Contours of coast of the World ocean constantly vary: in one places water absorbs a land, in others - concedes to it.
The Sea coast represents continent edge - a line of contact of the sea and a land. But in the majority of places of a strip of the earth leave in the sea: the underwater sites known under the name of a continental shelf. Extent of this shelf under water depends on a sea level which in turn varies depending on the sizes of glaciers on Northern and southern poles.
In the end of last Glacial age, about 10 000 years ago, on ice caps enormous weights of water have concentrated. Thereof the sea level was much more low, than today, and the land territory - is much more extensive. For example, then England and Europe, Siberia and Alaska were connected by land strips.
On a measure tajanja ices the sea level rose approximately on 7 mm a year. It seems that it is not a lot of, but for 10 000 years the difference has made 70 metres. Strait of Dover in La Manche has appeared about 8000 years ago. In the smallest place depth of passage makes only 36 m, and in Bering strait the minimum depth is no more 45 m. For all history of a planet such changes of contours of coastal lines occurred many times.
Drawing of coast was affected also by movements of the tectonic plates lying down under ocean and serving by the base to continents. For example, the Pacific plate leaves under water at the western coast of the South America. In this place at ocean the coast has a deep hollow, instead of a continental shelf. Mountain rocks break directly in the sea.
These large-scale changes occur very slowly - during millions years, - and to notice them it is possible only by means of geological researches. But some changes are shown literally in the face of under the influence of waves.
Waves and inflow
Waves in the sea are formed and change under the influence of inflow and a wind. Inflow arise under the influence of force of a lunar attraction: the Moon pushes forward the water which is directly under it. Otlivy arise in the places which are out of an operative range of a lunar attraction. Inflow and otlivy arise exclusively in a full moon and a new moon when the Moon and the Sun are on one or on opposite sides of the Earth, and their force of an attraction vzaimodopolnjajut each other. These especially active inflow known as spring, arise twice within calendar month. When the vector of a solar attraction is to the right of lunar, begin kvadraturnye inflow - lower, than usual, and observed twice a month when the Moon is in the first and third quarters.
On the majority of coasts it is observed on two inflow in the day, arising under the influence of a lunar orbit and rotation of the Earth. In some places, for example, in South East Asia, inflow occur only once a day. Recurrence of inflow is adhered by lunar months, that is the periods between the new moons, equal about 29,5 solar days.
In lakes and the seas as, say, in Great Lakes in the North America or in Mediterranean sea, tidal activity is lowest, and in Baltic sea inflow do not arise in general.
Tidal activity is accompanied nakatom and recoil of waves on coast. The big waves are formed under the influence of the winds blowing everywhere. The waves possessing destructive force, are a consequence of hurricanes and the other cataclysms shaking sea abyss. Especially destructive waves arise owing to underwater earthquakes. In Japanese such wave is called "чѕэрьш". If the tsunami hits about shoal near to coast the height of this wave can reach to 25 metres, and the destructions caused by it can be really awful.
Using abrasive properties of a sea ground, a wave "чрёшЁр¦=" coast also wash away a foot of sea rocks.
Under the influence of waves soft rocky breeds especially quickly collapse. When the rock foot appears is washed definitively away, the top part of rock falls off in the sea. Being rolled away, waves carry away with themselves breed splinters. The short and abrupt waves formed during local winter storm, wash off the ground most part. Long waves arise owing to remote storm in summer months. They, as a rule, put more stone and sand on coasts, than carry away with itself. As a result of it the shore is gradually formed.
On different sites of coast it is possible to observe both erosion, and accumulation. As an example it is possible to result southern coast of England. Near to Hastings rocks annually concede in Fejrlajte to the sea on some metres, forming a new gulf, and hardly further on the east stones namyli cape in the form of a human nose of Dandzhness ("эхёё" means "эюё").
the Corner under which waves are rolled on coast, influences character of erosion and adjournment. If waves wash coast under one corner, there is a so-called coastal deposit. Waves move sand and a pebble along a coast line, transferring them on the one hand a beach on another. Coastal deposits extend a beach strip, for example, on cape Cape Cod, the State of Massachusetts.
The Sea is able to create really delightful stone forms, washing away soft or not monolithic breed in rocks. For example, the sea is capable to form tunnels in rocks, "сѕЁш=і" vertical bowls and to cut through arches in a stone.
Drainage of the earths
The People living at the sea, constantly combat these elements, and most difficultly, perhaps, it is necessary Dutches. In this country even there is such saying: "Гхьы¦ God has created, the Netherlands has created уюыырэфч№". The fourth part of territory of the country which are below sea-level, has been flooded in the end of the Glacial age when the passage La Manche was formed. More than 1900 km of carefully equipped dams, dams and dunes block a way to water, preserving one of the best agricultural grounds in the world.
Opposition of the person and the sea lasts 700 years. Holland some of the most populated areas, including the cities of Amsterdam and Rotterdam, are located on the huge drained islands named polderami.
The Most large-scale project has begun in 1927 when Dutches have decided to drain huge harbour of Zuider Zee. Works on building of a 32-kilometre dam through an input in harbour have ended in 1932. Zuider Zee became fresh-water lake Ijsselmeer. Dams have surrounded five polderov which after drainage have turned to the new earths. Dutches obessolili the earth also began to cultivate it.
There are two principal views of islands. Some islands, for example, Great Britain and Ireland, have separated in due time from continent. They are located on a continental shelf and if the sea level has fallen, they again "тюёёюхфшэшышёі" with "сюыі°ющ чхьыхщ".
Other islands have arisen as consequence of volcanic activity. They represent the underwater mountains which foot is deeply on an ocean floor. As an example of such islands Hawaii can serve in Pacific ocean and the Azores in the north of Atlantic.
The romanticists Shrouded in an aura atolls, or coral islands, in warmer areas of Pacific ocean are special type of volcanic islands. The atoll, as a rule, is formed round an edge of the underwater volcanic crater which is on a surface of ocean or is superficial under water.
The Coral is a heap of skeletons of tiny sea beings, the polyps which millions live in warm shoal depth to 30 m. Limy skeletons of the died off polyps are heated up and form a stronghold named a reef. If the reef grows round a crater it forms a ring with a lagoon, or a pond with salty water inside. In a coral ring there are some breaks through which the lagoon is informed with ocean outside of a reef.
Fragments of corals form original limy sand. On it settle transferred by a wind and birds seeds from which then start up plant roots. The died off flora enriches soil - so in due course there is a blossoming island. On an atoll outer side the beaches washed by a surf sharply breaking in depth of ocean are formed. The beach strip on an inside of a lagoon is mainly wider also depth in the majority of lagoons is insignificant.
There is also other kind known under the name of a barrier reef. It arises in the sea and forms the long lagoon connecting it with coast. As well as in a case with an atoll, in a barrier reef there are breaks, is frequent opposite ustev the rivers.
The Greatest and well-known is the Big barrier reef reaching 2000 km along east coast of Queensland, Australia. The outer side of the reef which is cut through only by ten breaks, is on distance from 16 to 240 km from coast, and in a lagoon is scattered about 700 islands.