Why Is the Ocean Salty? Where the Salt Comes From
✦ Key takeaways
- Ocean salt comes mainly from the weathering of rocks on land, carried by rivers.
- Average seawater salinity is about 35 grams of salt per kilogram of water (about 3.5%).
- The most abundant ions are sodium and chloride, the components of table salt.
- The ocean stays salty because salt input is balanced by slow removal and deposition.
Taste seawater and you know its salty flavor at once. Yet paradoxically, the rivers that pour into the oceans are almost fresh. So where did all that salt come from? The answer is a geological story stretching over billions of years.
The first source: rock weathering and rivers
Rainwater dissolves a little carbon dioxide from the air, making it slightly acidic. When it falls on rocks and flows over them, it slowly breaks down their minerals and carries the dissolved ions — like sodium, calcium, and chloride — into streams, then rivers, then the ocean. River water tastes fresh because its salt concentration is tiny, but rivers deliver enormous volumes of water over millions of years, so the salt accumulates in the ocean.
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The second source: hydrothermal vents on the seafloor
Not all the salt comes from land. On the ocean floor there are hydrothermal vents where seawater seeps into the hot crustal rocks near plate boundaries, reacts with them chemically, and returns loaded with dissolved elements. This process adds some ions and removes others, helping shape the ocean's chemical makeup.
The third source: volcanoes and the seabed
Volcanic eruptions and undersea springs release gases and minerals that dissolve in water, including chlorine and sulfur. Over long geological ages, these sources, alongside rock weathering, made ocean water what it is today.
Why does the ocean stay salty rather than growing ever saltier?
One might think salinity climbs forever, but the ocean is in approximate balance: as much salt enters, salt also leaves by several routes. Some ions settle into seafloor minerals, some are used by living organisms to build shells and skeletons, and some shallow basins trap water that evaporates and leaves salt behind. This balance keeps average salinity relatively stable over long ages.
| Water body | Approximate salinity (g/kg) | Note |
|---|---|---|
| Fresh river water | Under 0.5 | Usually drinkable |
| Baltic Sea | 7 – 8 | A low-salinity sea |
| Average ocean water | 35 | The global reference |
| Red Sea | 40 | High evaporation, little rain |
| Dead Sea | About 340 | Among the saltiest bodies |
Numbers that put salinity in context
Average ocean salinity is about 35 parts per thousand — 35 grams of salt in every kilogram of water. If all of Earth's oceans dried up, the leftover salt would pile into a layer estimated at more than 45 meters thick over the seabed — enough to blanket the land in a thick crust. Sodium and chloride alone make up about 85% of the dissolved salts.
Takeaway
The ocean's saltiness is no mystery but the result of a long balance: land rocks crumble, rivers carry their ions, hydrothermal vents and volcanoes add their share, while deposition and living organisms restore equilibrium. It is the chemical memory of a planet that has been recycling its materials for billions of years.