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Why is the sea salty?

Waves breaking on a rocky coastWaves breaking on a rocky coast Most of the salt in the sea comes from the land around us. Rainwater contains some dissolved carbon dioxide from the surrounding air. This makes it a weak acid. Every time it rains, the rainwater passes through the soil and seeps through tiny holes in the rocks called pores. It dissolves a tiny amount of the mineral salts, such as sodium chloride (common salt), that make up the rocks. The rainwater eventually washes into the rivers, which flow into the sea. Warmth from the sun causes water from the surface of the sea to evaporate—but the salt stays in the sea. Over time, the salt in the seawater becomes more and more concentrated. Because of the huge volume of the seas and oceans, it’s taken hundreds of millions of years for the salt content to build to its present level.

Turtle “crying” salty tearsTurtle “crying” salty tearsThe Amazon flowing into the AtlanticThe Amazon flowing into the AtlanticThe concentration of salt in seawater (its salinity) is, on average, about 35 parts per thousand, or approximately 35 grams per litre of water. It is not the same everywhere. The Mediterranean Sea, for example, is saltier than the Atlantic Ocean near the mouth of the River Amazon, where fresh river water mixes with seawater.

Hydrothermal ventsHydrothermal vents

The salt in seawater doesn’t only come from the land. Hot water jetting out from hydrothermal vents—cracks in the ocean floor running along the crests of oceanic ridges—also contains mineral salts. Seawater seeps into the ocean floor, where it is heated by magma (melted rock). The hot water dissolves minerals from the rocks before gushing back into the ocean through the vents. Underwater volcanoes also add to the salt in seawater in a similar way.

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The oceans contain around 50 million billion tonnes of salt—enough to cover the Earth's land surface with layer more than 160 m (500 ft) thick. That is about the height of a 40-storey skyscraper.

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