The pyramid-shaped ice formation you see here growing from a bird bath in England is a natural variation on a phenomenon known as "ice spikes." Most spikes are thin lances growing from ice cube trays and, occasionally, small ponds. Ice pyramids are more rare - and harder to form.

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Ice Spikes

If you go out one morning, just as the temperature has really started to drop, you might be forgiven for thinking that tiny aliens landed in your birdbath during the night. Birdbaths, as still pools of water that are left out in freezing conditions, are the most likely place to see inverted ice pyramids.

How in the hell could this form naturally? This paper, by Miles Chen a the University of Toronto, explains the process. Regular ice spikes form because, at just the right temperature, the sides and top of a body of water - usually water in an ice cube tray - freeze first. As they freeze they expand, putting pressure on the water in the middle. If there is a tiny hole in the ice forming at the surface of the still-hardening cube, the liquid water is pushed upwards. The water pushed up through the hole forms a little frozen mound on the top of the ice cube. This little mound also has a hole in its center, through which more water is pushed, and the whole thing builds up into a spike.

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Ice Pyramids

Ice pyramids form through a variation on the process. Chen discovers that the water doesn't freeze continuously, moving from the sides of the container to the middle. It freezes in what can best be described as "sheets." These sheets hang down vertically from the surface of the water. Sometimes they can be parallel to each other, as if someone were taking orderly slices from the water. Other times they can form at all angles to each other.

Inverted pyramids are formed when these sheets are at just the right angle to each other. Essentially, they form the shape, or the mold, for the pyramid under the water. In the meantime, the surface of the water freezes in roughly the shape of the pyramid base. At that point, the only way for the pyramid to go is up and out of the water. The only important part is that the "tip" of the pyramid doesn't freeze over, so more water can be forced into the pyramid "mold." As they ice sheets keep freezing and expanding, the pyramid is pushed up and up. Because the sheets are freezing, the underwater mold is getting smaller, the pyramid eventually tapers off to a point.

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Eventually, the entire thing is frozen in place, waiting for people to be astonished by it the next morning. If you have a pond, a bird bath, or any small body of water that can freeze over, keep an eye out for an ice spike. Under the right conditions, the "spikes" can be pyramids, "cubes," or even vases.

[Via Ice Spike Formation Induced by Dendritic Ice Sheets.]