Why Is the Ocean Salty? Where Sea Salt Really Comes From

That sharp, salty taste when you get a mouthful of seawater is the result of a process that has been running on our planet for millions of years. But where does the salt come from — and why are rivers fresh when they flow into the same sea?

How much salt is in seawater?

On average, about 3.5% of ocean water is dissolved salt, or roughly 35 grams per litre. Most of it comes from the two elements that make up table salt:

• Chloride: about 55%
• Sodium: about 30%
• The rest is ions such as sulphate, magnesium, calcium and potassium.

Source one: rocks

The story starts with rain. As raindrops fall, they dissolve a little carbon dioxide from the air and become slightly acidic — enough to gently but constantly wear away rock.

As rainwater flows over and through rocks, it dissolves minerals. They break apart into tiny electrically charged particles called ions. Streams and rivers collect these ions and carry them to the sea.

River water contains salt too, but so little that we cannot taste it. The real difference happens in the ocean.

Evaporation: the water leaves, the salt stays

When the Sun warms the sea surface, water evaporates into the atmosphere. Dissolved salts cannot go with it, so they stay behind. The evaporated water later falls as rain, weathers more rock and brings fresh salts back to the sea.

This cycle has run without pause for millions of years. Salt delivered by rivers accumulates in the sea year after year, and that is how seawater reached its present saltiness.

Source two: the ocean floor

Some of the salt comes straight from the bottom of the ocean:

• Hydrothermal vents: Seawater seeps into cracks in the seafloor, is heated deep in the crust and dissolves minerals from surrounding rock before gushing back into the ocean.
• Undersea volcanoes: Gases and minerals released during eruptions also enter seawater. Much of the ocean’s chloride is thought to come from volcanic sources like these.

Is the sea getting saltier?

If salt is added every year, you might expect the oceans to keep getting saltier. In fact, ocean salinity has been roughly stable for hundreds of millions of years, because salt also leaves the sea:

• It becomes part of sediments that settle on the seafloor.
• In shallow, warm seas, evaporation leaves thick layers of salt. Much of the rock salt mined today is the remains of ancient seas.
• Some ions are used by marine life to build shells and skeletons.

Because inputs and outputs balance, the ocean behaves like a giant steady-state system.

Why aren’t all seas equally salty?

Salinity varies with evaporation, rainfall and how much fresh water flows in:

The Baltic and Black Sea are less salty because large rivers pour in plenty of fresh water while little seawater is exchanged with the ocean. In the Mediterranean, the warm climate means evaporation exceeds rainfall, so it is saltier than the open ocean. The Dead Sea has no outlet: water leaves only by evaporating, so the salt becomes extremely concentrated — which is why people float so easily there.

Why can’t we drink seawater?

The body must keep the salt level in the blood within a narrow range, and seawater is about four times saltier than blood. To get rid of that extra salt, your kidneys would need more water than the seawater contains. Drinking seawater therefore does not quench thirst; it makes dehydration worse.

In short

Sea salt comes from rocks, from hot springs on the ocean floor and from volcanoes. Rivers deliver it, the Sun evaporates the water and the salt stays behind. Over millions of years, with salt arriving and leaving in balance, this cycle has given the ocean the saltiness we taste today.