The sea is salty mainly because rainwater, which is slightly acidic due to dissolved carbon dioxide, weathers rocks on land, releasing mineral salts into rivers and streams that carry them into the ocean. Over millions of years, these salts have accumulated in the ocean since water evaporates but leaves the salts behind. Sodium and chloride ions, which are the main components of common salt, make up most of the dissolved salts in seawater. Additionally, salts are also released from underwater volcanic activity and hydrothermal vents on the ocean floor. Some minerals are used by marine life, but many salts remain and build up in concentration over time, resulting in the ocean's characteristic salinity of about 3.5% by weight.

How Salt Reaches the Ocean

• Rain reacts with carbon dioxide to form carbonic acid, which dissolves minerals from rocks.
• These minerals wash into rivers and eventually the ocean.
• Evaporation removes pure water but leaves salts behind, causing salt concentration to increase.

Sources of Salt

• Weathering of rocks on land (primary source).
• Underwater volcanic eruptions and hydrothermal vents.
• Salt domes and geological processes under the ocean.

Why Isn’t Fresh Water Salty?

• Rivers and lakes get diluted constantly by rainfall.
• Seawater accumulates salts because salts are not removed as quickly and evaporation leaves salts behind.

The balance between salt input (from land and seafloor sources) and removal (by geological and biological processes) has kept ocean salinity relatively stable over millions of years.