Geography
Why is the sea salty when the rivers feeding it are not?
- Rivers carry a little salt in, and only water leaves
- Salt rises from the seabed
- Sea creatures make it
The answer is Rivers carry a little salt in, and only water leaves. Only water evaporates, so the salt stays behind and builds up.
Rivers are not quite fresh - they carry a very small amount of dissolved mineral picked up from the rock they run over. They deliver it to the sea, and then only the water leaves again, by evaporation. The salt has nowhere to go, so over hundreds of millions of years it has accumulated.
The sea is therefore a residue, and the process still runs. It is also balanced: salt leaves at roughly the rate it arrives, taken out by sea spray, by minerals settling on the seabed and by hydrothermal vents, which is why the ocean is not steadily getting saltier now.
Lakes show the same effect where they have no outlet. The Dead Sea and the Great Salt Lake are salty for exactly this reason - water comes in and only evaporates out - while a lake with a river running out of it stays fresh because the salt is carried onward. The Dead Sea is nearly ten times saltier than the ocean, which is why swimmers float so high in it.