Why do ice cubes grow spikes?
The short explanation is this: as the ice freezes fast under supercooled conditions, the surface can get covered except for a small hole. Water expands when it freezes. As freezing continues, the expanding ice under the surface forces the remaining water up through the hole and it freezes around the edge forming a hollow spike. Eventually, the whole thing freezes and the spike is left.
A slightly longer explanation: the form of the ice crystals depends on the cooling rate and hence on the degree of supercooling. Large supercooling favors sheets which rapidly cover the surface, with some sheets hanging down into the water like curtains. These crystalites tend to join at 60 degrees and leave triangular holes in the surface. Hence, spikes often have a triangular base. The sides of the spike are sometimes a continuation of pre-existing subsurface crystalites, and can extend from the surface at steep angles.
See
www.physics.utoronto.ca/~smorris/edl/icespikes/icespikes....
Photo by Mary de Bruyn
