Icicles in reverse

Ice cube spike

About a year ago, in late winter, Blackburnian called me into the kitchen to show me a bizarre ice cube he’d discovered in the tray in the freezer. It looked like a normal cube, except that at one side, sticking out of the top at a slight angle, was a narrow spike of ice about an inch long. It was the strangest thing, looking a little like the little back-splash of water when a droplet hits the surface, frozen in ice. But obviously that wasn’t possible, so my hypothesis was that a hair had fallen into the cube while it was still liquid, and the water molecules had frozen up its length in a capillary action (where the chemical bonding properties of the water molecule cause it to crawl up a surface a little). Although I could see no hair inside, I’m blonde and figured it was just hidden. After all, what else could cause such a bizarre structure?

This afternoon I opened the freezer door to pull out tonight’s dinner to thaw, and happened to notice a freshly-frozen ice cube tray on the freezer door. And within this tray was – you guessed it – another ice cube spike. Well, what are the odds that another invisible hair had fallen into the cube tray? Particularly since I so rarely fill it, and Blackburnian has dark hair. So this time I did some poking around. Sure enough, it seemed to be a fairly common observation – under the right conditions.

Ice cube spike
(last year’s ice spike)

Water begins freezing at the edges of its container (the banks of a pond, or the sides of the ice cube tray), where the solid acts as a catalyst (remember the frost?). As the top freezes inward, it creates a little hole in the surface of the ice. At the same time, the ice cube is also freezing along the sides and bottom (doesn’t happen in a pond, but in small, contained bodies of water where the cold reaches all sides, all sides freeze at the same time). Ice is less dense than water, and takes up more space than the water does when it melts. So as the ice cube is freezing from the bottom and sides, the still-unfrozen water inside is getting squeezed out through the hole in the top of the surface.

Like an icicle grows through the addition of more ice as the water droplets running down the outside freeze, so the ice spike grows as more water is pushed up through the hole and freezes at the edges, creating a narrow tube or straw, through which more water is pushed and subsequently freezes. Some ice spikes can grow quite long if the pressure is slow and steady, and the temperatures mild enough to result in slow freezing speeds of the tube. The tube closes off when either the inside of the cube is completely frozen (no more water to force out), the temperature gets too cold (the water on the spike freezes faster than in the cube and seals it off), or the walls of the tube narrow beyond a certain point (where the water coming up forms a bridge between the walls instead of adding to the walls as it freezes).

Ice cube spike

You’d think that given the nature of an ice cube this ought to be seen more frequently, but it actually takes fairly specific conditions to produce the effect. For one, the water needs to be distilled, or low in dissolved solids (such as salts and minerals, typically found in most well or city water). The salts and minerals in the water will act as a catalyst to freezing so that the ice isn’t growing only from the walls of the tray (and therefore isn’t creating the same pressure on the water inside), although not all of these dissolved solids affect ice formation in the same way. We use a Brita filter, and it’s possible that it had removed enough of the minerals out of these couple ice cubes to encourage the spike to form.

Your freezer also needs to ideally be just below freezing – cool enough to encourage the water to freeze, but not so cold that it will freeze quickly. However, ice spikes will also form under “supercooling” situations, where lukewarm or room-temperature water is placed in a very cold environment (-10 to -20 oC, such as outdoors in winter, or in a freezer, particularly chest freezers which maintain colder temperatures) and is brought to freezing rapidly. In these cases a slightly different sort of ice crystal forms on the surface creating sheets of thin ice, including some that jut into the water below. These crystals, like those in snowflakes, form 60 degree angles creating a triangular opening in the surface, and the sheets hanging into the water form the base of the ice spike as it grows, often resulting in strange angles depending on the way the sheet was hanging.

The Discovery Channel’s show Daily Planet aired a segment a few years ago on the phenomenon after it was sent in by a viewer in Courtenay, BC, who discovered a spike in her birdbath. This site, by Professor Stephen Morris of the University of Toronto, also has a lot of information and links to photos and other sites.

You can try making your own ice spikes simply by freezing distilled water (not tap water!) in an ordinary ice cube tray in your freezer. Good luck and have fun!