Rainwater and Minerals
The rainwater which falls on land contains some carbon dioxide dissolved from the surrounding air, making it slightly acidic in nature. This acid weathers rocks as it flows over them, dissolving the rocks and creating electrically charged particles (ions). These ions primarily consist of sodium and chlorine which are salty in nature and once they reach oceans and seas – are the reason why the water is salty.
Underwater Volcanoes and Hydrothermal Vents
Some of these minerals are used up by organisms living in the water, while the remainder is left to build up over time. It is millions of years of accumulation which has led to oceans and seas being as salty as they are today. Underwater volcanoes and hydrothermal vents present on the ocean and sea beds also release salts into the water.
Evaporation and Salinity
The process of evaporation also plays a role in adding salinity to the water. Water evaporates, leaving the dissolved salts behind. The salt content in oceans and seas is regulated by the rivers which flow out of them – constantly balancing the level of salt. Isolated bodies, such as the Dead Sea, have grown increasingly salty as minerals have been constantly added over millions of years, without being drained out, creating hypersaline water.