Theme "Lithosphere"

in 7th grade

K.S. LAZAREVICH

How to conduct literate,
interesting and meaningful lessons
on upcoming topics

The boundaries of the lithosphere

The course of geography in the 7th grade begins with the fact that students return to topics that seemed to be studied in the 6th grade - the lithosphere, atmosphere, hydrosphere. This beginning of the course already shows how unreliable, unsteady the knowledge gained in the first year of geography. And for the 7th grade, these topics are quite complicated, but there is no need to talk about the 6th grade. We will try to analyze the difficulties that are encountered in the first topics of the 7th grade. At the same time, we will return to the textbooks of the previous year of study, clarify and correct some of the provisions found there.

Term lithosphere has been used in science for a long time - probably since the middle of the 19th century. But it acquired its modern significance less than half a century ago. Even in the geological dictionary of the 1955 edition it is said:
LITHOSPHERE - the same as Earth's crust.
In the dictionary of the 1973 edition and in subsequent ones we already read:
The LITHOSPHERE ... in the modern sense includes the earth's crust ... and the rigid upper part of the Earth's upper mantle.

We draw the reader's attention to the wording: the upper part of the upper mantle. Meanwhile, in one of the textbooks in the figure it is indicated: "The lithosphere (the earth's crust and upper mantle)", and according to the figure it turns out that the entire mantle, which is not part of the lithosphere, is the lower one (Krylova 6, p. 50, fig. 30 ). By the way, in the same textbook in the text (p. 49) and in the textbook for the 7th grade (Krylova 7, p. 9) everything is correct: it is said about the upper part of the mantle. Upper mantle is a geological term for a very large layer; the upper mantle has a thickness (thickness) of up to 500, according to some classifications - over 900 km, and the lithosphere includes only the upper ones from several tens to two hundred kilometers. All this is difficult not only for students, but also for teachers. It would be better to abandon the term at school altogether lithosphere, limiting itself to mentioning the earth's crust; but here lithospheric plates arise, and there is no way without the lithosphere. Perhaps rice will help. 1, it is easy to redraw it in an enlarged form. Speaking of the lithosphere, one must firmly remember that it includes the earth's crust and the upper, relatively thin layer of the mantle, but not the upper mantle- the last term is much broader.

Layers of the lithosphere

The earth's crust, with tenacity worthy of a better application, is continued in all textbooks to be divided into three layers - sedimentary, granite and basalt. And it's time to change the record.
Most of the information about the deep structure of the Earth was obtained from indirect, geophysical data - from the propagation velocities of seismic waves, from changes in the magnitude and direction of gravity (insignificant, perceptible only by very accurate instruments), from magnetic properties and the magnitude of the electrical conductivity of rocks. The mass of dense rocks in the same volume is greater than less dense rocks, they create an increased gravitational field. In dense rocks, shock waves travel faster (recall that sound travels noticeably faster in water than in air). Passing through rocks with different physical properties, waves are reflected, refracted, and absorbed. Waves are transverse and longitudinal, the speed of their propagation is different. Explore the passage of natural shock waves during earthquakes, create these waves artificially, producing explosions.
From all these data, a picture of the distribution over the area and in depth of rocks with different physical properties is formed. On its basis, a model of the structure of the Earth's interior is created: rocks are selected whose physical properties more or less coincide with the properties determined using indirect methods, and these rocks are mentally placed at the appropriate depth. When it is possible to drill a well to a depth previously inaccessible, or to obtain some other reliable data, this model is confirmed in whole or in part. It happens that it is not confirmed at all, you have to build a new one. After all, it is by no means excluded that rocks lie at depth that we do not meet on the surface at all, or that at depth, at high temperature and pressure, the properties of rocks well known to us will change beyond recognition.
In 1909, the Serbian geophysicist Andrei Mohoro'vich noticed that at a depth of 54 km, the velocities of seismic waves increase sharply, abruptly. Subsequently, this jump was traced throughout the globe at depths from 5 to 90 km and is now known as the Mohorovichich boundary (or surface), in short, the Moho boundary, even shorter, the M boundary. The M surface is considered the lower boundary of the earth's crust. An important feature of this surface is that, in general terms, it is like a mirror reflection of the relief of the earth's surface: it is higher under the oceans, lower under the continental plains, lower than everything under the highest mountains (these are the so-called mountain roots).
This feature of the earth's crust, probably, will not be difficult to explain to schoolchildren by letting several pieces of wood of different shapes, preferably heavy, so that they go into the water by 2 / 3 - 3 / 4, float in a transparent vessel with water; those of them that protrude above the water will also be deeper submerged (Fig. 2).

Rice. 2. Experience explaining the ratio between the upper and lower boundaries of the earth's crust

According to the traditional concept of the structure of the earth's crust, which can be read in any textbook, it is customary to distinguish three main layers in the composition of the earth's crust. The upper of them is composed mainly of sedimentary rocks and is called sedimentary. The two lower layers are called "granite" and "basalt". Accordingly, two types of the earth's crust are distinguished. continental crust contains all three layers and has a thickness of 35-50 km, under the mountains up to 90 km. In the oceanic crust, the sedimentary layer has a much smaller thickness, and the middle, "granite" layer is absent; the thickness of the oceanic crust is 5–10 km (Fig. 3). Between the "granite" and "basalt" layers lies the Konrad boundary, named after the Austrian geophysicist who discovered it; it is not mentioned in school textbooks.

