- these are slow, uneven vertical (lowering or raising) and horizontal tectonic movements of vast areas of the earth's crust, changing the height land and the depths of the seas. They are sometimes also called secular oscillations of the earth's crust.

Causes

The exact reasons for the movements of the earth's crust have not yet been sufficiently elucidated, but one thing is clear that these vibrations occur under the influence of the internal forces of the Earth. The original reason all movements of the earth's crust - both horizontal (along the surface) and vertical (mountain building) - is thermal mixing of a substance in the planet's mantle.

In the distant past, on the territory where Moscow is now located, the waves of a warm sea splashed. This is evidenced by the thickness of marine sediments with the remains of fish and other animals buried in them, which now lie at a depth of several tens of meters. And at the bottom of the Mediterranean Sea, not far from the shore, scuba divers found ruins ancient city.

These facts indicate that the earth's crust, which we are accustomed to consider motionless, is experiencing slow ups and downs. On the Scandinavian Peninsula, you can currently see mountain slopes eroded by the sea surf on such high altitude, where the waves do not reach. At the same height, rings are set into the rocks, to which boat chains were once tied. Now from the surface of the water to these rings there are 10 meters, or even more. This means that we can conclude that the Scandinavian Peninsula is currently slowly rising. Scientists have calculated that in some places this rise is occurring at a rate of 1 cm per year. Material from the site

But the western coast of Europe is sinking at about the same speed. To prevent ocean waters from flooding this part of the continent, people built dams along the seashore that stretched for hundreds of kilometers.

Slow movements of the earth's crust occur over the entire surface of the Earth. Moreover, the period of rise is replaced by a period of decline. Once upon a time, the Scandinavian Peninsula was sinking, but in our time it is experiencing an uplift.

Due to movements of the earth's crust, volcanoes are born,

The earth's crust consists of lithospheric plates. Each lithospheric plate is characterized by continuous movement. People don't notice such movements because they happen extremely slowly.

Causes and consequences of crustal movement

We all know that our planet consists of three parts: the earth's core, the earth's mantle, and the earth's crust. The core of our planet contains many chemical substances that continuously enter into chemical reactions with each other.

As a result of such chemical, radioactive and thermal reactions, vibrations occur in the lithosphere. Due to this, the earth's crust can move vertically and horizontally.

History of the study of crustal movements

Tectonic movements were studied by ancient scientists. The ancient Greek geographer Strabo first proposed the theory that individual areas of land are systematically rising. The famous Russian scientist Lomonosov called the movements of the earth's crust as long-term and insensitive earthquakes.

However, a more detailed study of the processes of movement of the earth's crust began at the end of the 19th century. American geologist Gilbert classified movements of the earth's crust into two main types: those that create mountains (orogenic) and those that create continents (epeirogenic). Both foreign and domestic scientists studied the movement of the earth's crust, in particular: V. Belousov, Yu. Kosygin, M. Tetyaev, E. Haarman, G. Stille.

Types of crustal movement

There are two types tectonic movements: vertical and horizontal. Vertical movements are called radial. Such movements are expressed in the systematic raising (or lowering) of lithospheric plates. Often, radial movements of the earth's crust occur as a consequence of strong earthquakes.

Horizontal movements represent displacements of lithospheric plates. According to the opinion of many modern scientists, all existing continents were formed as a result of the horizontal displacement of lithospheric plates.

The significance of the movement of the earth's crust for humans

Movements of the earth's crust today threaten the lives of many people. A striking example is the Italian city of Venice. The city is located on a section of a lithospheric plate that is subsiding at a high rate.

Every year, the city sinks under water - a process of transgression occurs (long-term advance of sea water onto land). There are cases in history when, due to the movement of the earth's crust, cities and towns went under water, and after some time they rose again (the process of regression).

The position of the earth's crust between the mantle and the outer shells - the atmosphere, hydrosphere and biosphere - determines the influence of external and internal forces of the Earth on it.

The structure of the earth's crust is heterogeneous (Fig. 19). The upper layer, whose thickness varies from 0 to 20 km, is complex sedimentary rocks– sand, clay, limestone, etc. This is confirmed by data obtained from studying outcrops and drill hole cores, as well as the results of seismic studies: these rocks are loose, the speed of seismic waves is low.

Rice. 19. Structure of the earth's crust

Below, under the continents, is located granite layer, composed of rocks whose density corresponds to the density of granite. The speed of seismic waves in this layer, as in granites, is 5.5–6 km/s.

Under the oceans there is no granite layer, but on the continents in some places it comes out to the surface.

Even lower is a layer in which seismic waves propagate at a speed of 6.5 km/s. This speed is typical for basalts, therefore, despite the fact that the layer is complex different breeds, he is called basalt.

The boundary between granite and basalt layers is called Conrad surface. This section corresponds to a jump in the speed of seismic waves from 6 to 6.5 km/s.

Depending on the structure and thickness, two types of bark are distinguished - 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, the granite layer is completely absent in many places, 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.

Mantle. This is 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 more than half of the Earth's volume. The mantle material is in a superheated state and experiences enormous pressure from the overlying lithosphere. The mantle has a great influence on the processes occurring on Earth. Magma chambers arise 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.

Core. There are two parts in the core: the outer, to a depth of 5 thousand km, and the inner, to the center of the Earth. The outer core is liquid, since transverse waves do not pass through it, while the inner core is solid. The substance of the core, especially the inner one, is highly compacted and its density corresponds to metals, which is why it is called metallic.

§ 17. Physical properties and chemical composition of the Earth

TO physical properties The lands are attributed temperature regime(internal heat), density and pressure.

Internal heat of the Earth. By modern ideas The earth after its formation was a cold body. Then the decay of radioactive elements gradually warmed it up. However, as a result of the radiation of heat from the surface into the near-Earth space, it cooled. A relatively cold lithosphere and crust were formed. Temperatures are still high at great depths today. An increase in temperatures with depth can be observed directly in deep mines and boreholes, during volcanic eruptions. Thus, pouring volcanic lava has a temperature of 1200–1300 °C.

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 fluctuations up to 30 m. Below this layer lies a zone of constant temperatures, where they always remain unchanged 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. Thus, in Russia it ranges from 1.4 m in Pyatigorsk to 180 m on the Kola Peninsula.

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 50,000 °C.

