Cheat sheet: The structure and origin of the continents. The structure of the earth's crust of the continents and the bottom of the oceans

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. The upper layer is composed of relatively loose sedimentary rocks, the middle one is called granite, and the lower one is 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 the 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 Paleozoic sedimentation took place under marine conditions, he laid down from the horizontal line - the assumed 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 of the 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 folded belts are mobile sections of the earth's crust, the geological history of which was characterized by intense sedimentation, repeated folding processes and strong volcanic activity. Thick 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.

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.

Geosyncline is a mobile zone of high activity, significant dissection, characterized in the early stages of its development by the predominance of intense subsidence, and in the final stages - by intense 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 are becoming 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)

1. Formation of continents and oceans

A billion years ago, the Earth was already covered with a solid shell, in which continental protrusions and oceanic depressions stood out. Then the area of ​​the oceans was about 2 times the area of ​​the continents. But the number of continents and oceans has changed significantly since then, and so has their location. Approximately 250 million years ago, there was one continent on Earth - Pangea. Its area was approximately the same as the area of ​​all modern continents and islands combined. This supercontinent was washed by an ocean called Panthalassa and occupied all the rest of the space on Earth.

However, Pangea turned out to be a fragile, short-lived formation. Over time, the currents of the mantle inside the planet changed direction, and now, rising from the depths under Pangea and spreading in different directions, the substance of the mantle began to stretch the mainland, and not compress it, as before. Approximately 200 million years ago, Pangea split into 2 continents: Laurasia and Gondwana. The Tethys Ocean appeared between them (now it is the deep-water parts of the Mediterranean, Black, Caspian Seas and the shallow Persian Gulf).

The currents of the mantle continued to cover Laurasia and Gondwana with a network of cracks and disintegrate them into many fragments that did not remain in a certain place, but gradually diverged in different directions. They were driven by currents within the mantle. Some researchers believe that it was these processes that caused the death of dinosaurs, but this question remains open for now. Gradually, between the diverging fragments - the continents - the space was filled with mantle matter, which rose from the bowels of the Earth. Cooling down, it formed the bottom of the future oceans. Over time, three oceans appeared here: the Atlantic, the Pacific, and the Indian. According to many scientists, the Pacific Ocean is the remnant of the ancient ocean of Panthalassa.

Later, new faults engulfed Gondwana and Laurasia. From Gondwana, the land first separated, which is now Australia and Antarctica. She began drifting to the southeast. Then it split into two unequal parts. The smaller one - Australia - rushed to the north, the larger one - Antarctica - to the south and took a place inside the Antarctic Circle. The rest of Gondwana split into several plates, the largest of them being African and South American. These plates are now diverging from each other at a rate of 2 cm per year (see Lithospheric Plates).

Faults also covered Laurasia. It split into two plates - North American and Eurasian, which make up most of the Eurasian continent. The emergence of this continent is the greatest cataclysm in the life of our planet. Unlike all other continents, which are based on one fragment of the ancient continent, Eurasia consists of 3 parts: Eurasian (part of Laurasia), Arabian (Gondwana ledge) and Hindustan (part of Gondwana) lithospheric plates. Approaching each other, they almost destroyed the ancient Tethys ocean. Africa is also involved in the formation of the image of Eurasia, the lithospheric plate of which, although slowly, is approaching the Eurasian one. The result of this convergence are the mountains: the Pyrenees, the Alps, the Carpathians, the Sudetes and the Ore Mountains (see Lithospheric Plates).

The convergence of the Eurasian and African lithospheric plates is still going on, this is reminiscent of the activity of the volcanoes Vesuvius and Etna, disturbing the tranquility of the inhabitants of Europe.

The convergence of the Arabian and Eurasian lithospheric plates led to crushing and crushing into folds of rocks that fell on their way. This was accompanied by the strongest volcanic eruptions. As a result of the convergence of these lithospheric plates, the Armenian Highland and the Caucasus arose.

The convergence of the Eurasian and Hindustan lithospheric plates made the entire continent shudder from the Indian Ocean to the Arctic, while Hindustan itself, which originally broke away from Africa, suffered little. The result of this rapprochement was the emergence of the highest highlands in the world of Tibet, surrounded by even higher chains of mountains - the Himalayas, the Pamirs, the Karakorum. It is not surprising that it is here, in the place of the strongest compression of the earth's crust of the Eurasian lithospheric plate, that the highest peak of the Earth is located - Everest (Chomolungma), rising to a height of 8848 m.

The "march" of the Hindustan lithospheric plate could lead to a complete split of the Eurasian plate, if there were no parts inside it that could withstand pressure from the south. Eastern Siberia acted as a worthy "defender", but the lands located to the south of it were crumpled into folds, crushed and moved.

So, the struggle between continents and oceans has been going on for hundreds of millions of years. The main participants in it are the continental lithospheric plates. Every mountain range, island arc, deepest oceanic depression is the result of this struggle.

2. Structure of continents and oceans

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 (ocean moving belts),

aseismic areas (ocean basins).

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

The second group of structural elements is 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 a mobile geosynclinal belt. This is the area of ​​maximum dissection of the earth's surface. This includes:

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

Island arcs 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 Kurile-Kamchatka ridge, the Aleutian Islands, etc. From the ocean side, island arcs are replaced by deep-water trenches, which are deep-water depressions 1500–4000 km long and 5–10 km deep. 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 a zone of concentration of earthquake sources and is called the Wadati-Zavaritsky-Benioff zone.

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 shape of structures and bodies of sedimentary strata,

a sharp change in the 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 the listed 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:

A geological formation is a more complex stage of development following a rock. It is a natural association of rocks associated with 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.

Type I 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.