But research over the past two decades has shown that this well-proportioned, easy-to-remember scheme does not fit well with reality. "Granite" and "basalt" layers consist mainly of igneous and metamorphic rocks. At the Konrad boundary, there is an abrupt increase in seismic wave velocities. Such an increase in velocities can be expected during the transition of waves from rocks with a density of 2.7 to rocks with a density of 3 g/cm 3 , which approximately corresponds to the densities of granite and basalt. Therefore, the overlying layer was called "granite", and the underlying "basalt". But note: these names are in quotation marks everywhere. Geophysicists did not consider these layers to be composed of granite and basalt, they only talked about some analogy. However, even many geologists could not resist the temptation to believe that the "granite" layer is really from granite, and the "basalt" layer is from basalt. What can we say about the authors of school textbooks!
Korinskaya, p. 20, fig. 8. Signatures to the conventional signs: “A layer of sedimentary rocks. layer of granite. layer of basalt.
Petrova, p. 47-48. “We are entering the granite layer of the Earth. Granite ... was formed from magma in the thickness of the earth's crust ... We are entering a layer of basalt - a rock of deep origin. (By the way, this is not true: basalt is not deep, but outflowing rock.)
Finarov, p. 15 and Krylova 7, p. 10, fig. 1 - the granite and basalt layers are named without quotes, and the student clearly sees that they consist of these rocks.
The necessary reservation is made only in one textbook, but is it sufficient to draw attention to it?
“In the mainland [crust] lies a layer called granite. It is composed of igneous and metamorphic rocks, similar in composition and density to granites ... The lower layer of the earth's crust is a layer conventionally called basalt; it ... consists of rocks whose density is close to that of basalts” (Krylova, Gerasimova, p. 10).
One of the tasks of the Kola superdeep well was to reach the Konrad boundary, which, according to geophysical data, lies in this place at a depth of 7-8 km. And perhaps the most important geological result of drilling was the proof of the absence of the Konrad boundary in its geological understanding: in which rocks the well went above the boundary established by geophysicists, in the same rocks it passed several kilometers below it.

And the geophysical fate at the Konrad boundary turned out to be not as glorious as that at the Mohorovichich boundary. In some places it was singled out confidently, in other places - less confidently (whether she was alone, or not alone), somewhere they were not found at all. There was a need to abandon the terms "granite layer" and "basalt layer", even if in quotation marks, and to recognize that the Conrad boundary does not exist. The modern model of the structure of the earth's crust looks much more complicated than the classical three-layer model (Fig. 4). It still has continental and oceanic crust. Characteristic features of the continental crust can be considered a significant (tens of kilometers) thickness, an increase in density from top to bottom - gradual or spasmodic; the sedimentary layer within the continental crust is usually thicker than within the oceanic one. The oceanic crust is much thinner, more homogeneous in composition; in relation to it, one can speak of a basalt layer without quotes, since the ocean floor is composed mainly of basalts.

For more details see: I.N. Galkin. Into the ocean behind the bark//Geography, No. 42/97, p. 6-7, 13.
** For more details see: T.S. Mints, M.V. Mints. Kola Superdeep//Geography, No. 33/99, p. 1-4.

Theory of lithospheric plates

This theory is usually very attractive to students. She is elegant and seems to explain everything. Many of the perplexities that arise among scientists in connection with it relate to issues so complex that it is not even worth talking about them at school (for example, which of the non-specialists will be able to assess the legitimacy of the doubts that arise in connection with the redistribution of the heat flow from the bowels of the Earth to the surface? ). But students must be told that there are unresolved problems in this theory, which, perhaps, will force them to reconsider it - most likely not entirely, but in some details.
According to the texts of textbooks, schoolchildren can conclude that plate tectonics is a refinement of Alfred Wegener's hypothesis, which peacefully replaced it. Actually it is not. Wegener has continents composed of a relatively light substance, which he called sial(silicium-aluminum), as if floating on the surface of a heavier substance - sima(silicium-magnesium). At first, the hypothesis captivated almost everyone, it was accepted with enthusiasm. But after 2-3 decades, it turned out that the physical properties of the rocks do not allow such navigation, and a fat cross was put on the theory of continental drift. And as often happens, the baby was thrown out with the water: the theory is bad, which means that the continents cannot move at all. Only by the 60s, that is, only 40-45 years ago, when the global system of mid-ocean ridges had already been discovered, they built a practically new theory, in which only a change in the relative position of the continents remained from Wegener's hypothesis, in particular, an explanation of the similarity of the outlines of the continents on both sides of the Atlantic.
The most important difference between modern plate tectonics and Wegener's hypothesis is that Wegener continents moved along the substance that made up the ocean floor, in the modern theory plates are involved in the movement, which include areas of both land and the ocean floor; The boundaries between plates can run along the bottom of the ocean, and on land, and along the boundaries of continents and oceans.
The movement of lithospheric plates occurs along the asthenosphere - a layer of the upper mantle that underlies the lithosphere and has viscosity and plasticity. Mention of the asthenosphere in the texts of textbooks could not be found, but in one textbook, not only the asthenosphere, but also “the layer of the mantle above the asthenosphere” is signed on the figure (Finarov, p. 16, Fig. 4). It is not worth mentioning the asthenosphere in the lessons, the structure of the upper layers of the Earth is already quite complicated.
The textbooks explain that along the axes of the mid-ocean ridges, the areas of lithospheric plates gradually increase. This process has been named spreading(English spreading expansion, distribution). But the surface of the globe cannot increase. The emergence of new sections of the earth's crust on the sides of the mid-ocean ridges must be compensated for by its disappearance somewhere. If we believe that lithospheric plates are sufficiently stable, it is natural to assume that the disappearance of the crust, as well as the formation of a new one, should occur at the boundaries of approaching plates. In this case, there can be three different cases:
- two parts of the oceanic crust are approaching;
- a section of the continental crust is approaching a section of the oceanic;
- two sections of the continental crust are approaching.
The process that occurs when parts of the oceanic crust approach each other can be schematically described as follows: the edge of one plate rises somewhat, forming an island arc; the other goes under it, here the level of the upper surface of the lithosphere decreases, and a deep-water oceanic trench is formed. Such are the Aleutian Islands and the Aleutian Trench framing them, the Kuril Islands and the Kuril-Kamchatka Trench, the Japanese Islands and the Japanese Trench, the Mariana Islands and the Mariana Trench, etc.; All this in the Pacific Ocean. In the Atlantic - the Antilles and the Puerto Rico Trench, the South Sandwich Islands and the South Sandwich Trench. The movement of plates relative to each other is accompanied by significant mechanical stresses, therefore, in all these places, high seismicity and intense volcanic activity are observed. The sources of earthquakes are located mainly on the surface of contact between two plates and can be at great depths. The edge of the plate, which has gone deep, plunges into the mantle, where it gradually turns into mantle matter. The subducting plate is heated, and magma is melted out of it, which erupts in island arc volcanoes (Fig. 5).

