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Expansion of an oceanic bottom

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the Data, allowed to solve one of the greatest riddles of the nature, has been received in 60th of the XX-th century, and not on a land as it would be possible to expect, and under thickness of water - on an ocean floor.

Today we know about the Earth much from this that was not clearly scientific still any 40 years ago. To us the reasons of earthquakes and eruptions of volcanoes are known. We know, why in bowels covered the richest deposits of oil formed of remains of tiny sea animals and plants are snow and ices of Alaska. We understand also, whence in the north of Europe the coal major fields the hardened tropical vegetation was which source undertook.

Drifting continents

Answers to all these riddles have been received, when in 60th the majority of scientists have accepted, at last, the theory of the drifting continents which have been put forward by German meteorologist Alfredom Vegenerom still in 1915 Why scientific community so much time was required to agree with its theory?

On it there were some reasons. First, Vegener was not the geologist. By words Edward Bullarda's geophysics, it was a member not that trade union and consequently geologists have not taken it seriously. Secondly, though Vegener considered that continents dvizhutsja, or drift, he has not offered any explanation to the reasons of this process.

the Oceanic bark

In 60th years the convincing data which has allowed, finally, "ѕ=тхЁфш=і" has been obtained; the theory of drifting continents and to establish the mechanism of this phenomenon. And proofs have been found not on continents, and in oceans and under their bottom.

The Top firm cover of the Earth - litosfera - represents a firm layer in the thickness about 100 km and includes earth crust (oceanic and continental) and the top part of a cloak (a layer which is directly under earth crust). The border has undressed between earth crust and a cloak is known as a surface of Mohorovichicha (in abbreviated form Moho), named so in honour of its pioneer, the Yugoslavian geologist Andrey Mohorovichicha (1857-1936).

The Oceanic bark strongly differs from the continental. It is much more thin and was generated almost completely for last 200 million years - very small term in the history of the Earth numbering 4 600 million of years. Unlike it, a continental bark much more thickly, and its age in separate places reaches 3 000 million years.

median-oceanic ridges

Though mostly the surface of an oceanic bottom flat, here is allocated with two elements of a relief: ridges and a trench. The median-oceanic ridge is a mountain chain in length 80 000 km and the Atlantic ridge was peaks in height to 4500 m. Among the first investigated sites of this underwater ridge. It lasts from Iceland in the north to tiny volcanic Tristan da Cunha Islands in the south, then bends around cape of Kind Hope and incorporates to ridges of the Indian and Silent oceans. As well as in other elements of system, in the Atlantic ridge allocate central riftovuju a valley in width about 50 km and depth of 2 km. On all extent of a median-oceanic ridge underwater volcanoes dominate.

In places oceanic ridges leave on a surface in the form of islands. Largest of islands such is Iceland. Far from ridges there are the separate volcanic tops named underwater mountains. Many of them rise over a surface of ocean and form the islands as which example the Hawaiian chain serves.

Underwater ridges have been found out in 50th by sea geologists of the Colombian university (USA). They also have found out that the thickness of earth crust under oceans makes only 6-7 km whereas under continents it reaches 30-40 km. The given fact has served as the first instructions that to an ocean box "ьюыюцх" the continental.

the New theory

In 1950 of Harry Gess, the professor of Princeton university (USA), on the basis of these two opening has put forward the theory spredinga (expansions) of an oceanic bottom according to which the oceanic bottom is constantly moved apart in the parties from underwater ridges. By its calculations, the new oceanic bottom was created with speed of several centimetres in a year - quickly enough that all deep oceanic bed was generated for last 200 million years.

At such rates or the Earth exclusively quickly increased in sizes (that, obviously, not so), or something occurred to a new oceanic bark. Gess considered that the oceanic bark collapsed with the same speed, as was formed. But where and how?

Soon it has received the answer. Along coastal lines of oceans passes a number of deep-water trenches. So, one of them lasts along all Pacific coast of the South America, and the deepest is Mariana trench depth of 11 022 m, located near to island Guam in Pacific ocean. Gess considered that the oceanic bark in these trenches falls back to a cloak.

He has come out with the assumption of communication of movement of a bark with konvektsionnymi currents in a cloak. Konvektsionnye currents are circular motions in a liquid or a plastic material, like what it is possible to see in boiling porridge. They arise under the influence of ascending thermal streams. The theory of professor Gess proves to be true the fact that intensity of a thermal stream, or speed of lifting of heat, from a kernel of the Earth to a surface in areas of ridges is very great. It decreases in process of removal from them and reaches a minimum in trenches. But for the theory proof it was required more facts, and they were given by the Earth. Our planet is a huge magnet, and its magnetic field forces an arrow of a compass to specify in the north. At formation from streams of a volcanic lava new breed always is a little magnetised according to a magnetic field of the Earth.