Type II 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. A geosyncline is a mobile extended area of ​​a deep trough of the earth's crust. It is characterized by the accumulation of thick sedimentary strata, prolonged volcanism, and a sharp change in the direction of tectonic movements with the formation of folded structures.

Geosynclines are divided into:

The continental type of the earth's crust is oceanic. Therefore, the ocean floor itself includes the depressions of the ocean floor located behind the continental slope. These huge depressions differ from the continents not only in the structure of the earth's crust, but also in their tectonic structures. The most extensive areas of the ocean floor are deep-sea plains located at depths of 4-6 km and ...

And depressions with sharp elevation changes, measured in hundreds of meters. All these features of the structure of the axial band of the median ridges should obviously be understood as a manifestation of intense blocky tectonics, and the axial depressions are grabens, and on both sides of them the median ridge is broken into raised and lowered blocks by ruptures. The whole set of structural features that characterize ...

The primary basalt layer of the Earth was formed. The Archean was characterized by the formation of primary large water bodies (seas and oceans), the appearance of the first signs of life in the aquatic environment, the formation of the ancient relief of the Earth, similar to the relief of the Moon. Several epochs of folding occurred in the Archaean. A shallow ocean was formed with many volcanic islands. An atmosphere has formed containing vapors...

Water in the South Equatorial Current is 22 ... 28 ° С, in the East Australian in winter from north to south it changes from 20 to 11 ° С, in summer - from 26 to 15 ° С. The Circumpolar Antarctic, or the Western Wind Current, enters the Pacific Ocean south of Australia and New Zealand and moves in a sublatitudinal direction towards the coasts of South America, where its main branch deviates to the north and, passing along the coasts ...

1. Deep structure of the Earth

The geographic shell interacts, on the one hand, with the deep matter of the planet, on the other hand, with the upper layers of the atmosphere. The deep structure of the Earth has a significant impact on the formation of the geographic envelope. The term "structure of the Earth" usually denotes its internal, i.e., deep structure, starting from the earth's crust and up to the center of the planet.

The mass of the Earth is 5.98 x 10 27 g.

The average density of the Earth is 5.517 g/cm3.

The composition of the earth. According to modern scientific concepts, the Earth consists of the following chemical elements: iron - 34.64%, oxygen - 29.53%, silicon - 15.20%, magnesium - 12.70%, nickel - 2.39%, sulfur - 1 .93%, chromium - 0.26%, manganese - 0.22%, cobalt - 0.13%, phosphorus - 0.10%, potassium - 0.07%, etc.

The most reliable data on the internal structure of the Earth are provided by observations of seismic waves, i.e., oscillatory movements of terrestrial matter caused by earthquakes.

A sharp change in the speed of seismic waves (recorded on seismographs) at a depth of 70 km and 2900 km reflects an abrupt increase in the density of matter at these limits. This gives grounds to isolate the following three shells (geospheres) in the inner body of the Earth: up to a depth of 70 km - the earth's crust, from 70 km to 2,900 km - the mantle, and from it to the center of the Earth - the core. The nucleus has an outer nucleus and an inner nucleus.

The Earth was formed about 5 billion years ago from some cold gas-dust nebula. After the mass of the planet reached its current value (5.98 x 10 27 g), its self-heating began. The main sources of heat were: firstly, gravitational compression, and secondly, radioactive decay. As a result of the development of these processes, the temperature inside the Earth began to rise, which led to the melting of metals. Since matter was strongly compressed in the center of the Earth and cooled by radiation from the surface, melting occurred mainly at shallow depths. Thus, a molten layer was formed, from which silicate materials, as the lightest, rose up, giving rise to the earth's crust. Metals remained at the melting level. Since their density is higher than that of undifferentiated deep matter, they gradually descended. This led to the formation of a metal core.

The CORE is 85-90% iron. At a depth of 2,900 km (the boundary between the mantle and the core), the substance is in a supersolid state due to the enormous pressure (1,370,000 atm.). Scientists suggest that the outer core is molten, while the inner core is in a solid state. The differentiation of terrestrial matter and the separation of the core is the most powerful process on Earth and the main, the first internal driving mechanism for the development of our planet.

The role of the core in the formation of the Earth's magnetosphere. The core has a powerful effect on the formation of the Earth's magnetosphere, which protects life from harmful ultraviolet radiation. In the electrically conductive outer liquid core of a rapidly rotating planet, complex and intense motions of matter occur, leading to the excitation of a magnetic field. The magnetic field extends into near-Earth space for several Earth radii. Interacting with the solar wind, the geomagnetic field creates the Earth's magnetosphere. The upper boundary of the magnetosphere is located at an altitude of about 90 thousand km. The formation of the magnetosphere and the isolation of terrestrial nature from the plasma of the solar corona was the first and one of the most important conditions for the origin of life, the development of the biosphere, and the formation of the geographic envelope.

The MANTLE consists predominantly of Mg, O, FeO and SiO2, which form magma. The composition of magma includes water, chlorine, fluorine and other volatile substances. In the mantle, the process of differentiation of matter proceeds continuously. Substances facilitated by the removal of metals rise towards the earth's crust, while heavier ones sink. Similar displacements of matter in the mantle are defined by the term "convection currents".

The concept of the asthenosphere. The upper part of the mantle (within 100-150 km) is called the asthenosphere. In the asthenosphere, the combination of temperature and pressure is such that the substance is in a molten, mobile state. In the asthenosphere, not only constant convection currents occur, but also horizontal asthenospheric currents.

The speed of horizontal asthenospheric currents reaches only a few tens of centimeters per year. However, over geological time, these currents led to the splitting of the lithosphere into separate blocks and to their horizontal movement, known as continental drift. The asthenosphere contains the foci of volcanoes and centers of earthquakes. Scientists believe that geosynclines form above descending currents, and mid-ocean ridges and rift zones form above ascending currents.

