Why Is Ocean Water Salty? Where Sea Salt Comes From
In short: the ocean is salty mainly because rain slowly dissolves minerals from rocks on land and rivers carry them to the sea, while hot water seeping through the sea floor adds more. The main ingredients are sodium and chloride, the same as table salt. Seawater holds about 35 g of salt per litre, against less than 1 g in freshwater.
The essentials
| Question | In short |
|---|---|
| Where does the salt come from? | Rocks on land, carried by rivers, and hot fluids from the sea floor |
| What is sea salt made of? | Mainly sodium and chloride, the main ingredients of table salt |
| How salty is seawater? | About 35 g per litre (3.5%), against less than 1 g per litre in freshwater |
| Is it the same everywhere? | No: it varies with rain, rivers and evaporation |
| Why does the ocean not keep getting saltier? | Salt arriving is largely balanced by salt and minerals leaving the water |
Ever swallowed a mouthful of seawater while swimming? Then you know the ocean is very salty. So where did all that salt come from, and why does it seem to stay at the same level? Here is a simple explanation of why ocean water is salty.
Why is the ocean salty? Where the salt comes from
Most of the ocean's salt comes from rocks on land, carried to the sea by rivers, with extra minerals added from the sea floor.
Rain is very slightly acidic, because it picks up carbon dioxide from the air. As it falls and runs over the land, it dissolves tiny amounts of salts and minerals from rocks. Streams and rivers carry them to the sea. Experts estimate that this brings in around four billion tonnes of dissolved minerals every year.
The sea floor adds its share. Seawater seeps into cracks in the Earth's crust, is heated by the hot rock below, dissolves minerals on the way and then rises back into the ocean through hydrothermal vents.
The two main ingredients are sodium and chloride, which together make sodium chloride: the salt in our kitchens. When seawater evaporates in the heat, only the water leaves. The salt stays behind in the ocean.
How salty is seawater?
Seawater holds about 35 g of salt per litre, around 3.5% of its weight, but the level varies from place to place.
On average, about 3.5% of seawater's weight is salt, which is roughly 35 g per litre. A litre of freshwater holds less than 1 g. This extra salt makes seawater denser than freshwater, which is why it is easier to float in the sea.
The figure is not the same everywhere. Salinity is lower near the equator and the poles, where there is a lot of rain or melting ice, and higher in between, where evaporation is strong. Where big rivers flow into the sea, the water is also less salty. Some seas are saltier than the open ocean, such as the Mediterranean.
Why doesn't the ocean get saltier and saltier?
Salt and minerals keep arriving, but they also keep leaving the water, so the overall balance holds.
Evaporation removes water but not salt, so you might expect the ocean to become saltier all the time. The reason it does not is that minerals also leave the water. Marine life uses dissolved minerals: for example, many sea creatures take up calcium to build their shells. Other minerals settle into the sediments on the sea floor. Rain and rivers, meanwhile, add large amounts of freshwater, which dilutes the salt.
Ocean currents also help. As we explain in our article on ocean currents, seawater is constantly on the move, at the surface and in the deep, and this mixing spreads salt towards less salty areas.
Want to bring a bit of the ocean into your home? Take a look at our Marine tide clock, which lets you keep an eye on the tide at your favourite beach.
Keep exploring: the difference between a sea and an ocean and the most beautiful salt marshes in France.
A little piece of ocean at home
Frequently asked questions
Why is the ocean salty?
Because rain dissolves minerals from rocks on land and rivers carry them to the sea, and because hot fluids from the sea floor add more. When seawater evaporates, the water leaves but the salt stays.
How much salt is there in seawater?
On average about 35 g per litre (around 3.5% of its weight), against less than 1 g per litre in freshwater. The exact level varies from one sea to another.
What is sea salt made of?
Mainly sodium and chloride, the main ingredients of table salt, along with smaller amounts of other minerals.
Why doesn't the ocean get saltier and saltier?
Because salt and minerals also leave the water: marine life uses some of them and others settle into the sea floor. Rain and rivers add freshwater too, and currents mix the water around.
Why isn't freshwater salty?
Rivers and lakes carry only small amounts of dissolved salt and are constantly topped up by rain. In the ocean, the salt is collected and kept, while the water evaporates.
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