Oceanic crust, plate tectonics, study methods - geology. The structure of the earth's crust

Earth's crust called the outer solid shell of the Earth, bounded from below by the surface of Mohorovichich, or Moho, which is distinguished by a sharp increase in the speed of elastic waves as they pass from the surface of the Earth into its depths.

Below the Mohorovichic surface is the following hard shell - upper mantle . The uppermost part of the mantle, together with the earth's crust, is a rigid and brittle solid shell of the Earth. - lithosphere (a rock). It is underlain by more plastic and pliable to deformation, less viscous layers of the mantle - asthenosphere (weak). In it, the temperature is close to the melting point of the mantle substance, but due to the high pressure, the substance does not melt, but is in an amorphous state and can flow, remaining solid, like a glacier in the mountains. It is the asthenosphere that is the plastic layer along which individual blocks of the lithosphere float.

The thickness of the earth's crust on the continents is about 30-40 km, under the mountain ranges it increases to 80 km (continental type of the earth's crust). Under the deep part of the oceans, the thickness of the earth's crust is 5-15 km (oceanic type of the earth's crust). On average, the sole of the earth's crust (the surface of Mohorovichich) lies under the continents at a depth of 35 km, and under the oceans at a depth of 7 km, i.e., the oceanic crust is about five times thinner than the continental crust.

In addition to differences in thickness, there are differences in the structure of the earth's crust of continental and oceanic types.

continental crust consists of three layers: upper - sedimentary, extending on average to a depth of 5 km; medium granite (the name is due to the fact that the speed of seismic waves in it is the same as in granite) with an average thickness of 10-15 km; the lower one is basalt, about 15 km thick.

oceanic crust also consists of three layers: upper - sedimentary to a depth of 1 km; medium-sized with a little-known composition, occurring at depths from 1 to 2.5 km; the lower one is basaltic with a thickness of about 5 km.

A visual representation of the nature of the distribution of land heights and depths of the ocean floor gives hypsographic curve (Fig. 1). It reflects the ratio of the areas of the solid shell of the Earth with different heights on land and with different depths in the sea. Using the curve, the average values ​​of the land height (840 m) and the average sea depth (-3880 m) are calculated. If we do not take into account the mountainous regions and deep-water depressions, which occupy a relatively small area, then two predominant levels are clearly distinguished on the hypsographic curve: the level of the continental platform with a height of about 1000 m and the level of the oceanic bed with elevations from -2000 to -6000 m. the zone is a relatively sharp ledge and is called the continental slope. Thus, the natural boundary separating the ocean and the continents is not the visible coastline, but the outer edge of the slope.

Rice. Fig. 1. Hypsographic curve (A) and generalized profile of the ocean floor (B). (I - underwater margin of the continents, II - transition zone, III - ocean floor, IV - mid-ocean ridges).

Within the oceanic part of the hypsographic (batygraphic) The curve distinguishes four main stages of the bottom topography: the continental shelf or shelf (0-200 m), the continental slope (200-2000 m), the ocean floor (2000-6000 m) and deep-water depressions (6000-11000 m).

Shelf (mainland)- underwater continuation of the mainland. This is an area of ​​the continental crust, which is generally characterized by a flat relief with traces of flooded river valleys, Quaternary glaciation, and ancient coastlines.

The outer boundary of the shelf is edge - a sharp inflection of the bottom, beyond which the continental slope begins. The average depth of the shelf crest is 130 m, however, in specific cases, its depth may vary. The width of the shelf varies in a very wide range: from zero (in a number of areas of the African coast) to thousands of kilometers (off the northern coast of Asia). In general, the shelf occupies about 7% of the area of ​​the World Ocean.

continental slope- the area from the edge of the shelf to the continental foot, i.e., before the transition of the slope to a flatter ocean bed. The average angle of inclination of the continental slope is about 6°, but often the steepness of the slope can increase up to 20-30 0 , and in some cases almost sheer ledges are possible. The width of the continental slope due to the steep drop is usually small - about 100 km.

The relief of the continental slope is characterized by great complexity and diversity, but its most characteristic form is submarine canyons . These are narrow gutters with a large angle of incidence along the longitudinal profile and steep slopes. The tops of underwater canyons often cut into the edge of the shelf, and their mouths reach the continental foot, where in such cases alluvial fans of loose sedimentary material are observed.

mainland foot- the third element of the topography of the ocean floor, located within the continental crust. The continental foot is a vast sloping plain formed by sedimentary rocks up to 3.5 km thick. The width of this slightly hilly plain can reach hundreds of kilometers, and the area is close to that of the shelf and continental slope.