2. The concept of the earth's crust. Hypotheses explaining the origin and development of the earth's crust

The Earth's crust is a complex of surface layers of the Earth's solid body. In the scientific geographical literature there is no single idea of ​​the origin and development of the earth's crust.

There are several hypotheses (theories) explaining the mechanism of formation and development of the earth's crust. The most reasonable hypotheses are the following:

• 1. The theory of fixism (from lat. fixus - motionless, unchanging) claims that the continents have always remained in the places they currently occupy. This theory denies any movement of continents and large parts of the lithosphere (Charles Darwin, A. Wallace and others).
• 2. The theory of mobilism (from Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has been especially confirmed in recent years in connection with the receipt of new scientific data in the study of the bottom of the World Ocean.
• 3. The concept of the growth of continents at the expense of the ocean floor assumes that the original continents were formed in the form of relatively small massifs, which now make up the ancient continental platforms. Subsequently, these massifs grew due to the formation of mountains on the ocean floor adjacent to the edges of the original land cores. The study of the bottom of the oceans, especially in the zone of mid-ocean ridges, gave reason to doubt the correctness of this concept.
• 4. The theory of geosynclines states that the increase in the size of land occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the earth's crust of the continents, is the basis of many modern scientific explanations.
• 5. The rotational theory bases its explanation on the proposition that since the figure of the Earth does not coincide with the surface of a mathematical spheroid and is rebuilt due to uneven rotation, the zonal bands and meridional sectors on a rotating planet are inevitably tectonically unequal. They react with varying degrees of activity to tectonic stresses caused by intraterrestrial processes.

Oceanic and continental crust. There are two main types of earth's crust: oceanic and continental. Its transitional type is also distinguished.

Oceanic crust. The thickness of the oceanic crust in the modern geological epoch ranges from 5 to 10 km. It consists of the following three layers:

• 1) the upper thin layer of marine sediments (thickness is not more than 1 km);
• 2) middle basalt layer (thickness from 1.0 to 2.5 km);
• 3) the lower gabbro layer (about 5 km thick).

Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic one. Its average thickness is 35-45 km, and in mountainous countries it increases to 70 km. It consists of the following three layers:

• 1) the lower layer (basalt), composed of basalts (about 20 km thick);
• 2) the middle layer (granite), formed mainly by granites and gneisses; forms the main thickness of the continental crust, does not extend under the oceans;
• 3) the upper layer (sedimentary) with a thickness of about 3 km.

In some areas, the thickness of precipitation reaches 10 km: for example, in the Caspian lowland. In some regions of the Earth, there is no sedimentary layer at all and a layer of granite comes to the surface. Such areas are called shields (eg Ukrainian Shield, Baltic Shield).

On the continents, as a result of the weathering of rocks, a geological formation is formed, called the weathering crust.

The granite layer is separated from the basalt layer by the Konrad surface. At this boundary, the speed of seismic waves increases from 6.4 to 7.6 km/sec.

The boundary between the earth's crust and the mantle (both on the continents and on the oceans) runs along the Mohorovichic surface (Moho line). The speed of seismic waves on it jumps up to 8 km/h.

In addition to the two main types of the earth's crust (oceanic and continental), there are also areas of a mixed (transitional) type.

On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out in it. In East Asia, in the region of island arcs (the Kuril Islands, the Aleutian Islands, the Japanese Islands, and others), the earth's crust of a transitional type is widespread. Finally, the earth's crust of the mid-ocean ridges is very complex and still little studied. There is no Moho boundary here, and the material of the mantle rises along faults into the crust and even to its surface.

The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than "the earth's crust". In the lithosphere, modern science includes not only the earth's crust, but also the uppermost mantle up to the asthenosphere, i.e., to a depth of about 100 km.

The concept of isostasy. The study of the distribution of gravity has shown that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density. Heavy oceanic crust is thinner than lighter continental crust.

Isostasy is not even a balance, but a striving for balance, continuously disturbed and restored again. So, for example, the Baltic Shield after the melting of continental ice of the Pleistocene glaciation rises by about 1 cm per year. The area of ​​Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero balance line is currently running slightly south of 600 N. Modern St. Petersburg is about 1.5 m higher than St. Petersburg during the time of Peter the Great. As the data of modern scientific research show, even the heaviness of large cities is sufficient for the isostatic fluctuation of the territory under them. Therefore, the earth's crust in the areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror reflection of the Moho surface (the soles of the earth's crust): the elevated areas correspond to depressions in the mantle, and the lower ones correspond to a higher level of its upper boundary. So, under the Pamirs, the depth of the Moho surface is 65 km, and in the Caspian lowland - about 30 km.

Thermal properties of the earth's crust. Daily fluctuations in soil temperature extend to a depth of 1.0 - 1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate - up to a depth of 20-30 m. At the depth where the influence of annual temperature fluctuations due to heating of the earth's surface by the Sun stops , there is a layer of constant soil temperature. It is called an isothermal layer. Below the isothermal layer deep into the Earth, the temperature rises. But this increase in temperature is already caused by the internal heat of the earth's interior. In the formation of climates, internal heat practically does not participate. However, it serves as the only energy basis for all tectonic processes.

The number of degrees by which the temperature increases for every 100 m of depth is called the geothermal gradient.

The distance in meters at which the temperature rises by 10°C is called the geothermal step. The value of the geothermal step depends on the relief, the thermal conductivity of rocks, the proximity of volcanic foci, the circulation of groundwater, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be as low as 5 m, and in geologically calm areas (on platforms) it can reach 100 m.

