Why is the Ocean Salty?

All water, even rain water, contains dissolved chemicals which scientists call "salts." But not all water tastes salty. Salt doesn't evaporate when water evaporates. The rain flows back down to the sea, adding a little bit of salt to it constantly. Over millions of years, this makes the sea salty.

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid. The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.

As the rains fall and water flows over the land, the water dissolves salt out of the rocks, washes the salt into streams, then rivers, and finally carries the salt to the sea. The salt stays in the sea because no water flows out of the sea just as no water flows out of the Great Salt Lake in Utah. When seawater evaporates to form clouds, almost all of the salt stays behind. The left-behind salt slowly accumulates until, over the eons, the seas became salty-now about three percent.