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Loki.

stalagmites?

One possible cause, however mundane, might be that as the water expands when becoming ice, the plastic tray does not and the ice is forced upward due to the tapered shape of the "pockets" in the tray.

Observe: 1) That particular cube has moved so that the far edge is above the tray surface, and the white opacity shows the ice no longer in contact with the mold on the near side. That suggests that the cube also has airspace at the bottom, and that may be the source of the water used to make the spike. 2) The cube on the near left has a slice of ice hovering over its surface. This indicates the tray was placed upside down after the initial freeze, and water collected on the top of the seal that commonly comes with this kind of ice tray. If one supposes that the ice tray was inverted and a warm object was placed on to or next to the spiked cube, one may envision a slow melting and refreezing of the water to form the spike.

Parenthetically, a visually comparable process is the generation of tin whiskers. In this case, tin alloy used for soldering will grow microscopic hairs, usually straight but sometimes curved. They are columns with stripes that likely reflect the part of the crystal domain exposed to the surface. This is a major issue as the EU seeks to remove lead from solder. At least 5 % lead is needed in the alloy to prevent tin whiskers from growing. If a circuit uses solder that grows tin whiskers, the whiskers can actually grow long enough to cause a short circuit and a failed circuit board. They have tried applying a conformal coating to stop the whiskers, and this often works, but sometimes the whiskers can still punch through!

Lol, its because of the expansion of the water molecules as they freeze and become a solid.. Basically the reason is as the water turns into ice it exceeds the shape of the form and as it is not all a solid yet the ice cube shifts in the form creating odd shapes out the top.

Anyone know why icicles sometimes form at an angle instead of straight downward? I saw a few houses with all the icicles at a weird angle this year. I've never noticed that before.

Personally I think it's because the water trapped an evil spell cast your way, thus saving your life, but others disagree.

[its.caltech.edu]

Ice Demons.

Water is one of the few chemicals that becomes less dense as it freezes. You know this because ice floats on water. Almost no other chemical does this.

We know that because it is less dense it takes up more volume for the same mass. Thus it expands when it freezes.

Slight jostling at the right time. The spear looks about the same height as the length of the cube.Can you replicate? When the ice has skimmed over the top you might try pull a piece away from the edge and see if it can be made to stand up. Ice generally freezes from the outside in and the top down so you might see pieces with that spear and a heavy base near a corner.