Why is the ocean salty?

Seawater is salty because rivers act as a slow, planet-sized delivery system for dissolved minerals. As rain falls, it absorbs a small amount of carbon dioxide from the atmosphere, making it slightly acidic. That weak acid slowly dissolves rock on land, picking up ions like calcium, magnesium, sodium, potassium, and chloride. USGS and NOAA both trace the dominant ingredients in seawater to weathering, not to the seafloor or to underground brine vents, although hydrothermal vents do add a meaningful share of magnesium and some other ions.

Rivers then carry those dissolved ions to the sea. When ocean water evaporates, pure H₂O leaves the surface as vapor and the salts stay behind, which is why the salinity of the open ocean averages about 35 parts per thousand, or 35 grams of dissolved salt per kilogram of seawater, according to NOAA. Over hundreds of millions of years that accumulation has built the salt concentration we see today. The ocean has not become a brine ocean because salt is also removed: ions precipitate out as minerals, get locked into sedimentary rock, and get cycled back into the mantle at subduction zones. The current salinity is a steady state, not an ever-rising number.

Most of the “salt” is not the table salt people sprinkle on food. Table salt is sodium chloride, and seawater does contain a lot of it, but the dissolved mix includes sulfate, magnesium, calcium, and potassium as well, plus smaller amounts of almost every other stable element. Of the 35 grams of salt in a kilogram of seawater, sodium and chloride together account for roughly 30 grams, with magnesium, sulfate, calcium, and potassium making up most of the rest. NOAA estimates that if you evaporated all the ocean water, the salts left behind would cover the continents to a depth of roughly 500 feet, equivalent to a layer about 150 meters thick across the land. That volume is a useful way to picture how much dissolved material has been moved by rivers since Earth’s oceans first formed.

Salinity also varies by region. The Red Sea and the Persian Gulf are well above the global average because evaporation is intense and freshwater inflow is small. The Baltic Sea and the open equatorial oceans are fresher because heavy rainfall and major rivers like the Amazon dilute the surface. The Dead Sea is the extreme outlier at roughly 340 parts per thousand, about ten times the open-ocean average, which is why its waters are dense enough that swimmers float with very little effort and why almost no fish survive there. Ice formation also spikes local salinity because freezing rejects salt from the ice, which is why polar surface waters can become dense enough to sink and drive deep-ocean circulation. Salinity, temperature, and density are tightly linked, and climate scientists track them closely because shifts in the freshwater balance — from melting ice sheets, for instance — can change ocean circulation patterns.

That is also why salt is spread on icy roads in winter: the dissolved ions lower the freezing point of water, which is the same chemistry that governs ocean phase changes. The reason sea ice can form at all while the ocean stays liquid down to about −1.8 °C is that the dissolved salt disrupts the orderly crystal lattice of pure water. Marine biologists also pay close attention to salinity because many organisms — coral reefs, oysters, salmon, and countless plankton species — are tuned to a narrow range, and sudden freshwater pulses from heavy rain or glacial melt can stress whole ecosystems.

For more on how water behaves, see the salt on ice explainer and the tide explainer, since salinity and tides both shape how the ocean works.