school energy&environment

How ice forms

Since most of the Earth’s ice does not come from frozen pure water, the ice we see  generally consists of  ice crystals mixed with a number of impurities, including dissolved salts absorbed in the ice crystal network (like sea water salts in sea ice), fragments of debris, atmospheric dusts, fragments of rock or soil trapped among the crystals, and minute air bubbles imprisoned during the freezing phases or as the snow was transformed into ice. The study of these impurities enables us to obtain important information about the formation processes and about where the ice comes from and even about  the composition and temperature of the atmosphere at the time of ice formation.
Snow transforms: glacier ice
The formation of the ice of a glacier begins with snow deposition. Snow, with its star or hexagon shaped crystals, contains a great quantity of air, and has a very low density (this is why we sink in so easily, furthermore because of this, snow has a great capacity to absorb sounds, a snowy landscape seems strangely “silent”).
As soon as it falls to the ground, snow begins a transformation that leads to the modification of the shape and size of the crystals and a progressive decrease in the number and size of the empty spaces, increasing the density. This transformation is  known to skiers, who well know the difference there is in skiing on powdery winter snow or in spring snow that  is transformed and granular! The main factor responsible for the transformation of snow is melting, by which  single crystals are enveloped in a film of water, which melts the sharp tips  giving them a more rounded shape. The variations in the shape and the presence of water in the gaps between the crystals provoke a gradual decrease in the empty spaces between the snow grains, also favoured by the compacting  effect of the weight of the overlying layers of snow. If there is a refreezing of the melted waters, the size of the pores decreases further and the larger crystals become bigger at the expense of the smaller ones, which disappear. The transformations are very rapid when the snow  is subjected to many cycles of melting and freezing, they are slower if the temperatures remain low : in the latter case the transformations occur by effect of sublimation, a process that requires a longer time (this is why plentiful snowfall  in winter  can lead to a high risk of avalanches, as persisting low temperatures do not allow the transformation and stabilization of the snow layers). Therefore snow is transformed into a not very compact mass of rounded ice crystals called old snow, or more elegantly known as névé or Firn (German term) if it remains for more than a year. Firn is characterized by a density  over 0.54 and porosity under 40% . The transformation of snow into Firn is more rapid when there are a greater number of  freezing and thawing cycles : approximately 4 months in the Andes, one year in the Alps, 4 years in Southern Alaska, twenty years in Greenland (source: Smiraglia, 1992). The size of the grains and the density increase and the porosity decreases with age. The transformation of Firn into glacier ice takes place when the empty spaces that are present no longer intercommunicate : ice becomes impermeable  and the air that is present remains trapped in bubbles between the crystals. When the mass of ice begins to flow , the air bubbles are further compressed and the density of the ice increases up to approximately 0.91 g/cm³ (compared to 1 for water). The transformation of Firn into ice is even slower and always depends on temperatures.