Ocean bed- the deepest part of the ocean floor, occupying more than 2/3 of the entire area of ​​\u200b\u200bthe World Ocean. The prevailing depths of the ocean floor range from 4 to 6 km, and the bottom relief is the most calm. The main elements of the relief of the ocean floor are oceanic basins, mid-ocean ridges and oceanic uplifts.

oceanic basins- extensive depressions of the ocean floor with depths of about 5 km. The leveled surface of the bottom of the basins is called abyssal (bottomless) plains, and it is due to the accumulation of sedimentary material brought from land. Abyssal plains in the World Ocean occupy about 8% of the ocean floor.

mid-ocean ridges- tectonically active zones in the ocean, in which the neoformation of the earth's crust occurs. They are composed of basalt rocks formed as a result of the entry of matter from the upper mantle from the bowels of the Earth. This led to the peculiarity of the earth's crust of the mid-ocean ridges and its allocation to the rift type.

ocean rises- large positive landforms of the ocean floor, not associated with mid-ocean ridges. They are located within the oceanic type of the earth's crust and are distinguished by large horizontal and vertical dimensions.

Separate seamounts of volcanic origin have been discovered in the deep part of the ocean. Seamounts with flat tops, located at a depth of more than 200 m, are called guyots.

Deep sea trenches (troughs)- zones of the greatest depths of the World Ocean, exceeding 6000 m.

The deepest depression is the Mariana Trench, discovered in 1954 by the Vityaz research vessel. Its depth is 11022 m.

From Wikipedia, the free encyclopedia

Age of the oceanic crust. Red shows the youngest sites, blue - the most ancient.

oceanic crust- type of earth's crust, common in the oceans. The crust of the oceans differs from the continents in its smaller thickness (thickness) and basaltic composition. It is formed at mid-ocean ridges and absorbed in subduction zones. Ancient fragments of oceanic crust preserved in folded structures on continents are called ophiolites. In the mid-ocean ridges, intense occurs, as a result of which easily soluble elements are taken out of it.

Annually, 3.4 km² of oceanic crust with a volume of 24 km³ and a mass of 7 × 10 10 tons of igneous rocks is formed in the mid-ocean ridges. The average density of the oceanic crust is about 3.3 g/cm³. The mass of the oceanic crust is estimated at 5.9 × 10 18 tons (0.1% of the total mass of the Earth, or 21% of the total mass of the crust). Thus, the average time of renewal of the oceanic crust is less than 100 Ma; the oldest oceanic crust, located in the ocean floor, was preserved in the basin Pigafetta in the Pacific Ocean and has a Jurassic age (156 million years).

The oceanic crust consists mainly of basalts and, being absorbed in subduction zones, turns into highly metamorphosed rocks - eclogites. Eclogites have a density greater than the most common mantle rocks, peridotites, and sink in depth. They linger at the boundary between the upper and lower mantle, at a depth of about 660 kilometers, and then penetrate into the lower mantle. According to some estimates, eclogites, which previously formed the oceanic crust, now make up about 7% of the mantle mass.

Relatively small fragments of the ancient oceanic crust can be excluded from the spreading-subduction circulation in closed basins closed as a result of the collision of the continents. An example of such a site can be the northern part of the Caspian Sea depression, the foundation of which, according to some researchers, is composed of Devonian oceanic crust.

Oceanic crust can creep on top of continental crust, as a result of obduction. This is how the largest ophiolite complexes of the type of the Semail ophiolite complex are formed.

The structure of the oceanic crust

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

• sedimentary rocks represented by deep oceanic sediments.
• basalt covers erupted under water.
• The dike complex consists of nested basalt dikes.
• layer of basic layered

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

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 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 the weathering of rocks, a geological formation is formed, called the weathering crust.

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

The boundary between the earth's crust and the mantle (both on the continents and on the oceans) runs along the 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. In East Asia, in the area 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". In 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 on the 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 - in essence, it is 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 the data of modern scientific research show, 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. On the whole, the relief of the earth's crust is a mirror image of the Moho surface, the sole of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to a higher level of its upper boundary. 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. a layer of constant soil temperature. It is called an 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 the geothermal gradient. The distance in meters at which the temperature rises by 1 0 C when lowered is called the geothermal step. 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.

There are significant differences in the structure of the earth's crust under the deep part of the ocean and on the continents. The thickness of the earth's crust on the continents is about 30-40 km, under the mountain ranges it increases to 80 km. Under the deep part of the ocean, the thickness of the earth's crust is 5-15 km. On average, the sole of the earth's crust lies under the continents at a depth of 35 km. and under the oceans at a depth of 7 km, i.e. The oceanic crust is about 5 times thinner than the continental crust.

In addition to the difference in thickness, there are significant differences in the structure of the earth's crust of continental and oceanic types.

The continental crust consists of three layers: the upper sedimentary layer, formed from the products of the destruction of crystalline rocks and extending on average to a depth of 5 km; medium granite (seismic wave velocity as in granite), consisting of crystalline and metamorphic rocks and having a thickness of 10-15 km; lower basalt, about 15 km thick.