3. Structural-tectonic principle of separating continents. The concept of the continents and parts of the world

Two qualitatively different types of the earth's crust - continental and oceanic - correspond to two main levels of planetary relief - the surface of the continents and the bed of the oceans. The selection of continents in modern geography is carried out on the basis of the structural-tectonic principle.

Structural-tectonic principle of allocation of continents.

The fundamental qualitative difference between the continental and oceanic crust, as well as some significant differences in the structure of the upper mantle under the continents and oceans, make it necessary to distinguish continents not according to their visible surroundings by oceans, but according to the structural-tectonic principle.

The structural-tectonic principle states that, firstly, the mainland includes a continental shelf (shelf) and a continental slope; secondly, at the heart of each continent there is a core or an ancient platform; thirdly, each continental block is isostatically balanced in the upper mantle.

From the point of view of the structural-tectonic principle, the mainland is an isostatically balanced array of the continental crust, which has a structural core in the form of an ancient platform, to which younger folded structures adjoin.

In total, there are six continents on Earth: Eurasia, Africa, North America, South America, Antarctica and Australia. Each continent contains one platform, and there are six of them at the heart of Eurasia: East European, Siberian, Chinese, Tarim (Western China, the Takla-Makan desert), Arabian and Hindustan. The Arabian and Hindustan platforms are parts of ancient Gondwana that joined Eurasia. Thus, Eurasia is a heterogeneous anomalous continent.

The boundaries between the continents are quite obvious. The border between North America and South America runs along the Panama Canal. The border between Eurasia and Africa is drawn along the Suez Canal. The Bering Strait separates Eurasia from North America.

Two rows of continents. In modern geography, the following two series of continents are distinguished:

• 1. Equatorial series of continents (Africa, Australia and South America).
• 2. Northern row of continents (Eurasia and North America).

Outside these rows remains Antarctica - the southernmost and coldest continent.

The current location of the continents reflects the long history of the development of the continental lithosphere.

The southern continents (Africa, South America, Australia and Antarctica) are parts ("fragments") of the Gondwana megacontinent that was united in the Paleozoic. The northern continents at that time were united into another megacontinent - Laurasia. Between Laurasia and Gondwana in the Paleozoic and Mesozoic was a system of vast marine basins, called the Tethys Ocean. This ocean stretched from North Africa (through southern Europe, the Caucasus, Western Asia, the Himalayas to Indochina) to modern Indonesia. In the Neogene (about 20 million years ago), an Alpine folded belt arose on the site of this geosyncline.

According to its large size, the supercontinent Gondwana, according to the law of isostasy, had a thick (up to 50 km) earth's crust, which was deeply immersed in the mantle. Under this supercontinent, convection currents were especially intense in the asthenosphere; the softened substance of the mantle moved very actively. This led first to the formation of a swelling in the middle of the continent, and then to its splitting into separate blocks, which, under the influence of the same convection currents, began to move horizontally. It is known that the displacement of a contour on the surface of a sphere is always accompanied by its rotation (Euler and others). Therefore, parts of Gondwana not only moved, but also unfolded in geographic space.

The first split of Gondwana occurred at the border of the Triassic and Jurassic (about 190-195 million years ago); Afro-America seceded. Then, on the border of the Jurassic and Cretaceous (about 135-140 million years ago), South America separated from Africa. On the border of the Mesozoic and Cenozoic (about 65-70 million years ago), the Hindustan block collided with Asia, and Antarctica moved away from Australia. In the present geological era, the lithosphere, according to scientists, is divided into six slab-blocks that continue to move.

The breakup of Gondwana successfully explains the form, geological similarity, and also the history of the vegetation cover and animal life of the southern continents. The history of the split of Laurasia has not been studied as carefully as Gondwana.

Patterns of the location of the continents. The current location of the continents is characterized by the following patterns:

• 1. Most of the land is located in the Northern Hemisphere. The northern hemisphere is continental, although even here land accounts for only 39%, and about 61% for the ocean.
• 2. The northern continents are quite compact. The southern continents are very scattered and fragmented.
• 3. The relief of the planet is anti-semitic. The continents are located in such a way that each of them on the opposite side of the Earth certainly corresponds to the ocean. This can best be seen in the comparison of the Arctic Ocean and Antarctic land. If the globe is set so that at one of the poles there is any of the continents, then at the other pole there will definitely be an ocean. There is only one minor exception: the end of South America is antipodal to Southeast Asia. Antipodality, since it has almost no exceptions, cannot be an accidental phenomenon. This phenomenon is based on the balance of all parts of the surface of the rotating Earth.

The concept of parts of the world. In addition to the geologically determined division of land into continents, there is also a division of the earth's surface into separate parts of the world that has developed in the process of the cultural and historical development of mankind. In total there are six parts of the world: Europe, Asia, Africa, America, Australia with Oceania, Antarctica. On one mainland of Eurasia there are two parts of the world (Europe and Asia), and two continents of the Western Hemisphere (North America and South America) form one part of the world - America.

The border between Europe and Asia is very conditional and is drawn along the watershed line of the Ural Range, the Ural River, the northern part of the Caspian Sea and the Kuma-Manych depression. Along the Urals and the Caucasus, there are lines of deep faults that separate Europe from Asia.

Area of ​​continents and oceans. The land area is calculated within the current coastline. The surface area of ​​the globe is approximately 510.2 million km 2. About 361.06 million km 2 is occupied by the World Ocean, which is approximately 70.8% of the total surface of the Earth. Approximately 149.02 million km 2 fall on land, i.e. about 29.2% of the surface of our planet.

