Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 238

the rate and depth of freezing (Fahey 1973, 1974). For example, snow is an excellent in-
sulator. Mountain surfaces covered with deep snow are relatively protected from freeze
and thaw as well as from deep frost penetration. In the Cascade Mountains of Wash-
ington, frost penetration is relatively unimportant because the landscape is covered by
a blanket of heavy snow in winter. In continental mountains, where there is less snow-
fall and temperatures are colder, the depth of freezing increases. This can pose special
problems for human settlements, as special care must be taken when building on sea-
sonally frozen ground.
Permafrost
Permafrost is defined as soil, bedrock, or any other material that has remained below
0°C continuously for two or more years (Ives 1974; Williams and Smith 1989; French
1996). This definition is based purely on thermal conditions and, in the strict sense of
the word, a glacier is considered a form of permafrost, since its temperatures are con-
tinually at or below freezing. Usually, however, the term permafrost is used to describe
the thermal state of the ground. Permafrost usually is superimposed by an active lay-
er, a layer that experiences seasonal freezing and thawing. The thickness of the active
layer depends on many variables, the most important of which are grain size, moisture
content, and temperature.
Approximately 25 percent of the Earth's surface is underlain by arctic, alpine, and
high-plateau permafrost (French 1996), including almost 80 percent of Alaska. Arctic
permafrost has been traditionally grouped into continuous and discontinuous zones, al-
though Ives (1974) argued for retention of the sporadic category. Continuous perma-
frost exists in the coldest areas; it is typically found in regions closer to the poles and
in continental interiors, such as the central Keewatin Region, Northwest Territories,
Canada. In Canada, the southern limit of continuous permafrost coincides with the −6
to −8°C mean annual air temperature (MAAT) isotherm, and discontinuous permafrost
ends at the −1°C MAAT isotherm. Discontinuous permafrost often contains a more ex-
tensive network of taliks, or unfrozen sections linked to some local variable such as
the presence of a former lake. Mountain permafrost can be categorized into continu-
ous permafrost, found at the highest elevations, but below the firn line; discontinuous
permafrost; and sporadic permafrost (Ives and Fahey 1971; Harris 1988; French 1996).
Mountain permafrost is much more complex than arctic permafrost. Slope, aspect, el-
evation, vegetation cover, soil type, and snow distribution all play important roles in the
formation of mountain permafrost. There is little precise information about permafrost
distribution in the contiguous United States (Ives and Fahey 1971; Péwé 1983), though
100,000 km 2 of mountain permafrost could exist, with the lower limit extending as low
as 2,500 m in Washington and as high as 3,500 m in Arizona.
The presence of permafrost may adversely affect engineering projects such as min-
ing, road building, well drilling, and the installation of structures such as ski lifts, power
lines, or cell phone communication towers. Special measures must be taken to maintain
the thermal balance of frozen ground. For instance, in order to minimize heave, roads
are often built on a well-drained, coarse layer that is underlain with an insulating ma-
terial such as peat. If these conditions are not met, the road will quickly buckle under
the force of periglacial processes. If ground ice is present, it may pose further prob-