Why is the ocean salty?
The ocean is salty because rain slowly wears down rocks on land, and rivers carry the dissolved minerals to the sea, where they build up as water evaporates.
Rainwater is slightly acidic because it picks up carbon dioxide from the air. As it falls on rocks, it slowly breaks them down and dissolves tiny amounts of minerals. Rivers carry these dissolved ions, especially sodium and chloride, into the ocean.
When seawater evaporates, the water leaves but the salts stay behind. Over hundreds of millions of years, this process has made the ocean about 3.5 percent salt by weight. Underwater volcanoes and hydrothermal vents on the seafloor add more minerals.
Why rivers are not salty
River water does contain dissolved minerals, but in very small amounts, and it is constantly replaced by fresh rain. The ocean is where the minerals collect. If all the salt in the sea were spread over the land, it would form a layer more than 150 meters thick. Some lakes with no outlet, like the Dead Sea, are even saltier than the ocean.
Did You Know?
The saltiest large body of seawater is the Red Sea, where hot sun evaporates a lot of water and few rivers flow in. The Baltic Sea, fed by many rivers, is among the least salty. Salty water is also denser, which drives currents that move heat around the planet.