What causes the water in the oceans to be salty?

Chris - This is quite simple and it comes down to the fact that we have a hydrological cycle. The sun puts energy into the Earth. Each square metre of the Earth's surface, on average, gets energy at the rate of about 1kW from the sun. This energy goes into the sea water, and the water molecules gain enough energy - sometimes - to evaporate. So you have water vapour, which leaves the sea and goes up to form clouds.

Those clouds then travel over to land. When they're forced to rise over things like high mountains then in order to rise they have to drop rain. They lose some mass in the form of water precipitation.

That fresh water comes down out of the clouds and lands on the ground. It goes into rivers and streams and picks up tiny amounts of minerals and salts, which it dissolves on its way, percolating through the ground. There's not that much there, so the water tastes fresh. You can just detect trace amounts of these chemicals in the river water and in pond water. 

As it makes its way down towards the sea it then takes with it those salts.

When the water then re-evaporates in the ocean, that's just fresh water evaporating. The salts get left behind. Over millions of years you then accumulate salts in the sea until you see the salinity that you see today.

The sea isn't actually going to get much saltier, because once you get to a certain threshold concentration you start to get other chemical reactions kicking in which limit the accumulation further of any more dissolved ions or salt. As a result it just contains at the level it's at.

Dave - Actually, where we get most of our salt from is the edges of the sea: very ancient shallow seas where there's lots of evaporation at the edge of a desert. The sea keeps flowing in, lots of water evaporates and the salt crystallises out. That often gets buried - there's huge amounts under the North Sea. There's quite a lot in Cheshire.

Chris - And you can go to Poland and some very famous salt mines which were these salt pans, weren't they? Absolutely amazing.