The distinct taste of ocean water comes from salt, a mineral. The ocean covers about 70% of Earth’s surface, holding 97% of all water on the planet. Understanding why this massive body of water is salty reveals fascinating natural processes.
Where Does Ocean Salt Come From?
Ocean salt primarily originates from two main sources: land runoff and openings in the seafloor. Rainwater, which is slightly acidic due to carbon dioxide dissolving in it, falls onto land. As this rainwater flows over rocks and soil, it slowly erodes them, dissolving tiny bits of minerals, including salts. These dissolved minerals are then carried by streams and rivers to the ocean. Billions of tons of dissolved salts are carried to the ocean by rivers globally each year.
Another significant source of ocean salt comes from deep within the Earth through underwater volcanoes and hydrothermal vents. These are cracks located on the seafloor. Ocean water seeps into these cracks, gets heated to very high temperatures by magma, and dissolves minerals from the surrounding crust. This hot, mineral-rich water then flows back into the ocean through the vents, continuously adding more dissolved salts.
Why the Ocean Stays Salty
The ocean remains salty because of a process called evaporation. The sun’s warmth heats the ocean’s surface water, turning it into water vapor. This water vapor rises into the atmosphere to form clouds, which eventually return to Earth as rain. Crucially, when water evaporates, the salt does not evaporate with it; the salt stays behind in the ocean.
This process happens continuously, over millions of years, leading to the accumulation of salt in the ocean. The ocean is like a giant basin or collection point. Water flows into it from rivers and rain, but only pure water leaves through evaporation, leaving the salt to build up over time. While the ocean’s saltiness has increased over geological time, it is currently in a balanced state where the amount of salt added is generally equal to the amount removed through various natural processes.
Why Rivers Aren’t Salty
Rivers, despite carrying dissolved salts to the ocean, are not salty themselves. This is because river water is constantly moving and is replenished by fresh water from rain and melting snow. The small amount of salt that rivers pick up from rocks and soil does not have enough time to concentrate or build up within the river system. It is continuously washed downstream towards the sea.
Rivers do contain some dissolved salts, but their concentration is very low compared to the ocean. For instance, the average river water has a salt content of about 0.012%, while ocean water contains approximately 3.5% dissolved salts. Unlike rivers, which are always flowing, the ocean acts as a large collection area where water evaporates but the salt remains, allowing it to accumulate over vast periods.