Why is the sea salty? Well, on average, seawater is made up of around 3.5% salt and other minerals (so the oceans are 96.5% water, 3.5% salt). Most of this is the ordinary table salt – sodium chloride – we use every day. So where does it all come from?
It is believed that minerals from within the Earth seeped into the seas through vents in the ocean floor (through the crust of the Earth in effect) when the oceans were first formed many years ago. Today, rivers and streams continually carry salt and other minerals to the ocean as land rocks get gradually eroded by water and the weather. When water evaporates from the sea (to fall again later as rain) the salt within the water is left behind where it concentrates. In addition, winds also carry particles which get deposited into the water and decomposed organisms (i.e. dead creatures and plants) also add some salinity.
The amount of salt in our seas has remained pretty consistent for a long time now, despite the regular and ongoing flow of minerals from rivers, etc. This is because some of the salt gets removed when it hits the sea bed. One theory is that it moves or gets forced back under our land masses.
So why aren’t our rivers salty then? Well, river water and streams can contain a small amount of salt but because there’s only a trace, we can’t really taste it. Rivers and streams are rushing or moving waters so salts don’t tend to accumulate or concentrate.
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