the Basic structure of a terrestrial relief is created by the forces concealed deeply in bowels of the Earth, and external processes indefatigably alter it day by day, smoothing mountains and cutting deep valleys.
The Science about forms of a terrestrial relief and their changes is called as geomorphology. Geologists know that an old epithet "тхёэ№х уюЁ№" it is far from true. Mountains are completely not eternal, though geological time of their formation and destruction can be measured hundreds millions years.
From the beginning of industrial revolution in the middle of XVIII century activity of the person also plays an important role in transformation of a face of the Earth, leading sometimes to unexpected results. Continents have found the present shape and a place on a planet owing to tectonics, i.e. Movements of the geological plates forming a firm external cover of the Earth. The most recent on moving time, including formation of a hollow of Atlantic ocean and connection of India with other part of Asia, have occurred within last 200 million years. For 4,6 billion years of the history our planet has undergone many other changes.
Numerous breaks and earth crust folds, and also powerful heaps of breeds from which ranges were generated became Result of all these movings, divergences and a convergence of huge files.
We Will result three vivid examples of recent mountain building, or orogeneza as it names geologists. The Alpes were generated as a result of collision of the African plate with European. The Himalayas have soared up to heavens when India has faced Asia. poddvig the Antarctic plate and a plate of Naska together forming a part of the Pacific hollow, under a plate upon which the South America is based, has left on one side continent a huge hem - the Andes.
All these ranges youngish. Physical and chemical processes which and continue to change today shape of the Earth, had not time to smooth their sharp peaks yet.
Formation of today's continents was affected by two more related forces. These are volcanoes and earthquakes. Earthquakes cause an enormous damage, seldom have long-term consequences. But at eruption of volcanoes to a surface rises and magma, forming new horizons of magmatic breed stiffens.
Aeration of rocks
Earth crust consists of rocks. Softer substances named soils are formed of them. The basic process changing shape of rocks, is called as aeration and occurs under the influence of atmospheric processes. There are two forms of aeration - mechanical at which the stone crumbles on slices, and chemical at which it decays.
In rocks often there are cracks and multilayered horizontal naplastovanija. In due course they rise on an earth surface, where pressure much more low. In process of pressure decrease the stone extends, and, accordingly, and all cracks in it.
Thanks to naturally formed cracks, connections and naplastovanijam the stone easily is exposed to influence of weather factors. For example, the water which has frozen in a crack extends and moves apart its edges. This process is called as frosty aeration. It is possible to name mechanical aeration and action of roots of plants which, sprouting in cracks, move apart them, as if wedges.
Chemical aeration occurs at intermediary of water which, being absorbed in rock or proceeding on a surface, brings in it chemical substances. For example, water oxygen enters reaction with iron containing in breed. At rain water there is the dioxide of carbon forming coal acid absorbed from air. This weak acid dissolves limestone, forming the characteristic karstic relief which has received the name from district in Yugoslavia. Water dissolves many minerals, and those, in turn, enter reaction with rocks, decomposing them. Not last role in this process is played also by atmospheric acids and salts.
From all processes changing shape of a terrestrial surface, we is better we know erosion, i.e. Destruction of breeds by water streams, the sea, a wind or ice.
River erosion is a combination of mechanical and chemical processes. Water not only moves breeds and even huge boulders, but as we saw, dissolves their chemical components. The rivers wash away pojmy and take out soil far in ocean where it settles at the bottom and in due course turns to sedimentary breeds.
The Sea indefatigably works on alteration of a coastal line, something cutting off in one places and increasing - in others. The wind transfers small particles like sand on improbably far distances. For example, from time to time it brings to Southern England sand from Sahara, covering roofs of cars and houses with the most thin layer of a reddish dust.
At landslips gravitation forces firm breeds to slip downhill, changing a lay of land. The landslip great bulk is made by the fragments of rocks formed as a result of aeration. Water operates as greasing, reducing a friction between particles.
Sometimes landslips dvizhutsja slowly, and sometimes rush with a speed of 100 km/s and more. The slowest landslip name kripom. For a year it crawls all some centimetres, and to notice it it is possible only in some years when walls, fences and trees will be inclined under the pressure of the creeping earth.
Oversaturation of soil or clay water can cause a mud stream, or sel. Happens that the earth for years strongly stays put, but enough small tremor to bring down it downhill. In a number of recent accidents like eruption of a volcano of Pinatubo on Philippines in June, 1991 the mud streams which have filled in many houses to the roof became the main reason of victims and destructions. Similar disasters occur and as a result avalanching - snow, stone or those and others.