The process of submerging one plate under another is called subduction(literally - pushing). This Latin term, like the English word "spreading" above, is widely used, both appear in popular literature, so teachers need to know them, but it hardly makes sense to introduce them in a school course.
When sections of the continental and oceanic crust move towards each other, the process proceeds approximately in the same way as in the case of a meeting of two sections of the oceanic crust, only instead of an island arc, a powerful chain of mountains is formed along the coast of the mainland. The oceanic crust is also submerged under the continental edge of the plate, forming deep-sea trenches, volcanic and seismic processes are just as intense. Magma that does not reach the earth's surface crystallizes, forming granitic batholiths (Fig. 6). A typical example is the Cordillera of Central and South America and the system of trenches running along the coast - Central American, Peruvian and Chilean.

When two sections of the continental crust approach each other, the edge of each of them experiences folding, faults, mountains are formed, and seismic processes are intense. Volcanism is also observed, but less than in the first two cases, since the earth's crust in such places is very thick (Fig. 7). This is how the Alpine-Himalayan mountain belt was formed, stretching from North Africa and the western tip of Europe through all of Eurasia to Indochina; it includes the highest mountains on Earth, high seismicity is observed along its entire length, and there are active volcanoes in the west of the belt.
Several textbooks contain diagrams of the position of the continents so many millions of years ago.

In one book (Krylova 7, p. 21, fig. 12) the location of the continents after 50 million years is given. If this textbook is used, it would be worth commenting on the scheme, saying beforehand that this is only a forecast, a very approximate one, which will be justified only if the general direction of movement of the plates is preserved, and there is no major restructuring of them. According to the forecast, the Atlantic Ocean, the East African Rifts (they will be filled with the waters of the World Ocean) and the Red Sea will expand significantly, which will directly connect the Mediterranean Sea with the Indian Ocean.

Thus, when checking whether schoolchildren remember well the topic “Lithosphere” in the 6th grade, it is necessary to simultaneously dispel some misconceptions that could arise. If you want to give students the basics of knowledge at a modern level, you will have to, while explaining new, more complex material, abandon the presentation of outdated information given in textbooks.
Here are the main theses that need to be stated and explained.
1. The lithosphere includes the earth's crust and the upper, relatively small part of the mantle.
2. The earth's crust is of two types - continental and oceanic.
3. The continental crust has a significant (tens of kilometers) thickness, its density increases downwards. The crust consists of sedimentary rocks (usually at the top), below are igneous and metamorphic rocks of various compositions.
4. The thickness of the oceanic crust is 5-10 km, it is composed mainly of basalts.
(When explaining the structure of the continental and oceanic crust, the "granite" and "basalt" layers, and even more so the Konrad boundary, should not be mentioned.)
5. The theory of plate tectonics came to replace Wegener's hypothesis only after the hypothesis was completely rejected.
6. According to Wegener's hypothesis, the continents moved along the denser matter that makes up the ocean floor.
7. According to the theory of lithospheric plates, large areas of the lithosphere with continental crust, or oceanic, or both, are involved in the movement.
Different types of interaction of lithospheric plates with different types of the earth's crust may or may not be considered by the teacher, depending on the degree of preparedness of the class. These examples are interesting, they can be illustrated on the physical map of the world, but they are not included in the compulsory program.

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Korinskaya - V.A. Korinskaya, I.V. Dushina, V.A. Shchenev. Geography of continents and oceans: Proc. for 7 cells. avg. school - M.: Enlightenment, 1993. - 287 p.
Krylova 6 - O.V. Krylov. Physical geography: Beginning. course: Proc. for 6 cells. general education institutions. - M.: Enlightenment, 1999 (and subsequent editions). - 192 p.
Krylova 7 - O.V. Krylov. Continents and oceans: Proc. for 7 cells. general education institutions. Moscow: Education, 1999 (and subsequent editions). - 304 p.
Krylova, Gerasimova - O.V. Krylova, T.P. Gerasimov. Geography of continents and oceans: Prob. textbook for 7 cells. general education institutions. - M.: Enlightenment, 1995. - 318 p.
Petrova - N.N. Petrov. Geography. Initial course. Grade 6: Proc. for general education textbook establishments. - M.: Bustard; DiK, 1997. - 256 p.
Finarov - D.P. Finarov, S.V. Vasiliev, Z.I. Shipunova, E.Ya. Chernikhov. Geography of continents and oceans: Proc. for 7 cells. general education institutions. - M.: Enlightenment, 1996. - 302 p.

