It varies, and the dependence of the composition of the crust on the nature of the relief and the internal structure of the territory is revealed. The results of geophysical research and deep drilling made it possible to identify two main and two transitional types of the earth's crust. The main types mark such global structural elements of the crust as continents and oceans. These structures are perfectly expressed on the Earth, and they are characterized by continental and oceanic types of crust.
The continental crust is developed under the continents and, as already mentioned, has different thicknesses. Within platform areas corresponding to continental ones, this is 35-40 km, in young mountain structures - 55-70 km. The maximum thickness of the earth's crust - 70-75 km - is established under the Andes. Two strata are distinguished in the continental crust: the upper - sedimentary and the lower - consolidated crust. The consolidated crust contains two different-velocity layers: the upper granite-metamorphic, composed of granites and gneisses, and the lower granulite-mafic, composed of highly metamorphosed basic rocks such as gabbro or ultrabasic igneous rocks. The granite-metamorphic layer was studied from cores of ultra-deep wells; granulite-mafic - according to geophysical data and dredging results, which still makes its existence hypothetical.
In the lower part of the upper layer, a zone of weakened rocks is found, not much different from it in composition and seismic characteristics. The reason for its occurrence is the metamorphism of rocks and their decompression due to the loss of constitutional water. It is likely that the rocks of the granulite-mafic layer are still the same rocks, but even more highly metamorphosed.
Oceanic crust is characteristic of. It differs from the continental one in power and composition. Its thickness ranges from 5 to 12 km, averaging 6-7 km. From top to bottom, three layers are distinguished in the ocean crust: the upper layer of loose marine sedimentary rocks up to 1 km thick; middle, represented by interlayering of basalts, carbonate and siliceous rocks, 1-3 km thick; the lower one, composed of basic rocks such as gabbro, often altered by metamorphism to amphibolites, and ultrabasic amphibolites, thickness 3.5-5 km. The first two layers were penetrated by drill holes, the third was characterized by dredging material.
The suboceanic crust is developed under the deep-sea basins of the marginal and internal seas (Chernoe, etc.), and is also found in some deep depressions on land (the central part of the Caspian Sea). The thickness of the suboceanic crust is 10-25 km, and it is increased mainly due to the sedimentary layer lying directly on the lower layer of the ocean crust.
Subcontinental crust is characteristic of arcs (Aleutian, Kuril, South Antilles, etc.) and continental margins. In structure it is close to the continental crust, but has a smaller thickness - 20-30 km. A feature of the subcontinental crust is the unclear boundary between layers of consolidated rocks.
Thus, the different types of crust clearly divide the Earth into oceanic and continental blocks. The high position of the continents is explained by a thicker and less dense crust, and the submerged position of the ocean floor is explained by a thinner, but denser and heavier crust. The shelf area is underlain by continental crust and is the underwater end of the continents.
Structural elements of the cortex. In addition to dividing into such planetary structural elements as oceans and continents, the earth's crust (and) reveals regions (tectonically active) and aseismic (quiet). The inner regions of the continents and the beds of the oceans—the continental and oceanic platforms—are calm. Between the platforms there are narrow seismic zones, which are marked by tectonic movements. These zones correspond to mid-ocean ridges and junctions of island arcs or marginal mountain ranges and deep-sea trenches on the ocean periphery.
The following structural elements are distinguished in the oceans:
- mid-ocean ridges are mobile belts with axial rifts such as grabens;
- oceanic platforms are calm areas of abyssal basins with uplifts complicating them.
On continents, the main structural elements are:
- mountain structures (orogens), which, like mid-ocean ridges, can exhibit tectonic activity;
- platforms are mostly tectonically quiet vast territories with a thick cover of sedimentary rocks.
Mountain structures are separated and bordered by low areas - intermountain troughs and depressions, which are filled with products of the destruction of ridges. For example, the Greater Caucasus is bordered by the West Kuban, East Kuban and Terek-Caspian foredeeps, and is separated from the Lesser Caucasus by the Rioni and Kura intermontane depressions.
But not all ancient mountain structures were involved in re-orogenesis. Most of them, after leveling, slowly sank, were flooded by the sea, and a layer of marine layers was layered on top of the relics of the mountain ranges. This is how the platforms were formed. In the geological structure of platforms there are always two structural-tectonic levels: the lower one, composed of metamorphosed remains of former mountains, which is the foundation, and the upper one, represented by sedimentary rocks.
