Last March a triangular ice spike appeared on our bird bath. I was reminded of it by Roy Spencer's recent post on ice spikes he has observed in Alabama. He also links to this laboratory study which used ice cubes in a tray, but presumably the mechanism is similar.
In the case of an ice spike forming in an ice cube tray, water first freezes at the surface, starting at the edges the cube, and the ice subsequently expands laterally until only a small hole in the ice surface remains. Then the continued freezing of water beneath the surface forces water up through the hole, where it freezes around the edge of the hole to form the beginnings of a hollow tube. Continued freezing forces water up through the tube, where it freezes around the rim and lengthens the tube. At some point the tube freezes shut and growth stops.