A section of the earth's crust. Report – Earth's crust

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Internal structure of the Earth

Characteristics of the Earth's shells. Tectonics of lithospheric plates and the formation of large relief forms. Horizontal structure of the lithosphere. Types of the earth's crust. Movement of mantle matter through mantle channels in the interior of the Earth. Direction and movement of lithospheric plates.

presentation, added 01/12/2011

Material composition and structure of the earth's crust

Descriptive characteristics of the stages of formation of the earth's crust and the study of its mineralogical and petrographic composition. Features of the structure of rocks and the nature of the movement of the earth's crust. Folding, ruptures and collisions of continental plates.

course work, added 08/30/2013

Plate theory

presentation, added 10/11/2016

Structural elements of the Earth's crust

Location of folded regions of the Earth's crust. Structure of the platform, passive and active continental margin. Structure of anticlise and syneclise, aulacogens. Mountain fold areas or geosynclinal belts. Structural elements of the oceanic crust.

presentation, added 10/19/2014

Tectonic movements of the earth's crust

Classification of the main types of tectonic deformations of the earth's crust: rifting (spreading), subduction, obduction, collisions of continental plates and transform faults. Determination of the speed and direction of movement of lithospheric plates by the geomagnetic field of the earth.

course work, added 06/19/2011

Material composition of the earth's crust

The main types of the earth's crust and its components. Compilation of velocity columns for the main structural elements of the continents. Determination of tectonic structures of the earth's crust. Description of syneclise, anteclise and aulacogen. Mineral composition of bark and rocks.

course work, added 01/23/2014

General characteristics of the tectonic structure of lithospheric plates in the Republic of Tatarstan

A brief history of the study of tectonics of the Republic of Tatarstan. General characteristics of uplifts, ruptures, and deformations of lithospheric plates. Description of modern movements of the earth's crust and the processes that determine them. Features of monitoring earthquake sources.

course work, added 01/14/2016

Mesozoic era

Triassic, Jurassic and Cretaceous periods of the Mesozoic era. The organic world of these periods. Structure of the earth's crust and paleogeography at the beginning of the era. History of the geological development of geosynclinal belts and ancient platforms (East European and Siberian).

abstract, added 05/28/2010

Microcontinents. Description of types of faults in the earth's crust

The origin and development of microcontinents, uplifts of the earth’s crust of a special type. The difference between the crust of the oceans and the crust of the continents. The sliding theory of ocean formation. Late synclinal stage of development. Types of faults in the earth's crust, classification of deep faults.

test, added 12/15/2009

Internal structure and heterogeneities of the Earth

General picture of the internal structure of the Earth. Composition of matter in the earth's core. Blocks of the earth's crust. Lithosphere and asthenosphere. The structure of the foundation of the East European Platform. Brief description of the deep structure of the territory of Belarus and adjacent regions.

test, added 07/28/2013

The largest structural elements of the earth's crust are continents And oceans, characterized by its different structure. These structural elements are distinguished by geological and geophysical characteristics. Not all the space occupied by ocean waters represents a single structure of the oceanic type. Vast shelf areas, such as those in the Arctic Ocean, have continental crust. The differences between these two largest structural elements are not limited to the type of crust, but can be traced deeper into the upper mantle, which is built differently under the continents than under the oceans. These differences cover the entire lithosphere, subject to tectonospheric processes, i.e. can be traced to depths of approximately 750 km.

On continents, there are two main types of crustal structures: calm, stable - platforms and mobile - geosynclines. In terms of area of ​​distribution, these structures are quite comparable. The difference is observed in the rate of accumulation and in the magnitude of the gradient of thickness changes: platforms are characterized by a smooth gradual change in thickness, and geosynclines are characterized by a sharp and rapid change. Igneous and intrusive rocks are rare on platforms; they are abundant in geosynclines. In geosynclines, flysch formations of sediments are underlying. These are rhythmically multilayered deep-sea terrigenous deposits formed during the rapid subsidence of a geosynclinal structure. At the end of development, geosynclinal areas undergo folding and turn into mountain structures. Subsequently, these mountain structures undergo a stage of destruction and gradual transition into platform formations with a deeply dislocated lower floor of rock deposits and gently lying layers in the upper floor.

Thus, the geosynclinal stage of development of the earth's crust is the earliest stage; then geosynclines die off and transform into orogenic mountain structures and subsequently into platforms. The cycle ends. All these are stages of a single process of development of the earth’s crust.

Platforms- the main structures of the continents, isometric in shape, occupying central regions, characterized by leveled relief and calm tectonic processes. The area of ​​ancient platforms on the continents approaches 40% and they are characterized by angular outlines with extended rectilinear boundaries - a consequence of marginal sutures (deep faults), mountain systems, and linearly elongated troughs. Folded areas and systems are either thrust onto platforms or border them through foredeeps, onto which folded orogens (mountain ranges) are in turn thrust. The boundaries of the ancient platforms sharply unconformably intersect their internal structures, which indicates their secondary nature as a result of the split of the Pangea supercontinent, which arose at the end of the Early Proterozoic.

For example, the East European Platform, defined within the boundaries from the Urals to Ireland; from the Caucasus, the Black Sea, the Alps to the northern reaches of Europe.

Distinguish ancient and young platforms.

Ancient platforms arose on the site of the Precambrian geosynclinal region. The East European, Siberian, African, Indian, Australian, Brazilian, North American and other platforms were formed in the late Archean - early Proterozoic, represented by a Precambrian crystalline basement and sedimentary cover. Their distinctive feature is the two-story structure.

Ground floor or foundation it is composed of folded, deeply metamorphosed rock strata, crushed into folds, broken by granite intrusions, with the widespread development of gneiss and granite-gneiss domes - a specific form of metamorphogenic folding (Fig. 7.3). The foundation of the platforms was formed over a long period of time in the Archean and Early Proterozoic and subsequently underwent very strong erosion and denudation, as a result of which rocks that previously lay at great depths were exposed.

