Frozen lake ice formation

Question

Yesterday I went walking on the Lagorai mountain, in northern Italy. Passing near the Lake Erdemolo, I noticed a small part of frozen water from the lake, separated from the rest (the lake was not frozen at all). I found a quite strange ice formation, which I cannot fully comprehend. At the borders of the pool there was a very thin layer of ice with no water underneath, and progressively going towards the center, more layers added up. A picture explains it better:

The layer at the center of the pool was very thick (a 5 kg rock hardly managed to brake it) and was covered with a thin layer of non-frozen water. Under this thick layer there was water.

Breaking the ice in various points I noticed that the various layers were supported by a lot of small columns and walls.

In the image the column in upside down. Those formations were collocated between the layers and the rocky ground.

I thought that the layers might be formed by an alternate solidification-evaporation process during which, once the first ice layer was formed, the water lowered its level (evaporating through the escape routes of the rocky ground). Maybe this can be caused by a higher outside temperature with respect to the ground one. I'm absolutely not sure about this explanation and, even so, I cannot figure out the cause of the columns between the layers. I guess it can depends of the velocity of the freezing process. If this is not too off-topic, can someone explain the cause of such formations?

Answer 1

This happens when, for whatever reason, the water level drops while the ice is forming. What caused it to drop here, this I cannot tell without having the bigger picture. I do not think evaporation plays a significant role, as the ice layer would be fairly effective at preventing it. I have often seen this kind of structures on puddles made by rain on non-saturated ground - so that they simply slowly drain into the ground after the rain is finished. Is this pool connected through groundwater to the main lake? If so, then the variations of its water level are driven by variations of the water level of the main lake (which can be due to variations in the inflow discharge).

While the water level drops, the ice layer remains in place (because it adheres to the sides of the pool), so it sags - with the middle touching the water, but the sides suspended above the water.

The mechanism responsible for the formation of "columns" and "walls" has, in all likelihood, something to do with surface tension - i.e. water sticking to the (upside-down) ice surface, then freezing (forming a bump or ridge), then sticking to that bump or ridge, then freezing in another layer, and so on.

Answer 2

You might be seeing evidence of 'frost heave'. Moisture in the soil will, in cold conditions, both freeze at the surface (adjacent to cold air), and wick up the existing ice crystal from its (relatively warm) base near the soil, to the cooler upper tip (where the newly arrived water freezes). Thus, ice crystals can be seen to grow upward... if such a field of crystal pillars is then covered with snow, you can (after a few freeze/thaw temperature changes) get a large mass of ice in the air, supported by a few pillars of ice that crystallized days ago from soil moisture.

Usually, the top layer of such a 'frost heave' patch is dirty.

Answer 3

As you say, the water level can go down, but it can also go up (almost all lakes have a source of water).

With that in mind, you can imagine a scenario where you start with a layer of ice above water (because the level lowered), and where a second layer of ice forms on top of the liquid water. Now you have two layers separated by air. But wait... the water level can raise again, filling the air gap. When this water freezes, it gives two layers bounded by ice. If you repeat this process with water getting up and down, you can form a wall as what you describe.

I think that the columns may come from the walls breaking in smaller pieces and then getting reattached to a layer of ice when one forms again on top of the water.