The transition of new snow to glacier ice is similar to the deposition, diagenesis, and metamorphism of a sediment to form a metamorphic rock. (And in a real sense, glacier ice is a metamorphic rock.)
New dry snow has low bulk density and high porosity, and enormous internal surface area. Crushing, compaction, and exchange of water between flakes and air at low temperatures by sublimation and deposition, and at temperatures near melting by melting, refreezing, evaporation, and condensation, tend to round grains, producing the nearly spherical grains of granular snow. Except in the coldest environments, this takes a few days to a few weeks. In the process, the snow settles and gets more compact, and small grains disappear at the expense of larger ones. The recrystallized granular snow, still friable and porous, is called firn when it is more than a year old. The firn is then converted to glacier ice as compaction due to increasing weight of overlying younger snow continues. Air is forced out, decreasing the intergranular space. Recrystallization proceeds by solution at points of contact and deposition in the interspaces.
When the permeability to air has reached almost zero, the firn has become glacier ice. This change occurs in a relatively narrow density range around 0.82–0.84 g/cm3, at depths of greater than 30 m. Crystal size has increased to about 1 cm. Firn is transformed into glacier ice in one year to a few hundred years, depending on temperature and rate of accumulation. Then, depending on both temperature and bottom slope, something like 50 m of ice is needed for the ice to flow plastically under its own weight (there’s more detail on flow of glacier ice in a later section). The ultimate density of glacier ice approaches 0.90 g/cm3, a bit short of the density of 0.917 g/cm3 of pure ice because of the inevitable residual presence of little air bubbles.