Hypotheses explaining the origin and development of the earth's crust
Concept of the earth's crust.
Earth's crust is a complex of surface layers solid Earth. In the scientific geographical literature there is no single idea about the origin and paths of development of the earth's crust.
There are several concepts (hypotheses) that reveal the mechanisms of formation and development of the earth’s crust, the most substantiated of which are the following:
1. The theory of fixism (from the Latin fixus - motionless, unchanging) states that the continents have always remained in the places that they currently occupy. This theory denies any movement of continents and large parts of the lithosphere.
2. The theory of mobilism (from the Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept was especially established in last years in connection with the receipt of new scientific data from the study of the bottom of the World Ocean.
3. The concept of continental growth at the expense of the ocean floor believes that the original continents formed in the form of relatively small massifs that now make up ancient continental platforms. Subsequently, these massifs grew due to the formation of mountains on the ocean floor adjacent to the edges of the original land cores. The study of the ocean floor, especially in the zone of mid-ocean ridges, has given reason to doubt the correctness of the concept of continental growth due to the ocean floor.
4. The theory of geosynclines states that the increase in land size occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the continental crust, forms the basis of many modern scientific explanations the process of origin and development of the earth's crust.
5. Rotation theory bases its explanation on the proposition that since the figure of the Earth does not coincide with the surface of a mathematical spheroid and is rearranged due to uneven rotation, zonal stripes and meridional sectors on a rotating planet are inevitably tectonically unequal. They react with varying degrees of activity to tectonic stresses caused by intraterrestrial processes.
There are two main types of earth's crust: oceanic and continental. A transitional type of the earth's crust is also distinguished.
Oceanic crust. The thickness of the oceanic crust in the modern geological era ranges from 5 to 10 km. It consists of the following three layers:
1) top thin layer marine sediments (thickness no more than 1 km);
2) middle basalt layer (thickness from 1.0 to 2.5 km);
3) lower layer of gabbro (thickness about 5 km).
Continental (continental) crust. The continental crust has more complex structure and greater thickness than the oceanic crust. Its thickness averages 35-45 km, and in mountainous countries it increases to 70 km. It also consists of three layers, but differs significantly from the ocean:
1) lower layer composed of basalts (thickness about 20 km);
2) the middle layer occupies the main thickness of the continental crust and is conventionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;
3) upper layer– sedimentary. Its thickness on average is about 3 km. In some areas the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth there is no sedimentary layer at all and a granite layer comes to the surface. Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).
On continents, as a result of the weathering of rocks, a geological formation is formed, called weathering crust.
The granite layer is separated from the basalt layer Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km/sec.
Border between earth's crust and the mantle (both on continents and oceans) passes along Mohorovicic surface (Moho line). The speed of seismic waves on it increases abruptly to 8 km/hour.
In addition to the two main types - oceanic and continental - there are also areas of mixed (transitional) type.
On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out. In East Asia, in the region of island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied. There is no Moho boundary here, and mantle material rises along faults into the crust and even to its surface.
The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than the "earth's crust". In the lithosphere, modern science includes not only the earth’s crust, but also the uppermost mantle to the asthenosphere, that is, to a depth of approximately 100 km.
The concept of isostasy . A study of the distribution of gravity showed that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from the Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density. Heavy oceanic crust is thinner than lighter continental crust.
Isostasy is, in essence, not even an equilibrium, but a desire for equilibrium, continuously disrupted and restored again. For example, the Baltic Shield, after the melting of continental ice of the Pleistocene glaciation, rises by about 1 meter per century. The area of Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero equilibrium line currently runs slightly south of 60 0 N latitude. Modern St. Petersburg is approximately 1.5 m higher than St. Petersburg during the time of Peter the Great. As data from modern scientific research, even the heaviness of large cities turns out to be sufficient for isostatic fluctuations of the territory beneath them. Consequently, the earth's crust in areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface, the base of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to more high level its upper limit. Thus, under the Pamirs the depth of the Moho surface is 65 km, and in the Caspian lowland it is about 30 km.
Thermal properties of the earth's crust . Daily fluctuations in soil temperature extend to a depth of 1.0 - 1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate to a depth of 20-30 m. At the depth where the influence of annual temperature fluctuations due to heating of the earth's surface by the Sun ceases, there is layer of constant soil temperature. It is called isothermal layer . Below the isothermal layer deep into the Earth, the temperature rises, and this is caused by internal warmth earth's bowels In the formation of climates internal heat does not participate, but it serves as the energetic basis of all tectonic processes.
The number of degrees by which the temperature increases for every 100 m of depth is called geothermal gradient . The distance in meters, when lowered by which the temperature increases by 1 0 C is called geothermal stage . The magnitude of the geothermal step depends on the topography, thermal conductivity of rocks, the proximity of volcanic sources, groundwater circulation, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be only about 5 m, and in geologically quiet areas (for example, on platforms) it can reach 100 m.
Earth's crust- the outer solid shell of the Earth (geosphere), part of the lithosphere, with a width from 5 km (under the ocean) to 75 km (under the continents). Below the crust is the mantle, which differs in composition and physical qualities- it is more compact and contains mainly refractory elements. The crust and mantle are divided by the Mohorovicic feature, or the Moho layer, where a sharp acceleration of seismic waves occurs.
There are continental (continental) and oceanic crust, as well as its transitional types: subcontinental and suboceanic crust.
Continental (mainland) crust consists of several layers. The top is a layer of sedimentary rocks. The thickness of this layer is up to 10-15 km. Beneath it lies a granite layer. The rocks that make it up are similar in their physical properties to granite. The thickness of this layer is from 5 to 15 km. Beneath the granite layer is a basalt layer, consisting of basalt and rocks whose physical characteristics resemble basalt. The thickness of this layer is from 10 km to 35 km. Consequently, the total thickness of the continental crust reaches 30-70 km.
Oceanic crust differs from the continental crust in that it does not have a granite layer, or it is very thin, therefore the thickness of the oceanic crust is only 6-15 km.
To determine the chemical composition of the earth's crust, only its upper parts are available - to a depth of less than 15-20 km. 97.2% of the total composition of the earth's crust is made up of: oxygen - 49.13%, aluminum - 7.45%, calcium - 3.25%, silicon - 26%, iron - 4.2%, potassium - 2.35 %, magnesium - 2.35%, sodium - 2.24%.
Other elements of the periodic table account for from 10ths to hundredths of a percent.
Sources:
The Earth's shell includes the earth's crust and the upper part of the mantle. The surface of the earth's crust has large irregularities, the main of which are the protrusions of the continents and their depressions - huge oceanic depressions. The existence and relative position of continents and ocean basins is associated with differences in the structure of the earth's crust.
Continental crust. It consists of several layers. The top is a layer of sedimentary rocks. The thickness of this layer is up to 10-15 km. Beneath it lies a granite layer. The rocks that make it up are similar in their physical properties to granite. The thickness of this layer is from 5 to 15 km. Below the granite layer is a basalt layer consisting of basalt and rocks, physical properties which resemble basalt. The thickness of this layer is from 10 km to 35 km. Thus, the total thickness of the continental crust reaches 30-70 km.
Oceanic crust. It differs from the continental crust in that it does not have a granite layer or it is very thin, so the thickness of the oceanic crust is only 6-15 km.
To determine the chemical composition of the earth's crust, only its upper parts are available - to a depth of no more than 15-20 km. 97.2% of the total composition of the earth's crust is made up of: oxygen - 49.13%, aluminum - 7.45%, calcium - 3.25%, silicon - 26%, iron - 4.2%, potassium - 2.35 %, magnesium - 2.35%, sodium - 2.24%.
Other elements of the periodic table account for from tenths to hundredths of a percent.
Most scientists believe that oceanic-type crust first appeared on our planet. Under the influence of processes occurring inside the Earth, folds, that is, mountainous areas, formed in the earth's crust. The thickness of the bark increased. This is how continental protrusions were formed, that is, the continental crust began to form.
In recent years, in connection with studies of the earth's crust of oceanic and continental types, a theory of the structure of the earth's crust has been created, which is based on the idea of lithospheric plates. The theory in its development was based on the hypothesis of continental drift, created at the beginning of the 20th century by the German scientist A. Wegener.
Types of the earth's crust Wikipedia
Site search:
Ocean chasms are primitive in composition and actually represent an upper differentiated layer of a coat dominated by a thin layer of pelagic sediment. In the oceanic crust, three layers are usually distinguished, of which the first (upper) sediment.
At the bottom of the sedimentary layer they are often thin and unstable metallic deposits dominated by iron oxides.
The lower part of the sediment usually consists of carbonate sediments at depths less than 4-4.5 km. With deeper recycling of carbonate, it usually does not precipitate due to their microscopic composition of the shells of single-chain organisms (foraminifera and colithopharids) at pressures above 400-450 ATM, immediately dissolved in sea water. For this reason, in marine basins at depths of more than 4-4.5 km, the upper part of the sedimentary layer mainly consists of only non-calcic sediments - dark red clays and silicate heat.
