Why is the sea salty?

If you’ve ever accidentally swallowed seawater, you know its taste well: intensely salty. It’s something we take for granted, but it holds a curious question. If rainwater and river water are fresh, where does all that salt in the ocean come from? The answer is a geological story spanning millions of years.

The salt comes from the land

Surprising as it may seem, the sea’s salt doesn’t originate in the sea: it comes from the rocks of the continents. The process, explained by NOAA, works like this:

1. The rain that falls isn’t entirely “pure”: mixing with carbon dioxide in the air, it becomes slightly acidic.
2. That acidic rain, falling on the land’s rocks, wears them down little by little and dissolves small amounts of their minerals and salts.
3. The rivers collect those dissolved minerals and carry them to the sea.

Thus, over a very long time, rivers have been carrying salts from the land to the ocean.

So why aren’t rivers salty?

Good question, and here’s the key. Rivers do carry dissolved salts, but in very small amounts, imperceptible to the taste. The difference from the sea comes down to two words: accumulation and evaporation.

• The river is in motion: it collects salts and carries them quickly to the sea, without them piling up.
• The sea is the final destination: there the salts stay and gather over millions of years.

The role of evaporation

Here comes the second ingredient. Seawater evaporates constantly from the Sun’s heat, forming clouds. But when it evaporates, only the water leaves: the salt stays behind.

It’s like a pot of salted water simmering: the water turns to vapor and disappears, but the salt remains, more and more concentrated. In the ocean, that cycle has been happening for billions of years: water with a little salt arrives, the water evaporates, the salt accumulates. The result is an ever-saltier sea.

An age-old balance

Today the ocean has a fairly stable salinity: about 3.5% salts (some 35 grams per liter). This is because the salt that arrives is balanced by what’s naturally removed (some gets trapped in sediments and rocks on the seafloor). It’s a delicate balance that has held for geological ages.

Not just common salt

When we say “salt,” we usually think of table salt (sodium chloride), and it’s true that it’s the most abundant in the sea. But seawater contains many other minerals dissolved in it: magnesium, calcium, potassium, and more. In fact, the ocean is a kind of “soup” with nearly every element of the periodic table in small proportions. That’s why seawater doesn’t just taste salty, but also slightly bitter.

Rivers that write into the sea

The next time you taste seawater, think of its incredible journey: every grain of that salt began in a rock on land, was dissolved by rain, carried by a river, and deposited in the ocean perhaps millions of years ago. The sea is salty because it’s the accumulated memory of the erosion of the continents. A taste with billions of years of history behind it.