Lithosphere. Earth's crust. 4.5 billion years ago, the Earth was a ball consisting of some gases. Gradually, heavy metals such as iron and nickel sank to the center and condensed. Light rocks and minerals floated to the surface, cooled and hardened.

The internal structure of the Earth.

It is customary to divide the body of the Earth into three main parts - lithosphere(earth crust) mantle and nucleus.

The core is the center of the earth , the average radius of which is about 3500 km (16.2% of the volume of the Earth). As suggested, it consists of iron with an admixture of silicon and nickel. The outer part of the core is in a molten state (5000 °C), the inner, apparently, is solid (subnucleus). The movement of matter in the core creates a magnetic field on Earth that protects the planet from cosmic radiation.

The core is changing mantle , which extends almost 3000 km (83% of the Earth's volume). It is believed that it is solid, at the same time plastic and red-hot. The mantle is made up of three layers: Golitsyn layer, Gutenberg layer and substrate. The upper part of the mantle, called magma , contains a layer with reduced viscosity, density and hardness - the asthenosphere, on which sections of the earth's surface are balanced. The boundary between the mantle and the core is called the Gutenberg layer.

Lithosphere

Lithosphere - the upper shell of the "solid" Earth, including the earth's crust and the upper part of the underlying upper mantle of the Earth.

Earth's crust - the upper shell of the "solid" Earth. The thickness of the earth's crust is from 5 km (under the oceans) to 75 km (under the continents). The earth's crust is heterogeneous. It distinguishes 3 layers – sedimentary, granite, basalt. The granite and basalt layers are so named because they contain rocks similar in physical properties to granite and basalt.

Compound the earth's crust: oxygen (49%), silicon (26%), aluminum (7%), iron (5%), calcium (4%); the most common minerals are feldspar and quartz. The boundary between the earth's crust and mantle is called moho surface .

Distinguish continental and oceanic earth's crust. Oceanic different from the continental (mainland) lack of granite layer and much lower power (from 5 to 10 km). Thickness continental crust on the plains 35-45 km, in the mountains 70-80 km. On the border of the continents and oceans, in the areas of the islands, the thickness of the earth's crust is 15-30 km, the granite layer is wedged out.

The position of the layers in the continental crust indicates different time of its formation . The basalt layer is the oldest, younger than it is granite, and the youngest is the upper, sedimentary, developing at the present time. Each layer of the crust was formed over a long period of geological time.

Lithospheric plates

The earth's crust is in constant motion. The first hypothesis about continental drift(i.e. the horizontal movement of the earth's crust) put forward at the beginning of the twentieth century A. Wegener. On its basis, created theory of lithospheric plates . According to this theory, the lithosphere is not a monolith, but consists of seven large and several smaller plates "floating" on the asthenosphere. The boundary regions between lithospheric plates are called seismic belts - these are the most "restless" areas of the planet.

The earth's crust is divided into stable and mobile sections.

Stable areas of the earth's crust - platforms- are formed at the site of geosynclines that have lost their mobility. The platform consists of a crystalline basement and a sedimentary cover. Depending on the age of the foundation, ancient (Precambrian) and young (Paleozoic, Mesozoic) platforms are distinguished. Ancient platforms lie at the base of all continents.

Mobile, highly dissected parts of the earth's surface are called geosynclines ( folded areas ). In their development, there are two stages : at the first stage, the earth's crust experiences subsidence, sedimentary rocks accumulate and metamorphize. Then the uplift of the earth's crust begins, the rocks are crushed into folds. There were several epochs of intensive mountain building on Earth: Baikal, Caledonian, Hercynian, Mesozoic, Cenozoic. In accordance with this, different areas of folding are distinguished.

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Comment

The lithosphere is the stone shell of the Earth. From the Greek "lithos" - a stone and "sphere" - a ball

The lithosphere is the outer solid shell of the Earth, which includes the entire earth's crust with part of the Earth's upper mantle and consists of sedimentary, igneous and metamorphic rocks. The lower boundary of the lithosphere is fuzzy and is determined by a sharp decrease in rock viscosity, a change in the propagation velocity of seismic waves, and an increase in the electrical conductivity of rocks. The thickness of the lithosphere on the continents and under the oceans varies and averages 25 - 200 and 5 - 100 km, respectively.

Consider in general terms the geological structure of the Earth. The third planet farthest from the Sun - the Earth has a radius of 6370 km, an average density of 5.5 g / cm3 and consists of three shells - bark, robes and i. The mantle and core are divided into inner and outer parts.

The Earth's crust is a thin upper shell of the Earth, which has a thickness of 40-80 km on the continents, 5-10 km under the oceans and makes up only about 1% of the Earth's mass. Eight elements - oxygen, silicon, hydrogen, aluminum, iron, magnesium, calcium, sodium - form 99.5% of the earth's crust.

According to scientific research, scientists were able to establish that the lithosphere consists of:

• Oxygen - 49%;
• Silicon - 26%;
• Aluminum - 7%;
• Iron - 5%;
• Calcium - 4%
• The composition of the lithosphere includes many minerals, the most common are feldspar and quartz.

