Oceanic and continental crust. Oceanic and continental crust: antipodes or different stages of lithosphere development? Thickness of the 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 solid body of the Earth. In the scientific geographical literature there is no single idea 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) claims 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 Latin mobilis - mobile) proves that the blocks of the lithosphere are in constant motion. This concept has been especially adopted in last years in connection with the receipt of new scientific data in the study of the bottom of the oceans.

3. The concept of the growth of continents at the expense of the ocean floor assumes that the original continents were formed in the form of relatively small massifs, which now make up the 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 mid-ocean ridges, gave reason to doubt the correctness of the concept of the growth of continents due to the ocean floor.

4. The theory of geosynclines states that the increase in the size of land occurs through the formation of mountains in geosynclines. The geosynclinal process, as one of the main ones in the development of the earth's crust of the continents, is the basis for many modern scientific explanations of the process of 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 rebuilt due to uneven rotation, zonal bands and meridional sectors on a rotating planet are inevitably tectonically unequal. They react with varying degrees of activity to tectonic stresses caused by intraterrestrial processes.

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

Oceanic Earth's 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 is not more than 1 km);

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

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

Continental (continental) crust. The continental crust has a more complex structure and greater thickness than the oceanic crust. Its average thickness is 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 (about 20 km thick);

2) the middle layer occupies the main thickness of the continental crust and is conditionally 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 regions of the Earth, the sedimentary layer is absent altogether and a granite layer comes to the surface. Such areas are called shields (eg Ukrainian Shield, Baltic Shield).

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

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

The boundary between the earth's crust and mantle (both on the continents and on the oceans) runs along Mohorovichic surface (Moho line). The speed of seismic waves on it jumps up to 8 km/h.

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

On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out in it. AT East Asia in the region of island arcs (the Kuril Islands, the Aleutian Islands, the Japanese Islands, and others), the earth's crust is of a transitional type. Finally, the earth's crust of the mid-ocean ridges is very complex and still little studied. There is no Moho boundary here, and the material of the mantle 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 earth's 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 has shown that all parts of the earth's crust - continents, mountainous countries, plains - are balanced by upper mantle. This balanced position 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. Heavy oceanic crust is thinner than lighter continental crust.

Isostasy is, in essence, not even an 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 balance line is currently running somewhat south of 60 0 N.L. Modern St. Petersburg is about 1.5 m higher than St. Petersburg during the time of Peter the Great. As data from modern scientific research, even the heaviness of large cities is sufficient for the isostatic fluctuation of the territory under them. Consequently, the earth's crust in the areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror reflection of the Moho surface, the soles of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to more high level its upper limit. So, under the Pamirs, 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 earth's surface The sun is a layer of constant temperature of the soil. 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 does not participate 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 of depth is called geothermal gradient . The distance in meters, when lowered by which the temperature rises by 1 0 C, is called geothermal stage . The value 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. In volcanic areas, the geothermal step can be only about 5 m, and in geologically calm areas (for example, on platforms) it can reach 100 m.

The earth's crust is a multi-layered 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 a crystalline foundation. In its structure lies the main differences between the continental and oceanic types of the earth's crust. On the continents, two thick layers are distinguished in the composition of the basement - “granite” and basalt. There is no "granite" layer under the abyssal bed of the oceans. However, the basalt basement of the ocean is by no means homogeneous in section; it is divided into the second and third layers.

Before ultra-deep and deep-water drilling, the structure of the earth's crust was judged mainly from geophysical data, namely, from 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 velocities of the passage of seismic waves change significantly. In the upper horizons, where weakly compacted sedimentary formations predominate, they are relatively small, while in crystalline rocks they increase sharply as their density increases.

After the velocities of seismic wave propagation in the rocks of the ocean floor were measured for the first time in 1949, it became clear that the velocity sections of the crust of the 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 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 located almost at the very bottom.

