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

Structure and age of the earth's crust

The main elements of the surface relief of our planet are continents and ocean basins. This division is not random; 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, and it differs sharply under the continents and the ocean floor. The thickest 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 thick layers, differing in their composition and density. The top layer is composed of relatively loose sedimentary rocks, the middle layer is called granite, and the bottom layer is called basalt. The names “granite” and “basalt” come from the similarity of these layers in composition and density to granite and basalt.

The earth's crust under the oceans differs from the continental crust 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 basaltic. There is a granite layer on the shelf; continental-type crust is developed here. The change from continental to oceanic crust 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 compared to the continental crust.

The age of the Earth is now estimated at approximately 4.2-6 billion years according to astronomical and radiometric data. The age of the oldest rocks of the continental crust studied by man is up to 3.98 billion years old (southwestern part of Greenland), and the rocks of the basalt layer are over 4 billion years old. There is no doubt that these rocks are not the primary substance of the Earth. The prehistory of these ancient rocks lasted many hundreds of millions, and perhaps billions of years. Therefore, the age of the Earth is approximately estimated to be up to 6 billion years.

Structure and development of the continental crust

The largest structures of the continental crust are geosynclinal fold belts and ancient platforms. They differ greatly from each other in their structure and history of geological development.

Before moving on to a 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” - Earth and “synclino” - deflection. It was first used by the American geologist D. Dana more than 100 years ago, while studying the Appalachian Mountains. He found that the marine Paleozoic sediments that make up the Appalachians have a maximum thickness in the central part of the mountains, much greater than on their slopes. Dana explained this fact absolutely correctly. During the period of sedimentation in the Paleozoic era, in place of the Appalachian Mountains there was a sagging depression, which he called a geosyncline. In its central part, subsidence was more intense than on the wings, as evidenced by the large thickness of sediments. Dana confirmed his conclusions with a drawing depicting the Appalachian geosyncline. Given that Paleozoic sedimentation occurred under marine conditions, he plotted down from a horizontal line—the assumed sea level—all the measured sediment thicknesses in the center and slopes of the Appalachian Mountains. The picture shows a clearly defined large depression in the place 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 development of the Earth. He established that folded mountain ranges formed in place of geosynclines. E. Og divided all areas of the continents into geosynclines and platforms; he developed the fundamentals of the study 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 identified, within which several geosynclinal areas are located. In our time, the doctrine of geosynclines has grown into a substantiated theory of 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, the geological history of which was characterized by intense sedimentation, repeated folding processes and strong volcanic activity. Thick layers of sedimentary rocks accumulated here, igneous rocks formed, and earthquakes often occurred. Geosynclinal belts occupy vast areas of continents, located between ancient platforms or along their edges in the form of wide stripes. 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 sedentary parts of the continents. Unlike geosynclinal belts, ancient platforms experienced slow oscillatory movements, sedimentary rocks of usually low thickness accumulated within them, there were no folding processes, and volcanism and earthquakes rarely occurred. Ancient platforms form sections of continents that are the skeletons of all continents. These are the most ancient parts of the continents, formed in the Archean and Early Proterozoic.

On modern continents there are from 10 to 16 ancient platforms. The largest are the 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, and sandy, carbonate and volcanic rocks are widely represented. In the sedimentary layer there are deposits of minerals such as coal, gas, and 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 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 movable belts.

A 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 uplift, 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 process of their evolution form the continental crust. There are two main stages in the development of mobile belts, regions and systems: geosynclinal and orogenic.

In the first of them, two main stages are distinguished: early geosynclinal and late geosynclinal.

Early geosynclinal the stage is characterized by 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 caused by compression processes, which are increasingly manifested in connection with the beginning of the closure of the ocean basin and the counter movement of lithospheric plates.

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

23. Platforms and stages of their development.

Platform, in geology - one of the main deep structures of the earth's crust, characterized by low intensity of tectonic movements, magmatic activity and flat topography. These are the most stable and calm areas 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, crushed into folds and broken by numerous faults.

2) Case. The upper structural floor is composed of gently lying non-metamorphosed layered strata - sedimentary, marine and continental deposits

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

1) Ancient platforms occupy about 40% of the continents' area

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

Stages of platform development.

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

2) Aulacogenic stage, which gradually follows from the previous one. Gradually aulacogens (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. As the syneclises grow, they cover the entire platform with a sedimentary cover, and its slab stage of development begins.

