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If you’ve ever surfed or swam in the ocean, you’re probably familiar with one of the ocean’s most potent qualities: it’s saltiness.
On average, the concentration of salt in the ocean hovers around 35 parts per thousand. If you were to remove all the salt from the oceans and spread it thick across Earth’s land surface, it was form a layer roughly 500 feet thick—the height of two Taj Mahal’s stacked on top of each other, according to the National Oceanic and Atmospheric Administration. Gulping up a mouthful of saline seawater can be gross, but how did all that salt get there in the first place?
You can thank the rocks on shore.
Rainwater contains small amounts of dissolved carbon dioxide from the atmosphere. This slightly acidic rain erodes rocks on land, and sends minerals and dissolved ions—including chloride and sodium—through rivers and streams into the ocean. These ions, in particular, make up more than 90 percent of all dissolved ions in the ocean and are responsible for its saltiness, according to the United States Geological Survey.
While the world’s rivers do have salt in them, they don’t taste salty like the ocean. That’s because the ocean acts as a repository for all of those minerals. There’s a higher concentration of them in the sea. Rivers play an important role in transporting salts and other minerals to the ocean, and discharge roughly 225 million tons of dissolved solids into the ocean each year, according to NOAA—but they aren’t the only source.
Salt can also seep up into the oceans through hydrothermal vents. These vents shoot dissolved minerals into the sea. As seawater seeps into the rocks on the seafloor and closer to Earth’s core, it begins to heat up. Then, it makes its way back toward the surface, where it flows out of hydrothermal vents. Curiously, in some cases, the salt that pours out of these spouts actually reacts with the basaltic rock on the seafloor and is removed from the water.
In regions of underwater volcanism, salt can be deposited into the ocean, too. As fresh lava emerges from the volcanoes on the seafloor, the hot rock reacts with the salty seawater, dissolving some of its minerals.
But the ocean isn’t uniformly salty. Some sections of the sea are saltier than others. For example, the seafloor of the Gulf of Mexico is filled with briny salt ponds caused by the dissolution of ancient layers of salt, according to NOAA. These pools of super-salty sludge can even host strange bacteria on the bottom of the sea floor—bacteria that scientists suspect could be found in other parts of the solar system.