The sea is salty, but the rain that fills it is fresh and the rivers running into it taste fresh too. So where does the salt come from? Rain is very slightly sour, and as it trickles over rock it dissolves a tiny bit of mineral and carries it down the rivers to the sea. Then the sun warms the surface and lifts the water back up into clouds as pure water vapour — but it leaves the salt behind. New rivers keep bringing more, and the sea keeps the lot, so over millions of years the salt has piled up. Run the cycle in the simulator and watch the saltiness climb.
Most people think the ocean is salty because rivers pour in salty water. In fact rivers carry only a faint, untasteable trace of dissolved minerals. The salt builds up because evaporation lifts away pure water and leaves the salt behind, over millions of years.
What's actually happening
It seems like a contradiction the first time you notice it. Rain falling on your hand is fresh. The river you can wade into is fresh. Yet pour both into the sea and the sea is salty enough to sting your eyes and float you higher than a pool. If the water arriving is fresh, the salt has to be coming from somewhere else, and the answer is hiding in the rock the water travels over on its way down.
Rainwater is not quite pure. As it falls it dissolves a little carbon dioxide from the air and turns very weakly acidic, and that mild acid is enough to nibble at the rocks and soil it runs across. It carries off dissolved minerals (most importantly sodium and chloride, the two halves of ordinary table salt) and rivers deliver this faint salty cargo to the sea. Any single river is so dilute you would never taste it. The trick is what happens next. The sun heats the ocean surface and evaporates water back into the sky to make clouds, but evaporation is picky: it lifts pure water vapour and leaves every grain of salt behind in the sea. The water leaves; the salt stays. Run the simulator and you can watch it happen — rivers feed a little salt in, the sun pulls clean water out, and the saltiness ratchets upward.
Do that for hundreds of millions of years and the numbers get serious. The ocean is now about 3.5 percent salt by weight, which works out to roughly 35 grams in every litre of seawater, enough that if you evaporated all the oceans the leftover salt would bury the continents under a layer many tens of metres thick. The sea is not actually getting much saltier today, though, because the salt has somewhere to go too: it settles into seafloor sediments and reacts with hot rock at deep-sea vents, so input and removal have drifted into rough balance. The ocean is salty because it is the bottom of the world's drain, and the one thing the drain can never wash away is the salt itself.
The sea is salty because it is the bottom of the drain: water can leave by evaporation, but the salt it carries cannot.
- 1Press Run cycle and let the sun evaporate water off the surface while the river trickles in — keep an eye on the saltiness readout.
- 2Turn the evaporation up and notice the saltiness climbs faster, because pure water leaves while the salt stays behind.
- 3Let it run a long while and watch the number settle high — you have just fast-forwarded millions of years of a draining world.
Common questions
Each river carries only a faint trace of dissolved salt, far too dilute to taste. The sea collects that trace from every river and then loses pure water to evaporation, concentrating the salt until it becomes noticeable.
Not really. Salt is also removed by settling into seafloor sediments and reacting with hot rock at deep-sea vents, so input and removal have drifted into rough balance rather than climbing forever.
The Dead Sea has no outlet, so its only way to lose water is evaporation. That has concentrated the salt to roughly ten times normal seawater, dense enough for swimmers to bob like corks.