Why is the sea salty?

A mouthful of ocean

If you have ever swallowed a mouthful of seawater at the beach, you know it tastes nothing like tap water. It is salty. How salty? On average, about 35 grams of every kilogram of seawater is dissolved salt. That is 3.5 per cent. Scientists call this saltiness salinity.

Most of that salt is sodium chloride, the same salt you sprinkle on chips. When salt dissolves, it breaks into pieces too tiny to see. That is why seawater still looks clear, even though your tongue can taste the salt. The sea also holds smaller amounts of other minerals, such as magnesium and sulfate.

Fresh water, the kind in most rivers and taps, has less than 1 gram of salt in every kilogram. Seawater has more than 35 times as much. All the salt in the ocean adds up to a huge amount. One estimate says that if you took it all out and spread it over the dry land of Earth, it would make a layer about 150 metres thick. That is as tall as a 40-storey building.

Rocks, rain and rivers

So where did all that salt come from? Mostly from rocks on land. Rain soaks up a little carbon dioxide from the air as it falls, the same gas that puts the bubbles in fizzy drinks. This makes rainwater very slightly acidic. When the rain lands, it slowly breaks down rock and dissolves tiny amounts of minerals from it.

The minerals travel as salts in creeks, then rivers, then out to sea. A river does not taste salty, because it carries only a tiny amount at any one time. But rivers have been flowing to the sea for a very, very long time.

Some salt comes up from below, too. In places on the seafloor, seawater seeps down through cracks and is heated by hot rock. It dissolves minerals from the rock, then gushes back out through openings called hydrothermal vents. Undersea volcanoes also add minerals. Scientists estimate that all the water in the world's oceans could pass through these vents every 10 to 20 million years.

The water leaves, the salt stays

Rivers bring salt in, but the sea does not just fill up with water. The Sun warms the surface, and some water evaporates: it turns into an invisible gas called water vapour and rises into the air. The salt cannot go with it. The vapour cools into clouds, falls as rain, and runs down the rivers to the sea again. This loop is the water cycle, and the salt gets left behind each time round.

That is why some seas are saltier than others. The Red Sea and the Mediterranean Sea lose lots of water to evaporation, so they are saltier than average. Near the equator, heavy rain makes the surface water a little fresher. So do river water and melting ice in cold places. NASA satellites have mapped the sea's saltiness. Their map shows values from 30 to 40 grams per kilogram, with 35 as the average.

You might wonder why the sea does not keep getting saltier and saltier. The answer is that salt also leaves it. Sea creatures use up some of the dissolved minerals to build hard parts, such as shells and coral skeletons. Some minerals react with hot rock on the seafloor and are locked away. Scientists think the salt coming in and the salt going out now roughly balance, so over a few million years the sea's saltiness stays about the same.

A lake with no way out

Now picture water that can never reach the sea. That is what happens in the Lake Eyre Basin, in the heart of Australia. Its creeks and rivers flow towards Kati Thanda–Lake Eyre in South Australia, but they do not reach the ocean. Kati Thanda is the name used by the Arabana people, the lake's traditional owners.

It is one of the driest places in the country. In an average year only about 140 millimetres of rain falls there, but the hot, dry air can evaporate about 2.5 metres of water. That is almost 18 times as much as the rain. So the water that does arrive soon disappears and leaves its salt behind. Geoscience Australia explains that the lake's salt is washed out of sediments that ancient seas laid down under the land.

Kati Thanda is Australia's largest salt lake. Most of the time it is a glittering white sheet of salt, like the one in the photo. After huge floods, water spreads across it and the crust begins to dissolve. It fills to the brim only a few times a century. Then the water dries up, and the salt crusts over again. A small fish, the Lake Eyre hardyhead, can survive in water about three times saltier than the sea.

Taking the salt out

People can make salt from the sea by copying Kati Thanda. At Dampier and Port Hedland in Western Australia, seawater is let into huge, shallow flats. The climate there is hot and dry, with little rain. Over months the Sun and wind evaporate the water, and a crust of salt crystals is left. Harvesters scoop it up. Dampier Salt is the world's biggest exporter of salt made this way. Most of it goes to factories, to help make things like chemicals, glass and paper.

We cannot drink seawater to stay alive. Our kidneys can only make wee that is less salty than seawater, so we would have to lose more water than we drank, and we would get thirstier and thirstier. But engineers can take the salt out first. In 2006, Perth opened the first big seawater desalination plant in Australia, at Kwinana.

The plant uses reverse osmosis. Seawater is pushed at high pressure through thin sheets called membranes. Their holes are so tiny that salt cannot squeeze through. About half of the seawater becomes fresh water. The rest, called brine, is about twice as salty as seawater. It is sent back out to sea through a long pipe with special mixers on the end, called diffusers, that spread it out quickly. Today, Perth's two desalination plants, at Kwinana and Binningup, together make around 30 to 40 per cent of the city's drinking water in a typical year, depending on how much rain falls.

📚 Words to know

Salinity

How much salt is dissolved in water. Seawater has an average salinity of about 35 grams of salt per kilogram.

Dissolve

To mix into a liquid so completely that you can no longer see the pieces, like salt in water.

Evaporation

When a liquid turns into a gas. Water evaporates into vapour, but the salt in it stays behind.

Hydrothermal vent

An opening in the seafloor where seawater heated by hot rock gushes out, carrying dissolved minerals.

Desalination

Taking the salt out of seawater to make fresh water that is safe to drink.

🧪 Try this: make a tiny sea, then take the salt back

1. Ask an adult to help. Stir 35 grams of table salt into 1 litre of warm tap water until the salt disappears. You have made water about as salty as the real sea. Dip a finger in and touch one drop to your tongue if you like, but do not swallow it.
2. Pour two tablespoons of your salty water onto a small dark plate. Pour two tablespoons of plain tap water onto a second plate. Put both plates in a sunny spot indoors, like a windowsill, where nobody will knock them.
3. Check each day. After a few days, or up to a week, the water is gone. The plate that held the salty water is left with white crystals. A magnifying glass will show tiny cube-shaped crystals. The tap-water plate should have only a faint film or a few specks. The salt stayed behind when the water evaporated, just as it does at Kati Thanda.
4. Challenge: do a fair test. Set up two more plates of salty water, one in the sun and one in the shade. Which dries faster? How does this help explain why the Pilbara is good for making salt?