Questions to consider:
1. Methods for studying the internal structure of the Earth.
2. Internal structure Earth.
3. Physical properties and chemical composition of the Earth.
4. History of the emergence and development of the earth’s shells. Movement of the earth's crust.
5. Volcanoes and earthquakes.
1. Methods for studying the internal structure of the Earth.
1) Visual observations of rock outcrops
Rock outcrop - this is the outcrop of rocks on the earth's surface in ravines, river valleys, quarries, mine workings, and on mountain slopes.
When studying an outcrop, attention is paid to what rocks it is composed of, what the composition and thickness of these rocks are, and the order of their occurrence. Samples are taken from each layer for further study in the laboratory to determine the chemical composition of the rocks, their origin and age.
2) Well drilling allows you to extract rock samples – core, and then determine the composition, structure, occurrence of rocks and construct a drawing of the drilled strata - geological section terrain. Comparison of many sections makes it possible to establish how the rocks are deposited and to draw up a geological map of the territory. The deepest well was drilled to a depth of 12 km. These two methods allow us to study the Earth only superficially.
3) Seismic exploration.
By creating an artificial earthquake wave with an explosion, people monitor the speed of its passage through various layers. The denser the medium, the greater the speed. Knowing these speeds and tracking their changes, scientists can determine the density of the underlying rocks. This method is called seismic sounding and helped to look inside the Earth.
2. Internal structure of the Earth.
Seismic sounding of the Earth made it possible to distinguish three parts of it - the lithosphere, the mantle and the core.Lithosphere (from Greek litos - stone and sphere - ball) - the upper, rocky shell of the Earth, including the earth's crust and the upper layer of the mantle (asthenosphere). The depth of the lithosphere reaches more than 80 km. The substance of the asthenosphere is in a viscous state. As a result, the earth's crust seems to float on a liquid surface.
The earth's crust has a thickness of 3 to 75 km. Its structure is heterogeneous (from top to bottom):
1 – sedimentary rocks (sand, clay, limestone) – 0-20 km. Loose rocks have low seismic wave speeds.
2 – granite layer (absent under the ocean) has a high wave speed of 5.5-6 km/s;
3 – basalt layer (wave speed 6.5 km/s);
There are two types of bark - mainland And oceanic. Under the continents, the crust contains all three layers - sedimentary, granite and basalt. Its thickness on the plains reaches 15 km, and in the mountains it increases to 80 km, forming “mountain roots”. Under the oceans, in many places the granite layer is completely absent and the basalts are covered with a thin cover of sedimentary rocks. In the deep-sea parts of the ocean, the thickness of the crust does not exceed 3-5 km, and the upper mantle lies below.
The temperature in the thickness of the crust reaches 600 o C. It mainly consists of silicon and aluminum oxides.
Mantle - an intermediate shell located between the lithosphere and the Earth's core. Its lower boundary supposedly lies at a depth of 2900 km. The mantle accounts for 83% of the Earth's volume. The temperature of the mantle ranges from 1000 O C in the upper layers up to 3700 O C in the lower ones. The interface between the crust and the mantle is the Moho (Mohorovicic) surface.
Earthquakes occur in the upper mantle, and ores, diamonds and other minerals are formed. This is where internal heat comes to the surface of the Earth. The material of the upper mantle constantly and actively moves, causing the movement of the lithosphere and the earth's crust. It consists of silicon and magnesium. The inner mantle constantly mixes with the liquid core. Heavy elements sink into the core, and light elements rise to the surface. The substance that makes up the mantle has completed the circuit 20 times. This process must be repeated only 7 times and the process of building the earth's crust, earthquakes and volcanoes will stop.
Core consists of an outer (up to a depth of 5 thousand km), liquid layer and an inner solid layer. It is an iron-nickel alloy. The temperature of the liquid core is 4000 o C, and the internal one is 5000 o C. The core has a very high density, especially the inner one, which is why it is solid. The density of the core is 12 times that of water.
3. Physical properties and chemical composition of the Earth.To physical properties Earth includes temperature (internal heat), density and pressure.
On the Earth's surface, the temperature is constantly changing and depends on the influx of solar heat. Daily temperature fluctuations extend to a depth of 1-1.5 m, seasonal - up to 30 m. Below this layer lies zone of constant temperatures, where they always remain the same
85 and correspond to the average annual temperatures of a given area on the Earth's surface.
The depth of the constant temperature zone is not the same in different places and depends on the climate and thermal conductivity of rocks. Below this zone, temperatures begin to rise, on average by 30 °C every 100 m. However, this value is not constant and depends on the composition of rocks, the presence of volcanoes, and the activity of thermal radiation from the bowels of the Earth.
Knowing the radius of the Earth, it can be calculated that in the center its temperature should reach 200,000 °C. However, at this temperature the Earth would turn into hot gas. It is generally accepted that a gradual increase in temperatures occurs only in the lithosphere, and that the source of the Earth’s internal heat is the upper mantle. Below, the temperature increase slows down, and in the center of the Earth it does not exceed 5000° WITH.
Density of the Earth. The denser the body, the greater the mass per unit volume. The standard of density is considered to be water, 1 cm 3 of which weighs 1 g, i.e., the density of water is 1 g/cm 3 . The density of other bodies is determined by the ratio of their mass to the mass of water of the same volume. From this it is clear that all bodies with a density greater than 1 sink, and those with less density float.
The density of the Earth is not the same in different places. Sedimentary rocks have a density of 1.5 - 2 g/cm 3, granite - 2.6 g/cm 3 , and basalts - 2.5-2.8 g/cm3. Average density Earth is 5.52 g/cm 3 . In the center of the Earth, the density of the rocks composing it increases and amounts to 15-17 g/cm 3 .
Pressure inside the Earth. The rocks located in the center of the Earth experience enormous pressure from the overlying layers. It is calculated that at a depth of only 1 km the pressure is 10 4 hPa, and in the upper mantle it exceeds 6 10 4 hPa. Laboratory experiments show that at this pressure, solids, such as marble, bend and can even flow, that is, they acquire properties intermediate between a solid and a liquid. This state of matter is called plastic. This experiment suggests that in the deep interior of the Earth, matter is in a plastic state.
Chemical composition Earth. IN Everything can be found on earth chemical elements tables of D.I. Mendeleev. However, their number is not the same, they are distributed extremely unevenly. For example, in the earth's crust, oxygen (O) makes up more than 50%, iron (Fe) less than 5% of its mass. It is estimated that the basalt and granite layers consist mainly of oxygen, silicon and aluminum, and the proportion of silicon, magnesium and iron increases in the mantle. In general, it is generally accepted that 8 elements (oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, hydrogen) account for 99.5% of the composition of the earth’s crust, and all others - 0.5%. Data on the composition of the mantle and core are speculative.
4. History of the emergence and development of the earth’s shells. Movement of the earth's crust.
About 5 billion years ago, the cosmic body Earth was formed from a gas-dust nebula. It was cold. Clear boundaries between the shells did not yet exist. Gases rose from the depths of the Earth in a stormy stream, shaking the surface with explosions.
As a result of strong compression, nuclear reactions began to occur in the core, which led to the release of a large amount of heat. The energy of the planet's core is heated. In the process of melting the metals of the subsoil, lighter substances floated to the surface and formed a crust, while heavier substances sank down. The frozen thin film sank in the hot magma and formed again. After a while, large masses of light oxides of silicon and aluminum began to accumulate on the surface, which no longer sank. Over time, they formed large masses and cooled. Such formations are called lithosphrenic plates(mainland platforms). They floated like giant icebergs and continue their drift on the plastic surface of the mantle.
2 billion years ago, a water shell appeared as a result of condensation of water vapor.
About 500-430 million years ago, there were 4 continents: Angaria (part of Asia), Gondwana, North American and European plates. As a result of plate movement, the last two plates collided, forming mountains. Euroamerica was formed.
About 275 million years ago, a collision between Euroamerica and Angaria occurred, and the Ural Mountains arose on the site. As a result of this collision, Laurasia emerged.
Soon, Laurasia and Gondwana united to form Pangea (175 million years ago), and then diverged again. Each of these continents broke up into fragments, forming modern continents.
Convection currents occur in the upper mantle under the influence of rising heat flows. Large deep pressure forces the lithosphere, consisting of individual blocks - plates, to move. The lithosphere is divided into approximately 15 large plates moving in different directions. When colliding with each other, their surface is compressed into folds and rises, forming mountains. Cracks form in other places ( rift zones) and lava flows, bursting out, fill the space. These processes occur both on land and on the ocean floor.
Video 1. Formation of the Earth and its lithospheric plates.
Movement of lithospheric plates.
Tectonics– the process of movement of lithospheric plates along the surface of the mantle. The movement of the earth's crust is called tectonic movement.
Study of rock structure, electronic topographic survey the ocean floor from space confirmed the theory of plate tectonics.
Video 2. Evolution of continents.
5. Volcanoes and earthquakes.
Vulcan –a geological formation on the surface of the earth's crust through which flows of molten rock, gases, steam and ash erupt. It is necessary to distinguish between magma and lava. Magma is liquid rock in the crater of a volcano. lava - flows of rock along the slopes of a volcano. Volcanic mountains form from cooled lava
There are about 600 on Earth active volcanoes. They form where the earth's crust is split by cracks and layers of molten magma lie close. High pressure forces it to rise. Volcanoes are either terrestrial or underwater.
A volcano is a mountain that has channel ending with a hole - crater. There may also be side channels. Through the volcano's channel, liquid magma flows from the magma reservoir to the surface, forming lava flows. If lava cools in the crater of a volcano, a plug is formed, which, under the influence of gas pressure, can explode, clearing the way for fresh magma (lava). If the lava is liquid enough (there is a lot of water in it), then it quickly flows down the slope of the volcano. Thick lava flows slowly and hardens, increasing the height and width of the volcano. The lava temperature can reach 1000-1300 o C and move at a speed of 165 m/s.
Volcanic activity is often accompanied by the release of large amounts of ash, gases and water vapor. Before the eruptionabove the volcano, a column of emissions can reach several tens of kilometers in height. A crater may form in place of the mountain after an eruption. gigantic size with a bubbling lake of lava inside - caldera.
Volcanoes form in seismically active zones: in places where lithospheric plates touch. In faults, magma comes close to the Earth's surface, melting rocks and forming a volcanic conduit. Trapped gases increase pressure and push the magma to the surface.
Shell structure of the Earth. Physical state (density, pressure, temperature), chemical composition, movement of seismic waves in the interior of the Earth. Terrestrial magnetism. Sources of internal energy of the planet. Age of the Earth. Geochronology.
The Earth, like other planets, has a shell structure. When seismic waves (longitudinal and transverse) pass through the body of the Earth, their velocities at some deep levels change noticeably (and abruptly), which indicates a change in the properties of the medium passed by the waves. Modern ideas about the distribution of density and pressure inside the Earth are given in the table.
Changes in density and pressure with depth inside the Earth
(S.V. Kalesnik, 1955)
|
Depth, km |
Density, g/cm 3 |
Pressure, million atm |
The table shows that in the center of the Earth the density reaches 17.2 g/cm 3 and that it changes with a particularly sharp jump (from 5.7 to 9.4) at a depth of 2900 km, and then at a depth of 5 thousand km. The first jump makes it possible to isolate a dense core, and the second - to subdivide this core into outer (2900-5000 km) and inner (from 5 thousand km to the center) parts.
Dependence of the speed of longitudinal and transverse waves from depth
|
Depth, km |
Longitudinal wave speed, km/sec |
Shear wave speed, km/sec |
|
60 (top) | ||
|
60 (bottom) | ||
|
2900 (top) | ||
|
2900 (bottom) | ||
|
5100 (top) | ||
|
5100 (bottom) | ||
Thus, there are essentially two sharp changes in velocities: at a depth of 60 km and at a depth of 2900 km. In other words, the earth's crust and inner core are clearly separated. In the intermediate belt between them, as well as inside the core, there is only a change in the rate of increase in speeds. It can also be seen that the Earth is in a solid state down to a depth of 2900 km, because Transverse elastic waves (shear waves) pass freely through this thickness, which are the only ones that can arise and propagate in a solid medium. The passage of transverse waves through the core was not observed, and this gave reason to consider it liquid. However, the latest calculations show that the shear modulus in the core is small, but still not equal to zero (as is typical for a liquid) and, therefore, the Earth's core is closer to a solid state than a liquid state. Of course, in this case the concepts of “solid” and “liquid” cannot be identified with similar concepts applied to the aggregate states of matter on earth's surface: High temperatures and enormous pressures prevail inside the Earth.
Thus, the internal structure of the Earth is divided into the crust, mantle and core.
Earth's crust - the first shell of the Earth’s solid body, has a thickness of 30-40 km. By volume it is 1.2% of the volume of the Earth, by mass - 0.4%, the average density is 2.7 g / cm 3. Consists mainly of granites; sedimentary rocks are of subordinate importance in it. The granite shell, in which silicon and aluminum play a huge role, is called “sialic” (“sial”). The earth's crust is separated from the mantle by a seismic section called Moho border, from the name of the Serbian geophysicist A. Mohorovicic (1857-1936), who discovered this “seismic section”. This boundary is clear and is observed in all places on Earth at depths from 5 to 90 km. The Moho section is not simply a boundary between rocks of different types, but represents a plane of phase transition between eclogites and gabbros of the mantle and basalts of the earth's crust. During the transition from the mantle to the crust, the pressure drops so much that gabbro turns into basalts (silicon, aluminum + magnesium - “sima” - silicon + magnesium). The transition is accompanied by an increase in volume by 15% and, accordingly, a decrease in density. The Moho surface is considered the lower boundary of the earth's crust. An important feature of this surface is that it is general outline It is, as it were, a mirror image of the relief of the earth's surface: under the oceans it is higher, under the continental plains it is lower, under the highest mountains it descends lowest (these are the so-called roots of the mountains).