The Most widespread form of a landslip is the mud landslip, or a collapse. Sometimes its traces can be noticed on the steep coast washed away by the river where the ground layer has broken away from a basis. The large landslip is capable to lead to considerable changes of a relief.
In mountains, gorges and on abrupt stony slopes rockfalls, especially there where the soft or destroyed rocks prevail are frequent. The weight which has slipped downwards forms a gentle slope at mountain bottom. Many hillsides are covered by long tongues shchebnistyh taluses.
Centuries-old climatic fluctuations too have led to considerable changes of a terrestrial relief. During last glacial age huge weights of water have been connected in ice polar caps. Northern cap was stretched far on the European continent and North America south. About 30% of a terrestrial land it has been covered by ice (today only 10%).
In a glacial age the sea level was approximately on 80 m below present. Thawing of ices has led to enormous changes of a relief. So, between Siberia and Alaska Bering strait has opened, the land site between Australia and New Guinea has left under water, and Ireland and Great Britain have appeared the islands separated from other Europe.
Glaciers - the ice rivers - are in high-mountainous areas of a planet and in the subpolar regions covered with ice. Every year glaciers of Greenland and Antarctic dump in ocean huge weights of ice, forming the icebergs representing terrible danger to navigation. Glaciers have played a glacial age a leading role in giving to a relief of northern regions of the Earth of shape familiar to us. Crawling a huge plane on a terrestrial surface, they cut off mountains and vytesyvali hollows of valleys. Under a heavy right hand of glaciers old mountains like that in the north of Scotland, have lost both former height, and sharpness of outlines. In many places glaciers absolutely cut off collected for millions years multimeter layers of rocks. In process of movement the glacier grasps in so-called. Area of accumulation set of rocky fragments. There get not only stones, but also water in the form of snow which, turning to ice, forms a glacier body.
Having passed border of a snow cover on a mountain slope, the glacier is displaced in a zone abljatsii, i.e. Gradual thawing and washing out. More close by the end of this zone the glacier starts to leave deposits of rocks on the earth. Them name moraines.
At glaciers as the rivers, have main channel and inflows. Glacial inflow runs into the main channel from the lateral valley laid to it. Its bottom is usually located above a bottom of the main channel. Completely thawn glaciers leave after themselves the main valley in the form of letter U and a little lateral, picturesque falls whence are overthrown. Such landscapes often meet in the Alpes.
From the geological point of view, lake are short-lived forms of a relief. In due course they are filled with deposits of the rivers running into them, their coast collapse, and water leaves. Glaciers have generated uncountable lakes in Asia, Europe and the North America, vytesav hollows in rocks, or having partitioned off valleys final moraines. The great variety of glacial lakes is in Canada and Finland.
Other lakes as, for example, the Crater lejk in Oregon (USA), are formed in craters of extinct volcanoes in process of their filling with water. And the Dead Sea between Israel and Jordan and the Siberian Baikal have arisen in the deep cracks of earth crust formed by prehistoric earthquakes.
the Anthropogenous relief
New forms of a relief are created by works of engineers and builders. A remarkable example of it we see in the Netherlands, whose people with pride declare that have created the country own hands. Thanks to powerful system of channels and dams, Dutches have managed to win from the sea about 40% of the territory.
The Requirement for fresh water and the hydroelectric power has forced people to construct many water basins, or artificial lakes. The lake Mid in American state Nevada was formed as a result of overlapping of the river Colorado by Hoover's dam. The lake Naser near to border of Egypt with Sudan has arisen in 1968 after erection of the Aswan dam on Nile. Regulation of annual high waters and regular maintenance with agriculture water was its main problem.
the Reverse of the medal
However the Aswan dam serves as a vivid example of that with the joke nature are bad: it does not suffer rash actions. The matter is that the dam has blocked annual deposits of the fresh silt fertilising agricultural grounds and as a matter of fact generated delta. Now silt collects behind a wall of the Aswan dam, threatening existence of lake, Naser. In a relief of Egypt it is possible to expect considerable changes.
New lines give to shape of the Earth erected by the person highway and railways, and also mine waste heaps. Destruction of a vegetative cover, whose root system fastens soils, leads to erosion. These unreasoned actions of the person threaten today with a trouble to pool of Amazon in the South America.