Platforms with a Precambrian foundation are considered ancient, while platforms with a Paleozoic and Early Mesozoic foundation are considered young. Young platforms are located between the ancient ones or border them. For example, between the ancient East European and Siberian platforms there is a young one, and on the southern and southeastern edge of the East European platform the young Scythian and Turanian platforms begin. Within the platforms, large structures of an anticlinal and synclinal profile, called anteclises and synclises, are distinguished.
So, the platforms are ancient denudated orogens, not affected by later (young) mountain-building movements.
In contrast to the quiet platform regions on Earth, there are tectonically active geosynclinal regions. The geosynclinal process can be compared to the work of a huge deep cauldron, where a new light continental crust is “cooked” from the ultrabasic and basic lithosphere material, which, floating up, builds up the continents in the marginal () and welds them together in intercontinental (Mediterranean) geosynclines. This process ends with the formation of folded mountain structures, in the arch of which they can work for a long time. Over time, the growth of mountains stops, volcanism dies out, the earth's crust enters a new cycle of its development: the leveling of the mountain structure begins.
Thus, where mountain ranges are now located, there used to be geosynclines. Large anticlinal and synclinal structures in geosynclinal regions are called anticlinoria and synclinoria.
Continents
Continents, or continents, are huge massifs-plates of relatively thick earth's crust (its thickness is 35-75 km), surrounded by the World Ocean, the crust under which is thin. Geological continents are somewhat larger than their geographical outlines, because have underwater extensions.
In the structure of continents, three types of structures are distinguished: platforms (flat forms), orogens (born mountains) and underwater margins.
Platforms
The platforms are distinguished by gently rolling, low-lying or plateau-like terrain. They have shields and a thick multi-layer cover. The shields are composed of very strong rocks, whose age ranges from 1.5 to 4.0 billion years. They arose at high temperatures and pressures at great depths.
The same ancient and durable rocks make up the rest of the platforms, but here they are hidden under a thick cloak of sedimentary deposits. This coat is called a platform cover. It can truly be compared to a furniture cover that protects it from damage. Parts of platforms covered with such a sedimentary cover are called slabs. They are flat, as if layers of sedimentary rocks had been ironed. About 1 billion years ago, layers of cover began to accumulate, and the process continues to the present day. If the platform could be cut with a huge knife, we would see that it looks like a layer cake.
SHIELDS have a round and convex shape. They arose where the platform slowly rose for a very long time. Strong rocks were subjected to the destructive action of air and water, and were influenced by changes in high and low temperatures. As a result, they cracked and crumbled into small pieces, which were carried away into the surrounding seas. The shields are composed of very ancient, highly altered (metamorphic) rocks, formed over several billion years at great depths at high temperatures and pressures. In some places, high temperatures caused the rocks to melt, which led to the formation of granite massifs.
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The structure of the planet on which we live has long occupied the minds of scientists. Many naive judgments and brilliant guesses were expressed, but until very recently no one could prove the rightness or wrongness of any hypothesis with convincing facts. And even today, despite the colossal successes of Earth science, primarily due to the development of geophysical methods for studying its interior, there is no single and final opinion about the structure of the inner parts of the globe.
True, all experts agree on one thing: the Earth consists of several concentric layers, or shells, inside of which there is a spherical core. The latest methods have made it possible to measure with great accuracy the thickness of each of these nested spheres, but what they are and what they consist of has not yet been fully established.
Some properties of the interior of the Earth are known for certain, while others can only be guessed at. Thus, using the seismic method, it was possible to establish the speed of passage of elastic vibrations (seismic waves) caused by an earthquake or explosion through the planet. The magnitude of this speed is, in general, very high (several kilometers per second), but in a denser medium it increases, in a loose medium it sharply decreases, and in a liquid medium such oscillations quickly die out.
Seismic waves can travel through the Earth in less than half an hour. However, upon reaching the interface between layers of different densities, they are partially reflected and return to the surface, where the time of their arrival can be recorded by sensitive instruments.
The fact that under the upper solid shell of our planet there is another layer was guessed back in ancient times. The ancient Greek philosopher Empedocles, who lived in the 5th century BC, was the first to say this. Watching the eruption of the famous volcano Etna, he saw molten lava and came to the conclusion that under the hard, cold shell of the earth's surface there was a layer of molten magma. A brave scientist died while trying to penetrate the crater of a volcano to better understand its structure.
The idea of the fiery-liquid structure of the deep earth's interior received its most striking development in the middle of the 18th century in the theory of the German philosopher I. Kant and the French astronomer P. Laplace. This theory lasted until the end of the 19th century, although no one was able to measure at what depth the cold solid crust ends and liquid magma begins. In 1910, the Yugoslav geophysicist A. Mohorovicic did this using the seismic method. While studying an earthquake in Croatia, he discovered that at a depth of 60-70 kilometers the speed of seismic waves changes sharply. Above this section, which was later called the Mohorovicic boundary (or simply “Moho”), the wave speed does not exceed 6.5-7 kilometers per second, while below it increases abruptly to 8 kilometers per second.