There are four types of the earth's crust; they correspond to the four largest forms of the Earth's surface. The first type is called mainland, its thickness is 30-40 km; under young mountains it increases to 80 km. This type of earth's crust corresponds in relief to continental protrusions (the underwater margin of the continent is included). The most common division is into three layers: sedimentary, granite and basalt. Sedimentary layer, up to 15-20 km thick, complex layered sediments(clays and shales predominate, sandy, carbonate and volcanic rocks are widely represented). granite layer(thickness 10-15 km) consists of metamorphic and igneous acidic rocks with a silica content of over 65%, similar in properties to granite; the most common are gneisses, granodiorites and diorites, granites, crystalline schists). The lower layer, the densest, 15-35 km thick, is called basalt for its resemblance to basalts. The average density of the continental crust is 2.7 g/cm3. Between the granite and basalt layers lies the Conrad boundary, named after the Austrian geophysicist who discovered it. The names of the layers - granite and basalt - are arbitrary; they are given according to the speed of passage of seismic waves. The modern name of the layers is somewhat different (E.V. Khain, M.G. Lomize): the second layer is called granite-metamorphic, because There are almost no granites in it; it is composed of gneisses and crystalline schists. The third layer is granulite-basite; it is formed by highly metamorphosed rocks.
Second type of earth's crust – transitional, or geosynclinal – corresponds to transition zones (geosynclines). Transition zones are located off the eastern shores of the Eurasian continent, off the eastern and western shores of North and South America. They have the following classical structure: a marginal sea basin, island arcs and a deep-sea trench. Under the basins of the seas and deep-sea trenches there is no granite layer; the earth's crust consists of a sedimentary layer of increased thickness and basalt. The granite layer appears only in island arcs. The average thickness of the geosynclinal type of the earth's crust is 15-30 km.
Third type - oceanic the earth's crust corresponds to the ocean floor, the thickness of the crust is 5-10 km. It has a two-layer structure: the first layer is sedimentary, formed by clayey-siliceous-carbonate rocks; the second layer consists of holocrystalline igneous rocks of basic composition (gabbro). Between the sedimentary and basaltic layers there is an intermediate layer consisting of basaltic lavas with interlayers of sedimentary rocks. Therefore, they sometimes talk about the three-layer structure of the oceanic crust.
Fourth type - riftogenic the earth's crust, it is characteristic of mid-ocean ridges, its thickness is 1.5-2 km. At mid-ocean ridges, mantle rocks come close to the surface. The thickness of the sedimentary layer is 1-2 km, the basalt layer in the rift valleys pinches out.
There are the concepts of “earth’s crust” and “lithosphere”. Lithosphere– the rocky shell of the Earth, formed by the earth’s crust and part of the upper mantle. Its thickness is 150-200 km, limited by the asthenosphere. Only the upper part of the lithosphere is called the earth's crust.
Mantle by volume it is 83% of the Earth's volume and 68% of its mass. The density of the substance increases to 5.7 g/cm3. At the boundary with the core, the temperature increases to 3800 0 C, the pressure - to 1.4 x 10 11 Pa. The upper mantle is distinguished to a depth of 900 km and the lower mantle to a depth of 2900 km. In the upper mantle at a depth of 150-200 km there is an asthenospheric layer. Asthenosphere(Greek asthenes - weak) - a layer of reduced hardness and strength in the upper mantle of the Earth. The asthenosphere is the main source of magma, where volcanic feeding centers are located and lithospheric plates move.
Core occupies 16% of the volume and 31% of the mass of the planet. The temperature in it reaches 5000 0 C, pressure – 37 x 10 11 Pa, density – 16 g/cm 3. The core is divided into an outer one, up to a depth of 5100 km, and an inner one. The outer core is molten and consists of iron or metallized silicates, the inner core is solid, iron-nickel.
The mass of a celestial body depends on the density of matter; mass determines the size of the Earth and the force of gravity. Our planet has sufficient size and gravity; it retains the hydrosphere and atmosphere. Metallization of matter occurs in the Earth's core, causing the formation of electric currents and the magnetosphere.
There are various fields around the Earth, the most significant influence on GO is gravitational and magnetic.
Gravity field on Earth it is the gravity field. Gravity is the resultant force between the force of attraction and the centrifugal force that occurs when the Earth rotates. Centrifugal force reaches its maximum at the equator, but even here it is small and amounts to 1/288 of the force of gravity. The force of gravity on earth mainly depends on the force of attraction, which is influenced by the distribution of masses inside the Earth and on the surface. The force of gravity acts everywhere on earth and is directed plumb to the surface of the geoid. The strength of the gravitational field decreases uniformly from the poles to the equator (at the equator the centrifugal force is greater), from the surface up (at an altitude of 36,000 km it is zero) and from the surface down (at the center of the Earth the gravity force is zero).
Normal gravitational field The shape of the Earth is what the Earth would have if it had the shape of an ellipsoid with a uniform distribution of masses. The real field strength at a specific point differs from normal, and a gravitational field anomaly occurs. Anomalies can be positive and negative: mountain ranges create additional mass and should cause positive anomalies, ocean trenches, on the contrary, negative ones. But in fact, the earth's crust is in isostatic equilibrium.
Isostasy (from the Greek isostasios - equal in weight) - balancing of the solid, relatively light earth's crust with a heavier upper mantle. The theory of equilibrium was put forward in 1855 by the English scientist G.B. Airy. Thanks to isostasy, an excess of mass above the theoretical equilibrium level corresponds to a shortage below. This is expressed in the fact that at a certain depth (100-150 km) in the asthenosphere layer, matter flows to those places where there is a lack of mass on the surface. Only under young mountains, where compensation has not yet fully occurred, are weak positive anomalies observed. However, the balance is constantly being disrupted: sediment is deposited in the oceans, and the ocean floor bends under its weight. On the other hand, mountains are destroyed, their height decreases, which means their mass decreases.
Gravity creates the shape of the Earth; it is one of the leading endogenous forces. Thanks to it, atmospheric precipitation falls, rivers flow, groundwater horizons are formed, and slope processes are observed. Gravity explains the maximum height of mountains; It is believed that on our Earth there cannot be mountains higher than 9 km. Gravity holds the gas and water shells of the planet together. Only the lightest molecules - hydrogen and helium - leave the planet's atmosphere. The mass pressure of matter, realized in the process of gravitational differentiation in the lower mantle, along with radioactive decay, generates thermal energy - a source of internal (endogenous) processes that rebuild the lithosphere.
The thermal regime of the surface layer of the earth's crust (on average up to 30 m) has a temperature determined by solar heat. This heliometric layer experiencing seasonal temperature fluctuations. Below is an even thinner horizon of constant temperature (about 20 m), corresponding to the average annual temperature of the observation site. Below the permanent layer, the temperature increases with depth - geothermal layer. To quantify the magnitude of this increase, two mutually related concepts. The change in temperature when going 100 m deeper into the ground is called geothermal gradient(varies from 0.1 to 0.01 0 S/m and depends on the composition of rocks, the conditions of their occurrence), and the plumb distance to which it is necessary to go deeper in order to obtain an increase in temperature by 1 0 is called geothermal stage(varies from 10 to 100 m/ 0 C).
Terrestrial magnetism - a property of the Earth that determines the existence of a magnetic field around it caused by processes occurring at the core-mantle boundary. For the first time, humanity learned that the Earth is a magnet thanks to the works of W. Gilbert.
Magnetosphere – a region of near-Earth space filled with charged particles moving in the Earth’s magnetic field. It is separated from interplanetary space by the magnetopause. This is the outer boundary of the magnetosphere.
At the heart of education magnetic field there are internal and external reasons. A constant magnetic field is formed due to electric currents arising in the outer core of the planet. Solar corpuscular flows form the Earth's alternating magnetic field. Magnetic maps provide a visual representation of the state of the Earth's magnetic field. Magnetic maps are compiled for a five-year period - the magnetic era.
The Earth would have a normal magnetic field if it were a uniformly magnetized sphere. To a first approximation, the Earth is a magnetic dipole - it is a rod whose ends have opposite magnetic poles. The places where the dipole's magnetic axis intersects with the earth's surface are called geomagnetic poles. Geomagnetic poles do not coincide with geographic ones and move slowly at a speed of 7-8 km/year. Deviations of the real magnetic field from the normal (theoretically calculated) are called magnetic anomalies. They can be global (East Siberian Oval), regional (KMA) and local, associated with the close occurrence of magnetic rocks to the surface.
The magnetic field is characterized by three quantities: magnetic declination, magnetic inclination and strength. Magnetic declination- the angle between the geographic meridian and the direction of the magnetic needle. The declination is eastern (+), if the northern end of the compass needle deviates east of the geographic one, and western (-), when the arrow deviates to the west. Magnetic inclination- the angle between the horizontal plane and the direction of the magnetic needle suspended on the horizontal axis. The inclination is positive when the north end of the arrow points down, and negative when the north end points up. The magnetic inclination varies from 0 to 90 0 . The strength of the magnetic field is characterized by tension. The magnetic field strength is low at the equator 20-28 A/m, at the pole – 48-56 A/m.
The magnetosphere has a teardrop shape. On the side facing the Sun, its radius is equal to 10 radii of the Earth; on the night side, under the influence of the “solar wind,” it increases to 100 radii. The shape is due to the influence of the solar wind, which, encountering the Earth’s magnetosphere, flows around it. Charged particles, reaching the magnetosphere, begin to move along magnetic power lines and form radiation belts. The inner radiation belt consists of protons and has a maximum concentration at an altitude of 3500 km above the equator. The outer belt is formed by electrons and extends up to 10 radii. At the magnetic poles, the height of the radiation belts decreases, and areas arise here in which charged particles invade the atmosphere, ionizing atmospheric gases and causing auroras.
The geographic significance of the magnetosphere is very great: it protects the Earth from corpuscular solar and cosmic radiation. Magnetic anomalies are associated with the search for minerals. Magnetic lines of force help tourists and ships navigate in space.
Age of the Earth. Geochronology.
The Earth arose as a cold body from an accumulation of solid particles and bodies like asteroids. Among the particles there were also radioactive ones. Once inside the Earth, they disintegrated there, releasing heat. While the size of the Earth was small, heat easily escaped into interplanetary space. But with the increase in the volume of the Earth, the production of radioactive heat began to exceed its leakage, it accumulated and heated the bowels of the planet, causing them to soften. The plastic state that opened up possibilities for gravitational differentiation of matter– floating of lighter mineral masses to the surface and gradual descent of heavier ones to the center. The intensity of differentiation faded with depth, because in the same direction, due to an increase in pressure, the viscosity of the substance increased. The earth's core was not captured by differentiation and retained its original silicate composition. But it thickened sharply due to the highest pressure, exceeding a million atmospheres.
The age of the Earth is determined using the radioactive method; it can only be applied to rocks containing radioactive elements. If we assume that all argon on Earth is a decay product of potassium-49, then the age of the Earth will be at least 4 billion years. Calculations by O.Yu. Schmidt gives an even higher figure - 7.6 billion years. IN AND. To calculate the age of the Earth, Baranov took the ratio between the modern amounts of uranium-238 and actinouranium (uranium-235) in rocks and minerals and obtained the age of uranium (the substance from which the planet later arose) of 5-7 billion years.
Thus, the age of the Earth is determined in the range of 4-6 billion years. The history of the development of the earth's surface has so far been able to be directly reconstructed in general terms only starting from those times from which the oldest rocks have been preserved, i.e. for approximately 3 - 3.5 billion years (Kalesnik S.V.).
The history of the Earth is usually divided into two eon: cryptozoic(hidden and life: no remains of skeletal fauna) and Phanerozoic(explicit and life) . Cryptose contains two eras: Archean and Proterozoic. The Phanerozoic covers the last 570 million years, it includes Paleozoic, Mesozoic and Cenozoic eras, which, in turn, are divided into periods. Often the entire period before the Phanerozoic is called Precambrian(Cambrian - the first period of the Paleozoic era).
Periods of the Paleozoic era:
Periods of the Mesozoic era:
Periods of the Cenozoic era:
Paleogene (epochs – Paleocene, Eocene, Oligocene)
Neogene (epochs – Miocene, Pliocene)
Quaternary (epochs - Pleistocene and Holocene).
Conclusions:
1. All manifestations of the internal life of the Earth are based on the transformation of thermal energy.
2. In the earth’s crust, the temperature increases with distance from the surface (geothermal gradient).
3. The heat of the Earth has its source from the decay of radioactive elements.
4. The density of the Earth’s substance increases with depth from 2.7 on the surface to 17.2 in the central parts. The pressure in the center of the Earth reaches 3 million atm. Density increases abruptly at depths of 60 and 2900 km. Hence the conclusion - the Earth consists of concentric shells that embrace each other.
