Earth's crust makes up the uppermost shell of the solid Earth and covers the planet with an almost continuous layer, changing its thickness from 0 in some areas of mid-ocean ridges and oceanic faults to 70-75 km under high mountain structures (Khain, Lomize, 1995). The thickness of the crust on the continents, determined by the increase in the velocity of the passage of longitudinal seismic waves up to 8-8.2 km/s ( Mohorovicic border, or moho border), reaches 30-75 km, and in oceanic depressions 5-15 km. The first type of earth's crust was named oceanic,second- continental.
oceanic crust occupies 56% of the earth's surface and has a small thickness - 5-6 km. Three layers are distinguished in its structure (Khain and Lomize, 1995).
First, or sedimentary, a layer no thicker than 1 km occurs in the central part of the oceans and reaches a thickness of 10–15 km at their periphery. It is completely absent in the axial zones of mid-ocean ridges. The composition of the layer includes clay, siliceous and carbonate deep-sea pelagic sediments (Fig. 6.1). Carbonate sediments occur no deeper than the critical depth of carbonate accumulation. Closer to the continent, an admixture of detrital material carried from land appears; these are the so-called hemipelagic sediments. The propagation velocity of longitudinal seismic waves here is 2–5 km/s. The age of the sediments of this layer does not exceed 180 Ma.
Second layer in its main upper part (2A) it is composed of basalts with rare and thin layers of pelagic
Rice. 6.1. Section of the lithosphere of the oceans in comparison with the average section of ophiolite allochthons. Below is a model for the formation of the main units of the section in the zone of oceanic spreading (Khain and Lomize, 1995). Symbols: 1 -
pelagic sediments; 2 – outflowing basalts; 3 – complex of parallel dikes (dolerites); 4 – upper (not layered) gabbroids and gabbro-dolerites; 5, 6 - layered complex (cumulates): 5 - gabbroids, 6 - ultramafic rocks; 7 – tectonized peridotites; 8 – basal metamorphic halo; 9 – basaltic magma change I–IV – successive change of crystallization conditions in the chamber with distance from the spreading axis
ical precipitation; basalts often have a characteristic pillow (in cross section) separation (pillow lavas), but there are also covers of massive basalts. In the lower part of the second layer (2B), parallel dolerite dikes are developed. The total thickness of the 2nd layer is 1.5–2 km, and the velocity of longitudinal seismic waves is 4.5–5.5 km/s.
third layer The oceanic crust consists of full-crystalline igneous rocks of basic and subordinately ultrabasic composition. In its upper part, rocks of the gabbro type are usually developed, and the lower part is composed of a "banded complex" consisting of alternating gabbro and ultra-ramafites. The thickness of the 3rd layer is 5 km. The velocity of longitudinal waves in this layer reaches 6–7.5 km/s.
It is believed that the rocks of the 2nd and 3rd layers were formed simultaneously with the rocks of the 1st layer.
Oceanic crust, or rather oceanic-type crust, is not limited in its distribution to the bed of the oceans, but is also developed in deep-water basins of marginal seas, such as the Sea of Japan, the South Okhotsk (Kuril) basin of the Sea of \u200b\u200bOkhotsk, the Philippine, Caribbean and many others
seas. In addition, there are serious grounds to suspect that in the deep depressions of the continents and shallow inland and marginal seas of the Barents type, where the thickness of the sedimentary cover is 10-12 km or more, it is underlain by oceanic-type crust; this is evidenced by the velocities of longitudinal seismic waves of the order of 6.5 km/s.
It was said above that the age of the crust of modern oceans (and marginal seas) does not exceed 180 Ma. However, within the folded belts of the continents, we also find a much older, up to the Early Precambrian, crust of the oceanic type, represented by the so-called ophiolite complexes(or just ophiolites). This term belongs to the German geologist G. Steinmann and was proposed by him at the beginning of the 20th century. to designate a characteristic "triad" of rocks commonly found together in the central zones of fold systems, namely serpentinized ultramafic rocks (layer 3 analog), gabbro (layer 2B analog), basalts (layer 2A analog), and radiolarites (layer 1 analog). The essence of this paragenesis of rocks was erroneously interpreted for a long time, in particular, gabbro and ultramafic rocks were considered intrusive and younger than basalts and radiolarites. Only in the 1960s, when the first reliable information about the composition of the oceanic crust was obtained, did it become obvious that ophiolites are the oceanic crust of the geological past. This discovery was of cardinal importance for a correct understanding of the conditions for the origin of the Earth's mobile belts.
Structures of the earth's crust of the oceans
Areas of continuous distribution oceanic crust expressed in the relief of the Earth oceanicdepressions. Within the oceanic basins, two major elements stand out: ocean platforms and oceanic orogenic belts. ocean platforms(or thalassocratons) in the bottom topography look like vast abyssal flat or hilly plains. To oceanic orogenic belts include mid-ocean ridges, having a height above the surrounding plain up to 3 km (in some places they rise in the form of islands above ocean level). Along the axis of the ridge, a zone of rifts is often traced - narrow grabens 12-45 km wide at a depth of up to 3-5 km, indicating the dominance of crustal extension in these areas. They are characterized by high seismicity, a sharply increased heat flow, and a low density of the upper mantle. Geophysical and geological data indicate that the thickness of the sedimentary cover decreases as it approaches the axial zones of the ridges, and the oceanic crust experiences a noticeable uplift.
The next major element of the earth's crust - transition zone between continent and ocean. This is the region of maximum dissection of the earth's surface, where island arcs, characterized by high seismicity and modern andesitic and andesite-basalt volcanism, deep-sea trenches and deep-water basins of marginal seas. The earthquake sources here form a seismic focal zone (the Benioff-Zavaritsky zone), plunging under the continents. The transition zone is the most
pronounced in the western part of the Pacific Ocean. It is characterized by an intermediate type of structure of the earth's crust.
continental crust(Khain, Lomize, 1995) is distributed not only within the continents themselves, i.e., land, with the possible exception of the deepest depressions, but also within the shelf zones of continental margins and individual areas within oceanic microcontinent basins. Nevertheless, the total area of development of the continental crust is smaller than that of the oceanic one, and accounts for 41% of the earth's surface. The average thickness of the continental crust is 35-40 km; it decreases towards the margins of the continents and within microcontinents and increases under mountain structures up to 70-75 km.
Generally, continental crust, like the oceanic one, has a three-layer structure, but the composition of the layers, especially the two lower ones, differs significantly from those observed in the oceanic crust.
1. sediment layer, commonly referred to as a sedimentary cover. Its thickness varies from zero on shields and smaller uplifts of platform foundations and axial zones of folded structures to 10 and even 20 km in platform depressions, frontal and intermountain troughs of mountain belts. True, in these depressions the crust that underlies the sediments and is usually called consolidated may already be closer in character to oceanic than to continental. The composition of the sedimentary layer includes various sedimentary rocks of predominantly continental or shallow marine, less often bathyal (again, within deep depressions) origin, and also, far
not everywhere, covers and sills of basic igneous rocks forming trap fields. The velocity of longitudinal waves in the sedimentary layer is 2.0-5.0 km/s with a maximum for carbonate rocks. The age range of the rocks of the sedimentary cover is up to 1.7 billion years, i.e., an order of magnitude higher than that of the sedimentary layer of modern oceans.
2. Upper layer of consolidated crust protrudes onto the day surface on shields and arrays of platforms and in the axial zones of folded structures; it was penetrated to a depth of 12 km in the Kola well and to a much shallower depth in wells in the Volga-Ural region on the Russian Plate, on the US Midcontinent Plate and on the Baltic Shield in Sweden. A gold mine in South India went through this layer up to 3.2 km, in South Africa - up to 3.8 km. Therefore, the composition of this layer, at least its upper part, is generally well known; the main role in its composition is played by various crystalline schists, gneisses, amphibolites and granites, in connection with which it is often called granite-gneiss. The speed of longitudinal waves in it is 6.0-6.5 km/s. In the basement of young platforms of Riphean-Paleozoic or even Mesozoic age, and partly in the inner zones of young folded structures, the same layer is composed of less strongly metamorphosed (greenschist facies instead of amphibolite) rocks and contains less granites; hence it is often referred to here granite-metamorphic layer, and typical velocities of longitudinal wills in it are of the order of 5.5-6.0 km/s. The thickness of this layer of the crust reaches 15-20 km on platforms and 25-30 km in mountain structures.
3. The lower layer of the consolidated crust. Initially, it was assumed that between the two layers of the consolidated crust there is a clear seismic boundary, which received the name of the Konrad boundary after its discoverer, a German geophysicist. The drilling of the wells just mentioned cast doubt on the existence of such a clear boundary; sometimes, instead of it, seismic reveals not one, but two (K 1 and K 2) boundaries in the crust, which made it possible to distinguish two layers in the lower crust (Fig. 6.2). The composition of the rocks that make up the lower crust, as noted, is not well known, since it has not been reached by boreholes, and is exposed fragmentarily on the surface. Based
Rice. 6.2. Structure and thickness of the continental crust (Khain and Lomize, 1995). BUT - the main types of the section according to seismic data: I-II - ancient platforms (I - shields, II
Syneclises), III - shelves, IV - young orogens. K 1 , K 2 -surfaces of Konrad, M-surface of Mohorovichich, velocities are indicated for longitudinal waves; B - histogram of continental crust thickness distribution; B - generalized strength profile
general considerations, V. V. Belousov came to the conclusion that, on the one hand, rocks that are at a higher stage of metamorphism should prevail in the lower crust and, on the other hand, rocks of a more basic composition than in the upper crust. So he called this layer of bark gra-zero-basic. Belousov's assumption is generally confirmed, although outcrops show that not only basic, but also acidic granulites are involved in the composition of the lower crust. At present, most geophysicists distinguish between the upper and lower crust on a different basis - according to their excellent rheological properties: the upper crust is rigid and brittle, the lower one is plastic. The velocity of longitudinal waves in the lower crust is 6.4-7.7 km/s; belonging to the crust or mantle of the lower part of this layer with velocities of more than 7.0 km/s is often disputable.
Between the two extreme types of the earth's crust - oceanic and continental - there are transitional types. One of them - suboceanic crust - It is developed along the continental slopes and foothills and, possibly, underlies the bottom of the basins of some not very deep and wide marginal and inland seas. Suboceanic crust is thinned up to 15-20 km and permeated with dykes and sills of basic igneous rocks.
bark. It was discovered by a deep-water drilling at the entrance to the Gulf of Mexico and exposed on the Red Sea coast. Another type of transitional cortex is subcontinental- is formed when the oceanic crust in ensimatic volcanic arcs turns into a continental one, but does not yet reach full “maturity”, having a lower thickness, less than 25 km, and a lower degree of consolidation, which is reflected in lower seismic wave velocities - no more than 5.0-5.5 km/s in the lower crust.
