Seawater consists of an average of about 2.5 percent sodium chloride and a total of 1 percent of the other elements. The encyclopedia says that seawater is salty because rain water wears away rocks on land and dissolves minerals containing sodium and chlorine in the rocks and soil. Rivers carry the minerals to the sea. Material released by volcanoes and undersea springs also contributes salt to the ocean. Evaporation increases the ocean's salinity by removing fresh water from the ocean's surface, leaving the salts.
Precipitation lowers the ocean's salinity by adding fresh water.
In addition to carrying salt to the ocean, rivers also carry fresh water, which lowers the salinity of seawater near river mouths. The oldest method of obtaining salt is by evaporating seawater. As seawater is moved through a series of evaporating ponds, the various minerals evaporate before the salt does, leaving a product that is 95 to 98 percent sodium chloride, the encyclopedia says. Salt also is mined from rock salt, the huge deposits beneath the ground. This method requires that about half of the salt be left behind as pillars.