5. The earth's crust is composed primarily of rocks such as granites, which are underlain by rocks such as basalts. The age of the earth is determined to be 4-6 billion years.
Remember! What do you know about the internal structure of the Earth, about the types of structure of the earth's crust? What are platforms and geosynclines? What are the differences between ancient and young platforms? Using the map “Structure of the Earth’s Crust” in the atlas “Geography of Continents and Oceans”, determine the patterns of location of ancient platforms and folded belts of different ages. What do you know about relief, mountains and plains, under the influence of what processes is the relief of the Earth formed?
The earth has a complex internal structure. The structure of the Earth is judged mainly on the basis of seismic data - by the speed of the waves that occur during earthquakes. Direct observations are possible only to a small depth: the deepest wells penetrated just over 12 km of the earth's thickness (Kola superdeep).
There are three main layers in the structure of the Earth (Fig. 15): the earth's crust, mantle and core.
Rice. 15. Internal structure of the Earth:
1 - earth's crust, 2 - mantle, 3 - asthenosphere, 4 - core
Earth's crust on the Earth's scale it is a thin film. Its average thickness is about 35 km.
Mantle extends to a depth of 2900 km. Inside the mantle, at a depth of 100-250 km under the continents and 50-100 km under the oceans, a layer of increased plasticity of matter begins, close to melting, the so-called asthenosphere. The base of the asthenosphere is located at depths of about 400 km. The earth's crust, together with the upper solid layer of the mantle above the asthenosphere, is called the lithosphere (from the Greek lithos - stone). The lithosphere, unlike the asthenosphere, is a relatively fragile shell. It is divided by deep faults into large blocks called lithospheric plates. The plates slowly move horizontally along the asthenosphere.
Core is located at depths from 2900 to 6371 km, i.e. the radius of the core occupies more than half the radius of the Earth. It is assumed, according to seismological data, that in the outer part of the core substances are in a molten mobile state and that due to the rotation of the planet, electric currents that create Earth's magnetic field; the inner part of the kernel is hard.
With depth, pressure and temperature increase, which in the core, according to calculations, is about 5000°C.
The layers of the Earth have different material composition, which is associated with the differentiation of the primary cold substance of the planet under conditions of its strong heating and partial melting. It is assumed that in this case the heavier elements (iron, nickel, etc.) “sank”, and the relatively light ones (silicon, aluminum) “floated”. The former formed the core, the latter - the earth's crust. Gases and water vapor were simultaneously released from the melt, which formed the primary atmosphere and hydrosphere.
Age of the Earth and geological chronology
The absolute age of the Earth, according to modern concepts, is assumed to be 4.6 billion years. The age of the oldest rocks on Earth - granite gneisses discovered on land - is about 3.8-4.0 billion years.
The events of the geological past in their chronological sequence are represented by a single international geochronological scale(Table 1). Its main time divisions are eras: Archean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. The oldest interval of geological time, including the Archean and Proterozoic, is called Precambrian It covers a huge period of time - almost 90% of the entire geological history of the Earth. Next highlighted Paleozoic (« ancient life"") era (from 570 to 225-230 million years ago), Mesozoic(“average life”) era (from 225-230 to 65-67 million years ago) and Cenozoic(“new life”) era (from 65-67 million years ago to the present day). Within eras, smaller time periods are distinguished - periods.
N. Kelder in the book “Restless Earth” (Moscow, 1975) gives the following interesting comparison for a clear idea of geological time: “If we conventionally take a megacentury (10 8 years) as one year, then the age of our planet will be equal to 46 years. Biographers know nothing about the first seven years of her life. Information relating to a later “childhood” is recorded in the most ancient rocks of Greenland and South Africa... Most of the information from the history of the Earth, including such an important moment as the emergence of life, refers to the last six years... Until the age of 42, her continents were practically lifeless. In the 45th year of life - just a year ago - the Earth was decorated with lush vegetation. At that time among
Table 1.
Geochronological scale
| Era | |||
| (continued - | Periods | Folding | Typical organisms |
| ity, million years) | |||
| Cenozoic | Quaternary | The emergence of man | |
| (65+3) | Neogene | Cenozoic | The flourishing of fauna |
| (alpine) | hoarders and birds | ||
| Paleogene | Bloom covered | ||
| seed plants | |||
| Mesozoic | Chalky | Mesozoic | The appearance of birds |
| (170+5) | Jurassic | The heyday of the giants | |
| reptiles | |||
| Triassic | Flowering of gymnosperms | ||
| ny plants | |||
| Paleozoic | Permian | Late Paleo- | sea corals, |
| (340+10) | zoya (hercyn- | trilobites, large | |
| sky) | amphibians | ||
| Coal- | |||
| ny | |||
| Devonian | Early Paleo- | Flowering of the club mosses | |
| Silurian | zoyskaya (kale- | and ferns | |
| Donskaya) | |||
| Ordovician | |||
| Cambrian | |||
| Baikalskaya | |||
| Proterozoic | Blue-green algae, primitive marine animals | ||
| (~2000) | generally accepted | ||
| divisions | |||
| Archaea | No | ||
| (~ 2000) |
Animals were dominated by giant reptiles, particularly dinosaurs. Approximately the same period marks the beginning of the collapse of the last giant supercontinent.
Dinosaurs disappeared from the face of the Earth eight months ago. They were replaced by more highly organized animals - mammals. Somewhere in the middle of last week, some apes transformed into ape-like people in Africa, and at the end of the same week, a series of the last grandiose glaciations hit the Earth. A little more than four hours have passed since a new genus of highly organized animals, later known as Homo sapiens, began to obtain food by hunting wild animals; and only an hour totals his experience of farming and the transition to a sedentary lifestyle. The flowering of the industrial power of human society occurs at the last minute...”
Composition and structure of the earth's crust
The earth's crust consists of igneous, sedimentary and metamorphic rocks. Igneous rocks are formed during the eruption of magma from the deep zones of the Earth and its hardening. If magma intrudes into the earth's crust and slowly solidifies under high pressure conditions at depth, intrusive rocks(granite, gabbro, etc.), when it pours out and quickly solidifies on the surface - effusive(basalt, volcanic tuff, etc.). Many minerals are associated with igneous rocks: titanium-magnesium, chromium, copper-nickel and other ores, apatites, diamonds, etc.
Sedimentary rocks are formed directly on the earth's surface in different ways: either due to the vital activity of organisms - organogenic rocks(limestone, chalk, coal, etc.), or during the destruction and subsequent deposition of various rocks - clastic rocks(clay, sand, boulder loams, etc.), or due to chemical reactions, usually occurring in aquatic environment, - rocks of chemical origin(bauxite, phosphorite, salt, ores of some metals, etc.). Many sedimentary rocks are valuable minerals: oil, gas, coal, peat, bauxite, phosphorites, salts, iron and manganese ores, various building materials, etc.
Metamorphic rocks arise as a result of changes (metamorphism) of various rocks found at depth, under the influence high temperatures and pressure, as well as hot solutions and gases rising from the mantle (gneiss, marble, crystalline schists, etc.). In the process of rock metamorphism, various minerals are formed: iron, copper, polymetallic, uranium and other ores, gold, graphite, gems, refractories, etc.
The earth's crust is composed mainly of crystalline rocks of igneous and metamorphic origin. However, it is heterogeneous in composition, structure and power. Distinguish two main types of earth's crust: continental And oceanic. The first is characteristic of continents (continents), including their underwater margins to a depth of 3.5-4.0 km below the level of the World Ocean, the second - oceanic basins (ocean bed).
Continental crust consists of three layers: sedimentary with a thickness of 20-25 km, granite (granite-gneiss) and basalt. Its total thickness is about 60-75 km in mountainous areas, 30-40 km in plains.
Oceanic crust also three-layer. On top lies a thin (on average about 1 km) layer of loose marine sediments of siliceous-carbonate composition. Beneath it is a layer of basalt lavas. There is no granite layer between the sedimentary and basalt layers (unlike the continental crust), which is confirmed by numerous drill holes. The third layer (according to dredging data) consists of igneous rocks - mainly gabbro. The total thickness of the oceanic crust is on average 5-7 km. In some places on the bottom of the World Ocean (usually along large faults), even rocks of the upper mantle protrude to the surface. They are also what makes up the island of Sao Paulo off the coast of Brazil.
Thus, the oceanic crust, both in composition and thickness, as well as in age (it is no older than 160-180 million years), differs significantly from the continental crust. Along with these two main types of earth's crust, there are several options transitional type crust.
continents, including their underwater outskirts, and oceans are the largest structural elements of the earth's crust. Within their boundaries, the main area belongs to quiet platform areas, the smaller one belongs to mobile geosynclinal belts (geosynclines). The evolution of the structure of the earth's crust proceeded mainly from geosynclines to platforms. But partly this process turns out to be reversible due to the formation of rifts (rift - English, crack, fault) on the platforms, their further opening (for example, the Red Sea) and transformation into the ocean.
Geosynclines - vast mobile, highly dissected areas of the earth's crust with tectonic movements of varying intensity and direction. There are two major stages in the development of geosynclines.
The first - the main stage in duration - characterized by immersion and sea mode. At the same time, in a deep sea basin, predetermined by deep faults, a thick (up to 15-20 km) thickness of sedimentary and volcanic rocks accumulates. The outpouring of lavas, as well as the intrusion and solidification of magma at different depths, is most typical for the internal parts of geosynclines. Metamorphism, and subsequently folding, also manifests itself more energetically here. In the marginal parts of the geosyncline, predominantly sedimentary strata accumulate, magmatism is weakened or even absent.
The second stage of development of geosynclines - shorter in duration - characterized by intense upward movements, which the latest tectonic hypotheses associate with the convergence and collision of lithospheric plates. Due to lateral pressure, energetic crushing of rocks into complex folds and intrusion of magma occurs with the formation of mainly granite. At the same time, the primary thin ocean crust, thanks to various deformations of rocks, magmatism, metamorphism and other processes, turns into a more complex composition, thick and hard continental (mainland) crust. As a result of the uplift of the territory, the sea retreats, first archipelagos of volcanic islands are formed, and then a complex folded mountainous country.
Subsequently, over tens to hundreds of millions of years, the mountains are destroyed, a large area of the earth’s crust is covered with a cover of sedimentary rocks and turns into a platform.
Platforms - extensive, most stable, predominantly flat blocks of the earth's crust. They usually have an irregular polygonal shape caused by large faults. The platforms have a typically continental or oceanic crust, and are accordingly divided into mainland And oceanic. They correspond to the main, flat stages of the relief of the earth's surface on land and the bottom of the ocean. Continental platforms have a two-tier structure. The lower tier is called the foundation. It consists of metamorphic rocks crumpled into folds, permeated with solidified magma, broken into blocks by faults. The foundation was formed during the geosynclinal stage of development. Upper tier - sedimentary cover - composed predominantly of sedimentary rocks of later age, lying relatively horizontally. The formation of the cover corresponds to the platform stage of development.
Areas of platforms where the foundation is immersed to a depth under the sedimentary cover are called slabs. They occupy the main area on the platforms. The places where the crystalline foundation emerges to the surface are called shields. There are ancient and young platforms. They differ, first of all, in the age of the folded foundation: in ancient platforms it was formed in the Precambrian, more than 1.5 billion years ago, in young ones - in the Paleozoic.
There are nine large ancient Precambrian platforms on Earth. The North American, East European and Siberian platforms form the northern row, the South American, African-Arabian, Indian, Australian and Antarctic platforms form the southern row. Until the mid-Mesozoic, the platforms of the southern series were part of a single supercontinent Gondwana. Occupies an intermediate position Chinese platform. There is an opinion that all ancient platforms are fragments of a huge single Precambrian massif of continental crust - Pangea.
Ancient platforms are the most stable blocks in the composition of continents, therefore they are their basis, a rigid skeleton. They are separated five geosynclinal belts, arose at the end of the Precambrian in connection with the split of Pangea. Three of them - North Atlantic, Arctic and Ural-Okhotsk - completed their development mainly in the Paleozoic. Two - Mediterranean (Alpine-Himalayan) and Pacific - partially continue their development in the modern era.
Within the geosynclinal belts, its various parts completed their development in different tectonic epochs. In the geological history of the last billion years, several tectonic cycles (epochs): Baikal cycle confined to the end of the Proterozoic - the beginning of the Paleozoic (1000-550 million years in absolute terms), Caledonian - early Paleozoic (550-400 million years), Hercynian- late Paleozoic (400-210 million years), Mesozoic(210-100 million years) and Cenozoic, or alpine(100 million years - until now). Accordingly, on land they distinguish areas of the Baikal, Caledonian, Hercynian, Mesozoic and Cenozoic (Alpine) folds. They are often called Baikal, Caledonian and other fold belts.
The conditions of occurrence of rocks within the earth's crust are reflected in the overview tectonic map of the world. It highlights areas whose folded structure formation was completed at different stages of folding. They are better studied and more reliably shown within the land. Ancient platforms and the folded belts (areas) framing them of different ages are depicted in certain colors. The ancient platforms (nine large and several small) are painted in reddish tones: brighter on the shields, less bright on the slabs. Areas of Baikal folding are shown in blue-blue, Caledonian - lilac, Hercynian - brown, Mesozoic - green and Cenozoic - yellow.