The area of ​​modern continents is characterized by the following values:

Eurasia - 53.45 km2, including Asia - 43.45 million km2, Europe - 10.0 million km2;

Africa - 30.30 million km2;

North America - 24.25 million km2;

South America - 18.28 million km2;

Antarctica - 13.97 million km2;

Australia - 7.70 million km2;

Australia with Oceania - 8.89 km2.

Modern oceans have an area of:

the Pacific Ocean - 179.68 million km2;

Atlantic Ocean - 93.36 million km2;

Indian Ocean - 74.92 million km2;

Arctic Ocean - 13.10 million km2.

Between the northern and southern continents (in accordance with their different origin and development) there is a significant difference in the area and nature of the surface. The main geographical differences between the northern and southern continents are as follows:

• 1. Eurasia is incomparable in size with other continents, which concentrates more than 30% of the land of our planet.
• 2. The northern continents have a significant shelf area. The shelf is especially significant in the Arctic Ocean and the Atlantic Ocean, as well as in the Yellow, Chinese and Bering Seas of the Pacific Ocean. The southern continents, with the exception of the underwater continuation of Australia in the Arafura Sea, are almost devoid of a shelf.
• 3. Most of the southern continents fall on ancient platforms. In North America and Eurasia, ancient platforms occupy a smaller part of the total area, and most of it falls on the territories formed by the Paleozoic and Mesozoic mountain building. In Africa, about 96% of its territory falls on platform sites and only 4% - on the mountains of the Paleozoic and Mesozoic age. In Asia, only 27% of the territory is occupied by ancient platforms and 77% by mountains of various ages.
• 4. The coastline of the southern continents, formed mostly by tectonic faults, is relatively straight; there are few peninsulas and mainland islands. The northern continents are characterized by an exceptionally winding coastline, an abundance of islands, peninsulas, often reaching far into the ocean. Of the total area, islands and peninsulas account for about 39% in Europe, North America - 25%, Asia - 24%, Africa - 2.1%, South America - 1.1% and Australia (excluding Oceania) - 1.1% .
• 4. Vertical dismemberment of land

Each of the main planetary levels - the surface of the continents and the oceanic bed - is divided into a number of secondary levels. The formation of both primary and secondary levels occurred in the process of long-term development of the earth's crust and continues at the present geological time. Let us dwell on the modern division of the continental crust into high-altitude steps. Steps are counted from sea level.

• 1. Depressions - land areas lying below sea level. The largest depression on Earth is the southern part of the Caspian lowland with a minimum elevation of -28 m. Inside Central Asia there is an extremely dry Turfan depression with a depth of about -154 m. The deepest depression on Earth is the Dead Sea basin; The shores of the Dead Sea lie 392 m below sea level. Depressions occupied by water, the levels of which lie above sea level, are called cryptodepressions. Typical examples of cryptodepressions are Lake Baikal and Lake Ladoga. The Caspian Sea and the Dead Sea are not crypto-depressions, because the water level in them does not reach the level of the ocean. The area occupied by depressions (without cryptodepressions) is relatively small and amounts to about 800 thousand km2.
• 2. Lowlands (low plains) - land areas lying at an altitude of 0 to 200 m above sea level. Lowlands are numerous on every continent (with the exception of Africa) and cover a larger area than any other land stage. The total area of ​​all the low plains of the globe is about 48.2 million km2.
• 3. Hills and plateaus lie at an altitude of 200 to 500 m and differ in the prevailing forms of relief: on the hills the relief is rugged, on the plateau it is relatively flat. Elevations above the lowlands rise gradually, and the plateau rises in a noticeable ledge. Hills and plateaus differ from each other and geological structure. The area occupied by highlands and plateaus is about 33 million km2.

Mountains are located above 500 m. They may be of different origins and ages. Mountains are classified according to height as low, medium and high.

• 4. Low mountains rise no higher than 1,000 m. Usually, low mountains are either ancient ruined mountains or foothills of modern mountain systems. The low mountains occupy about 27 million km2.
• 5. Medium mountains have a height of 1,000 to 2,000 m. Examples of medium-altitude mountains are: the Urals, the Carpathians, Transbaikalia, some ridges of Eastern Siberia and many other mountainous countries. The area occupied by medium mountains is about 24 million km2.
• 6. High (alpine) mountains rise above 2,000 m. The term "alpine mountains" is often applied only to mountains of Cenozoic age, lying at an altitude of more than 3,000 m. High mountains account for about 16 million km2.

Below ocean level, the continental lowland, flooded with water, continues - the shelf, or the continental shelf. Until recently, according to the same conditional account as the steps of the land, the shelf was called underwater plains with depths of up to 200 m. Now the shelf boundary is drawn not along a formally chosen isobath, but along the line of the actual, geologically determined end of the continental surface and its transition to the continental slope . Therefore, the shelf continues in the ocean to different depths in each sea, often exceeding 200 m and reaching 700 and even 1,500 m.

At the outer edge of the relatively flat shelf, there is a sharp break in the surface to the continental slope and the continental foot. Shelf, slope and foot together form the underwater margin of the continents. It continues on average to a depth of 2,450 m.

The continents, including their underwater margin, occupy about 40% of the Earth's surface, while the land area is about 29.2% of the total Earth.

Each continent is isostatically balanced in the asthenosphere. There is a direct relationship between the area of ​​the continents, the height of their relief and the depth of immersion in the mantle. The larger the area of ​​the continent, the greater its average height and thickness of the lithosphere. The average height of the land is 870 m. The average height of Asia is 950 m, Europe is 300 m, Australia is 350 m.

