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

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

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 In the consolidated crust there are two layers of different speeds: the upper granite-metamorphic (according to outdated ideas, this is a granite layer), composed of granites and gneisses, and the lower granulite-mafic (according to outdated ideas, this is a basalt layer), composed of highly metamorphosed basic rocks of the gabbro type or ultrabasic igneous rocks.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

In the lower part of the upper layer, a zone of weakened rocks is found, which differs little from it in composition and seismic characteristics. The reason for its occurrence is the metamorphism of rocks and their decompaction due to the loss of constitutional water. It is likely that the rocks of the granulite-mafic layer are all the same rocks, but even more highly metamorphosed.

oceanic crust characteristic of the oceans. It differs from the continental one in thickness and composition. Its thickness ranges from 5 to 12 km, averaging 6-7 km. From top to bottom, three layers are distinguished in the oceanic crust: the upper layer of loose marine sedimentary rocks up to 1 km thick; middle, represented by interbedding of basalts, carbonate and siliceous rocks, 1-3 km thick; the lower one, composed of basic rocks of the gabbro type, often metamorphosed to amphibolites, and ultrabasic amphibolites, thickness 3.5-5 km. The first two layers were drilled, the third one was characterized by dredging material.

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

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

Thus, various types of the earth's crust distinctly divide the earth into oceanic and continental blocks. The high position of the continents is explained by a more powerful and less dense earth's crust, and the submerged position of the ocean floor is explained by a thinner, but denser and heavier crust. The shelf area is underlain continental crust and is the underwater end of the continents.

Structural elements of the cortex

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

In the oceans, the following structural elements are distinguished:

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

On the continents, the main structural elements are:

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

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

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

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

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

So, platforms are ancient denuded orogens, not affected by later (young) orogeny movements.

As opposed to calm platform regions, there are tectonically active geosynclinal regions on Earth. The geosynclinal process can be compared to the work of a huge deep-seated cauldron, where ultrabasic and basic magma and lithosphere material are “boiled” new light continental crust, which, rising up, builds up continents in the marginal (Pacific) and welds them together in intercontinental (Mediterranean) geosynclines. This process ends with the formation of folded mountain structures, in the arched part of which volcanoes can work for a long time - the site. Over time, the growth of mountains stops, volcanism fades, the earth's crust enters a new cycle of its development: the alignment of the mountain structure begins.

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

Earth is a cosmic body that is part of the solar system. Considering the origin of the continents and oceans, it is worth touching on the issue of the origin of the planet.

How our planet was formed

The origin of the continents and oceans is the second question. The first is to explain the causes and method of the formation of the Earth. Its solution was dealt with by the pundits of antiquity. Many hypotheses have been put forward to explain their consideration - the prerogative of astronomy. One of the most common is the hypothesis of O.Yu. Schmidt, which states that our planet arose from a cold cloud of gas and dust. The particles that make up it, while rotating around the Sun, were in contact with each other. They stuck together, and the resulting lump increased in size, its density increased, and the structure changed.

There are other hypotheses explaining the appearance of planets. Some of them suggest that space bodies, including the Earth - the result of explosions in outer space high power, which led to the decay of stellar matter. Many scientists are still searching for the truth about the origin of the planet.

The structure of the earth's crust under the continents and oceans

Studying the origin of continents and oceans Grade 7 high school. Even students know that the top layer of the lithosphere is called the earth's crust. It is a kind of "cloak" that covers the seething bowels of the planet. If you compare it with others, it will seem like the thinnest film. Its average thickness is only 0.6% of the planet's radius.

The origin of the continents and depressions of the oceans that determine appearance Earth, it will become clearer if you first study the structure of the lithosphere. consists of continental and oceanic plates. The first consist of three layers (bottom-up): basalt, granite and sedimentary. Oceanic plates are devoid of the last two, so their thickness is much less.

Differences in the structure of plates

The question that geography studies (grade 7) is the origin of the continents and oceans, as well as the distinctive features of their structure. According to the vast majority of scientists, only oceanic plates originally arose on Earth. Under the influence of processes occurring in the bowels of the earth, the surface became folded, mountains appeared. The crust became thicker, ledges began to appear, which later turned into continents.

Further transformation of continents and oceanic depressions is not so unambiguous. Scientists' opinions on this issue divided. According to one hypothesis, the continents do not move, according to another, they are constantly moving.

