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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Types of bark. In different regions, the ratio between various rocks in the earth's crust is different, and a 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 in the Earth's topography, and they are characterized by continental and oceanic types of crust.

1 - water, 2 - sedimentary layer, 3 - interlayering of sedimentary rocks and basalts, 4 - basalts and crystalline ultrabasic rocks, 5 - granite-metamorphic layer, 6 - granulite-mafic layer, 7 - normal mantle, 8 - decompressed mantle.

Continental crust developed under the continents and, as already mentioned, has different thicknesses. Within the platform areas corresponding to the continental plains, 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 Himalayas and 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 layer (according to outdated ideas, this is a granite layer), composed of granites and gneisses, and the lower granulite-mafic layer (according to outdated ideas, this is a basalt layer), 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.

Ocean crust characteristic of the World Ocean. 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.

Suboceanic crust developed under the deep-sea basins of the marginal and inland seas (Black, Mediterranean, Okhotsk, etc.), and also found in some deep depressions on land (the central part of the Caspian basin). 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 characteristic of island 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, 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 floors 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 being divided into such planetary structural elements as oceans and continents, the earth's crust (and lithosphere) reveals seismic (tectonically active) and aseismic (quiet) regions. The inner regions of the continents and the beds of the oceans - continental and oceanic platforms - are calm. Between the platforms there are narrow seismic zones, which are marked by volcanism, earthquakes, and tectonic movements - the site. 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 - mobile belts with axial rifts such as grabens;
- oceanic platforms - calm areas of abyssal basins with uplifts complicating them.

On continents, the main structural elements are:

Mountain structures (orogens: from the Greek “oros” - mountain), which, like mid-ocean ridges, can exhibit tectonic activity;
- platforms - mostly tectonically calm vast territories with a thick cover of sedimentary rocks.

Mountain structures have a complex internal structure and history of geological development. Among them are orogens composed of young pre-Paleogene marine sediments (Carpathians, Caucasus, Pamir), and more ancient ones formed from Early Mesozoic, Paleozoic and Precambrian rocks that experienced folding movements. These ancient ridges were denudated, often to the base, and in recent times have experienced a secondary uplift. These are the revived mountains (Tian Shan, Altai, Sayans, ridges of the Baikal region and Transbaikalia).

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-mountain building. Most of them, after leveling, slowly sank, were flooded by the sea, and a layer of marine sediments was layered onto 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 West Siberian platform, 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 ultrabasic and basic magma and lithosphere material, which, as it floats up, builds up continents in the marginal (Pacific) and welds them together in intercontinental (Mediterranean) geosynclines. This process ends with the formation of folded mountain structures, in the arch of which volcanoes can operate for a long time - the site. 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.

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

Biosphere- a special shell of the earth, an 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. Lithosphere is the hardest shell of the earth:

Structure:

1. earth's crust

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

3. outer core

4. inner core

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

Core composition – Ni\Fe; core density – 12.5 kg/cm 3 ;

Kimberlites- (from the name of the city of Kimberley in South Africa), igneous ultrabasic brecciated rock of effusive appearance, producing explosion tubes. It consists mainly of olivine, pyroxenes, pyrope-almandine garnet, picroilmenite, phlogopite, less commonly zircon, apatite and other minerals included in the 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 end-member (omphacite) and grossular-pyrope-almandine garnet, quartz and rutile. The chemical composition of eclogites is identical to igneous rocks of basic composition - gabbro and basalts.

Structure of the earth's crust

Layer thickness = 5-70 km; highlands - 70 km, seabed - 5-20 km, average 40-45 km. Layers: sedimentary, granite-gneiss (not in the oceanic crust), granite-bosite (basalt)

The earth's crust is a complex of rocks that lie above the Mohorovicic boundary. Rocks are regular aggregates of minerals. The latter consist 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 the rocks in the field.

There are two types of 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-mafic, with a thickness of 30-40 km on the plains to 70-75 km under 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 ultrabasites. The continental crust is enriched in silica and light elements - Al, sodium, potassium, C, compared to the oceanic crust.

Continental (mainland) crust characterized by great thickness - on average 40 km, in some places 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). 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 observed under 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 conventionally: 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 basic composition. Therefore, it is also called granulite-mafic.

Oceanic crust differs sharply from the continental one. Over most of the ocean floor, its thickness ranges from 5 to 10 km. Its structure is also peculiar: under a sedimentary layer with a thickness ranging from several hundred meters (in deep-sea basins) to 15 km (near continents) lies 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 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.

