Continental crust. Oceanic and continental crust

Hypotheses explaining the origin and development of the earth's crust

The concept of the earth's crust.

Earth's crust Is a complex of surface layers of the Earth's solid body. In the scientific geographical literature, there is no unified understanding of the origin and development of the earth's crust.

There are several concepts (hypotheses) that reveal the mechanisms of formation and development of the earth's crust, the most justified of which are the following:

1. The theory of fixism (from Lat. Fixus - motionless, unchanging) asserts that the continents have always remained in the places they currently occupy. This theory denies any movement of continents and large parts of the lithosphere.

2. The theory of mobilism (from the Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has been especially confirmed in recent years in connection with the receipt of new scientific data in the study of the bottom of the World Ocean.

3. The concept of continental growth at the expense of the ocean floor assumes that the original continents were formed in the form of relatively small massifs, which now constitute ancient continental platforms. Subsequently, these massifs grew due to the formation of mountains on the ocean floor, adjacent to the edges of the original land cores. The study of the ocean floor, especially in the zone of the mid-ocean ridges, gave rise to doubts about the correctness of the concept of continental growth at the expense of the ocean floor.

4. The theory of geosynclines states that the increase in the size of the land occurs through the formation of mountains in the geosynclines. The geosynclinal process, as one of the main ones in the development of the earth's crust of continents, is the basis for many modern scientific explanations of the process of the origin and development of the earth's crust.

5. The rotational theory bases its explanation on the proposition that since the figure of the Earth does not coincide with the surface of a mathematical spheroid and is rearranged due to uneven rotation, zonal stripes and meridional sectors on a rotating planet are inevitably tectonically unequal. They With varying degrees activities respond to tectonic stresses caused by intraterrestrial processes.

There are two main types of the earth's crust: oceanic and continental. The transitional type of the earth's crust is also distinguished.

Oceanic crust. The thickness of the oceanic crust in the modern geological epoch ranges from 5 to 10 km. It consists of the following three layers:

1) the upper thin layer of marine sediments (thickness no more than 1 km);

2) middle basalt layer (thickness from 1.0 to 2.5 km);

3) the lower layer of gabbro (about 5 km thick).

Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic crust. Its capacity is on average 35-45 km, and in mountainous countries it increases to 70 km. It also consists of three layers, but differs significantly from the ocean:

1) the lower layer, composed of basalts (thickness about 20 km);

2) the middle layer occupies the main thickness of the continental crust and is conventionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;

3) the upper layer is sedimentary. Its average thickness is about 3 km. In some areas, the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth, the sedimentary layer is absent altogether and a granite layer emerges on the surface. Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).

On the continents as a result of weathering rocks a geological formation is formed, called weathering crust.

The granite layer is separated from the basalt layer Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km / sec.

The border between the earth's crust and the mantle (both on the continents and on the oceans) runs along Mohorovicic surface (Moho line). The speed of seismic waves on it abruptly increases to 8 km / h.

In addition to the two main types - oceanic and continental - there are also areas of mixed (transitional) type.

On continental shoals or shelves, the crust has a thickness of about 25 km and is generally similar to the continental crust. However, a layer of basalt can fall out in it. IN East Asia in the area of ​​island arcs (Kuril Islands, Aleutian Islands, Japanese Islands, etc.), the earth's crust is of a transitional type. Finally, the crust of the mid-oceanic ridges is very complex and so far little studied. There is no Moho boundary here, and the mantle material rises along faults into the crust and even to its surface.

The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than the "crust". Into the lithosphere modern science includes not only the earth's crust, but also the uppermost mantle to the asthenosphere, that is, to a depth of about 100 km.

The concept of isostasy ... The study of the distribution of gravity showed that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position of them is called isostasy (from Latin isoc - even, stasis - position). Isostatic equilibrium is achieved due to the fact that the thickness of the earth's crust is inversely proportional to its density. The heavy oceanic crust is thinner than the lighter continental crust.

Isostasy - in essence, it is not even equilibrium, but a striving for equilibrium, continuously disturbed and restored again. So, for example, the Baltic shield after the melting of continental ice of the Pleistocene glaciation rises by about 1 meter per century. The area of ​​Finland is constantly increasing due to the seabed. The territory of the Netherlands, on the contrary, is decreasing. The zero line of equilibrium is currently passing somewhat south of 60 0 N. Modern St. Petersburg is about 1.5 m higher than St. Petersburg during the time of Peter the Great. As the data of modern scientific research, even the severity of large cities is sufficient for the isostatic fluctuation of the territory below them. Consequently, the earth's crust in the zones of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface, the bottom of the earth's crust: elevated areas correspond to depressions in the mantle, lower ones - more high level its upper boundary. So, under the Pamir, the depth of the Moho surface is 65 km, and in the Caspian lowland - about 30 km.

