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The other day a friend of my dad's showed him a picture of an icicle that appeared to be "growing" upwards out of a crack in a sidewalk. There were no roofs, overhangs or vehicles parked near by... It looked like a stalagmite you would see in a cave. It looked something like this*:

ice stalagmite.

How could this happen?

* Note that this is not the actual icicle, I used this picture for an example.

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    $\begingroup$ en.wikipedia.org/wiki/Ice_spike $\endgroup$
    – Nick T
    Jan 9, 2015 at 21:48
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    $\begingroup$ @ruakh - No, it's an ice spike. They are cool, literally and figuratively. $\endgroup$
    – David Hammen
    Jan 10, 2015 at 1:21
  • $\begingroup$ @ruakh Well, since it was a sidewalk there likely weren't any vehicles nearby at any point. $\endgroup$
    – L.B.
    Jan 10, 2015 at 2:06
  • $\begingroup$ @ruakh Ah... Sorry about that :) I used this for an example. This isn't the original. $\endgroup$
    – L.B.
    Jan 10, 2015 at 2:14
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    $\begingroup$ I found one of these spikes growing from a puddle of frozen water on top of a plastic sheet. All the other puddles were flat but had pockets of air in the centre underneath. The one with the spike did not, so I think the air pushed up the water to escape somehow as the water froze from the edges inwards. $\endgroup$
    – user5228
    Jan 19, 2016 at 15:09

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If there is enough water underground, in a sealed reservoir (like under the concrete in the picture), as that water freezes, it will expand, putting pressure on what ever water is left unfrozen. That water will be squeezed out through what ever cracks it can find - in this case, upwards. Because the water is already near-freezing (perhaps even slightly below, if it's super cooled), it will freeze very quickly when it hits the air (which will be much colder than the ground, late at night). But if there is still water being pushed up from below, that ice will have to move out of the way (it's weaker than concrete). The friction of the flow of cool upwards might warm it just enough to keep it liquid, and the "stalagmite" will end up with a hollow core that the water is being pumped through, and that structure can just keep building on itself, until it is toppled by wind or gravity.

The picture is a bit grainy, but it looks like that's what's happened there. It also looks like the side of the hollow "stalagmite" has fallen off up the top, and released a bit of the remaining water (or maybe it just melted in the sun).

Edit: Hah, I knew I'd seen this somewhere before - in my freezer! This page describes how Ice spikes form - I basically got it right. It seems that they're more likely to form with de-mineralised water, and the temperature has to be within certain narrow limits.

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  • $\begingroup$ You can also get hoar frost which is caused by near freezing air but below freezing surfaces. It forms more like a crystal growth from ambient humidity. $\endgroup$
    – John
    Jan 1, 2017 at 22:45
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Ice can grow up from the ground in the form of needle ice. The formation requires sufficient moisture in the ground. According to the National Snow and Ice Data Center (NSIDC) site needle ice can grow up to 40 cm in length. Needle ice are thin crystals of ice and has a very distinct look. A search on Needle Ice provides a wealth of examples of needle ice, note that all pictures are not relevant!

Needle ice can also form from plants if suddenly subjected to very low temperatures and where moisture in the plant has limited escape possibilities.

The picture in the post is, however, not needle ice, it is clearly a form of icicle. In fact the common icicle is reminiscent to a stalactite forming from a cave ceiling. Corresponding to the stalactite, a stalagmite forms on the floor. The ice form on the picture is a stalagmite form, created by water dripping from a structure down onto the ground in sub-freezing temperatures (credit to geotheory).

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  • $\begingroup$ "Note that not all pictures are relevant"? Also, you might be better off including a good example image, than telling people to search. The picture is not a stalagmite - it's on concrete, out in the open - there's nothing to drip by. $\endgroup$
    – naught101
    Jan 12, 2015 at 2:56
  • $\begingroup$ The point is that features like this can result from objects, for example, vehicles, that have since been moved. The fact that it is on concrete and in the open is completely irrelevant. The similarity to a stalagmite concerns the process of gradual growth through addition of new material, in this case ice, on the surface, nothing else. $\endgroup$
    – Peter Jansson
    Jan 12, 2015 at 8:15
  • $\begingroup$ I agree that you could find an icicle in a car park, after a car dripped on the ground (although in most cases the car is likely to freeze first). I was referring to the "clearly a form of icicle" - to me, it doesn't look anything like one - an icicle would be somewhat conical, and would likely remain so while it melted. $\endgroup$
    – naught101
    Jan 12, 2015 at 10:57
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It's called an ice spike. Not fully understood, can form on any surface of frozen water. my curiosity was peaked when one formed in a cup of water in my car. Scientists can't really say why it happens yet.

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    $\begingroup$ According to this page the process of ice spike formation seems to be well understood. $\endgroup$
    – daniel.heydebreck
    Jan 2, 2017 at 12:19

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