Anyone who has visited a beach knows how salty the water is. As everyone knows, freshwater from rain, rivers, and even ice is not salty. 

Why are some of the world’s waters salty while others are not? 

We can deduce the solution from two clues.

Let’s begin with the first one. “Freshwater” is not wholly devoid of dissolved salt. Even rainfall contains dissolved remnants of chemicals taken up during transit through the atmosphere. The majority of the stuff that “washes out” of the atmosphere nowadays is pollution, but natural chemicals are also present.

Rainwater dissolves some minerals when it flows over soil and percolates through rocks, a process known as weathering. We drink this water, and because the salt concentration is too low, we can’t taste it. 

This water eventually reaches a stream and travels into lakes and oceans, carrying a modest load of dissolved minerals or salts. 

The amount of dissolved salts added to the ocean each year by rivers is a minuscule percentage of the total salt in the ocean. The dissolved salts carried by all of the world’s rivers would equal the salt in the ocean in around 200 to 300 million years.

Israel Dead Sea Salt
Israel Dead Sea Salt | Image Credit – Pixabay

The existence of salt lakes such as the Great Salt Lake and the Dead Sea provides a second hint of how the ocean became salty. Both are around ten times as salty as seawater. 

Why are these lakes more saline than most of the world’s lakes? Lakes are temporary water storage areas. Water is brought to the lakes by rivers and streams, and water is taken away from the lakes by other rivers. As a result, lakes are nothing more than water-filled depressions along a river channel. Water enters from one end and exits through the other.

There are no outlets for the Great Salt Lake, the Dead Sea, or other salt lakes. Only evaporation allows all of the water that comes into these lakes to depart. The dissolved salts are left behind when water evaporates.

A few lakes are salty because rivers brought salts to them, the lakes’ water evaporated, and the salts remained. The salt content of the lake water has risen to current levels after years of river inflow and evaporation. The water becomes salty as a result of the same process. 

Dissolved salts are carried to the ocean via rivers. The water from the oceans evaporates to fall as rain and feed the rivers, while the salts remain in the ocean. Because of the enormous volume of the oceans, it took hundreds of millions of years for the salt content to reach its current level.

Oceans represent around 70 percent of the total Earth’s surface, and saline water makes up roughly 97 percent of all water on and in the world; there’s a lot of salty water on our planet. 

According to some estimates, removing the salt from the ocean and spreading it equally across the Earth’s land surface would create a layer more than 500 feet (166 meters) deep, equivalent to a 40-story office building (NOAA). 

Where did all this salt come from, though? The salt in the water derives from land-based rocks. 

The salt in the ocean comes from:

  • Weathering of continents
  • Hydrothermal vents
  • Submarine Volcanoes

1. Weathering

River flowing over the rocks
River flowing over the rocks | Image Credit – Pixabay

Carbon dioxide in the surrounding air dissolves with the water molecules of raindrops, forming carbonic acid during the rainfall. As a consequence of the carbonic acid formation, the rain becomes slightly acidic. 

Rainfall physically erodes the rock, while acids break it down chemically and carry salts and minerals along as ions in a dissolved condition. The ions with the runoff are transferred to the ocean by streams and rivers.

Most of the dissolved ions are used by ocean species and then eliminated from the water. Those which are not used, Others are not used up and are left for extended periods, causing their concentrations to rise.

Sodium and chloride are the two most common ions found in seawater. Over 90% of all dissolved ions in saltwater are made up of these two. The salt concentration in saltwater (salinity) is around 35 parts per thousand.

In other words, dissolved salts account for about 3.5 percent of the weight of seawater. The weight of salt (as sodium chloride) in a cubic mile of saltwater is approximately 120 million tons. 

2. Hydrothermal Vents

Hydrothermal Vents
Hydrothermal Vent | Image Credit – Flickr

Rivers flowing to the ocean are not the only source of dissolved salts. Features on the crests of oceanic ridges were discovered about twenty years ago that changed our understanding of how the water became salty. 

These landforms, known as hydrothermal vents, are locations on the ocean floor where seawater that has seeped into the oceanic crust’s rocks has heated up and dissolved; some of the crust’s minerals have now flowed back into the ocean. The hot water brings a considerable amount of dissolved minerals with it. 

Hydrothermal vents are newly discovered phenomena on the crests of oceanic ridges that provide the oceans with dissolved minerals. 

These vents are the points on the ocean floor where seawater seeping through the oceanic crust’s rocks has heated up, dissolved some of the crust’s minerals, and then flowed back into the ocean. 

According to current estimates of the number of hydrothermal fluids pouring from these vents, the whole volume of the seas might seep through the oceanic crust in around 10 million years. 

As a result, this mechanism has a significant impact on salinity. Most of the other dissolved salts react with the rock and are eliminated from the water because of the reactions between saltwater and oceanic basalt, the rock that makes up the ocean crust.

The reactions between seawater and oceanic basalt (the rock that makes up the ocean crust) are not one-way, but some of the dissolved salts react with the rock and are subsequently eliminated from the seawater.

3. Submarine volcanism

Submarine volcanism
Submarine Volcano | Image Credit – Flickr

Submarine volcanism, or underwater volcanic eruptions, is the final mechanism that feeds salts to the oceans. Seawater reacts with hot rock, dissolving part of the mineral elements similar to the preceding process.

Will the waters keep getting saltier? It isn’t particular. In fact, for hundreds of millions, if not billions of years, the sea has had roughly the same salt level. 

The salt level in the water has reached a stable state. As rapidly as rivers and hydrothermal activities provide new salts, dissolved salts are being taken from saltwater to build new minerals at the ocean’s bottom.

Conclusion

When river water comes into contact with the rocks of the Earth’s crust, whether on land, in the ocean, or within the oceanic crust, some minerals dissolve and are taken to the ocean by the water. 

Because new minerals form on the seafloor at the same rate as salt is added, the salt concentration of seawater does not change. As a result, the sea’s salt content is constant.

Suraksha Pal is an Industrial Engineer currently pursuing my master’s degree in Renewable Energy Engineering at the Institute of Engineering, Pulchowk Campus. She has a keen interest in Renewable Energy and is passionate about sustainable development. She loves to express her views on these subjects through articles and blogs.