xkcd #3115: Unsolved Physics Problems
Title text:
'Tin pest' makes more sense to me. Tin just doesn't want to be locked down in a shape like that. I get it. But why would any metal want to grow hair??
Transcript:
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Source: https://xkcd.com/3115/
I think it's been figured out for a while now? Essentially most of the surface freezes, except for a small hole. The spike forms from that hole since the water is pushed out before freezing (on the outside) leaving a hollow spike.
The rate of freezing is similar to the rate of extrusion, a spike can form.