Thus, it turned out that directly under the lithosphere (crust) there is not molten magma at all, but, on the contrary, a hundred-kilometer layer, even denser than the crust. It is underlain by the asthenosphere (weakened layer), the substance of which is in a softened state.
Some researchers believe that the asthenosphere is a mixture of solid granules with liquid melt.
Judging by the speed of propagation of seismic waves, there are super-dense layers under the asthenosphere, down to a depth of 2900 kilometers.
It is difficult to say what this multilayered inner shell (mantle) located between the Moho surface and the core is. On the one hand, it has signs of a solid body (seismic waves propagate quickly in it), on the other, the mantle has undoubted fluidity.
It should be noted that the physical conditions in this part of the interior of our planet are completely unusual. There prevail high temperatures and colossal pressures of the order of hundreds of thousands of atmospheres. The famous Soviet scientist, academician D. Shcherbakov believes that the substance of the mantle, although solid, has plasticity. Perhaps it can be compared to shoe polish, which, under the blows of a hammer, breaks into fragments with sharp edges. However, over time, even in the cold, it begins to spread like a liquid and flow down a slight slope, and when it reaches the edge of the surface, it drips down.
The central part of the Earth, its core, is fraught with even more mysteries. What is it, liquid or solid? What substances does it consist of? Seismic methods have established that the core is heterogeneous and is divided into two main layers - outer and inner. According to some theories, it consists of iron and nickel, according to others - from super-densified silicon. Recently, the idea has been put forward that the central part of the core is iron-nickel, and the outer part is silicon.
It is clear that the best known of all geospheres are those that are accessible to direct observation and research: the atmosphere, hydrosphere and crust. The mantle, although it comes close to the earth's surface, is apparently not exposed anywhere. Therefore, there is no consensus even about its chemical composition. True, Academician A. Yanshin believes that some rare minerals from the so-called mer-richbite-redderite group, previously known only as part of meteorites and recently found in the Eastern Sayan Mountains, represent outcrops of the mantle. But this hypothesis still requires careful testing.
The earth's crust of the continents has been studied by geologists with sufficient completeness. Deep drilling played a big role in this. The upper layer of the continental crust is formed by sedimentary rocks. As the name itself shows, they are of aqueous origin, that is, the particles that formed this layer of the earth’s crust settled from aqueous suspension. The vast majority of sedimentary rocks were formed in ancient seas, less often they owe their origin to freshwater bodies of water. In very rare cases, sedimentary rocks arose as a result of weathering directly on land.
The main sedimentary rocks are sands, sandstones, clays, limestones, and sometimes rock salt. The thickness of the sedimentary layer of the crust varies in different parts of the earth's surface. In some cases it reaches 20-25 kilometers, but in some places there is no precipitation at all. In these places, the next layer of the earth’s crust emerges onto the “day surface” - granite.
It received this name because it is composed of both granites themselves and rocks close to them - granitoids, gneisses and micaceous schists.
The granite layer reaches a thickness of 25-30 kilometers and is usually covered on top by sedimentary rocks. The lowest layer of the earth's crust - basalt - is no longer accessible for direct study, since it does not reach the surface anywhere and deep wells do not reach it. The structure and properties of the basalt layer are judged solely on the basis of geophysical data. It is assumed with a high degree of certainty that this lower layer of crust consists of igneous rocks similar to basalts, originating from cooled volcanic lava. The thickness of the basalt layer reaches 15–20 kilometers.
Until recently, it was believed that the structure of the earth's crust is the same everywhere, and only in the mountains does it rise, forming folds, and under the oceans does it sink, forming giant bowls. One of the results of the scientific and technological revolution was the rapid development in the middle of the 20th century of a number of sciences, including marine geology. In this branch of human knowledge, many fundamental discoveries have been made that have radically changed previous ideas about the structure of the crust under the ocean floor. It was found that if under the marginal seas and near the continents, that is, in the shelf area, the crust is still to some extent similar to the continental crust, then the oceanic crust is completely different. Firstly, it has a very small thickness: from 5 to 10 kilometers. Secondly, under the ocean floor it consists not of three, but only of two layers - sedimentary, 1-2 kilometers thick, and basalt. The granite layer, so characteristic of the continental crust, continues towards the ocean only to the continental slope, where it breaks off.
