Freshwater Over-layering Saltwater

June 09, 2020


Photographer: James Van Gundy 
Summary Author: James Van Gundy 

The ice in a glacier is composed of water that originally fell as snow and thus contains relatively low levels of dissolved materials. When it melts, it forms liquid freshwater that has a density very close to one gram per cubic centimeter (1.0 g/cm3). Seawater on the other hand has an average density of about 1.03 g/cm3 due to the rather large quantity of salts that are dissolved in it.

When ice is calved from the front of a tidewater glacier, it falls into seawater, floats there for a while and then eventually melts. As it melts, it forms a stable layer of freshwater that in essence floats upon the denser seawater below. If there’s little wind or other disturbance to upset this stability, the layering may persist for quite a period of time until random diffusion eventually breaks it up. In the above photograph, small bits of ice from southeast Alaska’s Hubbard Glacier remain entrapped in the layer of freshwater that forms a visible boundary with the denser seawater. Photo taken on June 19, 2019.