Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 154

FIGURE 4.6 (A-E) Various classification schemes for new snow include eight to ten of the basic crys-
tal types shown above. The newest ICSI classification for solid precipitation considers those crys-
tals shown in panel E (spatial dendrites and capped columns) to be subtypes of more basic forms.
(Images courtesy of L. R. Dexter and K. Birkeland.)
EQUILIBRIUM METAMORPHISM
The first process is equilibrium metamorphism (referred to in older literature as equi-
temperature, ET, or destructive metamorphism) (Fig. 4.7). This process, commonly
called “rounding,” occurs when the snowpack is subfreezing (i.e., not melting) and free
of large vapor pressure and temperature variations. When these conditions are met,
grain geometry (crystal shape) and pressure contact between adjacent grains controls
the metamorphism. Points of grains are locations of higher vapor pressure, while grain
declivities are locations of lower vapor pressure. A vapor flow is set up that transfers
mass, molecule by molecule, from the tips of the grains to the branch junctions, leading,
in time, to a spherical form often referred to as rounded grains or rounds. Where grains
are in contact in these conditions, sintering (i.e., bonding) can take place through both