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

4) Activation stage. Epiplatform orogens ( mountain, mountain-fold structure that arose in place of a geosyncline)

1. Formation of continents and oceans

A billion years ago, the Earth was already covered with a durable shell, in which continental protrusions and oceanic depressions stood out. At that time, the area of ​​the oceans was approximately 2 times larger than the area of ​​the continents. But the number of continents and oceans has changed significantly since then, and their location has also changed. About 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, which occupied the rest of the space on Earth.

However, Pangea turned out to be a fragile, short-lived formation. Over time, the flow 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 continent, and not compress it, as before. About 200 million years ago, Pangea split into two continents: Laurasia and Gondwana. The Tethys Ocean appeared between them (now these are the deep-sea parts of the Mediterranean, Black, Caspian Seas and the shallow Persian Gulf).

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

Later, new faults covered Gondwana and Laurasia. The land that now makes up Australia and Antarctica was first separated from Gondwana. She began to drift 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 which are the African and South American plates. These plates are now moving away from each other at a rate of 2 cm per year (see Lithospheric plates).

Rifts also covered Laurasia. It split into two plates - the North American and Eurasian plates, 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 includes 3 parts: the Eurasian (part of Laurasia), Arabian (Gondwana protrusion) and Hindustan (part of Gondwana) lithospheric plates. By getting closer to each other, they almost destroyed the ancient Tethys Ocean. Africa also participates in shaping the appearance of Eurasia, whose lithospheric plate, although slowly, is moving closer to the Eurasian one. The result of this rapprochement are the mountains: the Pyrenees, the Alps, the Carpathians, the Sudetes and the Ore Mountains (see Lithospheric plates).

The rapprochement of the Eurasian and African lithospheric plates is still taking place; this is reminiscent of the activity of the Vesuvius and Etna volcanoes, which disturb the peace of the inhabitants of Europe.

The convergence of the Arabian and Eurasian lithospheric plates led to the crushing and folding of rocks along their path. This was accompanied by violent volcanic eruptions. As a result of the convergence of these lithospheric plates, the Armenian Highlands and the Caucasus arose.

The convergence of the Eurasian and Hindustan lithospheric plates caused the entire continent from the Indian Ocean to the Arctic to tremble, while Hindustan itself, which initially broke away from Africa, suffered little damage. The result of this rapprochement was the emergence of the highest plateau in the world, Tibet, surrounded by even higher mountain chains - the Himalayas, Pamirs, and Karakoram. 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 folded, fragmented 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 continental lithospheric plates. Every mountain range, island arc, deepest ocean trench is the result of this struggle.

2. The structure of continents and oceans

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

The continental and oceanic crusts differ in composition. This, in turn, leaves an imprint on the features of their development and structure.

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

In the structure of the oceans, areas are distinguished according to the degree of tectonic mobility, which is expressed in manifestations of seismic activity. On this basis they 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, width – up to 1000 km, height reaches 2–3 km from the ocean floor. In the axial part of such ridges, rift zones can be traced almost continuously. They are marked by high heat flow values. Mid-ocean ridges are considered to be areas of crustal extension or spreading zones.

The second group of structural elements are 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 large 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-sea trenches, 3 – deep-sea depressions of marginal seas.

Island arcs are long (up to 3000 km) mountain structures formed by a chain of volcanic structures with modern manifestations of andesite-basaltic volcanism. An example of island arcs is the Kuril-Kamchatka ridge, the Aleutian Islands, etc. From the ocean side, island arcs are replaced by deep-sea trenches, which are deep-sea 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 currents. The slopes of the gutters are stepped with different angles of inclination. No sediment was 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 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 the 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, let us define what a geological formation is. First of all, it is a real category. In the hierarchy of matter in the earth's crust, you know the following sequence:

A geological formation is a more complex stage of development following a rock. It represents natural associations of rocks, connected by the unity of their material composition and structure, which is determined by their common 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. We will consider examples of formations and the conditions for their formation when analyzing the development of the structural elements of continents.

There are two types of regions on continents.

Type I coincides with mountainous areas in which sedimentary deposits are folded and broken by various faults. Sedimentary strata are intruded by igneous rocks and metamorphosed.