Rice. 7.3. Principal section of the platform

1 - basement rocks; rocks of the sedimentary cover: 2 - sands, sandstone, gravelites, conglomerates; 3 - clays and carbonates; 4 - effusive; 5 - faults; 6 - shafts

Top floor platforms presented cover, or a cover, gently lying with a sharp angular unconformity on the basement of non-metamorphosed sediments - marine, continental and volcanogenic. The surface between the cover and the basement reflects the main structural unconformity within the platforms. The structure of the platform cover turns out to be complex and on many platforms, in the early stages of its formation, grabens and graben-like troughs will appear - aulacogens(avlos - furrow, ditch; gene - born, i.e. born of a ditch). Aulacogens most often formed in the Late Proterozoic (Riphean) and formed extended systems in the basement body. The thickness of continental and less commonly marine sediments in aulacogens reaches 5-7 km, and deep faults that bounded aulacogens contributed to the manifestation of alkaline, mafic and ultrabasic magmatism, as well as platform-specific trap magmatism (mafic rocks) with continental basalts, sills and dikes. Alkaline-ultrabasic is very important (kimberlite) formation containing diamonds in explosion pipe products (Siberian Platform, South Africa). This lower structural layer of the platform cover, corresponding to the aulacogenic stage of development, is replaced by a continuous cover of platform sediments. At the initial stage of development, the platforms tended to slowly sink with the accumulation of carbonate-terrigenous strata, and at a later stage of development they were marked by the accumulation of terrigenous coal-bearing strata. At the late stage of development of the platforms, deep depressions filled with terrigenous or carbonate-terrigenous sediments formed in them (Caspian, Vilyui).

During the process of formation, the platform cover repeatedly underwent restructuring of the structural plan, timed to coincide with the boundaries of geotectonic cycles: Baikal, Caledonian, Hercynian, Alpine. The areas of the platforms that experienced maximum subsidence are, as a rule, adjacent to the mobile area or system bordering the platform, which was actively developing at that time ( pericratonic, those. at the edge of the craton, or platform).

Among the largest structural elements of the platforms are shields and slabs.

The shield is a ledge surface of the crystalline foundation of the platform ( (no sedimentary cover)), which throughout the platform stage of development experienced a tendency to rise. Examples of shields include: Ukrainian, Baltic.

Stove They are considered either part of a platform with a tendency to subsidence, or an independent young developing platform (Russian, Scythian, West Siberian). Within the slabs, smaller structural elements are distinguished. These are syneclises (Moscow, Baltic, Caspian) - extensive flat depressions under which the foundation is bent, and anteclises (Belorusskaya, Voronezh) - gentle arches with a raised foundation and a relatively thinned cover.

Young platforms formed either on the Baikal, Caledonian or Hercynian basement, they are distinguished by a greater dislocation of the cover, a lower degree of metamorphism of the basement rocks and a significant inheritance of the structures of the cover from the structures of the basement. These platforms have a three-tiered structure: the foundation of metamorphosed rocks of the geosynclinal complex is covered by a layer of denudation products of the geosynclinal region and a weakly metamorphosed complex of sedimentary rocks.

Ring structures. The place of ring structures in the mechanism of geological and tectonic processes has not yet been precisely determined. The largest planetary ring structures (morphostructures) are the Pacific Ocean basin, Antarctica, Australia, etc. The identification of such structures can be considered conditional. A more thorough study of ring structures made it possible to identify elements of spiral, vortex structures in many of them).

However, it is possible to distinguish structures endogenous, exogenous and cosmogenic genesis.

Endogenous ring structures of metamorphic and igneous and tectonogenic (arches, ledges, depressions, anteclises, syneclises) origin, their diameters range from a few kilometers to hundreds and thousands of kilometers (Fig. 7.4).

Rice. 7.4. Ring structures north of New York

Large ring structures are caused by processes occurring in the depths of the mantle. Smaller structures are caused by diapiric processes of igneous rocks rising to the surface of the Earth and breaking through and uplifting the upper sedimentary complex. Ring structures are caused by both volcanic processes (volcanic cones, volcanic islands) and diapirism processes of plastic rocks such as salts and clays, the density of which is less than the density of the host rocks.

Exogenous ring structures in the lithosphere are formed as a result of weathering and leaching. These are karst sinkholes and sinkholes.

Cosmogenic (meteorite) ring structures - astroblemes. These structures are the result of meteorite impacts. Meteorites with a diameter of about 10 kilometers fall to the Earth with a frequency of once every 100 million years, smaller ones much more often. The crater structure has a bowl-shaped shape with a central rise and a shaft of ejected rocks. Meteor ring structures can have diameters ranging from tens of meters to hundreds of meters and kilometers. For example: Pribalkhash-Iliyskaya (700 km); Yucotan (200 km), depth - more than 1 km: Arizona (1.2 km), depth more than 185 m; South Africa (335 km), about 10 km across from the asteroid.

In the geological structure of Belarus one can note ring structures of tectonomagmatic origin (Orsha depression, Belarusian massif), diapiric salt structures of the Pripyat trough, volcanic ancient channels such as kimberlite pipes (on the Zhlobin saddle, the northern part of the Belarusian massif), an astrobleme in the Pleschenitsy area with a diameter of 150 meters.

Ring structures are characterized by anomalies of geophysical fields: seismic, gravitational, magnetic.

Rift the structures of continents (Fig. 7.5, 7.6) of small width up to 150 -200 km are expressed by extended lithospheric uplifts, the arches of which are complicated by subsidence grabens: Rhine (300 km), Baikal (2500 km), Dnieper-Donets (4,000 km), East African (6,000 km), etc.

Rice. 7.5. Section of the Pripyat continental rift

Continental rift systems consist of a chain of negative structures (troughs, rifts) of a ranked time of origin and development, separated by lithospheric uplifts (saddles). Rift structures of continents can be located between other structures (anteclises, shields), cross platforms and continue on other platforms. The structure of continental and oceanic rift structures is similar, they have a symmetrical structure relative to the axis (Fig. 7.5, 7.6), the difference lies in the length, degree of opening and the presence of some special features (transform faults, protrusions-bridges between links).

The oldest piece of the earth's crust has been discovered

7.6. Profile sections of continental rift systems

1-foundation; 2-chemogenic-biogenic sediments; 3- chemogenic-biogenic-volcanogenic formation; 4- terrigenous deposits; 5, 6-faults

Part (link) of the Dnieper-Donets continental rift structure is the Pripyat trough. The Podlasie-Brest depression is considered to be the upper link; it may have a genetic connection with similar structures in Western Europe. The lower part of the structure is the Dnieper-Donets depression, then similar structures Karpinskaya and Mangyshlakskaya and then the structures of Central Asia (the total length from Warsaw to the Gissar ridge). All links of the rift structure of the continents are limited by listric faults, have a hierarchical subordination in age of origin, and have thick sedimentary strata that are promising for containing hydrocarbon deposits.

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Stable sections of the earth's crust that rest on an ancient (Precambrian) crystalline foundation are called ancient platforms. The territory of Russia is located on two ancient platforms. In some places, the foundation of the platforms (many meters thick granite) goes directly to the surface, you can walk on it. Such places are called shields. The shields occupy small areas of the platforms. Most often, the foundation is hidden under the thickness of younger layers of the earth's crust. These parts of platforms are called plates. A young platform is also a stable section of the earth's crust, but its foundation is younger (formed in Paleozoic time). According to geologists, once upon a time two lithospheric plates with ancient platforms collided and were firmly “glued” together.

