Eurasia

Tides

The term tide is applied to the periodical rising and falling of the water of the ocean caused by the attraction of the sun and moon. Periodical alterations in the direction of the wind, and periodical variations in atmospheric pressure, may give rise to alterations in the level of the sea, but true tides are attributed (are due to) to astronomical causes. It is supposed that the attraction of the sun and moon may affect not only the waters of the ocean but also the solid crust of the earth, producing an alternating change in its shape, but so small as to be difficult of detection.

Anyone living at the seaside must have observed the gradual advance and retreat of the sea about twice in the 24 hours, or to be more exact, twice in 24 hours 50 minutes, the average interval between two successive high waters being 12 hours 25 minutes. The time of high-water thus changes from day to day, and is evidently related to the position of the moon, which passes the meridian on an average 50 minutes later on each succeeding day. The height to which the water rises varies also from day to day, the range from high-water to low-water being greatest about the time of full moon and new moon, when the tides are called “spring-tides”, and least about the time of the moon’s first and third quarters, when the tides are called “neap-tides”. The tide generating effect of the moon is more than double that of the sun, because of the very much greater distance of the sun, in spite of its greater mass. When the sun and the moon are both on the same side of the earth and when they are diametrically opposed to each other their tide-generating effects are additive, but when they are at right angles to each other the effects are subtractive, so that the spring-tides have a range three times greater than the neap-tides.

The origin of the largest of the continents-Eurasia – goes to the very beginning of our planet. Basically, it was represented by an archipelago of giant islands, rising above the surface of the vast seas which then covered much of the world. These islands were five in number: the Scandinavian shield in the northwest; the Siberian shield, largest of all, in the north; the Chinese shield in the east; the Thailand – Cambodjian in the southeast; and the Indian shield in the south.

The seas above which these islands rose in these earliest days of the earth’s formation have disappeared, but two of them, the Tethys and the Uralian existed for so long that they played an important role in the history of Old World flora and fauna.

The Tethys was the southern of these two enormous bodies of water: it reached from the Alps and the Mediterranean basin all the way to the Timor Sea in the Indonesian archipelago, covering the entire width of southern Asia, Turkey, Iran, the Himalayas and Vietnam. Its shorelines changed during the more than 560 million years of its existence. Some 36 million years ago the Tethys began to dry up, eventually leaving behind it some of the familiar contours of the lands we know today. Great upheavals of the earth’s crust gave rise to the Alps and the Himalayas, isolated its entire central area, and the two ends of this enormous sea became separated by thousands of miles of emerged land.

But the common parentage of the Mediterranean and the seas around Japan is still evident in the great resemblance of the fishes found in this widely separated waters: by contrast, the Mediterranean and the Red sea, separated by only a hundred miles or so of land, have quite different fishes, and it wasn’t until the Suez Canal was cut in 1869 that they began to mix.

The second and more northern sea, the Uralian ran north from the Tethys to what is now the Arctic Ocean and it separated the Siberian from the Scandinavian shields. About 240 million years ago, what were to be the Ural mountains began to emerge. There was a general rising of land which lifted the level of the continent – to be above the waves, giving Eurasia its shape. The Ural Sea disappeared – but only for a while. Succeeding upheavals and subsidences of the continental crust created new seas separating east from west more than once before the continent was again reunited. Thus Eurasia, as we know it today has existed only for about 25 million years.

Дата добавления: 2015-01-03