erode · dissolve · acidic · minerals · hydrothermal
The ocean is salty for two main reasons. First, when rain falls through the air it absorbs carbon dioxide and becomes slightly acidic. This acidic water erodes rocks on land and picks up dissolved minerals and salts. These minerals travel in rivers until they reach the ocean, where they slowly accumulate over millions of years. Second, hydrothermal vents on the ocean floor release hot water that is full of metals and minerals. Ocean organisms absorb some minerals like calcium, but salts stay behind in the water. Rivers and lakes also contain some salt, but their flowing water prevents it from accumulating. This is why the ocean keeps getting saltier over time.