The oldest piece of the earth's crust has been discovered

The place where they are “glued together” is the Ural Mountains, and another young platform has formed between the Ural Mountains and the Siberian Platform. It is all covered with a thick layer of sedimentary rocks. Its surface is a flat plain. Over the millions of years while the sedimentary cover of the platforms is being formed, magma in different places penetrates into the thickness of the earth’s crust through cracks in the foundation. On the territory of the Siberian Platform it formed traps - lava covers or lakes of solidified lava. How traps are formed is well shown in a multimedia textbook when the Siberian Platform approaches. Traps were not formed on the East European Platform, but there are intrusions - massifs of magma that did not break through to the surface and froze in the thickness of the earth's crust. On geological sections and maps they are indicated in red, like the foundation. Sometimes the destruction of rocks from above leads to the fact that cooled and crystallized intrusions come to the surface.

platforms

platforms

platform

The earth's crust within modern Russia was formed over a long period of time as a result of various geological processes. Therefore, its parts differ: firstly, in the structure, composition and occurrence of rocks, and secondly, in age and history of development.

According to the structural features, mobile and stable sections of the earth's crust are distinguished. Mountain structures are located on moving areas. They are composed of rocks crushed into folds, separated by splits into separate blocks. These blocks move in different directions at different speeds. As a result of these movements, mountain ranges and depressions separating them are formed. Intense movements of the earth's crust are often accompanied by earthquakes.

Most of the territory of Russia is occupied by stable sections of the earth's crust - platforms: East European, West Siberian and Siberian. The platforms have a two-tier structure. Their lower part is the foundation. These are the remains of collapsed mountain systems that previously existed on the site of modern platforms. Therefore, it consists of rocks crushed into folds. Loose sedimentary rocks (sedimentary cover) lie on top of the foundation. They were formed during the destruction of mountains and the slow subsidence of the foundation, when it was filled with sea waters. In some parts of the platforms there is no sedimentary cover. Such sections of platforms are called shields.

The rocks of fold belts and platforms have different ages, as they were formed over a long time.

The entire geological history of the Earth is divided into 5 large time periods - eras. The name of each era is given in accordance with the type of life characteristic of it: Archean (ancient life), Proterozoic (early life), Paleozoic (ancient life), Mesozoic (middle life), Cenozoic (new life). The length of eras varies greatly. In turn, eras are divided into smaller periods of time - periods. The names of periods most often come either from the names of those areas where the rocks formed during this period were first studied in detail, or from the names of the rocks themselves.

The age and time of formation of individual rocks can be determined in different ways. If the original occurrence of rocks is not disturbed by subsequent geological processes, then the layers that lie above are younger than those located below. They help determine the age of rocks and fossil remains of plants and animals. The more complex organisms are, the younger they are. Both of these methods allow one to estimate the relative age of rocks.

They learned to determine the absolute age of rocks only in the 20th century. To do this, evaluate the decay process of radioactive elements contained in rocks. The decay process occurs at a constant speed and does not depend on external conditions. Therefore, by the ratio of the content of a radioactive element in a rock and its decay products, it is possible to determine the absolute age of the rock in billions and millions of years.

The most ancient folded areas formed on the territory of Russia in the Archean and Proterozoic (2600-500 million years ago). They are composed of pre-Paleozoic rocks. They form the lower structural tier of the platforms - their folded foundation.

On the territory of Russia there are two ancient platforms - the East European and Siberian. Both of them have a two-tier structure: a folded foundation of crystalline and igneous rocks of Archean-Proterozoic age and a Paleozoic-Cenozoic sedimentary cover. The sedimentary rocks of the cover lie quietly, usually subhorizontally. Sedimentation was interrupted during periods of uplift and was replaced by demolition processes.

East European Platform it is limited in the east by the Ural folded structures, in the south by the young Scythian plate adjacent to the folded structures of the Caucasus, in the north it continues under the waters of the Barents Sea, and in the west it extends far beyond the borders of Russia. Within its borders there are two shields, one of which - the Baltic - extends into the territory of the Kola Peninsula and Karelia, the second - the Ukrainian - is completely outside Russia. The rest of the platform space is occupied by the Russian plate.

The shallow foundation is characteristic of the Voronezh anteclise (the first hundreds of meters) and some positive structures of the Volga-Ural arch. In syneclises (Moscow, Pechora, Baltic) the foundation is lowered by 2-4 km. The greatest depth of the foundation is characteristic of the Caspian syneclise (15-20 km).

East Siberian Platform- a large geological region in the northeast of the Eurasian plate, occupies the middle part of North Asia. This is one of the large, relatively stable ancient blocks of the Earth's continental crust, classified as ancient (pre-Riphean) platforms. Its foundation was formed in the Archean; subsequently, it was repeatedly covered by seas, in which a thick sedimentary cover was formed. Several stages of intraplate magmatism occurred on the platform, the largest of which was the formation of the Siberian traps at the Permian-Triassic boundary. Before and after the introduction of the traps, there were sporadic outbreaks of kimberlite magmatism, which formed large diamond deposits.

The Siberian platform is limited by zones of deep faults - marginal sutures, well-defined gravitational steps, and has polygonal outlines. The modern boundaries of the platform took shape in the Mesozoic and Cenozoic and are well expressed in relief. The western border of the platform coincides with the valley of the Yenisei River, the northern - with the southern edge of the Byrranga Mountains, the eastern - with the lower reaches of the Lena River (Verkhoyansk regional trough), in the southeast - with the southern end of the Dzhugdzhur ridge; in the south the border runs along faults along the southern edge of the Stanovoy and Yablonovy ridges; then, bending around from the north along a complex system of faults in Transbaikalia and Pribaikalia, it descends to the southern tip of Lake Baikal; the southwestern border of the platform extends along the Main East Sayan Fault.

The platform has an Early Precambrian, mainly Archean, foundation and a platform cover (Riphean-Anthropocene). Among the main structural elements of the platform, the following stand out: the Aldan shield and the Lena-Yenisei plate, within which the foundation is exposed on the Anabar massif, Olenyoksky and Sharyzhalgai uplifts. The western part of the plate is occupied by the Tunguska syneclise, and the eastern part by the Vilyui syneclise. In the south there is the Angara-Lena trough, separated from the Nyu depression by the Peleduy uplift.