On the continents, the crust is three-layered: sedimentary rocks cover granitic rocks, and granitic rocks lie on basalt ones. Under the oceans, the crust is "oceanic", two-layered; sedimentary rocks lie simply on basalts, there is no granite layer. There is also a transitional type of the earth's crust (island-arc zones on the outskirts of the oceans and some areas on the continents, such as the Black Sea).

The earth's crust is thickest in mountainous regions.(under the Himalayas - over 75 km), the middle one - in the areas of the platforms (under the West Siberian lowland - 35-40, within the boundaries of the Russian platform - 30-35), and the smallest - in the central regions of the oceans (5-7 km). The predominant part of the earth's surface is the plains of the continents and the ocean floor.

The continents are surrounded by a shelf - a shallow-water strip up to 200 g deep and an average width of about 80 km, which, after a sharp steep bend of the bottom, passes into the continental slope (the slope varies from 15-17 to 20-30 °). The slopes gradually level off and turn into abyssal plains (depths 3.7-6.0 km). The greatest depths (9-11 km) have oceanic trenches, the vast majority of which are located on the northern and western margins of the Pacific Ocean.

The main part of the lithosphere consists of igneous igneous rocks (95%), among which granites and granitoids predominate on the continents, and basalts in the oceans.

Blocks of the lithosphere - lithospheric plates - move along the relatively plastic asthenosphere. The section of geology on plate tectonics is devoted to the study and description of these movements.

To designate the outer shell of the lithosphere, the now obsolete term sial was used, which comes from the name of the main elements of rocks Si (lat. Silicium - silicon) and Al (lat. Aluminum - aluminum).

Lithospheric plates

It is worth noting that the largest tectonic plates are very clearly visible on the map and they are:

• Pacific- the largest plate of the planet, along the boundaries of which constant collisions of tectonic plates occur and faults form - this is the reason for its constant decrease;
• Eurasian- covers almost the entire territory of Eurasia (except Hindustan and the Arabian Peninsula) and contains the largest part of the continental crust;
• Indo-Australian- It includes the Australian continent and the Indian subcontinent. Due to constant collisions with the Eurasian plate, it is in the process of breaking;
• South American- consists of the South American mainland and part of the Atlantic Ocean;
• North American- consists of the North American continent, part of northeastern Siberia, the northwestern part of the Atlantic and half of the Arctic Oceans;
• African- consists of the African continent and the oceanic crust of the Atlantic and Indian oceans. It is interesting that the plates adjacent to it move in the opposite direction from it, therefore the largest fault of our planet is located here;
• Antarctic Plate- consists of the mainland Antarctica and the nearby oceanic crust. Due to the fact that the plate is surrounded by mid-ocean ridges, the rest of the continents are constantly moving away from it.

Movement of tectonic plates in the lithosphere

Lithospheric plates, connecting and separating, change their outlines all the time. This enables scientists to put forward the theory that about 200 million years ago the lithosphere had only Pangea - a single continent, which subsequently split into parts, which began to gradually move away from each other at a very low speed (an average of about seven centimeters per year ).

It is interesting! There is an assumption that due to the movement of the lithosphere, in 250 million years a new continent will form on our planet due to the union of moving continents.

When there is a collision of the oceanic and continental plates, the edge of the oceanic crust sinks under the continental one, while on the other side of the oceanic plate its boundary diverges from the plate adjacent to it. The boundary along which the movement of the lithospheres occurs is called the subduction zone, where the upper and plunging edges of the plate are distinguished. It is interesting that the plate, plunging into the mantle, begins to melt when the upper part of the earth's crust is squeezed, as a result of which mountains are formed, and if magma also breaks out, then volcanoes.

In places where tectonic plates come into contact with each other, there are zones of maximum volcanic and seismic activity: during the movement and collision of the lithosphere, the earth's crust collapses, and when they diverge, faults and depressions form (the lithosphere and the Earth's relief are connected to each other). This is the reason why the largest landforms of the Earth are located along the edges of the tectonic plates - mountain ranges with active volcanoes and deep-sea trenches.

Problems of the lithosphere

The intensive development of industry has led to the fact that man and the lithosphere have recently become extremely difficult to get along with each other: pollution of the lithosphere is acquiring catastrophic proportions. This happened due to the increase in industrial waste in combination with household waste and fertilizers and pesticides used in agriculture, which negatively affects the chemical composition of the soil and living organisms. Scientists have calculated that about one ton of garbage falls per person per year, including 50 kg of hardly decomposable waste.

Today, pollution of the lithosphere has become an urgent problem, since nature is not able to cope with it on its own: the self-purification of the earth's crust occurs very slowly, and therefore harmful substances gradually accumulate and eventually negatively affect the main culprit of the problem - man.

The internal structure of the Earth includes three shells: the earth's crust, mantle and core. The shell structure of the Earth was established by remote methods based on measuring the propagation velocity of seismic waves, which have two components - longitudinal and transverse waves. Longitudinal (P) waves associated with tensile (or compressive) stresses oriented in the direction of their propagation. Transverse (S) waves cause oscillations of the medium, oriented at right angles to the direction of their propagation. These waves do not propagate in a liquid medium. The main values ​​of the physical parameters of the Earth are given in fig. 5.1.

