In fact there is some salt in all water, though in what we call ‘fresh’ water the amounts are too small for us to notice. There is even a small amount of salt in rainwater, as there is of many other minerals. However, it’s clear that in the sea the salt levels are much higher.

Salt reaches the sea by a number of different processes. In one of these, the salt and other mineral contents of rocks are dissolved as rainwater permeates the rocks, and the minerals are eventually washed into the streams and rivers and carried down to the sea. Salt deposits don’t tend to build up in streams and rivers because they flow and the water, with its contents, is always moving and changing.

However, salt does sometimes build up in lakes, and some lakes like the Dead Sea in Israel (yes, it is a lake despite its name) are much saltier than any sea. This is because such lakes don’t have a channel, such as a river, that lets water flow out of them – only into them. And if a lake has no outlet, the water just stays there, along with all its mineral deposits, and in addition the water that does escape does so only by evaporation in the sun’s heat. When that happens the mineral (including salty) deposits are left behind and become very concentrated. This is also why you tend to find salt lakes in hot countries where the evaporation effect is very strong due to the sun’s heat. (The same is true of seas – the Red Sea and the Sargasso Sea, for example, have very high salt levels due to evaporation, while in the polar regions the water is much less salty. In the polar regions the salt content is further reduced by melting ice flowing into the water).

What happens in salt water lakes is what happens in the ocean, except that the lakes being shallower and more confined, the effect is more concentrated. In the sea as in salt lakes, the water also escapes by evaporation and leaves its salt and mineral contents behind. Of course this process has only happened over very, very long periods of time, and in fact the salt content of the sea increases so slowly that there has probably been very little change in it for several hundred million years.

In addition, there are areas on the sea bed known as hydrothermic vents. These show us points on the floor of the ocean where water has first penetrated through the rocks of the oceanic crust (the thick layer of rocks etc that cover the ocean floor) and then heated up, which increases the dissolving process of the mineral deposits in the rocks. The hot water then flows up into the ocean and releases large quantities of salt into the rest of the water.

Volcanoes under water also have the effect in some parts of the world of releasing more salt into the sea; again, more salt is released because the heat of the volcano causes more salt from the rocks to dissolve and spread out into the sea.

It is sometimes asked whether, as salt from rocks is constantly carried into the sea by different means, does that mean that the sea will get more and more salty, while the salt content of rocks gradually decreases? The answer is no, because new minerals are always forming, and the mineral content of rocks remains at about the same rate as the rate of dissolution into salty water.

You can learn more about this topic here.