Some researchers single out as special types two more varieties of oceanic crust, which have already been discussed above; this is, firstly, the oceanic crust of the internal uplifts of the ocean (Iceland, etc.) thickened up to 25-30 km, and, secondly, the oceanic-type crust, “built on” by a thick, up to 15-20 km, sedimentary cover (the Caspian depression and etc.).
The Mohorovichic surface and the composition of the upper manti. The boundary between the crust and the mantle, usually seismically quite clearly expressed by a jump in the velocities of compressional waves from 7.5-7.7 to 7.9-8.2 km / s, is known as the Mohorovichic surface (or simply Moho and even M), by name the Croatian geophysicist who established it. In the oceans, this boundary corresponds to the transition from the banded complex of the 3rd layer with a predominance of gabbroids to continuous serpentinized peridotites (harzburgites, lherzolites), less often dunites, in some places protruding to the bottom surface, and in the rocks of São Paulo in the Atlantic against the coast of Brazil and on about. Zabargad in the Red Sea, towering above the surface
ocean. The tops of the oceanic mantle can be observed in places on land as part of the bottoms of ophiolite complexes. Their thickness in Oman reaches 8 km, and in Papua New Guinea, perhaps even 12 km. They are composed of peridotites, mainly harzburgites (Khain and Lomize, 1995).
The study of inclusions in lavas and kimberlites from pipes shows that even under the continents, the upper mantle is mainly composed of peridotites, both here and under the oceans in the upper part, these are spinel peridotites, and below, garnet ones. But in the continental mantle, according to the same data, in addition to peridotites, eclogites, i.e., deeply metamorphosed basic rocks, are present in a subordinate amount. Eclogites may be metamorphosed relics of oceanic crust dragged into the mantle during subduction of this crust.
The upper part of the mantle is secondarily depleted in a number of components: silica, alkalis, uranium, thorium, rare earths, and other incoherent elements due to the smelting of basalt rocks from the earth's crust from it. This "depleted" ("depleted") mantle extends under the continents to a greater depth (covering all or almost all of its lithospheric part) than under the oceans, giving way to a deeper "non-depleted" mantle. The average primary composition of the mantle should be close to spinel lherzolite or a hypothetical mixture of peridotite and basalt in a ratio of 3: 1, called by the Australian scientist A. E. Ring-wood pyrolite.
At a depth of about 400 km, a rapid increase in the velocity of seismic waves begins; from here to 670 km
erased Golitsyn layer, named after the Russian seismologist B.B. Golitsyn. It is also distinguished as a middle mantle, or mesosphere - transition zone between the upper and lower mantle. The increase in the velocities of elastic vibrations in the Golitsyn layer is explained by an increase in the density of the mantle substance by about 10% due to the transition of some mineral species to others with a denser packing of atoms: olivine into spinel, pyroxene into garnet.
lower mantle(Khain and Lomize, 1995) starts from a depth of about 670 km. The lower mantle should be composed mainly of perovskite (MgSiO 3) and magnesia-wustite (Fe, Mg)O - products of further alteration of the minerals that make up the middle mantle. The core of the Earth in its outer part, according to seismology, is liquid, and the inner one is solid again. Convection in the outer core generates the Earth's main magnetic field. The composition of the core is accepted by the vast majority of geophysicists as iron. But again, according to experimental data, it is necessary to allow some admixture of nickel, as well as sulfur, or oxygen, or silicon, in order to explain the lower density of the core compared to that determined for pure iron.
According to seismic tomography, core surface is uneven and forms protrusions and depressions with an amplitude of up to 5-6 km. At the boundary of the mantle and core, a transitional layer with the index D "is distinguished (the crust is indicated by the index A, the upper mantle is B, the middle is C, the lower is D, the upper part of the lower mantle is D"). The thickness of layer D" in some places reaches 300 km.
Lithosphere and asthenosphere. Unlike the crust and mantle, distinguished by geological data (by material composition) and seismological data (by the jump in seismic wave velocities at the Mohorovichich boundary), the lithosphere and asthenosphere are purely physical concepts, or rather rheological ones. The initial basis for the allocation of the asthenosphere is a weakened, plastic shell. underlying a more rigid and fragile lithosphere, there was a need to explain the fact of the isostatic balance of the crust, discovered during measurements of gravity at the foot of mountain structures. It was originally expected that such structures, especially as grand as the Himalayas, should create an excess of gravity. However, when in the middle of the XIX century. appropriate measurements were made, it turned out that no such attraction was observed. Consequently, even large irregularities in the relief of the earth's surface are somehow compensated, balanced at depth so that significant deviations from the average values of gravity do not appear at the level of the earth's surface. Thus, the researchers came to the conclusion that there is a general desire of the earth's crust to balance due to the mantle; this phenomenon is called isostasis(Khain, Lomize, 1995) .
There are two ways to implement isostasy. The first is that mountains have roots immersed in the mantle, i.e., isostasy is provided by variations in the thickness of the earth's crust and the lower surface of the latter has a relief that is the opposite of that of the earth's surface; this is the hypothesis of the English astronomer J. Erie
(Fig. 6.3). On a regional scale, it is usually justified, since mountain structures really have a thicker crust, and the maximum thickness of the crust is observed in the highest of them (Himalayas, Andes, Hindu Kush, Tien Shan, etc.). But another mechanism for the implementation of isostasy is also possible: areas of elevated relief should be composed of less dense rocks, and areas of low relief, more dense; this is the hypothesis of another English scientist, J. Pratt. In this case, the sole of the earth's crust may even be horizontal. The balance of the continents and oceans is achieved by a combination of both mechanisms - the crust under the oceans and much thinner and noticeably denser than under the continents.
Most of the Earth's surface is in a state close to isostatic equilibrium. The greatest deviations from isostasy - isostatic anomalies - reveal island arcs and associated deep-sea trenches.
In order for the striving for isostatic equilibrium to be effective, i.e., under an additional load, the crust would sink, and when the load was removed, it would rise, it is necessary that a sufficiently plastic layer exist under the crust, capable of flowing from areas of increased geostatic pressure to areas reduced pressure. It was for this layer, originally identified hypothetically, that the American geologist J. Burrell proposed in 1916 the name asthenosphere, what does "weak shell" mean. This assumption was confirmed only much later, in the 60s, when seismic
Rice. 6.3. Schemes of isostatic equilibrium of the earth's crust:
a - by J. Erie, b - according to J. Pratt (Khain, Koronovsky, 1995)
logs (B. Gutenberg) discovered the existence at some depth under the crust of a zone of decrease or absence of an increase, natural with an increase in pressure, seismic wave velocity. Subsequently, another method of establishing the asthenosphere appeared - the method of magnetotelluric sounding, in which the asthenosphere manifests itself as a zone of lower electrical resistance. In addition, seismologists have identified another sign of the asthenosphere - increased attenuation of seismic waves.
The asthenosphere also plays a leading role in the movements of the lithosphere. The flow of asthenospheric matter drags lithospheric plates-plates along with it and causes their horizontal displacements. The rise of the surface of the asthenosphere leads to the rise of the lithosphere, and in the limiting case, to a break in its continuity, the formation of separation and subsidence. The outflow of the asthenosphere also leads to the latter.
Thus, of the two shells that make up the tectonosphere: the asthenosphere is an active element, and the lithosphere is a relatively passive element. Their interaction determines the tectonic and magmatic "life" of the earth's crust.
In the axial zones of the mid-ocean ridges, especially in the East Pacific Rise, the roof of the asthenosphere is located at a depth of only 3-4 km, i.e., the lithosphere is limited only to the upper part of the crust. As we move towards the periphery of the oceans, the thickness of the lithosphere increases due to
lower crust, but mainly the upper mantle and can reach 80-100 km. In the central parts of the continents, especially under the shields of ancient platforms, such as the East European or Siberian, the thickness of the lithosphere is already measured at 150-200 km or more (in South Africa, 350 km); according to some ideas, it can reach 400 km, i.e., here the entire upper mantle above the Golitsyn layer should be part of the lithosphere.
The difficulty of detecting the asthenosphere at depths of more than 150-200 km gave rise to doubts among some researchers about its existence under such areas and led them to an alternative view that the asthenosphere as a continuous shell, i.e., the geosphere, does not exist, but there is a series of disparate "asthenolenses ". We cannot agree with this conclusion, which could be important for geodynamics, since it is these areas that demonstrate a high degree of isostatic balance, because they include the above examples of areas of modern and ancient glaciation - Greenland, etc.
The reason why the asthenosphere is not easy to detect everywhere is obviously the change in its viscosity laterally.
The main structural elements of the earth's crust of the continents
On the continents, two structural elements of the earth's crust are distinguished: platforms and mobile belts (Historical Geology, 1985).
Definition:platform- a stable rigid section of the earth's crust of the continents, which has an isometric shape and a two-story structure (Fig. 6.4). Lower (first) structural floor – crystalline foundation, represented by highly deformed metamorphosed rocks cut through by intrusions. The upper (second) structural floor is gently sloping sedimentary cover, weakly dislocated and non-metamorphosed. The exits to the day surface of the lower structural floor are called shield. The areas of the foundation covered by the sedimentary cover are called stove. The thickness of the sedimentary cover of the plate is a few kilometers.
Example: two shields (Ukrainian and Baltic) and the Russian plate stand out on the East European platform.
Structures of the second floor of the platform (case) there are negative (deflections, syneclises) and positive (anteclises). Syneclises are saucer-shaped, and anteclises are inverted saucers. The thickness of deposits is always greater on the syneclise, and less on the anteclise. The dimensions of these structures in diameter can reach hundreds or a few thousand kilometers, and the fall of layers on the wings is usually a few meters per 1 km. There are two definitions of these structures.
Definition: syneclise - a geological structure, the fall of the layers of which is directed from the periphery to the center. Anteclise - a geological structure, the fall of the layers of which is directed from the center to the periphery.
Definition: syneclise - a geological structure in the core of which younger deposits emerge, and along the edges
Rice. 6.4. Platform structure diagram. 1 - folded foundation; 2 - platform cover; 3 Faults (Historical Geology, 1985)
- more ancient. Anteclise is a geological structure, in the core of which there are older deposits, and at the edges - younger ones.
Definition: deflection - an elongated (elongated) geological body, having a concave shape in cross section.