Type II coincides with flat areas in which sediments lie 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 in place of geosynclinal belts or geosynclines. A geosyncline is a mobile extended area of ​​deep depression 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:

Continental type of earth's crust is oceanic. Therefore, the ocean floor proper 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 changes in height, measured in hundreds of meters. All these structural features of the axial strip of the middle ridges should obviously be understood as a manifestation of intense block tectonics, with the axial depressions being grabens, and on both sides of them the middle ridge is divided into uplifted and downturned blocks by discontinuities. The entire 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 bodies of water (seas and oceans), the appearance of the first signs of life in the aquatic environment, and the formation of the ancient relief of the Earth, similar to the relief of the Moon. Several eras of folding occurred in the Archean. A shallow ocean with many volcanic islands formed. An atmosphere containing couples has formed...

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

1. Deep structure of the Earth

The geographic envelope interacts, on the one hand, with the deep substance of the planet, and on the other, 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 refers to its internal, i.e., deep structure, starting from the earth’s crust 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.

Composition of the Earth. According to modern scientific ideas, 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 comes from observations of seismic waves, that is, oscillatory movements of the earth's matter caused by earthquakes.

A sharp change in the speed of seismic waves (recorded on seismographs) at depths 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: to a depth of 70 km - the earth's crust, from 70 km to 2,900 km - the mantle, and from there to the center of the Earth - the core. The nucleus is divided into an outer core and an inner core.

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 increase, which led to the melting of metals. Since the matter was highly compressed in the center of the Earth and was cooled from the surface by radiation, melting occurred mainly at shallow depths. Thus, a molten layer was formed, from which silicate materials, being the lightest, rose upward, 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 sank. This led to the formation of a metallic core.

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

The role of the nucleus 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 movements of matter occur, leading to the excitation of a magnetic field. The magnetic field extends into near-Earth space over several Earth radii. Interacting with the solar wind, the geomagnetic field creates the Earth's magnetosphere. The upper boundary of the magnetosphere is at an altitude of about 90 thousand km. The formation of the magnetosphere and the isolation of the earth's 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 geographical envelope.

THE MANTLE consists mainly of Mg, O, FeO and SiO2, which form magma. Magma contains water, chlorine, fluorine and other volatile substances. The process of differentiation of matter continuously occurs in the mantle. Substances lightened by the removal of metals rise towards the earth's crust, while heavier substances sink. Such movements 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 volcanoes and earthquake centers. Scientists believe that geosynclines are formed above descending currents, and mid-ocean ridges and rift zones are formed 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 about the origin and paths of 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 the Latin fixus - motionless, unchanging) states that the continents have always remained in the places that they currently occupy. This theory denies any movement of continents and large parts of the lithosphere (Charles Darwin, A. Wallace, etc.).
• 2. The theory of mobilism (from the Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has become especially firmly established in recent years in connection with the acquisition of new scientific data from the study of the bottom of the World Ocean.
• 3. The concept of continental growth at the expense of the ocean floor believes that the original continents formed in the form of relatively small massifs that now make up 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. Study of the ocean floor, especially in the area of ​​mid-ocean ridges, has given reason to doubt the correctness of this concept.
• 4. The theory of geosynclines states that the increase in land size occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the continental crust, forms the basis of many modern scientific explanations.
• 5. The rotation 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 rearranged due to uneven rotation, the zonal stripes 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 era ranges from 5 to 10 km. It consists of the following three layers:

• 1) upper thin layer of marine sediments (thickness no more than 1 km);
• 2) middle basalt layer (thickness from 1.0 to 2.5 km);
• 3) lower layer of gabbro (thickness about 5 km).

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

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

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

On 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 Conrad 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 mantle (both on continents and oceans) runs along the surface of Mohorovicic (Moho line). The speed of seismic waves on it increases abruptly to 8 km/hour.

In addition to the two main types of the earth's crust (ocean and continental), there are also areas of 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 East Asia, in the region of island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied. There is no Moho boundary here, and mantle material 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 to the asthenosphere, that is, to a depth of about 100 km.

The concept of isostasy. A study of the distribution of gravity showed 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 the 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 an equilibrium, but a desire for equilibrium, continuously disrupted and restored again. 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 equilibrium line currently runs slightly south of 600 N latitude. Modern St. Petersburg is approximately 1.5 m higher than St. Petersburg during the time of Peter the Great. As data from modern scientific research show, even the heaviness of large cities is sufficient for isostatic fluctuations of the territory beneath them. Therefore, the earth's crust in areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface (the bottom of the earth's crust): elevated areas correspond to depressions in the mantle, lower areas correspond to a higher level of its upper boundary. Thus, under the Pamirs the depth of the Moho surface is 65 km, and in the Caspian lowland it is 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 - 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 ceases , 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 caused by the internal heat of the earth’s bowels. Internal heat practically does not participate in the formation of climates. 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, when lowered by which the temperature increases by 10C, is called the geothermal step. The magnitude of the geothermal step depends on the topography, thermal conductivity of rocks, proximity of volcanic sources, circulation of groundwater, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be only 5 m, and in geologically quiet areas (on platforms) it can reach 100 m.