Thermal properties of the earth's crust ... Daily fluctuations in soil temperature extend to a depth of 1.0-1.5 m, and annual fluctuations in temperate latitudes in countries with a continental climate to a depth of 20-30 m. At the depth where the influence of annual temperature fluctuations due to heating ceases the earth's surface By the sun, there is a layer of constant ground temperature. It is called isothermal layer ... Below the isothermal layer deep into the Earth, the temperature rises, and this is already caused by the internal heat of the earth's interior. Internal heat is not involved in the formation of climates, but it serves as the energy basis for all tectonic processes.

The number of degrees by which the temperature increases for every 100 m depth is called geothermal gradient ... The distance in meters, when lowering by which the temperature increases by 1 0 С is called geothermal stage ... The magnitude of the geothermal step depends on the relief, the thermal conductivity of rocks, the proximity of volcanic foci, the circulation of groundwater, etc. On average, the geothermal step is 33 m. on platforms), it can reach 100 m.

The earth's crust is a multilayer formation. Its upper part - the sedimentary cover, or the first layer - is formed by sedimentary rocks and sediments not compacted to the state of rocks. Below, both on the continents and in the oceans, lies the crystalline basement. In its structure, the main differences between the continental and oceanic types of the earth's crust lie. On the continents, the basement consists of two thick layers - "granite" and basalt. There is no “granite” layer under the abyssal oceans. However, the basaltic basement of the ocean is by no means homogeneous in the section; it is divided into the second and third layers.

Before superdeep and deep-sea drilling, the structure of the earth's crust was judged mainly by geophysical data, namely, by the velocities of longitudinal and transverse seismic waves. Depending on the composition and density of the rocks that make up certain layers of the earth's crust, the speed of passage of seismic waves varies significantly. In the upper horizons, where weakly compacted sedimentary formations predominate, they are relatively small, while in crystalline rocks they sharply increase as their density increases.

After the velocities of propagation of seismic waves in the rocks of the ocean floor were first measured in 1949, it became clear that the velocity sections of the crust of continents and oceans are very different. At a shallow depth from the bottom, in the basement under the abyssal basin, these velocities reached values ​​that were recorded on the continents in the deepest layers of the earth's crust. The reason for this discrepancy soon became clear. The fact is that the crust of the oceans turned out to be amazingly thin. If on the continents the thickness of the earth's crust is on average 35 km, and under the mountain-fold systems even 60 and 70 km, then in the ocean it does not exceed 5-10, rarely 15 km, and in some regions the mantle is almost at the very bottom.

The standard high-speed section of the continental crust includes the upper, sedimentary layer with longitudinal waves of 1-4 km / s, intermediate, "granite" - 5.5-6.2 km / s and the lower, basaltic - 6.1-7.4 km /from. Below, as it is believed, lies the so-called peridotite layer, which is already part of the asthenosphere, with speeds of 7.8–8.2 km / s. The names of the layers are arbitrary, since no one has yet seen real continuous sections of the continental crust, although the Kola superdeep borehole has penetrated into the depths of the Baltic shield already 12 km.

In the abyssal ocean basins, under a thin sedimentary mantle (0.5–1.5 km), where seismic wave velocities do not exceed 2.5 km / s, there is a second layer of oceanic crust. According to the American geophysicist J. Warzel and other scientists, it differs in surprisingly close velocity values ​​- 4.93-5.23 km / s, on average 5.12 km / s, and the average thickness under the oceans is 1.68 km ( in the Atlantic - 2.28, in the Pacific - 1.26 km). However, in the peripheral parts of the abyssal, closer to the margins of the continents, the thickness of the second layer increases rather sharply. Under this layer, a third crustal layer is distinguished with no less uniform velocities of propagation of longitudinal seismic waves equal to 6.7 km / s. Its thickness ranges from 4.5 to 5.5 km.

In recent years, it has become clear that the velocity sections of the oceanic crust are characterized by a greater scatter of values ​​than previously assumed, which is apparently associated with deep inhomogeneities existing in it [Pushcharovsky, 1987].

As you can see, the velocities of propagation of longitudinal seismic waves in the upper (first and second) layers of the continental and oceanic crust are significantly different.

As for the sedimentary cover, this is due to the predominance in its composition on the continents of ancient formations of the Mesozoic, Paleozoic and Precambrian age, which underwent rather complex transformations in the depths. The ocean floor, as mentioned above, is relatively young, and the sediments overlying the basement basalts are weakly compacted. This is due to the action of a number of factors that determine the effect of underconsolidation, which is known as the paradox of deep-sea diagenesis.

It is more difficult to explain the difference in the velocities of seismic waves during their propagation through the second ("granite") layer of the continental and the second (basalt) layer of the oceanic crust. Oddly enough, in the basalt layer of the ocean, these velocities turned out to be lower (4.82–5.23 km / s) than in the “granite” layer (5.5–6.2 km / s). The point here is that the velocities of longitudinal seismic waves in crystalline rocks with a density of 2.9 g / cm 3 are approaching 5.5 km / s. From this it follows that if the "granite" layer on the continents is really composed of crystalline rocks, among which the metamorphic formations of the lower stages of transformation prevail (according to the data of superdeep drilling on the Kola Peninsula), then in the composition of the second layer of the oceanic crust, in addition to basalts, formations with a density less than that of crystalline rocks (2–2.55 g / cm 3).

