A halocline is also a layer of separation between two water masses by difference in density, but this time it is not caused by temperature. It occurs when two bodies of water come together, one with freshwater and the other with saltwater. Saltier water is denser and sinks leaving fresh water on the surface.

Why does a halocline form?

Formation of pycnocline may result from changes in salinity or temperature. Because the pycnocline zone is extremely stable, it acts as a barrier for surface processes. Thus, changes in salinity or temperature are very small below pycnocline but are seasonal in surface waters.

What is the halocline and why does it appear as it does?

Haloclines are common in water-filled limestone caves near the ocean. Less dense fresh water from the land forms a layer over salt water from the ocean. For underwater cave explorers, this can cause the optical illusion of air space in caverns. Passing through the halocline tends to stir up the layers.