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Salt water, normally with a salinity of 35 parts per thousand (3.5%), in or coming from the sea or ocean. Although seawater contains more than 70 elements, most seawater salts are ions of six major elements: chloride, sodium, sulfate, magnesium, calcium, and potassium. The major sources of these salts are underwater volcanic eruptions, chemical reactions involving volcanic matter, and chemical weathering of rocks on the coasts. Seawater is believed to have had the same salinity for billions of years.