1. In the Archean and early Proterozoic, most of the foundation of the East Siberian Platform was formed.
2. At the end of the Proterozoic (Vendian) and the beginning of the Paleozoic, the platform was periodically covered by a shallow sea, resulting in the formation of a thick sedimentary cover.
3. At the end of the Paleozoic, the Paleo-Ural Ocean closed, the crust of the West Siberian Plain consolidated, and it, together with the East Siberian and East European platforms, formed a single continent.
4. In the Devonian there was an outbreak of kimberlite magmatism.
5. A powerful outbreak of trap magmatism occurred at the Permian-Triassic boundary.
6. During the Mesozoic, some parts of the platform were covered by epicontinental seas.
7. At the Cretaceous-Paleogene boundary, rifting and a new outbreak of magmatism, including carbonatite and kimberlite, occurred on the platform.

The foundation of the platform is composed of Archean, Proterozoic and Riphean rocks. The surface of the crystalline basement of the Siberian platform, like the Russian one, is very uneven; in some parts the foundation reaches the surface or is submerged to an insignificant depth, in others it is covered by a thick layer of sedimentary rocks. The foundation surface consists of a system of anteclises and syneclises. The largest basement uplifts are the Anabarskin massif, the Aldan shield, the Yenisei meganticlinorium, the Turukhansk uplift and the folded system of the Stanovoy Range. The largest subsidences are the Tunguska (5-6 km), the Vilyuiskaya (5-8 km), the Khatanga syneclise and the Angara-Lena trough, laid down at different times: the Tunguska - in the Lower Paleozoic, the Khatanga - in the Middle Paleozoic, the Vilyuiskaya - in Mesozoic. The thickness and completeness of the sedimentary complex section in individual parts of the platform varies widely. The most characteristic platform structures are gentle and dome-shaped folds of a northwestern direction, disturbed by fault dislocations of the Alpine cycle.
In the initial phases of the Hercynian cycle - Upper Devonian and Carboniferous - the Siberian platform was occupied by the sea on its northern margin. By the end of the Carboniferous period, the sea retreated, leaving vast swampy areas in which the Permian sandy-clayey coal-bearing sediments of the Tunguska basin accumulated, and lakes.
The final phases of the Hercynian folding were manifested by powerful trap eruptions over an area of ​​1.5 billion km2. Intrusive intrusions and effusive outpourings continued into the Triassic and possibly into the early Jurassic. The trap formation includes strata of tuffs, as well as andesites, porphyrites, and basalts. Effusives of basic, ultrabasic and alkaline composition predominate. In various parts of the platform there are kimberlites associated with explosion pipes. The thickness of the trap formation varies greatly. In areas of the platform that were flooded in the Carboniferous and Permian by the sea, thick strata of sedimentary rocks were deposited - limestones, marls, dolomites, clays, shales, and sandy sediments.
Precambrian structures are associated with gold ore deposits associated with granitoid intrusions (Yenisei, Lensky, Anabar regions), muscovite deposits (Mamsko-Vitimskoye), metamorphic iron ore deposits (Angara-Ilimsky region "Angaro-Pitsky basin"). Deposits of copper-nickel ores (Norilsk) and optical Iceland spar are also associated with trap eruptions.
The geotectonic structure of the platforms as a whole determines the main features of the modern surface topography of the Russian Plain, the West Siberian Lowland and the Central Siberian Plateau. Anteclises determine positive forms of relief; syneclises correspond to low-lying lowlands and plains. However, sometimes there is a discrepancy between the forms of modern relief, the position of river valleys and tectonic structures. For example, the Polesie lowland is located on the site of the Belorussian uplift, the Putorana uplift is located on the site of the synclinal structure of the platform foundation, etc. The Baikal folding occurred in the late Proterozoic - Lower Cambrian. The structures she created partially became part of the foundation of the platforms, consolidating more ancient blocks, and also adjoin the outskirts of the ancient platforms. They outline the Siberian platform from the north, west and south (Taimyr-Severozemelskaya, Baikal-Vitim and Yenisei-East Sayan regions). On the northeastern edge of the East European Platform is the Timan-Pechora-Barents Sea region. Apparently, at the same time, the Irtysh-Nadym block was formed, occupying a central position within the West Siberian Plain. Areas of Baikal folding E.E. Milanovsky (1983, 1987) refers to metaplatform areas.

In the Phanerozoic, along with ancient platforms and adjacent metaplatform areas, there are so-called mobile belts, three of which extend into the territory of Russia: the Ural-Mongolian, Pacific and Mediterranean. In their development, mobile belts go through two main stages: geosynclinal and postgeosynclinal, or epigeosynclinal folded belt, the change of which in different belts and even in different regions of a single belt occurred at different times and dragged on until the end of the Phanerozoic.

The features of the first stage have already been discussed when characterizing geosynclines. The tectonic regime of the second stage is significantly inferior in activity to the geosynclinal one, but at the same time exceeds the tectonic regime of the ancient platforms.

The Paleozoic Ural-Mongolian belt is located between the ancient East European and Siberian platforms and forms the southern frame of the latter. Deflections within this belt began in the Late Proterozoic, and in the Lower Paleozoic the Caledonian folding appeared here. The main phases of folding occur at the end of the Cambrian - the beginning of the Ordovician (Salairian), the middle - the Upper Ordovician, the end of the Silurian - the beginning of the Devonian. As a result of the Caledonian folding, mountain structures were created in the Western Sayan, Kuznetsk Alatau, Salair, in the eastern regions of Altai, in Tuva, in a significant part of Transbaikalia, in the southern regions of Western Siberia, adjacent to the western part of the Kazakh small hills, where the Caledonian folding was also final. In all these territories, Lower Paleozoic sediments are intensively folded and metamorphosed. The Precambrian base is often visible through their cover.

In the Upper Paleozoic (Late Devonian - Early Carboniferous and Late Carboniferous - Permian) Hercynian(Variscan) folding. It was the final one in the vast space of Western Siberia, consolidating the blocks that previously existed here, in the Ural-Novozemelskaya region, in the western regions of Altai, in the Tom-Kolyvan zone. It also appeared in the Mongol-Okhotsk zone.

Thus, by the end of the Paleozoic, an intracontinental folding zone formed within the Ural-Mongolian mobile belt, welding two ancient platforms into a single large structure, a rigid block, which became the core of the Eurasian lithospheric plate. There was also an increase in the area of ​​the platforms due to the emergence of folded structures along their southern edges.

Subsequently (in the Mesozoic), young EpiPaleozoic plates (quasi-cratons) were formed within the Ural-Mongolian belt, including the West Siberian plate, almost entirely located on the territory of Russia.

Stages of formation of the earth's crust in Russia

They are confined to areas that experienced general subsidence in the Meso-Cenozoic.