Magnetic helps

In the beginning of 60 scientists have made the important opening. At towing by research vessels of magnetic detectors on an ocean floor alternating strips of weak and strong magnetism have been found out. Onboard one of the courts investigating Indian ocean, there was a scientist of Cambridge university Drammond Metjuz.

He has told about opening to the post-graduate student of university Fredu Vajnu. Vajn has understood that magnetic strips are the important proof of a solvency of the theory of Gess about spredinge an oceanic bottom. In September, 1963 Vajn and Metjuz have offered a hypothesis about communication of alternating magnetic strips with periodic change of magnetic poles of the Earth when the magnetic north becomes the magnetic south.

Having applied the given theory to oceanic ridges, they have drawn a conclusion: at expansion of an oceanic bottom alternating blocks of opposite magnetised material disperse from the center of a ridge and lie down in parallel its crest.

Speed spredinga

At that time only few scientists recognised that the planet magnetic field really changes the polarity. Soon after publication of the theory of Vajna and Metjuza the group of scientists of geological service of the USA, headed by Alan Koksom, has revealed magnetic reversal traces in continental breeds practically on all continents and has found out that polarity of the breeds generated during the same period of time, is identical.

Defining age of underwater rocks near to the median-Atlantic ridge, scientists have found out that on its both parties the oceanic bottom is moved apart with a speed of 2 sm a year, i.e. Atlantic ocean extends with a speed of 4 sm a year, and today it on 80 m is wider, than at the time of Jesus Christ.

Speed spredinga At other oceans speed spredinga above, for example, in ju.-century of a part of Pacific ocean in one and a half time. Nevertheless, the ocean is not becomes wider, as at suburbs of continents the oceanic bottom falls to a cloak.

Red sea - one of the youngest on the Earth. As well as in Atlantic ocean, in it there is a median ridge, and speed spredinga here reaches ~2 sm year, therefore Arabian peninsula gradually keeps away from Africa. The same sea - a part of the big rupture in litosfere, lasting from East African riftovoj systems to a hollow of the Dead Sea in Israel. Scientists consider that in the future this system can turn to the sea.

Tectonics of plates

Today that fact is conventional that continents and an oceanic bottom are the separate plates moving on a surface of a cloak rather each other. The given process is known under the name tectonics of plates and describes structural features of earth crust. The term "=хъ=юэшър" means "юсЁрчютрэшх".


Plates represent mainly firm blocks consisting of earth crust and the top firm part of a cloak, in other words - litosfery. Under a firm cloak is astenosfera - the plastic or semifused part of a cloak which are lying down on depth between 100 km and 200 km from a terrestrial surface.

Exists about 15 large plates and a considerable quantity of the small. Plates are divided by oceanic ridges. Ridges and their valleys are zones of tectonic breaks. On either side of from a plate ridge keep away in opposite directions.

Oceanic a trench arise at collision of two plates. When one of plates falls and leaves in the bottom part of a cloak, it draws near under other plate. Such area of trenches is called as a zone subduktsii.

Borders of section of plates

There are three kinds of borders between plates. Besides two already considered by us (border on a ridge, or the constructive border, and zone border subduktsii) is still border transformnogo a break. It also name conservative border as in this case the weight litosfery remains invariable - does not grow and does not decrease.

Borders of section of plates Transformnyj the break is a rupture of constructive border at right angle. Oceanic ridges do not last a continuous thread - from time to time they deviate to the right or to the left when the plate changes a direction of the movement.

Borders of plates not always pass in parallel a joint of an oceanic and continental bark. Plates often include both kinds of a bark, and many of them are crossed at an angle. Some plates slide one on another at collision. The most known example is the area at the western coast of the North America where the border between plates is designated by a break the Dignity-andreas.

Compression or a stretching

When Vajn and Metjuz have proved for the first time that takes place spreding an ocean floor, it was supposed that the motive power moving plates, proceeds from oceanic ridges where the new bark is constantly formed.

The Subsequent researches have shown that the main motive power is not the stretching of a surface of oceanic ridges, and compression of zones subduktsii - places where the old bark is involved back in a cloak and is melted. In other words, litosfera it is stretched at oceanic ridges, instead of breaks up under the influence of internal forces.