Maybe you’ve seen a stalagmite-like ice structure jutting out of your ice cube tray, or witnessed a thumb of frozen water protruding from your outdoor birdbath, as in the photograph above. The frozen formation is known as an “ice spike.” Edwyn Anderton, a lecturer in science education, came across this one in his garden one cold morning in Sheffield, England. He says the spike measured almost four inches long.

Although ice spikes occur frequently, no one is entirely sure why, says Caltech physics professor Kenneth Libbrecht, who conducted a study on ice spikes in 2003. He enlisted a student to freeze plastic ice cube trays of water in dormitory freezers around campus, and the team investigated how variables such as temperature, air motion, and salinity affected ice spike formation.

Libbrecht says that ice spikes seem to manifest like this: As water freezes inside, say, an ice cube tray, the top of the liquid solidifies first, occasionally leaving one spot on the surface where ice doesn’t form right away. The water continues to freeze from the top down, and because water expands as it freezes, the ice eventually pushes the remaining liquid up through that hole—similar to lava rising through a volcano. Instead of gushing over the edge like lava, however, the water begins to freeze up through the hole, creating a spike.

Libbrecht’s experiment found that -7 degrees Celsius (close to 20 degrees Fahrenheit) was about the peak temperature for ice spike formation—any colder, and no spikes formed; any warmer, and the water wouldn’t solidify fast enough to create spikes. Air circulation—common in modern freezers to prevent frost—also contributed to higher odds for ice spike development.

The most influential variable in ice spike formation, however, is most likely the purity of the water itself, Libbrecht says. In his experiment, adding salt to distilled water reduced the likelihood of ice spiking, while regular tap water—which has dissolved salt in it—didn’t produce ice spikes at all. Pure distilled water, on the other hand, more regularly yielded ice spikes, though still unpredictably, Libbrecht says.

Curious if you can create an ice spike? Try your own experiment using ice cube trays or a water-filled bowl. (Hint: Remember what variables influenced ice spike formation in Libbrecht’s experiment.) See some photo submissions here.

Do you have an eye on a Sci Candy?

A photo is worth a thousand words—or maybe even a Sci Candy. Nominate an image that you’d like us to explore! Email scicandy@sciencefriday.com.