On the morning of December 7 of 2008, I saw a small yet distinct white patch on the ground amongst the dirty brown crunchy soil-ice columns in a rice field. If I hadn’t been looking straight at it, I would have missed it. Crouching down and clearing away some of the surrounding dirt-ice columns, I found it to be a white ice column resting on a small pebble, unlike the surrounding brown columns that rested on the soil. You can see this pebble and ice at the upper left in the image below.

The white ice “cap” detached easily and cleanly from the pebble (See the above image, upper right). I could put the cap back on the pebble, and it would stay in place, fitting snugly.

Looking around the field of ice, I spotted another white cap, though not as tall as the first one. On other mornings with frozen ground, I would visit the same field, sometimes finding several white caps and sometimes finding none. The longest columns were about an inch and a half (3-4 cm) and relatively rare; more common were just white glazes on pebble tops. I could see that this ice was not hoar frost – it was too thick and dense – and closely resembled the dirty ice that pushes dirt, plant debris, and pebbles up off the ground. Sometimes, the ice grew off the pebble at a sharp angle, like the top two cases in the image below. The view is from above for the pebble at the upper right, all others are side views.

I immediately started to wonder how the water built up on top of a rock. There had been neither rain nor snow on the previous night, so the water must have come from underground, as it does for the dirtier columns.

Before addressing the new observation, let's discuss how the more common dirty columns form. I understand that the water crystallizes slightly above the freezing level in the ground; above this level, the ground is below 0 degrees C (32 F), being cooled from above, and below this level, the wet ground exceeds 0 degrees. During the night, the freezing level will start at the ground surface (which is not ‘level’ in the sense of being flat and horizontal) and slowly works its way deeper underground. This latter motion of the freezing level may be the reason that dirt and rocks get trapped in the ice, but otherwise is unrelated to the upward growth of the ice. Though I am not clear on exactly how this growth happens, I know that it has been simulated in the laboratory by substituting a thin membrane filter for the soil. In those experiments, water can flow through the pores of the filter from the liquid below, but ice cannot grow through the pores from above, at least until the temperature becomes cold enough. So, for a period of time, water flows through the membrane and freezes, and as more water flows through, more ice gets pushed up. This phenomenon has nothing to do with the expansion of water upon freezing – it is purely an ice-growth and water-flow phenomenon. In the soil, the spaces between the grains act as the pores in a filter. A more recent idea is that the boundary between ice and any other material (including air, stone, and dirt grain) has a thin region in which the ice molecules can move around in a somewhat mobile state between that of ice and liquid water. I’m not clear on how this mobile water layer influences the formation of ice columns, but a lot of articles have been written on the topic, and if I spent the time, I could probably understand it.

When I first saw the thick ice on these pebbles, I first thought of the recent idea of a mobile water layer. The pebble, no matter how clean, would have a thin coating of water before any ice formed on it. After some of the water film on the pebble’s top froze, there would remain a mobile layer of water separating the ice from the stone. More water would flow along the interface from below to replace the water that had frozen, and this would push the ice up. I showed some pictures to Charles Knight at NCAR, he told me that he hadn’t seen the phenomenon himself, but in the same year another person had related a similar observation to him. He wondered if these pebbles were porous and suggested that I collect them.

This season, I had to look elsewhere for these pebbles, as the farmer covered up my previous spot with rice husks. The bottom two cases in the image above shows pebbles from two new places. On the left, ice on a large, soft, red rock in dirt, and on the right, a few rocks from a sandy creek bed. So it appears that the white ice-on-a-rock can form in a range of places and on a range of rock types. I still don’t understand how the ice gets there, but just yesterday morning, I saw ice that seemed to have come from water within a rock. As I was walking back from a morning icespotting outing, I saw something strange on the walkway.

The ice in the above rock looks like the ‘sap crystals’ that grow out of woody plant tissue – very fine white curly ice hairs. When I scraped some of them off, I saw that the ice seemed to be loosely attached to the flat rock surface.

This appears to be ice that formed from water that came through pores in the rock, as Charlie had suggested. This ice is finer and less dense than the previous white columns, so I wonder if the thick columns formed by the flow of mobile interfacial water between ice and stone, and the finer, less dense ice in the above image formed by water flowing within the rock.

- JN