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 - 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 ophiolitic 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, the total area of development of the continental crust is smaller than that of the oceanic crust, amounting to 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.
The main structural elements of the 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.
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 Earth’s mass.
Eight elements - oxygen, silicon, hydrogen, aluminum, iron, magnesium, calcium, sodium - form 99.5% of the earth's crust.
On continents, the crust is three-layered: sedimentary rocks cover granite rocks, and granite rocks overlie basaltic rocks. Under the oceans the crust is of the “oceanic”, two-layer type; sedimentary rocks simply lie on basalts, there is no granite layer. There is also a transitional type of the earth's crust (island-arc zones on the margins of the oceans and some areas on continents, for example).
The earth's crust is greatest in mountainous regions (under the Himalayas - over 75 km), average in platform areas (under the West Siberian Lowland - 35-40, within the Russian Platform - 30-35), and least in the central regions of the oceans (5-7 km).
The predominant part of the earth's surface is the plains of 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 steep bend of the bottom, turns into a continental slope (the slope varies from 15-17 to 20-30° ). The slopes gradually level out and turn into abyssal plains (depths 3.7-6.0 km). The oceanic trenches have the greatest depths (9-11 km), the vast majority of which are located on the northern and western outskirts.
The earth's crust formed gradually: first a basalt layer was formed, then a granite layer; the sedimentary layer continues to form to this day.
The deep strata of the lithosphere, which are studied by geophysical methods, have a rather complex and still insufficiently studied structure, just like 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 about 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 t/cm2, it increases to 13-17 g/cm3. The nature of the increase in the Earth's deep temperature has also been established. At a depth of 100 km it is approximately 1300 K, at a depth of approximately 3000 km -4800 K, and in the center of the earth's core - 6900 K.
The predominant part of the Earth's substance is in a solid state, but at the boundary of the earth's crust and the upper mantle (depths of 100-150 km) lies a layer of softened, pasty rocks. This thickness (100-150 km) is called the asthenosphere. Geophysicists believe that other parts of the Earth may also be in a rarefied state (due to decompression, 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 regarding its material composition.
Lithosphere. Earth's crust. 4.5 billion years ago, the Earth was a ball consisting of only gases. Gradually, heavy metals such as iron and nickel sank to the center and became denser. Light rocks and minerals floated to the surface, cooled and solidified.
Internal structure of the Earth.
It is customary to divide the body of the Earth into three main parts – lithosphere(earth's crust), mantle And core.
The core is the center of the Earth , the average radius of which is about 3500 km (16.2% of the Earth's volume). It is believed to consist of iron mixed with silicon and nickel. The outer part of the core is in a molten state (5000 ° C), the inner part is apparently solid (subcore). The movement of matter in the core creates a magnetic field on Earth that protects the planet from cosmic radiation.
The core is replaced mantle , which extends almost 3000 km (83% of the Earth's volume). It is believed to be hard, but at the same time plastic and hot. The mantle consists of three layers: Golitsyn layer, Guttenberg layer and substrate. The upper part of the mantle, called magma , contains a layer with reduced viscosity, density and hardness - the asthenosphere, on which sections of the earth's surface are balanced. The boundary between the mantle and core is called the Guttenberg layer.
Lithosphere
Lithosphere – the upper shell of the “solid” Earth, including the earth’s crust and the upper part of the underlying upper mantle of the Earth.
Earth's crust – the upper shell of the “solid” Earth. The thickness of the earth's crust ranges from 5 km (under the oceans) to 75 km (under the continents). The earth's crust is heterogeneous. It distinguishes 3 layers – sedimentary, granite, basalt. Granite and basalt layers are so named because they contain rocks similar in physical properties to granite and basalt.
Compound earth's crust: oxygen (49%), silicon (26%), aluminum (7%), iron (5%), calcium (4%); the most common minerals are feldspar and quartz. The boundary between the earth's crust and mantle is called Moho surface .