Earth's crust- a stony shell composed of a solid substance with an excess of silica, alkali, water and an insufficient amount of magnesium and iron. It separates from the upper mantle Mohorović border(Moho layer), on which there is a jump in the velocities of longitudinal seismic waves up to about 8 km/s. This boundary, established in 1909 by the Yugoslav scientist A. Mohorovic, is believed to coincide with the outer peridotite shell of the upper mantle. The thickness of the earth's crust (1% of the total mass of the Earth) averages 35 km: under young folded mountains on the continents it increases to 80 km, and under mid-ocean ridges it decreases to 6 - 7 km (counting from the surface of the ocean floor) .

Mantle is the largest shell of the Earth in terms of volume and weight, extending from the sole of the earth's crust to borders Gutenberg, corresponding to a depth of approximately 2900 km and taken as the lower boundary of the mantle. The mantle is subdivided into lower(50% of the Earth's mass) and top(eighteen%). According to modern concepts, the composition of the mantle is fairly homogeneous due to intense convective mixing by intramantle currents. There are almost no direct data on the material composition of the mantle. It is assumed that it is composed of a molten silicate mass saturated with gases. The propagation velocities of longitudinal and transverse waves in the lower mantle increase to 13 and 7 km/s, respectively. The upper mantle from a depth of 50-80 km (under the oceans) and 200-300 km (under the continents) to 660-670 km is called asthenosphere. This is a layer of increased plasticity of a substance close to the melting point.

Nucleus is a spheroid with an average radius of about 3500 km. There is also no direct information about the composition of the core. It is known that it is the most dense shell of the Earth. The core is also subdivided into two spheres: external, to a depth of 5150 km, which is in a liquid state, and internal - hard. In the outer core, the propagation velocity of longitudinal waves drops to 8 km/s, while transverse waves do not propagate at all, which is taken as proof of its liquid state. Deeper than 5150 km, the propagation velocity of longitudinal waves increases and transverse waves pass again. The inner core accounts for 2% of the mass of the Earth, the outer - 29%.

The outer "hard" shell of the Earth, including the earth's crust and the upper part of the mantle, forms lithosphere(Fig. 5.2). Its capacity is 50-200 km.

Rice. 5.1. Changes in physical parameters in the bowels of the Earth (according to S.V. Aplonov, 2001)

Rice. 5.2. The internal structure of the Earth and the propagation velocity of longitudinal (R) and transverse (S) seismic waves (according to S. V. Aplonov, 2001)

The lithosphere and the underlying mobile layers of the asthenosphere, where intraterrestrial movements of a tectonic nature are usually generated and realized, and earthquakes and molten magma are often located, are called tectonosphere.

The composition of the earth's crust. Chemical elements in the earth's crust form natural compounds - minerals, usually solids that have certain physical properties. The earth's crust contains more than 3,000 minerals, among which about 50 are rock-forming.

Regular natural combinations of minerals form rocks. The earth's crust is composed of rocks of different composition and origin. By origin, rocks are divided into igneous, sedimentary and metamorphic.

Igneous rocks formed by the solidification of magma. If this happens in the thickness of the earth's crust, then intrusive crystallized rocks, and when magma erupts onto the surface, effusive education. According to the content of silica (SiO2), the following groups of igneous rocks are distinguished: sour(> 65% - granites, liparites, etc.), medium(65-53% - syenites, andesites, etc.), main(52-45% - gabbro, basalts, etc.) and ultrabasic(<45% - перидотиты, дуниты и др.).

Sedimentary rocks arise on the earth's surface due to the deposition of material in various ways. Some of them are formed as a result of the destruction of rocks. it clastic, or plastic, rocks. The size of the fragments varies from boulders and pebbles to silty particles, which makes it possible to distinguish among them rocks of different granulometric composition - boulders, pebbles, conglomerates, sands, sandstones, etc. Organogenic rocks are created with the participation of organisms (limestone, coal, chalk, etc.). A significant place is occupied chemogenic rocks associated with the precipitation of a substance from solution under certain conditions.

metamorphic rocks are formed as a result of changes in igneous and sedimentary rocks under the influence of high temperatures and pressures in the bowels of the Earth. These include gneisses, schists, marble, etc.

About 90% of the volume of the earth's crust are crystalline rocks of igneous and metamorphic genesis. For the geographic envelope, a relatively thin and discontinuous layer of sedimentary rocks (stratisphere) plays an important role, which are in direct contact with various components of the geographic envelope. The average thickness of sedimentary rocks is about 2.2 km, the real thickness varies from 10-14 km in troughs to 0.5-1 km on the ocean floor. According to the studies of A.B. Ronov, the most common sedimentary rocks are clays and shale (50%), sands and sandstones (23.6%), carbonate formations (23.5%). An important role in the composition of the earth's surface is played by loess and loess-like loams of non-glacial regions, unsorted strata of moraines of glacial regions, and intrazonal accumulations of pebble-sand formations of water origin.

The structure of the earth's crust. According to the structure and thickness (Fig. 5.3), two main types of the earth's crust are distinguished - continental (continental) and oceanic. Differences in their chemical composition can be seen from Table. 5.1.

continental crust consists of sedimentary, granite and basalt layers. The latter is arbitrarily singled out because the velocities of seismic waves are equal to the velocities in basalts. The granite layer consists of rocks enriched in silicon and aluminum (SIAL), the rocks of the basalt layer are enriched in silicon and magnesium (SIAM). The contact between a granite layer with an average rock density of about 2.7 g/cm3 and a basalt layer with an average density of about 3 g/cm3 is known as the Konrad boundary (named after the German explorer W. Konrad, who discovered it in 1923).

oceanic crust two-layer. Its main mass is composed of basalts, on which lies a thin sedimentary layer. The thickness of the basalts exceeds 10 km; in the upper parts, layers of sedimentary Late Mesozoic rocks are reliably identified. The thickness of the sedimentary cover, as a rule, does not exceed 1-1.5 km.