Recently, another hypothesis of the structure of the earth's crust has been substantiated. The basis for it was the theory of the movement of continents, the author of which was A. Wegener at the beginning of the 20th century. At one time, he failed to answer legitimate questions about the forces that make continents drift.

Lithospheric plates

The upper layer of the mantle, together with the earth's crust, is the lithosphere. The origin of continents and oceans is closely related to the theory of plates that are able to move, and are not shackled monolithically. many cracks reaching the mantle. They break the lithosphere into huge areas with a thickness of 60-100 km.

Plate junctions coincide with oceanic ridges that run through the middle of the oceans. They look like huge trees. The border may be in the form of gorges running along the bottom of the ocean. Cracks also exist on the territory of the continents, they pass through mountain ranges (the Himalayas, the Urals, etc.). We can say that these are old scars on the body of the Earth. There are also relatively fresh faults, these include crevices in eastern Africa.

Found 7 huge blocks and dozens of small areas. The main number of plates capture the oceans and continents.

The movement of the plates of the lithosphere

Under the plates there is a rather soft and plastic mantle, which makes their drift possible. The hypothesis of the origin of the continents and oceans says that the blocks are set in motion due to the forces arising from the movement of the substance in the upper part of the mantle.

Strong currents directed from the center of the Earth cause ruptures in the lithosphere. You can see this type of faults on the continents, but most of them are located in the zone of mid-ocean ridges under the thickness ocean waters. In this place, the earth's crust is much thinner. Substances in the molten state rise from the depths of the mantle and, pushing apart the plates, increase the thickness of the lithosphere. And the edges of the plates are moved in opposite directions.

Pieces of the earth's crust move from the ridges at the bottom of the oceans to the troughs. The speed of their movement is 1-6 cm/year. These figures are obtained from satellite images taken in different years. The plates in contact move towards, along or diverge. Their movement along the upper layer of the mantle resembles ice floes on the water.

When two plates move towards each other (oceanic and continental), then the first, having made a bend, goes under the second. The result is deep trenches, archipelagos, mountain ranges. Examples: the islands of Japan, the Andes, the Kuril Trench.

When continental plates collide, folding is formed as a result of crushing of the edges containing sedimentary layers. So the Himalayan mountains appeared at the junction of the Indo-Australian and Eurasian plates.

Continental evolution

Why does geography study the origin of continents and oceans? Because the understanding of these processes is necessary for the perception of other information related to this science. The theory of lithospheric plates suggests that at first one single continent appeared on the planet, the rest was occupied by the World Ocean. The deep faults of the crust that appeared led to its division into two continents. Laurasia is located in the northern hemisphere, and Gondwana is in the southern hemisphere.

All new cracks appeared in the earth's crust, they led to the division of these continents. The continents that exist now, as well as the oceans: the Indian and the Atlantic, arose. The basis of modern continents are platforms - aligned, very ancient and stable areas of the crust. In other words, these are plates that formed a long time ago by geological standards.

In places where sections of the earth's crust collided, mountains turned out. On the separate continents traces of contact of several plates are visible. Their surface area gradually increased. In a similar way the Eurasian continent emerged.

Plate Movement Forecast

The theory of lithospheric plates involves calculations of their future movement. The calculations that were made by scientists indicate that:

• Indian and Atlantic Oceans will increase.
• The African continent will be shifted towards the northern hemisphere.
• The Pacific will get smaller.
• The Australian mainland will overcome the equator and join the Eurasian one.

According to forecasts, this will happen no earlier than in 50 million years. However, these results need to be refined. The origin of continents and oceans, as well as their movement, is a very slow process.

In the mid-ocean ridges, new lithospheric plates are being formed. The resulting oceanic-type crust smoothly diverges away from the fault. In 15 or 20 million years, these blocks will reach the mainland and go under it into the mantle that created them. The cycle of lithospheric plates closes on this.

seismic belts

Studying the origin of continents and oceans Grade 7 secondary school. Knowing the basics will help students understand more difficult questions by subject. The joints between the plates of the lithosphere are called seismic belts. These places clearly demonstrate the processes occurring at the boundary of the plates. The vast majority of volcanic eruptions and earthquakes are confined to these areas. Now there are about 800 volcanoes on the planet.

The origin of continents and oceans must be known for forecasting natural Disasters and prospecting for minerals. There is an assumption that different ores are formed in the places of plate contact as a result of magma entering the crust.

