How does ocean water become salty

The sea often appears blue because of the way light interacts with the water. White light is made up of many different visible colours ranging from red to violet — red has the longest wavelength, blue light the shortest. As water molecules are better at absorbing light with longer wavelengths, they absorb much of the red, orange, yellow and green light. The bluer colours, with shorter wavelengths, are less likely to be absorbed, giving the sea its blue hues. Shallow water often appears clear as there are fewer water molecules to absorb the light, so other colours are able to reach the sea floor and reflect.

The deeper you go, the more other colours are absorbed and the deeper blue the light becomes, until you reach the point where no visible light can reach, where it is completely dark. The colour of the water also depends on other factors, such as what particles are floating in it. In the beginning, the primeval seas were probably only slightly salty. But over time, as rain fell to the Earth and ran over the land, breaking up rocks and transporting their minerals to the ocean, the ocean has become saltier.

Sodium chloride is the main salt in seawater, and the same one you might have on your table at home. The rain water flows off the land and into the rivers and streams that lead all the way to the sea — carrying the dissolved salts along with it.

The salts in the seas have built up over billions of years, and seawater contains about times more dissolved salts than average river water. To put it another way, every one litre of seawater has 35 grams of salts dissolved in it, while a litre of freshwater would only have 0. Some salts can also enter the seas from hot vents on the deep ocean floor and from volcanoes on the land and in the sea. Since salt is always flowing from the land to the sea, you might think the sea is getting saltier.

But actually, some of this salt is removed by algae and animals that live in the sea, and some is deposited as sediment on the bottom of the ocean. So the salt going into the sea keeps a balance with the salt being deposited or removed. Thus, lakes are really only wide depressions in a river channel that have filled with water.

Water flows in one end and out the other. All the water that flows into these lakes escapes only by evaporation. When water evaporates, the dissolved salts are left behind. So a few lakes are salty because rivers carried salts to the lakes, the water in the lakes evaporated and the salts were left behind. After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels.

The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.