A characteristic feature of the evolution of the Earth is the differentiation of matter, the expression of which is the shell structure of our planet. The lithosphere, hydrosphere, atmosphere, biosphere form the main shells of the Earth, differing in chemical composition, thickness and state of matter.
Internal structure of the Earth
Chemical composition of the Earth(Fig. 1) is similar to the composition of other terrestrial planets, such as Venus or Mars.
In general, elements such as iron, oxygen, silicon, magnesium, and nickel predominate. The content of light elements is low. The average density of the Earth's substance is 5.5 g/cm 3 .
There is very little reliable data on the internal structure of the Earth. Let's look at Fig. 2. It depicts the internal structure of the Earth. The Earth consists of the crust, mantle and core.
Rice. 1. Chemical composition of the Earth
Rice. 2. Internal structure Earth
Core
Core(Fig. 3) is located in the center of the Earth, its radius is about 3.5 thousand km. The temperature of the core reaches 10,000 K, i.e. it is higher than the temperature of the outer layers of the Sun, and its density is 13 g/cm 3 (compare: water - 1 g/cm 3). The core is believed to be composed of iron and nickel alloys.
The outer core of the Earth has a greater thickness than the inner core (radius 2200 km) and is in a liquid (molten) state. The inner core is subject to enormous pressure. The substances that compose it are in a solid state.
Mantle
Mantle- the Earth’s geosphere, which surrounds the core and makes up 83% of the volume of our planet (see Fig. 3). Its lower boundary is located at a depth of 2900 km. The mantle is divided into a less dense and plastic upper part (800-900 km), from which it is formed magma(translated from Greek means “thick ointment”; this is the molten substance of the earth’s interior - a mixture of chemical compounds and elements, including gases, in a special semi-liquid state); and the crystalline lower one, about 2000 km thick.
Rice. 3. Structure of the Earth: core, mantle and crust
Earth's crust
Earth's crust - the outer shell of the lithosphere (see Fig. 3). Its density is approximately two times less than the average density of the Earth - 3 g/cm 3 .
Separates the earth's crust from the mantle Mohorovicic border(often called the Moho boundary), characterized by a sharp increase in seismic wave velocities. It was installed in 1909 by a Croatian scientist Andrei Mohorovicic (1857- 1936).
Since the processes occurring in the uppermost part of the mantle affect the movements of matter in the earth's crust, they are combined under the general name lithosphere(stone shell). The thickness of the lithosphere ranges from 50 to 200 km.
Below the lithosphere is located asthenosphere- less hard and less viscous, but more plastic shell with a temperature of 1200 ° C. It can cross the Moho boundary, penetrating into the earth's crust. The asthenosphere is the source of volcanism. It contains pockets of molten magma, which penetrates into the earth's crust or pours out onto the earth's surface.
Composition and structure of the earth's crust
Compared to the mantle and core, the earth's crust is a very thin, hard and brittle layer. It is composed of a lighter substance, in which about 90 natural chemical elements. These elements are not equally represented in the earth's crust. Seven elements - oxygen, aluminum, iron, calcium, sodium, potassium and magnesium - account for 98% of the mass of the earth's crust (see Fig. 5).
Peculiar combinations of chemical elements form various rocks and minerals. The oldest of them are at least 4.5 billion years old.
Rice. 4. Structure of the earth's crust
Rice. 5. Composition of the earth's crust
Mineral is a relatively homogeneous natural body in its composition and properties, formed both in the depths and on the surface of the lithosphere. Examples of minerals are diamond, quartz, gypsum, talc, etc. (You will find characteristics of the physical properties of various minerals in Appendix 2.) The composition of the Earth's minerals is shown in Fig. 6.
Rice. 6. General mineral composition of the Earth
Rocks consist of minerals. They can be composed of one or several minerals.
Sedimentary rocks - clay, limestone, chalk, sandstone, etc. - were formed by the precipitation of substances in the aquatic environment and on land. They lie in layers. Geologists call them pages of the history of the Earth, since they can learn about the natural conditions that existed on our planet in ancient times.
Among sedimentary rocks, organogenic and inorganogenic (clastic and chemogenic) are distinguished.
Organogenic Rocks are formed as a result of the accumulation of animal and plant remains.
Clastic rocks are formed as a result of weathering, destruction by water, ice or wind of the products of destruction of previously formed rocks (Table 1).