Typically, plates are formed over those areas of moving belts, in the structural plan of which blocks of ancient consolidation - middle massifs - play a significant role. Young plates do not always “fit” strictly into the contours of the moving belt. They can also overlap areas of ancient platforms adjacent to the moving belt (metaplatform areas), as is the case on the eastern margin of the West Siberian Plate. The cover of the young platforms is composed of sedimentary strata of Mesozoic-Cenozoic age. The thickness of the cover varies from several hundred meters - a kilometer in the marginal parts to 8-12 km in the most deeply submerged northern part of the West Siberian Plate.

Pacific Mobile Belt occupies a marginal position between the ancient Siberian platform and the oceanic lithospheric plate of the Pacific Ocean. It includes folded structures of the Northeast and Far East.

Some sections of this belt completed the period of geosynclinal development back in the Precambrian or Paleozoic and form middle massifs, the largest of which are the Kolyma and Bureinsky (original “microplatforms” having a shield and a slab); others experienced folding in the Mesozoic, others in the Cenozoic.

The Verkhoyansk-Chukotka folded region was created by Cimmerian folding (late Cimmerian, or Kolyma, late Jurassic - mid-Cretaceous). Along the southeastern edge of this region stretches the Okhotsk-Chukotka volcanogenic belt, which in the southern part of the Far East passes into the Primorsky volcanogenic belt, separating the mesozoids of this region from the region of the Pacific folding. Early and late Cimmerian folding manifested itself here, creating the Mesozoic structures of the Amur region and the central part of Sikhote-Alin, and the Larami sought (end of the Cretaceous - beginning of the Paleogene), which culminated in the formation of folded structures in Sikhote-Alin. The Koryak region was also created by the Laramie folding.

The mountain structures of Sakhalin and Kamchatka arose as a result of the Pacific folding, which appeared in the Oligocene and mainly in Neogene-Quaternary time, i.e. are at the orogenic stage of development. These are the youngest folded and volcanic mountains in Russia. The Kuril Islands have not yet completed geosynclinal development; These are modern island arcs with a deep-sea trench located next to them, clearly identifying the subduction zone of the Pacific lithospheric plate. Vast areas here are occupied by the oceanic crust. In fact, island arcs are characterized by early stages of the formation of the continental crust.

Continuing tectonic activity, especially along the eastern margin of this belt, is evidenced by intense volcanic activity, large amplitude of Quaternary uplifts and high seismicity of the region.

Mediterranean geosynclinal belt- one of the main mobile belts of the Earth that developed during the late Precambrian and Phanerozoic. The belt extends in the general latitudinal direction from the Atlantic to the Pacific Ocean, covering Central and Southern Europe, North-West Africa (Maghreb), the Mediterranean, the Caucasus, Western Asia, the Pamirs, Tibet, the Himalayas, the Indochina Peninsula, Indonesia and merging here with the Pacific geosynclinal belt ( western branch).

The origin of the belt, judging by the age of the most ancient ophiolites, dates back to the Late Proterozoic (Riphean); Most researchers believe that it occurred as a result of the destruction of the supercontinent, which at the beginning of the Riphean united the future Laurasia and Gondwana, namely the Eastern European, African-Arabian, Hindustan, Chinese-Korean and South Chinese (Yangtze) ancient platforms. In Central and Central Asia, the Mediterranean geosynclinal belt almost touches the Ural-Okhotsk belt, and in the area of ​​the British Isles - with the North Atlantic belt. The first stage of the development of the belt dates back to the late Riphean-Vendian - early Cambrian (in Western Europe it is called the Cadomian, to the east - the Baikal, Salair). The stage ended with folding, metamorphism (mainly greenschist facies) and moderate-scale granite formation. The resulting continental crust was not stable, surviving from subsequent destruction within Nubia, Arabia and Western Asia and in individual massifs in other parts of the belt (northern Armorican massif in France, North Caucasus massif, etc.). A new expansion with the formation of oceanic crust (Paleotethys) occurred in the Cambrian - Ordovician.

It is not yet clear whether this basin was partially inherited from the Riphean-Vendian or was entirely newly formed. At the beginning of the Devonian, the development of the northern periphery of the basin in Europe from southern Great Britain to Poland culminated in a new era of diastrophism; this Caledonian folded zone built up the East European Platform and the Great Britain Midland massif bordering the North Atlantic belt. In Asia, the Caledonian folded zone, the geosynclinal development of which began in the Vendian - Early Cambrian, covers the Qilianshan ridge and the northern slope of the Qinling ridge and adjoins the Sino-Korean platform from the south. In the Devonian, the zone of active subsidence shifts to the south, into Central Europe, the Iberian Peninsula, the Maghreb, the North Caucasus, the Northern Pamirs, Kunlun, and Central Qinling. Starting from the middle of the Early Carboniferous, it was involved in fold-thrust deformations (their first phases date back to the 2nd half of the Devonian), which created the Hercynian structures (see Hercynian folding). As a result, the western part of the belt experienced complete regeneration of the continental crust and drying; here Laurasia merged with Gondwana into a single supercontinent - Pangea.

In the east, in Asia, in the late Paleozoic there was only a new shift of the area of ​​maximum subsidence to the south, to the southern slope of the Greater Caucasus, to Central Afghanistan, the Pamirs and Tibet, as well as the Indochina Peninsula and partly Indonesia. The development of this zone - Mesotethys - ended with folding, granitization and mountain building at the end of the Triassic and the beginning of the Jurassic; the corresponding era is known in the west as Early Cimmerian, in the east as Indosinian. At the end of the Triassic - the beginning of the Jurassic, Eurasia again completely separated from Gondwana, and a new deep-sea basin with oceanic crust opened up - the Tethys proper, or Neo-Tethys, which extended in the west to Central America. Its axial zone is shifted even further to the south compared to the Paleo- and Mesotethys, in the east to the area of ​​Baikal consolidation. The first deformations of this belt date back to the end of the Jurassic - mid-Cretaceous (Late Cimmerian, Austrian eras); the main deformations - towards the end of the Eocene - the end of the Miocene, the main mountain formation - from the end of the Miocene. As a result of these processes, the Alpine-Himalayan folded mountain belt arose, stretching from the Pyrenees and Gibraltar to Indonesia. Active mountain building, seismic activity, and in the Mediterranean and Indonesia, volcanism continue in this belt in the modern era. Foredeep and intermountain troughs are distinguished by rich oil and gas potential; deposits of ferrous and non-ferrous metal ores are known in mountain structures. Simultaneously with mountain building in the Alpine-Himalayan belt, the formation of deep-sea depressions in the Mediterranean and Indonesia with oceanic-type crust took place.

