Why the Ocean Is Salty

If you have ever swallowed a mouthful of seawater at the beach, you know the ocean tastes nothing like a glass of water from the tap. It burns your throat and stings your eyes. But here is the strange part: almost all the water in the ocean got there from rivers, and river water is fresh. So where does the salt come from?

The answer starts on land, with rain. Rainwater is slightly acidic. As it falls through the air and soaks into the ground, it picks up a tiny bit of carbon dioxide and becomes a weak acid — far too weak to hurt you, but strong enough to slowly eat away at rocks. This slow chemical attack is called weathering. As rain trickles over stones and through soil, it dissolves microscopic amounts of minerals out of the rock. Among those minerals are sodium and chloride, the two pieces that, locked together, make ordinary table salt.

Each river carries this dissolved load downhill toward the sea. Any one bucket of river water has so little salt in it that you cannot taste it. But rivers have been doing this for billions of years, and every drop they deliver brings another sliver of mineral with it.

Here is the step that surprises most people. The ocean has no drain. Water leaves the ocean only by evaporation — the sun lifts pure water vapor into the sky to make clouds, and the salt gets left behind. The salt cannot evaporate. It cannot flow out somewhere else. It just stays. Imagine filling a pot on the stove with very slightly salty water and letting it simmer for hours, topping it up whenever it gets low. The pot never gets emptier, but the water inside gets saltier and saltier as the pure water boils off. The ocean is that pot, and the sun is the stove, and the rivers are the hand topping it up. Run that process for billions of years and you get the sea.

Rivers are not the only source. Down on the seafloor, in places where the Earth's crust is cracked and hot, seawater seeps into the rock, gets heated, and shoots back out carrying dissolved minerals from deep inside the planet. These cracks are called hydrothermal vents, and they add their own chemistry to the mix — especially elements rivers do not deliver as easily.

One more thing is worth noticing. The ocean is not getting saltier and saltier without limit. It reached a rough balance a long time ago. Salt is constantly being added by rivers and vents, but it is also being removed — sea creatures build shells out of some of it, and other minerals stick to particles and settle to the seafloor as sediment. Inputs and outputs are roughly matched, so the saltiness of the sea has held fairly steady for hundreds of millions of years.

So the ocean is salty for three reasons stacked on top of each other: rain quietly dissolves rock and rivers carry the result to the sea; the sea has no exit except evaporation, which leaves salt behind; and an enormous amount of time has passed. Each drink of seawater is, in a way, a sip of every rainstorm that ever fell on every continent.