Example: on the Russian Plate of the East European Platform stand out anteclises(Belarusian, Voronezh, Volga-Ural, etc.), syneclises(Moscow, Caspian, etc.) and troughs (Ulyanovsk-Saratov, Pridnestrovsko-Black Sea, etc.).
There is a structure of the lower horizons of the cover - av-lacogen.
Definition: aulacogene is a narrow elongated depression extending through the platform. Aulacogens are located in the lower part of the upper structural stage (sheath) and can be up to hundreds of kilometers long and tens of kilometers wide. Aulacogens are formed under conditions of horizontal extension. Thick strata of sediments accumulate in them, which can be folded into folds and are close in composition to the formations of miogeosynclines. Basalts are present in the lower part of the section.
Example: Pachelma (Ryazan-Saratov) aulacogene, Dnieper-Donetsk aulacogen of the Russian plate.
History of platform development. Three stages can be distinguished in the history of development. First- geosynclinal, on which the formation of the lower (first) structural element (foundation) takes place. Second- aulacogenous, which, depending on the climate, accumulates
red-colored, gray-colored or coal-bearing sediments in aulacogenes. The third- slab, on which sedimentation occurs over a large area and the upper (second) structural floor (slab) is formed.
The process of accumulation of precipitation, as a rule, occurs cyclically. Accumulates first transgressive maritime terrigenous formation, then carbonate formation (transgression maximum, Table 6.1). During regression in an arid climate, a saline red-flowered formation, and in a humid climate - paralytic coal-bearing formation. Precipitation is formed at the end of the sedimentation cycle continental formations. At any time, the stage can be interrupted by the formation of a trap formation.
Table 6.1. Sequence of slab accumulation
formations and their characteristics.
End of table 6.1.
For mobile belts (folded areas) characteristic:
linearity of their contours;
the enormous thickness of the accumulated deposits (up to 15-25 km);
consistency composition and thickness of these deposits along strike folded area and abrupt changes across its stretch;
the presence of peculiar formations- complexes of rocks formed at certain stages of development of these areas ( slate, flysch, spilito-keratophyric, molasse and other formations)
intense effusive and intrusive magmatism (large granite batholith intrusions are especially characteristic);
strong regional metamorphism;
7) strong folding, an abundance of faults, including
thrusts indicating the dominance of compression. Folded regions (belts) arise at the site of geosynclinal regions (belts).
Definition: geosyncline(Fig. 6.5) - a mobile area of the earth's crust, in which thick sedimentary and volcanogenic strata initially accumulated, then they were crushed into complex folds, accompanied by the formation of faults, the introduction of intrusions and metamorphism. There are two stages in the development of the geosyncline.
First stage(properly geosynclinal) characterized by a predominance of subsidence. Great rainfall in the geosyncline is the result of the stretching of the earth's crust and her bending. AT the first half of the firststages sandy-argillaceous and clayey sediments usually accumulate (as a result of metamorphism, they then form black argillaceous shales, released in slate formation) and limestones. The subsidence may be accompanied by ruptures along which mafic magma rises and erupts under subsea conditions. The resulting rocks after metamorphism, together with the accompanying subvolcanic formations, give spilit-keratophyric formation. Simultaneously with it, siliceous rocks and jaspers are usually formed.
oceanic
Rice. 6.5. Scheme of the structure of geosync-
molting in a schematic section through the Sunda Arc in Indonesia (Structural Geology and Plate Tectonics, 1991). Symbols: 1 - sediments and sedimentary rocks; 2 - volcano-
nic breeds; 3 - basement conti-metamorphic rocks
Specified formations accumulate at the same time, but in different areas. Accumulation spilito-keratophyric formations usually occur in the interior of the geosyncline - in eugeosynclines. For eugeo-synclines the formation of thick volcanic sequences, usually basic, and the intrusion of gabbro, diabases, and ultrabasic rocks are characteristic. In the marginal part of the geosyncline, along its border with the platform, there are usually miogeosynclines. Here, mainly terrigenous and carbonate strata accumulate; volcanic rocks are absent, intrusions are not typical.
In the first half of the first stage most of the geosyncline is sea with significantdepths. Evidence is provided by the fine granularity of sediments and the rarity of faunal finds (mainly nekton and plankton).
To middle of the first stage due to different sinking rates in different parts of the geosyncline, sections are formed relative uplift(intrageoantic-linali) and relative subsidence(intrageosyncline-whether). Small plagiogranite intrusions may occur at this time.
In second half of the first stage as a result of the appearance of internal uplifts, the sea becomes shallower in the geosyncline. now this archipelago separated by straits. Due to shallowing, the sea advances on adjacent platforms. Limestones accumulate in the geosyncline, thick sandy-clayey rhythmically built strata, forming flysch for-216
mation; there is an outpouring of lavas of medium composition, composing porphyritic formation.
To end of the first stage intrageosynclines disappear, intrageoanticlinals merge into one central uplift. This is a common inversion; it matches main phase of folding in the geosyncline. Folding is usually accompanied by the intrusion of large synorogenic (simultaneous with folding) granite intrusions. There is a crushing of rocks into folds, often complicated by overthrusts. All this causes regional metamorphism. At the site of intrageosynclines, synclinoria- complex structures of the synclinal type, and in place of the intrageoanticlinals - anticlinoria. The geosyncline "closes", turning into a folded area.
In the structure and development of the geosyncline, a very important role belongs to deep faults - long-lived ruptures that cut through the entire earth's crust and go into the upper mantle. Deep faults determine the contours of geosynclines, their magmatism, the division of the geosyncline into structural-facies zones that differ in the composition of sediments, their thickness, magmatism, and the nature of structures. Inside geosynclines are sometimes distinguished mid arrays, limited by deep faults. These are blocks of more ancient folding, composed of rocks of the base on which the geosyncline was laid. In terms of sediment composition and thickness, the median massifs are close to the platforms, but they are distinguished by strong magmatism and rock folding, mainly along the massif edges.
The second stage of the development of the geosyncline called orogenic and is characterized by a predominance of uplifts. Sedimentation occurs in limited areas along the periphery of the central uplift - in edge deflections, arising along the boundary of the geosyncline and the platform and partially overlapping the platform, as well as in intermountain troughs, sometimes formed inside the central uplift. The source of precipitation is the destruction of the constantly rising central uplift. In the first halfsecond stage this uplift probably has a hilly relief; when it is destroyed, marine, sometimes lagoonal sediments accumulate, forming lower molasse formation. Depending on climatic conditions, this may be coal-bearing paralytic or saline thick. At the same time, the intrusion of large granite intrusions - batholiths - usually occurs.
In the second half of the stage the rate of uplift of the central uplift increases sharply, which is accompanied by its splits and the collapse of individual sections. This phenomenon is explained by the fact that due to folding, metamorphism, and intrusions, the folded area (no longer a geosyncline!) becomes rigid and reacts with splits to the ongoing uplift. The sea leaves this territory. As a result of the destruction of the central uplift, which at that time was a mountainous country, continental coarse clastic strata accumulate, forming upper molasse formation. Splitting of the crest of the uplift is accompanied by terrestrial volcanism; usually these are felsic lavas, which, together with
subvolcanic formations give porphyry formation. Fissure alkaline and small acid intrusions are associated with it. Thus, as a result of the development of the geosyncline, the thickness of the continental crust increases.
By the end of the second stage, the folded mountainous area that arose at the site of the geosyncline collapses, the territory gradually levels off and becomes a platform. The geosyncline transforms from the area of accumulation of sediments into the area of destruction, from the mobile territory into the inactive rigid leveled territory. Therefore, the range of motion on the platform is small. Usually the sea, even shallow, covers vast areas here. This area no longer experiences such strong subsidence as before, therefore, the thickness of precipitation is much less (on average 2-3 km). The subsidence is repeatedly interrupted, so there are frequent breaks in sedimentation; then weathering crusts can form. There is also no vigorous uplift accompanied by folding. Therefore, the newly formed thin, usually shallow sediments on the platform are not metamorphosed and lie horizontally or slightly obliquely. Igneous rocks are rare and are usually represented by terrestrial outpourings of basaltic lavas.
In addition to the geosynclinal model, there is a model of lithospheric plate tectonics.
Model of lithospheric plate tectonics
Plate tectonics(Structural Geology and Plate Tectonics, 1991) is a model that was created to explain the observed pattern of the distribution of deformations and seismicity in the outer shell of the Earth. It is based on extensive geophysical data obtained in the 1950s and 1960s. The theoretical foundations of plate tectonics are based on two premises.
The outermost shell of the earth, called lithosphere, lies directly on the layer called acetenosphere, which is less durable than the lithosphere.
The lithosphere is divided into a number of rigid segments, or plates (Fig. 6.6), which are constantly moving relative to each other and whose surface area is also constantly changing. Most of the tectonic processes with intense energy exchange operate at the boundaries between the plates.
Although the thickness of the lithosphere cannot be measured with great accuracy, researchers agree that within the plates it varies from 70-80 km under the oceans to a maximum value of over 200 km under some parts of the continents, with an average value of about 100 km. The asthenosphere underlying the lithosphere extends down to a depth of about 700 km (the maximum depth of propagation of sources of deep-focus earthquakes). Its strength increases with depth, and some seismologists believe that its lower limit is
single lines - transform faults (shifts); Speckled areas of the continental crust that are undergoing active faulting (Structural Geology and Plate Tectonics, 1991)
It is located at a depth of 400 km and coincides with a slight change in physical parameters.
Borders between plates are divided into three types:
divergent;
convergent;
transform (with offsets along strike).
At the divergent boundaries of plates, represented mainly by rifts, a new formation of the lithosphere occurs, which leads to the expansion of the ocean floor (spreading). At convergent plate boundaries, the lithosphere sinks into the asthenosphere, i.e., is absorbed. At transform boundaries, two lithospheric plates slide relative to each other, and the substance of the lithosphere is neither created nor destroyed on them. .
All lithospheric plates are constantly moving relative to each other. It is assumed that the total area of all plates remains unchanged for a significant period of time. At a sufficient distance from the edges of the slabs, the horizontal deformations inside them are insignificant, which makes it possible to consider the slabs as rigid. Since displacements along transform faults occur along their strike, the movement of the plates must be parallel to modern transform faults. Since all this happens on the surface of the sphere, then, in accordance with Euler's theorem, each section of the plate describes a trajectory equivalent to rotation on the spherical surface of the Earth. For the relative movement of each pair of plates at any time, you can determine the axis, or pole of rotation. As you move away from this pole (up to the angular
distance of 90°) spreading rates naturally increase, but the angular velocity for any given pair of plates about their pole of rotation is constant. We also note that geometrically, the poles of rotation are unique for any pair of plates and are in no way connected with the pole of rotation of the Earth as a planet.
