Sea ice does not simply grow and melt. During its lifespan, it is very dynamic. Due to the combined action of winds, currents, water temperature, and air temperature fluctuations, sea ice expanses typically undergo a significant amount of deformation. Sea ice is classified according to whether or not it is able to drift, and according to its age.
Sea ice can be classified according to whether or not it is attached (or frozen) to the shoreline (or between shoals or to grounded icebergs). If attached, it is called landfast ice, or more often, fast ice (from fastened). Alternatively, and unlike fast ice, drift ice occurs further offshore in very wide areas, and encompasses ice that is free to move with currents and winds. The physical boundary between fast ice and drift ice is the fast ice boundary. The drift ice zone may be further divided into a shear zone, a marginal ice zone and a central pack. Drift ice consists of floes, individual pieces of sea ice 20 metres (66 ft) or more across. There are names for various floe sizes: small – 20 m (66 ft) to 100 m (330 ft); medium – 100 m (330 ft) to 500 m (1,600 ft); big – 500 m (1,600 ft) to 2,000 m (6,600 ft); vast – 2 kilometres (1.2 mi) to 10 km (6.2 mi); and giant – more than 10 km (6.2 mi). The term pack ice is used either as a synonym to drift ice, or to designate drift ice zone in which the floes are densely packed. The overall sea ice cover is termed the ice canopy from the perspective of submarine navigation.
New ice is a general term used for recently frozen sea water that does not yet make up solid ice. It may consist of frazil ice (plates or spicules of ice suspended in water), slush (water saturated snow), or shuga (spongy white ice lumps a few centimeters across). Other terms, such as grease ice and pancake ice, are used for ice crystal accumulations under the action of wind and waves.
Nilas designates a sea ice crust up to 10 centimetres (3.9 in) in thickness. It bends without breaking around waves and swells. Nilas can be further subdivided into dark nilas – up to 5 cm (2.0 in) in thickness and very dark, and light nilas – over 5 cm (2.0 in) in thickness and lighter in color.