The continental crust has a three-layer structure:

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

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

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

22. Structure and development of mobile belts.

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

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

The first one has two main stages: early geosynclinal and late geosynclinal.

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

Late geosynclinal the stage begins at the moment of complication of the internal structure of the mobile belt, which is due to compression processes, which become more and more pronounced in connection with the incipient closure of the ocean basin and the oncoming movement of lithospheric plates.

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

23. Platforms and stages of their development.

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

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

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

2) Cover. The upper structural stage is composed of gently sloping non-metamorphosed layered strata - sedimentary, marine and continental deposits.

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

1) ancient platforms occupy about 40% of the area of ​​the continents

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

Stages of platform development.

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

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

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

4) Stage of activation. epiplatform orogens ( mountain)

Continents and oceans are the largest elements in the structure of the Earth's crust. Speaking of oceans, one should keep in mind the structure of the crust within the areas occupied by the oceans.

The composition of the earth's crust is different between continental and oceanic. This, in turn, leaves an imprint on the features of their development and structure.

The boundary between the mainland and the ocean is drawn at the foot of the continental slope. The surface of this foot is an accumulative plain with large hills, which are formed due to underwater landslides and alluvial fans.

In the structure of the oceans, sections are distinguished according to the degree of tectonic mobility, which is expressed in manifestations seismic activity. On this basis, distinguish:

• seismically active areas(oceanic mobile belts),
• aseismic regions (ocean basins).

Mobile belts in the oceans are represented by mid-ocean ridges. Their length is up to 20,000 km, width - up to 1,000 km, height reaches 2-3 km from the bottom of the oceans. In the axial part of such ridges, one can almost continuously trace rift zones. They are marked with high values heat flow. Mid-ocean ridges are considered as areas of stretching of the earth's crust or zone spreading.

The second group of structural elements - ocean basins or thalassocratons. These are flat, slightly hilly areas of the seabed. The thickness of the sedimentary cover here is no more than 1000 m.

Another major element of the structure is the transition zone between the ocean and the mainland (continent), some geologists call it mobile geosynclinal belt. This is the area of ​​maximum dissection earth's surface. This includes:

1-island arcs, 2 - deep trenches, 3 - deep sea trenches marginal seas.

island arcs- these are extended (up to 3000 km) mountain structures formed by a chain of volcanic structures with modern manifestation basaltic andesite volcanism. An example of island arcs is the Kuril-Kamchatka ridge, the Aleutian Islands, etc. From the ocean side, island arcs are replaced deep sea trenches , which are deep depressions with a length of 1500-4000 km, a depth of 5-10 km. The width is 5-20 km. The bottoms of the gutters are covered with sediments, which are brought here by turbidity streams. The slopes of the gutters are stepped with different angles of inclination. No deposits were found on them.

The boundary between the island arc and the slope of the trench represents the zone of concentration of earthquake sources and is called the zone Wadati-Zavaritsky-Benioff.

Considering the signs of modern ocean margins, geologists, relying on the principle of actualism, conduct a comparative historical analysis of similar structures that formed in more ancient periods. These signs include:

• marine type of sediments with a predominance of deep-sea sediments,
• linear form of structures and bodies of sedimentary strata,
• abrupt change thickness and material composition of sedimentary and volcanic strata in a cross strike of folded structures,
• high seismicity,
• a specific set of sedimentary and igneous formations and the presence of indicator formations.

Of these signs, the latter is one of the leading ones. Therefore, we define what a geological formation is. First of all, it is a real category. In the hierarchy of the matter of the earth's crust, you know the following sequence:

Chem. element→ mineral → rock → geological formation

A geological formation is a more complex stage of development following a rock. It is a natural association of rocks, connected by the unity of the material composition and structure, which is due to the commonality of their origin or location. Geological formations are distinguished in groups of sedimentary, igneous and metamorphic rocks.

For the formation of stable associations of sedimentary rocks, the main factors are the tectonic setting and climate. Examples of formations and the conditions for their formation will be considered in the analysis of the development of structural elements of continents.

There are two types of regions on the continents.

I type is the same as mountainous areas, in which sedimentary deposits are crumpled into folds and broken by various faults. Sedimentary sequences are intruded by igneous rocks and metamorphosed.

II the type coincides with flat areas, on which deposits occur almost horizontally.

The first type is called a folded region or folded belt. The second type is called a platform. These are the main elements of the continents.