Table 1. Clastic rocks depending on the size of the fragments
|
Breed name |
Size of bummer con (particles) |
|
More than 50 cm |
|
|
5 mm - 1 cm |
|
|
1 mm - 5 mm |
|
|
Sand and sandstones |
0.005 mm - 1 mm |
|
Less than 0.005 mm |
Chemogenic Rocks are formed as a result of the precipitation of substances dissolved in them from the waters of seas and lakes.
In the thickness of the earth's crust, magma forms igneous rocks(Fig. 7), for example granite and basalt.
Sedimentary and igneous rocks when immersed to great depths under the influence of pressure and high temperatures undergo significant changes, becoming metamorphic rocks. For example, limestone turns into marble, quartz sandstone into quartzite.
The structure of the earth's crust is divided into three layers: sedimentary, granite, and basalt.
Sedimentary layer(see Fig. 8) is formed mainly by sedimentary rocks. Clays and shales predominate here, and sandy, carbonate and volcanic rocks are widely represented. In the sedimentary layer there are deposits of such minerals, like coal, gas, oil. All of them are of organic origin. For example, coal is a product of the transformation of plants of ancient times. The thickness of the sedimentary layer varies widely - from complete absence in some land areas to 20-25 km in deep depressions.
Rice. 7. Classification of rocks by origin
"Granite" layer consists of metamorphic and igneous rocks, similar in their properties to granite. The most common here are gneisses, granites, crystalline schists, etc. The granite layer is not found everywhere, but on continents where it is well expressed, its maximum thickness can reach several tens of kilometers.
"Basalt" layer formed by rocks close to basalts. These are metamorphosed igneous rocks, denser than the rocks of the “granite” layer.
The thickness and vertical structure of the earth's crust are different. There are several types of the earth's crust (Fig. 8). According to the simplest classification, a distinction is made between oceanic and continental crust.
Continental and oceanic crust vary in thickness. Thus, the maximum thickness of the earth’s crust is observed under mountain systems. It is about 70 km. Under the plains the thickness of the earth's crust is 30-40 km, and under the oceans it is thinnest - only 5-10 km.
Rice. 8. Types of the earth's crust: 1 - water; 2- sedimentary layer; 3—interlayering of sedimentary rocks and basalts; 4 - basalts and crystalline ultrabasic rocks; 5 – granite-metamorphic layer; 6 – granulite-mafic layer; 7 - normal mantle; 8 - decompressed mantle
The difference between the continental and oceanic crust in the composition of rocks is manifested in the fact that there is no granite layer in the oceanic crust. And the basalt layer of the oceanic crust is very unique. In terms of rock composition, it differs from a similar layer of continental crust.
The boundary between land and ocean (zero mark) does not record the transition of the continental crust to the oceanic one. The replacement of continental crust by oceanic crust occurs in the ocean at a depth of approximately 2450 m.
Rice. 9. Structure of the continental and oceanic crust
There are also transitional types of the earth's crust - suboceanic and subcontinental.
Suboceanic crust located along continental slopes and foothills, can be found in marginal and Mediterranean seas. It represents continental crust with a thickness of up to 15-20 km.
Subcontinental crust located, for example, on volcanic island arcs.
Based on materials seismic sounding - the speed of passage of seismic waves - we obtain data on the deep structure of the earth’s crust. Thus, the Kola superdeep well, which for the first time made it possible to see rock samples from a depth of more than 12 km, brought a lot of unexpected things. It was assumed that at a depth of 7 km a “basalt” layer should begin. In reality, it was not discovered, and gneisses predominated among the rocks.
Change in temperature of the earth's crust with depth. The surface layer of the earth's crust has a temperature determined by solar heat. This heliometric layer(from the Greek helio - Sun), experiencing seasonal temperature fluctuations. Its average thickness is about 30 m.
Below is an even thinner layer, characteristic feature which is a constant temperature corresponding to the average annual temperature of the observation site. The depth of this layer increases in continental climates.
Even deeper in the earth's crust there is a geothermal layer, the temperature of which is determined by the internal heat of the Earth and increases with depth.
The increase in temperature occurs mainly due to the decay of radioactive elements that make up rocks, primarily radium and uranium.