The standard velocity section of the continental crust includes the upper, sedimentary layer with a longitudinal wave velocity of 1–4 km/s, an intermediate, “granite” layer, 5.5–6.2 km/s, and a lower, basalt layer, 6.1–7.4 km /with. Below, as is believed, lies the so-called peridotite layer, which is already part of the asthenosphere, with velocities of 7.8-8.2 km/s. The names of the layers are conditional, since no one has yet seen real continuous sections of the continental crust, although the Kola superdeep well has already penetrated 12 km deep into the Baltic Shield.

an average of 5.12 km/s, and the average thickness under the ocean floor 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 continental margins, the thicknesses of the second layer increase quite sharply. Under this layer, a third layer of the crust stands out 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 was previously assumed, which is apparently associated with deep heterogeneities that exist in it.

As we can see, the propagation velocities 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 of ancient Mesozoic, Paleozoic and Precambrian formations in its composition on the continents, which have undergone rather complex transformations in the bowels. 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/cm3 approach 5.5 km/s. It follows from this that if the “granite” layer on the continents is indeed composed of crystalline rocks, among which metamorphic formations of the lower stages of transformation predominate (according to the data of ultra-deep drilling on the Kola Peninsula), then the composition of the second layer of the oceanic crust, in addition to basalts, should include formations with a density lower than that of crystalline rocks (2–2.55 g/cm3).

Indeed, during the 37th voyage of the drilling vessel "Glomar Challenger" the rocks of the oceanic basement were uncovered. The drill penetrated several basalt sheets, between which there were horizons of carbonate pelagic sediments. In one of the wells, an 80-meter stratum of basalts with limestone interbeds 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 ultramafic rocks - gabbro and ultramafic rocks, which probably already belong to the third layer of the oceanic crust.

Deep-sea drilling and the study of rift zones from manned underwater vehicles (UAVs) made it possible to elucidate in general terms the structure of the oceanic crust. True, it is impossible to assert with certainty that we know its complete and continuous section, not distorted by subsequent superimposed processes. At present, the upper, sedimentary layer, partially or completely exposed at almost 1000 points of the bottom, has been studied in most detail by the Glomar Challenger and Joydes Resolution drills. Much less explored is the second layer of the oceanic crust, which has been penetrated to a certain depth by a much smaller number of boreholes (a few 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 belong to tholeiite varieties that arose in underwater conditions. These are pillow lavas, often composed of hollow lava tubes and pillows. The sediments located between the 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 dike belt - a series of small igneous bodies (intrusions), closely fitted one to the other. The composition of these intrusions is basic to ultrabasic. These are gabbro and hyperbasite, which were formed not during the outpouring of magmas on the bottom surface, like basalts of the second layer, but in the depths of the crust itself. In other words, we are talking about magmatic melts that solidified near the magma chamber without reaching the bottom surface. Their "heavier" ultramafic composition indicates the residual nature of these magmatic melts. If we recall that the thickness of the third layer is usually 3 times the thickness of the second layer of the oceanic crust, then its definition as basaltic may seem like a great exaggeration.

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

Both varieties of the earth's crust are not antagonists. In the marginal parts of the young oceans, the Atlantic and Indian, the boundary between the continental and oceanic crust is somewhat "blurred" 8a due to the gradual thinning of the former in the region of transition from the continent to the ocean. On the whole, this boundary is tectonically calm, i.e., it does not manifest itself either as powerful seismic shocks, which occur here extremely rarely, or as volcanic eruptions.

However, this situation does not hold everywhere. AT pacific ocean The boundary between continental and oceanic crust is perhaps one of the most dramatic division lines on our planet. So what, after all, are these two varieties of the earth's crust antipodes or not? It seems that we can justifiably 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 of one type of crust to another. As will be shown below, the continental crust is formed in specific tectonic settings in active transition zones between the mainland and the ocean, and mainly as a result of the transformation of another type of earth's crust, called suboceanic. The oceanic substrate disappears in the Benioff zones, or is squeezed out like a flipper from a tube, to the edge of the continent, or turns into tectonic melange (crushed from ground rocks) in the areas of "collapsing" of the oceans.

The earth consists of several shells: atmosphere, hydrosphere, biosphere, lithosphere.

