Average ocean salinity
About 3.5 percent by weight, meaning roughly 35 grams of dissolved salts in every kilogram of seawater.
Earth & Oceans
Why Is Sea Water Salty?
The ocean is not salty because someone poured salt into it once. Rivers keep carrying dissolved minerals from rocks, while evaporation leaves the salts behind.
The short answer
Rain is slightly acidic. As it falls and flows over rocks, it dissolves tiny amounts of minerals including sodium and chloride. Those minerals wash into streams, streams flow into rivers, and rivers drain into the ocean. When ocean water evaporates, only water vapor rises into the atmosphere. The dissolved minerals stay behind. Rain forms, falls on land, picks up more minerals, and the cycle repeats. Over billions of years, the ocean has concentrated salt that has nowhere else to go. Today the ocean is about 3.5 percent salt by weight on average. About 85 percent of that dissolved material is sodium chloride. Hydrothermal vents on the seafloor also contribute minerals directly from inside Earth.
Where the salt comes from
Mostly from rock erosion carried by rivers over billions of years. Hydrothermal vents add additional minerals.
Myth: rivers are not salty because they flow fast
Rivers do carry dissolved minerals. They just keep moving, so the salt accumulates in the ocean instead.
Has ocean salinity changed over time
Ocean salinity has been relatively stable for hundreds of millions of years because salt inputs and removals roughly balance.
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