THE COMPACTION OF POLAR SNOW PACKS

As snow is deposited at the surface of a pack, compaction takes place in two stages. There is an initial period of settlement where the rate of volume decrease is dominated by thermal processes, reflecting the rapid metamorphism as branched crystals break down. This is followed b y further slower densification as pores collapse, which is largely cau sed by the overburden. The conventional assumption is that of a linear ly viscous relation between the rate of decrease of volume and the pre ssure, with the viscosity depending on the density and temperature. In view of the long time scales associated with the accumulation of pola r snow, compared with observed pore collapse times, an alternative vie w is that the densification takes place instantaneously, which can be described simply by a pressure-density-temperature relation. This, of course, may depend also on the particular snow structure which is dete rmined by the deposit conditions and subsequent metamorphism. Here, we investigate the special, and much simplified, case of a pressure-dens ity law, ignoring temperature influence, to demonstrate that such a la w is consistent with the same data used to infer the viscous law. The function relating density to pressure is determined from observed dens ity profiles with depth, assuming that the snow was deposited at a fix ed constant density rho0, but no restriction on the accumulation varia tion is necessary. The model is then used to predict the pressure, den sity and velocity fields for general surface conditions of deposit den sity and accumulation rate, to show how the density and velocity field s are influenced by surface conditions for this alternative model. The density profiles with depth are confirmed to be independent of time w hen the deposit density is held constant, and independent of the accum ulation rate variation.