These discoveries sharply intensified the interest of geologists in studying the ocean. There was hope of discovering outcrops of mysterious basalt, and perhaps even mantle, on the seabed. The prospects for underwater drilling, with the help of which one can reach deep layers through a relatively thin and easily surmountable layer of sediment, also look extremely tempting.
MAIN STRUCTURAL ELEMENTS OF THE EARTH'S CRUST: The largest structural elements of the earth's crust are continents and oceans.
Within the oceans and continents, smaller structural elements are distinguished; firstly, these are stable structures - platforms that can be found both in the oceans and on the continents. They are characterized, as a rule, by a leveled, calm relief, which corresponds to the same position of the surface at depth, only under continental platforms it is located at depths of 30-50 km, and under the oceans 5-8 km, since the oceanic crust is much thinner than the continental crust.
In the oceans, as structural elements, mid-ocean mobile belts are distinguished, represented by mid-ocean ridges with rift zones in their axial part, intersected by transform faults and which are currently zones spreading, i.e. expansion of the ocean floor and buildup of newly formed ocean crust.
On the continents, as structural elements of the highest rank, stable areas are distinguished - platforms and epiplatform orogenic belts, formed in the Neogene-Quaternary time in stable structural elements of the earth's crust after a period of platform development. Such belts include modern mountain structures of the Tien Shan, Altai, Sayan, Western and Eastern Transbaikalia, East Africa, etc. In addition, mobile geosynclinal belts that underwent folding and orogenesis in the Alpine era, i.e. also in Neogene-Quaternary times, they constitute epigeosynclinal orogenic belts, such as the Alps, Carpathians, Dinarides, Caucasus, Kopet Dag, Kamchatka, etc.
Structure of the Earth's crust of continents and oceans: The Earth's crust is the outer hard shell of the Earth (geosphere). Below the crust is the mantle, which differs in composition and physical properties - it is denser and contains mainly refractory elements. The crust and mantle are separated by the Mohorovicic boundary, where seismic wave velocities sharply increase.
The mass of the earth's crust is estimated at 2.8 1019 tons (of which 21% is oceanic crust and 79% is continental). The crust makes up only 0.473% of the Earth's total mass.
Oceanic bark: The oceanic crust consists mainly of basalts. According to the theory of plate tectonics, it continuously forms at mid-ocean ridges, diverges from them, and is absorbed into the mantle at subduction zones (the place where oceanic crust sinks into the mantle). Therefore, the oceanic crust is relatively young. Ocean. the crust has a three-layer structure (sedimentary - 1 km, basaltic - 1-3 km, igneous rocks - 3-5 km), its total thickness is 6-7 km.
Continental crust: The continental crust has a three-layer structure. The upper layer is represented by a discontinuous cover of sedimentary rocks, which is widely developed, but rarely very thick. Most of the crust is composed of the upper crust, a layer composed primarily of granites and gneisses that is low in density and ancient in 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. Average thickness 35 km.
Chemical composition of the Earth and the earth's crust. Minerals and rocks: definition, principles and classification.
Chemical composition of the Earth: consists mainly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%) ), calcium (1.5%) and aluminum (1.4%); the remaining elements account for 1.2%. Due to mass segregation, the interior is presumably composed of iron (88.8%), a small amount of nickel (5.8%), sulfur (4.5%)
Chemical composition of the earth's crust: The earth's crust is slightly more than 47% oxygen. The most common rock-component minerals in the earth's crust consist almost entirely of oxides; the total content of chlorine, sulfur and fluorine in rocks is usually less than 1%. The main oxides are silica (SiO2), alumina (Al2O3), iron oxide (FeO), calcium oxide (CaO), magnesium oxide (MgO), potassium oxide (K2O) and sodium oxide (Na2O). Silica serves mainly as an acidic medium and forms silicates; the nature of all major volcanic rocks is related to it.
Minerals: - natural chemical compounds arising as a result of certain physical and chemical processes. Most minerals are crystalline solids. The crystalline form is determined by the structure of the crystal lattice.
According to their prevalence, minerals can be divided into rock-forming minerals - which form the basis of most rocks, accessory minerals - often present in rocks, but rarely making up more than 5% of the rock, rare, the occurrence of which is rare or few, and ore minerals, widely represented in ore deposits.
Saints of minerals: hardness, crystal morphology, color, shine, transparency, cohesion, density, solubility.
Rocks: a natural collection of minerals of more or less constant mineralogical composition, forming an independent body in the earth’s crust.