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/s 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/cm3, and basalts have a density of more than 2 g/cm3. The average density of the Earth is 5.52 g/cm 3 - this is more than 2 times the density of granite. In the center of the Earth, the density of the rocks composing it increases and amounts to 15–17 g/cm3.

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 substances is called plastic. This experiment suggests that in the deep interior of the Earth, matter is in a plastic state.

Chemical composition of the Earth. In the Earth you can find all the chemical elements of D.I. Mendeleev’s table. 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.

§ 18. Movement of the earth's crust

The earth's crust only seems motionless, absolutely stable. In fact, she makes continuous and varied movements. Some of them occur very slowly and are not perceived by the human senses, others, such as earthquakes, are landslide and destructive. What titanic forces set the earth's crust in motion?

The internal forces of the Earth, the source of their origin. It is known that at the boundary of the mantle and lithosphere the temperature exceeds 1500 °C. At this temperature, matter must either melt or turn into gas. When solids transform into a liquid or gaseous state, their volume must increase. However, this does not happen, since the overheated rocks are under pressure from the overlying layers of the lithosphere. A “steam boiler” effect occurs when matter, seeking to expand, presses on the lithosphere, causing it to move along with the earth’s crust. Moreover, the higher the temperature, the stronger the pressure and the more active the lithosphere moves. Particularly strong pressure centers arise in those places in the upper mantle where radioactive elements, the decay of which heats the constituent rocks to even higher temperatures. Movements of the earth's crust under the influence of the internal forces of the Earth are called tectonic. These movements are divided into oscillatory, folding and bursting.

Oscillatory movements. These movements occur very slowly, imperceptibly for humans, which is why they are also called centuries-old or epeirogenic. In some places the earth's crust rises, in others it falls. In this case, the rise is often replaced by a fall, and vice versa. These movements can be traced only by the “traces” that remain after them on the earth’s surface. For example, on the Mediterranean coast, near Naples, there are the ruins of the Temple of Serapis, the columns of which were worn away by sea mollusks at an altitude of up to 5.5 m above modern sea level. This serves as absolute proof that the temple, built in the 4th century, was at the bottom of the sea, and then it was raised. Now this area of ​​land is sinking again. Often on the coasts of the seas there are steps above their current level - sea terraces, once created by the surf. On the platforms of these steps you can find the remains of marine organisms. This indicates that the terrace areas were once the bottom of the sea, and then the shore rose and the sea retreated.

The descent of the earth's crust below 0 m above sea level is accompanied by the advance of the sea - transgression, and the rise - by his retreat - regression. Currently in Europe, uplifts occur in Iceland, Greenland, and the Scandinavian Peninsula. Observations have established that the region of the Gulf of Bothnia is rising at a rate of 2 cm per year, i.e. 2 m per century. At the same time, the territory of Holland, Southern England, Northern Italy, the Black Sea Lowland, and the coast of the Kara Sea is subsiding. Sign of descent sea ​​coasts serves the formation of sea bays in the estuaries of rivers - estuaries (lips) and estuaries.

When the earth's crust rises and the sea retreats, the seabed, composed of sedimentary rocks, turns out to be dry land. This is how extensive marine (primary) plains: for example, West Siberian, Turanian, North Siberian, Amazonian (Fig. 20).

Rice. 20. The structure of primary, or marine, strata plains

Folding movements. In cases where rock layers are sufficiently plastic, under the influence of internal forces they collapse into folds. When the pressure is directed vertically, the rocks are displaced, and if in the horizontal plane, they are compressed into folds. The shape of the folds can be very diverse. When the bend of the fold is directed downward, it is called a syncline, upward - an anticline (Fig. 21). Folds form at great depths, i.e. at high temperatures and high pressure, and then under the influence of internal forces they can be lifted. This is how they arise fold mountains Caucasian, Alps, Himalayas, Andes, etc. (Fig. 22). In such mountains, folds are easy to observe where they are exposed and come to the surface.

Rice. 21. Synclinal (1) and anticlinal (2) folds

Rice. 22. fold mountains

Breaking movements. If the rocks are not strong enough to withstand the action of internal forces, cracks (faults) form in the earth's crust and vertical displacement of the rocks occurs. The sunken areas are called grabens, and those who rose - handfuls(Fig. 23). The alternation of horsts and grabens creates block (revived) mountains. Examples of such mountains are: Altai, Sayan, Verkhoyansk Range, Appalachians in North America and many others. Revived mountains differ from folded ones in both internal structure, and by appearance– morphology. The slopes of these mountains are often steep, the valleys, like the watersheds, are wide and flat. Rock layers are always displaced relative to each other.

Rice. 23. Revived fold-block mountains

The sunken areas in these mountains, grabens, sometimes fill with water, and then deep lakes are formed: for example, Baikal and Teletskoye in Russia, Tanganyika and Nyasa in Africa.

§ 19. Volcanoes and earthquakes

With a further increase in temperature in the bowels of the Earth, rocks, despite high pressure, melt, forming magma. This releases a lot of gases. This further increases both the volume of the melt and its pressure on the surrounding rocks. As a result, very dense, gas-rich magma tends to go where the pressure is lower. It fills cracks in the earth's crust, breaks and lifts the layers of its constituent rocks. Part of the magma, before reaching the earth's surface, solidifies in the thickness of the earth's crust, forming magma veins and laccoliths. Sometimes magma breaks out to the surface and erupts in the form of lava, gases, volcanic ash, rock fragments and frozen lava clots.

Volcanoes. Each volcano has a channel through which lava erupts (Fig. 24). This vent, which always ends in a funnel-shaped expansion - crater. The diameter of the craters ranges from several hundred meters to many kilometers. For example, the diameter of the Vesuvius crater is 568 m. Very large craters are called calderas. For example, the caldera of the Uzon volcano in Kamchatka, which is filled by Lake Kronotskoye, reaches 30 km in diameter.

The shape and height of volcanoes depend on the viscosity of the lava. Liquid lava spreads quickly and easily and does not form a cone-shaped mountain. An example is the Kilauza volcano in the Hawaiian Islands. The crater of this volcano is a round lake with a diameter of about 1 km, filled with bubbling liquid lava. The level of lava, like water in the bowl of a spring, then falls, then rises, splashing out over the edge of the crater.

Rice. 24. Volcanic cone in section

More widespread are volcanoes with viscous lava, which, when cooled, forms a volcanic cone. The cone always has a layered structure, which indicates that eruptions occurred many times, and the volcano grew gradually, from eruption to eruption.