Near the island arc and volcanic islands, lentils and intertwined volcanic dams and terrigenous landfills are often found near the deltas of large rivers in part of the sedimentary layers. In the open oceans, the thickness of the sediment layer increases from the central ocean reefs, where there is almost no sediment in their peripheral areas.
The average thickness of sediments is low and, according to A.P. Lisitsyn, it is close to 0.5 km, near continental edges of the Atlantic type and in areas of a large rectal delta, increasing to 10-12 km. This is due to the fact that almost all terrigenous materials that land due to floating sedimentation processes are practically emplaced in the coastal regions of the oceans and the continental slopes of the continents.
The other, or basaltic, layer of oceanic crust in the upper part consists of basaltic lavas of the Tolly composition (Fig.
5). Underwater lava will be an unusual shape corrugated pipes and pillows, so these pillows are lava. Below are doleitic berms, tholeiites of the same composition, the former are supply channels for which basaltic magma in tectonic areas is filled on the surface of the seabed.
The basalt layer of oceanic crust is exposed in many areas of the ocean floor, bordering the emblem of mid-ocean reefs and turning knife-edge defects. This layer has been discussed in detail as conventional methods of exploring the ocean floor in (mining, drilling survey samples) or using underwater manned vehicle, so that geologists take into account the geological structure of objects and carry out targeted selection of stone samples.
In addition, over the past twenty years, the surface of the basalt layer and its upper layers was discovered by a series of deep-sea drill holes, one of which also penetrated the soft lion layer and entered the lobular complexes of the dike complex. The total thickness of the basalt or other layer of oceanic crust is 1.5, sometimes 2 km, according to seismic data.
Figure 5. Structure of the rift belt of oceanic crust:
1 - ocean level; 2—precipitation; 3 - soft basaltic lava (layer 2a); 4—complex complex, dolerite (layer 2b); 5 - gabbro; 6 - layered complex; 7 - serpentinites; 8—lyrosolites of lithospheric plates; 9 - asthenosphere; 10 - isotherm 500 ° C (beginning of serpentinization).
Frequent findings within the framework of the main transformation errors involving gabbrotoleum show that the composition of the oceanic crust includes these dense and coarse rocks.
The structure of the ophiolite leaves in the land strips we know fragments the ancient oceanic crust that was removed in these areas at the edge of the former continents. Therefore, it can be concluded that the mound complex in the modern oceanic crust (as in the upper ophiolite) is below the main layer of ghabro properties that makes up the upper part of the oceanic crust of the third layer (3a layers). At a certain distance from the ridge in the middle of the sea reefs, according to seismic data, traces and the lower part of the crust lay.
Many findings in large convertible serpentinite defects responsible for the composition of hydrated peridotite and serpentinites, similar to the structure of ophiolite complexes, indicate that the lower part of the oceanic crust is composed of serpentinite.
According to seismic data, the thickness of the gabbro-serpentinite (third) layer of the oceanic crust reaches 4.5-5 km. Under ridge reefs in the middle of the ocean, the thickness of the oceanic crust usually decreases to 3-4 and even 2-2.5 km just below the river valley.
The total thickness of the oceanic crust without sedimentary layer, reaching 6.5-7 km. Below, the oceanic crust is covered with crystalline rocks of the upper layer, which form the subcrustal regions of lithospheric plates. Beneath the mid-ocean ridge, oceanic crust lies directly above the centers of basaltic hostages separated from hot-coat material (from the asthenosphere).
The area of the oceanic crust is approximately 3.0610 x 18 cm2 (306,000,000 km2), the average density of the oceanic crust (rain) is close to 2.9 g/cm3, therefore the cleared mass of the oceanic crust can be estimated (5.8-6 ,2) , where h1024
The volume and mass of the sedimentary layer of the deep-sea basins of the World Ocean, according to A.P. Lisitsyn, is 133 million km3 and about 0.1 × 1024 g.
Precipitation is concentrated on the continental shelf and the slope is slightly higher at about 190 million km3, approximately (0.4-0.45) 1024 depending on weight (including precipitation)
The ocean floor, which is the surface of the oceanic crust, has a characteristic relief.
In the abyssal trench, the ocean floor is at a depth of about 66.5 km, while the emblems of the middle ocean ridge, sometimes cutting steep grapes, the fever of the deep ocean depths decreased by 2-2.5 km.
In some places the ocean floor extends, for example, onto the surface of the Earth. Iceland and Afar Province (Northern Ethiopia). To the island arcs around the western edge Pacific Ocean, northeast of the Indian Ocean, in front of the arc of the Lesser Antilles and South Sandwich Islands in the Atlantic, and before the beginning of the active continental margin in the Central and South America, the oceanic crust bends and its surface sinks to a depth of 9 -10 km to go further into these structures and form in front of them and two longer narrow trenches.
Oceanic crust is formed in tectonic regions of central ocean reefs due to the separation of melt that occurs beneath the basalt from the hot layer (Earth's asthenospheric layers) and seepage onto the surface of the seafloor.
Every year in these areas, at least 5.5-6 km3 of basaltic melts rise from the astenosfera, pour out onto the seabed, and crystallize, forming the entire second layer of oceanic crust (including the volume of the gabbro layer implanted into the crust of basaltic melts increases to 12 km3) .
These magnificent tectonomagmatic processes, which are constantly developing under the ridge of the middle ocean, are uncontrollable on land and are accompanied by increased seismicity (Fig. 6).
Figure 6. Earth seismicity; earthquake location
Barazangi, Dorman, 1968
In rift regions, located on mid-ocean ridge reefs, the ocean floor expands and spreads.
Therefore, all such zones are marked by frequent, but earthquakes with little emphasis, with the predominant effect of interrupting the movement mechanisms. On the contrary, under the bends of islands and the active edges of continents, i.e.
In areas of panel subduction, there are usually more strong earthquakes are generated by the predominance of compression and shear mechanisms. According to the earthquake data, the subsidence of the oceanic crust and lithosphere occurs in the upper layer and mesosphere to a depth of about 600-700 km (Fig. 7). According to the same tomography, the subsidence of oceanic lithospheric plates was traced to a depth of about 1400-1500 km and, if possible, deeper - to the surface of the earth's core.
Figure 7. Structure of the underwater section of the plate on the Kuril Islands:
1 - asthenosphere; 2 - lithosphere; 3 - oceanic crusts; 4-5 - sedimentary-volcanogenic layers; 6—ocean sediments; isolines show seismic activity in A10 units (Fedotov et al., 1969); β is the Wadati-Benif morbidity aspect; α is the field of view of the plastic deformation region.
For the ocean floor, there are characteristic and fairly contrasting magnetic band anomalies, which are usually located parallel to the ridge in the middle of the ocean ridge (Fig.
8). The origin of these anomalies is related to the possibility of magnetization of ocean floor basalts by cooling by the Earth's magnetic field, thereby resembling the direction of this field during their unloading onto the surface of the ocean floor.
Taking into account that the geomagnetic field has repeatedly changed its polarity over a long period of time, the English scientist F. Vine and D. Matthews in 1963 were the first to identify individual irregularities, and suggest that different inclinations in the middle of the ocean reef about these anomalies symmetrical with their coats of arms. As a result, they were able to reconstruct the basic laws of plate motion in parts of the oceanic crust in the North Atlantic and to show that the ocean floor extends roughly symmetrically along the sides of mid-ocean ridge velocity on the order of several centimeters per year.
In the future, similar studies were carried out in all areas of the World Ocean, and this picture was confirmed everywhere. In addition, a detailed comparison of magnetic anomalies on the ocean floor with a reversal of the geo-chronology of the magnetization of continental rocks, the age of which was known from other sources, will contribute to the spread of Osipovka disturbances throughout the Cenozoic, Mesozoic, and then later.
Therefore, a new and reliable paleomagnetic method for determining the age of the ocean floor has emerged.
Figure 8. Map of magnetic field anomalies in the Reykjanes Ridge in the North Atlantic
(Heirtzler et al., 1966).
Positive anomalies are marked in black; AA—zero rift zone anomaly.
The use of this method led to the confirmation of previously expressed ideas regarding youth on the seafloor: the paleomagnetic receives everything without exception that only the oceans and the late cenozoic (Fig.
9). This conclusion was later fully confirmed by deep-sea drilling at many points on the ocean floor. In this case, the young age of the ocean cavities (Atlantic, Indian and Arctic) coincides with the bottom of their age, the era of the ancient Pacific Ocean, far beyond its bottom. Indeed, the Pacific Basin is at least late Proterozoic (perhaps even earlier) and the oldest areas of the ocean floor are less than 160 million years old, while most were created only in the Kenozoic, i.e.
younger than 67 million years.