In the areas of the Baikal, Caledonian and Hercynian folds, mountain structures were subsequently significantly destroyed. Over large areas, their folded structures were covered on top by continental and shallow-marine sedimentary rocks and acquired stability. In relief they are expressed as plains. These are the so-called young platforms(for example, West Siberian, Turanian, etc.). On a tectonic map they are depicted as lighter shades of the main color of the fold belt within which they are located. Young platforms, unlike ancient ones, do not form isolated massifs, but are attached to ancient platforms.
From a comparison of the physical and tectonic maps of the world, it follows that the mountains mainly correspond to mobile folded belts of different ages, the plains - to ancient and young platforms.
The concept of relief. Geological relief-forming processes
Modern relief is a set of irregularities of the earth's surface of different scales. They are called landforms. The relief was formed as a result of the interaction of internal (endogenous) and external (exogenous) geological processes.
Landforms vary in size, structure, origin, development history, etc. There are convex (positive) landforms(mountain range, hill, hill, etc.) and concave (negative) shapes(intermountain basin, lowland, ravines, etc.).
The largest forms of relief - continents and ocean basins and large forms - mountains and plains were formed primarily due to the activity of the internal forces of the Earth. Medium-sized and small relief forms - river valleys, hills, ravines, dunes and others, superimposed on larger forms, are created by various external forces.
Geological processes are based on different energy sources. The source of internal processes is heat generated during radioactive decay and gravitational differentiation of substances inside the Earth. The source of energy for external processes is solar radiation, which is converted on Earth into the energy of water, ice, wind, etc.
Internal (endogenous) processes
Various tectonic movements of the earth's crust are associated with internal processes, creating the main forms of the Earth's relief, magmatism, and earthquakes. Tectonic movements manifest themselves in slow vertical vibrations of the earth's crust, in the formation of rock folds and faults.
Slow vertical oscillatory movements - uplifts and subsidences of the earth's crust occur continuously and everywhere, alternating in time and space throughout geological history. They are specific to platforms. Associated with them is the advance of the sea and, accordingly, a change in the outlines of continents and oceans. For example, the Scandinavian Peninsula is currently slowly rising, but the southern coast of the North Sea is sinking. The speed of these movements is up to several millimeters per year.
Under folded tectonic disturbances of rock layers This means bending the layers without violating their continuity. Folds vary in size, with small ones often complicating large ones, in shape, in origin, etc.
TO rupture tectonic disturbances of rock layers relate faults. They can be different in depth (either within the earth’s crust, or dissect it and go into the mantle up to 700 km), in length, duration of development, without displacement of sections of the earth’s crust or with displacement of blocks of the earth’s crust in horizontal and vertical directions, etc. d.
Folded and fractured deformations (disturbances) of the layers of the earth's crust against the background of a general tectonic uplift of the territory lead to the formation of mountains. Therefore, folding and tearing movements are combined under the general name orogenic(from the Greek ogo - mountain, genos - birth), i.e. movements that create mountains (orogens).
During mountain building, the rate of uplift is always more intense than the processes of destruction and removal of material.
Folded and faulty tectonic movements are accompanied, especially in the mountains, by magmatism, rock metamorphism and earthquakes.
Magmatism associated primarily with deep faults crossing the earth's crust and extending into the mantle. Depending on the degree of penetration of magma from the mantle into the earth's crust, it is divided into two types: intrusive, when magma, without reaching the surface of the Earth, freezes at depth, and effusive, or volcanism, when magma breaks through the earth's crust and pours out onto the earth's surface. At the same time, many gases are released from it, the original composition changes, and it turns into lava The composition of lavas is very diverse. Eruptions occur either along fissures (this type of eruption was prevalent in the early stages of the Earth's formation) or through narrow channels at the intersection of faults, called vents.
During fissure eruptions, extensive lava sheets(on the Deccan plateau, on the Armenian and Ethiopian highlands, on the Central Siberian plateau, etc.). In historical times, significant lava outpourings occurred in the Hawaiian Islands and Iceland; they are very characteristic of mid-ocean ridges.
If magma rises through a vent, then during outpourings, usually multiple, elevations are formed - volcanoes with a funnel-shaped extension at the top called crater. Most volcanoes are cone-shaped and consist of loose eruption products interbedded with hardened lava. For example, Klyuchevskaya Sopka, Fuji, Elbrus, Ararat, Vesuvius, Krakatau, Chimbarazo, etc. Volcanoes are divided into active(there are more than 600 of them) and extinct. Most active volcanoes are located among young mountains of the Cenozoic folding. There are also many of them along large faults in tectonically mobile areas, including on the ocean floor along the axes of mid-ocean ridges. The main volcanic zone is located along the Pacific coast - Pacific Ring of Fire where there are more than 370 active volcanoes (in the east of Kamchatka, etc.).
In places where volcanic activity subsides, hot springs are typical, including periodically gushing ones - geysers, emissions of gases from craters and cracks, which indicate active processes in the depths of the bowels
Volcanic eruptions allow scientists to look tens of kilometers deep into the Earth and understand the secrets of the formation of many types of minerals. Employees of volcanological stations maintain a round-the-clock watch in order to promptly predict the onset of volcanic eruptions and prevent natural disasters associated with them. Usually the greatest damage is caused not so much by lava flows as by mud flows. They are caused by the rapid melting of glaciers and snow on the tops of volcanoes and the rainfall from powerful clouds onto fresh volcanic “ash”, consisting of debris and dust. The speed of mud flows can reach 70 km/h and spread over a distance of up to 180 km. Thus, as a result of the eruption of the Ruiz volcano in Colombia on November 13, 1985, lava melted hundreds of thousands of cubic meters of snow. The resulting mud flows swallowed the city of Armero with a population of 23 thousand people.
Endogenous processes are also associated earthquakes are sudden underground shocks, tremors and displacements of layers and blocks of the earth's crust. The sources of earthquakes are confined to fault zones. In most cases, the centers of earthquakes are located at a depth of the first tens of kilometers in the earth's crust. However, sometimes they lie in the upper mantle at a depth of 600-700 km, for example along the Pacific coast, in the Caribbean Sea and other areas. Elastic waves arising in the source, reaching the surface, cause the formation of cracks, its oscillation up and down, and displacement in the horizontal direction. Thus, along the most studied San Andreas fault in California (more than 1000 km long, running along the Gulf of California to the city of San Francisco), the total horizontal displacement of rocks from the moment of its formation in the Jurassic to the present is estimated at 580 km. The average rate of displacement is now up to 1.5 cm/year. Frequent earthquakes are associated with it. The intensity of earthquakes is assessed on a scale of twelve based on the deformation of the Earth's layers and the degree of damage to buildings. Hundreds of thousands of earthquakes are recorded on Earth every year, meaning we live on a restless planet. During catastrophic earthquakes, the topography changes in a matter of seconds, landslides and landslides occur in the mountains, cities are destroyed, and people die. Earthquakes on coasts and ocean floors cause waves - tsunami. The catastrophic earthquakes of recent decades include: Ashgabat (1948), Chilean (1960), Tashkent (1966), Mexico City (1985), Armenian (1988). Volcanic eruptions are also accompanied by earthquakes, but these earthquakes are limited in nature.
External (exogenous) processes
In addition to internal processes, the relief of the earth's surface is simultaneously affected by various external forces. The activity of any external factor consists of the processes of destruction and demolition of rocks (denudation) and deposition of material in depressions (accumulation). This is preceded by weathering - rock destruction process under the influence of sharp temperature fluctuations and freezing of water in rock cracks, as well as chemical changes in their composition under the influence of air and water containing acids, alkalis and salts. Living organisms also take part in weathering. There are two main types of weathering: physical And chemical. As a result of the weathering of rocks, loose deposits are formed that are convenient for movement by water, ice, wind, etc.
The most important external process on the earth's surface is the activity of flowing water . It is almost universal, except in polar regions and glaciated mountains, and is limited in deserts. Due to flowing water, there is a general lowering of the surface under the influence of the removal of soil and rocks, and erosional relief forms such as ravines, gullies, river valleys, as well as accumulative forms - alluvial cones of gullies and ravines, river deltas are formed.
Gullies are elongated depressions with steep, unturfed slopes and a growing peak. They are created by temporary watercourses. Their formation, in addition to natural factors (the presence of slopes, easily eroded soils, heavy precipitation, rapid snowmelt, etc.), is facilitated by people through their irrational activities (clearing forests and meadows, plowing slopes, especially from top to bottom, etc.).
Balki, unlike ravines, have stopped growing; their slopes are usually less steep, occupied by meadows and forests. Gully-gully relief is very typical for the Central Russian, Volga and other elevations. He dominates High Plains in the USA, on the Ordos plateau in China, etc. Gullies and gullies create difficulties for agricultural development of the territory, road and other construction, lower the groundwater level, and cause other negative consequences.
In the mountains, temporary mud and stone flows called Selami. The content of solid material in them can reach 75% of the total mass of the flow. Mudflows move huge amounts of debris to the foothills of the mountains. Mudflows are associated with catastrophic destruction of villages, roads, and dams.
A lot of constant, destructive work is carried out both in the mountains and on the plains rivers. In the mountains, using intermountain valleys and tectonic faults, they form deep narrow river valleys with steep slopes such as gorges, on which various slope processes develop that lower the mountains. On the plains, the rivers are also actively working, eroding the slopes and expanding the valley to tens of kilometers in width. Unlike mountain rivers, they have floodplain The slopes of river valleys on the plains usually have above-floodplain terraces - former floodplains, indicating periodic incision of rivers. Floodplains and river beds serve as the levels to which ravines and gullies are “attached”. Therefore, their decrease causes the growth and incision of ravines, an increase in the steepness of the adjacent slopes, soil erosion, etc.
Surface flowing waters over a long geological time are capable of producing enormous destructive work in the mountains and plains. It is with them that the formation of plains on the site of once mountainous countries is primarily associated.
Certain destructive work is carried out in the mountains and plains glaciers. They occupy about 11% of the land. More than 98% of modern glaciation occurs on the ice sheets of Antarctica, Greenland and the polar islands, and only about 2% on mountain glaciers. The thickness of cover glaciers is up to 2-3 km or more. In the mountains, glaciers occupy flat peaks, depressions on slopes and intermountain valleys. Valley glaciers remove from the mountains all the material that comes to its surface from the slopes, and that which it plows up when moving along the subglacial bed. The material transported by the glacier in the form of unsorted loam and sandy loam with boulders, the so-called moraine, is deposited at the edge of the glacier, and then carried to the foot of the mountains by rivers starting at the edge of the glaciers.
During the maximum Quaternary glaciation, the area of glaciers on the plains was three times larger than now, and mountain glaciers in subpolar and temperate latitudes descended to the foothills.
During the Quaternary glaciations, the centers and areas of glacial demolition were the Scandinavian Mountains, the Polar Urals, the northern Rocky Mountains, as well as the highlands of the Kola Peninsula, Karelia, the Labrador Peninsula, etc. Here there are glacially polished protrusions of hard crystalline rocks in the form of hills, which are called sheep's foreheads, oblong in the direction of glacier movement plowing basins and others. To the south, at a distance of 1000-2000 km from the centers of glaciation, there are areas of glacial sediment in the form of random hilly and ridge heaps that have survived to the present day. Consequently, on the plains the cover glaciers performed not only destructive, but also creative work.
Wind- a ubiquitous factor on Earth. However, his destructive and creative work is most fully manifested in the deserts. It is dry, there is almost no vegetation, there are a lot of loose loose particles - products of intense physical weathering caused by a sharp temperature change during the day. Landforms created by the wind are called aeolian(by name greek god Aeolus - lord of the winds). In rocky deserts, the wind not only blows away small particles formed due to destruction processes. The wind-sand flow wears away the rocks, gives them bizarre shapes and ultimately destroys them and levels the surface.
In sandy deserts the wind forms dunes - crescent-shaped hills moving at speeds of up to 50 m/year, as well as ridges, mounds and other aeolian forms fixed by vegetation. On the coasts of seas and rivers, the daytime breeze forms sandy hills - dunes(for example; on the coast of the Bay of Biscay in France, along the southern coast of the Baltic Sea, where they are overgrown with pine forests and heather).
In plowed steppe and semi-desert areas with unstable moisture, it is not uncommon dust storms, during which the top layer of soil, along with seeds, and sometimes seedlings, is torn off strong winds and is transported tens of kilometers from the demolition site and deposited in front of obstacles or in depressions where the force of the wind subsides.
A certain contribution to the change in the earth's surface is made by The groundwater, dissolving some rocks, permafrost, wave activity on sea coasts, and Human.
Thus, the Earth's topography is formed due to internal and external forces - eternal antagonists. Internal processes create the main irregularities on the Earth's surface, and external processes Due to the destruction of convex forms and the accumulation of material in concave forms, they strive to destroy them and level the earth's surface.
There is one interesting feature in the structure of our planet: we encounter the most complex and diverse structure in the surface layers of the earth’s crust; The deeper we descend into the bowels of the Earth, the simpler its structure turns out to be. One can, of course, express the suspicion that it only seems so to us, because the deeper we go, the more approximate and uncertain our information becomes. Apparently, this is not the case, and the simplification of structure with depth is an objective fact, independent of the degree of our knowledge.
We will begin our consideration from the top, with the most complex upper layers of the earth's crust. These layers, as we know, are studied primarily using direct geological methods.
Approximately two-thirds of the earth's surface is covered by oceans; one third falls on the continents. The structure of the earth's crust under the oceans and continents is different. Therefore, we will first consider the features of the continents, and then turn to the oceans.
