Why is the ocean salty?
Rain dissolves minerals from rock, rivers carry them to the sea, and water leaves while salt stays.
It starts with rain dissolving rock
Rainwater isn’t quite pure. As it falls, it absorbs carbon dioxide from the air and becomes faintly acidic, strong enough, over millions of years, to break down the minerals in exposed rock.
The freed-up ions, mostly sodium and chloride, plus magnesium, calcium, and sulfate, ride downhill in rivers. Freshwater tastes fresh only because its salt load is tiny and constantly flushed onward to the ocean.
Why the sea keeps the salt
The ocean is the planet’s lowest basin, water flows in but only leaves by evaporating. When the sun lifts water vapor into the sky, the salt stays behind. Repeat for a few billion years and the dissolved minerals build up.
What's actually dissolved in there
Sodium and chloride together make up about 85% of sea salt, which is why the ocean tastes like the stuff in your shaker. Underwater volcanic vents add their own minerals too, but the river-and-evaporation loop does most of the work.
The balance is roughly steady: salt arrives, and an equal share gets locked away into seafloor sediments and rock. The ocean isn’t getting saltier, it’s been near this concentration for a very long time.