The oceanic crust also consists of three layers: the upper sedimentary layer extending to a depth of 1 km; medium-sized with a little-known composition, occurring at depths of 1-2.5 km; lower basaltic, having an average thickness of about 5 km.

The boundary between the continental and oceanic types of the earth's crust passes, on average, along the isobath of 2000 m. At this depth, the granite layer is wedged out and disappears. The boundary between the continental and oceanic types of the earth's crust is not always clearly defined. Individual regions are characterized by a gradual transition from the earth's crust of an oceanic type to a continental one. So, for example, for the Far Eastern seas, the basin of the marginal sea adjoins the edge of the continental platform; the granite layer is absent, but the sedimentary layer is so developed that the total thickness of the earth's crust in the basins of the Far Eastern seas is 15-20 km (suboceanic type).

The border of the seas and oceans are bottom uplifts - island arcs. The earth's crust in the region of island arcs is similar in structure and thickness to the continental type and is called subcontinental.

The term "transitional zone" is used in a double sense: firstly, the transitional position of a certain zone between the mainland and the ocean is stated (in this sense, the continental slope with its foot can be considered a transitional zone), secondly, the genetic and historical meaning of this concept is emphasized, the zone where the transition occurs, the transformation of one state of the earth's crust into another.

The sea basin-island arc-deep-water trench complexes form areas of the transition zone. Comparison of these areas allows us to divide them into several types that make up a certain genetic series.

1. Vityazevsky type. The area including the Vityaz trench belongs to this type. It is characterized by: the absence of a clearly defined island arc, a relatively shallow trench depth, and weak seismicity.

2. Mariana type. Mariana transitional region. A clearly defined (mainly in the form of an underwater ridge) island arc, a very deep trench, intense seismicity and volcanism, a low thickness of the sedimentary layer in the trench and in the sea basin, which essentially does not differ from adjacent oceanic basins.

3. Kuril type. In many features, the transitional region is similar to the previous type, but it differs by a much greater isolation of sea basins, a suboceanic type of the earth's crust under their bottom, and a much larger size of the islands. There are areas with a subcontinental crust, island arcs are often double. The intensity of seismic and volcanic processes reaches its maximum. The depths of the trenches are very large. The thickness of the sedimentary layer in the trenches and basins increases noticeably.

4. Japanese type. Island arcs of different ages merge into single large massifs of island or peninsular land. Large-sized areas of a typical continental crust appear. The intensity of volcanism is greatly reduced, but the intensity of seismic processes is still very high. The bottoms of sea basins are composed of suboceanic crust with a thick sedimentary layer.

Two more varieties adjoin the type under consideration, which can be called Indonesian and East Pacific. They are united by a very significant participation of continental elements in the structure of the transitional region, a smaller (compared to the previous type) depth of the trenches, and often a decline in volcanic activity.

5. Mediterranean type. It is characterized by a further increase in the role of the continental crust. Suboceanic basins remain in the form of "windows", surrounded on all sides by the continental crust. The former island arcs are essentially young mountain structures that form the edge of the continent or its peninsula. Deep-sea trenches are either preserved as relics (the Hellenic Trench in the Mediterranean Sea), or they are absent.

The thickness of the suboceanic crust in the basins is very high; modern folded processes or the formation of diorite structures are possible in the loose cover (for example, the South Caspian, the Balearic Basin of the Mediterranean Sea). In transitional zones, one can also find typically oceanic crust (the bottom of the Philippine Sea) and typically continental crust (Japan Islands). The transition zones are characterized by high seismicity and high relief contrast: the tops of the island arcs rise to 3–4 km, and the sea depth in the trenches can reach 11 km. This indicates the intensity of tectonic movements of the earth's crust in the transition zones characteristic of geosynclinal regions, therefore this type of earth's crust is also called geosynclinal.

Within the oceanic crust, another type is distinguished - riftogenic, characteristic of the zones of mid-ocean ridges. The main feature of the structure of the oceanic crust in the zones of mid-ocean ridges is that the sedimentary cover at the bottom of the axial rift valleys is practically absent, and the thickness of the sedimentary layer increases with distance from the ridge. High seismicity, high values ​​of heat flow, and anomalies in geophysical characteristics also testify to the peculiarity of the structure of the oceanic crust of the riftogenic type.

Thus, within the limits of the World Ocean, the earth's crust is represented by continental and oceanic types, transitional (geosynclinal) and riftogenic.

The oceanic crust has a characteristic relief. In the abyssal basins, the ocean floor lies at a depth of about 6-6.5 km, while on the MOR ridges, sometimes dissected by deep gorges (rift valleys), its level is raised to about -2.5 km, and in some places the ocean floor comes out directly on the daytime surface of the Earth (for example, on the island of Iceland and in the province of Afar in Northern Ethiopia). In front of the island arcs surrounding the western periphery of the Pacific Ocean, the northeast Indian Ocean, in front of the arc of the Lesser Antilles and South Sandwich Islands in the Atlantic, as well as in front of the active continental margin in Central and South America, the oceanic crust sags and sinks to a depth of 9-10 km , going further under these structures and forming in front of them narrow and extended deep-water trenches.[ ...]

