Anyone who has gotten a mouthful of seawater at the beach knows the ocean is salty. But stop and think about it for a second. Rain is fresh water. Rivers and streams are fresh water. And they all flow into the sea. So where does all that salt come from?
Here's the short answer. Rain slowly wears away rocks on land, and rivers carry tiny amounts of dissolved salts and minerals down to the sea. When ocean water evaporates, the salt stays behind, so over enormous stretches of time it has built up. Some of the ocean's salt also came from the sea floor and from gases released by volcanoes and vents deep in the Earth.
How Salty Is It?
On average, seawater is about 3.5 percent salt. That means every liter of seawater, roughly a quart, contains about 35 grams of dissolved salts, a bit more than an ounce. Most of that is sodium and chloride, the same two ingredients that make up table salt.
The amount varies from place to place. Near river mouths, melting glaciers, or areas with heavy rain, the sea can be much less salty. The saltiest open sea is the Red Sea, where heat drives lots of evaporation, little rain falls, and few rivers flow in. Isolated bodies of water can be far saltier still. The Dead Sea is about ten times saltier than the ocean.
An Astronomer's Clever Idea
One of the first scientific explanations came from Sir Edmond Halley, the astronomer famous for the comet that bears his name. In 1715, he proposed that rainfall washed salt and minerals out of the ground, and rivers carried them to the sea, where they built up over time.
Halley noticed a clue. Lakes with no outlet to the ocean, such as the Dead Sea and the Caspian Sea, tend to be very salty. Water flows in, evaporates, and leaves the salt behind with nowhere to go. He called the process continental weathering.
The Rest of the Story
Halley turned out to be partly right. Scientists now know there's more to it. When the oceans first formed, sodium leached out of the ocean floor into the water. Chloride, the other main ingredient, came from deep inside the Earth. It escaped as a gas through volcanoes and hydrothermal vents on the sea floor and dissolved into the ocean.
Over time, sodium and chloride became the most abundant ingredients in sea salt. So the ocean's saltiness is a team effort: rocks on land, the sea floor, and the planet's fiery interior all contributed.
Why You Shouldn't Drink It
Swallowing a little seawater while swimming isn't harmful. But drinking it to stay hydrated backfires. To get rid of the extra salt, your body has to flush out more water than the seawater gave you, leaving you thirstier than before.
Survival manuals consistently warn against it. One look at 163 life raft voyages estimated a 39 percent risk of death for people who drank seawater, compared with 3 percent for those who didn't. It's the problem behind the famous line from Samuel Taylor Coleridge's poem The Rime of the Ancient Mariner: 'Water, water, everywhere, nor any drop to drink.'
So the next time you taste salt on your lips at the shore, you're tasting a story millions of years in the making. Every wave carries a little bit of ancient mountains, volcanoes, and sea floor, all mixed together into one big salty soup.
