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This morning I found a really strange ice formation in my garden. I can't figure out how it appeared, because there was nothing above. The night was particularly cold (Belgium).

To give an idea, it has the size of a common mouse (5 cm of Height and 2 cm for the base of the inverted pyramid).

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    $\begingroup$ @snoobdogg: You have an answer now so its not really relevant but in future I'd recommend if being asked for dimensions to give them in more standard units of measure. Centimetres or inches would have worked. Mice are a less commonly used unit and thus many people would have to look up the conversion of "common mouse" to units they are familiar with. I certainly am left still having no actual idea of how big this thing is. :) $\endgroup$ – Chris Feb 22 '18 at 11:31
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    $\begingroup$ and the common mouse in your part of the world may have different dimensions than common mice in other places! $\endgroup$ – Mr Lister Feb 22 '18 at 11:33
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    $\begingroup$ @Chris I will next open a new International Bureau of Mice and Measures, soon :) $\endgroup$ – snoob dogg Feb 22 '18 at 11:37
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    $\begingroup$ Please could you place an image of a mouse to scale beside one of the images? $\endgroup$ – camden_kid Feb 22 '18 at 16:28
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    $\begingroup$ How come nobody mentioned the banana as the proper unit? "banana for scale" is the correct quote to use. $\endgroup$ – WoJ Feb 22 '18 at 17:59
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Congratulations, you found an inverted pyramid ice spike, sometimes called an ice vase!

The Bally-Dorsey model of how it happens is that first the surface of the water freezes, sealing off the water below except for a small opening. If the freezing rate is high enough the expansion of ice under the surface will increase pressure (since the ice is less dense than the water and displaces more volume), and this forces water up through the opening, where it will freeze around the rim. As the process goes on a spike emerges.

If the initial opening or the crystal planes near it are aligned in the right way the result is a pyramid rather than a cylinder/spike.

The process is affected by impurities, the water has to be fairly clean. It also requires fairly low temperatures so freezing is fast enough (but not too fast).

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    $\begingroup$ How quickly does this grow in millimice/minute? $\endgroup$ – Barmar Feb 22 '18 at 21:04
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    $\begingroup$ I have the impression that nobody has "caught it in the wild" and measured it, but the linked papers seem to indicate that the growth happens in minutes to hours. It is likely pretty fast by ice standards. $\endgroup$ – Anders Sandberg Feb 22 '18 at 21:11
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    $\begingroup$ I saw a video, purportedly from Lake Michigan, of a so-called “ice wave,” where the expansion pushes ice onto the shore. $\endgroup$ – WGroleau Feb 23 '18 at 5:12
  • $\begingroup$ Is there a relationship to hoar frost? In very humid (absolute) conditions I've seen spikes form regularly on ice cube trays in the freezer i.e. every single ice cube has one. The fractal like spikes off of the main spike tend to not to survive long once the door is opened. I've never seen it when the air is dry. $\endgroup$ – JimmyJames Feb 23 '18 at 17:13

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