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More than %70 of the Earth is covered with water.The seas and oceans account for %97 of this water the oceans cover 361 million square kilometers.

The extent of the seas and oceans used to be different.Before the first geological era, all the lands on the Earth were connected together . The Urasia was on the northern hemesphere , and on the southern hemesphere there was the Gondwana.During some geological changes in the history of the planet Earth, the first breaking of the land took place in the first geological era and as a result the Thetis sea was formed;then gradually due to different geological events and also the important process of horizontal movement of the crust of the Earth the continents got separated from each other the separation theory of continents and hence , various oceans were formed.

The present oceans are :

1 - The Pacific ocean with the surface area of 177 million square kilometers and maximum depth of 11000 m.

2- The Atlantic ocean with the surface area of 106 million square kilometers and maximum depth of 8380 m.

3 - The Indian ocean with the surface area of 77 million square kilometers and maximum depth of 7450 m.

4 - The Arctic ocean with the surface area of 14 million square kilometers and maximum depth of 5450 m.

5 - The Antarctic ocean which lies on the southern hemesphere.

The main difference between the oceans and seas lies in their relative hugeness.Oceans are vast expanses of water which separate continents from each other,and their surface area can reach up to tens or hundreds of millions of square kilometers,while seas, which are generally located on the edges of oceans never exceed more than few million square kilometers.There are some cuts in a varieties of forms at the edges of oceans near where they meet the land which are, in some places,more or less deep;in fact, these cuts form seas and gulfs.

Seas are divided into two distinct groups according to their morphology :

1- the seas which are somehow connected with the oceans and are generally separated from them by a group of islands or peninsulas . The free connection between oceans and these seas is the main reason for the very close relationship which exists between the morine life and the water temperature and saltiness in these two environments. Examples of these are the Bering and the Barents and the Japan seas.2- the seas which have fully and deeply penetrated the land and formed semi-closed basins of water.These basins are separated from the oceans by under sea ridges. Connection between these seas and oceans is very difficult.The only way of connection is through the semi-deep and narrow straits, like the Black Sea and the Mediterranean Sea. Each sea is divided into four zones according to its depth :

1- The coastal or tidal zone littoral zone. This is a zone located between the flow and ebb of the sea.

2- The shallow zone neritic zone starts from the lower limit of the littoral zone and extends upto the depth of 200 m.This zone is also called the continental shelf.

3- The average depth zone of the sea bathgal zone which begins from the depth of 200 m ,and extends downward for 2000 to 3000 m.

4- The deep zone abyssal zone consists of some parts with the depth of more than 3000 m the lowest part of the Earth or the deepest part of the oceans of the world is near Japan and is called Marien graben with the depth of more than 11 km.

The bottom surface of the seas and oceans are not even or flat , but are similar to lands with heights and depressions such as valleys, rifts , ocean ridges , mountains subduction areas . Old volcanic cones or guyots and corals.

The surface of the seas are also constantly on the move and changing. Most of the waves seen on the surface of the seas are caused by winds. Of course single, dangerous waves are sometimes resulted from earthquakes and explosions happening under the seas. Studies show that the top surfaces of the oceans are also changing in the same way as level of the underground water . On a world scale these changes are called static changes . These phenomena are affected by such factors as newly formed oceanic water basin which are the result of the separation of continents and the melting of icebergs.

The degree of saltiness of ocean waters varies in different parts it varies according to time and location. Generally the degree of saltiness increases from the equator to the Cancer Orbit and the Capricorn Orbit , and decreases as it reaches the poles.It is worth mentioning that the degree of saltiness largely depends on the evaporation process. There are different gases dissolved in the waters of the oceans the most important one is oxygen.The main role of this gas is for the respiration of marine life.

Adapted from the book : "Water"

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