian.
Blocks floating through the asthenosphere are characterized by solidity and rigidity. 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. At the same time, the movement of lithospheric plates was carried out under the influence of an invisible force. As a result of the research, it was revealed that the blocks float passively over the material of the mantle. It is worth noting that their direction is vertical at first. The mantle material rises under the crest of the ridge. Then there is a spread in both directions. Accordingly, there is a divergence of lithospheric plates. This model representsthe ocean floor as a giant conveyor belt. It comes to the surface in the rift regions of the mid-ocean ridges. Then hides in deep sea trenches.
The divergence of lithospheric plates provokes the expansion of ocean 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 areas of subduction (underthrust) in deep-sea trenches.
Why do lithospheric plates move?
The reason is the thermal convection of the planet's mantle material. The lithosphere is stretched and uplifted, which occurs over ascending branches from convective currents. This provokes the movement of lithospheric plates to the sides. As the platform moves away from the mid-ocean rifts, the platform becomes compacted. It becomes heavier, its surface sinks down. This explains the increase in ocean depth. As a result, the platform plunges into deep-sea trenches. As the updrafts from the heated mantle die down, it cools and sinks to form pools that are filled with sediment.
Zones of collision of lithospheric plates are areas where the crust and the 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. are revealedalso zones of collision of lithospheric plates. The lifting speed can be up to tens of millimeters.
Horizontal large lithospheric plates are floating 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. Scandinavian Peninsula - 250 m in 25,000 years. The mantle material moves relatively slowly. However, earthquakes, volcanic eruptions and other phenomena occur as a result. This allows us to conclude that the material moving power is high.
Using the tectonic position of the plates, researchers explain many geological phenomena. At the same time, during the study, it turned out that the complexity of the processes occurring with the platform is much greater than it seemed at the very beginning of the emergence of the hypothesis.
Plate tectonics could not explain 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 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 andthe youngest direction of all the existing geosciences. However, the solution of new problems was carried out using not only seismic tomography. 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. The methods of isotope geochemistry were also used in the studies. 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. Methods of geomagnetism are used, with the help of which scientists are trying to uncover the causes and mechanism of reversals in the magnetic field.
Modern painting
The platform tectonics hypothesis continues to satisfactorily explain the process of development of the crust of the oceans and continents over at least the last three billion years. At the same time, there are satellite measurements, according to which the fact that the main lithospheric plates of the Earth do not stand still is confirmed. As a result, a certain picture emerges.
There are three most active layers in the cross section of the planet. The thickness 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 put forward in 1963 by Wilson about ascending mantle jets. This theory explained the phenomenon of intraplate magnetism. The resulting plumetectonics is becoming more and more 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. She descends into the core. The location of the lighter layer under the heavy one is unstable. In this regard, the accumulating material is collected periodically into fairly large blocks that float into the upper layers. The size of such formations is about a hundred kilometers. This material was the basis for the formation of the Earth's upper mantle.
The bottom layer is probably undifferentiated primary matter. 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 are uplifted in the lower mantle along the channels. In them, the temperature of the mass is quite high. At the same time, the viscosity is significantly reduced. The increase in temperature is facilitated by the release of a large amount of potential energy in the process of lifting matter into the region of gravity at 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 with a sufficiently hightemperature and significantly lighter than the surrounding elements.
Due to the reduced 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 the primary differentiation at the "core-mantle" level, the secondary one occurs. At shallow depths, light matter is partially subjected to melting. During differentiation, denser substances are released. They sink into the lower layers of the upper mantle. The lighter components that stand out rise accordingly.
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 intervals 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 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 occurs. In the latter case, the separation of components and their subsequent ascent are noted. They quickly pass through the asthenosphere. When they reach the lithosphere, their speed decreases. In some areas, matter forms accumulations of anomalous mantle. They lie, as a rule, in the upper layers of the planet.
Anomalous mantle
Its composition approximately corresponds to normal mantle matter. The difference between the anomalous 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 isostatic uplift. Due to the elevated temperature, the anomalous cluster has a lower density than the normal mantle. In addition, there is a slight viscosity of the composition.
In the process of entering the lithosphere, the anomalous mantle is rather quickly distributed along the sole. 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 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. Above submerged areas, the crust remains stable.
Traps
The process of cooling the upper mantle layer and the crust to a depth of about a hundred kilometers is slow. In general, it takes several hundred million years. In this regard, inhomogeneities in the thickness of the lithosphere, explained by horizontal temperature differences, have a rather large inertia. In the event that the trap is located not far from the upward flow of the anomalous accumulation from the depth, a large amount of the substance is captured very heated. As a result, a rather large mountain element is formed. In accordance with this scheme, high uplifts occur in the areaepiplatform orogeny in folded belts.
Description of processes
In the trap, the anomalous layer undergoes compression by 1-2 kilometers during cooling. The bark located on top is immersed. Precipitation begins to accumulate in the formed trough. Their heaviness contributes to even greater subsidence of the lithosphere. As a result, the depth of the basin can be from 5 to 8 km. At the same time, during the compaction of the mantle in the lower part of the bas alt layer, a phase transformation of the rock into eclogite and garnet granulite can be observed in the crust. Due to the heat flow leaving the anomalous substance, the overlying mantle is heated and its viscosity decreases. In this regard, there is a gradual displacement of the normal cluster.
Horizontal offsets
When uplifts form in the process of anomalous mantle reaching the crust on continents and oceans, the potential energy stored in the upper layers of the planet increases. To dump excess substances, they tend to disperse to the sides. As a result, additional stresses are formed. They are associated with different types of movement of plates and crust.
The expansion of the ocean floor and the floating of the continents are the result of the simultaneous expansion of the ridges and the sinking 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 directly under the crust. The lithosphere here has a much smaller thickness. At the same time, the anomalous mantle spreads in the area of high pressure - in bothsides from under the spine. At the same time, it quite easily breaks the ocean's crust. The crevice is filled with bas altic magma. It, in turn, is melted out of 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 mid-ridges, the anomalous mantle has reduced viscosity due to increased temperature. The substance is able to spread quite quickly. As a result, 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 areas are in the same ocean, then the process occurs at a relatively high speed. This situation is typical today for the Pacific Ocean. If the expansion of the bottom and the 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 resulting friction, there is a significant resistance to movement. As a result, the rate at which the bottom expands is reduced if there is no compensation for the mantle subsidence in the same area. Thus, the growth in the Pacific is faster than in the Atlantic.
