The structure and age of the earth's crust

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

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

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

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

The age of the Earth is now estimated at approximately 4.2-6 billion years according to astronomical and radiometric data. The oldest rocks of the continental crust studied by man are up to 3.98 billion years old (southwestern part of Greenland), and the rocks of the basalt layer are over 4 billion years old. Undoubtedly, these rocks are not the primary matter of the Earth. The prehistory of these ancient rocks lasted many hundreds of millions, and perhaps even billions of years. Therefore, the age of the Earth is approximately estimated at 6 billion years.

The structure and development of the earth's crust of the continents

The largest structures of the earth's crust of the continents are geosynclinal folded belts and ancient platforms. They differ greatly from each other in their structure and history of geological development.

Before proceeding to the description of the structure and development of these main structures, it is necessary to talk about the origin and essence of the term "geosyncline". This term comes from the Greek words "geo" - the Earth and "synclino" - a deflection. It was first used by the American geologist D. Dan more than 100 years ago, while studying the Appalachian mountains. He established that the marine Paleozoic deposits that make up the Appalachians have a maximum thickness in the central part of the mountains, much greater than on their slopes. Dan explained this fact quite correctly. During the period of sedimentation in Paleozoic era on the site of the Appalachian Mountains there was a sagging depression, which he called the geosyncline. In its central part, the sagging was more intense than on the wings, which is evidenced by the large thickness of the deposits. Dan confirmed his findings with a drawing depicting the Appalachian geosyncline. Considering that sedimentation in the Paleozoic occurred in maritime conditions, he laid down from the horizontal line - the estimated sea level - all the measured thicknesses of sediments in the center and on the slopes of the Appalachian mountains. The figure turned out to be a clearly expressed large depression at the site of the modern Appalachian Mountains.

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

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

Ancient platforms are the most stable and inactive parts of the continents. In contrast to the geosynclinal belts, the ancient platforms experienced slow oscillatory movements, sedimentary rocks, usually of small thickness, accumulated within them, there were no folding processes, and volcanism and earthquakes were rare. Ancient platforms form parts of the continents that are the backbones of all continents. These are the most ancient parts of the continents, formed in the Archean and early Proterozoic.

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

Continents

Continents, or continents, are huge slabs of relatively thick earth's crust (its thickness is 35-75 km), surrounded by the World Ocean, the crust under which is thin. Geological continents are somewhat larger than their geographic outlines, because. have underwater extensions.

In the structure of the continents, three types of structures are distinguished: platforms (flat forms), orogens (born mountains) and underwater margins.

Platforms

Platforms are distinguished by gently undulating, low-lying or plateau-like relief. They have shields and a thick layered case. The shields are composed of very strong rocks, the age of which is from 1.5 to 4.0 billion years. They arose at high temperatures and pressures at great depths.

The same ancient and durable rocks compose the rest of the platforms, but here they are hidden under a thick cloak of sedimentary deposits. This cape is called a platform cover. It really can be compared to a furniture cover that keeps it from damage. Parts of platforms covered with such a sedimentary cover are called slabs. They are flat, as if layers of sedimentary rock have been ironed out. About 1 billion years ago, cover layers began to accumulate, and the process continues to the present. If the platform could be cut with a huge knife, then we would see that it looks like a layer cake.

SHIELDS have a rounded and convex shape. They arose where the platform was slowly rising for a very long time. Strong rocks were exposed to the destructive action of air, water, they were influenced by the change of high and low temperatures. As a result, they cracked and crumbled into small pieces, which were carried away into the surrounding seas. The shields are composed of very ancient, highly altered (metamorphic) rocks, formed several billion years at great depths at high temperatures and pressures. In some places, the high temperature caused the rocks to melt, which led to the formation of granite massifs.

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Origin of the Earth. As you already know. Earth is a small cosmic body, part of the solar system. How was our planet born? Even scientists of the ancient world tried to answer this question. There are many different hypotheses. You will get acquainted with them when studying astronomy in high school.

From modern views on the origin of the Earth, the most common hypothesis is O. Yu. Schmidt about the formation of the Earth from a cold gas-dust cloud. The particles of this cloud, revolving around the Sun, collided, "stick together", forming clots that grew like a snowball.

There are also hypotheses for the formation of planets as a result of space disasters - powerful explosions caused by the decay of stellar matter. Scientists continue to look for new ways to solve the problem of the origin of the Earth.

