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Earth · Ocean Salinity

Why is sea water salty?

An underwater photograph of hydrothermal vents releasing mineral-rich plumes into the deep ocean.
Image courtesy of Submarine Ring of Fire 2006 Exploration, NOAA Vents Program.. Public domain. Original.

The short answer

Sea water is salty because minerals like sodium and chloride dissolve from rocks12. Rainwater becomes slightly acidic and washes these salts into rivers and streams, which carry them into the sea1. This process has built up salt levels over millions of years2. Additional salts enter the ocean from subsea volcanoes and hydrothermal vents2. The salt levels remain relatively constant because new salt is balanced by sediment deposits and removal by animals and algae14.

What the evidence shows

Land and Water Cycles

Rain contains small amounts of carbon dioxide and sulphur dioxide1. These substances make the rain slightly acidic and allow it to dissolve mineral salts from rocks1.

Rivers and streams transport these dissolved salts from the land into the sea1. Rivers carry almost 4 billion tonnes of salt to the ocean every year2.

Geological Sources

Some salts enter the sea from volcanoes on land and in the ocean1. Subsea volcanoes and hydrothermal vents also supply dissolved salts to the water2.

Certain salts, such as chloride, are moved by the water cycle and originally came from volcanoes1. Rocks located within the sea also contribute to the salt supply2.

Concentration and Balance

The concentration of salt in the sea has built up over millions of years2. The amount of salt in the ocean stays relatively constant on a scale of millions of years4.

Salt levels are balanced by deposition as sediment on the ocean floor or removal by algae and animals1. Evaporation increases salinity, while precipitation and runoff reduce it4.

Our take

This part is our own interpretation, not a sourced finding.

It is fascinating that the salt in our oceans is balanced by the constant removal of minerals by living organisms1. This shows that the sea is a dynamic system rather than just a static container of salt4.

What we can't say for sure

The source does not state the reason why the North Sea water is salty3.

Sources

  1. Sally Little. Curious Kids: why is the sea salty?. theconversation.com. other website, accessed 1 November 2026.
  2. Helen Phillips. I have always wondered: why is the sea salty?. theconversation.com. other website, accessed 1 November 2026.
  3. Authority control databases. Baltic Sea - Wikipedia. en.wikipedia.org. other website, accessed 1 November 2026.
  4. Authority control databases. Sea - Wikipedia. en.wikipedia.org. other website, accessed 1 November 2026.

Published 2 November 2026. Sources last checked 1 November 2026. Prepared with automated tools and checked against the sources named here: how we work. Spotted a mistake? Tell us.

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