Biosphere- a special shell of the earth, the area of vital activity of living organisms. It includes the lower part of the atmosphere, the entire hydrosphere and the upper part of the lithosphere. The lithosphere is the hardest shell of the earth:

Structure:

Earth's crust

mantle (Si, Ca, Mg, O, Fe)

outer core

inner core

center of the earth - temperature 5-6 thousand o C

The core composition is Ni\Fe; core density - 12.5 kg / cm 3;

Kimberlites- (from the name of the city of Kimberley in South Africa), an igneous ultrabasic brecciated rock of an effusive appearance that fills the explosion pipes. It consists mainly of olivine, pyroxenes, pyrope-almandine garnet, picroilmenite, phlogopite, less often zircon, apatite, and other minerals included in a fine-grained groundmass, usually altered by post-volcanic processes to a serpentine-carbonate composition with perovskite, chlorite, etc. d.

eclogite- metamorphic rock consisting of pyroxene with a high content of jadeite minal (omphacite) and grossular-pyrope-almandine garnet, quartz and rutile. In terms of chemical composition, eclogites are identical to the magmatic rocks of the basic composition - gabbro and basalts.

The structure of the earth's crust

Layer thickness =5-70 km; highlands - 70 km, seabed - 5-20 km, on average 40-45 km. Layers: sedimentary, granite-gneiss (none in the oceanic crust), granite-bosite (basalt)

The earth's crust is a complex of rocks lying above the Mohorovichic boundary. Rocks are natural aggregates of minerals. The latter are composed of various chemical elements. The chemical composition and internal structure of minerals depend on the conditions of their formation and determine their properties. In turn, the structure and mineral composition of rocks indicate the origin of the latter and make it possible to determine rocks in the field.

There are two types of the earth's crust - continental and oceanic, which differ sharply in composition and structure. The first, lighter, forms elevated areas - continents with their underwater margins, the second occupies the bottom of the oceanic depressions (2500-3000m). The continental crust consists of three layers - sedimentary, granite-gneiss and granulite-basite, with a thickness of 30-40 km on the plains to 70-75 km under the young mountains. The oceanic crust up to 6-7 km thick has a three-layer structure. Under a thin layer of loose sediments lies the second oceanic layer, consisting of basalts, the third layer is composed of gabbro with subordinate ultrabasic rocks. The continental crust is enriched in silica and light elements - Al, sodium, potassium, C, in comparison with the oceanic one.

Continental (mainland) crust characterized by high power - an average of 40 km, sometimes reaching 75 km. It consists of three "layers". On top lies a sedimentary layer formed by sedimentary rocks of different composition, age, genesis and degree of dislocation. Its thickness varies from zero (on shields) to 25 km (in deep depressions, for example, the Caspian one). Below lies the "granite" (granite-metamorphic) layer, consisting mainly of acidic rocks, similar in composition to granite. The greatest thickness of the granite layer is noted under the young high mountains, where it reaches 30 km or more. Within the flat areas of the continents, the thickness of the granite layer decreases to 15-20 km. Under the granite layer lies the third, “basalt”, layer, which also received its name conditionally: seismic waves pass through it at the same speeds with which, under experimental conditions, they pass through basalts and rocks close to them. The third layer, 10–30 km thick, is composed of highly metamorphosed rocks of predominantly mafic composition. Therefore, it is also called granulite-mafic.

Oceanic crust sharply different from the continental. Over most of the area of the ocean floor, its thickness varies from 5 to 10 km. Its structure is also peculiar: under a sedimentary layer with a thickness of several hundred meters (in deep-sea basins) to 15 km (near the continents), there is a second layer composed of pillow lavas with thin layers of sedimentary rocks. The lower part of the second layer is composed of a peculiar complex of parallel dikes of basaltic composition. The third layer of the oceanic crust, 4-7 km thick, is represented by crystalline igneous rocks of predominantly basic composition (gabbro). Thus, the most important specific feature of the oceanic crust is its low thickness and the absence of a granite layer.

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

After the velocities of seismic wave propagation in the rocks of the ocean floor were measured for the first time in 1949, it became clear that the velocity sections of the crust of the 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 mountain-fold systems even 60 and 70 km, then in the ocean it does not exceed 5-10, rarely 15 km, and in some areas the mantle is located almost at the very bottom.

The standard velocity section of the continental crust includes the upper, sedimentary layer with a P-wave velocity of 1–4 km/s, an intermediate, “granite” layer, 5.5–6.2 km/s, and a lower, basaltic layer, 6.1–7.4 km /with. Below, it is believed, lies the so-called peridotite layer, which is already part of the asthenosphere, with velocities of 7.8–8.2 km/s. The names of the layers are conditional, since no one has yet seen real continuous sections of the continental crust, although the Kola superdeep well has already penetrated 12 km deep into the Baltic Shield.