The structure of the continental and oceanic crust. The earth's crust is the most top part lithosphere. It is like a thin "veil", under which the restless earth's bowels are hidden. Compared to other geospheres, the earth's crust seems to be a thin film in which the globe is wrapped. On average, the thickness of the earth's crust is only 0.6% of the length of the earth's radius.

The appearance of our planet is determined by the protrusions of the continents and the depressions of the oceans filled with water. To answer the question of how they formed, one must know the differences in the structure of the earth's crust. You can see these differences in Figure 8.

1. What are the three layers that make up the earth's crust?
2. How thick is the crust on the continents? Under the oceans?
3. Highlight two features that distinguish the continental crust from the oceanic.

How to explain the differences in the structure of the earth's crust? Most scientists believe that oceanic-type crust first formed on our planet. Under the influence of processes occurring inside the Earth, folds, i.e., mountainous areas, formed on its surface. The thickness of the crust increased, ledges of the continents formed. There are a number of hypotheses regarding the further development of continents and ocean basins. Some scientists argue that the continents are motionless, while others, on the contrary, speak of their constant movement.

AT last years created a theory of the structure of the earth's crust, based on the concept of lithospheric plates and on the hypothesis of continental drift, created at the beginning of the 20th century. German scientist A. Wegener. However, at that time he could not find an answer to the question of the origin of the forces that move the continents.

Rice. 8. The structure of the earth's crust on the continents and under the oceans

Plates of the lithosphere. According to the theory of lithospheric plates, the earth's crust, together with part of the upper mantle, is not a monolithic shell of the planet. It is broken by a complex network of deep cracks that go to great depths and reach the mantle. These giant cracks divide the lithosphere into several very large blocks (plates) with a thickness of 60 to 100 km. The boundaries between the plates run along the mid-ocean ridges - giant swellings on the body of the planet or along deep-sea trenches - gorges on the ocean floor. There are such cracks and on land. They pass through mountain belts like the Alysh-Himalayan, Ural, etc. These mountain belts are like "seams in the place of healed old wounds on the body of the planet." On land there are also "fresh wounds" - the famous East African faults.

There are seven huge slabs and dozens of smaller slabs. Most plates include both continental and oceanic crust (Fig. 9).

Rice. 9. Plates of the lithosphere

The plates lie on a relatively soft, plastic layer of the mantle, along which they slide. forces, causing movement plates, arise during the movement of matter in the upper mantle (Fig. 10). Powerful ascending flows of this substance break the earth's crust, forming deep faults in it. These faults are found on land, but most of them are in the mid-ocean ridges at the bottom of the oceans, where the earth's crust is thinner. Here, molten matter rises from the bowels of the Earth and pushes the plates apart, building up the earth's crust. The edges of the faults move away from each other.

Rice. 10. Estimated movement of lithospheric plates: 1. Atlantic Ocean. 2. Mid-ocean ridge. 3. Immersion of plates in the mantle. 4. Ocean trench. 5. Andes. 6. Rise of matter from the mantle

Plates slowly move from the line of underwater ridges to the lines of trenches at a speed of 1 to 6 cm per year. This fact was established by comparing photographs taken with artificial satellites Earth. Neighboring plates approach, diverge or slide one relative to the other (see Fig. 10). They float on the surface of the upper mantle, like pieces of ice on the surface of water.

If plates, one of which has oceanic crust and the other continental crust, approach each other, then the plate covered by the sea bends, as it were, dives under the continent (see Fig. 10). In this case, deep-sea trenches, island arcs, and mountain ranges arise, for example, the Kuril Trench. Japanese islands, Andes. If two plates approach the continental crust, then their edges, together with all the sedimentary rocks accumulated on them, are crushed into folds. This is how the Himalayas were formed, for example, on the border of the Eurasian and Indo-Australian plates.

Rice. 11. Changing the outlines of the continents at different times

According to the theory of lithospheric plates, the Earth once had one continent surrounded by an ocean. Over time, deep faults arose on it and two continents formed - in the Southern Hemisphere Gondwana, and in the Northern Hemisphere - Laurasia (Fig. 11). Subsequently, these continents were also broken by new faults. Formed modern continents and new oceans - the Atlantic and Indian. At the base of modern continents lie the oldest relatively stable and leveled sections of the earth's crust - platforms, that is, plates formed in the distant geological past of the Earth. When the plates collided, mountain structures arose. Some continents have preserved traces of the collision of several plates. Their area gradually increased. So, for example, Eurasia was formed.

