Many rivers flow into the sea. The rivers are not salty but the sea is. Why?
Actually, the rivers are salty. Most of them just happen to contain a minute 0.005% salt which is too low for us to detect by tasting. Mineral salts always get washed into rivers when rain water flows through soil and weathers rocks.
Never mind the low salt. Now imagine all the rivers in the world dumping a little salt into the sea for many years. The sea gets salty.
But wait, how can the sea get salty from rivers? It’s the same water with 0.005% salt flowing into the sea.
Yes, it is. But the sea has a notable difference from rivers – it doesn’t flow into any other water body. Being the last place where all water flows to, the only way the sea loses some of its water is by evaporation. And it is this process that increases the concentration of salt in the sea; when the water evaporates, it leaves behind the salt.
The increase in salt concentration from evaporation explains why the Dead Sea is many times saltier than the ocean. The Dead Sea has no outlet through which it enters the ocean. In effect, this lake is also like a last stop where water flows and water is lost only through evaporation. After many years of rivers pouring in their low levels of salt, evaporation has increased the salt concentration. So salty is the Dead Sea now, that a tiny drop causes a burning sensation on the tongue. Drinking just a little amount of it could even result in death.
Other contributors to the saltiness of the sea are hydrothermal vents and submarine volcanoes. Hydrothermal vents are crevices on the ocean floor through which sea water has entered underlying rock in the earth’s crust, has become hotter and dissolved some more mineral salts. This water with its dissolved mineral salt, gets back into the seawater. Volcanoes under the sea which bring sea water into contact with molten rock, also result in more mineral salts getting dissolved in seawater.
Put these natural occurrences together for many, many years and the sea’s salt becomes a lot less of a mystery.