The oceanic crust is formed in the MOR rift zones due to the release of basaltic melts from the Earth's asthenospheric layer and the outpouring of tholeiitic basalts onto the ocean floor (see Fig. 1.2). Every year in these zones, at least 12 km3 of basalt melts rise from the asthenosphere, crystallize and pour onto the ocean floor, which form the entire second and part of the third layer of the oceanic crust. These grandiose tectono-magmatic processes, constantly developing under the MOR ridges, are unparalleled on land and are accompanied by increased seismicity.[ ...]

The oceanic crust is relatively simple in its composition and, in essence, represents the upper differentiated layer of the mantle, overlain from above by a thin layer of pelagic sediments. Over the past decades, thanks to seismic work in the World Ocean and the development of new seismic methods, generalizing models of the structure of the oceanic crust have been obtained and the main characteristics of its constituent layers have been identified. There are three main layers in the oceanic crust.[ ...]

The oceanic crust is much thinner than the continental crust and consists of two layers. Its minimum thickness does not exceed 5 - 7 km. The upper layer of the earth's crust here is represented by loose deep-sea sediments. Its thickness is usually determined as several hundred meters, and below is a basalt layer with a thickness of several kilometers.[ ...]

The layers of the oceanic crust are conditionally divided into primary magnetic and primary non-magnetic. The first group includes layer 2A (extrusive basalts), layer 2B (dyke complex), and layer 3A (intrusive isotropic gabbro). The second group includes the ST layer (cumulative gabbro and layered complex). Such division of rocks occurs in the process of differentiation of magma and crystallization of the residual melt. The degree of differentiation of the residual melt determines the amount and state of titanomagnetite, the main ferromagnetic mineral in extrusive rocks. Primary titanomagnetites are formed in the axial part of the MOR rift zone during the crystallization of basaltic melts and acquire magnetization when these basalts are cooled to the Curie temperature.[ ...]

Layer 2B of the oceanic crust is a complex of dikes similar in composition to the overlying basaltic layer 2A. The rocks of layer 2B are less accessible for study than the basalts of layer 2A, since they are exposed mainly in ophiolite complexes, in transform faults, and in rare deep-sea drilling holes (for example, well 504B on the southern flank of the Costa Rica ridge). Due to the low accessibility of the rocks of layer 2B, the knowledge of their petromagnetic properties is worse than for the basalts of layer 2A. The scatter in the values ​​of natural remanent magnetization and the Koenigsberg factor for these rocks is very large. Although their most realistic average values ​​vary, respectively, from 1.5 to 2 A/m and about 5 A/m.[ ...]

The earth's crust is not the same in composition, structure and thickness. There are continental, oceanic and intermediate crusts. The continental (mainland) crust covers a third of the globe, it is inherent in the continents, including their underwater margins, has a thickness of 35-70 km and consists of 3 layers: sedimentary, granite and basalt. The oceanic crust is located under the oceans, has a thickness of 5-15 km and consists of 3 layers: sedimentary, basalt and gabbro-serpentinite. The intermediate (transitional) crust has features of both continental and oceanic crust.[ ...]

Oceanic crust differs sharply from continental in the homogeneity of its composition. Under a thin layer of sediments, it is represented by tholeiitic basalts of practically unchanged chemical composition (see Table 1.2) at any point in the World Ocean. We can talk about the constancy of the composition of the oceanic crust in the same way as we talk about the constancy of the composition of sea water or the atmosphere. This is one of the global constants, which, together with the constant thickness of the oceanic crust, testifies to a single mechanism of its formation. In the crust, elevated contents of the main long-lived radioactive isotopes - uranium (232 3), thorium (MT) and potassium (K) are noted. The highest concentration of radioactive elements is characteristic of the "granite" layer of the continental crust. The content of radioactive elements in the oceanic crust is negligible.[ ...]

The second layer of the oceanic crust is basalt, in its upper part it is composed of pillow lavas of oceanic-type tholeiitic basalts (layer 2A). Below are dolerite dikes of the same composition (layer 2B) (Fig. 1.2). The total thickness of the basalt layer of the oceanic crust, according to seismic data, reaches 1.4-1.5, sometimes 2 km.[ ...]