Rice. 5.3. The structure of the earth's crust: 1 - basalt layer; 2 - granite layer; 3 - stratisphere and weathering crust; 4 - basalts of the ocean floor; 5 - areas with low biomass; 6 - areas with high biomass; 7 - ocean waters; 8 - sea ice; 9 - deep faults of continental slopes

The basalt layer on the continents and the ocean floor is fundamentally different. On the continents, these are contact formations between the mantle and the most ancient terrestrial rocks, as if the primary crust of the planet, which arose before or at the beginning of its independent development (possibly evidence of the "lunar" stage of the Earth's evolution). In the oceans, these are real basaltic formations, mainly of the Mesozoic age, which arose due to underwater outpourings during the expansion of lithospheric plates. The age of the first should be several billion years, the second - no more than 200 million years.

Table 5.1. Chemical composition of the continental and oceanic crust (according to S.V. Aplonov, 2001)

	Content, %
oxides	continental crust	oceanic crust
SiO2	60,2	48,6
TiО2	0,7	1.4
Al2O3	15,2	16,5
Fe2O3	2,5	2,3
FeO	3,8	6,2
MNO	0,1	0,2
MgO	3,1	6,8
CaO	5,5	12,3
Na2O	3,0	2,6
K2O	2,8	0,4

In some places there is transitional type the earth's crust, which is characterized by significant spatial heterogeneity. It is known in the marginal seas of East Asia (from the Bering Sea to the South China Sea), the Sunda Archipelago and some other regions of the globe.

The presence of different types of the earth's crust is due to differences in the development of individual parts of the planet and their age. This problem is extremely interesting and important from the point of view of the reconstruction of the geographic envelope. Previously, it was assumed that the oceanic crust is primary, and the continental crust is secondary, although it is many billions of years older than it. According to modern concepts, the oceanic crust arose due to the intrusion of magma along faults between continents.

The dreams of scientists about the practical verification of ideas on the structure of the lithosphere, based on remote geophysical data, came true in the second half of the 20th century, when deep and ultra-deep drilling on land and the bottom of the World Ocean became possible. Among the most famous projects is the Kola super-deep well, drilled to a depth of 12,066 m (drilling was stopped in 1986) within the Baltic Shield in order to reach the boundary between the granite and basalt layers of the earth's crust, and, if possible, its sole - the Moho horizon. The Kola super-deep well disproved many established ideas about the structure of the Earth's interior. The location of the Konrad horizon in this region at a depth of about 4.5 km, which was assumed by geophysical sounding, was not confirmed. The velocity of compressional waves changed (did not increase, but fell) at the level of 6842 m, where the volcanogenic-sedimentary rocks of the Early Proterozoic changed to amphibolite-gneiss rocks of the Late Archean. The "culprit" of the change was not the composition of the rocks, but their special state - hydrogenous decompaction, first discovered in the natural state in the Earth's thickness. Thus, another explanation of the change in the speeds and directions of geophysical waves became possible.

Structural elements of the earth's crust. The Earth's crust has been formed for at least 4 billion years, during which it has become more complex under. the influence of endogenous (mainly under the influence of tectonic movements) and exogenous (weathering, etc.) processes. Manifested with different intensity and at different times, tectonic movements formed the structures of the earth's crust, which form relief planets.

Large landforms are called morphostructures(e.g. mountain ranges, plateaus). Relatively small landforms form morphosculptures(for example, karst).

The main planetary structures of the Earth - continents and oceans. AT within the continents, large structures of the second order are distinguished - folded belts and platforms, which are clearly expressed in modern relief.

Platforms - these are tectonically stable sections of the earth's crust, usually of a two-tier structure: the lower one, formed by the most ancient rocks, is called foundation, upper, composed mainly of sedimentary rocks of a later age - sedimentary cover. The age of platforms is estimated by the time of formation of the foundation. Platform sections where the foundation is submerged under the sedimentary cover are called slabs(for example, Russian plate). The places where the rocks of the platform foundation come to the day surface are called shields(for example, the Baltic Shield).

At the bottom of the oceans, tectonically stable areas are distinguished - thalassocratons and mobile tectonically active bands - georiftogenals. The latter spatially correspond to mid-ocean ridges with alternating uplifts (in the form of seamounts) and subsidences (in the form of deep-water depressions and trenches). Together with volcanic manifestations and local uplifts of the ocean floor, oceanic geosynclines create specific structures of island arcs and archipelagos, expressed on the northern and western margins of the Pacific Ocean.

Contact zones between continents and oceans are divided into two types: active and passive. The first are the centers of the strongest earthquakes, active volcanism and a significant scope of tectonic movements. Morphologically, they are expressed by the conjugation of marginal seas, island arcs, and deep ocean trenches. The most typical are all the margins of the Pacific Ocean ("Pacific Ring of Fire") and the northern part of the Indian Ocean. The latter are an example of a gradual change of continents through the shelves and continental slopes to the ocean floor. These are the margins of most of the Atlantic Ocean, as well as the Arctic and Indian Oceans. We can also talk about more complex contacts, especially in the regions of development of transitional types of the earth's crust.

