1 December, 2000

Snow and Ice

While digging several snow pits I have become acquainted with the nature of snow at Siple Dome. Currently, the snow surface is wind packed due to a recent windstorm and the lack of new snow. The texture of snow changes as you go deeper. The surface snow is composed of very fine crystals or grains. Farther down the snow has recrystalized into larger grains. A couple meters (6.5 ft) down, the snow has a consistency like coarse, crumbly styrofoam. The snow grains are surrounded by small, interconnected air spaces. As the snow grains increase in size so do these pores.

The first time I went out cross-country skiing, I was suddenly startled by the sound of collapsing snow beneath my feet. It wasn't a hidden crevasse but rather a "snowquake." The snow did collapse but only a couple centimeters. The collapse usually travels outward five to ten meters (16 to 33 feet). I've experienced many of these events while walking or skiing around camp but they always come as a surprise.

Snowquakes result from the collapse of a porous level of snow called a hoar layer. Hoar forms when a layer of snow differs in temperature from the adjacent snow or air. This results in very large snow grains separated by large air spaces. These layers have little strength leading to their collapse when stepped on.

A. Last week's windstorm scoured and sculpted the surface snow into a variety of patterns. It reminds me of a windy beach made of fine-grained sand.

B. Jerry Bowling and I constructed a backlit snow pit. It consists of two pits separated by a thin wall of snow. The viewing pit is darkened with a covering of plywood. The structure of the snow in the wall is illuminated by the sunlight from the back side. In this image you can see four hoar layers. These layers typically form in late summer or autumn. The hoar layers at 18 and 29 centimeters both probably formed last year. The ones at 56 and 80 cm probably formed in 1998 and 1997 respectively. The floor of the pit, at 109 cm, was also a hoar layer, probably from 1996. Using these annual layers we can estimate that the yearly snow accumulation at Siple Dome is about 27 centimeters (11 in).