In the abyssal basins of the ocean, 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. Worzel and other scientists, it has surprisingly similar speeds - 4.93–5.23 km / s, an average of 5.12 km / s, and the average thickness under the ocean floor 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 continental margins, the thicknesses of the second layer increase quite sharply. Under this layer, a third layer of the crust stands out 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 thought, which is apparently associated with deep heterogeneities that exist in it (Pushcharovsky, 1987).

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

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 approach 5.5 km/s. It follows from this that if the "granite" layer on the continents is indeed composed of crystalline rocks, among which metamorphic formations of the lower stages of transformation predominate (according to the data of ultra-deep drilling on the Kola Peninsula), then the composition of the second layer of the oceanic crust, in addition to basalts, should include formations with a density less than that of crystalline rocks (2–2.55 g / cm 3).

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

Deep-sea drilling and the study of rift zones from manned underwater vehicles (UAVs) made it possible to elucidate in general terms the structure of the oceanic crust. True, it is impossible to assert with certainty that we know its complete and continuous section, not distorted by subsequent superimposed processes. At present, the upper, sedimentary layer, partially or completely exposed at almost 1000 points of the bottom, has been studied in the most detail by the Glomar Challenger and Joydes Resolution drills. Much less explored is the second layer of the oceanic crust, which has been penetrated to a certain depth by a much smaller number of boreholes (a few 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 belong to tholeiite varieties that arose in underwater conditions. These are pillow lavas, often composed of hollow lava tubes and pillows. The sediments located between the basalts in the central parts of the ocean consist of the remains of the smallest planktonic organisms with a carbonate or siliceous function.

Both varieties of the earth's crust are not antagonists. In the marginal parts of the 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 transition region from the continent to the ocean. On the whole, this boundary is tectonically calm, i.e., it does not manifest itself either as powerful seismic shocks, which occur here extremely rarely, or as volcanic eruptions.

However, this situation does not hold everywhere. In the Pacific, the boundary between continental and oceanic crust is perhaps one of the most dramatic dividing lines on our planet. So what, after all, are these two varieties of the earth's crust antipodes or not? It seems that we can justifiably 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 of one type of crust to another. As will be shown below, the continental crust is formed in specific tectonic settings in active transition zones between the mainland and the ocean, and mainly as a result of the transformation of another type of earth's crust, called suboceanic. The oceanic substrate disappears in the Benioff zones, or is squeezed out like paste from a tube to the edge of the continent, or turns into tectonic melange (crushed ground rocks) in the areas of "collapsing" oceans. However, more on that later.

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 - by granite or gneiss, the third consists of basalt, granulites and other metamorphic rocks.

continental crust

The thickness of the continental crust is about 35-45 km, sometimes it reaches 75 km (as a rule, in the areas of mountain ranges). The considered type of the Earth's crust covers approximately 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 that forms oceanic crust, - basalt. But besides him, its structure includes:

sedimentary rocks;
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, the age of its oldest sections is about 200 million years.

oceanic crust

The thickness of the oceanic crust is about 5-10 km, depending on the specific area of measurements. It can be noted that over time it almost does not change. 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 largely depends on age.

Comparison

The main difference between the continental crust and the oceanic crust, obviously, lies 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 - predominantly basalt.

The continental crust is much thicker and older. It is inferior to the oceanic in terms of the area covered by the earth's surface, but it is superior in terms of the volume occupied 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 fix the conclusions in a small table.

Table

continental crust	oceanic crust
Places on itself continents, land	Carries oceans and seas
Represented mainly by sedimentary rocks, granites, granulites	Composed predominantly of basalt
It has a thickness of up to 75 km, usually 35-45 km	Has a thickness usually within 10 km
The age of some parts of the continental crust reaches 4.4 billion years	The oldest parts of the oceanic crust are about 200 million years old.
Occupies about 40% of the Earth's surface	Occupies about 60% of the Earth's surface
Occupies about 70% of the volume of the earth's crust	Occupies about 30% of the volume of the earth's crust