The height of volcanic cones ranges from several tens of meters to several kilometers. For example, the Aconcagua volcano in the Andes has a height of 6960 m.

There are about 1,500 volcano mountains, active and extinct. Among them are such giants as Elbrus in the Caucasus, Klyuchevskaya Sopka in Kamchatka, Fuji in Japan, Kilimanjaro in Africa and many others.

Most active volcanoes are located around Pacific Ocean, forming the Pacific “Ring of Fire”, and in the Mediterranean-Indonesian belt. In Kamchatka alone, 28 active volcanoes are known, and in total there are more than 600. The distribution of active volcanoes is natural - they are all confined to mobile zones of the earth’s crust (Fig. 25).

Rice. 25. Zones of volcanism and earthquakes

In the Earth's geological past, volcanism was more active than it is now. In addition to the usual (central) eruptions, fissure eruptions occurred. From giant cracks (faults) in the earth's crust, stretching for tens and hundreds of kilometers, lava erupted onto the earth's surface. Continuous or patchy lava covers were created, leveling the terrain. The thickness of the lava reached 1.5–2 km. This is how they were formed lava plains. Examples of such plains are certain sections of the Central Siberian Plateau, the central part of the Deccan Plateau in India, the Armenian Highlands, and the Columbia Plateau.

Earthquakes. The causes of earthquakes are different: volcanic eruptions, mountain collapses. But the most powerful of them arise as a result of movements of the earth's crust. Such earthquakes are called tectonic. They usually originate at great depths, at the boundary of the mantle and lithosphere. The origin of an earthquake is called hypocenter or hearth. On the surface of the Earth, above the hypocenter, is epicenter earthquakes (Fig. 26). Here the strength of the earthquake is greatest, and as it moves away from the epicenter it weakens.

Rice. 26. Hypocenter and epicenter of earthquake

The earth's crust shakes continuously. Over 10,000 earthquakes are observed throughout the year, but most of them are so weak that they are not felt by humans and are recorded only by instruments.

The strength of earthquakes is measured in points - from 1 to 12. Powerful 12-point earthquakes are rare and are catastrophic in nature. During such earthquakes, deformations occur in the earth's crust, cracks, shifts, faults, landslides in the mountains and failures in the plains form. If they occur in densely populated areas, then great destruction and numerous casualties occur. The largest earthquakes in history are Messina (1908), Tokyo (1923), Tashkent (1966), Chilean (1976) and Spitak (1988). In each of these earthquakes, tens, hundreds and thousands of people died, and cities were destroyed almost to the ground.

Often the hypocenter is located under the ocean. Then a destructive ocean wave arises - tsunami.

§ 20. External processes transforming the surface of the Earth

Simultaneously with internal, tectonic processes, external processes operate on Earth. Unlike internal ones, which cover the entire thickness of the lithosphere, they act only on the Earth’s surface. The depth of their penetration into the earth's crust does not exceed several meters and only in caves - up to several hundred meters. The source of the forces causing external processes is thermal solar energy.

External processes are very diverse. These include the weathering of rocks, the work of wind, water and glaciers.

Weathering. It is divided into physical, chemical and organic.

Physical weathering- This is mechanical crushing, grinding of rocks.

It occurs when there is a sudden change in temperature. When heated, rock expands; when cooled, it contracts. Since the expansion coefficient of different minerals included in the rock is not the same, the process of its destruction intensifies. Initially, the rock breaks up into large blocks, which are crushed over time. Accelerated destruction of the rock is facilitated by water, which, penetrating into cracks, freezes in them, expands and tears the rock into separate parts. Physical weathering is most active where there is a sharp change in temperature, and hard igneous rocks come to the surface - granite, basalt, syenites, etc.

Chemical weathering- this is a chemical effect on rocks of various aqueous solutions.

In this case, in contrast to physical weathering, various chemical reactions, and as a result, a change in the chemical composition and, possibly, the formation of new rocks. Chemical weathering occurs everywhere, but is especially intense in easily soluble rocks - limestone, gypsum, dolomite.

Organic weathering is the process of destruction of rocks by living organisms - plants, animals and bacteria.

Lichens, for example, settling on rocks, wear away their surface with secreted acid. Plant roots also secrete acid, and in addition, the root system acts mechanically, as if tearing the rock. Earthworms passing through themselves inorganic substances, transform the rock and improve access to water and air.

Weathering and climate. All types of weathering occur simultaneously, but act with different intensities. This depends not only on the constituent rocks, but also mainly on the climate.

Frost weathering is most active in polar countries, chemical weathering in temperate countries, mechanical weathering in tropical deserts, and chemical weathering in the humid tropics.

The work of the wind. Wind is capable of destroying rocks and transporting and depositing solid particles. The stronger the wind and the more often it blows, the more work it can produce. Where rocky outcrops emerge on the Earth's surface, the wind bombards them with grains of sand, gradually erasing and destroying even the hardest rocks. Less stable rocks are destroyed faster, and specific, aeolian landforms– stone laces, aeolian mushrooms, pillars, towers.

In sandy deserts and along the shores of seas and large lakes, the wind creates specific relief forms - barchans and dunes.

Dunes- These are moving sandy hills of a crescent shape. Their windward slope is always gentle (5-10°), and the leeward slope is steep – up to 35–40° (Fig. 27). The formation of dunes is associated with the inhibition of the wind flow carrying sand, which occurs due to any obstacles - uneven surfaces, stones, bushes, etc. The force of the wind weakens, and sand deposition begins. The more constant the winds and the more sand, the faster the dune grows. The highest dunes - up to 120 m - were found in the deserts of the Arabian Peninsula.

Rice. 27. The structure of the dune (the arrow shows the direction of the wind)

The dunes move in the direction of the wind. The wind blows grains of sand along a gentle slope. Having reached the ridge, the wind flow swirls, its speed decreases, grains of sand fall out and roll down the steep leeward slope. This causes the entire dune to move at a speed of up to 50–60 m per year. As they move, dunes can cover oases and even entire villages.

On sandy beaches, blowing sands form dunes. They stretch along the coast in the form of huge sandy ridges or hills up to 100 m or more in height. Unlike dunes, they do not have a permanent shape, but they can also move inland from the beach. In order to stop the movement of the dunes, trees and shrubs, primarily pine trees, are planted.