The concept of a hypsometric (batygraphic) curve. The generalized profile of the earth's surface is represented by a hypsometric curve. The oceanic part of it is called the bathygraphic curve. The curve is constructed as follows. The dimensions of areas lying at different heights and depths are taken from hypsometric and bathygraphic maps and plotted in the system of coordinate axes: along the ordinate line, heights are plotted from 0 upwards, and depths downwards; along the abscissa line - areas in millions of square kilometers.

5. Relief and structure of the ocean floor. Islands

The average depth of the World Ocean is 3,794 m.

The bottom of the World Ocean consists of the following four planetary morphosculptural forms:

• 1) the underwater margin of the continents,
• 2) transition zones,
• 3) the bed of the ocean,
• 4) mid-ocean ridges.

The underwater margin of the continents consists of the shelf, the continental slope, the continental foot. It descends to a depth of 2,450 m. The earth's crust here has a continental type. The total area of ​​the underwater margin of the continents is about 81.5 million km2.

The continental slope plunges into the ocean relatively steeply, the slopes are on average about 40, but sometimes they reach 400.

The continental foot is a trough at the boundary of the continental and oceanic crust. Morphologically, this is an accumulative plain formed by sediments carried down from the continental slope.

Mid-ocean ridges are a single and continuous system that spans all oceans. They are huge mountain structures, reaching a width of 1-2 thousand km and rising above the ocean floor by 3-4 thousand km. Sometimes the mid-ocean ridges rise above the ocean level and form numerous islands (Iceland, Azores, Seychelles, etc.). In grandiosity, they significantly surpass the mountainous countries of the continents and are commensurate with the continents. For example, the Mid-Atlantic Ridge is several times larger than the largest terrestrial mountain system, the Cordilleras and the Andes. All mid-ocean ridges are characterized by increased tectonic activity.

The system of mid-ocean ridges includes the following structures:

• - Mid-Atlantic Ridge (stretches from Iceland along the entire Atlantic Ocean to the island of Tristan da Cunha);
• - Mid-Indian Ridge (its peaks are expressed by the Seychelles);
• - East Pacific Rise (extends south of the California Peninsula).

According to the relief and features of tectonic activity, mid-ocean ridges are: 1) rift and 2) non-rift.

Rift ridges (for example, the Mid-Atlantic) are characterized by the presence of a "rift" valley - a deep and narrow gorge with steep slopes (the gorge goes along the crest of the ridge along its axis). The width of the rift valley is 20-30 km, and the depth of the fault can be located below the ocean floor up to 7400 m (Romansh basin). The relief of the rift ranges is complex and rugged. All ridges of this type are characterized by rift valleys, narrow mountain ranges, giant transverse faults, intermountain depressions, volcanic cones, underwater volcanoes, and islands. All rift ridges are characterized by high seismic activity.

Non-rift ridges (for example, the East Pacific Rise) are characterized by the absence of a "rift" valley and have a less complex topography. Seismic activity is not typical for non-rift ridges. However, they are characterized by a common feature of all mid-ocean ridges - the presence of grandiose transverse faults.

The most important geophysical features of mid-ocean ridges are as follows:

• -increased amount of heat flow from the bowels of the Earth;
• -specific structure of the earth's crust;
• - magnetic field anomalies;
• -volcanism;
• - seismic activity.

The distribution of sediments that make up the upper layer of the earth's crust in mid-ocean ridges obeys the following pattern: on the ridge itself, sediments are thin or absent altogether; as the distance from the ridge increases, the thickness of the sediments (up to several kilometers) and their age increase. If in the cleft itself the age of the lavas is approximately 13 thousand years, then in 60 km it is already 8 million years. Rocks older than 160 million years have not been found at the bottom of the World Ocean. These facts testify to the constant renewal of the mid-ocean ridges.

Mechanisms of formation of mid-ocean ridges. The formation of mid-ocean ridges is associated with upper magma. The upper magma is a huge convection system. According to scientists, the formation of mid-ocean ridges causes the Earth's interior to rise. Lava flows outward along the rift valleys and forms a basalt layer. Joining the old crust, new portions of lava cause horizontal displacement of lithosphere blocks and expansion of the ocean floor. The speed of horizontal movements in different parts of the Earth varies from 1 to 12 cm per year: in the Atlantic Ocean - about 4 cm/year; in the Indian Ocean - about 6 cm / year, in the Pacific Ocean - up to 12 cm / year. These insignificant values, multiplied by millions of years, give enormous distances: in the 150 million years that have passed since the split of South America and Africa, these continents have separated by 5 thousand km. North America separated from Europe 80 million years ago. And 40 million years ago, Hindustan collided with Asia and the formation of the Himalayas began.

As a result of the expansion of the ocean floor in the zone of mid-ocean ridges, there is no increment of terrestrial matter at all, but only its overflow and transformation. The basalt crust, which grows along the mid-ocean ridges and spreads horizontally from them, travels thousands of kilometers over millions of years and, at some edges of the continents, sinks back into the bowels of the Earth, taking ocean sediments with it. This process explains the different ages of the rocks on the crest of the ridges and in other parts of the oceans. This process also causes continental drift.

Transitional zones include deep-sea trenches, island arcs, and marginal sea basins. In transitional zones, parts of the continental and oceanic crust are difficult to combine.

Deep ocean trenches are found in the following four regions of the Earth:

• - in the Pacific Ocean along the coasts of East Asia and Oceania: the Aleutian Trench, the Kuril-Kamchatka Trench, the Japanese Trench, the Philippine Trench, the Mariana Trench (with a maximum depth of 11,022 m for the Earth), the West Melanesian Trench, Tonga;
• - in the Indian Ocean - the Java Trench;
• - in the Atlantic Ocean - the Puerto Rican Trench;
• - in the Southern Ocean - South Sandwich.

