[ConcowMomma (OP)]

Why does my ice do this? There's a distinct spike!

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[chris]

Thank you for this lovely question - and photo - and welcome to the forum!

The reason that this happens is because the water in the icecube tray freezes first where it is in contact with the sides of the tray and also with the cold surrounding air.

This leaves a pool of unfrozen water in the centre of the forming ice-cube.

Remember that water expands as it freezes, so as more ice forms the remaining water has progressively less space to fit into.

This has the effect of squeezing the remaining water from the centre of the forming ice-cube up and out of the top of the cube; as the water squeezes out it freezes on contact with the surrounding ice, assembling an ice stalagmite.

Other ice-cube-related discussions and references from the Naked Scientists:
https://www.thenakedscientists.com/forum/index.php?topic=44990.0
https://www.thenakedscientists.com/forum/index.php?topic=839

[ConcowMomma (OP)]

I understand the nature of water.... but this is so strange! It's happened twice, and Ive never seen it before this.  If the outside is frozen, how?why? is it breaking through to allow the stalagmite to even form? 

[ConcowMomma (OP)]

Here's the first one.... the OP was the second occurrence.

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[ConcowMomma (OP)]

Different view

[ConcowMomma (OP)]

First occurrence