What makes sea water salty? Here’s the scientific answer

Salt is present in our kitchens, on our dining tables, and in the oceans. But why is seawater salty? The history of salt dates back thousands of years. For millions of years, minerals have been eroded from rocks, washed away as rivers, and eventually carried to the sea. Volcanic activity and hydrothermal vents add minerals from the Earth's crust to the water. This is the main reason why seawater becomes salty.

When freshwater from rivers and lakes mixes with seawater, it carries salts and minerals with it. These salts and minerals have numerous openings on the sea surface that lead to the interior of the ocean.

The primary source of salts dissolved in seawater and oceans is rocks on land. Because rainwater is slightly acidic, these rocks are constantly eroded, introducing ions into the water. Streams and rivers carry these ions to the ocean.

Another source of ocean salts comes from hydrothermal fluids exuded from ocean floor vents. Water from the ocean seeps into cracks in the seafloor and is heated by magma. The heat supports chemical processes in which the water loses oxygen, magnesium, and sulfates and gains metals like iron, zinc, and copper from the surrounding rocks.

Underwater volcanic eruptions spill minerals directly into the ocean. Water falls into the ocean, travels under cracks in the Earth's crust that are deep in the ocean, and is heated by contact with magma. This hot water dissolves salts and minerals from the rocks, just as hot water dissolves table salt or sugar much more easily. Seawater carries these dissolved elements into the ocean through vents.

The two most common ions found in seawater are chloride and sodium, which make up approximately 85% of the dissolved ions, while magnesium and sulfate make up another 10%.

Generally, salinity is lower near the equator and the poles and higher at mid-latitudes. Seawater generally has an average salinity of 35 parts per thousand, meaning it contains 3.5% dissolved salts by weight.

Many of the salts and minerals dissolved in the ocean are consumed by marine organisms. For example, organisms absorb iron, zinc, and copper from this water. Sodium and chloride, the main contents of table salt, accumulate over time and make the water in the ocean salty. Seawater has a salinity of about 3.5%, and it is denser than freshwater.

Salinity varies from one ocean to another. Salinity is lower near the equator and the poles. However, the level of salinity in some seas, such as the Mediterranean Sea, is higher than in other parts of the other ocean. Some lakes, such as Mono Lake in California and the Caspian Sea in Asia, are saline.

In such landlocked bodies of water, when salts evaporate, they are left behind, which builds up the level of salinity over time. Most of these salt lakes are located in arid regions with little rainfall and very high temperatures.

Therefore, the salt in the oceans comes from weathered rocks and hydrothermal vents. Rainwater erodes rocks and carries minerals to the ocean, while hot water from volcanic activity adds more minerals. As the water evaporates, the salts remain, increasing the salinity of the seawater.