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Asked by Einstein123jnr to Áine, Ciarán, Eoin, Lydia, Victoria on 7 Nov 2014.
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Ciarán O'Brien answered on 7 Nov 2014:
It actually didn’t start off terribly salty.
Falling rain and rivers break up rocks very, very slowly, and that’s where a lot of salty-tasting minerals come from. They’re dissolved in tiny amounts in rain and river water, and they all ended up in the oceans. When water evaporates from the oceans to form more rain, it leaves the salt behind, so you get fresh rain falling and feeding rivers, which dissolves more salt from the rocks, which also ends up in the sea, which is then left behind again as water evaporates to form new clouds… And so on. Over billions of years, the salty minerals from the land added up to the levels we have now.
Interestingly, the sea probably won’t get much saltier. Have you tried dissolving salt in a glass of water? There’s only so much that will dissolve before it just falls to the bottom. It’s the same with the sea, any more salt mostly just gets deposited as sediment on the sea floor along with all the other sediments. Exceptions like the Dead Sea (actually a lake) have very interesting and unique geology and features, which is why the salt levels are so much higher there.
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Lydia Bach answered on 9 Nov 2014:
Hi Einstein123jnr,
You might have noticed that when you go swimming on summer holiday. In every litre of sea water there are about six table spoons of salt, that’s a lot, but why? The salt comes from chemicals called minerals from rocks that have been slowly breaking down. But minerals also leach out from the ocean floor or are carried to the sea in rivers and can enter the ocean through volcanic eruptions.
Some oceans like the Dead Sea have much more salt in them – the Dead Sea has 62 tea spoons of salt per litre of water! – That means even bowling balls and people can float, because the large amount of salts give bowling balls and people a lot of buoyancy.
Check out this cool bowling ball experiment:
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Áine Broderick answered on 10 Nov 2014:
As rainwater passes through soil and rocks, it dissolves some of the minerals. This is the water we drink, and of course, we can’t taste the salt because its concentration is too low. Eventually, this water with its small amounts of salts reaches a stream or flows into lakes and the ocean.
The Great Salt Lake, Dead Sea, and other salt lakes have no outlets. All the water that flows into these lakes escapes only by evaporation. When water evaporates, the dissolved salts are left behind. So a few lakes are salty because rivers carried salts to the lakes, the water in the lakes evaporated and the salts were left behind. After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process makes the sea salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean. Because of the huge volume of the oceans, hundreds of millions of years of river input were required for the salt content to build to its present level.
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