Why is the ocean salty?
Rain slowly dissolves minerals from rocks on land, and rivers carry them to the sea. When seawater evaporates, the salt stays behind, so it has built up over a very long time.
Quick Answer
Most of the ocean’s salt comes from rocks on land. Rainwater picks up a little carbon dioxide from the air, which makes it slightly acidic, so over time it wears away rocks and releases mineral ions. Streams and rivers carry those ions to the sea. When seawater evaporates, the water leaves but the salts stay behind, so they build up. Hydrothermal vents on the seafloor add more dissolved minerals, and so do underwater volcanoes. Two ions, chloride and sodium, make up more than 90% of the dissolved salts, and salt accounts for about 3.5% of seawater’s weight. According to NOAA, if all that salt were spread over Earth’s land, it would form a layer more than 500 feet (166 meters) thick.
By the Numbers
Key Facts
- Main source
- Minerals worn from rocks on land by slightly acidic rain
- Delivery
- Rivers carry the dissolved minerals to the sea
- Why it builds up
- Evaporation removes water but leaves the salt behind
- Other sources
- Seafloor hydrothermal vents and underwater volcanoes
How salt gets into the sea
- Rain turns slightly acidic
Rain absorbs carbon dioxide from the air, forming a weak acid.
- Rocks slowly dissolve
That rain wears away rocks on land and releases mineral ions.
- Rivers deliver them
Streams and rivers carry the ions to the ocean.
- Evaporation leaves salt behind
The sun lifts water from the sea as vapor, but the salts stay.
- The seafloor adds more
Hydrothermal vents and underwater volcanoes add dissolved minerals too.
Why rivers aren’t salty
Rivers carry salts too, but only in tiny amounts, and their water keeps flowing, so the salt never builds up. In the ocean, it concentrates as water evaporates and the salts are left behind.
Salt from below
Seawater seeps into cracks in the seafloor, is heated by magma, and comes back out through hydrothermal vents. On the way, it loses some magnesium and sulfates and picks up metals such as iron, zinc and copper from the surrounding rock.
Myth vs. Fact
MythRivers must be salty, too.
FactThey do carry dissolved salts, but in concentrations far too low to taste.
MythOcean salt is just table salt.
FactSodium and chloride dominate, but seawater also holds magnesium, sulfate, calcium, potassium and more.
The Bottom Line
The ocean is salty because rain wears minerals out of rocks, rivers carry them to the sea, and evaporation leaves them behind. Add a little from seafloor vents, and you get seawater that’s about 3.5% salt.