Figure 9. Map of the ocean floor over millions of years
Larson, Pitman et al., 1985
The mechanism of modernization of the “bicycle” of the ocean floor with the constant immersion of sections of the old ocean crust and accumulated sediments on it in a coat under the island arches explains why during the life of the Earth’s ocean dams there was no time to fill the chasms.
In fact, at the current stage of filling of marine basins destroyed from terrestrial sediments 2210 x 16 g of sediment, the total volume of these wells is approximately 1.3710 x 24 cm 3, it will be completely bombarded with approximately 1.2 GA. We can now say with confidence that continents and ocean basins coexisted around 3.8 billion years ago, and there was no significant recovery of their depressions at that time. In addition, after drilling operations in all oceans, we now know for sure that there has been no sediment on the ocean floor for more than 160-190 million years.
However, this can only be observed in one case - in the case of an effective sediment removal mechanism in the ocean. This mechanism is now known as the rain extension process, based on island bows and active continental margins in subduction regions where these sediments melt and re-invade as granitoid intrusion into emerging continental crust in these zones.
This process of overflowing terrigenous sediments and reattaching their material to the continental crust is called sediment recycling.
Oceanic and continental crust
There are two main types of earth's crust: oceanic and continental. A transitional type of the earth's crust is also distinguished.
Oceanic crust. The thickness of the oceanic crust in the modern geological era ranges from 5 to 10 km. It consists of the following three layers:
1) upper thin layer of marine sediments (thickness no more than 1 km);
2) middle basalt layer (thickness from 1.0 to 2.5 km);
3) lower layer of gabbro (thickness about 5 km).
Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic crust.
Its thickness averages 35-45 km, and in mountainous countries it increases to 70 km. It also consists of three layers, but differs significantly from the ocean:
1) lower layer composed of basalts (thickness about 20 km);
2) the middle layer occupies the main thickness of the continental crust and is conventionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;
3) the top layer is sedimentary.
Its thickness on average is about 3 km. In some areas the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth there is no sedimentary layer at all and a granite layer comes to the surface.
Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).
On continents, as a result of the weathering of rocks, a geological formation is formed, called weathering crust.
The granite layer is separated from the basalt layer Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km/sec.
The boundary between the earth's crust and mantle (both on continents and oceans) runs along Mohorovicic surface (Moho line). The speed of seismic waves on it increases abruptly to 8 km/hour.
In addition to the two main types - oceanic and continental - there are also areas of mixed (transitional) type.
On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust.
However, a layer of basalt may fall out. In East Asia, in the region of island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied.
There is no Moho boundary here, and mantle material rises along faults into the crust and even to its surface.
The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than the "earth's crust".
In the lithosphere, modern science includes not only the earth’s crust, but also the uppermost mantle to the asthenosphere, that is, to a depth of approximately 100 km.
The concept of isostasy .
A study of the distribution of gravity showed that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from the Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density.
Heavy oceanic crust is thinner than lighter continental crust.
Isostasy is, in essence, not even an equilibrium, but a desire for equilibrium, continuously disrupted and restored again. For example, the Baltic Shield, after the melting of continental ice of the Pleistocene glaciation, rises by about 1 meter per century.
The area of Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero equilibrium line currently runs slightly south of 600 N latitude. Modern St. Petersburg is approximately 1.5 m higher than St. Petersburg during the time of Peter the Great. As data from modern scientific research show, even the heaviness of large cities is sufficient for isostatic fluctuations of the territory beneath them.
Consequently, the earth's crust in areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface, the base of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to a higher level of its upper boundary. Thus, under the Pamirs the depth of the Moho surface is 65 km, and in the Caspian lowland it is about 30 km.
Thermal properties of the earth's crust .
Daily fluctuations in soil temperature extend to a depth of 1.0 - 1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate to a depth of 20-30 m. At the depth where the influence of annual temperature fluctuations due to heating of the earth's surface by the Sun ceases, there is layer of constant soil temperature.
It is called isothermal layer . Below the isothermal layer deep into the Earth, the temperature rises, and this is caused by the internal heat of the earth's bowels. Internal heat does not participate in the formation of climates, but it serves as the energy basis for all tectonic processes.
The number of degrees by which the temperature increases for every 100 m of depth is called geothermal gradient . The distance in meters, when lowered by which the temperature increases by 10C is called geothermal stage .
The size of the geothermal step depends on the topography, thermal conductivity of rocks, proximity of volcanic sources, circulation of groundwater, etc. On average, the geothermal step is 33 m.
In volcanic areas, the geothermal step may be only about 5 m, but in geologically quiet areas (for example, on platforms) it can reach 100 m.
TOPIC 5. CONTINENTS AND OCEANS
Continents and parts of the world
Two qualitatively different types of the earth's crust - continental and oceanic - correspond to two main levels of planetary relief - the surface of the continents and the bed of the oceans.
Structural-tectonic principle of separation of continents.
The fundamentally qualitative difference between the continental and oceanic crust, as well as some significant differences in the structure of the upper mantle under the continents and oceans, oblige us to distinguish continents not according to their apparent surroundings by oceans, but according to the structural-tectonic principle.
The structural-tectonic principle states that, firstly, the continent includes a continental shelf (shelf) and a continental slope; secondly, at the base of every continent there is a core or ancient platform; thirdly, each continental block is isostatically balanced in the upper mantle.
From the point of view of the structural-tectonic principle, a continent is an isostatically balanced massif of the continental crust, which has a structural core in the form of an ancient platform, to which younger folded structures are adjacent.
There are six continents in total on Earth: Eurasia, Africa, North America, South America, Antarctica and Australia.
Each continent contains one platform, and at the base of Eurasia alone there are six of them: Eastern European, Siberian, Chinese, Tarim (Western China, Taklamakan Desert), Arabian and Hindustan. The Arabian and Hindu platforms are parts of ancient Gondwana, adjacent to Eurasia. Thus, Eurasia is a heterogeneous anomalous continent.
The boundaries between the continents are quite obvious.
The border between North America and South America runs along the Panama Canal. The border between Eurasia and Africa is drawn along the Suez Canal. The Bering Strait separates Eurasia from North America.
Two rows of continents . In modern geography, the following two series of continents are distinguished:
Equatorial series of continents (Africa, Australia and South America).
2. Northern series of continents (Eurasia and North America).
Antarctica, the southernmost and coldest continent, remains outside these ranks.
The modern location of the continents reflects the long history of the development of the continental lithosphere.
The southern continents (Africa, South America, Australia and Antarctica) are parts (“fragments”) of the single Paleozoic megacontinent Gondwana.
The northern continents at that time were united into another megacontinent - Laurasia. Between Laurasia and Gondwana in the Paleozoic and Mesozoic there was a system of vast marine basins called the Tethys Ocean. The Tethys Ocean stretched from North Africa, through southern Europe, the Caucasus, Western Asia, the Himalayas to Indochina and Indonesia.
In the Neogene (about 20 million years ago), an Alpine fold belt arose in the place of this geosyncline.
According to their large sizes supercontinent Gondwana. According to the law of isostasy, it had a thick (up to 50 km) crust, which sank deeply into the mantle. Beneath them, in the asthenosphere, convection currents were especially intense and the softened substance of the mantle was moving actively.
This led first to the formation of a bulge in the middle of the continent, and then to its splitting into separate blocks, which, under the influence of the same convection currents, began to move horizontally. As proven mathematically (L. Euler), the movement of a contour on the surface of a sphere is always accompanied by its rotation. Consequently, parts of Gondwana not only moved, but also unfolded in geographical space.
The first breakup of Gondwana occurred at the Triassic-Jurassic boundary (about 190-195 million years ago).
years ago); Afro-America seceded. Then, at the Jurassic-Cretaceous boundary (about 135-140 million years ago), South America separated from Africa. At the border of the Mesozoic and Cenozoic (about 65-70 million years ago)
years ago) The Hindustan block collided with Asia and Antarctica moved away from Australia. In the present geological era, the lithosphere, according to neomobilists, is divided into six plate blocks that continue to move.
The breakup of Gondwana successfully explains the shape of the continents, their geological similarity, as well as the history of the vegetation and animal world of the southern continents.
The history of the split of Laurasia has not been studied as thoroughly as Gondwana.
The concept of parts of the world .
In addition to the geologically determined division of land into continents, there is also a division of the earth's surface into separate parts of the world that has developed in the process of cultural and historical development of mankind. There are six parts of the world in total: Europe, Asia, Africa, America, Australia and Oceania, Antarctica. On one continent of Eurasia there are two parts of the world (Europe and Asia), and two continents of the Western Hemisphere (North America and South America) form one part of the world - America.
The border between Europe and Asia is very arbitrary and is drawn along the watershed line of the Ural ridge, the Ural River, the northern part of the Caspian Sea and the Kuma-Manych depression.
Deep fault lines that separate Europe from Asia run through the Urals and the Caucasus.