The amount of temperature increase in rocks with depth is called geothermal gradient. It varies within a fairly wide range - from 0.1 to 0.01 °C/m - and depends on the composition of rocks, the conditions of their occurrence and a number of other factors. Under the oceans, temperature increases faster with depth than on continents. On average, with every 100 m of depth it becomes warmer by 3 °C.
The reciprocal of the geothermal gradient is called geothermal stage. It is measured in m/°C.
The heat of the earth's crust is an important energy source.
The part of the earth's crust that extends to depths accessible to geological study forms bowels of the Earth. The Earth's interior requires special protection and wise use.
On continents and under the depths of the oceans, the structure of the earth's crust is different. In flat areas the thickness of the crust is about 40 kilometers; under mountain ranges it is even greater - up to 80 kilometers. Under the deep ocean, the thickness of the crust is less, from 5 to 15 kilometers. On average, the earth's crust lies at a depth of 35 km under the continents, and 7 km under the oceans. Each type has a different structure, which raises the question, what types of crust are the Pacific plate formed by?
Differences in the structure of continental and oceanic crust
In addition to differences in thickness, there are differences in the structure of the oceanic and terrestrial crust. The mainland consists of three layers: sedimentary (topmost), granite (middle layer) and basalt (bottom). Oceanic sedimentary and basaltic layers.
The boundary between continental and oceanic crust is not always visible; it is often blurred. For example, the edge of a continental platform may be adjacent to the outskirts of a basin of seas, where the structure of the earth’s crust is close to the oceanic type. In such places there is practically no granite layer, but the upper sedimentary layer is highly developed.
The boundaries of oceans and seas are represented by island arcs. The earth's crust in these areas is similar in structure and thickness to the continental type. And these are not all types.
Types of oceanic crust
What types of crust are the Pacific plate formed and what types exist? There are several categories of structures of oceanic types of crust.
- Oceanic-continental. This type is found on the shallows and represents a direct continuation of continental structures within the shelf. The thickness of the crust in this place is up to 35 kilometers. The structure of the shelf is the same as that of the continental type: there are basalt (lower), granite (middle) and sedimentary (upper, forming the surface of the planet) layers. But even with all three layers, the shelf crust has a thick sedimentary layer.
- Geosynclinal marine type. Found in sea depressions. This species underlies the Bering, Black, Okhotsk, Mediterranean, Caribbean Sea etc. This type of crust is characterized by gradual wedging out of the granite layer.
- Suboceanic. Located within the continental slope. In its lower part there is a decrease in the granite layer.
- Type of oceanic ridges and rises. It is characterized by complex terrain with faults. This type includes mid-ocean ridges and mountainous countries located in the Pacific Ocean.
Different types can form one slab. But the Pacific lithospheric plate is formed only by crust oceanic type.
Pacific Plate
The largest lithospheric plate is the Pacific. Since the development of the earth's crust, it has been in constant motion and gradually its size decreases.
In the south, the plate borders the Antarctic plate. The border between them runs along the Pacific-Antarctic ridge. In the north, the plate forms the Aleutian Trench, and in the west, the Mariana Trench.
The plate moves north, forming the San Andreas Fault.
Features of the Pacific Plate
Knowing what types of the earth's crust is formed, we can formulate its difference from the earth's crust.
The first and main difference is the absence of a granite layer. This type of slab has only two layers, while the mainland type has three. The slabs differ in age. The oceanic one is considered young, and the terrestrial one is considered older.
Knowing what types of crust the Pacific plate is formed from and what its thickness is, one can understand why it bends under the continental plate. The latter is thicker and more powerful, has a hard layer. But the oceanic type is considered soft and thin. The thickness is clearly visible in places where ridges form - the closer the oceanic ridge, the younger the section of the crust.
Scientists suggest that the growth occurs from the ridges to the continents, and then a lowering of the layers is observed under the weight of the continental type of crust. During this process, island arcs, grooves, protrusions, and deflections appear. Thus, two zones are distinguished: spreading and subduction. The first zone is the area where oceanic-type crust is formed, and the subduction zone is the place where the crust begins to subduct under the continental crust.
A striking example of the transition of crust from one type to another on the Pacific plate is the Mariana Trench. This is a transitional region with a clearly defined island arc, large trench depth and intense seismic activity.
