Why is the sea salty?
The early oceans were probably not salty. Erosion of local rocks, by rivers which flowed into them, provided the salts. All fresh water in rivers contains traces of salts and minerals, but these get concentrated in the oceans because they are left behind when water evaporates (to form clouds). This water falls as rain which eventually washes more minerals into the sea.
But if you measure the sea’s salt content now, and work out how salty it should have been after billions of years of minerals being washed into it, it is much less salty than expected. Someone tried to calculate the age of the Earth from how salty the seas were and got a figure close to 5,000 years – so something else is happening.
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