Indeed, during the 37th voyage of the drilling vessel Glomar Challenger, rocks of the oceanic basement were uncovered. The drill penetrated through several basalt covers, between which there were horizons of carbonate pelagic sediments. In one of the wells, an 80-meter stratum of basalts with interlayers of limestone was drilled, in the other - a 300-meter series of rocks of volcanic-sedimentary origin. Drilling of the first of these wells was stopped in ultrabasic rocks - gabbros and hyperbasites, which probably already belong to the third layer of the oceanic crust.

Deep-sea drilling and exploration of rift zones from underwater habitable vehicles (HVA) made it possible to clarify in general terms the structure of the oceanic crust. True, it cannot be asserted with certainty that we know its complete and continuous cut, not distorted by subsequent superimposed processes. The most detailed study is currently the upper, sedimentary layer, recovered partially or completely at almost 1000 points of the bottom with the drill "Glomar Challenger" and "Joydes Resolution". Much less studied is the second layer of the oceanic crust, which has been exposed to one depth or another by a much smaller number of wells (several dozen). However, it is now obvious that this layer was formed mainly by lava covers of basalts, between which various sedimentary formations of small thickness are enclosed. Basalts are classified as tholeiitic varieties formed underwater conditions. These are pillow lavas, often composed of hollow lava tubes and cushions. Sediments located between basalts in the central parts of the ocean consist of the remains of the smallest planktonic organisms with a carbonate or siliceous function.

Finally, the third layer of the oceanic crust is identified with the so-called dyke belt - a series of small magmatic bodies (intrusions) closely aligned with one another. The composition of these intrusions is basic to ultrabasic. These are gabbros and hyperbasites, which were formed not during the outpouring of magmas on the bottom surface, like the basalts of the second layer, but in the depths of the crust itself. In other words, we are talking about magma melts that have frozen near the magma chamber, never reaching the bottom surface. Their heavier ultrabasic composition indicates the residual character of these magmatic melts. If we remember that the thickness of the third layer is usually 3 times the thickness of the second layer of the oceanic crust, then defining it as basaltic may seem like a great exaggeration.

Similarly, the "granite" layer of the continental crust, as it turned out during the drilling of the Kola superdeep well, turned out to be not granite at all, at least in its upper half. As mentioned above, metamorphic rocks of lower and middle stages of transformation prevailed in the section traversed here. For the most part, they are altered at high temperatures and pressures existing in the bowels of the Earth, ancient sedimentary rocks. In this regard, a paradoxical situation has developed, which consists in the fact that we now know more about the oceanic crust than about the continental crust. And this despite the fact that the first has been intensively studied for two decades, while the second has been an object of research for at least one and a half centuries.

Both varieties of the earth's crust are not antagonists. In the marginal parts of young oceans, the Atlantic and Indian, the boundary between the continental and oceanic crust is somewhat "blurred" due to the gradual thinning of the first of them in the area of ​​transition from continent to ocean. On the whole, this boundary is tectonically calm, that is, it does not manifest itself either by powerful seismic shocks, which are extremely rare here, or by volcanic eruptions.

However, this is not the case everywhere. In the Pacific Ocean, the boundary between the continental and oceanic crust is perhaps one of the most dramatic dividing lines on our planet. So are these two varieties of the earth's crust antipodes or not? It seems that we can reasonably consider them as such. Indeed, despite the existence of a number of hypotheses suggesting the oceanization of the continental crust or, on the contrary, the transformation of the oceanic substrate into a continental one due to a number of mineral transformations of basalts, in fact, there is no evidence of a direct transition from one type of crust to another. As will be shown below, the continental crust is formed in specific tectonic settings in the active zones of the transition between the mainland and the ocean and mainly as a result of the transformation of another type of the earth's crust, called suboceanic. The oceanic substrate disappears in the Benioff zones, or is squeezed out, like a paste from a tube, to the edge of the continent, or turns into tectonic melange (crumbly from rubbed rocks) in the areas of the “collapse” of the oceans. However, more on that later.

Library
materials

Ltd The educational center

"PROFESSIONAL"

Discipline abstract:

"Geography"

On this topic:

“The internal structure of the Earth.

Continental and oceanic crust.

The origin of the mainland protrusions and

ocean trenches "

Executor:

Logunova Yulia Alexandrovna

Zvenigorod 2018

Content 2

Introduction 3

Internal structure of the Earth 4

1. Earth's crust

   Mantle

   Outer core

   Inner core

Continental and oceanic crust 7

1. Continental crust

   Oceanic earth crust

Origin of continental ledges and oceanic trenches 10

Conclusion 14

Bibliography 15

Introduction

The relevance of this topic is determined by the fact that the Earth is part of a system where the center is the Sun, which contains 99.87% of the mass of the entire system. The Earth is surrounded by a powerful shell of gas - the atmosphere. It is a kind of regulator of exchange processes between the Earth and the Cosmos. The earth's crust is the upper (stone) shell of the Earth, called the lithosphere (in Greek, "cast" - stone).