Plate tectonics is an effective model of processes occurring in the crust, as it is in good agreement with known observational data, provides an elegant explanation for previously unrelated phenomena, and opens up possibilities for prediction.
Wilson cycle(Structural Geology and Plate Tectonics, 1991). In 1966, Professor Wilson of the University of Toronto published a paper in which he argued that continental drift occurred not only after the early Mesozoic split of Pangea, but also in pre-Pangean times. The cycle of opening and closing of the oceans relative to adjacent continental margins is now called Wilson cycle.
On fig. 6.7 shows a schematic explanation of the basic concept of the Wilson cycle in the framework of ideas about the evolution of lithospheric plates.
Rice. 6.7a represents the beginning of the Wilson cycle– the initial stage of the breakup of the continent and the formation of the accretionary margin of the plate. known to be tough
Rice. 6.7. Scheme of the Wilson cycle of ocean development within the framework of the evolution of lithospheric plates (Structural geology and plate tectonics, 1991)
the lithosphere covers a weaker, partially molten zone of the asthenosphere - the so-called low-velocity layer (Figure 6.7, b) . As the separation of the continents continues, a rift valley develops (Fig. 6.7, 6) and a small ocean (Fig. 6.7, c). These are the stages of early ocean opening in the Wilson cycle.. Suitable examples are the African Rift and the Red Sea. With the continuation of the drift of separated continents, accompanied by symmetrical accretion of the new lithosphere on the margins of the plates, shelf sediments accumulate on the border of the continent with the ocean due to the erosion of the continent. fully formed ocean(Fig. 6.7, d) with a median ridge at the plate boundary and a developed continental shelf is called Atlantic type ocean.
From observations of oceanic trenches, their relationship with seismicity, and reconstruction from the pattern of oceanic magnetic anomalies around the trenches, it is known that the oceanic lithosphere dissects and sinks into the mesosphere. On fig. 6.7, d shown ocean with plate, which has simple margins of increment and absorption of the lithosphere, - this is the initial stage of the closure of the ocean in Wilson cycle. The division of the lithosphere in the vicinity of the continental margin leads to the transformation of the latter into the Orogen Andean type as a result of tectonic and volcanic processes occurring at the absorbing plate boundary. If this division occurs at a considerable distance from the continental margin towards the ocean, then an island arc of the type of the Japanese islands is formed. ocean absorptionlithosphere leads to a change in the geometry of the plates and at the end
ends to complete disappearance of the accretionary margin of the plate(Fig. 6.7, e). During this time, the opposite continental shelf may continue to expand, turning into an Atlantic-type semi-ocean. As the ocean shrinks, the opposite continental margin is eventually involved in the plate absorption regime and participates in the development accretionary orogen of the Andean type. This is the early stage of the collision of two continents (collisions) . At the next stage, due to the buoyancy of the continental lithosphere, the absorption of the plate stops. The lithospheric plate comes off below, under the growing Himalayan-type orogen, and comes final orogenic stageWilson cyclewith mature mountain belt, which is a seam between the newly joined continents. antipode Andean-type accretionary orogen is an Himalayan-type collisional orogen.
Earth's crust - the thin upper shell of the Earth, which has a thickness of 40-50 km on the continents, 5-10 km under the oceans and makes up only about 1% of the mass of the Earth.
Eight elements - oxygen, silicon, hydrogen, aluminum, iron, magnesium, calcium, sodium - form 99.5% of the earth's crust.
On the continents, the crust is three-layered: sedimentary rocks cover granitic rocks, and granitic rocks lie on basalt ones. Under the oceans, the crust is of an "oceanic", two-layer type; sedimentary rocks lie simply on basalts, there is no granite layer. There is also a transitional type of the earth's crust (island-arc zones on the outskirts of the oceans and some areas on the continents, for example).
The earth's crust has the greatest thickness in mountainous regions (under the Himalayas - over 75 km), the average - in the areas of platforms (under the West Siberian lowland - 35-40, within the boundaries of the Russian platform - 30-35), and the smallest - in the central regions of the oceans (5-7 km).
The predominant part of the earth's surface is the plains of the continents and the ocean floor. The continents are surrounded by a shelf - a shallow strip with a depth of up to 200 g and an average width of about SO km, which, after a sharp abrupt bend of the bottom, passes into the continental slope (the slope varies from 15-17 to 20-30 ° ). The slopes gradually level off and turn into abyssal plains (depths 3.7-6.0 km). The greatest depths (9-11 km) have oceanic trenches, the vast majority of which are located on the northern and western margins.
The earth's crust was formed gradually: first a basalt layer was formed, then a granite layer, the sedimentary layer continues to form at the present time.
The deep layers of the lithosphere, which are explored by geophysical methods, have a rather complex and still insufficiently studied structure, as well as the mantle and core of the Earth. But it is already known that the density of rocks increases with depth, and if on the surface it averages 2.3-2.7 g/cm3, then at a depth of close to 400 km it is 3.5 g/cm3, and at a depth of 2900 km ( boundary of the mantle and the outer core) - 5.6 g/cm3. In the center of the core, where the pressure reaches 3.5 thousand tons/cm2, it increases to 13-17 g/cm3. The nature of the increase in the deep temperature of the Earth has also been established. At a depth of 100 km, it is approximately 1300 K, at a depth of close to 3000 km -4800 K, and in the center of the earth's core - 6900 K.
The predominant part of the Earth's matter is in a solid state, but on the border of the earth's crust and upper mantle (depths of 100-150 km) lies a stratum of softened, pasty rocks. This thickness (100-150 km) is called the asthenosphere. Geophysicists believe that other parts of the Earth can also be in a rarefied state (due to decompaction, active radio decay of rocks, etc.), in particular, the zone of the outer core. The inner core is in the metallic phase, but today there is no consensus on its material composition.
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The internal structure of the Earth
Characteristics of the shells of the Earth. Tectonics of lithospheric plates and the formation of large landforms. Horizontal structure of the lithosphere. Types of the earth's crust. The movement of mantle matter through mantle channels in the bowels of the Earth. Direction and movement of lithospheric plates.
presentation, added 01/12/2011
The material composition and structure of the earth's crust
Descriptive characteristics of the stages of formation of the earth's crust and the study of its mineralogical and petrographic compositions. Features of the structure of rocks and the nature of the movement of the earth's crust. Folding, ruptures and collisions of continental plates.
term paper, added 08/30/2013
Theory of lithospheric plates
presentation, added 10/11/2016
Structural elements of the Earth's crust
The location of the folded regions of the Earth's crust. Structure of the platform, passive and active continental margin. Structure of anticlise and syneclise, aulacogenes. Mountain-folded areas or geosynclinal belts. Structural elements of the oceanic crust.
presentation, added 10/19/2014
Tectonic movements of the earth's crust
Classification of the main types of tectonic deformations of the earth's crust: rifting (spreading), subduction, obduction, collisions of continental plates and transform faults. Determination of the speed and direction of movement of lithospheric plates by the geomagnetic field of the earth.
term paper, added 06/19/2011
The material composition of the earth's crust
The main types of the earth's crust and its components. Compilation of speed columns for the main structural elements of the continents. Determination of tectonic structures of the earth's crust. Description of syneclise, anteclise and aulacogen. Mineral composition of the crust and rocks.
term paper, added 01/23/2014
General characteristics of the tectonic structure of the lithospheric plates of the Republic of Tatarstan
Brief history of the study of tectonics of the Republic of Tatarstan. General characteristics uplifts, ruptures, deformations of lithospheric plates. Description of modern movements of the earth's crust and the processes that determine them. Peculiarities of observation of earthquake sources.
term paper, added 01/14/2016
Mesozoic era
Triassic, Jurassic and Cretaceous periods of the Mesozoic era. The organic world of these periods. The structure of the earth's crust and paleogeography at the beginning of the era. History of the geological development of geosynclinal belts and ancient platforms (East European and Siberian).
abstract, added 05/28/2010
Microcontinents. Description of the types of faults in the earth's crust
Origin and development of microcontinents, uplifts of the earth's crust of a special type. The difference between the crust of the oceans and the crust of the continents. Sliding theory of ocean formation. Late synclinal stage of development. Types of faults in the earth's crust, classification of deep faults.
test, added 12/15/2009
Internal structure and inhomogeneities of the Earth
General picture of the internal structure of the Earth. The composition of the matter of the earth's core. Blocks of the earth's crust. Lithosphere and asthenosphere. Foundation structure of the East European platform. a brief description of deep structure of the territory of Belarus and adjacent regions.
test, added 07/28/2013
The largest structural elements of the earth's crust are continents and oceans, characterized by different structures. These structural elements are distinguished by geological and geophysical features. Not all the space occupied by the waters of the ocean is a single structure of the oceanic type. Vast shelf areas, for example, in the Arctic Ocean, have continental crust. The differences between these two major structural elements are not limited to the type of the earth's crust, but can be traced deeper into the upper mantle, which is built differently under the continents than under the oceans. These differences cover the entire lithosphere subject to tectonospheric processes, i.e. traced to depths of about 750 km.
On the continents, two main types of structures of the earth's crust are distinguished: calm stable - platforms and mobile - geosynclines. These structures are quite comparable in terms of their distribution area. The difference is observed in the rate of accumulation and in the magnitude of the gradient of thickness change: platforms are characterized by a smooth gradual change in thickness, while geosynclines are sharp and fast. On the platforms, igneous and intrusive rocks are rare; they are numerous in geosynclines. Flysch formations of sediments are underlying in geosynclines. These are rhythmically multilayered deep-water terrigenous deposits formed during the rapid subsidence of the geosynclinal structure. At the end of development, geosynclinal regions undergo folding and turn into mountain structures. In the future, these mountain structures go through a stage of destruction and a gradual transition to platform formations with a deeply dislocated lower floor of rock deposits and gently dipping layers in the upper floor.
Thus, the geosynclinal stage of the development of the earth's crust is the earliest stage, then the geosynclines die off and are transformed into orogenic mountain structures and subsequently into platforms. The cycle ends. All these are stages of a single process of development of the earth's crust.