Area of continents and oceans. Land area is calculated within the modern coastline. Surface area globe is approximately 510.2 million km 2. About 361.06 million km 2 is occupied by the World Ocean, which is approximately 70.8% of the total surface of the Earth. On land there are approximately 149.02 million.
km 2, which is about 29.2% of the surface of our planet.
Area of modern continents characterized by the following values:
Eurasia – 53.45 km2, including Asia – 43.45 million km2, Europe – 10.0 million km2;
Africa - 30, 30 million km 2;
North America – 24, 25 million km2;
South America – 18.28 million km2;
Antarctica – 13.97 million km2;
Australia – 7.70 million
Australia with Oceania - 8.89 km2.
Modern oceans have an area:
Pacific Ocean - 179.68 million km 2;
Atlantic Ocean - 93.36 million km 2;
Indian Ocean - 74.92 million km 2;
Arctic Ocean – 13.10 million km2.
Between the northern and southern continents, in accordance with their different origins and development, there is a significant difference in area and character of the surface.
The main geographical differences between the northern and southern continents are as follows:
1. Eurasia is incomparable in size with other continents, concentrating more than 30% of the planet’s landmass.
2.The northern continents have a significant shelf area. The shelf is especially significant in the Arctic Ocean and the Atlantic Ocean, as well as in the Yellow, Chinese and Bering Seas of the Pacific Ocean. The southern continents, with the exception of the underwater continuation of Australia in the Arafura Sea, are almost devoid of a shelf.
3. Most of the southern continents lie on ancient platforms.
IN North America and Eurasia, ancient platforms occupy a smaller part total area, and most of it falls on the territories formed by the Paleozoic and Mesozoic orogeny. In Africa, 96% of its territory is in platform areas and only 4% is in mountains of Paleozoic and Mesozoic age. In Asia, only 27% is on ancient platforms and 77% on mountains of various ages.
4. The coastline of the southern continents, formed mostly by rifts, is relatively straight; There are few peninsulas and mainland islands.
The northern continents are characterized by exceptionally sinuous coastline, an abundance of islands, peninsulas, often extending far into the ocean.
Of the total area, islands and peninsulas account for about 39% in Europe, North America - 25%, Asia - 24%, Africa - 2.1%, South America - 1.1% and Australia (excluding Oceania) - 1.1% .
Previous12345678910111213141516Next
The structure of the continental crust in different areas.
Continental crust or continental crust is the crust of the continents, which consists of sedimentary, granite and basalt layers.
The average thickness is 35-45 km, the maximum is up to 75 km (under mountain ranges). It is contrasted with oceanic crust, which is different in structure and composition. The continental crust has a three-layer structure. The upper layer is represented by a discontinuous cover of sedimentary rocks, which is widely developed, but rarely has great thickness. Most of the crust is made up of the upper crust, a layer composed mainly of granites and gneisses that is low in density and ancient in history.
Research shows that most of these rocks were formed a very long time ago, about 3 billion years ago. Below is the lower crust, consisting of metamorphic rocks - granulites and the like.
5. Types of ocean structures. The land surface of the continents makes up only one third of the Earth's surface. The surface area occupied by the World Ocean is 361.1 ml sq. km. The underwater margins of the continents (shelf plateaus and continental slope) account for about 1/5 of its surface area, the so-called.
“transitional” zones (deep-sea trenches, island arcs, marginal seas) – about 1/10 of the area. The remaining surface (about 250 ml sq. km.) is occupied by oceanic deep-sea plains, depressions and intra-oceanic rises separating them. The ocean floor differs sharply in the nature of seismicity. It is possible to distinguish areas with high seismic activity and aseismic areas.
The first are extended zones occupied by systems of mid-ocean ridges, stretching across all oceans. Sometimes these zones are called oceanic mobile belts. Mobile belts are characterized by intense volcanism (tholeiitic basalts), increased heat flow, sharply dissected topography with systems of longitudinal and transverse ridges, trenches, scarps, and shallow mantle surface.
Seismically inactive areas are expressed in relief by large ocean basins, plains, plateaus, as well as underwater ridges, limited fault-type ledges and intra-oceanic swell-like uplifts, crowned by cones of active and extinct volcanoes. Within areas of the second type there are underwater plateaus and uplifts with continental-type crust (microcontinents).
Unlike mobile oceanic belts, these areas, by analogy with the structures of continents, are sometimes called Thalassocratons.
6. The structure of the oceanic crust in structures of various types. Oceanic basins, as the largest negative structures on the surface of the earth's crust, have a number of structural features that allow them to be contrasted with positive structures (continents) and compared with each other.
The main thing that unites and distinguishes all ocean basins is the low position of the earth's crust within them and the absence of a geophysical granite-metamorphic layer characteristic of continents.
Movable belts stretch across all ocean basins—mountain systems of mid-ocean ridges with a high heat flow and elevated position of the mantle layer, which is not typical for continents. The system of mid-ocean ridges, the longest on the surface of the Earth, penetrates and thereby connects all ocean basins, occupying a central or marginal position in them. It is also characteristic that the tectonic structures of the ocean floor are often closely related to the structures of continents.
First of all, these connections are expressed in the presence of common faults, in the transitions of rift valleys of mid-ocean ridges into continental rifts (the Gulf of California and the Gulf of Aden), in the presence of large submerged blocks of continental crust in the oceans, as well as depressions with granite-free crust on continents, in transitions trap fields of continents on the shelf and ocean floor. The internal structure of oceanic basins is also different. Based on the position of the zone of modern spreading, one can contrast the trench of the Atlantic Ocean with the median position of the Mid-Atlantic Ridge with all other oceans in which the so-called.
the median ridge is shifted to one of the edges. The internal structure of the Indian Ocean basin is complex. In the western part it resembles the structure of the Atlantic Ocean, in the eastern part it is closer to the western region of the Pacific Ocean. Comparing the structure of the western region of the Pacific Ocean with the eastern part of the Indian Ocean, one notices their certain similarities: bottom depths, age of the crust (Cocos and Western Australian basins of the Indian Ocean, Western Pacific basin).
In both oceans, these parts are separated from the continent and the depressions of the marginal seas by systems of deep-sea trenches and island arcs. The connection between the active margins of the oceans and the young folded structures of the continents is observed in Central America, where the Atlantic Ocean is separated from the Caribbean Sea by a deep-sea trench and an island arc.
The close connection of the deep-sea trenches separating the ocean basins from the continental massifs with the structures of the continental crust can be seen in the example of the northern continuation of the Sunda deep-sea trench, which passes into the Pre-Arakan foredeep.
Structures of the margins of continents (oceans) and types of crust. 
8. Types of boundaries of continental blocks and oceanic basins. Continental masses and ocean basins can have two types of boundaries - passive (Atlantic) and active (Pacific). The first type is distributed along most of the Atlantic, Indian, and Arctic oceans. This type is characterized by the fact that through a continental slope of varying steepness with a system of stepped faults, ledges and a relatively flat continental foot, the closure of continental massifs with the region of abyssal plains of the ocean floor occurs.
In the zone of the continental foothills, systems of deep troughs are known, but they are smoothed out by thick layers of loose sediments. The second type of margins is expressed along the edge of the Pacific Ocean, along the northeastern edge of the Indian Ocean and on the edge of the Atlantic Ocean adjacent to Central America. In these areas, between the continental massifs and the abyssal plains of the ocean floor, there is a zone of varying width with deep-sea trenches, island arcs, and depressions of marginal seas.
Lithospheric plates and types of their boundaries. By studying the lithosphere, which includes the earth’s crust and upper mantle, geophysicists came to the conclusion that it contains its own inhomogeneities. First of all, these heterogeneities of the lithosphere are expressed by the presence of strip zones with high heat flow, high seismicity, and active modern volcanism crossing its entire thickness. The areas located between such strip zones are called lithospheric plates, and the zones themselves are considered as the boundaries of lithospheric plates.
In this case, one type of boundaries is characterized by tensile stresses (the boundaries of plate divergence), another type – compression stresses (the boundaries of convergence of plates), and a third – tension and compression that arise during shears.
The first type of boundaries are divergent (constructive) boundaries, which on the surface correspond to rift zones.
The second type of boundaries is subduction (when oceanic blocks are pushed under continental ones), obduction (when oceanic blocks are pushed onto continental ones), and collision (when continental blocks move). On the surface they are expressed by deep-sea trenches, marginal troughs, and zones of large thrusts, often with ophiolites (sutures).
The third type of boundaries (shear) is called transform boundaries. It is also often accompanied by intermittent chains of rift basins. Several large and small lithospheric plates are distinguished. Large plates include the Eurasian, African, Indo-Australian, South American, North American, Pacific, and Antarctic.
Small plates include the Caribbean, Scotia, Philippine, Cocos, Nazca, Arabian, etc.
10. Rifting, spreading, subduction, obduction, collision. Rifting is the process of the emergence and development of continents and oceans in the earth's crust, strip-shaped zones of horizontal extension on a global scale.
