Why Is the Sea Salty?
― The Salt Was Carried In by Rivers That Aren't Salty at All
Many people have had the experience of swimming in the sea, accidentally getting a mouthful of water, and being startled by how salty it is. Yet if you scoop up water from a river flowing into that same sea and taste it, it isn't salty at all. No matter how much non-salty water pours in, the sea stays salty. That strange contradiction holds the key to the answer.
A beach in summer. You trip at the water's edge and swallow a mouthful of seawater. Your mouth stays salty for a while, and there's even grit from the sand.
On the way home that day, you stop by a mountain stream and dip your hand in. It's cold, clear, and — of course — not salty at all.
That stream flows straight toward the sea. Non-salty water pours in day after day, yet the sea never gets diluted. Why not?
There are really only two reasons
River water also contains a tiny amount of minerals dissolved out of rock. It's far too dilute for your tongue to notice, but it has been pouring in for hundreds of millions of years.
What evaporates from the sea surface is water — the dissolved minerals are left behind. Water can return to the sky, but salt cannot. So it keeps accumulating in the sea.
In other words, the sea is "a container where dilute salt water comes in, and only fresh water goes out." Keep that exchange going long enough, and the contents slowly grow more concentrated. Let's look at each step.
1. Rivers carry in invisible salt
As rain falls, it dissolves a small amount of carbon dioxide from the air. That makes rainwater very slightly acidic, giving it the power to dissolve rock. Rainwater that soaks into the ground slowly dissolves minerals out of rock over long stretches of time. This process is called weathering.
The dissolved minerals become groundwater, and eventually rivers, heading for the sea. The amount of dissolved matter in river water varies by location, but it's often said to be around 0.1 grams per liter — roughly a hundredth or less of seawater's concentration, far too little for your tongue to detect.
But rivers never stop. In Japan alone, huge volumes of water flow into the sea with every rainfall. Rivers all over the world have kept carrying this "too-dilute-to-notice salt" for hundreds of millions of years. Look at Figure 1. This one-way flow from land to sea is the heart of the story.
2. Only water leaves the sea
Water is constantly evaporating from the sea surface. What rises into the sky is only water molecules — the dissolved salt is left behind in the sea. Salt simply can't pass through the exit that is evaporation.
The same thing happens in a kitchen. If you boil down dilute salt water in a pot, the steam is pure water, so the salt water left in the pot keeps getting more concentrated, until white crystals are finally left behind. What happens in the sea is exactly the same thing — just on a vastly different scale of size and time.
Another key point: the sea is a "dead end." Rivers always flow downhill, and the lowest point is the sea. No river flows onward from the sea. Water comes in, but it can only leave in the form of water vapor. So the dissolved minerals have nowhere left to go, and they stay put.
Surprisingly, that's thought not to be the case. The sea's salinity is believed to have stayed roughly constant for at least hundreds of millions of years. That's because dissolved minerals are gradually removed from the sea — taken up into seafloor mud or the shells of living things, left behind as layers of rock salt when shallow inland seas dry up, or soaked into cracks in the seafloor where they react with rock. The view is that, over the long run, the amount coming in balances the amount going out.
Of the minerals dissolved in seawater, sodium chloride — the same compound as table salt — makes up a little under 80%, with the rest being magnesium, calcium, sulfate compounds, and more. The bitterness of "nigari," the liquid left behind after table salt is extracted from seawater, is mainly due to this magnesium. That's also why seawater tastes not just salty but somewhat bitter too.
Summary
The sea is salty because of the buildup of a very dilute mineral load that non-salty rivers have kept carrying in. Water can evaporate and return to the sky, but dissolved minerals cannot. The sea is the final destination of the water cycle, having kept receiving what the land has slowly dissolved away, bit by bit.
Rivers carry in dilute salt, and the sun carries away only the water.
What's left behind is the taste of today's sea.
Even though it's the same seawater, the reason for its color runs on a completely different mechanism. Have a look at Why Is the Sea Blue? as well. For how salt affects water, see Why Does Ice Get Colder Than 0°C When You Sprinkle Salt on It?, and for how rainwater dissolves rock, see Why Do Stalactites in Limestone Caves Take Thousands of Years to Grow?. How snow keeps the ground warm is covered in Why Does It Stay Around 0°C Under the Snow Even When It's -20°C Outside?.
- Dissolve about 1½ teaspoons (around 7 grams) of table salt into 200 milliliters of water in a cup. This gets you close to the concentration of seawater.
- Spread just 1 teaspoon of that water on a dark-colored plate, spread the same amount of tap water on a separate plate, and leave both in the sun for half a day.
- Compare them once dry. The salt-water plate will have a clear white crystal residue, while the tap-water plate will show only a faint trace. You can see with your own eyes that only water left the plate.
Don't taste the crystals you end up with — throw them away. Grime from the plate will have mixed in. To avoid taking in too much salt, don't drink the salt water you made either.
Want to know more? ― Terms, formulas, and how this links to the textbookWe label exactly which level each part belongs to, from middle-school science through university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry / Basic Earth Science"
- High school+High-school advanced content, or textbook sidebar material
- UniversityNot covered in high school — university-level specialist content (geochemistry, oceanography)
- ResearchNot yet taught as settled fact even at university — an active research topic
Middle schoolTerms: this phenomenon has names
- Salinity: the amount of matter dissolved in one kilogram of seawater. The average for the open ocean is roughly 35 grams, or about 3.5% by proportion.
