Every litre of seawater contains about 35 grams of salt — roughly seven teaspoons worth. That's enough to make your mouth pucker if you accidentally swallow some while swimming. But where does all this salt actually come from?
The Great Rock Robbery
Rain might seem innocent, but it's actually a gentle thief. When rainwater falls, it picks up tiny amounts of carbon dioxide from the air, making it slightly acidic — like a very weak fizzy drink. This acidic water then flows over rocks and slowly dissolves minerals, including various types of salt.
Rivers carry these dissolved minerals downstream to the ocean, where they stay put. Unlike fresh water, which evaporates back into clouds, salt doesn't evaporate. It just keeps accumulating, year after year, century after century.
Think of the ocean like a giant bathtub that never gets emptied. Every day, someone pours in a tiny bit of salty water, but only the plain water evaporates away. After doing this for billions of years, you'd end up with quite a salty bath!
Underwater Salt Factories
Rivers aren't the only salt suppliers. Deep beneath the ocean, hydrothermal vents act like underwater geysers, spewing out hot, mineral-rich water. These vents dissolve salt directly from the ocean floor and pump it into the surrounding water.
Volcanic activity also contributes, releasing minerals that eventually become salt. Even underwater landslides expose new rock surfaces to the ocean's dissolving power.
The Ancient Recipe
The ocean has been collecting salt for roughly 3.8 billion years. Early on, it was much less salty than today, but the recipe has been perfecting itself ever since. Interestingly, the saltiness has remained fairly stable for millions of years because the ocean has reached a kind of balance — new salt comes in at roughly the same rate that salt gets removed through various geological processes.
Different seas have different salt levels too. The Dead Sea is so salty you can float on it like a cork, while the Baltic Sea is much less salty because it receives lots of fresh water from rivers and has limited connection to saltier oceans.
So next time you taste seawater, remember you're sampling a recipe that's been brewing since long before dinosaurs existed — a liquid time capsule of every mountain range that ever weathered away.
Every litre of seawater has about 35 grams of salt in it. That is roughly seven teaspoons worth. It is enough to make your face scrunch up if you swallow some while swimming. But where does all this salt actually come from?
How Rain Steals Salt From Rocks
Rain might seem harmless, but it is actually a sneaky thief. When rain falls, it soaks up tiny amounts of carbon dioxide from the air. This makes the rainwater slightly acidic. It is a bit like a very weak fizzy drink. This slightly fizzy water then flows over rocks. Slowly, it dissolves minerals out of the rocks. Salt is one of those minerals.
Rivers carry these dissolved minerals all the way to the ocean. Once there, the minerals stay in the water. Fresh water evaporates and turns back into clouds. But salt cannot evaporate. So it just builds up more and more, year after year.
Think of the ocean like a giant bath that never gets emptied. Every day, someone pours in a tiny bit of salty water. Only the plain water evaporates away. If you did this for billions of years, you would end up with a very salty bath indeed.
Underwater Salt Factories
Rivers are not the only thing adding salt to the ocean. Deep under the sea, there are cracks in the ocean floor called hydrothermal vents. They work like underwater geysers. They shoot out very hot water that is full of minerals. These vents dissolve salt straight from the ocean floor. Then they pump it into the water around them.
Volcanoes also help to add salt. They release minerals that eventually become salt. Even underwater landslides help. They uncover fresh rock for the ocean to dissolve.
A Very Old Recipe
The ocean has been collecting salt for about 3.8 billion years. In the beginning, it was much less salty than it is today. Over time, more and more salt built up. Interestingly, the saltiness has stayed fairly steady for millions of years. This is because the ocean has found a kind of balance. New salt arrives at roughly the same speed that old salt gets removed by natural processes in the ground.
Different seas have different amounts of salt too. The Dead Sea is so salty that you can float on top of it. It is like floating on a lilo without actually needing one. The Baltic Sea is much less salty. This is because lots of rivers pour fresh water into it. It is also not well connected to saltier oceans.
So next time you taste seawater, think about what you are actually tasting. That saltiness has been slowly building up since long before the dinosaurs existed. It is like a time capsule in liquid form, holding the story of billions of years of rain, rivers, and rocks.