Platforms- the main structures of the continents, isometric in shape, occupying the central regions, characterized by a leveled relief and calm tectonic processes. The area of ancient platforms on the continents approaches 40% and they are characterized by angular outlines with extended rectilinear boundaries - a consequence of edge seams (deep faults), mountain systems, linearly elongated deflections. The folded areas and systems are either thrust over the platforms or border on them through foredeeps, which in turn are thrust by folded orogens (mountain ranges). The boundaries of the ancient platforms sharply unconformably intersect their internal structures, which indicates their secondary nature as a result of the split of the Pangea supercontinent that arose at the end of the Early Proterozoic.
For example, the East European platform, identified within the borders from the Urals to Ireland; from the Caucasus, the Black Sea, the Alps to the northern borders of Europe.
Distinguish ancient and young platforms.
ancient platforms arose on the site of the Precambrian geosynclinal region. The East European, Siberian, African, Indian, Australian, Brazilian, North American and other platforms were formed in the late Archean - early Proterozoic, represented by the Precambrian crystalline basement and sedimentary cover. Their distinguishing feature is the two-story building.
lower floor, or foundation it is composed of folded, deeply metamorphosed rock strata, crumpled into folds, cut through by granite intrusions, with a wide development of gneiss and granite-gneiss domes - a specific form of metamorphogenic folding (Fig. 7.3). The foundations of the platforms were formed over a long period of time in the Archean and early Proterozoic and subsequently underwent very strong erosion and denudation, as a result of which rocks that had previously occurred at great depths were exposed.
Rice. 7.3. Principal section of the platform
1 - basement rocks; rocks of the sedimentary cover: 2 - sands, sandstone, gravelstones, conglomerates; 3 - clays and carbonates; 4 - effusives; 5 - faults; 6 - shafts
Top floor platforms presented case, or cover, flat-lying with a sharp angular unconformity on the basement of non-metamorphosed sediments - marine, continental and volcanogenic. The surface between the mantle and basement reflects the underlying structural unconformity within the platforms. The structure of the platform cover turns out to be complex, and on many platforms at the early stages of its formation, grabens, graben-like troughs - aulacogens(avlos - furrow, ditch; gene - born, i.e. born by a ditch). Aulacogens most often formed in the Late Proterozoic (Riphean) and formed extended systems in the basement body. The thickness of continental and, more rarely, marine deposits in aulacogens reaches 5–7 km, and deep faults that bounded aulacogens contributed to the manifestation of alkaline, basic, and ultrabasic magmatism, as well as platform-specific trap (mafic rocks) magmatism with continental basalts, sills, and dikes. Alkaline-ultrabasic is very important (kimberlite) formation containing diamonds in the products of explosion pipes (Siberian platform, South Africa). This lower structural layer of the platform cover, corresponding to the aulacogenous stage of development, is replaced by a continuous cover of platform deposits. At the initial stage of development, the platforms tended to slowly sink with the accumulation of carbonate-terrigenous strata, and at a later stage of development, it is marked by the accumulation of terrigenous coal-bearing strata. In the late stage of platform development, deep depressions filled with terrigenous or carbonate-terrigenous deposits (Caspian, Vilyui) formed in them.
The platform cover in the process of formation repeatedly underwent a structural restructuring, timed to coincide with the boundaries of geotectonic cycles: Baikal, Caledonian, Hercynian, Alpine. Platform sections that experienced maximum subsidence, as a rule, are adjacent to the mobile area or system bordering on the platform, which was actively developing at that time ( pericratonic, those. on the edge of the craton, or platform).
Among the largest structural elements of the platforms are shields and plates.
The shield is a ledge platform crystalline basement surface ( (no sedimentary cover)), which experienced a tendency to rise throughout the entire platform stage of development. Examples of shields include: Ukrainian, Baltic.
Stove they are considered either a part of a platform with a tendency to sag, or an independent young developing platform (Russian, Scythian, West Siberian). Smaller structural elements are distinguished within the plates. These are syneclises (Moscow, Baltic, Caspian) - vast flat depressions, under which the foundation is bent, and anteclises (Belarusian, Voronezh) - gentle vaults with a raised foundation and a relatively thinned cover.
Young platforms formed either on the Baikalian, Caledonian, or Hercynian basement, they are distinguished by a greater dislocation of the cover, a lower degree of metamorphism of the basement rocks, and a significant inheritance of the cover structures from the basement structures. These platforms have a three-tiered structure: the basement of metamorphosed rocks of the geosynclinal complex is overlain by a stratum of denudation products of the geosynclinal area and a weakly metamorphosed complex of sedimentary rocks.
Ring structures. The place of ring structures in the mechanism of geological and tectonic processes has not yet been precisely determined. The largest planetary ring structures (morphostructures) are the Pacific Ocean depression, Antarctica, Australia, etc. The identification of such structures can be considered conditional. A more thorough study of ring structures made it possible to identify elements of spiral, vortex structures in many of them.
However, structures can be distinguished endogenous, exogenous and cosmogenic genesis.
Endogenous ring structures metamorphic and magmatic and tectonic (arches, ledges, depressions, anteclises, syneclises) origin have diameters from units of kilometers to hundreds and thousands of kilometers (Fig. 7.4).
Rice. 7.4. Ring structures north of New York
Large ring structures are due to processes occurring in the depths of the mantle. Smaller structures are due to diapiric processes of igneous rocks rising to the Earth's surface and breaking through and uplifting the upper sedimentary complex. Ring structures are determined and volcanic processes(volcanic cones, volcanic islands), and the processes of diapirism of plastic rocks such as salts and clays, the density of which is less than the density of the host rocks.
exogenous ring structures in the lithosphere are formed as a result of weathering, leaching, these are karst funnels, failures.
Cosmogenic (meteorite) ring structures are astroblems. These structures result from meteorite impacts. Meteorites with a diameter of about 10 kilometers fall to the Earth with a frequency of once every 100 million years, smaller ones much more often. Meteoritic ring structures can have diameters from tens of meters to hundreds of meters and kilometers. For example: Balkhash-Ili (700 km); Yukotan (200 km), depth - more than 1 km: Arizona (1.2 km), depth more than 185 m; South Africa (335 km), from an asteroid with a diameter of about 10 km.
In the geological structure of Belarus, one can note ring structures of tectonomagmatic origin (Orsha depression, Belarusian massif), diapiric salt structures of the Pripyat trough, volcanic ancient channels of the kimberlite pipes(on the Zhlobin saddle, the northern part of the Belarusian massif), an astroblem in the Pleschenitsy region with a diameter of 150 meters.
Ring structures are characterized by anomalies of geophysical fields: seismic, gravitational, magnetic.
Rift structures of continents (Fig. 7.5, 7.6) of small width up to 150 -200 km are expressed by extended lithospheric uplifts, the arches of which are complicated by subsidence grabens: Rhine (300 km), Baikal (2500 km), Dnieper-Donetsk (4000 km), East African (6,000 km), etc.
Rice. 7.5. Section of the Pripyat continental rift
Continental rift systems consist of a chain of negative structures (troughs, rifts) with a ranged time of inception and development, separated by uplifts of the lithosphere (saddles). Rift structures of continents can be located between other structures (anteclises, shields), cross platforms, and continue on other platforms. The structure of continental and oceanic rift structures is similar, they have a symmetrical structure relative to the axis (Fig. 7.5, 7.6), the difference lies in the length, degree of opening and the presence of some special features (transform faults, protrusions-bridges between links).
Oldest part of the earth's crust found
7.6. Profile sections of continental rift systems
1-foundation; 2-chemogenic-biogenic sedimentary deposits; 3- chemogenic-biogenic-volcanogenic formation; 4 - terrigenous deposits; 5, 6-faults
A part (link) of the Dnieper-Donets continental rift structure is the Pripyat trough. The Podlasko-Brest depression is considered to be the upper link; it may have a genetic connection with similar structures in Western Europe. The lower links of the structure are the Dnieper-Donetsk depression, then the similar structures Karpinskaya and Mangyshlakskaya and further the structures of Central Asia (the total length from Warsaw to the Gissar Range). All links of the rift structure of the continents are limited by listric faults, have a hierarchical subordination according to the age of occurrence, and have a thick sedimentary stratum promising for the content of hydrocarbon deposits.
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Stable areas of the earth's crust, which rest on an ancient (Precambrian) crystalline foundation, are called ancient platforms. The territory of Russia is located on two ancient platforms. In some places, the foundation of the platforms (many meters of granite) goes directly to the surface, you can walk on it. Such places are called shields. Shields occupy small areas of the platforms. Most often, the foundation is hidden under the thickness of younger layers of the earth's crust. These parts of the platforms are called plates. A young platform is also a stable section of the earth's crust, but its foundation is younger (it was formed in the Paleozoic time). According to geologists, once two lithospheric plates collided with ancient platforms and firmly “glued” together.
Found the oldest part of the earth's crust
The place of their “gluing” is the Ural Mountains, and another young platform has formed between the Ural Mountains and the Siberian platform. It is covered with a thick layer of sedimentary rocks. Its surface is a flat plain. During those millions of years, while the sedimentary cover of the platforms is being formed, magma in different places penetrates into the thickness of the earth's crust through the basement cracks. On the territory of the Siberian platform, it formed traps - lava covers or lakes of solidified lava. How traps are formed is well shown in the multimedia textbook as the Siberian platform approaches. Traps have not formed on the East European Platform, but there are intrusions - magma massifs that did not break through to the surface and solidified in the thickness of the earth's crust. On geological sections and maps, they are indicated in red, like the foundation. Sometimes the destruction of rocks from above leads to the fact that cooled and crystallized intrusions come to the surface.
platforms
platforms
platform
The earth's crust within modern Russia was formed over a long period of time as a result of various geological processes. Therefore, its parts differ: firstly, in the structure, composition and occurrence of rocks, and secondly, in age and history of development.
According to the structural features, mobile and stable sections of the earth's crust are distinguished. Mountain structures are located on mobile sites. They are composed of rocks crumpled into folds, divided by splits into separate blocks. These blocks move in different directions at different speeds. As a result of these movements, mountain ranges and depressions separating them are formed. Intensive movements of the earth's crust are often accompanied by earthquakes.
Most of the territory of Russia is occupied by stable areas of the earth's crust - platforms: East European, West Siberian and Siberian. Platforms have a two-tiered structure. Their lower part is the foundation. These are the remains of the collapsed mountain systems that previously existed on the site of modern platforms. Therefore, it consists of rocks crumpled into folds. Loose sedimentary rocks (sedimentary cover) overlie the foundation. They were formed during the destruction of mountains and the slow sinking of the foundation, when it was flooded with the waters of the seas. There is no sedimentary cover in some parts of the platforms. Such sections of platforms are called shields.
The rocks of the folded belts and platforms have different ages, as they were formed over a long period of time.