Folded areas are formed at the site of geosynclinal belts or geosynclines. Geosyncline- this is a mobile extended area of ​​deep deflection of the earth's crust. It is characterized by the accumulation of thick sedimentary strata, prolonged volcanism, a sharp change in direction tectonic movements with the formation of folded structures.

Geosynclines are divided into:

1. Eugeosinklinal - represents inner part moving belt,

2. Miogeosyncline - the outer part of the mobile belt.

They are distinguished by the manifestation of volcanism, the accumulation of sedimentary formations, folded and discontinuous deformations.

There are two stages in the formation of the geosyncline. In turn, in each of the stages, the stages are distinguished, which are characterized by: certain type tectonic movements and geological formations. Let's consider them.

stages	Stages of tectonic movements	Movement sign	Formations in:
			Miogeosynclines	Eugeosynclines
	1. Early geosynclinal Lowering - relief irregularities are formed, by the end of the stage, a partial inversion i.e. relative descent and rise individual sections geosynclines 2.Late geosynclinal Shallowing of the sea, formation of island arcs and marginal seas	↓ ↔ → ←	Slate (black shale) sandy-clayey Flysch - rhythmic interbedding of sandy-silty sediments and limestones	Basaltic volcanism with siliceous sediments Differentiated: basalt-andesite-rhyolitic lavas and tuffs
	1.Early orogenic The formation of a central uplift and marginal deflections, the speed of movement is low. The sea is shallow 2.orogenic A sharp rise in the central rise with splits into blocks. Intermountain depressions in the middle massifs	→ ← → ←	Thin molasses -fine clastic rocks + saline and coal-bearing strata Rough molasse continental coarse sediments	Intrusion of granite batholiths Porphyritic: terrestrial alkaline andesite-iolite volcanism, stratovolcanoes

The time from the beginning of the origin of the geosyncline to the completion of its development is called the stage of folding (tectonic epoch). In the history of the formation of the earth's crust, several tectonic epochs are distinguished:

1. Precambrian, unites several epochs, among which we single out Baikal stage of folding, ended in the early Cambrian.

2. Caledonianfolding - occurred in the early Paleozoic, was maximally manifested at the end of the Silurian. The Scandinavian mountains, Western Sayan, etc.

3. Hercynianfolding - occurred in the late Paleozoic. It includes folded structures Western Europe, Ural, Appalachians, etc.

4. Mesozoic(Cimmerian) - covers the entire MZ . The Cordillera, Verkhoyansk-Chukotka folded regions were formed.

5. Alpinefolding - manifested itself in Cenozoic era and continues now. The Andes, Alps, Himalayas, Carpathians, etc.

After the completion of folding, a section of the earth's crust may again be involved in the next geosynclinal cycle. But in most cases, after the completion of mountain building, the epigeosynclinal stage of development of the folded area begins. Tectonic movements become slow oscillatory (huge areas experience slow subsidence or rise), as a result of which powerful strata of sedimentary formations accumulate. Magmatic activity takes on new forms. In this case, we are talking about the platform stage of development. And large areas of the earth's crust with a stable tectonic development regime are called platforms.

Platform features:

1-marine shallow, lagoonal and terrestrial types of sediments;

2-slope occurrence of layers,

3- sustained over large areas, the composition and thickness of deposits,

4-lack of metamorphism of sedimentary strata, etc.

Common in the structure of the platforms - there are always two floors: 1 - lower folded and metamorphosed, broken through by intrusions - called the foundation; 2 - upper, represents horizontally or gently sloping thick sedimentary strata, called a cover.

By the time of formation, the platforms are divided into ancient and young. The age of the platforms is determined by the age of the folded basement.

Ancient platforms are those in which the folded foundation is represented by granite-gneisses of the Archean-Proterozoic age. Otherwise, they are also called cratons.

The largest ancient platforms:

1-North American, 2-South American, 3-African-Arabian, 4-East European, 5-Siberian, 6-Australian, 7-Antarctic, 8-Indostan.

There are two types of structures on platforms - shields and slabs.

Shield- this is the section of the platform on which the folded foundation comes to the surface. In these areas, vertical uplift predominates.

Plate- part of the platform covered by a sedimentary cover. Slow vertical subsidence prevails here. In the structure of the plates, anteclises and syneclises are distinguished. Their formation is due to the uneven structure of the surface of the folded foundation.

