The father of modern chemistry, Antoine Lavoisier, gave us the first definitive answer some two hundred years ago. He stated that oceans are the “rinsings of the Earth.” He meant that salts are washed from the land into the ocean.
The rocks on land contain calcium carbonate (limestone), magnesium sulfate (Epsom salts), and sodium chloride (table salt). The process of weathering breaks down the minerals in these rocks and salts, and they dissolve in the water as rivers and streams wash the salts from the land into the ocean.
We can see evidence of this interaction of water and stone at most any cemetery. Many early headstones were made of marble. After a hundred years of wind and rain, the inscriptions are hard to read.
The salt in the ocean also comes from volcanic activity. While there is hardly any sulfur and chlorine in rocks, volcanoes spew these elements into the atmosphere, and they end up falling in the world’s oceans. Sulfur and chlorine add to the saltiness of the oceans.
Since weathering and volcanic eruptions continually happen, it might seem that the oceans should become more and more salty. But salt is constantly being removed by clams and other shellfish that use calcium carbonate to build their shells. So the salinity of the oceans has remained fairly consistent for a long time.
The Dead Sea, on the border of Israel and Jordan, is surrounded by the lowest land on Earth, and it’s also one of the world’s saltiest seas. It has a salinity of 31.5 percent, almost nine times that of the ocean. The Dead Sea has no outlet. The minerals that flow into the Dead Sea stay there for centuries. The majority of freshwater bodies have rivers flowing out of them that dispose of dissolved minerals. Not so for the Dead Sea.
Mono Lake, near Yosemite National Park in California, has a salinity of about 10 percent. Diverting of water to Los Angeles caused the high salt content, leading to loss of water due to evaporation that soon exceeded freshwater inflow from streams. The lake continued to get smaller and saltier until 1994, when the drawdown was finally halted.
The Great Salt Lake in Utah is the remains of a pluvial, or rainwater-filled, lake that covered much of Utah in prehistoric times (see page 78). Today, three rivers deposit their sediment in the lake; they leave behind more than a million tons of minerals each year.
The lake has no outlet, so water disappears by evaporation only. When water evaporates, the minerals are left behind. No need to worry about drowning in the Great Salt Lake; people float in it, because the water is denser than the human body.
From the book, "Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works"; Copyright © Larry Scheckel, 2013. Publishes December 17; available wherever books are sold.