The ocean is salty primarily because rainwater, which is slightly acidic, erodes minerals from rocks on land, releasing ions that rivers carry into the ocean. Sodium and chloride ions, the main components of table salt, accumulate in the ocean over millions of years because they do not get used up by marine life, unlike many other minerals. In addition, hydrothermal vents at the ocean floor release minerals and salts from heated water interacting with Earth's crust, further adding to the ocean's salt content. The continual process of mineral runoff and undersea volcanic activity has made seawater salty, with about 3.5% of seawater's weight coming from dissolved salts. This salt concentration has reached an equilibrium, so the ocean is not getting saltier over time.