– limited to the surface of the land or the bottom of the oceans. It also has a geophysical boundary, which is the section Moho. The boundary is characterized by the fact that the velocities of seismic waves sharply increase here. It was installed in $1909 by a Croatian scientist A. Mohorovicic ($1857$-$1936$).
The earth's crust is composed sedimentary, igneous and metamorphic rocks, and according to its composition it stands out three layers. Rocks of sedimentary origin, the destroyed material of which was redeposited into the lower layers and formed sedimentary layer The earth's crust covers the entire surface of the planet. It is very thin in some places and may be interrupted. In other places it reaches a thickness of several kilometers. Sedimentary rocks are clay, limestone, chalk, sandstone, etc. They are formed by sedimentation of substances in water and on land, and usually lie in layers. From sedimentary rocks one can learn about the natural conditions that existed on the planet, which is why geologists call them pages of Earth's history. Sedimentary rocks are divided into organogenic which are formed by the accumulation of animal and plant remains and inorganogenic, which in turn are divided into clastic and chemogenic.
Clastic rocks are a product of weathering, and chemogenic- the result of sedimentation of substances dissolved in the water of seas and lakes.
Igneous rocks make up granite layer of the earth's crust. These rocks were formed as a result of the solidification of molten magma. On the continents, the thickness of this layer is $15$-$20$ km; it is completely absent or very much reduced under the oceans.
Igneous substance, but poor in silica composes basaltic layer having a high specific gravity. This layer is well developed at the base of the earth's crust in all regions of the planet.
The vertical structure and thickness of the earth's crust are different, so several types are distinguished. According to a simple classification, there is oceanic and continental earth's crust.
Continental crust
Continental or continental crust is different from oceanic crust thickness and device. The continental crust is located under the continents, but its edge does not coincide with coastline. From a geological point of view, a real continent is the entire area of continuous continental crust. Then it turns out that geological continents are larger than geographical continents. Coastal zones of continents, called shelf- these are parts of continents temporarily flooded by the sea. Seas such as the White, East Siberian, and Azov seas are located on the continental shelf.
There are three layers in the continental crust:
- The top layer is sedimentary;
- The middle layer is granite;
- The bottom layer is basalt.
Under young mountains this type of crust has a thickness of $75$ km, under plains - up to $45$ km, and under island arcs - up to $25$ km. The upper sedimentary layer of the continental crust is formed by clay deposits and carbonates of shallow marine basins and coarse clastic facies in marginal troughs, as well as on the passive margins of Atlantic-type continents.
Magma invading cracks in the earth's crust formed granite layer which contains silica, aluminum and other minerals. The thickness of the granite layer can reach up to $25$ km. This layer is very ancient and has a considerable age - $3$ billion years. Between the granite and basalt layers, at a depth of up to $20$ km, a boundary can be traced Conrad. It is characterized by the fact that the speed of propagation of longitudinal seismic waves here increases by $0.5$ km/sec.
Formation basalt The layer occurred as a result of the outpouring of basaltic lavas onto the land surface in zones of intraplate magmatism. Basalts contain more iron, magnesium and calcium, which is why they are heavier than granite. Within this layer, the speed of propagation of longitudinal seismic waves is from $6.5$-$7.3$ km/sec. Where the boundary becomes blurred, the speed of longitudinal seismic waves increases gradually.
Note 2
The total mass of the earth's crust of the mass of the entire planet is only $0.473$%.
One of the first tasks associated with determining the composition upper continental crust, young science began to solve geochemistry. Since the bark consists of many different rocks, this task was quite difficult. Even in one geological body, the composition of rocks can vary greatly, and in different areas they can be distributed different types breeds Based on this, the task was to determine the general average composition that part of the earth's crust that comes to the surface on continents. This first estimate of the composition of the upper crust was made by Clark. He worked as an employee of the US Geological Survey and was engaged in the chemical analysis of rocks. In the course of many years of analytical work, he was able to summarize the results and calculate the average composition of rocks, which was close to granite. Job Clark was subjected to severe criticism and had opponents.
The second attempt to determine the average composition of the earth's crust was made by V. Goldshmidt. He suggested that moving along the continental crust glacier, can scrape and mix exposed rocks that will be deposited during glacial erosion. They will then reflect the composition of the middle continental crust. Having analyzed the composition of ribbon clays that were deposited in the last glaciation Baltic Sea, he got a result close to the result Clark. Different methods gave the same ratings. Geochemical methods were confirmed. These issues have been addressed and the assessments Vinogradov, Yaroshevsky, Ronov, etc..
