Structure and Physical State of Glaciers

Abstract

Within the first year, the snow flakes falling on a glacier surface are altered to sub-rounded grains, forming firn. Thereafter, in a dry snow zone, where the surface temperature remains below the melting point throughout the year, there are three steps in the metamorphic process of densification. In the first and most rapid step, the grains rearrange themselves with little change in size or shape until they are as tightly packed as they can get. At this point the maximum packing density of about 0.55 mg m⁻³ has been attained. For further densification, the process slows to modification of the grains by means of sintering—mass transfer between the grains, principally in the vapor phase, recrystallization by molecular diffusion, and plastic deformation of the grains. Grain modification continues until the permeability of the firn has been reduced to zero, which occurs at a density of about 0.83 mg m⁻³. At this point the firn has been transformed, by definition, into glacier ice. Further densification then proceeds by a still slower process—bulk compression; here squeezing leads to a reduction in the pore size. The final result is ice that contains air bubbles, but that nevertheless is able to attain a density very close to that of single-crystal ice. At pressures of the order of 10 MN m⁻², the gas is driven into the lattice of the ice and the bubbles disappear.