Snow and ice work. Snow, especially in the mountains, does a lot of work. Huge masses of snow accumulate on the mountain slopes. From time to time they fall from the slopes, forming avalanches. Such avalanches, moving at tremendous speed, capture rock fragments and carry them down, sweeping away everything in their path. Due to the terrible danger that avalanches pose, they are called “white death”.

The solid material that remains after the snow melts forms huge rocky mounds that block and fill intermountain depressions.

They do even more work glaciers. They occupy enormous areas on Earth - more than 16 million km 2, which is 11% of the land area.

There are continental, or cover, and mountain glaciers. Continental ice occupy vast areas in Antarctica, Greenland, and many polar islands. The ice thickness of continental glaciers varies. For example, in Antarctica it reaches 4000 m. Under the influence of enormous gravity, the ice slides into the sea, breaks off, and icebergs– ice floating mountains.

U mountain glaciers two parts are distinguished - areas of feeding or accumulation of snow and melting. Snow is accumulating in the mountains above snow line. The height of this line is not the same at different latitudes: the closer to the equator, the higher the snow line. In Greenland, for example, it lies at an altitude of 500–600 m, and on the slopes of the Chimborazo volcano in the Andes – 4800 m.

Above the snow line, snow accumulates, becomes compacted and gradually turns into ice. Ice has plastic properties and, under the pressure of the overlying masses, begins to slide down the slope. Depending on the mass of the glacier, its saturation with water and the steepness of the slope, the speed of movement ranges from 0.1 to 8 m per day.

Moving along the slopes of mountains, glaciers plow out potholes, smooth out rock ledges, widen and deepen valleys. The debris that the glacier captures during its movement, when the glacier melts (retreats), remains in place, forming a glacial moraine. Moraine- these are piles of fragments of rocks, boulders, sand, clay left by the glacier. There are bottom, lateral, surface, middle and terminal moraines.

Mountain valleys through which a glacier has ever passed are easy to distinguish: in these valleys the remains of moraines are always found, and their shape resembles a trough. Such valleys are called touches.

Work of flowing waters. Flowing waters include temporary rainfall and snowmelt water, streams, rivers and groundwater. The work of flowing waters, taking into account the time factor, is enormous. We can say that the entire appearance of the earth's surface is, to one degree or another, created by flowing water. All flowing waters are united by the fact that they perform three types of work:

– destruction (erosion);

– transfer of products (transit);

– relation (accumulation).

As a result, various irregularities are formed on the surface of the Earth - ravines, furrows on slopes, cliffs, river valleys, sand and pebble islands, etc., as well as voids in the thickness of rocks - caves.

The action of gravity. All bodies - liquid, solid, gaseous, located on the Earth - are attracted to it.

The force with which a body is attracted to the Earth is called gravity.

Under the influence of this force, all bodies tend to occupy the lowest position on the earth's surface. As a result, water flows arise in rivers, rainwater seeps into the thickness of the earth's crust, snow avalanches collapse, glaciers move, and rock fragments move down the slopes. Gravity - necessary condition actions of external processes. Otherwise, the weathering products would remain at the site of their formation, covering the underlying rocks like a cloak.

§ 21. Minerals and rocks

As you already know, the Earth consists of many chemical elements - oxygen, nitrogen, silicon, iron, etc. By combining with each other, chemical elements form minerals.

Minerals. Most minerals are composed of two or more chemical elements. You can find out how many elements are contained in a mineral by its chemical formula. For example, halite (table salt) is composed of sodium and chlorine and has the formula NCl; magnetite (magnetic iron ore) - from three molecules of iron and two oxygen (F 3 O 2), etc. Some minerals are formed by one chemical element, for example: sulfur, gold, platinum, diamond, etc. Such minerals are called native. About 40 native elements are known in nature, accounting for 0.1% of the mass of the earth’s crust.

Minerals can be not only solid, but also liquid (water, mercury, oil), and gaseous (hydrogen sulfide, carbon dioxide).

Most minerals have a crystalline structure. The crystal shape for a given mineral is always constant. For example, quartz crystals have the shape of a prism, halite has the shape of a cube, etc. If table salt is dissolved in water and then crystallized, the newly formed minerals will take on a cubic shape. Many minerals have the ability to grow. Their sizes range from microscopic to gigantic. For example, a beryl crystal 8 m long and 3 m in diameter was found on the island of Madagascar. Its weight is almost 400 tons.

According to their formation, all minerals are divided into several groups. Some of them (feldspar, quartz, mica) are released from the magma during its slow cooling at great depths; others (sulfur) - when lava cools quickly; third (garnet, jasper, diamond) - at high temperatures and pressure at great depths; the fourth (garnets, rubies, amethysts) are released from hot aqueous solutions in underground veins; fifths (gypsum, salts, brown iron ore) are formed during chemical weathering.

In total, there are more than 2,500 minerals in nature. For their determination and study, physical properties are of great importance, which include luster, color, the color of the trait, i.e., the trace left by the mineral, transparency, hardness, cleavage, fracture, and specific gravity. For example, quartz has a prismatic crystal shape, glassy luster, no cleavage, conchoidal fracture, hardness 7, specific gravity 2.65 g/cm 3 , has no features; Halite has a cubic crystal shape, hardness 2.2, specific gravity 2.1 g/cm3, glass luster, white color, perfect cleavage, salty taste, etc.

Of the minerals, the most famous and widespread are 40–50, which are called rock-forming minerals (feldspar, quartz, halite, etc.).

Rocks. These rocks are an accumulation of one or more minerals. Marble, limestone, and gypsum consist of one mineral, while granite and basalt consist of several. In total, there are about 1000 rocks in nature. Depending on their origin - genesis - rocks are divided into three main groups: igneous, sedimentary and metamorphic.

Igneous rocks. Formed when magma cools; crystal structure, do not have layering; do not contain animal or plant remains. Among igneous rocks, a distinction is made between deep-seated and eruptive. Deep rocks formed deep in the earth's crust, where magma is under high pressure and its cooling occurs very slowly. An example of a plutonic rock is granite, the most common crystalline rock composed primarily of three minerals: quartz, feldspar, and mica. The color of granites depends on the color of the feldspar. Most often they are gray or pink.