Distinguish continental And oceanic the earth's crust. Oceanic different from continental (mainland) absence of granite layer and significantly less powerful (from 5 to 10 km). Thickness continental crust on the plains is 35-45 km, in the mountains 70-80 km. At the border of continents and oceans, in the areas of islands, the thickness of the earth's crust is 15-30 km, the granite layer pinches out.
The position of layers in the continental crust indicates different times of its formation . The basalt layer is the oldest, younger than it is the granite layer, and the youngest is the upper, sedimentary layer, which is still developing today. Each layer of crust was formed over a long period of geological time.
Lithospheric plates
The earth's crust is in constant motion. The first hypothesis about continental drift(i.e. horizontal movement of the earth's crust) put forward at the beginning of the twentieth century A. Wegener. Created on its basis plate theory . According to this theory, the lithosphere is not a monolith, but consists of seven large and several smaller plates “floating” on the asthenosphere. The boundary areas between lithospheric plates are called seismic belts - these are the most “restless” areas of the planet.
The earth's crust is divided into stable and mobile areas.
Stable areas of the earth's crust - platforms- are formed on the site of geosynclines that have lost mobility. The platform consists of a crystalline basement and sedimentary cover. Depending on the age of the foundation, ancient (Precambrian) and young (Paleozoic, Mesozoic) platforms are distinguished. At the base of all continents lie ancient platforms.
Mobile, highly dissected areas of the earth's surface are called geosynclines ( folded areas ). In their development there are two stages : at the first stage, the earth’s crust experiences subsidence, sedimentary rocks accumulate and metamorphose. Then the earth's crust begins to rise, and the rocks are crushed into folds. There were several eras of intense mountain building on Earth: Baikal, Caledonian, Hercynian, Mesozoic, Cenozoic. In accordance with this, various folding areas are distinguished.
A characteristic feature of the evolution of the Earth is the differentiation of matter, the expression of which is the shell structure of our planet. The lithosphere, hydrosphere, atmosphere, biosphere form the main shells of the Earth, differing in chemical composition, thickness and state of matter.
Internal structure of the Earth
Chemical composition of the Earth(Fig. 1) is similar to the composition of other terrestrial planets, such as Venus or Mars.
In general, elements such as iron, oxygen, silicon, magnesium, and nickel predominate. The content of light elements is low. The average density of the Earth's substance is 5.5 g/cm 3 .
There is very little reliable data on the internal structure of the Earth. Let's look at Fig. 2. It depicts the internal structure of the Earth. The Earth consists of the crust, mantle and core.
Rice. 1. Chemical composition of the Earth
Rice. 2. Internal structure of the Earth
Core
Core(Fig. 3) is located in the center of the Earth, its radius is about 3.5 thousand km. The temperature of the core reaches 10,000 K, i.e. it is higher than the temperature of the outer layers of the Sun, and its density is 13 g/cm 3 (compare: water - 1 g/cm 3). The core is believed to be composed of iron and nickel alloys.
The outer core of the Earth has a greater thickness than the inner core (radius 2200 km) and is in a liquid (molten) state. The inner core is subject to enormous pressure. The substances that compose it are in a solid state.
Mantle
Mantle- the Earth’s geosphere, which surrounds the core and makes up 83% of the volume of our planet (see Fig. 3). Its lower boundary is located at a depth of 2900 km. The mantle is divided into a less dense and plastic upper part (800-900 km), from which it is formed magma(translated from Greek means “thick ointment”; this is the molten substance of the earth’s interior - a mixture of chemical compounds and elements, including gases, in a special semi-liquid state); and the crystalline lower one, about 2000 km thick.
Rice. 3. Structure of the Earth: core, mantle and crust
Earth's crust
Earth's crust - the outer shell of the lithosphere (see Fig. 3). Its density is approximately two times less than the average density of the Earth - 3 g/cm 3 .
