I've noticed when I make ice, sometimes an icicle spike will form sticking up about an inch or two high.?

Question

How does this occur? It's almost like a stalagmite forming in the ice cube tray. Usually it happens in only 2 or 3 of the cubes but I thought this was really strange.

Answer 1

When water freezes, it expands. If the water is enclosed so that it can't freely expand, it will tend to break the container. I'm sure you've seen what happens to bottles of soda or beer when it freezes. Your ice cube tray likely contained the freezing water so that it expanded upward. Under the right circumstances, The rising water could form a pipeline as the outer layer froze, allowing the liquid inside to expand upward.

Answer 2

There are two processes involved, supercooling and expansion. The water in the ice cube tray cools to the freezing level and as it cools further, it freezes from the outside in. As it freezes, it expands and forms a dome on the ice cube.

Sometimes, the water cools below 0°C and remains liquid. This is supercooled water and is quite common. Agitation, perhaps from the freezer turning itself on, causes the water to freeze rapidly from the outside in. The speed of the process and the rapid expansion forces a squirt of supercooled water upwards in the middle of the ice cube which freezes immediately. This is your stalagmite.

Supercooling is often seen in beer cans that have been placed in the freezer for quick chilling. If the cans remain too long, the temperature falls below 0°C but the beer remains liquid. It is supercooled. When the can is removed and particularly when it is opened, the beer freezes rapidly in the can.