Why in sea water?

Ocean salt primarily comes from rocks on land and openings in the seafloor. Salt in the ocean comes from two sources: runoff from the land and openings in the seafloor. Rocks on land are the major source of salts dissolved in seawater. Rainwater that falls on land is slightly acidic, so it erodes rocks.

You may know that the oceans cover about 70 percent of the of Earth's surface, and that about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on our planet. By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth's land surface it would form a layer more than 500 feet (166 meters) thick, about the height of a 40-story office building (Source: NOAA). But, where did all this salt come from? If you get into folk stories and mythology you will see that almost every culture has a story explaining how the oceans became salty. The answer is really very simple. Salt in the ocean comes from rocks on land. Here's how it works...

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. By the way, the concentration of salt in seawater (salinity) is about 35 parts per thousand. In other words, about 35 of 1,000 (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. And, just so you don't think seawater is worthless, a cubic mile of it also can contain up to 25 pounds of gold (at a concentration of 0.000005 parts per million). Before you go out and try alchemy on seawater, though, just think about how big a cubic mile is (1 cubic mile contains 1,101,117,147,000 gallons!).

Rivers and surface runoff are not the only source of dissolved salts. Hydrothermal vents are recently-discovered features on the crest of oceanic ridges that contribute dissolved minerals to the oceans. These vents are the exit point on the ocean floor from which sea water that has seeped into the rocks of the oceanic crust has become hotter, has dissolved some of the minerals from the crust, and then flows back into the ocean. With the hot water comes large amounts of dissolved minerals. Estimates of the amount of hydrothermal fluids now flowing from these vents indicate that the entire volume of the worlds oceans could circulate through hydrothermal vent systems every 10-20 million years. Thus, this process has a very important effect on salinity. The reactions between seawater and oceanic basalt, the rock of ocean crust, are not one-way, however; some of the dissolved salts react with the rock and are removed from seawater.

A final process that provides salts to the oceans is submarine volcanism, the eruption of volcanoes under water. This is similar to the previous process in that seawater is reacting with hot rock and dissolving some of the mineral constituents.