The entire geological history of the Earth is divided into 5 large time periods - eras. The name of each era is given in accordance with the type of life characteristic of it: Archean (earliest life), Proterozoic (early life), Paleozoic (ancient life), Mesozoic (middle life), Cenozoic ( new life). The length of the eras varies greatly. In turn, eras are subdivided into smaller periods of time - periods. The names of the periods most often come either from the names of those areas where the rocks formed during this period were first studied in detail, or from the names of the rocks themselves.
The age and time of formation of individual rocks can be determined in different ways. If the original occurrence of rocks is not disturbed by subsequent geological processes, then the layers that lie above are younger than those located below. They help to determine the age of rocks and fossil remains of plants and animals. The more complex organisms are, the younger they are. Both of these methods make it possible to estimate the relative age of rocks.
They learned to determine the absolute age of rocks only in the 20th century. To do this, evaluate the process of decay of radioactive elements contained in rocks. The decay process proceeds at a constant rate and does not depend on external conditions. Therefore, by the ratio of the content of the radioactive element in the rock and the products of its decay, it is possible to establish the absolute age of the rock in billions and millions of years.
The most ancient folded areas were formed on the territory of Russia in the Archean and Proterozoic (2600-500 million years ago). They are composed of pre-Paleozoic rocks. It is they who form the lower structural tier of the platforms - their folded foundation.
On the territory of Russia there are two ancient platforms - East European and Siberian. Both of them have a two-tier structure: a folded basement of crystalline and igneous rocks of the Archean-Proterozoic age and a Paleozoic-Cenozoic sedimentary cover. Sedimentary rocks of the cover lie quietly, usually subhorizontally. Sedimentation was interrupted during uplifts and was replaced by demolition processes.
East European platform it is bounded in the east by the Ural folded structures, in the south by the young Scythian plate, adjacent to the folded structures of the Caucasus, in the north it continues under the waters of the Barents Sea, and in the west it extends far beyond the borders of Russia. Within its boundaries there are two shields, one of which - the Baltic - enters the territory Kola Peninsula and Karelia, the second - Ukrainian - is completely outside of Russia. The rest of the platform space: occupied by the Russian plate.
The shallow bedding of the basement is characteristic of the Voronezh anteclise (the first hundreds of meters) and some positive structures of the Volga-Ural dome. In syneclises (Moscow, Pechora, Baltiyskaya), the foundation is lowered by 2-4 km. The greatest depth of the basement is typical for the Caspian syneclise (15-20 km).
East Siberian Platform- a large geological region in the northeast of the Eurasian plate, occupies the middle part of North Asia. This is one of the large, relatively stable ancient blocks of the Earth's continental crust, which are among the ancient (pre-Riphean) platforms. Its foundation was formed in the Archaean, subsequently it was repeatedly covered by seas, in which a powerful sedimentary cover was formed. Several stages of intraplate magmatism occurred on the platform, the largest of which is the formation of Siberian traps at the Permian–Triassic boundary. Before and after the emplacement of the traps, there were sporadic outbursts of kimberlite magmatism that formed large diamond deposits.
The Siberian platform is bounded by deep fault zones - marginal sutures, well-defined gravity steps, and has a polygonal outline. The modern boundaries of the platform took shape in the Mesozoic and Cenozoic and are well expressed in the relief. The western boundary of the platform coincides with the valley of the Yenisei River, the northern one with the southern margin of the Byrranga Mountains, the eastern one with the lower reaches of the Lena River (Verkhoyansk marginal trough), in the southeast with the southern tip of the Dzhugdzhur Ridge; in the south, the boundary runs along the faults along the southern margin of the Stanovoy and Yablonovy ridges; then, going around from the north along a complex system of faults in Transbaikalia and the Baikal region, it descends to the southern tip of Lake Baikal; the southwestern boundary of the platform extends along the Main East Sayan Fault.
On the platform, the Early Precambrian, mainly Archean, basement and platform cover (Riphean-Anthropogenic) stand out. Among the main structural elements of the platform stand out: the Aldan Shield and the Leno-Yenisei Plate, within which the basement is exposed on the Anabar massif, Olenyok and Sharyzhalgai uplifts. The western part of the plate is occupied by the Tungusskaya, and the eastern part by the Vilyui syneclise. In the south there is the Angara-Lena trough, separated from the Nyu depression by the Peledui uplift.
- During the Archean and the beginning of the Proterozoic, most of the basement of the East Siberian Platform was formed.
- At the end of the Proterozoic (Vendian) and the beginning of the Paleozoic, the platform was periodically covered by a shallow sea, resulting in the formation of a thick sedimentary cover.
- At the end of the Paleozoic, the Paleo-Ural Ocean closed, the crust of the West Siberian Plain consolidated, and it, together with the East Siberian and East European platforms, formed a single continent.
- In the Devonian, an outbreak of kimberlite magmatism.
- A powerful outbreak of trap magmatism occurred at the Permian–Triassic boundary.
- In the Mesozoic, some parts of the platform were covered by epicontinental seas.
- Rifting and a new outbreak of magmatism, including carbonatite and kimberlite, took place on the platform at the Cretaceous–Paleogene boundary.
The foundation of the platform is composed of Archean, Proterozoic and Riphean rocks. The surface of the crystalline basement of the Siberian platform, as well as the Russian one, is very uneven; in some parts, the foundation comes to the surface or is submerged to an insignificant depth, in others it is covered by a thick layer of sedimentary rocks. The foundation surface consists of a system of anteclises and syneclises. The largest basement uplifts are the Anabarska massif, the Aldan shield, the Yenisei meganticlinorium, the Turukhanskoe uplift and the folded system of the Stanovoy Range. The largest subsidences are Tungusskaya (5-6 km), Vilyuiskaya (5-8 km), Khatanga syneclises and the Angara-Lena trough, laid down at different times: Tungusskaya - in the lower Paleozoic, Khatanga - in the middle Paleozoic, Vilyuiskaya - in mesozoic. The thickness and completeness of the section of the sedimentary complex in separate parts of the platform varies widely. The most characteristic platform structures are flat and dome-shaped folds of the northwestern direction, disturbed by discontinuous dislocations of the Alpine cycle.
The Siberian platform in the initial phases of the Hercynian cycle - Upper Devonian and Carboniferous - on the northern margin was occupied by the sea. By the end of the Carboniferous period, the sea receded, leaving vast marshy spaces, in which the accumulation of Permian sandy-argillaceous coal-bearing deposits of the Tunguska basin, and lakes took place.
The final phases of the Hercynian folding were manifested by powerful trap eruptions over an area of 1.5 billion km2. The invasion of intrusions and outpourings of effusives continued into the Triassic and, possibly, into the early Jurassic. The tuff formation includes tuffs, as well as andesites, porphyrites, and basalts. The effusives of basic, ultrabasic and alkaline composition predominate. In various parts of the platform, there are kimberlites associated with explosion pipes. The thickness of the trap formation varies greatly. In the areas of the platform, flooded in the Carboniferous and Permian by the sea, thick strata of sedimentary rocks were deposited - limestones, marls, dolomites, clays, shales, sandy deposits.
The Precambrian structures are associated with gold deposits associated with granitoid intrusions (Yenisei, Lena, Anabar regions), a muscovite deposit (Mamsko-Vitimskoe), metamorphic iron ore deposits (Angara-Ilimsk region "Angara-Pitsky basin). Deposits of copper-nickel ores (Norilsk) and optical Icelandic spar are also associated with trap effusions.
The geotectonic structure of the platforms as a whole determines the main features of the modern topography of the surface of the Russian Plain, the West Siberian Lowland, and the Central Siberian Plateau. Anteclises determine positive landforms, while syneclises correspond to slightly hilly lowlands and plains. However, sometimes there is also a discrepancy between the forms of the modern relief, the position of river valleys and tectonic structures. For example, the Polesskaya lowland is located on the site of the Belorussian uplift, the Putorana uplift is on the site of the synclinal structure of the platform base, etc. The Baikal folding occurred in the Late Proterozoic - Lower Cambrian. The structures created by her partially became part of the foundation of the platforms, consolidating older blocks, and also adjoin the outskirts of the ancient platforms. They delineate the Siberian platform from the north, west and south (Taimyr-Severozemelskaya, Baikal-Vitim and Yenisei-East-Sayan regions). The Timan-Pechora-Barents Sea Region is located on the northeastern margin of the East European Platform. Apparently, at the same time, the Irtysh-Nadym block was formed, which occupies a central position within West Siberian Plain. Areas of Baikal folding E.E. Milanovsky (1983, 1987) refers to metaplatform areas.
In the Phanerozoic, along with ancient platforms and adjacent metaplatform areas, there are so-called mobile belts, three of which enter the territory of Russia: the Ural-Mongolian, Pacific and Mediterranean. In their development, mobile belts go through two main stages: geosynclinal and postgeosynclinal, or epigeosynclinal folded belt, the change of which in different belts and even in different areas single belt occurred at different times and dragged on until the end of the Phanerozoic.
The features of the first stage have already been discussed in the characterization of geosynclines. The tectonic regime of the second stage is significantly inferior in its activity to the geosynclinal one, but at the same time surpasses the tectonic regime of the ancient platforms.
The Paleozoic Ural-Mongolian belt is located between the ancient East European and Siberian platforms and forms the southern frame of the latter. Downturns within this belt began as early as the Late Proterozoic, and in the Lower Paleozoic, Caledonian folding manifested itself here. The main phases of folding occur at the end of the Cambrian - the beginning of the Ordovician (Salair), the middle - the Upper Ordovician, the end of the Silurian - the beginning of the Devonian. As a result of the Caledonian folding, mountain structures were created in the Western Sayan, Kuznetsk Alatau, Salair, in the eastern regions of Altai, in Tuva, in a significant part of Transbaikalia, in the southern regions Western Siberia, adjoining in the western part of the Kazakh uplands, where the Caledonian folding was also final. In all these territories, the Lower Paleozoic deposits are intensely folded and metamorphosed. A Precambrian base often peeps through their cover.
In the Upper Paleozoic (Late Devonian - Early Carboniferous and Late Carboniferous - Permian) hercynian(Varisian) folding. It was the final one in the vast expanse of Western Siberia, consolidating the blocks that previously existed here, in the Ural-Novaya Zemlya region, in the western regions of Altai, in the Tom-Kolyvan zone. It also appeared in the Mongolian-Okhotsk zone.
Thus, by the end of the Paleozoic, an intracontinental folding zone formed within the Ural-Mongolian mobile belt, soldering two ancient platforms into a single large structure, a rigid block that became the core of the Eurasian lithospheric plate. There was also an increase in the platform area due to the appearance of folded structures along their southern margins.