The main methods of studying the inner parts of our planet are geophysical, primarily observing the speed of propagation of seismic waves generated from explosions or earthquakes. Just as waves spread from a stone thrown into water in different directions along the surface of the water, elastic waves propagate in a solid substance from the source of the explosion.

The speed of the waves increases with increasing density of the substance. With a sharp change in the density of matter, the speed of the waves will change abruptly. As a result of studying the propagation of seismic waves through the Earth, it was found that there are several definite boundaries for the jump-like change in wave velocities. Therefore, it is assumed that the Earth consists of several concentric shells (geospheres).

The object of research is the internal structure of the Earth, as well as the influence of the oceans on the origin of continents.

The subject of research is the influence of the origin of continental protrusions and oceanic troughs with the internal structure of the Earth.

The purpose This work is to identify and study the movement of the earth's crust on the origin of continental ledges and oceanic depressions.

The internal structure of the Earth

The earth's crust is a term, although it entered natural science use in the Renaissance, long time was interpreted quite freely due to the fact that it was impossible to directly determine the thickness of the crust and study its deep parts. The discovery of seismic vibrations and the creation of a method for determining the speed of propagation of their waves in media of different densities gave a powerful impetus to the study of the earth's interior. With the help of seismographic studies at the beginning of the XX century. a fundamental difference in the speed of passage of seismic waves through the rocks that compose the earth's crust and the material of the mantle was discovered, and the boundary of their separation (the Mokhorovichich boundary) was objectively established. Thus, the concept of "earth's crust" has received a specific scientific justification.

1. Earth's crust

The Earth's crust is the outer solid shell of the Earth, the upper part of the lithosphere. The Earth is separated from the mantle by the surface of Mohorovichich. Distinguish the continental crust with a thickness of 35 - 45 km under the plains to 70 km in the mountains and oceanic - 5 - 10 km at the bottom of the seas and oceans. The age of the most ancient parts of the earth's crust is set at 3.54 billion years.

In the structure of the earth's crust of the oceanic type, the following layers are distinguished: unconsolidated sedimentary rocks (up to 1 km), volcanic oceanic, which consists of compacted sediments (1-2 km), basalt (4-8 km).

The earth's continental crust consists of the following shells: weathering crust, sedimentary, metamorphic, granite, basaltic.

1. Mantle

Mantle- part () located directly under and higher . In the mantle there is most of the earth's matter. The mantle is also found on other planets. The earth's mantle ranges from 30 to 2900 km from the earth's surface.

The border between the crust and the mantle is, or, for short, Moho. On it there is a sharp an increase in seismic velocities - from 7 to 8 - 8.2 km / s. This border is located at a depth of 7 (under the oceans) to 70 kilometers (under folded belts). The Earth's mantle is subdivided into an upper mantle and a lower mantle. The boundary between these geospheres is the Golitsyn layer, disposed at a depth of about 670 km.

The difference in the composition of the earth's crust and mantle - consequence their origin: the initially homogeneous Earth, as a result of partial melting, was divided into a fusible and light part - the crust and a dense and refractory mantle.

1. Outer core

The very first layer of the core that is in direct contact with the mantle is the outer core. Its upper border is at a depth of 2.3 thousand kilometers below sea level, and the lower one is at a depth of 2900 kilometers. In composition, it is no different from the underlying shells - the pressure of gravity is simply not enough for the red-hot metal to solidify. But his liquid state is the main trump card of the Earth in comparison with other inner planets of the solar system.

The fact is that it is the liquid part of the core that is responsible for the occurrence of magnetic field... Biologists believe that it was the active magnetic field that became the key to the survival of primitive unicellular creatures.

The outer core heats up the mantle - and in some places so strongly that the ascending flows of magma even reach the surface, causing volcanic eruptions.

1. Inner core

Inside the liquid shell is the inner core. It is the solid core of the Earth, with a diameter of 1220 kilometers. This part of the core is very dense - the average concentration of the substance reaches 12.8 - 13 g / cm 3 , which is twice the density of iron, and hot - the heat reaches 5-6 thousand degrees Celsius.

The high pressure at the center of the earth causes the metal to solidify at temperatures above its boiling point. At the same time, unusual crystals are formed that are stable even under normal conditions. It is believed that the inner core is a forest of many kilometers of iron and nickel crystals that run from south to north. In order to test this theory, Japanese scientists spent ten years creating a special diamond anvil - only in it it is possible to achieve such pressure and temperature as in the center of our planet.

Continental and oceanic crust

In the structure of the Earth, two types of the earth's crust are distinguished -continental crust and oceanic crust .

