At one time I read many books by Wells, Doyle, Verne, and each of these authors has a work describing underwater life. As a rule, it mentions the features of life on the ocean floor or penetrating through the earth's crust. Therefore, I wanted to figure out how the land differs from the bottom of the sea.
Continental crust is different from oceanic crust
Of course, the main difference between them will be their location: the first carries all the land and continents, and the second carries the seas, oceans, and indeed all bodies of water. But they also differ in other ways:
- the first consists of granulites, the second - of basalt;
- continental crust is thicker than oceanic crust;
- the terrestrial crust is inferior to the oceanic crust in area, but superior in total volume;
- The oceanic crust is more mobile and is capable of layering on the continental crust.
The process described in the last paragraph is called obduction and means the layering of tectonic plates one on top of another.
Main characteristics of the continental crust
This crust is also called continental, and it consists of 3 layers.
- Upper sedimentary - consists of rocks of the same name, different in origin, age, location. Typically its thickness reaches 25 km.
- Middle granite-metaphorical - formed from acidic rocks similar in composition to granite. The thickness of the layer varies from 15 to 30 km (its greatest thickness is recorded under the most high mountains).
- Lower basaltic - formed by metamorphosed rocks. Its thickness reaches 10–30 km.
It is noteworthy that the third layer is called “basalt” conventionally: seismic waves pass through it at the same speed as they would pass through basalt.
Parameters of the oceanic crust
Some scientists identify only 2 main ones, but, in my opinion, it is better to take a three-level interpretation of the structure of this cortex.
- The top layer is represented by sedimentary rocks and can reach a thickness of 15 km.
- The middle layer is composed of pillow lavas, its thickness does not exceed 20 km.
- The third layer consists of igneous rocks of basic composition, its thickness is 4–7 km.
The last layer is also called “gabbro” due to the crystalline structure of the rock.
Earth's crust- the outer solid shell of the Earth (geosphere), part of the lithosphere, with a width from 5 km (under the ocean) to 75 km (under the continents). Below the crust is the mantle, which differs in composition and physical qualities- it is more compact and contains mainly refractory elements. The crust and mantle are divided by the Mohorovicic feature, or the Moho layer, where a sharp acceleration of seismic waves occurs.
There are continental (continental) and oceanic crust, as well as its transitional types: subcontinental and suboceanic crust.
Continental (mainland) crust consists of several layers. The top is a layer of sedimentary rocks. The thickness of this layer is up to 10-15 km. Beneath it lies a granite layer. The rocks that compose it, in their own way physical properties similar to granite. The thickness of this layer is from 5 to 15 km. Beneath the granite layer is a basalt layer, consisting of basalt and rocks whose physical characteristics resemble basalt. The thickness of this layer is from 10 km to 35 km. Consequently, the total thickness of the continental crust reaches 30-70 km.
Oceanic crust differs from the continental crust in that it does not have a granite layer, or it is very thin, therefore the thickness of the oceanic crust is only 6-15 km.
To determine the chemical composition of the earth's crust, only its upper parts are available - to a depth of less than 15-20 km. 97.2% of the total composition of the earth's crust is made up of: oxygen - 49.13%, aluminum - 7.45%, calcium - 3.25%, silicon - 26%, iron - 4.2%, potassium - 2.35 %, magnesium - 2.35%, sodium - 2.24%.
Other elements of the periodic table account for from 10ths to hundredths of a percent.
Sources:
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:
Earth's crust
mantle (Si, Ca, Mg, O, Fe)
outer core
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 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-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 various compositions, ages, genesis and degrees 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.
I think that any person understands that one of the components of our planet is the crust. But few people know that there is a difference between the earth's crust on continents and in the oceans. I want to clarify what the differences are and why.
Oceanic crust
It is one of the types of ordinary earth's crust and is located within the oceans. But oceanic crust sometimes tends to creep directly onto the continental crust. The thickness of this crust is approximately seven kilometers, and it consists of the following layers:
- ocean sediments;
- basalt covers;
- mantle.
At the root of the oceanic crust there are most often formations that are obtained as a result of the crystallization of various melts, or they may initially be rocks that are found in the mantle. I would like to note that there are places where the thickness of the crust within the ocean is greater than usual. This occurs in areas where islands are located.
Continental crust
This crust is also part of the earth's crust and, accordingly, predominates in continental areas. Unlike the oceanic crust, the composition of the continental crust is characterized by a granite layer, sedimentary and other various layers. The thickness differs significantly from the crust in the oceans - it ranges from 35 to 45 kilometers, and is found up to 75 kilometers in mountainous areas. Despite the fact that the continental crust makes up almost 70 percent of the total volume of the earth's crust, it covers less than half of the entire surface of the planet (this is due to the fact that there is more water than land).
I want to note important fact that the continental crust is much older than the oceanic crust. If the second age is approximately 200 million years, then the continental age is approximately two and a half billion years old (but this includes approximately seven percent of the crust). That is, as a result, we can say that the main difference between one crust and another is in thickness (continental is larger), age (continental is also larger), composition (basaltic base in oceanic) and, of course, in location ( oceans and continents).
