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Answer: Ocean salt comes mainly from rocks on land: slightly acidic rain erodes rock and rivers carry the dissolved ions to the sea. Hydrothermal vents in the seafloor and undersea salt domes add more. Sodium and chloride make up about 85 percent of the dissolved ions.

From land

Rain absorbs a little carbon dioxide from the air, forming weak carbonic acid. It physically and chemically breaks down rock, and the released ions flow through streams and rivers to the ocean. Organisms use some ions and remove them; others stay and build up over time.

From the seafloor

Seawater seeps into cracks in the seafloor, is heated by magma, loses some oxygen, magnesium and sulfate, and picks up metals such as iron, zinc and copper before venting back out.

How much salt

About 97 percent of Earth's water is saline. USGS cites a NOAA estimate that the ocean's salt, spread over the land, would form a layer more than 500 feet thick.

Salinity varies

Salinity changes with temperature, evaporation and precipitation; it is generally lower near the equator and poles and higher in mid-latitudes.

Claims

• id: C1 claim: Salt in the ocean comes from runoff from land and openings in the seafloor. source: S1 evidence: "Salt in the ocean comes from two sources: runoff from the land and openings in"
• id: C2 claim: Rocks on land are the major source of salts in seawater. source: S1 evidence: "Rocks on land are the major source of salts dissolved in seawater."
• id: C3 claim: Chloride and sodium make up around 85 percent of dissolved ions. source: S1 evidence: "Together, they make up around 85 percent of all dissolved ions in the ocean."
• id: C4 claim: About 97 percent of Earth's water is saline. source: S2 evidence: "about 97 percent of all water on and in the Earth is saline"

Sources

• [S1] Why is the ocean salty?. NOAA. https://oceanservice.noaa.gov/facts/whysalty.html . License: US government work, public domain. Accessed 2026-11-04.
• [S2] Why is the Ocean Salty?. NOAA. https://www.usgs.gov/special-topics/water-science-school/science/why-ocean-salty . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_cgwz5zzh,
  author = {{iinventedlight}},
  title = {{Why the ocean is salty}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/why-the-ocean-is-salty/},
  note = {Permanent ID iil:note:cgwz5zzh. License: CC-BY-4.0.},
}

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