As a child, my family and I were ocean people. While other people went skiing, hiking, or on bike tours of Tuscany, we spent our summers swimming, deep-sea fishing, jet-skiing, snorkeling, and scuba diving. My dad taught us about coral reefs, how oysters make pearls, and about how the moon controlled the tides. He also explained to us why we should never drink salt water when we were thirsty.
As kids, we knew that salt water was different from fresh water, but one thing we never really talked about was exactly why the ocean is so salty. About 97 percent of the earth’s water is salt water. How did it get that way?
When It Drains, It Pours
It all begins with rain. Running water is the most powerful erosive force on earth, and when rain falls and washes over rocks and pieces of soil, it breaks them down, picking up tiny particles of minerals along the way. The rain eventually drains into rivers, which eventually drain into lakes, and all that water eventually makes its way to the ocean.
Technically, there’s no such thing as “fresh” water. All rain and water from rivers and lakes contains trace amounts of dissolved salts and minerals. It’s not usually enough for us to recognize as salt, but we can definitely detect the differences between fresh water from different parts of the world, because they all have different mineral contents. If you’ve ever had a glass of distilled water, you’ve probably noticed how flat and stale it seemed. That’s because salt and dissolved minerals are what give our fresh water its characteristic taste.
As lakes and rivers drain into the sea, the water evaporates into the atmosphere, but the salt stays behind. The amount of salt carried by rivers and lakes is extremely small, but after a few billion years of it draining into the oceans, eventually the salt built up into the concentrations it has today. The ocean also gets its minerals from undersea volcanoes. As they erupt, minerals spew out of the earth’s crust into the water. Most seawater is about 3 to 4 percent salt, but it varies by location. Close to the polar icecaps, melting freshwater glaciers dilutes the water, making it less salty; areas of open ocean tend to be much saltier. The United States Geological Survey estimates that if all the salt in the ocean could be extracted and spread over the earth’s land, it would form a layer more than 500 feet thick—the height of a forty-story building.
Even though salt continues to pour into the earth’s oceans every day, it’s not likely that the water will ever become any saltier than it is now. The seas have had approximately the same level of salinity (salt content) for hundreds of millions of years. The oceans have reached a state of equilibrium, where the same amount of dissolved salts that flow into seawater are removed from the water to form new minerals by other geothermal processes at the seafloor.
Beyond the Sea
The oceans aren’t the only places where salt water is found. There are several inland bodies of salt water, including the Dead Sea, Utah’s Great Salt Lake, the Caspian Sea, the Aral Sea, and several salt lakes in Australia and South America.
The proper name for these saline lakes is endorheic basins, or terminal lakes. While normal lakes drain into the ocean through rivers or underground seepage, which keeps the water moving and fresh, saline lakes have no natural outlet. Once the water arrives in the lake—accompanied by all its dissolved salts—it has nowhere to go. Water flows in, brought by rivers and tributaries, but the only way it leaves is through evaporation. Without any other methods of drainage, these lakes eventually become more and more saline as water continually evaporates and leaves its dissolved salts behind. Salt lakes occur most often in dry, arid climates, where water flow is low and constant heat causes massive evaporation. Salt lakes tend to be relatively young bodies of water, but because they’re so much smaller than the oceans, their salinity levels have risen extremely quickly. The Dead Sea, which straddles the border between Israel and Jordan, is more than eight times saltier than the ocean. The Great Salt Lake, the largest saline lake in North America, is about four times saltier.
While visitors to these saltwater lakes might find it fun to bob and float at the surface (because of increased buoyancy) and collect salt crystals from the shore, hypersaline lakes don’t support any life besides microscopic creatures and brine shrimp. There are no fish, no aquatic mammals, and no plants. The excessive mineral content prevents any living things from adapting to the water.
There are dozens of folktales and myths from every culture that try to explain why our oceans are salty. The reality might be far less exciting than a tale of a generous German giant, but the scientific explanation for our oceans’ saltiness is a fascinating glimpse into the history of our planet and how our environment is changing every day.