Why is the ocean salty?
Rivers carry dissolved rock into the sea, and then only the water leaves again — evaporation takes the water and leaves the salt behind, over billions of years.
Simple intuition
The plain reason, in everyday words
Rainwater is not quite pure. It picks up a little carbon dioxide from the air on the way down, which makes it faintly acidic — far too weak to notice, but strong enough that over a long time it dissolves a tiny amount of any rock it flows across. Every river is carrying that dissolved rock to the sea. Once there, the water has only one way out: it evaporates. And evaporation takes only the water. Whatever was dissolved in it stays behind. So the ocean is on the receiving end of a process that keeps adding dissolved minerals and never removes them the same way they arrived. Do that for a few billion years and you get a sea you cannot drink. River water is salty too, in fact — about three hundred times more weakly, which is why nobody notices.
The ocean gets saltier every year.
Salt is removed as well as added — into sediments, evaporite deposits and hot ocean crust — and the composition has been roughly stable for hundreds of millions of years. Inputs and outputs are close to balanced.
Rivers are fresh and the sea is salty, so the salt cannot come from rivers.
Rivers are salty too, at roughly a three-hundredth of the concentration. It is not noticeable per litre, but a continuous flow of slightly salty water into a basin that loses only pure water is exactly what produces a salty sea.
Sea salt is the same stuff as table salt.
Sodium chloride dominates, but seawater also carries magnesium, sulfate, calcium and potassium in significant amounts, which is why seawater tastes bitter rather than simply salty and why drinking it is harmful.
Undersea salt deposits dissolve into the sea and make it salty.
The causation runs the other way. Salt beds form when arms of the sea are cut off and evaporate to dryness, which removes salt from the ocean rather than adding it.
It is the clearest large-scale example of a reservoir with an asymmetric exit: things come in dissolved and leave as pure water, so whatever was dissolved accumulates. The same reasoning explains why lakes without outlets go salty, why kidneys have to work hard, and why concentration builds in any system where one component can leave and another cannot. It also introduces residence time, which is the tool for thinking about anything that flows in and out of a reservoir, from carbon in the atmosphere to money in an economy.
Who worked it out
Edmond Halley proposed in 1715 that ocean salt came from rivers, and suggested measuring the rate of increase to estimate the age of the Earth.
What problem forced it
John Joly carried the calculation out in 1899, dividing the ocean's sodium by the annual river delivery, and arrived at roughly 90 million years — far short of the true figure, because he assumed no salt was ever removed.
How it changed since
Twentieth-century ocean chemistry replaced that one-way picture with a balance of inputs and sinks, and the discovery of hydrothermal vents in the 1970s added a major removal route that no earlier budget had accounted for.
Why the Dead Sea is so much saltier
Same mechanism with the exit removed entirely, which shows what the ocean would be without its slow sinks.
How salinity drives ocean currents
Salt changes water's density, and density differences move enormous volumes of water around the planet.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.