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 ocean faults to 70-75 km under high mountain structures (Khain, Lomise, 1995). The thickness of the crust on the continents, determined by the increase in the speed of 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. First type of earth's crust was named oceanic,second- continental.

Ocean crust occupies 56% of the earth's surface and has a small thickness of 5–6 km. Its structure consists of three layers (Khain and Lomise, 1995).

First, or sedimentary, a layer no more than 1 km thick occurs in the central part of the oceans and reaches a thickness of 10–15 km at their periphery. It is completely absent from the axial zones of mid-ocean ridges. The composition of the layer includes clayey, siliceous and carbonate deep-sea pelagic sediments (Fig. 6.1). Carbonate sediments are distributed no deeper than the critical depth of carbonate accumulation. Closer to the continent there appears an admixture of clastic material carried from the land; these are the so-called hemipelagic sediments. The speed of propagation of longitudinal seismic waves here is 2–5 km/s. The age of sediments in this layer does not exceed 180 million years.

Second layer in its main upper part (2A) it is composed of basalts with rare and thin pelagic interlayers

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 ocean spreading zone (Khain and Lomise, 1995). Legend: 1 –

pelagic sediments; 2 – erupted basalts; 3 – complex of parallel dikes (dolerites); 4 – upper (not layered) gabbros and gabbro-dolerites; 5, 6 – layered complex (cumulates): 5 – gabbroids, 6 – ultrabasites; 7 – tectonized peridotites; 8 – basal metamorphic aureole; 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 covers of massive basalts also occur. 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 speed of longitudinal seismic waves is 4.5–5.5 km/s.

Third layer The oceanic crust consists of holocrystalline igneous rocks of basic and subordinate ultrabasic composition. In its upper part, rocks of the gabbro type are usually developed, and the lower part is made up of a “banded complex” consisting of alternating gabbro and ultra-ramafites. The thickness of the 3rd layer is 5 km. The speed 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 ocean-type crust, is not limited in its distribution to the ocean floor, but is also developed in deep-sea basins of marginal seas, such as the Sea of ​​Japan, the South Okhotsk (Kuril) basin of the Sea of ​​Okhotsk, the Philippine, Caribbean and many others

seas. In addition, there are serious reasons to suspect that in the deep depressions of continents and shallow internal and marginal seas such as the Barents, 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 million years. However, within the folded belts of the continents we also find much more ancient, up to the Early Precambrian, ocean-type crust, represented by the so-called ophiolite complexes(or simply ophiolites). This term belongs to the German geologist G. Steinmann and was proposed by him at the beginning of the 20th century. to designate the characteristic “triad” of rocks usually found together in the central zones of folded systems, namely serpentinized ultramafic rocks (analogous to layer 3), gabbro (analogous to layer 2B), basalts (analogous to layer 2A) and radiolarites (analogous to layer 1). The essence of this rock paragenesis has long been interpreted erroneously; in particular, gabbros and hyperbasites were considered intrusive and younger than basalts and radiolarites. Only in the 60s, when the first reliable information about the composition of the ocean crust was obtained, it became obvious that ophiolites are the ocean 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 moving belts.

Crustal structures of the oceans

Areas of continuous distribution oceanic crust expressed in the relief of the Earth oceanicdepressions. Within the ocean basins, two largest elements are distinguished: oceanic platforms And oceanic orogenic belts. Ocean platforms(or tha-lassocratons) in the bottom topography have the appearance of extensive abyssal flat or hilly plains. TO oceanic orogenic belts These include mid-ocean ridges that have a height above the surrounding plain of 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 3-5 km, indicating the dominance of crustal extension in these areas. They are characterized by high seismicity, sharply increased heat flow, and 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 is transition zone between continent and ocean. This is the area of ​​maximum dissection of the earth's surface, where there are island arcs, characterized by high seismicity and modern andesitic and andesite-basaltic volcanism, deep-sea trenches and deep-sea depressions of marginal seas. The sources of earthquakes here form a seismofocal zone (Benioff-Zavaritsky zone), plunging under the continents. The transition zone is most

clearly manifested 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, Lomise, 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 ocean basins-microcontinents. Nevertheless total area The development of the continental crust is less than that of the oceanic crust, and makes up 41% of the earth's surface. The average thickness of the continental crust is 35-40 km; it decreases towards the margins of continents and within microcontinents and increases under mountain structures to 70-75 km.

All in all, continental crust, like the oceanic one, has a three-layer structure, but the composition of the layers, especially the lower two, differs significantly from those observed in the oceanic crust.

1. sedimentary layer, commonly referred to as the 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, forward and intermountain troughs of mountain belts. True, in these depressions the crust underlying the sediments and usually called consolidated, may already be closer in nature 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 speed of longitudinal waves in the sedimentary layer is 2.0-5.0 km/s with a maximum for carbonate rocks. The age range of rocks in the sedimentary cover is up to 1.7 billion years, i.e., an order of magnitude higher than 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 discovered to a depth of 12 km in the Kola well and to a much smaller 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 passed 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, and therefore it is often called granite-gneiss. The speed of longitudinal waves in it is 6.0-6.5 km/s. In the foundation of young platforms, which have a Riphean-Paleozoic or even Mesozoic age, and partly in the internal zones of young folded structures, the same layer is composed of less strongly metamorphosed (greenschist facies instead of amphibolite) rocks and contains fewer granites; that's why it is often called here granite-metamorphic layer, and typical longitudinal velocities in it are of the order of 5.5-6.0 km/s. The thickness of this crustal layer reaches 15-20 km on platforms and 25-30 km in mountain structures.

