Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 185

A glacier is a mass of relatively slow-moving ice created by the accumulation of snow.
The transformation of snow into ice is basically a continuation of the processes of snow
metamorphism already discussed. These processes are accomplished by sublimation,
melting, refreezing, and compaction of the ice grains. Sublimation, melting, and refreez-
ing are most important when the snow is still near the surface in the active layer; com-
paction becomes more important after the snow has been buried under successive an-
nual accumulations (Martini et al. 2001; Benn and Evans 2010; Barry and Gan 2011).
First, newly fallen snow turns into pea-sized melt-freeze polycrystals of ice at the end
of the season (corn snow). The corn snow then becomes irn (also called névé ) as it
survives from one year to the next. As the snow is compressed further, the air spaces
between the particles are diminished and eventually close off to become bubbles. Once
this stage is reached, the snow has become glacial ice. The difference between firn
and glacial ice is not always clearly marked, but they can usually be differentiated by
the color and density of the material. If there are air spaces between the ice crystals
and the ice has a whitish color when viewed in mass, it is firn. If the material has a
massive structure with no air spaces between the ice crystals, and a vitreous appear-
ance reflecting and transmitting a blue or greenish color (because of the absorption of
red wavelengths), it is glacial ice and has attained densities between 0.700 and 0.914
(Seligman 1936: 118; Paterson 1994; Zwally and Li 2002).
Types of Glaciers
The full classification of glaciers, which includes both alpine (mountain) and continental
types, is quite involved, and we will consider only the most basic forms of purely moun-
tain glaciers here (Martini et al. 2001; Benn and Evans 2010). Mountain or alpine glaci-
ers range from small cirque glaciers occupying isolated depressions on mountain slopes
(Fig. 4.30) to major ice caps or icefields covering all but the highest peaks.
Cirque glaciers are found at almost all latitudes, while ice-cap glaciers are typically
found in subpolar and polar areas. Intermediate between these is the valley glacier,
which heads in an accumulation basin (the cirque) and extends downvalley for some dis-
tance (e.g., the main glaciers shown in Figs. 4.32 and 4.38). Where the ice is sufficient
to flow through the valley and accumulate at the base of the mountains, it may spread
out upon reaching the flats to form a spatulate tongue. This is a piedmont glacier, good
examples being the Malaspina Glacier, Alaska, and Skeiðarárjökull, Iceland.
The various forms of alpine glaciers result from both topography and climate. A gla-
cier cannot develop if the slopes are too steep, since the snow cannot accumulate, even
if climatic conditions are favorable. At the opposite extreme, it is unlikely that a glacier
would develop on an exposed level upland of limited size, because of wind and sun ex-
posure. Topography can be viewed as the initial mold into which the snow and ice must
fit, while climate determines at what level and to what extent glaciers develop in any
given topographic situation. In the simplest terms, all that is required for a glacier to
form is for more snow to fall than melts. This may be accomplished by combinations of
various environmental factors. Consider the differences in energy flux and temperature
versus precipitation regimes in mountains at various latitudes. Midlatitude mountains
receive heavy amounts of snow, but summers are relatively warm, resulting in quick
melting and relatively rapid hydro-logic turnover within the system. By contrast, polar