Anteclises- areas of the sedimentary cover formed above the ledges of the folded basement. Signs of anteclise: reduction in the thickness of the sedimentary cover, breaks and wedging out of layers towards the anteclise dome.

syneclise- large depressions above the areas of immersion of the surface of the folded foundation.

Both forms are characterized by gently sloping (not >5 o) occurrence of layers and isometric forms in plan. Along with this, on the plates allocate aulacogens are graben-like deflections. They appear at an early stage of development of the platform cover and represent a system of stepped deep faults, along which the basement rocks subside and the thickness of the sedimentary rocks of the cover increases.

The junction zones of geosynclinal and platform areas are of two types.

edge seam- a linear zone of deep faults along the edge of the platform, arising from mountain building processes in the adjacent geosyncline.

Edge (forward) deflection - a linear zone on the border of the platform and the geosynclinal belt, formed as a result of the lowering of the edge blocks of the platform and part of the wing of the geosyncline. In the section, the marginal trough is an asymmetric synclinal shape, in which the wing from the side of the platform is flat, and the wing adjacent to the folded belt is steep.

The platform formation process can be divided into two stages.

The first stage is the beginning of the subsidence of the folded orogenic area and its transformation into the foundation of the platform. The second stage covers the process of formation of the sedimentary cover, which occurs cyclically. Each cycle is divided into stages, which are characterized by their own tectonic regime and a set of geological formations.

Stages of tectonic movements	Sign	formations
1. Immersion of the foundation sections along the faults - the initiation and development of aulacogen with the accumulation of sediments in it		Basal, lagoonal-continental in aulacogenes
2. Slab - immersion of a significant part of the platform		Transgressive marine terrigenous (sands, clays - often bituminous, clay-carbonate)
3 Maximum transgression		Carbonate (limestones, dolomites with interlayers of sandy-argillaceous rocks)
4 Shallowing of the sea - the beginning of the regression		Salt-bearing, coal or red
5 General lift - continental mode		Continental

In the development of platforms, epochs of tectonic activation are distinguished, in which the fragmentation of platforms along faults and the revival of several types of magmatism took place. Let's point out 2 main ones.

1. Fissure eruptions with the formation of thick covers of basic rocks - the formation of a trap formation (Siberian platform).

2. Intrusions of alkaline - ultrabasic formation (kimberlite) with explosion pipes. Diamond deposits in South Africa and Yakutia are associated with this formation.

On some platforms, such processes of tectonic activity are accompanied by uplifting of crustal blocks and mountain building. Unlike folded regions, they are called regions epiplatform orogeny, or lumpy.

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

On the continents, two main types of structures of the earth's crust are distinguished: calm stable - platforms and mobile - geosynclines. These structures are quite comparable in terms of their distribution area. The difference is observed in the rate of accumulation and in the magnitude of the gradient of change in thicknesses: platforms are characterized by a smooth gradual change in thicknesses, and geosynclines - by a sharp and fast one. On the platforms, igneous and intrusive rocks are rare; they are numerous in geosynclines. Flysch formations of sediments are underlying in geosynclines. These are rhythmically multilayered deep-water terrigenous deposits formed during fast dive geosynclinal structure. At the end of development, geosynclinal regions undergo folding and turn into mountain structures. In the future, these mountain structures go through a stage of destruction and a gradual transition to platform formations with a deeply dislocated lower floor of rock deposits and gently dipping layers in the upper floor.

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

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

For example, the East European platform, identified within the borders from the Urals to Ireland; from the Caucasus, the Black Sea, the Alps to the northern borders of Europe.

Distinguish ancient and young platforms.

ancient platforms arose on the site of the Precambrian geosynclinal region. The East European, Siberian, African, Indian, Australian, Brazilian, North American and other platforms were formed in the late Archean - early Proterozoic, represented by the Precambrian crystalline basement and sedimentary cover. Them distinguishing feature- Two storey building.

lower floor, or foundation It is composed of folded, deeply metamorphosed rock strata, crumpled into folds, cut through by granite intrusions, with a wide development of gneiss and granite-gneiss domes - a specific form of metamorphogenic folding (Fig. 7.3). The foundations of the platforms were formed over a long period of time in the Archean and early Proterozoic and subsequently underwent very strong erosion and denudation, as a result of which rocks that had previously occurred at great depths were exposed.