3. The lower layer of the consolidated crust. It was initially assumed that there was a clear seismic boundary between the two layers of the consolidated crust, which was named the Conrad boundary after its discoverer, a German geophysicist. The drilling of the wells just mentioned has cast doubt on the existence of such a clear boundary; sometimes, instead, seismicity detects not one, but two (K 1 and K 2) boundaries in the crust, which gave grounds to distinguish two layers in the lower crust (Fig. 6.2). The composition of the rocks composing the lower crust, as noted, is not sufficiently known, since it has not been reached by wells, and is exposed fragmentarily on the surface. Based

Rice. 6.2. Structure and thickness of the continental crust (Khain, Lomise, 1995). A - main types of section according to seismic data: I-II - ancient platforms (I - shields, II

Syneclises), III - shelves, IV - young orogens. K 1 , K 2 -Conrad surfaces, M-Mohorovicic surface, velocities are indicated for longitudinal waves; B - histogram of the distribution of thickness of the continental crust; B - generalized strength profile

General considerations, V.V. Belousov came to the conclusion that the lower crust should be dominated, on the one hand, by rocks at a higher stage of metamorphism and, on the other hand, by rocks of a more basic composition than in the upper crust. That's why he called this layer of cortex gra-nullite-mafic. 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. Currently, most geophysicists distinguish the upper and lower crust on another basis - by their excellent rheological properties: the upper crust is hard and brittle, the lower crust is plastic. The speed of longitudinal waves in the lower crust is 6.4-7.7 km/s; belonging to the crust or mantle of the lower layers of this layer with velocities exceeding 7.0 km/s is often controversial.

Between the two extreme types of the earth's crust - oceanic and continental - there are transitional types. One of them - suboceanic crust - developed along the continental slopes and foothills and, possibly, underlies the bottom of the basins of some not very deep and wide marginal and internal seas. The suboceanic crust is a continental crust thinned to 15-20 km and penetrated by dikes and sills of basic igneous rocks.

bark It was exposed by deep-sea 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 in the case when the oceanic crust in ensimatic volcanic arcs turns into continental, but has not yet reached full “maturity”, having a reduced, less than 25 km, thickness and a lower degree of consolidation, which is reflected in lower velocities of seismic waves - no more than 5.0-5.5 km/s in the lower crust.

Some researchers identify two more types of ocean crust as special types, which were already discussed above; this is, firstly, the oceanic crust of the internal uplifts of the ocean thickened to 25-30 km (Iceland, etc.) and, secondly, the ocean-type crust, “built on” with a thick, up to 15-20 km, sedimentary cover (Caspian Basin and etc.).

Mohorovicic surface and composition of the upper manatii. The boundary between the crust and the mantle, usually seismically quite clearly expressed by a jump in longitudinal wave velocities from 7.5-7.7 to 7.9-8.2 km/s, is known as the Mohorovicic surface (or simply Moho and even M), named the Croatian geophysicist who established it. In the oceans, this boundary corresponds to the transition from a banded complex of the 3rd layer with a predominance of gabbroids to continuous serpentinized peridotites (harzburgites, lherzolites), less often dunites, in places protruding onto the bottom surface, and in the rocks of Sao Paulo in the Atlantic off the coast of Brazil and on o. Zabargad in the Red Sea, rising above the surface

the sea's fury. 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 Lomise, 1995).

The study of inclusions in lavas and kimberlites from pipes shows that beneath 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 are 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 minor quantities. Eclogites may be metamorphosed relics of oceanic crust, dragged into the mantle during the process of underthrusting this crust (subduction).

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 melting of basaltic rocks of the earth's crust from it. This “depleted” (“depleted”) mantle extends under the continents to a greater depth (encompassing all or almost all of its lithospheric part) than under the oceans, giving way deeper to the “undepleted” mantle. The average primary composition of the mantle should be close to spinel lherzolite or a hypothetical mixture of peridotite and basalt in a 3:1 ratio, named by the Australian scientist A.E. Ringwood pyrolite.

At a depth of about 400 km, a rapid increase in the speed 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 the middle mantle, or mesosphere - transition zone between the upper and lower mantle. The increase in the rates of elastic vibrations in the Golitsyn layer is explained by an increase in the density of the mantle material by approximately 10% due to the transition of some mineral species to others, with a more dense packing of atoms: olivine into spinel, pyroxene into garnet.

Lower mantle(Hain, Lomise, 1995) begins at a depth of about 670 km. The lower mantle should be composed mainly of perovskite (MgSiO 3) and magnesium wustite (Fe, Mg)O - products of further alteration of the minerals composing the middle mantle. The Earth's core in its outer part, according to seismology, is liquid, and the inner part is solid again. Convection in the outer core generates the Earth's main magnetic field. The composition of the core is accepted by the overwhelming majority of geophysicists as iron. But again, according to experimental data, it is necessary to allow for some admixture of nickel, as well as sulfur, or oxygen, or silicon, in order to explain the reduced core density compared to that determined for pure iron.

According to seismic tomography data, 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 the core, a transition layer with the index D is distinguished (the crust is designated by the index A, the upper mantle - B, the middle - C, the lower - D, the upper part of the lower mantle - 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 Mohorovicic boundary), the lithosphere and asthenosphere are purely physical, or rather rheological, concepts. The initial basis for identifying the asthenosphere is a weakened, plastic shell. underlying a more rigid and fragile lithosphere, there was a need to explain the fact of isostatic balance of the crust, discovered when measuring gravity at the foot of mountain structures. It was initially expected that such structures, especially those as grand as the Himalayas, would create an excess of gravity. However, when in the middle of the 19th century. corresponding measurements were made, it turned out that such attraction was not observed. Consequently, even large unevenness in the relief of the earth's surface is somehow compensated, balanced at depth so that at the level of the earth's surface there are no significant deviations from the average values ​​of gravity. Thus, the researchers came to the conclusion that there is a general tendency of the earth’s crust to balance at the expense of the mantle; this phenomenon is called isostasia(Hain, Lomise, 1995) .

