Pancake ice is a compelling natural phenomenon observed in the world’s coldest marine and freshwater environments, representing a distinct stage in the formation of solid ice sheets. It is defined by the unique shape of its individual pieces, which resemble small, circular floating trays or discs. This form of ice occurs in water that is turbulent and agitated, such as in polar seas or large, cold lakes with active currents. The presence of pancake ice signals a period of rapid freezing in conditions that are too rough for a smooth, continuous sheet of ice to form.
Defining the Appearance and Scale
Individual discs typically range in diameter from 30 centimeters to three meters, depending on the local wave energy and temperature. These ice pieces are relatively thin, generally measuring up to 10 centimeters in thickness.
Each disc possesses a distinct raised rim or lip around its perimeter, while the center remains flatter and smoother. This elevated edge forms a hardened barrier resulting from the continuous motion and impact of the pieces against one another.
The Mechanics of Formation
The process begins in supercooled water with the formation of frazil ice, which are tiny, needle-like ice crystals suspended throughout the water column. As these crystals rise and accumulate on the surface, they create a dense, soupy mixture known as grease ice, the initial material from which the discs emerge.
The key requirement for the transition to pancake ice is the presence of moderate to high wave activity or strong currents. Without this agitation, the grease ice would simply freeze into a smooth, continuous sheet called nilas. Instead, the waves break the initial slushy layer into small pieces and then constantly jostle them against each other.
Repeated collisions between these floating ice fragments lead to the gradual rounding of their edges. During each impact, the soft, slushy material is pushed up and freezes onto the rims of the discs. This continuous process builds the signature raised edges that protect the disc from being broken down further.
From Pancake Ice to Consolidated Ice
Pancake ice is frequently observed in the marginal ice zone, the dynamic outer edge of the polar ice pack where the waves are strongest. It is especially common in the Antarctic, where high wave energy from the Southern Ocean often prevents the formation of calmer, smoother ice types.
The discs are destined to consolidate into larger structures as the temperature drops and wave action subsides. They begin to freeze together, a process accelerated when one piece slides over another, known as rafting. This amalgamation creates larger, more stable masses called ice floes.
As the freezing continues, the consolidated floes lock together, forming vast, continuous sheets of thick ice. This progression from mobile pancake ice to solid sheets forms much of the seasonal pack ice, with the hardened rims contributing to the overall strength of the new sea ice cover.