Today's curiosity: Why ice makes a spike when it freezes?

I have been meaning to figure something out for several weeks now. Life stops me from beelining to a computer to search why I find at least one spike forming on my ice cube tray every single time I make ice. I am old school, I don't an automatic ice maker. Call it artisan hand crafted ice. 

Let's go to an expert for this curiosity solved. Unofficial Dr. Freeze, Professor Stephen Morris has a self proclaimed obsession with icicles. One could be guilty of worse things and coupled with my obsession with wintertime I could be a distant cousin in science family tree terms. He's even created The Icicle Atlas.  He can legit answer this ice spike question of mine.

We learned in grade school that water is one of the rare things that expands when frozen. That's part of what's going on here. If you had an automatic ice maker you'd be missing out on all this sciencey goodness. Spikes are forming inside your freezer and you don't even know it.

Have you ever tapped a tray after about 1-2 hours to see how frozen it was only to have your finger break the ice crust and plunge 1" deep into icy cold tray cube water?  So you know the top freezes first from those antics. The timing of the top layer freezing over with the possibility that there is an air pocket on that surface is how the icicle structure has the "doorway" to make the formation. As the interior freezes after the top crust freeze is formed the expansion of the freezing water below has an escape hatch through that tiny hole.  It slowly climbs out, freezing along the way, until the point is formed, hole closed over and it can expand no further in that direction.  Behold the glorious icicle spike and you didn't have to do anything to make it happen - other than keep your grubby finger out of it until it was done turning into an ice cube with spike bonus.

Recap of conditions to make this awesomeness happen:

  1. The nature of water expanding when frozen
  2.  Straight sided vessels are ideal conditions
  3. Top layer crusts over first
  4.  Air hole must be present on initial top frozen layer of cube
  5.  See 1. expanding freezing water enters the icicle portal
  6.  Bonus: Distilled water gives the best chance for a cube spike. The tiny amounts of salt in tap water prevent the ice crystals that create the air space for the spike to form. Another reason to use a Brita filter people.

While icicles are perfectly natural they are not common, hence why not all cubes in the tray form the delightful points. Wintery icicles that form as snow melts from fresh water dripping over it and refreezing is different than these that shoot upward from pressure. I now call them lucky cubes. How refreshing.

Curiosity solved.