There are two ways to implement isostasy. The first is that mountains have roots immersed in the mantle, i.e. isostasy is ensured by variations in the thickness of the earth's crust and the lower surface of the latter has a relief opposite to the relief of the earth's surface; this is the hypothesis of the English astronomer J. Airy

(Fig. 6.3). On a regional scale, it is usually justified, since mountain structures actually have thicker crust and the maximum thickness of the crust is observed at the highest of them (Himalayas, Andes, Hindu Kush, Tien Shan, etc.). But another mechanism for the implementation of isostasy is also possible: areas of increased relief should be composed of less dense rocks, and areas of lower relief should be composed of more dense ones; This is the hypothesis of another English scientist, J. Pratt. In this case, the base of the earth's crust may even be horizontal. The balance of continents and oceans is achieved by a combination of both mechanisms—the crust under the oceans is both 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—are found in island arcs and associated deep-sea trenches.

In order for the desire for isostatic equilibrium to be effective, i.e., under additional load, the crust would sink, and when the load is removed, it would rise, it is necessary that there be a sufficiently plastic layer under the crust, capable of flowing from areas of increased geostatic pressure to areas low pressure. It was for this layer, initially identified hypothetically, that the American geologist J. Burrell proposed the name asthenosphere, which means “weak shell”. 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 - by 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 increase, natural with an increase in pressure, in the speed of seismic waves. Subsequently, another method of establishing the asthenosphere appeared—the method of magnetotelluric sounding, in which the asthenosphere manifests itself as a zone of reduced 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 carries along lithospheric plates and causes their horizontal movements. The rise of the surface of the asthenosphere leads to the rise of the lithosphere, and in the extreme case, to a break in its continuity, the formation of a separation and subsidence. The latter also leads to the outflow of the asthenosphere.

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 mid-ocean ridges, especially on the East Pacific Rise, the top 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

the lower crust, and 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 has raised doubts among some researchers about its existence beneath such areas and led them to an alternative idea that the asthenosphere as a continuous shell, i.e., the geosphere, does not exist, but there is a series of disconnected “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 these include the above examples of areas of modern and ancient glaciation - Greenland, etc.

The reason that the asthenosphere is not easy to detect everywhere is obviously a change in its viscosity laterally.

Basic structural elements continental crust

On 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 continental crust, having an isometric shape and a two-story structure (Fig. 6.4). Lower (first) structural floor – crystalline foundation, represented by highly dislocated metamorphosed rocks, intruded by intrusions. The upper (second) structural floor is gently lying sedimentary cover, weakly dislocated and unmetamorphosed. Exits to the day surface of the lower structural floor are called shield. Areas of the foundation covered by sedimentary cover are called stove. The thickness of the sedimentary cover of the plate is a few kilometers.

Example: on the East European Platform there are two shields (Ukrainian and Baltic) and the Russian plate.

Structures of the second floor of the platform (cover) There are negative (deflections, syneclises) and positive (anteclises). Syneclises have the shape of a saucer, and anteclises have the shape of an inverted saucer. The thickness of sediments 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 the layers on the wings is usually a few meters per 1 km. There are two definitions of these structures.

Definition: syneclise is a geological structure, the fall of the layers of which is directed from the periphery to the center. Anteclise is 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 sediments emerge, and along the edges

Rice. 6.4. Platform structure diagram. 1 - folded foundation; 2 - platform case; 3 faults (Historical Geology, 1985)

- more ancient. Anteclise is a geological structure, in the core of which more ancient sediments emerge, and at the edges - younger ones.

Definition: trough is an elongated (elongated) geological body that has 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, Transnistria-Black Sea, etc.).

There is a structure of the lower horizons of the cover - av-lacogene.

Definition: aulacogen - a narrow, elongated depression extending across the platform. Aulacogens are located in the lower part of the upper structural floor (cover) and can reach a length of up to hundreds of kilometers and a width of tens of kilometers. Aulacogens are formed under conditions of horizontal extension. Thick layers of sediments accumulate in them, which can be crushed into folds and are similar in composition to the formations of miogeosynclines. Basalts are present in the lower part of the section.

Example: Pachelma (Ryazan-Saratov) aulacogen, Dnieper-Donets aulacogen of the Russian plate.

History of the development of platforms. The history of development can be divided into three stages. First– geosynclinal, on which the formation of the lower (first) structural element (foundation) occurs. Second- aulacogenic, on which, depending on the climate, accumulation occurs

red-colored, gray-colored or carbon-bearing sediments in av-lacogenes. Third– slab, on which sedimentation occurs over a large area and the upper (second) structural floor (slab) is formed.

The process of precipitation accumulation usually occurs cyclically. Accumulates first transgressive maritime terrigenous formation, then - carbonate formation (maximum transgression, Table 6.1). During regression under arid climate conditions, salt-bearing red-flowered formation, and in conditions of a humid climate - paralytic coal-bearing formation. At the end of the sedimentation cycle, sediments are formed continental formations. At any moment 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 movable belts (folded areas) characteristic:

linearity of their contours;

the enormous thickness of accumulated sediments (up to 15-25 km);

consistency composition and thickness of these deposits along strike folded area and sudden changes across its strike;

presence of peculiar formations- rock complexes formed at certain stages of development of these areas ( slate, flysch, spilito-keratophyric, molasse and other formations);

intense effusive and intrusive magmatism (large granite intrusions-batholiths are especially characteristic);

strong regional metamorphism;

7) strong folding, an abundance of faults, including

thrusts indicating the dominance of compression. Folded areas (belts) arise in place of geosynclinal areas (belts).

