Informopedia Explains: take a mouthful of seawater — not recommended — and you will taste it instantly. Every litre of the ocean holds about 35 grams of dissolved salt, roughly a heaped tablespoon. That is about 3.5% of seawater by weight, and it adds up to a staggering total: around 50 quadrillion tonnes of salt dissolved in the world’s oceans. If you could somehow scoop it all out and spread it over the land, the US Geological Survey calculates, it would bury the continents under a layer about 500 feet thick.
So where did it all come from? The short answer is that the ocean is the end of a delivery system that has been running for billions of years — and it has no way to send the goods back.
It starts with rain and rock
Rain is not pure water. As droplets form and fall, they absorb carbon dioxide from the air and turn very slightly acidic — a weak solution of carbonic acid. When that faintly acidic rain lands on rock, it attacks it: prying loose tiny quantities of minerals, ion by ion. Sodium, chloride, calcium, potassium, magnesium — the chemical ingredients of stone, dissolved into water.
Streams and rivers collect that mineral-laden water and carry it downhill. The amounts are so small that river water still tastes fresh. But the flow never stops, and the ocean is where it all ends up. The world’s rivers deliver on the order of four billion tonnes of dissolved salts to the sea every single year. The English astronomer Edmond Halley worked this out as long ago as 1715: rain washes minerals out of the ground, rivers carry them to the sea, and there they accumulate.
Some salt also arrives from below. Seawater seeps into cracks in the ocean floor, is superheated by magma at hydrothermal vents, and returns carrying minerals leached from the Earth’s crust. Submarine volcanoes add their share too. But the rivers do most of the work.
The same chemistry is at work in your own body — your tears are salty for the same reason the sea is.
Evaporation: the one-way filter
The reason the salt stays is evaporation. The Sun lifts pure water off the ocean’s surface as vapour, which falls again as fresh rain. The dissolved minerals cannot evaporate, so they are left behind. The ocean is a basin with an inlet and no outlet — water leaves only as vapour, and the salt has nowhere else to go.
That also explains why rivers stay fresh while the sea grows salty. A river is a conveyor belt, not a storage tank: fresh rain keeps flushing it through, and its small load of minerals is always moving downstream. The sea is the place where the conveyor belt ends. Salt only becomes obvious once the water finally stops.
Why the ocean is not getting saltier
Here is the puzzle Halley’s idea leaves behind. If rivers have been pouring four billion tonnes of salt into the sea every year for billions of years, why is the ocean not a solid block of salt by now? The answer is that salt is also constantly being removed — just as steadily as it arrives.
Some of it leaves as sea spray, blown onto land by the wind. Some settles out as sediment on the ocean floor, or is locked away by shell-building creatures that use calcium and other minerals to build their homes. Some is buried in layers of rock when shallow seas dry up. And some is carried down into the Earth’s mantle, where one tectonic plate slides beneath another. Input and output have been roughly in balance for a very long time, which is why the ocean’s salinity has held near its present value for hundreds of millions of years.
That same warm, salty water shapes the climate itself — it is the engine behind El Niño.
The seas that went too far
You can see the whole process in miniature in any lake with no outlet. Most lakes are fresh because a river flows in and another flows out, carrying the dissolved salts away before they can build up. But close the outlet, and the lake becomes a small ocean. The Caspian Sea — the largest lake on Earth — is salty for exactly this reason. The Great Salt Lake in Utah is saltier still.
And then there is the Dead Sea, which takes the process to its extreme. It sits at the lowest exposed land on the planet, its surface more than 400 metres below sea level, with no outlet at all. Water arrives, the desert sun evaporates it, and the salt stays. The result is a lake about 34% salt — nearly ten times as salty as the ocean — so dense that swimmers float like corks and almost nothing can live in it. The Red Sea and the Persian Gulf, where evaporation far exceeds rainfall, run saltier than the open ocean too, at more than 40 parts per thousand against the ocean’s 35.
The ocean’s salt is, in the end, a record of deep time: the slow dissolving of continents, grain by grain, carried downhill by rain and rivers, and left behind when the water rises back into the sky. Every wave that breaks on a beach is carrying the mountains with it — a little at a time, for billions of years.