Crustal fracturing is likely responsible for the reduced seismic wave values ​​in layer 2A of the oceanic crust. This layer, with a thickness of about 500 m, is characterized by a volume velocity of seismic waves of only 2.5–3.8 km/s, which is noticeably lower than the velocity characteristic of individual samples (5.6–6.0 km/s). Subsequently, the cracks are filled with sediments and sealed in the process of low-temperature diagenetic cementation. High-temperature metal-bearing solutions also tend to fill cracks with hydrothermal minerals. As these processes continue, the seismic velocity of layer 2A will increase (up to 5.5 km/sec), and it is difficult to distinguish the fractured zone from seismic wave velocities.[ ...]

The continental crust, both in structure and composition, differs sharply from the oceanic one: its thickness varies from 20-25 km under island arcs and areas with a transitional type of crust to 80 km under the young folded belts of the Earth, for example, under the Andes or the Alpine-Himalayan belt . The thickness of the continental crust under the ancient platforms is on average 40 km, and its mass is about 0.4% of the mass of the Earth.[ ...]

L. is different on the continents and under the oceans. The continental crust consists of a discontinuous layered shell and granite and even lower basalt layers located under it. The total thickness of the lithosphere is 35-45 km (up to 50-70 km in mountainous areas). The oceanic crust is 5-10 km thick and consists of a thin (on average less than 1 km) layer of sediments, under which there are basic rocks (basalt, gabbro).[ ...]

The surface of the earth's crust is formed due to three multidirectional influences: 1) endogenous, including tectonic and magmatic processes that create relief irregularities; 2) exogenous, causing denudation (leveling) of this relief due to the destruction and weathering of the rocks that make it up; and 3) sedimentary accumulation, hiding the unevenness of the basement relief and forming the uppermost layer of the earth's crust. There are two main types of the earth's crust: "basaltic" oceanic and "granitic" continental.[ ...]

The processes of generation of the oceanic crust and the formation of the thermal regime of the lithosphere, including the formation of a subaxial magma chamber, are closely related to the release of melt under the axial spreading zones due to adiabatic decompression during upwelling of mantle material, as well as to the mechanisms of melt migration from its segregation zones in the mantle to the axial generation zone. bark. Many models are devoted to the analysis of these mechanisms.[ ...]

As already noted, the oceanic lithosphere is the shell of the Earth, which is a cooled and completely crystallized substance of the earth's crust and upper mantle, underlain from below by hot and partially molten matter of the asthenosphere. It is natural to assume that oceanic lithospheric plates are formed due to cooling and complete crystallization of partially molten matter of the asthenosphere, just as it happens, for example, on a river when water freezes and ice forms. The analogy here is very deep - after all, the crystalline rocks of the lithosphere are essentially the same "silicate ice" for the partially molten silicate substance of the asthenosphere. The only difference is that ordinary ice is always lighter than water, while crystalline silicates are always heavier than their melt. In this case, further solution of the problem of the formation of lithospheric plates is not difficult, since the process of water crystallization is well studied.[ ...]

After the transformations of the oceanic crust, the growth of the ocean mass began again, but approximately 1 billion years ago it approached the modern one, and its growth rates slowed down significantly. The process of changing the mass of the hydrosphere due to degassing is closely related to the evolution of the Earth's interior and is determined by the growth rate of the planet's dense core due to the separation of iron compounds in it.[ ...]

In the process of remelting the oceanic crust after it is immersed in the bowels of the Earth, water plays an important role, since water-saturated silicate layers melt at temperatures of about 700 ° C, while dry ones at more than 1000 ° C.[ ...]

In the formation of new oceanic crust in slowly expanding ridges, two types of models are considered: in the first (dyke) model, the oceanic crust is formed by intrusion of a large number of dikes randomly distributed within the axial neovolcanic zone. The second model assumes that volcanic lava flows extend from both sides of the dikes, overlapping each other. In fact, there is a combination of both of these effects, as evidenced by observations at 37 N. latitude. MAR in the FAMOUS area. When drilling three OBBR wells in the Atlantic (332B, 395A, 418A), which penetrated more than 500 m into the basalt crust, anomalous dip and numerous inversions were found within one well. In most cases, the 500 m section did not entirely correspond to the known distribution of magnetic reversals. These results clearly contradicted the initial assumption made from observations of anomalies at the EPR that the magnetic sources are located in a layer about 1 km thick, and also contradicted the observed shape and sharp border between positive and negative anomalies studied with the Elvin ROV at the EPR. [...]

In the axial part of the mid-ocean ridges, the depth of the earthquake source rarely exceeds 5 km. At the same time, two types of earthquakes are clearly distinguished by the nature of the mechanism in the source. The sources of the first type are concentrated within narrow zones of seismic activity, stretching along the crest of the mid-ocean ridge. In these zones, the roles of small-focus earthquakes arise, the depth of the sources of which, as a rule, does not exceed a few kilometers from the bottom. The sources are dominated by mechanisms of subhorizontal extension in the direction perpendicular to the strike of the mid-ocean ridge spreading axis. Spreading is the process of growth of the newly formed oceanic crust in both directions from the growth axis.[ ...]

