Ask a grown-up: where does salt come from before it gets into the sea?
There are, on average, 35g of salt in every litre of seawater. It gets there because rain continuously washes it off the land as it erodes the rocks. Salt builds up in the oceans and, because it is very soluble, it tends to stay there a long time.
Three hundred years ago, scientists such as Edmond Halley (who predicted the comet named for him) thought the oceans must be getting saltier, and that if you measured the amount of salt and estimated the rate at which it came off the land, you could work out the age of the Earth. But this method does not work because salt does in fact sometimes come out of solution and go back into rock form, very slowly over geological time. For example, there is a thick layer of salt under the floor of the Mediterranean, because six million years ago it was cut off at the Strait of Gibraltar and evaporated. In general, the oceans' average salt content is in equilibrium and does not change over time.
Dr Ted Nield edits Geoscientist, the Geological Society's magazine, and is the author of Underlands.
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