Nature of Russia

Geography textbook for 8th grade

§ 6. Geological structure of the territory of Russia

• What structure does the lithosphere have?
• What phenomena occur at its plate boundaries?
• How are seismic belts located on Earth?

Structure of the earth's crust. The largest features of the country's relief are determined by the peculiarities of the geological structure and tectonic structures. The territory of Russia, like the whole of Eurasia, was formed as a result of the gradual convergence and collision of individual large lithospheric plates and their fragments.

The structure of lithospheric plates is heterogeneous. Within their boundaries there are relatively stable areas - platforms and mobile folded belts.

The oldest crust of the earth was formed by gravitational mixing

The location of the largest landforms - plains and mountains - depends on the structure of lithospheric plates. The plains are located on platforms.

Tectonic structures and the time of their formation are shown on tectonic maps, without which it is impossible to explain the patterns of location of the main relief forms.

Mountains formed in mobile folded belts. These belts arose at different times in the marginal parts of lithospheric plates when they collided with each other. Sometimes fold belts are found in the inner parts of a lithospheric plate. This is, for example, the Ural ridge. This suggests that once there was a boundary between two plates, which later turned into a single, larger plate.

The geological history of the Earth begins with the formation of the earth's crust. The oldest rocks indicate that the age of the lithosphere is more than 3.5 billion years.

The period of time corresponding to the longest (prolonged) stage of development of the earth's crust and the organic world is usually called the geological era. The entire history of the Earth is divided into five eras: Archean (ancient), Proterozoic (era of early life), Paleozoic (era of ancient life), Mesozoic (era of middle life), Cenozoic (era of new life). Eras are divided into geological periods. The names of periods most often come from the areas where the corresponding deposits were first found.

Geological chronology, or geochronology, is a branch of geology that studies the age, duration and sequence of formation of the rocks that make up the earth's crust.

Sciences that study the earth's crust

The diversity of modern relief is the result of long-term geological development and the influence of modern relief-forming factors, including human activity. Geology deals with the study of the structure and history of the Earth. Modern geology is divided into a number of branches: historical geology studies the patterns of the structure of the earth's crust over geological time; geotectonics is the study of the structure of the earth's crust and the formation of tectonic structures (folds, cracks, shifts, faults, etc.). Paleontology is the science of extinct (fossil) organisms and the development of the organic world of the Earth. Mineralogy and petrography study minerals and other natural chemical compounds. If the occurrence of rocks is not disturbed by crushing, folds, or breaks, then each layer is younger than the one on which it lies, and the topmost layer was formed later than all.

In addition, the relative age of rocks can be determined from the remains of extinct organisms.

They learned to determine the absolute age of rocks quite accurately only in the 20th century. For these purposes, the process of decay of radioactive elements contained in the rock is used.

Geochronological table contains information about the successive change of eras and periods in the development of the Earth and their duration. Sometimes the table indicates the most important geological events, stages in the development of life, as well as the most typical minerals for a given period, etc.

The table is built from the most ancient stages of the Earth's development to the modern one, so it needs to be studied from bottom to top. Using a geochronological table, you can obtain information about the duration and geological events in different eras and periods of the Earth's development.

Geological maps contain detailed information about what rocks are found in certain areas of the globe, what minerals lie in their depths, etc.

Rice. 15. Geological chronology. History of the development of the Earth

A geological map will allow you to get an idea of ​​the distribution of rocks of different ages throughout Russia. Please note that the most ancient rocks come to the surface in Karelia and Transbaikalia.

In the course of the geography of continents and oceans, you have already become acquainted with a map of the structure of the earth's surface, that is, with a tectonic map. By studying the tectonic map of Russia, you can obtain detailed information about the location and age of various tectonic structures within our country.

Rice. 16. Tectonic structures of the world

Compare the geological and tectonic maps and determine which tectonic structures the outcrops of ancient rocks are associated with.

Analysis of the tectonic map of Russia allows us to draw the following conclusions.

Areas with flat relief are confined to platforms - stable areas of the earth's crust, where folding processes have long ended. The most ancient of the platforms are the East European and Siberian. At the base of the platforms lies a hard foundation composed of igneous and highly metamorphosed rocks of Precambrian age (granites, gneisses, quartzites, crystalline schists). The foundation is usually covered with a cover of horizontally occurring sedimentary rocks, and only on the Siberian Platform (Central Siberian Plateau) are significant areas occupied by volcanic rocks - Siberian traps.

Using the map (Fig. 16), determine within which lithospheric plates the territory of Russia is located.

The outcrops of the foundation, composed of crystalline rocks, to the surface are called shields. In our country, the Baltic Shield on the Russian Platform and the Aldan Shield on the Siberian Platform are known.

Compare the tectonic and physiographic maps and determine what landforms are characteristic of the shields.

Rice. 17. Platform structure

Mountain areas have a more complex geological structure. Mountains are formed in the most mobile areas of the earth's crust, where, as a result of tectonic processes, rocks are crushed into folds and broken by faults and faults. These tectonic structures arose at different times - during the eras of Paleozoic, Mesozoic and Cenozoic folding. The youngest mountains of our country are located in the Far East, namely on the Kuril Islands and Kamchatka. They are part of the vast Pacific Volcanic Belt, or the Pacific Ring of Fire as it is called. They are characterized by significant seismicity, frequent strong earthquakes, and the presence of active volcanoes.

Rice. 18. Structure of the folded region

Information from geological and tectonic maps is necessary not only for geologists and geographers, but also for builders, as well as representatives of other professions.

Table 2. Main active volcanoes in Russia

To successfully work with these rather complex maps, you must first carefully study their legends.

Questions and tasks

1. What sciences study the history of the development of the Earth?
2. What information can be obtained from a geochronological table?
3. What is shown on a tectonic map?
4. Using a geochronological table, compose a story about the formation of the main forms of the surface of our country.
5. Determine from the geochronological table what era and period we live in; what geological events are currently taking place; what minerals are formed.

EARTH CRUST (a. earth crust; n. Erdkruste; f. croute terrestre; i. сorteza terrestre) - the upper solid shell of the Earth, limited below by the Mohorovicic surface. The term "earth's crust" appeared in the 18th century. in the works of M.V. Lomonosov and in the 19th century. in the works of the English scientist Charles Lyell; with the development of the contraction hypothesis in the 19th century. received a certain meaning arising from the idea of ​​cooling the Earth until the crust formed (American geologist J. Dana). Modern ideas about the structure, composition and other characteristics of the Earth’s crust are based on geophysical data on the speed of propagation of elastic waves (mainly longitudinal, V p), which at the Mohorovicic boundary increase abruptly from 7.5-7.8 to 8.1-8 .2 km/s. The nature of the lower boundary of the Earth's crust is apparently due to changes in the chemical composition of rocks (gabbro - peridotite) or phase transitions (in the gabbro - eclogite system).