Dynamics of the lithosphere. Ideas about the mechanism of formation of terrestrial structures are being developed by scientists of various directions, which can be combined into two groups. Representatives fixism they proceed from the statement about the fixed position of the Continents on the surface of the Earth and the predominance of vertical Movements in tectonic deformations of the layers of the earth's crust. Supporters mobilism the primary role is given to horizontal movements. The main ideas of mobilism were formulated by A. Wegener (1880-1930) as continental drift hypothesis. New data obtained in the second half of the 20th century made it possible to develop this direction to the modern theory neomobilism, explaining the dynamics of processes in the earth's crust by the drift of large lithospheric plates.

According to the theory of neomobilism, the lithosphere consists of plates (their number, according to various estimates, ranges from 6 to several dozen), which move in a horizontal direction at a speed of several millimeters to several centimeters per year. Lithospheric plates are drawn into motion as a result of thermal convection in the upper mantle. However, recent studies, in particular deep drilling, show that the asthenosphere layer is not continuous. If, however, the discreteness of the asthenosphere is recognized, then the established ideas about convective cells and the structure of the movement of crustal blocks, which underlie the classical models of geodynamics, should also be rejected. P. N. Kropotkin, for example, believes that it is more correct to speak of forced convection, which is associated with the movement of matter in the Earth's mantle under the influence of an alternate increase and decrease in the Earth's radius. Intensive mountain building in the last tens of millions of years, in his opinion, was due to the progressive compression of the Earth, which amounted to about 0.5 mm per year, or 0.5 km per million years, possibly with the general tendency of the Earth to expand.

According to the modern structure of the earth's crust, in the central parts of the oceans, the boundaries of the lithospheric plates are mid-ocean ridges with rift (fault) zones along their axes. Along the periphery of the oceans, in the transition zones between the continents and the bed of the ocean basin, geosynclinal mobile belts with folded-volcanic island arcs and deep-water trenches along their outer margins. There are three options for the interaction of lithospheric plates: discrepancy, or spreading; collision, accompanied, depending on the type of contacting plates, by subduction, eduction or collision; horizontal slip one plate relative to another.

Concerning the problem of the emergence of oceans and continents, it should be noted that at present it is most often solved by recognizing the fragmentation of the earth's crust into a number of plates, the separation of which caused the formation of huge depressions occupied by ocean waters. The diagram of the geological structure of the ocean floor is shown in fig. 5.4. The scheme of magnetic field reversals in ocean floor basalts shows amazing regularities of the symmetrical arrangement of similar formations on both sides of the spreading zone and their gradual ageing towards the continents (Fig. 5.5). Not only for the sake of fairness, we note the existing opinion about the sufficient antiquity of the oceans - deep ocean sediments, as well as relics of the basaltic oceanic crust in the form of ophiolites, are widely represented in the geological history of the Earth for the last 2.5 billion years. Blocks of the ancient oceanic crust and lithosphere, imprinted in a deeply submerged foundation of sedimentary basins - a kind of failures of the earth's crust, according to S.V. Aplonov, testify to the unrealized possibilities of the planet - "failed oceans".

Rice. 5.4. Scheme of the geological structure of the bed of the Pacific Ocean and its continental framing (according to A. A. Markushev, 1999): / - continental volcanism (a- separate volcanoes, b - trap fields); II - island volcanoes and continental margins (a - underwater, b- ground); III- volcanoes of underwater ridges (a) and oceanic islands (b); IV- marginal sea volcanoes (a - underwater, b - ground); V- spreading structures of the development of modern tholeiite-basalt underwater volcanism; VI- deep water trenches; VII- lithospheric plates (numbers in circles): 1 - Burmese; 2 - Asian; 3 - North American; 4 - South American; 5 - Antarctic; 6 - Australian; 7- Solomon; 8- Bismarck; 9 - Philippine; 10 - Mariana; 11 - Juan de Fuca; 12 - Caribbean; 13 - Coconut; 14 - Nazca; 15 - Skosha; 16 - Pacific; VIII- the main volcanoes and trap fields: 1 - Baker; 2 - Lassen Peak; 3-5- traps {3 - Colombia, 4 - Patagonia, 5 - Mongolia); 6 - Tres Virgines; 7 - Paricutin; 8 - Popocatepetl; 9 - Mont Pele; 10 - Cotopaxi; 11 - Taravera; 12 - Kermadec; 13 - Maunaloa (Hawaiian archipelago); 14- Krakatoa; 75- Taal; 16- Fujiyama; 17 - Theologian; 18 - Katmai. The age of basalts is given according to drilling data

Rice. 5.5. Age (million years) of the bottom of the Atlantic Ocean, determined by the magnetostratigraphic scale (according to E. Zeibol and V. Berger, 1984)

Formation of the modern appearance of the Earth. AT Throughout the history of the Earth, the location and configuration of continents and oceans have constantly changed. According to geological data, the continents of the Earth united four times. Reconstruction of the stages of their formation over the past 570 million years (in the Phanerozoic) indicates the existence of the last supercontinent - Pangaea with a fairly thick, up to 30-35 km continental crust, formed 250 million years ago, which broke up into gondwana, occupying the southern part of the globe, and Laurasia, united the northern continents. The collapse of Pangea led to the opening of the body of water, initially in the form paleo-pacific ocean and ocean Tethys, and later (65 million years ago) - modern oceans. We are now watching the continents drift apart. It is difficult to imagine what will be the location of modern continents and oceans in the future. According to S. V. Aplonov, it is possible to unite them into the fifth supercontinent, the center of which will be Eurasia. V. P. Trubitsyn believes that in a billion years the continents may again gather at the South Pole.