SE-Ed: Why Is the Ocean Salty?

If you've ever swallowed a mouthful of seawater, you'll know how horribly salty it is! Yet the rivers that flow into the ocean taste fresh. So where does all that salt actually come from? Surprisingly, the story begins with rain! As rain falls through the air, it absorbs a tiny amount of carbon dioxide. This makes it very slightly acidic. Over thousands of years, it slowly wears away the rocks, releasing tiny bits of minerals, including salt. Rivers then carry these minerals down to the sea. So why don't rivers taste salty too? The amount of salt in a river is tiny, and it's constantly being carried away. The ocean, on the other hand, has been collecting salt for hundreds of millions of years. And here's the clever part! When the sun heats the ocean, water evaporates, but the salt is left behind. That water later falls as rain, and the cycle starts again. Rain, rivers, sunshine, repeat — for millions of years! Rivers aren't the only source, though. Deep on the ocean floor, there are cracks called hydrothermal vents. Seawater seeps into the rock and gets heated by magma, then shoots back out, full of dissolved minerals! Underwater volcanoes add even more. Around three and a half percent of seawater is salt. Most of it is sodium and chloride, which make up table salt. That might not sound like much. But if you removed all the salt from the oceans and spread it evenly over the land, it would form a layer over one hundred and fifty metres deep! That's taller than a forty-storey building! Some seas are even saltier than the ocean. The Dead Sea is nearly ten times as salty, so swimmers float on top like corks! So will the oceans keep getting saltier forever? Probably not, because salt also leaves the water. It's slowly buried in the mud and rock of the seabed. Scientists believe this balance has stayed steady for a very long time. Just remember: drinking seawater only makes you thirstier!