Definition: geosyncline(Fig. 6.5) - a mobile region 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 a geosyncline.

First stage(actually geosynclinal) characterized by a predominance of subsidence. High precipitation rate in a geosyncline - this is result of stretching of the earth's crust and its deflection. IN first half firststages Sandy-clayey and clayey sediments usually accumulate (as a result of metamorphism, they then form black clayey shales, released in slate formation) and limestones. Subduction may be accompanied by ruptures through which mafic magma rises and erupts under submarine conditions. The resulting rocks after metamorphism, together with accompanying subvolcanic formations, give spilite-keratophyric formation. At the same time, siliceous rocks and jasper are usually formed.

oceanic

Rice. 6.5. Scheme of the geosync structure

linali on a schematic cross-section through the Sunda Arc in Indonesia (Structural Geology and Plate Tectonics, 1991). Legend: 1 – sediments and sedimentary rocks; 2 – volcano-

nic breeds; 3 – basement conti-metamorphic rocks

Specified formations accumulate simultaneously, But in different areas. Accumulation spilito-keratophyric formation usually occurs in the inner part of the geosyncline - in eugeosynclines. For eugeo-synclines Characterized by the formation of thick volcanogenic strata, usually of basic composition, and the introduction of intrusions of gabbro, diabase and ultrabasic rocks. In the marginal part of the geosyncline, along its border with the platform, there are usually located miogeosynclines. Mainly terrigenous and carbonate strata accumulate here; There are no volcanic rocks, and 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 mid first stage due to different rates of subsidence, areas are formed in different parts of the geosyncline relative rise(intrageoantic-linali) And relative descent(intrageosynclines). At this time, the intrusion of small intrusions of plagiogranites may occur.

In second half of the first stage As a result of the appearance of internal uplifts, the sea in the geosyncline becomes shallower. now this archipelago, separated by straits. Due to shallowing, the sea is advancing on adjacent platforms. Limestones, thick sandy-clayey rhythmically built strata, accumulate in the geosyncline, forming flysch for-216

mation; there is an outpouring of lavas of intermediate composition that make up porphyritic formation.

TO end of the first stage intrageosynclines disappear, intrageoanticlines merge into one central uplift. This is a general inversion; she matches main phase of folding in a geosyncline. Folding is usually accompanied by the intrusion of large synorogenic (simultaneous with folding) granite intrusions. Rocks are crushed into folds, often complicated by thrusts. All this causes regional metamorphism. In place of intrageosynclines there arise synclinorium- complexly constructed structures of the synclinal type, and in place of intrageoanticlines - anticlinoria. The geosyncline “closes”, turning into a folded area.

In the structure and development of a 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, and the division of the geosyncline into structural-facial zones that differ in the composition of sediments, their thickness, magmatism and the nature of the structures. Inside a geosyncline they sometimes distinguish middle massifs, limited by deep faults. These are blocks of more ancient folding, composed of rocks from the foundation on which the geosyncline was formed. In terms of the composition of sediments and their thickness, the middle massifs are similar to platforms, but they are distinguished by strong magmatism and folding of rocks, mainly along the edges of the massif.

The second stage of geosyncline development called orogenic and is characterized by a predominance of uplifts. Sedimentation occurs in limited areas along the periphery of the central uplift - in marginal deflections, arising along the border of the geosyncline and the platform and partially overlapping the platform, as well as in intermountain troughs that sometimes form inside the central uplift. The source of sediment is the destruction of the constantly rising central rise. First halfsecond stage this rise probably has a hilly topography; when it is destroyed, marine and sometimes lagoonal sediments accumulate, forming lower molasse formation. Depending on climatic conditions, this may be coal-bearing paralic or salty thickness. At the same time, the introduction of large granite intrusions - batholiths - usually occurs.

In the second half of the stage the rate of uplift of the central uplift sharply increases, which is accompanied by its splits and collapse of individual sections. This phenomenon is explained by the fact that, as a result of folding, metamorphism, and the introduction of intrusions, the folded region (no longer a geosyncline!) becomes rigid and reacts to ongoing uplift with rifts. The sea is leaving this area. 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. The splitting of the arched part of the uplift is accompanied by ground volcanism; usually these are lavas of acidic composition, which, together with

subvolcanic formations give porphyry formation. Fissure alkaline and small acidic 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 mountain area that arose on the site of the geosyncline is destroyed, the territory gradually levels out and becomes a platform. The geosyncline turns from an area of ​​sediment accumulation into an area of ​​destruction, from a mobile territory into a sedentary, rigid, leveled territory. Therefore, the range of movements on the platform is small. Usually the sea, even shallow, covers vast areas here. This territory no longer experiences such strong subsidence as before, therefore the thickness of the sediments is much less (on average 2-3 km). The subsidence is repeatedly interrupted, so frequent breaks in sedimentation are observed; then weathering crusts can form. There are no energetic uplifts accompanied by folding. Therefore, the newly formed thin, usually shallow-water sediments on the platform are not metamorphosed and lie horizontally or slightly inclined. 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 plate tectonics

Plate tectonics(Structural Geology and Plate Tectonics, 1991) is a model that was created to explain the observed pattern of distribution of deformations and seismicity in the outer shell of the Earth. It is based on extensive geophysical data acquired in the 1950s and 1960s. The theoretical foundations of plate tectonics are based on two premises.

The outermost layer of the Earth, called lithosphere, lies directly on a layer called actenosphere, 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 tectonic processes with intense energy exchange operate at the boundaries between plates.