In addition to continental and oceanic crust, there are various intermediate types of crust. For such types, when the "granite" layer in the crust is seismically weakly expressed, the terms subcontinental or suboceanic are used.[ ...]

Along the axial zones of the mid-ocean ridges in the oceans, numerous volcanic structures are traced, which, along with slotted extrusive apparatuses, are involved in the formation of a new oceanic crust of our planet. The formation process is accompanied by earthquakes, high heat flow, significant hydrothermal activity, ore formation, etc. This seismic volcanic zone with a length of about 70 thousand km can be traced in all the oceans of the Earth.[ ...]

The geodynamics of modern oceanic rifting is a new direction that allows, on the basis of a complex of geological and geophysical data, to present models of the deep structure of rift zones and the development of these zones on the Earth's surface, where the oceanic crust and lithosphere originate. This book is devoted to the study of deep processes that determine the structure of the ocean rift zones, the regularities of their modern morphostructural plan and anomalous geophysical fields, as well as the features of the distribution of deep-sea sulfide ores. The varying degree of knowledge and complexity of the deep structure of modern rift zones have caused different aspects of their structure and evolution to be elucidated with varying degrees of reliability. Therefore, where the processes are quite complex, and there is not much actual data, various geodynamic models were used. At the same time, attention was focused on those models that, in our opinion, are most adequate to the real situation.[ ...]

At present, the earth's crust is understood as the upper layer of the solid body of the planet, located above the seismic boundary. This boundary is located at different depths, where there is a sharp jump in the speed of seismic waves that occur during an earthquake. There are two types of the earth's crust - continental and oceanic. Continental is characterized by a deeper seismic boundary. At present, the term lithosphere, proposed by E. Suess, is more often used, which is understood as an area more extensive than the earth's crust.[ ...]

In total, during the movement of the oceanic crust through the zone of its active hydrothermal flushing (about 50 million years), approximately 6-1025 g of water flows, which is 40-45 times more than the volume of water in the ocean itself. Consequently, the complete circulation of oceanic waters through hydrothermal vents on the slopes of the MOR occurs in just 1-1.2 million years.[ ...]

The hard shell of the Earth - the earth's crust, composed of sedimentary and crystalline rocks, forms a continuous shell, 2/3 of which is covered by the waters of the oceans and seas. The greatest thickness of the earth's crust is 40-100 km, under the oceans its thickness is sharply reduced. According to physical properties, the earth's crust is divided into two types: continental and oceanic. The earth's crust of the continental type - plains and mountainous areas - is rich in silicon and aluminum, characteristic of rocks of the granite group. The thickness of the granite layer (sial) increases in the mountains. The oceanic type of the earth's crust is represented by rocks of the basalt type with a predominance of silicon and magnesium. Here, the granite layer is absent, and the thickness of the basalt layer (sima) reaches 15 km.[ ...]

A very important circumstance that distinguishes the earth's crust from other geospheres is the increased content in it of long-lived radioactive isotopes of uranium 232U, theory 238Th, potassium 40K, and their highest concentration was found in the "granite" layer of the continental crust. In the oceanic crust, radioactive elements are represented by "traces".[ ...]

There are two most common types of earth's crust: continental and oceanic. The continental type consists of three main layers - sedimentary, granite and basalt, and the oceanic - from sedimentary and basalt. However, some scientists dispute this classification of types of the earth's crust. They believe (Afanasiev et al.) that the crust is one, as a rule, consists of three layers and differs only in thickness.[ ...]

If we assume that t is 120 million years, then the average heat flux through the oceanic crust turns out to be 40Kc= 2.41-10 6 cal/cm-s.[ ...]

Based on the difference in composition and thickness, three types of the earth's crust are distinguished: 1) continental; 2) oceanic; 3) bark of transitional areas.[ ...]

Rift zones on the continents are areas of degradation of the continental crust, its transformation into the oceanic crust (Fig. 15). Rifting is currently considered by geologists as one of the most important processes in the development of the earth's crust, comparable in its significance to the geosynclinal process.[ ...]

Although the data are still insufficient, it can already be suggested that the crust at low spreading rates is subject to a greater tectonic effect (faults, cracks, etc.) than at high rates. Studies show that the area of ​​active faults extends 4–10 km away from the axis for ridges with high and medium spreading rates, and noticeably wider (30 km) for slowly spreading ridges (see Fig. 2.1). Outside the zone of active fault formation, the oceanic lithosphere can be considered as a relatively rigid body. The boundary of the zone of active faults thus marks the position of the edge of the plate boundary or the beginning of the area of ​​quasi-rigid behavior of the plates.[ ...]