In its upper fragile part, it manifests itself in the formation of rifts expressed in the form of large linear grabens, expansion cavities and related structural forms, and their filling with sediments and (or) products of volcanic eruptions, usually accompanying rifting.
In the lower, more heated part of the crust, brittle deformations during rifting are replaced by plastic stretching, leading to its thinning (formation of a “neck”), and with particularly intense and prolonged stretching, a complete rupture of the continuity of the pre-existing crust (continental or oceanic) and the formation of "gaping" of new oceanic-type crust.
The latter process, called spreading, proceeded powerfully in the late Mesozoic and Cenozoic within the modern oceans, and on a smaller (?) scale it periodically manifested itself in some zones of more ancient mobile belts.
Subduction is the movement of lithospheric plates of oceanic crust and mantle rocks under the edges of other plates (according to the concepts of Plate Tectonics).
Accompanied by the emergence of zones of deep-focus earthquakes and the formation of active volcanic island arcs.
Obduction is the pushing of tectonic plates composed of fragments of oceanic lithosphere onto the continental margin.
As a result, an ophiolite complex is formed. Obduction occurs when some factors disrupt the normal absorption of oceanic crust into the mantle. One of the mechanisms of obduction is the lifting of oceanic crust onto the continental margin when it enters the subduction zone of a mid-ocean ridge. Obduction is a relatively rare phenomenon and has occurred only periodically in Earth's history.
Some researchers believe that in our time this process occurs on the southwestern coast of South America.
Continental collision is the collision of continental plates, which always leads to crushing of the crust and the formation of mountain ranges. An example of a collision is the Alpine-Himalayan mountain belt, formed as a result of the closure of the Tethys Ocean and the collision with the Eurasian plate of Hindustan and Africa. As a result, the thickness of the crust increases significantly; under the Himalayas it reaches 70 km.
This is an unstable structure; its sides are intensively destroyed by surface and tectonic erosion. In the crust with a sharply increased thickness, granites are smelted from metamorphosed sedimentary and igneous rocks.
Structure and types of the earth's crust
All types of rocks occurring above the Moho boundary take part in the structure of the earth's crust. The ratio of different types of rocks in the earth's crust varies depending on the topography and structure of the Earth. In the relief of the Earth, continents and oceans are distinguished - structures of the first (planetary) order, significantly different from each other in geological structure and nature of development.
Within the continent, structures of the second order are distinguished - plains and mountain structures; in the oceans - underwater continental margins, beds, deep-sea trenches and mid-ocean ridges. The relief of the Earth's surface is dominated by two levels: continental plains and plateaus (heights less than 1000 m, occupying more than 70% of the land surface) and flat, relatively level spaces of the World Ocean bed, located at depths of 4-6 km below water level.
Initially, two main types of the earth's crust were distinguished - continental and oceanic, then two more were allocated - subcontinental and suboceanic, characteristic of continental-ocean transition zones and marginal and inland seas.
CONTINENTAL CRUST consists of three layers.
First- upper, represented by sedimentary rocks with a thickness of 0 to 5 (10) km within platforms, up to 15-20 km in tectonic troughs of mountain structures. Second- granite-gneiss or granite-metamorphic is 50% composed of granites, 40% - gneisses and other metamorphosed rocks. The thickness on the plains is 15-20 km, in mountain structures up to 20-25 km. Third— granulite-mafic (mafic is the main rock, granulite is a metamorphic rock of gneiss-like texture of a high (granulite) degree of metamorphism).
Thickness is 10-20 km within platforms and up to 25-35 km in mountain structures. The thickness of the continental crust within the platforms is 35-40 km, in young mountain structures 55-70 km, maximum under the Himalayas and Andes 70-75 km. The boundary between granite-metamorphic and granulite-mafic layers is called the Conrad section. Deep seismic sounding data showed that the Conrad surface is recorded only in certain places.
Research by N.I. Pavlenkova and other specialists, drilling data from the Kola superdeep well showed that the continental crust has a more complex structure than that presented above, and the interpretation of the data obtained by different authors is ambiguous.
Ocean crust. According to modern data, the ocean crust has a three-layer structure. Its thickness is from 5 to 12 km, on average 6-7 km.
It differs from the continental crust in the absence of a granite-gneiss layer. First(upper) layer of loose marine sediments ranging in thickness from a few hundred meters to 1 km. Second, located below, is composed of basalts with interlayers of carbonate and siliceous rocks.
Thickness from 1 to 3 km. Third, lower, has not yet been drilled. According to dredging data, it is composed of basic igneous rocks such as gabbro and partially ultrabasic rocks (pyroxenites). Thickness from 3.5 to 5 km.
SUB OCEAN TYPE OF GROUND CRUST confined to the deep-sea basins of the marginal and inland seas (southern basin of the Caspian, Black, Mediterranean, Okhotsk, Japanese, etc.).
Its structure is close to that of the ocean, but differs in the greater thickness of the sedimentary layer - 4-10 km, in some places up to 15-20 km. A similar structure of the crust is characteristic of some deep depressions on land - the central part of the Caspian lowland.
SUB-CONTINENTAL TYPE OF EARTH CRUST characteristic of island arcs (Aleutian, Kuril, etc.) and passive margins of the Atlantic type, where the granite-gneiss layer pinches out within the continental slope.
Its structure is close to that of the mainland, but it is less thick - 20-30 km.
Composition and state of matter in the Earth's mantle and core
Indirect, more or less reliable data on the composition are available for the layer IN(Gutenberg layer).
These are: 1) the outcrop of igneous intrusive ultrabasic rocks (peridotites) to the surface, 2) the composition of the rocks filling the diamond-bearing pipes, in which, along with peridotites containing garnets, there are eclogites, highly metamorphosed rocks similar in composition to gabbro, but with a density of 3 ,35-4.2 g/cm3, the latter could only form under high pressure. According to the study of intrusive bodies and experimental study it is assumed that the layer IN consists mainly of ultramafic rocks such as peridotites with garnets.
This breed was named by A.E. Ringwood in 1962 pyrolite.
State of matter in the layer IN
In layer IN using the seismic method, a layer of less dense, seemingly softened rocks, called asthenosphere(Greek
“asthenos” - weak) or waveguide. In it, the speed of seismic waves, especially transverse ones, decreases. The state of matter in the asthenosphere is less viscous, more plastic in relation to the layers above and below. The solid suprasthenospheric layer of the upper mantle together with the earth's crust is called lithosphere(Greek “lithos” - stone).
Horizontal movements of lithospheric plates are associated with this layer. The depth of the asthenosphere under continents and oceans varies. Research in recent decades has shown a more complex picture of the distribution of the asthenosphere under continents and oceans than before.
Under the rifts of the mid-ocean ridges, the asthenospheric layer is in some places located at a depth of 2-3 km from the surface. Within the shields (Baltic, Ukrainian, etc.), the asthenosphere was not detected by seismic methods to a depth of 200-250 km. Some researchers believe that the asthenospheric layer is discontinuous, in the form of asthenolenses. Nevertheless, there is indirect evidence of the presence of asthenosphere under the platform shields.
It is known that the Baltic and Canadian shields were subjected to powerful Quaternary glaciations. Under the weight of the ice, the shields sagged (like Antarctica and Greenland now). After the melting of the glaciers and the removal of the load, a rapid rise of the shields occurred in a relatively short period of time - the leveling of the disturbed equilibrium.
Here the phenomenon of isostasy is manifested (Greek “izos” - equal, “statis” - state) - a state of equilibrium of the masses of the earth’s crust and mantle.
According to V.E. Khain, the asthenosphere under the shields lies deeper than 200-250 km and its viscosity increases, so it is more difficult to detect using existing methods.
Data on the vertical heterogeneity of the asthenosphere were obtained. The depth of the base of the asthenosphere is estimated ambiguously. Some researchers believe that it descends to depths of 300-400 km, others that it covers part of layer C. Taking into account the endogenous activity of the lithosphere and upper mantle, the concept tectonosphere. The tectonosphere includes the earth's crust and upper mantle to depths of 700 km (where the deepest earthquake foci are recorded).
Composition and state of matter in layers C and D
Temperature and pressure increase with depth, and the substance transforms into denser modifications.
At depths of more than 400 (500) km, olivine and other minerals acquire the structure spinels, the density of which increases by 11% relative to olivine. At a depth of 700-1000 km, even greater compaction occurs and the spinel structure acquires a denser modification - perovskite. There is a sequential change of mineral phases:
pyrolite to a depth of 400(420) km,
spinel to a depth of 670-700 km,
perovskite to a depth of 2900 km.
There is another opinion regarding the composition and condition of the layers WITH And D.
It is assumed that iron-magnesium silicates decompose into oxides that are densely packed.
