The elements in ocean sea water are oxygen, hydrogen, sodium, and chloride. Sodium and chloride combine to form what we know as table salt. The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid. The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time. The two ions that are present most often in seawater are chloride and sodium. These two make up over 90% of all dissolved ions in seawater. The concentration of salt in seawater (its salinity) is about 35 parts per thousand; in other words, about 3.5% of the weight of seawater comes from the dissolved salts. In a cubic mile of seawater, the weight of the salt (as sodium chloride) would be about 120 million tons. When freshwater flows into the ocean, it carries salts and minerals with it. More salts and minerals are added from seafloor vents. Deep in the ocean, water seeps into cracks in the Earth’s crust. There, it’s heated by magma. Hot water dissolves salts and minerals from the rock. (This is similar to dissolving table salt or sugar in water. The process happens faster and the water holds more salt or sugar when it’s hot.) Sea water then flows through seafloor vents, carrying those dissolved elements up into the ocean’s waters. Many of those salts and minerals are used by ocean life. Iron, zinc, and copper, for example, are removed from water by organisms. Sodium and chloride, the main ingredients in table salt, are not. Because they are left behind, levels of salt in the ocean have built up over time. This is the reason sea water is salty.