Structure and age of the earth's crust

The main elements of the surface relief of our planet are continents and ocean basins. This division is not random; it is due to profound differences in the structure of the earth's crust under the continents and oceans. Therefore, the earth's crust is divided into two main types: continental and oceanic crust.

The thickness of the earth's crust varies from 5 to 70 km, and it differs sharply under the continents and the ocean floor. The thickest crust under the mountainous regions of the continents is 50-70 km, under the plains its thickness decreases to 30-40 km, and under the ocean floor it is only 5-15 km.

The earth's crust of the continents consists of three thick layers, differing in their composition and density. The top layer is composed of relatively loose sedimentary rocks, the middle layer is called granite, and the bottom layer is called basalt. The names “granite” and “basalt” come from the similarity of these layers in composition and density to granite and basalt.

The earth's crust under the oceans differs from the continental crust not only in its thickness, but also in the absence of a granite layer. Thus, under the oceans there are only two layers - sedimentary and basaltic. There is a granite layer on the shelf; continental-type crust is developed here. The change from continental to oceanic crust occurs in the zone of the continental slope, where the granite layer becomes thinner and breaks off. The oceanic crust is still very poorly studied compared to the continental crust.

The age of the Earth is now estimated at approximately 4.2-6 billion years according to astronomical and radiometric data. The age of the oldest rocks of the continental crust studied by man is up to 3.98 billion years old (southwestern part of Greenland), and the rocks of the basalt layer are over 4 billion years old. There is no doubt that these rocks are not the primary substance of the Earth. The prehistory of these ancient rocks lasted many hundreds of millions, and perhaps billions of years. Therefore, the age of the Earth is approximately estimated to be up to 6 billion years.

Structure and development of the continental crust

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

Before moving on to a description of the structure and development of these main structures, it is necessary to talk about the origin and essence of the term “geosyncline”. This term comes from the Greek words “geo” - Earth and “synclino” - deflection. It was first used by the American geologist D. Dana more than 100 years ago, while studying the Appalachian Mountains. He found that the marine Paleozoic sediments that make up the Appalachians have a maximum thickness in the central part of the mountains, much greater than on their slopes. Dana explained this fact absolutely correctly. During the period of sedimentation in the Paleozoic era, in place of the Appalachian Mountains there was a sagging depression, which he called a geosyncline. In its central part, subsidence was more intense than on the wings, as evidenced by the large thickness of sediments. Dana confirmed his conclusions with a drawing depicting the Appalachian geosyncline. Given that Paleozoic sedimentation occurred under marine conditions, he plotted down from a horizontal line—the assumed sea level—all the measured sediment thicknesses in the center and slopes of the Appalachian Mountains. The picture shows a clearly defined large depression in the place of the modern Appalachian Mountains.

At the beginning of the 20th century, the famous French scientist E. Og proved that geosynclines played a large role in the history of the development of the Earth. He established that folded mountain ranges formed in place of geosynclines. E. Og divided all areas of the continents into geosynclines and platforms; he developed the fundamentals of the study of geosynclines. A great contribution to this doctrine was made by Soviet scientists A.D. Arkhangelsky and N.S. Shatsky, who established that the geosynclinal process not only occurs in individual troughs, but also covers vast areas of the earth's surface, which they called geosynclinal regions. Later, huge geosynclinal belts began to be identified, within which several geosynclinal areas are located. In our time, the doctrine of geosynclines has developed into a substantiated theory of geosynclinal development of the earth's crust, in the creation of which Soviet scientists play a leading role.

Geosynclinal fold belts are mobile sections of the earth's crust, the geological history of which was characterized by intense sedimentation, repeated folding processes and strong volcanic activity. Thick layers of sedimentary rocks accumulated here, igneous rocks formed, and earthquakes often occurred. Geosynclinal belts occupy vast areas of continents, located between ancient platforms or along their edges in the form of wide stripes. Geosynclinal belts arose in the Proterozoic; they have a complex structure and a long history of development. There are 7 geosynclinal belts: Mediterranean, Pacific, Atlantic, Ural-Mongolian, Arctic, Brazilian and Intra-African.

Ancient platforms are the most stable and sedentary parts of the continents. Unlike geosynclinal belts, ancient platforms experienced slow oscillatory movements, sedimentary rocks of usually low thickness accumulated within them, there were no folding processes, and volcanism and earthquakes rarely occurred. Ancient platforms form sections of continents that are the skeletons of all continents. These are the most ancient parts of the continents, formed in the Archean and Early Proterozoic.

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

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 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 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, Kopetdag, 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 has great thickness. 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 connected with 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.