Earth's core
The issue is complex and controversial. A sharp drop in P-waves from 13.6 km/s at the base of layer D to 8-8.1 km/s in the outer core, and S-waves are completely extinguished. The outer core is liquid and does not have the shear strength of a solid. The inner core appears to be solid. According to modern data, the core density is 10% lower than that of an iron-nickel alloy.
Many researchers believe that the Earth's core consists of iron mixed with nickel and sulfur and possibly silicon or oxygen.
Physical characteristics of the Earth
Density
The average density of the Earth is 5.52 g/cm3.
The average density of the rocks is 2.8 g/cm3 (2.65 according to Palmer). Below the Moho boundary the density is 3.3-3.4 g/cm3, at a depth of 2900 km - 5.6-5.7 g/cm3, at the upper boundary of the core 9.7-10.0 g/cm3, in the center of the Earth - 12.5-13 g/cm3.
The density of the continental lithosphere is 3-3.1 g/cm3. The density of the asthenosphere is 3.22 g/cm3. The density of the oceanic lithosphere is 3.3 g/cm3.
Thermal regime of the Earth
There are two sources of the Earth's heat: 1.
received from the Sun, 2. carried out from the interior to the surface of the Earth. Warming by the Sun extends to a depth of no more than 28-30 m, and in some places a few meters.
At some depth from the surface there is constant belt temperature, in which the temperature is equal to the average annual temperature of a given area. (Moscow -20 m - +4.20, Paris - 28 m - +11.830). Below the constant temperature zone, there is a gradual increase in temperature with depth, associated with the deep heat flow. The increase in temperature with depth in degrees Celsius per unit length is called geothermal gradient, and the depth interval in meters at which the temperature rises by 10 is called geothermal stage. The geothermal gradient and step are different in different places on the globe.
According to B. Gutenberg, the limits of fluctuations differ by more than 25 times. This indicates different endogenous activity of the earth's crust, different thermal conductivity of rocks. The greatest geothermal gradient is noted in the state of Oregon (USA), equal to 1500 per 1 km, the smallest - 60 per 1 km in South Africa.
The average value of the geothermal gradient has long been assumed to be 300 per 1 km and the corresponding geothermal step is 33 m.
According to V.N. Zharkov, near the Earth’s surface the geothermal gradient is estimated at 200 per 1 km.
If we take both values into account, then at a depth of 100 km the temperature is 30,000 or 20,000 C. This does not correspond to the actual data. Lava flowing from magma chambers at these depths has a maximum temperature of 1200-12500 C. A number of authors, taking into account this kind of thermometer, believe that at a depth of 100 km the temperature does not exceed 1300-15000. With more high temperatures the mantle rocks would be completely melted and S-waves would not pass through them.
Therefore, the average geothermal gradient can be traced to a depth of 20-30 km, and deeper it should decrease. But the change in temperature with depth is uneven. For example: Kola well. We calculated a geothermal gradient of 100 per 1 km. Such a gradient was up to a depth of 3 km, at a depth of 7 km - 1200 C, at 10 km - 1800 C, at 12 km - 2200 C. More or less reliable data on temperature were obtained for the base of the layer IN — 1600 + 500 C.
Question about temperature change below the layer IN not resolved.
It is assumed that the temperature in the Earth's core is in the range of 4000-50000 C.
Earth's gravitational field
Gravity, or the force of gravity, is always perpendicular to the surface of the geoid.
The distribution of gravity on continents and in ocean areas is not the same at any latitude. Gravimetric measurements of the absolute value of gravity make it possible to identify gravimetric anomalies—areas of increasing or decreasing gravity.
An increase in gravity indicates a denser substance, a decrease indicates the occurrence of less dense masses. The magnitude of the acceleration due to gravity varies. On the surface, on average, 982 cm/s2 (at the equator 978 cm/s2, at the pole 983 cm/s2), with depth it first increases, then quickly decreases. At the boundary with the outer core 1037 cm/s2, in the core it decreases, in the F layer it reaches 452 cm/s2, at a depth of 6000 km - 126 cm/s2, in the center to zero.
Magnetism
The Earth is a giant magnet with a force field around it.
The geomagnetic field is dipole; the Earth's magnetic poles do not coincide with the geographic ones. The angle between the magnetic axis and the rotation axis is about 11.50.
A distinction is made between magnetic declination and magnetic inclination. Magnetic declination is determined by the angle of deviation of the magnetic compass needle from the geographic meridian. Declension can be Western or Eastern. The eastern declination is added to the measurement value, the western declination is subtracted. Lines connecting points on the map with the same declination are called zogonami (Greek.
“izos” - equal and “gonia” - angle). Magnetic inclination is defined as the angle between the magnetic needle and the horizontal plane. A magnetic needle, suspended on a horizontal axis, is attracted by the magnetic poles of the Earth, and therefore is not installed parallel to the horizon, forming a larger or smaller angle with it. In the northern hemisphere, the northern end of the arrow moves down, and in the southern hemisphere, vice versa. The maximum angle of inclination of the magnetic needle (900) will be at the magnetic pole; it reaches zero value in the area close to the geographic equator.
Lines connecting points on the map with the same inclination are called isoclines (Greek “wedge” - I tilt). The line of zero inclination of the magnetic needle is called the magnetic equator.
The magnetic equator does not coincide with the geographic equator.
The magnetic field is characterized by a tension that increases from the magnetic equator (31.8 A/m) to the magnetic poles (55.7 A/m). The origin of the Earth's constant magnetic field is associated with the action complex system electric currents that arise during the rotation of the Earth and accompany turbulent convection (movement) in the liquid outer core.
The Earth's magnetic field influences the orientation of ferromagnetic minerals in rocks (magnetite, hematite and others), which, during the process of solidification of magma or accumulation in sedimentary rocks, take on the orientation of the Earth's magnetic field existing at that time. Studies of the remanent magnetization of rocks have shown that the Earth's magnetic field has changed repeatedly in geological history: the north pole became south, and the south pole became north, i.e.
n e r s i (turnover) occurred. The magnetic inversion scale is used to subdivide and compare rock strata and determine the age of the ocean floor.
Previous12345678910111213Next
The oceanic crust is primitive in its composition and, in essence, represents the upper differentiated layer of the mantle, overlain by a thin layer of pelagic sediments. The oceanic crust is usually divided into three layers, the first of which (upper) is sedimentary.
At the base of the sedimentary layer there are often thin metal-bearing sediments that are not consistent along the strike, with a predominance of iron oxides. The lower part of the sedimentary layer is usually composed of carbonate sediments deposited at depths less than 4-4.5 km. At great depths, carbonate sediments, as a rule, are not deposited, since the microscopic shells of unicellular organisms composing them (foraminifera and cocolithopharids) easily dissolve in sea water at pressures above 400-450 atm. For this reason, in oceanic depressions at depths greater than 4-4.5 km, the upper part of the sedimentary layer is composed mainly only of carbonate-free sediments - red deep-sea clays and siliceous silts. Near island arcs and volcanic islands, lenses and layers of volcanic deposits are often found in the section of sedimentary strata, and terrigenous sediments are also found near deltas of large rivers. In the open oceans, the thickness of the sediment layer increases from the crests of mid-ocean ridges, where there is almost no precipitation, to their peripheral parts. The average thickness of sediments is small and, according to A.P. Lisitsyn, close to 0.5 km, but near the continental margins of the Atlantic type and in areas of large river deltas it increases to 10-12 km. This is due to the fact that almost all terrigenous material transported from land, thanks to avalanche sedimentation processes, is deposited in coastal areas of the oceans and on the continental slopes of continents.
The second, or basaltic, layer of oceanic crust in the upper part is composed of basaltic lavas of tholeiitic composition (Fig. 5). Erupting underwater, these lavas acquire fancy shapes corrugated pipes and pillows, which is why they are called pillow lavas. Below are dolerite dikes of the same tholeiitic composition, which are former supply channels through which basaltic magma in rift zones flowed onto the surface of the ocean floor. The basaltic layer of oceanic crust is exposed in many places on the ocean floor adjacent to the crests of mid-ocean ridges and the transform faults that feather them. This layer was studied in detail both by traditional methods of studying the ocean floor (dredging, sampling with soil tubes, photography), and with the help of underwater manned vehicles, allowing geologists to observe the geological structure of the objects under study and carry out targeted sampling of rocks. In addition, over the past 20 years, the surface of the basalt layer and its upper layers have been penetrated by numerous deep-sea drilling holes, one of which even penetrated the pillow lava layer and entered the dolerites of the dike complex. The total thickness of the basalt, or second, layer of the oceanic crust, judging by seismic data, reaches 1.5, sometimes 2 km.
Figure 5. Structure of the rift zone and oceanic crust:
1 - ocean level; 2—precipitation; 3—pillow basaltic lavas (layer 2a); 4—dike complex, dolerites (layer 2b); 5 - gabbro; 6 - layered complex; 7 - serpentinites; 8—lherzolites of lithospheric plates; 9 - asthenosphere; 10—isotherm 500 °C (beginning of serpentinization).
