Why is the sea salty?
Rivers deliver dissolved minerals to the ocean continuously. Evaporation removes the water and leaves the minerals behind. Over hundreds of millions of years, that one-way filter is enough.
The question is really two questions. Where does the salt come from, and why does it stay?
Where the salt comes from
Rainwater is naturally slightly acidic — carbon dioxide from the air dissolves into it to form weak carbonic acid, giving unpolluted rain a pH of around 5.6. That mild acid slowly dissolves the rock it flows over, releasing sodium, calcium, magnesium, potassium, chloride, sulphate and bicarbonate into solution.
Rivers carry that dissolved load to the sea. Every river on Earth is delivering minerals to the ocean right now. A second, smaller source is hydrothermal exchange at mid-ocean ridges, where seawater circulates through hot volcanic rock and exchanges ions with it.
Why the salt stays
Evaporation is a phase change that only water molecules undergo. Dissolved ions cannot enter the vapour, so when seawater evaporates, the water leaves and the salt does not. That water returns as rain, runs off into rivers, dissolves a little more rock, and delivers another load of minerals to the sea.
The ocean is the end of the line. Water can leave it; salt largely cannot. Over hundreds of millions of years, that one-way filter concentrates dissolved minerals to the roughly 35 grams per kilogram we measure today.
Why it is not getting saltier
If rivers keep adding salt, ocean salinity ought to keep rising. It does not, on any measurable modern timescale, because salts are also removed: buried in sediments, incorporated into minerals on the seafloor, taken up by organisms, exchanged at hydrothermal vents, and blown ashore as sea spray. Input and output are roughly in balance, and ocean salinity has been broadly stable for a very long time.
What sea salt is actually made of
| Ion | Share of dissolved solids | Approximate concentration (g/kg) |
|---|---|---|
| Chloride (Cl⁻) | 55.0% | 19.3 |
| Sodium (Na⁺) | 30.6% | 10.8 |
| Sulphate (SO₄²⁻) | 7.7% | 2.7 |
| Magnesium (Mg²⁺) | 3.7% | 1.3 |
| Calcium (Ca²⁺) | 1.2% | 0.41 |
| Potassium (K⁺) | 1.1% | 0.40 |
| All others | 0.7% | 0.25 |
These proportions are remarkably constant throughout the world ocean — the principle of constant proportions. Total salinity varies between regions; the ratios between the major ions barely do.
Note that sodium and chloride together account for over 85% of dissolved solids, which is why sea salt is essentially sodium chloride with impurities. But river water is not simply dilute seawater — it is typically dominated by calcium and bicarbonate rather than sodium and chloride, because the most soluble ions have long since been flushed to the sea while calcium is continually resupplied by carbonate weathering.
Why some seas are saltier than others
Salinity is set locally by the balance between evaporation, which concentrates, and precipitation and river inflow, which dilute. The Mediterranean loses more to evaporation than it gains from rivers and rain, so it is saltier than the Atlantic and draws a permanent inflow through the Strait of Gibraltar to make up the deficit. The Baltic receives enormous river inflow into a nearly enclosed basin and is barely brackish. The Dead Sea has no outlet at all, so everything delivered stays.
Sources
- United States Geological Survey — Water Science School — Earth’s water distribution. Open · US Government work — public domain
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted