Ashleigh Edmondston, of Ardgowan School, asks :-
The Room 12 class, at Hoon Hay School, asked:- Scott Goodhew, at New Windsor Primary School, asked:- Reece Evans, at Balclutha Primary School, asked:- A geography class at Manawatu College asked:-
Why is the sea salty? Why is lake water not salty but sea water is? Why is the ocean salty and mainly with sodium and potassium ions?
James White, a geologist at Otago University responded.
Salts in rocks and soils on land are slowly dissolved (chemically broken down) by water (rainwater, river water etc). Rivers then carry these salts to the oceans.
There is little salt dissolved in river water so it doesn't seem salty to us. Neither do fresh-water lakes which are fed by rivers at one end and drained by a river at the other. They are really just large, deep parts of freshwater rivers, like Lake Wakatipu.
Lakes that aren't drained by rivers do tend to be very salty, like Sutton Lake in Central Otago. The ocean is salty mostly because, over millions and millions of years, rivers have been carrying into it dissolved salts from the chemical breakdown of rocks and soils on land.
Once the water is in the ocean, some of it evaporates to form clouds, leaving the salt behind in the water. The clouds drift over land, making rain that feeds the rivers, carrying yet more salt to the ocean. The amount of salt in the ocean stays fairly constant even though the rivers continue to bring in salt because it is in chemical balance. If salt levels get too high, salt crystals form and separate from the water, taking away the excess salt.
Volcanoes of the ocean floor also contribute components of salts to the ocean. Even recycling of seawater salt is important, both short-term recycling when wind blows fine salt spray far inland, and long-term recyling of salt components present in marine sedimentary deposits now exposed on land.
Sodium and potassium are abundant in seawater primarily because of their solubility, but there is no shortage of minerals that contain these elements and can release them into river systems. In particular, they are important ions in feldspars, the most abundant minerals on earth.