3. Structural-tectonic principle of separation of continents. The concept of 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 identification of continents in modern geography is carried out on the basis of the structural-tectonic principle.

Structural-tectonic principle of separation of continents.

The fundamentally 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, oblige us to distinguish continents not according to their apparent surroundings by oceans, but according to the structural-tectonic principle.

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

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

There are six continents in total on Earth: Eurasia, Africa, North America, South America, Antarctica and Australia. Each continent contains one platform, and at the base of Eurasia alone there are six of them: Eastern European, Siberian, Chinese, Tarim (Western China, Taklamakan Desert), Arabian and Hindustan. The Arabian and Hindu platforms are parts of ancient Gondwana adjacent to 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 series of continents (Eurasia and North America).

Antarctica, the southernmost and coldest continent, remains outside these ranks.

The modern 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 single Paleozoic megacontinent Gondwana. The northern continents at that time were united into another megacontinent - Laurasia. Between Laurasia and Gondwana in the Paleozoic and Mesozoic there 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 fold belt arose in the place of this geosyncline.

According to its large size, the Gondwana supercontinent, according to the law of isostasy, had a thick (up to 50 km) crust, which was deeply buried in the mantle. Beneath 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 bulge in the middle of the continent, and then to its split into separate blocks, which, under the influence of the same convection currents, began to move horizontally. It is known that the movement of a contour on the surface of a sphere is always accompanied by its rotation (Euler et al.). Therefore, parts of Gondwana not only moved, but also unfolded in geographical space.

The first breakup of Gondwana occurred at the Triassic-Jurassic boundary (about 190-195 million years ago); Afro-America seceded. Then, at the Jurassic-Cretaceous boundary (about 135-140 million years ago), South America separated from Africa. At 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 plate blocks that continue to move.

The breakup of Gondwana successfully explains the shape, geological similarity, as well as the history of the vegetation and animal world of the southern continents. The history of the split of Laurasia has not been studied as thoroughly as Gondwana.

Patterns of the location of 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 here only 39% is land and about 61% is ocean.
• 2. The northern continents are located quite compactly. The southern continents are located very scattered and disconnected.
• 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 has a corresponding ocean. This can best be seen by comparing the Arctic ocean and Antarctic land. If the globe is installed so that any of the continents is at one of the poles, then there will definitely be an ocean at the other pole. There is only one minor exception: the end of South America antipodal to Southeast Asia. Antipodality, since it has almost no exceptions, cannot be a random 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 cultural and historical development of mankind. There are six parts of the world in total: Europe, Asia, Africa, America, Australia and Oceania, Antarctica. On one continent 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 arbitrary and is drawn along the watershed line of the Ural ridge, the Ural River, the northern part of the Caspian Sea and the Kuma-Manych depression. Deep fault lines that separate Europe from Asia run through the Urals and the Caucasus.

Area of ​​continents and oceans. Land area is calculated within the modern coastline. The surface area of ​​the globe is approximately 510.2 million km2. About 361.06 million km2 is occupied by the World Ocean, which is approximately 70.8% of the total surface of the Earth. The land area accounts for approximately 149.02 million km 2, 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:

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 (according to their different origins and development) there is a significant difference in area and surface character. The main geographical differences between the northern and southern continents are as follows:

• 1. Eurasia is incomparable in size with other continents, containing more than 30% of the landmass 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 lie on ancient platforms. In North America and Eurasia, ancient platforms occupy a smaller part of the total area, and most of them occur in areas formed by Paleozoic and Mesozoic orogeny. In Africa, about 96% of its territory is in platform areas and only 4% is in mountains of 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 extending 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 dissection of land

Each of the main planetary levels - the surface of the continents and the ocean floor - breaks down into a number of minor levels. The formation of both the main and minor levels occurred during the long-term development of the earth's crust and continues in the present geological time. Let us dwell on the modern division of the continental crust into high-altitude levels. The steps are counted from sea level.