On the surface of the Earth on continents, rocks of different ages are found in different places. Some areas of the continents are composed on the surface of the most ancient rocks - Archeozoic or, as they are more often called, Archean, and Proterozoic. Together they are called pre-Paleozoic or Precambrian rocks. Their peculiarity is that most of them are highly metamorphosed: clays have turned into metamorphic shales, sandstones into crystalline quartzites, limestones into marbles. A major role among these rocks is played by gneisses, i.e., schistose granites, as well as ordinary granites. The areas where these most ancient rocks come to the surface are called crystalline massifs or shields. An example is the Baltic Shield, which embraces Karelia, the Kola Peninsula, all of Finland and Sweden. Another shield covers most of Canada. Similar most of Africa is a shield, as is a large part of Brazil, almost all of India and all of Western Australia. All rocks of ancient shields are not only metamorphosed and have undergone recrystallization, but also very strongly crushed into small complex folds.
Other areas on the continents are occupied by predominantly younger rocks - Paleozoic, Mesozoic and Cenozoic in age. These are mainly sedimentary rocks, although among them there are also rocks of igneous origin, erupted on the surface in the form of volcanic lava or embedded and frozen at some depth. There are two categories of areas: on the surface of some, layers of sedimentary rocks lie very calmly, almost horizontally, and only rare and small folds are observed in them. In such places, igneous rocks, especially intrusive rocks, play a relatively small role. Such areas are called platforms. In other places, sedimentary rocks are strongly folded and riddled with deep cracks. Among them, intruded or extruded igneous rocks are often found. These places usually coincide with mountains. They're called folded zones, or geosynclines.
The differences between individual platforms and folded zones are in the age of the rocks lying quietly or folded into folds. Among the platforms, ancient platforms stand out, on which all Paleozoic, Mesozoic and Cenozoic rocks lie almost horizontally on top of a highly metamorphosed and folded “crystalline base” composed of Precambrian rocks. An example of an ancient platform is the Russian platform, within which all layers, starting with the Cambrian, lie generally very calm.
There are platforms on which not only Precambrian, but also Cambrian, Ordovician and Silurian layers are folded, and younger rocks, starting with Devonian ones, lie quietly on top of these folds on their eroded surface (as they say, “unconformably”). In other places, the “folded foundation” is formed, in addition to Precambrian, by all Paleozoic rocks, and only Mesozoic and Cenozoic rocks lie almost horizontally. The last two categories of platforms are called young. Some of them, as we see, were formed after the Silurian period (before that, folded zones existed here), and others - after the end of the Paleozoic era. Thus, it turns out that on the continents there are platforms of different ages, formed earlier or later. Before the platform was formed (in some cases - until the end of the Proterozoic era, in others - until the end of the Silurian period, in others - until the end of the Paleozoic era), a strong collapse of layers into folds occurred in the earth's crust, igneous molten rocks were introduced into it, sediments were subjected to metamorphization and recrystallization. And only after this did calm occur, and subsequent layers of sedimentary rocks, having accumulated horizontally at the bottom of sea basins, generally continued to remain calm in the future.
Finally, in other places all the layers are folded and penetrated by igneous rocks - even Neogene ones.
Saying that the platforms could have formed in different time, we also point out the different ages of the folded zones. Indeed, on ancient crystalline shields, the collapse of layers into folds, the intrusion of igneous rocks, and recrystallization ended before the beginning of the Paleozoic. Consequently, the shields are zones of Precambrian folding. Where the quiet bedding of the layers had not been disturbed since the Devonian period, the folding of the layers into folds continued until the end of the Silurian period, or, as they say, until the end of the early Paleozoic. Consequently, this group of young platforms is at the same time an area of Early Paleozoic folding. The folding of this time is called the Caledonian folding. Where the platform was formed from the beginning of the Mesozoic, we have zones of Late Paleozoic or Hercynian folding. Finally, the areas where all layers, up to and including the Neogene ones, are strongly folded are zones of the youngest, Alpine folding, which left only the layers formed in the Quaternary period unfolded.
Maps depicting the location of platforms and folded zones of different ages and some other features of the structure of the earth's crust are called tectonic (tectonics is a branch of geology that studies the movements and deformations of the earth's crust). These maps serve as a complement to geological maps. The latter are primary geological documents that most objectively illuminate the structure of the earth's crust. Tectonic maps already contain some conclusions: about the age of platforms and folded zones, about the nature and time of formation of folds, about the depth of the folded foundation under the quiet layers of platforms, etc. The principles of compiling tectonic maps were developed in the 30s by Soviet geologists, mainly Academician A.D. Arkhangelsky. After the Great Patriotic War, tectonic maps of the Soviet Union were compiled under the leadership of Academician N. S. Shatsky. These maps are taken as an example for the compilation of international tectonic maps of Europe, other continents and the entire Earth as a whole.
The thickness of sedimentary formations in those places where they lie calmly (i.e. on platforms) and where they are strongly folded is different. For example, Jurassic deposits on the Russian Platform are nowhere more than 200 meters thick or “thick,” while their thickness in the Caucasus, where they are strongly folded, reaches 8 kilometers in places. The deposits of the Carboniferous period on the same Russian platform have a thickness of no more than several hundred meters, and in the Urals, where the same deposits are strongly folded, their thickness in some places increases to 5-6 kilometers. This indicates that when sediments of the same age accumulated on the platform and in areas of the folded zone, the earth's crust flexed very little on the platform and flexed much more in the folded zone. Therefore, there was no room on the platform for the accumulation of such thick formations as could accumulate in deep troughs of the earth's crust in folded zones.
Within platforms and folded zones, the thickness of accumulated sedimentary rocks does not remain the same everywhere. It varies from site to site. But on platforms these changes are smooth, gradual and small. They indicate that during the accumulation of sediments, the platform sagged in places a little more, in places a little less, and wide gentle depressions (syneclises) were formed in its foundation, separated by equally gentle uplifts (anteclises). In contrast, in folded zones the thickness of sedimentary rocks of the same age varies from site to site very sharply, over short distances, sometimes increasing to several kilometers, sometimes decreasing to several hundred or tens of meters, or even disappearing. This indicates that during the accumulation of sediments in the folded zone, some areas sagged strongly and deeply, others sagged little or even did not sag at all, and others at the same time rose strongly, as evidenced by the coarse sediments found next to them, formed as a result of erosion of uplifted areas. It is significant that all these areas, which were intensely sagging and intensely uplifted, were narrow and located in the form of strips closely next to each other, which led to very large contrasts in the movements of the earth’s crust at close distances.
Bearing in mind all the indicated features of the movements of the earth's crust: very contrasting and strong subsidence and upliftment, strong folding, vigorous magmatic activity, i.e. all the features of the historical development of folded zones, these zones are usually called geosynclines, reserving the name “folded zone” only to characterize their modern structure, which was the result of all previous turbulent events in the earth’s crust. We will continue to use the term “geosyncline” when we are talking not about the modern structure of the folded zone, but about the features of its previous development.
Platforms and folded zones differ significantly from each other in the mineral resources that are located on their territory. On the platforms there is little igneous rock that has been intruded into quiet layers of sedimentary rock. Therefore, minerals of igneous origin are only rarely found on platforms. But in the calmly lying sedimentary layers of the platform, coal, oil, natural gases, as well as rock salt, gypsum, building materials, etc. are widespread. In folded zones, the advantage is on the side of igneous minerals. These are various metals that were formed in different stages solidification of magma chambers.
However, when we talk about the predominant confinement of sedimentary minerals to platforms, we must not forget that we are talking about layers that lie quietly, and not about those highly metamorphosed and crumpled crystalline rocks of the ancient “folded foundation” of platforms, which is best seen on “ shields." These basement rocks reflect the era when the platform was not yet here, but a geosyncline existed. Therefore, minerals found in the folded basement are geosynclinal in type, that is, predominantly igneous. Consequently, on the platforms there are, as it were, two floors of minerals: the lower floor is ancient, belonging to the foundation, geosynclinal; it is characterized by metal ores; the upper floor is the platform itself, belonging to the cover of sedimentary rocks quietly lying on the foundation; these are sedimentary, i.e., predominantly non-metallic minerals.
A few words must be said about folds.
Strong folding in folded zones and weak folding on platforms were mentioned above. It should be noted that we should talk not only about different intensities of folding, but also about the fact that folded zones and platforms are characterized by folds different types. In folded zones, folds are of a type called linear or complete. These are long narrow folds that, like waves, follow each other, adjoining each other in a circle and completely covering large areas. The folds have different shapes: some are round, others are sharp, some are straight, vertical, others are inclined. But they are all similar to each other, and most importantly, they cover the folded zone in a continuous sequence.
The platforms have folds of a different type. These are separate isolated uplifts of layers. Some of them are table-shaped or, as they say, chest-shaped or box-shaped, many have the appearance of gently sloping domes or shafts. The folds here are not elongated, as in the folded zone, into stripes, but are arranged in more complex shapes or scattered rather randomly. This is “intermittent” or dome-shaped folding.
Folds of intermittent type - chest elevations, domes and shafts - are found not only on the platform, but also on the edge of the folded zones. So there is to some extent a gradual transition from platform folds to those that are typical of fold zones.
On platforms and at the edges of folded zones, another unique type of folds occurs - the so-called “diapiric domes”. They are formed where thick layers of rock salt, gypsum or soft clays lie at some depth. Specific gravity rock salt less than the specific gravity of other sedimentary rocks (rock salt 2.1, sands and clays 2.3). Thus, lighter salt ends up under heavier clays, sands, and limestones. Due to the ability of rocks to slowly plastically deform under the influence of small mechanical forces (the phenomenon of creep, which was mentioned above), salt tends to float to the surface, piercing and pushing apart the overlying heavier layers. This is helped by the fact that salt under pressure is extremely fluid and at the same time durable: it flows easily, but does not break. The salt floats up in the form of columns. At the same time, it lifts the overlying layers, bends them dome-shaped and, protruding upward, causes them to split into separate pieces. Therefore, on the surface, such diapiric domes often have the appearance of a “broken plate.” In a similar way, diapiric folds are formed, in the “piercing cores” of which we find not salt, but soft clays. But clay diapiric folds usually do not look like round columns, like salt diapiric domes, but rather long elongated ridges.
Domes (including diapiric ones) and shafts found on platforms play a large role in the formation of oil and gas accumulations. In folded zones, mineral deposits are mostly confined to cracks.
Let us now turn to the deeper layers of the earth's crust. We will have to leave the area that we know from direct observations from the surface, and go to a place where information can only be obtained through geophysical research.
As already mentioned, metamorphic rocks of Archean age lie deepest within the visible part of the earth's crust. Among them, the most common are gneisses and granites. Observations show that the deeper the section of the earth's crust we observe on the surface, the more granites we encounter. Therefore, one can think that even deeper - several kilometers below the surface of crystalline shields or about 10 km below the surface of platforms and folded zones - we would encounter a continuous layer of granite under the continents. The upper surface of this granite layer is very uneven: it either rises to the day surface, or falls 5-10 km below it.
We can only guess the depth of the lower surface of this layer based on some data on the speed of propagation of elastic seismic vibrations in the earth's crust. The speed of movement of so-called longitudinal seismic waves in granites is on average about 5 km/sec.
In longitudinal waves, particle oscillations occur in the direction of wave movement: forward and backward. So-called transverse waves are characterized by oscillations across the direction of wave movement: up - down or right - left.
But in a number of places it was discovered that at a depth of 10, 15, 20 km, the speed of propagation of the same longitudinal seismic waves becomes greater and reaches 6 or 6.5 km/sec. Since this speed is too high for granite and is close to the speed of propagation of elastic vibrations, which characterizes a rock such as basalt in laboratory tests, the layer of the earth's crust with a higher speed of propagation of seismic waves is called basalt. In different areas it starts at different depths - usually at a depth of 15 or 20 km, but in some areas it comes much closer to the surface, and a well 6-8 km deep could reach it.
However, so far not a single well has penetrated into the basalt layer and no one has seen the rocks that lie in this layer. Are these really basalts? Doubts have been expressed about this. Some people think that instead of basalts we will find there the same gneisses, granites and metamorphic rocks that are characteristic of the overlying granite layer, but which at greater depths are strongly compacted by the pressure of the overlying rocks, and therefore the speed of propagation of seismic waves in them is greater. The solution to this issue is of great interest and not only theoretical: somewhere in the lower part of the granite and upper part of the basalt layers, processes of granite formation and the nucleation of those hot solutions and gases occur, from which various ore minerals crystallize higher up, as they move to the surface. To know what a basalt layer actually is means to better understand the processes of formation of metal ores in the earth's crust and the laws of their distribution. That is why the project of drilling ultra-deep wells to study the structure of the entire granite and at least the upper part of the basalt layer deserves all support.