The bed of the oceans, which accounts for about 73% of the total area of ​​the World Ocean, is occupied by deep-water (from 2,450 to 6,000 m) plains. In general, these deep-water plains correspond to oceanic platforms. Between the plains are mid-ocean ridges, as well as uplands and uplifts of another genesis. These uplifts divide the ocean floor into separate basins. For example, from the North Atlantic Ridge to the west is the North American Basin, and to the east - the Western European and Canary Basins. At the bottom of the ocean there are numerous volcanic cones.

Islands. In the process of the development of the earth's crust and its interaction with the World Ocean, large and small islands were formed. The total number of islands is constantly changing. Some islands appear, others disappear. For example, deltaic islands are formed and eroded, ice massifs are melting, which were previously taken for islands (“lands”). Sea spits acquire an island character and, conversely, the islands join the land and turn into peninsulas. Therefore, the area of ​​the islands is calculated only approximately. It is about 9.9 million km2. About 79% of all island land falls on 28 large islands. The largest island is Greenland (2.2 million km2).

IN The 28 largest islands in the world include the following:

• 1. Greenland;
• 2. New Guinea;
• 3. Kalimantan (Borneo);
• 4. Madagascar;
• 5. Baffin Island;
• 6. Sumatra;
• 7. UK;
• 8. Khonshu;
• 9. Victoria (Canadian Arctic Archipelago);
• 10. Ellesmere Land (Canadian Arctic Archipelago);
• 11. Sulawesi (Celebes);
• 12. South Island of New Zealand;
• 13. Java;
• 14. North Island of New Zealand;
• 15. Newfoundland;
• 16. Cuba;
• 17. Luzon;
• 18. Iceland;
• 19. Mindanao;
• 20. New Earth;
• 21. Haiti;
• 22. Sakhalin;
• 23. Ireland;
• 24. Tasmania;
• 25. Banks (Canadian Arctic Archipelago);
• 26. Sri Lanka;
• 27. Hokkaido;
• 28. Devon.

Both large and small islands are located either singly or in groups. Groups of islands are called archipelagos. Archipelagos can be compact (eg Franz Josef Land, Svalbard, Greater Sunda Islands) or elongated (eg Japan, Philippines, Greater and Lesser Antilles). Elongated archipelagos are sometimes called ridges (for example, the Kuril ridge, the Aleutian ridge). The archipelagos of small islands scattered across the expanses of the Pacific Ocean are united into the following three large groups: Melanesia, Micronesia (Caroline Islands, Mariana Islands, Marshall Islands), Polynesia.

By origin, all the islands can be grouped as follows:

I. Mainland Islands:

• 1) platform islands,
• 2) islands of the continental slope,
• 3) orogenic islands,
• 4) island arcs,
• 5) coastal islands: a) skerries, b) Dalmatian, c) fjord, d) spits and arrows, e) delta.

II. Independent islands:

• 1) volcanic islands, including a) fissure outpouring of lava, b) central outpouring of lava - shield and conical;
• 2) coral islands: a) coastal reefs, b) barrier reefs, c) atolls.

The mainland islands are genetically related to the mainland, but these connections are of a different nature, which affects the nature and age of the islands, their flora and fauna.

The platform islands lie on the continental shelf and geologically represent a continuation of the mainland. The platform islands are separated from the main land mass by shallow straits. Examples of platform islands are: British Isles, Svalbard Archipelago, Franz Josef Land, Severnaya Zemlya, New Siberian Islands, Canadian Arctic Archipelago.

The formation of the straits and the transformation of part of the continents into islands dates back to recent geological time; therefore, the nature of the island land differs little from the mainland.

The islands of the continental slope are also parts of the continents, but their separation happened earlier. These islands are separated from the adjacent continents not by a gentle trough, but by a deep tectonic fault. Moreover, the straits are oceanic in nature. The flora and fauna of the islands of the continental slope is very different from the mainland and is generally insular in nature. Examples of continental slope islands are: Madagascar, Greenland, etc.

Orogenic islands are a continuation of the mountainous folds of the continents. So, for example, Sakhalin is one of the folds of the Far Eastern mountainous country, New Zealand is a continuation of the Urals, Tasmania is the Australian Alps, the islands of the Mediterranean Sea are branches of the Alpine folds. The archipelago of New Zealand is also of orogenic origin.

Island arcs garland border East Asia, America and Antarctica. The largest region of island arcs is located off the coast of East Asia: the Aleutian ridge, the Kuril ridge, the Japanese ridge, the Ryukyu ridge, the Philippine ridge, etc. The second region of island arcs is located off the coast of America: the Greater Antilles, the Lesser Antilles. The third region is an island arc located between South America and Antarctica: the Tierra del Fuego archipelago, the Falkland Islands, etc. Tectonically, all island arcs are confined to modern geosynclines.

The mainland offshore islands have different origins and represent different types of coastline.

Independent islands have never been part of the continents and in most cases formed independently of them. The largest group of independent islands are volcanic.

Volcanic islands are found in all oceans. However, they are especially numerous in the zones of mid-ocean ridges. The size and features of volcanic islands are determined by the nature of the eruption. Fissure outpourings of lava create large islands, which are not inferior in size to platform ones. The largest island of volcanic origin on Earth is Iceland (103 thousand km2).

The main mass of volcanic islands is formed by eruptions of the central type. Naturally, these islands cannot be very large. Their area depends on the nature of the lava. The main lava spreads over long distances and forms shield volcanoes (for example, the Hawaiian Islands). The eruption of acidic lava forms a sharp cone of a small area.

