An overlapping blend of nila and smooth, snow-covered ice, with breakage around the edges, and fracturing on the surface.
—Credit: Ted Scambos, NSIDC
As the ocean water begins to freeze, small needle-like ice crystals called frazil form. These crystals are typically 3 to 4 millimeters (0.12 to 0.16 inches) in diameter. Because salt doesn't freeze, the crystals expel salt into the water, and frazil crystals consist of nearly pure fresh water. See also Salinity and Brine.
Sheets of sea ice form when frazil crystals float to the surface, accummulate and bond together. Depending upon the climatic conditions, sheets can develop from grease and congelation ice, or from pancake ice. These processes are described below.
In calm waters, frazil crystals form a smooth, thin form of ice, called grease ice for its resemblance to an oil slick. Grease ice develops into a continuous, thin sheet of ice called nilas. Initially, the sheet is very thin and dark (called dark nilas), becoming lighter as it thickens. Currents or light winds often push the nilas around so that they slide over each other, a process known as rafting. Eventually, the ice thickens into a more stable sheet with a smooth bottom surface, called congelation ice. Frazil ice cannot form in the relatively still waters under sea ice, so only congelation ice developing under the ice sheet can contribute to the continued growth of a congelation ice sheet. Congelation ice crystals are long and vertical because they grow much slower than frazil ice.
If the ocean is rough, the frazil crystals accummulate into slushy circular disks, called pancakes or pancake ice, because of their shape. A signature feature of pancake ice is raised edges or ridges on the perimeter, caused by the pancakes bumping into each other from the ocean waves. If the motion is strong enough, rafting occurs. If the ice is thick enough, ridging occurs, where the sea ice bends or fractures and piles on top of itself, forming lines of ridges on the surface. Each ridge has a corresponding structure, called a keel, that forms on the underside of the ice. Particularly in the Arctic, ridges up to 20 meters (60 feet) thick can form when thick ice deforms. Eventually, the pancakes cement together and consolidate into a coherent ice sheet. Unlike the congelation process, sheet ice formed from consolidated pancakes has a rough bottom surface.
Once sea ice forms into sheet ice, it continues to grow through the winter. When temperatures increase in spring and summer, the first-year ice begins to melt. If the ice does not grow thick enough over the winter, it will completely melt during the summer. If the ice grows enough during the winter, it thins during the summer but does not completely melt. In this case, it remains until the following winter, when it grows and thickens and is classified as multiyear ice.