This is a tranquil winter morning. Snow falls silently onto the Green, onto the Occom pond, and into the hectic life of Dartmouth students. The airy and delicate snow crystals will soon undergo a series of Metamorphisms. Metamorphism is a change in the crystalline forms of the snow due to changes in exterior chemical and physical conditions [2]. Within a few days, a little brown color Snow Clearer will shovel the accumulation away and we will not have the luck to witness the processes of metamorphisms that the snow crystals undergo elsewhere.
Snow constantly moves and metamorphoses to reduce its surface free energy (the increase in entropy energy of the snow when the area of surface increases by a unit) [1]. During the initial changes in the snow structure, dry snow obtains a greater density by packing the snow grains tighter through rearrangement of its crystals. However, when the rearrangement no longer serves densification purposes, snow grains begin to form stronger bonds between one another. Eventually, snow in deeper layers are packed so close with morphed bonds that spaces in between individual grains of the snow disappear and are replaced by separate bubbles instead. When this step is completed, the snow mass has successfully turned from permeable snow to impermeable ice.
Dr. Henri Bader, a scientist from the Cold Regions Science and Engineering Laboratory, identified four types of metamorphisms of snow when it is being converted to ice [3]. Soon after the snow is deposited on a surface, melting, sublimation and surface diffusion will cause the snow to loose it original crystalline structure. The resultant aggregation consists of crystals with relatively rounder shapes and smoother angles. This process is known as the process of Destructive Metamorphism.
Snow crystals on the ground near Beaver, Oregon.
Dry snow on glacier surfaces also undergoes Constructive Metamorphism. Uprising vapor carried by convective airflow from evaporated or sublimated snow crystals will condense on crystals of a lower temperature. Coarse-grained snow (a.k.a. “depth hoar” or “Schwimmschnee”) is then formed. Coarse-grained snow comprises large grains with loose connection between one another. Constructive metamorphism increases the mass of the snow grains and thus eliminates smaller grains.
Melt Metamorphism happens in “warm” snow, a rise in temperature will result in the melting of the snow crystals. Similar to the Destructive Metamorphism, the snow grains become rounded and in addition, they are covered by a thin film of water. Water molecules accumulate at the contacts between snow grains due to surface tension. Strong adhesion between the grains thus acts as a strong bond and may cause composite grains when the snow re-encounters a low temperature. The resultant grain forms from Melt Metamorphism in “warm” snow varies with the surface, slopes and the presence of impermeable layers [2].
The last type of Metamorphism is Pressure Metamorphism. It happens when snow grains are under compression or compaction. This metamorphism happens when the snow grains first begin to rearrange the crystals to close up the empty intergrain spaces. However, a maximum density between 0.50 – 0.55 g/cm3 can be reached in this way. When pressure on the grains is still increasing, actual deformation of the grains and their bonds takes place as a result. Similar to Melt Metamorphism, the stronger inter-grain bonds will eventually seal off the connected inter-snow grain airways and produce air bubbles in the ice. At this stage, the snow mass reaches a density of 0.8-0.83 g/cm3 and a dept from 40 to 150 meters [2].
Through these four Metamorphism processes, snow slowly morphed from the fluffy accumulation of grains into a tightly bonded aggregation of snow grains and eventually to ice under high pressure and extremely cold temperature. How the snow grain structure will turn out depends on the local temperature gradient, wind speed, ventilation and slope, etc. Deep down the snow surface, snow grains are constantly but stealthily changing, moving and morphing.
References:
[1] S.C. Colbeck, Snow Metamorphism and Classification (Hanover, NH, 1986).
[2] M. Mellor, F.J. Sanger, Ed., Snow and Ice on the Earth’s Surface (Hanover, NH, US Army Material Command Cold Regions Research & Engineering Laboratory, 1964) pp. 57-58.
[3] F.J. Sanger, Ed., Snow as a Material (Hanover, NH, U.S. Army Cold Regions Research and Engineering Laboratory, 1962) Part II, Sect B.