Salt water and fresh water seas
The contiguity and clash of two seas , two water currents , or two kinds of water have long occupied the minds of interpretors of Quran ; on the basis of the scientific knowledge of the time , they have given different interpretations in connection whit the these verses , such as metaphormic and figurative interpretations,the two Roman and Persian seas , and underground deposits and oceans.What is far sure is that wonderful sea currents such as the Gulf stream,which flows like a gigantic river that is 150 km.Wide and a few hundred meters deep , sparkles for ever in the midst of the Atlantic ocean , and what is more it does not mix whit its adjacent waters. This huge water current is one of the mysteries of creation and also a mgnificent scene of the incarnation of the Lord's greatness which follows its course apart from any interpretation.
Anyhow,many research studies concerning sea and water currents have so far been carried on.The results obtained from such studies are briefly taken up in two section: factors involved in the emergence of water currents,and kinds of water currents. Factors involved in the emergence of sea currents.
1- Tides : It has been proved that the Moon , like the Earth has a gravitational field , and exerts a force of attraction on the Earth. Considering the greatness of the size of the Earth in relation to that of the Moon, the hemisphere of the Earth which is closer to the Moon is more under the Moon's gravitational force than the other one. On the other hand,it is quite obvious that ocean water is closer to the Moon that the land at is bottom and is , therefore, more under the Moon's gravitational force. Consequently ocean water is convexly attracted towards the Moon.Because of the movement of the Earth and the Moon , this condition changes every few hours and cause currents known as flow and ebb currents.
2- The Earth's rotation : The Earth rotates around its axis once every 24 hours , and hence we , on the Earth , are turning towards the east at the speed of 82 kilometers per second.The tow elements of air and water however , are not attached to the Earth , therefore , they don't turn with the Earth at the same speed so they roll away in wave form causing sea currents and winds. These currents lean towards the right in the north hemisphere,and to the left in the south hemisphere this fact was first recognized and stated by Coriolis , a french engineer, and is therefore known as Coriolis force.
3- convection : The sea water around the pole becomes cold and as a result hevier , so it moves down; to take its place ,the warmer water on the surface of the sea moves towards the pole. The reverse of this process occurs in the equator and causes water currents . This exchange of warm equatorial water whit cold polar water is one of the most important water movements in oceans.
4 - Winds : Convectional currents of the air cause wind, and because of the cold air in the pole and the warm air in the equator , winds , on the surface of the seas are always blowing from the pole towards the equator,these winds are known as prevailing winds,they,inturn,cause water currents.
5 - Water pressure : When water passes a narrow passage,it moves faster Bernoullis' principle ; this fact is also true for sea water ,and cause water current.This phenomenon is more seen at the mouth of gulfs and straits.
6 - Water density : The relative density of sea water is the ratio of the weight of the unit volume of sea water is to the weight of distillted water of equal volume at 4 degree c. Density is effective in the formation of currents because it regulates the equilibrium condition between water masses in such away that the more dense water always flows at a lower level.For example in the Mediterranean sea which the surface sea water evaporates a lot , the density of water rises and as a result it moves downward to the bottom of the sea.This water is replaced by the lighter water entering the Mediterranean sea from Gibraltar strait;and the hevier water at the bottom of the Mediterranean sea passes the mouth of Gibraltar strait and enters the lower level of the Atlantic ocean waters; the same phenomenon occurs in the Baltic Sea too.
7 - Waves : Sea waves themselves cause water currents. As waves approach seashores , they deviate move until they become parallel to the shore. These waves are convergent at the two ends of the shore and divergent in the middle. Therefore ,it is expected that the waves be more intense and higher at the convergent site,and at the divergent site they be less intense and shorter . The difference in the intensity of these waves becomes a current towards the divergent site . Such currents are called wave made currents.
Adapted from the book : "Water"
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