Rice. 7.3. Principal section of the platform

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

Top floor platforms presented case, or cover, flat-lying with a sharp angular unconformity on the basement of non-metamorphosed sediments - marine, continental and volcanogenic. The surface between the mantle and basement reflects the underlying structural unconformity within the platforms. The structure of the platform cover turns out to be complex, and on many platforms at the early stages of its formation, grabens, graben-like troughs - aulacogens(avlos - furrow, ditch; gene - born, i.e. born by a ditch). Aulacogens most often formed in the Late Proterozoic (Riphean) and formed extended systems in the basement body. The thickness of continental and, more rarely, marine deposits in aulacogens reaches 5–7 km, and deep faults that bounded aulacogens contributed to the manifestation of alkaline, basic, and ultrabasic magmatism, as well as platform-specific trap (mafic rocks) magmatism with continental basalts, sills, and dikes. Highly importance has an alkaline-ultrabasic (kimberlite) formation containing diamonds in the products of explosion pipes (Siberian platform, South Africa). This lower structural layer of the platform cover, corresponding to the aulacogenous stage of development, is replaced by a continuous cover of platform deposits. On the initial stage The development of the platform tended to slowly sink with the accumulation of carbonate-terrigenous strata, and at a later stage of development it is marked by the accumulation of terrigenous coal-bearing strata. In the late stage of platform development, deep depressions filled with terrigenous or carbonate-terrigenous deposits (Caspian, Vilyui) formed in them.

Platform cover in the process of formation has repeatedly undergone restructuring structural plan, timed to the boundaries of geotectonic cycles: Baikal, Caledonian, Hercynian, Alpine. Platform sections that experienced maximum subsidence, as a rule, are adjacent to the mobile area or system bordering on the platform, which was actively developing at that time ( pericratonic, those. on the edge of the craton, or platform).

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

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

Stove they are considered either a part of a platform with a tendency to sag, or an independent young developing platform (Russian, Scythian, West Siberian). Smaller structural elements are distinguished within the plates. These are syneclises (Moscow, Baltic, Caspian) - vast flat depressions, under which the foundation is bent, and anteclises (Belarusian, Voronezh) - gentle vaults with a raised foundation and a relatively thinned cover.

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

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

However, structures can be distinguished endogenous, exogenous and cosmogenic genesis.

Endogenous ring structures metamorphic and magmatic and tectonic (arches, ledges, depressions, anteclises, syneclises) origin have diameters from units of kilometers to hundreds and thousands of kilometers (Fig. 7.4).

Rice. 7.4. Ring structures north of New York

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

exogenous ring structures in the lithosphere are formed as a result of weathering, leaching, these are karst funnels, failures.

Cosmogenic (meteorite) ring structures are astroblems. These structures result from meteorite impacts. Meteorites with a diameter of about 10 kilometers fall to the Earth with a frequency of once every 100 million years, smaller ones much more often. Meteoritic ring structures can have diameters from tens of meters to hundreds of meters and kilometers. For example: Balkhash-Ili (700 km); Yukotan (200 km), depth - more than 1 km: Arizona (1.2 km), depth more than 185 m; South Africa (335 km), from an asteroid with a diameter of about 10 km.

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

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

Rift structures of continents (Fig. 7.5, 7.6) of small width up to 150 -200 km are expressed by extended lithospheric uplifts, the arches of which are complicated by subsidence grabens: Rhine (300 km), Baikal (2500 km), Dnieper-Donetsk (4000 km), East African (6,000 km), etc.

Rice. 7.5. Section of the Pripyat continental rift

Continental rift systems consist of a chain of negative structures (troughs, rifts) with a ranged time of inception and development, separated by uplifts of the lithosphere (saddles). Rift structures of continents can be located between other structures (anteclises, shields), cross platforms, and continue on other platforms. The structure of continental and oceanic rift structures is similar, they have a symmetrical structure about 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).

Rice. 7.6. Profile sections of continental rift systems

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

A part (link) of the Dnieper-Donets continental rift structure is the Pripyat trough. The Podlasko-Brest depression is considered to be the upper link; it may have genetic connection with similar structures in Western Europe. The lower links of the structure are the Dnieper-Donetsk depression, then similar structures Karpinskaya and Mangyshlak and further structures Central Asia (total length from Warsaw to the Hissar Range). All links of the rift structure of the continents are limited by listric faults, have a hierarchical subordination according to the age of occurrence, and have a thick sedimentary stratum promising for the content of hydrocarbon deposits.