Based on their origin, rocks are divided into three groups: igneous(effusive (frozen at depth) and intrusive (volcanic, erupted)), sedimentary And metamorphic(rocks formed deep within the earth's crust as a result of changes in sedimentary and igneous rocks due to changes in physicochemical conditions). Igneous and metamorphic rocks make up about 90% of the volume of the earth's crust, however, on the modern surface of the continents, the areas of their distribution are relatively small. The remaining 10% comes from sedimentary rocks, occupying 75% of the earth's surface area.
Types of Earth's crust: oceanic, continental
The Earth's crust (the solid shell of the Earth above the mantle) consists of two types of crust and has two types of structure: continental and oceanic. The division of the Earth's lithosphere into the crust and upper mantle is quite conventional; the terms oceanic and continental lithosphere are often used.
Earth's continental crust
The continental crust of the Earth (continental crust, continental crust) which consists of sedimentary, granite and basalt layers. The continental crust has an average thickness of 35-45 km, with a maximum thickness of up to 75 km (under mountain ranges).
The structure of the continental crust “American style” is somewhat different. It contains layers of igneous, sedimentary and metamorphic rocks.
Continental crust has another name "sial" - because. granites and some other rocks contain silicon and aluminum - hence the origin of the term sial: silicon and aluminum, SiAl.
The average density of the continental crust is 2.6-2.7 g/cm³.
Gneiss is a (usually loose layered structure) metamorphic rock composed of plagioclase, quartz, potassium feldspar, etc.
Granite is “an acidic igneous intrusive rock. It consists of quartz, plagioclase, potassium feldspar and micas” (article “Granite”, link at the bottom of the page). Granites consist of feldspars and quartz. Granites have not been discovered on other bodies of the solar system.
Oceanic crust of the Earth
As far as is known, a granite layer has not been found in the Earth’s crust at the bottom of the oceans; the sedimentary layer of the crust lies immediately on the basalt layer. The oceanic type of crust is also called "sima", the rocks are dominated by silicon and magnesium - similar to sial, MgSi.
The thickness of the oceanic crust (thickness) is less than 10 kilometers, usually 3-7 kilometers. The average density of the sub-oceanic crust is about 3.3 g/cm³.
It is believed that oceanic is formed in mid-ocean ridges and absorbed in subduction zones (why is not very clear) - as a kind of transporter from the growth line in the mid-ocean ridge to the continent.
Differences between continental and oceanic types of crust, hypotheses
All information about the structure of the earth's crust is based on indirect geophysical measurements, except for individual surface injections with wells. Moreover, geophysical research is mainly research into the speed of propagation of longitudinal elastic waves.
It can be argued that the “acoustics” (the passage of seismic waves) of the continental-type crust differs from the “acoustics” of the oceanic-type crust. And everything else is more or less plausible hypotheses based on indirect data.
"... in structure and material composition, both main types of lithosphere are radically different from each other, and the “basalt layer” of geophysicists in them is the same only in name, as well as the lithospheric mantle. These types of lithosphere also differ in age - if within the continental segments, the entire spectrum of geological events is established starting from approximately 4 billion years, then the age of the rocks of the bottom of modern oceans does not exceed the Triassic, and the age of the proven most ancient fragments of the oceanic lithosphere (ophiolites in the understanding of the Penrose Conference) does not exceed 2 billion years (Kontinen, 1987; Scott et al., 1998). Within the modern Earth, the oceanic lithosphere accounts for ~60% of the solid surface. In this regard, the question naturally arises: has there always been such a ratio between these two types of lithosphere or has it changed over time? and in general - have they both always existed? Obviously, answers to these questions can be given both by the analysis of geological processes at the destructive boundaries of lithospheric plates and by the study of the evolution of tectono-magmatic processes in the history of the Earth."
“Where does the ancient continental lithosphere disappear?”, E.V. Sharkov
What then are these - lithospheric plates?
http://earthquake.usgs.gov/learn/topics/plate_tectonics/
Earthquakes and Plate Tectonics:
"...a concept which has revolutionized thinking in the Earth"s sciences in the last 10 years. The theory of plate tectonics combines many of the ideas about continental drift (originally proposed in 1912 by Alfred Wegener in Germany) and sea-floor spreading (suggested originally by Harry Hess of Princeton University)."
Additional information on the structure of the lithosphere and sources
The Earth's Crust
Earth's crust
Earthquake Hazards Program - USGS.
Earthquake Hazards Program - United States Geological Survey.
The map of the globe shows:
tectonic plate boundaries;
thickness of the earth's crust, in kilometers.
For some reason, the map does not show the boundaries of tectonic plates on the continents; boundaries of continental plates and oceanic plates - boundaries of the earth's crust of continental and oceanic types.
