Ice cubes forming ice spikes as water freezes

I’ve seen these many times over the years. Each time I was slightly curious, but never felt motivated to look it up – until today.

These formations are commonly referred to as “ice spikes”. They seem to grow most readily with distilled water and an air current. They can also form with regular tap water, although much less frequently. Every time I have personally witnessed these formations has been using regular tap water in regular ice cube trays.

The idea is that the edges, including the surface, freeze faster than the interior of the ice cube. Water expands as it freezes, so if the surface is (nearly) frozen over, pressure will build up from the interior of the cube, and water will force its way up. This can form a tunnel or tube that the water travels up until freezing along the rim of the tube (forming the ice spike).

CalTech with some good info:
http://www.its.caltech.edu/~atomic/snowcrystals/icespikes/icespikes.htm

More links pertaining to ice spikes:
http://www.physics.utoronto.ca/~smorris/edl/icespikes/icespikes.html
http://en.wikipedia.org/wiki/Ice_spike
http://www.halbertcicles.com/how.htm
http://www.sciforums.com/Unexplainable-Water-Phenomenon-t-46063.html