Later (in the Mesozoic), young epipaleozoic plates (quasicratons) formed within the Ural-Mongolian belt, including the West Siberian one, which is almost entirely located on the territory of Russia.
Stages of formation of the earth's crust in Russia
They are confined to areas that experienced a general subsidence in the Meso-Cenozoic.
Plates usually form over those areas of mobile belts, in the structural plan of which blocks of ancient consolidation play a significant role - the median massifs. Young slabs do not always fit strictly into the contours of the movable belt. They can also be superimposed on areas of ancient platforms adjacent to the mobile belt (metaplatform areas), as is the case on the eastern margin of the West Siberian Plate. The cover of young platforms is composed of sedimentary sequences of the Meso-Cenozoic age. The thickness of the cover ranges from several hundred meters - a kilometer in the marginal parts to 8-12 km in the most deeply subsided northern part of the West Siberian plate.
Pacific mobile belt occupies a marginal position between the ancient Siberian platform and the oceanic lithospheric plate of the Pacific Ocean. It includes folded structures of the North-East and the Far East.
Some sections of this belt completed the period of geosynclinal development as early as the Precambrian or Paleozoic and form median massifs, the largest of which are the Kolyma and Bureinsky (peculiar "microplatforms" having a shield and a plate); others experienced folding in the Mesozoic, others in the Cenozoic.
The Verkhoyansk-Chukotka folded region was created by Cimmerian folding (Late Cimmerian, or Kolyma, late Jurassic - mid-Cretaceous). The Okhotsk-Chukotka volcanic belt stretches along the southeastern margin of this region, which passes into the Primorsky volcanic belt in the southern part of the Far East, separating the mesozoids of this region from the region of Pacific folding. Early and late Cimmerian folding appeared here, which created the Mesozoic structures of the Amur region and the central part of the Sikhote-Alin, and the Larami searched (late Cretaceous - early Paleogene), culminating in the formation of folded structures in Sikhote-Alin. The Koryak region was also created by the Laramian folding.
The mountain structures of Sakhalin and Kamchatka arose as a result of the Pacific folding, which manifested itself in the Oligocene and mainly in the Neogene-Quaternary time, i.e. are at the orogenic stage of development. These are the youngest folded and volcanic mountains in Russia. The Kuril Islands have not yet completed their geosynclinal development; these are modern island arcs with a deep-water trench located next to it, clearly fixing the subduction zone of the Pacific lithospheric plate. Vast areas here are occupied by the oceanic crust. Actually, the island arcs are characterized by the early stages of the formation of the continental crust.
The ongoing tectonic activity, especially along the eastern margin of this belt, is evidenced by intense volcanic activity, a large amplitude of Quaternary uplifts, and a high seismicity of the region.
Mediterranean geosynclinal belt- one of the main mobile belts of the Earth, which developed during the late Precambrian and Phanerozoic. The belt stretches in the general latitudinal direction from the Atlantic to Pacific Ocean covering Central and Southern Europe, Northwest Africa (Maghrib), the Mediterranean, the Caucasus, Western Asia, the Pamirs, Tibet, the Himalayas, the Indochinese Peninsula, Indonesia and merging here with the Pacific geosynclinal belt (western branch).
The origin of the belt, judging by the age of the most ancient ophiolites, belongs to the Late Proterozoic (Riphean); most researchers believe that it occurred as a result of the destruction of the supercontinent, which at the beginning of the Riphean united the future Laurasia and Gondwana, namely the East European, African-Arabian, Hindustan, Chinese-Korean and South Chinese (Yangtze) ancient platforms. In Central and Central Asia, the Mediterranean geosynclinal belt almost touches the Ural-Okhotsk belt, and in the area of the British Isles - with the North Atlantic belt. The first stage of development of the belt refers to the late Riphean-Vendian - early Cambrian (in Western Europe it is called Kadom, to the east - Baikal, Salair). The stage ended with folding, metamorphism (mainly greenschist facies) and moderate granite formation. The resulting continental crust did not differ in stability, being preserved from subsequent destruction within Nubia, Arabia, and Western Asia and in separate massifs in other parts of the belt (the north of the Armorican massif in France, the North Caucasian massif, etc.). A new expansion with the formation of oceanic crust (Paleotethys) occurred in the Cambrian - Ordovician.
It is not yet clear whether this basin was partly inherited from the Riphean-Vendian or whether it was entirely newly formed. At the beginning of the Devonian, the development of the northern periphery of the basin in Europe from southern Great Britain to Poland ended with a new era of diastrophism; this Caledonian fold zone built up the East European platform and the Midland massif of Great Britain bordering the North Atlantic belt. In Asia, the Caledonian folded zone, whose geosynclinal development began as early as the Vendian - Early Cambrian, covers the Qilianshan Range and the northern slope of the Qinling Range and adjoins the Sino-Korean platform from the south. In the Devonian, the zone of active subsidence shifts to the south, to the limits of Central Europe, the Iberian Peninsula, the Maghreb, the North Caucasus, the Northern Pamirs, the Kunlun, and the Central Qinling. Starting from the middle of the Early Carboniferous, it is involved in fold-and-thrust deformations (their first phases date back to the second half of the Devonian), which created Hercynian structures (see Hercynian folding). As a result, the western part of the belt experienced complete regeneration of the continental crust and drainage; here Laurasia joined with Gondwana into a single supercontinent - Pangea.
In the east, in Asia, in the Late Paleozoic there was only a new shift of the area of maximum subsidence to the south, to the southern slope of the Greater Caucasus, to Central Afghanistan, the Pamirs and Tibet, as well as the Indochinese Peninsula and partly Indonesia. The development of this zone - Mesotethys ended with folding, granitization and mountain building at the end of the Triassic and the beginning of the Jurassic; the corresponding epoch is known in the west as the early Cimmerian, in the east as the Indo-Sinian. At the end of the Triassic - the beginning of the Jurassic, Eurasia again completely separated from Gondwana, a new deep-sea basin with oceanic crust opened up - the Tethys proper, or Neotethys, which extended in the west to Central America. Its axial zone is shifted even further south compared to the Paleo- and Mesotethys, in the east to the region of the Baikal consolidation. The first deformations of this belt date back to the end of the Jurassic - the middle of the Cretaceous (Late Cimmerian, Austrian eras); the main deformations - by the end of the Eocene - the end of the Miocene, the main mountain building - from the end of the Miocene. As a result of these processes, the Alpine-Himalayan folded mountain belt arose, stretching from the Pyrenees and Gibraltar to Indonesia. Active mountain building, seismic activity, and in the Mediterranean and Indonesia, volcanism continue in this belt into the modern era. The advanced and intermountain troughs are distinguished by rich oil and gas content; deposits of ores of ferrous and non-ferrous metals are known in mountain structures. Simultaneously with mountain building in the Alpine-Himalayan belt, the formation of deep-water basins of the Mediterranean and Indonesia with oceanic-type crust was going on.
Nature of Russia
Geography textbook for grade 8
§ 6. Geological structure of the territory of Russia
- What is the structure of the lithosphere?
- What phenomena occur at the boundaries of its plates?
- How are seismic belts located on Earth?
The structure of the earth's crust. The largest features of the country's relief are determined by the peculiarities of the geological structure and tectonic structures. The territory of Russia, like the whole of Eurasia, was formed as a result of the gradual convergence and collision of individual large lithospheric plates and their fragments.
The structure of lithospheric plates is heterogeneous. Within their limits there are relatively stable areas - platforms and mobile folded belts.
The oldest earth's crust was formed by gravitational mixing
The location of the largest forms of land relief - plains and mountains - depends on the structure of lithospheric plates. Plains are located on platforms.
Tectonic structures and the time of their formation are shown on tectonic maps, without which it is impossible to explain the patterns of distribution of the main landforms.
Mountains formed in mobile folded belts. These belts arose at different times in the marginal parts of the lithospheric plates when they collided with each other. Sometimes fold belts are found in the inner parts of the lithospheric plate. Such, for example, is the Ural Range. This suggests that once there was a boundary between two plates, which later turned into a single, larger plate.
The geological history of the Earth begins with the formation of the earth's crust. The oldest rocks indicate that the age of the lithosphere is more than 3.5 billion years.
The period of time corresponding to the longest (longest) stage in the development of the earth's crust and the organic world is commonly called the geological era. The entire history of the Earth is divided into five eras: Archean (ancient), Proterozoic (the era of early life), Paleozoic (the era of ancient life), Mesozoic (the era of middle life), Cenozoic (the era of new life). Eras are subdivided into geological periods. The names of the periods most often come from the localities where the corresponding deposits were first found.
Geological reckoning, or geochronology, is a branch of geology that studies the age, duration and sequence of formation of rocks that make up the earth's crust.
Sciences that study the earth's crust
The diversity of modern relief is the result of a long geological development and the impact of modern relief-forming factors, including human activity. Geology deals with the study of the structure and history of the development of the Earth. Modern geology is divided into a number of branches: historical geology studies the regularities in the structure of the earth's crust during geological time; geotectonics is the study of the structure of the earth's crust and the formation of tectonic structures (folds, cracks, shifts, faults, etc.). Paleontology is the science of extinct (fossil) organisms and the development of the organic world of the Earth. Mineralogy and petrography study minerals and other natural chemical compounds. If the occurrence of rocks is not disturbed by crushing, folds, ruptures, then each layer is younger than the one on which it lies, and the uppermost layer was formed later than all.
In addition, the relative age of rocks can be determined from the remains of extinct organisms.
It was only in the 20th century that they learned to determine the absolute age of rocks with sufficient accuracy. For these purposes, the process of decay of radioactive elements contained in the rock is used.
Geological table contains information about the successive change of eras and periods in the development of the Earth and their duration. Sometimes the table indicates the most important geological events, stages in the development of life, as well as the most typical minerals for a given period, etc.
The table is built from the most ancient stages of the development of the Earth to the modern one, so you need to study it from the bottom up. With the help of a geochronological table, one can obtain information about the duration and geological events in different eras and periods of the Earth's development.
Geological maps contain detailed information about what rocks are found in certain regions of the globe, what minerals lie in their bowels, etc.
Rice. 15. Geological chronology. The history of the development of the Earth
The geological map will allow you to get an idea of the distribution of rocks of various ages throughout Russia. Please note that the most ancient rocks come to the surface in Karelia and Transbaikalia.
In the course of the geography of continents and oceans, you have already become acquainted with a map of the structure of the earth's surface, that is, with a tectonic map. By studying the tectonic map of Russia, you can get detailed information about the location and age of various tectonic structures within our country.