Although the thickness of the lithosphere cannot be measured with great precision, researchers agree that within plates it varies from 70-80 km under the oceans to a maximum of over 200 km under some parts of the continents, with an average of about 100 km. The asthenosphere underlying the lithosphere extends down to a depth of about 700 km (the maximum depth for the distribution of sources of deep-focus earthquakes). Its strength increases with depth, and some seismologists believe that its lower limit is

single lines - transform faults (slip faults); areas of the continental crust that are subject to active faulting are speckled (Structural geology and plate tectonics, 1991)

Tsa is located at a depth of 400 km and coincides with a slight change in physical parameters.

Boundaries between plates are divided into three types:

divergent;

convergent;

transform (with displacements along strike).

At divergent plate boundaries, represented mainly by rifts, new formation of the lithosphere occurs, which leads to the spreading of the ocean floor (spreading). At convergent plate boundaries, the lithosphere is submerged into the asthenosphere, i.e., it is absorbed. At transform boundaries, two lithospheric plates slide relative to each other, and lithosphere matter is neither created nor destroyed on them .

All lithospheric plates continuously move relative to each other. It is assumed that the total area of ​​all slabs remains constant over a significant period of time. At a sufficient distance from the edges of the plates, horizontal deformations inside them are insignificant, which allows the plates to be considered rigid. Since displacements along transform faults occur along their strike, plate movement should be parallel to modern transform faults. Since all this happens on the surface of a 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 given time, an axis, or pole of rotation, can be determined. As you move away from this pole (up to the corner

distance of 90°), spreading rates naturally increase, but the angular velocity for any given pair of plates relative to their pole of rotation is constant. Let us 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 crustal processes because it fits well with known observational data, provides elegant explanations 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 breakup of Pangea, but also in pre-Pangean times. The cycle of opening and closing of oceans relative to adjacent continental margins is now called Wilson cycle.

In Fig. Figure 6.7 provides a schematic explanation of the basic concept of the Wilson cycle within the framework of ideas about the evolution of lithospheric plates.

Rice. 6.7, but represents beginning of the Wilson cycle– the initial stage of continental breakup and formation of the accretionary plate margin. 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 continents continue to separate, a rift valley (Fig. 6.7, 6) and a small ocean (Fig. 6.7, c) develop. These are the stages of early ocean opening in the Wilson cycle.. The African Rift and the Red Sea are suitable examples. With the continuation of the drift of separated continents, accompanied by the symmetrical accretion of new lithosphere on the margins of plates, shelf sediments accumulate at the continent-ocean boundary due to 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 ocean of the Atlantic type.

From observations of oceanic trenches, their relationship to seismicity, and reconstruction from patterns of oceanic magnetic anomalies around the trenches, it is known that the oceanic lithosphere is dismembered and subducted into the mesosphere. In Fig. 6.7, d shown ocean with stove, which has simple margins of lithosphere accretion and absorption, – this is the initial stage of ocean closure V Wilson cycle. The dismemberment of the lithosphere in the vicinity of the continental margin leads to the transformation of the latter into an Andean-type orogen as a result of tectonic and volcanic processes occurring at the absorbing plate boundary. If this dismemberment occurs at a considerable distance from the continental margin towards the ocean, then an island arc like the Japanese Islands is formed. Oceanic absorptionlithosphere leads to a change in the geometry of the plates and in the end

ends to complete disappearance of the accretionary plate margin(Fig. 6.7, f). During this time, the opposite continental shelf may continue to expand, becoming an Atlantic-type semi-ocean. As the ocean shrinks, the opposite continental margin is eventually drawn into the plate absorption mode and participates in the development Andean-type accretionary orogen. 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 breaks off below, under a growing Himalayan-type orogen, and advances final orogenic stageWilson cyclewith a mature mountain belt, representing the seam between the newly united continents. Antipode Andean-type accretionary orogen is Himalayan-type collisional orogen.

Types of bark. IN different regions the ratio between different rocks in the earth's crust is different, and a dependence of the composition of the crust on the nature of the relief and the internal structure of the territory is revealed. The results of geophysical research and deep drilling made it possible to identify two main and two transitional types of the earth's crust. The main types mark such global structural elements of the crust as continents and oceans. These structures are perfectly expressed in the Earth's topography, and they are characterized by continental and oceanic types of crust.

1 - water, 2 - sedimentary layer, 3 - interlayering of sedimentary rocks and basalts, 4 - basalts and crystalline ultrabasic rocks, 5 - granite-metamorphic layer, 6 - granulite-mafic layer, 7 - normal mantle, 8 - decompressed mantle.

Continental crust developed under the continents and, as already mentioned, has different thicknesses. Within the platform areas corresponding to the continental plains, this is 35-40 km, in young mountain structures - 55-70 km. The maximum thickness of the earth's crust - 70-75 km - is established under the Himalayas and the Andes. Two strata are distinguished in the continental crust: the upper - sedimentary and the lower - consolidated crust. The consolidated crust contains two different-velocity layers: the upper granite-metamorphic layer (according to outdated ideas, this is a granite layer), composed of granites and gneisses, and the lower granulite-mafic layer (according to outdated ideas, this is a basalt layer), composed of highly metamorphosed basic rocks such as gabbro or ultrabasic igneous rocks. The granite-metamorphic layer was studied from cores of ultra-deep wells; granulite-mafic - according to geophysical data and dredging results, which still makes its existence hypothetical.

In the lower part of the upper layer, a zone of weakened rocks is found, not much different from it in composition and seismic characteristics. The reason for its occurrence is the metamorphism of rocks and their decompression due to the loss of constitutional water. It is likely that the rocks of the granulite-mafic layer are still the same rocks, but even more highly metamorphosed.