The continental crust has three geological layers:

1) sedimentary;

2) granite;

3) basaltic.

The oceanic crust is younger than the mainland and consists of only two layers:

1) sedimentary;

2) basaltic.

1. Continental crust

Continental (continental) crust and consists of several layers. The upper is a layer of sedimentary rocks. The thickness of this layer is up to 10-15 km. There is a granite layer under it. The rocks that make it up are similar in their physical properties to granite. The thickness of this layer is from 5 to 15 km. Beneath the granite layer is a basalt layer composed of basalt and rocks, the physical properties of which are reminiscent of basalt. The thickness of this layer is from 10 km to 35 km. Thus, the total thickness of the continental crust reaches 30 - 70 km.

The main regularities in the distribution of magnetic minerals in continental the earth's crust:

" Lithological " - sedimentary rocks are almost always non-magnetic, magmatic - both magnetic and non-magnetic, depending on the tectonic setting and differentiation processes, mantle rocks - non-magnetic;

Tectonic – magmatic magnetic rocks belong to extension zones (spreading, island arcs, hot spots), and igneous non-magnetic rocks belong to compression zones (collisional, folded magmatism

" Magmatic " - "inside" the stretching zones is the process magmatic crystallization differentiation, which leads to the formation of two groups of rocks - the first is practically non-magnetic and weakly magnetic cumulates, the second is products differentiation - magnetic. Magnetic rocks are, as a rule, initially igneous rocks, mainly of basic composition, less often of intermediate and felsic composition.

Primarily - magmatic the distribution of magnetic minerals was not noticeably disturbed by subsequent metamorphism. Clusters of magnetic minerals are extremely rare other origin.

1. Oceanic earth crust

Oceanic earth crust differs from the continental crust in that it does not have a granite layer, or it is very thin, therefore the thickness of the oceanic crust is only 6 - 15 km.

Oceanic crust is a type of crust found in the oceans. The crust of the oceans differs from the continents in less thickness and basaltic composition. It forms in mid-ocean ridges and is absorbed in subduction zones. Ancient fragments of oceanic crust preserved in folded structures on the continents are called ophiolites. In the mid-oceanic ridges, there is an intense hydrothermal change in the oceanic crust, as a result of which readily soluble elements are removed from it.

The standard oceanic crust is 7 km long and has a strictly regular structure. From top to bottom, it is folded by the following complexes:

sedimentary rocks, represented by deep ocean sediments;

basalt covers, poured out under water (pillow lavas);

dike complex, consists of nested basalt dikes (a complex of parallel dikes);

layer of main layered intrusions;

mantle, represented by dunites and peridotites.

Dunites and peridotites are usually found at the bottom of the oceanic crust. These rocks can be formed both as a result of crystallization of melts and be primary mantle rocks. They can be distinguished by the orientation of the grains in the rock. In rocks that have passed the magmatic stage, the crystals are oriented arbitrarily. In mantle rocks flowing in convective cells, grains are oriented in accordance with their rheological properties.

A layer of layered intrusions forms in the mid-ocean ridge, in magma chambers located at a depth of 2-4 km. These masses are nested within each other.

The oceanic crust can be thicker in areas of plume magmatism. In such places there are oceanic islands and oceanic plateaus.

The oceanic crust can creep over the continental crust as a result of obduction (thrusting of tectonic plates).

Origin of continental ledges and oceanic trenches

To restore a picture of the past of the earth's surface great importance have questions about the origin of continents and oceanic depressions, the movement of continents. The nature of the location of the continents and oceans to a large extent determines the system of circulation of air masses and especially oceanic waters, which carry out the horizontal transfer of energy, water, mineral matter, etc.

There are a number of points of view regarding the origin of continents and oceans. Some of them have long been rejected. Others, to a greater or lesser extent, are confirmed by facts, the number of which has increased sharply over the past 30 years in connection with the active study of the oceans, the use of more advanced methods of studying the earth's crust, including remote sensing.

1. Origin of continental ledges

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

Before proceeding to the description of the structure and development of these main structures, it is necessary to talk about the origin and essence of the term "geosyncline". This term comes from the Greek words "geo" - Earth and "synclino" - deflection. It was first used by the American geologist D. Dan more than 100 years ago while studying the Appalachian Mountains. He found that the Paleozoic marine sediments that compose the Appalachians have a maximum thickness in the central part of the mountains, much greater than on their slopes. Dan explained this fact quite correctly. During the period of sedimentation in paleozoic era on the site of the Appalachian Mountains there was a sagging depression, which he called the geosyncline. In its central part, the subsidence was more intense than on the wings, as evidenced by the large thickness of the sediments. Dan confirmed his conclusions with a drawing on which he depicted the Appalachian geosyncline. Considering that Paleozoic sedimentation took place under marine conditions, he deposited downward from the horizontal line - the assumed sea level - all measured sediment thickness in the center and on the slopes of the Appalachian Mountains. The figure shows a clearly pronounced large depression on the site of the modern Appalachian Mountains.