Oceanic crust
Oceanic crust is located where the sea depth is more than $4$ km, which means that it does not occupy the entire space of the oceans. The rest of the area is covered with bark intermediate type. The oceanic crust is structured differently from the continental crust, although it is also divided into layers. It is almost completely absent granite layer, and the sedimentary one is very thin and has a thickness of less than $1$ km. The second layer is still unknown, so it is simply called second layer. Bottom, third layer - basaltic. The basalt layers of the continental and oceanic crust have similar seismic wave velocities. The basalt layer predominates in the oceanic crust. According to the theory of plate tectonics, oceanic crust is constantly formed at mid-ocean ridges, then it moves away from them and into areas subduction absorbed into the mantle. This indicates that the oceanic crust is relatively young. Largest quantity Subduction zones are typical for Pacific Ocean , where powerful seaquakes are associated with them.
Definition 1
Subduction- this is the lowering of rock from the edge of one tectonic plate into the semi-molten asthenosphere
In the case when the upper plate is a continental plate, and the lower one is an oceanic one, ocean trenches.
Its thickness in different geographical zones varies from $5$-$7$ km. Over time, the thickness of the oceanic crust remains virtually unchanged. This is due to the amount of melt released from the mantle at mid-ocean ridges and the thickness of the sedimentary layer at the bottom of the oceans and seas.
Sedimentary layer The oceanic crust is small and rarely exceeds a thickness of $0.5$ km. It consists of sand, deposits of animal remains and precipitated minerals. Carbonate rocks of the lower part are not found at great depths, and at depths greater than $4.5 km, carbonate rocks are replaced by red deep-sea clays and siliceous silts.
Basaltic lavas of tholeiitic composition formed in the upper part basalt layer, and below lies dike complex.
Definition 2
Dykes- these are channels through which basaltic lava flows to the surface
Basalt layer in zones subduction turns into ecgoliths, which plunge into depth because they have a high density of surrounding mantle rocks. Their mass is about $7$% of the mass of the entire Earth's mantle. Within the basalt layer, the velocity of longitudinal seismic waves is $6.5$-$7$ km/sec.
The average age of the oceanic crust is $100$ million years, while the oldest sections of it are $156$ million years old and are located in the depression Jacket in the Pacific Ocean. The oceanic crust is concentrated not only within the bed of the World Ocean, it can also be in closed basins, for example, the northern basin of the Caspian Sea. Oceanic The earth's crust has a total area of $306 million km sq.
The earth's crust is the upper part of the lithosphere. On a scale of everything globe it can be compared to the thinnest film - its power is so insignificant. But we don’t know even this uppermost shell of the planet very well. How can one learn about the structure of the earth’s crust if even the deepest wells drilled in the crust do not go beyond the first ten kilometers? Seismic location comes to the aid of scientists. By deciphering the speed of seismic waves passing through different media, it is possible to obtain data on the density of the earth's layers and draw conclusions about their composition. Under continents and ocean basins, the structure of the earth's crust is different.
OCEANIC CRUST
The oceanic crust is thinner (5-7 km) than the continental crust, and consists of two layers - lower basalt and upper sedimentary. Below the basalt layer is the Moho surface and the upper mantle. The topography of the ocean floor is very complex. Among the various landforms, the huge mid-ocean ridges stand out. In these places, the birth of young basaltic oceanic crust from mantle material occurs. Through a deep fault running along the peaks in the center of the ridge - a rift - magma comes to the surface, spreading in different directions in the form of underwater lava flows, constantly pushing the walls of the rift gorge in different directions. This process is called spreading.
Mid-ocean ridges rise several kilometers above the ocean floor, and their length reaches 80 thousand km. The ridges are cut by parallel transverse faults. They are called transformative. Rift zones are the most turbulent seismic zones on Earth. The basalt layer is overlain by layers of marine sedimentary deposits - silts and clays of various compositions.
CONTINENTAL CRUST
The continental crust occupies a smaller area (about 40% of the Earth's surface - approx.), but has a more complex structure and much greater thickness. Under high mountains its thickness is measured 60-70 kilometers. The structure of the continental crust is three-membered - basalt, granite and sedimentary layers. The granite layer comes to the surface in areas called shields. For example, the Baltic Shield, part of which is occupied by the Kola Peninsula, is composed of granite rocks. It was here that deep drilling was carried out, and the Kola superdeep well reached the 12 km mark. But attempts to drill through the entire granite layer were unsuccessful.