When magma erupts onto the surface, it forms erupted rocks. They are either a sintered mass, reminiscent of slag, or glassy, ​​in which case they are called volcanic glass. IN in some cases a fine-crystalline rock such as basalt is formed.

Sedimentary rocks. Cover approximately 80% of the entire surface of the Earth. They are characterized by layering and porosity. As a rule, sedimentary rocks are the result of the accumulation in the seas and oceans of the remains of dead organisms or particles of destroyed solid rocks carried from land. The accumulation process occurs unevenly, so layers of different thicknesses are formed. Fossils or imprints of animals and plants are found in many sedimentary rocks.

Depending on the place of formation, sedimentary rocks are divided into continental and marine. TO continental breeds include, for example, clays. Clay is a crushed product of the destruction of hard rocks. They consist of tiny scaly particles and have the ability to absorb water. Clays are plastic and waterproof. Their colors vary - from white to blue and even black. White clays are used to produce porcelain.

Loess is a rock of continental origin and widespread. It is a fine-grained, non-laminated, yellowish rock consisting of a mixture of quartz, clay particles, lime carbonate and iron oxide hydrates. Easily allows water to pass through.

Marine rocks usually form on the ocean floor. These include some clays, sands, and gravels.

Large group of sedimentary biogenic rocks formed from the remains of dead animals and plants. These include limestones, dolomites and some combustible minerals (peat, coal, oil shale).

Limestone, consisting of calcium carbonate, is especially widespread in the earth's crust. In its fragments one can easily see accumulations of small shells and even skeletons of small animals. The color of limestones varies, most often gray.

Chalk is also formed from the smallest shells - inhabitants of the sea. Huge reserves of this rock are located in the Belgorod region, where along the steep banks of rivers you can see outcrops of thick layers of chalk, distinguished by its whiteness.

Limestones that contain an admixture of magnesium carbonate are called dolomites. Limestones have wide application in construction. Lime for plastering and cement are made from them. The best cement is made from marl.

In those seas where animals with flint shells previously lived and algae containing flint grew, the tripoli rock formed. This is a light, dense, usually yellowish or light gray rock that is a building material.

Sedimentary rocks also include rocks formed by precipitation from aqueous solutions(gypsum, rock salt, potassium salt, brown iron ore, etc.).

Metamorphic rocks. This group of rocks was formed from sedimentary and igneous rocks under the influence of high temperatures, pressure, and chemical changes. Thus, when temperature and pressure act on clay, shales are formed, on sand - dense sandstones, and on limestone - marble. Changes, i.e. metamorphoses, occur not only with sedimentary rocks, but also with igneous rocks. Under the influence of high temperatures and pressure, granite acquires a layered structure and a new rock is formed - gneiss.

High temperature and pressure promote recrystallization of rocks. Sandstones form a very strong crystalline rock - quartzite.

§ 22. Development of the earth's crust

Science has established that more than 2.5 billion years ago, planet Earth was completely covered by ocean. Then, under the influence of internal forces, the uplift of individual sections of the earth's crust began. The uplift process was accompanied by violent volcanism, earthquakes, and mountain building. This is how the first land masses arose - the ancient cores of modern continents. Academician V. A. Obruchev called them "the ancient crown of the Earth."

As soon as the land rose above the ocean, external processes began to act on its surface. Rocks were destroyed, the products of destruction were carried into the ocean and accumulated along its outskirts in the form of sedimentary rocks. The thickness of the sediments reached several kilometers, and under its pressure the ocean floor began to bend. Such giant troughs of the earth's crust under the oceans are called geosynclines. The formation of geosynclines in the history of the Earth has been continuous from ancient times to the present. There are several stages in the life of geosynclines:

– embryonic– deflection of the earth’s crust and accumulation of sediments (Fig. 28, A);

– maturation– filling of the trough with sediments, when their thickness reaches 15–18 km and radial and lateral pressure arises;

– folding– the formation of folded mountains under the pressure of the internal forces of the Earth (this process is accompanied by violent volcanism and earthquakes) (Fig. 28, B);

– attenuation– destruction of the emerging mountains by external processes and the formation in their place of a residual hilly plain (Fig. 28).

Rice. 28. Scheme of the structure of the plain formed as a result of the destruction of mountains (the dotted line shows the reconstruction of the former mountainous country)

Since sedimentary rocks in the geosyncline area are plastic, as a result of the resulting pressure they are crushed into folds. Fold mountains are formed, such as the Alps, Caucasus, Himalayas, Andes, etc.

The periods when active formation of folded mountains occurs in geosynclines are called eras of folding. Several such eras are known in the history of the Earth: Baikal, Caledonian, Hercynian, Mesozoic and Alpine.

The process of mountain building in a geosyncline can also cover non-geosynclinal areas - areas of former, now destroyed mountains. Since the rocks here are hard and lack plasticity, they do not fold into folds, but are broken by faults. Some areas rise, others fall - revived block and folded block mountains appear. For example, during the Alpine era of folding, the folded Pamir mountains were formed and the Altai and Sayan mountains were revived. Therefore, the age of mountains is determined not by the time of their formation, but by the age of the folded base, which is always indicated on tectonic maps.

Geosynclines at different stages of development still exist today. Thus, along the Asian coast of the Pacific Ocean, in the Mediterranean Sea there is a modern geosyncline, which is going through a maturation stage, and in the Caucasus, in the Andes and other folded mountains the process of mountain formation is completing; Kazakh small hills are peneplain, rolling plain, formed on the site of the destroyed mountains of the Caledonian and Hercynian folds. The base of ancient mountains comes to the surface here - small hills - “witness mountains”, composed of durable igneous and metamorphic rocks.

Vast areas of the earth's crust with relatively low mobility and flat topography are called platforms. At the base of the platforms, in their foundations, lie strong igneous and metamorphic rocks, indicating the processes of mountain building that once took place here. Usually the foundation is covered by a thick layer of sedimentary rock. Sometimes basement rocks come to the surface, forming shields. The age of the platform corresponds to the age of the foundation. Ancient (Precambrian) platforms include the East European, Siberian, Brazilian, etc.

The platforms are mostly plains. They experience predominantly oscillatory movements. However, in some cases, the formation of revived block mountains is possible on them. Thus, as a result of the emergence of the Great African Rifts, individual sections of the ancient African platform rose and fell and the block mountains and highlands of East Africa, the volcano mountains Kenya and Kilimanjaro, were formed.