On modern continents, from 10 to 16 ancient platforms are distinguished. The largest are East European, Siberian, North American, South American, African-Arabian, Hindustan, Australian and Antarctic.

1. Formation of oceanic trenches

The ocean floor is the most important component complex system called the ocean. It represents depressions with a complex relief, separated by underwater uplifts and having a completely different structure of the underlying layers than the continents. Spaces hidden by ocean waters occupy most of the Earth's surface, therefore, knowledge of their structure helps to understand the structure of the entire planet.

Despite the fact that oceanological research has grown enormously over the past two decades and is now widely carried out, geological structure the ocean floor remains poorly understood.

It is known that the structures of the continental crust continue within the shelf, and in the zone of the continental slope the continental type of the earth's crust changes to the oceanic one. Therefore, the ocean floor itself includes the ocean floor depressions located behind the continental slope. These huge depressions differ from the continents not only in the structure of the earth's crust, but also in their tectonic structures.

The most extensive areas of the ocean floor are deep-water plains located at depths of 4-6 km and separated by seamounts. There are especially large deep-water plains in the Pacific Ocean. Along the edges of these vast plains are deep-water trenches - narrow and very long troughs stretching for hundreds and thousands of kilometers.

The depth of the bottom in them reaches 10-11 km, and the width does not exceed 2-5 km. These are the deepest areas on the surface of the Earth. Along the edges of these troughs are island chains called island arcs. These are the Aleutian and Kuril arcs, the islands of Japan, the Philippines, Samoa, Tonga, etc.

There are many different seamounts on the ocean floor. Some of them form real underwater mountain ranges and mountain chains, others rise from the bottom in the form of individual hills and mountains, and others appear above the ocean surface in the form of islands.

Of exceptional importance in the structure of the ocean floor are the mid-ocean ridges, which got their name because they were first discovered in the middle of the Atlantic Ocean. They are traced at the bottom of all oceans, forming a single system of uplifts at a distance of more than 60 thousand km. This is one of the most ambitious tectonic zones on Earth. Starting in the waters of the Arctic Ocean, it stretches in a wide ridge (700-1000 km) in the middle part of the Atlantic Ocean and, skirting Africa, passes into the Indian Ocean. Here this system of underwater ridges forms two branches. One goes to the Red Sea; the other goes around Australia from the south and continues in the south The Pacific to the shores of America. In the system of mid-oceanic ridges, earthquakes are often manifested and underwater volcanism is highly developed.

Today's scarce geological data on the structure of oceanic depressions do not yet allow solving the problem of their origin. So far, we can only say that different oceanic trenches have different origins and ages. The most ancient age is in the Pacific Ocean depression. Most researchers believe that it originated in the Precambrian and its bed is a remnant of the oldest primary earth's crust. The depressions of other oceans are younger, most scientists believe that they formed on the site of pre-existing continental massifs. The most ancient of them is the depression Indian Ocean, it is assumed that it originated in the Paleozoic era. The Atlantic Ocean appeared at the beginning of the Mesozoic, and the Arctic Ocean - at the end of the Mesozoic or at the beginning of the Cenozoic.

Conclusion

The study of the deep structure of the Earth is one of the largest and most urgent areas of geological sciences. The new stratification of the Earth's mantle makes it possible to approach the complex problem of deep geodynamics much less schematically than before. Difference in seismic characteristics earthly shells(geospheres), reflecting the difference in their physical properties and mineral composition, creates opportunities for modeling geodynamic processes in each of them separately. Geospheres in this sense, as it is now quite clear, have a certain autonomy. However, this extremely important topic is beyond the scope of this article. From further development seismotomography, like some other geophysical research, as well as the study of mineral and chemical composition depths will depend on significantly more substantiated constructions in relation to the composition, structure, geodynamics and evolution of the Earth as a whole.

At the same time, the study of the internal structure of the Earth is vitally important. It is associated with the formation and placement of many types of minerals, the relief of the earth's surface, the emergence of volcanoes and earthquakes. Knowledge about internal structure Lands are also needed for making geological and geographical forecasts.

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To ask questions.

In the structure of the Earth, researchers distinguish 2 types of the earth's crust - continental and oceanic.

What is the continental crust?

Continental crust, also called continental, is characterized by the presence of 3 different layers in its structure. The upper one is represented by sedimentary rocks, the second one is granite or gneisses, the third one consists of basalt, granulites and other metamorphic rocks.

Continental crust

The thickness of the continental crust is about 35-45 km, sometimes up to 75 km (as a rule, in the areas of mountain ranges). The considered type of the earth's crust covers about 40% of the Earth's surface. In terms of volume, it corresponds to approximately 70% of the earth's crust.

The age of the continental crust reaches 4.4 billion years.

What is the oceanic crust?