In general, the Earth's crust is characterized by vertical and horizontal heterogeneity (anisotropy), which reflects the different nature of its evolution in different parts of the planet, as well as its significant processing during the last stage of development (40-30 million years), when the main features of modern life were formed. face of the Earth. A significant part of the Earth's crust is in a state of isostatic equilibrium (see Isostasy), which, if disrupted, is restored quite quickly (104 years) due to the presence of the Asthenosphere. There are two main types of the Earth's crust: continental and oceanic, differing in composition, structure, thickness and other characteristics (Fig.). The thickness of the continental crust, depending on tectonic conditions, varies on average from 25-45 km (on platforms) to 45-75 km (in mountain-building areas), however, it does not remain strictly constant within each geostructural area.

In the continental crust, sedimentary (V p up to 4.5 km/s), “granite” (V p 5.1-6.4 km/s) and “basaltic” (V p 6.1-7.4 km/s) are distinguished. c) layers. The thickness of the sedimentary layer reaches 20 km; it is not distributed everywhere. The names of “granite” and “basalt” layers are arbitrary and are historically associated with the identification of the Conrad boundary separating them (V p 6.2 km/s), although subsequent studies (including ultra-deep drilling) showed some dubiousness of this boundary (and according to some data its absence). Both of these layers are therefore sometimes combined into the concept of consolidated crust. The study of outcrops of the “granite” layer within the shields showed that it includes rocks not only of the granite composition itself, but also various gneisses and other metamorphic formations. Therefore, this layer is often also called granite-metamorphic or granite-gneiss; its average density is 2.6-2.7 t/m3. Direct study of the “basalt” layer on continents is impossible, and the seismic wave velocities by which it is identified can be satisfied by both igneous rocks of basic composition (mafic rocks) and rocks that have experienced a high degree of metamorphism (granulites, hence the name granulite-mafic layer) . The average density of the basalt layer ranges from 2.7 to 3.0 t/m3.

The main differences between the oceanic crust and the continental one are the absence of a “granite” layer, significantly lower thickness (2-10 km), younger age (Jurassic, Cretaceous, Cenozoic), and greater lateral homogeneity. The oceanic crust consists of three layers. The first layer, or sedimentary layer, is characterized by a wide range of velocities (V from 1.6 to 5.4 km/s) and a thickness of up to 2 km. The second layer, or acoustic foundation, has an average thickness of 1.2-1.8 km and Vp 5.1-5.5 km/s. Detailed studies made it possible to divide it into three horizons (2A, 2B and 2C), with horizon 2A having the greatest variability (V p 3.33-4.12 km/s). Deep-sea drilling has established that horizon 2A is composed of highly fractured and brecciated basalts, which become more consolidated with increasing age of the oceanic crust. The thickness of the horizon 2B (V p 4.9-5.2 km/s) and 2C (V p 5.9-6.3 km/s) is not constant in different oceans. The third layer of oceanic crust has fairly close values ​​of V p and thickness, which indicates its homogeneity. However, its structure also shows variations in both speed (6.5-7.7 km/s) and power (from 2 to 5 km). Most researchers believe that the third layer of oceanic crust is composed of rocks mainly of gabbroic composition, and variations in velocities in it are determined by the degree of metamorphism.

In addition to the two main types of the Earth's crust, subtypes are distinguished based on the ratio of the thickness of individual layers and the total thickness (for example, transitional type crust - subcontinental in island arcs and suboceanic on continental margins, etc.). The earth's crust cannot be identified with the lithosphere, which is established on the basis of rheology and properties of matter.

The age of the oldest rocks of the Earth's crust reaches 4.0-4.1 billion years. The question of what was the composition of the primary Earth's crust and how it was formed during the first hundred million years is not clear. During the first 2 billion years, apparently, about 50% (according to some estimates, 70-80%) of all modern continental crust was formed, the next 2 billion years - 40%, and only about 10% accounted for the last 500 million .years, i.e. to the Phanerozoic. There is no consensus among researchers on the formation of the Earth's crust in the Archean and Early Proterozoic and the nature of its movements. Some scientists believe that the formation of the Earth's crust occurred in the absence of large-scale horizontal movements, when the development of rift greenstone belts was combined with the formation of granite-gneiss domes, which served as nuclei for the growth of the ancient continental crust. Other scientists believe that since the Archean, an embryonic form of plate tectonics was in operation, and granitoids formed above Subduction Zones, although there were no large horizontal movements of the continental crust yet. The turning point in the development of the Earth's crust occurred in the late Precambrian, when, under the conditions of the existence of large plates of already mature continental crust, large-scale horizontal movements became possible, accompanied by subduction and obduction of the newly formed lithosphere. Since that time, the formation and development of the Earth's crust has occurred in a geodynamic setting determined by the mechanism of plate tectonics.

The "little cortex" is usually identified with the sialic membrane; in other words, the earth’s crust includes “layers” of granite and basalt. In this case, the thickness, i.e., the thickness of the earth’s crust within the vast flat expanses of the continents, will be determined by a figure of the order of 40–50 km, under mountain ranges - up to 80 km, and disappears under the ocean.

Another option can be proposed: consider that the earth’s crust is the outer crystalline solid shell of the globe, within which the temperature varies from 0° at the surface to 1300–1500° at depth (i.e., it increases to the melting temperature of rocks). In this case, the thickness of the earth’s crust will be equal to 100–130 km, regardless of the composition of the rocks composing it and regardless of where we consider it - on the continent or in the ocean.

Whatever meaning we give to the term “earth’s crust,” we who live on the surface of the Earth are especially interested in the structure of its most superficial parts, which are composed primarily of sedimentary rocks.

By studying the composition, location and other features and properties of sedimentary rocks, we discover the following important circumstance.

Vast areas of plains - such as Russian or Siberian - are composed of a variety of sedimentary rocks on the surface, forming layers of low thickness and horizontal occurrence. Indeed, in any cliff, in a ravine, on the slope of a river-washed bank or in an artificial quarry, you can see similar rocks - sands or sandstones, clays or limestones, occurring in the form of clearly defined horizontal layers, spreading far to the sides, but quickly replacing each other in the vertical direction. By their origin, these rocks most often turn out to be marine, as evidenced by the fossilized remains of marine animals contained in them, for example, belemnites, ammonites, etc.; Often there are rocks of continental, terrestrial origin, as evidenced by the remains of plants of former times contained in them; such are, say, coal and peat.

