Seawater is salty but the river water that makes up the sea is not salty. Not only seawater but some lakes also have salty water, much saltier than seawater. Does sea make salt on its own? God must not be crazy enough to make all that water undrinkable adding salt to it even if God does exist!

Whys is the seawater salty?

Why Is The Seawater Salty?

Let’s check out the facts! The average salinity (salt content in water) of seawater is about 3.5 per cent. That means a litre of water that you take out from sea contains 35 grams of salt approximately. The composition is mainly sodium and chlorine ion including ions of magnesium, calcium, sulphate, potassium, etc. The sodium and chlorine ions are actually responsible for making the seawater salty.

Three causes of saltiness of seawater

The foremost cause of salinity of seawater is the river water! It dissolves the minerals from rocks during rain and it flows from upstream down to the sea, a phenomenon called weathering. The rain contains little amount of acid, so during rainfall, the rocks and minerals are corroded. The corroded minerals dissolve in the rainwater and it is responsible for providing minerals to sea. This theory was proposed by Sir Edmund Halley in 1715 A.D. Same things happen to various salty lakes since they do not have anywhere to drain water and concentration of salt increases with the evaporation of water. So, the lakes with mineral-rich rocks nearby can be much saltier.

Halley was not completely correct in his theory as the salinity was not completely the result of weathering. Besides Haley’s theory, there are two other phenomenons responsible for the salty water in the sea. The Earth’s interior releases hydrochloric acid in gaseous form and other gases continuously called the hydrothermal vents. These gases are the major chlorine source for the seawater which is responsible for making the water salty.

Besides hydrothermal vents inside the sea, there are submarine volcanoes from which a large mass of mineral salts are delivered to the sea from the interior of the Earth yearly. An estimated 30,000 seamounts are present across the globe. Most of these are extinct volcanoes but still, some are active. During an eruption, magma comes out with different minerals which dissolve in the sea to make the sea water salty.

Why isn’t river water salty then?

 Of course, the river water has minerals content but the concentration is too low to make it salty. It is like using a floor mat: no matter how clean your surrounding is, after a long time the mat is dusty. The mat is like the sea and your surrounding is like a river. As the river flows to the sea, the minerals reach the sea. The water evaporates and keeps on collecting the minerals to the sea through water cycle (evaporated water forms clouds and then the clouds fall to land in the form of rain, the rainwater eventually ending in the sea through rivers). This leads to an increase in the concentration of salt in seawater. It took millions of years for the seawater to be as salty as today.

Is seawater getting saltier as the day goes by?

 It actually does not. As the concentration of salt is increased it has been balanced by different activities. The evaporation of seawater leads to precipitation of the salts on the rocks called evaporite deposits, also the water burial through pores in the sea bed decreases the mineral content. Apart from this, the reaction of the minerals with sea-floor basalts (volcanic rock) can also lead to a decrease in dissolved salts. So, the salinity is in the state of equilibrium.

So we can say that the seawater is salty because of the mineral-rich land on which it rests. The minerals are deposited by three major sources; weathering of rocks during rain, hydrothermal vents and submarine volcanoes. The salinity took millions of years to reach this state and still it feels the sea has always been salty unless the truth reveals.

Ashwin Khadka is a PhD Scholar in Nano Energy and Thermofluid Lab in Korea University, Republic of Korea under Korean Government Scholarship Program. He has a Masters Degree in Physics from Tribhuvan University, Kathmandu, Nepal. He is a science enthusiast, researcher and writer.