HOW COME? The ocean is made of salt water

Why is the ocean made of salt water? asks A.J. Schiano, a student in Brookville

"Water, water everywhere, nor any drop to drink," wrote Samuel Taylor Coleridge, in his "Rime of the Ancient Mariner." At sea, the ocean stretches to the horizon. But the water is undrinkable, thanks to its heavy load of salt.

Leaving pollution aside, water from rivers and freshwater lakes is drinkable. So it's surprising to find out that our planet's salty seas get their salt courtesy of those same rivers.

And the oceans contain a staggering amount: If you were to weigh seawater, 100 pounds of water would contain about 3.5 pounds of dissolved salt. By contrast, the average river would contain a scant 8/100ths of an ounce of salt, less than half a teaspoon, in 100 pounds of water.

In fact, about 85 percent of the 50 quadrillion tons of solids in the Earth's seas are the minerals sodium and chloride (aka table salt). Taste a mouthful of ocean, and it's like sipping a glass of water with a teaspoonful of stirred-in salt.

Since fresh water from rivers continuously pours into the oceans all around the world, we might assume the oceans' salt becomes more diluted over time. It seems to make sense that in the distant future, we might find ourselves swimming in freshwater oceans -- with many "drops to drink."

But rivers dump billions of tons of solids into the oceans each year. Pouring rain and flowing rivers leach salt out of rocks and soil on land, eventually carrying it into the sea. Erupting volcanoes belch salts into the air, and much of it lands in the ocean. And more salts seep into seawater from volcanic vents, hidden on the ocean floor.

The result: Over billions of years, the salt deposited in the oceans has built up to its current enormous level (enough, if removed and dried, to bury our entire planet's land mass under 500 feet of white crystals).

But thanks mainly to the sun, the salt stays put. Day in and day out, the sun's heat evaporates water from the ocean's surface. Our star acts like a water distiller; water molecules fly off into the atmosphere, but nearly all the salt is left behind.

Other minerals seem to disappear. Rivers dump more calcium than chloride into the sea, but the oceans end up with about 40 times more chloride than calcium.

How come? Some ends up on the sea floor as calcium carbonate, the raw material of elaborate coral reefs. And much of the "missing" calcium is happily extracted by sea-dwelling animals, such as crabs, clams, oysters, lobsters, and shrimp, which use the hard mineral to grow their protective shells and carapaces.