Your turn
Each lab is one part of the video. Tap and slide, and see what happens.
Each lab isolates one step. They are simplified teaching sketches: sped up and not to scale.
Tap the buttons and slide the sliders.
Boil a litre of river water and a litre of seawater until they are dry. What is left?
Compare the dissolved load of fresh water (< 1 g/L) and seawater (~35 g/L).
Dry up the river water and the sea water. Which leaves more salt?
Simplified: real boiling takes much longer.
Tap the cloud. Rain nibbles the rock and carries ions away. Does the river get salty?
Weak carbonic acid in rain dissolves minerals into ions that runoff carries away.
Tap the cloud! The rain nibbles the rock.
Run the conveyor. Water leaves, but the salt stays. Watch the meter.
Evaporation removes only water, so the ocean concentrates the rivers' dissolved load.
Tap to run the river. Watch the sea get saltier.
Toy speed: the real ocean took millions of years.
Tap the chimney. Hot water gushes minerals up from the sea floor.
Seawater heated by magma leaches metals and jets out of the vent.
Slide the heat and the fresh water. How salty can your sea get?
Balance evaporation against rivers, rain and ice melt, then compare with real seas.
Slide the sun and the water. Make your sea extra salty!
“My sea” is a made-up toy rule; the other bars are real measurements.
Pick a way people get salt. Where does each one lead?
Solar salt, rock salt and solution mining: every route traces back to seawater.
Tap a button to see how people get salt.
River water tastes fresh. But the sea tastes salty! Where does the salt come from? It is a very long story, and it starts with rain.
Rain falls on rocks, high in the mountains. Very, very slowly, the rain nibbles the rocks. Tiny bits wash away. They are too small to see, and they make a tiny bit of salt in the water.
Rivers carry the tiny bits of salt down to the sea, every day. There is so much fresh rain in a river that it still tastes fresh.
The Sun warms the sea. The water floats up into the sky, but the salt stays behind! The water falls as rain again and brings more salt. For a very, very long time, the salt piled up. That is why the sea is salty.
The ocean's salt comes mostly from rocks on land.3 Rain soaks up a little carbon dioxide from the air, which makes it very slightly acidic. That weak acid slowly breaks down rock and dissolves tiny amounts of minerals, which travel as invisible charged particles called ions.1 Streams and rivers carry those ions to the sea.1
The trick is what happens next. When the Sun warms the sea, water evaporates, but the salt can't evaporate with it, so it stays behind.7 Rivers keep delivering more, and over millions of years the leftover salt built up until seawater held about 3.5% salt, roughly 35 grams in every litre.310 A smaller share comes up from the sea floor through hot springs called hydrothermal vents and from underwater volcanoes.3
Scientists point to two sources: runoff from the land and openings in the seafloor.3
That's why the explainer above calls the ocean a salt trap: water can get out, but salt can't. If you want to follow the water side of the loop, see how the water cycle works and why it rains.
Rivers do carry salt, just very little of it compared with the water. Rain keeps topping rivers up with fresh water, and the water keeps flowing downhill and out, so the salt never gets a chance to build up.47 Fresh water is defined as having less than 1,000 parts per million of dissolved salt; seawater has about 35,000.14
The ocean is different because it's the end of the line. It collects the salt from all the rivers that flow into it, and the only way water leaves is by evaporating.4 Lakes that have no outlet in dry places, like the Dead Sea, turn salty for exactly the same reason.710
On average, seawater has about 35 parts per thousand of salt: about 3.5% of its weight.31 Boil away a litre and you'd be left with about 35 grams of salt. Across the open ocean, salinity stays between about 33 and 37 grams per kilogram.11
| Ion | Share of the dissolved salt |
|---|---|
| Chloride + sodium | about 85–90%31 |
| Magnesium + sulfate | about 10%3 |
| Calcium, potassium and others | small amounts3 |
That's a lot of salt in total. According to the USGS, if you took all the salt out of the ocean and spread it evenly over the land, it would make a layer more than 166 metres (500 feet) thick, as tall as a 40-storey building.1
Probably not. The first oceans were likely only slightly salty and became saltier over time.4 But today, scientists estimate that about as much salt settles out onto the sea floor as the rivers bring in, so the ocean is roughly in balance.411