Lithospheric plates and their movement. The doctrine of geosynclines and platforms is called in science "fixism" since, according to this theory, large blocks of bark are fixed in one place. In the second half of the 20th century. many scientists supported theory of mobilism, which is based on the idea of ​​horizontal movements of the lithosphere. According to this theory, the entire lithosphere is divided into giant blocks - lithospheric plates - by deep faults reaching the upper mantle. Boundaries between plates can occur both on land and on the ocean floor. In the oceans, these boundaries are usually mid-ocean ridges. In these areas it was recorded a large number of faults - rifts along which the material of the upper mantle flows to the bottom of the ocean, spreading over it. In those areas where the boundaries between plates pass, mountain building processes are often activated - in the Himalayas, Andes, Cordillera, Alps, etc. The base of the plates is in the asthenosphere, and along its plastic substrate the lithospheric plates, like giant icebergs, slowly move in different directions. directions (Fig. 29). The movement of the plates is recorded by precise measurements from space. Thus, the African and Arabian shores of the Red Sea are slowly moving away from each other, which has allowed some scientists to call this sea the “embryo” of the future ocean. Space images also make it possible to trace the direction of deep faults in the earth's crust.

Rice. 29. Movement of lithospheric plates

The theory of mobilism convincingly explains the formation of mountains, since their formation requires not only radial, but also lateral pressure. Where two plates collide, one of them plunges under the other, and “hummocks”, i.e. mountains, are formed along the collision boundary. This process is accompanied by earthquakes and volcanism.

§ 23. Relief of the globe

Relief- this is a set of irregularities of the earth’s surface, differing in height above sea level, origin, etc.

These irregularities give our planet a unique appearance. The formation of relief is influenced by both internal, tectonic, and external forces. Thanks to tectonic processes, mainly large surface irregularities arise - mountains, highlands, etc., and external forces are aimed at their destruction and the creation of smaller relief forms - river valleys, ravines, dunes, etc.

All relief forms are divided into concave (depressions, river valleys, ravines, gullies, etc.), convex (hills, mountain ranges, volcanic cones, etc.), simply horizontal and inclined surfaces. Their size can be very diverse - from several tens of centimeters to many hundreds and even thousands of kilometers.

Depending on the scale, planetary, macro-, meso- and microforms of relief are distinguished.

Planetary objects include continental protrusions and ocean depressions. Continents and oceans are often antipodes. Thus, Antarctica lies against the Arctic Ocean, North America - against the Indian Ocean, Australia - against the Atlantic, and only South America– against Southeast Asia.

The depths of oceanic depressions vary widely. The average depth is 3800 m, and the maximum, noted in the Mariana Trench of the Pacific Ocean, is 11,022 m. The highest point of land - Mount Everest (Qomolungma) reaches 8848 m. Thus, the height amplitude reaches almost 20 km.

The prevailing depths in the ocean are from 3000 to 6000 m, and the heights on land are less than 1000 m. High mountains and deep-sea depressions occupy only a fraction of a percent of the Earth's surface.

The average height of the continents and their parts above ocean level is also different: North America - 700 m, Africa - 640, South America - 580, Australia - 350, Antarctica - 2300, Eurasia - 635 m, with the height of Asia 950 m, and Europe - only 320 m. Average land height 875 m.

Relief of the ocean floor. On the ocean floor, as on land, there are various landforms - mountains, plains, depressions, trenches, etc. They usually have softer outlines than similar landforms, since external processes proceed more calmly here.

The relief of the ocean floor includes:

– continental shelf, or shelf (shelf), – shallow part up to a depth of 200 m, the width of which in some cases reaches many hundreds of kilometers;

– continental slope– a rather steep ledge to a depth of 2500 m;

– ocean bed, which occupies most of the bottom with depths up to 6000 m.

The greatest depths were noted in gutters, or oceanic depressions, where they exceed 6000 m. The trenches usually stretch along continents along the margins of the ocean.

In the central parts of the oceans there are mid-ocean ridges (rifts): South Atlantic, Australian, Antarctic, etc.

Land relief. The main elements of land relief are mountains and plains. They form the macrorelief of the Earth.

Mountain called a hill that has a summit point, slopes, and a bottom line rising above the terrain above 200 m; an elevation up to 200 m high is called hill. Linearly elongated landforms with a ridge and slopes are mountain ranges. The ridges are separated by those located between them mountain valleys. Connecting with each other, mountain ranges form mountain ranges. A set of ridges, chains and valleys is called mountain node, or mountainous country, and in everyday life - mountains. For example, the Altai Mountains, the Ural Mountains, etc.

Vast areas of the earth's surface consisting of mountain ranges, valleys and high plains are called highlands. For example, the Iranian Plateau, the Armenian Plateau, etc.

The origin of mountains is tectonic, volcanic and erosive.

Tectonic mountains formed as a result of movements of the earth's crust, they consist of one or many folds raised to a considerable height. All the highest mountains in the world - the Himalayas, Hindu Kush, Pamir, Cordillera, etc. - are folded. They are characterized by pointed peaks, narrow valleys (gorges), and elongated ridges.

Blocky And fold-block mountains are formed as a result of the rise and fall of blocks (blocks) of the earth's crust along fault planes. The relief of these mountains is characterized by flat peaks and watersheds, wide, flat-bottomed valleys. These are, for example, the Ural Mountains, Appalachians, Altai, etc.

Volcanic mountains are formed as a result of the accumulation of products of volcanic activity.

Quite widespread on the Earth's surface eroded mountains, which are formed as a result of the dismemberment of high plains by external forces, primarily flowing waters.

By height, mountains are divided into low (up to 1000 m), medium-high (from 1000 to 2000 m), high (from 2000 to 5000 m) and highest (above 5 km).

The height of mountains can be easily determined from a physical map. It can also be used to determine that most of the mountains belong to the mid-altitude and high range. Few peaks rise above 7000 m, and all of them are in Asia. Only 12 mountain peaks, located in the Karakoram mountains and the Himalayas, have a height of more than 8000 m. Highest point planet is a mountain, or, more precisely, a mountain node, Everest (Qomolungma) - 8848 m.

Most of the land surface is occupied by flat areas. Plains- these are areas of the earth's surface that have a flat or slightly hilly topography. Most often the plains are slightly sloping.

Based on the nature of the surface, plains are divided into flat, wavy And hilly, but on vast plains, for example Turanian or West Siberian, one can find areas with various forms of surface relief.

