Why is the sea salty when rivers are fresh?
The salt in the ocean was mostly nibbled out of rocks on land by slightly acidic rain, one river at a time.
▶ Start the storyThe sea is salty because rivers have been delivering salt to it for a very long time, and the ocean keeps it. Rain that falls on land is slightly acidic, so it slowly eats away at rocks and frees tiny charged particles called ions. Streams and rivers carry those ions down to the sea. Living things in the ocean use up many of them, but others are not removed, so they build up.
Step 1: Slightly acidic rain
Falls on land and slowly erodes rocks
Step 2: Ions are freed
The charged building blocks of salts
Step 3: Rivers carry them
All the way down to the ocean
Step 4: The ocean keeps many
Some are used by living things; the rest build up
A second, hidden source is the seafloor itself: seawater seeps into cracks, is heated by magma, and comes back out of hydrothermal vents loaded with dissolved minerals. The result is water in which about 3.5% of the weight is dissolved salt, and around 85% of those dissolved ions are chloride and sodium, the two halves of table salt.
The first person to explain this scientifically was Edmond Halley. In 1715 he proposed that rain washes salt out of the ground and rivers carry it to the sea. He noticed a telling clue: lakes with no outlet to the ocean, like the Dead Sea and the Caspian Sea, are usually very salty. In such lakes the water escapes by evaporating, and the minerals stay behind.
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Recap
Rivers bring the salt, the ocean keeps it: about 3.5% of seawater's weight is dissolved salt.
Surprising fact · In 1899 John Joly tried to date the Earth from the salt in the sea and got 80 to 100 million years, because salinity has actually been stable for billions of years.
Sources (4)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.