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Comment from: Jon Nelson [Visitor]
Jon Nelson

Now, nearly a year later, I am not sure what my explanation in the third-to-last paragraph was. I might have been thinking that the depth of the paw-print holes was due to water draining.

But I think the more likely explanation is more obvious: The pawprints were made before the ice formed, and then the ice formed and pushed up around the indentations, just like the columnar ground ice in the last photo.

Two questions about this explanation:
1) why would ice grow up everywhere except at the depressions? and
2) Why then does this ice not look columnar?

The answer to 1) is that ground ice grows from a certain level in the soil, at an isotherm surface (i.e., a constant temperature) that is typically a little below freezing. This level must have been at the uppermost surface of the ground (thus explaining why that ice is so white - it could not push up much dirt because there was very little dirt above the isotherm). But in the pawprint depressions, the temperature was above the freezing temperature, meaning that no ice could grow from those spots. Ah! it seems so obvious now…

About 2), I don’t know why the ground ice is sometimes columnar and sometimes more of a continuous mesh, but I see both types quite often. Perhaps the mesh type happens when the columns essentially grow together, being multiply connected to the point of resembling a web-like structure. Maybe this happens more with a smoother, more uniform and flat, dirt surface.


12/23/10 @ 17:41

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