why is the ocean salty

The ocean is salty primarily because rain erodes rocks on land, carrying dissolved minerals into the sea, while hydrothermal vents and underwater volcanoes also contribute minerals from the Earth's crust.

Weathering of rocks. Rainwater absorbs carbon dioxide from the air, becoming slightly acidic. This acid breaks down rocks on land, releasing ions that flow into rivers and eventually reach the ocean.
Hydrothermal and volcanic activity. Seawater seeps into cracks in the ocean floor, where it is heated by magma. This hot water dissolves minerals from the surrounding crust and carries them back into the ocean through hydrothermal vents.
Accumulation of ions. Sodium and chloride, the main components of salt, are not easily removed by marine organisms or chemical precipitation, allowing them to accumulate in the ocean over millions of years.
Maintaining balance. Although rivers continuously add salt, the ocean's salinity remains stable because biological, chemical, and geological processes remove salt at a rate that balances the input.