It can be expected that in the center of the spreading segments, above the zone of maximum melt generation, the oceanic crust will reflect the presence of transient magma chambers and will show a clear structure of crustal layers. Near segment ends, where melt generation is least, the oceanic crust may be highly heterogeneous, reflecting the past presence of short-lived igneous bodies, or may consist only of a thin basaltic layer overlying mantle peridotites. In the latter case, the absence of a gabbro layer will reflect the absence of a magma chamber and imply a lateral movement of the basalt melt from the middle of the segment to its boundaries.[ ...]

The P-wave velocities within most of the ESL are lower than the normal velocities for layer 3 of the oceanic crust by 1 km/s. The lowest speeds (7 5 km/s) are confined to a narrow ([ ...]

Understanding the patterns and features of morphology, magmatism, and the distribution of disjunctive disturbances in the lithosphere and crust of different ages in the vicinity of the MOR is one of the fundamental problems of modern marine geotectonics. The urgency of this task is further enhanced by the fact that the formation of faults and cracks in the MOR rift zones is most directly related to hydrothermal activity and, consequently, to the distribution of deep-sea polymetallic sulfides. Obviously, the processes of accretion of the oceanic crust, as well as fault and fracture formation in rift zones, depend on geodynamic processes that control the formation and evolution of a wide variety of morphotectonic structures of different scale levels. Therefore, the problem of structure formation, apparently, should be considered in the context of the existing levels of geodynamic segmentation of the MOR.[ ...]

The largest and most complex geocomplexes of the Earth are continents and oceans. They are formed on the largest landforms - continental ledges and oceanic depressions of the Earth with various types of the earth's crust. The earth's crust of the continents, in contrast to the oceanic one, has a much greater thickness and a granite layer. The border between continents and oceans as geocomplexes runs along the coastline. The oceans as aquatic geocomplexes include the flooded part of the continents - the shelf, the continental slope and the bottom, composed of a basalt layer.[ ...]

The centers of the second type also extend in the form of rather narrow zones, as a rule, perpendicular to the general strike of the mid-ocean ridge spreading axis. In such foci, predominantly subhorizontal strike-slip faults in the direction orthogonal to the strike of the ridge prevail. Seismic focal zones with shear mechanisms in earthquake sources indicate subhorizontal displacement of plate edges. In the absolute majority of cases, each such seismic zone is located between two segments of the spreading axis. This zone fixes a living transform fault, which is a linear tectonic structure, when passing through which the growth of the new oceanic crust changes its direction (transforms) to the opposite. The depth of the sources along the transform faults of the mid-ocean ridges is usually small: in the absolute majority of cases, it does not exceed tens of kilometers. Seismically active zones extending in the axial region of the mid-ocean ridges mark the displacement of plate edges in rift cracks and along transform faults.[ ...]

From the point of view of tectonics, this is evidence of some isolation of accretionary processes that form mainly the lower part of the oceanic crust section (gabbro layer) from eruptive outpourings of basalt magmas, leading to the formation of layer 2A. In addition to the change in thickness due to reduced melt supply away from the localized zone of mantle upwelling, the structure of the oceanic crust under non-transform faults may differ significantly from the structure of the crust under the mid-segments.[ ...]

The relationships between the anomalous gravitational field and the relief of the Earth's surface described above in the most general form are equally valid for both continental and oceanic regions. A distinctive feature of the latter is that in the oceans, due to the relatively smaller thickness and greater homogeneity of the earth's crust and lithosphere, the effects of such relationships are more pronounced. This makes it possible to draw more substantiated conclusions about the geodynamics and structure of the oceanic lithosphere based on gravity data. Elucidation of the patterns of processes occurring in rift and transition zones, establishing the response of the oceanic lithosphere to external load and internal stress, and solving many other problems of modern geodynamics - in a joint analysis of the bottom topography and the gravity field.[ ...]

In recent years, works have appeared that contribute to the achievement of the third target task of studying the magnetic field of the ocean - revealing the nature of the magnetization of the layers of the oceanic crust. The results of these works, based on experimental studies of the petro-magnetic and magneto-mineralogical characteristics of rock samples, as well as the results of the interpretation of geomagnetic surveys, made it possible to propose and substantiate a generalized petromagnetic model of the oceanic lithosphere (Fig. 2.7).[ ...]

The work is of interest to geologists, petrographers, tectonists and geophysicists who are interested in the geology and petrology of metamorphic rocks, the problems of the relationship between continental and oceanic structures and the evolution of the earth's crust on continental margins.[ ...]

The same sinusoidal character is characteristic of the along-axis profiles of changes in anomalies in free air, mantle Bouguer anomalies, changes in the intensity of the axial magnetic anomaly, and changes in the thickness of the oceanic crust. The change in mantle Bouguer anomalies (MAB) indicates the presence of density inhomogeneities in the upper mantle. Reduced negative MAB values ​​are fixed above a more decompressed one, i.e. over the hotter mantle (isometric bulls-eye anomalies). Due to the fact that the boundary of the lithosphere is determined by the position of the melting isotherm, the lithosphere will be thinner where the melting isotherm will come closer to the surface, i.e. in hotter regions of the mantle. Therefore, lower MAB values ​​correspond to a thinner layer of the lithosphere. They, as a rule, are confined to the centers of the segments (see Fig. 3.36), which indicates a decrease in the thickness of the lithosphere towards the centers of the segments, i.e. the middle of each segment is usually a hotter area compared to its edges.[ ...]

