The ocean is salty due to several natural influences and processes. The primary source of salt in the ocean is rocks on land, which are eroded by rainwater that falls on land and flows into rivers and ultimately into the ocean. Rainwater dissolves minerals from rocks, and these minerals separate into ions, which are carried by runoff water and ultimately reach the ocean. The two most prevalent ions in seawater are chloride and sodium, which together make up around 85% of all dissolved ions in the ocean. Other sources of salt in the ocean include hydrothermal fluids that come from vents in the seafloor and underwater volcanic eruptions, which directly release minerals into the ocean. Salt domes, vast deposits of salt that form over geological timescales, are also found underground and undersea around the world and contribute to the oceans saltiness.
It is important to note that while rivers and streams flowing into the ocean do contain dissolved salts, they are not the primary source of salt in the ocean. The water in the ocean collects all of the salt and minerals from all of the rivers that flow into it, and it is estimated that rivers carry an estimated four billion tons of dissolved salts to the ocean annually. However, about the same tonnage of salt from ocean water probably is deposited as sediment on the ocean bottom, and yearly gains may offset yearly losses, so the ocean today probably has a balanced salt input and output.