Separates the earth's crust from the mantle Mohorovicic border(often called the Moho boundary), characterized by a sharp increase in seismic wave velocities. It was installed in 1909 by a Croatian scientist Andrei Mohorovicic (1857- 1936).
Since the processes occurring in the uppermost part of the mantle affect the movements of matter in the earth's crust, they are combined under the general name lithosphere(stone shell). The thickness of the lithosphere ranges from 50 to 200 km.
Below the lithosphere is located asthenosphere- less hard and less viscous, but more plastic shell with a temperature of 1200 ° C. It can cross the Moho boundary, penetrating into the earth's crust. The asthenosphere is the source of volcanism. It contains pockets of molten magma, which penetrates into the earth's crust or pours out onto the earth's surface.
Composition and structure of the earth's crust
Compared to the mantle and core, the earth's crust is a very thin, hard and brittle layer. It is composed of a lighter substance, which currently contains about 90 natural chemical elements. These elements are not equally represented in the earth's crust. Seven elements - oxygen, aluminum, iron, calcium, sodium, potassium and magnesium - account for 98% of the mass of the earth's crust (see Fig. 5).
Peculiar combinations of chemical elements form various rocks and minerals. The oldest of them are at least 4.5 billion years old.
Rice. 4. Structure of the earth's crust
Rice. 5. Composition of the earth's crust
Mineral is a relatively homogeneous natural body in its composition and properties, formed both in the depths and on the surface of the lithosphere. Examples of minerals are diamond, quartz, gypsum, talc, etc. (You will find characteristics of the physical properties of various minerals in Appendix 2.) The composition of the Earth's minerals is shown in Fig. 6.
Rice. 6. General mineral composition of the Earth
Rocks consist of minerals. They can be composed of one or several minerals.
Sedimentary rocks - clay, limestone, chalk, sandstone, etc. - were formed by the precipitation of substances in the aquatic environment and on land. They lie in layers. Geologists call them pages of the history of the Earth, since they can learn about the natural conditions that existed on our planet in ancient times.
Among sedimentary rocks, organogenic and inorganogenic (clastic and chemogenic) are distinguished.
Organogenic Rocks are formed as a result of the accumulation of animal and plant remains.
Clastic rocks are formed as a result of weathering, destruction by water, ice or wind of the products of destruction of previously formed rocks (Table 1).
Table 1. Clastic rocks depending on the size of the fragments
|
Breed name |
Size of bummer con (particles) |
|
More than 50 cm |
|
|
5 mm - 1 cm |
|
|
1 mm - 5 mm |
|
|
Sand and sandstones |
0.005 mm - 1 mm |
|
Less than 0.005 mm |
Chemogenic Rocks are formed as a result of the precipitation of substances dissolved in them from the waters of seas and lakes.
In the thickness of the earth's crust, magma forms igneous rocks(Fig. 7), for example granite and basalt.
Sedimentary and igneous rocks, when immersed to great depths under the influence of pressure and high temperatures, undergo significant changes, turning into metamorphic rocks. For example, limestone turns into marble, quartz sandstone into quartzite.
The structure of the earth's crust is divided into three layers: sedimentary, granite, and basalt.
Sedimentary layer(see Fig. 8) is 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 such mineral, like coal, gas, oil. All of them are of organic origin. For example, coal is a product of the transformation of plants of ancient times. The thickness of the sedimentary layer varies widely - from complete absence in some land areas to 20-25 km in deep depressions.
Rice. 7. Classification of rocks by origin
"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.
"Basalt" layer formed by rocks close to basalts. These are metamorphosed igneous rocks, denser than the rocks of the “granite” layer.
The thickness and vertical structure of the earth's crust are different. There are several types of the earth's crust (Fig. 8). According to the simplest classification, a distinction is made between oceanic and continental crust.
Continental and oceanic crust vary in thickness. Thus, the maximum thickness of the earth’s crust is observed under mountain systems. It is about 70 km. Under the plains the thickness of the earth's crust is 30-40 km, and under the oceans it is thinnest - only 5-10 km.