The shelf - the underwater margin of the continent - also has continental crust. The same applies to the large islands - New Zealand, the islands of Kalimantan, Sulawesi, New Guinea, Greenland, Sakhalin, Madagascar and others. Marginal seas and internal seas, such as the Mediterranean, Black, and Azov, are located on continental-type crust.
It is possible to talk about basalt and granite layers of the continental crust only conditionally. This means that the speed of passage of seismic waves in these layers is similar to the speed of their passage in rocks of basalt and granite composition. The boundary between granite and basalt layers is not very clearly defined and varies in depth. The basalt layer borders the Moho surface. The upper sedimentary layer changes its thickness depending on the surface topography. So, in mountainous regions it is thin or absent altogether, since the external forces of the Earth move loose material down the slopes - approx.. But in the foothills, on the plains, in basins and depressions it reaches significant thicknesses. For example, in the Caspian lowland, which is undergoing subsidence, the sedimentary layer reaches 22 km.
FROM THE HISTORY OF THE KOLA SUPERDEEP WELL
Since the start of drilling this well in 1970, scientists have set a purely scientific goal for this experiment: to determine the boundary between the granite and basalt layers. The location was chosen taking into account the fact that it is in the areas of the shields that the granite layer, not covered by the sedimentary one, can be passed “through and through”, which would allow one to touch the rocks of the basalt layer and see the difference. It was previously assumed that such a boundary on the Baltic Shield, where ancient igneous rocks come to the surface, should be located at a depth of approximately 7 km.
Over several years of drilling, the well repeatedly deviated from the specified vertical direction, intersecting layers with different strengths. Sometimes the drills broke, and then we had to start drilling again, using bypass shafts. The material that was delivered to the surface was studied by various scientists and constantly brought amazing discoveries. Thus, at a depth of about 2 km, copper-nickel ores were found, and from a depth of 7 km, a core was delivered (this is the name of a rock sample from a drill in the form of a long cylinder - approx. from the site), in which the fossilized remains of ancient organisms were discovered.
But, having traveled more than 12 km by 1990, the well never went beyond the granite layer. In 1994, drilling was stopped. The Kola superdeep well is not the only well in the world that was laid for deep drilling. Similar experiments were carried out in different places by several countries. But only Kola reached such marks, for which it was included in the Guinness Book of Records.
All described types of rocks participate in the structure of the earth's crust - igneous, sedimentary and metamorphic, occurring above the Moho boundary. Both within the continents and within the oceans, mobile belts and relatively stable areas of the earth's crust are distinguished. On continents, stable areas include vast flat spaces - platforms (East European, Siberian), within which the most stable areas are located - shields (Baltic, Ukrainian), which are outcrops of ancient crystalline rocks. Mobile belts include young mountain structures, such as the Alps, Caucasus, Himalayas, Andes and others (Figure 3.1).
Figure 3.1. Generalized profile of the ocean floor (according to O. K. Leontiev)
Continental structures are not limited only to continents; in some cases they extend into the ocean, forming the so-called underwater margin of continents, consisting of a shelf, up to 200 m deep, a continental slope with a foot to depths of 2500 -3000 m. Stable areas are also distinguished within the oceans - ocean platforms - significant areas of the ocean floor - vast abyssal (Greek "abyssos" - abyss) plains 4-6 km deep, and mobile belts, which include mid-ocean ridges and active margins of the Pacific Ocean with developed marginal seas (Okhotsk, Japanese etc.), island arcs (Kuril, Japanese, etc.) and deep-sea trenches (8-10 km deep or more).
At the first stages of geophysical research, two main types of the earth's crust were distinguished: 1) continental and 2) oceanic, sharply different from each other in the structure and thickness of the constituent rocks. Subsequently, two transitional types began to be distinguished: 1) subcontinental and 2) suboceanic (Figure 3.2).
Legend:
1 - water; 2 - sedimentary layer; 3 - granite layer; 4 - basalt layer of continental crust; 5 - basalt layer of oceanic crust; 6 - magmatic layer of oceanic crust; 7 - volcanic islands; 8.9 - mantle (ultrabasic igneous rocks).
