Ice spikes

An ice spike is a slender projection of ice that grows upward from a freezing water surface. Familiar examples appear on ice cubes, but spikes have also been observed on small outdoor containers and water bodies. Many are hollow or contain a central channel left by the liquid water that feeds their growth. The term is used for conspicuous projections; not every mound or irregular bulge on ice is an ice spike.

A well-supported general explanation is that freezing becomes partly confined beneath a developing surface ice layer. Because water expands by roughly 9% on freezing, continued ice formation can press remaining liquid upward through a small opening in the ice cover. If the water freezes around the edge and near the tip of that opening, the resulting tube can lengthen into a spike. This is often called the Bally–Dorsey explanation. Observations and laboratory work support this account of the basic process, while the detailed conditions that determine whether a spike forms, and the precise growth behavior in particular cases, are more complex and not fully settled.

Ice spikes are intermittent rather than an inevitable result of freezing. Experiments have examined temperature, airflow, water purity, salt content and the orientations of ice crystals at the surface. The last open patch in a developing ice cover, and how the surrounding crystals grow, can influence whether water is extruded into a narrow projection or instead floods across the surface. Results depend on experimental conditions, so there is no single recipe that guarantees a spike.

The opening in the surface ice and the crystal structure around it can influence a spike's cross-section and direction. Ice spikes are often hollow, and some have triangular or polygonal bases associated with the geometry of crystals around the opening. A spike can grow while water is supplied through its interior and freezes near its tip. The balance between the rate of water extrusion and ice growth affects whether its sides remain nearly parallel or taper.

Ice spikes are most often discussed as a laboratory or household freezing phenomenon, especially in ice-cube trays. Similar protrusions have been reported outdoors on birdbaths and other small volumes of water. Such examples depend on suitable freezing and surface conditions; observations of an unusual outdoor formation do not by themselves establish that it followed exactly the same growth pathway as an ice-cube spike.