If all the salt in the sea were spread evenly over the land, it would form a layer more than 500 feet [150 m] thick—about 45 stories high! Where does all that salt come from, especially considering that countless freshwater streams and rivers empty into the oceans? Scientists have discovered a number of sources.
One source is the ground beneath our feet. As rainwater seeps through soil and rocks, it dissolves tiny amounts of minerals, including salts and their chemical constituents, and carries them out to sea by means of streams and rivers (1). This process is called weathering. Of course, the concentration of salt in freshwater is very low, so we cannot taste it.
Another source is salt-forming minerals in the earth’s crust beneath the oceans. Water penetrates the seafloor through cracks, gets superheated, and returns to the surface with its cargo of dissolved minerals. Hydrothermal vents—some forming deep-sea geysers—disgorge the resulting chemical soup into the sea (2).
In a reverse process that has a similar end result, undersea volcanoes eject large amounts of hot rock into the oceans, where the rock releases chemicals into the water (3). An additional source of minerals is the wind, which carries particles from land out to sea (4). All these processes make seawater a solution of practically every element known. The major salt component, however, is sodium chloride—common table salt. It makes up 85 percent of the dissolved salts and is the primary reason why seawater tastes salty.
What Keeps Salt Levels Stable?
Salts are concentrated in the sea because the water that evaporates from the ocean is almost pure. The minerals are left behind. At the same time, more minerals continue to enter the oceans; yet, the salt level remains stable at about 35 parts per thousand of seawater. Evidently, then, salts and other minerals are being added and removed at about the same rate. This raises the question, Where do the salts go?
Many salt components are absorbed into the bodies of living organisms. For instance, coral polyps, mollusks, and crustaceans harvest calcium, a salt component, for their shells and skeletons. Microscopic algas called diatoms extract silica. Bacteria and other organisms consume dissolved organic matter. When these organisms die or are eaten, the salts and minerals in their bodies eventually settle to the seafloor as dead matter or feces (5).
Many salts not removed by biochemical processes are disposed of in other ways. For example, clay and other terrestrial materials that find their way into the oceans by means of rivers, land runoff, and volcanic fallout may bind certain salts and carry them down to the seafloor. Some salts also bind to rock. Thus, through a number of processes, much of the salt ends up being added to the seafloor (6).
Many researchers believe that geophysical processes complete the cycle, albeit over aeons of time. The earth’s crust is made up of gigantic plates. Some of these meet at subduction zones, where one plate plunges beneath its neighbor and sinks into the hot mantle. Usually, the denser oceanic plate sinks beneath its lighter continental neighbor, at the same time carrying its cargo of salty sediments with it like a great conveyor belt. In this way much of the earth’s crust slowly gets recycled (7). Earthquakes, volcanoes, and rift zones are three manifestations of this process.
Ocean salinity varies from place to place and sometimes from season to season. The saltiest unenclosed waters are in the Persian Gulf and the Red Sea, where evaporation is very high. Regions of ocean that receive freshwater from large rivers or much rainfall are less salty than average. So, too, is seawater near melting polar ice, which is frozen freshwater. Conversely, when ice forms, nearby seawater becomes more saline. Overall, though, ocean salinity is very stable.
Seawater also has a relatively stable pH, which is a measure of the acidity or alkalinity of a substance, 7 being neutral. The pH of seawater ranges between 7.4 and 8.3, which is slightly alkaline. (Human blood has a pH of about 7.4.) If the pH were to go out of this range, the oceans would be in big trouble. In fact, this is what some scientists now fear. Much of the carbon dioxide that humans are adding to the atmosphere ends up in the oceans, where it reacts with water to form carbonic acid. So human activity may be slowly acidifying the oceans.
Many of the mechanisms that keep seawater chemically stable are not completely understood. Still, what we have learned underscores the vast wisdom of the Creator, who cares about his handiwork.