• 1. Depressions are areas of land 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 ocean level, are called cryptodepressions. Typical examples of cryptodepression are Lake Baikal and Lake Ladoga. The Caspian Sea and the Dead Sea are not cryptodepressions, because the water level in them does not reach ocean level. The area occupied by depressions (without cryptodepressions) is relatively small and amounts to about 800 thousand km2.
• 2. Lowlands (low plains) - areas of land lying at an altitude of 0 to 200 m above sea level. Lowlands are numerous on every continent (except Africa) and occupy a larger area than any other level of land. The total area of ​​all the lowland 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 from each other in the prevailing forms of relief: on the hills the relief is rugged, on the plateau it is relatively flat. The hills rise above the lowlands gradually, and the plateau rises as a noticeable ledge. Hills and plateaus differ in each other and in their geological structure. The area occupied by hills and plateaus is about 33 million km2.

Above 500 m there are mountains. They can be of different origins and ages. By height, mountains are divided into low, medium and high.

• 4. Low mountains rise no higher than 1,000 m. Typically, low mountains are either ancient destroyed mountains or the foothills of modern mountain systems. Low mountains occupy about 27 million km2.
• 5. Medium mountains have a height of 1,000 to 2,000 m. Examples of medium-high mountains are: the Urals, the Carpathians, Transbaikalia, some ridges of Eastern Siberia and many other mountainous countries. The area occupied by medium-sized 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. The high mountains account for about 16 million km2.

Below ocean level, the continental lowland continues, flooded with water - the shelf, or continental shoal. Until recently, according to the same conventional account as the stages of 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 . The shelf therefore continues into the ocean to varying 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 towards the continental slope and continental foot. The shelf, slope and foot together form the underwater margin of the continents. It continues to an average depth of 2,450 m.

Continents, including their underwater margins, occupy about 40% of the Earth's surface, while the land area is about 29.2% of the total earth's surface.

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 - 300 m, Australia - 350 m.

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

5. Relief and structure of the bottom of the World Ocean. 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) underwater continental margins,
• 2) transition zones,
• 3) ocean bed,
• 4) mid-ocean ridges.

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

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

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

Mid-ocean ridges are a single and continuous system spanning 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 mid-ocean ridges rise above ocean level and form numerous islands (Iceland, Azores, Seychelles, etc.). In terms of grandeur, they significantly surpass the mountainous countries of the continents and are comparable to the continents. For example, the Mid-Atlantic Ridge is several times larger than the largest land mountain system, the Cordillera and Andes. All mid-ocean ridges are characterized by increased tectonic activity.

The mid-ocean ridge system 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 Islands);
• - East Pacific Rise (extends south of the California Peninsula).

According to the relief and characteristics 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 runs 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 7,400 m (Romanche Trench). The relief of rift ridges is complex and rugged. All ridges of this type are characterized by rift valleys, narrow mountain ranges, giant transverse faults, intermontane depressions, volcanic cones, submarine 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 less complex terrain. Seismic activity is not typical for non-rift ridges. However, they share a common feature of all mid-ocean ridges - the presence of enormous transverse faults.

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

• -increased 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 one moves away from the ridge, the thickness of the sediments increases (up to several kilometers) and their age. If in the cleft itself the age of the lavas is approximately 13 thousand years, then 60 km away it is already 8 million years old. Rocks older than 160 million years have not been found at the bottom of the World Ocean. These facts indicate the constant renewal of 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. Along rift valleys, lava flows out and forms a basalt layer. By joining the old crust, new portions of lava cause horizontal displacement of lithospheric blocks and expansion of the ocean floor. The speed of horizontal movements in different places of the Earth ranges 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 diverged 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 not an increase in earthly matter at all, but only its flow and transformation. The basaltic crust, growing along the mid-ocean ridges and spreading horizontally from them, travels thousands of kilometers over the course of millions of years and, at some edges of the continents, descends again into the bowels of the Earth, taking with it ocean sediments. This process explains the different ages of rocks on the crests of ridges and in other parts of the oceans. This process also causes continental drift.

Transition zones include deep-sea trenches, island arcs, and basins of marginal seas. In transition zones, areas of continental and oceanic crust are complexly combined.

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: Aleutian Trench, Kuril-Kamchatka Trench, Japanese Trench, Philippine Trench, Mariana Trench (with a maximum depth of 11,022 m for the Earth), Western 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 ocean floor, which accounts for about 73% of the total area of ​​the World Ocean, is occupied by deep-water plains (from 2,450 to 6,000 m). In general, these deep-sea plains correspond to oceanic platforms. Between the plains there are mid-ocean ridges, as well as hills and uplifts of other origins. These rises 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 are the Western European and Canary Basins. There are numerous volcanic cones on the ocean floor.