Ocean crust characteristic of the World Ocean. It differs from the continental one in power and composition. Its thickness ranges from 5 to 12 km, averaging 6-7 km. From top to bottom, three layers are distinguished in the ocean crust: the upper layer of loose marine sedimentary rocks up to 1 km thick; middle, represented by interlayering of basalts, carbonate and siliceous rocks, 1-3 km thick; the lower one, composed of basic rocks such as gabbro, often altered by metamorphism to amphibolites, and ultrabasic amphibolites, thickness 3.5-5 km. The first two layers were penetrated by drill holes, the third was characterized by dredging material.

Suboceanic crust developed under the deep-sea basins of the marginal and inland seas (Black, Mediterranean, Okhotsk, etc.), and also found in some deep depressions on land (the central part of the Caspian basin). The thickness of the suboceanic crust is 10-25 km, and it is increased mainly due to the sedimentary layer lying directly on the lower layer of the ocean crust.

Subcontinental crust characteristic of island arcs (Aleutian, Kuril, South Antilles, etc.) and continental margins. In structure it is close to the continental crust, but has a smaller thickness - 20-30 km. A feature of the subcontinental crust is the unclear boundary between layers of consolidated rocks.

Thus, Various types The earth's crust clearly divides the Earth into oceanic and continental blocks. The high position of the continents is explained by a thicker and less dense crust, and the submerged position of the ocean floor is explained by a thinner, but denser and heavier crust. The shelf area is underlain by continental crust and is the underwater end of the continents.

Structural elements of the cortex

In addition to being divided into such planetary structural elements as oceans and continents, the earth's crust (and lithosphere) reveals seismic (tectonically active) and aseismic (quiet) regions. The inner regions of the continents and the beds of the oceans - continental and oceanic platforms - are calm. Between the platforms there are narrow seismic zones, which are marked by volcanism, earthquakes, and tectonic movements - the site. These zones correspond to mid-ocean ridges and junctions of island arcs or marginal mountain ranges and deep-sea trenches on the ocean periphery.

The following structural elements are distinguished in the oceans:

- mid-ocean ridges - mobile belts with axial rifts such as grabens;
- oceanic platforms - calm areas of abyssal basins with uplifts complicating them.

On continents, the main structural elements are:

Mountain structures (orogens: from the Greek “oros” - mountain), which, like mid-ocean ridges, can exhibit tectonic activity;
- platforms - mostly tectonically calm vast territories with a thick cover of sedimentary rocks.

Mountain structures have complex internal structure and the history of geological development. Among them are orogens composed of young pre-Paleogene marine sediments (Carpathians, Caucasus, Pamir), and more ancient ones formed from Early Mesozoic, Paleozoic and Precambrian rocks that experienced folding movements. These ancient ridges were denuded, often to the base, and in modern times experienced a secondary uplift. These are the revived mountains (Tian Shan, Altai, Sayan Mountains, ridges of the Baikal region and Transbaikalia).

Mountain structures are separated and bordered by low areas - intermountain troughs and depressions, which are filled with products of the destruction of ridges. For example, the Greater Caucasus is bordered by the West Kuban, East Kuban and Terek-Caspian foredeeps, and is separated from the Lesser Caucasus by the Rioni and Kura intermontane depressions.

But not all ancient mountain structures were involved in re-orogenesis. Most of them, after leveling, slowly sank, were flooded by the sea, and a layer of marine sediments was layered onto the relics of the mountain ranges. This is how the platforms were formed. IN geological structure platforms, there are always two structural-tectonic levels: the lower one, composed of metamorphosed remains of former mountains, which is the foundation, and the upper one, represented by sedimentary rocks.

Platforms with a Precambrian foundation are considered ancient, while platforms with a Paleozoic and Early Mesozoic foundation are considered young. Young platforms are located between the ancient ones or border them. For example, between the ancient East European and Siberian platforms there is a young West Siberian platform, and on the southern and southeastern edge of the East European platform the young Scythian and Turanian platforms begin. Within the platforms, large structures of an anticlinal and synclinal profile, called anteclises and synclises, are distinguished.

So, the platforms are ancient denudated orogens, not affected by later (young) mountain-building movements.

In contrast to the quiet platform regions on Earth, there are tectonically active geosynclinal regions. The geosynclinal process can be compared to the work of a huge deep cauldron, where a new light continental crust is “cooked” from ultrabasic and basic magma and lithosphere material, which, as it floats up, builds up continents in the marginal (Pacific) and welds them together in intercontinental (Mediterranean) geosynclines. This process ends with the formation of folded mountain structures, in the arch of which volcanoes can operate for a long time - the site. Over time, the growth of mountains stops, volcanism dies out, the earth’s crust enters a new cycle of its development: the leveling of the mountain structure begins.

Thus, where mountain ranges are now located, there used to be geosynclines. Large anticlinal and synclinal structures in geosynclinal regions are called anticlinoria and synclinoria.

The continental crust has a three-layer structure:

1) Sedimentary layer formed mainly by sedimentary rocks. Clays and shales predominate here, and sandy, carbonate and volcanic rocks are widely represented. In the sedimentary layer there are deposits of minerals such as coal, gas, and oil. All of them are of organic origin.

2) “Granite” layer consists of metamorphic and igneous rocks, similar in their properties to granite. The most common here are gneisses, granites, crystalline schists, etc. The granite layer is not found everywhere, but on continents where it is well expressed, its maximum thickness can reach several tens of kilometers.

3) “Basalt” layer formed by rocks close to basalts. These are metamorphosed igneous rocks, denser than the rocks of the “granite” layer.

22. Structure and development of movable belts.

A geosyncline is a mobile zone of high activity, significant dissection, characterized in the early stages of its development by the predominance of intense subsidence, and in the final stages by intense uplift, accompanied by significant fold-thrust deformations and magmatism.

