Why is the ocean salty?
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tl;dr
Rivers deliver about 4 billion tons of dissolved rock salts to the sea each year, and evaporation takes back only pure water — so salt concentrated for billions of years, to today's 3.5%. Spread over land it would stand 166 m deep; today, deposits to the seafloor roughly balance the inflow.
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The ocean is salty because rivers have been delivering dissolved rock to it for billions of years, and the sun only ever takes the water back. Rain weathers rock into mineral salts, rivers carry them seaward — about 4 billion tons a year worldwide — and evaporation lifts pure water while every gram of salt stays behind.
A one-way ledger for salt
River water tastes fresh because its salt load is dilute and always moving through. The sea is where the load stops. Evaporation, the sun's half of the water cycle, distills only pure water into clouds; the salts concentrate in what remains. Undersea volcanoes and hot vents add their own minerals from below. Today the sea sits at about 3.5% salt — a teaspoon and a half in every glass.
How much salt is that?
Enough that if you took it all out and spread it over the world's land, it would bury every continent under a layer about 166 meters thick — the height of a 40-story building.
Why it isn't getting saltier
The surprise in the ledger: today's ocean is roughly in balance. Salt leaves too — settling into seafloor sediment, locking into minerals, drying into salt beds — at about the rate rivers add it. The sea spent billions of years earning its salt, but the account has largely stopped growing.
cited
Sources
- [01]Why is the ocean salty? — U.S. Geological Survey
“Rivers discharge an estimated 4 billion tons of dissolved salts to the ocean annually... if the salt in the ocean could be removed and spread evenly over the Earth's land surface it would form a layer more than 500 feet (166 meters) thick.”
- [02]Why is the ocean salty? — NOAA Ocean Service
“Salt in the ocean comes from rocks on land: rain erodes rock, rivers carry the dissolved ions to the sea, and hydrothermal vents and undersea volcanism add minerals from below.”
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If rivers carry salt, why don't they taste salty?
Their salt is dilute and passing through — a river is a conveyor, not a container. The sea is the end of the line, where billions of years of deliveries accumulated while the sun kept removing only the water.
Why are some seas saltier than others?
It's the local balance of evaporation versus fresh input. Hot, enclosed seas like the Red Sea evaporate hard and dilute little, so they run saltier; seas fed by big rivers or melting ice, like the Baltic, run fresher. The Dead Sea is the extreme: inflow but no outlet.
Could we just desalinate the ocean for drinking water?
We already do — thousands of plants worldwide do the sun's trick on purpose, distilling or filtering pure water out and leaving brine behind. It works, but pushing water through membranes costs serious energy, which is why it serves dry coastal regions rather than the whole planet.
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