At some distance from the MOR crests, according to seismic data, the lower part of this layer (ST layer) is also traced, most likely composed of serpentinites corresponding to hydrated peridotites (see Fig. 1.2). Judging by seismic data, the thickness of the gabbro-serpentinite third layer of the oceanic crust reaches 4.7-5 km. The total thickness of the oceanic crust, without a sedimentary layer, reaches 5-8 km and does not depend on age. Under the ridges of the MOR, the thickness of the oceanic crust is usually reduced to 3-4 km and even to 1.5-2 km (directly under the rift valleys).[ ...]

Soviet researchers discovered underwater ridges in the Arctic basin, named after Lomonosov, Mendeleev, and Gakkel, a prominent domestic oceanologist. A number of Soviet scientists, including the well-known oceanologist VV Dibner, noted the close relationship between the structure of the ocean floor and the adjacent areas of the mainland, in particular the Arctic Basin and the northeastern part of the Asian mainland. So, modern mountains in geosynclanal zones (for example, the Urals) are “degenerated” more ancient mountain formations. The result of the process of transformation and "degeneration" of pre-existing ridges are also hollows of the land of the type that the Aral Sea now fills, and on the ocean floor - depressions-troughs, for example, Novaya Zemlya or the St. Anna in the Arctic Ocean. It is assumed that at the next stage of the transformation of the earth's crust, new mountain ranges will arise. But no longer folded, like the former, “degenerate”, but volcanic (the underwater Gakkel ridge can serve as an example of them).[ ...]

The experimental results indicate that with an increase in the thickness of the brittle layer, the pattern of segmentation and the types of structures formed do not fundamentally change, with the exception of small-scale segments. In the process of development of the rift zone, during the mechanical destruction of the fragile layer of the oceanic crust during its extension, the general features of the fracture geometry are laid and the main morphostructural heterogeneities are formed, creating a natural multi-scale segmentation of the rift zone.[ ...]

Large overlaps can migrate along the rift axis, which is accompanied by the advancement of one branch of the axis and the retreat of the other. Their movement is recorded in U-shaped traces located at an angle to the rift axis, which stretch from the modern position of the overlaps to older sections of the crust (see Fig. 3.3, a). Traces are zones with a disturbed magnetic field, along which linear magnetic anomalies are displaced. These traces are characterized by an anomalous structure of the crust and relief, which is expressed in a deviation of 10-30° in the strike of linear uplifts and depressions compared to the “normal” sections of the ocean floor. Such traces represent the terminal segments of overlapping volcanic ridges that died as a result of the evolution of the PCS and cut-off parts of the central basin. In the areas of small overlaps, there are no deviations in discontinuities and relief, indicating the presence of Y-shaped traces.[ ...]

To explain the nature of the alternating and symmetric anomalous magnetic field of the ocean floor, F. Vine and D. Matthews suggested that the magnetic anomalies of the ocean are nothing more than a record of the Earth's magnetic field reversals in the geological past on a giant natural "tape" tape - the oceanic crust, which , freezing in a rift crack, breaks in it approximately in the middle and each half moves apart from its birthplace (Fig. 1.4). Knowing the order of alternation and the time of each reversal of the main magnetic field of the Earth, it is possible to compile a single scale of geomagnetic reversals, correlated with the geochronological scale, and determine the age of the ocean floor from the pattern of anomalies (Fig. 1.5). The geohistorical interpretation of the ocean's anomalous magnetic field, confirmed by deep-sea drilling data, has convincingly shown the geological youth of the ocean floor. The youngest rocks of modern age are located in rift cracks, and on the flanks of the MOR and in the areas of abyssal basins, the age of rocks reaches 80-100 million years. The oldest age of the oceanic crust does not exceed 160-170 million years, which is only 1/30 of the age of our planet.[ ...]

Intense gravity anomalies in free air (+190 mGal above the ridge and -90 mGal above the trench), as well as the characteristic shape of the gravity curve, indicate a clear violation of isostasy caused by dynamic compression of the edges of neighboring lithospheric plates. In the model shown in fig. 3.19.6, when choosing density parameters, seismic data obtained during the study of this area were used. Here, as in the case of the Barracuda Fault, we assumed that during compression, the layers of the underthrusting block are “pulled up” and the submerged block is partially submerged. A significant role in the subsidence of the last block is given to the load of sediments that bends the layers of the oceanic crust south of the Gorringe Ridge.