The main mineral forming oceanic crust, - basalt. But besides him, its structure includes:

1. sedimentary rocks;
2. layered intrusions.

In accordance with the widespread scientific concept, the oceanic crust is constantly formed due to tectonic processes. It is much younger than the mainland, its oldest sites are about 200 million years old.

The thickness of the oceanic crust is about 5-10 km, depending on the specific measurement site. It can be noted that it hardly changes over time. Among scientists, the approach is widespread, according to which the oceanic crust should be considered as belonging to the oceanic lithosphere. In turn, its thickness is largely dependent on age.

Comparison

The main difference between the continental crust and the oceanic crust is, obviously, in their location. The first places on itself continents, land, the second - oceans and seas.

The continental crust is represented mainly by sedimentary rocks, granites and granulites. Oceanic - mainly basalt.

The continental crust is much thicker and older. It is inferior to the oceanic in terms of the area of ​​coverage of the earth's surface, but surpasses in terms of the occupied volume in the entire earth's crust.

It can be noted that in some cases, the oceanic crust is capable of layering over the continental crust in the process of obduction.

Having determined what is the difference between the continental and oceanic crust, we will fix the conclusions in a small table.

Table

Continental crust or continental crust is the earth's crust of continents, which consists of sedimentary, granite and basalt layers. The average thickness is 35-45 km, the maximum is up to 75 km (under the mountain ranges). It is opposed to the oceanic crust, which is different in structure and composition. The continental crust has a three-layered structure. The upper layer is represented by a discontinuous sedimentary cover, which is widely developed, but rarely has a large thickness. Most of the crust is composed of the upper crust - a layer composed mainly of granites and gneisses, with a low density and ancient history... Research shows that most of these rocks were formed a very long time ago, about 3 billion years ago. Below is the lower crust, consisting of metamorphic rocks - granulites and the like.

5. Types of ocean structures. The land surface of the continents is only one third of the Earth's surface. The surface area occupied by the World Ocean is 361.1 ml square meters. km. The underwater margins of the continents (shelf plateaus and continental slope) account for about 1/5 of its surface area, on the so-called. “Transitional” zones (deep-sea trenches, island arcs, marginal seas) - about 1/10 of the area. The rest of the surface (about 250 ml sq. Km.) Is occupied by oceanic deep-water plains, depressions and intraoceanic uplifts separating them. The ocean floor differs sharply in the nature of seismicity. It is possible to identify areas with high seismic activity and aseismic areas. The first are extended zones occupied by systems of mid-oceanic ridges, stretching across all oceans. These zones are sometimes called oceanic mobile belts... Mobile belts are characterized by intense volcanism (tholeiitic basalts), increased heat flow, sharply dissected relief with systems of longitudinal and transverse ridges, troughs, ledges, shallow bedding of the mantle surface. Seismically low-active areas are expressed in the relief by large oceanic basins, plains, plateaus, as well as underwater ridges bounded by fault-type ledges and intraoceanic swell-like uplifts crowned with cones of active and extinct volcanoes. Within the regions of the second type, there are underwater plateaus and uplifts with continental crust (microcontinents). Unlike the moving ocean belts, these areas, by analogy with the structures of the continents, are sometimes called thalassocratons.

6. The structure of the oceanic crust in structures of different types. Oceanic trenches, as the largest negative structures of the earth's crust surface, have a number of structural features that allow them to be opposed to positive structures (continents) and to be compared with each other.

The main thing that unites and distinguishes all oceanic depressions is the low position of the earth's crust surface within them and the absence of a geophysical granite-metamorphic layer typical for continents. Moving belts - mountain systems of mid-ocean ridges with a high heat flow, an elevated position of the mantle layer, which is not typical for continents - stretch through all oceanic depressions. The system of mid-oceanic ridges, the longest on the Earth's surface, penetrates and thereby connects all oceanic depressions, occupying a central or marginal position in them. It is also characteristic that the tectonic structures of the oceanic floor are often closely related to the structures of the continents. First of all, these connections are expressed in the presence of common faults, in the transitions of rift valleys of the mid-oceanic ridges into continental rifts (Gulfs of California and Aden), in the presence of large submerged blocks of continental crust in the oceans, as well as depressions with granite-free crust on the continents, in the transitions trap fields of continents on the shelf and ocean floor. The internal structure of oceanic trenches is also different. According to the position of the zone of modern spreading, one can oppose the trough of the Atlantic Ocean with the median position of the Mid-Atlantic Ridge to all other oceans, in which the so-called. the median ridge is displaced to one of the edges. The internal structure of the Indian Ocean basin is complex. In the western part, it resembles the structure of the Atlantic Ocean, in the eastern part it is closer to the western region of the Pacific Ocean. Comparing the structure of the western Pacific with eastern part Indian, draws attention to their certain similarity: the depth of the bottom, the age of the crust (Coconut and Western Australian basins of the Indian Ocean, Western Pacific basin). In both oceans, these parts are separated from the continent and the depressions of the marginal seas by systems of deep-sea trenches and island arcs. The connection between the active margins of the oceans and the young folded structures of the continents is observed in Central America, where the Atlantic Ocean is separated from the Caribbean Sea by a deep-sea trench and island arc. The close connection of the deep-sea trenches separating the ocean depressions from the continental massifs with the structures of the continental crust is traced on the example of the northern continuation of the Sunda deep-sea trench, which turns into the Prearakan foredeep.

