Why is the ocean so salty?

The ocean’s salt comes from two sources — runoff from the land and openings in the seafloor.

Rocks that end up in the ocean water are the primary source. Rainwater, which is slightly acidic, erodes rocks over time. The eroding rocks release ions that eventually end up in the ocean after they pass through streams and rivers, according to the National Ocean and Atmospheric Administration (NOAA).

There are also hydrothermal fluids, which come from openings at the bottom of the ocean, explains NOAA. Ocean water heats up from magma at the Earth’s core when it seeps into cracks in the seafloor. The heat causes chemical reactions, including underwater volcanic eruptions, which release minerals into the ocean. This process also causes water to lose oxygen, magnesium and sulfates and pick up iron, zinc and copper.

Another explanation for the ocean’s saltiness, says NOAA, is salt domes, which are vast salt deposits formed over time.

Which ocean is the saltiest?

Of the world’s five ocean basins — Pacific, Atlantic, Indian, Southern and Arctic Oceans — the Atlantic Ocean has the highest salinity, says the National Weather Service. This is because salinity decreases near the equator and at both poles. The tropic’s heavy rainfall near the equator decreases the salinity as the fresh water falls into the salty ocean.

The National Weather Service says the Red Sea has the saltiest ocean water with a salinity level of about 40%, which is due to the region’s high evaporation rate.

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