Rocks on continents are eroded and contain salt, which is carried by rivers to the sea. In fact, freshwater rivers carry a certain amount of dissolved minerals. Most of these are common table salt, sodium chloride. When salt reaches the ocean, it tends to remain and accumulate there.
Sodium chloride is highly soluble in water, and the world’s interconnected oceans are vast, so this solution does not become saturated and the salt does not precipitate.
At the beginning of this century, it was thought that the age of the Earth could be estimated by comparing the salinity of all the rivers in the world with that of the oceans. The figures proposed by theorists were around 300 million years. In reality, the age of the Earth is about 4.5 billion years.
The reason for this discrepancy is relatively simple. The salty water vapor from the ocean enters the atmosphere, evaporates, dries up, is blown onto the continents, and is then reincorporated back into rivers, making their salt concentration too high to be accurately measured. If we could account for the amount of salt that is reincorporated, we could get closer to the accurate age of the Earth. The concentration of salt in the ocean varies slightly from place to place. For example, in tropical regions where there is heavy rainfall and near large river mouths, the salt concentration is lower. The average concentration is about 3.5%, with three-quarters of that in the form of sodium chloride.