Deep on the sea floor, seawater seeps down into cracks in the crust, where hot magma heats it. The hot water reacts with the rock, loses some substances such as magnesium and sulfate, and picks up metals such as iron, zinc and copper. Then it gushes back out through chimney-like hydrothermal vents.3 Vent water can be hotter than 700 °F (about 370 °C), yet it doesn't boil because of the enormous pressure down there.6 Scientists first found these vents in 1977, near the Galápagos Islands.6
The USGS estimates that all the water in the world's oceans could cycle through vent systems every 10 to 20 million years, so vents matter for the sea's chemistry.2 Underwater volcanic eruptions also release minerals straight into the ocean.3 Still, rocks on land remain the main source of sea salt.3
Saltiness (called salinity) changes with evaporation, rain, rivers and ice.8 Evaporation takes away water and makes the sea saltier; rain, rivers and melting ice add fresh water and make it less salty.11 Salinity is generally lower near the equator and the poles and higher in the belts in between.3 NASA's Aquarius satellite, launched in 2011, made the first global maps of sea-surface salinity from space, and they show the Atlantic is saltier than the Pacific and Indian Oceans.89
| Water | Salt (grams per kg, about the same as per litre) | Why |
|---|---|---|
| Parts of the Baltic Sea | 10 or less | lots of river water, little evaporation11 |
| Open ocean (average) | about 35 (range 33–37) | long-term balance311 |
| Red Sea (surface) | up to about 41 | hot, dry, high evaporation, few rivers11 |
| Dead Sea | about 270–330 | a lake with no outlet in a hot, dry place12 |
The Dead Sea is roughly eight to nine times saltier than the ocean.12 All that salt makes the water dense, so people float in it much more easily than in the ocean.1014 In the "Your turn" toy above, the bars for the Baltic, the ocean, the Red Sea and the Dead Sea are real figures. The "my sea" bar follows a simple made-up rule, just to show which way heat and fresh water push the salt.
Salt also changes how seawater behaves. Saltier, colder water is denser and sinks, which helps drive deep ocean currents.8 Salt is only one part of how the sea works: see also what causes waves , why ice floats and why the ocean is blue.
No. NOAA explains that your kidneys can only make urine that is less salty than seawater, so to get rid of the extra salt you'd have to lose more water than you drank. It makes you thirstier, not less thirsty.5
All the salt we use comes from the sea one way or another: either from today's seawater or from ancient seas that dried up long ago.12 There are three main ways to get it:
In the United States in 2023, rock salt made up about 46% of the salt sold, salt in brine 33%, vacuum-pan salt 11% and solar-evaporated salt 10%. Most of it isn't eaten at all: about 41% goes on icy roads, and only about 4% goes into food processing.15
Rain is very slightly acidic, so it slowly dissolves minerals out of rocks on land. Rivers carry those dissolved salts to the sea. When seawater evaporates, only the water leaves and the salt stays. Over millions of years the salt built up to about 3.5 percent of seawater.
Rivers and most lakes do carry a little salt, but rain keeps topping them up with fresh water and the water keeps flowing out, so the salt stays very dilute. The ocean is the end of the line: it collects the salt from all the rivers, and water only leaves it by evaporating. Lakes with no outlet, like the Dead Sea or Great Salt Lake, become salty for the same reason.
No. Boiling kills germs but doesn't remove salt; as water boils off, what's left gets even saltier. To make seawater drinkable you have to separate the water from the salt, for example by catching and cooling the steam (distillation). Drinking seawater makes you thirstier, because your kidneys have to use more water than you drank to get rid of the salt.
On average about 35 parts per thousand: roughly 35 grams of salt in every litre, or 3.5 percent by weight. Most of the open ocean ranges from about 33 to 37 grams per kilogram. Sodium and chloride, the two parts of table salt, make up most of it.
Probably not overall. Rivers bring in about 4 billion tons of dissolved salt each year, but scientists estimate that about the same amount settles out onto the sea floor, so the ocean is roughly in balance. Local saltiness does change as rainfall, evaporation and melting ice shift.
No. The ocean holds an enormous amount: spread over all the land, it would make a layer more than 166 metres (500 feet) thick. There are also huge underground beds of rock salt left by ancient seas.
The Dead Sea is a lake with no outlet in a hot, dry place. The Jordan River brings in salt, and water leaves only by evaporating, so the salt keeps concentrating. Its water is about 270 to 330 grams of salt per kilogram, roughly eight to nine times saltier than the ocean, which is why people float so easily.