Stephen Morris, professor of experimental nonlinear physics at the University of Toronto, maintains a Web page on just this topic. So we asked him to tackle the question:
Water is one of those rare materials that expands while it freezes. If a crust of ice with a small hole in it forms over liquid water, it can trap the liquid below, leaving it no room to expand during freezing. So, as the water begins to solidify, it is forced up through the hole and begins to freeze around the hole's edge, forming a hollow, water-filled spike.
Water keeps moving up the spike—creating a little self-made soda straw, which can grow quite long and thin. Eventually, all the water freezes and the spike becomes solid. The energy needed to lift the water up into the spike comes entirely from the expansion of the water while it is freezing.
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Some features that govern the way water crystallizes play important roles in the spike-formation process. The form of the ice crystals depends on the cooling rate and hence on the air temperature. Spikes typically form when the ambient air is well below 0 degrees Celsius, allowing for rapid freezing—an air temperature of about –7 degrees C (20 degrees Fahrenheit) turns out to be optimal.
Fast cooling favors crystals that look like sheets, which rapidly cover the surface, with some sheets hanging down into the water like curtains. These crystallites tend to join to one another at 60-degree angles, much like the arms of a snowflake do. They tend to leave triangular holes in the surface; hence, spikes often have a triangular base. The sides of the spike are sometimes a continuation of preexisting crystallites that are below the surface of the freezing water. This explains why some spikes can extend from the surface at steep angles. It is also possible to get structures that look like little inverted pyramids, or "ice vases," if the water happens to drain out before completely freezing.
Pure water works best for spike formation, as does a container with vertical sides, such as an ice tray. People often see spikes in birdbaths or pet drinking dishes that are left outside overnight. It is rather rare for them to form naturally—like on the surface of lakes or ponds—because the cooling rate of natural bodies of water is usually not rapid enough. They can, however, sometimes form on falling sleet pellets.