Such rocks have changed very little over time. Of course they are compacted; Compared to the original loose sediment from which they were formed, they acquired new features, but still the compaction process did not disrupt their structure, did not change the conditions of occurrence, and did not damage the fossils. In some cases the rocks retain their freshness to such an extent that they appear to have been deposited just now; These are, say, the Cambrian clays near Leningrad. These clays are at least 500 million years old, and they are so fresh and pliable, as if they were formed quite recently.

Among such calmly lying strata of little altered sedimentary rocks, igneous rocks are almost never found; here, among the plains, as a rule, there are no volcanoes, no geysers, no hot springs, or other manifestations of volcanic life; earthquakes do not occur here either.

All the properties described above are inherent in those parts of the earth’s crust that are called “platforms”. Within the platforms, tectonic movements are very weak. They are expressed only in the fact that the platform as a whole or its individual parts experience very slow, barely noticeable rises or subsidences, replacing each other over time, which leads either to the advance of the sea onto the land, or to a retreat. Hence the change in the composition of sediments accumulating on the platforms. This expresses the so-called oscillatory movements. Consequently, platforms should be understood as relatively stable, sedentary areas of the earth’s crust, within which low-thickness sediments accumulate, the layers lie in an undisturbed position, there are no manifestations of volcanism, there are no earthquakes, and there are no mountain ranges.

The exact opposite of platforms are the so-called “folded zones,” an example of which are mountain systems such as the Carpathians or the Caucasus. First of all, what surprises us here is the enormous thickness of the sedimentary rocks: if on platforms the thickness of sedimentary strata is measured in tens or, less often, hundreds of meters, then within the folded zones it is measured in many thousands of meters. How could such huge masses of sediments, and, as a rule, marine sediments, accumulate? We have no other explanation than to assume that, in parallel with the accumulation of sediments, the bottom of the corresponding basin sagged, thereby giving way to new portions of sediment. It follows that in the history of the development of the folded zone it is necessary to distinguish some early stage, characterized by the predominance of subsidence over uplifts. The dives were quite large in scale and quite long in time. Such an early stage in the development of a folded zone is called “geosynclinal,” and a section of the crust in this state is called a “geosyncline.” The geosynclinal regime usually persists for several periods (for example, for the Urals - throughout the Paleozoic, for the Caucasus - even longer) and leads to the accumulation of those huge thicknesses of sediment, which were mentioned above.

Then comes the second stage in the development of the geosyncline. Within its boundaries, various and highly intense motion processes begin to appear. First of all, these are tectonic movements themselves, which crush layers, lead to the formation of folds, sometimes enormous and very complex, to ruptures and movements of some areas relative to others. It is enough to look at the sections of bedrock, which appear in abundance before us in any mountainous country, to be convinced that it is almost impossible to find an undisturbed area here: everywhere the layers are crumpled (Fig. 14) and bent or stand vertically, and sometimes overturned and torn. Such tectonic disturbances are one of the main objects of study of that branch of geology called “tectonics.”

But it is not only tectonic disturbances in the layers that distinguish the folded zone. The rocks themselves have been changed so much that it is sometimes difficult to imagine what they were like before. Instead of limestone, marble appears, instead of sandstone - quartzite, instead of dense clay - crystalline slate, etc. This is reflected in the so-called processes of “metamorphism” (changes). They consist of the impact on rocks of high temperature and high pressure - both from the weight of the rocks lying above a given point, and from tectonic forces. As a result, the rocks recrystallize, acquire a different structure, new minerals appear in them, and almost nothing remains of their previous appearance. These are the rocks that are called metamorphic; they are widespread within folded zones.

Another feature of folded zones is the abundance of igneous rocks. Volcanic phenomena here are extremely diverse. Extensive intrusions of acidic or basic magma into the thickness of sedimentary rocks, which, after solidification of the magma, turn into huge buried crystalline bodies - “batholiths”; implantations that solidify closer to the surface and give mushroom-shaped forms - “laccoliths”; various veins, interlayer injections of magma, small-sized “stocks”, etc., up to ordinary volcanoes and underwater eruptions - these are the forms of manifestation of volcanic forces, countless in variety and scale, leading to the accumulation of igneous rock massifs in the thickness of the crust. The interaction between igneous and sedimentary rocks is an object of geological research, since important minerals often appear in contact between both.

The characteristics of the folded zone should be supplemented by the fact that the period of revival of tectonic movements ends, as a rule, with the general drying of this section of the geosyncline, its uplift and the formation of high mountains. In parallel with this, many earthquakes occur in the area of ​​the developing folded zone.

So, after a long stage of geosynclinal development, tectonic movements of high intensity, both oscillatory and fold-forming, begin to appear; Numerous folds and ruptures appear in the thickness of previously accumulated rocks, intense volcanic and seismic activity is noted; processes of metamorphism occur everywhere, and finally mountains are formed. The geosyncline thus turns into a folded zone.

Subsequently, all the processes described above die out, and the mountains, exposed to prolonged exposure to various external agents - rivers, wind, sunlight, frost, etc. - are destroyed, smoothed out and gradually disappear, giving way to a flat plain. Consequently, a platform appears in place of the previous geosyncline. The geosyncline passes through the stage of the folded zone into a platform.

Of course, geosynclines, folded zones and platforms can be of different ages. Thus, in Norway, the geosynclinal regime ceased at the beginning of the Paleozoic era (in the Silurian period). Throughout the Paleozoic, the Urals was a geosyncline; at the end of the Paleozoic era, tectonic movements manifested themselves here with great intensity, and, finally, from the middle of the Mesozoic era, a stable, sedentary platform formed in place of the Urals. In the Caucasus, the geosynclinal regime persisted longer, until the end of the Mesozoic era; Now the Caucasus is a typical folded zone, which is in the process of intensive development. Several million years will pass, processes of internal origin will subside, and the Caucasus will begin to turn into a platform. The Russian platform also once (a very long time ago, even before the Paleozoic) experienced an era of extremely strong movements, with abundant intrusions of igneous rocks and the strongest metamorphization of all strata, and by the beginning of the Paleozoic era a platform regime had already taken shape almost everywhere here. We see traces of the violent revolutions of the past in those rocks - metamorphic and igneous, which are exposed under the Paleozoic sedimentary cover in certain places on the Russian Platform - in Karelia, Ukraine, etc.

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.

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.

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, 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 (case) 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. Simultaneously with it, 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 the 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

Rice. 6.6. Earth's lithospheric plates and their active boundaries. Double lines indicate divergent boundaries (spreading axes); lines with teeth - convergent grains P.PIT

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.