Mobile geosynclinal belts are an extremely important structural element of the earth's crust. They are usually located in the transition zone from the continent to the ocean and in the process of their evolution form the continental crust. There are two main stages in the development of mobile belts, regions and systems: geosynclinal and orogenic.

In the first of them, two main stages are distinguished: early geosynclinal and late geosynclinal.

Early geosynclinal the stage is characterized by processes of stretching, expansion of the ocean floor through spreading and, at the same time, compression in the marginal zones

Late geosynclinal the stage begins at the moment of complication of the internal structure of the mobile belt, which is caused by compression processes, which are increasingly manifested in connection with the beginning of the closure of the ocean basin and the counter movement of lithospheric plates.

Orogenic stage replaces the late geosynclinal stage. The orogenic stage of the development of mobile belts consists of the fact that first, ahead of the front of growing uplifts, forward troughs appear in which thick strata of fine clastic rocks with coal- and salt-bearing strata - thin molasse - accumulate.

23. Platforms and stages of their development.

Platform, in geology - one of the main deep structures of the earth's crust, characterized by low intensity of tectonic movements, magmatic activity and flat topography. These are the most stable and calm areas of the continents.

In the structure of the platforms, two structural floors are distinguished:

1) Foundation. The lower floor is composed of metamorphic and igneous rocks, crushed into folds and broken by numerous faults.

2) Case. The upper structural floor is composed of gently lying non-metamorphosed layered strata - sedimentary, marine and continental deposits

By age, structure and development history continental platforms are divided into two groups:

1) Ancient platforms occupy about 40% of the continents' area

2) Young platforms occupy a significantly smaller area of ​​the continents (about 5%) and are located either along the periphery of ancient platforms or between them.

Stages of platform development.

1) Initial. Cratonization stage, is characterized by a predominance of uplifts and fairly strong final basic magmatism.

2) Aulacogenic stage, which gradually follows from the previous one. Gradually aulacogens (a deep and narrow graben in the basement of an ancient platform, covered by a platform cover. It is an ancient rift filled with sediments.) develop into depressions, and then into syneclises. As the syneclises grow, they cover the entire platform with a sedimentary cover, and its slab stage of development begins.

3) Slab stage. On ancient platforms it covers the entire Phanerozoic, and on young ones it begins from the Jurassic period of the Mesozoic era.

4) Activation stage. Epiplatform orogens ( mountain, mountain-fold structure that arose in place of a geosyncline)

Types of Earth's crust: oceanic, continental

The Earth's crust (the solid shell of the Earth above the mantle) consists of two types of crust and has two types of structure: continental and oceanic. The division of the Earth's lithosphere into the crust and upper mantle is quite conventional; the terms oceanic and continental lithosphere are often used.

Earth's continental crust

The continental crust of the Earth (continental crust, continental crust) which consists of sedimentary, granite and basalt layers. The continental crust has an average thickness of 35-45 km, with a maximum thickness of up to 75 km (under mountain ranges).

The structure of the continental crust “American style” is somewhat different. It contains layers of igneous, sedimentary and metamorphic rocks.

Continental crust has another name "sial" - because. granites and some other rocks contain silicon and aluminum - hence the origin of the term sial: silicon and aluminum, SiAl.

Average density continental crust - 2.6-2.7 g/cm³.

Gneiss is a (usually loose layered structure) metamorphic rock composed of plagioclase, quartz, potassium feldspar, etc.

Granite is “an acidic igneous intrusive rock. It consists of quartz, plagioclase, potassium feldspar and micas” (article “Granite”, link at the bottom of the page). Granites consist of feldspars and quartz. Granites on other bodies solar system not detected.

Oceanic crust of the Earth

As far as is known, a granite layer has not been found in the Earth’s crust at the bottom of the oceans; the sedimentary layer of the crust lies immediately on the basalt layer. The oceanic type of crust is also called "sima", the rocks are dominated by silicon and magnesium - similar to sial, MgSi.

The thickness of the oceanic crust (thickness) is less than 10 kilometers, usually 3-7 kilometers. The average density of the sub-oceanic crust is about 3.3 g/cm³.

It is believed that oceanic is formed in mid-ocean ridges and absorbed in subduction zones (why is not very clear) - as a kind of transporter from the growth line in the mid-ocean ridge to the continent.

Differences between continental and oceanic types of crust, hypotheses

All information about the structure of the earth's crust is based on indirect geophysical measurements, except for individual surface injections with wells. Moreover, geophysical research is mainly research into the speed of propagation of longitudinal elastic waves.

It can be argued that the “acoustics” (the passage of seismic waves) of the continental-type crust differs from the “acoustics” of the oceanic-type crust. And everything else is more or less plausible hypotheses based on indirect data.

"... in structure and material composition, both main types of lithosphere are radically different from each other, and the “basalt layer” of geophysicists in them is the same only in name, as well as the lithospheric mantle. These types of lithosphere also differ in age - if within the continental segments, the entire spectrum of geological events is established starting from approximately 4 billion years, then the age of the rocks of the bottom of modern oceans does not exceed the Triassic, and the age of the proven most ancient fragments of the oceanic lithosphere (ophiolites in the understanding of the Penrose Conference) does not exceed 2 billion years (Kontinen, 1987; Scott et al., 1998).Within the modern Earth, the oceanic lithosphere accounts for ~60% of the solid surface. In this regard, the question naturally arises - has there always been such a ratio between these two types of lithosphere or has it changed over time? and in general - have they both always existed? Answers to these questions, obviously, can be given both by the analysis of geological processes at the destructive boundaries of lithospheric plates, and by the study of the evolution of tectono-magmatic processes in the history of the Earth."
“Where does the ancient continental lithosphere disappear?”, E.V. Sharkov

What then are these - lithospheric plates?

http://earthquake.usgs.gov/learn/topics/plate_tectonics/
Earthquakes and Plate Tectonics:
"...a concept which has revolutionized thinking in the Earth"s sciences in the last 10 years. The theory of plate tectonics combines many of the ideas about continental drift (originally proposed in 1912 by Alfred Wegener in Germany) and sea-floor spreading (suggested originally by Harry Hess of Princeton University)."

