Why Is the Ocean Salty? Where All That Salt Actually Comes From
The ocean is salty mainly because rain slowly dissolves salts out of rocks on land and rivers carry them to the sea. According to the US National Oceanic and Atmospheric Administration (NOAA), salt in the ocean comes from two sources: runoff from land and openings in the seafloor.
Step one: rain wears down rock
The US Geological Survey (USGS) explains that rain contains some dissolved carbon dioxide from the air, which makes it slightly acidic. That acid, along with physical erosion, breaks down rock and carries salts and minerals away as dissolved ions into streams and rivers, and then to the ocean.
Step two: salt stays, and builds up
Many of the dissolved ions are used by living things in the ocean and removed from the water. Others are not used up, so they stay for long periods and their concentrations increase over time, says NOAA.
The seafloor adds more
NOAA also names hydrothermal vents. Seawater seeps into cracks in the seafloor, is heated by magma, and changes chemically as it picks up metals from the rock. It is then released through vents. Some salts come from underwater volcanic eruptions, and salt domes, large deposits formed over geological timescales, also contribute.
How salty is it?
The USGS gives the average salinity of seawater as about 35 parts per thousand, which means about 3.5% of its weight is dissolved salts. Sodium and chloride, the two ions in table salt, make up more than 90% of all dissolved ions in seawater. By some estimates, the salt in the ocean spread evenly over the Earth's land would form a layer more than 500 feet (166 meters) thick, the USGS says.
So why are rivers not salty?
NOAA addresses this directly in a separate explainer. It says the ocean's saltiness is the result of several natural influences and processes, and that river water entering the ocean is just one of them.
Quick answers
Does the ocean keep getting saltier? Ions that organisms do not use remain and build up over time, according to NOAA, but this guide does not claim a current rate.
Facts checked October 6, 2026.
Sources
- NOAA National Ocean Service, "Why is the ocean salty?": https://oceanservice.noaa.gov/facts/whysalty.html
- US Geological Survey, "Why is the ocean salty?": https://www.usgs.gov/faqs/why-ocean-salty
- NOAA National Ocean Service, "Why is the ocean salty, but rivers flowing into it are not?": https://oceanservice.noaa.gov/facts/riversnotsalty.html