Why Is the Sea Salty?

Step 1 of 5

Rain is faintly acidic and attacks rock.

Rainwater picks up carbon dioxide and becomes a very weak acid, which slowly frees mineral salts from the rocks it flows over.

Step 2 of 5

Rivers carry the salt to the sea.

River water tastes fresh because the salt is so dilute — but every river is delivering dissolved minerals to the ocean every single day.

Step 3 of 5

Evaporation takes only the water.

When the Sun lifts water from the sea it leaves every dissolved salt behind. Salt cannot ride along, so it accumulates.

Step 4 of 5

Billions of years of that adds up.

Seawater is now about 3.5 percent salt. In a bathtub of it, you would have several kilograms of salt.

Step 5 of 5

It is not going up forever.

Salt is also removed — trapped in seafloor sediments and in the minerals of ocean crust. Delivery and removal have roughly balanced for a long time.

The Dead Sea is nearly ten times saltier than the ocean, because water only leaves it by evaporating.