There are two main types of earth's crust: oceanic and continental. A transitional type of the earth's crust is also distinguished.
Oceanic crust. The thickness of the oceanic crust in the modern geological era ranges from 5 to 10 km. It consists of the following three layers:
1) top thin layer marine sediments (thickness no more than 1 km);
2) middle basalt layer (thickness from 1.0 to 2.5 km);
3) lower layer of gabbro (thickness about 5 km).
Continental (continental) crust. The continental crust has more complex structure and greater thickness than the oceanic crust. Its thickness averages 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) lower layer composed of basalts (thickness about 20 km);
2) the middle layer occupies the main thickness of the continental crust and is conventionally called granite. It is composed mainly of granites and gneisses. This layer does not extend under the oceans;
3) upper layer– sedimentary. Its thickness on average is about 3 km. In some areas the thickness of precipitation reaches 10 km (for example, in the Caspian lowland). In some areas of the Earth there is no sedimentary layer at all and a granite layer comes to the surface. Such areas are called shields (for example, Ukrainian Shield, Baltic Shield).
On continents, as a result of the weathering of rocks, a geological formation is formed, called weathering crust.
The granite layer is separated from the basalt layer Conrad surface , at which the speed of seismic waves increases from 6.4 to 7.6 km/sec.
The boundary between the earth's crust and mantle (both on continents and oceans) runs along Mohorovicic surface (Moho line). The speed of seismic waves on it increases abruptly to 8 km/hour.
In addition to the two main types - oceanic and continental - there are also areas of mixed (transitional) type.
On continental shoals or shelves, the crust is about 25 km thick and is generally similar to the continental crust. However, a layer of basalt may fall out. In East Asia in the region of island arcs ( Kurile Islands, Aleutian Islands, Japanese islands etc.) the earth's crust of transitional type. Finally, the crust of the mid-ocean ridges is very complex and has so far been little studied. There is no Moho boundary here, and mantle material rises along faults into the crust and even to its surface.
The concept of "earth's crust" should be distinguished from the concept of "lithosphere". The concept of "lithosphere" is broader than the "earth's crust". To 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 approximately 100 km.
The concept of isostasy . A study of the distribution of gravity showed that all parts of the earth's crust - continents, mountainous countries, plains - are balanced on the upper mantle. This balanced position is called isostasy (from the 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 desire for equilibrium, continuously disrupted and restored again. 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 equilibrium line currently runs slightly south of 60 0 N latitude. Modern St. Petersburg is approximately 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 turns out to be sufficient for isostatic fluctuations of the territory beneath them. Consequently, the earth's crust in areas of large cities is very mobile. In general, the relief of the earth's crust is a mirror image of the Moho surface, the base of the earth's crust: elevated areas correspond to depressions in the mantle, lower areas correspond to more high level its upper limit. Thus, under the Pamirs the depth of the Moho surface is 65 km, and in the Caspian lowland it is 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 of the earth's surface by the Sun ceases, there is layer of constant soil temperature. It is called isothermal layer . Below the isothermal layer deep into the Earth, the temperature rises, and this is caused by internal warmth earth's bowels In the formation of climates internal heat does not participate, but it serves as the energetic basis of 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 increases by 1 0 C is called geothermal stage . The magnitude of the geothermal step depends on the topography, thermal conductivity of rocks, the proximity of volcanic sources, groundwater circulation, etc. On average, the geothermal step is 33 m. In volcanic areas, the geothermal step can be only about 5 m, and in geologically quiet areas (for example, on platforms) it can reach 100 m.
TOPIC 5. CONTINENTS AND OCEANS
Continents and parts of the world
Two qualitatively various types The earth's crust - continental and oceanic - corresponds to two main levels of planetary relief - the surface of the continents and the bed of the oceans.
Structural-tectonic principle of separation of continents. The fundamentally qualitative difference between the continental and oceanic crust, as well as some significant differences in the structure of the upper mantle under the continents and oceans, oblige us to distinguish continents not according to their apparent surroundings by oceans, but according to the structural-tectonic principle.
The structural-tectonic principle states that, firstly, the continent includes a continental shelf (shelf) and a continental slope; secondly, at the base of every continent there is a core or ancient platform; thirdly, each continental block is isostatically balanced in the upper mantle.
From the point of view of the structural-tectonic principle, a continent is an isostatically balanced massif of the continental crust, which has a structural core in the form of an ancient platform, to which younger folded structures are adjacent.
There are six continents in total on Earth: Eurasia, Africa, North America, South America, Antarctica and Australia. Each continent contains one platform, and at the base of Eurasia alone there are six of them: Eastern European, Siberian, Chinese, Tarim (Western China, Taklamakan Desert), Arabian and Hindustan. The Arabian and Hindu platforms are parts of ancient Gondwana, adjacent to Eurasia. Thus, Eurasia is a heterogeneous anomalous continent.
The boundaries between the continents are quite obvious. The border between North America and South America runs along the Panama Canal. The border between Eurasia and Africa is drawn along the Suez Canal. The Bering Strait separates Eurasia from North America.