Basalt layer is the lower layer of the continental earth's crust. Below it is separated from the deeper parts of the Earth by a very sharp division called Mohorovicic section(named after the Yugoslav seismologist who discovered the existence of this section at the beginning of our century). At this Mohorovicic section (or Moho for short), the speed of longitudinal seismic waves changes sharply: above the section it is usually 6.5 km/sec, and immediately below it it increases to 8 km/sec. This section is considered the lower boundary of the earth's crust. Its distance from the surface, therefore, is the thickness of the earth's crust. Observations show that the thickness of the crust under the continents is far from uniform. On average it is 35 km, but under the mountains it increases to 50, 60 and even 70 km. Moreover, the higher the mountains, the thicker the earth’s crust: a large upward protrusion of the earth’s surface corresponds to a much larger downward protrusion; Thus, the mountains have, as it were, “roots” that descend deeply into the deeper layers of the Earth. Under the plains, on the contrary, the thickness of the crust is less than average. The relative role of granite and basalt layers in the section of the earth's crust also varies from region to region. It is especially interesting that under some mountains the “roots” are formed mainly due to an increase in the thickness of the granite layer, and under others - due to an increase in the thickness of the basalt layer. The first case is observed, for example, in the Caucasus, the second - in the Tien Shan. Further we will see that the origin of these mountains is different; this was also reflected in the different structure of the earth’s crust beneath them.
One property of the earth's crust, closely related to the "roots" of mountains, should be especially noted: this is the so-called isostasy, or equilibrium. Observations of the magnitude of the force of gravity on the surface of the Earth show, as we have seen, the presence of some fluctuations in this value from place to place, that is, the existence of certain anomalies of gravity. However, these anomalies (after subtracting the influence of the geographical and altitudinal position of the observation point) are extremely small; they can cause a person's weight to change by just a few grams. Such deviations from normal gravity are extremely small compared to those that could be expected, bearing in mind the topography of the earth's surface. In fact, if mountain ranges were a pile of superfluous masses on the surface of the Earth, then these masses would have to create a stronger attraction. On the contrary, over the seas, where instead of dense rocks the attracting body is less dense water, the force of gravity should weaken.
In reality there are no such differences. The force of gravity does not become greater in the mountains and less at sea; it is approximately the same everywhere, and the observed deviations from the average value are significantly less than the influence that uneven terrain or the replacement of rocks with sea water should have had. From here only one conclusion is possible: the additional masses on the surface forming the ridges must correspond to a shortage of masses at depth; only in this case the total mass and the general attraction of the rocks located under the mountains will not exceed the normal value. On the contrary, the lack of masses on the surface in the seas must correspond to some heavier masses at depth. The above changes in the thickness of the crust under the mountains and plains precisely correspond to these conditions. The average density of the earth's crust is 2.7. Under the earth's crust, immediately below the Moho section, the substance has a higher density, reaching 3.3. Therefore, where the earth’s crust is thinner (under lowlands), a heavy subcrustal “substrate” comes closer to the surface and its attractive influence compensates for the “lack” of mass on the surface. On the contrary, in the mountains, an increase in the thickness of the light crust reduces the overall force of attraction, thereby compensating for the increase in attraction caused by additional surface masses. Conditions are created under which the earth's crust seems to float on a heavy substrate, like ice floes on water: a thicker ice floe sinks deeper into the water, but also protrudes higher above it; a thinner ice floe sinks less, but also protrudes less.
This behavior of ice floes corresponds to the well-known law of Archimedes, which determines the equilibrium of floating bodies. The earth’s crust also obeys the same law: where it is thicker, it goes deeper into the substrate in the form of “roots”, but also protrudes higher on the surface; where the crust is thinner, the heavy substrate moves closer to the surface, and the surface of the crust is relatively lowered and forms either a plain or the bottom of the sea. Thus, the state of the cortex corresponds to the equilibrium of floating bodies, which is why this state is called isostasy.
It should be noted that the conclusion about the equilibrium of the earth's crust in relation to its gravity and substrate is valid if we take into account the average thickness of the crust and the average height of its surface for large areas - several hundred kilometers in diameter. If we clarify the behavior of much smaller sections of the earth's crust, we will discover deviations from equilibrium, discrepancies between the thickness of the crust and the height of its surface, which are expressed in the form of corresponding anomalies in gravity. Let's imagine a large ice floe. Its balance, like a body floating on water, will depend on its average thickness. But in different places the ice floe can have very different thickness, it can be corroded by water and its lower surface can have many small pockets and bulges. Within each pocket or each bulge, the position of the ice in relation to the water can differ greatly from the equilibrium: if we cut out the corresponding piece of ice from the ice floe, it will either sink deeper than the surrounding ice floe or float above it. But in general, the ice floe is in equilibrium, and this equilibrium depends on the average thickness of the ice floe.
Under the earth's crust we enter the next, very powerful shell of the Earth, called Earth's mantle. It extends inland for 2900 km. At this depth there is the next sharp division in the Earth's substance, separating the mantle from Earth's core. Inside the mantle, as it deepens, the speed of propagation of seismic waves increases and at the bottom of the mantle reaches 13.6 km/sec for longitudinal waves. But the increase in this speed is uneven: it is much faster in the upper part, to a depth of about 1000 km, and extremely slow and gradual at greater depths. In this regard, the mantle can be divided into two parts - the upper and lower mantle. Nowadays, more and more data are accumulating indicating that this division of the mantle into upper and lower is of great fundamental importance, since the development of the earth's crust is apparently directly related to the processes occurring in the upper mantle. The nature of these processes will be discussed further. The lower mantle apparently has little effect directly on the earth's crust.
The substance that makes up the mantle is solid. This confirms the nature of the passage of seismic waves through the mantle. There are differences of opinion regarding the chemical composition of the mantle. Some people think the upper mantle is made of a rock called peridotite. This rock contains very little silica; its main component is the mineral olivine - a silicate rich in iron and magnesium. Others suggest that the upper mantle is much richer in silica and has a composition similar to basalt, but the minerals that make up this deep basalt are denser than those of the surface basalt. For example, in deep basalt, garnets play a significant role - minerals with a very dense “packing” of atoms in crystal lattice. Such deep basalt, obtained as a result of compression of ordinary surface basalt, is called eclogite.
There are arguments for both points of view. In particular, the second point of view is confirmed by the huge number of basalts that were and are now pouring out during volcanic eruptions, very uniform in their chemical composition. Their source can only be in the upper mantle.
If this point of view turns out to be correct, then we must consider that at the Moho section there is not a change in the chemical composition of a substance, but a transition of a substance of the same chemical composition into a new, denser, “deep” state, into another, as they say , "phase". Such transitions are called "phase transitions". This transition depends on the change in pressure with depth. When a certain pressure is reached, ordinary basalt transforms into eclogite and less dense feldspars are replaced by more dense garnets. Such transitions are also influenced by temperature: increasing it at the same pressure complicates the transition of basalt to eclogite. Therefore, the lower boundary of the earth's crust becomes mobile, dependent on temperature changes. If the temperature rises, then some of the eclogite turns back into ordinary basalt, the crustal boundary drops, and the crust becomes thicker; in this case, the volume of the substance increases by 15%. If the temperature decreases, then at the same pressure, part of the basalt in the lower layers of the crust transforms into eclogite, the crust boundary rises, the crust becomes thinner, and the volume of material that has passed into a new phase decreases by 15%. These processes can explain the oscillations of the earth's crust up and down: as a result of its thickening, the crust will float and rise, but as its thickness decreases, it will sink and sag.
However, the question of the chemical composition and physical state of the upper mantle will be finally resolved, apparently, only as a result of ultra-deep drilling, when drill holes, having passed through the entire crust, will reach the substance of the upper mantle.
An important feature of the structure of the upper mantle is the “softening belt” located at a depth of between 100 and 200 km. In this belt, which is also called asthenosphere, the speed of propagation of elastic vibrations is slightly less than above and below it, and this indicates a slightly less solid state of the substance. In the future we will see that the “softening belt” plays a very important role in the life of the Earth.
In the lower mantle, the material becomes much heavier. Its density apparently increases to 5.6. It is assumed that it consists of silicates, very rich in iron and magnesium and poor in silica. It is possible that iron sulfide is widespread in the lower mantle.
At a depth of 2900 km, as indicated, the mantle ends and begins Earth's core. The most important feature of the core is that it allows longitudinal seismic vibrations to pass through, but is impenetrable to transverse vibrations. Since transverse elastic vibrations pass through solids, but quickly fade away in liquids, while longitudinal vibrations pass through both solids and liquid bodies, it should be concluded that the Earth's core is in a liquid state. Of course, it is not nearly as liquid as water; it is a very thick substance, close to a solid state, but still much more fluid than the substance of the mantle.
Inside the core there is also inner core, or nucleolus. Its upper boundary is located at a depth of 5000 km, i.e. at a distance of 1370 km from the center of the Earth. Here there is a not very sharp section, at which the speed of seismic vibrations quickly drops again, and then, towards the center of the Earth, begins to increase again. There is an assumption that the inner core is solid and that only the outer core is liquid. However, since the latter prevents the passage of transverse vibrations, the question of the state of the inner core cannot yet be finally resolved.
There has been much debate about the chemical composition of the nucleus. They continue to this day. Many still adhere to the old point of view, believing that the Earth's core consists of iron with a small admixture of nickel. The prototype of this composition is iron meteorites. Meteorites are generally considered either as fragments of previously existing and disintegrated planets, or as remaining “unused” small cosmic bodies from which planets were “assembled” several billion years ago. In both cases, meteorites should seem to represent the chemical composition of one or another shell of the planet. Stone meteorites probably correspond to the chemical composition of the mantle, at least the lower one. Heavier, iron meteorites correspond, as many think, to the deeper interior - the core of the planet.
However, other researchers find arguments against the idea of an iron composition of the core and believe that the core must consist of silicates, in general the same as those that make up the mantle, but that these silicates are in a “metallic” state as a result of the enormous pressure in the core on upper limit core it is equal to 1.3 million atmospheres, and in the center of the Earth 3 million atmospheres). This means that under the influence of pressure, the silicate atoms were partially destroyed and individual electrons broke off from them, which were able to move independently. This, as in metals, determines some of the metallic properties of the core: high density; electrical and thermal conductivity reaching 12.6 in the center of the Earth.
Finally, there is an intermediate point of view, which is now beginning to prevail, namely, that the inner core is iron, and the outer one is composed of silicates in a metallic state.
According to modern theory, the Earth's magnetic field is associated with the outer core. Charged electrons move in the outer core at a depth of between 2900 and 5000 km, describing circles or loops, and it is their movement that leads to the emergence of a magnetic field. It is well known that Soviet rockets launched to the Moon did not detect a magnetic field on our natural satellite. This is quite consistent with the assumption that the Moon does not have a core similar to the Earth’s.
Let us now consider the structure of the earth's interior under the oceans.
Although for Lately Since the International Geophysical Year, the ocean floor and the depths of the Earth under the oceans have been studied extremely intensively (the numerous voyages of the Soviet research ship Vityaz are well known); we still know the geological structure of the ocean territories much worse than the structure of the continents. It has been established, however, that at the bottom of the oceans there are no shields, platforms and folded zones similar to those known on the continents. Based on the bottom topography in the oceans, the largest elements can be identified as plains (or basins), oceanic ridges and deep-sea ditches.
Plains occupy wide spaces at the bottom of all oceans. They are almost always located at the same depth (5-5.5 km).
Ocean ridges are broad, undulating ridges. The Atlantic Ridge is especially characteristic. It stretches from north to south, exactly along the midline of the ocean, curving parallel to the shores of the bordering continents. Its crest is usually located at a depth of about 2 km, but individual peaks rise above sea level in the form of volcanic islands (Azores, St. Paul, Ascension, Tristan da Cunha). Iceland with its volcanoes is located right on the continuation of the underwater ridge.
The underwater ridge in the Indian Ocean also extends in the meridional direction along the ocean's midline. At the Chagos Islands this ridge branches. One of its branches goes straight to the north, where in its continuation in the Bombay region huge frozen flows of volcanic basalts are known (Deccan Plateau). The other branch heads northwest and is lost before entering the Red Sea.
The Atlantic and Indian submarine ridges are connected. In turn, the Indian Ridge connects with the East Pacific Underwater Ridge. The latter extends in a latitudinal direction south of New Zealand, but at the meridian of 120° west longitude it turns sharply to the north. It approaches the shores of Mexico and here it is lost in the shallow waters before entering the Gulf of California.
A series of shorter submarine ridges occupy the central Pacific Ocean. Almost all of them extend from southeast to northwest. At the top of one such underwater ridge are the Hawaiian Islands, at the tops of others are numerous archipelagos of smaller islands.
An example of an underwater oceanic ridge is also the Lomonosov Ridge, discovered by Soviet scientists in the Arctic Ocean.
Almost all large underwater ridges are interconnected and form, as it were, unified system. It is still unclear the relationship of the Lomonosov Ridge with other ridges.
Deep ocean trenches are narrow (100-300 km) and long (several thousand kilometers) trenches in the ocean floor, within which maximum depths are observed. It was in one of these potholes, the Mariana, that the Soviet expedition ship “Vityaz” found the greatest depth of the World Ocean, reaching 11,034 m. Deep-sea potholes are located along the periphery of the oceans. Most often they border island arcs. The latter in a number of places are a characteristic feature of the structure of transition zones between the continents and the ocean. Island arcs are especially widespread along the western periphery of the Pacific Ocean - between the ocean, on the one hand, and Asia and Australia, on the other. From north to south, the arcs of the Aleutian, Kuril, Japanese, Bonino-Marian, Philippine, Tonga, Kermadec and New Zealand islands descend like garlands. Almost all of these arcs are bordered on the outer (convex) side by deep-sea potholes. The same pothole borders the Antilles island arc in Central America. Another pothole borders the side Indian Ocean island arc of Indonesia. Some potholes, located on the periphery of the ocean, are not associated with island arcs. This is, for example, the Atacama Pothole off the coast of South America. The peripheral position of deep-sea potholes is, of course, not accidental.