Rice. 8. Types of the earth's 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
The difference between the continental and oceanic crust in the composition of rocks is manifested in the fact that there is no granite layer in the oceanic crust. And the basalt layer of the oceanic crust is very unique. In terms of rock composition, it differs from a similar layer of continental crust.
The boundary between land and ocean (zero mark) does not record the transition of the continental crust to the oceanic one. The replacement of continental crust by oceanic crust occurs in the ocean at a depth of approximately 2450 m.
Rice. 9. Structure of the continental and oceanic crust
There are also transitional types of the earth's crust - suboceanic and subcontinental.
Suboceanic crust located along continental slopes and foothills, can be found in marginal and Mediterranean seas. It represents continental crust with a thickness of up to 15-20 km.
Subcontinental crust located, for example, on volcanic island arcs.
Based on materials seismic sounding - the speed of passage of seismic waves - we obtain data on the deep structure of the earth’s crust. Thus, the Kola superdeep well, which for the first time made it possible to see rock samples from a depth of more than 12 km, brought a lot of unexpected things. It was assumed that at a depth of 7 km a “basalt” layer should begin. In reality, it was not discovered, and gneisses predominated among the rocks.
Change in temperature of the earth's crust with depth. The surface layer of the earth's crust has a temperature determined by solar heat. This heliometric layer(from the Greek helio - Sun), experiencing seasonal temperature fluctuations. Its average thickness is about 30 m.
Below is an even thinner layer, the characteristic feature of which is a constant temperature corresponding to the average annual temperature of the observation site. The depth of this layer increases in continental climates.
Even deeper in the earth's crust there is a geothermal layer, the temperature of which is determined by the internal heat of the Earth and increases with depth.
The increase in temperature occurs mainly due to the decay of radioactive elements that make up rocks, primarily radium and uranium.
The amount of temperature increase in rocks with depth is called geothermal gradient. It varies within a fairly wide range - from 0.1 to 0.01 °C/m - and depends on the composition of rocks, the conditions of their occurrence and a number of other factors. Under the oceans, temperature increases faster with depth than on continents. On average, with every 100 m of depth it becomes warmer by 3 °C.
The reciprocal of the geothermal gradient is called geothermal stage. It is measured in m/°C.
The heat of the earth's crust is an important energy source.
The part of the earth's crust that extends to depths accessible to geological study forms bowels of the earth. The Earth's interior requires special protection and wise use.
Introduction…………………………………………………………………………………..2
1. Structure of the Earth……………………………………………………………….3
2. Composition of the earth’s crust………………………………………………………...5
3.1. State of the Earth………………………………………………………...7
3.2.State of the earth’s crust……………………………………………………...8
List of used literature………………………….………………10
Introduction
The Earth's crust is the outer hard shell of the Earth (geosphere). Below the crust is the mantle, which differs in composition and physical properties - it is denser and contains mainly refractory elements. The crust and mantle are separated by the Mohorovicic boundary, or Moho for short, where there is a sharp increase in seismic wave velocities. On the outside, most of the crust is covered by the hydrosphere, and the smaller part is exposed to the atmosphere.
There is a crust on most 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. Structure of the Earth
Most of the Earth's surface (up to 71%) is occupied by the World Ocean. The average depth of the World Ocean is 3900 m. The existence of sedimentary rocks whose age exceeds 3.5 billion years serves as evidence of the existence of vast bodies of water on Earth already at that distant time. On modern continents, plains are more common, mainly low-lying ones, and mountains - especially high ones - occupy a small part of the planet's surface, as well as deep-sea depressions at the bottom of the oceans. The shape of the Earth, as is known, is close to spherical, but with more detailed measurements it turns out to be very complex, even if we outline it with a flat ocean surface (not distorted by tides, winds, currents) and the conditional continuation of this surface under the continents. The irregularities are maintained by the uneven distribution of mass in the Earth's interior.