Frequent finds of gabbro tholeiitic inclusions within large transform faults indicate that the oceanic crust also includes these dense and coarse-crystalline rocks. The structure of ophiolite covers in the folded belts of the Earth, as is known, are fragments of ancient oceanic crust, thrust in these belts onto the former edges of the continents. Therefore, we can conclude that the dike complex in the modern oceanic crust (as well as in the ophiolite nappes) is underlain by a layer of gabbro, which makes up the upper part of the third layer of the oceanic crust (layer 3a). At some distance from the crests of the mid-ocean ridges, judging by seismic data, the lower part of this crustal layer can also be traced. Numerous findings in large transform faults of serpentinites, corresponding in composition to hydrated peridotites and ophiolite complexes similar in structure to serpentinites, suggest that the lower part of the oceanic crust is also composed of serpentinites. According to seismic data, the thickness of the gabbro-serpentinite (third) layer of the oceanic crust reaches 4.5-5 km. Under the crests of mid-ocean ridges, the thickness of the oceanic crust is usually reduced to 3-4 and even 2-2.5 km directly below the rift valleys.
The total thickness of the oceanic crust without the sedimentary layer thus reaches 6.5-7 km. Below, the oceanic crust is underlain by crystalline rocks of the upper mantle, which make up the subcrustal sections of lithospheric plates. Beneath the crests of mid-ocean ridges, the oceanic crust lies directly above pockets of basaltic melts released from the hot mantle (from the asthenosphere).
The area of the oceanic crust is approximately equal to 3.0610 × 18 cm 2 (306 million km 2), the average density of the oceanic crust (without precipitation) is close to 2.9 g/cm 3, therefore, the mass of the consolidated oceanic crust can be estimated at (5.8 -6.2)x10 24 g. The volume and mass of the sedimentary layer in the deep-sea basins of the world ocean, according to A.P. Lisitsyn, is respectively 133 million km 3 and about 0.1 × 10 24 g. The volume of sediment concentrated on the shelves and continental slopes, somewhat larger - about 190 million km 3, which in terms of mass (taking into account the compaction of sediments) is approximately (0.4-0.45) 10 24 g.
The ocean floor, which is the surface of the oceanic crust, has a characteristic topography. In abyssal basins, the ocean floor lies at depths of about 66.5 km, while on the crests of mid-ocean ridges, sometimes dissected by steep gorges and rift valleys, ocean depths decrease to 2-2.5 km. In some places, the ocean floor reaches the surface of the Earth, for example, on the island. Iceland and in the Afar province (Northern Ethiopia). In front of the island arcs surrounding the western periphery of the Pacific Ocean, the northeastern Indian Ocean, in front of the arc of the Lesser Antilles and South Sandwich Islands in the Atlantic, as well as in front of the active continental margin in Central and South America, the oceanic crust bends and its surface plunges to depths of up to 9 -10 km, going further under these structures and forming narrow and extended deep-sea trenches in front of them.
The oceanic crust is formed in the rift zones of mid-ocean ridges due to the separation of basaltic melts from the hot mantle (from the asthenospheric layer of the Earth) occurring beneath them and their outpouring onto the surface of the ocean floor. Every year in these zones, at least 5.5-6 km 3 of basaltic melts rise from the asthenosphere, pour out onto the ocean floor and crystallize, forming the entire second layer of the oceanic crust (taking into account the gabbro layer, the volume of basaltic melts introduced into the crust increases to 12 km 3) . These enormous tectonomagmatic processes, constantly developing under the crests of mid-ocean ridges, have no equal on land and are accompanied by increased seismicity (Fig. 6).
Figure 6. Seismicity of the Earth; earthquake placement
Barazangi, Dorman, 1968
In rift zones located on the crests of mid-ocean ridges, stretching and spreading of the ocean floor occurs. Therefore, all such zones are marked by frequent but shallow-focus earthquakes with a predominance of rupture displacement mechanisms.
In contrast, under island arcs and active continental margins, i.e. in zones of plate underthrust, stronger earthquakes usually occur with the dominance of compression and shear mechanisms. According to seismic data, the subsidence of the oceanic crust and lithosphere can be traced in the upper mantle and mesosphere to depths of about 600-700 km (Fig. 7). According to tomography data, the subsidence of oceanic lithospheric plates has been traced to depths of about 1400-1500 km and, possibly, deeper - right up to the surface of the earth's core.
Figure 7. The structure of the plate underthrust zone in the Kuril Islands area:
1 - asthenosphere; 2 - lithosphere; 3 - oceanic crust; 4-5—sedimentary-volcanogenic strata; 6—ocean sediments; isolines show seismic activity in A 10 units (Fedotov et al., 1969); β is the angle of incidence of the Wadati-Benief zone; α is the angle of incidence of the plastic deformation zone.
The ocean floor is characterized by characteristic and fairly contrasting banded magnetic anomalies, usually located parallel to the crests of mid-ocean ridges (Fig. 8). The origin of these anomalies is associated with the ability of the basalts of the ocean floor, when cooling, to be magnetized by the Earth's magnetic field, thereby remembering the direction of this field at the moment of their outpouring onto the surface of the ocean floor. Taking into account now that the geomagnetic field has repeatedly changed its polarity over time, the English scientists F. Vine and D. Matthews, back in 1963, were the first to date individual anomalies and show that on different slopes of mid-ocean ridges these anomalies turn out to be approximately symmetrical in in relation to their ridges. As a result, they were able to reconstruct the basic patterns of plate movements in individual areas of the oceanic crust in the North Atlantic and show that the ocean floor is moving approximately symmetrically away from the crests of the mid-ocean ridges at speeds of the order of several centimeters per year. Subsequently, similar studies were carried out in all areas of the World Ocean, and everywhere this pattern was confirmed. Moreover, a detailed comparison of magnetic anomalies of the ocean floor with the geochronology of magnetization reversal of continental rocks, the age of which was known from other data, made it possible to extend the dating of the anomalies to the entire Cenozoic, and then to the late Mesozoic. As a result, a new and reliable paleomagnetic method for determining the age of the ocean floor was created.
Figure 8. Map of magnetic field anomalies in the area of the submarine Reykjanes Ridge in the North Atlantic
(Heirtzler et al., 1966). Positive anomalies are indicated in black; AA—zero anomaly of the rift zone.
The use of this method led to the confirmation of previously expressed ideas about the comparative youth of the ocean floor: the paleomagnetic age of all oceans without exception turned out to be only Cenozoic and Late Mesozoic (Fig. 9). Subsequently, this conclusion was brilliantly confirmed by deep-sea drilling at many points on the ocean floor.
It turned out that the age of the basins of the young oceans (Atlantic, Indian and Arctic) coincides with the age of their bottom, while the age of the ancient Pacific Ocean significantly exceeds the age of its bottom. Indeed, the Pacific Ocean basin has existed at least since the late Proterozoic (maybe earlier), and the age of the most ancient sections of the bottom of this ocean does not exceed 160 million years, while most of it was formed only in the Cenozoic, i.e. younger than 67 million years.
Figure 9. Map of the age of the ocean floor in millions of years
from Larson, Pitman et al., 1985
The “conveyor” mechanism of renewal of the ocean floor with the constant immersion of older sections of the oceanic crust and sediments accumulated on it into the mantle under island arcs explains why, during the life of the Earth, ocean basins never had time to be filled with sediments. Indeed, at the current rate of filling ocean basins with terrigenous sediments carried from land, 2.210 × 16 g/year, the entire volume of these basins, approximately equal to 1.3710 × 24 cm 3, would be completely filled in approximately 1.2 billion years. Now we can say with great confidence that the continents and ocean basins have existed together for about 3.8 billion years and no significant filling of their depressions has occurred during this time. Moreover, after drilling in all oceans, we now know for sure that there is no sediment on the ocean floor older than 160-190 million years. But this can only be observed in one case - if there is an effective mechanism for removing sediment from the oceans. This mechanism, as is now known, is the process of sediments being pulled under island arcs and active continental margins in plate subduction zones, where these sediments are melted and reattached in the form of granitoid intrusions to the continental crust forming in these zones. This process of melting terrigenous sediments and reattaching their material to the continental crust is called sediment recycling.
Concept of the earth's crust.
Earth's crust
3) the top layer is sedimentary. Its thickness on average is about 3 km. In some areas the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth there is no sedimentary layer at all and a granite layer comes to the surface.
Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).
weathering crust.
Conrad surface
On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out. In East Asia, in the region of island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied.
There is no Moho boundary here, and mantle material rises along faults into the crust and even to its surface.
The concept of isostasy
isothermal layer
geothermal gradient geothermal stage
Read also:
The Earth's shell includes the earth's crust and the upper part of the mantle.
The surface of the earth's crust has large irregularities, the main of which are the protrusions of the continents and their depressions - huge oceanic depressions. The existence and relative position of continents and ocean basins is associated with differences in the structure of the earth's crust.