Depending on the height above sea level, the plains are divided into low-lying(up to 200 m), sublime(up to 500 m) and high (plateaus)(over 500 m). Elevated and high plains are always heavily dissected by water flows and have a hilly topography, while low-lying ones are often flat. Some plains are located below sea level. Thus, the Caspian lowland has a height of 28 m. Closed basins of great depth are often found on the plains. For example, the Karagis depression has an elevation of 132 m, and the Dead Sea depression has an elevation of 400 m.

Elevated plains bounded by steep escarpments separating them from the surrounding area are called plateau. These are the plateaus of Ustyurt, Putorana, etc.

Plateau- flat-topped areas of the earth's surface can have a significant height. For example, the Tibet plateau rises above 5000 m.

Based on their origin, there are several types of plains. Significant land areas are occupied by marine (primary) plains, formed as a result of marine regressions. These are, for example, the Turanian, West Siberian, Great Chinese and a number of other plains. Almost all of them belong to the great plains of the planet. Most of them are lowlands, the terrain is flat or slightly hilly.

Stratified plains- These are flat areas of ancient platforms with almost horizontal occurrence of layers of sedimentary rocks. Such plains include, for example, the East European. These plains mostly have hilly terrain.

Small spaces in river valleys are occupied by alluvial (alluvial) plains, formed as a result of leveling the surface with river sediments - alluvium. This type includes the Indo-Gangetic, Mesopotamian, and Labrador plains. These plains are low, flat, and very fertile.

The plains are raised high above sea level - lava sheets(Central Siberian Plateau, Ethiopian and Iranian Plateaus, Deccan Plateau). Some plains, for example the Kazakh small hills, were formed as a result of the destruction of mountains. They are called erosive. These plains are always elevated and hilly. These hills are composed of durable crystalline rocks and represent the remains of the mountains that were once here, their “roots”.

§ 24. Soil

The soil– this is the upper fertile layer of the lithosphere, which has a number of properties inherent in living and inanimate nature.

The formation and existence of this natural body cannot be imagined without living beings. The surface layers of rock are only the initial substrate from which various types of soils are formed under the influence of plants, microorganisms and animals.

The founder of soil science, Russian scientist V.V. Dokuchaev, showed that

the soil is an independent natural body formed on the surface of rocks under the influence of living organisms, climate, water, relief, and also humans.

This natural formation has been created over thousands of years. The process of soil formation begins with the settlement of microorganisms on bare rocks and stones. Feeding on carbon dioxide, nitrogen and water vapor from the atmosphere, using mineral salts of rock, microorganisms release organic acids as a result of their vital activity. These substances gradually change the chemical composition of rocks, making them less durable and ultimately loosening the surface layer. Then lichens settle on such rock. Unpretentious to water and nutrients, they continue the process of destruction, while simultaneously enriching the rock with organic substances. As a result of the activity of microorganisms and lichens, the rock gradually turns into a substrate suitable for colonization by plants and animals. The final transformation of the original rock into soil occurs due to the vital activity of these organisms.

Plants absorb carbon dioxide from the atmosphere and water and minerals, create organic compounds. As plants die, they enrich the soil with these compounds. Animals feed on plants and their remains. The products of their vital activity are excrement, and after death their corpses also end up in the soil. The entire mass of dead organic matter accumulated as a result of the vital activity of plants and animals serves as a food supply and habitat for microorganisms and fungi. They destroy organic substances and mineralize them. As a result of the activity of microorganisms, complex organic substances are formed that make up soil humus.

Soil humus is a mixture of stable organic compounds formed during the decomposition of plant and animal residues and their metabolic products with the participation of microorganisms.

In the soil, primary minerals decompose and clay secondary minerals form. Thus, the cycle of substances occurs in the soil.

Moisture capacity is the soil's ability to hold water.

Soil with a lot of sand does not retain water well and has low moisture holding capacity. Clay soil, on the other hand, holds a lot of water and has a high moisture holding capacity. In case of heavy rainfall, water fills all the pores in such soil, preventing air from passing deeper. Loose, lumpy soils retain moisture better than dense soils.

Moisture permeability- This is the ability of the soil to pass water.

The soil is permeated with tiny pores - capillaries. Water can move through capillaries not only downwards, but also in all directions, including from bottom to top. The higher the capillarity of the soil, the higher its moisture permeability, the faster water penetrates the soil and rises up from deeper layers. Water “sticks” to the walls of the capillaries and seems to creep upward. The thinner the capillaries, the higher the water rises through them. When the capillaries reach the surface, the water evaporates. Sandy soils have high moisture permeability, while clay soils have low permeability. If after rain or watering a crust (with many capillaries) has formed on the surface of the soil, the water evaporates very quickly. When loosening the soil, capillaries are destroyed, which reduces water evaporation. It’s not for nothing that loosening the soil is called dry watering.

Soils can have different structures, that is, they can consist of lumps of different shapes and sizes into which soil particles are glued. The best soils, such as chernozems, have a fine-lumpy or granular structure. By chemical composition soils can be rich or poor in nutrients. An indicator of soil fertility is the amount of humus, since it contains all the basic elements of plant nutrition. For example, chernozem soils contain up to 30% humus. Soils can be acidic, neutral and alkaline. Neutral soils are most favorable for plants. To reduce acidity, they are limed, and gypsum is added to the soil to reduce alkalinity.

Mechanical composition of soils. Based on their mechanical composition, soils are divided into clayey, sandy, loamy and sandy loam.

Clay soils have high moisture capacity and are best provided with batteries.

Sandy soils low moisture capacity, well permeable to moisture, but poor in humus.

Loamy– the most favorable in terms of their physical properties for agriculture, with average moisture capacity and moisture permeability, well provided with humus.

Sandy loam– structureless soils, poor in humus, well permeable to water and air. To use such soils, it is necessary to improve their composition and apply fertilizers.

Soil types. The most common soil types in our country are: tundra, podzolic, sod-podzolic, chernozem, chestnut, gray soil, red soil and yellow soil.

Tundra soils are located in the Far North in the permafrost zone. They are waterlogged and extremely poor in humus.

Podzolic soils common in the taiga under coniferous trees, and sod-podzolic– under coniferous-deciduous forests. Broadleaf forests grow on gray forest soils. All these soils contain enough humus and are well structured.

