Why the ocean is salty — Earth science, 11–13 years
Rocks release dissolved minerals into water, rivers carry them to the sea and evaporation leaves much of the salt behind.
What salinity means
Salinity tells us how much dissolved salt is in water. Average seawater has about 35 grams of dissolved salts per kilogram of seawater, roughly 3.5 percent by mass. It is not just table salt: magnesium, calcium, potassium and other substances are mixed into the water too.
Where the salt came from
Rainwater picks up tiny amounts of minerals as it passes through soil and rock. Streams and rivers carry these dissolved particles towards the ocean. Water then evaporates from the sea but leaves the minerals behind, so salt can build up over very long times, while sea-floor reactions and living things also remove some.
Evaporation by the numbers
Start with 1 kilogram of seawater containing about 35 grams of dissolved salts and 965 grams of water. If the water evaporates completely, the 35 grams of salts remain in the dish. They may form crystals, but the salt has not vanished; it is simply no longer spread through liquid water.
The common mix-up
It is reasonable to think that every part of the ocean must have exactly the same salinity, because waves mix the water. In reality, evaporation, rainfall, river water, melting ice and freezing ice change local values. Currents mix these differences, but they do not erase them everywhere or immediately.
Where it appears
Salinity helps scientists understand ocean density, because saltier water is generally denser than fresher water. Together with temperature, density drives some deep ocean movements and affects which organisms can live in a place. Desalination plants also remove salts from seawater to make fresh water, but this needs energy.
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