7. Structures of continental margins (oceans) and types of crust.

8. Types of boundaries of continental blocks and oceanic depressions. Continental massifs and oceanic depressions can have two types of boundaries - passive (Atlantic) and active (Pacific). The first type is distributed along the framing of most of the Atlantic, Indian, Arctic oceans. For this type, it is characteristic that through a continental slope of one or another steepness with a system of stepped faults, scarps and a relatively gentle continental foot, the continental massifs merge with the area of ​​abyssal plains of the ocean floor. In the zone of the continental foot, systems of deep troughs are known, but they are smoothed out by thick strata of loose sediments. The second type of margins is expressed around the Pacific Ocean, along the northeastern edge of the Indian Ocean and on the edge of the Atlantic Ocean adjacent to Central America. In these areas, between the continental massifs and the abyssal plains of the ocean floor, there is a zone of varying width with deep-sea trenches, island arcs, and depressions of the marginal seas.

9. Lithospheric plates and types of their boundaries. Studying the lithosphere, which includes the earth's crust and the upper mantle, geophysicists came to the conclusion that it contains their own inhomogeneities. First of all, these inhomogeneities of the lithosphere are expressed by the presence of strip zones crossing it over the entire thickness with a high heat flow, high seismicity, and active modern volcanism. The areas located between such strip zones are called lithospheric plates, and the zones themselves are considered as the boundaries of the lithospheric plates. In this case, one type of boundaries is characterized by tensile stresses (boundaries of separation of plates), another type - compression stresses (boundaries of convergence of plates), and the third - tension and compression arising from shears. The first type of boundaries is divergent (constructive) boundaries, which correspond to rift zones on the surface. The second type of boundaries is subduction (when oceanic blocks move beneath continental ones), obductional (when oceanic blocks thrust onto continental ones), collisional (when continental blocks move). On the surface, they are expressed by deep-sea trenches, marginal troughs, zones of large thrust faults, often with ophiolites (sutures). The third type of boundaries (shear) was named transform boundaries. It is also often accompanied by discontinuous chains of rift basins. Several large and small lithospheric plates are distinguished. Large plates include the Eurasian, African, Indo-Australian, South American, North American, Pacific, Antarctic. Small plates include the Caribbean, Scotia, Philippine, Coconut, Nazca, Arabian, etc.

10. Rifting, spreading, subduction, obduction, collision. Riftogenesis is a process of emergence and development in the earth's crust of continents and oceans that are stripe-like in terms of zones of horizontal extension on a global scale. In its upper fragile part, it manifests itself in the formation of rifts expressed in the form of large linear grabens, expansion cavities and related structural forms, and their filling with sediments and / or products of volcanic eruptions, usually accompanying rifting. In the lower, more heated part of the crust, brittle deformations during rifting give way to plastic stretching, leading to its thinning (the formation of a "neck"), and with especially intense and prolonged stretching - and a complete rupture of the continuity of the pre-existing crust (continental or oceanic) and the formation in the formed the "gaping" of the new oceanic type crust. The latter process, called spreading, powerfully proceeded in the Late Mesozoic and Cenozoic within the modern oceans, and on a smaller (?) Scale periodically manifested itself in some zones of more ancient mobile belts.

Subduction - subduction of the lithospheric plates of the oceanic crust and mantle rocks under the edges of other plates (according to the concepts of Plate Tectonics). It is accompanied by the emergence of zones of deep focus earthquakes and the formation of active volcanic island arcs.

Obduction is the thrusting of tectonic plates, made up of fragments of the oceanic lithosphere, onto the continental margin. As a result, an ophiolite complex is formed, which occurs when any factors disrupt the normal absorption of the oceanic crust into the mantle. One of the mechanisms of obduction is the lifting of the oceanic crust to the continental margin when it enters the subduction zone of the mid-ocean ridge. Obduction is a relatively rare phenomenon and has occurred only periodically in Earth's history. Some researchers believe that in our time this process takes place on the southwestern coast of South America.

Continental collision is a collision of continental plates that always leads to crustal collapse and the formation of mountain ranges. An example of a collision is the Alpine-Himalayan mountain belt formed as a result of the closure of the Tethys Ocean and the collision with the Eurasian plate of Hindustan and Africa. As a result, the thickness of the crust increases significantly, under the Himalayas it is 70 km. This is an unstable structure, its sides are intensively destroyed by surface and tectonic erosion. In the crust with a sharply increased thickness, granites are melted from metamorphosed sedimentary and igneous rocks.