- Weathering: the process by which rock gradually breaks down through water, air, and temperature change, releasing its mineral components.
- Residence time: a rough measure of how long, on average, a given component stays in the sea from the time it enters until it leaves.
Middle schoolHigh schoolCheck it with a formula: how many kilograms of salt are in a full bathtub of seawater?
Being told "3.5%" doesn't give you much of a feel for how concentrated that is. Translating it into something familiar gives a clear number. All you need is multiplication and division.
| In symbols | m = V × ρ × S |
| In words | Weight of dissolved salt = volume of seawater × density of seawater × salinity (weight of salt per kilogram of seawater) |
| Where the formula comes from | Salinity is defined as a concentration: "weight of dissolved salt ÷ total weight of seawater." Multiply volume by density to get the weight of the seawater, then multiply that by salinity to get back to the weight of salt. |
| What the symbols mean | m is the weight of salt (grams), V is volume (liters), ρ is density (kilograms per liter), and S is salinity (grams per kilogram) |
| About the units | Weight is in kilograms, the amount of salt in grams, and volume in liters |
| Salinity dissolved in 1 kilogram of seawater | said to be around 35 grams |
| Weight of 1 liter of seawater | said to be around 1.03 kilograms |
| Typical home bathtub volume | around 200 liters as a benchmark |
| Weight of a bathtub filled with seawater (kilograms) | 200 × 1.03 = 206 |
| Salinity dissolved in that (grams) | 206 × 35 = 7210 |
| Converting to kilograms | 7210 ÷ 1000 = 7.21 |
Turn a full bathtub into seawater, and the calculation says you'd have about 7 kilograms of dissolved salt.
If a bag of table salt at the supermarket holds 1 kilogram, that works out to 7 bags' worth dissolved in a single bathtub. Pick it up, and it's a seriously heavy amount. Even though the sea looks clear, that's how much salt it's holding. And it took the rivers hundreds of millions of years to carry in enough salt to reach that concentration.
High schoolHigh school+What's dissolved isn't "grains of salt" — it's ions that have come apart
High schoolWhen table salt dissolves in water, it splits into sodium ions and chloride ions, each surrounded by water molecules. There aren't "grains of salt" floating around in seawater. When seawater is evaporated and the water decreases, the ions reach a concentration where they meet again, and crystals appear. That's exactly what happens in salt fields, and in dried-up salt lakes.
High school+The reason only water leaves through evaporation is that ions are bound tightly to water molecules and can't fly off into the gas phase the way a single water molecule can. Turning an ion into gas takes an enormously larger amount of energy. That's precisely why the vast exit that is the sea surface is open to water alone.
UniversityWhat decides the sea's salinity isn't "how much comes in" — it's the balance
In geochemistry, the sea is treated as a single container, and the amount entering and leaving is tallied for each component. This way of thinking is called mass balance. Components that are hard to remove, like sodium, have extremely long residence times, often estimated at tens of millions to around 100 million years. Components that settle out quickly, like aluminum, have much shorter residence times. The reason seawater's composition is nearly the same everywhere, regardless of location, is explained by the fact that the residence times of these components are far longer than the time it takes for the whole ocean to get mixed together. Residence time is defined by the formula "amount present in the sea ÷ amount entering or leaving per year."
📖 Going further: Seawater (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- How salty was the ancient sea? There's a view that the sea, in its earliest days, was considerably more concentrated than today, but estimates vary widely among researchers. Since none of that ancient seawater survives, it can only be inferred indirectly, from fluid trapped inside rock and similar clues.
- How much salt is exchanged through cracks in the seafloor? The process by which seawater circulates through cracks in seafloor rock is hard to measure from outside, and large uncertainty is said to remain in the mass-balance estimates.
- How does a change in salinity feed back into climate? Salinity affects the weight of seawater and is part of what drives deep ocean currents. How that circulation would change if melting ice diluted the salinity is a topic being actively studied right now.
In other words, this article too only describes things "as currently understood." Behind the familiar topic of the sea's taste lies a planet-scale exchange that hasn't been fully measured yet.
Links to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: aqueous solutions / balance in nature | The early explanation that only water leaves through evaporation |
| High school | Basic Chemistry / Basic Earth Science (dissolution and ions, the water cycle) | The section on dissolving as ions |
| High school+ | Chemistry (properties of vapor and solutions) | The explanation of why ions can't become gas |
| University | Geochemistry / Oceanography (mass balance and residence time) | How the sea's salinity is kept constant |
| Research | Paleoceanography / research on ocean circulation | Ancient salinity and its link to climate |
| ― | Everyday connections | Nigari and tofu, salt production in salt fields, why metal near the sea rusts easily |
- Japan Meteorological Agency, "Ocean Observation and Diagnostic Information" (explanatory pages on sea surface temperature, salinity, and related topics)
- The Chemical Society of Japan (ed.), Kagaku Binran, Kiso-hen (化学便覧 基礎編, "Chemistry Handbook, Basic Edition"), Maruzen Publishing (on the composition of seawater's major constituents)
- W. S. Broecker and T.-H. Peng, "Tracers in the Sea", Lamont-Doherty Geological Observatory, 1982.
- J. I. Drever, "The Geochemistry of Natural Waters", Prentice Hall.
※This article is a general-audience science explainer. The figures given are approximate, meant to help convey the underlying mechanism. Seawater's salinity varies by location and season. When out at the coast, please follow local signage and the guidance of fire departments, municipal authorities, and similar bodies.