Why Is the Ocean Salty? Rocks, Seafloor Vents and Salt Domes Explained
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "Why is the ocean salty?", checked October 29, 2026.
Two main sources
1. Rocks on land (the biggest source). Rain is slightly acidic, so it slowly erodes rocks and releases ions. Streams and rivers carry those ions to the sea. Ocean life uses some of them, but others stay behind and build up over time.
2. Openings in the seafloor. Seawater seeps into cracks and is heated by magma. It loses oxygen, magnesium and sulfates, and picks up metals such as iron, zinc and copper. Then it flows back out through hydrothermal vents. Underwater volcanic eruptions also release minerals directly.
Salt domes, huge underground and undersea salt deposits, add salt too. They're common on the continental shelf of the northwestern Gulf of America.
What's dissolved in seawater
- Chloride and sodium make up about 85% of the dissolved ions.
- Magnesium and sulfate add about another 10%.
- Everything else is present in very small amounts.
How salty, and where
Average salinity is about 35 parts per thousand. It changes with temperature, evaporation and rainfall. It's generally lower near the equator and the poles and higher at mid-latitudes.
Related: why the ocean is blue, how beach sand forms, what is sea foam.