Additional information on the structure of the lithosphere and sources

The Earth's Crust
Earth's crust
Earthquake Hazards Program - USGS.
Earthquake Hazards Program - United States Geological Survey.
On the map Globe shown:
borders tectonic plates;
thickness of the earth's crust, in kilometers.
For some reason, the map does not show the boundaries of tectonic plates on the continents; boundaries of continental plates and oceanic plates - boundaries of the earth's crust of continental and oceanic types.

Structure and age of the earth's crust

The main elements of the surface relief of our planet are continents and ocean basins. This division is not random; it is due to profound differences in the structure of the earth's crust under the continents and oceans. Therefore, the earth's crust is divided into two main types: continental and oceanic crust.

The thickness of the earth's crust varies from 5 to 70 km, and it differs sharply under the continents and the ocean floor. The thickest crust under the mountainous regions of the continents is 50-70 km, under the plains its thickness decreases to 30-40 km, and under the ocean floor it is only 5-15 km.

The earth's crust of the continents consists of three thick layers, differing in their composition and density. Upper layer it is composed of relatively loose sedimentary rocks, the middle one is called granite, and the lower one is called basalt. The names “granite” and “basalt” come from the similarity of these layers in composition and density to granite and basalt.

The earth's crust under the oceans differs from the continental crust not only in its thickness, but also in the absence of a granite layer. Thus, under the oceans there are only two layers - sedimentary and basaltic. There is a granite layer on the shelf; continental-type crust is developed here. The change from continental to oceanic crust occurs in the zone of the continental slope, where the granite layer becomes thinner and breaks off. The oceanic crust is still very poorly studied compared to the continental crust.

The age of the Earth is now estimated at approximately 4.2-6 billion years according to astronomical and radiometric data. The age of the oldest rocks of the continental crust studied by man is up to 3.98 billion years old (southwestern part of Greenland), and the rocks of the basalt layer are over 4 billion years old. There is no doubt that these rocks are not the primary substance of the Earth. The prehistory of these ancient rocks lasted many hundreds of millions, and perhaps billions of years. Therefore, the age of the Earth is approximately estimated to be up to 6 billion years.

Structure and development of the continental crust

The largest structures of the continental crust are geosynclinal fold belts and ancient platforms. They differ greatly from each other in their structure and history of geological development.

Before moving on to a description of the structure and development of these main structures, it is necessary to talk about the origin and essence of the term “geosyncline”. This term comes from the Greek words “geo” - Earth and “synclino” - deflection. It was first used by the American geologist D. Dana more than 100 years ago, while studying the Appalachian Mountains. He found that the marine Paleozoic sediments that make up the Appalachians have a maximum thickness in the central part of the mountains, much greater than on their slopes. Dana explained this fact absolutely correctly. During the period of sedimentation in the Paleozoic era, in place of the Appalachian Mountains there was a sagging depression, which he called a geosyncline. In its central part, subsidence was more intense than on the wings, as evidenced by the large thickness of sediments. Dana confirmed his conclusions with a drawing depicting the Appalachian geosyncline. Given that Paleozoic sedimentation occurred under marine conditions, he plotted down from a horizontal line—the assumed sea level—all the measured sediment thicknesses in the center and slopes of the Appalachian Mountains. The picture shows a clearly defined large depression in the place of the modern Appalachian Mountains.

At the beginning of the 20th century, the famous French scientist E. Og proved that geosynclines played a big role in the history of the development of the Earth. He established that folded mountain ranges formed in place of geosynclines. E. Og divided all areas of the continents into geosynclines and platforms; he developed the fundamentals of the study of geosynclines. A great contribution to this doctrine was made by Soviet scientists A.D. Arkhangelsky and N.S. Shatsky, who established that the geosynclinal process not only occurs in individual troughs, but also covers vast areas of the earth's surface, which they called geosynclinal regions. Later, huge geosynclinal belts began to be identified, within which several geosynclinal areas are located. In our time, the doctrine of geosynclines has grown into a substantiated theory of geosynclinal development of the earth's crust, in the creation of which Soviet scientists play a leading role.

Geosynclinal fold belts are mobile sections of the earth's crust, the geological history of which was characterized by intense sedimentation, repeated folding processes and strong volcanic activity. Thick layers of sedimentary rocks accumulated here, igneous rocks formed, and earthquakes often occurred. Geosynclinal belts occupy vast areas of continents, located between ancient platforms or along their edges in the form of wide stripes. Geosynclinal belts arose in the Proterozoic; they have a complex structure and a long history of development. There are 7 geosynclinal belts: Mediterranean, Pacific, Atlantic, Ural-Mongolian, Arctic, Brazilian and Intra-African.

Ancient platforms are the most stable and sedentary parts of the continents. Unlike geosynclinal belts, ancient platforms experienced slow oscillatory movements, sedimentary rocks of usually low thickness accumulated within them, there were no folding processes, and volcanism and earthquakes rarely occurred. Ancient platforms form sections of continents that are the skeletons of all continents. These are the most ancient parts of the continents, formed in the Archean and Early Proterozoic.

On modern continents there are from 10 to 16 ancient platforms. The largest are the East European, Siberian, North American, South American, African-Arabian, Hindustan, Australian and Antarctic.