Speaking about the geological structure of the ocean floor, first of all it should be noted that in the open ocean the thickness of the loose sediments accumulated on the bottom is small - no more than a kilometer, and often less. These sediments consist of very thin calcareous silts, formed mainly by microscopically small shells of unicellular organisms - globigerina, as well as so-called red deep-sea clays containing tiny grains of iron and manganese oxides. Recently, in many places, at great distances from the coast, entire strips of sediments of clastic origin - sands - have been discovered. They were clearly brought to these areas of the oceans from coastal areas and their existence indicates the presence of strong deep-sea currents in the oceans.
Another feature is the huge and widespread development of traces of volcanic activity. At the bottom of all oceans it is known a large number of huge cone-shaped mountains; these are extinct ancient volcanoes. There are many ocean floors and active volcanoes. From these volcanoes, only basalts erupted and are erupting, and at the same time they are very monotonous in their composition, the same everywhere. Along the periphery of the oceans, on island arcs, other lavas containing more silica are known - andesites, but in the middle parts of the oceans volcanic eruptions are only basaltic. In general, in the middle parts of the oceans, almost no other solid rocks are known except basalts. Oceanographic dredging has always lifted only basalt fragments from the bottom, with the exception of some sedimentary rocks. It is also worth mentioning the deep, huge latitudinal cracks, several thousand kilometers long, cutting through the bottom of the northeastern part of the Pacific Ocean. Sharp ledges in the ocean floor can be traced along these cracks.
The deep structure of the earth's crust in the ocean is much simpler than under the continents. In the oceans there is no granite layer and loose sediments lie directly on a basalt layer, the thickness of which is much less than on the continents: usually it is only 5 km. Thus, the solid part of the earth's crust in the oceans consists of one kilometer of loose sediments and five kilometers of basalt layer. The fact that this layer actually consists of basalt is much more likely for the oceans than for continents, given the widespread distribution of basalts on the ocean floor and on oceanic islands. If we add to this five kilometers of the average thickness of the layer of ocean water, then the depth of the lower boundary of the earth's crust (Moho section) under the oceans will be only 11 km - much less than under the continents. Thus, the oceanic crust is thinner than continental crust. Therefore, American engineers began drilling through the entire earth’s crust in the ocean, from a floating drilling rig, hoping there it would be easier to reach the upper layers of the mantle and find out their composition.
There is evidence to suggest that the oceanic crust is becoming thicker under submarine ridges. There its thickness is 20-25 km and it remains basaltic. Interestingly, the crust has an oceanic structure not only under the open oceans, but also under some deep seas: basaltic crust and the absence of a granite layer have been established under the deep part of the Black Sea, under the South Caspian Sea, under the deepest trenches of the Caribbean Sea, under the Sea of Japan and in other places. Seas of intermediate depths have intermediate structure crust: underneath it is thinner than typical continental crust, but thicker than oceanic crust; it has both granite and basalt layers, but the granite layer is much thinner than on the continent. Such intermediate crust is observed in shallow areas of the Caribbean Sea, the Sea of Okhotsk and other places.
The structure of the mantle and core under the oceans is generally similar to their structure under the continents. The difference is observed in the upper mantle: the “softening belt” (asthenosphere) under the oceans is thicker than under the continents; Under the oceans, this belt begins already at a depth of 50 km and continues to a depth of 400 km, while on the continents it is concentrated between 100 and 200 km of depth. Thus, differences in structure between continents and oceans extend not only throughout the entire thickness of the earth's crust, but also into the upper mantle to a depth of at least 400 km. Deeper - in the lower layers of the upper mantle, in the lower mantle, in the outer and inner core - no changes in the structure in the horizontal direction, no differences between the continental and oceanic sectors of the Earth have yet been found.
In conclusion, let's say a few words about some general properties of the globe.
The globe radiates heat. A constant flow of heat flows from the interior of the Earth to the surface. In this regard, there is a so-called temperature gradient - an increase in temperature with depth. On average, this gradient is taken to be 30° per 1 km, i.e., with a deepening of 1 km, the temperature increases by 30° Celsius. This gradient, however, varies very widely from place to place. Moreover, it is only correct for the most superficial parts of the earth's crust. If it remained the same all the way to the center of the Earth, then in the inner regions of the Earth the temperature would be so high that our planet would simply explode. Now there is no doubt that with depth the temperature rises more and more slowly. In the lower mantle and core it increases very slightly and in the center of the Earth, apparently, does not exceed 4000°.
Based on the temperature gradient near the surface, as well as the thermal conductivity of rocks, it is possible to calculate how much heat flows from the depths outward. It turns out that every second the Earth loses 6 ∙ 10 12 calories from its entire surface. Recently, quite a lot of measurements have been made of the size of the Earth's heat flow in different places -on the continents and at the bottom of the oceans. It turned out that on average the heat flow is 1.2 ∙ 10 -6 cal/cm 2 per second. In some of the most common cases, it fluctuates between 0.5 and 3 ∙ 10 -6 cal/cm 2 per second, and there are no differences in heat release on the continents and in the ocean. However, against this uniform background, anomalous zones were discovered - with very high heat transfer, 10 times higher than the normal heat flow. Such zones are underwater ocean ridges. Especially many measurements were made on the East Pacific Ridge.
These observations challenge geophysicists interest Ask. It is now quite clear that the source of heat inside the Earth is radioactive elements. They are present in everyone rocks, in all the material of the globe and during their decay they release heat. If we take into account the average content of radioactive elements in rocks, assume that their content in the mantle is equal to their content in stony meteorites, and the content in the core is considered equal to the content in iron meteorites, then it turns out that the total amount of radioactive elements is more than sufficient to form the observed flow heat. But it is known that granites contain on average 3 times more radioactive elements than basalts, and accordingly should generate more heat. Since the granite layer is present in the earth's crust under the continents and absent under the oceans, one might assume that the heat flow on the continents should be greater than on the ocean floor. In reality this is not the case, in general the flow is the same everywhere, but at the bottom of the oceans there are zones with abnormally high thermal flow. In the following we will try to explain this anomaly.
The shape of the Earth, as you know, is a sphere, slightly flattened at the poles. Due to oblateness, the radius from the center of the Earth to the pole is 1/300th shorter than the radius directed from the center to the equator. This difference is approximately 21 km. On a globe with a diameter of 1 m, it will be a little more than one and a half millimeters and is practically invisible. It was calculated that a liquid ball the size of the Earth, rotating at the same speed, would take this form. This means that, thanks to the property of creep, which we discussed above, the material of the Earth, subjected to a very long-term influence of centrifugal force, was deformed and took on such an equilibrium shape that (of course, much faster) a liquid would take.
The inconsistency of the properties of the Earth's substance is interesting. Elastic vibrations caused by earthquakes propagate in it as in a very solid body, and in the face of long-acting centrifugal force the same substance behaves like a very mobile liquid. Such inconsistency is common for many bodies: they turn out to be solid when a short-term force acts on them, an impact similar to a seismic shock, and they become plastic when the force acts on them slowly, gradually. This property has already been mentioned when describing the collapse of layers of hard rocks into folds. However, recently data has appeared that suggests that the Earth’s substance adapts to the action of centrifugal force with some delay. The fact is that the Earth is gradually slowing down its rotation. The reason for this is sea tides caused by the attraction of the Moon. There are always two bulges on the surface of the World Ocean, one of which faces the Moon, and the other in the opposite direction. These bumps move across the surface due to the rotation of the Earth. But due to the inertia and viscosity of water, the crest of the bulge facing the Moon is always a little late, always slightly shifted in the direction of the Earth’s rotation. Therefore, the Moon attracts the wave not perpendicular to the earth's surface, but along a slightly inclined line. It is this tilt that causes the Moon’s gravity to slightly slow down the Earth’s rotation. There is very little braking. Thanks to this, the day increases by two thousandths of a second every 100 years. If this rate of deceleration remained unchanged throughout geological time, then in the Jurassic period the day was shorter by one hour, and two billion years ago - at the end of the Archean era - the Earth rotated twice as fast.
Along with the slowdown of rotation, the centrifugal force should also decrease; therefore, the shape of the Earth should change - its flatness should gradually decrease. However, calculations show that the currently observed shape of the Earth corresponds not to the current speed of its rotation, but to the one that was approximately 10 million years ago. The substance of the Earth, although fluid under conditions of long-term pressure, has significant viscosity, high internal friction and therefore is subject to new mechanical conditions with a noticeable delay.
In conclusion, let us point out some interesting consequences of earthquakes. The vibrations caused by ordinary earthquakes have different periods. Some earthquakes have a short period - about a second. Registration of such vibrations is extremely important for studying earthquakes that occurred near a seismic station, that is, local earthquakes. With distance from the source of the earthquake, such vibrations quickly fade. On the contrary, oscillations with a long period (18-20 sec.) spread far; during a large earthquake, they can pass right through the globe or go around it along the surface. Such vibrations are recorded at many seismic stations and are convenient for studying distant earthquakes. It is with the help of long-period oscillations that the Moscow seismic station can record earthquakes occurring in South America or the Philippines.
In recent years, oscillations caused by earthquakes have been discovered with very long periods of approximately an hour. Ultra-long seismic waves, for example, were formed by a powerful earthquake in Chile in 1960. Such waves, before dying out, circle the globe seven to eight times, or even more.
Calculations show that ultralong waves are caused by vibrations of the entire globe. The energy of some earthquakes is so great that they seem to rock the entire globe, causing it to pulsate as a whole. True, the amplitude of such oscillations is insignificant: far from the source of the earthquake, it can only be noticed by sensitive instruments and completely fades away within a few days. However, the phenomenon of “trembling” of the entire Earth as a whole cannot but produce an impression. The general vibrations of the entire Earth have proven useful in determining some of the physical properties of the globe.
Methods for studying the internal structure and composition of the EarthMethods for studying the internal structure and composition of the Earth can be divided into two main groups: geological methods and geophysical methods. Geological methods are based on the results of direct study of rock strata in outcrops, mine workings (mines, adits, etc.) and wells. At the same time, researchers have at their disposal the entire arsenal of methods for studying the structure and composition, which determines the high degree of detail of the results obtained. At the same time, the capabilities of these methods in studying the depths of the planet are very limited - the deepest well in the world has a depth of only -12262 m (Kola Superdeep in Russia), even smaller depths are achieved when drilling the ocean floor (about -1500 m, drilling from the board of the American research vessel Glomar Challenger). Thus, depths not exceeding 0.19% of the radius of the planet are available for direct study.
Information about the deep structure is based on the analysis of indirect data obtained geophysical methods, mainly the patterns of changes with depth in various physical parameters (electrical conductivity, mechanical quality factor, etc.) measured during geophysical research. The development of models of the internal structure of the Earth is based primarily on the results of seismic research, based on data on the patterns of propagation of seismic waves. At the source of earthquakes and powerful explosions, seismic waves—elastic vibrations—emerge. These waves are divided into volume waves - propagating in the bowels of the planet and “transparent” them like X-rays, and surface waves - propagating parallel to the surface and “probing” the upper layers of the planet to a depth of tens to hundreds of kilometers.
Body waves, in turn, are divided into two types - longitudinal and transverse. Longitudinal waves, which have a high propagation speed, are the first to be recorded by seismic receivers; they are called primary or P-waves ( from English primary - primary), slower transverse waves are called S-waves ( from English secondary - secondary). Transverse waves are known to have important feature– they spread only in solid media.
At the boundaries of media with different properties, waves are refracted, and at the boundaries of sharp changes in properties, in addition to refracted ones, reflected and exchanged waves arise. Shear waves can have a displacement perpendicular to the plane of incidence (SH waves) or a displacement lying in the plane of incidence (SV waves). When crossing the boundaries of media with different properties, SH waves experience normal refraction, and SV waves, in addition to refracted and reflected SV waves, excite P waves. This is how a complex system of seismic waves arises, “transparent” the bowels of the planet.
By analyzing the patterns of wave propagation, it is possible to identify inhomogeneities in the bowels of the planet - if at a certain depth an abrupt change in the speeds of propagation of seismic waves, their refraction and reflection is recorded, we can conclude that at this depth there is a boundary of the inner shells of the Earth, differing in their physical properties.
The study of the paths and speed of propagation of seismic waves in the bowels of the Earth made it possible to develop a seismic model of its internal structure.
Seismic waves, propagating from the earthquake source deep into the Earth, experience the most significant abrupt changes in speed, are refracted and reflected on seismic sections located at depths 33 km And 2900 km from the surface (see figure). These sharp seismic boundaries make it possible to divide the planet's interior into 3 main internal geospheres - the earth's crust, mantle and core.
The earth's crust is separated from the mantle by a sharp seismic boundary, at which the speed of both longitudinal and transverse waves increases abruptly. Thus, the speed of shear waves increases sharply from 6.7-7.6 km/s in the lower part of the crust to 7.9-8.2 km/s in the mantle. This boundary was discovered in 1909 by the Yugoslav seismologist Mohorovicic and was subsequently named Mohorovicic border(often briefly called the Moho boundary, or M boundary). The average depth of the boundary is 33 km (it should be noted that this is a very approximate value due to different thicknesses in different geological structures); at the same time, under the continents, the depth of the Mohorovichichi section can reach 75-80 km (which is recorded under young mountain structures - the Andes, Pamirs), under the oceans it decreases, reaching a minimum thickness of 3-4 km.
