Fresh vs. Salt Water: Unmixable Conditions Explained

[flatmaster]

oceanic water obviously has a higher salt concentration and is more dense than fresh water. What specific conditions keep these two from mixing? On discovery, I see underwater caves where salty and fresh waters don't mix. What's behind this? Should we integrate their schools?

 

[Borek]

They mix, but slowly, as different densities help them keep separated and the only process that takes place - diffusion - is slow and occurs only on the boundary between fresh and salty water. They will mix much faster when stirred, as the surface between both waters becomes much larger.

 

[flatmaster]

How slowly is slowly?

 

[Borek]

Good question, one that I don't know answer to. It will depend on many factors - difference in salinity, thickness of the layers, temperature. Probably minutes to years depending on the circumstances.