Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 155

vapor diffusion and solid diffusion, forming continuous ice “necks” connecting adjacent
grains and hence producing a mechanically strong snowpack (Colbeck 1982, 1983).
KINETIC METAMORPHISM
The second process is the kinetic metamorphic process (referred to in older literature
as temperature gradient, TG, or constructive metamorphism) (Fig. 4.8). In this process,
commonly called “faceting,” the snowpack is also subfreezing (i.e., not melting) but,
unlike equilibrium metamorphism, this process is dominated by large vapor pressure
and temperature variations across sections of the snowpack (usually in a vertical dir-
ection). When temperature gradients become greater than approximately 10°C per m
(5.5° F/ft), depending on the layer temperature, snow density, and other factors, vapor
diffusion can occur (Akitaya 1974). An example of such conditions can be found in a
shallow snowpack with a warm ground interface below and a cold air interface above.
Water vapor flowing through the pores between the individual grains via this mechan-
ism leads to metamorphism. Grain bodies serve as areas of vapor deposition (i.e., the
change of state from a gas directly to a solid), while the grain contacts receive little
deposition. As a result, grains can become very large, with angular and stepped edges
growing into the direction of the vapor flow (Sturm and Benson 1997). These growth
forms are often referred to as angles or facets and can become completely three-dimen-
sional cup crystals if sufficient space is available. It is interesting to note that these
kinetic crystals are relatively strong in compressive strength (top to bottom loading),
but very weak in shear strength (sideways loading). The rate of grain growth over-
powers the sintering (bonding) effect, resulting in larger grains with fewer bonds per
unit volume and a correspondingly weaker layer (Colbeck 1982, 1983). Several sub-
types of this process occur, depending on the location and source of the vapor and
temperature gradients (i.e., rates of temperature change). Steep temperature gradients
near the ground (a common condition in cold mountains with low snowfall) can lead
to weak zones lower in the snowpack, called depth hoar (McClung and Schaerer 2006:
57); temperature gradients driven by a variety of sources in the upper snowpack result
in at least three types of near-surface faceting (Birkeland 1998), including radiation re-
crystallization (LaChapelle and Armstrong 1976). Temperature gradients immediately
above the surface can lead to surface hoar formation (Fig. 4.9). Surface hoar forms due
to atmospheric rather than snowpack processes, and its formation and persistence can
be complicated by factors such as proximity to trees or exposure to sun and wind (Lutz
and Birkeland 2011). In all cases, faceting forms weak layers of varying thickness and
location within the snowpack, a key ingredient for many avalanches.