An even sharper seismic boundary separating the mantle and core is recorded at depth 2900 km. At this seismic section, the P-wave speed drops abruptly from 13.6 km/s at the base of the mantle to 8.1 km/s at the core; S-waves - from 7.3 km/s to 0. The disappearance of transverse waves indicates that the outer part of the core has the properties of a liquid. The seismic boundary separating the core and mantle was discovered in 1914 by the German seismologist Gutenberg and is often called Gutenberg border, although this name is not official.
Sharp changes in the speed and nature of the passage of waves are recorded at depths of 670 km and 5150 km. Border 670 km divides the mantle into the upper mantle (33-670 km) and the lower mantle (670-2900 km). Border 5150 km divides the core into an outer liquid (2900-5150 km) and an inner solid (5150-6371 km).
Significant changes are also noted in the seismic section 410 km, dividing the upper mantle into two layers.
The obtained data on global seismic boundaries provide the basis for considering a modern seismic model of the deep structure of the Earth.
The outer shell of the solid Earth is Earth's crust, bounded by the Mohorovicic boundary. This is a relatively thin shell, the thickness of which ranges from 4-5 km under the oceans to 75-80 km under continental mountain structures. The upper crust is clearly visible in the composition of the central crust. sedimentary layer, consisting of unmetamorphosed sedimentary rocks, among which volcanics may be present, and underlying it consolidated, or crystalline,bark, formed by metamorphosed and igneous intrusive rocks. There are two main types of earth's crust - continental and oceanic, fundamentally different in structure, composition, origin and age.
Continental crust lies under continents and their underwater margins, has a thickness from 35-45 km to 55-80 km, 3 layers are distinguished in its section. The top layer is usually composed of sedimentary rocks, including a small amount of weakly metamorphosed and igneous rocks. This layer is called sedimentary. Geophysically, it is characterized by low P-wave speeds in the range of 2-5 km/s. The average thickness of the sedimentary layer is about 2.5 km.
Below is the upper crust (granite-gneiss or “granite” layer), composed of igneous and metamorphic rocks rich in silica (on average, corresponding in chemical composition to granodiorite). The speed of P-waves in this layer is 5.9-6.5 km/s. At the base of the upper crust, a Conrad seismic section is distinguished, reflecting an increase in the speed of seismic waves during the transition to the lower crust. But this section is not recorded everywhere: in the continental crust, a gradual increase in wave velocities with depth is often recorded.
The lower crust (granulite-mafic layer) is characterized by a higher wave speed (6.7-7.5 km/s for P-waves), which is due to a change in the composition of the rocks during the transition from the upper mantle. According to the most accepted model, its composition corresponds to granulite.
Rocks of various geological ages take part in the formation of the continental crust, up to the most ancient ones, about 4 billion years old.
Ocean crust has a relatively small thickness, on average 6-7 km. In its context at its very general view 2 layers can be distinguished. The upper layer is sedimentary, characterized by low thickness (on average about 0.4 km) and low P-wave speed (1.6-2.5 km/s). The lower layer is “basaltic” - composed of basic igneous rocks (at the top - basalts, below - basic and ultrabasic intrusive rocks). The speed of longitudinal waves in the “basalt” layer increases from 3.4-6.2 km/s in basalts to 7-7.7 km/s in the lowest crustal horizons.
The age of the oldest rocks of modern oceanic crust is about 160 million years.
Mantle It is the largest inner shell of the Earth in terms of volume and mass, bounded above by the Moho boundary and below by the Gutenberg boundary. It consists of an upper mantle and a lower mantle, separated by a boundary of 670 km.
According to geophysical features, upper mania is divided into two layers. Upper layer - subcrustal mantle- extends from the Moho boundary to depths of 50-80 km under the oceans and 200-300 km under the continents and is characterized by a smooth increase in the speed of both longitudinal and transverse seismic waves, which is explained by the compaction of rocks due to the lithostatic pressure of the overlying strata. Below the subcrustal mantle to the global interface of 410 km there is a layer of low velocities. As the name of the layer suggests, the velocities of seismic waves in it are lower than in the subcrustal mantle. Moreover, in some areas there are lenses that do not transmit S-waves at all, which gives grounds to state that the mantle material in these areas is in a partially molten state. This layer is called the asthenosphere ( from Greek "asthenes" - weak and "sphair" - sphere); the term was introduced in 1914 by the American geologist J. Burrell, in English-language literature often referred to as LVZ - Low Velocity Zone. Thus, asthenosphere- This is a layer in the upper mantle (located at a depth of about 100 km under the oceans and about 200 km or more under the continents), identified on the basis of a decrease in the speed of seismic waves and having reduced strength and viscosity. The surface of the asthenosphere is well established and sharp decline resistivity(up to values of about 100 Ohm . m).
The presence of a plastic asthenospheric layer, which differs in mechanical properties from the solid overlying layers, gives grounds for identifying lithosphere- the solid shell of the Earth, including the earth's crust and subcrustal mantle located above the asthenosphere. The thickness of the lithosphere ranges from 50 to 300 km. It should be noted that the lithosphere is not a monolithic rock shell of the planet, but is divided into separate plates that are constantly moving along the plastic asthenosphere. Foci of earthquakes and modern volcanism are confined to the boundaries of lithospheric plates.
Below the 410 km section, both P- and S-waves propagate everywhere in the upper mantle, and their speed increases relatively monotonically with depth.
IN lower mantle, separated by a sharp global boundary of 670 km, the speed of P- and S-waves monotonically, without abrupt changes, increases, respectively, to 13.6 and 7.3 km/s up to the Gutenberg section.
In the outer core, the speed of P waves sharply decreases to 8 km/s, and S waves completely disappear. The disappearance of transverse waves suggests that the Earth's outer core is in a liquid state. Below the 5150 km section there is an inner core in which the speed of P waves increases and S waves begin to propagate again, indicating its solid state.
The fundamental conclusion from the Earth velocity model described above is that our planet consists of a series of concentric shells representing an iron core, a silicate mantle, and an aluminosilicate crust.
Geophysical characteristics of the Earth
Distribution of mass between inner geospheres
The bulk of the Earth's mass (about 68%) falls on its relatively light but large-volume mantle, with about 50% in the lower mantle and about 18% in the upper. The remaining 32% of the Earth's total mass comes mainly from the core, with its liquid outer part (29% of the Earth's total mass) being much heavier than the solid inner part (about 2%). Only less than 1% of the planet's total mass remains on the crust.
Density
The density of the shells naturally increases towards the center of the Earth (see figure). The average density of the bark is 2.67 g/cm3; at the Moho boundary it increases abruptly from 2.9-3.0 to 3.1-3.5 g/cm 3 . In the mantle, the density gradually increases due to compression of the silicate substance and phase transitions (rearrangement of the crystalline structure of the substance during “adaptation” to increasing pressure) from 3.3 g/cm 3 in the subcrustal part to 5.5 g/cm 3 in the lower parts of the lower mantle . At the Gutenberg boundary (2900 km), the density almost doubles abruptly - up to 10 g/cm 3 in the outer core. Another jump in density - from 11.4 to 13.8 g/cm 3 - occurs at the boundary of the inner and outer core (5150 km). These two sharp density jumps have different natures: at the mantle/core boundary, a change in the chemical composition of the substance occurs (transition from the silicate mantle to the iron core), and the jump at the 5150 km boundary is associated with a change in the state of aggregation (transition from the liquid outer core to the solid inner core) . In the center of the Earth, the density of matter reaches 14.3 g/cm 3 .
Pressure
The pressure in the Earth's interior is calculated based on its density model. The increase in pressure with distance from the surface is due to several reasons:
compression due to the weight of the overlying shells (lithostatic pressure);
phase transitions in shells of homogeneous chemical composition (in particular, in the mantle);
differences in the chemical composition of the shells (crust and mantle, mantle and core).
At the base of the continental crust, the pressure is about 1 GPa (more precisely 0.9 * 10 9 Pa). In the Earth's mantle the pressure gradually increases; at the Gutenberg boundary it reaches 135 GPa. In the outer core, the pressure gradient increases, and in the inner core, on the contrary, it decreases. The calculated pressure values at the boundary between the inner and outer cores and near the center of the Earth are 340 and 360 GPa, respectively.
Temperature. Sources of thermal energy
The geological processes occurring on the surface and in the interior of the planet are primarily caused by thermal energy. Energy sources are divided into two groups: endogenous (or internal sources), associated with the generation of heat in the bowels of the planet, and exogenous (or external to the planet). The intensity of the flow of thermal energy from the subsurface to the surface is reflected in the magnitude of the geothermal gradient. Geothermal gradient– temperature increase with depth, expressed in 0 C/km. The “reverse” characteristic is geothermal stage– depth in meters, when diving to which the temperature will increase by 1 0 C. average value The geothermal gradient in the upper part of the crust is 30 0 C/km and ranges from 200 0 C/km in areas of modern active magmatism to 5 0 C/km in areas with a quiet tectonic regime. With depth, the value of the geothermal gradient decreases significantly, averaging about 10 0 C/km in the lithosphere, and less than 1 0 C/km in the mantle. The reason for this lies in the distribution of thermal energy sources and the nature of heat transfer.
Sources of endogenous energy are the following.
1. Energy of deep gravitational differentiation, i.e. heat release during the redistribution of a substance by density during its chemical and phase transformations. The main factor in such transformations is pressure. The core-mantle boundary is considered as the main level of release of this energy.
2. Radiogenic heat, arising from the decay radioactive isotopes. According to some calculations, this source determines about 25% of the heat flow emitted by the Earth. However, it is necessary to take into account that increased contents of the main long-lived radioactive isotopes - uranium, thorium and potassium - are observed only in the upper part of the continental crust (isotopic enrichment zone). For example, the concentration of uranium in granites reaches 3.5 10 -4%, in sedimentary rocks - 3.2 10 -4%, while in the oceanic crust it is negligible: about 1.66 10 -7%. Thus, radiogenic heat is an additional source of heat in the upper part of the continental crust, which determines the high value of the geothermal gradient in this area of the planet.
3. Residual heat, preserved in the depths since the formation of the planet.
4. Solid tides, caused by the attraction of the Moon. The transition of kinetic tidal energy into heat occurs due to internal friction in rock strata. The share of this source in the total heat balance is small - about 1-2%.
In the lithosphere, the conductive (molecular) mechanism of heat transfer predominates; in the sublithospheric mantle of the Earth, a transition occurs to a predominantly convective mechanism of heat transfer.
Calculations of temperatures in the interior of the planet give the following values: in the lithosphere at a depth of about 100 km the temperature is about 1300 0 C, at a depth of 410 km - 1500 0 C, at a depth of 670 km - 1800 0 C, at the boundary of the core and mantle - 2500 0 C, at a depth of 5150 km - 3300 0 C, in the center of the Earth - 3400 0 C. In this case, only the main (and most probable for deep zones) heat source was taken into account - the energy of deep gravitational differentiation.
Endogenous heat determines the course of global geodynamic processes. including the movement of lithospheric plates
On the surface of the planet, the most important role is played by exogenous source heat - solar radiation. Below the surface, the influence of solar heat is sharply reduced. Already at a shallow depth (up to 20-30 m) there is a zone of constant temperatures - a region of depths where the temperature remains constant and is equal to the average annual temperature of the region. Below the belt of constant temperatures, heat is associated with endogenous sources.
Earth Magnetism
The Earth is a giant magnet with a magnetic force field and magnetic poles that are located close to the geographic ones, but do not coincide with them. Therefore, in the readings of the magnetic compass needle, a distinction is made between magnetic declination and magnetic inclination.
Magnetic declination is the angle between the direction of the magnetic compass needle and the geographic meridian at a given point. This angle will be greatest at the poles (up to 90 0) and smallest at the equator (7-8 0).
Magnetic inclination– the angle formed by the inclination of the magnetic needle to the horizon. As you approach the magnetic pole, the compass needle will take a vertical position.
It is assumed that the emergence of a magnetic field is due to systems of electric currents arising during the rotation of the Earth, in connection with convective movements in the liquid outer core. The total magnetic field consists of the values of the Earth's main field and the field caused by ferromagnetic minerals in the rocks of the earth's crust. Magnetic properties characteristic of ferromagnetic minerals, such as magnetite (FeFe 2 O 4), hematite (Fe 2 O 3), ilmenite (FeTiO 2), pyrrhotite (Fe 1-2 S), etc., which are minerals and are established by magnetic anomalies. These minerals are characterized by the phenomenon of residual magnetization, which inherits the orientation of the Earth's magnetic field that existed during the formation of these minerals. Reconstruction of the location of the Earth's magnetic poles in different geological epochs indicates that the magnetic field periodically experienced inversion- a change in which the magnetic poles changed places. The process of changing the magnetic sign of the geomagnetic field lasts from several hundred to several thousand years and begins with an intensive decrease in the strength of the main magnetic field of the Earth to almost zero, then reverse polarity is established and after some time there follows a rapid restoration of tension, but of the opposite sign. The North Pole took the place of the South Pole and, vice versa, with an approximate frequency of 5 times every 1 million years. The current orientation of the magnetic field was established about 800 thousand years ago.