In the forest-steppe and steppe zones there are chernozem soils. They were formed under steppe and grassy vegetation and are rich in humus. Humus gives the soil a black color. They have a strong structure and high fertility.

Chestnut soils located further south, they form in drier conditions. They are characterized by a lack of moisture.

Serozem soils characteristic of deserts and semi-deserts. They are rich in nutrients, but poor in nitrogen, and there is not enough water.

Krasnozems And zheltozems are formed in the subtropics under humid and warm climates. They are well structured, quite moisture-absorbing, but have a lower humus content, so fertilizers are added to these soils to increase fertility.

To increase soil fertility, it is necessary to regulate not only the nutrient content in them, but also the presence of moisture and aeration. The topsoil should always be loose to provide air access to the roots of the plants.

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The earth's crust only seems motionless, absolutely stable. In fact, she makes continuous and varied movements. Some of them occur very slowly and are not perceived by the human senses, others, such as earthquakes, are landslide and destructive. What titanic forces set the earth's crust in motion?

The internal forces of the Earth, the source of their origin. It is known that at the boundary of the mantle and lithosphere the temperature exceeds 1500 °C. At this temperature, matter must either melt or turn into gas. When solids transform into a liquid or gaseous state, their volume must increase. However, this does not happen, since the overheated rocks are under pressure from the overlying layers of the lithosphere. A “steam boiler” effect occurs when matter, seeking to expand, presses on the lithosphere, causing it to move along with the earth’s crust. Moreover, the higher the temperature, the stronger the pressure and the more active the lithosphere moves. Particularly strong pressure centers arise in those places in the upper mantle where radioactive elements are concentrated, the decay of which heats the constituent rocks to even higher temperatures. Movements of the earth's crust under the influence of the internal forces of the Earth are called tectonic. These movements are divided into oscillatory, folding and bursting.

Oscillatory movements. These movements occur very slowly, imperceptibly for humans, which is why they are also called centuries-old or epeirogenic. In some places the earth's crust rises, in others it falls. In this case, the rise is often replaced by a fall, and vice versa. These movements can be traced only by the “traces” that remain after them on the earth’s surface. For example, on the Mediterranean coast, near Naples, there are the ruins of the Temple of Serapis, the columns of which were worn away by sea mollusks at an altitude of up to 5.5 m above modern sea level. This serves as absolute proof that the temple, built in the 4th century, was at the bottom of the sea, and then it was raised. Now this area of ​​land is sinking again. Often on the coasts of the seas there are steps above their current level - sea terraces, once created by the surf. On the platforms of these steps you can find the remains of marine organisms. This indicates that the terrace areas were once the bottom of the sea, and then the shore rose and the sea retreated.

The descent of the earth's crust below 0 m above sea level is accompanied by the advance of the sea - transgression, and the rise - by his retreat - regression. Currently in Europe, uplifts occur in Iceland, Greenland, and the Scandinavian Peninsula. Observations have established that the region of the Gulf of Bothnia is rising at a rate of 2 cm per year, i.e. 2 m per century. At the same time, the territory of Holland, Southern England, Northern Italy, the Black Sea Lowland, and the coast of the Kara Sea is subsiding. A sign of the subsidence of sea coasts is the formation of sea bays in the estuaries of rivers - estuaries (lips) and estuaries.

When the earth's crust rises and the sea retreats, the seabed, composed of sedimentary rocks, turns out to be dry land. This is how extensive marine (primary) plains: for example, West Siberian, Turanian, North Siberian, Amazonian (Fig. 20).

Rice. 20. The structure of primary, or marine, strata plains

Folding movements. In cases where rock layers are sufficiently plastic, under the influence of internal forces they collapse into folds. When the pressure is directed vertically, the rocks are displaced, and if in the horizontal plane, they are compressed into folds. The shape of the folds can be very diverse. When the bend of the fold is directed downward, it is called a syncline, upward - an anticline (Fig. 21). Folds form at great depths, i.e. at high temperatures and high pressure, and then under the influence of internal forces they can be lifted. This is how they arise fold mountains Caucasian, Alps, Himalayas, Andes, etc. (Fig. 22). In such mountains, folds are easy to observe where they are exposed and come to the surface.

Rice. 21. Synclinal (1) and anticlinal (2) folds

Rice. 22. fold mountains

Breaking movements. If the rocks are not strong enough to withstand the action of internal forces, cracks (faults) form in the earth's crust and vertical displacement of the rocks occurs. The sunken areas are called grabens, and those who rose - handfuls(Fig. 23). The alternation of horsts and grabens creates block (revived) mountains. Examples of such mountains are: Altai, Sayan, Verkhoyansk Range, Appalachians in North America and many others. Revived mountains differ from folded ones both in internal structure and in appearance - morphology. The slopes of these mountains are often steep, the valleys, like the watersheds, are wide and flat. Rock layers are always displaced relative to each other.

Rice. 23. Revived fold-block mountains

The sunken areas in these mountains, grabens, sometimes fill with water, and then deep lakes are formed: for example, Baikal and Teletskoye in Russia, Tanganyika and Nyasa in Africa.

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To detect and record all types of seismic waves, special seismograph devices are used. Some seismographs are sensitive to horizontal movements, others to vertical ones. The waves are recorded by a vibrating pen on a moving paper tape. There are also electronic seismographs (without paper tape).

During the late Han era, imperial astronomer Zhang Heng (78-139) invented the world's first seismoscope, which detected small earthquakes over long distances. This device has not survived to this day. Its design can be judged from an incomplete description in the Hou Han shu (History of the Second Han). Modern reconstruction of a seismograph made by Zhang Heng in 132 AD

Snakes, especially poisonous ones, in anticipation of an approaching earthquake, leave their inhabited holes within a few days. Lizards and ants do the same. Some scientists tend to explain this indisputable fact by the high sensitivity of the skin to temperature changes in the soil.

Earthquakes can be predicted by the behavior of plankton, says a group of scientists from India and the United States. They found that before strong underwater shocks, the smallest ocean plants actively turn green. According to the BBC, this conclusion is confirmed by satellite images taken shortly before four recent disasters - in the Indian state of Gujarat, the Andaman Islands, Algeria and Iran.

1)§ 18, read, retell 2) p. 49 answers to questions orally 3) On the k/k, mark with shading the areas that are characterized by earthquakes. 4) Workbook (page).