Rice. 16. Tectonic structures of the world
Compare the geological and tectonic maps and determine to which tectonic structures the outcrops of the most ancient rocks are confined.
Analysis of the tectonic map of Russia allows us to draw the following conclusions.
Areas with a flat relief are confined to platforms - stable areas of the earth's crust, where folding processes have long ended. The most ancient of the platforms are East European and Siberian. At the base of the platforms lies a rigid foundation composed of igneous and highly metamorphosed rocks of the Precambrian age (granites, gneisses, quartzites, crystalline schists). The foundation is usually covered with a cover of horizontal sedimentary rocks, and only on the Siberian Platform (Central Siberian Plateau) are significant areas occupied by volcanic rocks - Siberian traps.
On the map (Fig. 16), determine within which lithospheric plates the territory of Russia is located.
The outcrops of the foundation, composed of crystalline rocks, to the surface are called shields. In our country, the Baltic Shield on the Russian Platform and the Aldan Shield on the Siberian Platform are known.
Compare tectonic and physical-geographical maps and determine what relief forms are characteristic of shields.
Rice. 17. Platform structure
Mountainous regions are distinguished by a more complex geological structure. Mountains are formed in the most mobile parts of the earth's crust, where, as a result of tectonic processes, rocks are crushed into folds, broken by faults and faults. These tectonic structures arose at different times - in the eras of the Paleozoic, Mesozoic and Cenozoic folding. The youngest mountains of our country are located in the Far East, namely the Kuril Islands and Kamchatka. They are part of the vast Pacific volcanic belt, or the Pacific Ring of Fire, as it is called. They are distinguished by significant seismicity, frequent strong earthquakes, and the presence of active volcanoes.
Rice. 18. Structure of the folded area
The information of geological and tectonic maps is necessary not only for geologists and geographers, but also for builders, as well as representatives of other professions.
Table 2. Main active volcanoes in Russia
To successfully work with these rather complex maps, one must first carefully study their legends.
Questions and tasks
- What sciences are engaged in the study of the history of the development of the Earth?
- What information can be obtained from a geochronological table?
- What is shown on a tectonic map?
- Using a geochronological table, write a story about the formation of the main forms of the surface of our country.
- Determine from the geochronological table in which era and period we live; what geological events are currently taking place; what minerals are formed.
Introduction…………………………………………………………………………..2
1. The structure of the Earth ………………………………………………………………….3
2. The composition of the earth's crust……………………………………………………………...5
3.1. State of the Earth …………………………………………………………....7
3.2. The state of the earth's crust………………………………………………………...8
List of used literature……………………………………………………………………………………………………………………………………………………………………………………………………………………….
Introduction
The Earth's crust is the outer solid shell of the Earth (geosphere). Below the crust is the mantle, which differs in composition and physical properties - it is denser, contains mainly refractory elements. The crust and mantle are separated by the Mohorovichic boundary, or Moho for short, on which there is a sharp increase in seismic wave velocities. From the outside, most of the crust is covered by the hydrosphere, and the smaller part is under the influence of the atmosphere.
There is a crust on most of the terrestrial planets, the Moon and many satellites of the giant planets. In most cases, it consists of basalts. The Earth is unique in that it has two types of crust: continental and oceanic.
1. Earth structure
Most of the Earth's surface (up to 71%) is occupied by the oceans. The average depth of the World Ocean is 3900 m. The existence of sedimentary rocks, whose age exceeds 3.5 billion years, is proof of the existence of vast reservoirs on Earth already at that distant time. On modern continents, plains are more common, mostly low-lying, and mountains - especially high ones - occupy an insignificant part of the planet's surface, as well as deep-sea depressions at the bottom of the oceans. The shape of the Earth, which is known to be close to spherical, turns out to be very complex with more detailed measurements, even if we describe it as a flat surface of the ocean (not distorted by tides, winds, currents) and the conditional continuation of this surface under the continents. Irregularities are maintained by the uneven distribution of mass in the bowels of the Earth.
One of the features of the Earth is its magnetic field, thanks to which we can use the compass. The magnetic pole of the Earth, to which the north end of the compass needle is attracted, does not coincide with the geographic North Pole. Under the influence of the solar wind, the Earth's magnetic field is distorted and acquires a "tail" in the direction from the Sun, which extends for hundreds of thousands of kilometers.
The internal structure of the Earth is primarily judged by the peculiarities of the passage through the various layers of the Earth of mechanical vibrations that occur during earthquakes or explosions. Valuable information is also provided by measurements of the magnitude of the heat flux emerging from the depths, the results of determinations of the total mass, moment of inertia and polar compression of our planet. The mass of the earth is found from experimental measurements the physical constant of gravity and the acceleration of gravity. For the mass of the Earth, a value of 5.967 1024 kg was obtained. On the basis of a whole complex of scientific research, a model of the internal structure of the Earth was built.
The solid shell of the Earth is the lithosphere. It can be compared to a shell covering the entire surface of the Earth. But this "shell" seemed to be cracked into pieces and consists of several large lithospheric plates, slowly moving one relative to the other. The vast majority of earthquakes are concentrated along their boundaries. Upper layer lithosphere is the earth's crust, the minerals of which consist mainly of silicon and aluminum oxides, iron oxides and alkali metals. The earth's crust has an uneven thickness: 35-65 km on the continents and 6-8 km under the ocean floor. The upper layer of the earth's crust consists of sedimentary rocks, the lower layer of basalts. Between them is a layer of granites, characteristic only of the continental crust. Under the crust is the so-called mantle, which has a different chemical composition and greater density. The boundary between the crust and the mantle is called the Mohorovich surface. In it, the speed of propagation of seismic waves increases abruptly. At a depth of 120-250 km under the continents and 60-400 km under the oceans lies a mantle layer called the asthenosphere. Here the substance is in a state close to melting, its viscosity is greatly reduced. All lithospheric plates seem to float in the semi-liquid asthenosphere, like ice floes in water. Thicker sections of the earth's crust, as well as areas consisting of less dense rocks, rise in relation to other sections of the crust. At the same time, an additional load on a section of the crust, for example, due to the accumulation of a thick layer of continental ice, as occurs in Antarctica, leads to a gradual subsidence of the section. This phenomenon is called isostatic leveling. Below the asthenosphere, starting from a depth of about 410 km, the "packing" of atoms in mineral crystals is compacted under the influence of high pressure. A sharp transition was detected by seismic research methods at a depth of about 2920 km. Here begins the earth's core, or, more precisely, the outer core, since in its center there is another one - the inner core, the radius of which is 1250 km. The outer core is obviously in a liquid state, since transverse waves that do not propagate in a liquid do not pass through it. The existence of a liquid outer core is associated with the origin of the Earth's magnetic field. The inner core appears to be solid. At the lower boundary of the mantle, the pressure reaches 130 GPa, the temperature there is no higher than 5000 K. In the center of the Earth, the temperature may rise above 10,000 K.
2. The composition of the earth's crust
The earth's crust consists of several layers, the thickness and structure of which are different within the oceans and continents. In this regard, oceanic, continental and intermediate types of the earth's crust are distinguished, which will be described later.
According to the composition, three layers are usually distinguished in the earth's crust - sedimentary, granite and basalt.
The sedimentary layer is composed of sedimentary rocks, which are the product of the destruction and redeposition of the material of the lower layers. Although this layer covers the entire surface of the Earth, in some places it is so thin that one can practically speak of its discontinuity. At the same time, sometimes it reaches a power of several kilometers.
The granite layer is composed mainly of igneous rocks formed as a result of solidification of molten magma, among which varieties rich in silica (acidic rocks) predominate. This layer, which reaches a thickness of 15-20 km on the continents, is greatly reduced under the oceans and may even be completely absent.
The basalt layer is also composed of igneous matter, but poorer in silica (basic rocks) and having a high specific gravity. This layer is developed at the base of the earth's crust in all regions of the globe.
The continental type of the earth's crust is characterized by the presence of all three layers and is much more powerful than the oceanic.
The earth's crust is the main object of study of geology. The earth's crust consists of very diverse rocks, consisting of no less diverse minerals. When studying a rock, first of all, its chemical and mineralogical composition is studied. However, this is not enough for a complete knowledge of the rock. The same chemical and mineralogical composition can have rocks of different origin, and, consequently, different conditions of occurrence and distribution.
Under the structure of the rock understand the size, composition and shape of the constituent mineral particles and the nature of their connection with each other. Different types of structures are distinguished depending on whether the rock is composed of crystals or an amorphous substance, what is the size of the crystals (whole crystals or their fragments are part of the rock), what is the degree of roundness of the fragments, the mineral grains that form the rock are completely unrelated to each other, or they are soldered with some kind of cementing substance, directly grown together with each other, sprouted each other, etc.
Texture is understood as the relative position of the components that make up the rock, or the way they fill the space occupied by the rock. An example of textures can be: layered, when the rock consists of alternating layers different composition and structures, shale, when the rock easily breaks up into thin tiles, massive, porous, solid, bubbly, etc.
The form of occurrence of rocks is understood as the shape of the bodies formed by them in the earth's crust. For some rocks, these are layers, i.e. relatively thin bodies bounded by parallel surfaces; for others - cores, rods, etc.
The classification of rocks is based on their genesis, i.e. way of origin. There are three major groups of rocks: igneous, or igneous, sedimentary and metamorphic.
Igneous rocks are formed in the process of solidification of silicate melts located in the bowels of the earth's crust under high pressure. These melts are called magma (from the Greek word for "ointment"). In some cases, magma penetrates into the thickness of the rocks lying above and solidifies at a greater or lesser depth; in others, it solidifies, pouring out on the Earth's surface in the form of lava.
Sedimentary rocks are formed as a result of the destruction of pre-existing rocks on the Earth's surface and the subsequent deposition and accumulation of the products of this destruction.
Metamorphic rocks are the result of metamorphism, i.e. transformations of pre-existing igneous and sedimentary rocks under the influence of a sharp increase in temperature, an increase or change in the nature of pressure (change from all-round pressure to oriented), as well as under the influence of other factors.
3.1. State of the Earth
The condition of the earth is characterized by temperature, humidity, physical structure and chemical composition. Human activity and the functioning of the flora and fauna can improve and worsen the indicators of the state of the earth. The main processes of impact on the land are: irretrievable withdrawal from agricultural activities; temporary withdrawal; mechanical impact; addition of chemical and organic elements; involvement in agricultural activities of additional territories (drainage, irrigation, deforestation, reclamation); heating; self-renewal.