Continental crust. It consists of several layers. The top is a layer of sedimentary rocks. The thickness of this layer is up to 10-15 km. Beneath it lies a granite layer. The rocks that make it up are similar in their physical properties to granite. The thickness of this layer is from 5 to 15 km. Beneath the granite layer is a basalt layer, consisting of basalt and rocks whose physical properties resemble basalt. The thickness of this layer is from 10 km to 35 km. Thus, the total thickness of the continental crust reaches 30-70 km.
Oceanic crust. It differs from the continental crust in that it does not have a granite layer or it is very thin, so the thickness of the oceanic crust is only 6-15 km.
To determine the chemical composition of the earth's crust, only its upper parts are available - to a depth of no more than 15-20 km. 97.2% of the total composition of the earth's crust is made up of: oxygen - 49.13%, aluminum - 7.45%, calcium - 3.25%, silicon - 26%, iron - 4.2%, potassium - 2.35 %, magnesium - 2.35%, sodium - 2.24%.
Other elements of the periodic table account for from tenths to hundredths of a percent.
Most scientists believe that oceanic-type crust first appeared on our planet.
Under the influence of processes occurring inside the Earth, folds, that is, mountainous areas, formed in the earth's crust. The thickness of the bark increased. This is how continental protrusions were formed, that is, the continental crust began to form.
In recent years, in connection with studies of the earth's crust of oceanic and continental types, a theory of the structure of the earth's crust has been created, which is based on the idea of lithospheric plates. The theory in its development was based on the hypothesis of continental drift, created at the beginning of the 20th century by the German scientist A. Wegener.
Types of the earth's crust Wikipedia
Site search:
Hypotheses explaining the origin and development of the earth's crust
Concept of the earth's crust.
Earth's crust is a complex of surface layers of the Earth’s solid body. In the scientific geographical literature there is no single idea about the origin and paths of development of the earth's crust.
There are several concepts (hypotheses) that reveal the mechanisms of formation and development of the earth’s crust, the most substantiated of which are the following:
1. The theory of fixism (from the Latin fixus - motionless, unchanging) states that the continents have always remained in the places that they currently occupy. This theory denies any movement of continents and large parts of the lithosphere.
2. The theory of mobilism (from the Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has become especially firmly established in recent years in connection with the acquisition of new scientific data from the study of the bottom of the World Ocean.
3. The concept of continental growth at the expense of the ocean floor believes that the original continents formed in the form of relatively small massifs that now make up ancient continental platforms. Subsequently, these massifs grew due to the formation of mountains on the ocean floor adjacent to the edges of the original land cores. The study of the ocean floor, especially in the zone of mid-ocean ridges, has given reason to doubt the correctness of the concept of continental growth due to the ocean floor.
4. The theory of geosynclines states that the increase in land size occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the earth's crust, forms the basis of many modern scientific explanations of the process of origin and development of the earth's crust.
5. Rotation theory bases its explanation on the proposition that since the figure of the Earth does not coincide with the surface of a mathematical spheroid and is rearranged due to uneven rotation, zonal stripes and meridional sectors on a rotating planet are inevitably tectonically unequal. They react with varying degrees of activity to tectonic stresses caused by intraterrestrial processes.
There are two main types of earth's crust: oceanic and continental. A transitional type of the earth's crust is also distinguished.
Oceanic crust. The thickness of the oceanic crust in the modern geological era ranges from 5 to 10 km. It consists of the following three layers:
1) upper thin layer of marine sediments (thickness no more than 1 km);
2) middle basalt layer (thickness from 1.0 to 2.5 km);
3) lower layer of gabbro (thickness about 5 km).
Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic crust. Its thickness averages 35-45 km, and in mountainous countries it increases to 70 km. It also consists of three layers, but differs significantly from the ocean:
1) lower layer composed of basalts (thickness about 20 km);
2) the middle layer occupies the main thickness of the continental crust and is conventionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;
3) the top layer is sedimentary. Its thickness on average is about 3 km.
In some areas the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth there is no sedimentary layer at all and a granite layer comes to the surface. Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).
On continents, as a result of the weathering of rocks, a geological formation is formed, called weathering crust.
The granite layer is separated from the basalt layer Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km/sec.
The boundary between the earth's crust and mantle (both on continents and oceans) runs along Mohorovicic surface (Moho line). The speed of seismic waves on it increases abruptly to 8 km/hour.
In addition to the two main types - oceanic and continental - there are also areas of mixed (transitional) type.
On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out. In East Asia, in the region of island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied. There is no Moho boundary here, and mantle material rises along faults into the crust and even to its surface.
The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than the "earth's crust". In the lithosphere, modern science includes not only the earth’s crust, but also the uppermost mantle to the asthenosphere, that is, to a depth of approximately 100 km.
The concept of isostasy . A study of the distribution of gravity showed that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from the Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density. Heavy oceanic crust is thinner than lighter continental crust.
Isostasy is, in essence, not even an equilibrium, but a desire for equilibrium, continuously disrupted and restored again. For example, the Baltic Shield, after the melting of continental ice of the Pleistocene glaciation, rises by about 1 meter per century. The area of Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero equilibrium line currently runs slightly south of 60 0 N latitude. Modern St. Petersburg is approximately 1.5 m higher than St. Petersburg during the time of Peter the Great. As data from modern scientific research show, even the heaviness of large cities is sufficient for isostatic fluctuations of the territory beneath them. Consequently, the earth's crust in areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface, the base of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to a higher level of its upper boundary. Thus, under the Pamirs the depth of the Moho surface is 65 km, and in the Caspian lowland it is about 30 km.
Thermal properties of the earth's crust . Daily fluctuations in soil temperature extend to a depth of 1.0 - 1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate to a depth of 20-30 m. At the depth where the influence of annual temperature fluctuations due to heating of the earth's surface by the Sun ceases, there is layer of constant soil temperature. It is called isothermal layer . Below the isothermal layer deep into the Earth, the temperature rises, and this is caused by the internal heat of the earth's bowels. Internal heat does not participate in the formation of climates, but it serves as the energy basis for all tectonic processes.
The number of degrees by which the temperature increases for every 100 m of depth is called geothermal gradient . The distance in meters, when lowered by which the temperature increases by 1 0 C is called geothermal stage . The magnitude of the geothermal step depends on the topography, thermal conductivity of rocks, the proximity of volcanic sources, groundwater circulation, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be only about 5 m, and in geologically quiet areas (for example, on platforms) it can reach 100 m.
⇐ Previous234567891011Next ⇒
In the structure of the Earth, researchers distinguish 2 types of the earth's crust - continental and oceanic.
What is the continental crust?
Continental crust, also called continental, is characterized by the presence of 3 different layers in its structure. The upper one is represented by sedimentary rocks, the second one is granite or gneisses, the third one consists of basalt, granulites and other metamorphic rocks.
Continental crust
The thickness of the continental crust is about 35-45 km, sometimes reaching 75 km (usually in mountainous areas). The type of crust in question covers approximately 40% of the Earth's surface. In terms of volume, it corresponds to approximately 70% of the Earth's crust.
The age of the continental crust reaches 4.4 billion years.
What is the oceanic crust?
Main mineral forming oceanic crust, - basalt. But besides this, its structure includes:
- sedimentary rocks;
- layered intrusions.
According to the prevailing scientific concept, the oceanic crust is constantly formed due to tectonic processes. It is much younger than the mainland, the age of its oldest sections is about 200 million years.
Oceanic crust The thickness of the oceanic crust is about 5-10 km, depending on the specific measurement site. It can be noted that over time it remains almost unchanged. A common approach among scientists is that the oceanic crust should be considered as belonging to the oceanic lithosphere. In turn, its thickness largely depends on age.
Comparison
The main difference between the continental crust and the oceanic crust is, obviously, their location. The first contains continents, land, the second - oceans and seas.
The continental crust is represented mainly by sedimentary rocks, granites and granulites. Oceanic - mainly basalt.
The continental crust is much thicker and older. It is inferior to the oceanic one in terms of the area covering the earth's surface, but superior in terms of the volume it occupies throughout the entire earth's crust.
It can be noted that in some cases the oceanic crust is capable of being layered on top of the continental crust in the process of obduction.
Having determined what the difference is between the continental and oceanic crust, we will record the conclusions in a small table.
Table
| Continental crust | Oceanic crust |
| Contains continents and land | Hosts oceans and seas |
| Represented mainly by sedimentary rocks, granites, granulites | Consists predominantly of basalt |
| Has a thickness of up to 75 km, usually 35-45 km | Has a thickness usually within 10 km |
| The age of some parts of the continental crust reaches 4.4 billion years | The oldest sections of oceanic crust are about 200 million years old. |
| Occupies about 40% of the Earth's surface | Occupies about 60% of the Earth's surface |
| Occupies about 70% of the volume of the earth's crust | Occupies about 30% of the volume of the earth's crust |
