Frazil ice

Frazil ice is a collection of loose, randomly oriented ice crystals, a millimeter or sub-millimeter in size and of varied shapes such as elliptical disks, dendrites, and needles. It forms in winter in turbulent open water in rivers, lakes, and reservoirs, where wave and current action supercools the water column and keeps the crystals from floating to the surface, and it also forms in oceans under windy, wave-driven conditions, appearing on the downwind side of leads and in polynyas, where it can accumulate at the surface into grease ice.