Two rows of continents . In modern geography, the following two series of continents are distinguished:
1. Equatorial series of continents (Africa, Australia and South America).
2. Northern series of continents (Eurasia and North America).
Antarctica, the southernmost and coldest continent, remains outside these ranks.
The modern location of the continents reflects the long history of the development of the continental lithosphere.
The southern continents (Africa, South America, Australia and Antarctica) are parts (“fragments”) of the single Paleozoic megacontinent Gondwana. The northern continents at that time were united into another megacontinent - Laurasia. Between Laurasia and Gondwana in the Paleozoic and Mesozoic there was a system of vast marine basins called the Tethys Ocean. The Tethys Ocean stretched from North Africa, through southern Europe, the Caucasus, Western Asia, the Himalayas to Indochina and Indonesia. In the Neogene (about 20 million years ago), an Alpine fold belt arose in the place of this geosyncline.
According to their large sizes supercontinent Gondwana. According to the law of isostasy, it had a thick (up to 50 km) crust, which sank deeply into the mantle. Beneath them, in the asthenosphere, convection currents were especially intense and the softened substance of the mantle was moving actively. This led first to the formation of a bulge in the middle of the continent, and then to its splitting into separate blocks, which, under the influence of the same convection currents, began to move horizontally. As proven mathematically (L. Euler), the movement of a contour on the surface of a sphere is always accompanied by its rotation. Consequently, parts of Gondwana not only moved, but also unfolded in geographical space.
The first breakup of Gondwana occurred at the Triassic-Jurassic boundary (about 190-195 million years ago); Afro-America seceded. Then, at the Jurassic-Cretaceous boundary (about 135-140 million years ago), South America separated from Africa. At the border of the Mesozoic and Cenozoic (about 65-70 million years ago), the Hindustan block collided with Asia and Antarctica moved away from Australia. In the present geological era, the lithosphere, according to neomobilists, is divided into six plate blocks that continue to move.
The breakup of Gondwana successfully explains the shape of the continents, their geological similarities, as well as history vegetation cover and fauna of the southern continents.
The history of the split of Laurasia has not been studied as thoroughly as Gondwana.
The concept of parts of the world . In addition to the geologically determined division of land into continents, there is also a division of the earth's surface into separate parts of the world that has developed in the process of cultural and historical development of mankind. There are six parts of the world in total: Europe, Asia, Africa, America, Australia and Oceania, Antarctica. On one continent of Eurasia there are two parts of the world (Europe and Asia), and two continents of the Western Hemisphere (North America and South America) form one part of the world - America.
The border between Europe and Asia is very arbitrary and is drawn along the watershed line of the Ural ridge, the Ural River, the northern part of the Caspian Sea and the Kuma-Manych depression. Deep fault lines that separate Europe from Asia run through the Urals and the Caucasus.
Area of continents and oceans. Land area is calculated within the modern coastline. Surface area globe is approximately 510.2 million km 2. About 361.06 million km 2 is occupied by the World Ocean, which is approximately 70.8% of the total surface of the Earth. The land area accounts for approximately 149.02 million km2, which is about 29.2% of the surface of our planet.
Area of modern continents characterized by the following values:
Eurasia - 53.45 km 2, including Asia - 43.45 million km 2, Europe - 10.0 million km 2;
Africa - 30, 30 million km 2;
North America - 24, 25 million km 2;
South America - 18.28 million km 2;
Antarctica - 13.97 million km 2;
Australia - 7.70 million km 2;
Australia with Oceania - 8.89 km 2.
Modern oceans have an area:
Pacific Ocean - 179.68 million km 2;
Atlantic Ocean - 93.36 million km 2;
Indian Ocean - 74.92 million km 2;
Arctic Ocean - 13.10 million km 2.
Between the northern and southern continents, in accordance with their different origins and development, there is a significant difference in area and character of the surface. The main geographical differences between the northern and southern continents are as follows:
1. Eurasia is incomparable in size with other continents, concentrating more than 30% of the planet’s landmass.
2.The northern continents have a significant shelf area. The shelf is especially significant in the Arctic Ocean and the Atlantic Ocean, as well as in the Yellow, Chinese and Bering Seas Pacific Ocean. The southern continents, with the exception of the underwater continuation of Australia in the Arafura Sea, are almost devoid of a shelf.
3. Most of the southern continents lie on ancient platforms. IN North America and Eurasia, ancient platforms occupy a smaller part total area, and most of it falls on the territories formed by the Paleozoic and Mesozoic orogeny. In Africa, 96% of its territory is in platform areas and only 4% is in mountains of Paleozoic and Mesozoic age. In Asia, only 27% is on ancient platforms and 77% on mountains of various ages.
4. The coastline of the southern continents, formed mostly by rifts, is relatively straight; There are few peninsulas and mainland islands. The northern continents are characterized by exceptionally sinuous coastline, an abundance of islands, peninsulas, often extending far into the ocean. Of the total area, islands and peninsulas account for about 39% in Europe, North America - 25%, Asia - 24%, Africa - 2.1%, South America– 1.1% and Australia (without Oceania) – 1.1%.