Figure 3.2 - Structure diagram various types earth's crust
Continental type of earth's crust
Continental type of earth's crust. The thickness of the continental crust varies from 35-40 (45) km within platforms to 55-70 (75) km in young mountain structures. The continental crust continues into the submarine margins of the continents. In the shelf area, its thickness decreases to 20-25 km, and on the continental slope (at a depth of about 2.0-2.5 km) it pinches out. The continental crust consists of three layers. The first - uppermost layer is represented by sedimentary rocks, with a thickness of 0 to 5 (10) km within platforms, up to 15-20 km in tectonic troughs of mountain structures. The velocity of longitudinal seismic waves (Vp) is less than 5 km/s. The second - traditionally called "granite" layer is 50% composed of granites, 40% - gneisses and other varying degrees metamorphosed rocks. Based on these data, it is often called granite-gneiss or granite-metamorphic. Its average thickness is 15-20 km (sometimes in mountain structures up to 20-25 km). Seismic wave speed (Vp) - 5.5-6.0 (6.4) km/s. The third, lower layer is called "basalt". On average chemical composition and the speed of seismic waves, this layer is close to basalts.
However, it is suggested that it is composed of basic intrusive rocks such as gabbro, as well as metamorphic rocks of amphibolite and granulite metamorphic facies; the presence of ultrabasic rocks is not excluded. It is more correct to call this layer granulite-mafic (mafic is the main rock). Its thickness varies from 15-20 to 35 km. Wave propagation speed (Vp) 6.5-6.7 (7.4) km/s. The boundary between granite-metamorphic and granulite-mafic layers is called the Conrad seismic section. For a long time The prevailing idea was that the Conrad boundary existed everywhere in the continental crust. However, subsequent deep seismic sounding data showed that the Conrad surface is not expressed everywhere, but is recorded only in certain places. Naturally, new interpretations of the structure of the continental crust arise. Thus, N.I. Pavlenkova and others proposed a four-layer model (Fig. 3.3). This model identifies an upper sedimentary layer with a clear velocity boundary, designated Ko. The lower parts of the earth's crust are combined into the concept of a crystalline foundation, or consolidated crust, within which three layers are distinguished: upper, intermediate and lower, separated by the boundaries K1 and K2. There is sufficient stability of the K2 boundary - between the intermediate and lower floors. The upper floor is characterized by a vertically layered structure and differentiation of individual blocks in composition and physical parameters. For the intermediate floor, thin horizontal layering and the presence of individual plates with a reduced seismic wave speed (Vp) - 6 km/s (with a total speed in the layer of 6.4-6.7 km/s) and an anomalous density are noted.
Based on this, it is concluded that the intermediate layer can be classified as a weakened layer, along which horizontal movements of the substance are possible. Currently, other researchers are paying attention to the presence of individual lenses in the continental crust with relatively (0.1-0.2 km/s) reduced seismic wave velocities at depths of 10-20 km, with a lens power of 5-10 km. It is assumed that these zones (or lenses) are associated with strong fracturing and water content in the rocks.
S. R. Taylor's data also indicate that within the continental crust there is no single layer with a reduced velocity, but discontinuous layering is noted. All of the above indicates the great complexity of the continental crust and the ambiguity of its interpretation. Quite convincing evidence of this is the data obtained during drilling of the ultra-deep Kola well, which has already reached a depth of over 12 km. According to preliminary seismic data, in the area where the well was laid, the boundary between the “granite” and “basalt” layers should be encountered at a depth of about 7 km. In reality, there was no geophysical “basalt” layer. At this depth, under a thick metamorphosed volcanogenic-sedimentary strata of Proterozoic age, plagioclase gneisses, granite-gneisses, and amphibolites were discovered - rocks of the medium-temperature stage of metamorphism, the percentage of which increases with depth. What caused the change in the speed of seismic waves (from 6.1 to 6.5-6.6 km/s) at a depth of about 7 km, where the presence of a geophysical “basalt” layer was assumed? It is possible that this is due to amphibolites and their role in changing the elastic properties of rocks. It is also possible that the boundary indicated earlier (before drilling the well) is not associated with a change in the composition of the rocks, but with an increase in the stress field caused by intense deformations and repeated manifestations of metamorphism.
