Have you heard about firn? No? Well, then maybe you don’t know as much about snow as you thought you do. At least that’s how it was for me before my research taught me more about glaciers.
The transition from snow to ice is easy. It all starts with the fall of single snowflakes in an environment cold enough for it not to melt – the first step in the formation of glacier ice. Once the snow settled it is exposed to changes and the little crystals of the fresh powder snow soon transforms to a different material due to climatic conditions.
Here we are, you just learned what firn represents.
Snow is actually called firn, once the fallen snow has changed and before it becomes dense glacier ice. Consequently, firn represents the intermediate product of the transformation of snow to ice.
How snow changes into ice and the time it takes depends on temperature and precipitation and can take up to millennia. With a change in temperature and/or precipitation, either as snow or as rain, the crystals start to deform, with the first stage of deformation classified as grain settling and packing [Herron&Langway, 1980]. Due to the settling and packing of the crystals alone, the snow gets compacted and its density changes. During that first stage the initial density of snow, which is around 300 kg/m3, changes to the critical density of about 550 km/m3. That process is further influenced by temperature and overburden pressure, induced by new snow-layers supporting the compression of previous grains.
Once the snow grains have reached a critical density a slower process of compaction starts, that results in embedded air being pressed out of the layer until a density of around 830 kg/m3 is reached. At this density the pore close-off depth is reached and still existing bubbles are trapped in the ice. Thereafter, these air bubbles undergo continuing compression until the density of 917 kg/m3 within the ice is obtained.
As climate plays an important role in the transformation, different mechanisms are found in different areas and even within one single glacier.
In Antarctica alone, there are different stages as the central part of the continent has temperatures well below the freezing point throughout the year, while coastal areas and the Antarctic Peninsular are more temperate with melting and refreezing periods. Percolating melt water plays an important role, as it influences the transformation of snow grains with a change in temperature (melt water is warmer than snow/firn) and by altering the snow structure.
Therefore, the transition from snow to ice happens much faster in temperate regions than in dry and cold areas.
The Vallée Blanche glacier in the European Alps, for example, shows the firn-ice transition at a depth of 32 m, corresponding to an age of 13 years, while Site-A in Greenland shows the transition in a depth of around 80 m with an age of 185 years. Observations at Byrd in West Antarctica shows the transition zone in a depth of 64 m with an age of 280 year, at the South Pole in 115 m depth after around 1020 years and in Vostok at a depth of 95 m after 2500 years.
Well, that’s all very interesting, but why is it important?
As mentioned in the basics article about satellite missions the compaction of snow influences the satellite altimetry measurements by changing the ice height without loosing actual mass.
The idea behind understanding the densification of firn is to develop a firn compaction model that can be applied to the satellite observations to subtract the firn compaction component. As you can imagine this task is rather difficult as so many elements affect a mechanism that is barely understood.
However, there have been models established to estimate firn densification for glaciers, which show good agreement with firn core observations [e.g. Arthern et al. 2010, Ligtenberg et al. 2011]. The idea behind these models is to estimate density profiles for glaciers in the most accurate way possible. To calculate the density, knowledge about grain growth of snow grains, the amount of pressure from overburden layers and the temperature gradient in the snow and firn is needed.
More on that topic that can be read in detail in the book “The physics of glaciers” by Cuffey and Paterson, 2012.