One of the features of the Earth is its magnetic field, thanks to which we can use a compass. The Earth's magnetic pole, 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 “trail” in the direction from the Sun, which extends for hundreds of thousands of kilometers.
The internal structure of the Earth is, first of all, judged by the characteristics of the passage of mechanical vibrations through the various layers of the Earth that occur during earthquakes or explosions. Valuable information is also provided by measurements of the magnitude of the heat flow 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 of the physical constant of gravity and the acceleration of gravity. For the mass of the Earth, the value obtained is 5.967 1024 kg. Based on 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” seems to have cracked into pieces and consists of several large lithospheric plates, slowly moving one relative to the other. The overwhelming number of earthquakes is concentrated along their boundaries. The upper layer of the 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 there 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 mantle is called the Mohorovic 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 layer of mantle 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 a 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 relative to other sections of the crust. At the same time, additional load on a section of the crust, for example, due to the accumulation of a thick layer of continental ice, as happens in Antarctica, leads to a gradual subsidence of the section. This phenomenon is called isostatic equalization. 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. The sharp transition was discovered by seismic research methods at a depth of about 2920 km. This is where the earth's core begins, or, more precisely, the outer core, since at 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, which do not propagate in liquid, do not pass through it. The origin of the Earth's magnetic field is associated with the existence of a liquid outer core. 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. Composition of the earth's crust
The earth's crust consists of several layers, the thickness and structure of which vary within the oceans and continents. In this regard, oceanic, continental and intermediate types of the earth's crust are distinguished, which will be described further.
Based on their composition, the earth's crust is usually divided into three layers - sedimentary, granite and basalt.
The sedimentary layer is composed of sedimentary rocks, which are the product of destruction and redeposition of material from the lower layers. Although this layer covers the entire surface of the Earth, it is so thin in places 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 the solidification of molten magma, among which varieties rich in silica (acidic rocks) predominate. This layer, which reaches a thickness of 15-20 km on continents, is greatly reduced under the oceans and may even be completely absent.
The basalt layer is also composed of igneous material, but it is poorer in silica (basic rocks) and has a higher specific gravity. This layer is developed at the base of the earth's crust in all areas of the globe.
The continental type of the earth's crust is characterized by the presence of all three layers and is much thicker than the oceanic one.
The earth's crust is the main object of study of geology. The earth's crust consists of a very diverse range of rocks, consisting of equally diverse minerals. When studying a rock, first of all, its chemical and mineralogical composition is examined. However, this is not enough to fully understand the rock. Rocks of different origins and, consequently, different conditions of occurrence and distribution may have the same chemical and mineralogical composition.
The structure of a rock is understood as the size, composition and shape of the mineral particles composing it 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 the size of the crystals is (whole crystals or fragments of them are included in the rock), what the degree of roundness of the fragments is, whether the mineral grains forming the rock are completely unrelated to each other or they are soldered together with some kind of cementing substance, directly fused with each other, sprouted each other, etc.
Texture refers to the relative arrangement of the components that make up the rock, or the way they fill the space occupied by the rock. Examples of textures can be: layered, when the rock consists of alternating layers of different composition and structure, schistose, when the rock easily breaks up into thin tiles, massive, porous, solid, bubbly, etc.
The form of occurrence of rocks refers to the shape of the bodies they form 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. method of origin. There are three large groups of rocks: igneous, or igneous, sedimentary and metamorphic.
Igneous rocks are formed during the solidification of silicate melts located in the depths 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 underlying rocks and solidifies at a greater or lesser depth, in others it solidifies, pouring out onto the surface of the Earth 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. transformation 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 confining pressure to oriented pressure), 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 activities and the functioning of flora and fauna can improve or worsen the state of the earth. The main processes of impact on land are: irreversible withdrawal from agricultural activities; temporary seizure; mechanical impact; addition of chemical and organic elements; involvement of additional territories in agricultural activities (drainage, irrigation, deforestation, reclamation); heating; self-renewal.