Coral islands are the waste products of coral polyps, diatoms, foraminifera and other marine organisms. Coral polyps are quite demanding on habitat conditions. They can only live in warm waters with a temperature of at least 200C. Therefore, coral structures are distributed only in tropical latitudes and go beyond them only in one place - in the area of ​​Bermuda, washed by the Gulf Stream.

Depending on their location in relation to modern land, coral islands are divided into the following three groups:

• 1) coastal reefs,
• 2) barrier reefs,
• 3) atolls.

Coastal reefs begin directly at the coast of the mainland or island in the low tide and border it in the form of a wide terrace. Near the mouths of the rivers and near the mangroves, they are interrupted due to the low salinity of the water.

Barrier reefs are located at some distance from the land, separated from it by a strip of water - a lagoon. The largest reef at present is the Great Barrier Reef. Its length is about 2,000 km; the width of the lagoon ranges from 35 to 150 km at a depth of 30-70 m. Coastal and barrier reefs border almost all the islands of the equatorial and tropical waters of the Pacific Ocean.

Atolls are located among the oceans. These are low islands in the form of an open ring. The diameter of the atoll ranges from 200 m to 60 km. Inside the atoll there is a lagoon up to 100 m deep. The depth of the strait between the lagoon and the ocean is the same. The outer slope of the atoll is always steep (from 9 to 450). The slopes facing the lagoon are flat; they host a variety of organisms.

The genetic relationship of the three types of coral structures is still an unresolved scientific problem. According to the theory of Charles Darwin, barrier reefs and atolls are formed from coastal reefs with the gradual subsidence of islands. At the same time, the growth of corals compensates for the lowering of its base. A lagoon appears on the site of the top of the island, and the coastal reef turns into a ring atoll.

The earth consists of several shells: atmosphere, hydrosphere, biosphere, lithosphere.

Biosphere- a special shell of the earth, the area of ​​vital activity of living organisms. It includes the lower part of the atmosphere, the entire hydrosphere and the upper part of the lithosphere. The lithosphere is the hardest shell of the earth:

Structure:

1. earth's crust

2. mantle (Si, Ca, Mg, O, Fe)

3. outer core

4. inner core

center of the earth - temperature 5-6 thousand o C

The core composition is Ni\Fe; core density - 12.5 kg / cm 3;

Kimberlites- (from the name of the city of Kimberley in South Africa), an igneous ultrabasic brecciated rock of an effusive appearance that fills the explosion pipes. It consists mainly of olivine, pyroxenes, pyrope-almandine garnet, picroilmenite, phlogopite, less often zircon, apatite, and other minerals included in a fine-grained groundmass, usually altered by post-volcanic processes to a serpentine-carbonate composition with perovskite, chlorite, etc. d.

eclogite- metamorphic rock consisting of pyroxene with a high content of jadeite minal (omphacite) and grossular-pyrope-almandine garnet, quartz and rutile. In terms of chemical composition, eclogites are identical to the igneous rocks of the basic composition - gabbro and basalts.

The structure of the earth's crust

Layer thickness =5-70 km; highlands - 70 km, seabed - 5-20 km, on average 40-45 km. Layers: sedimentary, granite-gneiss (not in the oceanic crust), granite-bosite (basalt)

The earth's crust is a complex of rocks lying above the Mohorovichic boundary. Rocks are natural aggregates of minerals. The latter are composed of various chemical elements. The chemical composition and internal structure of minerals depend on the conditions of their formation and determine their properties. In turn, the structure and mineral composition of rocks indicate the origin of the latter and make it possible to determine the rocks in the field.

There are two types of the earth's crust - continental and oceanic, which differ sharply in composition and structure. The first, lighter, forms elevated areas - continents with their underwater margins, the second occupies the bottom of the oceanic depressions (2500-3000m). The continental crust consists of three layers - sedimentary, granite-gneiss and granulite-mafic, with a thickness of 30-40 km on the plains to 70-75 km under the young mountains. The oceanic crust up to 6-7 km thick has a three-layer structure. Under a thin layer of loose sediments lies the second oceanic layer, consisting of basalts, the third layer is composed of gabbro with subordinate ultrabasic rocks. The continental crust is enriched in silica and light elements - Al, sodium, potassium, C, in comparison with the oceanic one.

Continental (mainland) crust characterized by high power - an average of 40 km, sometimes reaching 75 km. It consists of three "layers". On top lies a sedimentary layer formed by sedimentary rocks of different composition, age, genesis and degree of dislocation. Its thickness varies from zero (on shields) to 25 km (in deep depressions, for example, the Caspian one). Below lies the "granite" (granite-metamorphic) layer, consisting mainly of acidic rocks, similar in composition to granite. The greatest thickness of the granite layer is noted under the young high mountains, where it reaches 30 km or more. Within the flat areas of the continents, the thickness of the granite layer decreases to 15-20 km.
Under the granite layer lies the third, “basalt”, layer, which also received its name conditionally: seismic waves pass through it at the same speeds with which, under experimental conditions, they pass through basalts and rocks close to them. The third layer, 10-30 km thick, is composed of highly metamorphosed rocks of predominantly mafic composition. Therefore, it is also called granulite-mafic.

Oceanic crust sharply different from the continental. Over most of the area of ​​the ocean floor, its thickness varies from 5 to 10 km. Its structure is also peculiar: under a sedimentary layer with a thickness of several hundred meters (in deep-sea basins) to 15 km (near the continents), there is a second layer composed of pillow lavas with thin layers of sedimentary rocks. The lower part of the second layer is composed of a peculiar complex of parallel dikes of basaltic composition. The third layer of the oceanic crust, 4-7 km thick, is represented by crystalline igneous rocks of predominantly basic composition (gabbro). Thus, the most important specific feature of the oceanic crust is its low thickness and the absence of a granite layer.