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The oceans were salted by the earth’s freshwater rivers

Posted in Geography, Geology, Minerals, Rivers, Science, Sea on Wednesday, 18 April 2012

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This edited article about the sea originally appeared in Look and Learn issue number 689 published on 29 March 1975.

seafront waves, picture, image, illustration

Waves crashing on the seafront by Clive Uptton

Most of us think that the sea consists entirely of water. But in every hundred pounds weight of sea water there are about three and a half pounds of solid materials. Most of them are salts of one kind or another.

If all the salts could be taken out of all the world’s oceans and spread over the continents, they would form a crust several feet thick.

Another surprising thing is that sodium chloride, which is the chemists’ name for the salt we sprinkle on our food, makes up only three-quarters of the salts dissolved in sea water.

If you extracted all the salts from a bucket of sea water, you would find that for every hundred parts of solids you got there would be: 77.75 parts of sodium chloride; 11 parts of magnesium chloride; 4.75 parts of magnesium sulphate or Epsom salts; 3.5 parts of calcium carbonate or limestone; and a quarter part of magnesium bromide. Another strange thing about the ocean is that if it were not for freshwater rivers, sea water itself would be fresh.

Scientists believe that when the oceans were first formed countless millions of years ago, the sea water was fresh.

As rivers pass over the land on their way to the sea, they wash out of the ground various kinds of salts. These are carried by the rivers into the oceans.

When we talk about “freshwater” rivers we must remember that no river is absolutely fresh. It contains a small percentage of salt which is being carried down to the sea, but the percentage is so small that it cannot be detected by taste.

The heat of the sun evaporates water from the sea; that is, turns it into a kind of invisible vapour. When sea water is evaporated, the salts in it are left behind.

You can prove this yourself by boiling away salt water in a pan. The water will all disappear as steam, and leave the salt at the bottom of the pan.

As the sun has been evaporating sea water for millions of years without taking any of the salts away, the ocean is getting more salty all the time.

This saltiness increases at a fairly regular rate, so that it is possible to work out how long ago it is since the sea consisted of fresh water only.

Some oceans are saltier than others, and all parts of any particular ocean are not equally salty. The surface of the sea is generally more salty than it is lower down.

In the Arctic and the Antarctic, where there are no great rivers to carry salts to the sea, the ocean is least salty. But the Atlantic is one of the saltiest seas because into it flow scores of rivers from the American and European continents.

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