One source of salts in the ocean comes from minerals on land dissolving in rainwater and streams, which eventually make their way into the ocean. Since salts do not tend to evaporate along with the water in the ocean, their concentration can build up over time. Another source of salts in the ocean are hydrothermal vents; seawater can flow into these vents, where it becomes warm and dissolves minerals before flowing back out. Underwater volcano eruptions also contribute to the presence of minerals in seawater.

The majority of the salt ions in the ocean are sodium and chloride—these make up about 90% of dissolved ions in seawater. The remainder of the ions present are mainly magnesium, sulfate, and calcium. The concentration of salts in seawater is fairly high, and, on average, seawater is about 3.5% salt by weight.