An ice spike is an upward-facing icicle that forms as a body of water freezes. Ice spikes can form in natural environments or can be made artificially by freezing distilled water in plastic ice cube trays.
Water expands when it freezes. If there is already a thin sheet of surface ice over the body of water, further freezing can force water out and upwards through a crack or weak point in the sheet. This can produce a tube-like structure where water emerges at the tip, progressively lengthening the tube.[1] Tube formation stops when the tip freezes and seals.
Ice spikes rarely form when freezing "normal" non-distilled water because impurities in the water act as an ice nucleus so the water freezes before an ice spike can form.
The formation of ice spikes is related to the shape of the water body, the concentration of dissolved impurities, air temperature and air circulation above the water.[2]