The doctrine of lithospheric plates makes it possible to look into the future of the Earth. It is assumed that in about 50 million years the Atlantic and Indian oceans, Quiet will decrease in size. Africa will move north. Australia will cross the equator and come into contact with Eurasia. However, this is only a forecast that needs to be clarified.

Scientists came to the conclusion that in places of rupture and stretching of the earth's crust in the middle ridges, a new oceanic crust is formed, which gradually spreads in both directions from the deep fault that gave rise to it. At the bottom of the ocean, it's like a giant conveyor belt. It transports young blocks of lithospheric plates from their place of origin to the continental margins of the oceans. The speed of movement is small, the path is long. Therefore, these blocks reach the coast in 15–20 Ma. Having passed this path, the plate descends into a deep-water trench and, “diving” under the continent, plunges into the mantle from which it was formed in the central parts of the median ridges. Thus the circle of life of each lithospheric plate closes.

Map of the structure of the earth's crust. Ancient platforms, folded mountainous areas, the position of mid-ocean ridges, fault zones on land and the ocean floor, ledges of crystalline rocks on the continents are shown on the thematic map "The structure of the earth's crust".

Seismic belts of the Earth. border areas between lithospheric plates called seismic belts. These are the most restless mobile areas of the planet. Most are concentrated here active volcanoes, occurs at least 95% of all earthquakes. Seismic areas stretched for thousands of kilometers and coincide with areas of deep faults on land, in the ocean - with mid-ocean ridges and deep-sea trenches. There are more than 800 active volcanoes on Earth, spewing a lot of lava, gases and water vapor onto the surface of the planet.

Knowledge of the structure and history of the development of the lithosphere is important for the search for mineral deposits, for making forecasts of natural disasters that are associated with the processes occurring in the lithosphere. It is assumed, for example, that it is at the boundaries of plates that ore minerals are formed, the origin of which is associated with the intrusion of igneous rocks into the earth's crust.

1. What is the structure of the lithosphere? What phenomena occur at the boundaries of its plates?
2. How are seismic belts located on Earth? Tell us about earthquakes and volcanic eruptions known to you from radio and television messages. newspapers. Explain the reasons for these phenomena.
3. How should one work with a map of the structure of the earth's crust?
4. Is it true that the distribution of the continental crust coincides with the land area? 5. Where do you think new oceans could form on Earth in the far future? New continents?

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 of 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 celebrated 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 of the earth's surface. This includes:

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

island arcs- these are extended (up to 3000 km) mountain structures formed by a chain of volcanic structures with a modern manifestation of 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 oceanic 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 structures and bodies of sedimentary strata,
• sudden change in power and material composition 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 last one 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 rocks, connected by the unity of the material composition and structure, which is due to the commonality of their origin or co-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 the type coincides with mountainous regions, in which sedimentary deposits are folded into folds and broken up 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 - outer part moving 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, stages are distinguished, which are characterized by: a certain type of tectonic movements and geological formations. Let's consider them.

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 the folded structures of Western Europe, the Urals, the Appalachians, etc.

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

5. Alpinefolding - manifested itself in the 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-aged on large areas 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 geosyncline wing. In the section, the marginal foredeep is an asymmetric synclinal shape, in which the wing from the side of the platform is flat, while 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.

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

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

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

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

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

Distinguish ancient and young platforms.

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

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

Rice. 7.3. Principal section of the platform

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

Top floor platforms presented case, or cover, flat-lying with a sharp angular unconformity on the basement of non-metamorphosed sediments - marine, continental and volcanogenic. The surface between the mantle and basement reflects the underlying structural unconformity within the platforms. The structure of the platform cover turns out to be complex and on many platforms on early stages its formations, grabens will appear, 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 aulacogenes 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.

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

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

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

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

Young platforms formed either on the 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 cover structures from the basement structures. These platforms have a three-tier 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 depression Pacific Ocean, Antarctica, Australia, etc. The identification of such structures can be considered conditional. A more thorough study of ring structures made it possible to identify elements of spiral, vortex structures in many of them.

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

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

Rice. 7.4. Ring structures north of New York

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

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

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

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

Rice. 7.5. Section of the Pripyat continental rift

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

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 a genetic connection with similar structures in Western Europe. The lower links of the structure are the Dnieper-Donetsk depression, then similar structures Karpinskaya and Mangyshlakskaya and further the structures of Central Asia (the total length from Warsaw to the Gissar Range). All links of the rift structure of the continents are limited by listric faults, have a hierarchical subordination according to the age of occurrence, and have a thick sedimentary stratum promising for the content of hydrocarbon deposits.