Islands. In the process of 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, delta islands are formed and eroded, and ice masses that were previously mistaken for islands (“lands”) are melting. Sea spits acquire an island character and, conversely, 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 the entire island landmass is located on 28 large islands. The largest island is Greenland (2.2 million km2).

IN The 28 largest islands on the globe include the following:

• 1. Greenland;
• 2. New Guinea;
• 3. Kalimantan (Borneo);
• 4. Madagascar;
• 5. Baffin Island;
• 6. Sumatra;
• 7. Great Britain;
• 8. Honshu;
• 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 (for example, Franz Josef Land, Spitsbergen, Greater Sunda Islands) or elongated (for example, Japanese, Philippine, Greater and Lesser Antilles). Elongated archipelagos are sometimes called ridges (for example, the Kuril ridge, the Aleutian ridge). 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 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 lava outpouring, b) central lava outpouring - shield and conical;
• 2) coral islands: a) coastal reefs, b) barrier reefs, c) atolls.

Mainland islands are genetically connected to the continents, but these connections are of a different nature, which affects the nature and age of the islands, their flora and fauna.

Platform islands lie on the mainland shallows and geologically represent a continuation of the mainland. The platform islands are separated from the main landmass by shallow straits. Examples of platform islands are: the British Isles, the Spitsbergen archipelago, Franz Josef Land, Severnaya Zemlya, the New Siberian Islands, the Canadian Arctic archipelago.

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

Orogenic islands are a continuation of the mountain 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 New Zealand archipelago is also of orogenic origin.

Island arcs garland around 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 the 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.

Mainland coastal islands have different origins and represent different types of coastline.

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

There are volcanic islands in all oceans. However, there are especially many of them in the zones of mid-ocean ridges. The size and features of volcanic islands are determined by the nature of the eruption. Fissure lava outpourings create large islands, not inferior in size to platform islands. 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). An 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 in terms of living conditions. They can live only in warm waters with a temperature of at least 200C. Therefore, coral structures are common only in tropical latitudes and extend 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 off the coast of the mainland or island at low tide and border it in the form of a wide terrace. Near river mouths and near mangroves, they are interrupted due to low salinity of the water.

Barrier reefs are located at some distance from land, separated from it by a strip of water - a lagoon. The largest reef currently available is the Great Barrier Reef. Its length is about 2,000 km; The width of the lagoon ranges from 35 to 150 km with a depth of 30-70 m. Coastal and barrier reefs fringe 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 shape 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 gentle; They are inhabited by a variety of organisms.

The genetic relationship of the three types of coral structures is an unresolved scientific problem. According to Charles Darwin's theory, barrier reefs and atolls are formed from coastal reefs during the gradual sinking of islands. In this case, the growth of corals compensates for the lowering of its base. A lagoon appears in place 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, an 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. 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

Core composition – Ni\Fe; core density – 12.5 kg/cm 3 ;

Kimberlites- (from the name of the city of Kimberley in South Africa), igneous ultrabasic brecciated rock of effusive appearance, producing explosion tubes. It consists mainly of olivine, pyroxenes, pyrope-almandine garnet, picroilmenite, phlogopite, less commonly zircon, apatite and other minerals included in the 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 end-member (omphacite) and grossular-pyrope-almandine garnet, quartz and rutile. The chemical composition of eclogites is identical to igneous rocks of basic composition - gabbro and basalts.

Structure of the earth's crust

Layer thickness = 5-70 km; highlands - 70 km, seabed - 5-20 km, 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 that lie above the Mohorovicic boundary. Rocks are regular aggregates of minerals. The latter consist 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 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 ultrabasites. The continental crust is enriched in silica and light elements - Al, sodium, potassium, C, compared to the oceanic crust.

Continental (mainland) crust characterized by great thickness - on average 40 km, in some places reaching 75 km. It consists of three "layers". On top lies a sedimentary layer formed by sedimentary rocks of various compositions, ages, genesis and degrees of dislocation. Its thickness varies from zero (on shields) to 25 km (in deep depressions, for example, the Caspian). 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 observed under 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 conventionally: 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 basic composition. Therefore, it is also called granulite-mafic.

Oceanic crust differs sharply from the continental one. Over most of the ocean floor, its thickness ranges from 5 to 10 km. Its structure is also peculiar: under a sedimentary layer with a thickness ranging from several hundred meters (in deep-sea basins) to 15 km (near continents) lies 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 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.